Abstract:
An over-center clamping mechanism is disclosed having a pivot guide to guide the pivot of one of the link arm members of the mechanism which comprises a piezoelectric actuator. The actuator is arranged to withdraw the pivot guide from the plane of the action of the mechanism, thereby allowing the guided pivot free movement of the plane of action which has the effect of collapsing the mechanism when is a clamped state.

Description:
TECHNICAL FIELD 
     The present invention relates to controlled mechanisms where at least one mode of control is exercised by a piezo ceramic device, 
     More particularly, the present invention relates to an over-centre mechanism where a piezo-ceramic device is used to control the mechanism whereby to collapse the mechanism when locked. 
     BACKGROUND ART 
     The use of over-centre mechanisms is known as a means of applying high clamping forces to electrical contacts, valve faces and frictional holding devices. The geometry of over-centre clamping mechanisms is also known, wherein the linkages in the mechanism must pass from one mode in which they are free to rotate, past the centreline of the plane of action and into a mode where they are urged to rotate by the reaction in the plane of action but are stopped by some mechanical means. 
     In order to readily collapse the mechanism, the means of stopping rotation can be withdrawn or otherwise collapsed, allowing spring forces to push the system open. Alternatively, the mechanism can be pulled back to rotate in the direction in which it was originally set. These actions are readily achieved with solenoids or fluidic pistons. The magnitude of the force necessary to cause the mechanism to collapse is generally lower than that required to set the system. 
     Electro-strictive piezo ceramic actuators are also known for the operation of mechanical devices, having the benefit of high speed, compact size and low power. Piezo devices are made in two basic forms. One type of device relies upon the basic change in dimension of the material. The change is measured in low parts per thousand, so devices of this type are generally suited only for micro positioning or ultrasonic transduction. 
     The other type of device acts like a bimetallic strip where the ceramic is bonded to a thin strip of metal to create a bending action. Although these devices have more movement it is still generally less than 1 mm and the output force is very low. Both types of devices are available in multi-layer forms which increase motion and reduce operating voltage. 
     In order for a piezo actuator to be used for the release of an over-centre mechanism suited to mass production it is necessary to combine higher force with higher displacement. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an over-centre mechanism in which a locked condition of the mechanism is released by an electrically actuable pivot guide releasing a pivot of a link of the mechanism by moving the pivot guide in a direction out of the plane of motion of the linkage, thereby allowing free motion of the pivot on the plane of motion and collapsing the mechanism. 
     In order to meet the above object according to the present invention there is provided an over-centre clamping mechanism comprising an actuating lever member arranged to rotate about a fixed pivot, a link arm member rotatably attached to said actuating lever member by a first movable pivot, and an actuable pivot guide means arranged to constrain the movement of a second movable pivot provided on said link arm member within a plane of action, wherein said actuable pivot guide means is electrically actuable to move out of said plane of action to release said second movable pivot whereby to collapse the clamping mechanism when said clamping mechanism is set. 
     The actuable pivot guide means may further comprise a piezo-ceramic bender actuator arranged to bend in a direction away from the plane of motion of the mechanism in order to allow the guide pivot to be released. 
     Furthermore, in a first mode of operation the guide means may constrain the guided pivot to move along a line on the plane of motion of the mechanism, and in a second mode of operation when the guide means have moved out of the plane of motion, the guided pivot may be free to move on the plane of motion. 
     The over-centre mechanism may further comprise a carrier arm arranged to rotate about a second fixed pivot point in response to the action of an action point of the link arm member there against, in order to force a free end of the carrier arm against a fixed portion in order to lock the mechanism. 
     The carrier arm and the fixed portion may each be provided with electrical contacts thereon, arranged to contact when the mechanism is locked. 
     Instead of being of single piece construction, the link arm member may comprise two link arms, one connected between the first movable pivot on the actuating lever member and the guided pivot, and the other connected between the guided pivot and a third pivot provided on the carrier arm member. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     In order that the present invention be more readily understood embodiments thereof will now be described by way of example with reference to the accompanying drawings, in which: 
     FIGS. 1 a ,  1   b  and  1   c  show a sequence of operation of a first embodiment of the present invention; 
     FIG. 2 shows a perspective diagrammatic view of how to implement the embodiment shown in FIG. 1; 
     FIGS. 3 a ,  3   b  and  3   c  show a sequence of operation of a modification to the first embodiment shown in FIG. 1; 
     FIG. 4 shows a perspective diagrammatic view of how to implement the modified embodiment; and 
     FIG. 5 shows a perspective diagrammatic view of a particular arrangement of the electrically actuable guide means used in the preferred embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description, the mechanism is shown operating a set of contacts, but the same principles apply to releasing a gas valve, removing a clamping force, or in any other application where an over-centre mechanism is used and is required to be released quickly and automatically. 
     Referring to FIG. 1, an over-centre mechanism is provided in which main pivots  11  and  51  are fixedly mounted to a suitable backplane which is not shown. An actuating lever  10  is rotatably connected to the main pivot  11  and to a link arm  20  by means of another pivot  22 . FIG. 1 a  shows the mechanism in its starting position. 
     The link arm  20  is constrained to move in a straight line by guide pivot  21  on the arm  20  being received in a groove whose sides are formed by a fixed member  40  associated with the backplane, and by a feature  30  on a suitably shaped actuator  31 . Rotation of the actuating lever  10  results in rotation and lateral movement of the link arm  20 , with the mechanism being constructed such that the pivot  22  can be rotated to a small angle beyond the horizontal centreline through the fixed pivot  11 . The output point, in this case is a rotatable contact carrier  50  pivotally mounted to the fixed pivot  51  and biassed to the “off” position. The moving contact carrier  50  is positioned to bear against the free end  25  of the link arm  20  so that it rotates about the fixed pivot  51  as the actuating lever  10  is operated. When the actuating lever is approaching the horizontal centreline, a contact  50   a  on the moving contact carrier  50  is brought into contact with a fixed contact  55  and pressure is applied to the arrangement as the mechanism moves to the horizontal and beyond. This situation is shown in FIG. 1 b . The surface of the moving contact carrier  50  which is engaged by the link arm free end  25  has upon it an angled portion  70  which imparts a rotary moment to the link arm  20  in the direction of the arrow  5  in FIG. 1 b . This rotary moment causes the link arm guide pivot  21  to bear against the upper wall of the constraining groove, which is formed by the feature  30  on an electrically actuable actuator  31 , such as a piezoelectric actuator. A particular arrangement of a suitable piezo electric actuator is shown in FIG. 5, wherein the feature  30  is in the form of a groove cut or otherwise formed at or near to one end of the actuator plate  31 . The actuator plate  31  is made from piezo-ceramic material arranged to bend in the direction of the arrow B in response to a suitable electric signal. Although here the feature  30  is shown as a groove, the feature  30  may also be any other shape which acts as a pivot guide, such as, for example, an extended platform formed on the plate  31 . 
     The mechanism is collapsed by the feature  30  moving in the direction of the arrow A on FIG.  2 . This is equivalent to the actuator moving into the plane of the paper in FIG.  1 . The length of the guide pivot  21  is such that the motion of the feature  30  causes it to be freed from the constraint of the feature. This freedom permits the guide pivot  21  to move upwards under the action of the return force on the contact carrier  50  until the guide pivot  21  rises above the level of the pivot  22  whereupon a return spring  80  connected to the link arm  20  causes the rotary moment  5  to be exerted upon the link arm  20  and for the link arm to rotate around the link pivot  22 . The moving contact carrier  50  is constructed such that upwards movement of the link arm  20  causes the two parts to disengage such that the moving contact  51  can return to its starting position under the power of a suitable spring which is not shown. The contacts will still open if the actuating lever is prevented from returning to its starting position and the speed of the opening is independent of the speed of the linkages. 
     The mechanism is reset by virtue of spring  80  which pulls the link and actuating lever down until the link arm guide pivot  21  is restored to within the constraint of the feature  30  once the piezo actuator has returned to its original position. 
     Control of high fault currents is achieved through the use of conveniently shaped and located arc splitter plates  65  as shown in FIG. 2 or other suitable devices, such as series positioned positive temperature co-efficient resistors. 
     FIGS. 3 a  to  3   c  show a modification in which the link arm  20  is replaced by two link arms  26 ,  27  which are pivotally linked at the guide pivot  21  and in which the second pivot  27  is pivotally linked to the moving contact carrier  50 . The motion of the setting lever  10  forces the first link  26  along the groove created by the backplane and the feature  30  of the piezo as before, which in turn causes the second link  27  to act upon the moving contact causing it to rotate with respect to its pivot  51 . The angle of the second link is constrained to always be below the horizontal centreline of the constraining groove, but a spring force is provided for urging the two link arms  26 ,  27  apart so that there is always a force trying to push the pivot  21  of the linkage upwards. 
     The first pivot point  22  passes beyond the centrelne of the actuating lever centre  11  and so is locked. Release is achieved by allowing the guide pivot  21  to move upwards which is done by the motion of the piezo actuator  30  as described earlier. This design is more robust because the moving parts are permanently linked, but there is a higher component count. 
     FIG. 4 shows a  3 D sketch view of the overall arrangement with the two linkages. 
     As is apparent from FIGS. 2 and 4, the groove formed by the feature  30  in the piezo device  31  and the feature  40  associated with the backplane need not both be on the same side of the mechanism; one feature can be on one side with the other feature on the other side.