Capacitance measuring circuit for a capacitive sensor

Details Capacitance measuring circuit for a capacitive sensor Capacitance measuring circuit for a capacitive sensor

1998-12-28

2001-10-23

Metjahic, Safet (Department: 2858)

Electricity: measuring and testing

Impedance, admittance or other quantities representative of...

Lumped type parameters

C324S629000

Reexamination Certificate

active

06307385

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a measuring circuit for a capacitive sensor, and more particularly to a capacitance measuring circuit producing a signal having a period that varies as a function of the charge and discharge times of the capacitive sensor. The circuit can be advantageously used in combination with a capacitive sensor installed on a rotating shaft, to measure the vibration or the circularity of the shaft.
BACKGROUND
Capacitive sensors and probes are used in a great number of applications, for example to monitor a fluid level in a tank, to measure the distance between two moving parts of an object, to measure the vibration or wear of a rotating shaft, etc.
Known in the art are U.S. Pat. Nos. 4,261,397 (Guy)—4,311,959 (Riessland et al.)—4,347,741 (Geiger)—4,661,797 (Schmall)—4,675,670 (Lalonde et al.)—4,677,275 (Schmall)—4,682,272 (Furlong et al.)—4,766,368 (Cox)—4,912,662 (Butler et al.)—5,012,196 (Baranski)—5,014,011 (Colvin)—5,153,525 (Hoekman et al.)—5,166,679 (Vranish et al.)—5,235,217 (Kirton)—5,237,284 (Van Der Valk)—5,326,983 (Hejazi)—5,399,979 (Henderson et al.)—5,410,297 (Joseph et al.)—5,583,443 (McMurtry et al.)—5,610,528 (Neely et al.), showing examples of various types and configurations of capacitive sensors used in a multitude of applications.
Usually, in the case of a reading of a rotating shaft with a capacitive sensor, inductive technology involving EDDY currents is used. The drawback of this technique is that the user must calibrate or interpret the measurements of the sensor according to the type of material forming the shaft. Furthermore, a density variation of the material on the circumference of the shaft causes a variation of the measurement which is difficult to discriminate from the real circularity or vibrations to be measured. Other kind of applications involving the use of capacitive sensors are also subjected to the above drawback. Furthermore, the distance between the capacitive sensor and the reading/measuring apparatus usually causes some problems due to the impedance of the link between them.
SUMMARY
An object of the invention is to provide a circuit and a method that are insensitive to the type or the density variation of the conductive material involved in the process.
A subsidiary object of the invention is to provide such a circuit that is simple in construction, thus highly cost-competitive, yet is highly reliable and efficient.
According to the present invention, there is provided a circuit for measuring a capacitance of a capacitive sensor, comprising charging circuit means for charging the capacitive sensor with a voltage, discharging circuit means for discharging the capacitive sensor, trigger means for triggering the charging circuit means and the discharging circuit means in alternation into play as a potential of the capacitive sensor drops and rises to predetermined lower and upper levels respectively, and output means for providing a signal having a low level during operation of one of the charging and discharging circuit means and a high level during operation of the other one of the charging and discharging circuit means, whereby the signal has a period defined by a same level repetition which is indicative of the capacitance of the capacitive sensor.
According to the present invention, there is also provided a method for measuring a capacitance of a capacitive sensor, comprising the steps of charging the capacitive sensor with a voltage and discharging the capacitive sensor in alternation as a potential of the capacitive sensor drops and rises to predetermined lower and upper levels respectively, and providing a signal having a low level during one of the charging and discharging steps and a high level during the other one of the charging and discharging steps; whereby the signal has a period defined by a same level repetition which is indicative of the capacitance of the capacitive sensor.
The measuring circuit according to the invention can be compared in some ways to an oscillator circuit generating a signal having a period that varies as a function of the charge and discharge times of the capacitive sensor between two predetermined voltage thresholds. The charging and discharging of the capacitive sensor is preferably carried out through a set of resistors connected to the oscillator circuit having a first output controlling the charge and the discharge of the capacitive sensor, and a second output generating the measurement signal. The plates of the capacitive sensor are connected to a follower circuit, whose output is connected to the oscillator circuit. The second output of the oscillator circuit can be connected to a processing circuit to make the measurement linear.


REFERENCES:
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Microelectronic Circuits of Sedra/Smith, the fourth edition pp. 1009-1011.*
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