Linearized capacitance sensor system
First Claim
1. A capacitive sensing circuit comprisinga first and a second capacitor with one end of each capacitor connected to a junction, at least said second capacitor being a sensing capacitor having a capacitance which varies in accordance with physically-induced change in plate position and which has a non-linear capacitance characteristic,first switching means for applying signals to non-common ends of said first and second capacitors in alternate switching cycles for developing a flow of charge through said capacitors such that the voltage present at the junction varies in functional dependence on the capacitance values of said first and second capacitors,a third capacitor having one end coupled to said junction,second switching means for coupling a signal to the non-common end of said third capacitor to develop a charge effective to substantially null the charge variation due to said non-linear capacitance characteristic of the sensing capacitor, andfeedback means responsive to the voltage at said junction for providing a feedback signal to said first switching means to charge at least said second capacitor and maintain stationary the voltage present at the junction.
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Abstract
A capacitive sensing circuit and system includes a variable capacitance sensing element as an active element in a circuit to measure changes in capacitance of the sensing element. A switching network in alternate cycles energizes a capacitor to develop a charge which effectively linearizes the sensor'"'"'s capacitance response. In a push-pull circuit with two active sensing elements, a multi-channel switching network energizes two fixed correction capacitors to provide third or higher order correction, significantly extending the effective scale of the sensor. In a preferred embodiment, a single feedback signal is applied to all capacitors in different switching cycles. Examples include weight and pressure sensing systems.
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Citations
12 Claims
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1. A capacitive sensing circuit comprising
a first and a second capacitor with one end of each capacitor connected to a junction, at least said second capacitor being a sensing capacitor having a capacitance which varies in accordance with physically-induced change in plate position and which has a non-linear capacitance characteristic, first switching means for applying signals to non-common ends of said first and second capacitors in alternate switching cycles for developing a flow of charge through said capacitors such that the voltage present at the junction varies in functional dependence on the capacitance values of said first and second capacitors, a third capacitor having one end coupled to said junction, second switching means for coupling a signal to the non-common end of said third capacitor to develop a charge effective to substantially null the charge variation due to said non-linear capacitance characteristic of the sensing capacitor, and feedback means responsive to the voltage at said junction for providing a feedback signal to said first switching means to charge at least said second capacitor and maintain stationary the voltage present at the junction.
Specification