Computer graphics processing and selective visual display system – Display peripheral interface input device – Touch panel
Reexamination Certificate
1998-06-04
2001-06-12
Jankus, Almis R. (Department: 2675)
Computer graphics processing and selective visual display system
Display peripheral interface input device
Touch panel
C345S173000, C178S018050, C178S018110
Reexamination Certificate
active
06246394
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to simplified resistive touch screen systems including a resistive touch screen and an ADC (analog-to-digital converter) for digitizing x and y coordinates of touch points applied to the resistive touch screen, and to switch circuitry that energizes such resistive touch screens; the invention relates more particularly to circuitry and a method for avoiding the effects of variations, especially thermal drift of the resistances of switches in the switching circuitry, on the calibration of the ADC to the touch screen.
U.S. Pat. No. 5,717,321 (Kerth et al.) issued Feb. 10, 1998, is believed to constitute the closest prior art. The Kerth et al. patent acknowledges that there are two conventional techniques for energizing a resistive touch screen assembly (hereinafter “touch screen”), either by connecting a voltage source across the resistive touch screen or by forcing a current through it. The Kerth et al. patent teaches (1) that applying a constant voltage across the resistive touch screen wastes power because typically there is a wide range in the resistivity of the resistor layers of a resistive touch screen; (2) that a class A driver is used which must be biased for the lowest acceptable resistance of the touch screen; and (3) that the resulting excess bias current above that required for touch screens that have higher resistance is wasted.
To overcome these problems, Kerth et al. teach that it is desirable to use feedback from the output of an ADC connected to the touch screen to control a current DAC that supplies current to energize the touch screen to cause a voltage across it to equal a desired reference voltage.
Referring to
FIG. 6
of U.S. Pat. No. 5,717,321 by Kerth et al., if the stylus
71
is pressed on the upper surface of touch screen
70
, that brings the resulting “touch point” of a conductive sheet in contact with a resistive layer. The voltage of that touch point, and hence the voltage of the conductive sheet, represents the x or the y coordinate of the tip of the stylus. Thus, accurate position information for the point of contact can be obtained by measuring resistive voltage division levels along an x axis of an x resistive sheet and a y axis of a y resistive sheet, respectively, and converting such analog voltages to digital numbers which then represent the x and y coordinates of the present touch point (at which the tip of the stylus is pressed on the touch screen).
If the values of touch screen resistance, contact resistance, and/or switch resistance change, for example with respect to temperature, age, etc., then the resistive divider output voltage produced at the touch point caused by the current I
T
also changes. Therefore, the touch screen becomes “uncalibrated” relative to the analog-to-digital converter
78
, which converts the analog voltage at the touch point to a digital number D
OUT
.
The circuit shown in
FIG. 6
of U.S. Pat. No. 5,717,321 attempts to deal with this problem by providing a switch
76
that connects the −REF terminal of analog-to-digital converter
78
to ground. A switch
73
also is provided to connect the +V
REF
terminal of analog-to-digital converter
78
to the output of current DAC
72
. Current DAC
72
forces a current I
S
through the “y” axis resistance of resistive sheet
74
of touch screen assembly
70
. Switches
80
and
79
likewise “energize” resistive sheet
75
of touch screen
70
. A complex feedback circuit controls the current output by current DAC
72
. Note that the voltage produced at the output of current DAC
72
is a “floating” voltage that is not nearly as stable as the voltage that would be produced by a typical voltage reference circuit or a typical power supply. The Kerth et al. feedback arrangement substantially eliminates the effect of variation of the switch resistances of switches
73
and
76
(that are used to energize “y” resistive sheet
74
of touch screen
70
) or
79
and
80
(that are used to energize “x” resistive sheet
75
) on the full scale digital output number produced by analog-to-digital output
78
. The feedback also eliminates the effects of variations in screen resistance and contact resistance on the full scale digital output number.
However, the feedback arrangement of Kerth et al. fails to provide calibration or compensation of resistivity variations across the resistive sheets
74
and
75
that result in accurate midpoint output voltages thereof. The Kerth et al. feedback arrangement also fails to provide any compensation of digitizing errors due to thermal drift in the current DAC
72
.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide a simple, inexpensive touch screen digitizing system in which a maximum or “full scale” analog input produced by a touch screen is calibrated to the full scale digital output of an analog-to-digital converter irrespective of thermal drift in resistances of switches coupling the touch screen to the analog-to-digital converter.
It is another object of the invention to provide a touch screen digitizing system of the type described in which the touch screen produces an analog signal having a large dynamic range.
It is another object of the invention to eliminate the effect of thermal drift in the resistances of switches coupling a touch screen to reference voltage inputs of an analog-to-digital converter on a full scale digital output produced by the analog-to-digital converter in response to a “full scale” touch point of the touch screen.
It is another object of the invention to provide a touch screen digitizing system which avoids inaccuracies that would be caused by thermal drift of a current DAC in a feedback system of the type in
FIG. 6
in U.S. Pat. No. 5,717,321 by Kerth et al.
It is another object of the invention to generate a ratio metric touch screen touch point measurement value that is insensitive to touch screen resistance changes, voltage supply changes, and/or reference voltage changes.
Briefly described, and in accordance with one embodiment thereof, the invention provides a touch screen digitizing system including a touch screen unit including a first resistive sheet with opposed x+ and x− terminals and a second resistive sheet with opposed y+ and y− terminals, and an analog-to-digital converter having first and second reference input terminals. A first switch is coupled between a first reference voltage and the x− terminal, and a second switch is coupled between the x+ terminal and a second reference voltage for energizing the first resistive sheet. A third switch is coupled between the first reference voltage and the y− terminal, and a fourth switch is coupled between the y+ terminal and the second reference voltage for energizing the second resistive sheet. Switching circuitry couples an input of the analog-to-digital converter to the y+ terminal while the first resistive sheet is energized and the second resistive sheet is not energized, and also couples the input to the x+ terminal while the second resistive sheet is energized and the first resistive sheet is not energized. The first and third switches are N-channel MOS transistors, and the second and fourth switches are P-channel MOS transistors. A microprocessor generates control signals that are respectively applied to the various gate electrodes of the P-channel transistors and the N-channel transistors and to the switching circuitry and to a convert input of the analog-to-digital converter.
REFERENCES:
patent: 4918262 (1990-04-01), Flowers et al.
patent: 5283559 (1994-02-01), Kalendra et al.
patent: 5327163 (1994-07-01), Hashimoto et al.
patent: 5355149 (1994-10-01), Casebolt
patent: 5644308 (1997-07-01), Kerth et al.
patent: 5717321 (1998-02-01), Kerth et al.
patent: 5736949 (1998-04-01), Ong et al.
patent: 5877458 (1999-03-01), Flowers
Kalthoff Timothy V.
Rundel Bernd M.
Awad Amr
Burr-Brown Corporation
Cahill Sutton & Thomas P.L.C.
Jankus Almis R.
LandOfFree
Touch screen measurement circuit and method does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Touch screen measurement circuit and method, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Touch screen measurement circuit and method will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2458585