CA1195406A

CA1195406A - Proximity switch

Info

Publication number: CA1195406A
Application number: CA000388457A
Authority: CA
Inventors: Geoffrey J. Harris
Original assignee: Square D Co
Current assignee: Schneider Electric USA Inc
Priority date: 1980-10-29
Filing date: 1981-10-21
Publication date: 1985-10-15
Also published as: DE3177316T2; DE3177316D1; MX150894A; EP0395770A3; EP0388477A3; IT1145125B; AU573521B2; EP0063573A1; US4456834A; WO1982001630A1; EP0063573B1; EP0388477B1; AU7648981A; IT8168399A0; DE3177266D1; EP0388477A2; EP0395770A2

Abstract

ABSTRACT
"Proximity Switch" (Fig . 1) The switch is of the type, known per se, in which an induction coil is driven by an oscillator. The coil is initially positioned in an end cap and secured therein by potting with epoxy resin .
This allows the coil and cap to be handled as a unit during manufacture without damage to the coil. The switch is calibrated and adjusted automatically by abrading away part of a resistive layer on a circuit substrate . The circuitry of the switch has its lower potential side coupled to the housing via a capacitor, by means of a conductive adhesive tape trapped between the housing and an end closure.

Description

Briti~h Patent Application of 5QUARE D COMPANY
. .
"Pxoximity Switch"
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 o~ 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, ~nd operate satisf~ctorilyO 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 a~ epoxy resin. The act o fillin~
can move the coil ~rom its mounting, ox otherwise damage the coil.
One object of the present invention is thus to provide a manufacturing method which eliminates or mitigate~ damage to the coil during manufac-ture. To this end one aspect o~ the invention provide~, in a method of manufacturing a proximity switch having an induction coil and an oscillator circuitJ the steps of providing an end cap for receipt of the coil;
positioning the coil in the end cap;
potting the coil in the end cap by at least partly filling the latter with a curable potting compound, thereby forming an easily handled unit, and subsequently connecting the coil to the oscillator circuit and mounting the end cap u.nit 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 sensitivi~y 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 10 . and labour intensive, and the accuracy achieved is likely to vary. The use of a tri~ming potentiometer is also disadvantageous when one wishes to reduce the size of the circuitry by using miniature or micro-miniature components~
Accordingl~, 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 cixcuit components, the improvements comprising forming sa.id resistor as a resistive layer on a substrate; and abrading said layer to adjust said resistox until a desired sensitivity is obtained.
~ 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 inter~ere with the induction coil or other parts of the circuitO 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 c~ble entxy, the switch further including means coupling the lower potential side of the circuit to the housing via a capacitor to suppress propaga~ion o~ noise on the housing, said means being connected to the housing adjacent the closure.
An embodiment o~ the invention will now be described, by way of example, with reference to the accompanying drawings, in which:-10Figure 1 is a cross-sectional side elevati~n of a proximity switch in accordance with the invention, Figure 2 is a plan view,.partl~ in section, of part of the switch of F.igure l;
Figure 3 is an end view ir, the direction III
of Figure 2 of the end cap of the switch;
Figure 4 is.an enlarged plan view of a thick film substrate carrying the cixcuitry of the switch;
Figure 5 diagra~natically illustrates one step in tha manufacture of the switch; and 20Figure 6 is a circuit diagram of the circuitry of Figure 4.
Referring particularly to Figure 1, the switch has a metal cylindrical housing 10 which i~ externally threaded at 12 or mounting to a ~upport, e.g. by nuts (not shown). A moulded plastics end cap 14 closes one end of the housing 10 and mounts an induction coîl 16.
The circuitry of the switch, to be described below, compri.ses micro-mini.ature components not shown in Figures 1 and 2 on a thicX-film printed substrate 18. The substrate 18 is positioned on a p~tics carrier 20 b~ integral moulded studs 22~ and secured by adhesive to integral moulded - pillars 24. The oth r end of the housing 10 is closed by a metal cap 26 which has a central aperture fox the encry o~ a two-wire supply cable 28 with cable sleeve 30.
35The assembly of the switch will now be de~cribed, ~)5~

with particular reference to Figures 1 and 2. The coil 16 is first positioned in the end cap 14 and is secured in position and potted by epoxy resin 32. Suitab]y a low viscosity two-part resin is used, such as 3M ~o~ 280, and is cured at 100 C. This gives a very low initial viscosity, which assists in obtaining penetration by the resin around the coil and in allowing air bubbles to escape. To the same end, ribs or protrusions 34 (best seen in Figure 3) are provided on the inner sur~ace v~
the end cap 14 to hold the coil 16 away from the surface and allow the resin to penetrate.
This procedure has the advantage that, after curing, the coil and end cap form a monolithic unit which can be handled during the remaining assembly operations easily and conveniently, and without risk of damaging or displacing the coil.
Mean~hile, the substrate 1~ has the circuit components connected thereon by conventional techniques, for example reflow soldering, and is then secured to the carrier 20 as described above. The carrier 20 is provided with integr~lly moulded pins 36 which are snapped into cooperating holes in the end cap 14. This provides a mechanical lock between the carrier and the end cap, through a hinged connection which is of col~venience when manipulatin~ the assembl~ and fitting it into jigs and the like. The ends of the coi~ are now soldered to pads P6, P7 and P8 (Figure 4) on the substrate 18. The wires 40,42 o~ the cable ~8 ar~ likewise soldered to pads Pl and P2 (Figure ~.
It is next necessary to adjust the circuitry to match the particular coil and other component tolerances in order to obtain the re~uired sensitivity. This is achieved by trimming a resistor R10 (Figure 6) whil~ the assembly is energised in the vicinity of a standard taxget. In accordance with an important feature o~ the invention, the resistor R10 is provided in the form of a resistive layer on the substrate as seen in Figure 4, and is trimmed by the appaxatus illustrated in Figure 5.
Referring to Figuxe 5, the assembly o~ substrate 18, carrier 20 and end cap 14 is clamped in a jig shown schematically at 46 and positioned on a work urface 48 at a given distance from target 50. The cable 28 is connected to a control unit 52. An airbrasive unit 54 is started and abrades the layer R10, the ~irbrasive unit 54 moving in the direction indicated under the control of the control unit 52, for example by being mounted on a motorised trolley (not shown). When the abrasion has increased the resistance o the layer R10 to the correct value, the proximity switch circuit operates and the re~ulting signal operates the control unit 52 to de-energise the airbrasive unit 54. Thus the trimming o~ the resistor is automatic and sel~-correcting.
The airbrasive unit 54 suitably pro~ides aluminium oxide particles in an air stream from a noæzle 0.005"
~0.127 mm~ by 0~002"(0 051 mm).
After trimming the resistor layer RlO, the substrate 18 and the components th~reon are coated with a resilient layer (omitted in the drawings). The layer is preferably of soft silicon rubbert e.g. Dow Corning 3140, extruded over the substrate in a layer approximately 3 mm thick. This layer ~cts to obviate harmful efects on the microminiature components of str~sses arising from subse~uent potting with and curing of further epoxy resin.
The parts thus far assembled are now united with the housing 10. A suitable adhesive such as "Loctite"
638" is applied to the periphery o the end cap 14, and the assembly is intxoducted into the housing lO until the cap 14 is a push fit in the end of the housing. The switch is then filled with a two-component transparent epoxy resin , and the rnetal cap 26 put in place, the further resin then being cured at 65 C. The cap 26 has a coarse internal screw thxead 56 to provide a mechanical bond with th~
res~n. The further resin is not shown in Figures 1 and

2 for clarity of the other component parts.
Vaxious ~orms of circuitry suitable for use in the switch are known in the artO The circuitry of Figure 6 is shown as one example only and, since it is not essential to the present invention, will not be descri~ed in detail. In essence it is a two-wire d.c.
circuit with all of the components shown in Fiyure 6 being mounted on the substrate 18 except the coil 16 and a light-emitting diode (l.e.d.) D2. Transistors Tl and T2 and associated biasing components form an oscillator driving the coil 16. Transistors T3 and T4 provide a snap-action level detector which, when a metal object loads the coil 16, drives into conduction output transistor T6. A regulated power supply for the oscillator and level detector is provided by ~ener diode ~2 and associated components T5, R7, R8, T7, T8 and R9. To ensure that a constant voltage appears across ~2, it is essential to maintain a constant current through/undex various conclitions of supply voltage and external load current. The current into ~2 is regulated by tha current regulator T7 and T8.
Any additional current is bypassed away from ~2 by T5.
Thus when a metal object is in proximity to the switch, T6 conducts to pexmit current flvw across the wires 40 and 42 to drive a device controlled by the switch either directly or via a relay or the likeO The l.e~d. D2 is illuminated when the swi~ch is "on". As seen in Figures 1 and 2, the lOe.d. D2 is mounted on a post 21 integral with the carrier 20 to be visible externally of the switch via an aperture 11 in the housing 10.
The items Pl-P8 in Figure 6 are connection pads -; 35 on th~ substrate 18.

~ 9 Si~

In accordance with another important feature o~
the invention, the lower potential side of the circuitry is ~onnected to the housing 10 via a capacitor C4 (Figures 4 and 60. It has been found that electrical nois~ on the cable 28 can be induced onto the met~l casing at the point of entry, and propagate along the housing causing spurious ef~ects in the coil 16 or other circuit components.
To eliminate or mitigate this, the capacitor C4 is connected tG the housin~ 10 adjacent the cable entry. The connection is effected, reerring to Figure 2, by providing a wire 58 having one end soldered to the substrate 18; to the other end is attached a tape 60. The tape 60 is a known electrically-conductive tape of copper foil having one side coated with an electrically-conductive pressure-sensitive adhesive.
During assembly o~ the switch, the tape 60 is arranged to protrude through the rear end of the housing 10; it can be held in this position by pressing the adhesive surface against the housing. When the m~tal cap 26 is insexted, the tape 60 is trapped and tightly gripped between the cap 26 and the housing 10, thus establishing an electrical connection between both of these and the capacitor C40 Subsequently the protxudin~ end of the tape 60 is simply torn of to leave the external sur~ace of the switch flush.
The ~act that this connection is at the rear of the switch adjacent the cable entry is significant, since it prevents noise signals being established on the housing via the cable. External noise induced directly onto the metal housing is similarly eliminated or mitigated by effectively putting the housing and circuit at the same potential with respect to noise fre~uPncies~
~ There has thus been described a proximity swltch which significantly improves ease and repeatability of manufacture.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a method of manufacturing a proximity switch having an induction coil and an oscillator circuit, the steps of:
providing an end cap for receipt of the coil;
positioning the coil in the end cap;
potting the coil in the end cap by at least partly filling the latter with a curable potting compound, thereby forming an easily handled unit; and subsequently connecting the coil to the oscillator circuit and mounting the end cap unit on a housing which encloses the oscillator circuit.

2. The method of claim 1, in which the potting compound is an epoxy resin compound which is cured at a high temperature to facilitate outgassing of entrapped air.

3. The method of claim 1 or claim 2, in which the inner end face of the end cap is provided with protrusions to hold the coil clear of said face.

4. The method of claim 1, in which the oscillator circuit and other circuitry is mounted on a substrate, and including the steps of mounting the substrate on a carrier and forming a mechanical lock between the carrier and the end cap.

5. The method of claim 4, in which said mechanical lock also forms a hinged connection.

6. The method of claim 1, including the further step of filling the housing with further potting compound after mounting the end cap unit.

7. A proximity switch including:
a cylindrical housing;
an end cap received in one end of the housing;
an induction coil positioned in the end cap and secured therein by a potting compound; and circuitry, including at least an oscillator circuit, positioned in the housing, the oscillator circuit being connected to the coil.

8. The switch of claim 7, including further potting compound filling the housing.

9. The switch of claim 7 or claim 8, in which the circuitry comprises micro-miniature components mounted on a thick film substrate, the substrate is secured to a plate-like carrier, and the carrier is hinged to the end cap.

10. 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;
the switch further including means coupling the lower potential side of the circuitry to the housing via a capacitor to suppress propagation of noise on the housing, said means being connected to the housing adjacent the closure.

11. The switch of claim 10, in which the circuitry and said capacitor are mounted on a substrate positioned in the housing, and said means comprises an electrically conductive adhesive tape secured to the substrate and trapped between the housing and the closure.

12. 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 improvement comprising:
forming said resistor as a resistive layer on a ?bstrate; and abrading said layer to adjust said resistor until a desired sensitivity is obtained.

13. The method of claim 12, in which said layer is abraded by air-borne abrasive particles blown from a nozzle.

14. The method of claim 13, in which the switch is held at a given distance from a standard metal target and is connected to a control circuit, and the nozzle is moved automatically across the layer under the control of the control circuit until the proximity switch operates.