GB2182747A - Actuator device - Google Patents

Abstract

An actuator utilises two resilient members 3, 4 composed of a shape memory alloy to move an actuation arm 1 between two or three rest positions. A shape memory alloy is a material which can be deformed below a certain critical temperature, but above that temperature it reverts to its original as manufactured configuration. The two resilient members act in opposition to each other, and each is raised in turn above its critical temperature by an electric current to achieve operation of the actuator arm 1 which cooperates with a pair of latching magnets 6, 7. <IMAGE>

Description

SPECIFICATION Actuator This invention relates to actuators which are capable of providing a degree of mechanical movement in a reliable and repetitive fashion.

Actuators often utilise electromagnetic solenoids or the like in order to achieve movement in a mechanism but suffer from the disadvantage of being relatively heavy and having a degree of complexity which may impair their reliability. Alternative actuators based on the use of what are sometimes termed Shape Memory Alloy (SMA) have been proposed for a two-state actuator, but such actuators require either the continued application of a considerable amount of energy to hold the actuator in one of its two possible states or the use of secondary bias springs. In general terms a shape memory alloy is a material which has two states which can be termed a relaxed state and an activated state, and it adopts one or other of the two states depending on whether its temperature is above or below a critical value.Presently available shape memory alloys of this kind are such that a considerable mechanical force can be exerted by the alloy when it is above a critical temperature at which the material adopts a strong austenitic structure. Below this critical temperature the material is a relatively weak martensitic crystal structure and can be easily deformed.

This invention seeks to provide an improved actuator which utilises shape memory alloy material.

According to this invention an actuator device includes two resiliant members composed of a shape memory alloy both of which are coupled to a common moveable actuation member, the resilient members being mounted in opposition to each other so that the moveable actuation member is caused to adopt one or other of two rest positions in dependence on which one of the resilient members is activated.

This is in contrast to known arrangements using shape memory alloy in which a resilient member composed of the shape memory alloy acts against a conventional mechanical spring.

The moveable actuation member may have additional rest positions located intermediate of the two rest positions mentioned above, but in the preferred embodiment of this invention, only two rest positions are provided. By activation of the shape memory alloy is meant that the alloy is at such temperature as to exert a strong mechanical force which tends to return its shape to its original manufactured shape, so that the moveable actuation member is forced to one rest position against the relatively weak opposing resistance of the non-activated resilient member which is in its relaxed state. When the resilient member which is activated to produce the required movement of the moveable actuation member is returned to its relaxed state by altering its temperature, the force acting on the actuation member due to both resilient members will have only a small imbalance.To ensure therefore that the actuation member does not move until one or other of the resilient members is next activated, it is very desirable to provide a restraining latch. The latch can take many forms, but a magnetic latch is particularly convenient as a small permanent magnet can readily exert sufficient force to overcome any minor imbalance in the forces exerted by the two relaxed resilient members. It is in fact, preferred to provide a magnetic latch which exerts a significantly greater restraining force than the minimum which is required to achieve the condition mentioned above.A shape memory alloy can be damaged if its shape is forceably altered whilst it is in the relatively strong austenitic structure and accordingly the mechanical latch is made sufficiently great so that activation of a second resilient member whilst the first is still in its activated state, does not cause movement of the moveable actuation member.

One suitable shape memory alloy is one composed of titanium and nickel. It has good ductility, corrosion resistance and a high electrical resistivity which makes it suitable for direct electrical heating. Its critical temperature is typically between 20 c and 80"c, and below the critical temperature the material is a relatively weak martensitic crystal structure which can be easily deformed. Above this critical temperature, the material adopts the strong austenitic structure and attempts to return to its original "as manufactured" configuration.

The invention is further described by way of example, with reference to the accompanying drawings in which Figure 1 illustrates one form of an actuator which is suitable for incorporation in a waveguide switch and Figure 2 shows such a switch in diagrammatic form.

Referring to the drawing, an actuator arm 1 is mounted on a central pivot 2 and is linked at its ends by means of two coiled springs 3 and 4 to an attachment bracket 5. The two springs 3 and 4 constitute resilient members and are composed of a shape memory alloy, typically titanium nickel, and whilst in their activated state both have the shape shown for spring 3-spring 4 is shown in its relaxed state. The shape of spring 3 is the natural shape which the springs have when originally manufactured, and they return naturally to this shape when they are heated above a critical temperature, but below this temperature they may be readily and easily deformed. Thus, spring 4 is extended beyond its natural length since it is in its relaxed state.It is convenient for the springs 3 and 4 to be coils as they can then readily allow for minor departures from their optimum length when installed in the actuator mechanism.

The actuator arm 1 is arranged to co-operate with a pair of latching magnets 6 and 7 each having an associated end stop 8, 9.

Two heat sinks 10 and 11 are positioned along the central axis 12 of the mechanism such that a spring is held closely adjacent to it when in its contracted state. An electrical circuit (not shown) is connected to each end of the springs so as to permit an electrical current to be passed through each spring 3,4 so as to heat it when required. Preferably a lock-out switch (not shown) is provided such that current can be passed through only one of the springs at a time.

In operation the actuator arm 1 can adopt the rest position shown in the drawing, or the actuator can be operated to tilt the arm 1 about the pivot 2 so as to bring the arm into contact with the end stop 9. The arm will remain in the position as shown whilst both springs 3 and 4 are below the critical temperature, so that both are in their weak state.

When it is desired to operate the actuator, an electric current is passed through the spring 4 to heat it above the critical temperature. When this temperature is reached, the spring adopts its active "strong" state and reverts to its contracted state, thereby pulling the actuator arm against the end stop 9. The electric current is switched off as the arm 1 moves and the spring 4 cools rapidly as it is brought into close proximity with the heat sink 10 which is arranged to snugly surround it so as to contact it. The presence of the heat sink 10 to achieve rapid cooling is fairly important if the mechanism is operative in a vacuum in which heat dissipation would otherwise be by radiation alone.

The magnet 7 is sufficiently strong to retain the arm 1 in position whilst both springs 3 and 4 are relaxed and it is also sufficiently strong to prevent the spring 3 moving the arm 1 in the event that the spring 3 is activated before the spring 4 has cooled below its critical temperature. This prevents either spring being damaged by forceably altering its shape whilst in its activated state. The provision of the heat sinks therefore enables the operating rate of the actuator mechanism to be greatly increased.

In a preferred embodiment the arm 1 is firmly attached to a rotatable boss or the like which is centred on the axis of the pivot 2 so that actuation produces a rotational movement of the boss through an angle of about 90".

For some purposes it is desirable to have an intermediate rest position at mid-way between the two end rest positions of the arm 1 so that it can be moved through angular steps of 45". This is achieved by having a small indent associated with the rotatable boss and arranging that only a short electrical heating pulse is applied to whichever of the two springs is to be activated so that movement of the arm is arrested by the indent. A subsequent heating pulse, whose length is not critical, is applied to complete or reverse the movement of the arm to bring it to an end rest position.

Figure 2 shows a waveguide switch in which the actuator is used to produce a required rotational movement. Refering to Figure 2 a conductive housing 20 is provided with four waveguide channels 21 22 23 24 which terminate at a central cylindrical space within which a rotatable boss 25 is mounted. This boss 25 fits closely within the housing 20 and contains three waveguide channels 26,27 28 which serve to link different combinations of waveguide channels 21,22,23,24 as the boss 25 is rotated through an angle of 90" in angular steps of 45".

Claims (8)

1. An actuator device including two resilient members composed of a shape memory alloy both of which are coupled to a common moveable actuation member, the resilient members being mounted in opposition to each other so that the moveable actuation member is caused to adopt one or other of two rest position in dependence on which one of the resilient members is activated.

2. An actuator device as claimed in claim 1 and wherein a latch is provided to hold the actuation member in its current rest position until one or other of the resilient members is next activated.

3. An actuator device as claimed in claim 2 and wherein a latch of a magnetic nature is associated with each of the said two rest positions.

4. An actuator device as claimed in any of the preceding claims and wherein an additional rest position is provided intermediate of said two rest positions.

5. An actuator device as claimed in any of the preceding claims and wherein a resilient member is activated by passing an electric current through it so as to raise its temperature above its critical value.

6. An actuator device as claimed in any of the preceding claims and wherein a heat sink is provided and positioned so as to be in close proximity to the position adooted by a resilient member when it adopts its original configuration.

7. An actuator as claimed in any of the preceding claims and which includes a waveguide switch so as to cause switching thereof.

8. An actuator device substantially as illustrated in and described with reference to Figure 1 of the accompanying drawing.