This invention relates to shape memory devices, and concerns in particular novel switch-like devices that employ shape memory elements actuable by some combination of conditions other than those ambient conditions directly surrounding the device.

It has been known since the sixties that certain alloys, now referred to as "Shape Memory Alloys" (SMAs), or "Form Memory Alloys" (FMAs), exhibit the rather strange property of "remembering" the shape in which they were first formed into some simple article, such as a coil or strip, despite the fact that the article has been subsequently, and apparently permanently, deformed out of that shape, this memory allowing the deformed article to return automatically to its original shape under the right ambient conditions. For example, a coil of such an alloy formed hot, and allowed to cool in its coil shape, can while cool be pulled straight, and will remain so as long as it stays cool - but if it is warmed up sufficiently it will reach a temperature at which it returns without outside assistance to its original, "hot" coil shape.

It is not necessary here to explain in detail the reasons for this behaviour - which is now so well understood as to be discussed in Textbooks - though perhaps it will help if it is observed that the memory effect is linked with the crystal structure of the alloy, which in turn depends upon the way in which some of the alloy constituents are dissolved in others, and to note firstly that this solubility is temperature

dependent, and secondly that different crystal forms have different sizes and shapes. As a result, if an article of alloy which is formed into a specific shape at a high temperature (above its Transi ion Temperature, Tz.) , having one sort of crystal structure (austenitic) , - and which is cooled in that shape down below its Transition Temperature such that it takes on a different crystal structure ( artensitic) , is then subjected to shape-changing plastic deformation, it has a set of crystal-internal (i.ntracrysta1) deformation-caused stress forces that are trying to return it to its undeformed shape. However, so long as the crystal structure remains martensitic, σrystal-to-crystal slip plane forces - intercrystal forces - prevent these i-?tracrystal shape-restoring forces having any significant effect. But, if the article is heated to the point where the structure reverts to austenitic then the intracrystal stress forces are increased as the structure changes, and become sufficient to overcome the restoration-preventing slip plane forces, and so the article returns to its original undeformed shape, sometimes almost instantaneously, sometimes progressively over a range of temperature increments.

Ordinarily, this transformation is a one-way process, as the strong intraσrystal forces cause the stresses in the material to be relieved. However, through a sequence of heating and cold deformation it is possible to fix the relationship of the austenitic and martensitic phases so that the transition to the austenitic phase elastically deforms the martensite rather than allowing it to be plasticly altered. The result of this "training" is a two-way effect, with the article having one shape when hot and another when cold. Hii

The life of the shape memory effect is limited, as small shifts in the crystal arrangement may occur to diminish the shape-changing forces, but with modern materials a memory metal element can reliably be expected to be able to undergo several thousand - even several tens of thousand - shape changes before any significant effect is noticeable. There are also limits imposed upon the absolute amount of deformation which can be imposed, and upon the temperature changes that the element can withstand, but these matters need not be considered here.

Various different metals, and their alloys, show the memory effect. That first investigated and actually used was an alloy of nickel and titanium, known as Ni tinol , but more recently the materials finding most use are copper alloys, typically, copper/aluminium, σopper/aluminium/nickel , σopper/zinc/tin and copper/ zinc/aluminium.

A Paper in which there is discussed both memory metals generally and their use in various devices (such as thermostatic central heating radiator valves, and as shrink-fit pipe couplings) is that by Michael and Hart, reported in The Metal l urgist and Ma teri als Technologi s t following its reading at the Institution's Meeting on 30th September 1982.

The devices using memory metals are in general switch-like devices wherein the shape change as a memory metal element is caused to pass through its Transition Temperature and so changes its shape is employed to drive or trigger some action, the devices often including means, such as bias pre-loading, enabling the actual change of shape to have an apparent effect at some other temperature distant from the Transition one. A well-known example of such a use is in

thermostatically-controlled central heating radiator valves, where the ambient room temperature controls the opening or closing of a valve (in the hot water supply to the radiator) driven by a memory metal element (and the actual temperature at which the valve is operated is determined by the amount of bias pre-load restraining the element from operating and actuating the valve, which amount can be pre-set by the user).

There are many memory element devices, and a feature that the majority of them seem to have in common is that they are caused to operate directly by the temperature of the environment into which they are placed - by exposure of the memory element to the ambient temperature. In the radiator valve mentioned above, for instance, the element is exposed to the room temperature air around it (because it is that which is to be modified by operation of the radiator) ; it is the surroundings which supply (or remove) the heat energy necessary to cause the memory element to heat up (or cool down), and thus to change its shape. More recently, however, there has been proposed a rather different type of device - thus, rather than the memory element being affected directly by the ambient conditions that the device itself (or some equipment controlled by the device) seeks. to regulate, it has been suggested that the element could be controlled indirectly, by some parameter other than that which the device itself seeks to control. More specifically, in this type of arrangement there is employed a self- contained memory metal device - an actuator of some sort employing a memory metal element to cause operation thereof as a result of its shape changing when suitably heated - that is in effect separated from the ambient temperature conditions, being housed within its own "micro-environment", and being provided with heating

means the action of which is at least primarily unrelated to the ambient temperature. Unfortunately, even these indirect devices, while undoubtedly superior to the earlier direct devices, have serious drawbacks which have, until now, stunted the design of memory- metal-using devices, and limited the range of control systems that could usefully incorporate such a device. For example, it is usual that the shape memory element be heated by a current through the element itself or vi a an adjacent heating element transferring its heat by conduction. In the former case the wire of the element must be of small diameter (so as to have a suitable electrical resistance that will produce the required rapid heating effect), while in the latter the interface between the heater and the memory element, which is critical, also necessitates small thin sections - and since the force created by a memory element is a function of its section the need for a small element inevitably results in a small relatively weak device. Now, larger devices are notionally possible, but then there is the serious problem of the heat capacity of the memory element, which creates a considerable delay both before the element is activated and before, on cooling, it re-sets. Moreover, the time all this takes to happen - and especially for the element to cool - is significantly affected by the ambient temperature.

The present invention proposes a possible solution to this difficulty by deploying the shape memory element within a micro-environment and then effecting its operation through the controlled application of both heating and cooling regimes, such that the working of the device is quite independent of ambient conditions on both the strong and weak cycles, so enabling larger and hence more powerful elements to operate with rapidity.

In one aspect, therefore, this invention provides a memory metal device containing, in combination within a housing, a memory metal element, a switchable element heating means, and a complementary switchable element cooling means, such that operation of the device is independent of the ambient temperature, and the rate of both heating and cooling of the element can be controlled to pass through the element's transition temperature range.

The memory metal device of the invention is a self- contained device - that is to say, it is a device which incorporates both its own memory element heating means and its own element cooling means rather than relying on the ambient atmosphere to provide the element-actuating heat. It is at least nominally independent of the surrounding temperature, and does not need, or respond to, changes in that temperature to make it operate. Indeed, as will become clearer hereinafter, it may be that the device, while naturally containing as its "active" part, a temperature-sensitive, heat-responsive memory element, is in no way connected with the surroundings' temperature, whether to be controlled by it or to effect control over it.

The device of the invention may be for any purpose, taking any physical form necessary therefor. For the most part, though, it will not surprisingly be a device which includes an output member, and provides actual physical movement of that output member, as its memory element changes shape, in response to whatever conditions are selected to cause the heating means to operate so as to effect that memory element shape change, the movement of the output member then being linked up to some further equipment to produce the

desired effect. One particularly convenient shape change is an elongation (see the further discussion hereinafter) that can be used to drive an abutting (but desirably not actually attached) piston-like output member to or fro - in and out of the device's housing, say - which output member is then operatively connected to, and thus drives in its turn, almost any physical apparatus (such as a lever switch, a rotary switch, a clamping mechanism, a valve or a lock hasp).

Various forms of output connection are possible. For example, a rotary connection may be achieved by having the output member drive a coarse-pitch internally-εcrew-threaded nut along a correspondingly- coarse-pitσh externally-screw-threaded rod - a "fast" screw - under circumstances where the nut is prevented from rotating (but can translocate - move longitudinally) while the rod is prevented from translocating but is free to rotate around its axis; pushing the nut back and forth along the rod drives the rod to rotate, so converting the longitudinal motion of the memory metal element into the rotational movement of the rod - which may then in its turn be connected on to whatever item of apparatus it is to operate. In another example the end of the output member projects out beyond the housing and has a piston-like cup attached thereto (and possibly flexibly sealed to the housing by a rubber skirt or something similar). However, an especially preferred form of output connection is to a Bowden-type cable (in which a stiff but flexible wire forms a longitudinally-movable core within a stiff but flexible outer sheath, or casing. Bowden cables, as typically met in a bicycle's cable brakes, are well known per se . They are commonly used to "transfer" force/movement at one end over a distance, and around corners and obstructions, to effect some action at the other end,

the transfer being achieved either by clamping the sheath stationary and moving the core wire or vice versa (either method may be employed with the memory metal device of the invention, though it may be more convenient to move the sheath than the core) . This use of a Bowden cable arrangement enables the device of the invention (via its output member) to be operatively linked to a remote effector the positioning of which relative to the device itself is of almost no consequence provided it be within the reach of the (length of the) cable. A Bowden cable arrangement is discussed in more detail hereinafter with reference to the accompanying Drawings.

The device of the invention comprises the combination of a memory metal element together with its associated heating and cooling means all contained within a housing, this housing thus providing the combination with its own micro-environment that in effect shuts it off from the ambient conditions around the device. The housing may take any convenient form - and in fact it may be a notional housing, formed perhaps by a framework within which the combination is mounted, rather than a sealed container, so that strictly speaking the combination is separated from but not truly sealed off from the ambient conditions - but will in general most preferably be an actual container, or canister, into which the combination is effectively sealed (with its output member operatively projecting therefrom), the container being shaped, and the combinatio-n being mounted therein, as appropriate.

^" Within the housing of the inventive device is the combination of a memory metal element and its heating/ cooling means. The memory metal element may take any conceivable form that can usefully be linked up, via a

suitable output member, to the world outside the housing (so as to drive or control some equipment). The preferred elements, though, have it in common that they are, in their deformed state, in some way bent or curved, so that as they undergo the temperature-induced shape change back to their original shape they straighten - or attempt to straighten (or become less curved) - so as to cause an increase in at least one of their dimensions that can be used to drive the output member. Spirals and helixes are typical of this concept, and indeed the helical spring, changing in overall length as it reacts to temperature changes, is by far the most convenient form for the memory metal element to have. Such a spring may expand and/or contract when it reverts to its original shape, and so may be regarded as either a compression spring or a tension spring, as appropriate.

The memory metal element may be mounted in its housing in any appropriate manner, and may in particular be associated with pre-settable biassing means whereby the point within a temperature range at which the element operates is adjustable. One advantageous mounting arrangement has the element butted up against a reaction surface, with a force measuring device, such as a strain gauge, between the two so that the output force applied by the output member driven by the element can be determined (conveniently electronically), and perhaps used in some feedback manner to control the heating means and thus the actuation of the element itself.

Combined with the memory metal element - that is to say, in operative association with the element within the housing so as to be able when working to transfer heat energy thereto to cause the element to change its shape - is the element heating means. This, too, can

take many forms, and the source of the heat (and, indeed, its removal; see hereinafter) may be mounted into the device or it may be quite separate therefrom (this latter arrangement is particularly useful when a number of devices are being controlled simultaneously in a system). For example, the source can be a simple radiated heat receptor member conductively or reflectively linked to the memory metal element (so as to pick up IR radiation from a source thereof external to the device and deliver it to the element to heat it). Another possibility is an electrically-powered heating element (such as a resistive heater; a much preferred form of this is a halogen bulb designed for heat/IR output rather than visible light output, a typical such bulb being that type available for use in cooker hot plates), when the device will conveniently include electrical power supply means feeding the heater. A third variety of heating means is a supply of hot fluid (such as air or water, or some hydraulic liquid such as a glycol or a hydrocarbon or silicone oil, preferably having a low specific heat and a high thermal conductivity so as rapidly to transfer all its heat to the memory element, pumped from a housing-external source through the housing as and when required) , when the device will similarly include fluid supply means feeding the relevant fluid, at the required temperature, into - and then out of - the housing. This idea of a supply of hot fluid, much like hot water to a radiator in a domestic central heating system, is of particular use in cases where there are several memory metal devices arranged in some common group, all of which are to act together, and hot working fluid is provided to them all in a circuit. Operation of a selection valve on each separate device might allow hot fluid to enter a sealed chamber holding one or more memory element, and

when it does so the device will operate rapidly to provide its force in whichever configuration is intended.

The heating means is associated with cooling means, so that the memory element can positively be cooled subsequent to its heating to ensure that it returns to its "cold" shape within an acceptable time. Various forms of cooling system are possible, including Peltier- effect arrangements, but generally the use of a circulating cooling fluid is preferred. As in the case of the heating means, one particularly convenient cooling means arrangement is a supply of cold fluid pumped from an external source to and through the element-containing housing as and when required. When using a circulating hot fluid it will, as inferred above, normally be a matter of merely pumping in cold fluid after the hot fluid; when the heating means is a electrically-resistive element it will usually be necessary to provide an independent cold fluid source and supply (in one particularly preferred embodiment, as discussed further hereinafter with reference to the accompanying Drawings, there is employed just such a combination of electrical heating and circulating fluid cooling) .

In the device of the invention the heating means is independent of the ambient temperature - by which is meant that regardless of what the device has for its purpose the operation of the device, by the temperature- caused shape change of its memory metal element, is not directly caused by the ambient temperature (though it may be indirect ly caused thereby, as observed below). For example, one device might operate to drive a Bowden- cable link to pull open a door upon receipt of an electrical signal from a pressure pad or IR-or

ultrasound detection system, the detector system providing an electrical output fed to and powering a halogen lamp heating unit within the device; the device operates without regard to the ambien t temperature, even though its own, micro-climate temperature is changed to cause it to work. Again, an IR-detector system might gather IR radiation and focus it upon the memory metal element (or upon a thermally-conductive receptor conductively linked to the element), the device then opening a valve to allow water therethrough to feed a pipe and spray arrangement. And, as intimated above, there might be a device in which the ambient temperature indirectly causes memory metal element shape change; the rise in temperature of the ambient surroundings might trigger some form of detection apparatus that, in its turn, powers the heating means of a device of the invention so as to cause the memory metal element to change its shape and initiate the actuation of a ventilation system (by, say, mechanically opening the windows) .

Although memory metal elements have a long life, and can go through many (thousands of) shape change cycles without any significant diminution of their properties, nevertheless they do get "tired", working less well, and so it is desirable to be able to replace them without too much difficulty. Similarly, it is likely that the heating means will also need replacing in time, particularly if it is of the electrically- resistive sort (such as a halogen bulb), so that that too should desirably be relatively easily changeable. In fact, it is a preferred feature of the inventive devices that the memory metal element/heating means combination be an actual, physical combination - the two being mounted together to form an integral unit (where the element is a helical spring and the heater is a

halogen bulb it is very convenient to have the latter disposed in the central space within the helix of the former) - and, moreover, that this combination, or unit, be removably retained within the housing, so that it can easily be taken out or inserted, and thus be replaced, as a whole without disturbing either the device's micro-environment or the links therefrom to either the output means or the heating means connections. There will be many ways in which this unit replaceabi1ity can be achieved, but one particularly useful way is to have the unit in the form of a plug-in module that can be secured into the device housing from the rear (that is, from the side opposite that of the output member); such a plug may conveniently be mounted into the housing by a screw or bayonet fitting, and if necessary will be a sealing fit thereinto.

The operation of the device of the invention is effected by suitably varying the temperature of the memory metal element such that it passes through its Transitional Temperature, and so changes shape. It will get hotter if more heat is provided by the heating means (or if less heat energy is taken away by any heat sink arrangement) - and, conversely, it will get colder if less heat (or more cooling) is provided. In a practical case it is usual to start, at a neutral (or stand by) state, with sufficient heat from the heater to take the element to just below its Transition Temperature, and with sufficient cooling from the heat sink to maintain it there. Then, when there occurs some external factor intended to trigger the device off (and make it, in its turn, effect some action), either the heating is increased or the cooling is reduced - but in either event the temperature change required to cause the

desired memory element shape change is small, so that the device reacts almost instantaneously (whereas if, as is possible, the element had to be heated up from cold, this would incur a delay that might be unacceptable) .

An embodiment of the invention is now described, though by way of illustration only, with reference to the accompanying Drawings in which:

Figure 1 shows a cutaway isometric view of a memory metal device of the invention.

The memory metal device (generally 11) of Figure 1 comprises a double-walled tubular housing (12) within which is mounted, as an integral unit, a helical spring¬ like memory element (13) having secured within its central, axial, space a tubular heating means (14: a can containing a halogen bulb 15 designed for maximum IR output). The unit is secured to a force-measuring platform (16) which in turn is attached to a plug (17) that is mounted into the otherwise open end (18) of housing 12, to which it is releasably secured by a cammed locking mechanism (not shown). The electrical power leads to the lamp 15 and to the force measuring platform 16 are led out through the can to termination pads (not shown) around the edge of the plug 17, and when the plug is securely locked into place these pads in turn make electrical contact with corresponding pads (not shown, but connectable either to a prime power source, such as mains electricity, or to some suitable electronic measuring equipment, as appropriate) on the inner surface of the open end 18 of the housing.

The housing 12 is double walled - an inner tube (21) sealed into an outer tube (22) to define a space (23) therebetween - and in order to provide a satisfactory thermal gradient across the element 13 this volume 23 is in use supplied with cooling fluid (not shown) via input and output ports (24, 25) disposed

along and around the housing to result in an acceptable flow of fluid through all parts of the volume 23.

At the end (31) of the housing distant the plug end 18 there is a housing co-axial tubular effector mounting (32), and within this mounting there is the device's effector, or output member (33). This output member is an elongate, rod-like device, with a piston¬ like head (34) at the inboard end of its shank that in use engages with the non-mounted end of the helical element 13 so that, as the element 13 moves in and out so the output member also moves in and out, coupling the element movement to whatever the outboard end of the shank may be operatively attached to. In this particular embodiment the shank, which is split into two side-by-side portions (35) with spline-like guidance and bearing ribs (as 36) , is in use located up against the end of the outer sheath (37) of a Bowden cable (generally 38), with the cable's core wire (39) retained in and by a between-shank-portion bridge piece (40; the exact way in which this is achieved is not shown) . In operation, then, as the memory metal element 13 pushes the piston-headed output member 33 out and in (assisted by a return spring, not shown, between the inner face of the housing end 31 and the Outboard face of the piston head 34) , so the relative positioning of the Bowden cable core 39 within its sheath 38 changes, making the cable appear longer or shorter, and there is actuated whatever equipment is attached, at some distant position, to the other end of the Bowden cable.

The operation of the device of the invention is effected by suitably varying the temperature of the coiled element 13. It will get hotter if more current is provided to the halogen lamp 15 or if less coolant is supplied to the intra-housing volume 23 - and,

conversely, it will get colder is less current or more coolant is provided. In a practical case it is usual to start, at a neutral state, with sufficient heat from the heater 14 to take the element 13 to just below its Transition temperature, and with sufficient cooling from the cooled housing to maintain it there. Then, when there occurs some external factor intended to trigger the device off (and make it, in its turn, effect some action), either the heating is increased or the cooling is reduced - but in either event the temperature change required to cause the desired memory element shape change is small, so that the device reacts almost instantaneously.