WO2004053781A2

WO2004053781A2 - Contact sensitive device

Info

Publication number: WO2004053781A2
Application number: PCT/GB2003/005268
Authority: WO
Inventors: Nicholas Patrick Roland Hill; Darius Martin Sullivan
Original assignee: New Transducers Limited
Priority date: 2002-12-06
Filing date: 2003-12-03
Publication date: 2004-06-24
Also published as: JP2006509301A; GB0228512D0; EP1570414A2; CA2506789A1; TW200424916A; TWI308709B; KR20050088103A; BR0317049A; WO2004053781A3; MXPA05006040A; TWI348115B; AR042333A1; JP4613068B2; KR101121891B1; AU2003288416A1; CN100405266C; CN1720498A; TW200817987A

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 member from the measured bending wave signals. The processor calculates a phase angle for each measured 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.

Description

TITLE: CONTACT SENSITIVE DEVICE

DESCRIPTION

TECHNICAL FIELD

The invention relates to contact sensitive devices.

BACKGROUND ART

Visual displays often include some form of touch sensitive screen. This is becoming more common with the emergence of 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 frequency waves on the surface of a glass screen, and their attenuation by the contact of a finger is used to detect the touch location. This technique is "time-of-flight" , where the time for the disturbance to reach one or more sensors is used to detect the location. Such an approach is possible when the medium behaves in a non-dispersive manner i.e. the velocity of the waves does not vary significantly over the frequency range of interest . In WO01/48684 and PCT/GB2002/003073 to the present applicant, two contact sensitive devices and methods of using the same are proposed. In both applications, the device comprises a member capable of supporting bending wave vibration and a sensor mounted on the member for measuring bending wave vibration in the member and for transmitting 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 . 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 dispersion relation describes the dependence of the in-plane velocity of the waves on the frequency of the waves .

Bending waves provide advantages, such as increased robustness and reduced sensitivity to surface scratches, etc. However, bending waves are dispersive i.e. the bending wave velocity, and hence the "time of flight", is dependent on frequency. In general, an impulse contains a broad range of component frequencies and thus if the impulse travels a short distance, high frequency components will arrive first. In WO01/48684 and PCT/GB2002/003073 a correction to convert the measured bending wave signal to a propagation signal from a non-dispersive wave source may be applied so that techniques used in the fields of radar and sonar may be applied to detect the location of the contact.

DISCLOSURE OF INVENTION According to one aspect of the invention, there is provided a contact sensitive device comprising a member capable of supporting bending waves, three sensors mounted on the member 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 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. 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 vibration in the member, applying a contact to the member at a 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 angle for each measured bending wave signal, calculating a phase difference between the phase angles of at least two pairs of sensors 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 calculations or processing steps of the method. Reflected waves may be suppressed by placing an absorber in contact with the 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 that bending wave energy is strongly absorbed in a frequency

band around a chosen frequency ω₀. The impedance of the absorber may be selected to be both resistive and compliant.

The impedances may be selected to satisfy the following equation:

Z_τ = -iZ_B(ω₀)

where Z_τ is the termination impedance of the absorber and Z_B 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 polyvinylchloride . For example the foam may be a soft PVC predominantly closed cell foam such as MIERS™ 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 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 4655. The absorber may extend substantially around the periphery of the member. The absorber may act as a mounting 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 member. The pattern may be periodic, or quasi-periodic with a statistically well-defined 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 anti-glare surface finish or an etched 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 band-pass filter with a pass-band centred at the chosen

frequency ω₀ and having a bandwidth Δω. The bandwidth Δω of the filter is preferably chosen to address the Doppler effect whereby a bending wave arrives at a point with a different frequency from its original frequency. Accordingly, the bandwidth preferably obeys the relationship :

Δω»2/(-»₀)v_rnax

where v_max is the maximum lateral velocity of the contact across the surface, e.g. if the contact is provided by a stylus, V_max is the maximum velocity that a user is capable of moving the stylus .

The phase of each filtered signal may be measured by comparison with a reference signal. The reference signal may

have a frequency _y₀ • The measured phase is the average phase difference between the input and reference signals, optimally

measured over the interval 2π/Δω. Alternatively, the reference signal may be derived from a filtered signal from a second sensor in which case, the measured phase is the phase difference between two input signals.

The phase differences may be calculated at intervals of

2π/Δω which may be intervals of less than 10ms. The reference and input signals may be fed to a phase detector. Output from the phase detectors may be fed through low-pass filters having

frequency cut-offs of approximately Δω/2, then through digitisers and finally through a processor to calculate the

phase angle θ.

The instantaneous phases, θ,{t) and θ_m χf) , of two measured

bending wave signals may satisfy the phase difference equation equation :

Δ0_to = θ, - θ_m = k(ω₀ )Ax_Im + 2mx_lm

where Δxι_m = xi - _m (X_m and i being the distance from the contact location to each sensor labelled m and 1

respectively) , and k {ω) is the wavevector. This equation may be satisfied if the path length difference between two sensors is less than the coherence length of the bandpass filter, which is defined as

2πω₀

^Xc =

Δrø (-»₀)

The coherence condition is therefore |ΔΛ_/m|«_c . If the

coherence condition is not satisfied, the above phase equation may not be satisfied.

Thus, values of n^ and the phase angle difference are required to determine the location of the contact . The shape of the member may be selected to constrain the magnitude of

----XTi_π to values less than half of one wavelength, ie.

< π/k{ω₀) . In this case, where all possible values of ^lm

satisfy the condition ' ^ / V o _^ there is only one value of

nim which is the integer n^ satisfying ' ^{lm lm}< 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 n_lm may be used to generate a series of path length differences thereby defining a series of discrete hyperbolic curves on the surface of the member, denoting possible locations 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 at which a large number of the hyperbolae intersect or nearly intersect. This point is likely to be the true location of the contact .

Where n_lm is unknown, the minimum number of series of hyperbolae required to determine the contact location is three 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 minimum number of sensors is three.

Alternatively, where -i_m 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 sensors. Although multiple phase angle 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 processing the bending wave signals by at least two band-pass filters having different pass-band frequencies. For example, using two band-pass filters having frequencies θo+<x>δ and ω₀-cos,

the phase angle differences Δθ_a, Δθ_b from two sensors may be defined as

Aθ_α = k(ω₀ + ω_s )Ax + 2πn_α

Δθ_b = k{ω₀ - ω_δ )Δx + 2πn_b

where Δx is a single path-length difference defined by the contact and the position of the sensors.

Therefore the values of n_a and n_b may be selected so that the measured phase angle differences infer similar values of the path-length difference. There may be only one combination of values (n_a, n_b) for which this is possible. In this case the true value of the path-length difference may be determined. The correct combination (n_a, n_b) may be determined as the combination of values that minimise the expression:

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 may be used to determine the location of the contact.

Alternatively, where nim 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 summarised in Figure 11) . Thereafter it may be assumed that the contact moves more slowly than the bending waves and hence that the phase angle differences change by small increments

over the timescale Δt. Accordingly, each value of n may be 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 n, for example by a state-space estimator such as the well known Kalman filter. The sequence having the maximum measure of likelihood is selected.

The state-space estimator provides an estimate of the internal state of a system of which noisy measurements are made. A necessary input to the state-space 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 other state-space estimators may provide a measure of likelihood that a sequence of observed, noisy measurements are consistent with the model of the system state .

A state-space estimator may therefore be employed to take

a sequence of a pair of path-length differences (say Δ-?-ϊ and

Ax₃₄) taken at different times (say t_x, t₂, t₃,...), to estimate the system-state, i.e. the position and velocity of the contact, at those times. Moreover, the overall likelihood of those values of path-length difference being consistent with the model of the system may be evaluated. Where the sequence of path-length differences are obtained from a sequence of phase-angle differences and a set of integers {n = n (t₁) , n { t₂) , n(t₃) ,...) , the measure of likelihood generated by the state-space estimator may be used to infer the likelihood that the correct values of n were chosen. It follows that a method for choosing the correct sequence of integers, n, is to find the sequence for which the state-space estimator gives the maximum measure of likelihood.

As mentioned above, the state space estimator uses some statistical description of the evolution of the system state. 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. 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 may be useful to assume that any contact on the member occurs within the discrete areas corresponding to the buttons . 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) 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 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 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 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 non-transparent , for example having a printed pattern. The member may have uniform thickness. Alternatively, the member may have a more complex shape, for example a curved surface and/or variable 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 movement of the contact . The contact may be in the form 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 stylus may have a flexible tip, e.g. of rubber, which generates bending waves in the member by applying 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 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) . 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 frequency band of the measurements from the sensors. The audio signal may thus be filtered, for example, the audio band may be limited to frequencies below 20kHz, and the vibration measurements may be limited to frequencies above 20kHz. A sensor may have dual functionality and act as the emitting transducer. 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 inertial 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 touch sensitive according to the present invention. In a laptop, the touchpad which functions as a mouse controller may be replaced by a continuous moulding which is a contact sensitive device according to the invention. Alternatively, the contact sensitive device may be a display screen, e.g. a liquid crystal display screen comprising liquid crystals which may be used to excite or sense bending waves . The display screen may present information relating to the contact .

BRIEF DESCRIPTION OF DRAWINGS The invention is diagrammatically illustrated, by way of example, in the accompanying drawings, in which :-

Figure 1 is a schematic plan view of a touch sensitive device according to one aspect of the invention;

Figure 2 is a schematic perspective view of the device of Figure 1 ;

Figure 3 is a schematic side view of a one-dimensional beam;

Figure 4a is a graph showing the amplitude of the reflection coefficient against frequency (Hz) , the amplitude is unitless since it is a ratio;

Figure 4b is a graph showing the phase (in radians) of the reflection coefficient against frequency (Hz) ;

Figures 5a and 5b are schematic perspective views of alternative touch sensitive devices;

Figure 6 is a flowchart of a method of finding the location of a contact according to the invention; Figure 7a is a schematic block diagram of apparatus used for calculating phase angles;

Figure 7b is a schematic block diagram of apparatus used with that of Figure 7a;

Figures 8a to 8d are plan views of apparatus according to 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 alternative method of calculating the location of the contact;

Figure 11 is a flow chart showing a method of calculating the location of the contact using the dispersion corrected correlation function;

Figure 11a is a graph of dispersion corrected correlation function against time, and Figure 12a is a schematic block diagram 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 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 text 20 or other matter on the touch sensitive plate 12.

The transparent touch sensitive plate 12 is a member, e.g. an acoustic device, capable of supporting bending wave vibration. As shown in Figure 2, four sensors 16 for measuring bending wave vibration in the plate 12 are mounted on the underside thereof. The sensors 16 are in the form of piezoelectric vibration sensors and are mounted one at each corner of the plate 12. At least one of the sensors 16 may also act as an emitting transducer for exciting bending wave vibration in the plate. In this way, the device may act as a combined loudspeaker and contact sensitive device.

A mounting 22 made of foamed plastics is attached to the 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 surface . The mechanical impedance of the mounting and plate are selected so as to minimise reflections of bending 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 waveguide 34 in the form of a beam which terminates at an edge mounting 36 having a termination impedance. An incident wave 38 travelling down the waveguide 34 is reflected by the mounting 36 to form a reflected wave 40. The incident and reflected waves are plane waves travelling in the direction 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 torque resistance; whereby the bending moment is equal to zero at the edge and (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 Z_τ is the termination impedance of the mounting and Z_B is the mechanical impedance of the end of the waveguide, given by

_ x BkHω) , _Λ ^z'^{w =} ^ ^{(1 + ,)}

where k {ω) , is the wavevector which may be expressed in

terms of the bending stiffness, -5, and mass per unit area, μ, of the panel ,

Thus, the reflection coefficient is determined by the ratio of the impedances at the end of the waveguide and the mounting. Furthermore, the impedance of the waveguide is proportional to the square root of frequency and is both real

and reactive in equal weights (i.e. π/4 phase angle). Accordingly, the reflection coefficient is likely to be strongly frequency dependent .

The reflection coefficient vanishes, i.e. bending wave

energy is strongly absorbed in a frequency band around co₀, if the following condition is satisfied:

Z_τ =-iZ_B(ω₀)

Thus, the termination impedance of the mounting must have both real and imaginary components, or, equivalently, the mounting should be both resistive and compliant.

The plate may be, for example, 1 mm thick polycarbonate sheet which has mass per unit area, μ=1.196 kg m^"2 and bending stiffness, B=0.38 Nm. The equations above can be used to calculate the impedances of the plate and absorber required to strongly absorb bending wave energy around the chosen angular frequency ω₀ = 2π(900 Hz).

The impedance, per unit width for a 1mm beam approximation of the plate is

Z_B(ω₀) = (1+i) 33.8 N s πf². The properties of the absorber which provide the desired absorption are thus : Resistance per unit width,

Re(Z_r)=Im[Z_fl (ω₀)]= 33.8Nsm^-2.

Stiffness per unit width,

-i1m(z_τ)ω₀ = Re[Z_B(ω₀)]ω₀ = 1.91x10^s Nm^"2 .

The reflection coefficient is a unitless complex number. Figures 4a and 4b are graphs showing the amplitude and phase

of the reflection coefficient R(ω) varying with frequency. The amplitude of the reflection coefficient is zero and its

phase is reversed for ω₀ 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 stylus 18 is drawn across 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 raised crossed lines and in Figure 5b, the surface pattern 29 is a random relief pattern.

In the embodiments of Figures 2, 5a and 5b, as the contact moves over the rough surface of the member, bending waves radiate isotropically in the member from the point of 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{ω; x) . At distances

larger than the wavelength, λ - 2πlk{ω) , the transfer function

can be approximated as,

where A is a constant and Jc(ω) , is the wavevector defined

previously. Although H{ω; x) strictly only applies to bending waves on an infinite plate, since the mounting is strongly absorb bending wave vibrations, the relationship is satisfied. The transfer function shows that where a source of bending waves emits a purely sinusoidal frequency with angular

frequency, -y₀, the phase difference Δθ₁₂ between displacements at two locations which are at distances, x and x₂ , from the point of contact for the source is :

exp(-^'Δ#₁₂)

- x₂)]

This implies the following relationship between the phase

angle difference, the path length difference Ax = (x_x -x₂) and an integer n₁₂.

Aθ_u = θ -θ₂ = Ar(-y₀)Δx₁₂ + 2πn '.12 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 W^t) and W-{t) ,

b) Calculate the phase angles #_;(t) and θ_j(f) of the measured

bending wave signals W_t{t) and W_j{t) ,

c) Calculate the difference between the two phase angles θ₍{t)

and θ_j{t)

d) Calculate the location of the contact from

k(ω₀)Δx_/7 - Θ_JJ - 2τm_ij

Figure 7a shows a schematic block diagram of a device for

calculating the phase angle θ_j of a bending wave signal W_j (t) measured by one of the sensors. The signal W_j (t) is a random signal and is thus uncorrelated over long time scales . The signal is first amplified by an amplifier 42 and then processed by an analogue band-pass filter 44 with a pass-band

centred at ω₀ and a bandwidth of Δω.

A moving source of bending waves may demonstrate the

Doppler effect, whereby a bending wave which has frequency ω₀ 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 ω₀ -k(ω₀)v . The maximum angular frequency

shift between bending waves at two different points on the

member is therefore 2&(-y₀)v_rrιa). , where V_max is the maximum velocity of the moving source. If the angular frequency shift becomes larger than the width of the band pass filter, the phase difference equation above does not hold. Accordingly,

the bandwidth Δω of the filter 44 is set to be greater than this maximum frequency shift and thus obeys the relationship:

Aω » 2k(ω₀)v_πm

After processing by the filter 44, the resulting filtered signal W'_j (t) is an amplitude and phase modulated carrier with

frequency ω₀ and is defined by:

where A_j (t) and θ_j (t) are the amplitude and phase of the

signal. Both fluctuate over a timescale -dt determined by the

bandwidth of the filter, namely At = 2π/Aω The maximum frequency at which independent phase angle measurements may be

taken from the output of the bandpass filter is — . Since a

Δt touch sensor typically provides an updated measurement of the contact position every 10ms, the condition for the minimum

frequency of positional measurement is Δt<10ms .

The filtered signal W'_j (t) is then passed simultaneously 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 ω_Q but a phase difference of π/2 are also fed to the two phase detectors . The outputs of the phase detectors are passed through low-pass filters 48 each having

frequency cut-offs of approximately Δω/2. The outputs of the

low-pass filters are proportional to cos ( #,J and sin(#_jj respectively. These outputs are then digitised by digitisers

50 and processed by processor 52 to give the phase angle θj .

Figure 7b shows how the reference signals used in Figure 7a may be generated. A second bending wave signal Wi ( t) is measured at a second sensor. The signal is fed through an amplifier 42 and analogue band-pass filter 44 to generate a 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 phase by π/2. The phase shifted signal is used as the reference signal to the second phase detector 46.

Figures 8a to 8d show how the phase angle differences and hence the path length differences 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 the plate 12. Figure 8a shows the three hyperbolic curves 26 which are generated using three different values of nχ_m 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 which are generated by the phase angle difference and different values of ι_m for two other pairs of sensors. Figure 8d shows all the hyperbolic curves created by the sensors. The contact location 24 is the point of intersection of three hyperbolic curves, one from each pair of sensors. From the contact location 24, the correct value of nι_m may be inferred.

A method of inferring n is implemented using the embodiment shown in Figure 9. The bending wave signal W {t)

measured by each sensor is simultaneously processed by two band-pass 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 pass-band frequencies whereby two phase angle differences, one for each pass-band frequency, are provided by each pair of sensors.

The phase angle differences Δθ_a, Δθ_b from the sensors may be defined as

Aθ_α = k(ω₀ + ω_δ )Ax + 2πn_α

Aθ_b = k(ω₀ - ω_s )AX + 2πn_b

The correct combination (n_a, n_b) may be determined as the combination of values that minimise the expression:

The path length difference may then be estimated as

Another pair of sensors may then be used to determine a second path length difference. Each path length difference defines a hyperbolic curve on the panel . The intersection point of these two hyperbolic curves is the location of the contact. As in Figures 8a to 8d, the hyperbolae 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 shows an alternative method for calculating the location of the contact from the equation above, namely

i. Measure a pair of bending wave signals W^t) and W-(t) ,

one signal being measured by a sensor; ii . Calculate the dispersion corrected correlation function of the two signals using the method described in Figures 11 and 11a; iii. Calculate the initial position of the contact using the dispersion corrected correlation function, as described in Figures 11 and 11a;

iv. Remeasure bending wave signals W_t{t) and W {t) ;

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 nι_m which minimises the change in the path length difference; viii.Plot the hyperbola defined by k{ω₀ )Ax_iJ = Aθ_y - 2πn_iJ

ix. Repeat steps (iv) to (viii) , remeasuring the bending wave

signals at regular intervals Δt, for example Δt =2π/Δω . At step (viii) a minimum of two hyperbolae from different pairs of sensors are required 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 determined. Two phase angle differences may be generated by using two sensors and splitting the signal into two frequency bands as described in Figure 9. Alternatively, multiple sensors may be used so that multiple phase angle differences may be calculated using different pairs of sensors . Figure 11 shows a method of calculating the dispersion corrected correlation function to reveal 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: (a) Measure two bending wave signals W_λ{t) and W₂{t) ;

(b) Calculate the Fourier transform of W (t) and W₂{t) to

arrive at W_x(ω) and W₂ ω) and hence the intermediate function

W_x {ω) W₂ {ω) ; where W₂'{ω) is the complex conjugate Fourier

transform, t represents time ω is 2πf where f is frequency. (c) Calculate a second intermediate function M(ω) which is a

function of W{ώ)W₂ {ω)

(d) and (e) at the same time as performing steps (a) to (c) ,

the frequency stretching operation f(ω) =

is

calculated using the predetermined panel dispersion relation

(f) M(ω) and f{ω) = v{μ/By co are combined to arrive at the

dispersion corrected correlation function:

G(t) =— r M[f(ω)]exp(iωt)dω ; and

(g) the dispersion corrected correlation function is plotted against time with a peak occurring at time t₁₂ as shown in

Figure 11a;

(h) Δ 12 i calculated from tι₂; Δx₁₂ is the path-length difference between the path lengths Xi and x₂ from the first and second sensors to the contact .

(i) Δx₁₂ 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 contact. Thus the ways of generating more hyperbolae discussed above apply equally to this method.

The second intermediate function M(ω) may simply be

W ω)W₂ ^* ω) which gives a standard dispersion corrected correlation function. Alternatively, M(ω) may be selected from the following functions which all yield phase equivalent functions to the standard dispersion corrected correlation function:

., jfw . WfiW

W_x{ω)W₂ ^*{ω)

_b)

c) is a real valued

function

d) M{ώ) = W_i{ω)W₂ ω)ψ(ω) where ψ{c ) is a real valued

function

Alternatively, M(ω) may be the function ∑>{ω) which is

the Fourier transformation of the correlation function D(t) :

D{t) = ^ W_l{t + f)W₂{f)df

The steps are calculate D(t); calculate D{ω) and apply a frequency stretching operation to arrive at the dispersion

corrected correlation function: O(t)=— -D[ ^"(ω)]exp(zfi>t)-f-y .

2π ^∞

Alternatively, at step (f) the following dispersion corrected correlation function may be calculated:

ΦΛfM] ^« (ⁱ«)r^fα>

where u ) = ∑ J Ip≠ M«Φ[- ) ^χj }

where and

are the Fourier transformation and

complex conjugate Fourier transformation of two measured

bending wave signals

and

is the path-

length difference.

A sensor may act as both the first and second sensor whereby the dispersion corrected correlation function is an autocorrelation function. The autocorrelation function may be calculated applying the same steps for the dispersion corrected correlation function using P,(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 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 to the member 106 to generate an acoustic output. Before being 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 a large frequency band. The signal from the contact and the audio signal sum to give a combined signal which is passed through a high pass filter 114 to remove the signal above the threshold frequency f₀. The filtered signal is then passed to a digitiser 116 and onto a processor 118.

Claims

1. A contact sensitive device comprising a member capable of supporting bending waves, three sensors mounted on the member 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 angle for each measured 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.

2. A contact sensitive device according to claim 1 comprising an absorber at the edges of the member whereby reflected waves are suppressed.

3. A contact sensitive device according to claim 2 , wherein the mechanical impedance of the absorber and the member are selected so as to minimise reflections of bending waves from the edges of the member.

4. A contact sensitive device according to claim 3 , wherein the impedances are selected so that bending wave energy is strongly absorbed in a frequency band around a chosen

frequency ω₀.

5. A contact sensitive device according to claim 4, wherein the impedances are selected to satisfy the following equation:

Z_τ = -iZ_B(ω₀)

where Z_τ is the termination impedance of the absorber and Z_B- is the mechanical impedance of the edge of the member.

6. A contact sensitive device according to claim 4 or claim 5, comprising a band-pass filter for filtering each measured bending wave signal, the filter having a pass-band centred at

the chosen frequency ω₀ and a bandwidth of Δω.

7. A contact sensitive device according to claim 6, wherein

the bandwidth Δω of the filter obeys the relationship:

Aω » 2k{ω₀)v_mΑ

where v_max 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 preceding claims, wherein the member comprises a raised pattern on its surface whereby a contact drawn across the surface provides a force to the member to generate bending waves in the member.

10. A contact sensitive device according to claim 9, wherein the pattern is random whereby a contact travelling over the 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 anti-reflective coating, an anti-glare surface finish or an etched finish.

12. A contact sensitive device according to any one of the preceding claims, comprising at least two band-pass filters which have different pass-band frequencies and which simultaneously process the bending wave signals measured by a pair of sensors whereby a phase angle difference for each pass-band 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 preceding claims, comprising means for determining the initial location of the contact using the dispersion corrected correlation function of pairs of measured bending wave signals and means for determining subsequent locations of the contact using the phase angle difference between pairs of measured bending wave signals .

15. A contact sensitive device according to any of the preceding claims , wherein the phase angle determining means comprises a phase detector.

16. A contact sensitive device according to claim 15, wherein the processor comprises a low-pass filter and a digitiser for determining the phase angles .

17. A contact sensitive device according to any one of the preceding claims, wherein the member is an acoustic radiator and an emitting transducer is mounted to the member to excite bending wave vibration in the member to generate an acoustic output .

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.

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 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 angle for each measured bending wave signal, calculating a phase difference between the phase angles of at least two pairs of sensors and determining the location of the contact from the at least two calculated phase differences.

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 mechanical impedances of the absorber and the member so as to minimise reflections of bending waves from the edges of the member.

23. A method according to claim 22, comprising selecting the impedances so that bending wave energy is strongly absorbed in

a frequency band around a chosen frequency ω₀.

24. A method according to claim 23, comprising selecting the impedances to satisfy the following equation Z_τ = -iZ_B(ω₀)

where Z_τ is the impedance of the absorber and Z_B is the 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 band-pass

filter having a pass-band centred at the chosen frequency ω_σ

and a bandwidth of Δ_ω.

26. A method according to any one of claims 20 to 25, comprising applying the phase difference equation:

Aθ_lm = θ, -θ_m = k(ω₀ )Ax_lm + 2πn_lm

to determine the location of the contact, where θ_± is the phase angle of a measured bending wave signal, xi is the

distance from the contact location to each sensor, Δxι_m=xι -x_m

is the path length difference of two sensors, k {ω) is the wavevector and χ_m is an unknown integer.

27. A method according to claim 26, comprising selecting the

member to constrain the magnitude of Axχ_m to values less than one half of a wavelength so that nι_m 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 signals and selecting a value of nι_m which minimises change in the path length difference.

29. A method according to claim 26, comprising selecting a series of values of nι_m, combining the series of values with each phase angle difference to define a series of path length differences, plotting the series of graphs of the path length differences, and inferring the true value of nχ_m 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 angle differences from the pairs of phase angles, 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 each sensor into at least two discrete frequency bands and calculating a phase angle difference for a pair of sensors for each frequency band.