Capacitance transducing method and apparatus

• US 4,661,768 A
• Filed: 09/11/1985
• Issued: 04/28/1987
• Est. Priority Date: 09/14/1983
• Status: Expired due to Fees

• Current Assignee: Johnson Service Company
• Original Assignee: Johnson Service Company
• Inventors: Carusillo, Steven J.
• Primary Examiner(s): Krawczewicz, Stanley T.
• Application Number:
  US06/774,796
• Time in Patent Office: 594 Days
• Field of Search: 324/60 R, 324/60 C, 324/60 CD, 73/724
• US Class Current: 324/678
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24

Abstract

A signal generating circuit useful in determining the value of a capacitor includes a charging source having a resistive element and a plurality of switches, each switch being positionable between a first, open position and a second, closed position in response to logic signals. The switches are thereby controllable for sequentially connecting a first reference capacitor of known value and a second capacitor to the resistive element for charging said capacitors to a potential. An oscillating comparator network establishes a trigger reference voltage and generates a clocking pulse when the sequentially detected charge potential of the capacitors is equal to the trigger reference voltage. A logic network is connected to the comparator network for generating the logic signals in response to the clocking pulse and the logic network includes a network for resetting the signal generating circuit. Also included is means for determining a first time required to charge the first capacitor from an initially-depleted state to the trigger reference voltage and for determining a second time required to charge the second capacitor from an initially-depleted state to the trigger reference voltage. A method for determining the value of a capacitance is also disclosed.

Claims

1. A signal generating circuit useful in determining the value of a capacitor and including:

a charging source including a resistive element;

a plurality of switches, each switch being positionable between a first, open position and a second, closed position in response to logic signals, said switches thereby being controllable for sequentially connecting a first reference capacitor of known value and a second capacitor to said resistive element for charging said capacitors to a potential;

an oscillating comparator network for establishing a trigger reference voltage and generating a clocking pulse when the sequentially detected charge potential of said capacitors is equal to said trigger reference voltage;

a logic network connected to said comparator network for generating said logic signals in response to said clocking pulse, said logic network including a network for resetting said signal generating circuit, and;

means for determining a first time required to charge said first capacitor from an initially-depleted state to said trigger reference voltage and for determining a second time required to charge said second capacitor from an initially-depleted state to said trigger reference voltage.

2. The invention set forth in claim 1 wherein said resetting network includes a capacitor chargeable to a reset voltage, thereupon grounding a terminal of said comparator network for re-initializing said clocking pulse.

3. An electrical circuit useful in determining the value of a capacitor including:

a first bilateral switch for flowing a charging current through a resistive element to an initially-depleted first capacitor of known value, thereby charging said first capacitor to the potential of a voltage reference over a resulting first time;

a second bilateral switch for flowing a charging current through said resistive element to an initially depleted second capacitor, thereby charging said second capacitor to said potential over a resulting second time;

means for determining said times;

said voltage reference including a reference comparator for establishing a trigger reference voltage and having a first comparator input terminal for sensing a voltage source at a potential and a second comparator input terminal for sequentially sensing the charge potential of said capacitors and means for generating an intermittent clocking pulse of logic "1" state when said charge potential is equal to said trigger reference voltage;

said electrical circuit further including a logic network for generating logic signals in response to said clocking pulse, said bilateral switches being positionably responsive to said logic signals, said logic network having a reset capacitor chargeable to a reset voltage for re-initializing said clocking pulse in the event said pulse becomes set at a logic "1" state.

4. A method for determining the value of a capacitor and including the steps of:

providing a source of charging voltage having a resistive element coupled in series therewith;

connecting an initially-depleted reference capacitor to said resistive element at a first time for charging said reference capacitor to a reference voltage;

generating a first clocking signal at a pulsed logic "1" state when said reference capacitor is charged to said reference voltage;

connecting an initially-depleted second capacitor to said resistive element at a second time for charging said second capacitor to said reference voltage;

generating a second clocking signal at a pulsed logic "1" state when said second capacitor is charged to said reference voltage;

measuring the elapsed times between said first time and said first clocking signal and between said first clocking pulse and said second clocking signal;

computing a duty cycle utilizing said elapsed times;

providing a reset network for detecting when a clocking signal is maintained at a logic "1" state, said reset network thereupon coacting with a logic network for grounding a terminal of an oscillating comparator network, thereby switching said clocking signal to a logic "0" state.