Abstract:
A bi-stable microswitch ( 1 ) including a pair of contacts ( 4, 5 ) and an armature ( 10,11 ) movable between a first position and a second position to selectively break or make the pair of contacts, the armature being latched in the second position by a magnetic path including a permanent magnet ( 3 ) and a magnetizable element ( 7 ) having a first temperature, wherein the armature is resiliently biased towards the first position when latched, and is movable from the second position to the first position upon heating of the magnetizable element to above the first temperature.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to microswitch arrays and microswitch array elements for switching electrical signal lines. The invention is applicable to the switching of telecommunications signal lines and it will be convenient to hereinafter describe the invention in relation to that exemplary, non limiting application. 
     Switching arrays are used in telecommunication applications, when a large number of telecommunication signal lines are required to be switched. Generally, such switching arrays are provided by the permanent connection of copper pairs to “pillars” or underground boxes, requiring a technician to travel to the site of the box to change a connection. 
     In order to remotely alter the copper pair connections at the box without the need for a technician to travel to the site, there have been proposed switching arrays consisting of individual electro mechanical relays wired to printed circuit boards. However, this type of array is complex, requires the addition of various control modules and occupies a considerable amount of space. Further, current must be continuously provided through the relay coil in order to maintain the state of the relay. Since in many applications switching arrays elements are only rarely required to be switched, this results in an undesired power consumption. 
     It would therefore be desirable to provide a switching array and switching array element which ameliorates or overcomes one or more of the problems of known switching arrays. 
     It would also be desirable to provide a bi-stable broad band electrically transparent switching array and switching array element adapted to meet the needs of modern telecommunications signal switching. 
     It would also be desirable to provide a switching array and switching array element tat facilitates the remotely controllable, low power bi-stable switching of telecommunication signal lines. 
     SUMMARY OF THE INVENTION 
     With this in mind, one aspect of the present invention provides a bi-stable microswitch including a pair of contacts and an armature movable between a first position and a second position to selectively break or make the pair of contacts, the armature being latched in the second position by a magnetic path including a permanent magnet and a magnetisable element having a first Curie temperature wherein the armature is resiliently biased towards the first position when latched, and is movable from the second position to the first position upon heating of the magnetisable element to above the first Curie temperature. 
     Conveniently, the armature may include a fist section having a first thermal expansion coefficient and a second section having a second thermal expansion coefficient causing movement of the armature from the first position to the second position upon heating of the armature. Such an armature is known as a thermal bimorph actuator. As an example of materials suitable for the fabrication of the armature, the first section may be at least partially formed of permalloy whilst the second section may be at least partially formed of invar. 
     The bi-stable microswitch may further include a fist heating device formed on or proximate the armature. A second heating device may also be formed on or proximate the magnetisable element. One or more of the first and second heating devices may include an electrical resistance element. 
     Alternatively, heat may be applied to at least one of the armature and the magnetisable element by means of electromagnetic radiation. For example, microwave or other radiation may be applied by non-contact means from a remote location. 
     The magnetisable element may be at least partially formed from a NiCu alloy, such as thermalloy, the composition of the alloy being adjusted to set the first Curie temperature. 
     Conveniently, the permalloy may at least partially constitute the pair of contacts. The pair of contacts may be formed in or on an electrically isolating substrate. The magnetisable element may be formed in the substrate, and separated from the pair of contacts by an electrically isolating layer formed in or on the substrate. The pair of contacts and the magnetisable layer may be formed by micro machining techniques, involving such steps as etching or electro forming. The armature may comprise a cantilever ovehanging the pair of contacts. The armature may also be formed by micromachining techniques, such as electro forming. 
     Another aspect of the present invention provides an array of bi-stable microswitches as described hereabove. Each of the microswitches may be at least partly formed in a common substrate by micro machining techniques. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following description refers in more detail to the various features of the switching array and switching array element of the present invention. To facilitate an understanding of the invention, reference is made in the description to the accompanying drawings where the invention is illustrated in a preferred but non limiting embodiment. 
     In the drawings: 
     FIG. 1 is a perspective diagram illustrating one embodiment of a bi-stable microswitch according to the present invention; 
     FIG. 2 is a circuit diagram showing one embodiment of a control circuit for the interconnection of two heating elements forming part of the bi-stable microswitch of FIG. 1; 
     FIG. 3 is a diagram showing one embodiment of a switching array including bi-stable microswitches of the type shown in FIG. 1; 
     FIG. 4 is a perspective diagram illustrating a second embodiment of a bi-stable microswitch according to the present invention; 
     FIG. 5 is a perspective diagram illustrating a third embodiment of a bi-stable microswitch according to the present invention; 
     FIG. 6 is a circuit diagram showing a second embodiment of a control circuit for the control of the two heating elements forming part of the bi-stable microswitch of FIG. 1; and 
     FIG. 7 is a circuit diagram showing an embodiment of an array of control circuits for control of heating elements forming part of an array of bi-stable microswitches according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, there is shown generally a microswitch  1  formed in an electrically inert substrate  2 , such as glass or silicon. Apertures are formed by etching or other micromachining techniques in the substrate  2 . Silk screening techniques are then used to apply a slurry of magnetic particles and binding into the apertures formed in the substrate. The orientation of these magnetic particles is then fixed and the slurry set in order to form permanent magnet  3 . The electro-deposited permalloy elements  4  and  5  form a pair of contacts of the microswitch  1 . A coating of Au, permalloy or like material is then formed on the upper surfaces of the pair of contacts  4  and  5 . It can be seen from FIG. 1 that the pair of contacts  4  and  5  project from one surface of the substrate  2 . 
     An insulating dielectric layer  6  is then formed on the other surface of the substrate  2 . The dielectric layer  6  may be formed from SiO 2 , SiN 2 , polyamide or like material. A layer  7  of thermalloy or other magnetisable material is then electro formed on the dielectric layer  6 . The composition of the thermalloy layer  7  is adjusted to set the Curie temperature of the layer. A further dielectric layer may then be formed on the thermalloy layer  7 , and electrical contacts a″ and b″ formed on the surface of that dielectric layer. An electrical resistance element  8 , such as an NiCr heating coil, is also applied to the surface of that dielectric layer by vapour deposition or like technique. 
     Electro deposition techniques are then used to form a column  9  and a cantilever  10  of invar. A cantilever  11  of permalloy is then electroformed on the permalloy cantilever  10 . An “adhesion” layer may be applied to the invar cantilever  10  prior to the electroforming of the permalloy cantilever  11 . 
     Another dielectric layer may then be formed on the cantilever  11 , and contacts a′ and b′ then formed on the upper surface of that dielectric layer. A heating coil  12  is also formed by vapour deposition on that dielectric layer. 
     The heating coils  8  and  12  may be connected in parallel as shown in FIG.  2 . In this arrangement, diodes  13  and  14  are respectively connected in series with the heating coils  12  and  8  in order that the application of a positive potential difference between common terminals A and B induces the flow of electrical current in only one heating coil at a time (See FIG.  2 ). 
     The operation of the bi-stable microswitch  1  will now be explained, Initially the microswitch  1  is in the stable state shown in FIG.  1 . The microswitch will remain in this state indefinitely until a positive potential difference is applied across the terminals A and B. This causes a current flow it through the heating coil  12 , causing the temperature in the cantilevers  10  and  11  to rise. The invar cantilever  10  and permalloy cantilever  11  form two sections, each having a different thermal expansion coefficient from the other, of a same microswitch armature. Such an armature is known as a thermal bimorph actuator. 
     Due to the different thermal expansion coefficients of its two sections, the heat generated from the heating coil  12  will cause the actuator to deflect downwards until it comes into close proximity with the pair of contacts  4  and  5 . This completes a magnetic circuit consisting of the permalloy/invar actuator, the permanent magnet  3 , the thermalloy layer  7  and the pair of contacts  4  and  5 . The inclusion of permanently magnetic material in the magnetic circuit will cause the actuator to latch into contact with the pair of contacts  4  and  5 . The pair of contacts  4  and  5  will thus remain indefinitely short-circuited. It should be noted that the pair of contacts  4  and  5  are electrically isolated from the magnetic circuit by the insulating dielectric layer  6 . 
     To release the armature, a negative potential difference is applied between the terminals A and B, thus causing the flow of a current i 2  through the heating coil  8 . This heats the thermalloy layer  7 . The thermalloy layer  7  is an alloy of NiCu whose Curie temperature can be determined by the composition of the alloy. Typically, the Curie temperature may be set at approximately 150° C. When the temperature of the thermalloy layer  7  reaches the Curie temperature, the permeability of the thermalloy layer  7  drops to unity, thus breaking the magnetic circuit. As a result, the contact latching force drops to a small value insufficient to retain the armature in contact with the pair of contacts  4  and  5 . As the armature is not being heating and caused to deflect downwards, the resilient biasing of the armature towards the position shown in FIG. 1 causes the armature to return to the stable state shown in that figure. 
     It will be noted that the bi-stable switch  1  shown in FIG. 1 has two stable states with the pair of contacts  4  and  5  being indefinitely open in one state and indefinitely closed in the other state. It does not require the supply of electrical power in either of these two stable states. Electrical power only needs to be provided for a short period, typically a few milliseconds, to cause a transition from one state to the other. Advantageously, the magnetic latching in the closed state results in the microswitch being resistant to vibration, since the magnetic force attracting the actuator to the pair of contacts  4  and  5  increases inversely as any gap therebetween decreases. 
     Although the embodiment illustrated in FIGS. 1 and 2 relies upon the use of heating devices formed on or proximate the armature and the layer  7  of magnetisable material, in alternative embodiments heat may be applied to at least one of the these elements by means of electromagnetic radiation or lasers. For example, microwave or other radiation may be applied by non contact means from a remote location. 
     A microswitch of the type illustrated in FIG. 1 can easily be fabricated to have a “foot print” of less than 1 millimeter×5 millimeters, and is amenable to fabrication using batch processing, standard photolithography, electroforming and other micromachining processes. 
     Moreover, such micromachining techniques facilitate the fabrication of a microswitch array of elements such as the microswitch illustrated in FIG.  1 . FIG. 3 illustrates one example of a microswitch array  20  including bi-stable microswitch elements  21  to  24  each identical to the microswitch  1  shown in FIG.  1 . In the example illustrated, control lines  25  and  26  are respectively connected to terminals A and B of the bi-stable microswitch element. Application of a potential difference between the control lines  25  and  26  in the manner described in relation to FIG. 2 causes the selective short circuiting of the pair of contacts  27  and  28 , thus interconnecting signal lines  29  and  30 . Other microswitch elements within the array  20  operate in a functionally equivalent manner. 
     FIG. 4 shows a second embodiment of a microswitch according to the present invention. In this embodiment, a microswitch  40  is again formed in a silicon substrate  41  from micromachining techniques. The microswitch  40  includes a thermal bimorph actuator  42  comprising a first layer  43  of silicon onto which is deposited a second layer  44  of permalloy. In use, the silicon/permalloy cantilever is thermally actuated by a heating coil formed on the upper surface of the permalloy layer, as was the case in the microswitch illustrated in FIG.  1 . 
     The microswitch  40  also includes a permanent magnet  45  interposed between two co-planar layers  46  and  47  of a thermalloy. Two columns  48  and  49  are formed at distal locations on the upper surface of the thermalloy layers  46  and  47  on either side of the permanent magnet  45 . 
     Metallic layers  50  and  51  are respectively deposited on the upper surfaces of the permalloy columns  48  and  49 . Metallic columns  52  and  53  connect the metallic layers  50  and  51  with the opposing surface of the substrate  41  in order to provide electrical connections for the microswitch  40 . In addition, an electrical resistance element  8  is applied to the under surface of the microswitch  40  in order to apply heating to the thermalloy layers  46  and  47 . 
     Heating of the bimorph actuator  42  causes the actuator to deflect downward until an end portion of the actuator  42  comes into contact with the metal surfaces directly above the permalloy columns  48  and  49 . This completes a magnetic circuit consisting of the permanent magnet  45  and co-planar thermalloy layers  46  and  47 , the permalloy columns  48  and  49 , the metal layers  50  and  51  and the permalloy end portion of the bimorph actuator  42 . It will be noted that this embodiment magnetic flux from the permanent magnet  45  no longer flows along the entire length of the cantilever, as was the case in the microswitch illustrated in FIG. 1, but only through the end portion of the cantilever. In order to release the microswitch, the thermalloy layers  46  and  47  are heated until the Curie temperature is reached, and the magnetic circuit broken, thus releasing the armature  42  which is then able to return to its at rest position as shown in FIG.  4 . 
     FIG. 5 shows a variant in which the orientation of the permanent magnet  45 , thermalloy co-planar layers  46  and  47 , and permalloy columns  48  and  49  remain the same, but the orientation of the silicon/permalloy bimorph cantilever  42 , and in particular the permalloy only end portion of the cantilever  42 , has been rotated through 90 degrees. Otherwise, the operation of the microswitches  40  and  60  is identical. 
     FIG. 6 shows a control circuit  70  for enabling selective operation of the microswitch  40 . This control circuit, which can be implemented using TTL logic directly fabricated into the silicon substrate  41 , includes two AND gates  71  and  72 . The output of the AND gate  71  is connected to a heating coil  73  deposited on the bimorph actuator  42 , whereas the output of the AND gate  72  is connected to a heating coil  74  acting to heat the thermalloy co-planar layer  46  and  47 . The electrical contacts provided by the metallic columns  52  and  53  of the microswitch  40  are respectively connected to signal lines  75  and  76 . The AND gate  71  includes three inputs, respectively connected to the control lines  76  and  77 , and a bimorph/thermalloy selection line  78 . The AND gate  72  includes three inputs, respectively connected to the control lines  76  and  77 , and also an inverting input connected to the bimorph/thermalloy selection line  78 . 
     The microswitch  70  remains in a bi-stable state controlled by the logical high or low signal of the bimorph/thermalloy selection line  78 . Accordingly, upon the placement of a logically high signal on the control lines  76  and  77 , and the placement of a logically high signal on the bimorph/thermalloy selection line  78 , a logically high output is placed at the output of the AND gate  71 , causing current to flow through the heating coil  73  and the consequent operation of the actuator  42 . Accordingly, the actuator  42  is brought into contact with the two metallic contacts  52  and  53  to thereby interconnect signal lines  75  and  76 . 
     Upon the placement of a logically low signal on the bimorph/thermalloy selection line  78 , the output of the AND gate  72  goes high, and a current is caused to flow through the heating coil  74 . The thermalloy layers  46  and  47  are then heated to above the Curie temperature, so that the magnetic circuit is broken and the actuator  42  caused to return to its at rest position in which contact is broken with the metallic contacts  52  and  53  and the signal line  75  and  76  are disconnect. 
     FIG. 7 shows an implementation of the control circuit using steering diodes as shown in FIG.  2 . In this arrangement, an array of healing coils  80  to  88  and associated steering diodes  89  to  97  are provided, each heating coil/diode pair acting to heat the bimorph actuator of a separate microswitch. Rows of adjacent heating coils/diode pairs are interconnected by control lines  98  to  100 , whilst columns of adjacent heating coils/diode pairs are interconnected by control lines  101  to  103 . Selective operation of control switches  104  to  106  in the control lines  98  to  100 , and control switches  107  to  109  in the control lines  101  to  103 , selectively interconnect a positive power source to ground through one of the bimorph actuator heating coils, thus causing activation of that selected actuator. 
     Similarly, further heating coils  110  to  118  and associated steering diodes  119  to  127  act to heat the thermalloy layers of individual microswitches in the array. Control lines  128  to  130  interconnect rows of adjacent heating coils/diode pairs, whilst columns of adjacent heating coil/diode pairs are interconnected by the control lines  101  to  103 . Control switches  131  to  133  selectively connect control lines  128  to  130  to a negative power supply. Selective operation of the control switches  131  to  133  and control switches  107  to  109  cause current to flow through a selected heating coil/diode pair, and the heating of the thermalloy layers of a selected microswitch. 
     Finally, it is to be understood that various modifications and/or additions may be made to the microswitch array and microswitch element without departing from the ambit of the present invention described herein.