GB2149962A - Contactless switch - Google Patents

Abstract

A coded switch uses as its transducer elements thin silicon monocrystal strips 1, each bearing a piezo-resistive strain gauge whose resistance depends on its flexure condition. Each such strip has a small permanent magnet 2 on its free end, the magnet being, for instance, plastics loaded with a samarium-cobalt magnetically hard material. The switch is controlled by a rotatable disc 4 whose edge is recorded with a magnetic pattern. Thus as the disc 4 is rotated the silicon strip flexes towards and away from the disc, thus producing variations in the resistance of the strain gauage. These variations are detected by an amplifier chip. Several such transducer arrangements can be provided on the same shaft. <IMAGE>

Description

SPECIFICATION Contactless switch The present invention relates to contactless electrical switches of the type in which the switching function is performed by a solid state semiconductor element.

A switch of the above type is described and claimed in our Application No. 8312776 (J.C.

Greenwood et al 54-53-1), and a switch according to that application is shown in the accompanying Fig. 1. The switch assembly is between front and back plates 11 and 12, and the front plate has a boss 13, in which an operating spindle 1 4 is located. This spindle carries a detent disc 1 5 and a code disc 16, which discs have axial openings 1 7 and 1 8 which receive the spindle 1 4. Relative rotation between the discs and spindle is prevented by flats 1 9 on the spindle, which engage registration flats in the di'sc openings.

The detent disc 1 5 is in an opening in a detent plate 20 and engages inwardly directed detent pawls such as 21 on the detent plate 20 to give the necessary detent action.

The code disc 1 6 has one or more rows of circumferential projections 22, which engage resilient cam followers 23 on a support body 24. These cam followers 23 transmit motion from the code disc to piezo-resistive transducer elements 25 on a transducer block 26.

Alignment of the assembly is facilitated by pins 27 extending from the back plate 1 2 via openings such as 28, and on which pins the static components are located.

In the assembled switch rotation of the code disc 1 6 via the shaft 1 4 causes corresponding deflection and release of the cam followers 23 by the projections 22 on the code disc. This in turn flexes the transducer element 25. Each of the elements 25 has diffused into its surface a bridge of a piezoresistive element, and the resistance changes in these bridges due to the flexing of the transducer elements are sensed by a chip 29 coupled to the elements 25 and the sensed signals are fed to logic decoding circuitry (not shown) which translates the switch output signals into corresponding current paths.

Although two code discs and associated transducer element are shown it will be appreciated that any number of discs and transducers may be mounted within the switch assembly to provide a ganged switching arrangement.

An object of the invention is to produce an improved switch of the same general type as that shown in Fig. 1.

According to the invention, there is provided a contactless electrical switch, which includes a transducer element movable between two different positions, which element carries a magnet and a piezo-resistive strain gauge the electrical resistance of which depends on the position of the transducer element, and control means magnetised in accordance with a preset pattern, wherein movement of the control means causes a sequence of movements of the transducer element, and hence a sequence of changes in the electrical resistance of the strain gauge.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which: Figure 2 is an end-on schematic view illustrating the principle of operation of one code disc and switching means embodying the invention.

Figure 2A is a preferred arrangement for the switching member of the device of Fig. 1.

Figure 3 is a plan view of a switch of the type shown in Fig. 2.

Figures 4 and 5 show two cantilever systems usable in a switch embodying the invention on an enlarged scale, Fig. 4 being the horizontal system of Fig. 1, shown for comparison with a vertical system of Fig. 5.

Figure 6 shows magnetic states and codes used in a three-track switch.

The major part of the structure of the present switch is similar to that of the switch shown in Fig. 1. Hence Figs. 2, 3 4 and 5 are confined to the features which differ from those of the switch as described herein. From the operating point of view, the switch of Fig.

2 can be regarded as being a magnetic equivalent of the switch of Fig. 1.

As in the switch of Fig. 1, there can be one or more silicon (or other suitable material) transducer elements, and Fig. 2 illustrates one such element and its associated parts. The monocrystalline thin silicon cantilever transducer 1 carries a small permanent magnet 2 at the free end of the cantilever, the other end thereof being secured, as shown. As a high efficiency magnet of low mass is preferred, samarium cobalt is used. This takes the form of a discrete magnet about half a millimetre square by about 0.15 mm thick.

In the versions described and which have been tested, a plastics material loaded with samarium cobalt has been used successfully.

The transducer 1-2 is mounted as shown between two end stops 3, which protect the transducer and control the maximum deflection. The transducer is mounted in close proximity to a magnetically coded rotor 4, so that when poles are aligned on the rotor and transducer, the cantilever deflects and an output change occurs at a strain gauge bridge diffused into the silicon surface near the root of the cantilever. This bridge is connected via metal (e.g. aluminium) tracks on the silicon transducer element to wire bonds 5. These connect to a printed circuit board which carries circuitry (not shown) for detecting and responding to the changes in the resistance of the strain gauge. Unlike the switch of Applicar tion No. 8312776, the present arrangement does not need an amplifier since the voltages generated are longer. This produces the switch output.

The magnetised rotor 4 is a plastics disc or drum which has either bonded or moulded on its surface a layer of plastics loaded with magnetically hard ferrite, or such alternative materials as samarium cobalt alloy. Another alternative is to use a drum coated with magnetic alloy. It is recorded at the manufacturing stage with the appropriate codings.

In some cases it has been found necessary to overcome mechanical resonance in the transducer element. This is done in the way shown in Fig. 2A, where the transducer 1 moves between stops 7 and 8, which stops also function as damping elements by suitable choice of material. It will be seen that the stop between the magnet 2 and the rotor is made as thin as possible so as not to undly increase the reluctance of the air gap.

As will be seen from Fig. 3, for the device shown, four tracks are recorded and actuate four cantilever transducers so as to produce a four-bit parallel output code for each position of the switch.

Referring to Fig. 3, the printed circuit board 6 has two outside tracks connected respectively to two strips 9 from which the current supply for the strain gauge bridges is derived.

Between these strips will be seen the four pairs of tracks, one pair for each of the transducers.

In Fig. 4 we see the transducer 1 in two positions, one shown in dashed lines, between the two stops 3, and the recorded surface 4 of the coded disk. This does not show the improved end stop damping arrangement of Fig. 2A.

Fig. 5 shows the vertical canilever system, in which the cantilever transducer 10 is vertically-mounted with respect to the coded disk 11.

In Figs. 4 and 5 the associated recordings on the coded disk are shown for the binary code 101010, and will produce deflections in the transducer, and thus an output which is + - + - + -, and so on. The output voltage could be all of one polarity but of different amplitudes, changing amplitude in either a positive or negative sense about the zero deflection voltage level.

Fig. 6 shows the recorded magnetic codes on the code disk or drum for three cantilever transducers corresponding to three binary levels of a simple binary code 202521. The three upper waveforms represent cantilever transducer deflection and also cantilever strain gauge output voltage. The detent positions are drawn for a horizontal cantilever system and would need to be displaced half a pitch in order to represent the vertical cantilever case.

Track 1 of the magnetic code in the lower half of Fig. 6 corresponds to the situation shown on Fig. 1. On track 2 it can be seen that a half pitch pole reversal is required to ensure the correct polarity of the output of cantilever 2. A reversal is necessary for every 2 detent positions. In order to correct track 3 we need 3 intermediate reversals for every 4 detent positions and this correction process must be extended into the higher order tracks for more complex codes.

The advantages of this arrangement described herein are: (a) Minimum number of parts.

(b) Magnets allow large clearances to be used, hence design may have wide tolerances.

(c) Coding is very simple, as any code may be provided by a very simple recording process.

(d) The direct output voltage is large compared with the direct output voltage of Hall effect sensors.

Claims (4)

1. A contactless electrical switch, which includes a transducer element movable between two different positions, which element carries a magnet, and a piezo-resistive strain gauge the electrical resistance of which depends on the position of the transducer element, and control means magnetised in accordance with a preset pattern, wherein movement of the control means causes a sequence of movements of the transducer element, and hence a sequence of changes in the electrical resistance of the strain gauge.

2. A contactless electrical switch, which includes a transducer element formed of a pliable piezo-resistive material movable between two different positions, which element is supported at one of its ends, a piezo resistive strain gauge on said transducer ele ment whose electrical resistance differs for said two different positions of the transducer element, a small permanent magnet located at the other end of the transducer element, and a movable control element located adjacent to the other end of the transducer element, wherein the movable control element is mag netised in accordance with a desired pattern, such that movement of the control element causes the magnet on the transducer element to be alternatively attracted to and repelled from the control element, so that the transducer element is moved between its said two positions, and wherein each said movement between the two positions causes a detectable change in the resistance-of the strain gauge.

3. A contactless switch as claimed in claim 2, wherein the or each said movable control element is a rotatable disc whose outer rim carries the magnetic pattern.

4. A contactless switch substantially as described with reference to Figs. 2, 3 and 4, or Fig. 5 of the accompanying drawings.