Title of Invention  A PASSIVE CONTACT SENSITIVE DEVICE AND A METHOD OF DETERMINING INFORMATION RELATING TO A CONTACT ON A PASSIVE CONTACT SENSITIVE DEVICE. 

Abstract  A contact sensitive device comprises a member (12) capable of supporting bending waves, three sensors (16) mounted on the member (12) for measuring bending wave vibration in the member, whereby each sensor (16) determines a measured bending wave signal and a processor which calculates a location of a contact on the membe from the mesured bending wave signals. The processor calculates a phase angle for each mesured bending wave signal and a phase difference between the phase angles of least two pairs of sensors so that at least two phase differences are calculated from which the location of the contact is determined 
Full Text  TITLE: CONTACT SENSITIVE DEVICE 10 Description 15 TECHNICAL FIELD The invention relates to contact sensitive devices BACKGROUND Visual display often include some from of touch sensitive screen. This is becoming more common with the 20 emergence on the next generation of portable multimedia devices such as palm top computers. The most established technology using waves to detect contact is Surface AcoustiC Wave (SAW), which generates high freguency waves on the surface of a glass screen, and their attenuation by the 25 contact of a finger is used to detect the touch location.This technique is "timeofflight", where the time for the WO 2004/053781 PCT/GB2003/005268 2 disturbance to reach one or more sensors is used to detect the location. Such an approach is possible when the medium beheves in a nondispersive manner i.e the velocity of the waves dose not Vary significantly over the f requency range of interest . 5 In WO01/48684 and PCT/GB2O02/OO3O73 to the present applicant, two contact sensitive devices and methods of using the Same are proposed. In both applications, the device coinprises a member capable of supporting bending wave vibration and a sensor mounted on the member for measuring 10 bending wavs vibiration in the member and for transmiting a signal to a processor whereby information relating to a contact made on a surface on the member is calculated from the change in bending wave vibration in the member created by the contact. 15 By bending wave vibration it is meant an excitation, for example by the contact ,which imparts some out of plane displacement to the member . Many materials bend , some with pure bending with a perfect square root dispersion relation and some with a mixture of pure and shear bending. The 20 dispersion relation describes the dependence of the inplane velocity of the waves on the frequency of the waves. Bending waves provide advantagss, such as incressed robustness and reduced, sensitivity to surface scratches, etc. However, bending waves are dispersive i.e. the bending wave 25 velocity, and hence the "time of flight", is dependent on frequency. In general, an impulse contains a broad range of WO 2004/053781 PCT/GB2003/005268 3 component frequencies and thus if the impulse travels a short distance, high frequency components will arrive first. In W001/48684 and PCT/GB2002/O03073 a correction to convert the measured bending wavs signal to a propagation signal from a 5 nondispersive wave source may be applied so that techniques used in the fields of radar and sonar may be applied so that techniques the location of the contact. DISCLOSURE OF INVENTION According to one aspect of the invention, there is 10 Provided a contact sensitive device comprising a member capable of supporting bending waves, three sensors mounted on the member for measuring wave vibration in the member whereby each sensor determines a measured bending wave signal and a processor which calculates a location of a contact on 15 the member from the measured bending wave signals, charecterised in that the processor calculates a phase angle for each measured bending wave signal, then calculates a phase difference between the phase angles of least two pairs of sensors from which the location of the contact is determined. 20 According to a second aspect of the invention, there is provided a method of determining information relating to a contact on a contact sensitive device comprising the steps of providing a member capable of supporting bending waves and three sensors mounted on the member for measuring bending wave 25 vibration in the member, applying a contact to the member at a location, using each sensor to determine a measured bending WO 2004/053781 PCT/GB2003/005268 4 wave signal and calculating the location of a contact from the measured bending wave signal characterised by calculating a phase angle for each measured bending wave signal, calculating a phase difference between the phase angel of at least two 5 pairs of sensor and determining the location of the contact from the at least two calculated phase differences. The following features may be applied to both the device and the method with the processor being adapted to provide many of the calculation or processing steps of the method. 10 Reflected waves may be suppressed by placing an absorber in contact with edges of the member.The mechanical impedance of the absorber and member may be selected so as to minimise reflections of bending waves from the edges of the member. In particular, the impedances may be selected such 15 that bending wave energy is strongly absorbed in a frequency band around a chosen frequency . The impedance of the absorber may be selected to be both resistive and compliant. The impedances may be selected to satisfy the following equation. 20 ZT = iZE() where Zr is the termination impedance of the obsorber and ZB is the mechanical impedance of the edge of the member. The absorber may be made from foamed plastics which may have open or closed cells and may be polyurethane or 25 polivinylchloride. For example the from may be a short PVC WO 2004/053781 PCT/GB2003/005268 , 5 predominantly closed cell from such as MIERS1w or a medium to high density, open cell polyurethane foam. Another class of foams that has been found to be suitable are acrylic closed cell foams. These may have a high degree of damping and 5 relatively high stiffness. Such properties are particularly suited to edge termination of stiff heavy materials such as glass. Examples include 3M serial numbers 4956, 4910, 4950 and 4555. The absorber may extend substantially around the periphery of the member. The absorber0 may act as a mounting 10 which supports the member in a frame or to another surface. The member may comprise a raised pattern on its surface whereby a contact drawn across the surface provides a variable force to the member to generate bending waves in the members. The pattern may be periodic, or quasiperiodic with a 15 statistically welldefind spatial distribution of undulations. The pattern may be random whereby a contact travelling over the surface of the member generates a random bending wave signal. The random relief pattern may be an anti reflective coating, an antiglare surface finish or an etched 20 finish such as those which are found on many known transparent panels placed in front of electronic displays. Each measured bending wave signal may be processed by a bandpass filter with a passband centred at the chosen frequency and having a bandwidth The bandwidth of 25 the filter is preferably chosen to address the Doppler effect WO 2004/053781 PCT/GB2003/005268 6 whereby a bending wave arrives at a point with a different frequency from its original frequency. Accordingly, the bandwidth, preferably obeys the relationship: 5 where vmax is the maxmimum lateral velocity of the contact across the surface, e.g. if the contact is provided by a stylus, vmax is the maximum velocity that a user is capable of moving the stylus. ' The phase of each filtered signal may be measured by 10 comparison with a reference signal. the reference signal may have a frequency . The measured phase is the average phase difference between the input and reference signals, optimally measured over the interval . Alternatively, the reference signal may be derived from a filtered signal from a second 15 sensor in which case, the measured phase is the phase difference between two input signals. The phase differences may be calculated at intervale of which may be intervals of less than 10ms. The reference and input signals may be fed to a phase detector. Output from 20 the phase detectors may be fed through lowpass filters having frequency cutoffs of approximately , then through digitisers and finally through a processor to calculate the phase angle . The instantaneous phases, and , of two measured 25 bending wave signals may satisfy the phase difference equation WO 2004/053781 PCT7GB2003/005268 equation: where being the distance from the contact location to each sensor lebelled m and 1 5 respectively) , and is the wavevector. This equation may be satisfied if the path lenth difference between two sensor is less than the coherence length of the bandpass filter, which is defined as 10 The coherence condition is therefore If the coherence condition is not satisfiecl, the above phase equation may not be satisfied. Thus, values of and the phase angle difference are required to determine the location of the contact. The Shape 15 of the member may be selected to constrain the magnitude of to values less than half of one wavelength, ie. In this case, where all possible values of satisfy the condition ,there is only one value of which is the integer satisfying 20 Alternatively, n may be estimated or inferred in some way. Each phase angle difference in combination with the range of possible values of the integer im may be used to generate a series of path length difference thereby difining a series WO 2004/053781 PCT/GB2003/005268 8 of discrete hyperbolic corves on the surface of the member, denoting possible location of the contact. The location of the contact may be determined by plotting each hyperbola defined by each path length difference and selecting a point 5 at which a large number of the hyperbolae interaect or nearly interest. This point is likely to be the true location of the contact. where is unknown, the minimum number of series of hyperbolae required to determine the contact location is three 10 and the likelihood of determining the correct location of the contact, is increased by increasing the number of hyperbolae to be plotted. Multiple sensors may be used whereby a phase angle difference may be calculated for each pair of sensors thus generating multiple hyperbolae. In this embodiment, the 15 minimum number of sensore is three. Alternatively, where is unknown, the measured bending wave signal from each sensor may be divided into two or more discrete frequency bands whereby a phase angle difference may be calculated for each frequency band and for each pair of 20 sensor. Although multiple phase angel differences may be calculated from a single pair of sensors, the phase angle differences at different frequencies are derived from the same path length difference. Thus the minimum number of sensors is three. The dividing of the frequency bands may be achieved by 25 processing the bending wave signals by at least two bandpass filters having different passband frequency. For exmaple, WO 2004/053781 PCT/GB2003/005368 9 using two bandpass filters having frequencies the phase angle differences from two sensors may be defined as 5 where is a single pathlength difference defined by the contact and the position of the sensors. Therefore the values of na and nb may be selected so that the measured phase angle differences infer similar values of 10 the pathlength difference. There may be only one combination of values for which this is possible. In this case the true value of the pathlength difference may be detetermined. The correct combination may be determined as the combination of values that minimise the expression: 15 The path length difference may then be estimated, as : Where this process is repeated with two pairs of sensors, two path length differences may be determined, which in turn 20 may be used to determine the location of the contact. Alternatively, where is unknown, an initial determination of the location of the contact may be made using the methods taught in WO01/48684 and PCT/GB2002/003073 (as WO 2004/053781 PCT/GB2003/005268 10 summarised in figure 111. Thereafter it may be assumed that the coneace moves more slowly than the bending waves and hence that the phase angle differences change by small increments over the timescale . Accordingly, each value of may be 5 chosen to minimise the change in path length difference. Measured phase angle differences may contain random errors which may result in the selection of incorrect values of n. This error may be alleviated by evaluating the likelihood of successive sequences of for example by a 10 state spaee estimator such as the well known Kalman filter. The sequence having the maximum measure of likelihood its selected. The statespace estimator provides an estimate of the internal state of asystem of a which noisy measurements are 15 made. A necessary input to the statespace estimator is a statistical description of the evolution of the system state. An example of such a state is the set of coordinates that describes the position and velocity of an object in contact with the member, It is widely known that the Kalman filter and 20 other statespace estimatore my provide a measure of likelihood that a sequence of observed, noisy measurements are consistent with the model of the system state. A statespace estimator may therefore be employed to take a sequence of a pair of pathlength differences (say and 25 taken at different times (say , to estimate WO2004/os3781 PCT/GB2003/005262 11 the systemstate,i.e. the position and velocity of the contact, at those time, Moroever, the overall likelihood of those values of pathlength difference being consisteng with the model of the system may be evaluated. 5 Where the sequence Of pathlength differences are of integers , the measure of likelihood generated by the statespace estimator may be used to infer the likelihood that the correct values of n were 10 chosen. It follows that a method for choosing the correct sequence of integers, n, is to find the sequence for which the statespace estimator gives the maximam measure of likelihood, As mentioned above, the state space estimator uses some statistical description of the evolution of the system state. 15 A suitable model for the motion of the contact may be a simple random walk. Alternatively, the model may employ a detailed statistical description of how the user moves the stylus or finger. One example is a statistical description of how the user moves a pen while writing text or individual characters. 20 The processor may further be adapted to include in the determination procedure any available information about where the contact can be expected. For example, if the member is an input device for a graphical user interface where the user is presented with a choice of ' buttons' to press, it msy be 25 useful to assume that any contact on the member occurs within the discrete areas corresponding to the buttons. WO 2004/053781 PCT/GB2003/005268 12 Alternatively, a map of the probability at which a contact is likely to occur and which is based on the expected behaviour of the user may be used. The device may comprise a software application with a graphical user interface (GUI) 5 which interacts with the operating system by means of an application program interface (API) in which the API is adapted to generate the probability map. The probability map may be based on the location , size, and frequency of use of objects presented by the graphical user interface. The 10 probability map may also be based on information about the relative likelihood of the various GUI elements being activated. The following characteristics may apply to all embodiments of the invention. The device may comprise means 15 for recording measured bending wave signals from the or each sensor over time as the contact moves across the member. The information relating to the contact may be calculated in a central processor. The sensors may be mounted at or spaced from an edge of the member. The sensors may be in the form of 20 sensing transducers which may convert bending wave vibration into an analogue input signal. The member may be in the form of a plate or panel. The member may be transparent or alternatively nontransparent, for example having a printed pattern. The member may have 25 uniform thickness. Alternatively, the member may have a more complex shape , for example a curved surf see and /or variable WO 2004/053781 PCT/GB2003/005268 13 thickness. The device may be a purely passive sensor with the bending wave vibration and hence the measured bending wave signals being generated by an initial impact or by frictional 5 movement of the contact . The contact may be in the from of a touch from a finger or from a stylus which may be in the form of a hand—held pen. The movement of a stylus on the member may generate a continuous signal which is affected by the location, pressure and speed of the stylus on the member. The 10 stylus may have a flexible tip e.g. of rubber, which generete bending waves in the member by applyig a variable force thereto.The variable force may be provided by tip Which alternatively adheres to or slips across a surface of The member. As the tip moves across of the member a tensile 15 force may be created which at a certain threshold, causes any adhesion between the tip and the member to break, thus allowing the tip to slip across the surface. The bending waves may have frequency components in the ultrasonic region (>20 kHz). 20 The member may also be an acoustic radiator and an emitting transducer may be mounted to the member to excite bending wave vibration in the member to generate an acoustic output. The frequency band of the audio signal of the transducer preferably differs from and does not overlap the 25 frequency band of the measurements from the sensor. The audio signal may thus be filtered, for example, the audio band WO 2004/53781 PCT/GB2003/005268 14 may be limited to frequency below 20kHz.and the vibration measurements may be limited to frequencies above 20kHz. A sensor may have dual functionaliy and act as the emmiting transducer. 5 The or each emitting transducer or sensor may be a bender transducer which is bonded directly to the member, for example a piezoelectric transducer. Alternatively, the or each emitting transducer or sensor may be an inertial transducer which is coupled to the member at a single point. The inertia 10 transducer may be either electrodynamic or piezoelectric. A contact sensitive device according to the invention may be included in a mobile phone, a laptop or a personal data assistant. For example, the keypad conventionally fitted to a mobile phone may be replaced by a continuous moulding which is 15 touch sensitive according to the present invention. In a laptop, the touchpad which functions as a mouse controller may be replaced by a cotinuous moulding which is a contact sensitive device according to the invention. Alternatively, the contact sensitive device may be a display screen, e.g. a 20 liquid crystal display screen comprising liquid crystal which may be used to excite or sense bending waves. The display screen may present information relating to the contact. BRIEF DESCRIPTION OF DRAWIHGS The invention is diagrammatically illustrated, by way of 25 example, in the accompanying drawings, in which: Figure 1 is a schematic plan view of atouch senditive WO 2004/053781 PCT/GB2003/005268 15 device accordin to one aspect of the invention; Figure 2 is a schematic perspective view of the device of Figure 1; Fxgure 3 is a schamatic side view of a one dimensional 5 beam; Figure 4a is a graph showing the amplitude of the reflection coefficxent against frequency (Hz), the amplitude is unitless since it is a ratio; Figure 4b is a graph showing the phase (in radians) of 10 the reflection coefficient against frequency (Hz); Figures 5a and 5b are schematic perspective views of alternatives touch sensitive devices; Figure 6 is a flowchart of a method of finding the location of a contact according to the invention 15 Figure 7a is a schematic block diagram of apparatus used for calculating phase angles, Figure 7b is a schematic block diagrame of apparatus used with that of Figure 7a; Figures 8a to 8d are plan views of apparatus according to 20 the invention showing the hyperbolae of path length differences; Figure 9 is a schematic block diagram of alternative apparatus used for calculating phase angles; Figure 10 is a flow chart showing an alternatives method 25 of calculating the location of the contact; Figure 11 is a flow chart showing a method of calculating WO 2004/053781 PCT/GB2003/005368 16 the location of the contact using the dispertion corrected correlation function; Figure 11a is a graph of dispertion corrected correlation function against time ,and 5 Figure 12a is a schematic block diagrame of a contact sensitive device which also operates as a loudspeaker, and Figure 12b is a method of separating audio signal and measured bending wave signal in the device of Figure 12a. DETAILED DESCRIPTION 10 Figure 1 shows a contact sensitive device 10 comprising a transparent touch sensitive plate 12 mounted in front of a display device 14. The display device 14 may be in the form of a television, a computer screen or other visual display device. A stylus 18 in the form of a pen is used for writing 15 text 20 or other matter on the touch sensitive plate 12. The transparent touch sensitive olate 12 is a member, e.g. an acoustic device, capable of supporting bending wave vibration. As shown in figure 2, four sensor 16 for measuring being have vibration in the plate 12 are mounted 20 on the underside thereof. The sensors 16 are in the form of piezoelectric vibration sensor and are mounted one at each corner of the plate 12. At least one of the sensor 16 may also act as an emitting transducer for exciting bending wave vibration in the plate. in this way, ths dsvics may act as a 25 combined loudspeaker and contact sensitive device. A mounting 22 made of foamed plastics is attached to the WO2004/053781 PCT/GB2003/005268 17 underside of and extends substantially around the periphery of the plate 12. The mounting 22 has adhesive surfaces whereby the member may be securely attached to any surfacs. The mechanical impedance of the mounting and plate are selected so 5 as to minimise reflections of having waves from the plate edges. The relationship between mechanical impedance of the mounting and the plate may be approximated by considering the one dimensional model shown in Figure 3. The model comprises a 10 waveguide 34 in the form of a beam which terminates at an edge mounting 36 having a termination impedence. An incident waves 38 travelling down the waveguide 34 is reflected by the mounting 36 to form a reflected waves 40. The incident and reflected Waves are plane waves travelling in the direction 15 perpendicular to the edge. Assuming the mounting 36 Satisfies the following boundary conditions: (i) the termination impedance only couples into the lateral velocity, i.e. it does not provide any torgue registance; whereby the bending moment is equal to zero at the edge and 20 (ii) The ratio of the lateral shear force and the velocity at the edge is equal to the terminal impedance; The reflection coefficient at the mounting is given by where Zr is the termination impedance of the mounting and 25 ZB is the mechanical impedance of the end of the waveguide. WO 2004/053781 PCT/GB2003/005268 18 given by wheics K (w) , is the wavevector which may be expressed in terms of the bending Stiffness, B, and pass per unit area, , 5 of the panel, Thus, the reflection confficient is determind by the ratio of the impedances at the end of the waveguide and the mounting. Furthermore, the impedance of the waveguide is 10 proportional to the square root of frequency and is both real and reactive in equal weights (i.e. n/4 phase angle) . Accordingly, the reflection coefficient is likely to be strongly frequency dependent. The reflection coefficient vanishes, i.e. bending wave 15 energy is strongly absorbed in a frequency band around if the following condition is satisfied: ' Thus, the termination impedance of the mounting must have both real and imaginary components, or, equivalent, the 20 mounting should be both resistive and compliant. The plate may be, for example, 1 mm thick polycarbonate sheet which has mass per unit area, and bending stiffness, B—0 = 38 Nm. The exquations above can be used WO 2004/053781 PCT/GB2003/005268 19 to calculate the impedances of the plate and absorber required to strongly absorb bending wave energy around the chosen angular frequency . The impedance, per unit width for a 1mm beam 5 approximation of the plate is The properties of the absorber which provide the desired absorption are thus: Resistance per unit width, The reflection coefficient is a unitless complex number. Figures 4a and 4b are graphs showing the amplitude and phass 15 of the reflection coefficient R(w) varying with frequency. The amplitude of the reflection cofficient is zero ano its phase is reversed for approximately equal to 900HZ. In Figures 5a and 5b, the plate 12 has uniform surface roughness in the form of a raised surface pattern 28,29. The 20 stulus 18 is drawn accross the surface along a path 30 and as it crosses a raised part or line of the pattern it generates bending waves 32 in the member. Thus contact from the stylus 18 provides a source of bending wave vibration in the member. In Figure 5a, the surface pattern 28 is a periodic pattern of 25 raised crossed lines and in Figure 5b, the surface pattern 29 WO2004/053781 PCT/GB20O3/005268 20 is a random relief pattern. In the embodiments of Figures 2, 5a and 5b, as the contact moves ovsr the rough surface of the member r bending waves radiate isotropically in the member from the point of 5 contact. The displacement of the member at a distance , x, from the point of contact is related to the displacement at the point of contact by a transfer function H distance larger than the wavelength, , the transfer function can be approximated as 10 Where A is a constant and K(is the wavevector defined previously. Although strictly only applies to bending wavps on an infinite platet , sines the mounting is strongly absorb bending wave vibration, the relationship is satisfied. 15 The transfer function shows that where a source of bending frequency, between displacements at two locations which at distances, X1 and k2, from the point of contact for the source is: 20 This implies the following relationship between the phase angle difference, the path length difference and an integer n12. WO 2004/053781 PCT/GB3003/OO5268 21 Figure 6 shows the steps in the method for using this equation to determine the contact location: a) Measure a bending wave signal with each sensor to give measured bending wave signals , 5 b) Calculate the phase angles of the measured bending wave signals c) Calculate the difference between the two phase angles ,(t) and d) Calculate the location of the contact from 10 Figure 7a shows a schematic block diagram of a device for calculating the phase angle ) measured by one of the sensors. The signal Wj (t) is a random signal and is thus uncorrelated over long time scale. The 15 signal is first amplified by an amplifier 42 and then processed by an analogue bandpass filter 44 with a passband centred at A moving source of bending waves inay demonstrate the Doppler effect, whereby a bending wave which has frequency 20 and is emitted by a source moving at velocity v towards a point on a member arrives at that point with a different frequency defined by . The maximum angular frequency shift between bending waves at two different points on the member is therefore is the maximum WO 2004/053781 PCT/GB2003/005268 22 velocity of the moving source. If the angular frequency shift becocomes larger than the width of the band pass filter, the phase difference equation above does not hold. Accordingly, the bandwidth of the filter 44 is set to be greater than 5 this maximum frequency shift and thus obsys the relationship; After processing by the filter 44, the resulting filtered signal W'j(t) is an amplitude and phase modulated carrier with frequency" and is defined by: 10 where are the amplitude and phase of the signal. Both fluctuate over a timescale determined by the bandwidth of the filter, namely . The maximum frequency at which independent phase angle measurements may be 15 taken from the output of the bandpass filter is . Sice a touch sensor typically provides an updated measurement of the contact position every 10ms, the condition for the minimum frequency of positional measurment is . The filtered signal W'j(t) is then passed simultaneously 20 to two analogue phase detectors 46. Such detectors are well known in the art, for: example, see p644 of "The Art of Electronics" by Horowitz and Hill. Reference signals each having frequency but a phase difference of n/2 are also fed to the two phase detectors. The output of the phase WO 2O04/053781 PCT/GB2003/005268 23 detectors are passed through lowpass filter 48 each having frequency cutoffs of approximately . The outputs of the lowpass filters are proportional to cos respectively. These outputs are then digitised by digitisers 5 50 and processed by processor 52 to give the phase angle Figure 7b shows how the reference signals used in Figure 7a may be generated. A second bending wave signal Wi (t) is measured at s second sensor. The signal is fed through an amplifier 42 and analogue bandpass filter 44 to generate a 10 filtered signal W' j(t). The filtered signal W' j(t) forms the reference signal which is fed directly to one phase detector 46. The filtered signal is also fed to the second phase detector 46 via a device which shifts its pgase by The phase shifted signal is used as the reference signal to the 15 second phase detector 46. Figures 8a to 8d show how the phase angle differences and hence the path length differencss may be used to calculate the location of the contact. The equation in step (d) of Figure 6 defines a hyperbolic curve which can be overlaid on 20 the plate 12. Figure 8a shows the three hyperbolic curves 26 which ars generated using three different values of nim and the calculated phase angle difference for a pair of sensors 16 mounted one on each end of the short sides of the plate 12. Similarly Figures 8b and 8c show the hyperbolic curves 26 25 which are generated by the phase angle difference and WO 2O04/053781 PCT/GB2003/005268 24 different values of for two other pairs of sensors. Figure 8d shows all the hyperbolic curves created by the sensor . The contact location 24 is the point of intersection of three hyperbolic curves , one from each pair of sensors. 5 Prom che contact location 24, the correct value of may be inferred.  A method of inferring n is implemented using the embodiment shown in figure 9. The bending wave signal W1(t) measured by each sensor is sirnultaneously processed by two 10 bandpass filters 48,54. Two phase angles, one for each filter, are calculated, for example as described in Figure 7. The filters 48, 54 have slightly different passband frequencies whereby two phase angle differences, one tor each passband frequency, are provided by each pair of sensors. 15 The phase angle differences from the sensors may be defined as where is a single pathlength difference defined by 20 the contact and the position of the sensors. The correct combination (na, nb) may ba determined as the combination of values that minimise the expression: The path length difference may then be estimated as: WO 2O04/053781 PCT/GB2003/005268 25 Another pair of sensors may than be used to determine a second path length difference. Each path length difference defines a hyperbolic curve on the pannel. The intersection 5 point of these two hyperbolic curves is the location of the contact. As in Figure 8a to 8b, the hyperbolic are plotted and the point at which the largest number of hyperbolae intersect is likely to be the true location of the contact. Figure 10 shown an alternative method for calculating the 10 location of the contact from the equation above, namely i. Measure a pair of bending wave signals W1(t) and Wj(t), one signal being measured by a sensor ; ii. calculate the dispersion corrected correlation function of the two signal using the method described in Figure 15 11 and lla; lii Calculate the inItial position of the contact using the dispersion corrected correlation , as described in Figures 11 and lla; iv. Remeasure bending wave signals w1(t) and wj(t): 20 v. Calculate the phase angle of each signal  for example, as described in Figures 7a and 7b; vi. calculate the difference between, the phase angles: vii. Select the value of n1m which minimises the change in the path length difference: WO 2O04/053781 PCT/GB2003/005268 26 viii .plot the hyperbola defined by ix. Report steps (iv) to (viii), remeasuring the bending wave signal at regular intervals 5 At step (viii) a minimum of two hyperbolae from different pairs of sensors are requireed to determine the position of the contact. Therefore the entire process must be performed simultaneously for at least two pairs of sensors. Thus a minimum number of two phase angle differences must be 10 determined. Two phase angle differences may be generated by using two sensor and splitting the signal into two frequency bands as described in Figure 9. Alternatively, multiple sensors may be used so that niultiple phase angle differences may be calculated using different pairs of sensors. 15 Figure 11 shows a method of calculating the dispersion corrected correlation function to travel the difference in path length between the contact location and the sensors. The method set out below summarises the information in PCT/GB2002/003073. The method comprises the following steps; 20 (a) Measure two bending wave signals W1(t) and W2(t)to (b) Calculate the Fourier transform of W1(t) and W2(t) to arrive at and hence the intermediate function is the complex conjugate Fourier transform, t represents time is 2nf where f is frequency. WO 2O04/053781 PCT/GB2003/005268 27 (c)Calculate a second intermidiate function which is a function of (d) and (e) at the same time as performing steps (a) to (c) the frequency stretching operation is 5 calculated using the predetermined pannel dispersion relation are combined to arrive at the dispersion corrected correlation function; 10 (9) the dispereion corrected correlation function is plotted against time with a peak occuring at time t12 as shown in Figure lla, difference between the path lengths x1 and x2 from the first 15 and second sensors tc the contact. (i) x12 defines a hyperbolae which may be plotted as in Figure 7 to calculate the location of the contact. As with the method of Figure 10, a minimum of two hyperbolae are required to determine the location of the 20 contact, Thus the ways of generating more hyperbolae discussed above apply equally to this method. The second intermediate function May simply be WO 2O04/053781 PCT/GB2003/005268 28 correlation function. Alternatively, may be selected from the following functions which all yield phase equivalent function to the standard dispersion corrected correlation function: 10 function Alternatively, may be the function which is the Fourier transformation of the correlation function D(t) : The steps are calculate D(t) ; calculate and apply a 15 frequency stretching operation to arrive at the dispersion corrected correlation function Alternatively, at step (f) the following dispersion corrected correlation function may be calculated: 20 where WO 2O04/053781 PCT/GB2003/005268 29 where are the Fourier transformation and complex conjugate fourier transformation of two measured bending wave signals is the path 5 length difference. A sensor may act as both the first and second sensor whereby the dispersion cprrected correlation function is an autocorrelation function. The autocorrelation function may be calculated applying the same steps for the dispersion 10 corrected correlation function using W1(t) W2(t) . Figure 12a shows a contact sensitive device which also operates as a loudspeaker. Figure 12b shows a method for partitioning the audio signal and measured signal into two distinct frequency bands sO that the contribution of the audio 15 signal to the processed measured signal is suppressed. The device comprises a member 106 in which bending waves are generated by an emitting transducer or actuator 108 and the contact. The emitting transducer applies an audio signal is filtered by a low the member 106 to Generate an acoustic output. Before being 20 applied to the member, the audio signal is filtered by a low pass filter 112 which as shown in Figure 12b, removes the audio signal above a threshold frequency f0. As shown in Figure 12b, the contact generates a signal which has a power output which is substantially constant over WO 2O04/053781 PCT/GB2003/005268 30 a large frequency band. The signal from the contact and the audio signal sum to give a combined signal which is passed through high pass filter 114 to remove the Signal abovs the thresholod frequency fn. The Filtered signal is than passed to 5 a digitiser 116 and onto a processor 118. 08122004 GB03O5268 31 CLAIMS 1. A contact sensitive device coprising a member capable of supporting bending waves, three sensors mounted on the member 5 for measuring bending wave vibration in the member, whereby each sensor determines a measured bending wave signal and a processor which calculates a location of a contact on the member from the measured bending wave signals, characterised in that the processor calculates a phase difference for each measured 10 bending wave signal by comprison to a reference signal and in that the processor calculates a phase difference between the phases of the measured bending wave signal from a first pair of sensor with and at least one other phase difference between the phases of the measured bending 15 wave signal from at least one other pair of sensor so that at least two phase differences are calculated from which the location of the contact is determind. 2. A contact sensitive device according to claim 1 consprising an absorber at the edges of the member whereby 20 reflected waves are suppressed. 3. A contact sensitive device according to claim 2, wherein the mechanical irnpedance of the absorber and the member are selected so as to minimise reflections of bending waves from the edges of the member. 25 4 . A, contact sensitive device, according to claim 3, wherein 08122004 GB0305268 32  the impedances are selected so that bending wave energy is strongly absorbed in a frequency band around a chosen frequency . 5. A contact sensitive device according to claim 4, wherein 5 the impedances are selected to satisfy the following equation: where ZT is the termination impedance of the absorber and Zb is the mechanical impedance of the edge of the member. 6. A contract sensitive device according to claim 4 or claim 10 5, comprising a bandpass filter for filtering each measured bending wave signal, the filter having a passband centred at the chosen frequency 7. A contact sensitive device according to claim 6, wherein. the bandwidth of the filter obeys the relationship: whace vmax is the maximum lateral velocity of the contact. 8.A contact sensitive device according to any one of claims 2 to 7, wherein the absorber is made from foamed plastics. 9 A contact sensitive device according to any one of the 20 preceding claims, wherein the member comprises a raised pattern on its surfacewhereby a contact drawn accross the surface provides a force to the member to generate bending wavws in the member. 10. A contact sensitive device according to claim 9, wherein 25 the pattern is random whereby a contact travelling over the  O8122004 GB030526 33 surface of the member generates a random bending wave signal. 11. A contact sensitive device according to claim 10,wherein the pattern is formed from an antireflective coating, an anti glare surface finish or an etched finish. 5 12. A contact sensitive device according to any one of the preceding claims, comprising at least two baudpass filters which have defferent passband frequencies and which simultaneously process the bending wave signal measured by a pair of sensors whereby a phase difference for each passband 10 frequency is provided by a pair of sensors. 13. A contact sensitive device according to any one of the preceding claims, comprising four sensors on the member. 14. A contact sensitive device according to any one of the precectino claims, comprising means for determining the initial 15 location of the contact using the dispersion corrected correlation function of pairs of measured bending wave signals and means for determining subsequent location of the contact using the phase difference between pairs of measured bending wave signals. 20 15. A contact sensitive device according to any one of the preceding claims, wherein the phase determining means comprises a phase detector. 16.A contact sensitive device according to claim 15, wherein the processor comprises a lowpass filter and a digitiser for 25 determining the phases. 17. A contact sensitive device according Lo any one of the 08122004 GB0305268 34 preceding claims, wherein the member is an acoustic radiator and an emitting transducer is mounted to the member to exite bending wave vibration in the member to generate an acoustic output. 5 18. A contact sensitive device according to claim 17, comprising means ensuring that the acoustic output and measured bending wave signals are in discrete frequency bands, 19.A contact sensitive device according to any one of the preceding claims, wherein the member is transparent. 10 20. A method of determining information relating to a contact on a contact sensitive device comprising the steps of providing a member capable of supporting bending waves and three sensors mounted on the member for measuring bending wave vibration in the member, applying a contact to the member at a 15 location, using each sensor to determine a measured bending wave signal and calculating the location of a contact from the measured bending wave signal characterised by calculating a phase for each measured bending wave signal, calculating a phase difference of the measured 20 bending wave signals from a first pair of sensors with , calculating at least one other phase disserence between the phases of the measured bending wave signals from at least one other pair of sensors and determining the location of the contact from the at least two calculated phase 25 differences. 08122004 GB0305268 35 21, A method according to claim 20, comprising suppressing reflected waves by placing an absorber at the edges of the member. 22.A method according to claim 21, comprising selecting the 5 mechanical impedances of the absorber and the rnember so as to minimise reflections of bending wave from the edges of the member. 23. A method according to claim 22 , comprising selecting the impedanees so that bending wave energy is strongly absorbed in 10 a. frequency band around a chosen frequency 24 , A method according to claim 23, comprising selecting the impedances to setisfy the following+ equation where ZT is the impedance of the absorber and ZB is the 15 impedance of the edge of the member, 25. A method according to claim 23 or claim 24, comprising filtering each measured bending wave signal by a bandpass filter having a passband centred at the chosen frequency and a bandwidth of . 20 26. A method according to any one of claims 20 to 25, comprising, applying the phase difference equation: to determine the location of the contact , where the phase of a measured bending wave signal, xi is the distance 25 from the contact location to each sensor, is the O8122004 GB0305268 36 path length difference of two sensors, is the wavevectL and nim is an unknown integer. 27. A method according to claim 26, comprising selecting the member to constrain the magnitude of to values less than 5 one half of a wavelength so that is determined from 28. A method according to claim 26, comprising determining an initial location of the contact using the dispersion corrected correlation function of a pair of measured bending wave 10 signals and selecting a value of which minimises change in the path length difference. 29. A method according to claim 26, comorising selecting a series to value of combining the series of path length each phase difference to difine a series of path length 15 defferences plotting the series of graphs of the path length defferences, and inffering the true value of from a point at which a large number of the graphs intersect. 30. A method according to any one of claims 20 to 29 , comprising calculating multiple phase differences from the 20 pairs of phases, plotting a graph of each path . length difference and selecting a point at which a large number of the hyperbolae intersect to be the location of the contact. 31. A method according to any one of claims 20 to 30, comprising dividing the measured bending wave signals from 25 each ssnsor into at least two discrete frequency bands and 08122004 GB0305268 37 calculating a phase difference for a pair of sensors for each frequency band. A contact sensitive device comprises a member (12) capable of supporting bending waves, three sensors (16) mounted on the member (12) for measuring bending wave vibration in the member, whereby each sensor (16) determines a measured bending wave signal and a processor which calculates a location of a contact on the membe from the mesured bending wave signals. The processor calculates a phase angle for each mesured bending wave signal and a phase difference between the phase angles of least two pairs of sensors so that at least two phase differences are calculated from which the location of the contact is determined 

Patent Number  213799  

Indian Patent Application Number  00891/KOLNP/2005  
PG Journal Number  03/2008  
Publication Date  18Jan2008  
Grant Date  16Jan2008  
Date of Filing  16May2005  
Name of Patentee  NEW TRANSDUCERS LIMITED  
Applicant Address  CYGNET HOUSE, KINGFISHER WAY HINCHINGBROOKE BUSINESS PARK HUNTINGDON CAMBS PE29 6FW UNITED KINGDOM.  
Inventors:


PCT International Classification Number  G06K 11/00  
PCT International Application Number  PCT/GB2003/005268  
PCT International Filing date  20031203  
PCT Conventions:
