ELECTROGRAPHIC SENSOR FOR DETERMINING PLANAR COORDINATES

• US 3,798,370 A
• Filed: 04/17/1972
• Issued: 03/19/1974
• Est. Priority Date: 04/17/1972
• Status: Expired due to Term

• Current Assignee: George S. Hurst
• Original Assignee: George S. Hurst
• Inventors: Hurst, George S.
• Primary Examiner(s): Cooper, William C.
• Application Number:
  US05/244,629
• Time in Patent Office: 701 Days
• Field of Search: 178/18,19,20
• US Class Current: 178/18.05
• CPC Class Codes: G01B 7/004 for measuring coordinates of points
• CPC Class Codes: G01L 1/205 using distributed sensing elements
• CPC Class Codes: G06F 3/045 using resistive elements, e.g. a single continuous surface or two parallel surfaces put in contact

Abstract

An electrographic sensor for determining planar coordinates with good resolution, e.g., about 0.1 mm, and an overall accuracy of about 0.4 mm. A rectangular single sheet of extremely uniform resistive material has a row of small electrodes arranged along each edge with discrete resistors connected between adjacent electrodes of each row so as to form resistor networks along each edge of the resistive sheet. A switching circuit applies a voltage across the resistive sheet by applying one polarity to both ends of the resistor network of one edge and the opposite polarity to both ends of the resistor network at an opposite edge. At a desired time interval, voltage is switched to the second set of resistor networks so as to produce orthogonal electric fields in the resistive material during mutually exclusive time intervals. The sensor is contacted with probe at selected points to produce voltage signals which are proportional to the coordinates of any such points. Specific embodiments are described for punched-card reading, the preprogrammed interpretation of graphical data, and the movement of a probe across the sensor to produce continuous contacting for many applications.

Claims

1. An electrographic sensor unit for use in determining the x and y planar coordinates of a point, which comprises:

a rectangular sheet of resistive material having a uniform electrical resistivity throughout the sheet;

corner spot electrodes in each corner of the resistive sheet in electrical contact therewith;

a plurality of spaced-apart edge spot electrodes along each edge of the resistive sheet in electrical contact therewith;

a plurality of discrete first resistors connected between adjacent of all of edge spot electrodes; and

a plurality of discrete second resistors connected between the corner spot electrodes and adjacent edge spot electrodes whereby the first and second resistors form series resistor networks along each edge of the resistive sheet.

2. The sensor of claim 1 wherein each of the edge and corner spot electrodes is small with respect to the spacing therebetween;

wherein the edge spot electrodes along each edge of the resistive sheet are equally spaced from each other of that edge and from the adjacent corner spot electrodes;

wherein all of the first resistors are of equal resistance value; and

wherein all of the second resistors are equal and each have a resistance value greater than the value of each of the first resistors.

3. The sensor of claim 2 wherein the corner and edge spot electrodes are circular and their diameter is about 1/16 inch;

the spacing therebetween is from about 1 inch to about 2 inches;

the resistivity of the resistive sheet is about 2,000 ohms per square;

the first resistors are each of a value of about 50 ohms with a precision of at least 1.0 percent; and

the second resistors are each about 75 ohms with a precision of at least 1.0 percent.

4. The sensor of claim 1 wherein each of the edge spot electrodes is individually displaced toward the center of the resistive sheet, from lines joining the corner spot electrodes, an effective distance such that application of an electrical potential across the resistive sheet by opposite pairs of the series resistor networks produces equal potential lines substantially parallel to the lines joining the corner spot electrodes whenever the equiPotential lines are at least one spot electrode separation distance from those lines joining corner spot electrodes.

5. The sensor of claim 1 further comprising:

a voltage source having first and second output leads;

switches connected between the voltage source leads and the corner spot electrodes on the resistive sheet;

means for operating the switches sequentially whereby during a first time interval the first output lead of the voltage source is connected to both ends of one of a first pair of opposite series resistor networks along one edge of the resistive sheet and the second output lead of the voltage source is simultaneously connected to both ends of the other of the first pair of opposite series resistor networks along the opposite edge of the resistive sheet and whereby a second pair of opposite series resistor networks along the remaining edges of the resistive sheet function as voltage dividers during the first time interval, and during a second and mutually exclusive time interval the first output lead of the voltage source is connected to both ends of one of the second pair of opposite series resistor networks and the second output lead of the voltage source is simultaneously connected to both ends of the other of the second pair of opposite series resistor networks and the first pair of opposite series resistor networks function as voltage dividers thereby producing orthogonal electric fields having uniform equipotential lines in the resistive sheet;

a conductive probe for electrically contacting the surface of the resistive sheet at a point whose x and y planar coordinates are to be determined; and

output means connected between the conductive probe and one corner spot electrode responsive to a potential difference between that corner spot electrode and the contacted point on the resistive sheet whereby separate electrical output signals are derived during the mutually exclusive time intervals that are accurately related to the x and y planar coordinate of the contacted point on the resistive sheet.

6. The sensor of claim 5 wherein the conductive probe includes a normally-open pressure sensitive switch in series with the probe and the output means whereby signals are obtained from the output means only when a preset pressure is exceeded between the probe and the surface of the resistive sheet to thereby close the pressure sensitive switch.

7. The sensor of claim 1 further comprising:

a layer of a deformable insulation in contact with substantially all of one surface of the resistive sheet; and

a sheet of conductive material spaced from the resistive sheet by the layer of the deformable insulation.

8. The sensor of claim 7 wherein the layer of deformable insulation is a fabric net, the threads thereof being about 0.004 in. in diameter and the threads being spaced apart about 0.05 to about 0.2 in.

9. The sensor of claim 7 wherein the layer of deformable insulation is a cured self-healing dielectric gel having a thickness of from about 0.002 to about 0.005 in.

10. The sensor of claim 7 wherein the conductive material is a conductive metallic sheet.

11. The sensor of claim 7 wherein the conductive material is a conductive plastic sheet.

12. The sensor of claim 7 further comprising:

a voltage source having first and second output leads;

switches connected between the voltage source leads and the corner spot electrodes on the resistive sheet;

means for operating the switches sequentially whereby during a first time interval the first output lead of the voltage source is connected to both ends of one of a first pair of opposite series resistor networks along one edge of the resistive sheet and the second output lead of the voltage source is simultaneously connected to both ends of the other of the first of series opposite pair resistor networks along the opposite edge of the resistive sheet and whereby a second pair of opposite series resistor networks along the remaining edgEs of the resistive sheet function as voltage dividers during the first time interval, and during a second and mutually exclusive time interval the first output lead of the voltage source is connected to both ends of one of the second pair of opposite series resistor networks and the second output lead of the voltage source is simultaneously connected to both ends of the other of the second pair of opposite series resistor networks and the first pair of opposite series resistor networks function as voltage dividers thereby producing orthogonal electric fields having uniform equipotential lines in the resistive sheet;

means for electrically contacting the resistive sheet and the sheet of conductive material at a point whose x and y planar coordinates are to be determined; and

output means connected between the sheet of conductive material and one corner spot electrode responsive to a potential difference between that corner spot electrode and the sheet of conductive material whereby separate electrical output signals are derived during the mutually exclusive time intervals that are accurately related to the x and y planar coordinate of the contacted point on the resistive sheet.

13. The sensor of claim 12 wherein the means for contacting the resistive sheet and the sheet of conductive mateirial is a pointed probe for pressing the resistive sheet into contact with the sheet of conductive material at a point by deforming the layer of deformable insulation at that point.

14. The sensor of claim 13 further comprising pressure sensitive means connected to the output means whereby output signals are produced only when pressure between the resistive sheet and the sheet of conductive material exceeds a preselected value.

15. The sensor of claim 14 wherein the pressure sensitive means comprises an operational amplifier, with an applied bias, connected between the resistive sheet and the sheet of conductive material to compare the contact resistance between the resistive sheet and the sheet of conductive material as pressure is applied by the probe with a preselected resistance value equivalent to the bias whereby the potentials proportional to the x and y planar coordinates at a point are applied to the output means only when the contact resistance is less than the preselected value.

16. The sensor of claim 14 wherein the pressure sensitive means comprises a normally open pressure sensitive electrical switch within the probe connected in series with the output means whereby output signals are produced only when the pressure applied by the probe exceeds a preselected value to thereby close the pressure sensitive switch.