Patent Publication Number: US-6707657-B1

Title: Integrated electrically actuated mechanical release mechanism

Description:
TECHNICAL FIELD 
     The present invention relates to an integrated electrically actuated mechanical release mechanism, and more particularly to such a mechanism when used as part of an electrical safety device such as a residual current circuit breaker. 
     BACKGROUND OF THE INVENTION 
     Various mechanisms are known in the art which provide electrical safety protection from a variety of potential electrical faults. Unfortunately each mechanism is usually designed to provide protection from a particular type of electrical fault. Various relevant types of electrical fault which may occur are, for example, a gross over-current condition such as may occur with a short circuit, an unbalanced fault current condition in the connections leading to and from the electrical supply, or a small fault current which, although insufficient to trip any short circuit protective mechanism, may still be damaging to sensitive electronic components in any device to which the safety mechanism is applied. Typically, a separate detection and actuation mechanism has previously been required for each type of fault, meaning that electrical safety could only be guaranteed within particular electrical regimes. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an integrated unitary electrically actuated release mechanism which integrates multiple level electrical protection into a single system. 
     In order to meet the above object, the present invention provides a integrated electrically actuable mechanical release mechanism comprising first fault detection means arranged to detect a first fault condition in an electric circuit; second fault detection means arranged to detect a second fault condition in the electrical circuit; third fault detection means arranged to detect a third fault condition in the electrical circuit; and means for breaking the electrical circuit in response to detection of any of the first, second or third fault conditions. 
     The first fault condition is preferably a low fault current condition which is insufficient to trip a short-circuit protection mechanism. The second fault current condition may preferably be a current imbalance between two or more parts of a circuit. The third fault current condition may preferably be a gross over-current condition such as those associated with a short-circuit condition. 
     In a preferred embodiment, the first fault detection means preferably comprises a bimetallic strip arranged to bend in response to the occurrence of the first fault condition in the circuit. Furthermore, the second fault detection means preferably includes an active material bender arranged to bend in response to the occurrence of the second fault condition. Moreover, in the preferred embodiment the third fault detection means preferably comprises a coil wound around a core, the core being ejected from within the coil on the occurrence of the third fault condition. 
     Preferably, the active material bender is a piezo active material bender as disclosed in our earlier international application no WO-A-98/40917, the relevant features of which necessary for a full understanding of the present invention being incorporated herein by reference. 
     The active material bender may by manufactured from a plurality of laminar members which are stacked one on top of the other to produce a low profile device. 
     A drive circuit is further provided for the active material bender which includes a toroidal transformer having primary and secondary coils arranged thereon adapted to detect current imbalances in two or more parts of the electrical circuit. The transformer is preferably further arranged to saturate at a level of current imbalance less that indicative of a second fault condition, the saturation of the core resulting in a high-voltage low-power output drive signal which can be used to drive the active material bender. 
     All of the detection means (the bimetallic member, the active material bender, and the coil) are preferably line independent, in that the energy of the fault current is used to actuate the detection means. Furthermore, all the faults to be detected are preferably current-driven. 
     The present invention has a primary advantage in that it provides an integrated unitary actuator which provides electrical protection from a variety of different electrical faults. Electrical safety can therefore be maintained over a wide range of electrical operating conditions. 
    
    
     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, presented by way of example only, and by reference to the accompanying drawings in which: 
     FIG. 1 shows a diagrammatic arrangement of the integrated mechanism of the present invention; 
     FIG. 2 shows an exploded perspective view of a particularly preferred construction of the piezo active material bender used in the integrated mechanism of the present invention; 
     FIG. 3 shows a cross-section of the preferred construction of the active material bender mechanism along the line A—A of FIG.  2  and looking in the direction of the arrows; 
     FIG. 4 shows a block diagram of the integration of the active material bender mechanism with a mechanism for opening electrical contacts used in the present invention; and 
     FIG. 5 shows a preferred drive circuit to be used with the preferred active material bender of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the integrated electrically actuated mechanical release mechanism of the present invention will now be described with reference to FIG.  1 . 
     With reference to FIG. 1, an electrical contact  94  is arranged to receive electric current from a mains supply via associated electric contacts (not shown). The electrical contact  94  may be switched in and out of contact with the mains contacts as required. Various mechanisms and arrangements for achieving this are well known in the art. 
     The electric current from the electrical contact  94  is fed to a switching means  80  which contains appropriate switches and connections to feed the necessary electric current to each of the three detection means of the present invention. 
     A first detection means comprises a bimetallic strip  82  composed of at two or more laminated plates of different material, each material having a different expansion coefficient with respect to temperature. The strip  82  is secured at a first end, and provided with an actuator  84  arranged at the opposite end. Connections are provided from the switching means  80  to one end of the strip. 
     A second detection means comprises a piezo active material bender  62  composed of piezo-active materials such as piezo-ceramics. A feature of all active materials is that they are relatively inefficient, having coupling factors between the electrical driving means and the actual mechanical output of fractions of a percent. Consequently, actuators which use such materials require high drive fields. In order to provide such a high field, connections are provided between the bimetal strip  82  to a toroidal transformer  70 , and from there to a voltage multiplier  61 . The connections are such that the current flows through the bimetal strip before flowing to the transformer, and thence to the voltage multiplier. The toroidal transformer provides a high-voltage low power signal to the multiplier in a manner to be described later, and the voltage multiplier multiplies the transformed voltage and feeds it to one end of the active material bender  62  Further connections are then provided from the far end of the active material bender back to the switching means  80  to complete the circuit. The active material bender is further provided with an actuator  86  and a movable tip  44  which moves in response to the applied fields from the current, as described later. 
     A third fault detection means comprises a coil  88  arranged so as to be wrapped around a metal core  90 . Connections are provided from each end of the coil to the switching means  80  to allow current to be fed to the coil. The operation of such a coil in detecting fault conditions is as follows. In the presence of a sudden short circuit, the sudden increase in current flow is accompanied by a sudden increase in magnetic flux around the wire in which the current is flowing. The increased magnetic flux when enhanced by virtue of the wire being wrapped into a coil acts upon the metal core  90  to eject the core from within the coil. The ejection need not be total but may easily be detected and used to actuate a tripping mechanism. 
     As part of a tripping mechanism to open the electrical contact  94  in the event of a fault being detected which relates to a short circuit, the present invention further provides a short circuit plunger  92 . This is arranged to act in response to detection of a short circuit by the coil  88  and core  90 , and may even form part of or be directly attached to the core  90 , although this need not necessarily be the case. Whatever arrangement is used to fire the plunger, the operation of the plunger in breaking contact of the electrical contact  94  with the mains contacts must be as fast as possible, lest the high current in the event of a short circuit cause the electrical contact  94  to melt or fuse to the mains contacts. 
     In use, the arrangement of the present invention acts to detect multiple types of electrical fault. As already discussed above, the coil arrangement  88  is used to detect short circuits which cause a gross-over current condition, and which require a very fast response to prevent severe or irreparable damage. The bimetal strip is used to detect relatively small fault currents which are not of immediate danger. The piezo material bender in combination with the toroidal transformer is used to detect current imbalances which are indicative of a fault condition. 
     The bimetal strip relies on the heating effect of the current passing through the strip to cause differential expansion of each layer of the strip and hence cause the strip to bend, and hence has a relatively slow response. The actuator  84  presses upon the piezo material  62  in the event of the bimetallic strip bending due to a fault current, and causes the piezo material to flex. This flexing may be detected mechanically through the arrangement of the piezo material as described later, or electrically through the current generated by the flexing piezo. Whichever mechanism is used, the detected flexing is used to trip the tripping mechanism to cause the electrical contact  94  to be opened. 
     A detailed description of a preferred construction of a tripping mechanism incorporating the piezo active material bender element will now be undertaken by way of example and with reference to FIGS. 2 and 3. 
     The tripping mechanism according to the preferred embodiment and 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 utilised. 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. However, where this preferred construction is used in the integrated actuator of the present invention, it is preferable that the alloys forming part of the active material bender, and/or of the other laminated layers be matched to the alloys of the bimetal strip, in order to cancel dimensional changes or flexing due to natural changes in ambient temperature. 
     The tripping 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. 
     FIG. 3 illustrates a cross-section of the various layers when assembled. With reference to FIG. 3, 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 0.35 mm. 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 FIG. 3 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 FIG. 3, 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 FIG.  2  and in the direction of the arrow C in FIG. 3, 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 the firing mechanism of the plunger  76 . 
     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. 
     Due to the small engagement depth and release force, it is possible to exploit the large motion of the planar bimorph to create a system which operates from a low power. 
     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. 
     A preferred drive circuit used to detect the second fault condition relating to current imbalances in the live and neutral lines, and to generate a corresponding drive signal for the active material bender will now be described with reference to FIG.  5 . 
     In FIG. 5, the drive circuit for the active material bender comprises the toroidal transformer  70  mentioned earlier, the transformer having a first primary coil  66  arranged to carry a load current i 1  from the live contact of a voltage source  64  such as, for example, the mains, to a load  74 . As will be apparent from the earlier description, in the preferred embodiment the current from the mains is first passed through the switching means  80 , and then through the bimetallic strip  82  prior to being fed to the toroidal transformer. The first primary coil  66  consists of a single turn around the toroidal coil of the transformer  70 . A second primary coil  72  is further provided consisting of a single turn of the toroidal coil, arranged to carry a current i n  from the load  74  back to the neutral contact of the voltage source  64 , and preferably via the switching means  80 , although this need not necessarily be the case. 
     In addition to the first and second primary coils, a secondary coil  68  comprising a plurality of turns is further provided on the core of the toroidal transformer  70 , an induced output voltage E across the secondary coil  68  being fed to a diode bridge rectifier  76  for rectification, the rectified output drive voltage from the secondary coil  68  then being passed to a voltage multiplier  61  for voltage multiplication of the output drive voltage E to an operating voltage V. In the preferred drive circuit, a smoothing capacitor  78  is further provided connected across the output of the diode bridge rectifier  76  in order to smooth the rectified voltage prior to multiplication in the voltage multiplier  61 . The voltage multiplier  61  may be any convenient multiplication means or circuit elements as will be apparent to the man skilled in the art. 
     Although within the preferred drive circuit described above and shown in FIG. 5 the output drive voltage E from the secondary coil is shown as being rectified by the diode bridge rectifier  76  prior to multiplication in the multiplier  61 , this order is not essential to the operation of the present invention, and it may of course be possible that the order of the rectifier  76  and the multiplier  61  be reversed, in that the AC voltage spikes output from secondary coil may be multiplied by the multiplier  61  prior to rectification by the bridge rectifier  76 . 
     The drive circuit as described above operates as one of the sensing means in the present invention to detect the second fault condition, being a current imbalance, and in particular the provision of the transformer allows for accurate current imbalance sensing, as will be explained more fully below. Preferably, the primary coils  66  and  72  comprise only a single turn, while the secondary coil  68  has a large number of turns, and typically more than 1000. High permeability materials such as Nickel Iron are used to increase the overall inductance of the system. 
     The drive circuit of the active material bender having the aforementioned construction operates in the following manner. The voltage created across the secondary coil  74  of the toroidal transformer  70  is the back-EMF to oppose any change in the currents flowing in either of the primary coils, and is given by the well known equation E=−Ldi/dt. Under normal circumstances i.e. in the absence of a fault condition, the respective electric currents flowing in the live coil  66  (I 1 ) and the neutral coil  72  (I N ) are equal and opposite, so the magnetic fields associated with the respective currents cancel out, and thus there is little or no current induced in the secondary coil  68 . 
     If a proportion of the incoming current I 1  begins to flow out of the load due to a fault condition such as, for example, a short circuit or a human in danger of electrocution, the magnetic fields associated with the respective currents flowing through the two primary coils cease to be equal and opposite, resulting in an induced voltage in the secondary coil  68 . Initially, the induced waveform is sinusoidal with the same frequency and phase as the voltage supply  64  to match the fault current, but as the fault current increases the toroidal transformer is arranged to saturate and the output voltage waveform E across the secondary coil  74  becomes spiked. In traditional electro-mechanical relays this is a disadvantage, because the power delivered decreases. Piezo electric and electrostrictive materials however are distinctive in requiring very low power but being demanding of a high electric field in order to operate. As mentioned above, the voltage output of an inductor is calculated by the equation E=−L di/dt, where E is the Voltage, L is the system inductance and di/dt is the rate of change of current over time. The saturation of the magnetic core results in a very high di/dt and so the voltage across the secondary coil goes up. The drive circuit used in the preferred embodiment of the present invention utilizes this behaviour in order to generate an initially high voltage from the toroidal transformer. Preferably, the magnetic core of the transformer is designed to saturate at a point around 50% of the trip value, being the level of current imbalance between the two primary coils indicative of a fault condition. 
     In the drive circuit shown in FIG. 5, the induced voltage waveform E across the secondary coil is preferably rectified in a bridge diode circuit  76 , and smoothed with a smoothing capacitor  78 . The thus rectified and smoothed signal is then fed to a voltage multiplier circuit  61  for multiplication by a convenient factor such as two or three up to an operating level V. The output signal V is then used to drive the active material bender. 
     As an alternative drive circuit, an oscillator circuit and appropriate control chip may be further provided arranged to control the switching of the current through the secondary coil on the transformer. Such operation is similar to that of a switched mode power supply, where the sudden switching off of the current in an inductor is used to create a high voltage pulse, where the timing of the disconnection is governed by the voltage across a reference resistor. If such switching is undertaken very rapidly using the oscillator circuit, then high voltages can be created. By controlling the frequency and duty cycle of the oscillator then the necessary operating voltages can be obtained from the toroidal transformer. In order to achieve a suitable voltage it is also necessary to rectify the toroid output and the also rectify the output from the drive chip. This is done using a diode bridge rectifier in both cases. 
     FIG. 4 shows a block diagram illustrating how the drive circuit and the tripping mechanism incorporating the active material bender may be integrated together. More particularly, with reference to FIG. 4, a pair of contact switches  63  are provided in the live circuit between the toroidal transformer  70  and the load  74  arranged to break the live circuit and thus prevent current flowing through the coils of the toroidal transformer  70 . The contacts  63  are mechanically linked to the tripping mechanism incorporating the active material bender labelled  62  in the diagram. More specifically, preferably the contacts  63  are mechanically linked to the extension  32  of the slider  18  of the electrical switching mechanism and arranged so that the electrical contacts  63  are opened when the slider  18  is released from its latched position within the profiled channel  30  such that the extension  32  projects a substantial amount beyond the end of the channel  30 . The contacts  63  may be directly mounted upon the extension  32  of the slider  18 , or a mechanical linkage or further mechanism may be provided between the slider  18  and the electrical contacts  63 . 
     Preferably, the electrical contacts  63  are the same contacts as those contacts  94  opened by the plunger mechanism, in which case a mechanical linkage or further mechanism is provided between the slider  18  and the firing mechanism of the plunger  92 , arranged to operate such that the plunger is fired to open the contacts  94  when the slider is released from within the profiled channel  30 . 
     In operation, therefore, assuming the electrical contacts  63  ( 94 ) are closed and current is flowing through the load  74 , the toroidal transformer  70  acts to detect any current imbalances between the current i 1  and i n  flowing in the respective live and neutral lines by virtue of outputting the output drive voltage E being the back EMF across the secondary coil, this back EMF is then rectified if required and fed to the voltage multiplier  61  for multiplication up to the operating voltage V, the operating voltage V being arranged to be placed across the piezo-bimorph of bender  16  as appropriate in order to actuate the piezo-bimorph  16  to bend out of the plane of action of the pawl  40  thus releasing the slider  18  from the profiled channel  40 , thus firing the plunger  92  to open the contacts. 
     The operation of the bimetallic strip in detecting low-current fault conditions will now be explained in more detail. 
     As mentioned previously, the actuator  84  of the bimetallic strip  82  acts to press upon the active material bender in the presence of a low-current fault condition. As will now be understood from the description of the tripping mechanism of FIG. 2 given above, preferably the actuator  84  of the strip  82  is arranged to move the piezo bimorph  16  of the tripping mechanism out of the plane of action of the pawl  40 , thus releasing the slider  18  from the channel  30 . As explained above, the release of the slider  18  from the channel preferably causes the plunger to fire, thus breaking the circuit. The tripping mechanism of FIG. 2 can therefore be caused to release by either the bimetallic strip detecting a low current fault condition, or the active material bender and associated drive circuit detecting a current imbalance. This has the advantage that the same tripping mechanism can be effectively used to detect and act upon two different types of current-driven fault. 
     As will be apparent from the foregoing, the present invention therefore presents an integrated actuator which may be used to detect multiple electrical fault conditions, and which combines at least three different detection mechanisms into an integrated mechanism.