Slack SMA wire in an SMA actuator assembly

Abstract

A method for detecting the degree of relaxation of multiple length segments of a shape memory alloy wire, wherein at least one of the length segments remains relaxed, the method comprising: heating the length segment of the shape memory alloy wire to contract the length segment of the shape memory alloy wire; measuring the electrical properties of the length segment of the shape memory alloy wire during the contraction of the length segment of the shape memory alloy wire; and deriving a measure of the degree of relaxation of the length segment of the shape memory alloy wire from the measured electrical properties.

Description

[0001] The present technology relates to shape memory alloy (SMA) actuator assemblies in which slack lengths of SMA wire are used. A first aspect of the present technology relates to the manufacture of such SMA actuator assemblies. A second aspect of the present technology relates to the control of such SMA actuator assemblies.

[0002] SMA actuators are known for use in handheld electronic devices such as cameras and mobile phones. Such actuators may be used, for example, in miniature cameras to achieve focus, zoom or optical image stabilization (OIS). For example, International Patent Publication No. WO2007 / 113478 discloses an SMA actuator arrangement for a camera that provides autofocus using a single length of SMA wire, and WO2013 / 175197 and WO2014 / 083318 disclose a compact SMA actuator arrangement for a camera that provides OIS using four lengths of SMA wire. In addition, WO2011 / 104518 discloses an SMA actuator arrangement that includes eight lengths of SMA wire that can achieve both autofocus and OIS. In each of these disclosures, each length of SMA wire is retained between a stationary component and a movable component that is movable relative to the stationary component, and the preferred method of securing is crimping, wherein the crimping portion is crimped around the length of SMA wire to form a crimp that retains the length of SMA wire.

[0003] In a first aspect of the present technology, the manufacture of such an SMA actuator assembly is considered. In the prior art examples described above, it has been assumed that it is necessary to attach a length of SMA wire under tension during manufacture so that its length and tension can be accurately known in an unpowered state. In such an actuator, the length of SMA wire is under greater tension in an unpowered state. However, the present technology is directed to manufacturing an SMA actuator assembly using a length of SMA wire that is slack, such as temporarily slack during manufacture, and / or slack in the absence of a drive signal or energy supplied by a suitable heat source in a manufactured SMA actuator assembly to increase the available travel.

[0004] In this case, it is desirable to measure the slack of an SMA actuator assembly to control the manufacturing process and / or evaluate a newly manufactured actuator assembly. However, due to the uncontrolled path and small size of the length of SMA wire, it is difficult to measure the slack. One might consider applying tension to the length of SMA wire to pull it, but the tension itself would affect the measurement results.

[0005] The energy provided to the length of the SMA wire can take the form of a drive signal, such as an electric current passing through the length of the SMA wire, thereby resistively heating the length of the SMA wire. Alternatively or additionally, the length of the SMA wire can be heated by a suitable heat source. The use of an external heat source allows the SMA wire to be measured and the SMA actuator to be calibrated during the manufacturing process. The heat source can transfer heat to the length of the SMA wire by conduction, convection or by radiation. For example, the heat source can be one or more of a contact heater such as a hot plate, an oven, a fan heater and an electromagnetic radiation source such as a laser.

[0006] According to a first aspect of the present technology, there is provided a method for detecting the degree of relaxation of multiple length segments of a shape memory alloy wire, wherein at least one of the length segments thereof remains relaxed, the method comprising: heating at least one length segment of the shape memory alloy wire to shrink the length segment of the shape memory alloy wire; during the shrinkage of the length segment of the shape memory alloy wire, measuring the electrical properties of the length segment of the shape memory alloy wire; and deriving a measure of the degree of relaxation of the length segment of the shape memory alloy wire from the measured electrical properties. In some embodiments, the method detects the degree of relaxation of multiple length segments of the shape memory alloy wire that remain relaxed.

[0007] The method provides an accurate and reliable way of detecting the degree of slack of lengths of SMA wire based on the recognition that the degree of slack is represented by the electrical characteristics experienced by the lengths of SMA wire when driven to contract from a relaxed state. More specifically, this can be understood as follows.

[0008] When a length segment of SMA is driven to contract from a relaxed state, the temperature of the length segment of SMA wire is initially lower than the starting point of the phase transition temperature. Therefore, the initial heating causes the resistance to increase as the resistivity of the SMA material increases. When the length segment of SMA wire reaches the phase transition temperature, it changes the phase from the martensite state to the austenite state, resulting in a decrease in resistivity. This results in a decrease in length and an increase in area. These factors together lead to a maximum resistance, and then a decrease in resistance. The resistance continues to decrease until the length segment of the SMA wire is mechanically opposed, at which point the resistance is minimum. It has been recognized that as the degree of relaxation of the length segment of the SMA wire increases in the initial relaxed state, the magnitude of the maximum resistance increases, while the magnitude of the minimum resistance decreases. The relaxation in the SMA wire of one length segment or more length segments can be referred to as the amount of tension in the SMA wire in the unenergized state. A relaxed wire may mean that the wire has a certain degree of free movement, or it may mean that the unenergized wire has been relaxed.

[0009] Therefore, the resistance of multiple lengths of shape memory alloy wire can be used as an electrical characteristic to derive a measure of the degree of relaxation. In that case, the measure can be the maximum resistance when each length of shape memory alloy wire begins to contract, or the minimum resistance when the length of shape memory alloy wire stops contracting. However, by combining these two factors, a higher accuracy is obtained, so that the measure is the difference between the maximum resistance when the length of shape memory alloy wire begins to contract and the minimum resistance when the length of shape memory alloy wire stops contracting.

[0010] However, none of these specific metrics or the use of resistance are essential, and other metrics of the degree of relaxation may be derived from resistance or other electrical properties based on similar considerations of the electromechanical properties of a relaxed SMA wire subjected to a drive signal or energy provided by a suitable heat source. For example, the rate of change of resistance, current, or another electrical property may be used to assess whether a length segment of an SMA wire is in a phase change.

[0011] This can be understood to be related to the known fact that during normal actuation, the change in resistance of a length of SMA wire is related to the change in length of the SMA wire, but during such actuation, the length of SMA wire is in tension rather than relaxation.

[0012] The step of heating the length of shape memory alloy wire may be performed by providing a drive signal sufficient to cause contraction or energy provided by a suitable heat source to the length of shape memory alloy wire. Such a method is easy to perform because it only requires the electrical connection of circuitry to the length of SMA wire, which circuitry may be, for example, external circuitry or may also be an additional function of the control circuitry of the length of SMA wire assembled with the actuator assembly.

[0013] Alternatively, the step of heating the length of shape memory alloy wire may be performed by externally heating the length of shape memory alloy wire, for example by placing the length of shape memory alloy wire in a hot environment such as an oven or on a hot plate or by laser heating.

[0014] This method can be applied to multiple lengths of SMA wire where at least one of the lengths is held slack in various assemblies and subassemblies during the manufacturing process. This method can be applied to multiple lengths of SMA wire where at least one of the lengths is held slack in various assemblies and subassemblies during the manufacturing process.

[0015] In one type of example, at least one of the lengths of shape memory alloy wire may maintain slack between two connecting portions that are maintained in a fixed relative position. Such a connecting portion may, for example, be a crimped portion that is crimped around the corresponding length of the shape memory alloy wire. In this case, when the length of SMA wire stops contracting, it will experience minimal resistance because the slack has been taken up and the connecting portion provides mechanical resistance to further contraction.

[0016] As an example, a length of shape memory alloy wire may be held in a shape memory alloy subassembly including at least one body portion integrally formed with a pair of crimp portions formed from a sheet of material.

[0017] In another type of example, a length of shape memory alloy wire may be held between a stationary component and a movable component that is movable relative to the stationary component in an SMA actuator assembly, and at least one length of the shape memory wire may be slack in the absence of a drive signal or energy provided by a suitable heat source. For example, at least one length of the SMA wire may be held slack between crimped portions crimped around a corresponding length of the shape memory alloy wire. Such crimped portions may be mounted on the stationary component and the movable component separately, or may be mounted at a stationary point by a length of the SMA wire that is also connected to the stationary component by hooking onto a hook-shaped feature on the movable component (or vice versa). This method may be applied to multiple lengths of SMA wire, each length held slack between corresponding crimped portions.

[0018] As an example, the SMA actuator assembly may be an optical device, wherein the movable component is an optical element, such as a lens element including at least one lens, or any other type of optical element.

[0019] Here, the following steps may be performed while the movable component is held in a fixed position relative to the stationary component: providing a drive signal or energy provided by a suitable heat source to the length of shape memory alloy wire, and measuring the electrical characteristics of the multiple lengths of shape memory alloy wire. In this case, the length of SMA wire will experience minimal resistance when it stops contracting because the slack has been taken up and mechanical resistance to further contraction is provided by the device holding the movable component.

[0020] Alternatively, the following steps may be performed while the movable component is allowed to move relative to the stationary component: providing a drive signal or energy provided by a suitable heat source to the length of shape memory alloy wire and measuring the electrical characteristics of the plurality of shape memory alloy wires. In this case, the length of SMA wire will experience minimal resistance when it stops contracting because the slack has been taken up and the lengths of SMA wire provide mechanical resistance relative to each other against further contraction.

[0021] The multiple lengths of SMA wire may be relative or non-relative, for example, the multiple lengths of SMA wire may be at an angle to each other. The degree of slack in the multiple lengths of SMA wire may be derived individually, differently or together with respect to the lengths of shape memory alloy wire. Preferably, the degree of slack in respect to the lengths of shape memory wire may be derived together and the total degree of slack or the average degree of slack in the actuator assembly may be determined therefrom.

[0022] In one form of the method, when the electrical characteristic is the resistance of each of the plurality of shape memory alloy wires, a measure of the degree of relaxation of each of the plurality of shape memory alloy wires can be derived and the measure of the degree of relaxation is taken as the difference between the resistance of the shape memory alloy wire before a drive signal is provided or energy is provided by a suitable heat source and the resistance of the shape memory alloy wire when all of the plurality of shape memory alloy wires are tensioned.

[0023] As an example, the total slack in the actuator assembly may be determined based on the sum of the slack of the length segments of the shape memory alloy wire, or the average slack in the actuator assembly may be determined based on the average slack of the length segments of the shape memory alloy wire.

[0024] The total amount of slack in the actuator assembly can be viewed as a metric related to the total reduction in resistance measured in all SMA wires between the start of heating of all SMA wires and their full tension. Similarly, the average amount of slack in the actuator assembly corresponds to the average reduction in resistance measured in all SMA wires between the start of heating and their full tension. Both values ​​can be used to monitor the manufacturing process, where the metric applied by each wire is related not only to the slack, but also to the position of the actuator assembly when the metric is obtained.

[0025] For example, a reference position and an offset of resistance, power or temperature may be applied in an SMA actuator assembly to reach a known mechanical opposition point. In an SMA actuator assembly having multiple shape memory wires, such a reference position may not be independently applicable to each individual wire, especially where differences between length segments of the SMA wires are used to control the degrees of freedom of the actuator assembly. The method according to the invention enables the average or total slack in the actuator system to be derived when determining the position of the actuator assembly at the center of its travel.

[0026] The difference between the resistance reduction measured from multiple lengths of wire corresponds to the difference in available travel of the individual wires before they become slack. Thus, by centering the actuator assembly at a position where the wires have reached their maximum extension, all SMA wires may contract by the same amount. For example, during actuation, the resistance reduction of the SMA wires may be similar.

[0027] Another form of the method is applicable to the case where the plurality of shape memory alloy wires include substantially opposed shape memory alloy wires. In that case, a measure of the degree of relaxation of the relative length segments of the shape memory alloy wires can be derived individually, differently, or together. Derivation of the degree of relaxation of the relative length segments of the SMA wire together simplifies the method because only the resistance at the beginning and end of contraction of the length segment of the SMA wire needs to be measured, without the need to continuously monitor the resistance to detect the maximum resistance or the minimum resistance.

[0028] The method may be applied to a length of SMA wire at any stage in the manufacturing process. The method is particularly suitable for holding a length of SMA wire slack between crimped portions crimped around the length of SMA wire, but is more generally applicable to lengths of SMA wire held in any other manner.

[0029] In one example, the method can be applied to a length of SMA wire that holds slack between respective crimping portions that are crimped around the length of SMA wire and are respectively mounted on a stationary component and a movable component that is movable relative to the stationary component in an SMA actuator assembly (e.g., an optical element, which can be a lens element including at least one lens).

[0030] In another example, the method can be applied to hold slack in a length of SMA wire between respective crimping portions crimped around the length of SMA wire in an SMA subassembly including at least one body portion integrally formed with a pair of crimping portions formed from a sheet of material. Such an SMA subassembly can be used to manufacture an SMA actuator assembly.

[0031] The measure of the degree of slack can be used in manufacturing in various ways. In one example, the measure of the degree of slack can be used for quality control. In another example, the method can further include adjusting the degree of slack of the length of SMA wire in response to the derived measure. In that case, the length of SMA wire can initially retain slack between the crimped portions that are partially crimped around the length of SMA wire, in which case the method can further include fully crimping the crimped portions around the respective lengths of SMA wire after the step of adjusting the degree of slack of the length of SMA wire.

[0032] Optionally, the method measures the relaxation degree of the length segments of the shape memory alloy wire simultaneously or measures the relaxation degree of each length segment sequentially.

[0033] In addition, according to the first aspect of the present technology, a device for measuring the degree of relaxation of multiple length segments of a shape memory alloy wire is provided, wherein at least one length segment thereof remains relaxed, the device comprising: a drive circuit, which is arranged to provide a drive signal sufficient to cause the length segment of the shape memory alloy wire to contract or energy provided by a suitable heat source; a measuring circuit, which is arranged to measure the electrical characteristics of the length segment of the shape memory alloy wire during the contraction of the length segment of the shape memory alloy wire; and a processor, which is arranged to derive a measure of the degree of relaxation of the length segment of the shape memory alloy wire from the measured electrical characteristics.

[0034] In a second aspect of the present technology, control of an SMA actuator assembly is contemplated. A known technique for controlling an SMA actuator assembly is resistance feedback closed loop control. In this case, energy in the form of a drive signal or energy from a suitable heat source is provided to the length of shape memory alloy wire while measuring the resistance of the length of shape memory alloy wire, and a closed loop control loop controls the energy provided based on a target signal representing a target position of a movable component and a feedback signal representing the measured resistance.

[0035] In the prior art examples described above, it has been assumed that it is necessary to attach the length of SMA wire under tension during manufacturing so that its length and tension can be accurately known in the unpowered state. In such an actuator, the length of SMA wire is under greater tension in the unpowered state. On the other hand, in an SMA actuator assembly using a length of SMA wire, it is particularly advantageous to keep the length of SMA wire slack when no energy is provided. This can, for example, simplify manufacturing and / or increase the available travel.

[0036] However, a particular problem of such control arises when the SMA actuator assembly comprises a plurality of SMA wires in a substantially opposite arrangement, wherein the SMA wires, when energy is applied thereto, drive the movable part to move in one or more degrees of freedom in opposite directions relative to the stationary part. In particular, it may be difficult to control the degree of relaxation of the individual length segments of the SMA wire during manufacture, resulting in the length segments of the SMA wire often having actual lengths that differ from their nominal lengths in an unpredictable manner. This makes it difficult to accurately control the position of the movable part near a known center position. This is important for uniformity of performance between the parts, and repeatability of position is required for certain applications (e.g., in the case of cameras that use information from multiple cameras to form a single image). It is desirable to maximize the actuator stroke in a selected direction, and therefore it is necessary to know the center of the actuator stroke. This is a particular problem for a plurality of SMA wires arranged in opposition, because the position of the degree of freedom or each degree of freedom depends on the degree of relaxation of all the SMA wires that drive the movement in opposite directions in that degree of freedom.

[0037] According to a second aspect of the present technology, a method for controlling a shape memory alloy actuator assembly is provided, the shape memory alloy actuator assembly comprising a stationary component, a movable component movable relative to the stationary component, and a plurality of shape memory alloy wires, the plurality of shape memory alloy wires being held between the stationary component and the movable component and being arranged to drive the movable component to move relative to the stationary component in substantially opposite directions with one or more degrees of freedom when energy provided thereto is applied, the plurality of shape memory alloy wires being relaxed when no energy in the form of a drive signal or heat provided by a suitable heat source is provided, the method comprising: measuring the shape memory alloy wires when no energy is provided an electrical characteristic of a length segment of the shape memory alloy wire, the electrical characteristic depending on the degree of relaxation of the length segment of the shape memory alloy wire; and when measuring the resistance of the length segment of the shape memory alloy wire, providing energy in the form of a drive signal to the length segment of the shape memory alloy wire, and / or providing energy provided by a suitable heat source, wherein a closed-loop control loop controls the provided energy based on a target signal representing a target position of the movable part relative to the stationary part and a feedback signal representing the measured resistance of the length segment of the shape memory alloy wire, wherein one or more signals in the closed-loop control loop are adjusted by an offset derived from the measured electrical characteristic and compensate for the relaxation of the length segment of the shape memory alloy wire.

[0038] Thus, the method includes adjusting one or more signals in the closed-loop control loop by an offset. By deriving the offset from the measured electrical characteristics, the offset can be selected to compensate for the degree of relaxation of the length of the shape memory alloy wire. This is based on the recognition that, in a manner similar to the first aspect of the present technology, the degree of relaxation is represented by the electrical characteristics experienced by the length of the SMA wire when it is driven to contract from a relaxed state.

[0039] When the length segment of the SMA is driven to contract from a relaxed state, the temperature of the length segment of the SMA wire is initially below the starting point of the phase transition temperature. Therefore, the initial heating causes the resistance to increase as the resistivity of the SMA material increases. When the length segment of the SMA wire reaches the phase transition temperature, it changes phase from the martensite state to the austenite state, resulting in a decrease in resistivity. This results in a decrease in length and an increase in area. These factors together result in a maximum resistance being experienced, and then the resistance decreases. The resistance continues to decrease until the length segment of the SMA wire is subjected to mechanical resistance, at which point the minimum resistance is experienced. Therefore, changing the degree of relaxation of the length segment of the SMA wire in the initial relaxed state causes the characteristics of the resistance to change and other electrical characteristics.

[0040] The measured electrical characteristic may be the resistance of the length of the shape memory alloy wire. This is convenient because the feedback signal represents the measured resistance, and this simplifies the derivation of the offset.

[0041] Advantageously, the deflection can be derived from the resistance of the length of shape memory alloy wire measured when the length of shape memory alloy wire is at ambient temperature (e.g., when an initial drive signal or energy provided by a suitable heat source is first provided to the length of shape memory alloy wire. Since it is not necessary to drive the length of SMA wire for a full movement cycle, a measure of relaxation can be obtained very quickly from the measurements.

[0042] This approach can be applied to a variety of different types of control loops, some examples of which are listed below.

[0043] The target signal may represent the target position with respect to one or more degrees of freedom.

[0044] In that case, the closed-loop control loop may include: a feedback transformation block arranged to transform the feedback signal to represent a resistance measure about one or more degrees of freedom; an error detector arranged to detect an error signal about one or more degrees of freedom, the error signal representing an error between the target signal and the transformed feedback signal; and a controller arranged to control the energy provided based on the error signal. This is an example of a closed-loop control loop that provides control within the "space" of one or more degrees of freedom of movement. In that case, any of the feedback signal before transformation, the feedback signal after transformation, the target signal, or the error signal may be adjusted by an offset.

[0045] As an alternative, where the target signal represents a target position with respect to one or more degrees of freedom, the closed-loop control loop may include: a target transformation block arranged to transform the target signal to represent a target position with respect to a length segment of the shape memory alloy wire; an error detector arranged to detect an error signal with respect to the length segment of the shape memory alloy wire, the error signal representing an error between the transformed target signal and the feedback signal; and a controller arranged to control the energy provided based on the error signal. This is an example of a closed-loop control loop that provides control within the "space" of the length segment of the SMA wire. In that case, any of the feedback signal, the target signal before transformation, the target signal after transformation, or the error signal may be adjusted by an offset.

[0046] As another alternative, the target signal may represent a target position with respect to the length segment of the shape memory alloy wire.

[0047] In that case, the closed-loop control loop may include: an error detector arranged to detect an error signal about the length segment of the shape memory alloy wire, the error signal representing the error between the target signal and the feedback signal; and a controller arranged to control the energy provided based on the transformed error signal. This is another example of a closed-loop control loop that provides control within the "space" of the length segment of the SMA wire. In that case, any of the feedback signal, the target signal or the error signal may be adjusted by an offset.

[0048] Furthermore, according to a second aspect of the present technique, there is provided a control circuit implementing a method similar to that described above.The control circuit may be provided as part of an SMA actuator assembly.

[0049] Both aspects of the present technology can be applied to an optical assembly, wherein the movable part is a lens element comprising at least one lens, which lens may have a diameter of at most 20 mm, preferably at most 15 mm, preferably at most 10 mm. When the movable part is a lens element, the stationary part may optionally mount an image sensor on which the lens element is arranged to focus an image.

[0050] In other alternatives, the SMA actuator assembly can be an optical assembly, wherein the movable component is an optical element other than a lens element, such as a diffractive optical element, a filter, a prism, a mirror, a reflective optical element, a polarizing optical element, a dielectric mirror, a metallic mirror, a beam splitter, a grid, a pattern plate or a grating, which can be a diffraction grating.

[0051] More generally, the present technology can be generally applied to any type of device including a stationary component and a movable component that is movable relative to the stationary component. As non-limiting examples, the actuator assembly can be any of the following devices or can be disposed in any of the following devices: a smartphone, a camera, a foldable smartphone, a foldable smartphone camera, a foldable consumer electronic device, an image capture device, a 3D sensing device or system, a servo motor, a consumer electronic device, a mobile computing device, a mobile electronic device, a laptop computer, a tablet computing device, an e-reader (also known as an e-book reader or e-book device), a computing accessory or computing peripheral device (e.g., a mouse, a keyboard, a headset, an earphone, an earbud, etc.), a security system, a gaming system, a gaming accessory (e.g., a controller, an earphone, a wearable controller, etc.), an augmented reality system, an augmented reality device, a virtual reality system, a virtual reality device, a wearable device (e.g., a watch, a smart watch, a fitness tracker, etc.), a drone (in the air, on the surface of the water, underwater, etc.), an aircraft, a spacecraft, a submersible, a vehicle, and an autonomous vehicle. It should be understood that this is a non-exhaustive list of example devices.

[0052] The SMA actuator assemblies described herein may be used in devices / systems suitable for: image capture, 3D sensing, depth mapping, aerial surveying, ground surveying, surveying in or from space, hydrographic surveying, underwater surveying, scene detection, collision warning, security, facial recognition, augmented and / or virtual reality, vehicles, advanced driver assistance systems for autonomous vehicles, gaming, gesture control / recognition, robotic devices, robotic device controls, contactless technology, home automation, medical devices, and tactile feedback.

[0053] For a better understanding, embodiments of the present technology will now be described by way of non-limiting examples with reference to the accompanying drawings, in which:

[0054] Figure 1is a diagram of an apparatus for measuring the degree of slack of a length segment of an SMA wire;

[0055] Figure 2 is a flow chart of the method for measuring the degree of relaxation;

[0056] Figure 3 is a graph of the temperature resistance of a length segment of the SMA wire;

[0057] Figure 4 and Figure 5 is a schematic diagram of the type of components that can hold length segments of SMA wire;

[0058] Fig. 6A is a schematic diagram of an assembly that can hold multiple lengths of SMA wire;

[0059] Figure 6B and Figure 6C is a schematic diagram of an assembly capable of holding multiple lengths of SMA wire, wherein the SMA wire is in an energized and deenergized state, respectively;

[0060] Fig.6D is Figure 6B and 6C A graph of the resistance in two SMA wires measured during initial excitation of an assembly;

[0061] Figure 7 is a top view of an SMA subassembly including a single body portion;

[0062] Figure 8 and Fig. 9 is a perspective view of the crimping part in a partially crimped state and a fully crimped state;

[0063] Fig.10 and Fig.11 is a perspective view of the SMA actuator assembly acting as a camera;

[0064] Fig.12 is a plan view of an SMA actuator assembly including four lengths of SMA wire;

[0065] Fig.13 and Fig.14 Can be used to control Fig.12 Schematic diagrams of two alternative forms of control circuits for an SMA actuator assembly;

[0066] Fig.15 is a flow chart of a method of controlling an SMA actuator assembly; and

[0067] Fig.16 is used in Fig.14 and Fig.15 Schematic diagram of an adder that provides an offset in a control circuit.

[0068] Figure 1 An apparatus 100 is shown that performs a method of measuring the degree of slack of a length of an SMA wire 20 that is held slack. For ease of description, Figure 1 A single length of SMA wire 20 is shown, but the device 100 may alternatively be connected to multiple lengths of SMA wire 20 .

[0069] The apparatus 100 may be used during the manufacturing process of the SMA actuator assembly 30, examples of which are described below. The method may be applied at any stage of the manufacturing process where a length of SMA wire 20 is held slack. Examples in which a length of SMA wire 20 is held slack and the apparatus 100 may be used are described below, and the examples generally involve a length of SMA wire being held by a connecting portion in the form of a crimping portion 10 crimped around the length of SMA wire 20. However, these examples are non-limiting, and the apparatus and method may be generally applicable to holding one or more lengths of SMA wire 20 slack in any assembly or subassembly.

[0070] The apparatus 100 may be a test circuit connected to a length of SMA wire 20 for the specific purpose of performing the method, or may be implemented by the functionality of a control circuit connected to a length of SMA wire 20 for the purpose of driving under normal use.

[0071] The apparatus 100 is arranged as follows.

[0072] The device 100 includes a drive circuit 101 connected to a length of SMA wire. The drive circuit 101 applies a signal to the length of SMA wire 20. In some cases, the signal is a drive signal that is sufficient to cause contraction of at least one length of the shape memory alloy wire. In other cases, the signal may be a test signal for measuring the resistance of the SMA wire 20. Such a test signal may not have sufficient power to significantly heat the SMA wire 20.

[0073] The drive circuit 101 may be a constant voltage current source or a constant current current source. For example, in the latter case, the constant current may be about 120 mA. The drive (dDrive) signal may be in any suitable form, such as a continuous signal or a pulse width modulated (PWM) signal.

[0074] The device 100 also includes a measurement circuit 102 that measures an electrical characteristic of the length of the SMA wire 20. The electrical characteristic may be the resistance of the length of the SMA wire 20. In the case where the drive circuit 101 is a constant current source, the detection circuit 102 may be a voltage detection circuit that is operable to detect the voltage across the length of the SMA wire 20 as a measure of its resistance. In the case where the drive circuit 101 is a constant voltage current source, the detection circuit 102 may be a current detection circuit that is operable to detect the current flowing through the length of the SMA wire 20 as a measure of its resistance. For greater accuracy, the detection circuit 102 may include a voltage detection circuit and a current detection circuit that are operable to detect both the voltage and the current across each length of the SMA wire and derive the resistance R as a ratio thereof. n Regardless of which measure of resistance is detected, the measured resistance is represented by an output signal from measurement circuit 102, which may typically be a digital signal or an analog signal that may subsequently be converted to a digital signal.

[0075] The device 100 also includes a processor 103 that receives the electrical characteristics measured by the measurement circuit 102. The processor 103 executes a program configured to derive a measure of the degree of slack of the length of SMA wire 20 from the measured electrical characteristics, as described in more detail below.

[0076] In operation, the apparatus 100 is used to perform a method of detecting the degree of slack in a length of SMA wire 20. The method Figure 2 is shown in and executed as follows.

[0077] In step S1 , the length of SMA wire is heated to cause contraction of the length of SMA wire 20. This may be done in any suitable way.

[0078] In a first option, step S1 is performed by operating the drive circuit 101 to apply a drive signal to the SMA wire 20 that heats the SMA wire 20 sufficiently to cause said contraction. The drive signal may increase in power over time such that the temperature of the length of the SMA wire 20 increases from its uncontracted initial state to a state where contraction ceases due to the mechanical resistance experienced by the length of the SMA wire 20.

[0079] In a second option, step S1 is performed by externally heating the SMA wire 20. External heating may be provided by any suitable means, such as placing the SMA wire 20 in a hot environment such as an oven, placing the SMA wire 20 on a hot plate or by laser heating.

[0080] In step S2 , the measurement circuit 102 measures an electrical characteristic, such as resistance, of the length of SMA wire 20 during contraction of the length of SMA wire 20 caused by the heating in step S1 .

[0081] In the first option of step S1 in which heating is performed by operating the drive circuit 101 to apply a drive signal, the measurement result in step S2 can be derived from the drive signal when the drive signal is provided (for example during a pulse of a PWM signal) or from an additionally provided test signal.

[0082] In the second option of step S1 in which the heating is performed by external heating, step S2 can be performed by applying a test signal and deriving a measurement result from the test signal when the test signal is provided.

[0083] In step S3 , the processor 103 derives a measure of the degree of slack in the length of SMA wire 20 from the electrical characteristics measured in step S2 .

[0084] In the case where the measured electrical property is resistance, the measure of the degree of relaxation can be (a) the maximum resistance when at least one length segment of the shape memory alloy wire begins to contract, (b) the minimum resistance when at least one length segment of the shape memory alloy wire stops contracting, (c) the difference between the maximum resistance when at least one length segment of the shape memory alloy wire begins to contract and the minimum resistance when at least one length segment of the shape memory alloy wire stops contracting, which achieves additional accuracy, or (d) the difference between the resistances at two predetermined temperatures on either side of the phase transition temperature.

[0085] These are good measures of the degree of slack in a length of SMA wire 20 for the following reasons. To illustrate, Figure 3 It shows how the resistance of a length of SMA wire 20 varies with temperature when a drive signal is applied from a cold state.

[0086] At ambient temperature, the length of SMA wire 20 has an initial resistance 116. When the length of SMA wire 20 is driven to contract from a relaxed state, the temperature of the length of SMA wire 20 is initially below the onset of the phase change temperature (which is a typical property of alloys used as SMA actuators). Therefore, it must be heated to tighten the slack in the system and reach the operating point of the actuator. The initial heating causes the resistance of the length of SMA wire 20 to increase as the resistivity of the SMA material increases. This is shown by portion 111 of the curve. There is also some thermal expansion, but the change in resistivity dominates the change in resistance.

[0087] As the length of SMA wire 20 reaches the phase transition temperature, it changes phase from the martensite state to the austenite state, resulting in a decrease in resistivity. This results in a decrease in length and an increase in area. These factors together result in a maximum resistance 112. Thereafter, the resistance decreases, as shown in portion 113 of the curve.

[0088] The resistance continues to decrease until the length of SMA wire is subjected to mechanical resistance. At this point, a minimum resistance 114 is experienced. Thereafter, the resistance increases, as shown in portion 115 of the curve. If the length of SMA wire fully transitions to the austenitic state, a similar minimum resistance will result, but in a practical actuator assembly, the length of SMA wire 20 is configured so that mechanical resistance is first encountered, such as from an opposing length of SMA wire. If a resilient biasing element (such as a spring) is provided, the mechanical resistance will result in a change in the gradient of the resistance versus temperature curve. Unless there is a resilient biasing element, this may not result in a minimum resistance, but in that case, a change in the gradient can be used instead of a minimum resistance.

[0089] The degree of relaxation of the length of the SMA wire 20 in the initial relaxed state affects the magnitude of the maximum resistance 112 and the magnitude of the minimum resistance 114. Specifically, as the degree of relaxation increases, the magnitude of the maximum resistance 112 increases while the magnitude of the minimum resistance 114 decreases.

[0090] The increase in the magnitude of the maximum resistance 112 may be understood as the length of the SMA wire 20 experiencing a greater resistivity change prior to transitioning with increased initial relaxation.

[0091] The reduction in the magnitude of the minimum resistance 114 can be understood as the length of the SMA wire 20 experiencing a greater change in resistivity and length with increasing initial relaxation.

[0092] These effects can be understood to be related to the known fact that during normal actuation, changes in resistance of a length of SMA wire 20 are related to changes in length of the SMA wire, although during such actuation, the length of SMA wire 20 is in tension rather than in relaxation. For example, the rate of change of resistance, current, or another electrical characteristic can be used to assess whether a length of SMA wire 20 is in a phase change.

[0093] Nonetheless, none of these specific measures or the use of resistance is essential. Indeed, other measures of relaxation can be derived from resistance or from other electrical properties based on similar considerations of the electromechanical properties of a relaxed SMA wire subjected to a drive signal that causes the SMA wire to contract.

[0094] The metric may be a relative metric, such as where the metric is resistance. Alternatively, the metric may be calibrated for a given SMA material and initial diameter of the length of SMA wire 20.

[0095] Some examples of slack length segments of SMA wire 20 to which the apparatus 100 may be applied are as follows.

[0096] Figure 4 An example of a subassembly 1 is shown in which a length of SMA wire 20 is held slack between two connection portions in the form of crimping portions 10 that are crimped around the length of SMA wire 20. The crimping portions 10 are both mounted on a common component 120, thereby holding the lengths of SMA wire 20 in fixed relative positions. In this case, when the length of SMA wire 20 stops contracting, it will experience minimal resistance because the slack has been taken up and the crimping portions 20 provide mechanical resistance against further contraction. The common component 120 provides a strong mechanical harness, so when tension is taken up, the resistance reaches a well-defined minimum.

[0097] The subassembly 1 may be adapted to hold multiple lengths of SMA wire 20, each of which is slack between two connecting portions mounted on the common component 120 to hold the lengths of SMA wire 20 in fixed relative positions. In that case, the device 100 may measure the degree of slack of each length of SMA wire 20 simultaneously or sequentially.

[0098] Figure 5 An example of an SMA actuator assembly 30 is shown in which a single length of SMA wire 20 is held slack between two connecting portions in the form of crimping portions 10 that are crimped around the length of SMA wire 20. The crimping portions 10 are respectively mounted on a stationary component 121 and a movable component 122 that is movable relative to the stationary component 121, thereby holding the length of SMA wire 20 between the stationary component 121 and the movable component 122, although as an alternative, the crimping portions 20 can be mounted on the stationary point 121, wherein the length of SMA wire 20 is connected to the stationary component 122 by being hooked onto a hook feature on the movable component (or vice versa).

[0099] Figure 6 shows a diagram similar to Figure 5An example of an SMA actuator assembly 30 of the invention is shown, except that there are two lengths of SMA wire 20, but each length is connected in the same manner so that they maintain slack between the stationary component 121 and the movable component 122. In this example, the lengths of SMA wire 20 are opposed, thereby driving the movable component 122 to move in opposite directions. In use, as each SMA wire 20 contracts until they are in tension with each other, thereafter, the movement is achieved by allowing one or more lengths of the SMA wire 20 to extend, being stretched by one or more of the other lengths of the SMA wire 20.

[0100] Although for simplicity, the relative lengths of the SMA wire 20 are shown in FIG6 as being parallel to the direction of movement, in practice the lengths of the SMA wire 20 may be tilted relative to the direction of movement to increase gain. Furthermore, the example of FIG6 may be modified to include any plurality of lengths of the SMA wire 20 in any geometric configuration.

[0101] exist Figure 5 6, steps S1 and S2 may be performed while the movable member 122 is held in a fixed position relative to the stationary member 121. In this case, minimal resistance is experienced when the length of SMA wire 20 stops contracting because the slack has been taken up and mechanical resistance to further contraction is provided by the means of holding the movable member 122.

[0102] 6 , steps S1 and S2 may be performed while the movable component 122 is allowed to move relative to the stationary component 121. In this case, the minimum resistance of each length of SMA wire 20 is experienced when the length of SMA wire 20 stops contracting because the slack has been taken up and the lengths of SMA wire 20 provide mechanical resistance relative to each other against further contraction.

[0103] A measure of the degree of slack in the opposing SMA wires can be derived individually, differentially, or together with respect to the length segments of the opposing SMA wires. As each SMA wire 20 contracts until they are in tension with each other, thereafter, movement is achieved by allowing one or more wires to stretch, being stretched by one or more other wires in the system. In order to achieve the initial tensioned state, the wires must all first contract from their cold state. In such a case, the measure summed (or similarly averaged) across the opposing length segments of the SMA wires 20 can be used as a measure of the amount of slack in the entire system.

[0104] In such an example where slack is maintained in multiple lengths of the SMA wire 20 between the stationary component 121 and the movable component 122, the degree of slack in each length of the SMA wire depends on the position of the movable component 122 relative to the stationary component 121. This means that the point evaluated as the minimum resistance 114 is selected to be the same for all lengths of the SMA wire 20.

[0105] Figure 6B , Figure 6C and Fig.6D This feature is further illustrated. When power is applied to each SMA wire, the SMA wires 20a, 20b contract until they oppose each other. As described, the resistance in each of the wires 20a, 20b initially increases due to the increase in temperature. As contraction begins, the resistance begins to decrease until the SMA wires 20a, 20b begin to oppose each other. From this point on, the resistance of the SMA wires 20a, 20b begins to increase.

[0106] Depending on the actuator's usage history, its orientation, or other factors that may affect the operation of the SMA wires 20a, 20b, the contraction rate in each SMA wire 20a, 20b of the system may be different. In this way, the actuator position where slack is tightened can be changed, for example, when the wire is fully tensioned.

[0107] like Figure 6B , Figure 6C and Fig.6D As shown, the same amount of power is supplied to a pair of SMA wires 20a, 20b. Compared with the second SMA wire 20b, the first SMA wire 20a is arranged to be connected in series with a parasitic resistance, or it is adjacent to an object that reduces its temperature rise. Fig.6D As shown, initially a higher level of resistance may be measured in the first SMA wire 20a due to the parasitic resistance (see curve 220a). As the power is increased, the first SMA wire 20a may decrease at a lower rate or by a smaller amount than the SMA wire 20b without parasitic resistance (see curve 220b). Fig.6D As can be seen in the graph, the first SMA wire 20a has a lower degree of slack, e.g., a smaller difference between the maximum and minimum values ​​in its measured resistance, than the second SMA wire 20b. However, the underlying reason for the smaller difference is actually due to the smaller contraction of the first SMA wire 20a when a given power is provided. If the system is to achieve optimal performance, all SMA wires 20a, 20b in the actuator must be the same distance from their limits at the center of the actuator.

[0108] Thus, the total amount of slack in the actuator can be used as a metric related to the total reduction in resistance in all of the SMA wires 20. Likewise, the average reduction in resistance corresponds to the average slack in the actuator. Both the total and average amounts of slack can be used to monitor the manufacturing process in situations where the metric applied by each wire is related not only to the slack, but also to the actuator position at the time the metric was obtained.

[0109] like Fig.6D As shown, the difference between the resistance reduction determined by the two wires can correspond to the difference in the available travel before those wires become slack. This allows the actuator to be placed in a position where the SMA wires each reach a position centered between their maximum extensions. In this way, an offset can be applied to the definition of the center position, allowing all SMA wires to contract by the same amount during startup, that is, the resistance in all SMA wires can be reduced to the same extent during startup. Optionally, based on the offset, the movable part can be placed in a center position, wherein at this center position, when receiving a corresponding drive signal, the contraction amount of multiple length segments of the smart memory alloy wire is substantially the same.

[0110] The power supply to each SMA wire may be any suitable power supply known to the skilled person, for example a suitable current source and / or a linearized driver (CLIN).

[0111] and Figure 5 Compared to the single SMA wire actuator shown in , where the wire length is fixed by a mechanical constraint, multiple or opposing SMA wire actuators can have their center position defined at a location where the total amount of slack pulled on all wires matches the mechanical constraint.

[0112] In principle, different lengths of the SMA wire 20 may reach the transition temperature at different points in time, for example if they are heated quickly and in different thermal environments, or if they are heated by, for example, providing different drive signals. In this case, the individual maximum resistances 112 can be evaluated to ensure that the correct starting point is used for each length of the SMA wire 20. If these points are significantly different, the rate of change and therefore the absolute temperature of each length of the SMA wire 20 can be inferred from the point at which the phase change is observed to start, since this is proportional to the temperature of the length of the SMA wire 20. However, many designs of the SMA actuator assembly 30 use lengths of SMA wire 20 with the same nominal length and slack, and the parts are typically assembled and tested in a controlled temperature environment, which reduces this problem.

[0113] In one advantageous case of using resistance as the measured electrical characteristic, the measure of the degree of relaxation derived for each of the plurality of lengths of the SMA wire 20 is the difference between the resistance of the length of the SMA wire 20 before the drive signal is provided and the resistance of the length of the SMA wire 20 when all of the plurality of lengths of the SMA wire 20 are tensioned. The measurement is simplified because there is no need to evaluate maxima and minima. This approach allows for rapid measurements because the resistance only needs to be measured at the beginning and end of heating.

[0114] Typically, reaching the operating point is achieved by providing a similar drive signal to each length of SMA wire 20, which is pre-calibrated to ensure that the length of SMA wire 20 is in tension. The difference in average resistance of the length of SMA wire 10 between the initial relaxed state and the final tensioned state includes the increase in resistance due to heating to the start of the phase change. This is similar for each length of SMA wire 20 if made of the same SMA material.

[0115] From this point on, the phase change and contraction cause the resistance of each length of SMA wire 20 to decrease until the length of SMA wire 20 is tensioned. Therefore, the method adds an offset when evaluating each maximum resistance, which offset depends on the initial ambient temperature. If this initial ambient temperature is not controlled, but known, the offset can be calibrated and the method used in an environment where the temperature is not controlled but measured, provided that the temperature is below the starting point of the phase change of the actuator material.

[0116] One effect that cannot be compensated for is the change in initial temperature. In some materials, there is a significant phase lag with temperature changes, which can cause differences in behavior between heating and cooling. This is not a problem if each length of SMA wire 20 and its surroundings have the same thermal history and starting point in a controlled temperature environment. However, if the initial thermal environment is not well controlled and measurements are taken when off rather than on, hysteresis can cause the metric to change.

[0117] If there is a possibility that the movable member 122 or the length of SMA wire 20 may be snagging, causing resistance to be encountered prematurely, then it is best to test in the off state when the movement of the movable member 122 may have been released from the initial snagging. If the actuator is operated only between the highest and lowest temperatures reached in this test, the off and on variations are interchangeable.

[0118] The method provides an accurate measurement of the degree of relaxation that can be performed very quickly, making it suitable for application at any stage in the manufacturing process in a mass production environment, including as a final check to verify correct assembly of a manufactured SMA actuator assembly.

[0119] In some applications of the method, such as for quality control of manufactured products, a measure of the degree of slack of the length of SMA wire 20 is output as data, such as for display or storage. In such cases, the following steps of the method may be omitted.

[0120] In other applications of the method, a measure of the degree of slack of the length of SMA wire 20 is used to control the manufacturing process, for example by performing steps S4 and S5. Figure 7 As shown and described below, the crimping portion 10 partially crimps around a length of SMA wire 20. In this manner, the crimping portion 10 holds the length of SMA wire 20 with a sufficiently low compressive force to allow the length of SMA wire 20 to move along its length to change slack.

[0121] In step S4 , a force is applied along the length of the length of the SMA wire 20 to adjust the degree of slack in the length of the SMA wire 20 .

[0122] In step S5, the crimping portion 10 is fully crimped around the length of SMA wire 20. In this way, the crimping portion 10 holds the length of SMA wire 20 with a sufficiently high compression force to resist the tension generated in the length of SMA wire 20 under application of a drive signal in normal use.

[0123] A conventional crimping tool may be used to accomplish step S5. Thus, the crimping portions 10 hold the length of SMA wire 20 in its relaxed state between the crimping portions 10. The body portion 3 of the SMA subassembly 1 holds the crimping portions 10 and maintains its length when the length of SMA wire 20 is subsequently tensioned, as described below.

[0124] Some products that include lengths of SMA wire 20 and to which the present method may be applied will now be described. These products are described by way of non-limiting examples.

[0125] Figure 7 An SMA subassembly 1 is shown, which includes a fret 2 having a body portion 3. The fret 2 is formed of a sheet material as a flat strip. The material of the fret 2 may be a metal such as phosphor bronze, steel, or a laminate containing a conductive component.

[0126] The plate 2 also includes a pair of crimping portions 10 formed integrally with the main body portion 3 formed from the same sheet of material. In particular, the main body portion 3 includes an elongated portion 4 and a transversely projecting arm 5 at the end of the elongated portion 4, and the crimping portions 10 are formed by tabs on the ends of the arms 5. Thus, the crimping portions 10 are separated by the main body portion 3.

[0127] The SMA subassembly 1 may have a similar construction and arrangement to the plate disclosed in WO2016 / 189314.

[0128] The crimping sections 10 are partially or completely crimped around the length of SMA wire 20 such that they hold the length of SMA wire 20. Thus, the crimping sections 10 crimp the length of SMA wire 20 to provide a mechanical and electrical connection.

[0129] In contrast to the length of SMA wire in WO2016 / 189314 which is in tension in the plate, the length of SMA wire 20 is slack, the term "slack" being used herein in the general sense that the length of SMA wire has zero tension. The length of SMA wire 20 may be made of any suitable SMA material, such as Nitinol or another titanium alloy SMA material.

[0130] The body portion 3 is sacrificial and removable from the crimping portion 10, for example by mechanical or laser cutting.

[0131] like Figure 8 As shown, the crimping portion 10 may be crimped partially around the length of SMA wire 20. In that case, the crimping portion 10 holds the length of SMA wire 20 with a sufficiently low compression force to allow the length of SMA wire 20 to move along its length to change slack or introduce tension.

[0132] Alternatively, if Fig. 9 As shown, the crimping portion 10 can be completely crimped around the length of SMA wire 20. In that case, the crimping portion 10 holds the length of SMA wire 20 with a sufficiently high compression force to resist the tension generated in the length of SMA wire 20 under the application of a drive signal in normal use.

[0133] Fig.10 An example of an SMA actuator assembly 30 is shown which is a camera arranged as follows.

[0134] The SMA actuator assembly 30 includes a stationary component 121 having an image sensor 33 mounted thereon. The stationary component 121 includes a base 34 as a rigid plate. The image sensor 33 is fixed to the front side of the base 34. The stationary component 121 also includes a chassis 36 protruding from the base 34, and the chassis 36 may be a molded component. The chassis 36 has a central hole 37 aligned with the image sensor 33.

[0135] The SMA actuator assembly 30 further comprises a lens element 122 which is located in the hole 37 and comprises a lens holder 42 which holds a lens 41, although alternatively there may be a plurality of lenses. The lens 41 may be made of glass or plastic. The SMA actuator assembly 30 is a micro-optical device in which the diameter of the lens 41 is at most 20 mm, preferably at most 15 mm, more preferably at most 10 mm.

[0136] The lens element 122 has an optical axis O aligned with the image sensor 33 and is arranged to focus an image on the image sensor 33. The lens element 122 also has a protrusion 43 formed on one side protruding laterally from the optical axis O.

[0137] The SMA actuator assembly 30 also includes a suspension system 50 that supports the lens element 122 on the stationary component 121. The suspension system 50 is configured to guide the movement of the lens element 122 relative to the stationary component 121 along the optical axis O, while constraining the movement of the lens element 122 relative to the stationary component 121 in other degrees of freedom. This relative movement of the lens element 122 changes the focus of the image on the image sensor 33, for example, to provide autofocus or zoom. Therefore, in this example, the stationary component 121 is a stationary component, and the lens element 122 is a movable component that is movable relative to the stationary component 121 along the optical axis O. The terms "stationary" and "movable" refer to this relative movement.

[0138] In this example, the suspension system 50 comprises a bearing arrangement comprising a plurality of rolling bearings 51. Each rolling bearing 51 comprises a bearing surface 52 on the stationary part 121, in particular on the chassis 36, and a bearing surface 53 on the lens element 122, in particular on the lens holder 42. Each rolling bearing 51 further comprises a ball 54 arranged between the bearing surface 52 and the bearing surface 53. The ball 54 thus serves as a rolling bearing element, although other types of rolling bearing elements, such as rollers, may also be used as an alternative.

[0139] As an alternative, rolling bearing 51 may be replaced by a planar bearing including bearing surfaces on each of stationary component 121 and lens element 122 that conform to and support each other to guide relative movement.

[0140] More generally, the term "suspension system" may refer to a bearing or an arrangement of bearings. Unless the context requires otherwise, the term "bearing" is used herein as follows. The term "bearing" as used herein includes the terms "plain bearing", "plane bearing", "rolling bearing", "ball bearing", "roller bearing" and "flexure bearing". The term "bearing" as used herein generally refers to any element or combination of elements used to constrain motion to only the desired motion and reduce friction between moving parts. The term "plain bearing" is used to refer to a bearing in which a bearing element slides on a bearing surface, and includes a "plane bearing". The term "rolling bearing" is used to refer to a bearing in which a rolling bearing element, such as a ball or roller, rolls on a bearing surface. In an embodiment, the bearing may be disposed on a nonlinear bearing surface or may include a nonlinear bearing surface.

[0141] In some embodiments of the present technology, more than one type of bearing element may be used in combination to provide a bearing function. Thus, the term "bearing" as used herein includes any combination of, for example, plane bearings, ball bearings, roller bearings, and flexure bearings.

[0142] The SMA actuator assembly 30 also includes two lengths of SMA wire 20 (at Fig.10 One of these can be seen in the figure, which is arranged as follows to drive movement of the lens element 122 along the optical axis O. Each length of SMA wire 20 is connected at one end to the stationary component 121 and at the other end to the lens element 122 by a crimping portion 10 (which is the crimping portion 10 of the SMA subassembly, as described in more detail below).

[0143] The lengths of SMA wire 20 have an angled V-shaped arrangement similar to that disclosed in WO2007 / 113478. That is, each length of the SMA wire 20 is inclined with the same sense and the same acute angle θ relative to a plane perpendicular to the optical axis O (the direction of movement in this example). The angle θ relative to the plane perpendicular to the optical axis O is selected to provide a gain between the length change of the length of the length of the SMA wire 20 and the movement along the optical axis O while also reducing the height of the projection along the optical axis, and the angle θ is typically in the range of from a lower limit of 5 degrees or more preferably 8 degrees to an upper limit of 20 degrees, preferably 15 degrees, and more preferably 12 degrees. There is also an angle of 90 degrees between the lengths of the SMA wire 20 when viewed along the optical axis O (the direction of movement in this example).

[0144] In an alternative simpler arrangement, one of the lengths of SMA wire 20 may be omitted.

[0145] The SMA actuator assembly 30 further includes a compression spring 45 connected between the base 34 of the stationary component 121 and the lens element 122 and serving as a resilient biasing element for the length of SMA wire 20. Thus, when the length of SMA wire 20 cools, the compression spring 45 drives the SMA wire 20 to move in opposite directions ( Fig.10 As a result, the temperature of the length of SMA wire 20 and thus the position of the lens element 122 along the optical axis O can be controlled by controlling the power of the drive signal.

[0146] The length of SMA wire 20 is arranged to be relaxed without providing a drive signal thereto. It has been appreciated through experimentation and analysis that it is not necessary to maintain the length of SMA wire 20 in a tensioned state in an unpowered state. However, the length of SMA wire 20 is configured such that a tension suitable for driving the SMA actuator assembly 30 can be applied to the length of SMA wire by applying a suitable drive signal to heat the wire and cause it to contract. This can be achieved by controlling the degree of relaxation of the length of SMA wire 20.

[0147] Additionally, this aspect of the relaxed length of SMA wire in the unpowered state provides significant advantages. If the length of SMA wire is under tension in the unpowered state, the SMA actuator assembly would typically lose a significant amount of its theoretical travel, such as about 50 μm to 100 μm in a typical optical device. This is important because the achievable travel is typically a limiting factor in the miniaturization of SMA actuator assemblies. On the other hand, by providing a length of SMA wire 20 that is relaxed in the unpowered state, the length of the length of SMA wire 20 is increased, thereby increasing the travel capability of the SMA actuator assembly 30, possibly to its theoretical maximum.

[0148] A control circuit implemented in an IC chip (not shown) generates a drive signal and provides it to a length of SMA wire 20 connected to the control circuit. The control circuit receives a target signal representing a target position of the lens element 122 along the optical axis O and generates a drive signal having a power selected to drive the lens element 122 to the desired position. The power of the drive signal can be linear or can be varied using pulse width modulation.

[0149] Fig.11 An example of an SMA actuator assembly 30 is shown, which is similar to Fig.10 The camera shown in WO2007 / 113478, but with the following modifications to provide an angled V arrangement of the type disclosed in WO2007 / 113478. Rather than providing two lengths of SMA wire as separate pieces of SMA wire 20, each length being as Fig.10Instead of being connected at each end by a crimping portion 10, a single length of SMA wire 20 is connected at each end to the stationary component 121 by a crimping portion 10 and connected to the lens element 122 by hooking onto a hook feature 44 formed on the protrusion 43. Thus, the two portions of the length of SMA wire 20 on either side of the protrusion 43 form corresponding lengths of SMA wire 22 having the same construction and therefore the same function and operation, as shown. Fig.10 Two length segments of SMA wire 20 in.

[0150] Fig.12 There is shown an SMA actuator assembly 120 comprising four lengths of SMA wire 20 similar to that described in WO 2013 / 175197 and WO 2014 / 083318. The SMA actuator assembly 120 is arranged as follows.

[0151] The SMA actuator assembly 120 comprises a stationary part 121 and a lens element 122 comprising at least one lens 123, which in this example is a movable part. The lens 123 may be made of glass or plastic. The SMA actuator assembly 120 is a micro-optical device, wherein the lens 123 has a diameter of at most 20 mm, preferably at most 15 mm, more preferably at most 10 mm. The lens element 123 has an optical axis O aligned with an image sensor 124 fixed to the stationary part 121 and is arranged to focus an image on the image sensor 124.

[0152] The SMA actuator assembly 120 also includes a suspension system 125 that supports the lens element 123 on the stationary component 121. In this example, the suspension system 125 includes a plurality of rolling bearings 126, which may have an arrangement as disclosed in WO2014 / 083318. The suspension system 125 is configured to guide the movement of the lens element 122 relative to the stationary component 121 perpendicular to the optical axis O, so that the movement has three degrees of freedom, namely two translational degrees of freedom perpendicular to the optical axis O and one rotational degree of freedom about the optical axis O. The terms "stationary" and "movable" refer to this relative movement.

[0153] The suspension system 125 constrains movement of the lens element 122 relative to the stationary component 121 in other degrees of freedom, namely, translational freedom along the optical axis O and two rotational degrees of freedom about axes orthogonal to the optical axis O. This relative movement of the lens element 122 can be used to provide OIS.

[0154] However, the suspension system 125 may have any other form performing the same function, for example being formed by a beam as disclosed in WO 2013 / 175197 or by a flexure bearing.

[0155] The SMA actuator assembly 120 also includes four lengths of SMA wire 20. Each length of SMA wire 20 is connected at one end to a stationary component 121 and at the other end to a lens element 122 via a crimping portion 10 (which is a crimping portion 10 of an SMA subassembly, as described in more detail below).

[0156] The four lengths of SMA wire 20 are arranged as follows. The lengths of SMA wire 20 each extend perpendicular to the optical axis O, although as an alternative, the lengths of SMA wire 20 may be arranged obliquely at a non-zero angle, preferably a small angle, relative to a plane perpendicular to the optical axis O. The four lengths of SMA 20 are arranged along respective sides of the lens element 122, wherein a pair of lengths of SMA actuator wire 20 on opposite sides of the lens element 122 are perpendicular to another pair of lengths of SMA actuator wire 20 on opposite sides of the lens element 122.

[0157] Within each pair of those lengths of the SMA actuator wire 20, the lengths of the SMA wire 20 are opposed so that when a drive signal is provided, the lens element 122 is driven to move in opposite directions. In use, within each pair, as each SMA wire 20 contracts until they are in tension with each other, thereafter, the movement is achieved by allowing one or more of the lengths of the SMA wire 20 to extend, being stretched by one or more of the other lengths of the SMA wire 20.

[0158] Thus, the two pairs of lengths of the SMA actuator wire 20, when differentially contracted, each drive translational movement of the lens element 122 in one of two orthogonal directions perpendicular to the optical axis O. Additionally, the two pairs of lengths of the SMA actuator wire 20, when co-contracted, each drive rotational movement of the lens element 122 about the optical axis O, but in opposite directions between the two pairs, so that all four lengths of the SMA actuator wire 20, when differentially contracted between the two pairs, drive rotational movement of the lens element 122 about the optical axis O. Further details are explained in WO2014 / 083318.

[0159] Each length of SMA wire 20 is arranged to be slack without providing a drive signal thereto. It has been realized through experimentation and analysis that it is not necessary to maintain the length of SMA wire 20 in a tensioned state in an unpowered state. However, the length of SMA wire 20 is configured so that a tension suitable for driving the SMA actuator assembly 120 can be applied to the length of SMA wire 20 by providing a suitable drive signal to heat the wire and cause it to contract. This can be achieved by controlling the degree of slack of the length of SMA wire 20.

[0160] Additionally, this provision of a relaxed length of SMA wire 20 in an unpowered state provides significant advantages. If the length of SMA wire is under tension in an unpowered state, the SMA actuator assembly would typically lose a significant amount of theoretical travel, such as about 50 μm to 100 μm in a typical optical device. This is important because achievable travel is typically a limiting factor in the miniaturization of SMA actuator assemblies. On the other hand, by providing a length of SMA wire 20 that is relaxed in an unpowered state, the length of the length of SMA wire 20 is increased, thereby increasing the travel capability of the SMA actuator assembly 120, possibly to its theoretical maximum value.

[0161] Although as an example in Fig.10 , Fig.11 and Fig.12 Specific SMA actuator assemblies 30 and 120 are shown in WO2013 / 175197, but the SMA subassembly 1 can be used to make other types of SMA actuator assemblies. In one alternative, the SMA actuator assembly can be a camera that provides OIS of the type disclosed in WO2013 / 175197, or a camera that provides two functions of the type disclosed in WO2011 / 104518. In other alternatives, the SMA actuator assembly can be an optical device in which the movable element is a lens element, but there is no image sensor. In other alternatives, the SMA actuator assembly can be an optical device in which the movable component is an optical element other than a lens element. In other examples, the SMA actuator assembly can be a type of device that is not an optical device and in which the movable element is not an optical element.

[0162] Fig.13 and Fig.14 shows the control Fig.12 Two alternative forms of control circuitry 130 for an SMA actuator assembly 120 are shown in FIG. 1 . In each case, the control circuitry 130 includes the following components.

[0163] The control circuit 130 includes drive circuits 131 connected to the corresponding lengths of the SMA wire 20. Each drive circuit 131 generates a drive signal according to the control signal and provides the drive signal to the corresponding SMA wire 20. The drive circuit 131 can be a constant voltage current source or a constant current source. For example, in the latter case, the constant current may be about 120mA.

[0164] The drive signal is a pulse width modulated (PWM) drive signal. Therefore, the control signal controls the power of the drive signal by controlling the PWM duty cycle. The drive circuit 131 generates a drive signal with a bias power that controls the average temperature of the SMA wires 20 to cause contraction and ensure that they are all in a tensioned state. The control signal controls the power of the drive signal relative to the bias power. The bias power can have a predetermined value, or can be set according to the ambient temperature measured by an ambient temperature sensor (not shown).

[0165] The control circuit 130 also includes a respective resistance measurement circuit 132 arranged to detect the resistance of the respective SMA wire 20. Where the drive circuit 131 is a constant current source, the resistance measurement circuit 132 may be a voltage resistance measurement circuit operable to detect a voltage across the SMA wire 20 as a measure of its resistance. Where the drive circuit 132 is a constant voltage current source, the resistance measurement circuit 132 may be a current detection circuit operable to detect a voltage across a length of the SMA wire 20 as a measure of its resistance. For greater accuracy, the resistance measurement circuit 132 may each include a voltage detection circuit and a current detection circuit operable to detect both a voltage and a current across the length of the SMA wire 20 and derive a measure of resistance as a ratio thereof. Regardless of which measure of resistance is detected, the measured resistance is represented by an output signal from the resistance measurement circuit 132, which may typically be a digital signal or an analog signal that may subsequently be converted to a digital signal.

[0166] The control circuit further includes a closed-loop control circuit 140 that controls the drive signal based on a target signal representing a target position of the lens element 122 relative to the stationary component 121. The target signal represents the target position with respect to the degree of freedom of movement of the lens element 122 (i.e., the degree of freedom space). Fig.12 In the example of the SMA actuator assembly 120 shown, the target position of the lens element 122 is expressed in terms of two translational degrees of freedom and one rotational degree of freedom, for example, as a position (x, y, θ).

[0167] The position of the lens element 122 can also be expressed in wire space as the position with respect to each length segment of the SMA wire 20, such as (l1, l2, l3, l4), where l n is the length of the nth length segment of the SMA wire 20. The transformation T can be used to transform the position from the wire space to the degree of freedom space, T -1is the inverse transformation from degrees of freedom space to line space. In a simple example comprising two opposing co-linear length segments of SMA wire 20, the transformation T is given by [1 -1] and simply represents the difference between the lengths of the length segments of SMA wire 20. Fig.12 In the example of the SMA actuator assembly 120 shown, the transformation T is given by:

[0168]

[0169] The closed-loop control circuit 140 applies closed-loop control using the signal from the resistance measurement circuit 131 as a feedback signal, the signal being representative of the measured resistance with respect to the length of the SMA wire 20 .

[0170] The closed-loop control circuit 140 may be arranged as follows. In general, the closed-loop control circuit 140 may be implemented in analog or digital form, in which case the signals discussed below are analog or digital signals, respectively.

[0171] Fig.13 and Fig.14 Two different versions of the closed-loop control circuit 140 are shown. Fig.13 and Fig.14 In the equation, D represents the target signal, R represents the feedback signal, and E represents the error signal. These signals in the line space are represented by the subscript w, while these signals in the degree of freedom space are represented by the subscript n. Therefore, the target signal in the degree of freedom space is D n , and so on.

[0172] exist Fig.13 In the form shown, the closed control loop 140 comprises an error detector 141 which derives an error signal E in line space. w Therefore, the feedback signal R in line space w is directly provided to the error detector 141, but the closed-loop control circuit 140 includes a target transformation block 142 which transforms T -1 The target signal D in the degree of freedom space n Transformed into the target signal D in line space w , which represents the target position with respect to the length segment of the shape memory alloy wire 20. The target signal D in the wire space derived therefrom is w is provided to the error detector 141. The error detector 141 detects the error signal E about the length segment of the SMA wire 20. w , the error signal represents the transformed target signal D w The feedback signal R in line space w The error between .

[0173] The error signal E in line space wis provided to the controller 144, which derives a control signal based on this. The control signal is provided to the drive circuit 131, thereby based on the error signal E in the line space w To control the drive signal.

[0174] The controller 144 can be configured to generate an error signal E w and power.

[0175] One option is to apply a gain in each degree of freedom represented by the transformation G. In this case, the controller 144 may therefore transform T -1 GT is applied to the error signal E in line space w .

[0176] Another option is to apply gain only to each length of the SMA wire 20 so that each error signal E in the wire space w Only the power supplied to the corresponding length of the SMA wire 20 is affected. It is not true that the total power is only through this option, so when this option is applied, the drive circuit 132 is controlled together to make the total power supplied to all lengths of the SMA wire 20 constant by scaling the power after applying the gain.

[0177] As an alternative, the closed-loop control circuit 140 can be in the online space D w In this case, the target transform block 142 is omitted.

[0178] exist Fig.14 In the form shown, the closed control loop 140 includes an error detector 145 which derives an error signal E in the degree of freedom space. n Therefore, the target signal D in the degree of freedom space is n is directly provided to the error detector 144, but the closed-loop control circuit 140 includes a feedback transformation block 146 which transforms the feedback signal R in line space by transforming T w Transformed into the feedback signal R in the degree of freedom space n , which represents the resistance measure with respect to the degrees of freedom. The feedback signal R in the degree-of-freedom space thus derived is n is provided to the error detector 144. The error detector 144 detects the target signal D represented in the degree of freedom space with respect to the degree of freedom. n The transformed feedback signal R n The error signal E n .

[0179] The error signal E in the degree of freedom space nis provided to the controller 147, which derives a control signal based on this. The control signal is provided to the drive circuit 131, thereby based on the error signal E in the degree of freedom space n To control the drive signal.

[0180] The controller 147 can be used to generate an error signal E n and power.

[0181] One option is to apply a gain in each degree of freedom represented by the transformation G. In this case, the controller 147 may therefore transform T -1 G is applied to the error signal E in the degree of freedom space n .

[0182] Another option is to only apply gain to each length of the SMA wire 20. The total power is not correct with only this option, so when this option is applied, the drive circuit 132 is controlled together to make the total power provided to all lengths of the SMA wire 20 constant by scaling the power after applying the gain.

[0183] exist Fig.13 and Fig.14 In the embodiment of the present invention, the feedback signal is a signal from the resistance measuring circuit 132, which is the actual measured resistance, so the target signal is also in resistance units. However, as long as the feedback signal and the target signal represent resistance, they can be converted. One possibility is to convert the feedback signal and the target signal into length units, because the length and resistance of the length segment of the SMA wire 20 correspond to each other.

[0184] Fig.15 Shows the control Fig.12 The method of the SMA actuator assembly 120 shown in FIG. 1 is equally applicable to other SMA actuator assemblies that employ relative lengths of SMA wire 20 that are slack in the absence of a drive signal. The method is performed as follows. The method may be implemented by software executed by a processor 150 that forms part of the control circuit 130 and controls other components of the control circuit.

[0185] In step T1 , the drive circuit 132 is controlled to initially provide a drive signal. This causes the lengths of SMA wire 20 to contract, which removes slack and places those lengths in tension with each other prior to operation of the closed-loop control circuit 140 .

[0186] In step T2, when the signal is driven in step T2, the resistance measurement circuit 132 measures the resistance of the length of SMA wire 20. As described below, in the absence of the drive signal, those resistances depend on the degree of relaxation of the length of SMA wire 20. Step T2 is performed while the lens element 122 is allowed to move relative to the stationary component 121.

[0187] In step T3 , an offset for the closed control loop 140 for compensating for slack in the length of SMA wire 20 is derived from the measurements taken in step T1 .

[0188] In a first option, in step T3, the offset is derived from the resistance of the length of SMA wire 20 measured when the drive signal is first applied, before the length of SMA wire 20 has had time to heat, so the offset is a measure of the resistance at ambient temperature. This option is simple to implement.

[0189] In a second option, in step T3, the offset is derived from another characteristic of the resistance that depends on the degree of relaxation of the length of SMA wire 20 in the absence of a drive signal as the power of the drive signal is increased over time to increase the temperature of the length of SMA wire 20 from its uncontracted initial state to a state where the relaxation of all the lengths of SMA wire 20 are tightened and the lengths of SMA wire 20 are in tension with each other.

[0190] In step T4, the drive circuit 132 is controlled to provide a drive signal under the control of the closed-loop control circuit 140. However, the signal in the closed-loop control is adjusted by the offset derived in step T3 to compensate for the slack of the length of SMA wire 20.

[0191] In step T4, an offset may be adjusted to various signals within the closed control loop 140 to provide compensation.

[0192] use Fig.13 In the form of the closed-loop control circuit 140 shown, the offset can be adjusted by the feedback signal R w (at point A), target signal D before transformation n (at point B), the transformed target signal D w (at point C) or error signal E w Since the operations performed by the error detector 141 and the target transformation block 142 are linear, applying the offsets at these various points A through D has the equivalent effect of providing compensation for the degree of slack in the length of the SMA wire 20, although the form of the offsets is calculated based on the locations at which they are applied.

[0193] use Fig.14In the form of the closed-loop control circuit 140 shown in FIG. 1 , the offset can be adjusted by the feedback signal R before the conversion. w (at point E), the transformed feedback signal R n (at point F), target signal D n (at point H) or the error signal E n Since the operations performed by the error detector 145 and the feedback transformation block 146 are linear, applying the offsets at these various points A through D has the equivalent effect of providing compensation for the degree of slack in the length of the SMA wire 20, although the form of the offsets is calculated based on the locations at which they are applied.

[0194] For each signal at one of the points AF, H or I to which the offset is applied, by providing in the line Fig.16 The offset is implemented by an adder 149. The adder 149 adds the offset to the signal in question to condition the signal.

[0195] The derivation of the offset in step T4 will now be discussed. Since all lengths of the SMA wire 10 are slack in their cold state in the absence of a drive signal, they must each contract to remove this slack and reach an initial tensioned state when a drive signal is applied. During this contraction, reference Figure 3 , the resistance of the length segment of the SMA wire 20 changes as follows, Figure 3 It shows how the resistance of a length of SMA wire 20 varies with temperature when a drive signal is applied from a cold state.

[0196] The initial heating causes the resistance of the length of the SMA wire 20 to increase as the resistivity of the SMA material increases. This is shown by portion 111 of the curve. There is also some thermal expansion, but the change in resistivity dominates the change in resistance. As the length of the SMA wire 20 reaches the phase transition temperature, it changes phase from the martensite state to the austenite state, resulting in a decrease in resistivity. This results in a decrease in length and an increase in area. These factors together lead to a maximum resistance 112. Thereafter, the resistance decreases, as shown in portion 113 of the curve. After the slack is tightened, the resistance continues to decrease until the relative lengths of the SMA wire 20 experience mechanical resistance to each other. At that moment, a minimum resistance 114 is experienced. Thereafter, the resistance increases, as shown in portion 115 of the curve. If the length of the SMA wire is completely transitioned to the austenite state, a similar minimum resistance will be generated, but in a practical actuator assembly, the length of the SMA wire 20 is configured so that the mechanical resistance from the relative length of the SMA wire 20 is first encountered.

[0197] The degree of relaxation of the length segment of the SMA wire 20 in the initial relaxation state affects the magnitude of the maximum resistance 112 and the magnitude of the minimum resistance 114. Specifically, as the degree of relaxation increases, the magnitude of the maximum resistance 112 increases and the magnitude of the minimum resistance 114 decreases. The increase in the magnitude of the maximum resistance 112 can be understood as the length segment of the SMA wire 20 undergoing a greater change in resistivity before the transition with the increased initial relaxation. The decrease in the magnitude of the minimum resistance 114 can be understood as the length segment of the SMA wire 20 undergoing a greater change in resistivity and length with the increased initial relaxation.

[0198] Thus, as mentioned above for the second option of step T3, the offset can be derived from such characteristics of the resistance that depend on the degree of relaxation, such as the magnitude of the maximum resistance 112, the magnitude of the minimum resistance 114, or the difference therebetween. More generally, other measures of the degree of relaxation can be derived from test results of the resistance or other electrical characteristics based on similar considerations of the electromechanical properties of a relaxed SMA wire subjected to a drive signal that causes the SMA wire to contract.

[0199] Because the SMA actuator assembly includes relatively long lengths of SMA wire 20, the following additional considerations should also be taken into account.

[0200] In contrast, in a rigid mechanical fixture, as the tension is tightened, the resistance will reach a well-defined minimum, after which the wire resistance will increase as the temperature increases. In this case, the difference between the maximum and minimum resistance seen across all wires, averaged over all wires, can be taken as a measure of the amount of slack in the system. This difference will be calibrated for a given material, path length, and initial wire diameter.

[0201] In contrast, in an SMA actuator assembly where the lengths of SMA wire 20 are opposed, the drop in resistance of each length of SMA wire 20 depends in part on the amount of slack in that length of SMA wire 20 and the slack in the opposing length of SMA wire 20, which affects the position of the moving component relative to the stationary component when all lengths of SMA wire 20 are tensioned. This would require that the point at which the minimum resistance 114 is measured be the same for all lengths of SMA wire 20.

[0202] It is also possible that different lengths of the SMA wire 20 reach the transition temperature at different points in time, for example, if they are heated quickly and in different thermal environments, or if they are heated by providing a drive signal that passes through the length of the SMA wire 20. In this case, the individual maximum resistances 112 must be evaluated to ensure that the correct starting point for each wire is evaluated. If these points are significantly different, the rate of change for each wire can be inferred from the point at which the phase change begins, and thus the absolute temperature can be inferred, because the point at which the phase change begins is proportional to the wire temperature.

[0203] However, this measurement can be simplified by deriving the offset from the initial resistance 116 of the length of SMA wire 20 measured at ambient temperature when the drive signal is first provided, as in the second option of step T3 above. Measuring the resistance of all lengths of SMA wire 20 at a known initial temperature simplifies the process because no maxima and minima need to be evaluated. For individual lengths of SMA wire 20, the drop in resistance due to phase change indicates the amount that the wire can extend. Therefore, for relative lengths of SMA wire 20, when their resistance changes at start-up are equal, all lengths of SMA wire 20 should have equal travel.

[0204] In this case, when an offset is applied to the feedback signal R in line space at point A or E in the closed control loop 140 w , the offset may simply be the negative of the initial resistance 116. Given the linear operation within the closed control loop 140, when the offset is applied to other signals at other points BD, F, H, or I in the closed control loop 140, the offset may be derived from the negative initial resistance 116 to provide the same effect.

[0205] Typically, the operating point is reached by applying a drive signal to the length of the SMA wire 20, which has been pre-calibrated to ensure that the system is in tension, but the wire resistance can also be evaluated and its rate of change over time or current can be used to evaluate whether the wire is in a phase change state. Regardless of which solution is used, the difference in average resistance between the cold state of the length of the SMA wire 20 in the absence of a drive signal and the state when the length of the SMA wire 20 is in tension reflects the increase in resistance caused by heating, which causes the wire resistance to increase to the starting point of the phase change. This is generally similar for all lengths of the SMA wire 20. From this point on, the phase change and length change cause the resistance of each length of the SMA wire 20 to decrease until they become tensioned. This approach is suitable because most SMA actuator assembly designs use multiple wires with the same nominal length and degree of slack, and the components are typically assembled and tested in a controlled temperature environment.

[0206] Therefore, the method adds an offset to evaluate a single maximum, which offset depends on the initial ambient temperature. If this initial ambient temperature is not controlled, but known, the offset can be calibrated and the method can be used in environments where the temperature is not controlled but measured, provided that this temperature is below the starting point of the phase change of the actuator material.

[0207] Since the initial resistance 116 is measured immediately upon application of the initial drive signal, a very fast test is provided. The most accurate performance is achieved when the lengths of SMA wire 20 are at the same temperature when measuring the initial resistance 116. In some SMA materials, there is a significant phase lag with temperature changes, which can cause differences in behavior between turning on and off (increasing and decreasing wire temperature). This is not a problem if all lengths of SMA wire 20 have the same thermal history and starting point in a controlled temperature environment.

[0208] In SMA actuator assemblies, moving parts may bind when open, causing tension to be reached prematurely, so it is best to perform the test in the closed state after movement of the parts has released the initial binding. If the actuator is operated only between the highest and lowest temperatures reached in this test, the closed and open variations are interchangeable.

[0209] In a related method of the present technology, a non-transitory data carrier is provided that carries processor control code to implement any of the methods described herein.

[0210] As will be appreciated by those skilled in the art, the present technology may be embodied as a system, a method or a computer program product.Therefore, the present technology may take the form of a complete hardware embodiment, a complete software embodiment or an embodiment combining software and hardware aspects.

[0211] Furthermore, the present technology may take the form of a computer program product embodied in a computer readable medium having computer readable program code embodied thereon. The computer readable medium may be a computer readable signal medium or a computer readable storage medium. The computer readable medium may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared or semiconductor system, apparatus or device, or any suitable combination of the foregoing.

[0212] Computer program code for performing operations of the present technology may be written in any combination of one or more programming languages, including object-oriented programming languages ​​and conventional procedural programming languages. Code components may be embodied as procedures, methods, etc., and may include subcomponents that may take the form of instructions or sequences of instructions at any level of abstraction, from direct machine instructions of a native instruction set to high-level compiled or interpreted language constructs.

[0213] Embodiments of the present technology also provide a non-transitory data carrier carrying code, which, when implemented on a processor, causes the processor to perform any of the methods described herein.

[0214] The present technology further provides processor control code to implement the above methods, for example, on a general-purpose computer system or on a digital signal processor (DSP). The present technology also provides a carrier carrying processor control code to implement any of the above methods at runtime, especially on a non-transitory data carrier. The code can be provided on a carrier such as a disk, a microprocessor, a CD-ROM or a DVD-ROM, a programming memory such as a non-volatile memory (e.g., flash memory) or a read-only memory (firmware), or on a data carrier such as an optical or electrical signal carrier. The code (and / or data) for implementing the embodiments of the technology described herein may include source code, object code or executable code in a conventional programming language (interpreted or compiled), such as C or assembly code, code for setting or controlling an ASIC (application-specific integrated circuit) or an FPGA (field programmable gate array), or a hardware description language such as Verilog (RTM) or VHDL (very high speed integrated circuit hardware description language). As will be understood by those skilled in the art, such code and / or data may be distributed between multiple coupled components that communicate with each other. The technology may include a controller including a microprocessor, a working memory, and a program memory coupled to one or more components of the system.

[0215] It will also be clear to those skilled in the art that all or part of the logic methods according to embodiments of the present technology may be suitably embodied in a logic device comprising logic elements that perform the steps of the above methods, and such logic elements may include components such as logic gates in a programmable logic array or an application specific integrated circuit. Such a logic arrangement may further be embodied in an enabling element for temporarily or permanently establishing a logic structure in such an array or circuit using, for example, a virtual hardware descriptor language that may be stored and transmitted using a fixed or transmittable carrier medium.

[0216] In one embodiment, the present technology can be implemented in the form of a data carrier having functional data thereon, the functional data comprising a functional computer data structure to enable the computer system to perform all the steps of the above-described method when loaded into a computer system or network and operated on it.

[0217] Further embodiments of the present technology are set forth in the following numbered clauses:

[0218] 1. A method for measuring the degree of relaxation of at least one length segment of a shape memory alloy wire that is held relaxed, the method comprising: heating at least one length segment of the shape memory alloy wire to contract the at least one length segment of the shape memory alloy wire; measuring electrical properties of the at least one length segment of the shape memory alloy wire during the contraction of the at least one length segment of the shape memory alloy wire; and deriving a measure of the degree of relaxation of the at least one length segment of the shape memory alloy wire from the measured electrical properties.

[0219] 2. The method of clause 1, wherein the step of heating at least one length of shape memory alloy wire comprises providing a drive signal to the at least one length of shape memory alloy wire sufficient to cause the contraction.

[0220] 3. The method of clause 1, wherein the step of heating at least one length of shape memory alloy wire comprises externally heating at least one length of shape memory alloy wire.

[0221] 4. A method according to any of the preceding clauses, wherein the electrical characteristic represents the electrical resistance of at least one length segment of the shape memory alloy wire.

[0222] 5. The method of clause 4, wherein the metric is the difference between a maximum resistance of at least one length of the shape memory alloy wire when it begins to contract and a minimum resistance of at least one length of the shape memory alloy wire when it stops contracting.

[0223] 6. The method of clause 4, wherein the metric is a minimum resistance of at least one length segment of the shape memory alloy wire when it stops contracting.

[0224] 7. The method of clause 4, wherein the metric is a maximum resistance at which at least one length segment of the shape memory alloy wire begins to contract.

[0225] 8. A method according to any preceding clause, wherein at least one or each length of shape memory alloy wire is held slack between two connected portions which are held in a fixed relative position.

[0226] 9. The method of clause 8, wherein the connecting portion is a crimped portion crimped around a length of shape memory alloy wire.

[0227] 10. The method of clause 9, wherein the length of shape memory alloy wire is held in a shape memory alloy subassembly comprising at least one body portion integrally formed with a pair of crimp portions formed from a sheet of material.

[0228] 11. A method according to any one of clauses 1 to 8, wherein, in the shape memory alloy actuator assembly, at least one length of shape memory alloy wire is held slack between a stationary component and a movable component movable relative to the stationary component.

[0229] 12. The method according to clause 11, wherein at least one length of the shape memory alloy wire is held slack between crimping portions crimped around the length of the shape memory alloy wire and mounted on the stationary component and the movable component, respectively.

[0230] 13. A method according to clause 11 or 12, wherein the steps of heating the length of shape memory alloy wire and measuring the electrical properties of the length of shape memory alloy wire are performed while the movable part is maintained in a fixed position relative to the stationary part.

[0231] 14. A method according to clause 11 or 12, wherein the steps of heating the length of shape memory alloy wire and measuring the electrical properties of the length of shape memory alloy wire are performed while the movable part is allowed to move relative to the stationary part.

[0232] 15. A method according to clause 14, wherein the at least one length segment of the shape memory alloy wire includes multiple length segments of the shape memory alloy wire, each of which maintains slack between the stationary part and the movable part, the electrical characteristic is the resistance of each of the multiple shape memory alloy wires, and the measure of the degree of slack is derived with respect to each of the multiple shape memory alloy wires and is the difference between the resistance of the shape memory alloy wire before heating the multiple shape memory alloy wires and the resistance of the shape memory alloy wire when all of the multiple shape memory alloy wires are tensioned.

[0233] 16. A method according to any one of clauses 11 to 14, wherein the at least one length of shape memory alloy wire comprises a plurality of lengths of shape memory alloy wire, each of the plurality of lengths being held slack between the stationary component and the movable component.

[0234] 17. The method of clause 16, wherein the plurality of shape memory alloy wires comprises opposing shape memory alloy wires, and the measure of the degree of relaxation is derived with respect to the opposing shape memory alloy wires individually, differentially, or together.

[0235] 18. A method according to any of clauses 11 to 17, wherein the movable component is an optical element.

[0236] 19. The method according to clause 18, wherein the movable component is a lens element comprising at least one lens.

[0237] 20. The method of any of the preceding clauses, further comprising adjusting a degree of relaxation of the length of shape memory alloy wire in response to the derived metric.

[0238] 21. A method according to claim 20, wherein the length segment of the shape memory alloy wire is initially maintained slack between the crimped portions partially crimped around the length segment of the shape memory alloy wire, and the method further comprises, after the step of adjusting the degree of slack of the length segment of the shape memory alloy wire, completely crimping the crimped portions around the length segment of the shape memory alloy wire.

[0239] 22. A device for measuring the degree of relaxation of at least one length segment of a shape memory alloy wire that is maintained relaxed, the device comprising: a drive circuit, the drive circuit being arranged to provide a drive signal to at least one length segment of the shape memory alloy wire sufficient to cause the at least one length segment of the shape memory alloy wire to contract; a measurement circuit, the measurement circuit being arranged to measure the electrical properties of at least one length segment of the shape memory alloy wire during the contraction of the at least one length segment of the shape memory alloy wire; and a processor, the processor being arranged to derive a measure of the degree of relaxation of the at least one length segment of the shape memory alloy wire from the measured electrical properties.

[0240] 23. A method for controlling a shape memory alloy actuator assembly, the shape memory alloy actuator assembly comprising a stationary component, a movable component movable relative to the stationary component, and a plurality of shape memory alloy wires, the plurality of shape memory alloy wires being held between the stationary component and the movable component and being arranged to drive the movable component to move relative to the stationary component in one or more degrees of freedom in opposite directions when a drive signal is applied thereto, the plurality of shape memory alloy wires being relaxed in the absence of the drive signal, the method comprising: measuring the length of the length of the shape memory alloy wire in the absence of the drive signal; an electrical characteristic that depends on the degree of relaxation of the length segment of the shape memory alloy wire; and providing a drive signal to the length segment of the shape memory alloy wire when measuring the resistance of the length segment of the shape memory alloy wire, wherein a closed-loop control loop controls the drive signal based on a target signal representing a target position of the movable part relative to the stationary part and a feedback signal representing the measured resistance of the length segment of the shape memory alloy wire, wherein one or more signals in the closed-loop control loop are adjusted by an offset that is derived from the measured electrical characteristic and compensates for the relaxation of the length segment of the shape memory alloy wire.

[0241] 24. The method of clause 23, wherein the measured electrical property is the electrical resistance of the length of shape memory alloy wire.

[0242] 25. The method of clause 26, wherein the offset is derived from a resistance of the length of shape memory alloy wire measured when the length of shape memory alloy wire is at ambient temperature.

[0243] 26. The method of clause 25, wherein the offset is derived from a resistance of the length of shape memory alloy wire measured when the drive signal is first provided to the length of shape memory alloy wire.

[0244] 27. A method according to any of clauses 22 to 26, wherein the step of measuring the electrical property of the length of shape memory alloy wire is performed while the moveable component is allowed to move relative to the stationary component.

[0245] 28. A method according to any of clauses 22 to 27, wherein the target signal represents a target position with respect to one or more degrees of freedom.

[0246] 29. A method according to claim 28, wherein the closed-loop control loop includes: a target transformation block, which is arranged to transform the target signal to represent the target position of the length segment of the shape memory alloy wire; an error detector, which is arranged to detect an error signal with respect to the length segment of the shape memory alloy wire, the error signal representing the error between the transformed target signal and the feedback signal; and a controller, which is arranged to control the drive signal based on the error signal.

[0247] 30. The method of clause 29, wherein the one or more signals adjusted by the offset are any of a feedback signal, a target signal before transformation, a target signal after transformation, or an error signal.

[0248] 31. A method according to any one of clauses 23 to 27, wherein the target signal represents a target position with respect to a length segment of the shape memory alloy wire, and the closed-loop control loop includes: an error detector arranged to detect an error signal with respect to the length segment of the shape memory alloy wire, the error signal representing an error between the target signal and a feedback signal; and a controller arranged to control the drive signal based on the transformed error signal.

[0249] 32. The method of clause 31, wherein the one or more signals adjusted by the offset are any of a feedback signal, a target signal, or an error signal.

[0250] 33. A method according to clause 28, wherein the closed-loop control loop includes: a feedback transformation block arranged to transform a feedback signal to represent a resistance measurement with respect to one or more degrees of freedom; an error detector arranged to detect an error signal with respect to one or more degrees of freedom, the error signal representing an error between a target signal and a transformed feedback signal; and a controller arranged to control a drive signal based on the error signal.

[0251] 34. A method according to clause 33, wherein the one or more signals adjusted by the offset are any of a pre-transformed feedback signal, a post-transformed feedback signal, a target signal or an error signal.

[0252] 35. A method according to any of clauses 23 to 34, wherein the movable component is an optical element.

[0253] 36. The method of clause 35, wherein the movable component is a lens element comprising at least one lens.

[0254] 37. A control circuit for controlling a shape memory alloy actuator assembly, the shape memory alloy actuator assembly comprising a stationary component, a movable component movable relative to the stationary component, and a plurality of shape memory alloy wires, the plurality of shape memory alloy wires being held between the stationary component and the movable component and being arranged to drive the movable component to move in one or more degrees of freedom in opposite directions relative to the stationary component when a drive signal is applied thereto, the plurality of shape memory alloy wires being relaxed in the absence of the drive signal; the control circuit comprising: a drive circuit arranged to provide a drive signal to a length segment of the SMA wire; a resistance measurement circuit, the resistance measurement circuit a measurement circuit arranged to measure the resistance of the length segment of the shape memory alloy wire; and a closed-loop control loop arranged to control a drive signal based on a target signal representing a target position of the movable component relative to the stationary component and a feedback signal representing the measured resistance of the length segment of the shape memory alloy wire, wherein one or more signals in the closed-loop control loop are adjusted by an offset, which is derived from a measured electrical characteristic of the length segment of the shape memory alloy wire, the electrical characteristic depends on the degree of relaxation of the length segment of the shape memory alloy wire in the absence of the drive signal, and the offset compensates for the relaxation of the length segment of the shape memory alloy wire.

[0255] 38. An SMA actuator assembly comprising: a stationary component; a movable component movable relative to the stationary component; a plurality of shape memory alloy wires, which are held between the stationary component and the movable component and are arranged to drive the movable component to move in opposite directions relative to the stationary component with one or more degrees of freedom when a drive signal is applied thereto, and the plurality of shape memory alloy wires are relaxed in the absence of a drive signal; and a control circuit according to claim 37.

[0256] Those skilled in the art will appreciate that, although the foregoing has described what are considered to be the best modes for carrying out the present technology and other suitable modes, the present technology should not be limited to the specific configurations and methods disclosed in the description of the preferred embodiments. Those skilled in the art will recognize that the present technology has a wide range of applications, and that the embodiments may be modified in many ways without departing from any inventive concept as defined in the appended claims.

Claims

1. A method of controlling a shape memory alloy actuator assembly, the shape memory alloy actuator assembly comprising a stationary component, a movable component movable relative to the stationary component, and a plurality of shape memory alloy wires, the plurality of shape memory alloy wires being held between the stationary component and the movable component and being arranged to drive the movable component to move in one or more degrees of freedom in substantially opposite directions relative to the stationary component when provided energy is applied thereto, the plurality of shape memory alloy wires being relaxed when no provided energy is applied, The method comprises: measuring an electrical property of the length of shape memory alloy wire in the absence of supplied energy, the electrical property being dependent on a degree of relaxation of the length of shape memory alloy wire; as well as providing energy to heat the length of shape memory alloy wire while measuring the resistance of the length of shape memory alloy wire, wherein a closed-loop control circuit controls the provided energy based on a target signal representing a target position of the movable component relative to the stationary component and a feedback signal representing the measured resistance of the length of shape memory alloy wire, Wherein, one or more signals in the closed-loop control circuit are adjusted by an offset, which is derived from the measured electrical characteristics and compensates for the relaxation of the length section of the shape memory alloy wire.

2. The method according to claim 1, wherein: The provided energy for heating the length of shape memory alloy wire includes one or more of a drive signal or energy provided by a heat source.

3. The method according to claim 1, wherein: The electrical characteristic measured is the electrical resistance of the length of shape memory alloy wire.

4. The method according to claim 2, wherein: The electrical characteristic measured is the electrical resistance of the length of shape memory alloy wire.

5. The method according to claim 3 or 4, wherein: The deflection is derived from a measured resistance of the length of shape memory alloy wire when the length of shape memory alloy wire is at ambient temperature.

6. The method according to claim 3 or 4, wherein: The deflection is derived from a resistance of the length of shape memory alloy wire measured when the provided energy is first provided to the length of shape memory alloy wire.

7. The method according to any one of claims 1 to 4, wherein: The step of measuring an electrical characteristic of the length of shape memory alloy wire is performed while the movable component is allowed to move relative to the stationary component.

8. The method according to any one of claims 1 to 4, wherein: The target signal represents the target position with respect to the one or more degrees of freedom.

9. The method according to claim 8, wherein: The closed-loop control circuit comprises: a target transformation block arranged for transforming the target signal to represent the target position with respect to the length segment of shape memory alloy wire; an error detector arranged to detect an error signal with respect to the length of shape memory alloy wire, the error signal being indicative of an error between a transformed target signal and the feedback signal; and A controller is arranged to control the energy provided based on the error signal.

10. The method according to claim 9, wherein: The one or more signals adjusted by the offset are any one of the feedback signal, the target signal before transformation, the target signal after transformation, or the error signal.

11. The method according to any one of claims 1 to 4 and 9 to 10, wherein: The target signal represents the target position with respect to the length of shape memory alloy wire, and the closed-loop control circuit comprises: an error detector arranged to detect an error signal with respect to the length of shape memory alloy wire, the error signal being indicative of an error between the target signal and the feedback signal; and A controller is arranged to control the provided energy based on the transformed error signal.

12. The method according to claim 11, wherein: The one or more signals adjusted by the offset are any one of the feedback signal, the target signal, or the error signal.

13. The method according to claim 7, wherein: The closed-loop control circuit comprises: a feedback transformation block arranged to transform the feedback signal to represent a measure of resistance with respect to the one or more degrees of freedom; an error detector arranged to detect an error signal with respect to the one or more degrees of freedom, the error signal being representative of an error between the target signal and a transformed feedback signal; and A controller is arranged to control the energy provided based on the error signal.

14. The method according to claim 13, wherein: The one or more signals adjusted by the offset are any one of a feedback signal before transformation, a feedback signal after transformation, the target signal or the error signal.

15. The method according to any one of claims 1-4, 9-10 and 12-14, further comprising: Based on the offset, the movable component is placed at a center position, wherein at the center position, the contraction degree in the plurality of length sections of the smart memory alloy wire is substantially the same when receiving the corresponding provided energy.

16. A control circuit for controlling a shape memory alloy actuator assembly, the shape memory alloy actuator assembly comprising a stationary component, a movable component movable relative to the stationary component, and a plurality of shape memory alloy wires, the plurality of shape memory alloy wires being held between the stationary component and the movable component and being arranged to drive the movable component to move in one or more degrees of freedom in opposite directions relative to the stationary component when supplied energy is applied thereto, the plurality of shape memory alloy wires being relaxed when no energy is supplied thereto, The control circuit comprises: a drive circuit arranged to provide energy to heat the length of SMA wire; a resistance measurement circuit arranged to measure the resistance of the length of shape memory alloy wire; and a closed-loop control circuit arranged to control the energy provided in dependence on a target signal indicative of a target position of the movable component relative to the stationary component and a feedback signal indicative of a measured resistance of the length of shape memory alloy wire, Wherein, one or more signals in the closed-loop control loop are adjusted by an offset, which is derived from the measured electrical characteristics of the length segment of the shape memory alloy wire, the electrical characteristics depend on the degree of relaxation of the length segment of the shape memory alloy wire in the absence of provided energy, and the offset compensates for the relaxation of the length segment of the shape memory alloy wire.

17. The control circuit according to claim 16, wherein: The provided energy for heating the length of shape memory alloy wire includes one or more of a drive signal or heat provided by a heat source.

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