Proximity switch with continuously operable test circuit respons

Details Proximity switch with continuously operable test circuit respons Proximity switch with continuously operable test circuit respons

1985-03-27

1987-03-24

Strecker, Gerard R.

Electricity: measuring and testing

Magnetic

Displacement

307116, 324202, 324236, 331 65, 340514, H03K 17945, H01H 3500, G01B 714, G08B 2900

Patent

active

046528199

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The invention relates to a self-testing proximity switch having an oscillator; a demodulator; a trigger stage; a clocked test pulse source feeding the oscillator with test pulses having positive and negative amplitudes sufficiently high to trigger the trigger stage from either one of its switching states to the other for the duration of a single pulse in order to simulate a damping reduction or damping of the oscillator; and a test circuit responding to the level changes in the output signal of the trigger stage occurring in response to the test pulses, and serving to thereby generate a test output signal.
2. Description of the Related Art
A self-testing proximity switch of the kind having a selectively damped oscillator has been described in European laid-open patent application No. 0 035 225. The proper functioning of this proximity switch is tested by determining whether the oscillator is damped by a test pulse or whether the oscillation amplitude of the oscillator is reduced to zero by the test pulse, to simulate the approach of an element to be detected toward the active surface of the proximity switch.
This type of functional test fails to detect faults which result in the output signal of the proximity switch indicating a damped state of the oscillator even though the oscillator is not actually damped by an element brought sufficiently close to the active surface of the proximity switch. This disadvantage is eliminated in the proximity switch disclosed in German laid-open patent application No. 3 007 929 and further described in Hermle et al. U.S. Pat. No. 4,433,309, wherein oscillator damping reduction is also used for test purposes. In these proximity switches, the internal operating parameters of the oscillator are influenced to cause a change in the oscillation of the oscillator which corresponds to a certain change in position of an element, for example, a trip cam, that influences the resonant circuit externally. In other words, an external damping or damping reduction is simulated by internally influencing the internal operating parameters. These kind of proximity switches use so-called breakaway oscillators, wherein the oscillation breaks away (i.e., terminates) in the event of external or internal damping.


SUMMARY OF THE INVENTION

In view of the above, the primary object of the invention is to enable functional testing of a proximity switch without substantially influencing the internal operating parameters of the oscillator, regardless of the oscillating state of the oscillator, i.e., both in the damped and damping reduced state of the oscillator.
Briefly, in accordance with the invention, a self-testing proximity switch as recited above in the field of the invention is provided wherein the oscillator operates in an analog manner such that the amplitude of oscillation is constantly variable in response to damping, and in which the test pulses of the test pulse source are fed in the form of bipolar double pulses to a terminal of the oscillator at which the supply voltage and/or the supply current to the oscillator are combined with the pulses. In other words, the supply current and/or voltage to the oscillator is moduated by the test pulses, which in turn modulates the amplitude of oscillation in an analog fashion.
The invention is based on the general thought that with the aid of test pulses in the form of bipolar double pulses at a supply voltage terminal of the oscillator (either with the positive single pulse or with the negative single pulse of the double pulse depending on the oscillation state of the oscillator and the corresponding switching state of the trigger stage), it is possible to bring about a significant change in the amplitude of the oscillation of the oscillator, thus effecting, in any case, a signal change at the output of the trigger stage. These signal changes at the output of the trigger stage are then evaluated with the aid of a test circuit which, consequently, only generates an output signal indicating prope

REFERENCES:
patent: 3514627 (1970-05-01), Bridgeman
patent: 4433309 (1984-02-01), Hermle et al.
patent: 4502042 (1985-02-01), Wuhrl et al.
patent: 4525699 (1985-06-01), Buck et al.

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