Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Patent
1992-08-28
1994-05-10
Strecker, Gerard R.
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
324132, 324610, 324673, 307359, 73765, G01R 2726
Patent
active
053111400
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to the technical field of circuits for measuring in the general sense, and adapted to determining variations in the capacitance of a capacitor that is variable.
PRIOR ART
Numerous measurement circuits have already been proposed in the prior art for detecting variations in capacitance, and suitable for delivering an electrical signal representative of the magnitude being measured. Such a signal is used for application to electronic processing systems for the purpose, for example, of automatic monitoring, measuring, regulation, or control.
Such measurement circuits are conventionally constituted by an electrical circuit called a conditioner which is directly associated with the capacitive sensor to deliver the electrical signal representative of variations in the capacitance of the sensor, which variations are related to changes in the physical magnitude to be measured. Such circuits for measuring the capacitance of a capacitor generally also include a linearizing circuit associated with the conditioner for the purpose of correcting the lack of linearity in the sensor or optionally also in the conditioner, in the event that the sensor or the conditioner departs from linearity in its operating range so that sensitivity can not be considered as being constant, given the accuracy required of the measurements. For example, lack of linearity exists when using a capacitive sensor constituted by a cantilevered beam that includes a sensitive element forming a moving plate placed in a distance relationship relative to a stationary plate on a support on which the sensitive element is fixed.
It should initially be observed that the conditioners at present in use have components that prevent them from being integrated and they are suitable for operating over a very limited temperature range only.
A first type of linearization circuit makes use of digital linearization techniques requiring the use of an analog-to-digital converter, a memory for attributing a stored value to each value of the electrical signal which is used for addressing the memory, and a digital-to-analog converter. Although quick and simple to implement, such linearization circuits are not cheap since accuracy is directly related to memory capacity.
A second type of linearization circuit makes use of analog techniques with diodes. The use of such circuits is limited in practice to linearizing an electrical signal for which the error curve slopes in a single and predetermined direction.
The present invention therefore seeks to remedy the above-mentioned drawbacks by providing a circuit for measuring variations in the capacitance of a capacitor, that presents good lack of sensitivity to stray capacitance and to electrical disturbances, and that is suitable for miniaturization as an integrated circuit.
The invention also seeks to provide a measurement circuit suitable for correcting the lack of linearity in an electrical signal for which the error slopes in various directions.
The invention also seeks to provide a measurement circuit suitable for correcting the lack of linearity that occurs between variations in the capacitance of the sensor and the electrical signal delivered, and that is relatively cheap in cost.
SUMMARY OF THE INVENTION
To achieve the above-specified objects, the present invention provides a circuit for measuring variations in the capacitance of a variable capacitor constituting, in particular, a sensor, the circuit comprising:
an oscillator generating an alternating signal;
a detection bridge comprising first and second branches connected to the oscillator, the first branch including the variable capacitor connected in series with a variable capacitance diode, while the second branch comprises a capacitor connected in series with a variable capacitance diode;
a differential stage having its inputs connected to respective ones of the branches at their common points between their capacitors and their variable capacitance diodes;
a DC bias circuit for biasing the common point of
REFERENCES:
patent: 4241303 (1980-12-01), Thompson
patent: 4673869 (1987-06-01), Michael
patent: 4684886 (1987-08-01), Doyle
Do Diep
Societe d'Applications Generales d'Electricite et de Mecanique S
Strecker Gerard R.
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