Electricity: measuring and testing – Magnetic – Displacement
Patent
1989-12-04
1991-09-03
Strecker, Gerard R.
Electricity: measuring and testing
Magnetic
Displacement
32420719, 324234, 324654, 324657, G01R 2726, G01B 714
Patent
active
050457861
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a circuit for measuring a variable inductance and in particular the variable inductance of an inductive displacement transducer. Circuits associated with such transducers are disclosed in, for example, DE-A-2924093 and DE-A-3119162.
In existing circuits a number of problems arise. For example low pass output filters are required which introduce unwanted phase shifts. In addition, there are often stability problems.
The present invention seeks to overcome or reduce at least one of the above problems.
According to the present invention there is provided an inductance evaluating circuit comprising an oscillator for generating a signal having a stable frequency, a series connection of a known fixed inductance and a variable inductance, signal-processing means connected to the common connection point of the two inductances to provide an output signal representative of the value of the variable inductance, the oscillator signal is processed into two signals having the same frequency at opposite phase, and being supplied by two feeding paths to the ends of the inductances remote from the common connection point, and one of the feeding paths incorporating a controllable signal amplifying device for controlling the amplitude of the oscillator signal according to output of the signal-processing means.
The above circuit has the following advantages: It provides an output signal with a low residual ripple so that a low frequency filter is not required. There are no problems with stability, the circuit is not sensitive to phase shifts and there are only a few component sub-assemblies which affect the accuracy of the circuit.
When used in connection with an inductive displacement transducer, high-resolution measurement may be achieved. The displacement may be converted by the transducer into an inductance ratio L/Lo which is proportional to displacement and is then converted by the signal-processing circuit into an electric output signal which may take any desired form, e.g. an analogue signal, a binary value or a mark/space ratio.
In a preferred embodiment the signal-processing means comprises an amplification control circuit comprising means for rectifying the signal from the common connection point, and means for integrating the output of the rectifying means. The rectifying means may comprise a switch which is switchable by a synchronising output derived from the oscillator signal to receive an input signal directly or via inverting means. The amplification control circuit may further comprise a delay element between the rectifying means and the integrating means, and the output of the integrating means may be supplied to a sample-and-hold circuit the operation of which is controlled by the synchronising output derived from the oscillator signal or from timing signal generating means which also control the oscillator.
Such a control circuit provides both the output signal U.sub.act of the whole circuit and a control signal V for ensuring that changes in the value of inductance L are quickly and accurately followed and that any errors are quickly corrected.
In a preferred embodiment, between the common connection point and the amplification control circuit there is connected a circuit for matching the signal at the common connection point to a reference d.c. voltage level. The matching circuit is preferably constituted by a differential amplifier circuit. This has the advantage of compensating any offset voltages present.
A voltage divider may be connected between the oscillator and the input of the differential amplifier circuit. This has the advantage of detecting any breaks in a cable or a short-circuiting to ground of the common connection point.
A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, of which:
FIG. 1 is a circuit diagram of a prior art arrangement;
FIG. 2 is a circuit diagram of an arrangement in accordance with the present invention; and
FIG. 3 is a signal diagram indicating the
REFERENCES:
patent: 3491289 (1970-01-01), Petrini
patent: 3684961 (1972-08-01), Muir
patent: 4050011 (1977-09-01), Mori et al.
patent: 4095181 (1978-06-01), Harris et al.
patent: 4667158 (1987-05-01), Redlich
Robert & Bosch GmbH
Strecker Gerard R.
Striker Michael J.
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