Abstract:
This invention describes a method and apparatus for filtering noise from a measurement of the X and Y coordinates of a resistive digitizer. The method applies to both four and five wire resistive digitizers biased with a DC voltage. The same filtering and measurement apparatus can be used on both types of digitizers with inclusion of an extra signal pin to accommodate the sense lead of the five wire digitizer. This approach involves connecting the signal to be read to a filter, reading the filtered voltage, and disconnecting the signal from the filter before disconnecting bias voltage from the planes of the digitizer. A separate filter is used for the X and Y coordinate signals and each filter voltage can be read at any time before the next measurement. A reset voltage is available to an established reference on the filter capacitor when a “pen up” status is detected.

Description:
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 coordinate and then the Y coordinate. The voltages to which each of the capacitors in the two filters were charged are maintained on the capacitors until the next coordinate measurement or until being reset by a reference voltage. The coordinate voltages can be read form the filters at any time after the filter is properly charged until the next coordinate measurement and a reset voltage is applied. 
     Filters other than R-C filters can be used with the four or five wire digitizers. The only requirements is that the filters sufficiently reduce the noise picked up by the digitizer and that resulting values the X and Y coordinate voltages be maintained long enough so the measurements can be made. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This invention will be described with reference to the accompanying drawings, wherein: 
     FIG. 1 is a schematic diagram of connections to the X plane of a four wire digitizer; 
     FIG. 2 is a schematic diagram of connections to the Y plane of a four wire digitizer; 
     FIG. 3 a  is a schematic diagram of connections to a five wire digitizer; 
     FIG. 3 b  is a table showing the various voltage connections to the five wire digitizer panel; 
     FIG. 4 is a flow diagram of a method to filter and read coordinate voltages for a four wire digitizer; and 
     FIG. 5 is a flow diagram of a method to filter and read coordinate voltages for a five wire digitizer. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIG. 1 is shown a schematic of the connections of a four wire digitizer in which a resistive X plane Dx  10  is powered and the coordinate position is read from the floating Y plane of the digitizer by an ADC  11 . Connected to the “X+” terminal  12  is a voltage V  14  through a switch SAx  13 . Connected to the “X−” terminal  15  is circuit ground  17  through a switch SCx  16 . The “Y−” terminal  18  of the floating Y plane is connected to a filter input Fx  20  through a switch SBx  19 . Since the Y plane is floating, the “Y+” terminal could also be used to connect the signal on the Y plane to the filter input Fx  20  through switch SBx  19  or both the “Y+” and the “Y−” terminals could be connected the filter input Fx  20  through switch SBx  19 . The filter consists of a resistor Rx  21  connected in series with a capacitor Cx  22 . The filtered voltage on the capacitor Cx  22  is connected to the ADC  11  through a filtered voltage input terminal INx  23  and an analog selector  24 . A reference voltage Vref  25  is connected to the filter Fx  20  through a voltage follower  26  and a switch SDx  27 . 
     Continuing to refer to FIG. 1, power is connected to the X plane of the digitizer Dx  10  by closing switches SAx  13  and SCx  16 . A voltage V  14  is connected by means of switch SAx  13  to the “X+” digitizer terminal  12  and the Ref. HI terminal of the ADC  11  through an analog selector  28 . A ground voltage  17  is connected by means of switch SCx  16  to the “X−” digitizer terminal  15  and the Ref. LO terminal of the ADC  11  through an analog selector switch  29 . A voltage representing the location of a pen contacting the planes of the digitizer is induced into the floating Y plane and is connected to the measurement circuitry through the “Y−” terminal  18 . This voltage could also be connected through the “Y+” terminal. A switch SBx  19  is closed connecting the voltage on “Y−”  18  to the filter input Fx  20 . After the filter capacitor Cx  22  is charged, the voltage across the capacitor is read by the ADC  11  and the switch SBx  19  is opened. 
     Continuing to refer to FIG. 1, the voltage on the filter capacitor Cx  22  is maintained until the next measurement of the X plane coordinate voltage. This can allow the voltage on the capacitor Cx  22  to be read any time after the capacitor Cx  22  is filly charged and until the next X plane measurement is initiated or a reset is initiated using a reference voltage Vref  25 . A reference voltage Vref  25  can be connected to the filter input Fx  20  by momentarily closing switch SDx  27  to reset the capacitor Cx  22  to the reference voltage Vref  25  in preparation for the next reading of an X coordinate. If the voltage follower  26  has enough drive capability, the reference voltage could be momentarily connected to the capacitor Cx  22  of the X filter to reset the capacitor voltage. 
     In FIG. 2 is shown a schematic of the connections of a four wire digitizer in which a resistive Y plane Dy  30  is powered and the coordinate position is read from a floating X plane of the digitizer by an ADC  11 . Connected to the “Y+” terminal  32  is a voltage V  14  through a switch SAy  33 . Connected to the “Y−” terminal  35  is circuit ground  17  through a switch SCy  36 . The “X−” terminal  38  of the floating X plane is connected to a filter input Fy  40  through a switch SBy  39 . Since the X plane is floating, the “X+” terminal could also be used to connect the signal on the X plane to the filter input Fy  40  through switch SBy  39  or both the “X+” and the “X−” could be connected the filter input Fy  40  through switch SBy  39 . The filter consists of a resistor Ry  41  connected in series with a capacitor Cy  42 . The filtered voltage on the capacitor Cy  42  is connected to the ADC  11  through a filtered voltage input terminal INy  43  and an analog selector  24 . A reference voltage Vref  25  is connected to the filter Fy  20  through a voltage follower  26  and a switch SDy  37 . 
     Continuing to refer to FIG. 2, power is connected to the Y plane of the digitizer Dy  30  by closing switches SAy  33  and SCy  36 . A voltage V  14  is connected by means of switch SAy  33  to the “Y+” digitizer terminal  32  and the Ref. HI terminal of the ADC  11  through an analog selector  28 . A ground voltage  17  is connected by means of switch SCy  36  to the “Y−” digitizer terminal  35  and the Ref. LO terminal of the ADC  11  through an analog selector switch  29 . A voltage representing the location of a pen contacting the planes of the digitizer is induced into the floating X plane and is connected to the measurement circuitry through the “X−” terminal  38 . This voltage could also be connected through the “X+” terminal since the X plane is floating. A switch SBy  39  is closed connecting the voltage on “X−” terminal  38  to the filter input Fy  40 . After the filter capacitor Cy  42  is charged, the voltage across the capacitor is read by the ADC  11  and the switch SBy  39  is opened. 
     Continuing to refer to FIG. 2, the voltage on the filter capacitor Cy  42  is maintained until the next measurement of the Y plane coordinate voltage. This can allow the voltage on the capacitor Cy  42  to be read any time after the capacitor Cy  42  is fully charged and until the next Y plane measurement is initiated or a reset is initiated using a reference voltage Vref  25 . A reference voltage Vref  25  can be connected to the filter input Fy  40  by momentarily closing switch SDy  37  to reset the capacitor Cy  42  to the reference voltage Vref  25  in preparation for the next reading of an Y coordinate. If the voltage follower  26  has enough drive capability, the reference voltage could be momentarily connected to the capacitor Cy  42  of the Y filter to reset the capacitor voltage. 
     Referring to FIG. 3 a,  connections to filter and read coordinate voltages for a five wire digitizer is shown The circuitry to filter and read the five wire digitizer is similar to that used with the four wire digitizer with the addition of a sense terminal  60 , an analog selector  61  for delivering power the digitizer panel, and the renaming of the terminals connecting power to the five wire digitizer to lower right LR  62 , lower left LL  63 , upper right UR  64  and upper left UL  65 . In a five wire digitizer there is a sense plane and a resistive coordinate plane. The resistive coordinate plane is powered from the corners with the upper left being the “0,0” coordinate. A sense terminal  60  is connected to the sense plane of the five wire digitizer which in turn is connected to a filter input Fx  20  through a switch SBx  19  and to a filter input Fy  40  through switch SBy  39 . The filter for filtering X coordinate voltages consists of a resistor Rx  21  connected in series with a capacitor Cx  22  to circuit ground The filtered voltage on the capacitor Cx  22  is connected to the ADC  11  through INx  23  and an analog selector  24 . The filter for filtering Y coordinate voltages consists of a resistor Ry  41  connected in series with a capacitor Cy  42  to circuit ground. The filtered voltage on the capacitor Cy  42  is connected to the ADC  11  through INy  43  and an analog selector  24 . A reference voltage Vref  25  is connected to the input of the X filter Fx  20  through a voltage follower  26  and a switch SDx  27 , and to the input of the Y filter Fy  40  through a voltage follower  26  and a switch SDy  37 . 
     Continuing to refer to FIG. 3 a,  when an X coordinate voltage is being measured, the analog selector  61  connects a voltage V  14  to the LR terminal  62  and the UR terminal  64 , and ground to the LL terminal  63  and the UL terminal  65 . The sense terminal  60  connected to the sense plane has an X coordinate voltage that is connected to the X filter input Fx  20  by means of a switch SBx  19 . The X filter consists of a resistor Rx  21  and a capacitor Cx  22 . After the capacitor Cx  22  has been charged with the X coordinate voltage, the voltage on the capacitor Cx  22  is read by the ADC  11  through terminal INx  23  and an analog selector  24 . After the ADC  11  reads the voltage on capacitor Cx  22 , the switch SBx  19  is opened. The voltage on capacitor Cx  22  can be reset to the reference voltage Vref  25  by momentarily closing switch SDx  27 . If the capacitor voltage is not reset, then the X coordinate voltage is maintained on the capacitor Cx  22  until the next X coordinate measurement; therefore an X coordinate measurement by the ADC  11  could be made anytime before the next X coordinate measurement or a reset of the capacitor voltage by the reference voltage Vref  25  through the voltage follower  26  and switch SDx  27 . 
     Continuing to refer to FIG. 3 a,  when a Y coordinate voltage is being measured, the analog selector  61  connects a voltage V  14  to the LR terminal  62  and the LL terminal  63 , and ground to the UR terminal  64  and the UL terminal  65 . The sense terminal  60  connected to the sense plane has a Y coordinate voltage that is connected to the Y filter input Fy  40  by means of a switch SBy  39 . The X filter consists of a resistor Ry  41  and a capacitor Cy  42 . After the capacitor Cy  42  has been charged with the Y coordinate voltage, the voltage on the capacitor Cy  42  is read by the ADC  11  through terminal INy  43  and an analog selector  24 . After the ADC  11  reads the voltage on capacitor Cy  42 , the switch SBy  39  is opened, and the voltage on capacitor Cy  42  can be reset to the reference voltage Vref  25  by momentarily closing switch SDy  37 . If the capacitor voltage is not reset, then the Y coordinate voltage is maintained on the capacitor Cy  42  until the next Y coordinate measurement; therefore a Y coordinate measurement by the ADC  11  could be made anytime before the next Y coordinate measurement or a reset of the capacitor voltage by the reference voltage Vref  25  through the voltage follower  26  and switch SDy  37 . When a Y coordinate voltage is on the sense terminal  60 , switch SBy  62  is closed and SBx  61  is open. The Y coordinate voltage is connected to the input of the Y filter Fy  67 . After the capacitor Cy  69  has been charged with the Y coordinate voltage, the voltage on the capacitor Cy  69  is read by the ADC  72  through terminal INy  70  and the analog selector  71 . The voltage on capacitor Cy  69  can be reset to the reference voltage Vref  73  by momentarily closing switch SDy  76 . 
     In FIG. 3 b  a table is shown that has the various voltages to be connected to the resistive coordinate plane of a five wire digitizer. The voltages are connected to the four corners of the five wire digitizer as noted by LR (upper left), LL (lower left), UR (upper right) and UL (upper left). When measuring an X coordinate, the LR and UR corners are connected to a voltage V, and the LL and UL corners are connected to ground G. When measuring a Y coordinate, the LR and LL corners are connected to a voltage V, and the UR and UL corners are connected to ground G. For touch detection, TD, all corners are connected to a voltage V. 
     Shown in FIG. 4 is a method for filtering coordinate voltages using a four terminal digitizer. When a pen is detected in the down position  80 , a voltage is applied across the “X+” and “X−” terminals of the digitizer  81 . The the word “down” as used in the previous sentence implies contact of the pen with the digitizer regardless of the orientation of the digitizer. Next the “Y−” terminal is connected to the X filter input  82 . The Y panel of the digitizer is not powered and is floating with respect to voltage bias. The voltage representing the position of a digitizing pen is superimposed on the Y plane at the point of contact with the pen. The filtered voltage across the filter capacitor is read by an ADC with respect to the “X−” terminal  83  which is ground after the capacitor is fully charged. After the filtered voltage is read, the “Y−” terminal is disconnected from the X filter input  84 . Then the bias voltage is disconnected from the “X+” and the “X−” terminals  85  and a voltage bias is applied across the “Y+” and “Y−” terminals of the digitizer  86 . The “X−” terminal, which is now floating with respect to applied power, is now connected to the Y filter input  87 , and the voltage representing the Y location of the pen is superimposed onto the floating X plane. When the capacitor of the Y filter is charged up with the Y coordinate voltage from the floating X plane, the voltage on the capacitor is read with respect to the “Y−” terminal of the digitizer  88 . The “X−” terminal is disconnected for the Y filter  89 , and the bias voltage is disconnected from the “Y+” and “Y−” terminals of the digitizer  90 . When the pen is detected in the up position (up meaning not in contact with the digitizer panels)  91 , Vref is momentarily connected to the X filter  92  to establish the same starting voltage on the X filter capacitor for the next measurement. Next Vref is momentarily connected to the Y filter  93  to establish a starting voltage on the Y filter capacitor, and the process returns to the beginning step waiting for the pen to be down in contact with the digitizer  80 . If the pen is not up  94 , the process continues with a return to the beginning step, and since the pen is down  80  continues on to applying a voltage across the X terminals of the digitizer  81 . It should be noted that all steps in the process may take varying time delays for voltages and signals to stabilize. Adequate time delays are assumed to exist in each of the steps of the process. 
     Shown in FIG. 5 is a method for filtering coordinate voltages using a five terminal digitizer. When a pen is detected in the down position  100 , a voltage is connected to the UR (upper right) and LR (lower left), and ground is connected to UL (upper left) and LL (lower left) terminals of the digitizer  101 . The the word “down” as used here implies contact of the pen with the digitizer regardless of the orientation of the digitizer. Next a sense terminal is connected to the X filter input  102 . The filtered voltage across the filter capacitor is read by an ADC with respect ground  103 . After the filtered voltage is read, the sense terminal is disconnected from the X filter input  104 . Then the bias voltage and ground is disconnected from the Up, LR, UL and LL terminals  105  and a voltage bias is connected to LR and LL and ground is connected to UR and UL terminals of the digitizer  106 . Next a sense terminal is connected to the Y filter input  107 . When the capacitor of the Y filter is charged up with the Y coordinate voltage, the voltage on the capacitor is read with respect to ground  108 . The sense terminal is then disconnected from the Y filter  109 , and the bias voltage and ground is disconnected from LR, LL, UR and UL terminals of the digitizer  110 . When the pen is detected in the up position (up meaning not in contact with the digitizer panels)  111 , Vref is momentarily connected to the X filter  112  to establish the same starting voltage on the X filter capacitor for the next measurement. Next Vref is momentarily connected to the Y filter  113  to establish a starting voltage on the Y filter capacitor, and the process returns to the beginning step waiting for the pen to be down in contact with the digitizer  100 . If the pen is not up  114 , the process continues with a return to the beginning step, and since the pen is down  100 , the process continues by applying a voltage across the X terminals of the digitizer  81 . It should be noted that all steps in the process may take varying time delays for voltages and signals to stabilize. Adequate time delays are assumed to exist in each of the steps of the process. 
     While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.