CA1234895A - Switch device - Google Patents

Abstract

SWITCH DEVICE
Abstract of the Disclosure A contactless electrical switch has a switch element comprising a piezo-resistive transducer (25).
Coded toothed cams (16) operated via a shaft (14) deflect the transducer (25) causing corresponding changes in stress and hence resistance. The output of the transducer is amplified and fed to logic circuitry which determines a corresponding current path.

Description

1234~3~5 70334-:L0 ~ his invention relates to electrical switches, and in particular to switches of the contactless type wherein the switching function is performed by a solid state semiconductor element~
The perormance o conventional electrlcal swltch contacts is sub~ct to deg~ac1ation cau~ed by a numbe~ o~ ~ctor~.
In particular contact we~r c~n arise from mechanicaL wiping o~ the contacts and from arcing between the contacts during operation o the switch. Contaminant materials can form deposited coatings on the contact sur~aces causing unacceptably high resistance.

Furthermore, unless precautions are taken, contact bounce can occur on closing the switch giving rise to electrical noise. This problem renders conventional switches inappropriate for use in conjunction with integrated circuits where a significant mismatch in lifetime and reliability are apparent.
The object of the present invention is to minimize or to overcome these disadvantages.
According to one aspect of the invention there is provided a contactless electrical switch, including two or more flexible piezo-resistive transducer elements, toothed cams one for each said element and rotatahle on a common shaft, and logic means responsive to resistance changes caused by deflection of the elements by the cam teeth for enabling corresponding current paths, and wherein each said cam is so profiled that rotation of the shaft generates a digital code characteristic of the degree of rotation o-f the shaft.

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~Z3~ 5 Typically the piezo-resi~tive material is silicon although other materials can of course be used. Single crystal silicon has a tensile modulus comparable with -that of steel and exhibits I10 plastic flow even up to the fracture point. Plexible switch elements constructed from this material can there~ore tolerate relatively large de1ections and hence a very long wor~lng life.
Embodiments of the invention will now be described with reference to the accompanying drawings in which:
Fig. l is an exploded view of a rotary switch assembly incorporating a piezo-resistive switch element;
and Figs. 2 and 3 show respectively plan and sectional views of the switch element of the assembly o Fig. l.
Referring to Fig. l, the switch assembly is mounted between a front plate ll and a back plate 12, the front plate being provided with a boss 13 in which an , ~

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operating spindle 14 is located. This spindle carries a detent disc 15 and a code disc 16, these discs having axial openings 17 and 18 respectively that engage the ~` spindle 14. Relative rotation between the discs and `- ` 5 spindle is prevented by flats 19 on the spindle which flats engage corresponding registration flats in the disc openings.
The detent disc 15 is disposed in an opening in a detent plate 2~ and engages one or rnore inwardly directed detent pawls 21 mounted on a detent plate 210 to provide the necessary detent action of the switch.
The code disc 16 has one or more rows of circumferential projections 22 which, in use, engages ; corresponding resilient ca~ followers 23 carried on a ~` 15 support body 24. These cam followers 23 transmit motion from the code disc to piezoresistive transducer elements 25 supported on a transducer block 26.
Alignment of the assembly may be facilitated by pins 27 extending from the back plate 12 and on which the : . ~..~s,~. ~,.~
various static components are located via corresponding openings 28.
i In the assembled switch rotation of the code disc 16 via the shaft 14 causes corresponding deflection ` and release of the cam followers 23 by the projections 22 ~ 25 on the disc. This in turn causes flexure of the ``r~ ' transducer elements 25. The resistance changes caused by flexure of the transducer elements are sensed by an i~ amplifier chip 29 coupled to the elements 25 and the ~- ~ amplified signals are fed to logic decoding circuitry ;,` ~ , 30 (not shown) which translates the switch output signals into corresponding current paths.
~; Although two code discs and associated -.`' ~ : transducer element are shown in ~ig. 1 it will be appreciated that further discs and transducers may be . ~ 35 mounted within the switch assembly to provide a gauged ` swltching arrangement.
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Figs. 2 and 3 show the transducer element in detail. The element 25 comprises a mounting po~tionl3l from which one or more resilient flexible armsl32 extend, each arml32 having piezo-resistive bridge arrangementl33 formed thereon. Connection to each bridge is effected - via conductive tracks 134, e.g. of aluminium, which connect to terminal padsl35. Flexin~ of an arml32 c~u~es resistance changes in the corresponding bridgel33 thus altering the balance condition of that bridge. It is this change that is detected by the amplifier 29 (Fig. 1).
Although Fig. 2 shows a three element transducer it will be clear that similar arrangements can be made with other numbers of elements including one.
( Typically the transducer element is formed as an integral component from single crystal silicon. The mounting portion may be about C.25 mm thick and the arms 0.025 mm thick, i.e. a thickness ratio of 10 to 1.
Advantageously the thickness ratio is between 8 to 1 and 12 to 1. The bridges 33 may be diffused into the arms at - 20 the point of maximum strain which is adjacent the point .,!
- of attachment of the arm to the mountins member. The briage resistors are, in the unstressed condition of the device, substantially equal in value and are typically in the range ~ k ohm to 10 k ohm.
It is preferred to obtain maximum~voltage change - ~ at the bridge output within the strain limits imposed by the transducer material (~pproximately 0.3% for silicon). The transducer element may therefore be deflected in both a positive and a negative direction about its undeflected position and in each case the strain limits need not be exceeded. Thus to obtain --~ maximum output from the bridge the cantilever must be deflected from its maximum negative deflection position to its maximum positive deflection position. To achieve this in practice a cantilever leaf spring 31 (Fig. 1) may be provided on the transducer block 26 and positioned abutting projection 32 such that its free end is in line ~ "' .A . .~. ' ;
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; ~ with the free end of the transducer cantilever 2S. ~.fter , .. ..
` assembly of the transducer block into the switch body, this spring is caused to deflect, by a projec~ion 33 on the code disc follower body, it then forces the ~nd of each transducer cantilever into contact with the corresponding code disc follower which in turn is forced into engagement with the code wheei serrations. '~his deflects the canti1ever away from its unstressed position. Rotation of the switch spindle then causes the follower to deflect between two oppositely stressed conditions in response to the coded serrations. The resultant resistance change of each cantilever resistance - bridge 133 is amplified by the operational amplifier chip ! mounted on or adjacent the transducer block. By ensuring ~; - 15 that the leaf spring is clear of the transducer cantilever until after the block is assembled in the switch body the thin relatively fragile cantilever is protected from breakage due to excessive applied force.

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Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A contactless electrical switch, including two or more flexible piezo-resistive transducer elements, toothed cams one for each said element and rotatable on a common shaft, and logic means responsive to resistance changes caused by deflection of the elements by the cam teeth for enabling corresponding current paths, and wherein each said cam is so profiled that rotation of the shaft generates a digital code characteristic of the degree of rotation of the shaft.

2. A switch as claimed in claim 1 wherein the transducer elements are mounted on a carrier disposed adjacent the toothed cam.

3. A switch as claimed in claim 2, wherein said carrier is provided with means whereby the transducer elements are urged away from an unstressed condition.

4. A switch as claimed in claim 3, wherein said means comprises a leaf spring mounted on said carrier.

5. A switch as claimed in claim 1 wherein said transducer elements comprise an integral structure formed from single crystal silicon.

6. A switch as claimed in claim 5, wherein the transducer elements comprise a plurality of cantilever springs extending from and integral with a silicon body.

7. A switch as claimed in claim 6, wherein the ratio of the thickness of the cantilever springs to that of the body is between 8 to 1 and 12 to 1.

8. A switch as claimed in claim 1, 2 or 3, wherein an amplifier is provided on or adjacent said transducer whereby resistance changes resulting from operation of the switch are sensed.