Gentle-bevel flat acoustic wave touch sensor

An acoustic wave touch sensor comprising an acoustic wave transducer, for transducing an acoustic wave; a substrate, having a front surface. A surface bound acoustic wave interface, having at one least bevel edge, is provided, having an transmitted acoustic wave signal attenuation of less than about 3 dB. The acoustic wave transducer is mounted to transmit the acoustic wave through the interface into the substrate, so that the energy of the acoustic wave appears at the surface. The acoustic wave transducer is mounted on a beveled region of the substrate, inclined to the front surface. The beveled edge has an effective radius which is small as compared to the wavelength and a bevel angle such that the acoustic wave is attenuated by less than about 3 dB on transmission across the edge. In this manner, means are provided to recess transducers for improved product robustness and compactness of design.

FIELD OF THE INVENTION 
The present invention relates the field of acoustic wave touch sensors, and 
more particularly to the field of acoustic wave touchscreens for use as 
computer input devices. 
BACKGROUND OF THE INVENTION 
Acoustic touchscreen systems are used as computer input devices, especially 
as absolute touch position detecting systems for use in front of computer 
displays. Such devices have excellent much position reconstruction and 
further are advantageous due to availability of various acceptable 
substrate materials, including glass, which provides both durability and 
optical clarity. The substrate may be planar, or conform to another shape. 
Thus, acoustic touchscreens may employ curved glass substrates, providing 
a constant touchscreen to cathode ray tube (CRT) spacing where a curved 
glass CRT is employed. 
A number of flat panel display technologies are also of growing importance, 
such as active-matrix liquid-crystal displays (AMLCDs); other 
liquid-crystal display (LCD) technologies; electroluminescent (EL) 
displays; plasma displays; and field-effect displays (FEDs). 
The substrate material for an acoustic touch sensor may be glass, e.g., 
soda-lime glass or borosilicate glass. However, other known materials may 
be used where mechanical and optical properties are suitable, such as 
aluminum, ceramic. Of course, for use as a pointing device input in 
conjunction with a graphic user interface, the touchscreen is preferably 
transparent and superimposed in front of the interface image. 
In many such applications involving flat panel displays, a compact 
mechanical sensor design is desired to allow efficient and aesthetically 
appealing enclosures. Transducers used with traditional acoustic 
touchscreens, however, pose a particular mechanical design challenge in 
this regard. Known acoustic transducer types include wedge and edge 
transducers; for example, see FIGS. 2A through 2D of U.S. Pat. No. 
5,162,618. 
Known systems have therefore sought to provide the transducer reoriented or 
displaced from the top surface of the substrate, for increased mechanical 
clearance and protection of the brittle transducer components. One method 
provides a sharp bevel angle, e.g. with a 33.degree. bevel slope, near a 
lateral edge of the substrate, on which a transducer wedge is applied. 
These systems are subject to acoustic effects when an acoustic wave 
crosses the interface between the touch surface of a substrate and a 
bounding beveled surface. These known systems employ quasi-Rayleigh waves, 
a type of "surface bound wave". 
A surface bound wave includes the classes of quasi-Rayleigh waves, Love 
waves and other waves in which the opposing substrate surface does not 
significantly affect the waves' motion. In contrast, Lamb waves and other 
plate waves involve the boundary condition on the opposing face, and 
therefore operate according to different principles across a beveled 
interface than surface bound waves. In appropriate circumstances, both 
surface bound waves and other waves may be surface acoustic waves. Here we 
define "surface acoustic waves" as acoustic waves with sufficient power 
density at the surface to detect a touch. 
For example, FIG. 1 shows a typical known wedge transducer, which includes 
mechanical elements 2, 3 extending forward of the front surface 8 of the 
substrate 1. These known wedge transducers 2, 3 are advantageous because 
they provide a relatively high coupling efficiency, highly directional 
coupling, and low coupling to parasitic acoustic modes. Coupling wedges 2 
are typically made of plastic, and mounted to a glass plate substrate 1. 
The transducer 2, 3, which is generally a piezoelectric element 3 with 
electrodes covering the two large area opposing faces, is bonded to the 
plastic wedge 2, and the piezoelectric element 3 with wedge 2 is then 
bonded to the glass touch plate substrate 1, with the piezoelectric 
element electrodes electrically connected to the electrical circuitry. In 
a wedge transducer 2, 3, the piezoelectric element 3 vibrates to produce a 
compressional bulk wave that propagates in the wedge 2, which in turn is 
transduced through the wedge 2-substrate 1 interface to impart a 
quasi-Raleigh wave. 
For most acoustic touchscreens presently manufactured, the wedge 2 extends 
above the surface 8 of the plate, and therefore the rear or inactive side 
of the substrate and its edges remain free of circuitry or critical 
elements. However, the wedge 2 and transducer 3 assembly may interfere 
with structures in front of the touchscreen; this is often the case in 
which an acoustic sensor is used with a flat panel display. Furthermore, 
the brittle piezoelectric element 3 is relatively exposed and subject to 
breakage during handling and touch/display system assembly; this issue is 
particularly important for heavy substrates such as the approximately 1/2 
inch thick glass used in Elo TouchSystems' SecureTouch.TM. products. 
One known solution to this forward extension problem is shown in FIGS. 2 
and 3. Here the forward extension problem is addressed by beveling the 
glass substrate 5 with an angled bevel 9 so that the acoustic transducer 
2, 3 does not extend forward of the substrate front surface 8 to any great 
extent. 
The motivation for this type of beveling was to recess the transducer on an 
angled surface behind the front glass surface. Thus, the bevel angle was 
large, and the bevel itself extended to the lateral edge of the substrate. 
Such products include certain prior art Elo Touchsystems, Inc. P/N 
002800A-8121 (33.degree.) and 002801A-8121 (33.degree.), shown in FIG. 3. 
These known systems have a bevel angle of about 33.degree., and were 
designed with a radiused blending region 4 which is provided to limit 
acoustic losses from the beveled interface. 
The beveled glass design shown in FIG. 3 proved to be difficult to 
manufacture. Grinding the four individual corner regions is a costly labor 
intensive process. In an effort to address these problems, prior art 
touchscreens were also produced with continuous 25.degree. bevels on the 
two sides with transducers. Continuous beveling is a known process which 
allows automation and requires less manual labor than selectively beveling 
a portion of an edge. The resulting manufacturing process is more widely 
available. These known continuous bevels allow protected routing of 
transducer wires. However, the resulting touchscreens retained at least 
two drawbacks. 
As shown in FIG. 2, the known beveling process produces a "knife edge" 7. 
For the glass substrate, this resulted in a sharp edge that was 
inconvenient to handle and subject to chipping. This becomes more of an 
problem for a continuous bevel. Efforts to blunt this knife edge with, 
e.g., an intended 0.020.+-.0.010 inch vertical edge, resulted in chipping 
flaws along the edge, as well as true knife edge portions due to excess 
manufacturing variations, which serve as nucleation sites for glass 
fracture in an oven processing step. Secondly, the brittle piezoelectric 
element 3 of these known 25.degree. angle bevel systems can not fit within 
the confines of the beveled portion, and thus protrudes either forward or 
laterally from the substrate. This leads to mechanical clearance problems 
or transducer breakage risk. 
Both examples of prior art described above share a further disadvantage. 
These known designs include a gently curved or blended interface 4 between 
the bevel surface 9 and the top surface 8 of the glass substrate 1. This 
rounding is motivated by acoustic reasons; without this rounding, the 
prior art teaches that a significant amount of acoustic power is lost, 
attributed to reflections and mode conversion at the discontinuity. 
Ideally, with rounding, surface acoustic waves travel across the interface 
with minimal reflection, much like ocean waves travel across the rounded 
globe without reflection. However, approximating this ideal is 
problematic, increasing the cost and labor required for manufacture, and 
resulting in deficiencies in acoustic property reproducibility. The 
blending of the bevel and top-surface interface adds cost to the 
manufacturing process and limits mass production capacity. 
Examples are also known without blending at interface 4, resulting in a 
substantial loss of signal amplitude for the aforementioned 25.degree. and 
33.degree. embodiments. This is undesirable. It may directly result in 
reduced touch sensitivity of the touchscreen. Alternately it may reduce 
the level of electromagnetic interference that the touch system can 
tolerate, or the degree of signal loss due to aging effects that can be 
endured. Signal amplitude is a measure of touchscreen quality. Without 
blending at interface 4, the resulting product is of reduced quality. 
While available commercial acoustic touchscreen products employ wedge or 
edge transducers bonded directly to the substrate containing the front 
touch surface, a number of proposals have been made to separate the 
transducer from the touch surface. FIGS. 15 and 16 of U.S. Pat. Nos. 
5,177,327, 5,243,148, 5,260,521, and 5,329,070 and FIGS. 12 and 13 of U.S. 
Pat. No. 5,072,427 propose means to place an edge transducer remote from 
the glass substrate with the aid of a connecting metal acoustic coupling 
element. These proposals, however, do not address the extent of acoustic 
losses due to the remote placement of the sensor, nor define acceptable 
limits. 
Adler, U.S. Pat. No. 4,642,423 discloses a transducer displaced from the 
substrate, mounted on an aluminum coupling element. A transducer generates 
a wave which propagates in a metal element having the reflective grating 
and a feathered edge, bonded to the top surface of the glass substrate. 
The acoustic wave transfers to the glass substrate. Adler teaches that 
care must be taken to ensure an efficient transition of the waves from the 
reflective grating onto the surface, proposing a feathered, or beveled, 
edge, near the wave transference portion. 
Acoustic touch position sensors are known to include a touch panel or plate 
having an array of transmitters positioned along a first edge of a 
substrate for simultaneously generating parallel surface acoustic waves 
that directionally propagate through the panel to a corresponding array of 
detectors positioned opposite the first array on a second edge of the 
substrate. Another pair of arrays is provided at right angles to the first 
set. Touching the panel at a point causes an attenuation of the waves 
passing through the point of touch, thus allowing an output from the two 
sets of arrays indicative of the coordinates of the touch. This type of 
acoustic touch position sensor is shown in U.S. Pat. No. 3,673,327, 
incorporated herein by reference. 
The substrate, in many embodiments, is preferred to be transparent because 
this allows efficient and effective use of the touch sensor as a panel 
placed in front of a visual display device, such as a cathode ray tube, 
electroluminescent display, or liquid crystal display. Glass substrates 
are therefore preferred, such as soda-lime glass or a lower acoustic loss 
glass such as borosilicate glass. In other applications, such as where a 
graphic overlay may be provided on a touch surface, ceramic or metals such 
as aluminum may be used as the substrate. 
Acoustic touch position sensors are also known wherein a single transducer 
or pair of transducers 6 per axis is provided, which produces a surface 
acoustic wave which is reflected, by a reflective grating 40 having 
elements set at 45.degree. to the beam, at right angles over the length of 
the grating to produce a surface acoustic wave pattern propagating through 
an active area of the substrate. The position of a touch in the active 
area is determined by, e.g., reflecting the waves back to the originating 
transducer or providing an opposing reflective grating 40 which directs 
the surface acoustic wave pattern along an axis of the grating toward a 
receiving transducer system 6, which records the time of arrival of an 
attenuation of the wave pattern, which corresponds to a position along the 
axis of the arrays. The touch, in this case, may include a finger or 
stylus, perhaps indirectly through a cover sheet, pressing against the 
surface. Other types of configurations for collecting the sensing signal 
are also known. 
The reflective array 40 is formed of acoustically partially reflective 
structures, which may be an inscribed or raised surface feature, or a 
feature having differing wave propagation characteristics which forms a 
partial barrier. These structural elements may, in theory, be formed on 
any portion of the substrate where there is a significant wave energy. 
Thus, if a wave has surface energy, surface features may be used. If wave 
energy is completely buried, then these barriers must intrude into the 
material of the substrate. Thus, for waves having surface energy, these 
reflecting arrays may be formed on the surface, and where wave energy is 
present on both sides of the substrate, these reflecting arrays may be 
formed on one or both sides of the substrate. Because the touch sensor is 
generally placed in front of a display device, and the reflective array is 
not generally optically invisible, the reflective arrays are generally 
placed at the periphery of the substrate, outside of the active sensing 
area, and are hidden and protected under a bezel. 
The wave pattern of known acoustic touch sensors is dispersed along the 
axis of the transmitting reflective array, traverses the substrate and is 
recombined, e.g., into an axially propagating wave, dispersed in time 
according to the path taken across the substrate, by another reflective 
grating, and is directed to a receiving transducer in a direction 
antiparallel to the transmitted wave, which receives the wave and converts 
it into an electrical signal for processing. Thus, according to this 
system, two transducers per axis are required. Because of the antiparallel 
path, the time delay of a perturbation of the electrical signal 
corresponds to a distance traveled by the wave, which in turn is related 
to the axial distance from the transducer along the reflecting arrays 
traveled by the wave before entering the active area of the substrate. The 
location of touch is determined by detecting an attenuated signal as 
compared to a standard received waveform. Thus, for each axis, a distance 
may be determined, and with two orthogonal axes, a unique coordinate for 
the attenuation determined. Acoustic touch position sensors of this are 
shown in U.S. Pat. Nos. 4,642,423, 4,644,100, 4,645,870, 4,700,176, 
4,746,914, 4,791,416 and Re. 33,151, incorporated herein by reference. 
For each axis, a standard signal is provided to the transducer by 
interfacing a piezoelectric transducer with the sheet-like substrate, 
outside the active area, to produce a wave, propagating along an axis. For 
example, quasi-Rayleigh waves are generally coupled through the surface 
portion of the substrate, on the side which is intended to be touch 
sensitive. The reflective array in the path of the wave, for redirecting 
the wave into the touch sensitive region, includes a series of spaced 
surface interruptions, having a separation distance of an integral number 
of wavelengths of the wave produced by the transmitting transducer, angled 
45.degree. to the axis, i.e., the direction of wave propagation. The 
reflective array thus produces a reflected surface acoustic wave 
propagating at 90.degree. to the original angle of transmission, through 
the active area of the substrate. 
Where the wave energy in a substrate is in the form of surface bound waves, 
the rear surface is insensitive to touch and mounting apparatus. 
To receive the sensing wave, it is generally considered desirable to 
provide a single transducer for transducing the wave into an electrical 
signal in which the touch position is encoded by temporal fluctuations in 
the signal. While a transducer extending the full length of the substrate 
could be provided, this requires a large transducer. Instead, the art 
teaches an inverse of the transmission technique, multiplexing the sensing 
wave into an acoustic wave directed toward a small receiving transducer. 
Thus, in an area outside the active area, the waves are again reflected by 
an otherwise identical reflecting array having spaced interruptions at a 
mirror image angle, thereby multiplexing the spatially dispersed signal 
into a single waveform pattern, propagating antiparallel to the 
transmitted surface acoustic wave, which is detected by another 
transducer. Known systems, have employed excitation frequencies of 5.0 and 
5.5 Mhz. The thickness of the sheet-like member, if used to propagate 
quasi-Rayleigh waves is typically in the range from 0.09 inches to 0.125 
inches. A known zero order horizontally polarized shear wave touch sensor 
employs a substrate having a thickness of about 0.040 inches. 
The art also teaches the use of a single transducer for both transmitting 
the wave and receiving the sensing wave, with a single reflective array 
employed to disperse and recombine the wave. Such systems therefore employ 
a reflective edge opposite the reflective array. As a result, the surface 
wave passes through the active region twice, with consequent increased 
wave absorption by the touch but also increased overall signal attenuation 
due to the reflection and additional pass through the active region of the 
substrate. For example, a quasi-Rayleigh wave may be reflected off an edge 
of the substrate parallel to the axis of the transmission reflective 
grating and reflected by half-wavelength spaced reflectors back through 
the substrate to the reflective array and retrace its path back to the 
transducer. The transducer, in this case, is configured to act as both 
transmitter and receiver at appropriate time periods. A second transducer, 
reflective array and reflective edge are provided for an axis at right 
angles to allow determination of the orthogonal touch coordinate. 
A related system provides for a single transducer which produces a sensing 
wave for detecting touch on two axes, which both produces the acoustic 
wave and also receives the wave from both axes. In this case, the area in 
which touch is to be sensed is generally oblong, such that the longest 
characteristic along one path is shorter than the shortest characteristic 
delay along the second path. 
Adler, U.S. Re. 33,151, incorporated herein by reference, relates to a 
touch-sensitive system for determining a position of a touch along an axis 
on a surface. An acoustic wave generator is coupled to a sheet-like 
substrate to generate a burst of waves, which are deflected into an active 
region of the system by an array of wave redirecting gratings. Surface 
waves traversing the active region are in turn redirected along an axis by 
gratings to an output transducer. The redirecting gratings are oriented at 
45.degree. to the axis of propagation. A location of touch is determined 
by analyzing a selective attenuation of the received waveform in the time 
domain, each characteristic delay corresponding to a locus on the surface. 
The grating elements are placed at a 45.degree. angle and spaced at 
integral multiples of the acoustic wavelength with dropped elements to 
produce an approximately constant surface wave power density over the 
active area. Thus the spacing between grates decreases with increasing 
distance along the axis of propagation from the transducer, with a minimum 
spacing of one wavelength of the transmitted wave. U.S. Pat. No. 4,746,914 
also teaches use of reflecting elements which vary in height to control a 
ratio of reflected wave power to unreflected wave power. 
Brenner et al., U.S. Pat. No. 4,644,100 relates to a touch sensitive system 
employing surface acoustic waves, responsive to both the location and 
magnitude of a perturbation of the surface waves, determining an amplitude 
of a received wave and comparing it to a stored reference profile. 
SUMMARY AND OBJECTS OF THE INVENTION 
The present invention provides a method for mounting an acoustic transducer 
to a substrate and a resulting mounted transducer system, in which the 
transducer is mounted, separated from a perturbation sensitive portion of 
the substrate, by an abrupt bevel, thus increasing the mounting options 
for the transducer. By providing an acceptable bevel, the location of the 
transducer, including its mounting structure, may be significantly 
relocated from a plane of a touch sensitive surface, with signal 
attenuation controlled, and a minimum area lost. 
The prior art address the transducer mounting clearance problem in two 
notable fashions. First, a relatively sharp bevel angle, e.g., one which 
would otherwise produce unacceptable signal losses, is provided with a 
blended bevel region, thus avoiding a sharp discontinuity. This structure 
generally results in a substantial region of the substrate occupied with 
the blending. Second, a moderate bevel angle, producing marginally 
unacceptable signal losses, is provided with a sharp bevel edge. This 
structure, however, results in a substrate having a sharp, brittle lateral 
edge. 
According to the one aspect of the present invention, having a positive 
angle bevel for recessed mounting of the transducer behind the front face 
of a touchscreen, two technical features may be provided: 
avoidance of a knife-edge at a beveled substrate edge; and 
providing a gentle unblended bevel angle, having a low transmission loss. 
Preferably, a continuous bevel is provided on the substrate edge. 
It is therefore an object of the present invention to provide an acoustic 
wave touch sensor comprising a substrate, having a front surface, and a 
bevel having a surface sloping at an angle with respect to the front 
surface, forming an edge with the front surface; and an acoustic wave 
transducer, mounted to the substrate to transduce an acoustic wave, having 
a wavelength and traveling in the substrate, the edge having an effective 
radius which is small as compared to the wavelength and a bevel angle 
being selected such that the acoustic wave is attenuated by less than 
about 3 dB on transmission across the edge. The bevel of a monolithic 
substrate preferably has an angle less than 25.degree. with respect to the 
front surface, and more preferably has an angle of about 16.degree. with 
respect to the front surface. 
It is therefore also an object according to the present invention to 
provide an acoustic wave touch sensor comprising an acoustic wave 
transducer, for transducing an acoustic wave; a substrate, having a front 
surface, and an acoustic wave interface, the interface having at least one 
abrupt acoustic impedance mismatch, the interface having an transmitted 
acoustic wave signal attenuation of less than about 3 dB; the acoustic 
wave transducer being mounted to transduce the acoustic wave through the 
interface into the substrate, so that energy of the acoustic wave appears 
at the surface; wherein the interface comprises a region for mounting the 
acoustic wave transducer which is displaced or inclined from the front 
surface. In one embodiment, the interface comprises a beveled edge. The 
beveled edge may be provided having an effective radius which is small as 
compared to the wavelength and a bevel angle such that the acoustic wave 
is attenuated by less than about 3 dB on transmission across the edge. 
It is also an object according to the present invention to provide an 
acoustic wave touch sensor having an interface comprising a pair of edges, 
spaced to cause destructive interference of a reflected portion of the 
acoustic wave, the pair of edges each having an effective radius which is 
small as compared to the wavelength and bevel angles such that the 
acoustic wave is attenuated by less than about 3 dB on transmission across 
the pair of edges. 
It is another object according to the present invention to provide an 
acoustic sensor wherein the substrate comprises a plurality of edges, each 
of the edges being spaced to produce reflections which sum to effectively 
destructively interfere, the plurality of edges each having an effective 
radius which is small as compared to the wavelength and bevel angles such 
that the acoustic wave is attenuated by less than about 3 dB on 
transmission across the plurality of edges. 
It is a further object according to the present invention to provide an 
acoustic sensor, wherein the substrate comprises a plurality of edges, a 
net attenuation of the acoustic wave across the plurality of edges being 
less than an acoustic loss across an edge having a largest attenuation 
loss. 
According to one aspect of the invention, the acoustic wave transducer 
comprises a piezoelectric element mounted on a wedge, the acoustic wave 
being a quasi-Rayleigh wave. The substrate may be formed of glass, e.g., 
soda lime glass or a low loss glass such as borosilicate glass. 
In another aspect of the invention, the front surface has a perimeter edge 
having a length, the bevel being approximately parallel to and 
approximately the same length as the perimeter edge. Therefore, an 
electrical wire connected to the transducer, disposed adjacent to the 
bevel, may lie flat without interference of structures in front of the 
front surface. Advantageously, the transducer is disposed on the bevel, 
totally or partially behind the front surface. Also, advantageously, in 
the plan view, the transducer may be disposed on the bevel totally within 
the outer boundary of the substrate. 
According to one aspect of the invention, a reflective array is provided, 
the reflective array redirecting portions of the acoustic wave across the 
front surface. Both the reflective array and the transducer may be 
disposed on the bevel, the array redirecting portions of the acoustic wave 
across the bevel edge to the front surface. Alternatively, the transducer 
may be disposed on the bevel, the reflecting array being disposed on the 
front surface and redirecting portions of the acoustic wave across the 
front surface. 
According to an aspect of the present invention, a touch on the front 
surface perturbs the acoustic wave, and the same or a different transducer 
may receive the perturbed acoustic wave to sense a touch characteristic. 
A coversheet may be provided, parallel to the front surface, fastened in 
proximity to the bevel, to protect the surface or modify the acoustic 
properties of a touch. The mounting for the coversheet, however, need not 
have a physical attachment to the substrate. 
The bevel preferably has a lateral aspect which as adapted to resist 
fracture, within the constraints imposed. In brittle substrates, a 
susceptibility to fracture may be considered related to a minimum 
thickness and aspect ratio, especially near a free edge. Thus, for glass 
and other brittle substrates, a knife edge, with a resulting long, thin 
free edge, is avoided, and therefore such substrates preferably have 
lateral edges with a substantial thickness with respect to the substrate. 
An outer edge surface may be provided, the bevel being formed between the 
outer edge surface and the front surface, the outer edge surface being 
substantially perpendicular to the front surface. Preferably, this lateral 
edge has a thickness of at least about 0.030.+-.0.010 inch. In another 
embodiment, a lateral wall of the bevel is provided, the lateral wall, 
with the bevel, forming a trench in the substrate. This avoids a free 
edge. Such a trench provides allows recessing of the wiring as well as the 
transducer itself. 
According to a further embodiment, the acoustic wave transducer comprises 
an electro-acoustic device mounted to the beveled portion of the 
substrate, the acoustic wave transducer emitting either a surface bound 
wave which crosses the bevel edge, or, an acoustic wave that is reflected 
or mode converted by a reflective array into a surface bound wave which 
crosses the bevel edge to the front surface. 
According to a still further object of the invention, the front surface is 
non-planar, and may be cylindrical, spherical, ellipsoidal or other shape. 
Further objects will become apparent through a review of the detailed 
description and drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred embodiments will now be described with respect to the 
Figures, in which corresponding reference numerals relate to corresponding 
structures. 
Continuous Bevel on Substrate Edge 
The bevel 29, 30 preferably extends the entire length of each side 
containing the mounted transducers 26, 27, as shown in FIG. 7. This 
provides a lower manufacturing cost, due to mass production capability, 
when compared to individually and selectively beveled substrates 5 of the 
prior art. 
In addition to a cost advantage, the continuous bevel 29, 30 also allows 
for placement of wiring to and from the transducers on the bevel surface. 
This reduces susceptibility to wire damage relative to wire muting on the 
glass edge or top surface. 
The transducer 20, 21 may be situated to direct an acoustic beam directly 
at the beveled edge, as shown in FIGS. 6B and 6C, with a reflective array 
41 formed on the touch surface of the substrate. The transducer 20, 21 may 
also direct an acoustic beam parallel to the bevel 29, 30, with the 
reflective arrays 51 formed on the bevel 29, 30 and redirecting rays 
across the bevel edge 11 and onto the touch surface 16, as shown in FIG. 
8. In each case, the acoustic wave traverses the edge. Of course, known 
methods of redirecting the waves back toward the emitting transducer, or 
alternatively towards a separate receiving transducer on a beveled 
surface, may be employed, which would cause the waves to traverse the edge 
multiple times. Because of the defined limited angle bevel according to 
the present invention, which in this example is a gentle angle, less than 
25.degree., the signals losses are reduced without recourse to costly 
manufacturing processes. 
Alternatively, the transducer may be aimed at a corresponding transducer on 
an opposing edge of the substrate, to directly detect an attenuation of 
the acoustic wave due to a touch. See U.S. Pat. No. 3,673,327, 
incorporated herein by reference. Depending on the application, one or 
more of the substrate edges is provided with a continuous bevel. 
The continuous bevel 29, 30 may also be advantageously employed to recess 
the transducer 20, 21 behind a coversheet 55 and to properly locate the 
coversheet. See FIG. 10. For example, the bevel 12 may be advantageously 
employed as a mesa structure to decrease a distance between the coversheet 
55 and the touch surface 16, while simultaneously facilitating coversheet 
attachment by a mechanical system 56 extending behind the front surface 16 
of the substrate. Advantageously, the bevel 12 additionally allows the 
transducer 20, 21 to be located behind the front surface of the substrate, 
allowing an arbitrarily small gap between the coversheet and the touch 
sensitive surface without mechanical interference from the transducers. 
Spacing elements may be used to assure a minimum spacing. 
The ability to reduce transducer breakage during handling and installation 
by recessing the transducer is particularly advantageous for touchscreen 
designs utilizing more massive substrates. For example, Elo TouchSystems' 
SecureTouch.TM. products address market demand for more robust touch 
system solutions by using glass substrates of approximately 0.5" 
thickness. The substrate can be annealed glass, tempered or chemically 
hardened glass, or laminated (safety) glass. In all cases, the resulting 
touchscreen is more massive, resulting in potentially greater forces on 
transducers from accidental impacts. The ability to cost-effectively 
recess transducers without significant signal loss is of significant 
benefit. The scope of the present invention thus includes touchscreens 
comprising such thick substrates. 
Avoidance of Knife-Edge 
In contrast to certain prior beveled glass substrate designs, the present 
invention avoids a knife edge 7 at the lateral aspects of the substrate, 
as shown in FIGS. 2, 4A, 5A and 6A of the prior art, as compared to FIGS. 
4B, 5B, 6B, 6C, and 6D of the present invention, which have a flat 
lateral-most edge surface 13, 13'. For example, a preferred embodiment 
includes a 0.125 inch thick glass sheet with a 16.degree. bevel on the 
front surface, extending about 0.35 inches, with a minimum substrate 
thickness of about 0.030 inches. This 0.030 inch thick lateral edge 
substantially improves fracture resistance, and is easier to handle. 
Experience of the assignee hereof with 25.degree. bevel angle sensors with 
a nominal edge thickness of 0.020.+-.0.010 inches proved that the minimum 
thickness was difficult to maintain, resulting in knife edge portions, and 
that this free edge was subject to nicks, all of which serve as nucleation 
sites for glass fracture in an oven process for curing a glass frit, 
applied to the substrate. 
A knife-edge 7 has several disadvantages relative to the present design. As 
shown by the dotted lines 15, 14 in FIGS. 4A and 4B, variations in the 
manufacturing process creating the bevel 9, 12 alter the outside 
dimensions of the finished product for the knife-edge 7 design, but not 
the present bevel design. 
The sharp knife-edge 7 is more susceptible to damage in handling. The 
knife-edge 7 is also less safe to handle. These issues become increasingly 
important when the bevel is continuous along the edge. 
A knife-edge 7 also provides nucleation sites for glass fractures. This is 
expected to lead to reduced performance with respect to, for example, a 
known Underwriters Laboratories ball drop test. 
Further, glass-flit reflectors may be heat cured on the substrates by 
passing the substrates through a conveyor oven. The resulting temperature 
cycling stresses the glass. With continuous knife-edge 7 bevels, a 
significant yield loss has been observed to occur, due to glass fractures 
in the heat curing process, a consequence of the fracture nucleation sites 
inherent in a knife edge 7. 
According to one aspect of the present invention, when a directional 
transducer is employed, the substrate behind the transducer is not an 
important factor in the sensor system. Therefore, the substrate need not 
end with the bevel 12, and may, in fact, continue. For example, the bevel 
12 may be formed as a trench structure with a beveled wall, having a 
substrate portion which extends past the bevel 12. Such a trench may be 
advantageous for protecting sensor cables, the transducers themselves, or 
for mounting. 
Gentle Bevel 
The present invention, having a defined and limited angle bevel 12, 
eliminates the need to seam, blend, or otherwise round 4 the interface 
between the bevel 9 surface and the top surface 8 of the glass, as shown 
in FIGS. 2, 5A and 6A of the prior art as compared to 4B, 5B, 6B, 6C and 
6D of the present invention. On a monolithic or homogeneous substrate with 
a single bevel edge, the bevel angle is gentle, and has an upper limit of 
less than 25.degree., applicable for broad classes of surface bound waves. 
For a given transducer height, a reduced bevel angle may require a wider 
bevel to keep the transducer behind the front surface. This potential 
increase in substrate size due to the gentle bevel 12 angle may be 
compensated for by the elimination of the need for a blending region. A 
gentle bevel 12 angle allows low loss transmission of the acoustic signals 
through this interface, with no further blending, seaming or rounding of 
the bevel to the front surface of the substrate. 
Therefore, two important advantages result from the use of a gentle bevel 
12 with an unblended interface 11: first, labor, cost, and process 
variation associated with interface 11 rounding are eliminated; and 
second, the sensor design layout space required for rounded interface 4 is 
eliminated. This latter aspect compensates, at least in part, for the 
reduced bevel angle, as shown in FIGS. 5A and 5B. 
In a preferred embodiment of the present invention, the vertical edge 13 is 
0.030 inches in height, the beveled surface 12 is 0.350 inches wide, the 
substrate 10 is 0.125 inches thick soda-lime glass, and the bevel angle is 
16.degree., continuous along an edge of a rectangular substrate. 
Quantification of Limited Bevel Angle 
The bevel angle is considered acceptable if signal loss from an unrounded 
bevel/top-surface interface (typically twice in the acoustic path from 
transmit to receive transducer) is sufficiently small. 
Analog signal budgets for acoustic touchscreen systems are complex and vary 
greatly in quantitative detail for different applications. However as a 
rough rule of thumb, a 6 dB loss, that is a factor of two in amplitude, is 
typically within the analog-signal budget margin. For losses greater than 
6 dB due to changes in sensor design, it becomes likely that compensating 
changes in other components in the analog system will be required, and 
therefore such losses are generally avoided. Therefore, such sufficiently 
small signals losses are generally less than about 6 dB for the entire 
acoustic path, which typically involves two beveled interfaces. Therefore, 
an acceptable loss across a single interface is about 3 dB. 
According to tests performed by inventors hereof, a 16.degree. bevel angle 
between the bevel surface and the active touch surface, for a monolithic 
soda lime glass substrate, results in significantly less than a 6 dB 
signal loss over the acoustic path. A 25.degree. bevel angle showed a 
measured signal loss of about 8 dB for a signal path including two beveled 
interfaces. Quadratic extrapolation (see below) from 25.degree. bevel data 
implies a 14 dB signal loss for a 33.degree. bevel angle substrate over 
the acoustic path. Therefore, as employed herein, a bevel angle less than 
25.degree. for a monolithic glass substrate is considered acceptable, 
while larger angles result in higher, and undesirable signal losses. In 
order to gain significant advantages in mounting the transducer, with 
limited loss of surface to the bevel, and limited signal loss, it is 
preferred that the bevel angle be between about 6.degree. and less than 
25.degree., and more preferably between about 10.degree. and about 
20.degree., and most preferably about 16.degree.. 
The above data were collected at an operating frequency of 5.53 MHz. As 
long as the effective radius of the bevel edge remains small compared to 
the acoustic wavelength, scaling-law considerations allow us to apply 
experimental results to other operating frequencies, or equivalently 
wavelengths. 
The above data were collected using soda-lime glass as a substrate which 
has a ratio of Poisson's ratio of approximately 0.26. These experimental 
results also apply to a good approximation to other materials with similar 
Poisson's ratios, such as borosilicate glass (0.24), aluminum (0.35), 
stainless steel (0.3). Poisson's ratio can be determined from the ratio of 
shear and longitudinal bulk wave velocities as follows (see, e.g., 
textbook by B. A. Auld): 
EQU Poisson's ratio=1-2*(V.sub.S /V.sub.1).sup.2 !/2*(1-(V.sub.S 
/V.sub.1).sup.2 ! 
Materials of practical interest as touch panel substrates, including 
borosilicate glass, aluminum and steel, thus have Poisson's ratios similar 
to soda-lime glass. 
The relevance of Poisson's ratio is as follows. There are three independent 
material properties, which along with boundary conditions, quantitatively 
determine the interaction of an incident acoustic wave with a bevel edge. 
However, by scaling with respect to size and time, or equivalently scaling 
wavelength and acoustic velocity of all acoustic modes simultaneously 
(keeping ratios of velocities fixed), there is only one independent 
material property that can affect the dB attenuation at the bevel edge. 
That parameter is Poisson's ratio, which determines the ratios of the 
velocities of the various acoustic modes. For a given bevel angle, 
assuming a sharp bevel edge, two materials with similar Poisson's ratios 
will have substantially the same dB transmission loss at the interface. 
More mathematically, Poisson's ratio, or equivalently, the ratio of shear 
to longitudinal bulk-wave velocities, uniquely determines the ratio of the 
wave-function depth for a Rayleigh wave to the wavelength. This 
depth-to-wavelength ratio is the key parameter determining transmission 
for a given bevel angle. 
Furthermore, the depth-to-wavelength ratio for Rayleigh waves in an 
isotropic medium varies little throughout the entire range of physically 
possible values of Poisson's ratio, namely zero to one-half. Calculations 
(based on equations in B. A. Auld's textbook on "Acoustic Fields and Waves 
in Solids") of the ratio of the root-mean-square depth of acoustic power, 
or equivalently, acoustic kinetic energy, to wavelength, as a function of 
Poisson's ratio, reveal that this ratio is limited to a range of 0.31 to 
0.48. Soda-lime glass is approximately in the middle of this range. The 
depth-to-wavelength ratio for soda-lime glass thus applies, within 25%, to 
all isotropic media. 
The transmission loss at the bevel edge is predominately due to phase 
incoherence in the wave-function overlap integral between incident and 
transmitted surface-bound waves. Polarization direction mismatch is 
typically a lesser effect. The degree of the phase incoherence, and hence 
the transmission loss, depends on the bevel angle times the effective 
wave-function depth divided by the wavelength. Given that the 
depth-to-wavelength ratio varies little between Rayleigh waves in 
differing isotropic media, the above experimental results for soda-lime 
glass generalize accordingly, and are expected to generally apply for a 
broad range of materials useful as acoustic touchscreen substrates. 
Furthermore, for surface bound waves, other than Rayleigh waves, for which 
substrate design leads to a root-mean-square depth-to-wavelength ratio of 
less than 0.5, a bevel angle of less than 25.degree. may also be also 
acceptable, in terms of signal losses. Generalizing further, a bevel angle 
is acceptable if it is less than 25.degree. times half the surface bound 
wave wavelength divided by the root-mean-square depth of the surface bound 
wave's power density. For example, a lowest order Love wave whose acoustic 
power is strongly confined near the surface with an appropriate layered 
substrate structure can support a bevel angle greater than 25.degree.. 
FIG. 9 shows the reflection and transmission of an incident wave with unit 
amplitude by an abrupt edge 11 of angle .theta.. R is the amplitude of the 
reflected wave, and T is the amplitude of the transmitted wave. R has to 
be an odd function of .theta., since the amplitude for the superposition 
of two reflected waves, one by an edge 11 of angle .theta., the other by 
an edge 11 of angle -.theta., equals zero (the amplitude of the wave 
reflected by an edge with .theta.=0). Consider the mathematical limit of 
zero distance between +.theta. and -.theta. edges. Thus, for small angles, 
R is proportional to .theta.. The parameters .alpha. and the phases 
.phi..sub.R and .phi..sub.T, are real constants. 
T may be calculated assuming negligible coupling to bulk waves and 
conservation of energy (.vertline.R.vertline..sup.2 
+.vertline.T.vertline..sup.2 =1). Thus, the signal attenuation in decibels 
for an acoustic path crossing two edges is approximately 
##EQU1## 
Taylor expanding the attenuation equation, gives, 
##EQU2## 
which is a quadratic scaling law for small bevel angles. For larger 
angles, dB loss as a function of angle might not be strictly quadratic; 
however, attenuation remains a strongly increasing function of angle. 
For clarity of presentation, possible coupling to bulk waves or other modes 
at the bevel edge has been neglected. However, the resulting scaling-law 
result remains valid. Note that the amplitudes for the generation of all 
parasitic modes, like the reflection amplitude, is zero if the angle is 
zero, is an odd function of angle, and hence is either linear or 
negligible for small angles. Hence the power removed from the incident 
acoustic beam, from both reflection and parasitic mode conversions, grows 
quadratically with angle. Limitation of the bevel angle greatly reduces 
acoustic losses at bevel edges. This applies equally to positive and 
negative angle bevels. 
Multiple bevel angles 45, 46 may also be provided in accordance with the 
present invention, as shown in FIG. 6D. However, when such multiple edges 
45, 46 are provided, they are preferably spaced to provide destructive 
interference of reflected surface bound wave or parasitic modes at the 
acoustic operating frequency of the sensor design, thereby reducing losses 
substantially. Because of this interference factor, the total angle for 
multiple edge 45, 46 structures may differ from that permissible for 
single edge 11 structures, and in fact a greater acceptable bevel angle 
may be possible under such circumstances. Such multiple bevel angles with 
controlled spacing may be formed on glass substrates by grinding, or on 
metal substrates, e.g., aluminum, by machining, extrusion, and other known 
metal fabrication techniques. 
According to the present invention, a transducer-wedge 20, 21 assembly may 
extend slightly in front of the substrate surface, causing a so-called 
"transducer bump". In order to minimize the transducer bump, a lower 
profile transducer 20', 21' may be employed, which has a smaller height. 
Therefore, when employing a transducer such as commonly employed in known 
beveled sensors, see, FIG. 2 of the prior art, a limited angle bevel 
design as shown in FIG. 6B may result. By reducing the height of the 
transducer 20', 21', a lower profile transducer system results, as shown 
in FIG. 6C. By reducing acoustic losses with a limited angle bevel, 
greater compromises may be made with respect to transducer performance yet 
achieve adequate performance. 
As stated above, where brittle substrates are employed, fracture is a risk, 
and therefore a minimum aspect ratio of a region near a free edge is 
preferably maintained to avoid fracture. Example brittle substrates 
include glass. Thus, knife edges are blunted or avoided. Ductile 
substrates, on the other hand, have a low fracture risk, and the lateral 
edges need not be blunted. Example ductile substrates include aluminum and 
steel. Thus, for brittle substrates such as glass, a minimum substrate 
thickness of about 0.010 inches, including tolerances is maintained, and 
more preferably with a minimum 0.020 inch thickness, with the free edges 
preferably rounded or chamfered with about a 0.005 inch seam. 
According to another aspect of the present invention, the substrate need 
not have a decreasing substrate thickness in the region of the bevel, and, 
for example, a bent structure may be provided, allowing the substrate to 
maintain a constant thickness. Such a constant thickness substrate may 
also be employed to assure sufficient substrate thickness for surface 
bound waves in cases in which the reflective array is placed on the 
beveled surface. For example, an aluminum substrate may be provided having 
a constant thickness bend at the lateral regions, thus allowing flush 
front mounting with the entire front surface usable. 
Advantageously, a beveled edge with substrate thinning may be employed with 
acoustic plate waves such as Lamb and shear waves. In this case, the 
bottom surface of the substrate plays a role in confining the acoustic 
power in a region close to the top surface. Consider again the case in 
which the reflective arrays are on the beveled surface. While the 
preferred acoustic modes crossing the bevel edge are surface bound modes, 
there is little restriction on the nature of the acoustic modes 
propagating between the transducers and the, perhaps mode converting, 
reflectors. 
The bevel and/or beveled edge may also form part of an acoustically 
functional structure, for example selectively reflecting or filtering 
various wave modes, such as parasitics generated at the interface. For 
example, the mode selectivity of a reflector due to its orientation can be 
enhanced by increasing the velocity dispersion of undesired higher-order 
plate-wave modes via the reduced substrate thickness in the region of the 
reflective array. 
Further, in various embodiments, the acoustic properties of the bevel may 
be advantageously used in forming or tuning the acoustic transducer, to 
impart or receive acoustic waves from the substrate. In this case, the 
form, location, and mounting of an electro-acoustic transducing element 
may be provided so that the system formed by the transducing element, the 
bevel and the substrate functionally cooperate to provide an efficient and 
useful acoustic system. 
Designs according to the present invention may also find applications with 
non-flat displays. In particular, such a limited angle bevel may be 
implemented with a cylindrically curved sensor, for example, to be used 
with SONY Trinitron .RTM. CRTs. 
While the previous embodiments refer to a positive bevel angle, the bevel 
angle may be negative. That is, the bevel edge may be in the form of 
concave alteration of the substrate surface. This geometry can also be 
used to recess transducers for protection from impact damage. The 
quadratic scaling law for dB loss as a function of angle leads to the 
conclusion that positive and negative bevel angles of the same magnitude 
will have similar transmission losses in a single substrate. Accordingly, 
the acoustic path may cross the negative-bevel interface between the 
transducers and the arrays. 
It should be understood that the preferred embodiments and examples 
described herein are for illustrative purposes only and are not to be 
construed as limiting the scope of the present invention, which is 
properly delineated only in the appended claims.