HK1140529A - Actuator comprising elements made of shape memory alloy with broadened range of working temperatures - Google Patents

Description

Actuator comprising an element made of a shape memory alloy having an enlarged operating temperature range

The invention relates to an actuator comprising an element made of a shape memory alloy, which actuator retains its functionality over a larger temperature range than similar known actuators.

Shape memory alloys are well known in the art as a combination of the acronym "SMA". Although various compositions of SMA are widely known, in practice only those compositions of Ni-Ti, preferably comprising 54-55.5% by weight of nickel, the remainder being titanium (other minor components are also permissible).

It is known that mechanical parts made of Ni-Ti alloys are able to interconvert between two shapes due to temperature variations which cause phase transformations in the alloy microstructure. The stable phase at higher temperatures is called austenite and the stable phase at lower temperatures is called martensite. According to the hysteresis cycle in the temperature deformation diagram, a transition occurs between the two phases, characterized by four temperature values: when heated, it reaches a temperature A at the beginning of the transformation from the martensite phase stable low temperature to the austenite phase_sThen reaches a temperature A at the completion of the transformation into austenite_f(A_f＞A_s) (ii) a When cooled, starting from a temperature at which the austenite phase is stable, reaches a temperature M at which the transformation into the martensite phase begins_sThereafter reaching a temperature M at the completion of such a transition_f(M_f＜M_s). These hysteresis cycle diagrams are shown, for example, in patents US4,896,955 and EP 807276.

Devices or parts comprising a moving element made of SMA (hereinafter referred to as SMA actuators for the sake of simplicity) are known and studied mainly in the automotive field, to replace actuators employing electric motors, for example in the lock of a car; in the following description, particular reference is made to SMA elements having a linear shape, but the actuator of the invention can also be used with such strip-shaped or other similarly shaped elements. In the SMA actuators proposed so far, the wire heating is usually obtained by passing an electric current therethrough; the subsequent deformation is spontaneous and exerts a suitable strength capable of transmitting a movement to the moving part to which it is coupled. The reverse transformation to the martensitic phase occurs due to the natural cooling of the wire when the current is switched off, thereby facilitating the return to the starting state of the shape by applying a force, such as by a biasing spring or the like.

Heretofore, the automotive industry mandated that there have been limits to the use of SMA actuators in the industry, requiring moving parts having a life of at least 50,000 cycles (e.g., open-closed in the case of a locking mechanism) in the temperature range of-20 ℃ to +80 ℃. SMA wires made of Ni-Ti alloys, usually M_f< 80 ℃, as a result, the resulting martensite transition is rather difficult, if not impossible, to perform for a complete operating cycle of the actuator.

It is known that by increasing the load applied to an SMA wire, it is possible to shift its hysteresis curve towards higher temperatures; thus, in principle it would be possible to obtain an SMA actuator that functions correctly even at 80 ℃, by suitably pre-tensioning the wire; however, a constant heavy load may have the disadvantage of quickly weakening the wire, thus breaking it or always disabling the actuator after thousands of cycles.

It is an object of the present invention to provide an SMA actuator which overcomes the above problems.

This object is achieved with an actuator comprising:

-a first element made of a shape memory alloy capable of changing its shape when heated, the fixed first and second ends of which are connected to a controlled mechanical part;

-means for heating said first element of shape memory alloy; and

-biasing means enabling said first element to return to its original shape when cooled, these means having a first end connected to the second end of said first element and a second end connected to an internal restraint of the actuator,

the method is characterized in that: the actuator further comprises M of said first element of shape memory alloy when the external temperature exceeds_fTime of value is suitableMeans incorporated into the internal restraint of the movement actuator and increasing the load applied to the biasing means.

The present invention is based on the following observations: even when the requirements of the end application impose higher test temperatures, these are not constant operating temperatures of the SMA actuator; typically in the final application, the actuator will have to perform only some of its cycles at high temperatures, while performing other cycles at low temperatures. It is therefore possible to design and employ an actuator in which the load applied to the functional SMA element can vary with temperature, such that its hysteresis cycle is generated above the external temperature for each value thereof. According to the invention, a small load is applied to a functional SMA element at a lower external temperature so as not to force such an element excessively and inefficiently, and a heavier load is applied when the external temperature exceeds a threshold value, such as +80 ℃.

The means to increase the load applied to the biasing means, and hence indirectly to the functional SMA element, may be different. In principle, even manual means can be foreseen, such as the lever being moved by the operator when it is observed that the actuator no longer works correctly. More commonly, the means of increasing the load react spontaneously to an increase in external temperature; for example, it is possible to employ a motor connected to a temperature sensor, a metal part that stretches a sufficient length (e.g., spirally wound) when the temperature rises, or a bimetal part that changes its shape when its threshold temperature is reached.

In a preferred embodiment of the invention, the means for moving the internal restraint of the actuator and increasing the load applied to the biasing means is formed as a second shape memory element in thermal equilibrium with the surrounding environment and has a fixed end and an end coupled to the internal restraint of the actuator; the second SMA element must be sized to exert greater strength during transformation due to heating relative to the first SMA element. The invention in this preferred embodiment will be mentioned below.

The invention will now be illustrated with reference to the accompanying drawings, in which,

figure 1 schematically illustrates an actuator according to a first embodiment of the invention; in particular, in part a) of the figure, the actuator is shown in a low temperature state, and in part b), the actuator is shown in a high temperature state;

FIG. 2 schematically illustrates a possible alternative embodiment of the present invention; also in this case, in part a) of the figure, the actuator in a low temperature state is shown, and in part b), the actuator in a high temperature state is shown.

Fig. 1a schematically shows an actuator according to the invention in a cold state, i.e. at a low outside ambient temperature, e.g. a temperature T below 50 deg.c_alAnd the time of the lower operation.

The actuator 10 comprises an SMA wire 11 forming a first element of an SMA as defined previously and having a first end (on the left-hand side in the figure) connected to a fixed component, schematically shown as a wall 12, and a second end connected to a controlled mechanical component (not shown), for example by means of a hook 13; the controlled mechanical part may be of any type that performs its function by movement that can be translational or rotational, for example it may be part of a lock; in the figure, the hook 13 is illustrated performing its action by moving towards the left-hand side, as indicated by the arrow. The second end of the wire 11 is coupled to a first end of a biasing means which, when cooled, contributes to the wire 11 returning to a stable shape or size at low temperatures; also illustratively, the biasing means includes a conventional spring 14. The second end of the spring is fixed to a slider 15 forming an internal limiter of the actuator. The slider 15 is housed inside a cylinder 16, which cylinder 16 is in turn fixed to a wall 17 outside the actuator and forms an external fixed limit for the actuator. On the opposite side of the slider 15 with respect to the spring 14, a second SMA element 18 is fixed, this element 18 being housed inside the cylinder; in particular, the second SMA element has a first end fixed to the slider 15 and a second end fixed to the bottom of the cylinder; the element 18 is shown in the figure as a bulletThe spring, but any other shape is possible, for example it may be a strip or a wire with a diameter larger than the diameter of the wire 11. The SMA elements in the actuator 10, i.e. the wires 11 and the elements 18, are treated during the manufacturing stage to contract when heated. The wire 11 is connected to means for its heating; in the figure, the means, indicated by conductors 19, 19', are connected to a power supply (not shown) in order to heat the wire 11 by means of an electric current and thereby cause a phase change thereof. Instead, the element 18 is in thermal equilibrium with the ambient environment. The wire 11, spring 14 and element 18 are dimensioned such that the tensile strength exerted by the element 18 is greater than the tensile strength of the wire 11, which in turn is greater than the tensile strength exerted by the spring 14. The load on the wire 11 corresponds to the tension of the spring 14; since the applied load is small, the temperature M for the Ni-Ti alloy wire_fAbout 65 deg.C, at the indicated temperature T_al(50 ℃) the whole hysteresis cycle is generated above the external temperature and the actuator is able to work correctly, while the wire 11 is heated by the current in the wire itself and cools naturally.

Fig. 1b shows the actuator when the external temperature rises and reaches a value of e.g. about 80 c. At this temperature, the element 18 in equilibrium with external heat undergoes a phase change and changes its shape by shrinking; the slider 15 is moved to the right-hand side in the figure (the original position of the slider is shown in chain line), thereby increasing the tension on the spring 14; this involves an increased load on the wire 11, which is then moved to the high temperature of the hysteresis cycle of the wire 11, without any movement of the part to be controlled, since the wire 11 is inextensible. By suitably dimensioning the element 18 and the spring 14, this movement causes M to be moved_f> 80 c, thus again bringing the system to the state resulting from the entire hysteresis cycle being above the external temperature, and the wire 11 is able to change from the austenitic phase to the martensitic phase by natural cooling, so that the actuator 10 is able to function correctly also at 80 c.

When the external temperature is reduced, the element 18 returns to the martensitic phase, returning the whole system to the configuration shown in fig. 1a, thus reducing the load on the line of weakness 11, avoiding weakening or breaking of the actuator caused by a constant high load if it is always kept in the configuration shown in fig. 1 b.

Figure 2 shows a possible alternative embodiment of the actuator of the invention. Fig. 2a shows the actuator in its cold configuration. The actuator 20 is rigidly fixed to a fixed limiter 27 (a wall outside the system) and comprises a wire 21 made of SMA, the wire 21 having a first fixed end (for example coupled to the wall 22) and a second end connected to a slider 25 ', the slider 25' in turn being coupled to a controlled mechanical component (not shown) by means of a hook 23. The wire 21 can be heated by means 29, 29' (illustrated in the figures as electrical conductors to supply current through the wire 21 itself) and has been treated in the manufacturing stage to shrink when heated. The slider 25' is also coupled to a first end of a spring 24 (the spring 24 is shown in cross-section in this example for clarity of the drawing) which provides the biasing means of the system. At each given temperature, the slider 25' has a position determined by the strength of the spring 24 operating under compression. The actuator further comprises a second element made of SMA, represented in the figures as a second spring 28 (shown in cross section for the sake of clarity of the figures), but which may take any functionally equivalent shape. A second SMA element 28 has a first end fixed to the inner wall of the cylinder 26 integral with the limiter 27 and a second end fixed to a second slider 25 free to move within the cylinder 26 and also fixing the second end of the spring 24 thereto. The second slider 25 forms an internal restraint for the actuator. The cylinder 26 contains the entire assembly of elements 24, 25' and 28. The element 28 has been treated at its manufacturing stage to expand upon heating. Also in this case, the elements 21, 24 and 28 are dimensioned so that the pulling force exerted by the element 28 is greater than the pulling force of the wire 21, which in turn is greater than the pulling force exerted by the spring 24. When the temperature outside the actuator enables the wire 21 to complete a complete cycle of hysteresis, the system then operates solely on the basis of the contraction of the wire 21 due to its heating by the means 29, 29' and the elongation of the wire 21 due to natural cooling.

Fig. 2b shows the configuration of the actuator 20 once the external temperature rises, in particular up to the value T at which the wire 21 cannot return to the martensitic phase by natural cooling. In this case, the second SMA element 28 undergoes its phase change by elongating and pushing the second slider 25 to the right-hand side in the drawing; this causes the spring 24 to compress, which in turn moves the first slider 25' to the right, increasing the load on the wire 21 and bringing it again to the condition in which its entire hysteresis cycle is above the external temperature, thus enabling the actuator 20 to function correctly also in this second condition of higher temperature.

The invention has been described herein in two possible embodiments thereof, but it is apparent that many variations are possible to those skilled in the art, while still remaining within the scope of the invention itself; for example, depending on the specific construction of the actuator, each of two SMA elements independent of each other may be treated to contract or elongate when heated; each of the two SMA elements, independent of each other, may be linear, strip-shaped, spring-shaped or other functional shape suitable for the specific purpose; the biasing means, shown here always as a common spring, may be of any shape functionally suitable for the purpose; and the geometrical relations between the various parts of the actuator can be varied at will, provided that the general conditions indicated in the broadest definition of the invention, corresponding to the main claims, are fulfilled.

Claims (4)

1. An actuator (10; 20) employing an element made of a shape memory alloy, the actuator (10; 20) comprising:

-a first element (11; 21) made of a shape memory alloy, the first element (11; 21) being able to change its shape when heated and having a first end coupled to the controlled mechanical component and a fixed second end;

-means (19, 19 '; 29, 29') for heating said first element made of shape memory alloy; and

-biasing means (14; 24) for returning said first element to its original shape during cooling, the biasing means having a first end connected to a second end of said first element and a second end connected to an internal limiter (15; 25) of the actuator;

it is characterized in that the preparation method is characterized in that,

the actuator further comprises means able to move said internal restraint of the actuator in order to exceed, at external temperature, the value M of complete transformation into the martensitic phase of the first element made of shape memory alloy_fIncreasing the load applied to the biasing means.

2. An actuator according to claim 1, wherein the means for increasing the load are housed in the housing element (16; 26).

3. An actuator according to claim 1, wherein the means for moving the internal restraint is selected from a manually operated device, a motor connected to a temperature sensor, a metal member which elongates on heating or a bimetallic element.

4. Actuator according to claim 1, wherein the means for moving the internal restraint are formed by a second element (18; 28) made of a shape memory alloy in thermal equilibrium with the surrounding environment, dimensioned to exert a strength during the transition due to heating greater than that exerted by said first element made of a shape memory alloy, and having a first fixed end and a second end coupled to said internal restraint of the actuator.