Capacitive sensor and method of measuring changes in capacitance

• US 5,585,733 A
• Filed: 06/07/1995
• Issued: 12/17/1996
• Est. Priority Date: 09/10/1992
• Status: Expired due to Fees

• Current Assignee: Sarnoff Corporation (SRI International, Inc.)
• Original Assignee: David Sarnoff Research Center, Inc.
• Inventors: Paglione, Robert W.
• Primary Examiner(s): Regan, Maura K.
• Assistant Examiner(s): BROWN, GLENN WARNER
• Application Number:
  US08/480,362
• Time in Patent Office: 559 Days
• Field of Search: 324/658, 324/676, 324/678, 324/679, 324/690, 340/870.37
• US Class Current: 324/678
• CPC Class Codes: G01B 7/02 for measuring length, width or thickness G01B7/004, G01B7/12 take precedence
• CPC Class Codes: G01D 5/2405 by varying dielectric
• CPC Class Codes: G01N 27/228 Circuits therefor measuring capacitance per se G01R27/26
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24

Abstract

An apparatus and method for measuring the variation in the capacitance of a capacitive sensor. The apparatus and method include means for constantly applying a constant electrically current to an electrode of the capacitor and means for generating a first series of timing pulses. The voltage on the capacitor is compared to a reference voltage and an electrical signal is generated when the voltage on the capacitor reaches a first voltage which exceeds the reference voltage. The first series of timing pulses emitted by the generating means from the time that the charging of the capacitor beings until the electrical signal is generated is counted and an output signal is generated corresponding to the number of timing pulses counted. An application of the sensor to the measurement of the change in length of a telescoping device such as a shock absorber is also disclosed.

Claims

1. Apparatus for measuring the capacitance of a capacitor comprising:

means for providing a continuous source of constant electrical current to an electrode of the capacitor;

separate means for generating a first series of timing pulses;

comparator means for comparing the voltage on the capacitor to a reference voltage;

means for generating an electrical signal when the comparator means indicates that the voltage on the capacitor reaches a first voltage which just exceeds the reference voltage;

means for counting the first series of timing pulses emitted by its generating means from the time that the charging of the capacitor beings until the electrical signal is generated;

means for generating an output signal corresponding to the number of timing pulses counted during the time required to charge the capacitor to the first voltage;

means for converting the total count of the first timing pulses to an output signal representative of the magnitude of the capacitor; and

the comparator being connected to the counting means so that the output pulse from the comparator resets the counting means.

2. The apparatus of claim 1 further comprising multiplexing means for alternately connecting an electrode of the capacitor to the means for providing the constant electrical current to the capacitor and to the reference voltage.

3. The apparatus of claim 2 wherein the multiplexing means alternately connects the electrode of the capacitor to the means for providing the constant electrical current to the capacitor or to the reference voltage in response to a second series of timing pulses.

4. The apparatus of claim 3 wherein the second series is a submultiple of the first series.