Proximity switch

Abstract: The switch is of the type, know per se , in which an inductioncoil is driven by an oscillator. The oscillator is monitored by a level detector whose sensitivity is set by a resistance (R10) in the form of a resistive layer on a substrate (18) (Figure 4). The switch is calibrated and adjusted automatically by abrading away part of the resistive layer (Figure 5).

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 a 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-minature components. GB-A-2 011 086 discloses a proximity switch in which the sensitivity is adjusted using a variable resistor. US-A-3 829 962 discloses a system for automatically calibrating the frequency of an oscillator by abrading a resistive layer on a substrate. Accordingly, the invention provides, in a method of making a proximity switch including an induction coil, an oscillator driving the coil, a detector circuit and a resistor which is adjusted during manufacture to compensate for tolerance in the coil and other circuit components, the improvements comprising: forming said resistor as a resistive layer on a substrate; abrading said layer to adjust said resistor until a desired sensitivity is obtained , said layer being abraded by air-borne abrasive particles blown from a nozzle; and holding the switch at a given distance from a standard metal target and connecting it to a control circuit, with the nozzle being moved automatically across the layer under the control of the control circuit until the proximity switch operates. An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a cross-sectional side elevation of a proximity switch; Figure 2 is a plan view, partly in section, of part of the switch of Figure 1; Figure 3 is an end view in the direction III of Figure 2 of the end cap of the switch; Figure 4 is an enlarged plan view of thick film substrate carrying the circuitry of the switch; Figure 5 diagrammatically illustrates one step in the manufacture of the switch in accordance with the invention; and Figure 6 is a circuit diagram of the circuitry of Figure 4. Referring in particular to Figure 1, the switch has a metal cylindrical housing 10 which is externally threaded at 12 for mounting to a support, eg 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 microminiature components not shown in figures 1 and 2 on a thick-film printed substrate 18. The substrate 18 is positioned on a plastics carrier 20 by integral moulded studs 22, and secured by adhesive to integral moulded pillars 24. The other end of the housing 10 is closed by a metal cap 26 which has a central aperture for the entry of a two-wire supply cable 28 with cable sleeve 30. The assembly of the switch is described in more detail in our copending European Application 81902798.8 (publication No. WO 82/01630). Briefly, the coil 16 is first positioned in the end cap 14 and is secured in position and potted by epoxy resin 32. Ribs or protrusions 34 (best seen in Figure 3) are provided on the inner surface of the end cap 14 to hold the coil 16 away from the surface and allow the resin to penetrate. Meanwhile, the substrate 18 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 integrally 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 convenience when manipulating the assembly and fitting it into jigs and the like. The ends of the coil are now soldered to pads P6, P7 and P8 (Figure 4) on the substrate 18. The wires 40, 42, of the cable 28 are likewise soldered to pads P1 and P2 (Figure 4). It is next necessary to adjust the circuitry to match the particular coil and other component tolerances in order to obtain the required sensitivity. This is achieved by trimming a resistor R10 (Figure 6) while the assembly is energised in the vicinity of a standard target. In accordance with 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 apparatus illustrated in Figure 5. Referring to Figure 5, the assembly of substrate 18, carrier 20 and end cap 14 is clamped in a jig shown schematically at 46 and positioned on a work surface 48 at a given distance from target 50. The cable 28 is connected to a control unit 52. An airbrasive 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 of the layer R10 to the correct value, the proximity switch circuit operates and the resulting signal operates the control unit 52 to de-energise the airbrasive unit 54. Thus the trimming of the resistor is automatic and self-correcting. The airbrasive unit 54 suitably provides aluminium oxide particles in an air stream from a nozzle 0.005" (0.127 mm) by 0.002" (0.051 mm). After trimming the resistor layer R10, the substrate 18 and the components thereon are coated with a resilient layer (omitted in the drawings). The layer is preferably of soft silicon rubber eg Dow Corning 3140, extruded over the substrate in a layer approximately 3 mm thick. This layer acts to obviate harmful effects on the microminiature components of stresses arising from subsequent 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 of the end cap 14, and the assembly is introducted into the housing 10 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 metal cap 26 put in place, the further resin then being cured at 65°C. The cap has a coarse internal screw thread 56 to provide a mechanical bond with the resin. The further resin is not shown in figures 1 and 2 for clarity of the other component parts. Various forms of circuitry suitable for use in the switch are known in the art. The circuitry of Figure 6 is shown as one example only and, since it is not essential to the present invention, will not be described in detail. In essence it is a two-wire d.c. circuit with all of the components shown in Figure 6 being mounted on the substrate 18 except the coil 16 and a light-emitting diode (1.e.d.) D2. Transistors T1 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 Zener diode Z2 and associated components T5, R7, R8, T7, T8 and R9. To ensure that a constant voltage appears across Z2, it is essential to maintain a constant current through Z2 under various conditions of supply voltage and external load current. The current into Z2 is regulated by the current regulator T7 and T8. Any additional current is by-passed away from Z2 by T5. Thus when a metal object is in proximity to the switch, T6 conducts to permit current flow across the wires 40 and 42 to drive a device controlled by the switch either directly or via a relay or the like. The 1.e.d. D2 is illuminated when the switch is "on". As seen in Figures 1 and 2, the 1.e.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 P1-P8 in Figure 6 are connection pads on the substrate 18. The lower potential side of the circuitry is connected to the housing 10 via a capacitor C4, wire 58 and tape 60 (Figures 4 and 6), as more fully described in our European Divisional Application No. EP-A-0395 770 filed concurrently herewith.

Claims: A method of making a proximity switch including an induction coil (16), an oscillator (T1, T2) driving the coil, a detector circuit (T3, T4) and a resistor (R10) which is adjusted during manufacture to compensate for tolerances in the coil and other circuit components; characterised by the combination of: forming said resister (R10) as a resistive layer on a substrate (18); abrading said layer to adjust said resistor until a desired sensitivity is obtained, said layer being abraded by air-borne abrasive particles blown from a nozzle; and holding the switch at a given distance from a standard metal target (50) and connecting it to a control circuit (52), with the nozzle being moved automatically across the layer under the control of the control circuit until the proximity switch operates.

Inventors: HARRIS GEOFFREY JOHN, HARRIS, GEOFFREY JOHN
Applicants: SQUARE D CO, SQUARE D COMPANY
Application year: 1981
Main classification: H03K17
IPC codes: H05K13, B29C70, H03K17