BACKGROUND OF THE INVENTION In our earlier international application WO-A-98/40917, we have disclosed a novel form of electrically activated mechanical release mechanism which we have termed an actuator which improves the displacement behaviour of active material benders using materials such as piezo-ceramics. A feature of all active materials is that they are relatively inefficient, having coupling factors between the driving means and the output of fractions of a percent. Consequently, actuators which use such materials tend to require high fields be they magnetic or electric. However, we have found that despite this disadvantage, an actuator utilising an active material bender can result in a product having good mechanical properties.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an actuator utilising an active material bender which can be manufactured in an efficient and cost effective manner without impairing the mechanical reliability of the system.

In order to meet the above object, according to the present invention there is provided an electrically actuable mechanical release mechanism comprising a planar frame member provided with a profiled channel; a planar slide member arranged to be received within the profiled channel; latching means arranged to latch the planar slide member within the profiled channel; and an electrical actuator means arranged to control said latching means to latch or release the slide member.

Preferably, the electrical actuator means comprises a planar active

material bender as disclosed in our earlier international application no.

W098/40917, the necessary features of which required for understanding the present invention being incorporated herein by reference.

In a preferred embodiment, the planar active material bender is arranged to be laminated to the planar frame member in order to produce a low profile device. In such a construction, the active material bender when acting as the electrical actuator is further arranged to move out of the plane of action of the latching means upon actuation.

Furthermore, in the particularly preferred embodiment, the latching means comprises a rotatable pawl arranged to latch the slide member when held in a first position by the electrical actuator. When the electrical actuator comprises the active material bender, as the active material bender moves out of the plane of action of the latching means upon actuation, the rotatable pawl is freed to rotate to allow the slide member to release.

In the preferred embodiment, a spring means is further provided arranged to bias the slide member out of the profiled channel provided in the frame member.

It is a feature of the present invention that a relatively large mechanical movement in the form of the release of the slide member can be obtained from a relatively small movement of the electrical actuator means. This has the advantage that the mechanism of the present invention is particularly suitable for use with active material benders using material such as piezo-ceramics which require high drive fields to produce relatively little movement. It is a further advantage of the present invention that the relatively large movement of the slide member upon release can be used to trigger a further mechanism such as, for example, a circuit breaker mechanism.

BRIEF DESCRIPTION OF THE DRAWINGS Further features and advantages of the present invention will become apparent from the following description of a preferred embodiment thereof,

described by way of example only, and with reference to the accompanying drawings in which:- Figure 1 shows an exploded perspective view of the release mechanism according to the present invention; and Figure 2 shows a cross-section of the assembled release mechanism of the present invention along the line A-A shown in Figure 1 and looking in the direction of the arrows.

DESCRIPTION OF THE PREFERRED EMBODIMENT The electrically actuable mechanical release mechanism shown in the accompanying drawings is constructed from a number of layers of sheet material. The relative thicknesses of the different layers are chosen having regard to the different functions to be performed by the layers and this also applies to the material utilise. For ease of handling in this particular construction, the material is metal strip in which the thickness is readily controlled to acceptable limits by the fabrication process. Thicknesses of 0.15 millimetres to 0.2 millimetres have been found to be suitable but other thicknesses can be used as can other materials for certain of the layers. It is not necessary for the layers to be metal or conductive and in fact, in some instances it may well be an advantage for the layers to be insulative or self lubricating by being manufactured from a suitable plastics material.

The release mechanism according to the preferred embodiment of the present invention comprises a substrate 10 to which are attached a stack of other layers the stack comprising a frame 12, a spacer 14, and a planar bimorph layer 16 in that order from the substrate 10. A slider element 18 is further provided arranged to slide within a profiled channel 30 formed in the frame 12 and the slider is formed with an extension 32 which extends beyond the open end of the profiled channel 30 in the frame 12.

The slider 18 is formed with a slot 34 provided in the extension 32, the slot being arranged to receive a spring 36, with one end of the spring being

located on a spring seat 37 provided with the slot, with the other end of the spring 36 in engagement with a spring seat 38 provided on one of the other layers, and in this case the spacer layer 14. The slider member is capable of being latched against the action of the spring 36 by means of a rotatable pawl 40. The pawl 40 is mounted for rotation by means of a bearing 41 provided in the preferred embodiment on the spacer 14 but which may also be provided on the substrate 10.

The spacer is also further provided with an aperture 42 through which the operable, movable tip 44 of the piezo bimorph extends in order to control the rotation of the pawl 40 and thus the release or latching of the slider 18.

Before describing the operation of the above described mechanism, it is important to understand that the profiled channel 30 in the frame 12 is specially shaped so that the slider 18, although being largely movable linearly in the direction of the arrow X under the action of the spring 36 is also capable of slight lateral or rotational motion. Also, the profiled channel narrows near the open end 64 of the channel so as to restrict the stroke of the slider which is formed with protrusions 46 wider than the narrow open end of the channel 64. Also, the pawl 40 has a semi circular portion 48 arranged to be received in a corresponding portion 50 of the profile channel so as to be capable of angular movement in the direction of the arrow A (shown as clockwise within the drawing) within the profile channel. The pawl is further formed with a shaped recess 52 arranged to receive a correspondingly-shaped projection 54 on the end of the slider 18 remote from the spring 36. The shape and size of the meeting projection 54 and recess 52 are carefully designed to provide a specific burst force and the slider is also provided with an additional angled latching surface 56 arranged to slidably engage a corresponding angled latching surface 58 provided on the frame 12. The angles of the respective latching surfaces 56 and 58 are such that the force exerted by the spring 36 upon the slider 18 when the slider 18 is latched causes the latching surface 56 to press against the latching surface 58, the reaction force generated by the latching surface 58 causing a turning moment to be applied to the slider 18 in

the direction of the arrow B, shown as anticlockwise on the drawing.

Figure 2 illustrates a cross-section of the various layers when assembled. With reference to Figure 2, it will be seen that the piezo-bimorph 16 is provided with a pin member 44 which extends through aperture 42 provided in the spacer to engage with the pawl 40. Typically, the pin member 44 corresponds to the depth of the spacer 14 and the slider 18, and this is typically 0.35mm. The pin member 44 is provided on the operating end of the piezo-bimorph 16 such that when the piezo-bimorph 16 is actuated the pin member 44 is moved out of the plane of rotation of the pawl 40 in the direction of the arrow C to such an extent that the pawl 40 becomes free to rotate in the direction of the arrow A. Within Figure 2 the pawl 40 is shown mounted on a bearing 41 (not shown) provided on the spacer 14, although it will also be possible to provide the bearing 41 on the substrate 10.

Turning now to the operation of the mechanism, let us assume that the various layers are all assembled, stacked one on top of the other as shown in Figure 2, with the slider in position in the channel 30 such that the latching surface 56 is in engagement with the latching surface 58 on the frame 12 and the spring 36 is thus in compression between the spring seats 37 and 38. The surfaces 56 and 58 are angled such that the spring force is converted into a rotational force as indicated by the arrow B. This rotation is restricted by virtue of the projection 54 on the slider 18 being restrained by the recess 52 in the pawl 40. Movement of the pawl 40 in the direction of the arrow A is restricted by virtue of the pin member 44 provided on the moveable end of the piezo bimorph 16.

When the mechanism is to be actuated, an electrical signal is applied to the piezo bimorph 16 which causes the bimorph to flex in such a way that the pin 44 is pulled upwards, out of the plane of the paper in Figure 1 and in the direction of the arrow C in Figure 2, and out of an engagement with the pawl 40.

The shape of the meeting surfaces of the projection 54 and recess 52 in combination with the shape of the meeting surfaces 56 and 58 under the action of

the force exerted by the spring 36 causes the slider 18 to start to pivot in the direction of the arrow B which in turn forces the pawl 40 to rotate in the direction of the arrow A until such time as the pawl 40 releases the projection 54 which permits free movement of the slider 18 firstly in an arcuate direction in the direction of the arrow B and subsequently in the direction of the arrow X so that the extension 32 of the slider 18 can be used to activate a further mechanism or apparatus, such as a circuit breaker mechanism.

In order to reset and relatch the mechanism, it is assumed that there is no electric signal applied to the bimorph 16 so that the pin 44 is in its down most position. By moving the slider 18 against the spring 36 in the direction opposite to the direction X, the spring 36 is compressed and the slider is moved past the latching projection 58 to permit the projection 54 on the end of the slider to be received in the recess 52 in the pawl. The pawl is resiliently biased by a slight spring force in a direction opposite to the direction of the arrow A so as to permit the projection 54 to be captured by the recess and the pin 44 to hold the capture position.

It will be apparent from the above description that the reaction force generated by the latching surfaces 56 and 58 due to the compression of the spring 36, the burst force of projection 53 and recess 52, and the return spring force of the pawl 40 must all be carefully balanced in order to achieve correct operation.

More particularly, whilst it will be apparent to the skilled reader that a large degree of variations can be accommodated, it will be appreciated that the sum of the return spring force acting to return the pawl 40 to the latch position with the burst force generated by the angled surfaces of the recess 52 and projection 54 must be less than the reaction force generated by the angled latching surfaces 56 and 58 under the compression of the spring 36 in order for the slider 18 to be released when the piezo-bimorph member 16 is actuated. If this condition is not adhered to, then the reaction force of the latching surfaces 56 and 58 will not overcome the burst force of the recess projection and the return force on the pawl,

and the slider will not release.

It will be appreciated that the above construction is capable of being manufactured to any dimensions. In fact, it is very suitable for micro-machining techniques due to the laminar nature of the structure. Furthermore, while the electrical actuator of the present invention has been described as a piezo-bimorph, other electrical actuators may be used, and in particular electric relays, armatures or moving-coil magnets.