Bi-stable microswitch including magnetic latch

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.