BACKGROUND OF THE INVENTION

Acoustic touch position sensors are known to include a touch plate and two or more transducers each of which imparts a surface acoustic wave that propagates along an axis on which a reflective grating is disposed to reflect portions of the surface acoustic wave along plural parallel paths of differing lengths. The reflective gratings associated with the respective transducers are disposed on perpendicular axes so as to provide a grid pattern to enable coordinates of a touch on the plate to be determined. Acoustic touch position sensors of this type are shown in United States

Patent Nos. 4,642,423, 4,644,100, 4,645,870, 4,700,176, 4,746,914 and 4,791,416.

Acoustic touch position sensors utilizing surface acoustic waves as taught by the above- mentioned patents have a number of problems which

are more readily understood when the nature of the surface acoustic wave used in these sensors is considered. Surface acoustic waves are shown in FIGS. 1A-D propagating in the X direction. Surface acoustic waves have an X component and a Z component such that the particles of a surface acoustic wave move elliptically in the X, Z plane. Although surface acoustic waves have a Z component, the disturbance of particles in the plate created by a surface acoustic wave decays rapidly in the -Z direction so that the wave energy is essentially confined to the surface of the plate. Surface acoustic waves have no Y component so that there is no disturbance of the touch plate particles in the Y direction.

Because the surface acoustic waves described in the above-mentioned patents are confined to only a single surface, i.e. the top touch surface of the touch plate, these surface acoustic waves are actually Rayleigh waves or more precisely guasi-Rayleigh waves since true Rayleigh waves exist only in an infinitely thick propagating medium. A Rayleigh/quasi-Rayleigh wave is shown more particularly in FIG. ID. In order to provide such a wave, the thickness of the touch plate must be at least three to four times the wavelength of the wave imparted into the substrate wherein the length of the touch plate is also limited. If the thickness of the touch plate is for example two wavelengths or less. Lamb waves will be generated in the touch plate instead of Rayleigh waves. Lamb waves are dispersive waves, varying in phase and group velocities. A touch plate in accordance with the teachings of the above-mentioned patents would not work in such a thin plate because Rayleigh or quasi-Rayleigh waves cannot exist therein. However, for a panel having a thickness that is at least

three to four times the wavelength of the wave propagating therein, nearer the source of the wave, i.e. the transducer, the symmetric and anti¬ symmetric Lamb waves appear to be in phase. As seen in FIGS. IE and IF, the symmetrical and anti- symmetrical Lamb waves are not confined to a single surface of the touch plate but extend through the plate to the opposite surface thereof. When in phase, however, the symmetric and anti-symmetric Lamb waves add to produce a quasi-Rayleigh wave as can be seen from a comparison of FIGS. IE and IF to FIG. ID. As the Lamb waves travel farther from the transducer, due to the differing phases and velocities of the Lamb waves, there is a complete transference of wave energy from the top surface of the touch plate on which the transducer is mounted, to the bottom surface of the touch plate. This transference of energy between top and bottom surfaces occurs at regularly spaced intervals making a touch plate having a dimension large enough for this transference of energy to occur unsuitable for a touch position sensor.

From the above it is seen that touch position sensors as shown in the above-mentioned patents utilizing surface acoustic waves and more particularly quasi-Rayleigh waves, as is necessary for the sensors to operate, are limited to relatively thick panels, i.e. panels having a thickness of three to four times the wavelength of the surface acoustic wave propagating therein. Although the wavelength of the propagating wave may be reduced by reducing the frequency of the drive signal applied to the transducer, as the wavelength of the wave is reduced, transference of energy between the top and bottom surfaces of the touch plate occurs closer to the transducer so as to limit the size of the touch late.

Further, because surface acoustic waves are confined to the surface of the touch plate, contaminants or other materials abutting the plate may create shadows or blind spots extending along the axes of the plate that intersect the contaminant or abutting material. The blind spots or shadows are created by a total or near total absorption of the wave energy by the contaminant or abutting material so that the touch position sensor cannot detect a touch if one of its coordinates is on a blinded axis. Substantial losses in wave energy over time as a result of air damping of the surface acoustic wave is also significant since surface acoustic waves are confined to the surface of the touch plate. The energy losses due to air damping further limit the size of the touch plate.

As shown in FIGS. 1A and C, surface acoustic waves are imparted into a touch plate utilizing a transducer mounted on a wedge that is in turn mounted on the touch surface of the plate wherein the transducer vibrates in the direction shown to produce a compressional bulk wave that propagates in the wedge to impart a surface acoustic wave in the touch plate. This type of wave generating device has several drawbacks. Because the device must convert a compressional bulk wave to a surface acoustic wave, the efficiency of the device is not as high as if the waves in the wedge were of the same type as those imparted into the plate. Also, because the wedge extends above the plate, it must be accommodated for in mounting the plate. Wedges are typically made of plastic thus creating a difficulty in bonding the wedge to a glass late. Further, the transducer must be bonded to the wedge and the wedge then bonded to the touch plate. Because problems with reliability increase with the number of bonds reσuired. this surface

acoustic wave generating device is not as reliable as other wave generating devices requiring fewer bonds.

Although acoustic waves other than surface acoustic wave can propagate in a solid, such waves including Lamb waves and shear waves, heretofore these other acoustic waves were thought to be unsuitable for a touch position sensor. Lamb waves were thought unsuitable because they are dispersive, varying in phase and velocity, so as to interfere with one another. Shear waves were thought unsuitable because a touch on a plate in which shear waves are propagating absorbs only a small percentage of the total shear wave energy whereas a touch on a plate in which a surface acoustic wave is propagating absorbs a much greater percentage of the surface acoustic wave energy. More particularly, the percentage of total energy absorbed by a given touch is ten times greater for a surface acoustic wave than it is for a shear wave. Since shear waves are not nearly as responsive to touch as surface acoustic waves, shear waves were not thought practical for a touch position sensor. SUMMARY OF THE INVENTION in accordance with the present invention, the disadvantages of prior acoustic touch position sensors as discussed above have been overcome. The touch position sensor of the present invention utilizes a shear wave that propagates in a substrate along a plurality of paths of differing lengths and differing positions wherein a touch on the substrate forms a perturbation the time of occurrence of which is sensed to determine the position of the touch on the substrate. Shear waves have several unexpected advantages over surface acoustic waves which compensate for the lower percentage of total energy

absorbed by a touch. One such unexpected advantage is that a shear wave can generate a much higher amplitude signal with a higher signal to noise ratio than can be generated by a surface acoustic wave so that with the aid of signal processing a shear wave touch position sensor results wherein the sensor is at least as sensitive to touch as a surface acoustic wave touch position sensor.

In fact, it is because shear waves are not confined to the surface of the substrate, as are surface acoustic waves, but extend throughout the entire thickness of the substrate that several advantages result from a shear wave touch position sensor. One advantage is that contaminates or other materials abutting the surface of a shear wave touch position sensor do not result in blind spots or significant shadows extending along the axes that intersect the contaminate or matter. Therefore, shear wave touch position sensors are suitable for use in environments that surface acoustic wave sensors may not be. Shear wave touch position sensors are also sensitive to a touch on both the top and bottom surfaces of the substrate whereas surface acoustic wave sensors are sensitive to touch only on the surface of the substrate on which the transducer is mounted. Further, the losses due to air damping are much less in a shear wave touch position sensor than in a surface acoustic wave touch position sensor so that the shear waves can travel much greater distances than a surface acoustic wave.

Another major advantage of a shear wave touch position sensor is that virtually the only limit on the thinness of the touch plate is its structural integrity making it extremely practical for applications where the weight of the touch sensor must be minimized. In fact, it is desirable

that the thickness of the substrate in a shear wave touch position sensor be less than that capable of supporting a Rayleigh wave since Rayleigh waves are more fractionally sensitive than shear waves. The shear wave touch position sensor of the present invention includes a substrate capable of propagating a shear wave; first means for reflecting portions of a shear wave along first paths of differing lengths, at least a portion of the first paths being substantially in parallel and axially spaced; means for propagating a shear wave along the first paths wherein a touch on the substrate forms a perturbation in the shear wave; and means for sensing the occurrence of a perturbation in a shear wave propagated along the first paths to determine the position of a touch on the substrate relative to an axis intersecting the first parallel paths.

In order to determine the position of a touch along a second axis generally perpendicular to the first axis, there is provided a second means for reflecting portions of a shear wave along second paths of differing lengths, at least a portion of the second paths being substantially in parallel, axially spaced and generally perpendicular to the first parallel path portions. A touch on the substrate surface forms a perturbation in a shear wave propagating along the second path the occurrence of which is sensed to determine the position of a touch along an axis intersecting the second paths. From the determined positions of the touch relative to the axes intersecting the first and second paths, the coordinates of the touch are known. In one embodiment of the present invention, the reflecting means includes a first array of reflective elements for reflecting a shear

wave along the first parallel path portions to a second array of reflective elements disposed on an opposite side of the substrate. In this embodiment, the first array of reflective elements is disposed on an axis perpendicular to a first transducer that imparts a shear wave into the substrate. The second array of reflective elements is disposed on an axis perpendicular to a second transducer that receives the shear wave to provide a signal representative thereof from which the position of a touch may be determined.

In another embodiment, the reflecting means includes a single array of reflective elements disposed along an axis perpendicular to the side of the substrate on which a transducer is mounted. The reflecting means also includes a reflective edge of the substrate, the reflective edge being positioned opposite to the array of reflective elements. The transducer imparts a shear wave for propagation along the axis of the array of reflective elements, each of the elements reflecting portions of the shear wave to the opposite edge of the substrate which in turn reflects the shear waves back to the reflective array. The reflective elements of the array reflect the shear waves propagating from the reflective edge of the substrate back to the transducer which, upon sensing the shear waves reflected back, produces a signal representative thereof. One transducer to transmit and receive shear waves may be provided for each axes for which a touch coordinate is to be determined. Alternatively, a single transducer may be provided for transmitting and receiving a shear wave that propagates on two coordinate axes wherein a means is provided that intersects both axes for reflection of a shear wave propagating along the first axis to the second axis and visa versa. This latter embodiment

is made possible because shear waves like to bounce off of reflective edges of the substrate with no appreciable loss of energy. This is in contrast to a surface acoustic wave which does not like to bounce and which is unable to travel great distances in a substrate due to the appreciable losses associated therewith.

In accordance with the present invention the transducers may be bonded directly on to a side of the substrate such that an electrical signal induces a shear wave in the transducer that is coupled thereby directly to the substrate. Further, a conductive frit may be used to bond each of the transducers to the substrate wherein the frit provides a contact for the transducer. The frit may be used to reduce the number of wires necessary for the operation of the sensor.

These and other objects, advantages and novel features of the present invention, as well as details of an illustrated embodiment thereof, will be more fully understood from the following description and the drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1A is a perspective view of a prior art surface acoustic wave propagating plate;

FIG. IB is a greatly exaggerated per¬ spective view of a surface acoustic wave traveling in the prior art plate of FIG. 1A;

FIG. 1C is a side cross sectional view of the prior art plate shown in FIG. 1A illustrating the nature of the waves generated in the plate;

FIG. ID is an illustration of a Rayleigh wave, a symmetrical Lamb wave and an anti-symmetric Lamb wave; FIG. 2A is a perspective view of a shear wave propagating plate in accordance with the present invention;

FIG. 2B is a greatly exaggerated perspective view of a shear wave traveling in the plate of FIG. 2A;

FIG. 3 is a perspective view of a shear wave touch position sensor in accordance with a first embodiment of the present invention;

FIG. 4 is an illustration of the variable height reflective elements forming a reflective array as shown in FIG. 3; FIG. 5 is a block diagram illustrating the signal processing portion of the touch position sensor illustrated in FIG. 3;

FIG. 6 is a flow chart illustrating the position determining operation of the sensor of the present invention;

FIG. 7 is a flow chart illustrating the touch scan routine called by the software routine illustrated in FIG. 6.

FIG. 8 is a graph illustrating the X and Y waveforms generated by the touch position sensor of FIG. 3;

FIG. 9 is a graph illustrating the difference in fractional sensitivity of a zeroth order horizontally polarized shear wave as compared to a surface acoustic wave;

FIG. 10 is a top view of a second embodiment of the touch position sensor in accordance with the present invention;

FIG. 11 is a third embodiment of the shear wave touch position sensor in accordance with the present invention;

FIG. 12 is a graph illustrating the waveform generated by the sensor shown in FIG. 11 as compared to the waveform generated by the sensor shown in FIG. 3;

FIG. 13 is a top view of the touch position sensor shown in FIG. 3 with spurious mode sup-pressor reflectors disposed thereon;

FIG. 14 is a top view of the touch position sensors shown in FIG. 3 with absorbing strips mounted thereon;

FIG. 15 is a partial perspective view of a shear wave propagating substrate coupled by an elongated flexible connector to a transducer; and FIG. 16 is a side view of the transducer, substrate and connector illustrated in FIG. 15 wherein the transducer is shielded.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The touch position sensor of the present invention includes a substrate 10 that is capable of propagating a shear wave 12 as shown in FIG. 2A. The substrate 10 may be formed of tempered or frosted glass, plastic, metal or ceramic. The substrate 10 may further be formed as a flat plate as shown or the substrate may be curved. In order to induce a shear wave propagating in the X direction, a piezoelectric transducer 14 is bonded on to an edge of the substrate perpendicular to the axis of propagation, X. The transducer 14 is responsive to a drive signal for vibrating along the Y axis wherein a shear wave 12 is induced in the transducer 14 and coupled thereby directly to the substrate 10. As shown in FIG. 2B, the shear wave 12 imparted into the substrate 10 is not confined to a single surface of the substrate 10, but extends throughout the entire thickness of the substrate 10. The particles of the shear wave move only in the Y direction. The shear wave 12 has no displacement components in the X or Z directions. It is noted that shear waves may be symmetric or anti-symmetric about the median plane. In the preferred embodiment of the present invention a nondispersive mode shear

wave is employed. More particularly, this nondispersive mode is the zeroth order of the following plate solution, eq. 1 U _y = Arr(cos Bx) exp i(γz - ωt) . This shear wave is designated herein as the Zeroth Order Horizontally Polarized Shear wave or a Zohps wave.

A touch position sensor 16 in accordance with a first embodiment of the present invention is shown in FIG. 3 having a pair of transmitting and receiving transducers 18, 20 and 22, 24 associated with each axis for which a coordinate is desired to be determined. Although the touch position sensor 16 has four transducers 18, 20 and 22, 24 respectively associated with the X axis and Y axis so that both X and Y coordinates of a touch may be determined, if only a single coordinate is desired, for example a coordinate along the X axis, then the transducers 22 and 24 associated with the Y axis may be eliminated.

Returning to FIG. 3, in accordance with the present invention, the piezoelectric transducers 18, 20, 22 and 24 are bonded on to the sides 26 and 32 of the substrate 10 by means of a conductive frit. The conductive frit forms a contact that may extend around the corner of the substrate 10 between the transducers 20 and 22 to eliminate the need for separate ground wires connected to these two adjacent transducers. The height of the transducers 18, 20, 22 and 24 and the height of the conductive frits bonding the transducers to the substrate 10 are equal to the thickness of the substrate 10 so that only the fundamental mode of the shear wave is generated in the substrate 10. The piezoelectric transducers 18, 20, 22 and 24 are thin so as not to protrude outwardly from the sides 26 and 32 of the substrate to any appreciable extent. Further, since

the transducers do not protrude above or below the top or bottom surfaces 40 and 42 of the substrate 10, the substrate 10 may be mounted in any fixture accommodating a plate of the same size without making special accommodations for the transducers.

In order to define the X axis, the X transmitting transducer 18 is bonded on to an edge 26 of the substrate 10 wherein the edge 26 is perpendicular to the X axis. The transmitting transducer 18 vibrates along the Y axis to impart a shear wave that travels along the X axis to an array 28 of reflective elements as described in detail below. Each element of the reflective array 28 is disposed at an approximately 45° angle so as to reflect a portion of the shear wave incident thereto in the Y direction to a corresponding reflective element disposed in a reflective array 30. The array 30 of reflective elements is disposed along an axis that is parallel to the axis along which the reflective array 28 is disposed. Each of the reflective elements in the array 30 is disposed at a 45° angle with respect to the axis of the array 30 so as to reflect a shear wave propagating in the Y direction from the reflective array 28 to the receiving transducer 20. The receiving transducer is bonded to the side 26 of the substrate 10 perpendicular to the axis of the array 30 so as to sense shear waves reflected thereby to the array 30 to provide a signal representative of the shear waves.

Similarly, in order to define the Y axis, the Y transmitting transducer 22 is bonded on an edge 32 of the substrate 10 wherein the edge 32 is perpendicular to the Y axis. The transmitting transducer 22 vibrates along the X axis to impart a shear wave that travels along the Y axis to an array 34 of reflective elements as described in detail

below. Each element of the reflective array 34 is disposed at an approximately 45° angle so as to reflect a portion of a shear wave incident thereto in the X direction to a corresponding reflective element disposed in a reflective array 36. The array 36 of reflective elements is disposed along an axis that is parallel to the axis along which the reflective array 34 is disposed. Each of the reflective elements in the array 36 is disposed at a 45° angle with respect to the axis of the array 36 so as to reflect a shear wave propagating in the X direction from the reflective array 34 to the receiving transducer 24. The receiving transducer 24 is bonded to the side 32 of the substrate 10 perpendicular to the axis of the array 36 so as to sense shear waves reflected thereto by the array 36 to provide a signal representative of the shear waves.

The reflective elements in the arrays 28 and 30 define a number of paths of differing lengths such that shear waves reflected by each successive element in the array 28 follow paths to the receiving transducer 20 that are progressively longer. Portions of each of the paths defined by the reflective arrays 28 and 30 extend in parallel across the substrate 10 in the Y direction, each parallel path portion defining an X coordinate. Similarly, the reflective elements in the arrays 34 and 36 define a number of paths of differing lengths such that shear waves reflected by each successive element in the array 34 follow paths to the receiver 24 that are progressively longer. Portions of each of the paths defined by the arrays 34 and 36 extend in parallel across the substrate 10 in the X direction, each parallel path portion defining a Y coordinate.

The X and Y signals generated by the respective receiving transducers 20 and 24 are depicted in FIG. 8 wherein reflective arrays 28, 30, 34 and 36 of variable height are employed to provide X and Y signals whose amplitudes remain substantially constant with time as discussed below. With regard to the X axis signal, if a shear wave is generated by the transducer 20 beginning at time t„, the first shear wave received by the transducer 20 occurs at a time equal to 2t _α + t, where t, is the time it takes a shear wave to travel from the substrate side 26 to the first reflective element in the array 28 and also the time that it takes the shear wave to travel from the first reflective element in the array 30 to the side 26 where it is sensed by the transducer 20. In the equation, t ₂ represents the time it takes a shear wave to travel across the substrate 10 in the Y direction. The shear wave portion reflected by the last element in the reflective array 28 and received by the last element in the reflective array 30 is received by the transducer 20 at a time equal to the 2tτ + t, + 2t, wherein t, represents the time it takes a shear wave to travel in the X direction between the first element of the reflective array 28 and the last element of the reflective array 28 as well as the time it takes a shear wave to travel in the X direction between the last element of the array 30 and the first element thereof. Similarly, if the transducer 22 generates a shear wave at time t„, the receiving transducer 24 receives the first shear wave reflected by the arrays 34 and 36 at a time 2tj. + t, and the receiving transducer 24 receives the last shear wave reflected by the arrays 34, 36 at time 2t, + t ₃ + 2t ₂ . Each value of t _y between 2t _x + t ₂ and 2t, + t ₂ + 2t, represents a coordinate along the X axis; whereas, each value of t„ between 2^ + t ₃ and

2ti + t ₃ + 2t ₂ represents a coordinate along the Y axis. It is noted that in the preferred embodiment the time at which the drive signal is applied to the Y axis transmitting transducer 22 is at a time subsequent to the application of the drive signal to the X axis transmitting transducer 18 and also subsequent to the time that the X axis receiving transducer 20 receives the last shear wave reflected by the arrays 28 and 30. A touch on the top surface 40 or on the bottom surface 42 of the substrate 10 will absorb a portion of the energy in the shear waves passing underneath or above the touched position. This partial absorption of energy creates a perturbation in the shear wave whose energy is absorbed, the perturbation being reflected in the amplitude of the signals generated by the receiving transducers 20 and 24. For example, the coordinates of a touch on the top or bottom surfaces of the substrate 10 are represented by the times of occurrence of the perturbations in the X and Y transducer signals depicted respectively at t _τx , t^ in FIG. 8.

The control system of the touch position sensor as shown in FIG. 5 controls the application of the drive signals to the transducers 18 and 22 and determines the coordinates of a touch on the substrate 10 from the times of occurrence t _τx and t _τy of the signal perturbations representing the touch. The touch panel 70 as shown in FIG. 5 is comprised of the substrate 10, the X and Y transmitting transducers 18 and 20, the X and Y receiving transducers 20 and 24 and the reflective arrays 28, 30, 34 and 36. A host computer 72 that may include a microprocessor or the like initiates a scan cycle of the touch panel 70 by instructing a controller 74. The controller 74 is responsive to an initiate scan cycle instruction from the computer 72 to apply

a drive signal to the X transmitting transducer 18 through an X driver 76 wherein the timing of the controller 74 is determined by a clock/oscillator 78. The drive signal applied to the transducer 18 is a burst drive signal in the form of a sine wave the number of cycles of which is equal to the width of the array 28 divided by a constant. The controller 74 also sets an X/Y switch 80 to the X position to couple the X receiving transmitter 20 to an R.F. amplifier 82. As the shear waves reflected by the arrays 28 and 30 are sensed by the transducer 20, the transducer 20 generates an X axis signal representative thereof that is coupled to the amplifier 82 through the switch 80. The amplified X axis signal output from the amplifier 82 is applied to a demodulator 84 that removes the burst drive signal from the amplified X axis signal to provide an envelope waveform such as depicted in FIG. 8. The output of the demodulator 84 is coupled to a threshold device 86 that provides an output signal which follows the input if the input to the device 86 is above the threshold thereof. The threshold device 86 does not however follow the input signal if the input is below the threshold thereof. The output of the threshold device 86 is applied to an analogue to digital converter 88 the output of which is coupled by a buffer 90 to an internal bus 91. The controller 74 stores the digital data output from the analogue to digital converter 88 in a static RAM 92 such that a value t _x representing the amplitude of the X axis signal at each point in time as sampled by the analogue to digital converter 88 is stored in a location in the static RAM 92 representing the point in time. After the X axis data is stored in the static RAM 92, the controller 74 controls the Y driver 76 to apply a burst drive signal to the Y axis transmitting transducer 22 of

the touch panel 70. The controller 74 also changes the state of the X/Y switch 80 so that the Y receiving transducer 24 is coupled to the R.F. amplifier 82. The digital data representing the Y axis signal as output from the analogue to digital converter 88 is likewise stored in the static RAM 92 such that a value t _y representing the amplitude of the Y axis signal at each point in time as sampled by the analogue to digital converter 88 is stored in a location in the static RAM representing the point in time.

During an initialization process, the host computer 22 is responsive to the values stored in the static RAM 92 for an untouched panel 70 to set the gain on the R.F. amplifier 82 via a buffer 94 the output of which is coupled to a digital to analogue converter 96. The threshold device 86 in combination with the automatic gain control provided by the feedback loop 98 shifts the zero level of the base band response to increase the difference in amplitude of the transducer's output signal representing a touched point and an untouched point so that a touched point may be more easily detected. This technique is possible since the signal to noise ratio of the Zohps waves generated is extremely high. This feature thus compensates for the difference in fractional sensitivity of a Zohps wave as compared to a surface acoustic wave as illustrated in FIG. 9. The operation of the host computer 72 in determining the position of a touch on the touch panel 70 is illustrated in FIG. 6. During the initialization of the system, a scan cycle is performed for an untouched panel 70 with the X and Y amplitude values stored in the static RAM 92 for the times t _x „ and t _y . During the initialization process the X and Y amplitude values for each sampled point

in time t _xo and t _yo is read out from the static RAM 92 and stored in a RAM 101 of the host computer 72. After the initialization is performed, at a block 100 the host computer 72 sets the values of t _* -, and t _^y egual to zero and the variables X and Y equal to 1. Thereafter, at block 102, the computer 72 calls a touch scan routine as shown in FIG. 7. The touch scan routine is a terminate and stay resident routine that is stored in the RAM 101 of the host computer 72. The host computer 72 in accordance with the touch scan routine at a block 104 sets the automatic gain control value for the R.F. amplifier 82 for the X axis to the value determined upon initialization. Thereafter, at block 106 the host computer 72 initiates a scan burst for the X axis by instructing the controller 74. After the X axis values for times t _x are stored in the static RAM 92, the computer 72 at a block 107 sets the automatic gain control value for the Y channel and at block 108 instructs the controller 74 to initiate a scan for the Y axis. After the Y axis values for times t„ are stored in the static RAM 92, the computer 72 at block 110 reads each of the amplitude stored values for times t _y and t _y in the static RAM 92 into a terminate and stay resident area of the RAM 101. Thereafter at block 112, the computer 72 returns to the routine depicted in FIG. 6.

After the X and Y axis amplitude values for times t _x and t _y are read from the static RAM 92 into the RAM 101 of the host computer, the host computer 72 at block 114 determines a difference value tv _D from the difference between the amplitude value stored for t _x wherein x was initialized to 1 at block 100 and the amplitude value stored for t _x i.e. for x = 1, t ₁₀ where At _lc represents the amplitude value stored for the first sampled time during the initialization routine. Thereafter, at block 116

- 20 - the computer determines whether the difference value t _xr . is greater than a threshold value and if it is, the computer 72 at block 118 determines whether the difference value t _xP is greater than D _y which represents the greatest difference value detected for the X axis. If t _XD is greater than D, the computer 72 at block 120 sets D _x equal to the difference value fc,,-, and sets the time of occurrence t„ _v of the difference value equal to t _x . At block 122 the computer 72 increments x by one and if x is not greater than N, the number of sampled time points for the X axis, as determined by the computer 72 at block 124, the computer 72 returns to block 114 to determine the next difference value. After difference values are determined at block 114 for each point in time sampled by the analogue to digital converter 88 and for which amplitude values are stored in the RAM 101, the computer 72 at block 126 determines whether t_^ _x , the time of occurrence of the greatest amplitude difference D _x , is equal to zero or not. If t^ _x is equal to zero indicating that no touch is detected on the X axis, the computer 72 exits the routine at a block 127. If however, the value of t _+x is not equal to zero indicating a touch the time of occurrence of which is equal to t _< . _x , the computer 72 goes to block 128. At block 128, the computer 72 compares the amplitude stored at time t„ to the initialization value stored for that same point in time t„ ₀ and stores the difference there between as ty _D . At block 130, the computer 72 compares t _yr , to a threshold and if t _yt , is greater than the threshold the computer 72 at block 132 compares t _yD to D _y , the value of the greatest difference calculated at block 128 for the Y axis signal. Thereafter, at block 134 if t _yD was determined to be greater than D _y at block 132, the computer 72 at block 134 sets D _v eσual to t _vn and the time of

occurrence t„ _y of the greatest difference signal D _y equal to t _y . At block 136 the computer 72 increments the variable y by one and at block 138 compares y to the number Z of sample points for the Y axis signal. If y is less than or equal to Z the computer 72 returns to block 128. If y is greater than Z indicating that a difference signal has been calculated for each sampled point on the Y axis, the computer 72 at block 140 determines the X and Y coordinates of a touch from the values of _*x and t„ _y . Thereafter at block 142 the computer 72 exits the routine.

The reflective arrays 28, 30, 34 and 36 may be formed of metal and bonded on to the top surface 40 of the substrate 10. However, in the preferred embodiment of the present invention, the reflective arrays are formed by silk screening frits on the top surface 40 of the substrate 10. The reflective array frits may be formed during the same cycle as the conductive frits used to bond the transducers to the substrate 10. As discussed above, each reflective element of the arrays 28, 30, 34 and 36 is disposed at a 45° angle with respect to its associated transducer 18, 20, 22 and 24. Preferably, the spacing between adjacent reflective elements is equal to one wavelength of the shear wave imparted into the substrate 10 by the respective transducer. The width of each reflective array 18, 20, 22 and 24 is equal to the width of the transducer where the drive signal applied to the transducer is a sine wave, the number of cycles of which is equal to the array width divided by a constant as discussed above.

In order to increase the ability of each reflective array 28, 30, 34 and 36 to discriminate between shear waves and Lamb waves, the number of reflectors in each array is selected to be as high

as possible since the quality of array discrimination is proportional to the number of reflectors in a given acoustic path at right angles to the axis of the array. This is achieved by maintaining the spacing between the elements of each array equal. In prior art surface acoustic wave sensors, in order to provide a constant power density of the surface acoustic waves reflected by the reflective arrays, a "finger withdrawal" method has been employed. This method increases the power reflectivity at points along the array as the distance between the points along the array and the respective transducer increases. The "finger withdrawal" method is such that selected reflective elements in the array are eliminated; however, this method diminishes the array's ability to discriminate between the wavelengths of different types of waves. Further the "finger withdrawal" method limits the size of a touch plate because the larger the plate the larger the spacings in the array. In accordance with the preferred embodiment of the present invention, a different technique is employed to increase the power reflectivity at points along the array as the distance of the points along the array from the respective transducer increases wherein a variable height reflective array is provided as shown in FIG. 4. The height of each element in the reflective array is such that the power reflectivity per unit length of a reflective array, σ(x) is given by eg. 2 σ(x) = ;

(1 + α/σ e* ^(l ~ - 1 the ratio of the height of the array at x to the height of the first array element (x = o) is eg. 3 h(κ) = f l + /σ e ^aL - 1 - ^" ; and h(o) ( 1 + a/σ _r _) e ^aii ~ ^x - 1

the ratio of the heights of the last array element and the first array element is eq. 4 hfL) = ( 1 + a/σ _τ ) e ^a - - 1 ¹² ; h(o) /σ _t where represents the power absorbtivity of the array per unit length, x is a variable representing the distance from the start of the array and L represents the length of the array. To design a variable height array, a practical value for the ratio of the maximum to minimum height, h(L)/h(o), is determined and substituted into equation 4 to determine σ_,. Thereafter the values of h(o) and σ _t are substituted into equation 3 to determine the height of the array as a function of distance. With variable height arrays, the waveforms shown in FIG. 6 are obtainable wherein the amplitude of the shear waves as reflected by the array elements is maintained substantially constant across the array in the absence of a touch. It is noted that other techniques may be employed to maintain the power density of the reflected waves substantially constant while maintaining the spacing between reflective elements equal to maximize the number of reflectors in the array. One alternative technique is to employ variable density reflective elements in the reflective arrays wherein the reflective elements extend through the thickness of the substrate and are formed of a material the density of which may be varied so as to increase the power reflectivity of the array elements as the distance between the elements and the respective transducer increases.

A second embodiment of the touch position sensor of the present invention is shown in FIG. 10 and includes a single transducer for transmitting and receiving the shear waves associated with each axis, the coordinates of a touch on which is to be

determined. Further, instead of having two reflective arrays for each axis as the embodiment de ^p icted in FIG. 3, the touch position sensor shown in FIG. 8 includes a single reflective array 28, 34 for each axis wherein the side 32, 44 of the substrate 10 opposite to each array 28, 34 is machined to provide a reflective edge. Because shear waves like to bounce, the reflective edge 32, 44 of the substrate 10 reflects the shear waves propagating perpendicular thereto without any appreciable loss in energy.

More particularly, the transducer 18 is coupled to a transmit/receive switch 146 that is controlled by the controller 74 to couple the X driver 76 or burst generator to the transducer 18 during a first time period to apply the drive signal thereto. The transducer 18 is responsive to the drive signal to impart a shear wave into the substrate 10 that propagates along the axis of the array 28. The reflective elements of the array 28 reflect portions of the shear wave incident thereto across the substrate 10 in the Y direction to the reflective edge 32 of the substrate 10. The substrate edge 32 reflects the shear waves propagating perpendicular thereto back to the reflective array 28 which in turn reflects the shear waves back to the transducer 18. After the drive signal is applied to the transducer 18, the controller changes the state of the transmit/receive switch 146 to the receive position wherein the transducer 18 is coupled to the R.F. amplifier 82 so that shear waves sensed by the transducer are coupled to the position detection circuitry.

Similarly, the transducer 20 is coupled to a transmit/receive switch 148 that is controlled by the controller 74 to couple the Y driver 76 to the transducer 20 during a second time period to apply

the drive signal thereto. • The transducer 20 is responsive to the drive signal to impart a shear wave into the substrate 10 that propagates along the axis of the array 34. The reflective elements of the array 34 reflect portions of the shear wave incident thereto across the substrate 10 in the X direction to the reflective edge 44 of the substrate 10. The substrate edge 44 reflects the shear waves propagating perpendicular thereto back to the reflective array 34 which in turn reflects the shear waves back to the transducer 20. After the drive signal is applied to the transducer 20, the controller changes the state of the transmit/receive switch 148 to the receive position wherein the transducer 20 is coupled to the R.F. amplifier 82 so that shear waves sensed by the transducer are coupled to the position detection circuitry.

A third embodiment of the touch position sensor of the present invention is shown in FIG. 11 and includes a single transducer for transmitting and receiving the shear waves associated with two perpendicular axes the coordinates of a touch on which are to be determined. In this embodiment, two reflective arrays are employed, a first reflective array 28 extending along an axis perpendicular to the side 26 on which the transducer 18 is mounted and a second reflective array 36 extending along an axis perpendicular to the axis of the first array 28 and adjacent to the end of the array 28. In order to couple a shear wave propagating along the axis of the reflective array 28 to the reflective array 36, the corner of the substrate 10 intersecting the axes of the arrays 28 and 36 is cut in order to provide a reflective edge 46 that is disposed at a 45° angle with respect to the adjacent sides 44 and 48 of the substrate 10. In response to a drive signal from the driver 76, the transducer 18 imparts a shear

wave into the substrate 10 that propagates along the axis of the array 28. The reflective elements of the array 28 reflect portions of the shear wave along a plurality of paths parallel to the Y axis to the side 32 of the substrate 10 wherein the side 32 is machined to provide a reflective edge. The side 32 of the substrate 10 reflects the shear waves propagating perpendicular thereto back to the array 28 which in turn reflects the shear waves from the side 32 back to the transducer 18. When the shear wave propagating along the axis of the reflective array 28 meets the reflective edge 46, the edge 46 reflects the shear wave along the axis of the second array 36. The elements of the second array 36 reflect portions of the shear wave along parallel paths across the substrate in the -X direction to the opposite side 26 of the substrate 10 which is machined to provide a second reflective edge. The substrate side 26 reflects the shear waves propagating perpendicular thereto back to the second reflective array 36 which in turn reflects the shear waves to the reflective edge 46. The reflective edge 46 then reflects the shear waves back to the transducer 18. The transducer 18 senses the shear waves reflected back and provides a signal representative thereof. This mode of operation is designated the triple transit mode. In the triple transit mode, the amplitude of the signal provided by the transducer 18 is reduced as compared to the amplitude of a signal generated by the transducer 20 as shown in FIG. 3, this difference in amplitude being depicted in FIG. 12. However, the triple transit mode touch panel is still sensitive to touch, a touch actually absorbing a greater percentage of the total shear wave energy so as to provide a touch position sensor that is actually more fractionally sensitive than the touch sensor

shown in FIG. 3. Because of the increase in fractional sensitivity of the triple transit mode sensor, the threshold device 86 may be eliminated. It is noted, that in the preferred embodiment the transducer 18 is positioned on the side of the substrate 10 that is perpendicular to the axis of the longest reflective array so that there are no overlapping path lengths associated with the X array and the Y array. Although the spacing of the elements in the reflective arrays and the number of elements in each of the arrays is selected so as to minimize Lamb wave interference, FIGS. 13 and 14 illustrate methods of further reducing Lamb wave interference. More particularly, in FIG. 13 spurious mode (Lamb wave) suppressor strip reflectors 50 are positioned adjacent to each of the reflective arrays 28, 30, 34 and 36, the strip reflectors 50 extending parallel to the axis of the associated array. The spacing between the reflector strips is selected to be equal to one quarter of a wavelength of the wave to be suppressed. In an alternative embodiment shown in FIG. 14, a strip of an absorbing material 52 is disposed adjacent to each of the reflective arrays 28, 30, 34 and 36 on both the top surface 40 and the bottom surface 42 of the substrate 10 in order to absorb spurious Lamb wave energy. In still a further embodiment, reflective arrays may be formed on the bottom surface 42 of the substrate wherein half of the reflectors on the bottom substrate are aligned with the reflectors in the arrays on the top surface 40 of the substrate to suppress first order antisymmetric Lamb waves and the remaining bottom reflective arrays elements are shifted from the upper array elements by a value equal to one half of the wavelength of the first order symmetric Lamb wave to suppress this type of Lamb wave. The bottom

array is thus positioned to reject both the first order symmetric and antisymmetric Lamb wave modes.

FIGS. 15 and 16 illustrate a flexible connector 60 that forms a Zohps mode transmission line. More particularly, the flexible connector 60 is formed of metal wherein the thickness of the connector bonded to the substrate 10 is equal to the thickness of the substrate, the thickness of the connector 60 reducing to a desired thinness a short distance from the edge of the substrate to which the connector is bonded, the distance being on the order of a few wavelengths of the Zohps wave. This embodiment has the advantage that there are no transducers or electrical wiring in the vicinity of the touch plate or substrate 10 so that the transducers may be brought inside the controller through a slit or the like in the wall of the controller's housing. Because the transducers are remote from the touch plate 10 electromagnetic radiation may be shielded from the transducer thus reducing pickup. Further, any radiation emanating from the transducer may be similarly shielded from external pickup. The flexible connector strip may be made as thin as 5 mils. Further a plastic sheathing may cover the connector 60 since the sheathing will not significantly dampen the Zohps wave propagating therein.

Since changes may be made in the above described apparatus without departing from the scope of the present invention, it is intended that the above description and drawing be interpreted as illustrative and not in a limiting sense.

What is claimed and desired to be secured by Letters Patent is: