The present invention relates to detent mechanisms, in particular to such mechanisms which may be used as a control means to retain or to release components as desired.

The use of detent mechanisms is common for retaining mechanical parts in a particular relationship to each other. In general terms such a mechanism will comprise a resiliently mounted pawl which, under the influence of its resilient mounting, will engage a recess on a movable member. That movable member will be retained in position by the detent pawl until such time as a force acting on the movable member to move that member past the pawl rises to a value sufficient to move the detent aside to allow the movable member to pass. Such assemblies are commonly found in car gear boxes and as latches for cupboard doors.

Generally such detent mechanisms provide a fixed holding force or at best a force which can be periodically adjusted by means of increasing the pre¬ load on a spring which is typically provided in the resilient pawl mounting. As a result, in use, such devices provide a pre-determined resilient force which is either overcome or not according to the force applied to the movable member. This means that such mechanisms cannot be used as a control means to retain or release components depending on known external parameters.

It is also well known to use piezo-electric materials, usually ceramics, in actuators which are essentially an elongate bar which is caused to bend to generate a deflection of one end relative to the other. The bending of the bar is generated by utilizing the dimensional change which occurs in a

piezo-electric material when the material is subject to an electric field usually generated by application of a voltage to the material. This dimensional change is very small, of the order of one micron in 25mm, but the forces generated within the material are great.

This dimensional change is used to generate a deflection as mentioned above by mounting piezo- electric material on a substrate which is not affected by the applied electric field to form the bar. When the device is subjected to an electric field the piezo-electric material alters in size while the substrate does not, thereby causing the bar to flex or bend.

A device as described above having one piece of piezo-electric material is commonly termed a unimorph device. In a bi-morph device the amount of deflection at the tip of the cantilever is increased by the use of two-pieces of piezo-electric material mounted either side of a substrate, which react differently to the same electric field. That is, one material expands when the other contracts and a greater movement is achieved, especially if the ceramics and substrate are made thin. However, losses are present through each layer and the whole system is strained by the bending.

It is also well known to use other materials in devices which are mechanically similar to those discussed above, i.e. actuators which are caused to flex. For instance magneto-strictive materials can be used very similarly to the piezo- electric materials discussed above, and a device using such materials is activated by a magnetic, rather than electric, field.

Further the use of dimensional change in piezo-electric or magneto-strictive materials as above is mechanically similar to the action of a

bimetallic strip caused to bend by changes in the temperature of the device.

The present invention, which is discussed in detail below, may utilize any such device, in which flexing is generated by differentional reactions to changes in ambient conditions, where ambient conditions are taken herein to refer to applied voltage in the case of piezo-electric materials, magnetic fields in the case of magneto- strictive materials and temperature in the case of bi-metallic actuators. In such devices, because the basic effect is a dimensional change caused by a change in ambient conditions and the required flexing is generated utilizing differential dimensional changes, there are inevitably stresses caused within the device and between the layers when the device is actuated.

There is a trade-off between the deflection generated in a device of this type and the strength of the device. Thin films of active materials generate the greatest movement but cause the overall device not to be particularly robust. In particular ceramics tend to be brittle and thus devices incorporation such materials cannot withstand significant flexing or impact. The best performance which can typically be achieved, by a practical piezo-electric bi-morph design is 0.2mm of deflection at the end of 25mm cantilever.

There is also a trade-off between the desirability to make actuator devices compact and the deflection which can be achieved with a single flexing bar, that is, clearly the greatest end deflection will be achieved with a long bar, while accommodating a long bar will increase the overall size of a device into which the actuator is incorporated.

Flexing actuators as discussed above can be

usefully incorporated into electro-mechanical latch or switch mechanisms. In particular the small movement generated in the actuator as described above can be used to release a further member to move under the influence of a force, for instance applied by a spring. However, in such devices the parts must be accurately manufactured and positioned in order to ensure proper operation of the latch mechanism. In particular it may be the case that if the parts are positioned to ensure that the latched configuration is secure than there is uncertainty as to whether the actuator can move sufficiently to ensure release of the device.

The present invention provides a mechanism comprising a fixed member, a movable member and resilient mounting means resiliently mounting said movable member with respect to said fixed member, in which the resilient mounting means comprises a first resilient member having a first, predetermined stiffness and a second resilient member having a second, variable stiffness, the stiffness of the resilient mounting means being a combination of the first and second stiffnesses, and the stiffness of the second resilient member being controllable by varying ambient conditions.

Preferably the second resilient member is a piezo-electric actuator which can be controlled by the application of electric fields.

The present invention can therefore provide a detent mechanism having a resiliently mounted pawl as outlined above. The force providing the resilient mounting can be altered in use in response to external parameters such that the pawl can be used either to retain or release a component which is subject to a known force.

This invention therefore provides a retain/release mechanism which is simple in design

and uses relatively few components and the device can be made quite small. This has particular applicability to small devices such as miniature circuit breakers (MCBs), portable residual current devices (RCDs) and robotic arms.

In RCDs it is further desirable for the retain/release mechanism to be readily changed between operation in two modes, the first mode being to retain a mechanism on application of a power signal and the second being to release a mechanism on application of a power signal. Normally these two modes of operation require separate mechanisms for implementation or require the replacement of a number of components in a single mechanism.

An embodiment of the present invention therefore further provides a retain/release mechanism which can be easily adjusted between these two modes of operation.

More specifically the present invention may be implemented as follows. A detent pawl is resiliently mounted by a spring, typically a leaf spring which provides a first force to bias the pawl to engage a recess in a movable member which has applied to it a further spring force which acts to move the movable member past the detent pawl. A second resilient means is provided which applies a second force to the detent pawl in addition to that provided by the spring mentioned above. The stiffness of the second resilient means can be altered by the application of external energy. Thus the invention can be arranged such that the first force provided by the spring means plus the residual stiffness in the second resilient means in its un- actuated state is insufficient to retain the movable member against the further spring force by engagement of the pawl with the recess. However when the second resilient means is actuated by the application of

external energy the total force on the detent pawl is sufficient to retain the movable member.

Alternatively the invention may be arranged such that the resilient force applied to the detent pawl by the mechanical spring alone is sufficient to retain the movable member and the second resilient means can be energized to provide a force which reduces the total force applied to the detent pawl thereby releasing the movable member.

Further the invention may be implemented in a device which can be easily, for instance by the movement of a single component, be switched between these two modes of operation.

As mentioned above, the second resilient member is preferably a piezo ceramic assembly which is actuated by the application of an electric field. However the second resilient member in this invention may be any of the alternative actuator devices mentioned above or may be any other biasing device whose stiffness can be altered by the application of external energy.

In order that the present invention can be better understood, preferred embodiments of the invention will now be described with reference to the accompanying drawings in which:

Fig. 1 illustrates an exploded view of a device according to a first preferred embodiment of the invention.

Fig. 2 illustrates a schematic cross section of the assembled device in the first embodiment;

Fig. 3 is a schematic cross-sectional diagram of a second preferred embodiment of this invention in the latched condition;

Fig. 4 illustrates part of the device as shown in Fig. 3 but in the released position; and

Figs 5a, 5b and 5c illustrate in schematic form the inclusion of the first embodiment in various devices.

The first preferred embodiment is a device which, in general terms, uses a piezo-electric actuator to cause relative rotation between two parts.

As shown in Figs 1 and 2, the device comprises an actuator 10, pivoting member 20 and base member 30.

As can be seen in Figure 1 base member 30 comprises base section 32 and walls 34 which together form a U-shaped cross section. Each wall 34 has formed therein a circular hole 36 and a window 38.

Pivoting member 20 comprises arms 22 joined at one end by cross member 24 and at the other by pins 26 and 27. On the outside of each arm 22 is formed pin 25 and each arm has formed therein window 28. On assembly, pivoting member 20 sits within the U-shape of base member 30 with pins 25 extending through holes 36, such that pivoting member 20 pivots with respect to base member 30 about pins 25. When so assembled windows 28 coincide with windows 38. The relative positions of pin 25 and pins 26 and 27 can be seen in Figure 2.

Piezo-electric actuator 10 comprises two arms 12a, 12b each of which comprises substrate 13 and piezo-electric material 14. The arms are joined at one end, as shown, by an end section with lugs 16 extending sideways. The piezo-electric material 14 is selected such that when actuated by an electric field it expands while the substrate 13 does not. As will be seen from Figure 1 therefore when the piezo- electric material is actuated the two arms 12a, 12b tend to bend away from each other.

On assembly actuator member 10 fits between

arms 22 of pivoting member 20. The ends of arms 12a and 12b lie between pins 26 and 27 with the end of arm 12a resting on pin 27 and the end of arm 12b resting on pin 26. Lugs 16 pass through windows 28 and windows 38. Windows 28 are shaped to permit a vertical movement, in the sense illustrated in Figure 1, of lugs 16 within the windows. Windows 38 are shaped such that lugs 16 rest on the lower edge of windows 38.

Finally there is provided leaf spring 40 as shown in Figure 2 fixed to the under-side of base section 32 of base member 30 the end of which engages with groove 29 in the end section 24 of pivoting member 20.

The operation of the device will be described with respect to Figure 2. As mentioned above when the piezo-electric actuator member 10 is activated by an electric field the tendency is for arms 12a and 12b to bend away from each other. As can be seen in Figure 2 such actuation in the device when assembled causes arm 12a to apply an upwards force Fl to the under-side of pin 27 and the end of arm 12b to apply a downwards force F2 to the top of pin 26. (A reaction force is of course also applied to the lower edge of windows 38 by lugs 16.) Thus an anti-clockwise moment is applied to pivoting member 20 about pivot point 25 and pivoting member 20 is rotated anti-clockwise with respect to base member 30. In Figure 2, assuming base member 30 is fixed, this will cause end section 24 of pivoting member 20 to rise thus flexing spring 40. The movement is restricted by spring 40 and therefore the device acts to move end section 24 between two positions, one lower relaxed position when actuator member 10 is not activated and a raised position when actuator member 10 is activated.

An advantage of this embodiment is that the use of the lever action of pivoting member 20 causes the movement of end section 24 to be significantly greater than the movement generated between the ends of arms 12a, 12b. This facilitates the incorporation of this device into latching or detent mechanisms.

In the second embodiment of this invention illustrated in Figs. 3 and 4 movable member 110 is constrained to move vertically in the sense of Fig. 3 in a channel 112 formed in frame 120. Member 110 is biased downwards in the sense of Fig. 3 by a force Fll applied by helical spring 114.

Also attached to frame 120 is leaf spring 130 having at its lower end a detent pawl 132. Leaf spring 130 is configured to bias detent pawl 132 to the right in the sense of Fig. 3. There is also provided a piezo ceramic hairpin assembly illustrated schematically at 140. This assembly 140 is mounted at its left end on another part of frame 120 and has its right hand end attached also to pawl 132. Thus both spring 130 and assembly 140 apply horizontal forces to pawl 132.

The device is illustrated in Figure 3 in its latched condition, that is with detent pawl 132 engaged with recess 116 in member 110. In this condition, due to the angles of surfaces 116a and 132a which are in engagement, force Fll which is applied by spring 114 will apply a force F12 on pawl 132 to the left in the sense of Fig. 3. If the total force F13 applied to pawl 132 towards the right by spring 130 and assembly 140 is greater than force F12 member 110 will be retained in its position illustrated in Fig. 3. However if force F13 reduces to be less than force F12 pawl 132 will be deflected to the left in the sense of Fig. 3 and member 110

will be released to its position shown in Fig. 4.

The device further comprises an adjustable piece 180, shown in cross-section in Fig. 3, which is movable so that it is either in contact with leaf spring 130, as illustrated in Fig. 3, or not in contact with leaf spring 130. This movement can be used to provide two modes of operation for the device as described in detail below. The adjusting piece may be provided in any suitable form, such as a pin, wedge or cam mounted on frame 120.

In the first mode of operation adjusting piece 180 is not in contact with leaf spring 130 and plays no part in the operation of the mechanism and therefore can be assumed not present in Fig. 3. The piezo ceramic assembly 140 is arranged such that when it is activated it applies a greater force to the right than when it is not actuated. The leaf spring 130 is designed to have a stiffness such that when the piezo ceramic assembly 140 is not actuated the total force F13 acting to the right on pawl 120 is less than force F12 acting on the pawl to the left. However when the assembly 140 is actuated the total force F13 acting to the right on pawl 132 is greater than force F12 acting to the left. Thus if member 110 is placed in its position shown in Fig. 3 and member 140 is actuated member 110 will be retained in this position. However if member 140 subsequently becomes de-actuated member 110 will be allowed to move to its release position shown in Fig. 4. Also in the absence of actuation of assembly 140 it will be impossible to re-set the device to leave member 110 held in its position in Fig. 3.

If the device of this invention is incorporated in a switch mechanism and member 110 causes the opening of contacts in its Fig. 4 position the device in this mode will be "fail-safe". That

is if actuation of the device is not present the device will switch off, if it is on, and will not be able to switched on, if it is off, until such time as the assembly 140 is re-actuated.

In the second mode of operation adjusting piece 180 is positioned in its location illustrated in Fig. 3. The effect of this is that it shortens the effective length of the leaf spring 130 such that its because the flexure of a cantilever beam of uniform section is proportional to cube of the length for any given load. With this increased stiffness the force applied towards the right on pawl 132 by leaf spring 130 plus the force applied to the right by assembly 140 in its de-actuated state is larger than F12 and therefore member 110 will be retained in its Fig. 3 position when assembly 140 is not actuated. In this mode when assembly 140 is actuated it applies a force on pawl 132 to the left thereby decreasing total force F13 to be lower than force F12 and releasing member 110. Thus, electrically this is the reverse operation to that of the first mode.

It will be appreciated that in this embodiment a simple mechanical movement effects a change in the electrical characteristics of the device and there is no necessity to alter any other mechanical parts.

If this device is incorporated in a switch device having associated electrical circuitry, for instance mounted on a printed circuit board, the movement of adjusting piece 180 can be arranged to make electrical changes to the circuitry so that it too operates in two modes. This may be simply by operation of a micro-switch on the circuit board. This facility has particular manufacturing advantages in a situation where two similar devices, having similar switch mechanisms, are being made. A single

manufacturing line be used to make the switch mechanism and the actual characteristics of the device selected by the positioning of adjusting piece 180.

Alternatively the device may be arranged such that a user of the device, properly instructed, can change the operation of the device between its two modes.

The piezo electric flexible members incorporated in the above described embodiments may be made using any well known piezo-electric material. The substrate on which the piezo-electric material is mounted would typically be Beryllium copper, Phosphour bronze or stainless steel. The thickness of the substrate would typically be in the range 0.2 to 0.25mm and the thickness of the piezo-electric material in the range 0.25 to 0.35mm. These are typical values and do not exclude the use of other values appropriate to the use of other materials and or particular applications.

Figures 5a, 5b and 5c illustrate in schematic form the incorporation of the first described embodiment above into various mechanisms. In each of these diagrams the piezo-electric device as a whole, as illustrated in Figure 2, is designated 50. To assist understanding the piezo electric actuator 10 is sketched in the device 50 and as described above in Figures 1 and 2, rotation takes place around point 25. Base member 30 is not shown in Figures 5a, 5b and 5c but it, or an equivalent, would of course be present in an actual device.

Figure 5a shows the incorporation of the device into a latch mechanism. In addition to the device 50 the latch mechanism comprises a latch arm 51 which is arranged to pivot around point 52. Latch arm 51 is biased to pivot in an anti-clockwise

direction from its position as shown by spring 53. As can be seen end 24 of the device 50 is shaped to be positioned beneath the end of latch arm 51 distal from pivot point 52 in either its deactivated or activated state. The latch arm 51 is therefore retained in its illustrated position. Depending on the actual implementation deactivation or activation will cause the device 50 to pivot moving end 24 away from latch arm 51 therefore permitting latch arm 51 to fall under the influence of spring 53.

Such a latch mechanism may be incorporated into a circuit breaker as illustrated by the contact pair 54 in the diagram which would be brought into contact with latch arm 51 in its raised position, and moved apart when latch arm 51 falls. This arrangement provides a relatively large contact gap, therefore enabling this device to be incorporated into safety circuit breakers.

Typically, the overall device 50 would be 25mm long with the distance between pivot point 25 and the upper end of activator 10 being approximately 20mm. With such a device the deflection of end 24 for releasing latch arm 51 would be of the order of 0.5mm.

Figure 5b illustrates the inclusion of device 50 in a relay designed to open and close a contact pair again designated 54. The relay mechanism simply comprises a pivoting arm 55 arranged to pivot around point 55a under the action of device 50 which opens and closes contacts 54. Such a mechanism provides a relatively small contact gap which is sufficient for incorporation in a switching relay.

Figure 5c illustrates piezo ceramic relay incorporating device 50 in which contacts 54 are opened and closed directly by movement of the end of

device 50. Depending on the particular application involved and the construction of a relay incorporating the device, the contact gap 0.5mm which would be provided by such a device is quite sufficient, as it is known to reduce arcing by the provision of an inert atmosphere around the contacts.