Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Patent
1997-05-30
1999-09-28
Regan, Maura
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
324671, G01R 2726
Patent
active
059594560
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to transducers, and in particular to a capacitance transducer for providing distance or position information about a target relative to the transducer.
1. Discussion of Prior Art
Typically, a capacitance position transducer is used to provide information about the position (or proximity) of an object (or target) relative to the transducer. A conventional transducer includes a single electrode plate for forming a capacitive coupling with the target. The amount of capacitive coupling is inversely proportional to the distance between the electrode plate and the target. The capacitance, and hence the distance, can be detected by means of an oscillator whose frequency is dependent on the capacitance value.
A typical oscillator circuit 1 is shown in FIG. 1 of the accompanying drawings. The electrode plate 3 is coupled to the inverting input of an operational amplifier 5. Negative feedback is provided via a first resistor 7. The positive input of the operational amplifier 5 is fed by a potential divider formed by two resistors 9,11 coupled in series across the output of the amplifier 5.
The transducer operates because the target (not shown) is substantially earthed, or is coupled relative to earth. The transducer effectively responds to the relatively small capacitance between the electrode plate 3 and the target, in series with the capacitance of the target to earth.
With reference to FIG. 2, a problem can arise if more than one such transducer 1 is used in conjunction with a single target 13, for example, to measure the distances between the transducers 1 and the target 13 around the target 13. The problem is caused by cross talk between the individual transducer capacitor signals. Further, if C.sub.T represents a typical value of each transducer capacitance, and C.sub.E represents the target to earth capacitance, then for less than 1% cross talk the value of C.sub.E must be at least one hundred times the value of C.sub.T ; even this factor may not be sufficient in practice.
SUMMARY OF THE INVENTION
The present invention has been devised to overcome problems associated with the use of more than one capacitance transducer with a single target.
Accordingly, the present invention provides a transducer for providing distance or position information about a target relative to the transducer, the transducer comprising a first resistor and a first electrode for forming a first capacitive coupling with a target, a second resistor and a second electrode for forming a second capacitive coupling with the target, means for applying signals of opposite phase or of opposite polarity to the respective resistor and electrode combinations and processing means for processing the voltages appearing across the electrodes to produce an output indicative of the distance or position of the target relative to the transducer.
When applying signals of opposite phase or of opposite polarity, no net signal is applied to the target. Problems associated with cross talk are, therefore, avoided, which is a marked improvement over the prior art transducer mentioned above. Further, a secondary advantage is provided in that common mode rejection occurs causing the circuit to ignore any signal on the target.
Preferably the processing means combine the voltages appearing on the two electrodes to produce an output indicative of the distance or position of the target from the transducer. Other ways of processing the voltages appearing across the electrodes may, of course, alternatively be used.
The means for applying signals preferably comprise an oscillator for producing equal and opposite waveforms to be applied to the respective first and second resistor and electrode combinations.
Preferably a head of the transducer carries the first and second electrodes in a first plane and a tail of the transducer substantially perpendicular to the first plane is adapted to connect the transducer to a power source.
The processing means may be mounted in the head of the transducer
REFERENCES:
patent: 3924177 (1975-12-01), Lemineur et al.
patent: 5140274 (1992-08-01), Wertz
patent: 5394969 (1995-03-01), Harbaugh
patent: 5602486 (1997-02-01), Novak
Jayawant Bhalchandra Vinayak
Whorlow Raymond John
British Nuclear Fuels PLC
Regan Maura
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