WO2018102009A1

WO2018102009A1 - Position or orientation determination based on duty-cycled frequency multiplexed electromagnetic signals

Info

Publication number: WO2018102009A1
Application number: PCT/US2017/052523
Authority: WO
Inventors: Advait Jain; Murphy Stein; Sherk Chung; Saket PATKAR
Original assignee: Google LLC
Current assignee: Google LLC
Priority date: 2016-11-29
Filing date: 2017-09-20
Publication date: 2018-06-07
Anticipated expiration: 2019-05-29
Also published as: GB201715626D0; DE202017105884U1; DE102017122456A1; GB2557401B; CN108120440A; EP3548912A1; GB2557401A

Abstract

A three-axis magnetic source [110] generates frequency multiplexed electromagnetic signals for transmission at a first power and a second power during respective first and second subsets of time intervals [406, 407] defined by a duty cycle. The first power is higher than the second power. A position or an orientation of a three-axis magnetic sensor [105] relative to the three-axis magnetic source is determined based on the frequency multiplexed electromagnetic signals received by the three-axis magnetic sensor at the first power during the first subset of time intervals. In some cases, the three-axis magnetic source includes a set of orthogonal coils [111, 112, 113] and a power supply [125] alternating higher and lower powers to drive the set of orthogonal coils to generate frequency multiplexed electromagnetic signals.

Description

POSITION OR ORIENTATION DETERMINATION BASED ON DUTY-CYCLED FREQUENCY MULTIPLEXED ELECTROMAGNETIC SIGNALS

BACKGROUND

[0001 ] The position or orientation of a three-axis magnetic source can be tracked using a corresponding three-axis magnetic sensor that detects variations in a magnetic field produced by the three-axis magnetic source. For example, the three- axis magnetic source can be implemented as three orthogonal coils that generate magnetic fields in response to electric currents applied to the coils. A three-axis magnetic source can be operated with pulsed direct current (DC) in which currents are applied to each of the three orthogonal coils in sequence (resulting in multiplexed time intervals). This operational mode can be referred to as a DC-pulsed time multiplexed mode in which three orthogonal coils in the three-axis magnetic sensor generate electric currents in response to the magnetic fields provided by the three- axis magnetic source in the multiplexed time intervals. One example of a technique for tracking the position or orientation of a three-axis magnetic source in the DC- pulsed time multiplexed mode is described in Raab, et al. (I EEE Transactions on Aerospace and Electronic Systems, vol. AES-15, No. 5, September 1979, page 709), which is incorporated herein by reference in its entirety. Frequency multiplexing using alternating current (AC) can also be used to allow the three-axis magnetic source to transmit electromagnetic signals concurrently in different frequency bands using the three orthogonal coils. This operational mode can be referred to as an AC frequency multiplexed mode. For example, in the AC frequency multiplexed mode, the orthogonal coils are driven by oscillating currents having different frequencies corresponding to the frequency bands. The three-axis magnetic sensor is configured to perform frequency demultiplexing of the electric currents generated by the frequency multiplexed magnetic fields in the coils of the three-axis magnetic sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0002] The present disclosure may be better understood, and its numerous features and advantages made apparent to those skilled in the art by referencing the accompanying drawings. The use of the same reference symbols in different drawings indicates similar or identical items. [0003] FIG. 1 is a block diagram of a system for determining a position or orientation of a three-axis magnetic sensor relative to a three-axis magnetic source according to some embodiments.

[0004] FIG. 2 is a block diagram of a system for determining a position or orientation of a three-axis magnetic sensor relative to a three-axis magnetic source during a first subset of time intervals and estimating the position or orientation during a second subset of time intervals using measured accelerations and gyroscopic orientations according to some embodiments.

[0005] FIG. 3 is a block diagram of a system including a three-axis magnetic sensor that implements a single amplifier and a three-axis magnetic source that implements a single amplifier according to some embodiments.

[0006] FIG. 4 shows timing relationships between signals in a position or orientation tracking system according to some embodiments.

[0007] FIG. 5 shows timing relationships between signals in a position or orientation tracking system that performs noise monitoring during a low power transmission mode of a three-axis magnetic source according to some embodiments.

[0008] FIG. 6 is a plot illustrating values of a Y-coordinate of a position of a three- axis magnetic sensor determined based on frequency multiplexed signals transmitted by a three-axis magnetic source during a first subset of time intervals and

measurements of accelerations and gyroscopic orientations performed during a second subset of time intervals according to some embodiments.

[0009] FIG. 7 is a plot illustrating values of an X-coordinate of a position of a three-axis magnetic sensor determined based on frequency multiplexed signals transmitted by a three-axis magnetic source during a first subset of time intervals and measurements of accelerations and gyroscopic orientations performed during a second subset of time intervals according to some embodiments.

SUMMARY OF THE INVENTION

[0010] In accordance with a first aspect, there is provided a three-axis magnetic source comprising: a set of orthogonal coils; and a power supply, wherein a first power and a second power are alternately applied to drive the set of orthogonal coils to generate frequency multiplexed electromagnetic signals during time intervals defined by a duty cycle, the first power being higher than the second power.

Therefore, signal-to-noise may be improved without requiring higher currents or larger coils.

[001 1 ] Optionally, the three-axis magnetic source may further comprise: a signal generator to generate signals at a plurality of frequencies; and at least one amplifier to amplify the signals and provide the amplified signals to drive the set of orthogonal coils to generate electromagnetic signals at the plurality of frequencies, wherein the at least one amplifier is powered by using the first power and the second power.

[0012] Optionally, the three-axis magnetic source may further comprise: a switching circuit to convey power from the power supply to the at least one amplifier, wherein the switching circuit is configured to switch states during alternating time intervals defined by the duty cycle so that the first power is provided to the at least one amplifier during a first subset of the time intervals and the second power is provided to the at least one amplifier during a second subset of the time intervals.

[0013] Optionally, the switching circuit interrupts power transmission from the power supply to the at least one amplifier during the second subset of the time intervals. [0014] Optionally, the set of orthogonal coils may include three orthogonal coils and each of three amplifiers provide amplified signals having different frequencies to a corresponding orthogonal coil.

[0015] Optionally, the three-axis magnetic source may further comprise: a multiplexer to selectively provide one of the signals at the plurality of frequencies from the signal generator to the at least one amplifier in a time-multiplexed manner; and a de-multiplexer to provide amplified signals at the plurality of frequencies from the at least one amplifier to a selected one of the set of orthogonal coils.

[0016] Optionally, the three-axis magnetic source may further comprise: [0017] an inertial measurement unit to measure acceleration and gyroscopic orientation of the three-axis magnetic source; and an interface to convey values of measurements performed by the inertial measurement unit.

[0018] Optionally, an indication of noise on at least one of the plurality of frequencies may be received via the interface, and wherein the signal generator generates signals based on the indication of the noise such that the signals are generated for frequencies having noise levels below a threshold.

[0019] In accordance with a second aspect, there is provided a three-axis magnetic sensor comprising: a set of orthogonal coils; and a processor to determine at least one of a position or an orientation of the three-axis magnetic sensor relative to a three-axis magnetic source based on frequency multiplexed electromagnetic signals alternately received at a first power and a second power by the set of orthogonal coils from the three-axis magnetic source during time intervals defined by a duty cycle, the first power being higher than the second power. [0020] Optionally, the three-axis magnetic sensor may further comprise: at least one amplifier to amplify signals generated by the set of orthogonal coils in response to the frequency multiplexed electromagnetic signals; and an analog-to-digital converter to sample the amplified signals and provide a digital representation of the amplified signals to the processor. [0021 ] Optionally, the frequency multiplexed electromagnetic signals may be received at the first power during a first subset of time intervals and the frequency multiplexed electromagnetic signals are received at the second power during a second subset of time intervals.

[0022] Optionally, the second power may be below a detection threshold at the three-axis magnetic sensor.

[0023] Optionally, the processor determines the at least one of the position or the orientation of the three-axis magnetic sensor during the first subset of time intervals based on the frequency multiplexed electromagnetic signals received during the first subset of time intervals, and further comprising: an inertial measurement unit to measure acceleration and gyroscopic orientation of the three-axis magnetic sensor, and wherein the processor is to compute the at least one of the position or the orientation of the three-axis magnetic sensor during the second subset of time intervals based on the measured acceleration and gyroscopic orientation.

[0024] Optionally, the processor is to measure noise on a plurality of frequencies used by the frequency multiplexed signals during the second subset of time intervals, and wherein the processor is to select frequencies having noise levels below a threshold for reception of the frequency multiplexed signals, and further comprising: an interface to convey an indication of noise values on the plurality of frequencies.

[0025] Optionally, the set of orthogonal coils includes three orthogonal coils and each of three amplifiers amplify signals from a corresponding orthogonal coil.

[0026] Optionally, the three-axis magnetic sensor may further comprise: a multiplexer to selectively provide one of the signals from the set of orthogonal coils to the at least one amplifier in a time-multiplexed manner; and a de-multiplexer to provide amplified signals from the at least one amplifier to the analog-to-digital converter.

[0027] According to a third aspect, there is provided a method, comprising:

generating, at a three-axis magnetic source, frequency multiplexed electromagnetic signals for transmission at a first power and a second power during respective first and second subsets of time intervals defined by a duty cycle, the first power being higher than the second power; and determining at least one of a position or an orientation of a three-axis magnetic sensor relative to the three-axis magnetic source based on the frequency multiplexed electromagnetic signals received by the three- axis magnetic sensor at the first power during the first subset of time intervals.

[0028] Optionally, generating the frequency multiplexed electromagnetic signals comprises reducing a power supplied to at least one amplifier in the three-axis magnetic source during the second subsets of time intervals.

[0029] Optionally, reducing the power supplied to the at least one amplifier comprises reducing the flow of power from a power supply to the at least one amplifier. [0030] Optionally, the method may further comprise: measuring acceleration and gyroscopic orientation of at least one of the three-axis magnetic source or the three- axis magnetic sensor.

[0031 ] Optionally, the method may further comprise: computing at least one of a position or an orientation of the three-axis magnetic sensor relative to the three-axis magnetic source during the second subset of time intervals based on the measured acceleration and gyroscopic orientation of the at least one of the three-axis magnetic source or the three-axis magnetic sensor.

[0032] Optionally, the method may further comprise: measuring noise at a plurality of frequencies used by the frequency multiplexed electromagnetic signals during the second subset of time intervals.

[0033] Optionally, the method of claim may further comprising: selecting, based on the measured noise, a subset of the plurality of frequencies for transmission of the frequency multiplexed electromagnetic signals at the first power during the first subset of time intervals.

[0034] Optionally, selecting the subset of the plurality of frequencies comprises selecting a subset of the plurality of frequencies having a measured noise that is below a threshold.

[0035] It should be noted that any feature described above may be used with any particular aspect or embodiment of the invention. In particular, any feature or combination of features of the source may be used with any feature or combination of features of the sensor.

DETAILED DESCRIPTION

[0036] In conventional position or orientation electromagnetic tracking systems, a three-axis magnetic source is separated from a three-axis magnetic sensor by a distance (r) within which the magnitude of the magnetic field produced at the three- axis magnetic sensor is determined by the near-field approximation. The magnitude of the magnetic field at the three-axis magnetic sensor therefore falls off at a rate of r ³. Consequently, the signal-to-noise ratio (SNR) of the electromagnetic signal detected by the three-axis magnetic sensor decreases rapidly as the distance (r) increases. The decreasing SNR causes a rapid degradation in performance of the determined position or orientation, as well as increased jitter in the determined position or orientation. Some conventional position or orientation tracking systems compensate for the decreasing SNR by increasing the current supplied to the three orthogonal coils in the three-axis magnetic source so that the coils generate stronger electromagnetic fields. However, this approach increases power consumption and requires physically larger coils, both of which can be significant drawbacks in battery- powered systems. [0037] The power consumed by a three-axis magnetic source in a frequency multiplexed position or orientation tracking system that tracks the position or orientation of a three-axis magnetic sensor is reduced by alternately providing higher and lower powers to drive a set of orthogonal coils in the three-axis magnetic source to generate frequency multiplexed electromagnetic signals during time intervals defined by a duty cycle. Some embodiments of the three-axis magnetic source include three orthogonal coils, a power supply (such as a battery), a signal generator to generate signals at one or more frequencies, one or more amplifiers to amplify the signals generated by the signal generator and drive currents in the orthogonal coils based on the amplified signals. The power supply supplies a first (higher) power, which can include power supplied to the signal generator and the one or more amplifiers, during a first time interval and reduces the power to a second (lower) power during a second time interval. For example, the power supply can reduce or interrupt power supplied to the one or more amplifiers during the second time interval. [0038] The three-axis magnetic sensor includes three orthogonal coils, one or more amplifiers to amplify signals generated by the three orthogonal coils in response to the electromagnetic field generated by the three-axis magnetic source, and an analog-to-digital converter (ADC) to generate digital signals from the amplified signals. A processor (which can be implemented in the three-axis magnetic sensor or external to the sensor) determines the position or orientation of the three-axis magnetic sensor relative to the three-axis magnetic source based on the digital signals. In some embodiments, inertial measurement units (IMUs) are included in the three-axis magnetic source or sensor. Duty cycling of the three-axis magnetic source reduces the effective sampling rate of the three-axis magnetic sensor. In order to increase the sampling rate, measurements generated by an accelerometer or gyroscope in the I MU are used to compute positions or orientations in the time intervals corresponding to the low-power mode of the three-axis magnetic source.

[0039] In some embodiments, the signal generator generates signals at different frequencies for each of the three orthogonal coils in the three-axis magnetic source, which can therefore concurrently or simultaneously generate electromagnetic fields at the different frequencies using the three orthogonal coils. Three amplifiers are used to amplify the signals provided to the three orthogonal coils in this case. The three- axis magnetic sensor concurrently or simultaneously receives the electromagnetic signal at the different frequencies and the processor associated with the three-axis magnetic sensor determines its position or orientation relative to the three-axis magnetic source using the sampled electromagnetic signals. In some embodiments, the signal generator in the three-axis magnetic source time multiplexes the frequency multiplexed signals, which allows the number of amplifiers in the source to be reduced. For example, instead of three amplifiers, the time multiplexed signals from the signal generator can be provided to a single amplifier for amplification. The amplified signals are then demultiplexed and provided to the corresponding orthogonal coils for transmission as a frequency multiplexed electromagnetic signal. In some embodiments, the three-axis magnetic sensor measures ambient

electromagnetic noise during the low-power mode of the three-axis magnetic source. The measurements can be used to detect excessive noise on channels

corresponding to the frequencies of the signals provided by the signal generator and to switch to channels that have lower noise levels. Power boosting at the three-axis magnetic source can also be used to improve the SNR in some cases while consuming equal or less power than the equivalent frequency-multiplexed system that transmits continuously without duty cycling the supplied power.

[0040] FIG. 1 is a block diagram of a system 100 for determining a position or orientation of a three-axis magnetic sensor 105 relative to a three-axis magnetic source 1 10 according to some embodiments. Both the three-axis magnetic sensor 105 and the three-axis magnetic source 1 10 can be either fixed or mobile and so the relative position or orientation of the devices varies in response to motion of one or more of them. For example, the three-axis magnetic source 1 10 can be mounted on a fixed structure such as a wall or a portion of a vehicle and the three-axis magnetic source 1 15 can be mounted on (or incorporated within) a movable object such as a head mounted display, a game controller, and the like. In this case, changes in the relative position or orientation are determined primarily by motion of the three-axis magnetic source 1 15. For another example, the three-axis magnetic source 1 10 can be mounted on a movable object such as a helmet or head mounted display and the three-axis magnetic sensor 105 can also be mounted on a movable object such as a game controller. In this case, changes in the relative position or orientation are determined by motion of both the three-axis magnetic source 1 10 and the three-axis magnetic sensor 105.

[0041] The three-axis magnetic source 1 10 includes a set of orthogonal coils that generate electromagnetic fields in response to applied currents. In the illustrated embodiment, the set of orthogonal coils includes a coil 1 1 1 that is symmetric about an axis that points out of the plane of the drawing, a coil 1 12 that is symmetric about an axis that is parallel to a horizontal direction in the plane of the drawing, and a coil 1 13 that is symmetric about an axis that is parallel to a vertical direction in the plane of the drawing. The coils 1 1 1 , 1 12, 1 13 are collectively referred to herein as "the coils 1 1 1 -1 13." The coils 1 1 1 -1 13 are depicted as separate structures in FIG. 1 . However, the coils 1 1 1 -1 13 can also be implemented as a single structure.

Furthermore, persons of ordinary skill in the art should appreciate that some embodiments of the three-axis magnetic source 1 10 can include coils that are not necessarily orthogonal to each other. [0042] The three-axis magnetic source 1 10 also includes a signal generator 1 15 that generates signals having a plurality of frequencies for transmission on corresponding channels in a frequency multiplexed system. The signal generator can be implemented as a single multi-channel signal generator that switches between different frequencies or a plurality of single-channel signal generators that each generate a signal corresponding to a different one of the plurality of frequencies. The signals generated by the signal generator 1 15 are provided to an amplifier element 120, which amplifies the signals and provides the amplified signals to drive currents in the coils 1 1 1 -1 13 to generate electromagnetic signals that are transmitted by the three-axis magnetic source 1 10. The illustrated embodiment of the amplifier element 120 includes three amplifiers 121 , 122, 123, which are collectively referred to herein as "the amplifiers 121 -123." Each of the amplifiers 121 -123 provides an amplified signal to drive a current in a corresponding one of the coils 1 1 1 -1 13. The electromagnetic signals 124 generated by the coils 1 1 1 -1 13 have different frequencies corresponding to the frequencies of the signals generated by the signal generator 1 15. The coils 1 1 1 -1 13 are therefore able to transmit the electromagnetic signals 124 concurrently as frequency multiplexed electromagnetic signals.

[0043] A power supply 125 supplies power used by the signal generator 1 15 to generate signals and the amplifiers 121 -123 to amplify signals that are provided to the coils 1 1 1 -1 13. The power supply 125 is linked to the amplifier element 120 by a switching circuit 130 that controls the amount of power that flows from the power supply 125 to the amplifier element 120. The switching circuit 130 can therefore increase or decrease the amount of power that is provided to the amplifier element 120. Some embodiments of the switching circuit 130 have a first state in which a higher amount of power is provided to the amplifier element 120 and a second state in which a lower amount of power is provided to the amplifier element 120. For example, the second state can be an open state of the switching circuit 130, in which case the flow of power to the amplifier element 120 is interrupted and substantially no power is provided to the amplifier element 120. For another example, the second state can be an idle state of the switching circuit 130, in which case a reduced amount of power is provided to the amplifier circuit 120. In some embodiments, the power provided to the signal generator 1 15 by the power supply 125 can also be reduced or interrupted in the second state to further reduce the overall power consumption of the three-axis magnetic source 1 10.

[0044] The switching circuit 130 shown in FIG. 1 is depicted as a separate circuit that is external to both the amplifier element 120 and the power supply 125.

However, some embodiments of the switching circuit 130 can be implemented within one or more of these elements. For example, the amplifier element 120 can implement an internal switching circuit that can change states to cause the amplifier element 120 to transition from a high power mode to a low (or zero) power mode in which the amplifier element 120 draws less (or no) power from the power supply 125. For another example, the power supply 125 can implement an internal switching circuit that can change states from a high power mode to a low power mode to cause the power supply 125 to provide less (or no) power to the amplifier element 120.

[0045] The SNR of signals received by the three-axis magnetic sensor 105 drops at a rate that is proportional to the cube of the distance between the three-axis magnetic sensor 105 and the three-axis magnetic source 1 10. Consequently, the accuracy of the position or orientation determination can degrade rapidly in response to an increase in the distance between the three-axis magnetic sensor 105 and the three-axis magnetic source 1 10. Some embodiments of the power supply 125 provide power at a level that can be modified so that the provided power can be increased in scenarios in which the distance between the three-axis magnetic sensor 105 and the three-axis magnetic source 1 10 is relatively large and decreased in scenarios in which the distance between the three-axis magnetic sensor 105 and the three-axis magnetic source 1 10 is relatively small. The variable level of power supplied by the power supply 125 can be used to balance competing demands for low power consumption and high accuracy in the determination of the relative position or orientation of the three-axis magnetic sensor 105. [0046] The amplification gain of the amplifiers 121 -123 is determined, at least in part, by the amount of power provided to the amplifier element 120. For example, the gain of the amplifiers 121 -123 is higher when a higher amount of power is provided to the amplifier element 120 and the gain of the amplifiers 121 -123 is lower when a lower amount of power is provided to the amplifier element 120. The transmission power of the electromagnetic signals 124 generated by the coils 1 1 1 -1 13 is therefore determined, at least in part, by the amount of power provided to the amplifier element 120. For example, the transmission power of the electromagnetic signals 124 generated by the coils 1 1 1 -1 13 is higher when a higher amount of power is provided to the amplifier element 120 and the transmission power of the electromagnetic signals 124 generated by the coils 1 1 1 -1 13 is lower when a lower amount of power is provided to the amplifier element 120. [0047] The state of the switching circuit 130 can be changed according to a duty cycle. Some embodiments of the switching circuit 130 alternate between a first state that provides a higher power to the amplifier element 120 and a second state that provides a lower power to the amplifier element 120 in successive time intervals determined by the duty cycle. For example, if the duty cycle has a period of 100 milliseconds (ms), the switching circuit 130 can alternate between the first state during a first time interval that has a first duration of 10 ms and the second state during a second time interval that has a second duration of 90 ms. The periodicity and the durations of the time intervals of the duty cycle can be fixed or variable. For example, a duty cycle that has a period of 100 ms can be subdivided into first time intervals of 50 ms and second time intervals of 50 ms. For another example, the periodicity of the duty cycle can be increased to 200 ms. Thus, the power supplied to the amplifier element 120 to amplify signals that drive the current in the coils 1 1 1 -1 13 alternates between a first power and a second power. In response to the duty-cycled current provided by the amplifiers 121 -123, the set of orthogonal coils 1 1 1 -1 13 generates frequency multiplexed electromagnetic signals at higher and lower transmission powers during time intervals defined by the duty cycle.

[0048] The three-axis magnetic sensor 105 includes a set 135 of orthogonal coils that produce signals, e.g. electric currents, in response to the electromagnetic signals 124 generated by the coils 1 1 1 -1 13. The set 135 of orthogonal coils is depicted as a single structure in FIG. 1 . However, the orthogonal coils can also be implemented as separate structures such as the coils 1 1 1 -1 13. Furthermore, persons of ordinary skill in the art should appreciate that some embodiments of the three-axis magnetic source 1 10 can include coils that are not necessarily orthogonal to each other. The signals generated by the set 135 of orthogonal coils are provided to an amplifier element 140, which includes the amplifiers 141 , 142, 143 (collectively referred to herein as "the amplifiers 141 -143"). For example, each of three orthogonal coils in the set 135 is connected to a corresponding one of the amplifiers 141 -143. The amplified signals are provided to an analog-to-digital converter (ADC) 145 that samples the amplified signals and generates digital signals representative of the amplified signals. The digital signals are provided to a processor 150. Although the processor 150 is depicted as an integral element of the three-axis magnetic sensor 105, some embodiments of the processor 150 are implemented external to the three- axis magnetic sensor 105 and receive digital samples over a communication link.

[0049] The processor 150 determines a position or orientation of the three-axis magnetic sensor 105 relative to the three-axis magnetic source 1 10 based on the frequency multiplexed electromagnetic signals 124 received from the three-axis magnetic source 1 10. As discussed herein, the electromagnetic signals 124 are alternately received at a first (higher) power and a second (lower) power by the set 135 of orthogonal coils from the three-axis magnetic source 1 10 during time intervals defined by a duty cycle. The processor 150 uses the electromagnetic signals 124 received at the first power (in corresponding first time intervals) to determine the relative position or orientation of the three-axis magnetic sensor 105. The processor 150 does not directly convert the electromagnetic signals 124 received at the second power (in corresponding second time intervals) into its relative position or orientation. In some embodiments, the second power is below the detection threshold for the processor 150, in which case the electromagnetic signals 124 are not detected by the three-axis magnetic sensor 105. Some embodiments of the three-axis magnetic sensor 105 are configured to monitor frequencies corresponding to the channels used to convey the frequency multiplexed signals to detect noise on the channels during the low power mode. The detected noise levels can then be used to select subsets of the channels for transmitting the frequency multiplexed electromagnetic signals 124, as discussed herein.

[0050] FIG. 2 is a block diagram of a system 200 for determining a position or orientation of a three-axis magnetic sensor 205 relative to a three-axis magnetic source 210 during a first subset of time intervals and estimating the position or orientation during a second subset of time intervals using measured accelerations and gyroscopic orientations according to some embodiments. The three-axis magnetic source 210 includes a set of orthogonal coils 21 1 -213, a signal generator 215, an amplifier element 220 that includes amplifiers 221 -223, a power supply 225, and a switch 230. The structural elements of the three-axis magnetic source 210 operate in a manner that is similar to the corresponding structural elements in the three-axis magnetic source 1 10 shown in FIG. 1 . The three-axis magnetic sensor 205 includes a set 235 of orthogonal coils, an amplifier element 240 that includes amplifiers 241 -243, an ADC 245, and a processor 250. The structural elements of the three-axis magnetic sensor 205 operate in a manner that is similar to the corresponding structural elements in the three-axis magnetic sensor 105 shown in FIG. 1 . [0051] The three-axis magnetic sensor 205 and three-axis magnetic source 210 differ from the corresponding three-axis magnetic sensor 105 and three-axis magnetic source 1 10 shown in FIG. 1 by incorporating inertial measurement units (IMUs) 255, 260 that measure accelerations and gyroscopic orientations of the three- axis magnetic sensor 205 and the three-axis magnetic source 210, e.g., using accelerometers and gyroscopes incorporated in the IMUs 255, 260. Some embodiments also include interfaces (IF) 265, 270 that are used to convey information such as measured accelerations and gyroscopic orientations between the three-axis magnetic sensor 205 and the three-axis magnetic source 210. Although IMUs 255, 260 are depicted in both the three-axis magnetic sensor 205 and the three-axis magnetic source 210, some embodiments may only include an IMU in one of these elements. For example, the IMU 250 may not be included in the three-axis magnetic source 210 if the three-axis magnetic source 210 is not expected to be mobile during operation.

[0052] As discussed herein, a processor such as the processor 250 determines the relative position or orientation of the three-axis magnetic sensor 205 based on the electromagnetic signals 224 that are received while the three-axis magnetic source 210 is transmitting in a high power mode. Receiving the electromagnetic signals 224 in a subset of the time intervals indicated by a duty cycle reduces power

consumption. However, limiting the high power mode transmissions of the electromagnetic signals 224 to a subset of the time intervals indicated by the duty cycle also reduces the number and frequency of determinations of the relative position or orientation. Some applications require higher numbers or frequencies of determinations of the relative position or orientation.

[0053] In order to fill in values of the relative position or orientation between the values determined based on measurements of the electromagnetic signals 224 in the high power mode, the processor 250 computes values of the relative position or orientation of the three-axis magnetic sensor 205 using the accelerations and gyroscopic orientations measured by one or more of the IMUs 255, 260. For example, the processor 250 can compute the relative position or orientation by "dead reckoning" from a previous value of the relative position or orientation (such as a value determined based on measurements of the electromagnetic signals 224 received in the high power mode) using the measured accelerations and gyroscopic orientations. The IMUs 255, 260 can measure the accelerations and gyroscopic orientations at a higher frequency so that the relative position or orientation can be computed at a higher frequency. For example, if the IMUs 255, 260 capture data at a frequency of 100 Hz, the captured data can be used to generate values of the relative position or orientation at a frequency of 100 Hz.

[0054] In some embodiments, the values of the relative position or orientation determined based on the electromagnetic signals 224 and the measured

accelerations and gyroscopic orientations can be smoothed by integrating over the values using a Kalman filter, a proportional-integral-derivative (PID) controller, or other smoothing or filtering process. The following pseudocode represents a Kalman filtering process that can be implemented in some embodiments of the processor 250.

Kalman Filter pseudo code. main():

while True:

event = poll_event()

switch event:

Controller IMU: // 100 Hz

ProcessControllerlmuMeasurement(timestamp, acceleration,

angular_ velocity)

fused_pose = GetControllerPose()

Controller Pose: // 10 Hz

ProcessControllerPoseMeasurement(timestamp, position,

orientation)

HMD Pose: // 200 Hz

ProcessHmdMeasurement(timestamp, position, orientation) end while

Class EmlmuFusion:

self.fused_pose = None

self, measurement = None

GetControllerPose(self):

return self.fused_pose

ProcessControllerlmuMeasurement(self, timestamp, acceleration, angular_ velocity):

if self.measurement is None:

self.fused_pose = lntegrateUptoTimestamp(timestamp,

acceleration, angular_ velocity)

else:

measurement_timestamp = self.measurement[1 ]

acceleration_i, angular_velocity_i =

lnterpolateAtTimestamp(measurement_timestamp,

acceleration, angular_ velocity) prediction = lntegrateUptoTimestamp(measurement_timestamp, acceleration_i,

angular_velocity_i)

self.fused_pose = KalmanFiltering(prediction,

self.measurement[0])

self.measurement = None

ProcessHmdMeasurement(self, timestamp, position, orientation): self.latest_hmd_pose = Pose( position, orientation)

ProcessControllerPoseMeasurement(self, timestamp, position, orientation):

world_space_measurement =

TransformToWorld(position, orientation, self.latest_hmd_pose) self.measurement = (world_space_measurement, timestamp)

}

[0055] FIG. 3 is a block diagram of a system 300 including a three-axis magnetic sensor 305 that implements a single amplifier and a three-axis magnetic source 310 that implements a single amplifier according to some embodiments. The three-axis magnetic source 310 includes a set of orthogonal coils 31 1 -313, a signal generator 315, a power supply 325, a switch 330, an IMU 355, and an interface 365. The structural elements of the three-axis magnetic source 310 operate in a manner that is similar to the corresponding structural elements in the three-axis magnetic source 210 shown in FIG. 2. The three-axis magnetic sensor 305 includes a set 335 of orthogonal coils, an ADC 345, a processor 350, an IMU 360, and an interface 370. The structural elements of the three-axis magnetic sensor 305 operate in a manner that is similar to the corresponding structural elements in the three-axis magnetic sensor 205 shown in FIG. 2. [0056] The three-axis magnetic sensor 305 and the three-axis magnetic source 310 differ from the three-axis magnetic sensor 205 and the three-axis magnetic source 210 by implementing a single amplifier 320, 340 to amplify signals received from the signal generator 315 and the set 335 of orthogonal coils, respectively. The signals are multiplexed onto the single amplifiers 320, 340 in a time-multiplexed manner and then de-multiplexed for provision to the coils 31 1 -313 or the ADC 345. For example, the three-axis magnetic source includes a multiplexer 321 that multiplexes signals having different frequencies onto the amplifier 320 in a time multiplexed manner. A de-multiplexer 322 then de-multiplexes the amplified signals and provides them to the coils 31 1 -313 to generate electromagnetic signals 324 at the corresponding frequencies. For another example, the three-axis magnetic sensor includes a multiplexer 341 that multiplexes the signal generated by the orthogonal coils in the set 335 onto the amplifier 340 in a time multiplexed manner. A demultiplexer 342 then de-multiplexes the amplified signals and provides them to the ADC 345. Implementing the single amplifiers 320, 340 can reduce the power consumption of the three-axis magnetic sensor 305 and the three-axis magnetic source 310 because amplifiers typically consume significantly less power than multiplexers or de-multiplexers. [0057] Some embodiments of the three-axis magnetic source 310 do not include the multiplexer 321 . Instead, the signal generator 315 is configured to output a signal corresponding to a single frequency channel at a time. The signal generator 315 therefore generates the signals at the plurality of frequencies in successive time intervals, thereby removing the need for the multiplexer 321 . Some embodiments of the three-axis magnetic sensor 310 do not include the de-multiplexer 342. Instead, the multiplexed amplified signal is provided to a single channel ADC 345, which provides digital samples representative of the multiplexed amplified signal to the processor 350. The processor 350 performs de-multiplexing on the digitized samples.

[0058] FIG. 4 shows timing relationships 400 between signals in a position or orientation tracking system according to some embodiments. The timing

relationships 400 represent timing relationships between signals in some

embodiments of the system 100 shown in FIG. 1 , the system 200 shown in FIG. 2, or the system 300 shown in FIG. 3. Time increases from left to right in FIG. 4.

[0059] A signal 405 indicates a duty cycle for transmitting electromagnetic signals from a three-axis magnetic source. The signal 405 indicates a high power mode of operation of the three-axis magnetic source during a first subset of time intervals (such as the time interval 406) and a low power mode of operation of the three-axis magnetic source during a second subset of the time intervals (such as the time interval 407). Some embodiments of the signal 405 are used to control switching circuitry that controls the flow of power from a power supply to one or more amplifiers in the three-axis magnetic source. For example, the signal 405 can be applied to the switching circuitry 130 shown in FIG. 1 . Thus, the signal 405 is used to control a transmission power for the electromagnetic signals transmitted by the three-axis magnetic source.

[0060] A signal 410 represents one channel of a frequency multiplexed

electromagnetic signal that is selectively transmitted according to the duty cycle represented by the signal 405. The signal 410 includes relatively high power signals transmitted during time intervals 406 corresponding to a high power mode of the three-axis magnetic source. In the illustrated embodiment, an amplitude of the signal 410 is substantially zero during time intervals 407 corresponding to a low power mode of the three-axis magnetic source, e.g., because the power supplied to the one or more amplifiers is interrupted during the low power mode. However, the amplitude of the signal 410 can be non-zero during the time intervals 407 if the power supply to the one or more amplifiers is reduced to a non-zero value to place the amplifiers in an idle mode.

[0061] A signal 415 represents a channel of the frequency multiplexed

electromagnetic signal that is received by a three-axis magnetic sensor. The signal 415 includes a relatively high power signal received during the time intervals 406. For example, the amplitude of the received signal 415 exceeds a detection threshold for the three-axis magnetic sensor. The signal 415 includes noise received during the time intervals 407. In the illustrated embodiment, the amplitude of signals transmitted by the three-axis magnetic source are below the detection threshold when received by the three-axis magnetic sensor during the time intervals 407.

Thus, substantially all of the received signal 415 in the time intervals 407 is noise. The noise detected by the three-axis magnetic sensor during the time intervals 407 can be ambient noise such as electromagnetic noise produced by fluorescent lights, refrigerators, or other electronic devices.

[0062] The open blocks 420 (only one indicated by a reference numeral in the interest of clarity) represent the values of the position or orientation of the three-axis magnetic sensor relative to the three-axis magnetic source. Each of the open blocks 420 is determined based on a corresponding portion of the received signal 415 during the corresponding time interval 407. For example, a processor in the three- axis magnetic sensor can calculate the relative values of the position or orientation represented by the block 420. The sets 425 (only one indicated by a reference numeral in the interest of clarity) of crosshatched blocks represent relative values of the position or orientation of the three-axis magnetic sensor that are computed by dead reckoning from the values indicated by the block 420 based on measurements of accelerations and gyroscopic orientations performed by the three-axis magnetic source or the three-axis magnetic sensor. [0063] FIG. 5 shows timing relationships 500 between signals in a position or orientation tracking system that performs noise monitoring during a low power transmission mode of a three-axis magnetic source according to some embodiments. The timing relationships 500 represent timing relationships between signals in some embodiments of the system 100 shown in FIG. 1 , the system 200 shown in FIG. 2, or the system 300 shown in FIG. 3. Time increases from left to right in FIG. 5.

[0064] A signal 505 indicates a duty cycle for transmitting electromagnetic signals from a three-axis magnetic source. The signal 505 indicates a high power mode of operation of the three-axis magnetic source during a first subset of time intervals (such as the time interval 506) and a low power mode of operation of the three-axis magnetic source during a second subset of the time intervals (such as the time interval 507). Some embodiments of the signal 505 are used to control switching circuitry that controls the flow of power from a power supply to one or more amplifiers in the three-axis magnetic source, e.g., by applying the signal 505 to the switching circuitry. Thus, as discussed herein, the signal 505 is used to control a transmission power for the electromagnetic signals transmitted by the three-axis magnetic source.

[0065] A signal 510 represents one channel of a frequency multiplexed

electromagnetic signal that is selectively transmitted according to the duty cycle represented by the signal 505. The signal 510 includes relatively high power signals transmitted during time intervals 506 corresponding to the high power mode of the three-axis magnetic source. In the illustrated embodiment, an amplitude of the signal 510 is substantially zero during time intervals 507 corresponding to a low power mode of the three-axis magnetic source, e.g., because the power supplied to the one or more amplifiers is interrupted during the low power mode. However, the amplitude of the signal 510 can be non-zero during the time intervals 507 if the power supply to the one or more amplifiers is reduced to a non-zero value to place the amplifiers in an idle mode or other reduced power mode of operation. [0066] A signal 515 represents a channel of the frequency multiplexed

electromagnetic signal that is received by a three-axis magnetic sensor. The signal 515 includes a relatively high power signal received during the time intervals 506. For example, the amplitude of the received signal 515 exceeds a detection threshold for the three-axis magnetic sensor. The signal 515 includes noise received during the time intervals 507. In the illustrated embodiment, the amplitudes of signals transmitted by the three-axis magnetic source are below the detection threshold when received by the three-axis magnetic sensor during the time intervals 507. Thus, substantially all of the received signal 515 in the time intervals 507 is noise. The noise detected by the three-axis magnetic sensor during the time intervals 507 can be ambient noise such as electromagnetic noise produced by fluorescent lights, refrigerators, or other electronic devices. [0067] A processor in the three-axis magnetic sensor performs a sequence of activities 520 in synchronization with the generation, transmission, and reception of the frequency multiplexed signals 510, 515. In the illustrated embodiment, the processor calculates (at 525, only one calculation box indicated by reference numeral in the interest of clarity) the relative position or orientation of the three-axis magnetic sensor based on the electromagnetic signals 515 that are received during the time intervals 506 corresponding to transmission by the three-axis magnetic source in the high power mode. The three-axis magnetic sensor continues to monitor (at 530, only one monitoring box indicated by a reference numeral in the interest of clarity) the magnetic fields during the time intervals 507 to determine the amplitude of ambient noise on the frequency channels used by the frequency multiplexed signals 510, 515. In some embodiments, monitoring the magnetic fields during the time intervals 507 includes measuring a baseline reading of the electromagnetic fields for a channel and, when the duty cycle is high, using a difference between the high reading of the signals received on the channel during the time interval 506 and the baseline reading to compute the position or orientation.

[0068] The processor compares the magnitude of the noise levels detected during the time intervals 507 to a threshold. If the noise level in one or more of the channels used by the frequency multiplexed signals 510, 515 exceeds the threshold, the three- axis magnetic sensor is able to select another channel that is to be used for transmission during the time intervals 506 by the three-axis magnetic source. Some embodiments of the processor compare the magnitudes of the noise levels detected on the different channels to each other. For example, the processor can rank the different channels based on the relative values of the noise levels detected on each of the channels in the time intervals 507. The ranking can then be used to select channels for transmission of frequency multiplexed signals during the time intervals 506, e.g., by giving the highest priority to the channels that have the lowest noise levels. The three-axis magnetic sensor can transmit a signal to the three-axis magnetic source indicating the channels that have been selected for transmission of the frequency multiplexed signals in the time intervals 506. In response to receiving the indication, the three-axis magnetic source changes the modulation frequency for one or more of the set of orthogonal coils so that the three-axis magnetic source transmits the frequency multiplexed signal at the frequency corresponding to the selected channel in one or more subsequent time intervals 506. Some embodiments of the three-axis magnetic source transmit an acknowledgment message to notify the three-axis magnetic sensor that the requested channel change has occurred.

[0069] FIG. 6 is a plot 600 illustrating values of a Y-coordinate of a position of a three-axis magnetic sensor determined based on frequency multiplexed signals transmitted by a three-axis magnetic source during a first subset of time intervals and measurements of accelerations and gyroscopic orientations performed during a second subset of time intervals according to some embodiments. The vertical axis in the plot 600 indicates the value of the Y-coordinate and the horizontal axis in the plot 600 indicates time increasing from left to right.

[0070] The line 605 indicates the value of the Y-coordinate determined based on frequency multiplexed signals that are sampled at a sampling rate of 200 Hz. The line 605 therefore indicates a ground truth reference value of the Y-coordinate.

[0071] The line 610 indicates values of the Y-coordinate determined based on frequency multiplexed signals that are duty-cycled (i.e., only transmitted during a high-power mode that is determined by a duty cycle of the three-axis magnetic source) at a frequency of 10 Hz. The values indicated by the lines 605, 610 are equal when the sampling time corresponds to the high-power mode transmission time. However, the values indicated by the lines 605, 610 diverge significantly during time intervals corresponding to the low power transmission mode of the three-axis magnetic source.

[0072] The line 615 indicates values of the Y-coordinate determined based on the duty-cycled frequency multiplexed signals and measurements of acceleration and gyroscopic orientation of the three-axis magnetic sensor. The lines 605, 615 are nearly indistinguishable, which indicates that supplementing the duty-cycled frequency multiplexed signals with measurements of acceleration and gyroscopic orientation maintains the accuracy of the position or orientation determination while also reducing the power consumed by the system.

[0073] FIG. 7 is a plot 700 illustrating values of an X-coordinate of a position of a three-axis magnetic sensor determined based on frequency multiplexed signals transmitted by a three-axis magnetic source during a first subset of time intervals and measurements of accelerations and gyroscopic orientations performed during a second subset of time intervals according to some embodiments. The vertical axis in the plot 700 indicates the value of the X-coordinate and the horizontal axis in the plot 700 indicates time increasing from left to right. [0074] The line 705 indicates the value of the X-coordinate determined based on frequency multiplexed signals that are sampled at a sampling rate of 200 Hz. The line 705 therefore indicates a ground truth reference value of the Y-coordinate.

[0075] The line 710 indicates values of the X-coordinate determined based on frequency multiplexed signals that are duty-cycled (i.e., only transmitted during a high power mode that is determined by a duty cycle of the three-axis magnetic source) at a frequency of 10 Hz. The values indicated by the lines 705, 710 are equal when the sampling time corresponds to the high power mode transmission time. However, the values indicated by the lines 705, 710 diverge significantly during time intervals corresponding to the low power transmission mode of the three-axis magnetic source.

[0076] The line 715 indicates values of the X-coordinate determined based on the duty-cycled frequency multiplexed signals and measurements of acceleration and gyroscopic orientation of the three-axis magnetic sensor. The lines 705, 715 are very similar, with a slight overestimation of the value of the X-coordinate in portions of the line 715, which indicates that supplementing the duty-cycled frequency multiplexed signals with measurements of acceleration and gyroscopic orientation maintains the accuracy of the position or orientation determination while also reducing the power consumed by the system.

[0077] In some embodiments, certain aspects of the techniques described above may implemented by one or more processors of a processing system executing software. The software comprises one or more sets of executable instructions stored or otherwise tangibly embodied on a non-transitory computer readable storage medium. The software can include the instructions and certain data that, when executed by the one or more processors, manipulate the one or more processors to perform one or more aspects of the techniques described above. The non-transitory computer readable storage medium can include, for example, a magnetic or optical disk storage device, solid state storage devices such as Flash memory, a cache, random access memory (RAM) or other non-volatile memory device or devices, and the like. The executable instructions stored on the non-transitory computer readable storage medium may be in source code, assembly language code, object code, or other instruction format that is interpreted or otherwise executable by one or more processors.

[0078] A computer readable storage medium may include any storage medium, or combination of storage media, accessible by a computer system during use to provide instructions and/or data to the computer system. Such storage media can include, but is not limited to, optical media (e.g., compact disc (CD), digital versatile disc (DVD), Blu-Ray disc), magnetic media (e.g., floppy disc, magnetic tape, or magnetic hard drive), volatile memory (e.g., random access memory (RAM) or cache), non-volatile memory (e.g., read-only memory (ROM) or Flash memory), or microelectromechanical systems (MEMS)-based storage media. The computer readable storage medium may be embedded in the computing system (e.g., system RAM or ROM), fixedly attached to the computing system (e.g., a magnetic hard drive), removably attached to the computing system (e.g., an optical disc or Universal Serial Bus (USB)-based Flash memory), or coupled to the computer system via a wired or wireless network (e.g., network accessible storage (NAS)).

[0079] Note that not all of the activities or elements described above in the general description are required, that a portion of a specific activity or device may not be required, and that one or more further activities may be performed, or elements included, in addition to those described. Still further, the order in which activities are listed are not necessarily the order in which they are performed. Also, the concepts have been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present disclosure as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present disclosure. [0080] Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any feature(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature of any or all the claims. Moreover, the particular embodiments disclosed above are illustrative only, as the disclosed subject matter may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. No limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope of the disclosed subject matter. Accordingly, the protection sought herein is as set forth in the claims below.

Claims

WHAT IS CLAIMED IS:

1 . A three-axis magnetic source [1 10] comprising:

a set of coils [1 1 1 , 1 12, 1 13]; and

a power supply [125], wherein a first power and a second power are

alternately applied to drive the set of coils to generate frequency multiplexed electromagnetic signals during time intervals [406, 407] defined by a duty cycle, the first power being higher than the second power.

2. The three-axis magnetic source of claim 1 , further comprising:

a signal generator [1 15] to generate signals at a plurality of frequencies; and at least one amplifier [121 ] to amplify the signals and provide the amplified signals to drive the set of coils to generate electromagnetic signals at the plurality of frequencies, wherein the at least one amplifier is powered by using the first power and the second power.

3. The three-axis magnetic source of claim 2, further comprising:

a switching circuit [130] to convey power from the power supply to the at least one amplifier, wherein the switching circuit is configured to switch states during alternating time intervals defined by the duty cycle so that the first power is provided to the at least one amplifier during a first subset of the time intervals and the second power is provided to the at least one amplifier during a second subset of the time intervals.

4. The three-axis magnetic source of claim 3, wherein the switching circuit interrupts power transmission from the power supply to the at least one amplifier during the second subset of the time intervals.

5. The three-axis magnetic source according to any of claims 2 to 4, wherein the set of coils includes three coils and each of three amplifiers [121 , 122, 123] provide amplified signals having different frequencies to a corresponding coil.

6. The three-axis magnetic source according to any of claims 2 to 5, further comprising:

a multiplexer [321 ] to selectively provide one of the signals at the plurality of frequencies from the signal generator to the at least one amplifier in a time-multiplexed manner; and

a de-multiplexer [322] to provide amplified signals at the plurality of

frequencies from the at least one amplifier to a selected one of the set of coils.

7. The three-axis magnetic source according to any of claims 2 to 6, further comprising:

an inertial measurement unit [355] to measure acceleration and gyroscopic orientation of the three-axis magnetic source; and

an interface [365] to convey values of measurements performed by the inertial measurement unit.

8. The three-axis magnetic source of claim 7, wherein an indication of noise on at least one of the plurality of frequencies is received via the interface, and wherein the signal generator generates signals based on the indication of the noise such that the signals are generated for frequencies having noise levels below a threshold.

9. A method, comprising:

generating, at a three-axis magnetic source [1 10], frequency multiplexed

electromagnetic signals for transmission at a first power and a second power during respective first and second subsets of time intervals defined by a duty cycle, the first power being higher than the second power; and

determining at least one of a position or an orientation of a three-axis magnetic sensor [105] relative to the three-axis magnetic source based on the frequency multiplexed electromagnetic signals received by the three- axis magnetic sensor at the first power during the first subset of time intervals.

10. The method of claim 9, wherein generating the frequency multiplexed

electromagnetic signals comprises reducing a power supplied to at least one amplifier [121 ] in the three-axis magnetic source during the second subsets of time intervals.

1 1 . The method of claim 10, wherein reducing the power supplied to the at least one amplifier comprises reducing the flow of power from a power supply to the at least one amplifier.

12. The method of claim 9, further comprising:

measuring acceleration and gyroscopic orientation of at least one of the three- axis magnetic source or the three-axis magnetic sensor.

13. The method of claim 12, further comprising:

computing at least one of a position or an orientation of the three-axis

magnetic sensor relative to the three-axis magnetic source during the second subset of time intervals based on the measured acceleration and gyroscopic orientation of the at least one of the three-axis magnetic source or the three-axis magnetic sensor.

14. The method according to any of claims 10 to 13, further comprising:

measuring noise at a plurality of frequencies used by the frequency

multiplexed electromagnetic signals during the second subset of time intervals.

15. The method of claim 14, further comprising:

selecting, based on the measured noise, a subset of the plurality of

frequencies for transmission of the frequency multiplexed electromagnetic signals at the first power during the first subset of time intervals.