GB2068545A - Temperature-responsive actuating elements - Google Patents

Abstract

A heat operated actuator comprises a plunger 4 which is slidably urged towards its operative position (shown in Fig. 2) by a first spring 12 of shape-memory-effect material when heat is applied to the spring. A bias spring 10 urges the plunger towards the retracted position. A latching member 24 is shown urged into engagement with an annular recess 20 in the plunger, so as to hold it in the operative position by a second SME spring 34 which acts against the force of another bias spring 28 when heat is applied to it. A heater for each of the SME springs is connected in parallel across a power supply via a switch 48, the heater for the first SME spring having a microswitch 62 connected in circuit with it. The microswitch is actuated by the latching member to disconnect the heater from the supply when the latch is engaged. <IMAGE>

Description

SPECIFICATION Heat operated actuator This invention relates to an actuator device operated by heat.

According to the present invention there is provided a heat operated actuator comprising an actuator member, a first heat-activated device arranged to move the actuator member, when heated, from a first to a second portion against the action of spring means; latching means adapted to engage and hold the actuator when it has reached the second position, the latching means being movable between its engaged and disengaged positions by means of a second heat-activated device acting to urge it in one direction and spring means acting in the opposite direction; and control means for supplying energy to the heatactivated devices so as to operate the actuator.

In one embodiment of the invention the arrangement is such that the second heatactivated device is arranged to urge the latching means away from the engaged position when actuated, the control means being arranged to supply energy only to the first heat activated device when switched "ON" and to supply energy only to the second heat activated device when switched "OFF".

Thus one form of the invention comprises a heat operated actuator comprising an actuator member, a first heat-activated device arranged to move the actuator member, when heated, from a first to a second position against the action of spring means; spring operated latching means adapted to engage and hold the actuator when it has reached the second position; a second heatactivated device for retracting the latching means; and control means having first and second positions which is so arranged that (a) upon switching to the first position, energy is supplied only to the first heat-activated device, and the supply of energy is cut off when the latching means is activated, and (b) upon switching to the second position, energy is supplied only to the second heat-activated device, and the supply is cut off when the latching means has reached the retracted position.

In an alternative embodiment of the invention the second heat-activated device is arranged to urge the latching means into the engaged position when activated, the control means being arranged to supply energy to both of the heat-activated devices when the control means is switched to the ON position. The arrangement may be such that the supply of energy to the first heat-activated device is cut off when the latch has engaged the actuator member.

Preferably, the control means includes an electrical switching circuit, and the heat-activated devices are heated by a supply of electrical energy.

The energy may be supplied directly to the heatactivated device, or it may be supplied to a heater element which contacts or is adjacent to the heatactivated device.

Preferably, the actuator member is in the form of a piston which slides in a fixed cylinder and has a plunger which extends from the end of the cylinder to engage with the device to be actuated.

The latching means may comprise a plunger which.can pass through an aperture in the side of the cylinder to engage the actuator member.

When the actuator and the latching means are of substantially cylindrical form, the heat-activated devices may conveniently be compression springs made of "shape memory effect" alloy having an elastic modulus which varies significantly with temperature in a reversible manner over a transition temperature range. Preferably, the spring is heated by means of a "PTC" (positive temperature co-efficient) heating element mounted adjacent to it, through which an electrical current is passed. These devices have the advantage that their electrical resistance is low when they are at ambient temperature, rises steeply as the temperature increases, and then levels off to a stable value so that the heat output rapidly reaches a constant level.

Some embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a heat operated actuator according to one embodiment of the invention in the "OFF" position; Figure 2 shows the actuator of Figure 1 in the "ON" position; and Figure 3 shows a heat-operated actuator according to another embodiment of the invention.

Referring to Figure 1 ,the actuator comprises a cylinder 2 in which a piston 4 is slidable. An end portion 6 of the piston, which is of reduced diameter, extends through an axial bore in the end wall 8 of the cylinder to engage with a device to be actuated (not shown). The piston 4 is normally urged to the inward position, as shown in full lines, by means of a return spring 10 located between the end wall of the cylinder and the annular end face of the larger diameter portion of piston 4. A spring 12 of "shape memory effect" alloy is positioned between the other end face 1 4 of the piston 4 and a heater device 1 6 mounted at the corresponding end 1 8 of the cylinder.

Because the compression spring 12 is made of shape memory effect alloy, it has a modulus of elasticity which varies significantly with temperature in a reversible manner over a transition temperature range. Such an alloy is well known, the alloy preferred for use in the present invention having a composition by weight of the order of: copper 70% zinc 26% aluminium 4% the actual proportions of the constituents differing in minor respects from those figures according to the temperature requirements. However other suitable alloys may be used instead.

The alloy is heat-treated to bring it into a condition in which it exhibits a martensitic transformation when subject to temperature change in the transition range. The characteristic displayed by the alloy is a progressive increase in stiffness of the spring 12 as the temperature rises through the range. At or below the lower end of that range, the stiffness is low and bias spring 10, which does not show a significant change in elastic modulus with temperature, urges spring 12 into the condition shown in Figure 1 where the turns of the spring are forced into engagement with one another. As the temperature of spring 12 increases, the stiffness also increases with the result that the turns of the spring are progressively spaced apart.Thus, when energy is supplied to the heater element 16, which is a positive temperature co-efficient device, in a manner to be explained below, the piston 4 will be moved to the right as shown in Figure 1, since the stiffness of spring 12 will increase relative to that of spring 10.

The piston 4 is also formed with an annular recess 20, which registers with an aperture 22 in the side wall of the cylinder 2 when the piston is in its outermost position (Figure 2). A spring operated plunger 24 is mounted in a housing 26 on the outside of the cylinder, so as to engage in the aperture 22, and thus to enter the annular recess 20, when the piston 4 has moved to the position shown in Figure 2. The plunger 24 is urged into the engaged position by means of a bias spring 28 which does not show a significant change in elastic modulus with temperature. The bias spring 28 engages between an end wall 30 of the housing 26 and a fixed collar 32 on the plunger 24, and a second heat-activated device, in the form of an SME spring 34, is located between collar 32 and the inner end wall 36 of the housing 26.A sleeve-like heating element 38 is mounted in the bore of housing 26, so as to surround the SME spring 34. With this arrangement, the SME spring can be caused to expand, so as to retract the plunger 24, when electrical energy is supplied to the PTC element 38, as explained below.

The outer end 40 of plunger 24 is arranged to co-operate with an aperture 42 in the end wall 30 of the housing 26, so that, in the retracted position of the plunger 24, the portion 40 engages the actuating members of a pair of microswitches 44 and 46. Microswitch 44 has normally closed contacts and microswitch 46 has normally open contacts, so that, in the position shown in Figure 1, in which the plunger 24 is retracted, the contacts of microswitch 44 are open and those of microswitch 46 are closed. A switch 48 having changeover contacts 50 and 52 is connected between one terminal of a power supply and an input terminal of each of the microswitches, so that, in the initial position, that is to say before the device is operated, the switch connects the power supply to microswitch 44 via the switch contact 50.Since the contacts of microswitch 44 are open and microswitch 46 is isolated from the supply by switch 48, no current passes to any part of the actuator device.

When the movable member of switch 48 is moved so as to connect the supply to contact 52, current flows through the closed contacts of microswitch 46 to terminal 54 of the heater device 16, whose other terminal 56 is permanently connected to the other side of the power supply. As a result, the SME spring 1 2 is heated, so that it expands pushing the piston 4 outwardly, against the substantially constant force of return spring 10, so as to operate the device which is to be controlled by the actuator.

When the piston 4 reaches the position shown in Figure 2, the plunger 24 is urged into the position shown in this figure by the spring 28. The retraction of the outer end 40 of the plunger causes the contacts of microswitch 46 to open, breaking the circuit to the heater 16, so that the SME spring 12 begins to cool down, leaving the piston held in the outer position by virtue of the engagement of plunger 24 in annular recess 20.

The retraction of the outer end 40 of plunger 24 also causes the contacts of microswitch 44 to close and thus, when the switch member 48 is returned to the initial position, as indicated by the broken line in Figure 2, so that contact 50 of the switch is again connected to the power supply, current flows through the contacts of microswitch 44 to terminal 58 of the PTC heater 38, which surrounds the SME spring 34 in the plunger housing. The other terminal 60 of the heater element 38 is permanently connected to the other side of the power supply and thus the SME spring 34 is heated and caused to expand so that it urges the plunger 24 towards the retracted position against the substantially constant force of the return spring 28.This causes the plunger 24 to return to the position shown in Figure 1, allowing the piston 4 to return to its initial position and also operating the actuating members of the microswitches so as to again isolate heater 38 from the power supply.

The arrangement of Figure 3 is mechanically similar to that of Figure 1, except that the positions of the springs 28 and 36 relative to the plunger 24 are reversed (corresponding parts have been given the same reference in both embodiments). Thus when the actuator is to be moved from the rest position shown (which corresponds to the position shown in Figure 1 for the first embodiment), energy is supplied to the heater 38 of the SME spring 34 as well as to the heater 16 of the SME spring 1 2. Consequently the two heaters can be simply connected in parallel 5 across the power supply by means of a normal onoff switch 48. This enables the device to be controlled by a two wire connection rather than the three wire connection required for the arrangement of Figures 1 and 2. In order to ensure that the device will be released rapidly when it is switched to the OFF position, the embodiment shown includes a microswitch 62, operated by the end 40 of plunger 24, which is arranged to disconnect the heater element 1 6 from the power supply when the plunger 24 has reached the engaged position, so that the spring 12 will have cooled down by the time the control switch is moved to the "OFF" position. It will however be appreciated that this microswitch is not essential in all applications.

In these embodiments of the invention, the heat activated devices are SME springs, but it will be appreciated that other types of heat-activated devices, such as bimetallic elements, could be used. It will also be appreciated that alternative heating means could be used in place of PTC elements, such as resistive heaters or resistive coatings on the SME springs themselves.

Similarly, instead of the pair of single pole microswitches used in Figure 1, a single doublepole changeover switch could be used. The device of the invention can be used in various applications as a substitute for a solenoid: for example it can be used to operate the louvres of a ventilating fan.

Claims (12)

1. A heat operated actuator comprising an actuator member, a first heat-activated device arranged to move the actuator member, when heated, from a first to a second position against the action of spring means; latching means adapted to engage and hold the actuator when it has reached the second position, the latching means being movable between its engaged and disengaged positions by means of a second heatactivated device acting to urge it in one direction and spring means acting in the opposite direction; and control means for supplying energy to the heat-activated devices so as to operate the actuator.

2. An actuator according to claim 1 in which the actuator member comprises a plunger.

3. An actuator according to claim 2 in which the actuating means comprises a second plunger which is movable in a direction perpendicular to the direction of movement of the first plunger and is adapted to engage a recess in the body of the actuator member.

4. An actuator according to any preceding claim in which each of the heat-activated device's comprises a spring of shape-memory-effect ("SME") material.

5. An actuator according to claim 4 in which each heat-activated device is arranged to be activated by respective electrical heating means.

6. An actuator according to any preceding claim in which the second heat-activated device is arranged to urge the latching means toward the engaged position.

7. An actuator according to claim 6 in which the two heating means are connected in parallel across a power supply via a manually-operable onoff switch.

8. An actuator according to claim 7 in which the first heating means is connected to the switch via a switching device actuated by the latching means, the arrangement being such that the first heating means is disconnected from the power supply by the switching device whenever the latching means is in the engaged position.

9. An actuator according to any of claims 1 to 5 in which the second heat-activated device is arranged to urge the latching means away from the engaged position, the control means being arranged to supply energy only to the first heating means when switched to the "ON" position, to supply energy only to the second heating means device when switched to the "OFF" position so as to withdraw the latching means, and to disconnect the supply of energy when the actuator member has returned to its first position.

10. An actuator according to claim 9 in which the control means includes an on-off switch with changeover contacts and at least one switching device actuated by the latching means, the latching means being arranged to operate the switching device so as to complete the energy supply circuit only between one contact of the onoff switch and the first heat-activated device when the latching device is disengaged, and to complete the energy supply circuit only between the other contact of the on-off switch and the second heating device when the latching device is engaged.

11. A heat-operated actuator substantially as herein described with reference to Figure 1 and Figure 2 of the accompanying drawings.

12. A heat-operated actuator substantially as herein described with reference to Figure 3 of the accompanying drawings.