GB2254694A

GB2254694A - Tennis ball monitor

Info

Publication number: GB2254694A
Application number: GB9207232A
Authority: GB
Inventors: Alexander Roger Deas
Original assignee: SYSTEM PRODUCTS Ltd
Current assignee: SYSTEM PRODUCTS Ltd
Priority date: 1991-04-02
Filing date: 1992-04-02
Publication date: 1992-10-14
Also published as: GB9207232D0; GB9106828D0

Abstract

An apparatus for monitoring ball games, e.g. tennis has at least one sensor AS for detecting the acoustic signal from a ball-surface striking event. A 120-500 Hz band-pass filter BPN rejects signals outwith the spectral region of the signals produced by a ball and surface. A processor means R, I eg a rectifier and integrator operates mathematically on at least one signal parameter from the filtered detected signal to produce a characteristic. A store is provided for at least one value of a characteristic corresponding to a known event; and comparator C determines the difference between the stored and derived characteristics. Output device OD registers parameters such as the occurrence of an event, or the speed or spin of the ball. Spin is derived from the oscillation of the main frequency component. <IMAGE>

Description

BALL MONITOR The present invention relates to apparatus for monitoring a ball game such as tennis and in particular providing data relating to events involving the striking of a surface by a ball.

In a game such as tennis there arises a need for monitoring various ball-surface striking events such as a ball being struck by a tennis racquet, a ball striking the net cord (resulting in a "lett), or the ball striking the ground outside the base or side line. Such ball-surface striking events happen very quickly though and are relatively complex so, that it is very difficult to measure the parameters of these events such as position and speed of the ball, and indeed whether or not an event - such as a net call (ball striking the net cord) has actually occurred.

It is an object of the present invention to avoid or minimize one or more of the above problems or disadvantages.

It has now been found that by monitoring the acoustic signal produced by the ball or the surface in a ball-surface striking event using one or more acoustic signal sensors and analysing it with the aid of data from previously recorded events, it is possible to extract various useful event parameters such as the occurrence of a "let" or the position and speed of the ball as well as more complex parameters such as the spin of the ball.

The present invention provides an apparatus suitable for use in monitoring ball games which apparatus comprises at least one acoustic signal sensor means for detecting an acoustic signal emitted by a ball-surface striking event and producing a detected signal related thereto; band-pass filter means formed and arranged for filtering out from said detected signal at least some of those signals outwith the spectral region of a desired one of the signals produced by an actual ball and a surface involved in said event so as to produce a filtered detected signal; processor means formed and arranged for operating mathematically on at least one signal parameter from the filtered detected signal related to the ball-surface striking event so as to produce a mathematical characteristic associated with said filtered detected signal; store means for storing at least one value of at least one characteristic corresponding to a known event; comparator means for determining the difference between a said mathematical characteristic obtained from said filtered detected signal with a said stored characteristic so as to produce a comparator result; and an output device for communicating said comparator result so as to register at least one event parameter of a said ball-surface striking event.

Thus, although ball-surface striking events normally produce a complex mixture of sounds including various resonances from the surface being struck e.g. tennis racquet,as well as from the ball itself as it recovers its shape after the event, and various background noises from the players, spectators, weather etc., the present invention enables the recovery of various data which can be useful in monitoring compliance with the rules of the game e.g. whether a ball strikes the ground outside the permitted area and/or monitoring players' performance e.g. by recording the speed of the ball when it has been struck and the spin applied to the ball.

It will be understood that various forms of sensor means may be used for detecting acoustic signals such as microphones of various kinds and piezo electric devices, for picking up acoustic signals transmitted from the ball and/or surface through the air or via solids e.g.

along the net cord and a post or other net cord support.

Depending on the nature of the particular desired parameters(s) of the ball-surface striking event to be monitored, one or more sensors may be used. Thus for example in the case of a simple "let" detector a single sensor would suffice, and in the case where the parameter of interest comprises the spin or speed of the ball, then two sensors would be required. If it is desired to determine the position of the striking event, then at least 3 sensors would be required. Of course by using 3 (or more) sensors then each of the previously mentioned parameters could also be determined.

As previously noted the original acoustic signal generated by the ball-surface striking event is relatively complex with at least two principal components generated by the ball and the surface impinged upon by the ball and the detected signal will additionally include other components from the players, any spectators, wind etc. In order to facilitate the necessary processing of the detected signal there is used a band-pass filter to filter out substantially "noise" i.e., those signals outwith the spectral region of a particular signal to be used to obtain the desired parameter of the ball-surface striking event.Thus, for example, in the case of a "let" where the desired parameter is simply the occurrence of a striking event between the ball and the net-cord, there is most conveniently used the signal generated in the net cord itself (as distinct from for example any signal generated on the ball which may be transmitted to and through the net cord), and accordingly there is used a band-pass filter formed and arranged to pass only that signal generated in the net cord itself by the event, and filter out all other signals. Conveniently there is used in this case a band-pass filter having a transmission range from 120 to 500 Hz.

It should be understood that the band pass filter means may be implemented in any convenient form. Thus the band pass filter may be implemented to a greater or lesser extent in the actual physical construction of the acoustic signal sensor, in electrical circuitry, and/or in the processor means used for operating mathematically on the "filtered detected signal" i.e. in additional mathematical operations carried out on the detected signal.

The processor means may be formed and arranged for carrying out any suitable mathematical operation for producing the chosen mathematical characteristic associated with said filtered detected signal. Thus, for example, there may be used a processor means formed and arranged (or programmed) for rectifying and integrating the filtered detected signal from the net cord so as to produce a mathematical characteristic associated therewith whose maximum amplitude can subsequently be compared using the comparator means simply with a stored amplitude level characteristic corresponding to the minimum strength or magnitude of interaction between the ball and net cord in an official "let" event as defined by the rules of the game, to produce a comparator result which simply comprises the presence or absence of an amplitude corresponding to a "let".In another case there may be used a processor means formed and arranged (or programmed) for deconvolving the filtered detected signal with the aid of a reference signal (i.e. a similar filtered detected signal for a known ball-surface striking event) in generally known manner (see for example "Modern Spectrum Analysis" (New York, IEEE Press, 1978) by Childers and Donald) so as to produce a mathematical characteristic in the form of a deconvolution with a timing corresponding to detection of the event at the or a particular sensor. This timing can subsequently be compared using the comparator means with timings from reference signals and the differences determined (comparator results) used in computing event parameters such as the speed of the ball or the position of the ball at the time of the event.It will be understood that the comparator means may be formed separately from the processor means or integrally therewith, with suitable programming, as desired.

With reference to the stored values of the mathematical characteristics it will be noted that these may be obtained from known events previously monitored by the apparatus, or they may be incorporated in to the apparatus in the form of predetermined values obtained from other sources e.g. theoretical studies. The store means in which the values are stored may be any suitable memory means used for storing data. In the case where the stored values are obtained by monitoring of known events, then there would be used read-and-write memory means. Where predetermined values are used then read-only memory means may be used. It will further be appreciated that where predetermined values are used these may be stored separately or as part of a program used to effect the mathematic operations.

In the case of a net cord monitor or "let" monitor application, the output device may simply comprise a buzzer or other acoustic and/or visual alarm signal device. Where ball-surface striking event parameters such as speed or position of the ball are required then the output device includes further processing means for operating on the comparator results. Thus for example where it is desired to determine the position of the ball the output device includes processing means formed and arranged for using the timing differences obtained from the comparator results at two spaced apart sensors to derive a first hyperbolic position line and those obtained for the comparator results at a different pair of spaced apart sensors to derive a second hyperbolic position line, the position of the event being thereupon obtainable from the intersection of said hyperbolic position lines.Furthermore by comparing the doppler shift of the frequency of the key component with known reference values, the speed, and if desired also the direction, of the ball when it has been struck may be determined. In addition by monitoring the oscillation of the frequency of the key component over a period of time after it has been struck and again comparing this with suitable data from known events, the degree of spin applied to the ball may be measured.

In a preferred aspect the present invention provides a ball monitoring installation comprising a playing court with a net and an apparatus of the invention with said at least one sensor mounted at a position in the vicinity of the court for detecting a said acoustic signal. It will be understood that the position and mounting of the sensor(s) may be chosen so as to increase sensitivity in relation to the acoustic signal generated by certain ball-surface striking events and decrease it in relation to acoustic signals generated by other events. Thus for example by mounting a sensor inside a net cord support post sensitivity to acoustic signals generated by the net cord upon the ball striking the net-cord may be increased. Preferably there are provided at least three acoustic signal sensors.

Conveniently first and second sensors are disposed at opposite sides of the net and substantially spaced apart therefrom and a third sensor is provided in the vicinity of the net, for example, at a supporting post of the net.

In yet another aspect the present invention provides a method of monitoring a ball game comprising the steps of providing at least one acoustic signal sensor means and detecting an acoustic signal emitted by a ball-surface striking event and producing a detected signal related thereto; filtering out from said detected signal at least some of those signals outwith the spectral region of a desired one of the signals produced by an actual ball and a surface involved in said event so as to produce a filtered detected signal; operating mathematically on at least one signal parameter from the filtered detected signal related to the ball-surface striking event so as to produce a mathematical characteristic associated with said filtered detected signal; storing at least one value of at least one characteristic corresponding to a known event; determining the difference between a said mathematical characteristic obtained from said filtered detected signal with a said stored characteristic so as to produce a comparison result; and communicating said comparison result so as to register at least one event parameter of a said ball-surface striking event.

Further preferred features and advantages of the invention will appear from the following detailed description given by way of example of some preferred embodiments illustrated with reference to the accompanying drawings in which: Fig. 1 is a block diagram of a tennis net court monitoring apparatus of the invention; Fig. 2 is a blck diagram of an automatic tennis trainer apparatus of the invention; Figs. 3a and 3b show time-continuous series of the acoustic spectra of a tennis ball being served and include background noise of a typical tennis court; Figs. 4A and 4B show the effect of deconvolving the signal in Fig. 3 with the aid of a reference ball signal; Fig. 5 shows the product of deconvolving the signal emitted by the ball in Fig. 3 as recorded using three acoustic signal sensors separated spatially across a tennis court;; Fig. 6 is a schematic diagram showing the use hyperbolic position lines to determine the position on the tennis court of the ball upon impact in a ball-racquet striking event; and Fig. 7 shows the effects of the doppler shift in the acoustic signal emitted by the ball as detected at two sensors separated spatially across a tennis court.

In its simplest form the apparatus of the invention allows determination of the fact that a ball has struck a particular surface. In the first embodiment of Fig. 1 an acoustic signal sensor AS is mechanically coupled to the surface which is the net cord through the support post to which the net cord is secured. The detected signal output of the sensor, for example a microphone, is then band-pass filtered via filter means in the form of a band-pass network BPN, to produce a filtered detected signal output which maximises the signal created by a ball striking the surface and minimises the signals created by noise, i.e. all signals outwith the spectral region of the particular signal chosen for processing.

In some embodiments the mechanical-acoustic structure of the microphone may be employed to implement portions of the band-pass filter. In this embodiment the acoustic sensor may be a microphone or piezo electric disc in a housing attached to a tennis post.

The band-pass filter removes signals outwith the spectrum 120 Hz to 500 Hz, by way of suggestion only, or other narrow band as required depending on the acoustic structure of the sensor housing and post. The processor means can be in the form of an analogue computer formed and arranged (or programmed) to rectify and integrate the signal, the result of which triggers a buzzer to notify the umpire of a net court fault whenever the integrated signal exceeds the threshold set on a comparator C.

The above simple embodiment of the invention can be applied to detect automatically the let-in-service caused by a net court fault in the game of tennis or the like by attaching the acoustic sensor AS to the net post 1 used to support the net cord 2 on a tennis court 3 (see Fig. 6). The band-pass filter BPN is used to remove low frequency signals caused by the action of wind on the net and the high frequency signals such as those caused by contraction and expansion of the net cord 2. Such a net cord monitoring apparatus is a one dimensional tracking device which operates by determining that the ball has impinged on a plane of known and finite dimensions.

Fig. 2 shows schematically an apparatus of the invention somewhat similar to that of Fig. 1 with like parts being indicated by like reference indicia. In this embodiment the filter means BPN filters out signals outwith the range 100 Hz to 1.5 KHZ, with amplification of the remaining detected signal, normally with automatic gain control.The filtered detected signal is then processed mathematically using a digital computer including a processor means programmed to extract from suitable signal parameters the desired mathematical characteristics, comparator means to determine the differences of these from stored values of characteristics from known events to provide a comparator result, and output device means programmed to determine from the comparator results obtained for the detected signals from each of the different acoustic sensors, various parameters of the ball-surface striking event.

In the case of a tennis ball, these characteristics are the deconvolution or correlation of the signal with a reference signal (see below), the doppler shift on the signal centred around 1.1 KHz and the frequency of the oscillation of the envelope of the signal centred around 1.1 KHz. To locate the ball these characteristics are processed and compared with stored values of characteristics from reference signals, then the time difference between the comparators being activated by the signal from spatially separated sensors is used to locate the position of the ball on impact by triangulation. In more detail the timings can be applied to determine a hyperbolic position line 4 which is the locus of the hyperbola formed by the intersection of the spheroid conic surfaces of the flight time of the ball signal.By the use of two different pairs of sensors, using at least three sensors producing signals, each of which are filtered and deconvolved, the intersection of two hyperbolic position lines 4a and 4b can be computed to obtain the position of the ball.

Fig. 3A shows the spectra of a tennis ball being served, which has been determined using the discrete Fourier transform method and Fig. 3B shows the same spectra simplified using the Linear Prediction Coefficients method (see for example "Modern Spectrum Analysis" (New York, IEEE Press, 1978) by Childers and Donald, and J.

Makhoul in Proceedings of the IEEE vol 63 p. 561, 1975.

Fig. 3B shows the main acoustic signal components of a ball-tennis racquet striking event which is a service on an indoor tennis court, with an initially larger amplitude lower frequency signal generated by the tennis racquet and a higher frequency signal generated by the tennis ball. As may be seen in this figure, the frequency of the ball signal is subject to an oscillation due to the spin of the ball and further analysis of this oscillation allows investigation of the spin on the ball.

Fig. 4A shows the variation with time of the amplitude of the filtered detected signal obtained from the acoustic signal generated by the tennis ball in a striking event. The signal from Fig. 4A is then deconvolved in generally known manner (see Childers & BR< Donald) to provide a deconvolution as shown in Fig. 4B.

This deconvolution has a sharply defined clearly distinguishable peak at T which provides a precise timing measurement of the acoustic signal generated by the ball which can then be used, by comparison with other timing measurements, to determine event parameters of interest. Fig. 5 shows how the timing measurements provided by deconvolutions of filtered signals from three spaced apart sensors differ due to the differences in distances travelled by the acoustic signals to reach the respective sensors, thereby enabling the aeterminatlon or event parameters sucn as mall speea ana ball position.

With reference to the above Fig. 6 shows how the position of the ball 6 in a striking event is determined by the intersection of hyperbolic position lines as explained above and Fig. 7 shows the difference in frequency of the acoustic signal generated by the ball as detected at two different spaced apart sensors due to differences in doppler shifting of the signal due in turn to differences in the direction of ball travel at the moment of generation of the ball signal which occurs very shortly after the actual ball-racquet striking event. In more detail Fig. 7 shows the simplified frequency spectra obtained using linear prediction coefficients from the signals detected by two sensors positioned at opposite ends of the court in the vicinity of the baselines of the court. The spectra show a lower frequency racquet signal which is substantially the same in both cases due to its relatively limited movement, and a ball signal which has a frequency of 1289 Hz in one case and 1055 Hz in the other case. This frequency difference can be used to determine the ball speed by means of mathematical calculations in generally known manner. In this particular case the calculated ball speed was 117 Km per hour.

Claims

1. Apparatus suitable for use in monitoring ball games which apparatus comprises at least one acoustic signal sensor means for detecting an acoustic signal emitted by a ball-surface striking event and producing a detected signal related thereto; band-pass filter means formed and arranged for filtering out from said detected signal at least some of those signals outwith the spectral region of a desired one of the signals produced by an actual ball and a surface involved in said event so as to produce a filtered detected signal; processor means formed and arranged for operating mathematically on at least one signal parameter from the filtered detected signal related to the ball-surface striking event so as to produce a mathematical characteristic associated with said filtered detected signal; store means for storing at least one value of at least one characteristic corresponding to a known event; comparator means for determining the difference between a said mathematical characteristic obtained from said filtered detected signal and a said stored characteristic so as to produce a comparator result; and an output device for communicating said comparator result so as to register at least one event parameter of a said ball-surface striking event.

2. Apparatus according to Claim 1 wherein the processor means for operating mathematically upon the acoustic signal is adapted to deconvolve the filtered detected acoustic signal generated by the ball with the use of a stored reference signal comprising a similar filtered detected signal from a known ball-surface striking event.

3. Apparatus according to Claim 1 wherein the processor means is formed and arranged for operating mathematically upon the acoustic signal so as to apply a difference algorithm to the filtered detected signal.

4. Apparatus according to Claim 1 or Claim 2 wherein the output device means is formed and arranged for operating mathematically upon the comparator results generated by the ball via different sensors to determine at least one of the ball spin and speed from the doppler shift and envelope frequency, respectively, of the ball.

5. Apparatus according to any one of Claims 1 to 4 wherein there is used at least one acoustic signal sensor coupled mechanically to said surface impinged upon by the ball in said ball-surface striking event.

6. Apparatus according to any one of Claims 1 to 5 wherein there are provided at least three acoustic signal sensors.

7. A ball monitoring installation comprising a playing court with a net and an apparatus according to claim 1 with said at least one sensor mounted at a position in the vicinity of the court for detecting a said acoustic signal.

8. An installation according to claim 7 wherein there are provided at least three acoustic signal sensors in substantially spaced apart positions.

9. A method of monitoring ball games which method comprises the steps of providing at least one acoustic signal sensor means and detecting an acoustic signal emitted by a ball-surface striking event and producing a detected signal related thereto; filtering out from said detected signal at least some of those signals outwith the spectral region of a desired one of the signals produced by an actual ball and a surface involved in said event so as to produce a filtered detected signal; operating mathematically on at least one signal parameter from the filtered detected signal related to the ball-surface striking event so as to produce a mathematical characteristic associated with said filtered detected signal; storing at least one value of at least one characteristic corresponding to a known event; determining the difference between a said mathematical characteristic obtained from said filtered detected signal with a said stored characteristic so as to produce a comparison result; and communicating said comparison result so as to register at least one event parameter of a said ball-surface striking event.

10. A net cord monitoring apparatus substantially as described hereinbefore with particular reference to Figs. 1 and 6 of the accompanying drawings.

11. A ball monitoring apparatus substantially as described hereinbefore with particular reference to Figs. 2 to 7 of the accompanying drawings.