HK1134864B - Controlling inductive power transfer systems - Google Patents

Description

Controlling inductive power transfer systems

The application is a divisional application, the original application of which is the application number 200580015312.9, the application date is 5/11/2005, and the name of the invention is 'control induction power transmission system'.

The present invention relates to a controlled inductive power transfer system and method for powering, for example, portable electrical or electronic devices.

This application claims priority from co-pending applications GB0410503.7 filed on 11/2004 and GB 0502775.0 filed on 10/2005 by the applicant and the entire contents of each are incorporated herein by reference.

An inductive power transfer system suitable for powering portable devices may comprise two parts:

● primary unit having at least one primary coil through which the primary unit drives an alternating current to generate a time varying magnetic flux.

● A secondary device, separable from the primary unit, includes a secondary coil. When the secondary coil is placed in proximity to the time-varying magnetic flux generated by the primary coil, the varying magnetic flux induces an alternating current in the secondary coil, and thus power can be inductively transferred from the primary unit to the secondary device.

Typically, the secondary device provides the transmitted power to the external load, and the secondary device may be loaded into or carried by a host object (host object) that includes the load. For example, the host object may be a portable electrical or electronic device having a rechargeable battery or battery. In this case, the load may be a battery charger circuit for charging a battery or batteries. Alternatively, the secondary device may be integrated into such a rechargeable battery or accumulator, together with a suitable accumulator charger circuit.

One such inductive power transfer system is described in our british patent publication GB-a-2388716. A significant feature of such systems is the physical "openness" of the magnetic system of the primary unit-a substantial portion of the magnetic circuit passes through the air. This is essential in order to enable the primary unit to provide power for secondary devices of different shapes and sizes and for a plurality of secondary devices simultaneously. Another example of such an "open" system is described in GB-a-2389720.

There may be problems with such systems. The first problem is that the primary unit will not be 100% efficient. For example, even when there is no secondary device currently present, or there is no secondary device currently requiring charging, switching loss in the electronic apparatus and I in the primary coil²The R losses still consume power. This wastes energy. Preferably, in this case, the primary unit should enter a "standby mode" with low power consumption.

A second problem in such systems is the inability to mechanically prevent foreign objects from being placed in proximity to the primary coil, coupling with the coil. Foreign objects made of metal will induce eddy currents therein. These eddy currents tend to act to repel magnetic flux, but because the material has a resistance, the flowing eddy currents will create I²R losses, which will cause the object to heat up. There are two special cases where heating is significant:

● if the resistance of either metal is high, for example if it is impure or very thin.

● if the material is ferromagnetic, for example steel. This material has a high magnetic permeability, excites a high magnetic flux density in the material, induces large eddy currents and thus large I²And R is lost.

In the present application, such foreign objects causing power consumption are referred to as "parasitic loads". Preferably, the primary unit should enter an "off mode" when there is a parasitic load to avoid heating it up.

Various approaches to solving these two problems have been proposed in the prior art.

A solution to the first problem of not wasting power when no secondary device needs to be charged includes:

● in EP0533247 and US 6118249, the secondary device adjusts its inductive load during charging, resulting in a corresponding change in the power drawn from the primary unit. This indicates that the primary unit should not enter a standby state.

● in EP1022840, the primary unit changes the frequency of its driver, thereby changing the coupling coefficient with the tuned secondary unit. If the secondary unit does not acquire power, there is no difference in the acquired power while sweeping, so the primary unit enters a standby state.

● in US5536979 the primary unit only measures the power flowing in the primary coil and enters a pulsed standby state if it is below a threshold.

● in US5896278 the primary unit contains a detection coil which couples power back into the detection coil depending on the position of the secondary device. If the secondary device is not present, the primary unit enters a standby mode.

● in US5952814, the secondary device has a mechanical protrusion that fits into a slot in the primary unit, activating it.

● in US6028413, a primary unit drives two coils and has a corresponding two power receiving secondary coils in a secondary unit. The primary unit measures the power delivered from each primary coil and enters a standby mode if it is below a threshold.

A solution to the second problem of parasitic loads includes:

● as mentioned above, in EP1022840 the primary unit changes the frequency of its driver. In this system, the secondary device is tuned so that this frequency change will result in a change in the power drawn from the primary unit. If the load is replaced by a piece of metal, changing the frequency will not have the same effect and the primary unit will go into the off state.

● as described above, in US5952814 a key in the secondary device activates the primary unit. It is assumed that if a secondary device is present, this will physically repel any foreign object.

● as described above, in US6028413 a primary unit powers a secondary device by driving two primary coils. If the amount of power provided by the two coils is different, the primary unit assumes that the load is not a valid secondary device and enters a shut down mode.

These methods all assume a 1: 1 relationship between the primary unit and the secondary device. These approaches are therefore inadequate for systems such as those described in GB-a-2388716 where more than one secondary device may be present at the same time. For example, these methods will not work when there are two secondary devices, one of which requires charging and the other does not.

Some of these approaches also assume that the physical or electrical presence of a valid secondary device means that all foreign objects are physically repelled by the secondary device. This need not be the case, particularly when the secondary unit is disposable with respect to the primary unit, as described in GB-a-2388716.

According to a first aspect of the present invention there is provided a method of controlling inductive power transfer in an inductive power transfer system, wherein the inductive power transfer system comprises: a primary unit operable to generate an electromagnetic field; and at least one secondary device separable from the primary unit and adapted to couple with the field when the secondary device is proximate to the primary unit such that the secondary device can inductively receive power from the primary unit without requiring direct conductive contact with each other, the method comprising: setting the or each secondary device to an unloaded state in which any power inductively received by the secondary device is substantially prevented from being supplied to its actual load; and in the primary unit, measuring power drawn from the primary unit when the or each secondary device is set to said unloaded state, and limiting or stopping the supply of inductive power from the primary unit in dependence on the measured power.

Since the secondary device is set to an idle state during power measurement, it can be easily detected from the measured power if there is a considerable parasitic load. If so, the primary unit may enter an off mode. For example, if the measured power is greater than a threshold value in the unloaded state, the power supply will be limited or stopped.

This approach is convenient because the secondary device does not have to communicate its power requirements to the primary unit, or the primary unit does not have to perform any summation of the power requirements: it is known that since the secondary devices are in an unloaded state, their total power requirement is zero or at least a small value, which is induced by any parasitic load imposed on the primary unit by the secondary devices themselves.

According to a second aspect of the present invention there is provided a method of controlling inductive power transfer in an inductive power transfer system, the inductive power transfer system comprising: a primary unit operable to generate an electromagnetic field; and at least one secondary device separable from the primary unit and adapted to couple with the field when the secondary device is proximate to the primary unit such that power can be inductively received from the primary unit by the secondary device without requiring direct conductive contact with each other, the method comprising: receiving, in the primary unit, information relating to the power requirements of the secondary device concerned from the or each secondary device in a power demand state; and in the primary unit, measuring power drawn from the primary unit when supplying power to the or each secondary device having a power demand state, and limiting or stopping inductive power transfer from the primary unit in dependence on the measured power and the received power demand information.

In this case, the inductive power supply from the primary unit may be limited or stopped according to a difference between the measured power and a sum of respective power demands of the secondary devices having the power demand state. For example, the inductive power supply may be limited or stopped in case the measured power exceeds the sum more than a threshold value.

This approach has the advantage over the method of the first aspect that the secondary device does not have to be set to an idle state during power measurement. Thus, power may be continuously provided to the actual load of the secondary device.

Of course, in the method of the first aspect, the power measurement period may be very short, such that any interruption of the power supply to the load is not noticeable. If the interruption of the load is an issue, an energy storage device, such as a capacitor, may be provided in the secondary device to maintain the power supply to the actual load during the power measurement period.

In the method of the second aspect, the power demand information may be transmitted from each secondary device to the primary unit using any suitable communication method. A preferred method for the or each secondary device to transmit its power requirement information to the primary unit is the RFID method. Alternatively, the or each secondary device may transmit its power demand information to the primary unit by varying the load imposed on the primary unit by the secondary device.

It will be appreciated that the methods embodying the first and second aspects of the present invention provide different ways of detecting whether there is a substantial difference between the power drawn from the primary unit on the one hand and the power demanded by the secondary devices on the other hand, or, if there is more than one secondary device, the total power demanded by the secondary devices. After this detection, the inductive power supply from the primary unit may be limited or stopped.

In the method of the first and second aspects, the load imposed on the primary unit by the secondary device may be varied to pass signals or information from the secondary device to the primary unit. For example, the power requirement information required in the second aspect may be so transmitted.

An advantage of using load changes to communicate is that it may allow two or more, or possibly all, secondary devices to provide various items of information to the primary unit at the same time. For example, if any secondary device requires power, its load may be changed. If the primary unit detects that the total load is not or substantially not changed, it can be concluded that no secondary device requires power, and thus enters a standby mode. Similarly, the primary unit will detect the sum of any load changes. If the load variation from each individual secondary device is proportional to some analog quantity to be transferred to the primary unit (e.g. the power demand or parasitic load of the secondary device), then in the power measurement the sum of the individual analog quantities will be detected by the primary unit. This means that the sum can be obtained directly without additional processing in the primary unit which would be time consuming and/or costly to perform.

In addition to detecting when to enter the off mode, it may also be desirable to detect a condition for entering the standby mode. For example, in the method of the first aspect, the inductive power supply may be limited or stopped in case the measured power is less than a standby threshold (different from the shut-down threshold described above). Another possibility is for the or each said secondary device to report status information to the primary unit indicating whether the secondary device is in a no power demand state in which the actual load of the secondary device currently does not require power from the primary unit or a power demand state in which the actual load currently requires power from the primary unit. The primary unit then limits or stops the inductive power supply from the primary unit in dependence on the status information reported by the or each secondary device. For example, the primary unit may limit or stop the transmission of inductive power unless the status information reported by the at least one secondary device indicates that it is in the power demand state. Preferably, in order to respond to speed, two or more secondary devices report their respective status information to the primary unit simultaneously. As mentioned above, one convenient possibility is for the or each secondary device to report its said status information by varying the load imposed by it on the primary unit.

In general, two or more measurements of the power drawn from the primary unit may be performed in different measurement periods. If the secondary devices are synchronized with the primary unit, the secondary devices will behave differently from each other during a measurement cycle to enable the primary unit to detect two or more different conditions (where power limitation or cessation is appropriate).

A preferred embodiment has three measurement cycles. During the first cycle, each secondary device disconnects a dummy load. In the second cycle, each secondary device requiring power turns on its dummy load. The other secondary devices disconnect their dummy loads. In the third cycle, each secondary device turns on its dummy load. The primary unit can detect from the comparison of the power measurements in the three cycles whether there is a considerable parasitic load that needs to be switched off or no device that needs power, to stand by properly.

The load may also be varied during the measurement period. For example, the magnitude of the load change may be fixed, but the duration may be varied to provide information.

The primary unit may have registered a power requirement of at least one of said secondary devices. In this case, the power demand information transmitted from the secondary device may be only the information identifying the secondary device. The primary unit uses the identification information to retrieve the registered power requirements of the device. The identification information may be a code or type, model number, or serial number assigned to the secondary device. This can reduce the amount of information transmitted to the primary unit and improve response speed and reliability.

Although each secondary device in the power receiving state transmits power demand information to the primary unit in the method of the second aspect, the or each said secondary device not in the power demand state may also transmit such power demand information to the primary unit if desired. One possibility is for the power demand information transmitted by the or each said secondary device not in said power demand state to be indicative of a parasitic load imposed on the primary unit by the secondary device. This can then be used to make the shut down detection more reliable. The power demand information may also be a sum of the power demands of the actual load and the parasitic load of the secondary device when in the power demand state, and the power demand information may be only the power demand of the parasitic load when the secondary device is not in the power demand state.

In general, detection of conditions that limit and stop the supply of inductive power ideally takes into account any losses in the primary unit and the secondary device. There are a number of ways to achieve this.

One approach is to use first compensation information about the losses of the primary unit itself when performing said detection, to compensate for said losses. Some or all of the first compensation information may be obtained from measurements taken by the primary unit when the primary unit is effectively in electromagnetic shielding. The first compensation information may be stored in a calibration unit of the primary unit.

Another approach is to use second compensation information about the parasitic load imposed on the primary unit by the or each secondary device when performing said detection, to compensate for said parasitic load of the or each secondary device. The or each said secondary device preferably communicates its said second compensation information directly to the primary unit, or communicates other information to the primary unit from which it derives said second compensation information. As mentioned above, the secondary device may communicate its said second compensation information or its said other information to the primary unit by varying the load imposed by it on the primary unit.

A particularly convenient and effective way is for the or each said secondary device to have a dummy load representative of its said parasitic load which the secondary device imposes on the primary unit to vary the load imposed on the primary unit by the secondary device.

Part or all of the first compensation information and/or part or all of the second compensation information may be information stored in the primary unit during manufacture and/or testing of the primary unit.

It may be advantageous to change one or both of said first and second compensation information when one or more operating conditions (e.g. temperature) of the primary unit change. The secondary device may be used alone or in combination with another object. For example, the secondary device may be removed from the host object. If power can be supplied when it is removed from or installed into the host object, the parasitic load of the device itself may be very different from the parasitic load common to the device and the host object. To deal with this, the second compensation information may be changed depending on whether the apparatus is used alone or together with another object.

In many embodiments, the secondary device needs to operate synchronously with the primary unit. Preferably therefore, a synchronisation signal is transmitted from the primary unit to the or each said secondary device, or from the or each said secondary device to the primary unit, to synchronise the operation of the primary unit and the or each said secondary device. This is conveniently achieved by modulating the drive signal applied to the primary coil in the primary unit. Frequency, amplitude, or phase modulation, or a combination thereof, may be used.

Many different techniques may be utilized to measure the power drawn by the secondary device from the primary unit. In one technique, an electromagnetic field is generated by a primary coil driven by an electric drive unit, and electric power for the drive unit is supplied from a power supply of the primary unit to a power input terminal of the drive unit. The power drawn from the primary unit may be measured by temporarily disconnecting the power supply and detecting a change at the power input during disconnection. The change may be a voltage decay. The advantage of this technique is that there is no series resistance through which the current of the drive unit flows. Such series resistance consumes a considerable amount of power.

Preferably, energy is stored in an energy storage unit, such as a capacitor connected to the power input, so that power can be continuously provided to the power input when the power supply is disconnected.

Another method of measuring the power drawn is also available if the electric drive unit has a feedback circuit for controlling the driver current or power to the primary coil. In this case, the feedback signal in the feedback circuit may provide a measure of the power drawn without the need to incorporate a power measurement unit at all.

Another method of measuring power includes operating a circuit including the primary coil in a non-driving resonant (undriven resonance) condition during a measurement period in which application of a drive signal to the primary coil is suspended such that energy stored in the circuit decays during the period. One or more measurements of this energy decay are then made during the period and these measurements are used to measure the power drawn from the primary unit.

Two or more power measurements may be made under the same conditions and the results averaged to improve accuracy.

In operation, it may be desirable to set the field strength of the electromagnetic field to a value that is lower than the maximum value in the run mode. In the method of the second aspect, the primary unit has power demand information from each secondary device, so that the field strength can be easily set according to the power demanded by the secondary device or (if there is more than one secondary device) the total power demanded by the secondary device. In this way, a minimum power output for powering the secondary device can be found. However, there are other ways to achieve similar results. For example, a secondary device that does not obtain sufficient power may modulate its load in some manner. The primary unit may start operating at maximum power and reduce power until load modulation from the at least one secondary device is detected. This allows the minimum power to be determined in a simple and fast manner.

According to a third aspect of the present invention there is provided an inductive power transfer system comprising: a primary unit operable to generate an electromagnetic field; at least one secondary device separable from the primary unit and adapted to couple with said field when the secondary device is in proximity to the primary unit such that the secondary device can inductively receive power from the primary unit without direct conductive contact with each other; means for detecting whether there is a substantial difference between the power drawn from the primary unit on the one hand and the power demanded by the secondary devices on the other hand, or, if there is more than one secondary device, the total power demanded by the secondary devices; and means operable, following such detection, to limit or stop the supply of inductive power from the primary unit.

According to a fourth aspect of the present invention there is provided a primary unit for an inductive power transfer system further having at least one secondary device separable from the primary unit, the primary unit comprising: means for generating an electromagnetic field coupled with the at least one secondary device when the secondary device is proximate to the primary unit such that the secondary device can inductively receive power from the primary unit without direct conductive contact with each other; means for detecting whether there is a substantial difference between the power drawn from the primary unit on the one hand and the power required by the secondary devices on the other hand, or, if there is more than one secondary device, the total power required by the secondary devices; and means operable, following such detection, to limit or stop the supply of inductive power from the primary unit.

According to a fifth aspect of the present invention, there is provided a secondary device for an inductive power transfer system comprising a primary unit which generates an electromagnetic field, the secondary device comprising: a secondary coil adapted to couple with said field generated by the primary unit when the secondary device is in proximity to the primary unit such that the secondary device can inductively receive power from the primary unit without direct conductive contact with each other; load connection means connected to said secondary coil and adapted to connect the secondary device, when in use, to a load requiring power from the primary unit for providing such inductively received power to the load; detection means for detecting the synchronization signal transmitted by the primary unit; and control means responsive to detection of the synchronisation signal for setting the secondary means to an unloaded state in which any inductively received power is substantially prevented from being supplied to the load by the load connection means.

This may provide a secondary apparatus suitable for use in carrying out the method of the first aspect of the invention described above.

According to a sixth aspect of the present invention, there is provided a secondary device for use in an inductive power transfer system comprising a primary unit which generates an electromagnetic field, comprising: a secondary coil adapted to couple with said field generated by the primary unit when the secondary device is in proximity to the primary unit such that the secondary device can inductively receive power from the primary unit without direct conductive contact with each other; load connection means connected to said secondary coil and adapted to be connected to a load requiring power from the primary unit when the secondary means is in use, for providing such inductively received power to the load; and an RFID communication device operable to provide information regarding the power requirements of the secondary device to the primary unit using an RFID communication method.

This may provide a secondary device suitable for use in carrying out the method of the second aspect of the invention described above. In this case, the load connection device does not disconnect the actual load during the power measurement.

According to a seventh aspect of the present invention there is provided a method of controlling inductive power transfer in an inductive power transfer system, wherein the inductive power transfer system comprises: a primary unit operable to generate an electromagnetic field; and at least one secondary device separable from the primary unit and adapted to couple with the field when the secondary device is proximate to the primary unit such that the secondary device can inductively receive power from the primary unit without direct conductive contact with each other, the method comprising: in the information providing phase, allowing two or more secondary devices to simultaneously provide the primary unit with information respectively about the secondary devices concerned; and interpreting the information provided at the same time at the primary unit, and determining whether to limit or stop the supply of the inductive power from the primary unit based on the interpreted information.

Such an approach may allow information or signals to be quickly provided from the secondary device to enable a limitation or stop of the power supply to be quickly achieved.

In one embodiment, the information provided by each secondary device indicates whether the secondary device concerned is in a power demand state (in which the actual load of the secondary device requires power from the primary unit), and the primary unit determines whether the inductive power provided by it should be limited or stopped, unless the information provided by at least one secondary device indicates that it has said power receiving state during the information providing phase.

The information provided by each secondary device may represent an analog quantity of the associated secondary device. In this case, the primary unit can directly derive the sum of the respective analog quantities of the secondary devices from the simultaneously provided information.

The analog quantity may represent a parasitic load imposed on the primary unit by the secondary device itself.

The analog quantity may represent the power demand of the actual load of the secondary device.

The analog quantity may represent the total load imposed on the primary unit by the secondary device, including the actual load of the secondary device and the parasitic load imposed on the primary unit by the secondary device itself.

In one embodiment, each said secondary device provides its said information by varying the load imposed by it on the primary unit. For example, during the information provision phase, each of the secondary devices may have a dummy load which the secondary device selectively imposes on the primary unit. Preferably, the dummy load represents the analog quantity. Different dummy loads may be used to represent different analog quantities, such as power requirements and parasitic loads.

In one embodiment, each of said secondary devices has its said information providing phase at a time determined by the primary unit.

According to an eighth aspect of the present invention there is provided a method of controlling inductive power transfer in an inductive power transfer system, wherein the inductive power transfer system comprises: a primary unit operable to generate an electromagnetic field; and at least one secondary device separable from the primary unit and adapted to couple with the field when the secondary device is in proximity to the primary unit such that the secondary device can inductively receive power from the primary unit without direct conductive contact with each other, wherein the method comprises the steps of: in a reporting phase, the or each said secondary device reports information to the primary unit, wherein the information indicates whether the secondary device is in a no power demand state (in which the actual load of the secondary device currently does not require power from the primary unit) or a power demand state (in which the actual load currently requires power from the primary unit); and the primary unit determining from information reported by the or each secondary device during the reporting phase that its inductive power supply should be limited or stopped.

Preferably, at the time determined by the primary unit, the or each said secondary device has its said reporting phase.

In one embodiment, there are at least two secondary devices, and each of the secondary devices has its reporting phase simultaneously.

The or each said secondary device may report said information thereof by varying the load imposed by it on the primary unit. For example, during the reporting phase, the or each secondary device may have a dummy load which the secondary device selectively imposes on the primary unit.

In one embodiment, during said reporting phase, the or each said secondary device having said power demand state imposes said dummy load thereof, and during said reporting phase, the or each said secondary device having said no power demand state does not impose said dummy load thereof.

According to a ninth aspect of the present invention, there is provided a secondary device for use in an inductive power transfer system comprising a primary unit which generates an electromagnetic field, the secondary device comprising: a secondary coil adapted to couple with said field generated by the primary unit when the secondary device is in proximity to the primary unit such that the secondary device can inductively receive power from the primary unit without direct conductive contact with each other; load connection means connected to said secondary coil and adapted to be connected to a load requiring power of the primary unit when the secondary device is in use, for providing such inductively received power to the load; and communication means operable to communicate to the primary unit information about a parasitic load imposed on the primary unit by the secondary device.

Such a secondary device may transfer its parasitic load to the primary unit for use by the primary unit to compensate for the load. For example, when detecting a condition that limits or stops inductive power transfer from the primary unit, the parasitic load of the transfer may be used.

Any communication method may be used, and the method is not limited to load variation. For example, infrared or ultrasonic communication may be used. RFID may also be used.

In one embodiment, the communication device is operable to communicate the information by imposing a dummy load on the primary unit. The communication means may be operable to impose a first dummy load on the primary unit at a first time and to impose a second dummy load, different to said first dummy load, on the primary unit at a second time, the difference between said first and second dummy loads being set in dependence on said dummy loads. One of the first and second dummy loads may be zero.

According to a tenth aspect of the present invention, there is provided a portable electrical or electronic device comprising: a load that at least sometimes requires power from the primary unit; and a secondary device embodying the fifth, sixth, or ninth aspect of the invention as hereinbefore described, the load connection means of the secondary device being connected to the load for supplying such inductively received power to the load at the said time.

According to an eleventh aspect of the present invention there is provided a method of controlling inductive power transfer in an inductive power transfer system, wherein the inductive power transfer system comprises: a primary unit having a primary coil to which an electric drive signal is applied to generate an electromagnetic field; and further comprising at least one secondary device, separable from the primary unit, and having a secondary coil adapted to couple with said field when the secondary device is in proximity to the primary unit, such that power can be inductively transferred from the primary unit to the secondary device without direct conductive contact with each other, the method comprising: operating a circuit comprising said primary coil in a non-driven resonant condition during a measurement period, in which condition application of said drive signal to said primary coil is suspended so that energy stored in the circuit is attenuated during said period; during said period, making one or more measurements of such energy decay; and limiting or stopping inductive power transfer from the primary unit based on the one or more energy attenuation measurements.

This approach may enable either or both of parasitic load and standby detection in a reliable and cost-effective manner. This is particularly advantageous in systems where the open magnetic nature of the system and/or the multiple secondary devices can make it easier for the parasite to couple to the primary coil.

Reference will now be made, by way of example, to the accompanying drawings, in which:

FIG. 1 is a block diagram showing portions of an inductive power transfer system embodying the present invention;

FIG. 2 is a flow chart illustrating a first method of detecting a shutdown condition in accordance with the present invention;

fig. 3 is a flowchart for explaining a first method of detecting a standby condition according to the present invention;

FIG. 4 is a block diagram illustrating portions of an inductive power transfer system in accordance with a first embodiment of the present invention;

FIG. 5 shows waveform diagrams useful in explaining the operation of the system of FIG. 4;

FIG. 6 shows a waveform diagram showing the timing of various signals in the system of FIG. 4: fig. 6(a) shows the frequency of the AC voltage signal applied to the primary coil; fig. 6(b) shows the power drawn from the primary unit; fig. 6(c) shows the state of the switches in the primary unit; and figure 6(d) shows the voltage across the primary unit switch;

FIG. 7 is a schematic diagram showing the loads acquired during three different measurement operations;

FIG. 8 is a schematic diagram illustrating different modes of operation in the system of FIG. 4;

fig. 9 is a block diagram showing a part of a primary unit in a power transmission system according to a second embodiment of the present invention;

FIG. 10 shows how the current flowing through the primary coil varies in the normal, buffered and attenuated states that occur during power measurement in the system of FIG. 9;

FIG. 11 is a flow chart for explaining a second method of detecting a shutdown condition in accordance with the present invention; and

fig. 12 is a block diagram showing a part of a power transmission system according to a third embodiment of the present invention.

Fig. 1 shows a portion of an inductive power transfer system embodying the present invention. The system 1 comprises a primary unit 10 and at least one secondary device 30. The primary unit 10 has a primary coil 12, and an electric drive unit 14 connected to the primary coil 12 for providing an electric drive signal to the primary coil 12 to generate an electromagnetic field. The control unit 16 is connected to the electric drive unit 14. The control unit generates an AC voltage signal 106. The electric drive unit 14 takes the AC voltage signal 106 and converts it to an AC current signal in the primary coil 12 to generate an induced electromagnetic field in the vicinity of the primary coil 12.

The primary unit 10 may have any suitable form, but one preferred form is a flat platform with a power transmission surface on or near which each secondary device 30 may be placed. In this case, the field may be distributed over the power transmission region of the surface, as described in GB-a-2388716.

The secondary device 30 may be separate from the primary unit 10 and have a secondary coil 32, the secondary coil 32 being coupled to the electromagnetic field generated by the primary unit 10 when the secondary device 30 is in proximity to the primary unit 10. In this manner, power may be inductively transferred from the primary unit 10 to the secondary device 30 without requiring direct conductive contact with each other.

The primary coil 12 and the secondary coil 32 may have any suitable form, but may be, for example, copper wire wound around a high permeability former (e.g., ferrite or amorphous metal).

The secondary device 30 is typically connected to an external load (not shown-an actual load that may also be referred to herein as a secondary device) and provides inductively received power to the external load. The secondary device 30 may be enclosed in or carried by an object requiring power (e.g., a portable electrical or electronic device or a rechargeable battery or battery). More information about the design of the secondary device 30 and the objects that can be powered using the secondary device 30 can be found in GB-a-2388716.

The primary unit 10 in the system of fig. 1 also comprises a power measurement unit 100 connected to the control unit 16. Upon receiving the signal provided by the control unit 16, the power measurement unit 100 measures the power drawn by the electric drive unit 14. The power measurement unit 100 provides an output to the control unit 16 that is representative of the power drawn by the electric drive unit 14. The power drawn by the electric drive unit 14 represents the power drawn by the primary coil 12 and therefore also the power drawn by all secondary devices 30 plus other losses.

In the system of fig. 1, it is desirable to detect certain conditions and limit or stop the inductive power supply from the primary unit under those conditions.

One such condition is that there is a substantial parasitic load in the vicinity of the primary unit. In this case, the control unit 16 may enter a shutdown mode in which the driving of the primary coil 12 is reduced or stopped to prevent the parasitic load from heating up.

Another such condition is that there is no secondary device 30 of the system in the vicinity of the primary unit 10. Another such condition is that there is at least one secondary device 30 but none has a load that currently requires power. For example, the load does not require power when turned off or when the rechargeable battery or cells are fully charged. Under both conditions, the control unit 16 may enter a standby mode in which the driving of the primary coil 12 is reduced or stopped, preventing unnecessary power consumption in the primary unit 10.

Fig. 2 is a flow chart for explaining a first method of detecting the presence of a substantial parasitic load in the vicinity of a primary unit according to the present invention.

In this first approach, all secondary devices in the vicinity of the primary unit are sometimes intentionally set to an unloaded state when using the system of fig. 1. In this unloaded state, any power inductively received by the secondary device is prevented from being supplied to its actual load (the external load described above).

In step S2, in the case where all the secondary devices are in the unloaded state, the power measurement unit 100 in the primary unit measures the power that the secondary devices acquire from the primary unit. In step S3, the control unit 16 in the primary unit determines whether to limit or stop the inductive power supply from the primary unit according to the power measured in step S2.

In the simplest case, in step S3, the control unit 16 simply compares the measured power with a predetermined shut-off threshold. If the measured power exceeds the shut-down threshold, the control unit 16 determines that the inductive power supply from the primary unit should be limited or stopped. However, as described in more detail below, it is preferable to consider losses that inevitably occur in a power transmission system. In particular, these losses include losses present in the primary unit itself and/or any secondary device/host object. These losses include inefficiencies of the primary coil itself and any other components associated with the primary coil (e.g., the electrical drive unit), such as I in the effective series resistance of the copper of the coil or any tuning capacitor²And R is lost. Losses also include any magnetic losses in the primary unit and secondary device, e.g. magnetic losses as compared to the primary unitHysteresis losses in any of the coils associated with the element and/or secondary device. Thus, the control unit 16 may use first compensation information about the losses of the primary unit itself, in addition to the measured power, to compensate for those losses in step S3. Alternatively, or in addition, the control unit 16 may use second compensation information regarding the parasitic load imposed on the primary unit by the or each secondary device in addition to the measured power to compensate for the parasitic load of the or each secondary device in step S3.

If it is determined in step S3 that the power supply should be limited or stopped, the control unit 16 sets the primary unit to an off mode in which the inductive power supply from the primary unit is limited or stopped in step S4.

The primary unit will remain in the off mode until it is reset in some way. Such a reset may be initiated manually by the user of the primary unit, or alternatively, the control unit 16 may be periodically activated to again provide inductive power, and steps S1 through S3 repeated to determine whether to maintain the off mode.

In step S3, if the control unit 16 determines that it is not necessary to limit or stop the supply of electric power, the secondary device that needs power re-receives power from the primary unit in step S6. Then, after a predetermined interval, for example, the process returns to step S1 again.

Next, a first method of detecting a condition for entering the standby state will be described with reference to fig. 3.

In fig. 3, from time to time, each secondary device (if any) present in the vicinity of the primary unit 10 has a reporting phase, as shown in step S11. All the existing secondary devices may enter the reporting phase at the same time. Alternatively, each secondary device may individually enter the reporting phase in turn. In either case, in the reporting phase, each secondary device reports status information to the primary unit indicating whether the secondary device is in a no power demand state or a power demand state. In the no power demand state, the actual load of the secondary device currently does not require power from the primary unit. On the other hand, in a power demand state, the actual load does currently require power from the primary unit.

In step S12, the control unit 16 in the primary unit determines whether the supply of electric power from the primary unit should be limited or stopped, according to the status information reported in step S11. Specifically, unless at least one secondary device reports to the primary unit that it is in a power demand state in a reporting phase, the control unit 16 determines that the supply of the inductive power should be limited or stopped, and the process proceeds to step S13, and the primary unit is set to a standby mode in step S13. Of course, if no secondary device is present in the vicinity of the primary unit at all, so that no or no valid status information is received by the primary unit in step S11, the control unit 16 also sets the primary unit to standby mode.

Once the primary unit is set to the standby mode, it may be reset to the run mode again either manually by user intervention or automatically, as described above with respect to step S5 of the method of fig. 2.

If, in step S12, the control unit 16 determines not to limit or stop the inductive power supply based on the reported status information, the process returns to step S11, for example, after a predetermined interval. In this way, each secondary device that is present periodically has a reporting phase that reports its status information to the primary unit.

The methods of fig. 2 and 3 may be performed independently of each other. Preferably, however, the control unit 16 of the primary unit is capable of detecting both when to enter the off mode and when to enter the standby mode. This may be achieved by combining the methods of fig. 2 and 3, which will now be described with reference to fig. 4.

Fig. 4 shows a part of an inductive power transfer system according to a first embodiment of the invention. The system 1 has a primary unit 10 and a secondary device 30. Fig. 4 also shows a parasitic load 500 on the primary unit caused, for example, by a foreign object placed near the primary unit 10. In this case, it is assumed that the secondary device 30 is loaded into or carried by a host object (e.g., a portable electric or electronic device). As described above, the secondary device 30 and/or the host subject also inevitably imposes a "favorable" parasitic load 501 on the primary unit 10.

As previously described with reference to fig. 1, the primary unit 10 includes a primary coil 12, an electric drive unit 14, a control unit 16, and a power measurement unit 100. The electric drive unit 14 has an input connected to an output of the control unit 16 providing an AC voltage signal 106. An output node of the electric drive unit 14 is connected to the primary coil 12. The electric drive unit is connected to a power supply 105 via a power measuring unit 100. The power supply 105 provides direct current to the electric drive unit 14. The electric drive unit 14 presents a high input impedance to the AC voltage signal 106 such that substantially all of the load current is drawn from the power supply 105.

In this embodiment, the control unit 16 is a microprocessor. The microprocessor has an embedded digital-to-analog converter (not shown) to drive an output that provides the AC voltage signal 106. Alternatively, the control unit 16 and some or all of the other circuit elements of the primary unit may be implemented using an ASIC.

The control unit 16 in this embodiment is adapted to modulate the AC voltage signal 106 for transmitting the synchronization signal to the secondary device. The modulation is a frequency modulation of the AC voltage signal. Other modulation techniques such as amplitude or phase modulation may also be used. The control unit 16 is adapted to send a synchronization signal to any of the secondary devices 30 present. The secondary devices 30 change their load conditions in response to the synchronization signal. This information is used to detect conditions for entering the off and standby modes.

It is desirable that the power measurement unit 100 can operate without disconnecting the power supply to the primary coil 12, since this means that the power supply to the secondary device 30 is not interrupted, thereby reducing stray electromagnetic interference into the surrounding environment. This is challenging because of the large amount of noise present and the requirement to make measurements in a short time.

The power measurement unit 100 includes a switch 102 between the 0V supply terminal of the power source 105 and the ground terminal of the electric drive unit 14. The switch 102 is controlled by the control unit 16. The power measuring unit further comprises a capacitor 101 connected between the positive pole of the electric drive unit 14 and ground. The capacitor functions as a power storage unit. A differential amplifier 103 has an input on each side of the switch 102 and has an output connected to an analog-to-digital converter 104. The output of the analog/digital converter is connected to the control unit 16.

When switch 102 is closed, power measurement unit 100 is not operating and power is coupled directly to electric drive unit 14 by power supply 105. When the switch 102 is open, a power measurement is performed. Capacitor 101 is now disconnected from the 0V rail (rail) on power supply 105, but still retains its charge. At the same time, the electric drive unit 14 continues to draw current, thus discharging the capacitor 101. Thus, the voltage across the capacitor 101 is slightly attenuated, and the voltage at the point between the capacitor 101 and the switch 102 slightly rises to be higher than 0V. The reservoir capacitor 107 ensures that the positive supply voltage remains constant. The differential amplifier 103 measures the voltage across the switch 102, and the resulting measurement is converted into a digital signal by the analog/digital converter 104 and transmitted to the control unit 16. A small temporary voltage drop across the electric drive unit 14 does not have any significant effect on the power transferred to the secondary device 30.

As shown in FIG. 5, when the switch 102 is open, at time t₁And t₂Two measurement results are obtained respectively and are set as measurement V respectively₁And V₂. After the switch is opened, there is a delay t₁To stabilize transient effects. Then, the power P is obtained by:

wherein, V⁺Is the supply voltage, let V₁，V₂，＜＜V⁺. Advantageously, the supply voltage is sampled at the same point in the cycle to remove periodic disturbances in the voltage (also shown in fig. 5). The switch 102 is then closed again, reconnecting the power supply 105 to the electric drive unit 14.

Incidentally, an inductor may be used as the energy storage unit instead of the capacitor 101. In this case, the change measured by the circuit during the power-off may be a change in current, for example measured as a voltage drop across a series resistor.

In this embodiment, the primary unit 10 further comprises a calibration unit 29. The calibration power supply 29 stores compensation information about losses (e.g., electrical or magnetic losses) in the primary unit. Deliberately, losses in the primary unit may be calibrated and stored in the calibration unit 29 at the time of manufacture, and/or periodically thereafter. The calibration unit 29 provides the stored information to the control unit 16 so that the control unit 16 subtracts the losses from the total measurement result, thereby calculating a loss value caused only by the parasitic load. The calibration unit 29 may vary the compensation information to cope with variable losses in the primary unit, e.g. losses of temperature variations.

The secondary device 30 includes a secondary coil 32, a rectifier 34, a secondary control unit 36, a dummy load switch 38, a dummy load 40, a load switch 42, a storage unit 44, and an actual load 46. For example, each of the dummy load switch 38 and the load switch 42 may be a FET. The dummy load 40 is, for example, a resistor. In the present embodiment, the memory cell 44 is a capacitor, but an inductor may be used instead.

In this embodiment, the real load 46 is located outside of the secondary device 30 and is part of the host object. Which may be a battery charge controller for a lithium ion battery.

There is also a detection unit 200 for detecting the modulation imposed on the received AC signal. To detect the frequency modulated signal, the detection unit 200 may be a zero crossing detector, the detection unit 200 delivering a signal to the control unit each time the AC signal crosses zero volts. Thus, the control unit 36 may include an internal clock and counting circuitry (not shown). The clock and counter circuit may be used to measure the time interval between successive zero crossings, thereby deriving the frequency of the AC signal 106 imposed by the primary unit control unit 16. Thus, the secondary unit can detect frequency changes and respond by adjusting switches 42 and 38 to change its load condition.

Other forms of load detection circuit 200 may include a threshold detector for digital amplitude modulation or an analog-to-digital converter for multi-level amplitude modulation, or a phase detector for phase modulation, or any combination thereof.

Now, the operation of the system will be described.

In the "run mode" of the system, the host object integrated with the secondary device 30 is placed on the primary unit 10 or in the vicinity of the primary unit 10. Switch 102 is closed. The control unit 16 applies an AC voltage signal 106 to the electric drive unit 14. The electric drive unit 14 takes DC power from the power source 105, amplifies the AC voltage signal 106, and applies it to the primary coil 12.

In the operating mode, the primary coil 12 generates an electromagnetic field in the vicinity of the primary unit 10. The secondary coil 32 is coupled to the field and induces an alternating current in the coil by the field. Dummy load switch 38 is open and load switch 42 is closed. The rectifier 34 rectifies the alternating current induced in the secondary coil 32 and supplies the rectified current to the storage unit 44 and the actual load 46 through the load switch 42. In this way, power is inductively transferred from the primary unit 10 to the secondary device 30, and from the secondary device 30 to the load 46. In the run mode, the storage unit 44 stores power.

When in the run mode, the control unit 16 in the primary unit 10 initiates measurements from time to time. The measurement begins as the primary unit 10 sends a synchronization signal to the secondary device 30 by applying a momentary frequency change to the AC drive voltage signal 106. The secondary devices 30 receive the AC voltage signal and in each receiving secondary device the detection unit 200 together with the control unit 36 determines when a synchronization signal is present. In response to the synchronization signal, the secondary units momentarily change their load conditions for a set period of time, and the primary unit 10 measures the total load (drawn power) during that period of time.

During normal operation, the secondary device 30 uses the storage unit 44 to store energy from the primary unit 10. During the measurement, the actual load 46 is disconnected by opening the switch 42. The energy stored in the storage unit 44 of the secondary device decays gradually as it is transferred to the load. Assuming that the storage unit has sufficient capacity and is fully charged before the measurement begins, the storage unit may transfer continuous energy to the secondary device throughout the measurement so that the actual load 46 is not interrupted.

In this embodiment, the primary unit 10 initiates a series of three power measurements for the following purposes: 1) determining whether there is currently a parasitic metal that requires the primary unit to enter an off mode to prevent overheating, and 2) determining whether there is no device that requires any power so that the unit can enter a standby mode. The operating conditions of the primary unit 10 and the secondary device 30 are slightly different in each of a series of three measurements.

During the first measurement, the secondary control unit 36 opens the dummy load switch 38 so that the dummy load 40 is not connected to the secondary coil 32. Thus, the first measurement is a measurement of the power of any parasitic load 500 passing to foreign objects in the vicinity of the primary unit and the power of any parasitic load 501 imposed by the losses of the secondary device and/or its host object and any losses of the primary unit itself. Therefore, steps S1 to S3 corresponding to fig. 2 described above are executed during the first measurement.

During the second measurement, the secondary control unit 36 selectively closes the dummy load switch 38. The secondary control unit 36 determines whether to open or close the dummy load switch 38 during the second measurement based on the power demand of the actual load 46. If the load 46 does not now require any power, for example because it has a rechargeable battery that is currently fully charged, the dummy load switch 38 remains open during the second measurement. On the other hand, if the load 46 does now require power, the dummy load switch 38 is closed to connect the dummy load 40 to the primary coil 32.

During the second measurement, the control unit 16 causes another measurement of the real load (power load). If the second power measurement differs considerably from the first power measurement, the control unit 16 detects that a secondary device requiring power is currently located in the vicinity of the primary unit. Therefore, the operation during the second measurement corresponds to steps S11 and S12 of fig. 3 described above.

During the third measurement, the secondary control unit 36 always closes the dummy switch 38 to connect the dummy load 40 to the secondary coil 32.

Another power measurement is made in the primary unit by the control unit 16. In this case, the measurement result is the sum of the parasitic load 500, the parasitic load 501 of the secondary device and/or the host object, the primary unit loss, and the dummy load 40. Based on the difference between the first and third power measurements, the control unit calculates the values of all the dummy loads 40 present in all the secondary devices in the vicinity of the primary unit.

The timing of the various signals and measurements is shown diagrammatically in fig. 6 (not to scale). Fig. 6(a) shows the driving frequency applied to the primary coil 12; fig. 6(b) shows the load introduced by the secondary device 30; fig. 6(c) shows the state of the switch 102 in the primary unit 10; and fig. 6(d) shows the voltage across the switch 102.

For the first, second and third measurements, at the start of each measurement, the primary unit 10 first instantaneously changes the drive frequency 510, 511, 512 to the primary coil, respectively. Each secondary device 30 then isolates its actual load 513, 514, 515 and introduces a dummy load 514, 515 as the case may be. Within this time frame, the switches 102 in the primary unit are opened 516, 517, 518. In the window in which the switch is open, the voltage across the switch 102 ramps up 519, 520, 521. The voltage is sampled at several points in the window to measure power. In a first measurement, there is no dummy load 513; in a second measurement, each device is only connected to dummy load 514 if its actual load requires power; in a third measurement, a dummy load 515 is always connected.

The secondary device 30 distinguishes which of these measurements is by the order in which the measurements occur. If there is a gap of a few ms long since the last synchronization signal, the secondary device knows that it must be the first measurement. This may be determined by the secondary device counting the number of cycles of the received alternating current. The second and third measurement synchronization signals naturally follow in this order within a set number of cycles. To obtain more accurate measurements, each measurement may be averaged over multiple sequences.

Each dummy load 40 in the secondary device 30 in the system of this embodiment is set to a particular value (either at manufacture or during calibration or testing) such that this value represents the parasitic load 501 imposed by the secondary device in question and/or by its host subject.

Thus, the total dummy load of all present secondary devices, calculated by the control unit 16, may be used by the control unit 16 as second compensation information to compensate for the parasitic load 501 of the present secondary devices. For example, if the control unit 16 detects that there is a substantial parasitic load 500 in the vicinity of the primary unit when the measured power exceeds a certain threshold, the threshold may be increased by an amount based on the total parasitic load 501 of all the secondary devices present, so that the detection of the parasitic load 500 from a foreign object is not affected by the number of secondary devices present.

Figure 7 diagrammatically shows the load obtained by three measurements. The load obtained is the sum of the following losses: losses 543 associated with the primary coil in the primary unit (pad), parasitic loads 542 associated with external metal objects, metal 'friendly parasitics' 541 associated with the host object (portable device) to be powered, and current loads 540 associated with all secondary devices. The first measurement 530 includes all of these components (components) except the load 540. If no device requires power, the second measurement result 531 will be the same as the first measurement result 530 and the primary unit can be set to standby mode (S4 in fig. 3). However, if at least one device requires power, the second measurement 531 will be greater than the first measurement 530 and power is required. In the third measurement result, each secondary device 30 is connected to its dummy load. The dummy load 40 of each device is made equal to the 'friendly parasitics' of the device. By subtracting the first measurement from the third measurement, the result is a 'friendly parasitics' 541. The primary unit losses 543 are known (and stored in the calibration unit 29). To get a measurement of the total parasitic load 542 present, the calculated 'friendly parasitic' 541 and the known primary cell loss 543 may be subtracted from the first measurement 530. If the number exceeds a certain threshold, the unit may be set to the off mode (step S4 in FIG. 2).

A system embodying the invention is capable of measuring the load imposed on the primary unit sensitively (e.g. in the order of 50 mW). With this sensitivity, it can be ensured that very little power is coupled to the parasitic load 500, such as a foreign object.

Fig. 8 is a schematic diagram showing different modes of operation in the system of fig. 4 and conditions for switching between these different modes. The three operating modes are an operating mode, an off mode, and a standby mode.

In the run mode, the primary unit is in the normal state (driving condition) most of the time, but periodically performs a sequence of three measurements as described above. If no secondary device requires power as a result of the measurement sequence, the primary unit enters a standby mode. If as a result of the measurement sequence a large parasitic load 500 is present, the primary unit enters the off mode.

In the standby mode, the electric drive unit 14 is stopped most of the time, and therefore consumes little power. The primary unit periodically enters the normal mode and then performs a series of measurements in each detection period to check whether it should enter the run mode or the off mode. Otherwise, the standby mode is maintained.

The off mode is functionally the same as the standby mode. However, the two modes can be distinguished by some user interface feature, such as an LED, to prompt the user to remove any substantial parasitic load 500.

In addition to the first embodiment of the invention, there are many other possible embodiments and combinations of features that may be applied to advantage.

For example, as described in GB-a-2398176, there are other inductive power transfer systems which, instead of having a single primary coil 12, have multiple coils. In such a system, there may be two sets of coils arranged orthogonal to each other. Each of them can be driven with the same AC voltage signal, but in quadrature (i.e. 90 ° out of phase) so that the induced magnetic field rotates over time. This allows the secondary device 30 to be placed in any direction and still be able to receive power. The invention can be applied directly to such a structure. The electric drive unit 14 supplies AC current drive not only to the first coil but also to the second coil. The transmitted synchronization signal will appear on both coils. Furthermore, because the current measurement is performed by determining the current drawn from the power supply, the power measurement will be the sum of all the loads drawn, regardless of the proportion drawn by each coil. In such a 2-track (channel) rotation system, the orientation of the secondary device 30 is arbitrary. The secondary device 30 will therefore have a phase difference of +/-180 deg. with respect to the primary unit. Therefore, each secondary device 30 must lift (lift) its load on each side of the primary unit's measurement cycle for at least 1/2 cycles.

In addition to the three measurements described, a fourth measurement may be taken. This measurement is initiated by the control unit 16 in the primary unit 10 and causes the power measurement unit 100 to perform a power measurement, but no synchronization signal is sent to the primary coil 12. The secondary devices 30 do not change their load conditions, and therefore this is a power measurement while in an operational state. This measurement may be made at any time and need not be made during the measurement sequence of the first third measurement. This fourth measurement is used to determine whether the total load obtained is greater than the power specification (specification) of the device, thus setting the primary unit to an 'overload state'. The 'overload state' is functionally equivalent to the 'off state', but can be discerned by some user interface feature such as an LED.

Another possibility is to adapt the electric drive unit 14 to vary the intensity of the current it outputs into the primary coil in order to vary the field strength of the generated magnetic field. This will allow the field strength to be reduced for small loads, thereby conserving electrical power. This feature is implemented in a different way using the first and second measurements, not only to detect whether the device requires power, but also to set the required field strength. Instead of accessing the dummy load 40 if power is needed during the second measurement, the secondary device may access its dummy load if not enough power is available. The primary unit then regards the difference between the first and second measurements as a "power-insufficient" signal. The primary unit 10 may periodically increase the field to a maximum intensity and then gradually decrease it until the difference between the first and second measurements is greater than a certain threshold ("power-insufficient" signal). In this way, the primary unit will always operate at the lowest possible field strength.

In another embodiment, the secondary devices are adapted to dynamically change the value of their dummy load. This can be achieved, for example, by integrating a load whose value can be varied by the control device. A simple example is a resistor ladder (resistor ladder) with an array of switches, which can be set by a binary increment value. The load may be adapted to have a continuously variable value by using a transistor circuit or by integrating some other non-linear element. Another way to dynamically change the load is to adjust the switch 38 connected to the load so that the effective load is changed when the power measurements are averaged over the measurement time interval. The pulse width or duty cycle may be varied to change the payload value.

The ability to dynamically change the dummy load is useful for devices whose 'friendly parasitic' load may vary. For example, when a self-charging battery is charged alone, there may be a different 'friendly parasitic' load than when it is connected to a mobile phone for charging. The control unit 36 may detect whether a telephone is connected and adjust the dummy load accordingly. Alternatively, the phone may transfer its 'friendly parasitic' load to the battery. Other removable accessories that contribute to additional 'friendly parasitic' loads may also be detected to adjust the dummy load. This includes, for example, but is not limited to, removable camera accessories, a housing, and a speaker.

In addition to providing load demand information as well as parasitic information about the secondary device 30, this approach may be used to enable the primary unit 10 to infer other information about the secondary device 30. For example, the primary unit 10 may receive information regarding serial number, model number, power requirements, or other information stored in the secondary device. This can be achieved by dynamically changing the load synchronously or asynchronously. Amplitude modulation or pulse width modulation may be used. Multiple ' bits ' or ' symbols ' (where ' symbol ' represents multiple amplitude levels or pulse width durations, and thus is greater than one ') may be used.

In another embodiment, the primary unit 10 is able to transmit information to the secondary device 30, rather than a synchronization signal, by modulating the AC voltage signal 106 applied to the electric drive unit. This information may include, but is not limited to: information about the primary unit 10, such as charging cost, power capacity, encoding; information about the location of the primary unit, e.g., nearby equipment; and other information, such as advertising material. The secondary device 30 may receive such information through the detection element 200 and the control unit 36.

It will be apparent to those skilled in the art that not all of these features need be implemented at the same time to achieve advantage. For example, by using only the first and second measurements, a standby detection feature may be implemented. Similarly, by using only the first and third measurements, a parasitic detection feature may be implemented. By using only the fourth measurement, the overload detection feature can be implemented. Information about the secondary device 30 may be derived by the primary device without having to implement other features. Similarly, information may be transmitted from the primary unit to the secondary device without implementing other features. Additional features may be implemented using additional measurements. It should be understood that each measurement is labeled for identification purposes only and may be performed in any order.

In addition to the described method of transmitting a synchronization signal before each measurement and identifying each measurement in the order in which it occurs, there are other methods of identifying each measurement. These methods include, but are not limited to: sending a different synchronization signal before each measurement, whereby the synchronization signals may differ in frequency offset, amplitude, or phase; or only the first synchronization signal is sent and the timing of the other measurements is derived by a counter or internal clock in each secondary device counting the received signal periods. It is even possible to perform several measurements one after the other with substantially no gaps in between. Optionally, the measurement is initiated by the secondary device rather than by the primary unit. The secondary device may initiate 'preamble' dynamic load modulation, which the primary unit will detect and then synchronize to bring its power measurement into line with the timing of the secondary device adapting to its load condition. In order that the primary unit may provide power to more than one secondary device at the same time, the 'preamble' may include some unique identifier used so that each secondary device may be interrogated independently. The 'preamble' may also be used for communication from the primary unit to the secondary device to address each device individually.

As mentioned above, the dummy load may be used to represent a 'friendly parasitic' load of the host device. Of course, the ratio between the dummy load value and the friendly parasitic load to be transmitted is not limited to any particular value. For example, the dummy load may be two or three times or a non-integer multiple of the value of the 'friendly parasitic' load. As long as the ratio is known, the primary unit can derive all 'friendly parasitic loads'. Furthermore, if the device does not have any significant 'friendly parasitic' load, it may be desirable to 'assign' a special value to it so that it can be used to indicate whether the device needs to be charged. Ideally, more than one dummy load may be used. The first dummy load may be used for the second measurement and the second dummy load may be used for the third measurement. The first dummy load will be used for standby detection and the second dummy load will represent 'friendly parasitics'. This is particularly advantageous if the secondary device has a large varying parasitic load. The first dummy load may also be used to determine the power requirements of the secondary device that needs to be charged, rather than just making a standby decision. The dummy load value will be adapted to represent the power requirements of the particular device. The first and second dummy loads may be implemented by the above-described dynamically variable dummy loads alone, or using fixed loads, or a combination of both.

In addition to the power measurement methods and apparatus described above, it should be understood that there are many methods that may be used to detect the load on the primary coil or coils. The simplest power measurement may involve inserting a series resistance on one power supply rail (supply rail). The voltage across the resistor can be measured and the power derived from the observed voltage and the known resistance value. By this method it is desirable to incorporate a switch across the resistor so that during periods other than the measurement time, the resistor can be short circuited so that there is no unnecessary power dissipation in the resistor.

Another power measurement method is to measure the power in the electric drive unit. For example, the electrical drive to the coil or coils is ideally regulated by a feedback circuit. The feedback signal may be used to derive a power measurement.

The functions of transmitting the synchronization signal and power measurement may also be combined in a single element, as described in, for example, co-pending application GB0410503.7 (which the present invention claims priority) filed by the applicant on 11/5/2004. In this system, power measurement involves disconnecting the power supply to the primary coil and detecting the attenuation in the undriven resonant circuit. The act of disconnecting the power to the primary coil 12 also has the effect of modulating the signal in the primary coil, thereby receiving the signal in the secondary device 30.

Fig. 9 shows a second embodiment of a power transfer system according to the invention. The main difference between this embodiment and the first embodiment of fig. 4 is the method of performing power detection. The primary unit 110 includes a primary coil 112, an electric drive unit 114, a control unit 116, and an attenuation measurement unit 118. The electric drive unit 114 in this embodiment has a conventional half-bridge configuration, wherein a first switch 120 is connected between a first power supply line of the primary unit and an output node of the electric drive unit, and a second switch 121 is connected between the output node and a second power supply line of the primary unit. For example, the first and second switches 120 and 121 may be Field Effect Transistors (FETs).

The electric drive unit 114 also includes a drive controller 119 that applies control signals to the switches 121 and 122 to turn them on or off. The drive controller 119 has a control input connected to an output of the control unit 116. An output node of the electric drive unit 144 is connected to one side of the primary coil 112 through a capacitor 117.

In this embodiment, the control unit 116 is a microprocessor. Alternatively, the control unit 116 may be implemented using an ASIC, as well as some or all of the other circuit elements of the primary unit.

The attenuation measuring unit 118 includes a resistor 125 having a first node connected to one side of the switch 128 and a second node connected to a second power supply line. Resistor 125 is a low value resistor. The attenuation measurement unit 118 further comprises an operational amplifier 126 having an input connected to a first node of the resistor 125. The attenuation measurement unit 118 further includes an analog-to-digital converter (ADC)127 connected to the output of the operational amplifier 126. The output of the ADC127 is connected to the measurement input of the control unit 116.

The other side of the switch 128 is connected to the other side of the primary coil 112. The buffer unit 122 is connected in parallel with the switch 128. The buffer unit 122 includes a capacitor 123 and a resistor 124 connected in series with each other. The calibration unit 129 is identical to the calibration unit 29 in fig. 4.

Each secondary device in the present embodiment is substantially the same as the secondary device 30 in fig. 4, and thus a description thereof is omitted here and the secondary device is not shown in fig. 9.

The operation of the system of fig. 9 will now be described with reference to fig. 10.

Initially, the system is in a normal state in which the control unit 116 causes the electric drive unit 114 to provide a drive signal to the primary coil 112 to cause it to oscillate. It will be appreciated that in the run mode, the system is in this state almost all the time. The switch 128 is closed and the circuit comprising the capacitor 117 and the primary coil 112 forms a resonant tank.

The next state is the "buffered" state. Application of the drive signal to the primary coil 112 by the electric drive unit 114 is suspended under the control of the control unit 116. Drive controller 119 closes switch 121. The control unit 116 also opens the switch 128 when most of the energy in the resonant tank remains in the capacitor 117. The opening of the switch 128 puts the damping unit 122 in series with the resonant tank. The buffer unit 122 quickly dissipates all the energy remaining in the primary coil 112, stopping its resonance around one cycle. Most of the energy stored in the resonant tank is stored in the capacitor 117. The sudden stop of the cycle is detected by the detection unit 200 and the secondary control unit 36 in the secondary device 30. The secondary control unit 36 opens the load switch 42. Incidentally, it should be understood that the detection unit 200 in fig. 4 needs to be modified to detect an abrupt stop of the cycle in the buffer state in the present embodiment. A threshold detector (as described above) may be used as the detection unit in the present embodiment.

Thus, in this embodiment, the buffer status is used as a synchronization signal to the secondary device, although other forms (e.g., frequency or phase modulation) may also be used. As mentioned above, it is not always necessary to have a synchronization signal before each measurement.

The system then enters the decay state from the buffer state. The control unit 116 closes the switch 128 and removes the damping unit 122 from the resonant tank, so that the energy in the capacitor 117 flows into the resonant tank again. In the damping state, the resonant tank operates at an undriven resonant condition. The energy stored in the resonant tank decays during the decay state. In the present embodiment, the attenuation measuring unit 118 measures the attenuation of energy in the resonant tank by measuring the current flowing through the primary coil 112. The same current flows through resistor 125 and produces a voltage at the first node of the resistor. This voltage is buffered by operational amplifier 126 and converted to a digital signal by ADC 127. The generated digital signal is supplied to a measurement input of the control unit 116.

Fig. 10 shows how the current flowing through the primary coil 112 varies in the normal, buffer and decay states that occur during power measurement. In this embodiment, a digital signal representing the current flowing in the primary coil during the measurement period is received and processed in the control unit 16 to calculate a measurement of the rate of power decay in the resonant tank.

At resonance, the equation describing the energy stored in the resonant tank is:

where E is the energy, L is the inductance,is the peak current, C is the capacitance, andis the peak voltage.

Thus, if the inductance and peak current or if the capacitance and peak voltage are known, or a combination thereof, the energy stored in the resonant tank of the primary unit at any given moment can be calculated. The capacitance is generally known, the peak current and voltage can be measured by suitable circuitry, and the inductance can be derived by observing the natural resonant frequency during the measurement and applying the following formula:

power ofThe measured value P is given by the decay rate of the energy (and thus the loss) from the resonant tank and can be determined by the decay rate at time T₁Measurement E₁And at another time T₂Measurement E₂And (4) calculating.

Since at resonance the voltage and current in the resonant tank will be 90 degrees out of phase with each other, a convenient way to read one peak voltage is to trigger a measurement at the other zero crossing.

A second method for detecting a shut down condition according to the present invention will be described with reference to fig. 11. The method may be used in the system of fig. 1.

When using the system of fig. 1, each secondary device in a power demand state provides information to the primary unit about its own power demand from time to time. The power demand information may take many different forms. For example, the information may include a binary portion to represent "no power demand" or "power demand". In this case, where the binary portion is "power demand," supplemental information may be provided by the secondary device to represent the amount of power demand. Alternatively, the power demand information may simply represent the amount of power demand, and a "0" may be transmitted if the device does not need power at all. It is also possible that the primary unit already knows the power requirements of the secondary device. For example, it is known that all secondary devices of a particular type will have a particular power requirement. In this case, the power demand information may simply be a code (or some other identifying information) indicating the type of secondary device.

All secondary devices may provide power demand information to the primary unit simultaneously. Optionally, each secondary device individually provides its power demand information to the primary unit in turn.

The primary unit receives power demand information provided by each secondary device having a power demand state.

In step S22, the control unit 16 in the primary unit causes the power measurement unit 100 to measure the power acquired by the secondary device from the primary unit, as described previously. In practice, the measured power will also reflect all losses in the system.

In step S23, the control unit 16 determines whether the inductive power supply from the primary unit should be limited or stopped based on the power measured in step S22 and the power demand information received in step S21. For example, the control unit 16 calculates the sum of the individual power demands of all the secondary devices in the power demand state. This sum is compared with the measured power obtained at step S22. If the measured power exceeds the sum of the power requirements by more than a threshold, the control unit determines that there must be a substantial parasitic load in the vicinity of the primary unit. In this case, the process proceeds to step S24, where the primary unit enters an off mode and the inductive power supply from the primary unit is limited or stopped. As previously described with respect to the method of fig. 2, in step S25, the system may be reset manually or automatically.

If, in step S23, the control unit 16 determines that it is not necessary to limit or stop the supply of electric power, the process returns to step S21, for example, after a predetermined time interval.

To compensate for losses in the primary unit and/or the secondary device, the shutdown threshold used in step S23 may be adjusted. One way this can be achieved is that each secondary device (whether or not in a power demand state) also provides information to the primary unit about its "friendly parasitic" load. Similarly, the losses in the primary unit may be calculated using a calibration unit, as described with reference to fig. 4.

Fig. 12 shows part of a power transmission system according to a third embodiment of the invention. The system implements the closure detection method of fig. 11 using an RFID communication method.

The system of fig. 12 includes a plurality of secondary devices 600₁、600₂、...、600_n. The system of fig. 12 also includes a primary unit 700. The primary unit 700 includes an RFID unit 710, a control unit 720, and a power measurement unit 730. The control unit 720 generally corresponds to the control unit 16 described above with reference to fig. 1, and the power measurement unit 730 generally corresponds to the power measurement unit 100 described with reference to fig. 1.

The features of the secondary device 600 are generally the same as those of the secondary device 30 of fig. 4, except that the elements 38, 40, 42, 44, and 200 may be omitted. Instead of these elements, each secondary device 600 includes its own load measuring unit 610 and RFID unit 620. The load measurement unit 610 measures power supplied to an actual load (46 in fig. 4) of the secondary device. For example, the load measurement unit 610 may measure the current and/or voltage provided to the actual load 46 and may accumulate these measurements over time for averaging, e.g., the averaging period may be ten seconds.

The RFID unit 620 in each secondary device may communicate with the RFID unit 710 in the primary unit 700 using an RFID link 630. The load measurements generated by the load measurement unit 610 in each secondary unit are provided to the RFID unit 620 in the device and then transmitted to the RFID unit 710 in the primary unit via the corresponding RFID link 630. For example, the RFID unit 710 may poll the RFID unit 610 in each secondary device from time to time. In response, the RFID unit 620 that has been polled transmits its load measurement. The load measurement corresponds to the power demand information of step S21 in fig. 11.

As in step S22 in fig. 11, the power measurement unit 730 in the primary unit also measures the power acquired by the secondary device from the primary unit. Then, the control unit 720 determines whether the power supply to the secondary device should be limited or stopped according to the sum of the measured power and the load measurement result received from the secondary device. In particular, if the measured power from the power measurement unit 730 exceeds the sum of the load measurement results from the secondary devices by more than the turn-off threshold, the control unit 720 concludes that there must be a considerable parasitic load as in step S24 in fig. 11, and sets the primary unit to the turn-off mode.

The load measurements generated by each secondary device may also represent the total load from the secondary device, including the actual load of the secondary device and/or the host object and the amount of power required by all friendly parasitic loads. If the actual load does not require power, the load measurements may become representative of friendly parasitic loads only.

In the embodiment of fig. 12, some collision-resistant or collision-proof techniques are required. In one known collision avoidance technique, each RFID unit 620 has a unique code (or indeed a unique one based on statistics). The RFID unit 710 in the primary unit sends a signal requesting all RFID units 620 within a particular range to respond. RFID unit 620 sends their responses in an encoding (e.g., Manchester encoding) to enable RFID unit 710 to ascertain whether more than one device has responded. The primary unit gradually reduces the range until the code uniquely identifying each existing device can be identified. Typically, the range of encoding is halved in each iteration to return quickly (home in).

It will be appreciated that instead of RFID, any suitable communication link may be used to allow each secondary device to transmit its power requirement information to the primary unit. For example, infrared or ultrasonic communication may be used. Alternatively, each secondary device may vary the load it imposes on the primary unit to transmit power demand information. For example, each secondary device may impose a dummy load representing the power value required by its actual load. In this technique, all secondary devices in a power demand state can simultaneously force their respective dummy loads so that the primary unit can directly sum the power demands of all secondary devices in one measurement. Alternatively, the dummy load may represent the total load from the secondary device, including the actual load of the secondary device and/or the host object and the power value required by all friendly parasitic loads.

Claims (20)

1. A secondary device for use in an inductive power transfer system comprising a primary unit that generates an electromagnetic field, the secondary device comprising:

a secondary coil adapted to couple with the electromagnetic field generated by the primary unit when the secondary device is in proximity to the primary unit such that the secondary device can inductively receive power from the primary unit without direct conductive contact with each other;

a load connection unit connected to the secondary coil and adapted to be connected to a load requiring power from the primary unit to provide such inductively received power to the load when the secondary device is in use; and

a communication unit operable to communicate to the primary unit information about a friendly parasitic load imposed on the primary unit by the secondary device,

wherein the friendly parasitic load represents a parasitic load of a metal of the secondary device itself that inductively receives power.

2. The secondary device of claim 1, wherein the communication unit is operable to communicate the information by imposing a dummy load to the primary unit.

3. The secondary apparatus of claim 2, wherein the communication unit is operable to impose a first dummy load to the primary unit at a first time and a second dummy load different from the first dummy load to the primary unit at a second time, the difference between the first and second dummy loads being set in dependence on the friendly parasitic load.

4. The secondary device of claim 3, wherein one of the first and second dummy loads is zero.

5. The secondary apparatus of claim 2, wherein the dummy load has a value selected to convey the friendly parasitic load of the secondary apparatus.

6. The secondary device of claim 2, wherein the dummy load is a resistor having a resistance value selected as a proportion of the friendly parasitic load of the secondary device.

7. The secondary device of claim 2, wherein the dummy load is a feedback resistor having a resistance value representative of the friendly parasitic load of the secondary device; and

wherein the communication unit includes: a switch for selectively switching the feedback resistor connected in parallel to the secondary coil.

8. A portable electrical or electronic device comprising:

a load, at least sometimes requiring power from the primary unit; and

a secondary device, comprising:

a secondary coil adapted to couple with an electromagnetic field generated by the primary unit when the secondary device is in proximity to the primary unit such that the secondary device can inductively receive power from the primary unit without direct conductive contact with each other;

a load connection unit connected to the secondary coil and adapted to be connected to a load requiring power from the primary unit to provide such inductively received power to the load when the secondary device is in use; and

a communication unit operable to communicate to the primary unit information about a friendly parasitic load imposed on the primary unit by the secondary device;

wherein the load connection unit of the secondary device is connected to the load to provide such inductively received power to the load when required by the load,

wherein the friendly parasitic load represents a parasitic load of a metal of the secondary device itself that inductively receives power.

9. A portable electrical or electronic device according to claim 8, wherein the communications unit is operable to communicate the information by imposing a dummy load on the primary unit.

10. A portable electrical or electronic device according to claim 9, wherein the communications unit is operable to impose a first dummy load to the primary unit at a first time and a second dummy load different from the first dummy load to the primary unit at a second time, the difference between the first and second dummy loads being set in dependence on the friendly parasitic load.

11. The portable electrical or electronic device of claim 10, wherein one of the first and second dummy loads is zero.

12. The portable electrical or electronic device of claim 9, wherein the dummy load has a value selected to convey the friendly parasitic load of the secondary device.

13. The portable electrical or electronic device of claim 9, wherein the dummy load is a resistor having a resistance value selected as a proportion of the friendly parasitic load of the secondary device.

14. The portable electrical or electronic device of claim 9, wherein the dummy load is a feedback resistor having a resistance value representative of the friendly parasitic load of the secondary device; and

wherein the communication unit includes: a switch for selectively switching the feedback resistor connected in parallel to the secondary coil.

15. A method of controlling inductive power transfer in an inductive power transfer system, the inductive power transfer system comprising: a primary unit having a primary coil on which an electrical drive signal is applied to generate an electromagnetic field, further comprising at least one secondary device, separable from the primary unit, and having a friendly parasitic load and a secondary coil adapted to couple with the electromagnetic field when the secondary device is in proximity to the primary unit to enable power to be inductively transferred from the primary unit to the secondary device without being in direct conductive contact with each other, the method comprising:

receiving, in the primary unit, information from the secondary device representative of a friendly parasitic load of the secondary device;

detecting in said primary unit whether a substantial parasitic load is present in the vicinity of said primary unit, said detecting taking into account information received in said primary unit representing said friendly parasitic load; and

limiting or stopping inductive power transfer from the primary unit in accordance with the detecting step,

wherein the friendly parasitic load represents a parasitic load of a metal of the secondary device itself that inductively receives power.

16. The method of claim 15, wherein the receiving step comprises imposing a dummy load on the primary unit.

17. The method of claim 16, wherein the forcing step comprises applying a dummy resistor across the secondary coil.

18. The method of claim 17, wherein the receiving step includes measuring a load imposed on the primary unit.

19. The method of claim 18, wherein the measuring of the measuring step is performed during a ring down cycle.

20. The method of claim 16, further comprising the step of varying the dummy load imposed on the primary unit in accordance with one or more conditions of the primary unit.