WO2005019838A1

WO2005019838A1 - Water based speed measurement system

Info

Publication number: WO2005019838A1
Application number: PCT/GB2004/003477
Authority: WO
Inventors: Ralph Sanders; Steve Pointer; Jonathan Davies; Duncan E. Smith
Original assignee: Qinetiq Limited
Priority date: 2003-08-20
Filing date: 2004-08-12
Publication date: 2005-03-03
Also published as: GB0319560D0

Abstract

A system for the measurement of a body travelling in or under water is disclosed, comprising a transmitting means attachable to the body, and a receiving means. The transmitting means is adapted to produce a wideband transmit signal, and the receive means is adapted to process the received signal to resolve multipath signals received and to measure any Doppler frequencies added to the transmit signal as a result of relative movement of the body. In a preferred embodiment the transmit signal is an acoustic signal. The system is particularly suited to measure the speed of a swimmer to provide information for training purposes. The system has further utility in other applications such as measurement of the speed of boats or other water based vessels.

Description

Water Based Speed Measurement System

The present invention relates to a system suitable for measuring characteristics of a body travelling through or under water. It is particularly suited to the measurement of the speed of a body, and although principally aimed at sports and recreation use, the invention has utility in other areas, such as manoeuvring of shipping and research into water based animal characteristics. When used in sports and recreation, the body may be that of a swimmer.

Athletes use various types of coaching aids to improve their competition performance, and swimmers are no exception to this. Top level swimmers and triathletes require, for example, a pool-side device that accurately measures the speed of the swimmer as he or she moves through the water. Ideally, speed measurements are taken at frequent intervals so that, for example, the swimmer's performance can be analysed throughout the swim stroke.

There have been a number of attempts at providing such information for use by swimmers and their coaches in perfecting their technique and/or enhancing their performance. Existing systems for measuring the real-time speed of a swimmer are based, for example, on the use of accelerometers, force transducers and winches. Those known systems, however, have a variety of associated problems, such as poor accuracy, and an inherent inability to function for certain swimming strokes.

According to a first aspect of the present invention a system for the measurement of the speed of a body travelling in a liquid, the system comprising: a transmitter attachable to the body, comprising wideband signal generation means in communication with transducer means, to produce a wideband transmit signal; a receiver for reception of a signal generated by the transmitter, comprising transducer means for conversion of the received signal to an electronic signal, and processing means for processing the received signal to measure any Doppler frequency shift caused by relative movement between the body and the receiver, and to calculate from this Doppler frequency the relative speed of the body. A wideband transmit signal provides substantial advantages when the swimmer is training in a restricted body of water such as a swimming pool. The relatively shallow depths and strong reflections due to the geometry of a typical pool are inherently likely to give rise to multipath reflections of a signal emitted by the transducer. These can cause problems for narrow band systems, but in wideband systems the problems are more easily surmounted. This is because a wideband transmit signal can easily be associated with a narrow time of arrival, and hence a wideband transmit signal that is subject to multipath can be identified with a plurality of arrival times, all but the first of which can be rejected.

The system of the present invention aims to provide fast, accurate measurement of swimmer speed and, if desired, to be used to provide accurate synchronisation information for simultaneous video capture. Synchronised video is highly advantageous for a swimming coach, who can use the combined video and speed information to improve the swimmer's technique.

Preferably, the transmit signal is an acoustic signal. Alternatively, an electromagnetic (EM) signal may be used, providing that signal strengths of the received signal are sufficiently high, although high attenuation of EM signals in water make acoustic embodiments more practicable.

Preferably, wideband transmit signal is a digital signal. The digital signal may comprise a pseudo-random binary maximum likelihood (PRML) sequence having suitable properties, preceded by a Doppler estimation signal having two distinct pulses. Each such pulse should be of sufficient bandwidth such that expected multipath signals are resolvable, and of sufficient duration to provide a desired Doppler resolution. Characteristics such as correlation properties and timing characteristics of the pulses should be known to the receiver. Such a signal allows any Doppler frequency components present to be estimated, in a known manner, as is described in more detail below.

Preferably, the Doppler estimation signals precede the transmission of the modulated carrier. Conveniently, the Doppler estimation signals are wideband signals and may also be selected to have good correlation properties. Preferably the system is arranged to utilise a wideband transmit signal that has a single autocorrelation peak above a known first threshold, with autocorrelation sidelobes below a known second threshold. Such a signal is advantageous in applications subject to multipath propagation.

Preferably, the signal characteristics are selected from a group of signals that have cross correlation peaks all below a third threshold. Such a signal will be advantageous if more than one system according to the present invention is in use in close proximity at the same time.

The signal preferably comprises a digital information bitstream that has been encoded and modulated onto a carrier signal. The encoding process may be used to give the signal the correlation properties described above.

The encoding process may comprise partitioning the digital information bitstream into a succession of symbols, each comprising n bits, where n is greater than or equal to 1 , and replacing each such symbol with a corresponding code sequence of length m chips (bits), where m>n. The code sequences may comprise any suitable code sequences that have correlation properties as described above. A suitable code sequence may be a Gold code, a Kasami code, or a pseudo-random maximal length (PRML) sequence having known correlation properties. Other code sequences may also be suitable.

In noisy environments the value of m/n may be increased, which has the effect of increasing the processing gain available in the receiver, and hence the ability to overcome uncorrelated noise. This impacts the available data transfer rate however, so a suitable balance needs to be found depending upon the noise levels of the environment and the desired transmission data rates.

Although the Doppler estimation signal is used to provide an initial determination of measured Doppler, from which an initial determination of speed, and sequencing information, can rapidly be derived, a more accurate measurement of the Doppler signal, and hence of the speed of the body through the water, is obtained from subsequent analysis of the pseudo-random binary maximum likelihood sequence.

The Doppler information derived from the signals at the receiver is advantageously utilised to determine the speed of the transmitter device as it is carried through the water by the swimmer. The wideband nature of the transmitted signal provides robustness against the presence of multipath reflections particularly likely to be an issue in pool based training. Whereas narrowband signals will tend to fade in a multipath environment, wideband signals are much more resilient.

The Doppler estimation signals are preferably wideband and/or have good correlation properties. Preferably, but not necessarily, the Doppler estimation signals are identical. Suitably, a pair of chirp signals may be used, particularly described as immediately sequential identical chirps with linearly rising frequencies, but the characteristics, e.g. starting and/or end frequency, envelope shape, duration, etc. of each chirp could be varied and could be the same as or differ from the characteristics of the other chirp.

The signal produced by the transmitter may be transmitted as a substantially continuous stream of data, or it may be transmitted for short periods, for example corresponding to when the body, such as a swimmer, is in a suitable position to be analysed. The transmitter may be arranged to operate at certain times, or may be activated by an activation signal. Such an activation signal may originate remotely from the transmitter.

In one Example, the transmitted signal is transmitted for 5 seconds there is a delay of 0.3 to 0.5 seconds between transmitted signals. The duration of the signal is arbitrary and is selected to allow the swimmer to complete a desired number of swim strokes within the transmission period. Typically, measurements covering at least two or three strokes will be required, but the time taken to complete those stokes depends upon the particular swim stroke employed.

The signal transmitted by the transmitter may be arranged to contain information relating to the body to which it is attached. If the body is a human or animal swimmer then the information may be related to the medical state of the swimmer. For example, the heart rate or some other biometric of the swimmer may be encoded within the signal for later analysis. This information may comprise the digital information bitstream. The information may also comprise timing information that may be utilised to provide a timebase against which the motion of the swimmer may be assessed. Furthermore, the information derived from the signal at the receiver, including but not limited to the Doppler signal may be used to facilitate the synchronisation of strokes of a swimmer in a pool with video capture equipment arranged at the poolside. In one embodiment of the invention, a record of the motion of the device is overlaid on video imagery of the swimmer carrying the device in the pool. Advantageously, this allows a coach or other individual to provide detailed input to the swimmer's technique. The timebase information may be used to synchronise the video signal with the speed and other information received from the transmitter.

Suitably, when used in swimming coaching, the components of the transmitter are positioned in one or more pockets in a swim suit, but, alternatively, the transmitter may be held in a separate mounting belt or other suitable receptacle, thereby enabling the transmitter to be located at various positions on the swimmer's body.

It is has been found that the positioning of the at least one transmitter should be considered carefully, because speed-readings are most accurate when the at least one . transmitter transducer is in direct line of sight of the receiver device. The actual preferred position of the at least one transmitter transducer depends upon the swim stroke, for example, on the stomach or chest for front crawl or breaststroke, and on the back for backstroke. In general, it may be beneficial to position the one or more transmitter transducers close to the swimmer's centre of gravity. Suitably, the at least one transducer is positioned so as to reduce obscuration by bubbles during the swim stroke.

Preferably, two or more transmitter transducers are used so that at least one transmitter transducer remains visible even if one or more remaining transmitter transducers are obscured. If more than one transmitter transducer is used, each is preferably positioned at substantially the same distance from the receiver device, so as to prevent interference between them. Preferably, each transmitter transducer comprises a hydrophone.

Preferably, the at least one transmitter transducer should be slightly spaced from the swimmer's body, in order to prevent the body from damping the transmit signal. The transducer can be made to protrude from a swimsuit by inserting pads of a desired thickness into the pocket holding the transducer between the transducer and the body.

Advantageously, the receiver comprises a single acoustic hydrophone held in a casing designed to resist water ingress under pressures likely to be encountered in a swimming pool. Suitably, the hydrophone is capable of receiving the entire frequency band of the modulated signal with at least adequate fidelity. Means is provided for mounting the receiver onto the wall of the swimming pool, or, alternatively, suspending the receiver from the edge of the pool, which means is preferably detachable. A particularly suitable mounting means is a suction device. Alternatively, the receiver may be attached by ropes, cables, mounting brackets etc.

It has been found that particularly useful results can be achieved where the receiver is positioned at one end of the pool in the path of the swimmer, either in front of the swimmer or to the rear of the swimmer. It will be appreciated that a swimmer may execute one or more turns during the course of a swimming session, thereby altering the swim direction. The present invention functions whether the receiver is positioned in front of the swimmer or to the rear of the swimmer, but best results are obtained when the swimmer is moving toward the receiver

The receiver may be positioned at any depth in the pool. Typically, the receiver is positioned approximately halfway down the side of the pool so as to minimise multipath reflections and surface reflections, but it will be appreciated that the precise optimum positioning will depend on the size and geometry of the swimming pool. Alternatively, the receiver may be located at a position such that phase difference fluctuations in the measured speed are minimised. It should also be borne in mind when positioning the receiver that the Doppler information derived from the received signal will give the radial velocity of the body in relation to the receiver position, and so it may be desirable to discount the Doppler information received if the body is not travelling substantially radially to the receiver.

The receiver processing means may be integral with the transducer, or alternatively, mounted in a convenient location in communication with the transducer.

The receiver processing means may be embodied in hardware, software or any appropriate combination thereof.

Data from the receiver can be displayed on any suitable device, for example a liquid crystal or other display unit, or, alternatively, taken to a data capture device such as a computer. The data can be displayed as speed information or, for example, derived information such as distance or acceleration.

Optionally, the system can be connected to a video using synchronisation information provided by the receiver. According to a second aspect of the current invention there is provided method of measuring the speed of a body travelling in a liquid comprising the steps of: i) positioning a transmitter on body and arranging a receiver within a suitable range of the transmitter ii) transmitting a wideband transmit signal from the transmitter; iii) receiving the wideband signal and processing the signal to measure any Doppler frequencies added to the transmit signal iv) calculating the relative speed of the body from the measured Doppler frequencies.

In order to assist in understanding the invention, a specific embodiment thereof will now be described by way of example only and with reference to the accompanying drawings, in which :

Figure 1 schematically illustrates a preferred swimmer training aid according to the present invention;

Figure 2 diagrammatically illustrates in more detail a transmitter according to the present invention;

Figure 3 illustrates a block diagram of the major components making up the transmitter according to an embodiment of the current invention.

Figure 4 illustrates a block diagram of the major components making up the receiver according to an embodiment of the current invention.

Figure 5 illustrates a method by which a signal may be generated which allows convenient detection of Doppler on the signal with no knowledge at the receiver of the frequency of transmission.

Figure 6 shows an output display for an embodiment of the current invention, synchronised to a video recording of a swimmer. Figure 1 schematically illustrates a preferred sports training aid according to the present invention, and shows a transmitter 1 worn by a swimmer 2 and a receiver processor 3 coupled to a hydrophone 4 at the side of the pool. In operation an acoustic signal is transmitted by the transmitter 1 and received by the receiver processor 3 via the hydrophone transducer 4. The measured characterisics detected by the receiver, such as the speed of the swimmer are derived, and these characteristics passed to a computer 5 for display and storage. Alternatively, the receiver may pass raw demodulated information to the computer system 5 where the characteristics may be derived. A video camera 6 may also be coupled to the computer 5 to record video images of the swimmer, and-allow synchronised display of the video image with the measured characteristics. Such a synchronised display may be done "live" as the swimmer is swimming, or at a later time.

The receive hydrophone 4 is deployed at the end of the pool. A suction device is provided for attachment to smooth tiles, alternatively this may be removed and the hydrophone can be suspended from any convenient point. Experimentation may be used to establish the best depth for the receiver; a good starting point is typically halfway down the pool wall.

The transmission of sound in water is strongly affected by the presence of bubbles in the water. Because of this, when the swimmer is travelling away from the transmitter, signal reception may become intermittent as the increased bubble content behind the swimmer in the water tends to interrupt the acoustic transmissions. In the situations where results are obtained, however, they will represent the true relative velocity of the transmitter from the receiver. To help overcome this problem, a second receiver may be mounted at the other end of the pool, and data from this second receiver used when the swimmer is travelling away from the first receiver.

Figure 2 shows in greater detail the transmitter 1. The transmitter may. be attached to the swimmer by pockets in a close fitting suit, or may be attached with suitable straps, adhesive, or by any other suitable means. The close fitting suit may have multiple pockets into which can be placed transmit transducers 7. The transmitter transducers comprise two hydrophone balls connected to the transmit signal generator 8. The use of two transducers provides a greater chance that at least one will have a clear line of sight to the receiver system.. The optimal positions for the transmit transducers on the body can be established by trial and error, but it has been found that positioning close to the swimmer's centre of gravity is usually adequate.

The suit or strapping should not constrain the transducers unduly (e.g. by "pinning them to the body"), as this may prevent the transducers 7 from imparting a good acoustic signal to the surrounding water. To avoid this problem, small pads of differing thickness are provided, which can be positioned between the transducers and the body. A variety of thicknesses are provided, and experimentation may be used to determine which gives the best results. It may also be advantageous to make the transducers protrude from the suit to avoid them being pressed too hard into the body.

For best results, a line of sight should be maintained between the transmitter and the receiver. The use of two transmit transducers 7 therefore allows one to be obscured without losing the signal. It is preferable for at least one transducer to remain in the water throughout the stroke.

Figure 3 shows a top level block diagram of the components within the transmitter. A bitstream generator 9 generates a serial digital bitstream that encodes Doppler estimation signals along with a wideband signal having advantageous correlation properties. The digital bitstream is then passed to a modulator/upconvertor 10 where it is modulated onto a carrier for transmission at the desired frequency. The frequency band of operation of the present embodiment is between 9kHz and 15kHz. The modulated carrier from modulator 10 is then passed to a power amplifier 11, and from there to a filter/matching unit 12. The signal is then split into two, with each resultant signal being used to drive a transducer 13a, 13b.

Optionally, a sensor 14 may be used to provide a data signal to the bitstream generator to encode information into the signal relating to the swimmer. Typically, the sensor 14 will be a heart rate monitor.

Figure 4 shows a top level block diagram of a receiver suitable for use with the current invention. A hydrophone 15 is connected to an electronic amplifier 16. This in turn goes to a filter circuit 17 and then to a digitiser 18. The output of the digitiser 18 is fed to a processor 19. A video display unit 20 is connected to the processor 19. The amplifier 16, filter 17, digitiser 18 and processor 19 are all mounted in a single portable receiver body unit 21. In use the hydrophone 15 is positioned in a pool or other body of water, typically at a pool edge. A waterproof cable links the hydrophone to the receiver body 21. Signals received by the hydrophone 15 are passed up the processing chain described above until they reach the processor 19. The processor 19 processes the signals to extract Doppler information from the transmitted Doppler estimation signals, and also performs the correlations to extract a more accurate Doppler frequency estimate, and also recover any data that has been transmitted.

Figure 5 shows how the Doppler estimation signals are processed in order to extract an approximate estimate of the swimmer speed, in a system where the receiver has no knowledge of the transmitted signal frequency. Two chirp signals 22 are produced and transmitted by the transmitter. The top plot (a) of Figure 5 shows the chirp signals 22 as immediately successive chirps 22i and 22₂ each of duration T_c, the first chirp commencing at time T = 0. The middle plot (b) of frequency F against time T shows lines 23., and 23₂ corresponding to chirps 22-ι and 22₂as transmitted, and, in the absence of Doppler and multi-path propagation, as received.

When Doppler is present, but without multi-path propagation, the received signal is effectively a version of the transmitted signal which is compressed or expanded by an amount and in a direction determined by the effective relative motion between source and receiver. The lines 24 and 24₂ of Figure 5 illustrate this for the two chirps when the Doppler is of a sense giving waveform compression. It will be seen that the plots 22 are effectively moved as broadly indicated by the arrows Do to give the plots 24 which are of shorter duration and rising to higher terminal frequencies, the amount and sense of the movement being indicative of the magnitude and sense of the Doppler effect. Also, since the two chirps 24ι and 24₂ are immediately consecutive, the start of plot 24₂ is displaced from Tc by an amount and in a sense indicative of the magnitude and sense of the Doppler effect.

The magnitude of the complex correlation of the received signal against the original chirp waveform 22 produces the signal shown in plot (c) of Figure 5, where the main peaks 25-, and 25₂ correspond to the plots 24-, and 24₂ respectively. Instead of sharp peaks at T_c/2 and 3T_c/2 which would be produced from the plots 27, the peaks 29 are somewhat broadened and occur earlier by amounts ΔT and (ΔT + αTc) respectively. Any multipath signals, such as those shown at 26, produce secondary correlation peaks 27, which are generally smaller than the main correlation peak 25

An estimate of the Doppler on the signal can be obtained from the values of ΔT and/or α.

Following the transmission of the Doppler estimation signals, a signal is transmitted that is encoded with a PRML sequence taken from a set of PRML sequences each having a single high autocorrelation peak, and low cross correlation peaks, as described earlier. The PRML encoded signal may then be transmitted repeatedly, so making up the bulk of the transmitted sig nal .

If data is to be encoded on to transmitted signal, such as from a heart rate monitor, each data bit may be replaced by a single instance of the PRML sequence. If the data bit is a logic 1 then the data bit is replaced with an unmodified PRML sequence; if it is a logic 0, then the bit is replaced with an inverted PRML sequence. When correlated in the receiver processor with a stored copy of the PRML sequence the output of the correlation process can be compared against suitable thresholds to recover the original data transmitted.

Multiple systems may be employed simultaneously, as long as the respective PRML sequences chosen from the set of PRML sequences are different for each system in use.

Details of the correlation, and the extraction of a more refined Doppler signal present on the received signal as implemented in an embodiment of the current invention can be found on pages 13-19 of the Assignee's earlier International patent application,

"Communication System for Underwater Use", PCT/GB02/0 517, the whole contents of which are hereby included by reference.

Figure 6 shows how the output from a receiver according to the present invention may be utilised in combination with a signal from a video camera 6 (as shown in Figure 1 ). A video signal from a camera 6 suitably positioned to record movements of a swimmer is digitally recorded in a computer system 30 and encoded with a time stamp signal to allow synchronisation for later replay. At the same time as the video signal is being recorded, the computer system 30 also takes an input from the receiver 3, 4, and records this along with similar synchronisation signal. A program on the computer system can replay 31 the video signal in a standard manner onto a display 32, and superimpose, onto this, data recorded from the receiver 3, 4 in graphical form 33 to provide, for example, instantaneous speed information for the different parts of a swimming stroke, enabling a swimming coach to determine any notable characteristics of the swimmer's style.

The present invention has been described with specific reference to the measurement of the speed of a swimmer in a swimming pool. It will be clear to the skilled reader, however, that the invention can be used to measure the speed of a swimmer in any body of water, for example a lake or open sea. Furthermore, the present invention can be readily adapted to measure the speed of other moving bodies through water, for example boats, or animals. There may be particular advantage in using the present invention to control the docking sequence of large vessels such as tankers and ferries.

Claims

1. A system for the measurement of the speed of a body travelling in a liquid, the system comprising: a transmitter attachable to the body, comprising wideband signal generation means in communication with transducer means, to produce a wideband transmit signal; a receiver for reception of a signal generated by the transmitter, comprising transducer means for conversion of the received signal to an electronic signal, and processing means for processing the received signal to measure any Doppler frequency shift caused by relative movement between the body and the receiver, and to calculate from this Doppler frequency the relative speed of the body.

2. A system as claimed in claim 1 wherein the wideband transmit signal is an acoustic signal.

3. A system as claimed in claim 1 or claim 2 wherein the wideband transmit signal comprises, at least in part, at least two broadband Doppler estimation pulse signals each having predetermined correlation properties and wherein timing characteristics of each pulse is known to the receiver.

4. A system as claimed in any of claims 1 to 3 wherein the wideband transmit signal is a digital signal encoded with at least one of i) a pseudo-random maximal length sequence, ii) a Gold code, and iii) a Kasami code.

5. A system as claimed in any of claims 1 to 4 wherein the wideband transmit signal generation means is adapted to encode information relating to one or more characteristics of the body.

6. A system as claimed in any of claims 1 to 5 wherein the body is a swimmer.

7. A system as claimed in claim 6 wherein the information encoded is medical information relating to the swimmer.

8. A system as claimed in claim 7 wherein the medical information relates to a heart rate of the swimmer.

9. A system as claimed in any of the above claims wherein the system further includes a video camera and a display system adapted to display a video image of the body, and further adapted to simultaneously display visual information relating to any measured characteristics of the body.

10. A system as claimed in any of the above claims wherein the transmitter incorporates a heart rate monitor.

11. An transmitter system suitable for use with the system as claimed in claim 1.

12. A method of measuring the speed of a body travelling in a liquid comprising the steps of: i) positioning a transmitter on body and arranging a receiver within a suitable range of the transmitter ii) transmitting a wideband transmit signal from the transmitter; iii) receiving the wideband signal and processing the signal to measure any Doppler frequencies added to the transmit signal iv) calculating the relative speed of the body from the measured Doppler frequencies.