Pressure mapping system with capacitive measuring pad

• US 5,010,772 A
• Filed: 05/05/1989
• Issued: 04/30/1991
• Est. Priority Date: 04/11/1986
• Status: Expired due to Term

• Current Assignee: Purdue Research Foundation - Purdue University
• Original Assignee: Purdue Research Foundation (The Trustees of Purdue University (d/b/a Purdue University))
• Inventors: Graber, George P., Tacker, Willis A. Jr., Bourland, Joe D., Barbs, Charles F., Geddes, Leslie A.
• Primary Examiner(s): NOT, DEFINED
• Application Number:
  US07/347,921
• Time in Patent Office: 725 Days
• Field of Search: 73/172, 73/862.04
• US Class Current: 73/862.046
• CPC Class Codes: A61B 2562/0247 Pressure sensors
• CPC Class Codes: A61B 2562/046 in a matrix array
• CPC Class Codes: A61B 5/113 occurring during breathing
• CPC Class Codes: A61B 5/6892 Mats

Abstract

A capacitive measuring pad is constructed of transverse conductive strips separated by a compressible insulator to form a matrix of pressure sensitive capacitive nodes. The nodes are repetitively scanned in sequence by a microcomputer to measure their respective capacitances, from which measurements a pressure map is then derived. The resulting pressure map may be displayed on a color graphics monitor with different colors representing diffrent pressures. Node capacitance is found by measuring the response of the mode to a driving signal of a known voltage. This measurement is accomplished by connecting one of the node'"'"'s transverse conductive strips to the driving source and the node'"'"'s other conductive strips to a sense amplifier. In order to isolate the node of interest from the influence of surrounding nodes, all of the conductive strips except the two intersecting the selected node are connected to ground. Furthermore, the input impedance to ground of the sense amplifier is made negligibly small with respect to the other system impedance. In this way, only the conductive strip connected to the driving source has a voltage impressed on it, and the conductive strips of all other nodes in the system are maintained at ground potential, thus allowing an accurate measurement of the one capacitance.

Claims

1. A system for monitoring the distribution of forces on a surface, comprising:

a pad having first and second arrays of linear electrodes respectively disposed on opposite sides of a sheet of compressible insulating material, the electrodes of said first array being oriented in a direction other than the direction of orientation of the electrodes of said second array to thereby form a matrix of nodes each of which is located at an intersection of an electrode of one array with an electrode of the other array and which has a capacitance that varies in accordance with compression of the insulating material at said intersection;

means for sequentially selecting the electrodes of said first array and applying an electrical driving signal to the selected electrode;

means for sequentially selecting the electrodes of said second array;

means for sensing an electrical current of the selected electrode of said second array to thereby measure the capacitance of the associated node at the intersection of the two selected electrodes;

means for connecting each of the non-selected electrodes of said first array to a reference voltage to thereby isolate said sensed node from the effects of changes in capacitance of nodes other than said sensed node whose capacitance is being measured; and

means for connecting each of the electrodes of said second array, including the selected electrode of said second array, to said reference voltage to thereby further isolate said sensed node from effects of changes in capacitance of nodes other than said sensed node whose capacitance is being measured and to reduce the effects of cable capacitance on the current from said sensed node.

2. The system of claim 1, further including means for displaying a distribution of forces on said pad as a matrix of cells with each cell depicting the relative capacitance of an associated node on said pad.

3. The system of claim 2, wherein the capacitances of said nodes are in a range which is associated with a spectrum of colors and pressure gradients are represented on said display means by a change in color from one cell to another.

4. The system of claim 1, further including means for determining an absolute pressure on the pad at each node thereof in response to the measured capacitance of the node.

5. The system of claim 4, further including means for summing the pressures at all of the nodes to thereby determine the weight of an object on the pad.

6. The system of claim 1, wherein said means for sensing includes an amplifier which receives said electrical current of said selected electrode of said second array as an input signal and which exhibits an input impedance much smaller than the impedance of said nodes.

7. The system of claim 1, wherein each of the electrodes of said first array and said second array are connected to the reference voltage via resistors.

8. The system of claim 7, wherein said means for sensing includes an amplifier which receives said electrical current of said selected electrode of said second array as an input signal and which exhibits an input impedance much smaller than the impedance of said nodes and said resistors connecting the electrodes of said second array to said reference voltage.

9. The system of claim 7, wherein the resistance of said resistors connecting each of the electrodes of said first array to the reference voltage is much smaller than the impedance of each node.

10. The system of claim 1, wherein said pad further includes a third array of linear electrodes, the electrodes in said third array being oriented parallel to and in alignment with the electrodes of said first array and being disposed on a side of said second array opposite said first array, and also including a second sheet of compressible insulating material interposed between said second and third arrays of electrodes, the electrodes of said third array being sequentially selected in correspondence with those of said first array to thereby effectively form two capacitors connected in parallel with one another at each node.

11. The system of claim 10, wherein each array of linear electrodes comprises conductive strips disposed on a substrate, and wherein the conductive strips of at least said first and third arrays are disposed on opposite sides of their respective substrates in an alternating fashion.

12. The system of claim 11, wherein each aligned pair of strips in the first and third electrodes are disposed on the same side of their respective substrates so that, at each node, one of the strips of the pair is disposed on the side of its substrate which is away from said second array and the other strip of the pair is disposed on the side of its substrate which is closest to said second array.

13. The system of claim 1, wherein said reference voltage is ground.

14. Apparatus for sensing the distribution of pressure on a surface, comprising:

first and second sheets of compressible insulating material;

a first array of linear electrodes interposed between said sheets of material and being oriented in a first direction;

a second array of linear electrodes disposed on a side of said first sheet that is opposite said first array and being oriented in a second direction which intersects said first direction;

a third array of linear electrodes disposed on a side of said second sheet that is opposite said first array, the electrodes of said third array being parallel to and in substantial alignment with the electrodes of said second array, each electrode in said third array being electrically connected to a corresponding aligned electrode in said second array to thereby effectively form two capacitors connected in parallel at each node formed at the intersection of an electrode in said first array with an aligned pair of electrodes in said second and third arrays; and

means for sensing an electrical current of a selected electrode of said first array to thereby measure capacitance of an associated node, said electrodes of said first array, including said selected electrode being connected to a common reference voltage.

15. The apparatus of claim 14, wherein the electrodes of said second and third arrays are oriented perpendicular to the electrodes of said first array.

16. The apparatus of claim 14, wherein said insulating material comprises a foam.

17. The apparatus of claim 14, wherein each of said sheets of insulating material comprises two layers of material which are joined to one another in a manner which forms enclosing cells of air between them, each cell being associated with one of said nodes.

18. A system for monitoring the distribution of forces on a surface, comprising:

a pad having first and second arrays of electrodes and insulating material disposed between said first and second arrays to form a matrix of capacitive nodes each of which has a capacitance that varies in accordance with the degree of pressure on the pad at the location of the node;

means for scanning the electrodes of said first and second arrays to thereby successively address nodes in said matrix;

means for measuring the capacitance of an addressed node by sensing current of a scanned electrode of said second array; and

means for connecting each electrode of said second array to a reference potential for the driving signal to thereby render all electrodes of said second array at substantially zero volts to thereby isolate the measured capacitance from the effects of changes in capacitance of nodes other than the addressed node whose capacitance is being measured.

19. The system of claim 18, further including means for connecting each electrode of said first array which is not associated with said addressed node to said reference potential to thereby render all electrodes of said first array except for the addressed node at zero volts.

20. The system of claim 19, wherein said measuring means comprises means for applying a predetermined voltage to a selected electrode of said first array, means for measuring the current from a selected electrode of said second array, and an amplifier having an input impedance that is negligible compared to any of the impedances formed by said arrays such that the selected electrode of said second array is practically at zero volts.

21. The system of claim 20, wherein said reference potential is ground.