Abstract:
A pressure operated switch has a bistable or multi-stable latching mechanism disposed between a bellows unit and an electrical switch. The latching mechanism has two relatively axially moveable elements which extend and contract with subsequent operations of the bellows unit to provide the bistable action.

Description:
FIELD OF THE INVENTION 
     This invention relates to pressure switches. 
     BACKGROUND OF THE INVENTION 
     For safety reasons in electrically operated installations where water is present, it has been proposed to use pressure operated electrical switches which are actuated by a manually operable bellows unit connected to the pressure switch by a non-conductive small bore tube. The switch can then be located near the electrical circuitry which it controls, and the bellows unit can be located remotely from the switch. 
     Since pressure may be lost from the bellows unit, tube or switch if they remain pressurized for a long time, it has been proposed to employ a bistable mechanism in the switch, so that one pulse of pressure changes the state of the switch and a subsequent pulse reverts the switch to its original state. The previously proposed bistable mechanism involves use of a lever connected to a bellows of the pressure switch which is connected to a crank mounted by a ratchet on a shaft. Thus repeated operations of the bellows index round the shaft. The shaft has a detent mechanism to make its indexing more positive. One or more cams are mounted on the shaft and operate the push-buttons of one or more microswitches. Thus, as the shaft is indexed around, the states of the microswitches are changed. 
     Problems with this arrangement are that the mechanism is complicated, and despite the use of the detent mechanism, the shaft may not index around properly. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a pressure switch which does not have an unduly complicated bistable mechanism and yet which provides reliable and positive operation. 
     In accordance with one aspect of the invention, there is provided a pressure switch comprising chamber means defining a variable volume chamber to receive pressure pulses from an actuator, the variable volume chamber having a movable wall; an electrical switch having an actuator element movable to change the state of the switch; and a mechanism for transmitting motion from the movable wall to the actuator element to change the state of the switch upon a first movement of the movable wall and to change back the state of the switch upon a repeat movement of the movable wall; characterised in that the mechanism comprises first and second axially movable elements and latch means operable to hold the elements in a first relative axial position after one movement of the movable wall and to release the elements after a subsequent movement of the movable wall. 
     There follows a description by way of example of specific embodiments of the present invention, reference being made to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectioned plan view of a pressure switch assembly; 
     FIG. 2 is a perspective view of a latch element of the switch assembly; 
     FIG. 3 is a development of a cylindrical section surface denoted by chain-dot lines 3 in FIG. 1; and 
     FIG. 4 is a similar development but of a modified switch assembly. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a pressure switch assembly 10 has main body member 12 comprising a base plate portion 14 and an upstanding wall 16 formed with a stepped cylindrical opening 18 and an integral collar 20. 
     A mechanism 22 is fitted in the opening 18 and comprises a cup shaped member 24 having a central opening 26, a top-hat shaped member 28 disposed in the cup member 24 and having a central opening 30 therethrough, and a latch member 32 slidable in the opening 30 in the top-hat member 28. The mechanism 22 operates between a piston 34 and a switch actuator rod 36. 
     The piston 34 is covered with a diaphragm 38 the periphery of which sealingly engages between the collar 20 and cup member 24 on the one hand and a cover member 40 on the other hand. The cover member and collar 20 have complementary snap engaging formations 42 to hold the cover member 40 in place. The cover member 40 also has an integral nipple 44 for connecting a plastics tube in communication with a variable volume chamber 46 formed by the cover member 40 and the diaphragm 38. A compression coil spring 48 extends between the piston 34 and the top-hat member 28, and when pressure is applied via the nipple 44 the chamber 46 expands and the piston 34 moves to the right, as seen in FIG. 1, against the action of the spring 48. 
     The latch member 32 has a pointed end which engages in a recess 50 in one end of the actuator rod 36. The other end of the rod 36 has a square section portion 52 which is slidable between two guide elements 54 integral with the base plate portion 14. A compression coil spring 56 encircles the square section portion 52 and engages between the guide elements 54 and an integral collar 58 on the actuator rod 36 to urge the actuator rod to the left as viewed in FIG. 1 to hold the recess 50 in engagement with the pointed end of the latch member 32. 
     Referring to FIG. 3, the opening 30 through the top-hat member 28 has three splines 60 to define three axial passageways 62 spaced apart by 120 degrees. 
     Referring to FIG. 2, the latch member 32 has three radial projections 64 of triangular shape which are also spaced by 120 degrees and which can slide along the spline passageways 62 and move from one passageway to the next past the upper ends 66 of the splines 60 (as viewed in FIG. 3) upon rotation of the latch member 32. 
     Referring to FIG. 3, the ends 66 of the splines are specially shaped cams, as too is the axially facing end 68 on the inside of a central boss 70 of the cup member 24. FIG. 3 shows in dotted lines the progress of one of the triangular projections 64 as it travels from one spline passageway to the next. This is caused under the action of the piston 34 in the case of movement to the right as seen in FIG. 1 and upwardly as seen in FIG. 3 and the action of the springs 48, 56 which moves the latch member 32 to the left as seen in FIG. 1 and downwardly as seen in FIG. 3. 
     With the one projection 64 which is being described initially being in position P1 in FIG. 3, action of the piston moves upwardly to a position P2, where it engages a cam portion C1 which causes it to rotate until a limiting position P3 is reached. Upon release of pressure, action of the spring moves the projection to a position P4, where it engages cam portion C2 which causes it to rotate slightly until it reaches a limiting position P5. Action of the piston can then move the projection upwardly to a position P6 where it engages a cam portion C3, which causes it to rotate until a limiting position P7 is reached. Under action of the spring, the projection can then move downwardly to a position P8 where it engages a cam portion C4 which causes it to rotate until it reaches the next spline passageway at a position P9, whence the projection can move into the passageway to reach a position P10 which corresponds to original position P1 but in the next spline passageway angularly displaced by 120 degrees. 
     Referring to FIG. 1, a pair of change-over microswitches 72, 74 are mounted on the base plate portion 14 on opposite sides of the actuator rod 36. Each microswitch has a press-button 76, 78 engageable with a cam portion 80, 82 on the actuator rod. The cam portion 80 is arranged such that with the projections 64 in the positions P1/P10 and P9, the press-button 76 is not depressed and with the projections 64 in the positions P2 to P8, it is depressed. The cam portion 82 is arranged such that with the projections in the positions P1/P10, P2, P4, P5 and P9, the press-button 78 is not depressed sufficiently to operate the microswitch, but with the projections in the positions P3 and P6 to P8, it is depressed sufficiently to operate the microswitch. 
     It will be noted that positions P1/P10 and P5 are stable positions of the latch element 32. Thus, starting at position P1, a pulse of pressure applied to the chamber 46 will cause a change-over of the microswitch 72 and a momentary change-over of the microswitch 74. A further pulse will cause change-over of the microswitch 72 and a momentary change-over of the microswitch 74. 
     The actuator rod 36 may be modified so that the cam portion 82 is disposed at the same position along the rod as the cam portion 80. The arrangement will then provide change-over of each microswitch for each pulse of pressure applied to the chamber 46, without any momentary change-over, and thus act as a two-pole change-over switch. 
     FIG. 4 shows a modified arrangement of the cammed boss 70 and cammed top-hat member 28. With this arrangement a modified latching member is used having only two triangular projections 64. The FIG. 4 arrangement provides an extra stage of latching. From an initial position P21, the projection moves up to a position P22 and engages a cam portion C21, the projection turning until it reaches a limiting position P23. The projection can then move downwardly to a position P24 and then turn on a cam portion C22 until a stable limiting position P25 is reached. The projection can then move upwardly to a position P26 and then turn on a cam portion C23 until a limiting position P27 is reached, whence the projection can then move downwardly to a position P28 and turn on a cam portion C24 until a stable limiting position P29 is reached. The projection can then be moved upwardly to a position P30 and then be turned by a cam portion C25 until a limiting position P31 is reached. The projection can then move downwardly to a position P32 and can be turned by a cam portion C26 until it reaches a position P33 where the projection can enter the spline passageway 62 disposed 180 degrees from the initial position P21. 
     It will be noted that positions P21/P34, P25 and P29 are stable positions of the latch element 32 and are spaced up-and-down as seen in FIG. 4, or left and right as seen in FIG. 1. 
     The cam portions 80, 82 of the actuator rod are arranged so that, starting at the position P21, a first pulse of pressure applied to the chamber 46 will cause a change-over of the microswitch 72 (the position P25 being reached); a second pulse will cause a change-over of the microswitch 74 (the position P29 being reached); and a third pulse will cause both of the microswitches 72, 74 to change-over (reverting to their initial states, the position P34 being reached). 
     It will be appreciated that other modifications may be made to the arrangements described above. For example, a fourth stable state may be added corresponding to the second state, so that the following action is obtained: 
     
         ______________________________________State of      State of    State ofactuator rod 36         microswitch 72                     microswitch 74______________________________________1             0           02             1           03             1           14             1           01             0           0______________________________________ 
    
     one pressure pulse being needed to change the state of the actuator rod. 
     In a simplified version of the switching arrangement only one microswitch is used, and the mechanism provides a bistable action.