Distance measurement with capacitive sensor
AssigneeDigital Imaging Systems GmbH
InventorsHorst Knoedgen, Frank Kronmueller
Life of this patent
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Abstract
Circuits and methods to read out capacitive sensors for distance measurement used by a position control system having a high accuracy and low noise have been disclosed. The ratio or difference of the capacitances of two sensor capacitors is used to determine the distance of an object from a target position. A sense amplifier is using auto-zero methods to achieve a long term stability. A sample-and-hold circuit using double correlated sampling methods minimizes noise. Low cost capacitors can be used with the sample-and-hold circuit because not the absolute value of capacitances but only the ratio of capacitances are relevant. A high resolution is ensured also by significant over-sampling of the control loop.
Claims19
A method to read out capacitive sensors for distance measurement used by a position control system having a high accuracy and low noise comprising the following steps:
- (1) providing two sensor capacitors, wherein at least one plate of said sensor capacitor is fastened on an object whose position is to be controlled, a sense amplifier stage, a digital controller controlling a motor, a digital-to-analog converter and a comparator;
- (2) setting a reference voltage dependent upon a target position to be reached;
- (3) measuring sequentially voltages across said two sensor capacitors using said reference voltage as input voltage on a first terminal of each sensor capacitor;
- (4) checking if both voltages are equal and, if positive go to step (7), otherwise go to step (5);
- (5) moving said object in a direction according to difference of the voltages measured across both sensor capacitors;
- (6) measuring sequentially voltages across said two sensor capacitors using last voltage measured across each sensor capacitor as input voltage on a first terminal of each correspondent sensor capacitor and go to step (4);
- and
- (7) target position is reached.
A system to read out capacitive sensors for distance measurement used by a position control system having a high accuracy and low noise comprising:
- a digital controller, having inputs and outputs, wherein first inputs are signals from a comparator, a second input is a reference voltage defining a target position of an object, and a first output are control signals for a motor moving said object, a second output are two digital signals defining an actual position of said object, and a third output are signals to control the readout process chain;
- said motor;
- a digital-to-analog converter converting each one of said two digital output signals of said digital controller to an analog voltage;
- a multiplexing means modulating each of said analog voltage output of said digital-to-analog converters having two outputs, wherein alternately a first output is switched to a first plate of a first sensor capacitor and a second output is switched to a first plate of a second sensor capacitor;
- said two sensor capacitors having each one of their plates fastened on said object and one of their plates fastened on a fixed position;
- a sense amplifier stage having two inputs and an output, wherein a first input is connected to second plates of said sense capacitors and a second input is connected to a reference voltage and its output is connected to a sample-and-hold circuit;
- said sample-and hold-circuit having two branches, wherein each branch comprises capacitors and wherein a first branch is holding a voltage measured across said first sensor capacitor and a second branch is holding a voltage measured across said second sensor capacitor and wherein an output of said first branch is connected to an input of a first branch of a low pass filter and an output of said second branch is connected to a second branch of a low pass filter;
- said low-pass filter having two branches, wherein the output of the first branch is connected to a first input of a differential amplifier and the output of a second branch is connected to a second input of said differential amplifier;
- said differential amplifier having differential outputs wherein both differential outputs are the inputs of the comparator;
- and
- said comparator, comparing said differential inputs, having a digital output connected to said digital controller.
Dependencies are inferred from the claim text.