Bi-stable microswitch including shape memory alloy 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 “posts” 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 modem telecommunications signal switching.

It would also be desirable to provide a switching array and switching array element that 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 make or break the pair of contacts, the armature being latched in the second position by a shape memory alloy latch, wherein the shape memory alloy latch is caused to deform upon heating so as to permit the armature to return to the first position.

In one embodiment, the armature includes a shape memory alloy element causing movement of the armature from the first position to the second position upon heating of the armature.

The armature may be resiliently biased towards the first position when latched so that upon removal of the heat and the deformation of the shape memory alloy latch, the armature returns to the first position.

The bi-stable microswitch may further include a first heating device formed on or proximate the shape memory alloy latch. A second heating device may also be formed on or proximate the armature. 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 shape memory alloy latch by means of electromagnetic radiation. For example, laser, microwave or other radiation may be applied by non-contact means from a remote location.

Another aspect of the invention provides an array of bi-stable microswitches as described above. Each of the microswitches may be at least partly formed in a common substrate by micromachining 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 schematic diagram illustrating an embodiment of a bi-stable microswitch according to the present invention;

FIG. 2

is a circuit diagram showing the interconnection of two heating elements forming part of the bi-stable microswitch of

FIG. 1

;

FIG. 3

is one embodiment of a switching array including bi-stable microswitches of the type shown in

FIG. 1

;

FIG. 4

is a circuit diagram showing a second embodiment of a control circuit for the control of two heating elements forming part of the bi-stable microswitch of

FIG. 1

; and

FIG. 5

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 first embodiment of a microswitch

1

formed in an electrically inert substrate, such as glass or silicon.

The microswitch

1

comprises two non-conductive arms

2

and

3

, formed of silicon or like material, and an armature

4

. The arms

2

and

3

and the armature

4

project from a base member

5

. Metal contacts

6

and

7

are formed on facing surfaces of the arm

2

and the armature

4

so that in the stable state shown in

FIG. 1

, the contacts

6

and

7

touch. The contact

6

is connected to a terminal

8

and the contact

7

is connected to a terminal

9

. Accordingly, the touching of the contacts

6

and

7

establishes a short circuit between the terminals

8

and

9

.

Similarly, a pair of contacts

10

and

11

are formed on facing surfaces of the armature

4

and the arm

3

. The electrical contact

11

is connected to a terminal

12

. Touching of the contacts

10

and

11

establishes a short circuit between the terminals

9

and

12

.

In this embodiment, the shape memory element of the armature

4

has a lower transition temperature T

1

above which the armature is caused to move from the stable position shown in

FIG. 1

in the direction indicated by the arrow

13

, so as to cause the metal contacts

10

and

11

to touch. This is referred to as the second position. Armature is held in this position by a shape memory alloy latch

14

which acts like a spring pressing down on the armature

4

from above.

When the temperature of the shape memory alloy element falls to below the lower transition temperature T

1

, the armature

4

is resiliently bent towards the position indicated in

FIG. 1

but held in the second position by the downwards spring action of latch

14

.

The arm

3

of the bi-stable microswitch

1

includes a shape memory alloy latch

14

having an upper transition temperature T

2

where T

2

is greater than T

1

. When the temperature of the shape memory alloy latch

14

is below the upper transition temperature T

2

, the shape memory alloy latch

14

remains in the hook-like shape shown in FIG.

1

. However, when the temperature of the shape memory alloy latch

14

exceeds the upper transition temperature T

2

, the latch

14

is caused to deform upwards so as to permit the armature

4

to return to the stable position shown in FIG.

1

.

Electrical contacts a″ and b″ are formed on the surface of the shape memory alloy latch

14

and an electrical resistance element

15

, such as an NiCr heating coil, is applied to the surface of the shape memory alloy latch

14

by vapour deposition or like technique.

Contacts a′ and b′ are then formed on the lower surface of the armature

4

. A heating coil

16

is formed by vapour deposition on the armature.

The heating coils

15

and

16

may be connected in parallel as shown in FIG.

2

. In this arrangement, diodes

17

and

18

are respectively connected in series with the heating coils

15

and

16

in order that the application of a potential difference between common terminals A and B induces the flow of electrical current in only one heating coil at a time.

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 i

1

through the heating coil

16

, causing the temperature in the shape memory alloy element in the armature

4

to rise above the lower transition temperature T

1

.

The armature

4

is accordingly caused to deform in the direction of the arrow

13

so as to cause the electrical contacts

10

and

11

to touch. In so doing, the shape memory alloy latch

14

is momentarily deflected by the armature

4

, and, once the armature

4

has moved past, latches the armature

4

in place by engagement of the shape memory alloy latch

14

on the upper surface of the armature

4

.

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

15

. This heats the shape memory allow latch

14

. When the temperature of the latch

14

exceeds the upper transitions temperature T

2

, the shape memory alloy latch

14

is caused to deform upwards so as to permit the armature

4

to return to the stable position shown in FIG.

1

. Since negligible current is flowing through the heating coil

16

at this time, the armature

4

is no longer caused to deform in the direction of the arrow

13

. The armature

4

then returns to the stable position shown in

FIG. 1

due to its resilient biasing towards this position.

It will be noted that the bi-stable switch

1

has two stable states with the pair of contacts

10

and

11

being indefinitely open in a first state (shown in

FIG. 1

) and indefinitely closed in a second state. Similarly, the pair of contacts

6

and

7

is indefinitely closed in that first state and indefinitely opened in the second 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.

Although the embodiment illustrated in

FIGS. 1 and 2

relies upon the use of heating devices formed on or proximate the armature

4

and shape memory alloy latch

14

, in alternative embodiments heat may be applied to at least one of these elements by means of electromagnetic radiation. For example, laser, microwave or other radiation may be applied by non contact means from a remote location.

A microswitch of the type illustrated in

FIGS. 1 and 2

can easily be fabricated to have a “foot print” of less than 1 milimeter×5 millimeters, and is amenable to fabrication using batch processing, standard photolithography, electroforming and other micromachining processes.

Moreover, such micro machining techniques facilitate the fabrication of a microswitch array of elements such as the microswitch illustrated in

FIGS. 1 and 2

.

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 control circuit

70

for enabling selective operation of the microswitch

1

. 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 actuator

42

, whereas the output of the AND gate

72

is connected to a heating coil

74

. 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 open/close selection line

78

.

The microswitch

70

remains in a bi-stable state controlled by the logical high or low signal of the open/close 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 open/close 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 open/close selection line

78

, the output of the AND gate

72

goes high, and a current is caused to flow through the heating coil

74

causing actuator

42

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 disconnected.

FIG. 5

shows an implementation of the control circuit using steering diodes as shown in FIG.

2

. In this arrangement, an array of heating coils

80

to

88

and associated steering diodes

89

to

97

are provided, each heating coil/diode pair acting to heat the 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 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 “release” actuators 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 “release” actuators 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.