Electrical transmission or interconnection systems – Anti-induction or coupling to other systems – Magnetic or electrostatic field control
Patent
1982-06-25
1984-06-26
Goldberg, E. A.
Electrical transmission or interconnection systems
Anti-induction or coupling to other systems
Magnetic or electrostatic field control
307116, 174 35MS, H01H 4702, H01H 3500
Patent
active
044568347
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
This invention relates to proximity switches, and in particular to proximity switches including an oscillator and an induction coil and which operate by virtue of a metallic object in proximity to the switch causing the oscillation of the switch circuitry to alter.
Switches of this general type are well known in the art, and operate satisfactorily. However there are problems in manufacturing these switches economically in a manner which gives a good degree of uniformity. The induction coil used is rather sensitive, both electrically and mechanically. To obtain a high sensitivity it is necessary to use a coil wound from fine wire, which is susceptible to damage during assembly of the switch.
It is also conventional to fill the switch with a potting compound such as epoxy resin. The act of filling can move the coil from its mounting, or otherwise damage the coil.
One object of the present invention is thus to provide a manufacturing method which eliminates or mitigates damage to the coil during manufacture. To this end one aspect of the invention provides, in a method of manufacturing a proximity switch having an induction coil and an oscillator circuit, the steps of: a curable potting compound, thereby forming an easily handled unit; and end cap unit on a housing which encloses the oscillator circuit.
A further problem resides in the fact that variations in the coils and other circuit components are inevitable; to give switches with a uniform sensitivity it is therefore necessary to adjust the detector circuit during manufacture. This has previously been done by providing a trimming potentiometer in the circuit, and adjusting this by hand. Such adjustment is time consuming and labour intensive, and the accuracy achieved is likely to vary. The use of a trimming potentiometer is also disadvantageous when one wishes to reduce the size of the circuitry by using miniature or micro-miniature components.
Accordingly, the invention also provides, in a method of making a proximity switch including an induction coil, an oscillator driving the coil and a detector circuit, the detector circuit having a resistor which is adjusted during manufacture to compensate for tolerances in the coil and other circuit components, the improvements comprising: obtained.
A further problem encountered with known proximity switches is that electrical noise on the supply cable can be induced on the metallic housing of the switch and propagate along it to interfere with the induction coil or other parts of the circuit. To prevent or mitigate this, a further aspect of the invention provides a proximity switch including a metallic housing; an end cap at one end of the housing; an induction coil positioned within the end cap; circuitry, including at least an oscillator circuit connected to the coil, positioned within the housing; and a metallic closure for the other end of the housing, said closure including a cable entry; circuit to the housing via a capacitor to suppress propagation of noise on the housing, said means being connected to the housing adjacent the closure.
An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:
FIG. 1 is a cross-sectional side elevation of a proximity switch in accordance with the invention;
FIG. 2 is a plan view, partly in section, of part of the switch of FIG. 1;
FIG. 3 is an end view in the direction III of FIG. 2 of the end cap of the switch;
FIG. 4 is an enlarged plan view of a thick film substrate carrying the circuitry of the switch;
FIG. 5 diagrammatically illustrates one step in the manufacture of the switch; and
FIG. 6 is a circuit diagram of the circuitry of FIG. 4.
Referring particularly to FIG. 1, the switch has a metal cylindrical housing 10 which is externally threaded at 12 for mounting to a support, e.g. by nuts (not shown). A moulded plastics end cap 14 closes one end of the housing 10 and mounts an induction coil 16. The circuitry of the switch, to be described below, comprises micro-miniature components no
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patent: 4378504 (1983-03-01), Yamasaki
Goldberg E. A.
Guttman Richard T.
Jennings Derek
Schmeling William H.
Square D Company
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