WO2014131894A2 - System and method for tracking a range of a moving object - Google Patents

Abstract

A system for tracking a range of a moving object is provided, the system comprising: at least two spaced apart broadband ultrasonic transmitters, each of said transmitters having a fixed and known location relative to one another; a first controller component connected to each of said transmitters and arranged to cause said transmitters to periodically emit at respective separate frequencies a burst of ultrasound; a broadband ultrasonic receiver; a second controller component operatively connected to said ultrasonic receiver to receive ultrasound signals from said transmitters; the second controller having, for each velocity within a range of velocities, a synthesised version of each burst of ultrasound at a given frequency as it would be expected to be received at said receiver when said object is moving at said velocity relative to a transmitter; the second controller being operable to periodically: for each transmitter: cross-correlate a received ultrasound signal with respective synthesized versions of bursts of ultrasound which are expected to be received directly by said ultrasonic receiver within a given time period after transmission by said transmitter; select a synthesized version with a highest cross-correlation peak as the version most accurately corresponding to the velocity of the moving object; select a time delay of a cross-correlation peak for said selected synthesized version as an indicator of a time of flight of said transmitted burst between said transmitter and said moving object; and calculate a range between said moving object and said transmitter based on said time delay; and calculate according to said ranges calculated for each transmitter, a location of said moving object relative to the locations of said transmitters.

Description

System and method for tracking a range of a moving object

Field

The present invention provides a system and method for tracking a range of a moving object in a manner that is tolerant to Doppler effect. Background

There is considerable research and commercial interest in accurately tracking the position of objects moving in three dimensions. Accurate three dimensional tracking is important for many applications, including user interface devices, monitoring of exercises and tracking human activity. Over the years many different technologies have been proposed and evaluated, including using Radio Frequency (RF) reference signals, optical systems, video systems, magnetic trackers and ultrasonic devices. Ultrasonic systems have some attractive properties. Due to the comparatively low speed of sound in air, they have the potential for high accuracy, they have a wide beam angle, reducing infrastructure requirements, and they offer low power consumption.

Early work on ultrasonic Local Positioning Systems (LPSs) utilized narrowband signals due to the restrictions of commonly available piezoelectric ceramic transducers. Unfortunately, narrowband systems show poor robustness to noise and multipath reflections of signals. In addition only a single transmitter can send at time, limiting their accuracy in tracking fast movements.

N. Priyantha, A. Chakraborty, and H. Balakrishnan, "The Cricket location-support system," in Proc. Int. Conf. on Mobile Computing and Networking, 2000, pp. 32-43; and A. Harter, A. Hopper, P. Steggles, A. Ward, and P. Webster, "The anatomy of a context-aware application," in Proc. ACM/IEEE Int. Conf. on Mobile Computing and Networking (MobiCom). New York, NY, USA: ACM, 1999, pp. 59-68 disclose Cricket and Bats from MIT and Bell Labs respectively, the most well known indoor ultrasonic location systems. More recently, broadband ultrasonic transducers have been developed, for example, as disclosed in M. Hazas and A. Ward, "A novel broadband ultrasonic location system," in ACM Int. Conf. on Ubiquitous Computing (UbiComp), ser. Lecture Notes in Computer Science, G. Borriello and L. Holmquist, Eds. Springer Berlin / Heidelberg, 2002, vol. 2498, pp. 299-305. These transducers allow for high update rates and simultaneous transmission by multiple beacons. As well as Hazas et al, N. M. Vallidis, "Whisper: a spread spectrum approach to occlusion in acoustic tracking," Ph.D. dissertation, 2002; and J. Prieto, A. Jimenez, J. Guevara, J. Ealo, F. Seco, J. Roa, and F. Ramos, "Performance evaluation of 3D-LOCUS advanced acoustic LPS," Instrumentation and Measurement, IEEE Transactions on, vol. 58, no. 8, pp. 2385 -2395, Aug. 2009 disclose improving the accuracy of location estimation for static devices by means of Direct Sequence Spread Spectrum (DSSS) modulation.

Further improvements in static accuracy and robustness can be achieved by means of Frequency Hopped Spread Spectrum (FHSS) modulation coupled with minimum phase variance range estimation as disclosed in J. Gonzalez, and C. Bleakley, "High-precision robust broadband ultrasonic location and orientation estimation," IEEE Journal of Selected Topics in Signal Processing, vol. 3, no. 5, pp. 832-844, 2009; and M. Saad, C. Bleakley, and S. Dobson, "Robust high-accuracy ultrasonic range measurement system," IEEE Transactions on Instrumentation and Measurement, vol. 60, no. 10, pp. 3334- 3341 , 201 1.

All of these systems assume that the object to be tracked is quasi- static and make no allowance for Doppler shift arising from movement. However, the Doppler shift arising for human-scale movements (up to 3 m/s) is a significant percentage of the carrier frequency of the ultrasonic signal. For example, a 3 m/s movement causes a 262 Hz shift in a 30 kHz carrier and this Doppler shift is sufficient to cause conventional cross-correlation-based Time Of Arrival (TOA) estimators to fail.

In E. Foxlin, M. Harrington, and G. Pfeifer, "Constellation: a wide-range wireless motion- tracking system for augmented reality and virtual set applications," in Proc. Int. Conf. on Computer Graphics and Interactive Techniques (SIGGRAPH), 1998, pp. 371-378, an RF reference signal and ultrasonic signals from transmitters on the ceiling were exploited by receivers on a helmet to track head motion. D. Vlasic, R. Adelsberger, G. Vannucci, J. Barnwell, M. Gross, W. Matusik, and J. Popovic, "Practical motion capture in everyday surroundings," Graphics, ACM Transactions on, vol. 26, no. 3, July 2007 disclose a body- worn ultrasonic-inertial system. In both of these systems, the ultrasonic sub-system provides position estimates when the MD is static, or nearly static, and the inertial sub-system is used to interpolate between these positions when there is movement. No provision is made for dealing with ultrasonic Doppler shift. Since the systems are narrowband, only one transmitter sends at a time. H. Muller, M. McCarthy, and C. Randell, "Particle filters for position sensing with asynchronous ultrasonic beacons," in Location- and Context-Awareness, ser. Lecture Notes in Computer Science, M. Hazas, J. Krumm, and T. Strang, Eds. Springer Berlin / Heidelberg, 2006, vol. 3987, pp. 1-13 exploit Doppler shifts to eliminate the need for an RF reference signal. The system estimates MD velocity by measuring the ultrasonic Doppler shift. The estimated velocity is integrated to obtain 3D position estimates. The system has two main drawbacks: firstly, the initial position of the MD must be known and, secondly, small velocity errors accumulate to give large position errors due to integration. Again, update rate is limited by the use of narrowband transceivers. F. Alvarez, A. Alvaro Hernandez, J. Moreno, C. Perez, J. Urena and C. Marziani, "Doppler- tolerant receiver for an ultrasonic LPS based on Kasami sequences," Sensors & Actuators: A. Physical, vol. in press, 2012 discloses a three dimensional wideband Doppler-tolerant receiver for a LPS. This uses DSSS modulation with Kasami codes and 9 matched filters implemented in an FPGA and provides ranging accuracy between 8 mm and 25 mm.

M. Alloulah and M. Hazas, "An efficient CDMA core for indoor acoustic position sensing," in Indoor Positioning and Indoor Navigation (IPIN), 2010 International Conference on, Sept. 2010, pp. 1-5 and M. Alloulah, "Real-time tracking for airborne broadband ultrasound," Ph.D. dissertation, School of Computing and Communications, Lancaster University, U.K., June 2011 each disclose DSSS-based systems, but these focus on ID ranging, rather than 3D tracking. It is worth noting that the problem of designing ultrasonic Doppler-tolerant receivers has been previously considered in underwater communications, underwater positioning and in-air ultrasonic sonar applications. Underwater communications receivers estimate the Doppler- shift and then correct for it by interpolation prior to feeding the signal into a conventional static receiver such as disclosed in M. Johnson, L. Freitag, and M. Stojanovic, "Improved Doppler tracking and correction for underwater acoustic communications," in IEEE Int. Conf. on Acoustics, Speech, and Signal Processing (ICASSP-97), vol. 1, Apr. 1997, pp. 575- 578; B. Sharif, J. Neasham, O. Hinton, and A. Adams, "A computationally efficient Doppler compensation system for underwater acoustic communications," Oceanic Engineering, IEEE Journal of, vol. 25, no. 1 , pp. 52-61, Jan. 2000; B. Sharif, J. Neasham, O. Hinton, A. Adams, and J. Davies, "Adaptive Doppler compensation for coherent acoustic communication," Radar, Sonar and Navigation, IEE Proceedings, vol. 147, no. 5, pp. 239 -246, Oct. 2000; and B. Li, S. Zhou, M. Stojanovic, L. Freitag, and P. Willett, "Multicarrier communication over underwater acoustic channels with nonuniform Doppler shifts," Oceanic Engineering, IEEE Journal of, vol. 33, no. 2, pp. 198-209, Apr. 2008. This approach is not suitable for high accuracy location estimation since errors in Doppler shift estimation and interpolation lead to phase distortions in the signal.

Underwater LPSs use simple cross-correlation based methods for range estimation because errors due to the variability of the speed of sound in water and in time synchronization dominate, as discussed in N. Kussat, C. Chadwell, and R. Zimmerman, "Absolute positioning of an autonomous underwater vehicle using GPS and acoustic measurements," Oceanic Engineering, IEEE Journal of, vol. 30, no. 1, pp. 153 - 164, Jan. 2005. It is an object of the present invention to provide a system for tracking a mobile device which overcomes the limitations of the prior art.

Summary of the Invention According to a first aspect of the present invention there is provided a system for tracking a range of a moving object, the system comprising: at least two spaced apart broadband ultrasonic transmitters, each of said transmitters having a fixed and known location relative to one another; a first controller component connected to each of said transmitters and arranged to cause said transmitters to periodically emit at respective separate frequencies a burst of ultrasound; a broadband ultrasonic receiver; a second controller component operatively connected to said ultrasonic receiver to receive ultrasound signals from said transmitters; the second controller having, for each velocity within a range of velocities, a synthesised version of each burst of ultrasound at a given frequency as it would be expected to be received at said receiver when said object is moving at said velocity relative to a transmitter; the second controller being operable to periodically: for each transmitter: cross-correlate a received ultrasound signal with respective synthesized versions of bursts of ultrasound which are expected to be received directly by said ultrasonic receiver within a given time period after transmission by said transmitter; select a synthesized version with a highest cross-correlation peak as the version most accurately corresponding to the velocity of the moving object; select a time delay of a cross-correlation peak for said selected synthesized version as an indicator of a time of flight of said transmitted burst between said transmitter and said moving object; and calculate a range between said moving object and said transmitter based on said time delay; and calculate according to said ranges calculated for each transmitter, a location of said moving object relative to the locations of said transmitters. The system may additionally be operable to output an indication of the calculated location of the moving object.

In some embodiments, each burst of ultrasound x_k[n] from a transmitter k, is defined as follows: x_t [n] = g[n modL_h ]cos 2π

where:

g[J is a window function;

L_h is length in samples n of the burst h within a sequence comprising a block of Nb bursts; k is a frequency for said burst; and

F_s is a sampling frequency.

The window function g[] may be defined as follows:

g[n] = w[n] if 0 < n

= 0 otherwise where L is the length of a burst within a hop comprising L_h'=L_h+L_g samples_. The function w[n] may comprise one of a Hanning, Hamming or Blackman window.

The second controller may be arranged to adjust a time delay selected for a burst of ultrasound within a block of bursts as a function of respective time delays selected for each burst within said block of bursts of ultrasound.

The burst frequency for a transmitter may be chosen according to a frequency hopping spread spectrum (FHSS) protocol comprising N_b bursts within a repeating block of bursts. The first controller may additionally be arranged to cause said ultrsonic transmitters to concurrently transmit said bursts of ultrasound. Additionally or alternatively, each of said transmitters may be arranged to concurrently emit a burst of ultrasound at respective carrier frequencies spaced apart from one another. The first controller may be arranged to cause at least one of said transmitters k, to concurrently emit two bursts of ultrasound [«],¾ [«] , wherein each burst of ultrasound has a different frequency f_k ' f_k2■ The different frequencies , f_k may be orthogonal.

The object may be a mobile device and the broadband ultrasonic receiver may be included in the mobile device. In some such embodiments, the first and second controller components are implemented within a common controller, said common controller being operably connected to an RF receiver, said mobile device including an RF transmitter arranged to relay a received ultrasound signal to said RF transceiver, said second controller component being operable to determine said time of flight of said transmitted bursts of ultrasound based on a difference between said calculated time delays and respective transmitted times of said bursts.

In some embodiments in which the object to be tracked is a mobile device, the mobile device incorporates said second controller component, said first controller component being operably connected to a reference signal transmitter and said mobile device including a reference signal receiver, said first controller being operable to cause a reference signal to be transmitted substantially simultaneously with said bursts of ultrasound to enable said second controller component to determine said time of flight of said transmitted bursts of ultrasound based on a difference between said selected time delays and a received time of said reference signal. The reference signal may be one of: an RF or an IR or LED signal. In some embodiments, the broadband ultrasound receiver is remote from the mobile device. For example, the broadband ultrasonic transmitters and the broadband ultrasound receiver may be included within the same device. In such embodiments the received ultrasound signal comprises a reflection of one or more of the bursts of ultrasound emitted by the broadband ultrasonic transmitters, said reflection being generated by the one or more emitted bursts of ultrasound being reflected by the moving object.

In accordance with a further aspect of the invention, there is provided a method of tracking a range of a moving object comprising: causing at least three spaced apart broadband ultrasonic transmitters, each of said transmitters having a fixed and known location relative to one another, to periodically emit at respective separate frequencies a burst of ultrasound; receiving, at a broadband ultrasonic receiver, ultrasound signals from said transmitters; providing, for each velocity within a range of velocities, a synthesised version of each burst of ultrasound at a given frequency as it would be expected to be received at said receiver when said object is moving at said velocity relative to a transmitter; periodically, for each transmitter: cross-correlating a received ultrasound signal with respective synthesized versions of bursts of ultrasound which are expected to be received directly by said ultrasonic receiver within a given time period after transmission by said transmitter; selecting a synthesized version with a highest cross-correlation peak as the version most accurately corresponding to the velocity of the moving object; selecting a time delay of a cross- correlation peak for said selected synthesized version as an indicator of a time of flight of said transmitted burst between said transmitter and said moving object; and calculating a range between said moving object and said transmitter based on said time delay; and calculating according to said ranges calculated for each transmitter, a location of said moving object relative to the locations of said transmitters.

The object may comprise a mobile device including the broadband ultrasound receiver. In some embodiments, the broadband ultrasonic transmitters and the broadband ultrasound receiver are included within the same device; and receiving the ultrasound signal comprises receiving a reflection of one or more of the bursts of ultrasound emitted by the broadband ultrasonic transmitters, said reflection being generated by the one or more emitted bursts of ultrasound being reflected by the moving object.

In accordance with a further aspect of the invention, there is provided a mobile device comprising a carrier signal generator; a broadband ultrasonic receiver operably connected to an RF modulator, said RF modulator being operable to modulate said carrier signal as a function of a received ultrasonic signal to provide a modulated carrier signal; said modulator being operably connected to an RF transmitter to transmit said modulated carrier signal to a remote RF receiver.

The mobile device may further comprise one of an: inertial or magnetic sensor, said sensor being operably connected to a second modulator for further modulating said carrier signal according to the sensed movement of said mobile device.

The present invention provides a Doppler-tolerant approach to 3D tracking of a moving object, for example a Mobile Device (MD) or a human body or part thereof.

fitted with a wideband ultrasonic receiver. The 3D position of the MD can be determined based on ultrasonic signals transmitted by at least three spaced wideband ultrasonic beacons.

The beacon-MD ranges are estimated using a Doppler-tolerant receiver. Doppler-tolerance is achieved by synthesizing reference beacon signals with a range of Doppler shifts. The synthesized signal giving the strongest cross-correlation peak with the received signal is selected as the one corresponding to the true Doppler shift. A modified minimum phase variance method can then be applied to estimate the range with high accuracy under conditions of motion. Three ranges estimated for respective beacons can be fused to obtain a 3D location estimate using trilateration.

To ensure a high update rate, the beacons may transmit concurrently (or simultaneously). In some embodiments, in addition to the ultrasonic signals, the MD receives a RF timing reference signal from the beacons. Error detection and correction can be performed to deal with any intermittent ranging errors.

Embodiments of the invention employ an FHSS signal design that meets the conflicting requirements of good correlation properties, low probability of collision and high update rate. Elements of the signal used in embodiments of the invention are:

• guard times to prevent collisions between consecutive hops;

• random hop inversion to improve correlation properties; and

• closely spaced carrier frequencies with a concurrent separation criteria achieving good correlation properties and avoiding phase distortion.

Brief Description of the Drawings

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

Figure la depicts an exemplary situation in which an object to be tracked is located at a respective distance to each of a plurality of transmitters.

Figure lb is a schematic diagram of a system for tracking a range of a moving object according to an embodiment of the present invention;

Figure lc is a schematic diagram of a system for tracking a range of an object according to an embodiment of the present invention; Figure 2 shows a sample signal produced by beacons of the apparatus of Figure la;

Figure 3 shows the signal structure for the signals of Figure 2;

Figure 4 shows a sample cross-correlation between a received signal and a synthesised signal (solid), the Hilbert transform of this signal (dotted) and the absolute value of the analytic signal which is the summation of the real cross-correlation and the imaginary Hilbert transform (dashed); Figure 5 provides an overview of the processing steps performed by a controller for one embodiment of the invention; and Figure 6 shows schematically the components of a low cost mobile device for use within a second embodiment of the invention.

Description of the Preferred Embodiment

Referring now to Figure l a, there is shown an exemplary situation in which an object to be tracked is located at a respective range (or distance) from each of a plurality of beacons k=l . ..3. Embodiments of the invention provide a method of determining a respective range r (or distance) between each of the beacons and the object. Based on the determined ranges, localisation of the object is possible. Figure la depicts a single object to be tracked. However, it will be appreciated that the system may track multiple objects at the same time.

In particular, as will be discussed further in relation to Figures lb and lc, the beacons emit ultrasonic signals which are detected by a broadband ultrasonic receiver (not shown in Figure la). It will be appreciated that the number of beacons N_b comprised within the system may on the localisation required. In the exemplary embodiments of Figures la to lc, N_b=3 to allow for 3D localisation. However, fewer or more beacons may also be used. For example, in situations where 2D localisation is required, N_b=2 beacons might instead be used.The beacons are connected to a common controller (not shown in Figure l a). Where the beacons are mounted on a common physical substrate, the connection to the controller can be wired, otherwise, the connection may be wireless.

As will be described below with reference to the accompanying figures, embodiments of the invention provide a system for tracking an object in which:

• Each of the N_b beacons comprises a respective wideband ultrasonic transmitter. The transmitters are synchronized with a wired or wireless reference signal. If the beacons are not hardwired on a common substrate to a controller, they may be mutually synchronized, for example, with an RF signal, otherwise synchronization can be wired. • The system comprises a broadband ultrasonic receiver configured toreceive wideband ultrasonic signals. In particular, the broadband ultrasonic receiver is configured to receive at least part of the signals transmitted by each of the beacons. The relative positions of the beacons are fixed and known.

· The speed of sound in air in the LPS space is either known or the ambient temperature is measured and the speed of sound is inferred. For example, a temperature sensor may be included within (or in close proximity to) the beacons. The speed of sound may then be calculated based on readings from the temperature sensor.

• The LPS is situated such that airflow is negligible or the system measures the airflow and uses this measurement to compensates for the airflow in the range estimation process.

As depicted in figure lb, in some embodiments, the beacons and the ultrasonic receiver are configured such that at least a respective component of a signal transmitted by each of the beacons is reflected from the moving object and the reflected signal components are received by the ultrasonic receiver. In such embodiments, the ultrasonic receiver may be comprised within the same device as the transmitters.

In an exemplary embodiment, the moving object comprises the human body, such as a hand or thumb. As discussed in relation to figure la, the system may be configured to track multiple objects, for example, multiple hands and/or multiple fingers of a given hand at the same time (or during the same time priod). Beacons comprised within a user device, e.g. a smartphone, tablet, personal computer etc., trasnmit ultrasonic signals, some or all of which are reflected by the body part and received by an ultrasonic receiver also comprised within the uesr device. As discussed in more detail below, a processor within the user device is then operated to determine a location and/or gesture of the one or more body parts based on the signals received by the receiver.

As depicted in figure lc, in some embodiments the moving object is a Mobile Device (MD). As discussed in relation to figure la, the system may be configured to track multiple MDs simultaneously (or during the same time period). In such embodoiments, the ultrasonic receiver may be comprised within the MD so that the MD can receive wideband ultrasonic signals. For exampple, the MD (and the ultrasonic receiver comprised therein) may have a line of sight to Nb beacons. As explained below, where the line of sight is temporarily lost, the MD can use inertial sensing to track its movement until line of sight is restored.

In some embodiments, processing circuitry comprised within the MD is configured calculatethe position of the MD based on the signal received by the receiver. In some such embodiments, the MD receives an RF timing reference signal from the controller. However, this is not necessary if the position can be determined in a reference- free manner using Time Difference of Arrival (TDOA) or (Angle Of Arrival) such as disclosed in M. Saad, C. Bleakley, T. Ballal, and S. Dobson, "High accuracy reference-free ultrasonic location estimation," Instrumentation And Measurement, IEEE Transactions on, vol. 61, no. 6, pp. 1561-1570, June 2012. This discloses a reference- free ultrasonic indoor location system where a mobile device (MD) determines its own position based only on ultrasonic signals received at a compact sensor array and sent by a fixed independent beacon. No radio frequency or wired timing reference signal is used. The MD uses angle of arrival (AoA) to obtain an initial estimate of its own location and based on this, it estimates the timing offsets (TOs) between the MD clock and the beacon transmissions to determine its location relative to the beacons with high accuracy. Alternatively, the need for a timing reference signal can be eliminated by placing the MD in a known location at certain times during motion tracking such as disclosed in S.Y. Park, H.S. Ahn and W. Yu, "A simple object tracking system using ITDOA without time synchronization", Advanced Communication Technology, the 9th International Conference on, pp. 2026-2028, February 2007. Since the MD position is known at these times, the timing between the MD and beacons can be established and utlized thereafter. While these methods allow reference-free operation, use of a timing reference signal generally improves accuracy over reference-free methods. Note also that the references does not need to be RF, although that is probably the most convenient. Other possibilities include IR or LEDs.

The transmitters may be configured to transmit bursts of ultrasound according to an FHSS signal design. FHSS originates in telecommunications systems and involves a sinusoidal carrier whose frequency switches periodically, or hops, according to a known pseudorandom hopping pattern. The discrete-time signal transmitted by a beacon k can be described as:

where n is the sample number, f_k[i] is the carrier frequency of beacon k during hop i, L_h is the length of a hop in samples, F_s is the sampling frequency and quot(.) returns the integer quotient of the argument. If the transducers within the beacon and MD transmitter/receivers provide a channel bandwidth of B, the number of available carrier frequencies will be N_c=B/B_h+l where B_h is the bandwidth of each hop. The carrier frequencies are chosen pseudo-randomly from the set of all possible carriers. For a given beacon k, the carrier frequency sequence f_k[i] can be specified as:

m = F₀ + c_k i]B_k (2) where F₀ is the lowest carrier frequency, c_k[i] is the FHSS code sequence of hop i and c_k[i] is an integer in the range 0 to N_c-1.

Since in the present ranging application, the FHSS signal need not modulate data, the correlation properties of the signals can be improved by allowing negative carrier frequencies, i.e. phase inversions:

f_k[i] = sign(c_k[i])F_o + c_k[i]B_k (3) where the function sign(.) returns -1 or +1 depending on the sign of the argument and the modified carrier index c_k'[i] is now an integer in the range -N_c+1 to+N_c-l.

Since the phase of the carrier is used in estimation of the beacon range between the transmitting beacons and the object to be tracked (the 'beacon-object' range), it is important that that direct signals do not collide with reflections, i.e. multi-path, and that direct signals from different beacons do not collide with each other. FHSS modulation provides robustness to multi-path, i.e. collisions between direct and indirect signals, in two ways. Firstly, provided that hops are sufficiently short (L_h), the carrier hops away from a given frequency before its reflection arrives. Secondly, provided that the number of carrier frequencies (N_c) is sufficiently large, the probability of collisions between the current hop and previous hops is low.

Consider the direct path signals from two beacons. Conventionally, the code sequence is designed so that concurrent hops from any of the beacons do not use the same carrier. However, in the ranging case, significant phase distortion occurs if the frequency separation of concurrent hops is less than a minimum frequency separation B_s. This frequency separation requirement can be reduced by applying a windowing function w[n], e.g. a Hanning, Hamming or Blackman window, to the signals so as to reduce spectral leakage. Conventionally, the hop bandwidth 5_¾ would be set equal to the minimum frequency separation B_s. However, in the ultrasonic case this would lead to too few carrier frequencies. In simulation, it was found that concurrent Hanning windowed sine waves of duration 1.33 ms must be separated by at least 2 kHz in order to avoid significant error when their phase is estimated via the Fourier Transform. We refer to this as the frequency separation limit 5_/. Longer hops decrease the separation requirement but lead to self-collisions close to walls, e.g. a 1.33 ms duration gives 50% self-collision at 11cm from a wall. Ultrasonic piezoelectric transducers, such as those from Prowave, provide bandwidths in the range 1.5 - 3 kHz whereas an audio grade super tweeter, such as the Fostex FT17H, gives a useable bandwidth of roughly 8 kHz in the ultrasonic range. Even accounting for this, a carrier separation of B_s=2 kHz, a device bandwidth of 5=8 kHz and k=3 beacons allow only N_c=5 carrier frequencies giving an unacceptably high probability of collision under conditions of multipath and concurrent operation of all beacons.

To resolve this problem in the invention, the carrier separation B_s is allowed to be less than the minimum dictated by the fequency separation limit, i.e. B_S<B;. However, the hopping sequences are selected subject to the constraint that, at any point in time, the carrier frequencies emitted any pair of beacons must have a separation greater than the frequency separation limit, i.e. L/fc H ^~~ fm [i] \≥ Bi where k m _means that, provided that the moving object is approximately equidistance to all of the becaons, the direct path signals from the beacons to the moving object are separated by more than the frequency separation limit and so their phase can be accurately determined. Of course, multipaths can still collide with the direct signal and cause phase distortion. As is normal with FHSS modulation, the probability of multipath collision is reduced by the use of orthogonal pseudorandom hopping sequences. Thus, an exemplary embodiment of the system achieves accurate range estimation with k=3 beacons, a device bandwidth of 5=10 kHz, a channel separation of 5 =0.833 kHz, giving N_c=13, and a concurrent frequency separation limit of 5/=2 kHz. When the moving object is not equidistant to the beacons, there is a possibility of phase distortion arising due to collisions between consecutive hops from different beacons. These collisions arise from differences in the beacon-object ranges. Clearly, the maximum range difference is equal to the maximum inter-beacon separation. Where the beacons are closely spaced, the potential overlap between consecutive hops from different beacons is small. So to prevent collisions between direct consecutive hops from pairs of beacons, a silent guard time of length L_g samples is introduced between hops. In the case of closely spaced beacons, collisions such as these can be prevented by setting the guard time equal to the time of flight (TOF) of the largest beacon separation. Including the windowing function w[n] and guard time, the overall FHSS signal used in the preferred embodiment becomes:

where L_h'=L_h+L_g is the burst length in samples and g[n] is the guarded window function defined as:

g[n] = w[n] if O < n < L_h

= 0 otherwise

An illustration of a signal transmitted from a beacon is illustrated in Figure 2 and the overall signal structure is shown in Figure 3. A signal block is made up of Nb hops, where Nb is an integer. The length of each block is then Lb=NbL_h' samples. In this embodiment, the infrastructure indicates the start of each block by sending an RF pulse and the MD provides a location estimate for each block, giving an update rate

In other implementations explained below, the position estimate can be provided by the common controller at the same update rate.

The signal repeats every N_r blocks. The value of N_r is chosen such that signal reflections are sufficiently attenuated before the signal repeats.

In an exemplary embodiment of the system, the sampling rate is 96 kHz, the hop length is L_h = 128 samples, the guard time is L_g=64 samples, the number of hops per block is N«=10 and the period is N_r=2 blocks. This gives a maximum inter-beacon separation of 22.7 cm and a range update rate of 50 Hz. The pseudorandom hopping sequences were selected by generating a number of candiate sequences, evaluating their properties and selecting the best for use in the system. The system requires kN_r hopping sequences each containing N_b hops. Each hop is represented by an integer <¾'[/] in the range -N_c+ 1 to+N_c-l . A subset of the candidate sequences were derived from Costas arrays of length N_c, by concatenation. A Costas array is a permutation array containing the numbers \ :N_b, such as disclosed in J. Costas, "A study of detection waveforms having nearly ideal range-doppler ambiguity properties," in Proceedings of the IEEE, vol. 72, no. 8, Aug. 1984, pp. 996-1009. The remaining arrays were obtained by generating Costas arrays of order N_b using the Welch and/or Golomb constructions, see S. Golomb and H. Taylor, "Constructions and properties of Costas arrays," Proceedings of the IEEE, vol. 72, no. 9, pp. 1 143-1 163, Sep. 1984, and randomly replacing elements from the arrays with the numbers N_b+ l :N_c. In both cases, half of the hops were randomly negated to allow for phase inversion, as per Equation (3). Once the candidate sequences had been generated, they were evaluated. The beacon signal x_k'[k] corresponding to each candidate was synthesized according to Equation (4). The auto- and cross-correlation were calculated for all signals and pairs of signals. The kN_r candidate hopping sequences matching the separation criteria and yielding the best auto- and cross-correlation properties were selected for use in the system. In some embodiments, two bursts of ultrasound signal x_k[n] with different carrier frequencies f_k ^' are transmitted concurrently by one or more of the transmitters. The carrier frequencies are selected according to the frequency and time separation rules described above. In an exemplary embodiment, the ultrasound signal bursts are orthogonal in frequency and are used in the cross correlation, phase adjustment and minimum phase variance steps described below in the same way as successive hops.

The advantage of using a pair of concurrent carriers is that, during motion, successive hops may be subject to differential phase changes. As a consequence, the minimum phase variance step becomes less robust than in the static case. In the case of motion, applying the minimum phase variance to concurrent carriers provides more robust results since, provided that the frequency separation is small, both carriers are subject to similar phase changes. Referring to Figure 5, the receiver algorithm comprises the processing steps required to determine a location of the object to be tracked. The receiver algorithm involves several steps: signal synthesis, cross-correlation, phase adjustment, minimum variance search, error correction and trilateration explained below:

Signal Synthesis

Doppler shifted variants of the transmitted signal from each beacon are generated for velocities v from -v_m to +v_m in steps of v; according to:

(6) where ^■!· indicates Doppler temporal compression and† indicates Doppler frequency shift given by: n i= n{\ + -) (7)

c

/^†=/a+-) (8)

c

where c is the speed of sound.

Typically, the range chosen is from -3m/s to +3m/s at intervals of 0.2m/s. Although this can be varied according to the expected range of velocity of the object with 3 m/s being typical of the range of human movement.

The direction of the arrow is consistent with the object moving towards the beacon, a positive velocity. In the case that the object is moving away from the beacon, the velocity is negative. Quadrature components are also generated for use in the phase adjust step by replacing the cosine in the above equation with a sine function. These signals can be stored once synthesized or generated on the fly depending on the memory availability and processing capability of the device. Cross-correlation

In the moving case, the velocity of the object is initially unknown. In embodiments of the invention, the received signal is cross-correlated with all of the above synthesized signals for each beacon.

It should be appreciated that the received ultrasonic signal will typically comprise a composite of signals which were transmitted simultaneously from each beacon. However, cross correlating the composite received signal against the synthesised signal for a given beacon, inherently isolates the received signal corresponding to the beacon.

The cross-correlation for each beacon p_k[v,d\ is given by:

Lc-l

pk[v, d] =∑ y[n]x_k [v,n + d] (9)

-0

where y[n] is the received signal, L_c is the length of the cross-correlation and d is the delay in samples. The zero sample (n=0) is taken as the sample arriving simultaneously with the RF reference signal, where such a signal is used. The length of the cross-correlation is the sum of the block length and the maximum range in samples, i.e. L_c=Lb+L_m. The synthesized signal giving the strongest correlation peak is taken as the one corresponding to the correct velocity of the object relative to the particular beacon. (Where beacons are relatively closely spaced by comparison to their distance to the object, the calculated velocity for each beacon would be expected to be similar, however, this need not be the case.)

To eliminate the effect of phase ambiguity, the absolute value of the analytic signal obtained from the cross-correlation is calculated as disclosed in J. Gonzalez, and C. Bleakley, "High- precision robust broadband ultrasonic location and orientation estimation," IEEE Journal of Selected Topics in Signal Processing, vol. 3, no. 5, pp. 832-844, 2009 referred above. The real part of the analytic signal is the cross-correlation itself, while the imaginary part is the Hilbert transform of the cross-correlation:

p_k' [v, d] = \p_k[v, d] + jH[p_k[v, d]]\

(10) where j = V— Ϊ and H[.] indicates the Hilbert transform, i.e. a 90° phase shift. As illustrated in Figure 4, for each velocity, the best delay candidate < [v] is the delay of the cross- correlation peak d_peak or the delay of earliest similar peak d_epeak, if it exists. The earliest peak must be at least 70% of the maximum peak.

The selected beacon-object range d_k and velocity V_k are the candidate delay and velocity associated with the largest cross-correlation peak:

d_k = d_k[v_k] si.Vv : d_k = d_k[v_k]≥ d_k[v] (11)

To reduce computational complexity, the cross-correlation and Hilbert transformation functions (Eqs. 9 and 10) can be performed in the frequency domain: p_k' [v, d] = \F-^l [H_cF[x_k ^f [n]TF[y[n]]]\

(12) where H_c=[l,2,...,2,1,0,...,0] are the Hilbert coefficients, F[.] is the FFT, is the Inverse FFT and is the complex conjugate operator. Additionally, after initial acquisition, the velocities considered need only be the neighbors of the previous velocity and the lags considered in the cross-correlation need only be those close to the previous delay. Phase Adjustment

The previous cross-correlation stage provides an estimate of the beacon-object ranges. Under ideal conditions, this range estimate is accurate to the nearest integer sample. Phase adjustment refines this estimate to sub-sample accuracy. Based on the Time Of Arrival (TOA) of the RF pulse (n=0), where this is used, and the estimated cross-correlation delay d_k , the TOA of a start of the current block is known. For each hop in the block, the phase of the signal φ^ί] is calculated by multiplying by the received signal by the in-phase and quadrature components of the signal synthesized for that beacon and the selected velocity:

L_h-\ _λ ^ , , ,

[/^'] = arctan(∑ y[d_k + iL_b + n]x_k [v_k , iL_b + n] - j∑ y[d_k + iL_h + n])x_k ^" [v_k , iL_h + n] n=0 n=0

(13)

These phase estimates can be viewed as adjustments that must be applied to the raw integer delay. In order to apply them, they are converted to delays in samples: _k[i] =

2 ¾[ϊ] T

(14)

These delay adjustments are added to the integer delay estimate from the cross-correlation stage above to obtain a sequence of sub-sample delay estimates. These sub-sample delay estimates are compensated for motion over the duration of the block:

where c is the speed of sound in air. The sub-sample delay estimate d_k is obtained by averaging over the block:

Minimum Phase Variance

The previous phase adjustment stage provides high accuracy estimates of the beacon-object delays if the cross-correlation range estimate is correct to within plus or minus half a carrier wavelength. If greater adjustments are required, the method fails due to phase ambiguity. To circumvent this problem, the phase adjustment step is applied to candidate integer delays in the range d_k— d_sto d_k + d_s where 2d_s+l is the search size. The variance in the delay estimates

Ok is calculated for each candidate integer delay according to:

The candidate integer delay giving the minimum variance in the delay estimates is taken at the true integer delay. The associated sub-sample delay estimate d_k is taken as the sub-sample delay for that beacon. The beacon-object range is then calculated as: n = d_kc

(18)

In the case that the object is moving, it is assumed that the object's velocity relative to a beacon can be considered to be quasi-constant over the duration of a signal block. Based on this, the sub-sample delay estimates at each hop either increase or decreased in a linear fashion as the object moves away from or towards the beacon. This effect can be accounted for applying linear fitting to the sub-sample delay estimates and detrending using the gradient estimate prior to calculating the variance. An alternative to linear fitting is to use the velocity estimate from the cross-correlation stage as an estimate of the gradient of the linear trend.

Error Correction

Ranging errors are detected when the difference between a pair of beacon-object range estimates exceeds the beacon separation ¾:

A beacon which gives errors in two range differences is taken to be incorrect and its range is re-calculated. In the re-calculation, the coarse range estimate is taken as the peak of the cross- correlation between the longest non-erroneous range minus the beacon separation and the shortest non-erroneous range minus the beacon separation.

Trilateration

After the sub-sample ranges have been determined for all of the beacons, the 3D location of the object is determined by trilateration. A closed-form solution for example as disclosed in D. Manolakis, "Efficient solution and performance analysis of 3-d position estimation by trilateration," Aerospace and Electronic Systems, IEEE Transactions on, vol. 32, no. 4, pp. 1239 -1248, Oct. 1996 can be used to obtain an initial estimate of position. Starting from this initial estimate, a least squared error minimization routine can be used to refine the position estimate, for example, as disclosed in E. Dijk, C. van Berkel, R. Aarts, and E. van Loenen, "3-D indoor positioning method using a single compact base station," in Proc. IEEE Conf. on Pervasive Computing and Communications (PerCom), Mar. 2004, pp. 101 - 1 10. The routine minimizes the total squared error between the estimated ranges r^ and the ranges arising from the candidate 3D position. Once the error threshold has been reached, the error minimization routine terminates giving the final 3D co-ordinates of the object p[b] where b is the block number. The final 3D velocity and acceleration of the object are calculated based on the difference between the current 3D position and the previous position p[b- 1 ] . In some embodiments the receiver algorithm is implemented at or by the object itself. For example, in embodiments in which the object to be tracked is a mobile device (MD), the MD may be comprise a processor configured to implement the receiver algorithm. For example, in embodiments in which the object to be tracked is an MD comprising a wideband receiver, the MD may be configured to determine, based on the received ultrasonic signals together with an RF reference signal, a range of the MD to each of three beacons and from this to calculate the MD's position and velocity in three dimensional space. It will be appreciated that this requires the MD to incorporate some processing capability making the device relatively expensive to produce and/or resulting in an MD that is too large (or undesirably large for some applications.

Alternatively, the receiver algorithm may be implemented by a controller external to the object to the be tracked, wherein the controller comprises a processor (or processing circuitry) configured to perform the steps of the receiver algorithm.

In some such embodiments, the controller may be comprised within the same device as the transmitters and/or the receiver. For example, in an embodiment in which the object to be tracked is a human hand and/or fingers, the transmitters, the receiver and the controller may be comprised within a user device.

In embodiments in which the object to be tracked is an MD and the receiver algorithm is implemented by the controller, the signal received at the MD may be returned (or retransmitted) to the controller (or to a receiver operating in association or communication with the controller). In this manner, the receiver algorithm may be implemented within a static device. Accordingly, the MD and/or the processing performed by the MD may be simplified resulting in a reduction in power consumption, batter requirements, cost etc.

Figure 6 depicts an exemplary embodiment in which the receiver algorithm is performed by a central controller. In the embodiment of Figure 6, the MD comprises an ultrasonic receiver which is connected to an RF modulator. The modulator can be a frequency or amplitude modulator or both. The modulator output signal is fed to an RF transmitter (or possibly a transceiver) which transmits the modulated signal. In this embodiment, the beacons are connected to a central controller which, as well as synthesizing the ultrasonic signals to be transmitted by the beacons, is coupled to an RF receiver (or possibly a transceiver). The RF receiver is connected to a de-modulator which produces a replica of the ultrasonic signal originally received at the mobile device. This is digitized through an ADC and the central controller can therefore perform the above described steps of reference signal synthesis, cross-correlation, phase adjustment, minimum variance search, error correction and trilateration to determine the range of the mobile device relative to the beacons and thus determine its location. This can be readily provided to any application or device which wishes to track the mobile device. The mobile device can therefore be implemented with the minimum of circuitry, i.e. as little as a microphone and modulator, and as such as cheaply as possible.

In such embodiments, the MD may also include a frequency shifter (not shown) for shifting the received modulated signal which may typically vary between 30-50 kHz to 0-20k Hz before modulating the carrier signal which might have a frequency of 2.4 Ghz. In this case, the common controller is appropriately adapted to take into account this shifting.

Nonetheless, in certain embodiments (regardless of whether the receiver algorithm is performed by the MD itself or by the controller), the mobile device can also include inertial sensors and/or magnetic sensors, for example, accelerometers or gyroscopes implemented as MEMs devices, which may be used to track movement of the device if it is occluded from one or more of the beacons for a period of time. In addition, when the ultrasonic signals are not occluded, the interial and/or magnetic measurements may be fused with the ultrasonic position estimates, possibility by means of Kalman or particle filtering, to improve the accuracy of motion tracking. In the second embodiment, the inertial and/or magnetic sensors can also be connected via a second modulator to the mobile device RF transmitter (transceiver). This second modulator applies the other of FM or AM modulation to the ultrasonic signal allowing both the ultrasonic and inertial tracking signals from the mobile device to be simultaneously transmitted to the central controller.

Where an MD is equipped with an RF transceiver and a suitable controller, it can also receive data from the central controller and depending on the protocol implemented with the controller, respond to certain requests. So for example, the central controller might be able to communicate with the mobile device to have it move from an active mode to a sleep mode where it is not required to transmit movement information to rationalize mobile device power consumption.

Claims

Claims:

1. A system for tracking a range of a moving object, the system comprising:

at least two spaced apart broadband ultrasonic transmitters, each of said transmitters having a fixed and known location relative to one another;

a first controller component connected to each of said transmitters and arranged to cause said transmitters to periodically emit at respective separate frequencies a burst of ultrasound;

a broadband ultrasonic receiver;

a second controller component operatively connected to said ultrasonic receiver to receive ultrasound signals from said transmitters;

the second controller having, for each velocity within a range of velocities, a synthesised version of each burst of ultrasound at a given frequency as it would be expected to be received at said receiver when said object is moving at said velocity relative to a transmitter;

the second controller being operable to periodically:

for each transmitter:

cross-correlate a received ultrasound signal with respective synthesized versions of bursts of ultrasound which are expected to be received directly by said ultrasonic receiver within a given time period after transmission by said transmitter;

select a synthesized version with a highest cross-correlation peak as the version most accurately corresponding to the velocity of the moving object;

select a time delay of a cross-correlation peak for said selected synthesized version as an indicator of a time of flight of said transmitted burst between said transmitter and said moving object; and

calculate a range between said moving object and said transmitter based on said time delay; and

calculate according to said ranges calculated for each transmitter, a location of said moving object relative to the locations of said transmitters.

2. The system of claim 1 wherein each burst of ultrasound x_k[n] from a transmitter k, is defined as follows:

where:

g[] is a window function;

L_h is length in samples n of the burst h within a sequence comprising a block of Nb bursts; f is a frequency for said burst; and

F_s is a sampling frequency.

3. The system of claim 2 where said window function g[] is defined as follows:

g[n] = w[n] if O≤n < L_h

= 0 otherwise

where L is the length of a burst within a hop comprising L_¾'=L_h+L_g samples_.

4. The system of claim 3 wherein said function w[n] comprises one of a Hanning, Hamming or Blackman window.

5. The system of any one of claims 2 to 4, wherein said second controller is arranged to adjust a time delay selected for a burst of ultrasound within a block of bursts as a function of respective time delays selected for each burst within said block of bursts of ultrasound.

6. The system of any one of claims 2 to 5, wherein said burst frequency for a transmitter is chosen according to a frequency hopping spread spectrum (FHSS) protocol comprising Nb bursts within a repeating block of bursts.

7. The system of claim 6 wherein said first controller is arranged to cause said ultrsonic transmitters to concurrently transmit said bursts of ultrasound.

8. The system of claim 6 wherein each of said transmitters is arranged to concurrently emit a burst of ultrasound at respective carrier frequencies spaced apart from one another.

9. The system of any one of claims 2 to 8, wherein the first controller is arranged to cause at least one of said transmitters k, to concurrently emit two bursts of ultrasound x_k M,¾₂ \n\ , wherein each burst of ultrasound has a different frequency f_k , f_k .

10. The system of claim 9, wherein the different frequencies f_k , f_k are orthogonal.

11. The system of any one of the preceding claims, wherein the object is a mobile device and wherein the broadband ultrasonic receiver is included in the mobile device.

12. The system of claim 11 wherein said first and second controller components are implemented within a common controller, said common controller being operably connected to an RF receiver, said mobile device including an RF transmitter arranged to relay a received ultrasound signal to said RF transceiver, said second controller component being operable to determine said time of flight of said transmitted bursts of ultrasound based on a difference between said calculated time delays and respective transmitted times of said bursts.

13. The system of claim 11, wherein said mobile device incorporates said second controller component, said first controller component being operably connected to a reference signal transmitter and said mobile device including a reference signal receiver, said first controller being operable to cause a reference signal to be transmitted substantially simultaneously with said bursts of ultrasound to enable said second controller component to determine said time of flight of said transmitted bursts of ultrasound based on a difference between said selected time delays and a received time of said reference signal.

14. The system of claim 13 wherein said reference signal is one of: an RF or an IR or LED signal.

15. The system of any one of claims 1 to 10, wherein:

the broadband ultrasound receiver is remote from the mobile device; and

the received ultrasound signal comprises a reflection of one or more of the bursts of ultrasound emitted by the broadband ultrasonic transmitters, said reflection being generated by the one or more emitted bursts of ultrasound being reflected by the moving object.

16. The system of any one of the preceding claims, wherein the system is operable to output an indication of the calculated location of the moving object.

17. A method of tracking a range of a moving object comprising:

causing at least two spaced apart broadband ultrasonic transmitters, each of said transmitters having a fixed and known location relative to one another, to periodically emit at respective separate frequencies a burst of ultrasound;

receiving, at a broadband ultrasonic receiver, ultrasound signals from said transmitters;

providing, for each velocity within a range of velocities, a synthesised version of each burst of ultrasound at a given frequency as it would be expected to be received at said receiver when said object is moving at said velocity relative to a transmitter;

periodically, for each transmitter:

cross-correlating a received ultrasound signal with respective synthesized versions of bursts of ultrasound which are expected to be received directly by said ultrasonic receiver within a given time period after transmission by said transmitter; selecting a synthesized version with a highest cross-correlation peak as the version most accurately corresponding to the velocity of the moving object;

selecting a time delay of a cross-correlation peak for said selected synthesized version as an indicator of a time of flight of said transmitted burst between said transmitter and said moving object; and

calculating a range between said moving object and said transmitter based on said time delay; and

calculating according to said ranges calculated for each transmitter, a location of said moving object relative to the locations of said transmitters.

18. The method of claim 17, wherein the object comprises a mobile device including the broadband ultrasound receiver.

19. The method of claim 17, wherein

the broadband ultrasonic transmitters and the broadband ultrasound receiver are included within the same device; and receiving the ultrasound signal comprises receiving a reflection of one or more of the bursts of ultrasound emitted by the broadband ultrasonic transmitters, said reflection being generated by the one or more emitted bursts of ultrasound being reflected by the moving object.

20. A mobile device comprising a carrier signal generator; a broadband ultrasonic receiver operably connected to an RF modulator, said RF modulator being operable to modulate said carrier signal as a function of a received ultrasonic signal to provide a modulated carrier signal; said modulator being operably connected to an RF transmitter to transmit said modulated carrier signal to a remote RF receiver.

21. The mobile device of claim 20 further comprising one of an: inertial or magnetic sensor, said sensor being operably connected to a second modulator for further modulating said carrier signal according to the sensed movement of said mobile device.