Telegraphy – Systems – Position coordinate determination for writing
Reexamination Certificate
1998-03-09
2001-08-21
Saras, Steven (Department: 2675)
Telegraphy
Systems
Position coordinate determination for writing
C345S174000
Reexamination Certificate
active
06278068
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to resistive digitizers and in particular noise filtration and coordinate voltage measurement for four and five wire digitizers.
2. Description of Related Art
Resistive digitizers when placed on an LCD or CRT screen pick up noise that is superimposed on the coordinate voltage that is to be read. This type of digitizer has two resistively coated surfaces that come into contact when pressed. The noise cannot be easily filtered because the different terminals are multi-function and see different voltages at different times. For example, in a four wire digitizer a terminal can be used to connect power to the digitizer panels or to read the coordinate voltage of the other panel. A filter one might use on a power terminal would have time constant implications when switching between functional use of the terminal, and a filter one might use in a signal voltage would have a voltage drop implication when used as a power terminal
In U.S. Pat. No. 4,306,110 (Nelson et al.) is described an apparatus for determining planar coordinates. A four wire digitizer is discussed with the un-powered plane providing the location voltage through a high impedance conditioning circuit to a sample and hold circuit. As power is alternated between digitizer planes, un-powered plane is changed to read X-plane coordinate voltages and then Y-plane coordinate voltages. These voltages are connected to different inputs of the high impedance conditioning circuitry and further connected to sample and hold circuits at the output of the high impedance and conditioning circuits.
In U.S. Pat. No. 4,484,026 (Thornburg) a touch tablet is shown for entering data into a computer. The touch tablet is made of two resistive sheets in parallel and oriented orthogonal to each other. Conductive strips on two ends of each resistive sheet and having orthogonal orientation between sheets are electrically connected to the resistive material of the sheet. A voltage is applied between the strips on one resistive sheet, and a coordinate location is read from the un-powered sheet. By connecting the output amplifier to both strip on the un-powered resistive sheet, the sensitivity to noise is reduced and system performance is improved.
In U.S. Pat. No. 5,041,701 (Wolfe et al.) is described an edge linearization device for producing orthogonal electric fields in a resistive surface to be used in a contact input system. The system is capable of locating an object in contact with the resistive surface. In U.S. Pat. No. 5,083,118 (Kazama) is described a coordinate measuring apparatus to be mounted to a CRT or an LCD screen in which a fist layer is used as an electromagnetic screen and resistors of value approximately one hundred ohms are connected to one end of the resistive layers to help reduce noise. Shown in U.S. Pat. No. 5,191,175 (Protheroe et al.) is shown a self tuning digitizer control circuit in which a narrow bandpass filter is used to filter out extraneous noise picked up by a coordinate sensing stylus. In U.S. Pat. No. 5,365,253 (Cheng et al.) is shown a digitizer device with anti-noise capability. The digitizer operates at several hundred thousand Hertz and uses a hardware circuit controlled by a software program to eliminate outside noises.
Resistive digitizers have two resistively coated surfaces that come into contact when pressed. A standard four wire technique involves applying a voltage across one of the surfaces while reading the voltage at the point of contact through the other layer. A controller switches between the two resistive surfaces and measures the voltage corresponding to the X and Y coordinates. The noise induced into the resistive surfaces when the digitizer is placed on a CRT or an LCD screen can produce errors in the coordinate readings.
SUMMARY OF THE INVENTION
In this invention an R-C series filter is connected by a switch to the measurement terminal of a four or five wire resistive digitizer. Once the capacitor of the filter has been charged to the voltage value of the coordinate being measured, an ADC (analog to digital converter) measures the voltage across the capacitor and the switch disconnects the filter from the digitizer. The voltage is maintained on the capacitor until the next reading or until a reference voltage resets the capacitor voltage. Since the coordinate voltage is maintained on the filter capacitor after the switch disconnects the filter from the digitizer, the ADC could measure the voltage at anytime until the voltage is changed by a new measurement or a reset signal.
In a four wire digitizer a bias voltage is applied to first one resistive plane with the second plane floating, and then to a second plane with the first plane floating. When the bias voltage is applied to the X plane of the digitizer, the X coordinate location of the digitizer pen is measured by connecting the “Y−” terminal, or the “Y+” terminal, of the floating Y plane to an R-C filter. Either “Y” terminal can be used to measure the X coordinate voltage since the Y plane is floating when the X plane is powered. The X coordinate voltage is connected to the floating Y plane at the location of the digitizer pen. An ADC measures the X coordinate voltage across the capacitor with respect to the “X−” terminal of the X plane, and then the R-C filter is disconnected from the “Y−” terminal, or the “Y+” terminal. Since the X coordinate voltage is maintained on the R-C filter, the measurement of the X coordinate voltage by the ADC can be done after the R-C filter is disconnected from the terminal of the Y plane.
Next a voltage is applied to the Y plane of the digitizer, and the Y coordinate location of the digitizer pen is measured by connecting the “X−” terminal of the floating X plane to an R-C filter. Either “X” terminal can be used to measure the Y coordinate voltage since the X plane is floating when the Y plane is powered. The Y coordinate voltage is connected to the floating X plane at the location of the digitizer pen. An ADC measures the Y coordinate voltage across the capacitor of the filter with respect to the “Y−” terminal of the Y plane, and then the R-C filter is disconnected from the “X−” terminal. Since the Y coordinate voltage is maintained on the R-C filter, the measurement of the Y coordinate voltage by the ADC can be done after the R-C filter is disconnected from the terminal of the X plane.
The X and the Y filter voltages can be reset to a reference voltage to allow the capacitors of the filters to be charged from a known value. This is done at reset and initial powering of the digitizer controller, and when a pen up condition is detected. If there is adequate drive capability, the reference voltage can be connected directly to the capacitor of the filter; otherwise, the reference voltage is connected to the signal input of the filter.
In a five wire digitizer there is an X-Y voltage coordinate plane and a sense plane. Reference voltages are connected to the corners of the X-Y coordinate plane and are designated; UL upper left, LL lower left, UR upper right and LR lower right. To measure an X coordinate a first reference voltage is connected to UR and LR and a second reference voltage is connected to UL and LL. When a Y coordinate voltage is measured, the first reference voltage is connected to LL and LR and the second reference voltage is connected to UL and UR.
The sense plane of a five wire digitizer is floating with respect to the X-Y coordinate plane except when the digitizing pen forces contact between sense plane and the coordinate plane. At this point of contact the X or Y coordinate voltage is connected to the sense plane. A sense terminal of the sense plane is connected by means of a first switch to an R-C filter for X coordinate voltage values and by means of a second switch to an R-C filter for Y coordinate voltage values. The appropriate R-C filter is connected to the sense terminal by means of switches as the coordinate voltage plane is connected to voltages to measure the X
Ackerman Stephen B.
Anyaso Uchendu
Saile George O.
Saras Steven
Tritech Microelectronics, Inc.
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