Touch panel using modulated light

A touch panel system using modulated light beams to enable the system to detect when one or more of the light beams are blocked even in bright ambient light conditions. The system has a "touch sensitive" display surface with a defined perimeter. Surrounding the display surface are a multiplicity of light emitting elements and light receiving elements. These elements are located so that the light paths defined by selected pairs of light emitting and light receiving elements cross the display surface and define a grid of intersecting light paths. A scanning circuit sequentially enables selected pairs of the light emitting and light receiving elements, modulating the amplitude of the light emitted in accordance with a predetermined pattern. A filter generates a blocked path signal if the currently enabled light receiving element is not generating an output signal modulated in accordance with the predetermined pattern. Finally a computer is used to determine if an object is adjacent to the display surface and the location of the object, by determining if the filter is generating at least two blocked path signals corresponding to light paths which intersect each other within the perimeter of the display surface. A failure detection circuit for testing the LEDs and phototransistors, by checking for blocked LED/phototransistor pairs. When a blocked pair is found the phototransistor of the blocked pair is paired with an LED near the blocked LED and the block LED is paired with a photo-transistor near the blocked LED to determine if they have failed.

The present invention relates generally to touch sensitive screens, 
displays and panels, and particularly to a touch panel apparatus and 
method which is operable even in strong ambient light conditions. 
BACKGROUND OF THE INVENTION 
Touch sensitive panels and screens are commonly used in many types of 
computerized equipment. In some systems, a touch screen avoids the need 
for providing a keyboard. 
Referring to FIG. 1, a touch screen is typically used to allow the user to 
easily select one of a plurality of displayed items. The user makes his or 
her selection by touching the portion of the screen associated with the 
item to be selected. To clarify the boundaries of the areas associated 
with each item, the image on the screen may include boxes surrounding the 
displayed items. 
The uses of touch screens have grown increasingly sophisticated, allowing 
the user to draw pictures, manipulate menus, use a displayed keyboard for 
alphanumeric input, and so on. 
The terms "screen", "display", and "panel" are used synonymously herein. 
The present invention concerns the touch aspect of touch screens. 
Therefore, for this purpose it is unimportant how the image on the touch 
sensitive apparatus is displayed. The touch mechanism could even be used 
with a static image instead of with a display device. 
In most cases, the term "touch sensitive" is a misnomer. Most touch 
sensitive screens, including the present invention, sense the interruption 
of one or more light beams; they do not sense actual physical contact with 
the screen or panel. The display is surrounded by pairs of light emitting 
and light sensing elements. These pairs are individually enabled in a 
preselected pattern, and the position of any object (such as the user's 
finger) touching the screen is determined by looking at which lights are 
blocked by the object. 
A serious shortcoming in prior art touch screens is that their performance 
degrades in bright ambient light conditions, especially in sunlight. The 
source of this problem is as follows. In the prior art touch screens, the 
light detection system determines that the light traveling between a 
selected pair of emitting and receiving elements is blocked if the 
amplitude of the received light is below a threshold value. However, if 
the ambient light by itself causes the amount of light received by the 
screen's receiving elements to exceed the threshold value, then the system 
is unable to detect the presence of an object touching the screen. 
In the more sophisticated prior art touch screens, the system compares the 
signal level output by the light receiving elements in response to ambient 
light with the signal level output when the light of a selected light 
emitting element is added to the ambient light. For instance, in U.S. Pat. 
No. 4,243,879, the disclosed system samples the signal level generated by 
each receiving element in response to the ambient light just before it 
turns on the corresponding light emitting element and compares the 
resulting signal level with sampled level. 
This type of "calibrated threshold" prior art system suffers from very poor 
signal to noise ratios. In bright ambient light conditions the signal 
level attributable to the light emitting element will be very small, and 
therefore the incremental threshold for determining that light from the 
light emitting element is not blocked must be very low. However, the lower 
the threshold, the greater the chance that small ambient light 
fluctuations will drown out the signal from the light emitting element. 
If a "calibrated threshold" system uses an ADC (analog to digital 
converter) to quantify the intensity of the light being received, when the 
system is in very bright ambient light (e.g., direct sunlight) the 
quantified ambient light level will be so large that the ADC will not be 
able to distinguish between ambient light and the light from unblocked 
light beams. 
Also, the intensity of the light emitted by LEDs typically varies, from 
component to component, by a factor of up to ten to one. Also, the 
sensitivity of light receiving elements, which are usually 
phototransistors, vary even more than ten to one. In other words, the 
signal level generated by any two supposedly identical phototransistors, 
in response to the same light intensity level, can vary be even more than 
ten to one. While the problem of nonuniform components can be at least 
partially solved by sorting components, these variations generally force 
the prior art touch screens to use a fairly low incremental threshold for 
detecting unblocked light beams--which decreases the signal to noise ratio 
of those system. As a result, these touch screens often malfunction in 
bright ambient light conditions. 
After studying these problems and the prior art solutions, the inventor of 
the present invention concluded that the use of a threshold intensity 
level is inherently problematic. Therefore the present invention uses a 
different concept. 
In particular, the present invention modulates the light transmitted by the 
touch screen's LEDs, and then detects whether the light received by the 
screen's phototransistors includes a signal component that is modulated in 
the same way. If so, the light path is unblocked, otherwise the system 
concludes that light path is blocked. The inventor has found that this 
system works in all ambient light conditions, including bright, direct 
sunlight. 
It is therefore a primary object of the present invention to provide a 
touch screen apparatus using modulated light that is operable even in 
strong ambient light conditions. 
SUMMARY OF THE INVENTION 
In summary, the present invention is a touch panel system which uses 
modulated light beams to enable the system to detect when one or more of 
the light beams are blocked even in bright ambient light conditions. 
The system has a "touch sensitive" display surface with a defined 
perimeter. Surrounding the display surface are a multiplicity of light 
emitting elements and light receiving elements. These elements are located 
so that the light paths defined by selected pairs of light emitting and 
light receiving elements cross the display surface and define a grid of 
intersecting light paths. A scanning circuit sequentially enables selected 
pairs of the light emitting and light receiving elements, modulating the 
amplitude of the light emitted in accordance with a predetermined pattern. 
A filter generates a blocked path signal if the currently enabled light 
receiving element is not generating an output signal modulated in 
accordance with the predetermined pattern. Finally a computer is used to 
determine if an object is adjacent to the display surface and the location 
of the object, by determining if the filter is generating at least two 
blocked path signals corresponding to light paths which intersect each 
other within the perimeter of the display surface.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, there is shown a touch screen display system 20 having 
a display 22 which is "touch sensitive". The display 22 has a display 
surface 24 with a defined perimeter. 
Surrounding the display surface 24 are a multiplicity of light emitting 
elements (LEDs) 28 and light receiving elements (phototransistors) 30. 
These LED and phototransistor elements are located so that the light paths 
32 and 34 defined by selected pairs of LEDs and phototransistors cross the 
display surface 24 and define a grid of intersecting light paths. For a 
rectangular display such as the one shown in FIG. 1, each LED 28 is 
matched by or paired with a phototransistor horizontally or vertically 
across the display surface. 
The light emitting elements used in touch screens are typically infrared 
LEDs (light emitting diodes), although other light frequencies and 
components could be used. 
A computer 36, in conjunction with a scanning circuit 38, determines if an 
object is adjacent to the display surface 24 by sequentially enabling each 
of the LEDs 28 surrounding the display surface 24 and looking to see if 
the corresponding phototransistors 30 receive the light transmitted by the 
LED 28. If the light is received, it can be concluded that no object is 
blocking the light path defined by the LED 28 and its corresponding 
phototransistor 30. On the other hand, it can be concluded that an object 
is blocking the light path if the emitted light is not received. 
By sequentially enabling or energizing all of the LEDs and looking at the 
resulting DBlk signal, the computer 36 can determine the location of any 
object which is adjacent to (i.e., touching or almost touching) the 
display surface. At the risk of belaboring the obvious, if two or more 
intersecting light paths are being blocked by an object, the location of 
the object is the intersection of the blocked vertical and horizontal 
light paths. 
In the preferred embodiment, there are approximately five LEDs and 
phototransistors per inch around the perimeter 26 of the display surface 
24. Most touch screens will use between three and six elements per inch. 
In the preferred embodiment there are fifty-six LED and phototransistor 
pairs surrounding the display. The scanning circuit 38 is designed to 
handle up to sixty-four pairs. Due to the close spacing of elements, it is 
unusual for only one or more light paths to be blocked without an 
intersecting light path also being blocked; but if this happens the 
computer 36 will not be able to locate the object blocking the light 
beam(s). For instance, a piece of paper could be used to block several 
vertical light paths without blocking any of the horizontal light paths. 
In normal operation, the computer 36 addresses each LED/phototransistor 
pair with a single six bit address AdrLED. The scanning circuit 38 uses a 
multiplexer 40 to decode the address AdrLED and thereby energize one of 
the fifty-six LEDs 28 along the top and right hand side of the display 
surface 24. A demultiplexer 42 also decodes the address AdrLED and enables 
just one of the fifty-six phototransistors 30 along the display's 
perimeter 26 to be coupled to a filter/detector circuit 44. 
The filter/detector circuit 44 determines whether the selected 
phototransistor is receiving light from the selected LED and, if not, 
generates a blocked path signal DBlk. 
As explained above, to solve the problems associated with using touch 
screens in bright ambient light conditions, the amplitude of the light 
emitted by the selected LED is modulated in accordance with a 
predetermined pattern. In the preferred embodiment, the LED is driven by a 
sinusoidally varying current generated by an LED modulator circuit 46. 
In the preferred embodiment, the LED's amplitude is modulated at a 
frequency of 60 kilohertz. The inventor has found that modulation 
frequencies between 10 kilohertz and 500 kilohertz are effective in most 
ambient light conditions and are therefore preferred. High modulation 
frequencies are preferred because they permit faster testing for blocked 
light paths, and therefore faster scanning of the entire display and 
faster response to actions by the user of the system 20. With currently 
available low cost phototransistors, the maximum feasible modulation 
frequency is approximately 1 megahertz. 
Phototransistors generate output signals which correspond (and, in fact, 
are approximately proportional) to the amplitude of the light received by 
the phototransistor. The filter circuit 44 generates a blocked path signal 
if the currently enabled phototransistor is not generating an output 
signal modulated in the same way (i.e., in the preferred embodiment, at 
the same frequency) that the LED's amplitude is modulated. In other words, 
as long as a portion of the light received by the phototransistor is 
modulated at the same frequency as the currently enabled LED, then the 
system concludes that the currently enabled light path is not blocked. 
FIG. 2 is a block diagram of the scanning circuit 38 used in the preferred 
embodiment. The signal lines on the left side of this figure go to the 
computer 36 shown in FIG. 1. As will be understood by those skilled in the 
art, the computer 36 is a one chip microcomputer which includes an 
interface circuit for buffering the flow of signals in and out of the 
microcomputer. 
Address lines A5-A0 from the computer 36 are used to select both the LED 
and the phototransistor to be energized. As will be discussed below, by 
using a latch signal, RLatch, the computer can select a phototransistor 
with an address that is different than the currently selected LED. 
Normally, though, the selected LED and phototransistor will have the same 
address. 
Since six binary address lines are used, up to 64 separate 
LED/phototransistor pairs can be addressed. 
As shown in FIG. 3, the phototransistors are wired using a matrix of 
connectors having up to eight rows and eight columns. In the preferred 
embodiment shown in FIG. 3, only 56 phototransistors are used and 
therefore one row connector RxRow7 is not used. The LEDs are wired in a 
similar fashion using an eight by eight matrix of connectors. 
The prior art devices known to the inventor, such as the touch panel 
disclosed in U.S. Pat. No. 4,243,879, use a separate connector for each 
light emitting and light receiving element. 
By using a matrix of connectors in this fashion, instead of a separate 
connector for each elements, the number of connectors is substantially 
reduced (generally, for N elements, from N+1 connectors to 2.sqroot.N). 
This not only simplifies the design of the circuit, it also reduces the 
size of the printed circuit board needed. In particular, it allows the use 
of a printed circuit board which is small in width so that the width and 
length of the display device 22 can be as small as possible for a given 
display surface. In the preferred embodiment, the width of the border 
around the display surface 24 is less than one inch (0.375 inches for the 
printed circuit board, plus approximately 0.45 inches for the LED and 
phototransistor elements, an optical filter, and the external packaging of 
the display). 
Referring back to FIG. 2, the LED currently selected by the computer is 
energized as follows. Address lines A5-A3 are decoded by a multiplexer 50 
so that only one of the lines LR7-LR0 are pulled high. These lines LR7-LR0 
are coupled to the gates of eight FETs (field effect transistors) 52, 
thereby enabling only one of the FETs. The output of a sine wave current 
signal generator 46 is coupled to the drains of all eight FETs 52, and the 
sources of the FETs are coupled to LED drive lines LEDRow7-LEDRow0. Thus, 
one of the LED drive lines is driven by the sine wave signal from 
generator 46, and all of the other drive lines are left floating (i.e., 
isolated by the FETs). 
Address lines A2-A0 are decoded by multiplexer 54 which pulls one of the 
resulting eight lines LC7-LC0 low and leaves the others at a high voltage 
(i.c., Vcc, which is 5 volts). The current capacity of these lines is 
amplified by buffer 56, so that one of the lines LEDCo17-LEDCo10 is pulled 
low and absorbs the current flowing through the selected LED. 
The circuit for selecting one phototransistor is similar to the circuit for 
selecting one LED, except that Latch 58 can latch the address used to 
select the phototransistor. When the RLatch signal from the computer 36 is 
high, the Latch 58 is transparent--the address signals flow unimpeded from 
lines A5-A0 to lines R5-R0. However, when RLatch is low, the present state 
of the A5-A0 lines is latched and held on lines R5-R0 until RLatch is 
pulled high. 
Multiplexer 60 decodes address lines R5-R3 and pulls one of the eight lines 
RR7-RR0 high. These lines RR7-RR0 are coupled to the gates of eight FETs 
(field effect transistors) 62, thereby enabling only one of the FETs. The 
sources of the FETs are coupled to phototransistor collector lines 
RxRow7-RxRow0, and drains are all coupled to the input of the filter 
detector circuit 44. Thus, one of the phototransistor collector lines is 
coupled to connector line 63, and all of the other drive lines are left 
floating (i.e., isolated by the FETs). 
Address lines R2-R0 are decoded by multiplexer 64 which pulls one of the 
resulting eight lines RxCo17-RxCo10 low and leaves the others at a high 
voltage (i.c., Vcc, which is 5 volts). The selected line RxCo17-RxCo10 
which is pulled low absorbs the current flowing through the selected 
phototransistor. 
The selected phototransistor gets its current from the power supply node 
Vcc through a resistor R in the filter 44. This resistor R will typically 
have a low resistance, typically between 50 and 100 ohms. 
Phototransistors draw current corresponding to the amplitude of the light 
received. Thus if the light received by the selected phototransistor 
consists of a d.c. component from the ambient light surrounding the 
display 22 plus a pulsing or modulated light from the LED across the 
display, the phototransistor's current will have both a d.c. and an a.c. 
component--as schematically shown above line 63. Further, the a.c. 
component will vary at the same frequency as the frequency that the 
incoming light is modulated. 
Note that the current source for the phototransistor is given a low 
resistance so that the phototransistor will draw a readily detectable a.c. 
current even in bright ambient light conditions. 
The resulting voltage on line 63 is sensed and analyzed by the filter 
circuit 44. The signal on line 63 is a.c. coupled through capacitor C to a 
tuned amplifier 70. Amplifier 70 is a narrow band amplifier which 
selectively amplified signals at the frequency generated by the sine wave 
generator 46. Thus, to the extent that the voltage on line 63 varies at 60 
kilohertz (the modulation frequency output by generator 46) the tuned 
amplifier 70 will generate a sinusoidal output signal. If the light from 
the selected LED is blocked by an object touching the display surface 24, 
then the output of the amplifier 70 will be a flat grounded signal. 
After passing the output of amplifier 70 through a rectifier 72, this 
signal is integrated by an integrator circuit 74. If the received light 
includes the light from the enabled LED, the output of the integrator 74 
will rise; otherwise it will generate a flat output. Then the output of 
the integrator is compared by comparitor 76 with a reference signal 
V.sub.R (having a voltage of approximately one volt in the preferred 
embodiment) to determine if the received light includes the light from the 
enabled LED. If so, the output of the comparitor DBlk is high; otherwise 
DBlk is low, indicating that the light path defined by the selected 
LED/phototransistor pair is blocked. 
The integrator 74 needs to be reset each time that a new 
LED/phototransistor pair is enabled. In an alternate embodiment, the 
integrator 74 could be replaced a low pass filter. The low pass filter 
would pass a null signal if the output of the tuned amplifier 70 is null, 
and would pass a positive signal if the tuned amplifier 70 generates a 
sinusoidal output. The low pass filter has the disadvantage of being 
somewhat slow, but has the advantage that it need not be reset. 
In summary, the computer generates an address A5-A0 and thereby selects one 
LED and one phototransistor. The scanning circuit decodes the address and 
enables the selected LED/phototransistor pair. The enabled LED is driven 
by a current which is modulated at a selected frequency, and therefore the 
amplitude of light emitted from the selected LED is also modulated at this 
frequency. A filter circuit 44 analyzes the waveform of the current drawn 
by the selected phototransistor and generates a blocked path signal DBlk 
if the waveform does not contain an a.c. component which is modulated in 
the same way as the amplitude of the light from the selected LED. 
FIG. 4 is a schematic representation of the modulating oscillator used in 
the preferred embodiment. The square wave output of a monostable 80 is 
shaped by an RC pair 82 and the resulting signal is amplified by a simple 
current amplifier circuit. 
FIG. 5 depicts how the printed circuit board for the LED and 
phototransistors are made. As shown, a number of equal sized L shaped 
substrates are cut from a single circuit board, which is much more 
material efficient than cutting out one rim from one circuit board for 
each display. As shown in FIG. 1, each L shaped board is used to mount 
either the LEDs or the phototransistors for one display. Further, as noted 
above, the use of matrix connections allows the L's to be made with a 
width of just 0.375 inches. 
Referring to Table 1, the computer 36 checks for an object touching the 
display surface 24 by sequentially enabling all of the LED/phototransistor 
pairs around the display and testing the DBlk signal. If a blocked path is 
detected, the system checks to make sure that all the blocked paths are 
contiguous, because if more than one object is touching the screen it will 
often be impossible to determine the position of both objects. At the end 
of each complete scan of the screen, the routine generates a report based 
on the range of horizontal and vertical paths which were found to be 
blocked so that the computer 36 can use that information to determine what 
action the user is requesting. 
The procedures shown in Tables 1 and 2 are written using a high level 
"pseudocode" language that can be easily understood by anyone skilled in 
the art of computer programming. 
Referring to Table 2, if the user suspects that the touch screen is not 
operating properly, the user can run the procedure shown in Table 2 to 
check for failed components. Similarly, if computer 36 consistently sees 
that one LED/phototransistor pair appears to be blocked, even when no 
other pairs are blocked, it can check to see if either the LED or the 
phototransistor from that pair are malfunctioning. In the preferred 
embodiment, the computer 36 generates a display which asks the user to 
make sure that nothing is touching the screen 24, and then it runs the 
procedure shown in Table 2. 
This procedure checks for blocked LED/phototransistor pairs. When a blocked 
pair is found, the computer then uses the RLatch signal shown in FIG. 2 to 
pair the selected phototransistor with an LED near the selected LED (i.e., 
near the ed normally corresponding to the selected phototransistor), and 
to pair the selected LED with a phototransistor near the selected 
phototransistor (i.e., the phototransistor normally corresponding to the 
selected LED). If the system still generates a blocked path signal when 
the alternate LED is enabled, then the system concludes that selected 
phototransistor is malfunctioning because it much more likely that one 
phototransistor isn't working than that two LEDs have failed. Similarly, 
if the system generates a blocked path signal when the alternate 
phototransistor is enabled, then the system concludes that the selected 
LED has failed. 
While the present invention has been described with reference to a few 
specific embodiments, the description is illustrative of the invention and 
is not to be construed as limiting the invention. Various modifications 
may occur to those skilled in the art without departing from the true 
spirit and scope of the invention as defined by the appended claims. 
For instance, the geometric pattern of light emitting elements and light 
receiving elements could be changed in numerous ways. In some instances, 
especially nonrectangular displays, several light receiving elements could 
be paired with a single light emitting element, or vice versa. 
In other embodiments, more complicated methods of modulating the emitted 
light could be used. For instance, the light could be modulated in a 
predetermined sequence of pulses, each pulse being further modulated at a 
predetermined frequency. In another variation the modulation frequency 
could be automatically changed (e.g., using a tunable monostable or 
oscillator) if the system detects that the ambient light includes light 
modulated at the modulation frequency initially used by the system. 
TABLE 1 
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Pseudocode for Normal Touch Detection 
______________________________________ 
##STR1## 
LowX = HighX = LowY = HighY = 0 
Loop: For K = 1 to 56 
##STR2## 
Call Touch(K) 
Endif 
Endloop 
Report LowX, HighX, LowY, HighY 
Return 
Subroutine Touch: 
If K &lt; 44 vertical paths: K = 1 to 43 
If LowX = 0 first blocked column? 
LowX = HighX = K 
Return 
Endif 
If HighX = K - 1 are the blocked columns 
HighX = K contiguous? 
Else 
Report Multiple Hit 
Endif 
Else horizontal paths: K &gt; 43 
If LowY = 0 
LowY = HighY = K 
Return 
Endif 
If HighY = K - 1 
HighY = K 
Else 
Report Multiple Hit 
Endif 
Endif 
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TABLE 2 
______________________________________ 
Pseudocode for Detecting Defective LEDs and Phototransistors 
______________________________________ 
Display "Please Make Sure Nothing Is Touching The Display" 
Wait X seconds 
Loop: For K = 1 to 56 
##STR3## 
Call Check(K) 
Endif 
Endloop 
Return 
Subroutine Check: 
Begin Case 
Case (K= 1) J=2 
Case (K=44) J=45 
Otherwise J=K-1 
Endcase 
##STR4## 
Report Bad Phototransistor K 
Return 
Endif 
##STR5## 
Report Bad LED K 
Return 
Endif 
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