WO2009022089A1

WO2009022089A1 - Wireless position sensing in three dimensions using ultrasound

Info

Publication number: WO2009022089A1
Application number: PCT/GB2007/003126
Authority: WO
Inventors: Steven David Lavache
Original assignee: Performance Designed Products Ltd
Current assignee: Performance Designed Products Ltd
Priority date: 2007-08-16
Filing date: 2007-08-16
Publication date: 2009-02-19
Anticipated expiration: 2010-02-16
Also published as: GB2463853A; GB201002602D0

Abstract

The present invention provides a method of position sensing between a wireless mobile component carrying a mobile ultrasonic transducer and a fixed component carrying a plurality of fixed ultrasonic transducers in a predetermined spaced-apart relationship, the method comprising sending an rf trigger signal from the second rf transducer, sending an rf acknowledgement signal from the first rf transducer responsive to receipt of the rf trigger signal, receiving the rf acknowledgement signal at the fixed component and starting a plurality of timers corresponding to the respective plurality of fixed transducers generally simultaneously with receiving the acknowledgement signal, transmitting a signal from the mobile ultrasonic transducer responsive to the received trigger signal, receiving the signal transmitted by the mobile ultrasonic transducer at each fixed ultrasonic transducer and stopping the respective timer generally at the time of reception of an edge of the received signal, calculating the distance between the mobile ultrasonic transducer and each fixed ultrasonic transducer based on a predetermined constant representative of the speed of sound and the time taken for transit of the signal to each fixed transducer as measured by the respective timers, and performing trigonometric calculations using the calculated distances in order to determine the 3-dimensional position of the mobile component relative to the fixed component.

Description

WIRELESS POSITION SENSING IN THREE DIMENSIONS USING ULTRASOUND

The present invention relates to the sensing of position in three dimensions using ultrasound and in particular to the sensing of a mobile component relative to a fixed component; the mobile component being free from connection to the fixed component by wires or cables or the like.

Many attempts have been made to sense position using ultrasound. Typically an ultrasonic emission is made from one component and is received at three or more other components. The three or more other components have a fixed spatial relationship to one another and by measuring the time of flight of the transmission to the three or more receivers, a measure of distance may be made. By using three or more receivers, the differences in time of arrival and the consequent different distance measurements allow trigonometric calculations to be carried out to determine the three dimensional relationship between the mobile and fixed components. Thus far, the theory of ultrasound position sensing is well understood and stands up to close scrutiny.

However, the practicalities of achieving this theoretical goal present several significant obstacles. Firstly, the time of commencement of transmission must accurately be known in order to measure the time of flight of the signal. In a wireless embodiment, this must be achieved without interfering with the signal itself. Secondly, this type of apparatus is frequently used inside buildings. Thus the transmitted signals undergo reflections from internal walls and furniture surfaces and the receivers therefore must distinguish between original and reflected signals in order to accurately determine the time of flight. Thirdly, the absolute time of flight varies according to the characteristics of the air and in particular according to its temperature and pressure. Fourthly, it is desirable that the ultrasound emissions do not cause annoying emissions in the audible frequency range. Of course, these problems must also be solved within the usual constraints of cost and reliable operation.

The invention described below being intended for the computer and console games market is particularly sensitive to the latter constraints of cost and reliability. Thus the invention described below provides a particularly simple, robust and yet highly effective method and apparatus for determining position in three dimensions using ultrasound and without using a wired connection between a fixed component and a mobile component.

US 5,142,506 contains a detailed description of the problems outlined briefly above. The apparatus described therein includes a wired mobile component and seeks to overcome the problem of detecting arrival of the transmitted signal using a complex combination of the resonance characteristics of the transducers and complex circuitry in order to find a zero crossing following the second received cycle above a predetermined amplitude.

US 5,175,695 discloses position detecting apparatus in which arrival of the transmitted signal is carried out using a complex envelope detection and threshold circuit. Activation of a known mobile unit is carried out by transmitting a burst of ultrasound. Calibration is carried out using an air temperature sensor. This device represents a complex and expensive approach to the problems set out above.

EP 0312481 approaches the problem of activating a mobile component by the use of infrared transmissions. This, of course, adds complexity relative to the ultrasonic embodiment of US 5,175,695 and also requires a line of sight provision between the fixed and mobile components.

Hitherto, none of the prior art solutions to the problem of wireless three dimensional position sensing using ultrasound have produced a comprehensive and yet simple solution to the many difficulties which must be overcome in this field of technology.

According to one aspect of the present invention there is provided a method of position sensing between a wireless mobile component carrying a mobile ultrasonic transducer and a first rf transducer and a fixed component carrying a plurality of fixed ultrasonic transducers in a predetermined spaced-apart relationship, and a second rf transducer, comprising sending an rf trigger signal from the second rf transducer, sending an rf acknowledgement signal from the first rf transducer responsive to receipt of the rf trigger signal, receiving the rf acknowledgement signal at the fixed component and starting a plurality of timers corresponding to the respective plurality of fixed transducers generally simultaneously with receiving the acknowledgement signal, transmitting a signal from the mobile ultrasonic transducer responsive to the received trigger signal, receiving the signal transmitted by the mobile ultrasonic transducer at each fixed ultrasonic transducer and stopping the respective timer generally at the time of reception of an edge of the received signal, calculating the distance between the mobile ultrasonic transducer and each fixed ultrasonic transducer based on a predetermined value representative of the speed of sound and the time taken for transit of the signal to each fixed transducer as measured by the respective timers, and performing trigonometric calculations using the calculated distances in order to determine the 3-dimensional position of the mobile component relative to the fixed component.

The predetermined value representative of the speed of sound may be determined periodically using a calibration process as explained below.

Preferably the said respective timer is stopped within the first twenty received edges and preferably within the first ten received edges and more preferably at the first received edge. This helps to minimise the effects of reflections by stopping reception shortly after the signal is first received. Sine reflections will typically take a longer route back to the fixed component, they will usually be received after the directly transmitted signal.

In a preferred embodiment, the ultrasonic signal received at the fixed component is clipped for example, by using relatively high gain class C amplifiers or threshold adjustable comparators, in order to further simplify the detection of edges of the received signal. This avoids the need for complex additional signal processing or complex analogue signal processing as proposed in the prior art.

It will be appreciated by those skilled in the art that the system may be used in a reverse mode in which the plurality of ultrasonic transducers are located on the mobile component and the single ultrasonic transducer is located on the fixed component. Generally however, weight and battery power constraints will suggest that the plurality of transducers be located on the fixed component. Nevertheless it should be appreciated that the invention will work equally well in either configuration and in the context of the whole of this application, both alternatives are envisaged.

Multiple mobile components to be used with the same fixed component. When modulating an identity signal onto the signal transmitted by the one or more fixed transducers, only one of the mobile components may be arranged to respond. . By polling a plurality of mobile transducers in a desired order (which need not be evenly distributed between mobile components), it is possible to determine the position of a plurality of such mobile transducers relative to a single fixed component of the system. The mobile components may be addressed using different identity signals in rf and/or ultrasonic transmissions. EP 0152905 attempts to carry out something similar in a medical context. However it is to be noted that this system includes an umbilical cord attached to the person carrying the transducer and thus this system is not a wireless system. GB-B-2354073 describes a complicated system of addressing multiple mobile components using memory means to store relevant information about mobile transponders.

In a second aspect, the invention provides a method of position sensing between a wireless mobile component carrying a mobile ultrasonic transducer and a first rf transducer and a fixed component carrying a plurality of fixed ultrasonic transducers in a predetermined spaced-apart relationship, and a second rf transducer, comprising sending an rf trigger signal from the fixed component, receiving an rf acknowledgement signal at the fixed component and starting a plurality of timers corresponding to a respective plurality of fixed transducers generally simultaneously with receiving the acknowledgement signal, receiving an ultrasonic signal transmitted by a mobile ultrasonic transducer at each fixed ultrasonic transducer and stopping the respective timer generally at the time of reception of an edge of the received signal found, and calculating the distance between the mobile ultrasonic transducer and each fixed ultrasonic transducer based on a predetermined constant representative of the speed of sound and the time taken for transit of the signal to each fixed transducer as measured by the respective timers. In a third aspect the invention provides a method of position sensing between a wireless mobile component carrying a mobile ultrasonic transducer and a first rf transducer and a fixed component carrying a plurality of fixed ultrasonic transducers in a predetermined spaced-apart relationship, and a second rf transducer, comprising receiving an rf trigger signal and sending an rf acknowledgement signal from the first rf transducer responsive to the trigger signal, and transmitting an ultrasonic signal from the mobile ultrasonic transducer responsive to the received trigger signal.

The invention also includes computer program product and computer program aspects as set out in claims 5 to 9 appended hereto.

Embodiments of the invention will now be described by way of example with reference to the figures in which:

Figure 1 is a schematic block diagram of a mobile component in accordance with the invention;

Figure 2 is a schematic block diagram of a fixed component in accordance with the invention; and

Figure 3 is a flow chart showing the steps taken during position measurement.

With reference to Figure 1, a mobile component 2 has a battery power supply 4, a transducer controller 6 and a demodulator 8.

An ultrasonic transducer 10 is coupled to the^' transducer controller 6 and an RF transducer 82 is coupled to the demodulator 8.

The transducer 10 is arranged to have a wide dispersal angle typically greater than 30° and preferably greater than 60°; Typically the transducer is arranged to transmit at around 4OkHz. As is known in the art, such transducers are therefore suitable for reception at around the same frequency. The frequency choice is not critical although lower frequencies will tend to disperse more broadly which is favourable. However, with the hearing range of human ears sometimes extending close to 2OkHz, frequencies of around 25kHz should be considered to be a minimum usable frequency.

The transducer controller 6 serves to control the transmission, reception and idle modes of the transducers 10 and 82 and also to deal with the necessary amplification and buffering. The demodulator 8 is arranged to demodulate and recognise unique ID codes received by the transducer 82 as explained in more detail below.

The mobile component may also include one or more accelerometer and/or gyroscopic/compass sensors 84. This allows acceleration information and also twisting motions of the mobile component to be detected, measured and transmitted back to a fixed component 11 (described below) using wireless transmissions such as via the rf or ultrasonic transmissions 10 or 82.

With reference also to Figure 2, the fixed component 11 has a plurality of transducers 12 which typically will generally be of the same construction as the transducer 10. Typically four transducers 12 will be used which provides 4 possible triangulation solutions which may then be averaged or otherwise processed, for improved accuracy. Only three transducers may be used (providing a single triangulation solution) if lesser accuracy is required and this option is encompassed in the present application. More than four transducers may also be used; providing further increased accuracy but at greater manufacturing cost.

The fixed component 11 also includes an rf transducer 80 which is arranged to communicate with the transducer 82 as described in more detail below.

In use, the inputs of the transducers 12 are received by unit 14, and are then passed to a high gain amplifier in Class C operation 16 which serves to clip the incoming waveform to produce a generally square waveform which is then passed to edge detector 18. Transducer and timer control unit 20 serves to control the transmit, receive and idle modes of the transducers 12 and also to activate and deactivate a plurality of timers 22. Finally, a distance calculator 24 is used to perform the final calculations used for position sensing.

The detailed operation of these components is now described in connection with Figure 3.

As a starting point, all transducers are idle. Upon initialisation, a self-calibration phase is entered in which one of the transducers 12 is caused to transmit a signal which is received at the other two transducers 12. The transducers 12 are mounted on a structure in such a way that their spatial relationship is constant and also is known. By measuring the time taken for the signal to arrive at the other two transducers 12 using the transducer timer and control unit 20 and timers 22, an estimate representative of the speed of sound in the vicinity of the fixed unit 11 may be made. This allows the effects of temperature and pressure changes to be calibrated out of future calculations.

The use of one transmission received at two places provides two different results. Ultimately, these are averaged to arrive at a final calibration figure to be used for future calculation. Further optimally, another of the transducers 12 or indeed all three of the transducers 12 may be used in sequence to transmit a signal to be received at the remaining two of the transducers. This may provide up to six measurements which may be averaged. .

Once the initial calibration phase has completed, a measurement phase may then commence. Typically this will be used for a user calibration in which the origin for further measurements is set thereby allowing for placement of the fixed component relative to a comfortable position for the user to hold the mobile component. This may be achieved by asking the user via a display device such as a games console, to hold a mobile component at a particular position e.g. in front of their chest or above their head, and then taking a position measurement. That position may then be used as a reference point for subsequent relative position measurements.

With reference to Figure 3, in step 50, a trigger signal is transmitted by the fixed component via the rf transducer 80. When this is received by the mobile component (step 54), the mobile component looks for an identify signal in the trigger signal (step 56) to determine whether the trigger signal is intended for that mobile component. In a system with only one mobile component the identity signal may be omitted. If the correct identity signal is received then the transducers 12 start to transmit a short burst of ultrasound and generally concurrently, an rf acknowledgment is sent via transducer 82 (step 58). Typically the short burst of ultrasound will be eight cycles of ultrasound. When the acknowledgement signal is received at the fixed component 11 , the control unit 20 causes the timers 22 to start (step 59).

As explained above, the mobile component 2 receives the rf transmitted signal and.using demodulator 8, decodes any identity code (step 56). The identity code is used to allow for multiple mobile components to be used with the same fixed components and may for example be an 8bit address. The mobile component 2 determines whether the identity code indicates that it should perform ultrasonic transmission. The identity code may also be used to indicate certain modes within the mobile component which might for example be used to activate vibration equipment, sound on the mobile unit and/or lighting effects on the mobile component.

Let us assume that the identity code which has been transmitted is the correct one for an active mobile component. If the mobile component 2 is active, a burst of ultrasound is transmitted (step 58). This typically will be a burst of eight cycles.

The burst is received by each fixed transducer (step 60) at different times due to their different proximity to the mobile component. As each fixed transducer receives a signal, the transducer and timer control turns off the relevant timer (step 62). Once all the fixed transducers have received a signal (step 64) transmission from the mobile component is stopped by transmitting a stop signal and/or because a predetermined time period has elapsed, and distance calculator 24 calculates the relative position of the components using the values stored in the timers 22 against each transducer, as a measure of distance. The 3-dimensional position of the mobile unit is then determinable using trigonometric calculations. The stopping of ultrasonic transmission (step 66) from the mobile component may be achieved by an RF signal or causing the mobile component to look for signals transmitted by the transducers 12 between its own transmissions or may be by means of a timer (not shown) which for example causes a predetermined duration of ultrasonic energy to be transmitted.

Preferably the transducer and timer control 20 uses the first edge detected by the edge detectors 18 to trigger stopping of a timer 22 in relation to a particular transducer 12. The value of each timer will be representative of the transit time for the ultrasonic energy and will, given a constant speed of sound, therefore be representative of a distance between the mobile transducer and fixed transducers. These timer values may be further processed in the apparatus or passed back to another processing device via an interface such as a USB interface 26.

The USB interface 26 may also be used to control the apparatus. For example, calibration phases may be initiated via. the interface and control of the order and rate of polling of a plurality of mobile devices may be communicated in this way.

The edge detection is facilitated by the use of the clipping (using the Class C amplifier 16) on the received signal. It is also assumed that the first received signal is the direct line of sight signal since typically a reflected signal will take longer to arrive. Thus simply choosing the first edge avoids the need for complex calculations to achieve removal of multipath reflection. By repeating the cycle of Figure 3 rapidly, for example at least 10 to 20 times a second or more, the impression of continuous motion detection is given. The balance to be struck is between smooth detection and cost of hardware to undertake rapid cycling of the steps in the Figure.

If multiple mobile components were used, these are polled in series (unless of course a particular mobile component is known to have a lower requirement for responsiveness in which case it may be polled less frequently). Also, the mobile component may be simplified so that it does not look for a stop signal. It may simply transmit a predestined numbers such as eight, cycles and then automatically stop. Although some silence is required in order to allow reflections to die away, it will be noted that with eight cycles of 4OkHz ultrasound repeating at something of the order of 100Hz, there is sufficient time for approximately 100 mobile components to be serviced. In practice the number is more likely to be between 1 and 10. Furthermore, the choice of an eight bit identification code allows for 256 combinations which as discussed above may be formed as 256 different mobile components or may be used to address a smaller number of mobile components whilst activating and deactivating features on the mobile components.

Thus what has been presented above is a particularly simple and effective solution to the problem of three-dimensional position sensing in a wireless context and using ultrasound.

The accuracy of position determination is largely dictated by the resolution of the timers 22 which in turn is dictated by the clock frequency. Presently, a clock frequency of 6.5MHz has been found to be suitable. Enhanced resolution niay be achieved using higher clock frequencies but at the cost of requiring larger counters.

As a further enhancement, the acceleration of the mobile component may be used to adjust error averaging of the position detection. Generally, in order to minimise the effect of errors, the sensed position of the mobile component is averaged over several readings. This averages out errors and provides a consistent position reading over time. However, the averaging also introduces delay between movement of the mobile component and recognition of this by the fixed component. This is because the first few readings after the movement has been started will be reduced in effect by the averaging calculation using historical position measurements taken before the movement commenced. This delay or lag may be noticeable by the user when the mobile component is being moved rapidly. By using the accelerometer data to determine the rapidity of movement, this problem may be overcome by reducing the number of samples used for averaging when the mobile component is accelerating above a

^■ predetermined threshold or set of thresholds (which may correspond to respective variations in the number of samples used for averaging) and then increasing the number of samples averaged again when the acceleration of the mobile component is lower. This works well because accuracy of position detection is subjectively less important when rapid movement is occurring and in contrast an increased delay in position reading is less important when movement is slow.

Thus using this process, accuracy may be increased by averaging out errors, when the user is able to notice error i.e. during slow movement and the accuracy may be reduced as a trade-off for increased responsiveness during rapid movement by reducing the averaging window length. Thus the window length may be reduced as certain accelerometer thresholds are met and increased in length again as the accelerometer data suggest reduced acceleration. Hysterisis may also be used for the accelerometer threshold detection,

Claims

1. A method of position sensing between a wireless mobile component and a fixed component, wherein the wireless mobile component comprises a mobile

5 ultrasonic transducer and a first rf transducer and the fixed component comprises a plurality of fixed ultrasonic transducers in a predetermined spaced- apart relationship and a second rf transducer, the method comprising;

(a) sending an rf trigger signal from the second rf transducer,

(b) sending an rf acknowledgement signal from the first rf transducer 10. responsive to receipt of the rf trigger signal,

(c) receiving the rf acknowledgement signal at the fixed component and starting a plurality of timers corresponding to the respective plurality of fixed transducers generally simultaneously with receiving the acknowledgement signal,

15 (d) transmitting a signal from the mobile ultrasonic transducer responsive to

. the received trigger signal,

(e) receiving the signal transmitted by the mobile ultrasonic transducer at each fixed ultrasonic transducer and stopping the respective timer generally at the time of reception of an edge of the received signal,

20 (f) calculating the distance between the mobile ultrasonic transducer and each fixed ultrasonic transducer based on a predetermined value representative of the speed of sound and the time taken for transit of the signal to each fixed transducer as measured by the respective timers, and

25 (g) performing trigonometric calculations using the calculated distances in order to determine the 3-dimensional position of the mobile component relative to the fixed component.

2. A method according to claim 1 wherein the said respective timer is stopped 30 within the first twenty received edges and preferably within the first ten received edges and more preferably at the first received edge.

3. A method of position sensing between a wireless mobile component and a fixed component wherein the wireless mobile component comprises a mobile ultrasonic transducer and a first rf transducer and the fixed component comprises a plurality of fixed ultrasonic transducers in a predetermined spaced- apart relationship, and a second rf transducer, comprising;

(a) sending an rf trigger signal from the fixed component

(b) receiving an rf acknowledgement signal responsive to the trigger signal, at the fixed component and starting a plurality of timers corresponding to a respective plurality of fixed transducers generally simultaneously with receiving the acknowledgement signal,

(c) receiving an ultrasonic signal transmitted by a mobile ultrasonic transducer at each fixed ultrasonic transducer and stopping the respective timer generally at the time of reception of an edge of the received signal found, and (d) calculating the distance between the mobile ultrasonic transducer and each fixed ultrasonic transducer based on a predetermined value representative of the speed of sound and the time taken for transit of the signal to each fixed transducer as measured by the respective timers.

4. A method of position sensing between a wireless mobile component and a fixed component, wherein the wireless mobile component comprises a mobile ultrasonic transducer and a first rf transducer and the fixed component comprises a plurality of fixed ultrasonic transducers in a predetermined spaced- apart relationship and a second rf transducer, the method comprising; (a) receiving an rf trigger signal and sending an rf acknowledgement signal from the first rf transducer responsive to the trigger signal, and (b) transmitting an ultrasonic signal from the mobile ultrasonic transducer responsive to the received trigger signal.

5. A computer program product directly loadable into the internal memory of a digital computer, comprising software code portions for performing the steps of claim 1 when said product is run on a computer.

6. A computer program product directly loadable into the internal memory of a digital computer, comprising software code portions for performing the steps of claim 3 when said product is run on a computer.

7. A computer program product directly loadable into the internal memory of a digital computer, comprising software code portions for performing the steps of claim 4 when said product is run on a computer.

8. A computer program directly loadable into the internal memory of a digital computer, comprising software code portions for performing the steps of claim 1 when said program is run on a computer.

9. A computer program directly loadable into the internal memory of a digital computer, comprising software code portions for performing the steps of claim 3 when said program is run on a computer.

10. A computer program directly loadable into the internal memory of a digital computer, comprising software code portions for performing the steps of claim 4 when said program is run on a computer.

11. A method of position sensing between a wireless mobile component and a fixed component, wherein the wireless mobile component comprises a mobile ultrasonic transducer and a first rf transducer and the fixed component comprises a plurality of fixed ultrasonic transducers in a predetermined spaced- apart relationship and a second rf transducer, the method comprising;

(a) sending an rf trigger signal from the second rf transducer,

(b) starting a. plurality of timers corresponding to the respective plurality of fixed transducers,

(c) receiving the trigger signal at the first rf transducer, (d) transmitting a signal from the mobile ultrasonic transducer responsive to the received trigger signal,

(e) receiving the signal transmitted by the mobile ultrasonic transducer at each fixed ultrasonic transducer and stopping the respective timer generally at the time of reception of an edge of the received signal found within the first twenty received edges and preferably within the first ten received edges and more preferably at the first received rising edge,

(f) calculating the distance between the mobile ultrasonic transducer and each fixed ultrasonic transducer based on a predetermined constant representative of the speed of sound and the time taken for transit of the signal to each fixed transducer as measured by the respective timers, and

(g) performing trigonometric calculations using the calculated distances in order to determine the 3-dimensional position of the mobile component relative to the fixed component.

12. A computer program product directly loadable into the internal memory of a digital computer, comprising software code portions for performing the steps of claim 11 when said product is run on a computer.

13. A computer program directly loadable into the internal memory of a digital computer, comprising software code portions for performing the steps of claim 11 when said program is run on a computer.

14. An ultrasonic position sensor arrangement constructed and arranged substantially as described herein with reference to the drawings.