Control of sma haptic assembly

Abstract

There is provided a method of driving a shape memory alloy haptic assembly, the shape memory alloy haptic assembly comprising an actuator comprising a shape memory alloy arranged to provide a haptic effect upon actuation, the method comprising continuously providing a drive current to the actuator during a pre-heat period in which the temperature of the shape memory alloy is raised without causing the shape memory alloy to provide the haptic effect and during an actuation period in which the temperature of the shape memory alloy is raised so as to cause the shape memory alloy to provide the haptic effect. There is also provided a shape memory alloy haptic assembly.

Description

[0001] This invention relates to the use of shape memory alloy (SMA) wires to control actuators to generate relative motion between two components. In particular, it relates to the control of such actuators used in haptic assemblies.

[0002] SMA actuators are known for use in handheld electronic devices such as cameras and mobile phones. In particular, they can be used to provide haptic feedback, such as responding to a user selecting a specific area of ​​a screen or pressing a button. Such actuators typically function by using the contraction of SMA wires to induce relative movement between two parts. The SMA wires partially contact the two opposing parts, which are forced apart due to the change in the length of the SMA wires as they contract. The haptic feedback can be a 'click' sensation, vibration, or a similar form.

[0003] For the SMA to contract and thus cause movement of the actuator, it must be heated until it transitions between two crystalline states. To produce certain types of tactile feedback (such as a "click" sensation), the SMA contraction must occur within a relatively short timeframe (e.g., milliseconds). Generating this effect requires transferring energy to heat the SMA to induce the transition within a sufficiently short time, which in turn requires delivering relatively high power to the SMA. This is particularly problematic in mobile or wearable devices, where the battery's limited ability to sustain high power output may prevent it from providing power for the required duration.

[0004] One solution to this problem is to use a capacitor circuit that is charged before actuation. The capacitor circuit is designed to provide the continuous power output required to actuate the SMA wires. However, a capacitor capable of providing the necessary continuous power output would be physically large and therefore unsuitable for mobile devices. For example, the energy required to actuate the SMA haptic components in a mobile device is typically 20-30 mJ. A capacitor capable of storing 30 mJ at 5V (typical for mobile device power supplies) would require a capacitance of 2200 μF and a physical size of approximately 22.4 mm x 2.5 mm. This would be too large and impractical in mobile devices, where the area occupied by components is very important.

[0005] Therefore, it is necessary to control the SMA haptic component to actuate for a sufficiently short time to provide a haptic signal without requiring a large capacitor. The purpose of this invention is to solve this problem.

[0006] According to the present invention, a method for driving a shape memory alloy tactile assembly is provided, the shape memory alloy tactile assembly including an actuator comprising a shape memory alloy arranged to provide a tactile effect upon actuation, the method comprising continuously supplying a driving current to the actuator during a preheating cycle and an actuation cycle, wherein during the preheating cycle the temperature of the shape memory alloy rises without causing the shape memory alloy to provide a tactile effect, and during the actuation cycle the temperature of the shape memory alloy rises so that the shape memory alloy provides a tactile effect.

[0007] The desired tactile effect imposes constraints on how the shape memory alloy (SMA) is heated during the actuation cycle that provides the tactile effect, so that the SMA changes its crystalline state at the correct rate. However, the tactile effect is only provided when the SMA changes state, which is typically significantly above room temperature. Therefore, during the preheating cycle prior to providing the tactile effect (where the SMA temperature rises from ambient temperature), the constraints on how the SMA is heated are much less stringent. Dividing the heating into these two cycles thus allows for much greater flexibility in how power is supplied to the SMA, rather than simply heating it as quickly as possible, which can provide advantages in the design of other components of the tactile assembly.

[0008] In some embodiments, the power profile of the drive current relative to time during the preheating cycle differs from that during the actuation cycle. Providing a power profile with a different distribution during the preheating cycle than during the actuation cycle means that heating in the two cycles can be optimized for the different requirements of the two cycles.

[0009] In some embodiments, the drive current has a lower time-averaged power during at least a portion of the preheating cycle (preferably the entire preheating cycle) than during the actuation cycle. By preheating the SMA with lower power during at least a portion of the preheating cycle, the power requirement of the actuator in that cycle is reduced, and higher power is only required during the actuation cycle when the SMA changes state. Therefore, a sustained high power output from the power supply is not required over an extended cycle, but actuation still occurs for a sufficiently short time to generate a tactile signal. This reduces the burden on the power supply and reduces the need for additional power components (e.g., capacitors).

[0010] In some embodiments, during the preheating cycle, the temperature of the shape memory alloy is raised to the transition temperature at which the shape memory alloy begins its phase transformation. This minimizes the additional energy that must be supplied to the SMA during actuation to induce its phase transformation, as any further heating above the transition temperature will initiate a phase transformation in the SMA. During the preheating cycle, the temperature of the shape memory alloy may be raised to a temperature close to the transition temperature at which the shape memory alloy begins its phase transformation (e.g., a few degrees or fractions of a degree difference from the transition temperature). In particular, in some embodiments, during the actuation cycle, the temperature of the shape memory alloy is raised to a temperature below the transition temperature at which the shape memory alloy begins its phase transformation. In some embodiments, during the actuation cycle, the temperature of the shape memory alloy is raised to a temperature above the transition temperature at which the shape memory alloy begins its phase transformation, thereby enabling the SMA to complete its phase transformation within the desired cycle, thus correctly providing the tactile effect.

[0011] In some embodiments, a preheating cycle begins in response to a demand signal for a tactile signal. By initiating heating only upon receiving a demand signal, the SMA wire does not remain at an elevated temperature for an extended period, thereby reducing energy consumption.

[0012] In some embodiments, the length of the preheating cycle is at most 80 ms, preferably at most 60 ms, more preferably at most 50 ms, even more preferably at most 40 ms, and most preferably at most 20 ms. Limiting the duration of the preheating cycle prevents the user from perceiving actuation delays, which would degrade the user experience.

[0013] In some embodiments, the preheating cycle begins before a demand signal is received in response to the tactile signal, and the actuation cycle begins in response to the demand signal. Preheating before receiving the demand signal allows the actuator to respond to the demand signal more quickly.

[0014] In some embodiments, a preheating cycle is initiated in response to the occurrence of a precursor event. This allows the actuator to respond quickly upon receiving a demand signal, while preventing the SMA from remaining at an elevated temperature over an extended cycle, thereby reducing energy consumption.

[0015] In some embodiments, the preheating cycle includes an initial heating cycle that raises the temperature of the shape memory alloy and a holding cycle that maintains the temperature of the shape memory alloy within a predetermined temperature range. This means that the SMA can be maintained at the raised temperature to allow the actuator to respond quickly to demand signals.

[0016] In some embodiments, the drive current is supplied to the actuator from the same current source during both the preheating cycle and the actuation cycle. This eliminates the need for two separate sources, which would increase the size and complexity of the system including the components.

[0017] In some embodiments, the drive current is supplied to the actuator from the same current source during both the preheating and actuation cycles, the current source being pulse-width modulated (PWM) with different distributions during the preheating and actuation cycles. PWM allows for regulation of the supplied current and prevents excessive current draw from the power supply (especially during the preheating cycle).

[0018] In some embodiments, drive current is supplied to the actuator from the same current source during both the preheating and actuation cycles, the current source having different voltages during the preheating and actuation cycles. A control voltage provides an alternative way to control the power of the drive current supplied during both cycles.

[0019] In some embodiments, drive current is supplied to the actuator from a current source during the preheating cycle and at least partially from a capacitor element during the actuation cycle. Using a capacitor element during at least partial actuation means that the current source does not necessarily provide the full power required during the actuation cycle; in many embodiments, the total power required during the actuation cycle will, on average, be higher than the total power required during the preheating cycle. Since the capacitor element is only used during actuation, it can be made smaller than in prior art devices.

[0020] In some embodiments, the method further includes charging the capacitor element during a preheating cycle. This ensures that the capacitor element is ready for actuation, rather than being kept constantly charged, which would result in increased energy consumption.

[0021] In some embodiments, the method further includes detecting a measurement of the actuator's resistance and controlling the supply of drive current during a preheating cycle based on the detected resistance measurement. The resistance measurement indicates the temperature of the SMA and allows for precise control of the drive current to heat the SMA to the desired temperature during the preheating cycle. Using the resistance measurement eliminates the need to provide a specific temperature sensor.

[0022] In some embodiments, the control of the supply of drive current during the preheating cycle is a control of the length of the preheating cycle. The length of the preheating cycle can be adjusted according to the initial temperature of the SMA, since reaching the desired temperature may require more or less energy.

[0023] In some embodiments, the method further includes controlling the length of the actuation cycle based on the length of the preheating cycle. Depending on the ambient temperature, more or less energy may need to be supplied to the SMA to fully switch on and fully actuate the actuator. The preheating cycle required to reach the desired temperature indicates the ambient temperature and the heat loss rate of the SMA, and can therefore be used to determine the necessary length of the actuation cycle.

[0024] In some embodiments, the control of the supply of drive current during the preheating cycle is the control of the power of the drive current during the preheating cycle. Controlling the power allows the length of the preheating cycle to remain consistent while appropriately adjusting the energy delivered.

[0025] In some embodiments, the method further includes sensing the ambient temperature with a temperature sensor and controlling the supply of drive current during a preheating cycle and / or an actuation cycle based on the sensed ambient temperature. This allows the distribution of power supplied during the preheating cycle and / or an actuation cycle to be adjusted according to how much energy must be supplied to achieve the desired heating effect.

[0026] In some embodiments, the control of the supply of drive current during the preheating cycle and / or actuation cycle is a control of the length of the preheating cycle and / or actuation cycle. The length of the preheating cycle and / or actuation cycle can be adjusted according to the initial temperature of the SMA wire, as more or less heating may be required to achieve the desired heating effect.

[0027] In some embodiments, the drive current supplied to the actuator during one or both of the preheating cycle and the actuation cycle is pulse-width modulated. The method further includes sensing the ambient temperature with a temperature sensor and controlling the duty cycle of the pulse-width modulation based on the sensed ambient temperature. By adjusting the modulation duty cycle, the preheating cycle can be kept constant to provide a predictable response time.

[0028] In some embodiments, the control of the supply of drive current during the preheating cycle and / or the actuation cycle is the control of the power of the drive current during the preheating cycle and / or the control of the power of the drive current during the actuation cycle. Controlling the power allows the length of the preheating cycle and / or the actuation cycle to remain consistent while appropriately regulating the energy delivered.

[0029] In some embodiments, the actuation cycle is up to 10 ms long, preferably up to 5 ms, or more preferably up to 2 ms. This ensures that the actuation occurs quickly enough to provide the user with the desired sensation.

[0030] According to a second aspect of the present invention, a shape memory alloy tactile component is provided that implements a method similar to that of the first aspect of the present invention.

[0031] Embodiments of the invention will now be described by way of non-limiting example with reference to the accompanying drawings, in which:

[0032] Figure 1 The SMA actuator assembly is shown;

[0033] Figure 2 This is a graph illustrating an example of the control of the actuator according to the present invention;

[0034] Figure 3 This is a graph illustrating another example of the control of the actuator according to the invention;

[0035] Figure 4 A schematic diagram of the components according to the present invention is shown;

[0036] Figure 5 This is a flowchart of the method according to the present invention;

[0037] Figure 6 This is a circuit diagram of the components according to the present invention;

[0038] Figure 7 It is shown Figure 6 A graph showing how the circuit can be controlled.

[0039] One type of shape memory alloy tactile component intended for use in this invention includes an actuator 2, for example in Figure 1 The actuator shown or described as in WO 2019 / 162708 A1, which is incorporated herein by reference (see, for example, WO 2019 / 162708 A1) Figure 1 Actuator 2 includes shape memory alloy (SMA) 10, which is arranged to provide a tactile effect upon actuation. Figure 1 The actuator 2 includes a first body 4 and a second body 6, each body having one or more teeth 8. The SMA 10 is in the form of a wire extending along a tortuous path between the first body 4 and the second body 6. Upon actuation, the SMA 10 contracts, thereby driving the first body 4 and the second body 6 apart. The relative movement of the first body 4 and the second body 6 provides a tactile effect. A first aspect of the method is a method of driving a shape memory alloy tactile assembly, the shape memory alloy tactile assembly including the actuator 2, for example... Figure 1 Actuator 2.

[0040] The actuator 2, including the SMA 10, is typically driven by one or more current pulses that transfer energy to the SMA 10. In embodiments where the SMA 10 is in the form of a wire, energy is transferred in the form of resistance heating due to the current flowing in the SMA wire. This energy is used to perform two processes. The first portion of the transferred energy heats the SMA 10, raising its temperature without causing a change in its shape and / or size, a change that occurs when the SMA 10 reaches its transformation temperature, at which point the SMA 10 begins to transform from one crystalline state to another, for example, from a martensite state to an austenite state. The second portion of the transferred energy provides energy to further raise the temperature of the SMA 10, causing a phase transition between the two states. This phase transition causes a change in the shape of the SMA 10 performing the actuation. For example, in the case where the SMA 10 is a wire, the phase transition may cause the wire to shrink in length.

[0041] The performance of SMA actuator 2 is typically limited by the time required to transfer the second portion of energy, even the time required for SMA 10 to undergo a phase transition between two crystalline states and actuate actuator 2. This energy transfer time may be limited by the characteristics of the power supply used to drive actuator 2 (e.g., its maximum continuous power output). As mentioned above, this limitation can be overcome by storing energy in a capacitor. However, a capacitor capable of storing all the energy required to heat SMA 10 and cause its transition would be physically large and therefore potentially incompatible with some applications, such as mobile phone handsets or wearable devices. Furthermore, keeping the capacitor in a charged state ready to actuate actuator 2 is inefficient because energy is lost due to current leakage through the capacitor.

[0042] To provide examples of actual parameters for an SMA haptic component intended for use in mobile devices, some appropriate values ​​for the component used in a mobile phone would be as follows:

[0043] The resistance of the SMA in the actuator is 5-7Ω;

[0044] The total energy required to activate the actuator is 17.5 mJ, including 10.5 mJ to preheat the SMA to the transition temperature and 7 mJ to cause the SMA to undergo a phase transition between its two crystalline states.

[0045] The power supply has a 5V rail, which is a step-up voltage from the battery, between 1.8V and 5V; and

[0046] The battery is limited to providing a maximum of 3A over a long period of time.

[0047] In this example, if the battery is simply connected to actuator 2, the initial current drawn will be Amperes. This represents a significant portion of the battery's maximum output, and therefore, if the battery is to sustain this current for the entire cycle requiring 17.5 mJ, it can impact the performance of other components in the mobile phone. This is especially true if there are multiple components, and if it is necessary or expected to actuate multiple actuators simultaneously. In such cases, the battery may not even be able to supply enough current to all components at the same time. Furthermore, in some mobile devices, the battery may not even be able to deliver a sufficiently high current to a single component.

[0048] Therefore, the battery output to the SMA actuator may be reduced, making the actuation cycle too long to provide a satisfactory tactile signal to the user. If a capacitor is used to store the required energy and then the energy is supplied from the capacitor, the capacitor would need to be physically very large to store the required 17.5 mJ of energy.

[0049] To address this issue, shape memory alloy tactile components (e.g.) are provided. Figure 4 The shape memory alloy tactile component shown includes: an actuator 2 comprising a shape memory alloy 10 arranged to provide a tactile effect upon actuation, for example... Figure 1 The diagram shows a drive circuit 20, which is arranged to continuously supply drive current to the actuator 2 during a preheating cycle and during an actuation cycle. During the preheating cycle, the temperature of the shape memory alloy 10 rises without causing the shape memory alloy to provide a tactile effect; during the actuation cycle, the temperature of the shape memory alloy 10 rises to cause the shape memory alloy to provide a tactile effect. A corresponding method is provided, which includes supplying drive current to the actuator 2 in the same manner as the drive circuit 20. The tactile effect will occur after the SMA 10 reaches its transition temperature, at which the SMA 10 undergoes a phase transition between crystalline states. At this point, the SMA 10 will begin to apply force to actuate the actuator 2. However, the tactile effect may not occur immediately when the SMA 10 reaches or exceeds (e.g., a predicted) transition temperature. For example, a variable preload force may exist on the actuator 2 due to external forces such as a user's finger on the actuator 2 and / or external constraints. Therefore, SMA 10 must undergo a sufficient phase change so that the force generated by SMA will move in part of actuator 2 and exceed the preloaded force before the tactile effect occurs.

[0050] By dividing the energy supply to the SMA 10 into two cycles, greater flexibility is provided in how energy is delivered in each cycle, where the requirements for how energy needs to be delivered to achieve the desired effect may differ substantially. Specifically, the power distribution of the drive current relative to time during the preheating cycle differs from that during the actuation cycle. For example, the drive current may have a first constant value during the preheating cycle and a different second constant value during the actuation cycle, such as... Figure 2 As shown. Alternatively, during both the preheating cycle and the actuation cycle, the drive current can vary in amplitude or in its voltage or other properties, but with different distributions (e.g., voltage over time or current amplitude over time).

[0051] Specifically, the drive current has a lower time-averaged power during at least a portion of the preheating cycle than during the actuation cycle. Preheating the SMA 10 with lower power during at least a portion of the preheating cycle means that the energy required to heat the SMA 10 to induce actuation is provided over a longer period than if the maximum current were drawn from the current source 24. This reduces the requirements on the current source 24. In some embodiments, the drive current has a lower time-averaged power during at least 50%, preferably at least 75%, and more preferably at least 90% of the preheating cycle than during the actuation cycle. In some embodiments, the drive current has a lower power during the entire preheating cycle than during the actuation cycle. This provides the maximum benefit of reduced power consumption during the preheating cycle.

[0052] In some embodiments, during the preheating cycle, the temperature of the shape memory alloy 10 is raised to the transition temperature at which the shape memory alloy 10 begins its phase transformation. This can be advantageous because it minimizes the amount of energy that needs to be transferred during the actuation cycle. This allows the actuation cycle to be minimized as much as possible, or it can mean reducing the load on the current source 24 during the actuation cycle.

[0053] In some embodiments, during the actuation cycle, the temperature of the shape memory alloy 10 is raised to a temperature above the transition temperature, at which point the shape memory alloy 10 begins a phase transition. This ensures that the SMA undergoes a complete phase transition, thereby providing the tactile effect as intended.

[0054] In some embodiments, the preheating cycle includes an initial heating cycle and a holding cycle, in which the temperature of the shape memory alloy 10 is increased, and in the holding cycle, the temperature of the shape memory alloy 10 is maintained within a predetermined temperature range. The predetermined temperature range may be below the transition temperature at which the SMA 10 begins to transform from one crystalline state to another. The SMA 10 may be maintained within a temperature range with a lower limit (e.g., at most 5°C lower, preferably at most 2°C lower, more preferably at most 1°C lower). The size of the temperature range is preferably small, such that the response of the SMA 10 after the holding cycle is predictable and consistent. For example, the size of the temperature range may be at most 1°C, preferably 0.5°C, more preferably 0.2°C. More preferably, the SMA 10 may be maintained at the transition temperature. This optional holding cycle allows the temperature of the SMA 10 to remain at or near the transition temperature, enabling rapid actuation of the actuator 2 when needed. The power of the drive current during the holding cycle may be lower than the power of the drive current during the initial heating cycle.

[0055] In some embodiments, the drive circuit 20 includes a current source 24 and is arranged to provide drive current from the current source 24 to the actuator 2 during both the preheating and actuation cycles. In other words, the same current source 24 is used to provide drive current for both preheating and actuation. In the case of using an SMA haptic component in a mobile device, the current source 24 may be a battery. In this context, the term current source is used to refer to any current source, not necessarily a power source configured to provide a constant current output. The current source 24, such as a battery in a mobile device, may not be able to provide a sufficiently high current to provide the total energy required to heat the SMA 10 and cause the SMA 10 to transition between states in a sufficiently short time to produce a satisfactory haptic signal, or a continuously high current may place unacceptably high demands on the output of the current source 24. However, a battery may be able to provide a current higher than its maximum continuous output current over a short period of time. In the example above, the battery may be able to output a current of 1A within 2ms that would be required to transfer 7mJ of energy to cause a phase transition in the SMA 10. Therefore, although the same current source 24 is used for both the preheating cycle and the actuation cycle, the actuation cycle used for the tactile signal is still reduced.

[0056] Control of the drive current can be provided, for example, by switching the drive current to actuator 2 on and off at frequencies in the order of Hz, kHz, or MHz, i.e., by modulating the drive current. In some embodiments, drive circuit 20 is arranged to provide pulse-width modulated drive current to actuator 2 during a preheating cycle and an actuation cycle, wherein the modulation has different distributions in the preheating cycle and the actuation cycle. The modulation can vary in terms of the duty cycle of the pulse width modulation (i.e., the percentage of time the pulse length between the beginnings of consecutive pulses). During the actuation cycle, the duty cycle increases relative to the preheating cycle to provide a larger average current. In some embodiments, the drive current is provided continuously during the actuation cycle, i.e., the pulse width modulation in the actuation cycle has a 100% duty cycle. Using pulse width modulation allows for variation of the power supplied by drive circuit 20 without changing the voltage of current source 24. It also allows the average current supplied from current source 24 to remain below the maximum permissible continuous output current of the power supply, where, as described above, the voltage of current source 24 and the resistance of the actuator would result in drawing excessive current if no further control is provided.

[0057] In some embodiments, the drive circuit 20 is arranged to provide drive current to the actuator 2 at different voltages during at least a portion of the preheating cycle and the actuation cycle. This means that instead of changing the modulation of the drive current, or in addition to changing the modulation of the drive current, the power of the drive current is controlled by changing the voltage of the current source 24.

[0058] In some embodiments, the drive circuit 20 includes a current source 24 and a capacitor element 26, and is arranged to provide drive current to the actuator 2 from the current source 24 during the preheating cycle and at least partially from the capacitor element 26 during the actuation cycle. The capacitor element 26 can be of any suitable type (e.g., an electrolytic capacitor or a ceramic capacitor). This provides an alternative method for increasing the power delivered to the actuator 2 during the actuation cycle. This can be advantageous in cases where the current source 24 is a battery that is unable or not permitted to provide a current higher than the maximum continuous current for a short period of time. In such cases, even the continuous drive current during the actuation cycle (i.e., modulation with a 100% duty cycle) may not be sufficient to actuate the actuator 2 for a sufficiently short time to provide a satisfactory tactile signal, and therefore the capacitor element 26 is used to provide the higher power required during the actuation cycle. However, since the SMA 10 has already been preheated by the drive current from the current source 24 during the preheating cycle, the energy that must be provided to generate the tactile signal during the actuation cycle is reduced. Therefore, capacitor element 26 can be manufactured to a much smaller size than it would be possible if it were to provide all the energy needed to heat SMA 10 to its transition temperature and cause its phase transition. In the example given above, the 7 mJ of energy required during the actuation cycle can be provided entirely from capacitor element 26 while still generating a 2 ms haptic signal. Therefore, capacitor element 26 can be much smaller than it would be necessary if it were to provide the full 17 mJ. In another embodiment, the energy required during the actuation cycle can be provided by a combination of power from current source 24 and power from capacitor element 26. This will further reduce the energy that needs to be stored in capacitor element 26, further reducing the physical size of capacitor element 26.

[0059] In some embodiments, capacitor element 26 is charged prior to a preheating cycle. In an alternative embodiment, drive circuitry 20 is arranged such that current source 24 charges capacitor element 26 during the preheating cycle. In either case, drive circuitry 20 is configured to discharge capacitor element 26 via actuator 2 during an actuation cycle. Charging capacitor element 26 during the preheating cycle can reduce energy loss due to leakage current through capacitor element 26 compared to embodiments where capacitor element 26 is charged prior to the preheating cycle. However, charging capacitor element 26 during the preheating cycle may also increase the length of the preheating cycle required to heat SMA 10 to the transition temperature and charge capacitor element 26.

[0060] Figure 2 A simplified graph illustrating the control of the SMA component is shown. Preheating cycles can be executed at different times in response to different triggering events. In some embodiments, the preheating cycle begins in response to a demand signal based on a tactile signal. Figure 2 In the graph, this corresponds to the demand signal triggered at time S1. After the demand signal, a drive current is supplied to actuator 2, and the temperature of SMA 10 rises. At time S2, after the preheating cycle, the drive current increases, and SMA 10 undergoes a phase transition, generating a tactile signal.

[0061] As is well known, a delay of up to 80ms between requesting and providing a tactile signal is imperceptible to the user. This time period can be used as a preheating cycle to heat the SMA 10 to its transition temperature. In this case, the preheating cycle will begin in response to a demand signal such as a button press event. The actuation cycle will immediately follow the preheating cycle. In some embodiments, the length of the preheating cycle is up to 80ms, preferably up to 60ms, more preferably up to 50ms, more preferably up to 40ms, and most preferably up to 20ms.

[0062] In some embodiments, the preheating cycle begins before a demand signal for a tactile signal, and the actuation cycle begins in response to the demand signal. This may be desirable if a delay between the demand signal and actuation would be unacceptable, for example, if a delay in the length of the preheating cycle would be perceptible to the user. In this embodiment, the demand signal will... Figure 2 The preheating cycle occurs at time S2. In some embodiments, the preheating cycle begins in response to the occurrence of a precursor event. In this embodiment, the precursor event will occur during time S2. Figure 2 The time S1 occurs. A precursor event can correspond to an event that is expected to immediately precede the event that will provide a tactile signal for feedback. For example, a precursor event could be the detection of contact with a button by a contact sensor (e.g., a capacitive sensor), or the detection of a small force applied to the button using a force sensor, where the small force is less than a threshold indicating the need for actuation. In this case, the preheating cycle can include an initial heating cycle and a holding cycle, during which the temperature of the SMA 10 rises, and during the holding cycle, the temperature of the shape memory alloy 10 is maintained within a predetermined temperature range. For example, as described above, the SMA 10 can be maintained at a temperature within a range above or slightly below the transition temperature (e.g., at most 5°C lower, optionally at most 2°C lower, optionally at most 1°C lower). In such an embodiment, if no demand signal is received within a specific time period after the precursor event, the drive circuit 20 can stop supplying drive current to the actuator 2 and allow the temperature of the SMA 10 to drop below the transition temperature. If the precursor event stops without a demand signal, for example, if contact with the button is removed without the button being actuated, the drive circuit 20 may also stop providing drive current.

[0063] exist Figure 3Another example of control of an SMA component is shown, wherein the power of the drive current is lower during the preheating cycle than during the actuation cycle, only for a portion of the preheating cycle. In this example, the preheating cycle includes an initial heating cycle and a holding cycle, and the SMA 10 is not heated to the transition temperature during the initial heating cycle, but rather to a temperature within a predetermined range below the transition temperature. The SMA 10 is then held at this temperature below the transition temperature during the holding cycle. In response to a demand signal at time S2, the power of the drive current is increased to heat the SMA 10 to the transition temperature, and then the actuation cycle begins, where the drive current is maintained at a higher power. This embodiment still provides some benefit because high power is not required for most of the preheating cycle. However, it is preferred that the power of the drive current is lower throughout the preheating cycle than during the actuation cycle because... Figure 3 Examples of this could lead to an increased delay after the demand signal, because the SMA 10 needs to be heated to the transition temperature for the remaining time before actuation can occur.

[0064] In some embodiments, the shape memory alloy tactile component further includes a temperature sensor 30 arranged to sense ambient temperature, and a drive circuit 20 arranged to control the supply of drive current during a preheating cycle and / or an actuation cycle based on the sensed ambient temperature. The temperature sensor 30 may include a thermistor, thermocouple, or similar device. The amount of energy supplied to the SMA 10 during the preheating cycle can be calculated based on the ambient temperature, since the amount of energy required to heat the SMA 10 to its transition temperature will depend on the temperature of the SMA 10 before it is heated (i.e., the ambient temperature). The drive circuit 20 can use the ambient temperature to determine how much energy is needed to heat the SMA 10 to its transition temperature and accordingly control the supply of drive current during the preheating cycle. Furthermore, the ambient temperature will determine how quickly heat is lost from the SMA 10 when it is at its transition temperature. For example, if the ambient temperature is further below the transition temperature, the SMA 10 will radiate heat more rapidly. Therefore, if the ambient temperature is low, a larger proportion of the power supplied by the drive current to the actuator 2 will be lost to the surrounding environment. By measuring the ambient temperature, the drive circuit 20 can control the supply of drive current during the actuation cycle to provide the SMA 10 with all the energy required for it to undergo a complete phase transition between the two crystalline states.

[0065] In some embodiments, the control of the supply of drive current during the preheating cycle and / or the actuation cycle is the control of the length of the preheating cycle and / or the length of the actuation cycle. If the ambient temperature necessitates the transfer of more total energy during the preheating cycle and / or the actuation cycle, the cycle length can be adjusted accordingly. In some embodiments, the control of the supply of drive current during the preheating cycle and / or the actuation cycle is the control of the power of the drive current during the preheating cycle and / or the power of the drive current during the actuation cycle. The energy transferred can also be regulated using the power of the drive current. In embodiments using pulse width modulation, the power of the drive current can be controlled by changing the voltage of the current source 24, the amplitude of the drive current, or by controlling the duty cycle of the pulse width modulation applied to the drive current. By controlling the power of the drive current, it is possible to maintain a uniform length of the preheating cycle within a certain temperature range. This may be desirable if the actuation of the component interacts with other functions in the device, and variations in the response time of the actuator 2 would make such interaction difficult. For example, power can be increased at lower ambient temperatures to provide the required amount of energy over the same time length.

[0066] In an embodiment where the drive circuit 20 provides drive current during the hold period, the drive circuit 20 is arranged to control the drive current during the hold period based on the ambient temperature. Since the heat loss rate is higher at lower ambient temperatures, a larger drive current will be required to maintain the temperature of the SMA 10 at the transition temperature during the hold period.

[0067] An alternative method to determine how long to heat the SMA 10 to reach the transition temperature would be to monitor the resistance of actuator 2, so that drive circuit 20 can indicate when the SMA 10 reaches the transition temperature. Figure 7 As shown, during the phase transition between the two crystalline states of SMA 10, the resistance of SMA 10 decreases. This is because as SMA 10 contracts, its diameter increases while its length decreases, which in turn reduces the resistance of the SMA wires. To detect when SMA 10 reaches its transition temperature, it is sufficient to detect the point where the SMA resistance reaches its maximum value and begins to decrease. This will not require accurately determining the absolute value of the SMA resistance, and therefore the advantage of resistance sensing is that the resistance measurement does not need to be accurate to determine when SMA 10 reaches its transition temperature.

[0068] In some embodiments, the shape memory alloy tactile component further includes a resistance measurement circuit 28 arranged to detect a measured value of the resistance of the actuator 2, and a drive circuit 20 arranged to control the supply of drive current during a preheating cycle based on the detected resistance measurement. This eliminates the need for a temperature sensor 30, although a temperature sensor 30 may still be provided in some embodiments. The time required to reach the maximum value of the SMA resistance can still be used to estimate the ambient temperature. In some embodiments, the control of the supply of drive current during the preheating cycle is a control of the length of the preheating cycle. The drive circuit 20 can also use the resistance measurement to determine the temperature of the SMA 10 before supplying the drive current, and thereby estimate the ambient temperature. This can be used to calculate the required length of the preheating cycle in the same manner as when measuring the ambient temperature using the temperature sensor 30. In some embodiments, the control of the supply of drive current during the preheating cycle is a control of the power of the drive current during the preheating cycle. As described above, this can be achieved using control of the voltage of the current source 24, the amplitude of the drive current, or the duty cycle of the pulse width modulation applied to the drive current. By controlling the power of the drive current, it is possible to maintain a uniform length of the preheating cycle within a certain temperature range. As mentioned above, this may be desirable if the actuation of the component needs to interact with other functions in the device, and if variations in the response time of actuator 2 would make such interaction difficult. For example, the power could be increased at a lower ambient temperature to provide the required amount of energy over the same time period.

[0069] During the holding cycle, it may be necessary to maintain the charge on capacitor element 26 and / or the temperature of SMA 10. This can be achieved using the same circuitry used in the initial heating cycle to charge capacitor element 26 and preheat SMA 10. The drive current required to maintain the temperature of SMA 10 and the charge on capacitor element 26 during the holding cycle will be lower than the drive current used to raise the temperature of SMA 10 during the initial heating cycle. As mentioned above, the drive current during the holding cycle may also depend on the ambient temperature.

[0070] In some embodiments, the drive circuit 20 is arranged to control the length of the actuation cycle based on the length of the preheating cycle. As mentioned above, factors such as ambient temperature can affect how much power is lost from the drive current supplied to the actuator 2, and therefore how long it takes to heat the SMA 10 to its transition temperature. Therefore, if the preheating cycle is long (e.g., due to a lower ambient temperature), the drive circuit 20 can determine that the actuation cycle should be long enough to supply all the energy required for the SMA 10 to undergo a complete phase transition. If the actuation cycle is not adjusted, the SMA 10 may not undergo a complete phase transition at lower ambient temperatures, where a higher proportion of the power supplied to the actuator 2 is lost to the surrounding environment.

[0071] In some embodiments, the length of the actuation cycle is at most 10 ms, preferably at most 5 ms, or more preferably at most 2 ms. During the actuation cycle, it is desirable for the drive circuit 20 to draw power from the current source 24 at the maximum permissible rate. If the components include a capacitor element 26, it is also desirable to discharge as much energy as possible from the capacitor element 26 to deposit the maximum amount of available energy into the SMA 10 during the actuation cycle. This can be achieved by minimizing the RC time constants of the capacitor element 26 and the SMA 10. In some embodiments, the RC time constants of the capacitor element 26 and the SMA 10 are less than 3.33 ms, optionally less than 1.67 ms, and optionally less than 0.667 ms. For example, if the actuation cycle will have a duration of ≤2 ms, the RC time constant is preferably ≤0.667 ms.

[0072] Figure 6 The diagram shows a block diagram of an example SMA haptic component. Vin is the power supply from current source 24. For applications in mobile phone headsets, the current source typically has a voltage of 5V and can continuously provide ≤3A. Figure 6 In the example, capacitor element 26 is capacitor C. Capacitor C stores some of the energy needed to generate a tactile signal during the actuation cycle.

[0073] exist Figure 6In this example, closing switch S1 applies a preheating drive current to SMA 10, which is in the form of a wire. For example, assuming Vin is 3V and the SMA wire has a resistance of 6Ω, the current source delivers 1.5W of power. This is typically sufficient to heat the SMA wire to the transition temperature in <20ms, a time short enough that it is imperceptible to the user. Vin should be specified based on the SMA resistance to deliver the maximum possible power to the SMA wire during the preheating cycle without exceeding the current capability of current source 24. Alternatively or additionally, the SMA wire can be designed with a resistance optimized based on power supply specifications. Alternatively, if the voltage of current source 24 and the resistance of the actuator result in drawing more current than current source 24 can continuously provide, S1 can be switched on and off to apply pulse-width modulation to the drive current from current source 24, and a capacitor can be placed on current source 24 to allow the drawn current to be regulated. When the transition temperature is reached, switch S1 can be opened to disable the preheating drive current.

[0074] exist Figure 6 In the example circuit, S1 is open and S2 is closed during the actuation cycle. During the actuation cycle, the boost power supply continues to operate to charge capacitor C. The boost power supply is designed to draw the maximum allowable current from current source 24 during the actuation cycle. The boost voltage, the capacitance of capacitor C, and the resistance of actuator 2 are chosen such that capacitor C discharges as much as possible during the actuation cycle. Figure 7 As shown, switch S3 is controlled to attempt to adjust Vboost when S2 is closed. Since the power demand on the boost power supply during the actuation cycle reaches or exceeds the power that the boost power supply can deliver, Vboost is expected to decrease significantly, as... Figure 7 As shown.

[0075] Data from the example embodiment shows that approximately half of the total energy supplied to the SMA conductor needs to be used to heat the SMA conductor to its transition temperature, and approximately half of the total energy needs to be used to achieve a complete phase transition of the SMA conductor so that actuator 2 moves over its full displacement. Therefore, for a typical embodiment designed for use in a mobile device, approximately 10 mJ must be stored in capacitor element 26 if it is to provide all the required energy during the actuation cycle. Figure 6 In the example, capacitor element 26 is provided by capacitor C. The capacitor is charged using a boost power supply. Using a boost power supply allows the voltage of capacitor C to be higher than the voltage of current source 24, thereby storing more energy in a capacitor of the same physical size than would be possible if the voltage of the current source were used directly. Figure 6A simple DC-DC boost power supply topology is shown, using inductors, diodes, capacitors, and switches. Other known boost topologies can be used, such as voltage multipliers using multiple capacitors and without inductors. Table 1 below illustrates the capacitor sizes required as Vboost increases, based on commonly available circuit components.

[0076]

[0077] Table 1: Capacitor parameters as a function of Vboost

[0078] This indicates a size advantage at higher voltage operation due to the need for smaller capacitance. Furthermore, charging capacitor element 26 with a higher voltage reduces the RC time constant of capacitor element 26 and the SMA wires, which is likely desirable as discussed above. In some embodiments, when capacitor element 26 is fully charged, the voltage across capacitor element 26 is at least 5V, optionally at least 9V, and optionally at least 15V. Using the exemplary components according to Table 1, a voltage of 15V or higher is required to provide a 2ms actuation cycle using the power delivered by capacitor element 26. However, the voltage across capacitor element 26 cannot be arbitrarily high, as higher voltages would impact the chip design of the drive circuit 20, requiring a larger silicon process.

[0079] Resistance sensing can be achieved using a series current sensing resistor connected in series with the SMA wire (e.g., in...). Figure 6 The current through the SMA wire can be determined by measuring the voltage across the current-sensing resistor (which has a known resistance). The resistance of the SMA wire can also be calculated by measuring the voltage across it. Alternatively, a current source (in...) Figure 6 The image shown (Imeas) can be used to transfer a known current into an SMA conductor. Measuring the voltage across the SMA conductor then allows the calculation of the conductor's resistance. Figure 6 In this configuration, the current-sensing resistor is placed on the high-voltage side of the SMA conductor. Alternatively, the current-sensing resistor can be placed on the low-voltage side of the SMA conductor. In this case, the current-sensing resistor will be present in the circuit when the SMA conductor is driven by the high voltage stored in the capacitor, thus reducing the power delivered to the actuator. In this case, the value of the current-sensing resistor is preferably minimized. The advantage of a low-side resistor is that it reduces the sensing voltage and the common-mode input voltage, which is beneficial for the design of boost power supplies.

[0080] Here are some additional examples of energy and time scales provided through illustration. A first actuator with an SMA wire of 5.7Ω resistance is driven by a 5.2V current source for a 5.5ms cycle to generate a tactile signal. In this case, the total energy transferred is... The actuator moves in the last 2ms, and therefore requires 9.5mJ of energy during the actuation cycle. A second device with an SMA wire of 7.5Ω resistance is driven by a 9.8V current source for 5ms cycles to generate a tactile signal. In this case, the total energy transferred is The actuator moves in the last 1.8 ms, and therefore requires 23 mJ of energy during the actuation cycle.

[0081] The first aspect of the method is a method for driving a shape memory alloy tactile component, the shape memory alloy tactile component including an actuator 2, for example... Figure 1 The actuator 2. The method may include steps substantially corresponding to the configuration of the drive circuit 20 described above, and embodiments of the method are as follows: Figure 5 The method includes step S10: providing a drive current from current source 24 to actuator 2 during an initial heating cycle, during which the temperature of shape memory alloy 10 rises. Step S10 begins before a demand signal for a tactile signal occurs in response to the occurrence of a precursor event. Simultaneously with step S10, the method includes step S11: detecting a measured value of the resistance of actuator 2 and sensing the ambient temperature using temperature sensor 30, and controlling the provision of drive current during the initial heating cycle based on the detected resistance measurement and the sensed ambient temperature. The method also includes step S12: charging capacitor element 26 during a preheating cycle.

[0082] The method further includes step S14: providing a drive current to the actuator 2 during a holding period, during which the temperature of the shape memory alloy 10 is maintained within a predetermined temperature range. In step S16, the holding period ends and an actuation period begins in response to the demand signal. In step S16, the method includes providing a drive current to the actuator from the current source 24 and from the capacitor element 26 during the actuation period, during which the temperature of the shape memory alloy rises to allow the shape memory alloy to provide a tactile effect. Simultaneously with step S16, the method includes step S15: detecting a measured value of the resistance of the actuator 2 and sensing the ambient temperature using the temperature sensor 30, and controlling the provision of the drive current during the actuation period based on the sensed ambient temperature.

Claims

1. A method of driving a shape memory alloy haptic assembly, the shape memory alloy haptic assembly comprising an actuator, the actuator comprising a shape memory alloy arranged to provide a haptic effect upon actuation, The method comprises: providing a drive current to the actuator continuously during a pre-heating period, in which the temperature of the shape memory alloy is raised without causing the shape memory alloy to provide the haptic effect, and during an actuation period, in which the temperature of the shape memory alloy is raised so as to cause the shape memory alloy to provide the haptic effect, wherein the drive current has a lower time-averaged power during at least a portion of the pre-heating period than during the actuation period.

2. The method of claim 1, wherein, the drive current has a lower time-averaged power during at least 50% of the pre-heating period than during the actuation period.

3. The method of claim 1, wherein, the drive current has a lower time-averaged power during the entire pre-heating period than during the actuation period.

4. The method of any one of claims 1-3, wherein, during the pre-heating period, the temperature of the shape memory alloy is raised to a transition temperature at which the shape memory alloy begins to undergo a phase change, or to a temperature close to the transition temperature.

5. The method of any one of claims 1-3, wherein, during the actuation period, the temperature of the shape memory alloy is raised to a temperature above a transition temperature at which the shape memory alloy begins to undergo a phase change.

6. The method of any one of claims 1-3, wherein, the pre-heating period is initiated in response to a demand signal for a haptic signal.

7. The method of claim 1, wherein, the pre-heating period has a length of at most 80 ms.

8. The method of any one of claims 1-3, wherein, the pre-heating period is initiated prior to a demand signal for a haptic signal, and the actuation period is initiated in response to the demand signal.

9. The method of claim 8, wherein, the pre-heating period is initiated in response to an occurrence of a precursor event.

10. The method of any one of claims 1-3, 7, and 9, wherein, the pre-heating period comprises an initial heating period, in which the temperature of the shape memory alloy is raised, and a hold period, in which the temperature of the shape memory alloy is held within a predetermined temperature range.

11. The method of any one of claims 1-3, 7, and 9, wherein, the drive current is provided to the actuator from the same current source during the pre-heating period and during the actuation period.

12. The method of claim 11, wherein, the drive current is provided to the actuator from the same current source during the pre-heating period and during the actuation period, the same current source being pulse width modulated by a modulation having a different profile in the pre-heating period and the actuation period.

13. The method of claim 11, wherein, the drive current is provided to the actuator from the same current source during the pre-heating period and during the actuation period, the same current source having a different voltage in the pre-heating period and the actuation period.

14. The method of claim 12, wherein, the drive current is provided to the actuator from the same current source during the pre-heating period and during the actuation period, the same current source having a different voltage in the pre-heating period and the actuation period.

15. The method of any one of claims 1-3, 7, and 9, wherein, the drive current is provided to the actuator from a current source during the pre-heating period, and is provided to the actuator at least partly from a capacitive element during the actuation period.

16. The method of claim 15, further comprising charging the capacitive element during the pre-heating period.

17. The method according to any of claims 1-3, 7, 9, 12-14 and 16, further comprising detecting a measure of electrical resistance of the actuator, and controlling the provision of drive current during the pre-heat period based on the detected measure of electrical resistance.

18. The method of claim 17, wherein, The control of the provision of drive current during the pre-heat period is control of the length of the pre-heat period.

19. The method according to claim 18, further comprising controlling the length of the actuation period based on the length of the pre-heat period.

20. The method of any one of claims 18-19, wherein, The control of the provision of drive current during the pre-heat period is control of the power of the drive current during the pre-heat period.

21. The method according to any of claims 1-3, 7, 9, 12-14, 16 and 18-19, further comprising sensing an ambient temperature with a temperature sensor, and controlling the provision of drive current during the pre-heat period and / or during the actuation period based on the sensed ambient temperature.

22. The method of claim 21, wherein, The control of the provision of drive current during the pre-heat period and / or during the actuation period is control of the length of the pre-heat period and / or the length of the actuation period.

23. The method of claim 21, wherein, The control of the provision of drive current during the pre-heat period and / or during the actuation period is control of the power of the drive current during the pre-heat period and / or control of the power of the drive current during the actuation period.

24. The method of claim 22, wherein, The control of the provision of drive current during the pre-heat period and / or during the actuation period is control of the power of the drive current during the pre-heat period and / or control of the power of the drive current during the actuation period.

25. The method of any one of claims 1-3, 7, 9, 12-14, 16, 18-19, and 22-24, wherein, The length of the actuation period is at most 10 ms.

26. The method of claim 7, wherein, The length of the pre-heat period is at most 60 ms.

27. The method of claim 7, wherein, The length of the pre-heat period is at most 50 ms.

28. The method of claim 7, wherein, The length of the pre-heat period is at most 40 ms.

29. The method of claim 7, wherein, The length of the pre-heat period is at most 20 ms.

30. The method of claim 25, wherein, The length of the actuation period is at most 5 ms.

31. The method of claim 25, wherein, The length of the actuation period is at most 2 ms.

32. A shape memory alloy haptic assembly comprising: an actuator comprising a shape memory alloy arranged to provide a haptic effect upon actuation; and a drive circuit arranged to provide a drive current to the actuator continuously during a pre-heat period in which the temperature of the shape memory alloy is raised without causing the shape memory alloy to provide the haptic effect, and during an actuation period in which the temperature of the shape memory alloy is raised so as to cause the shape memory alloy to provide the haptic effect, wherein the drive current has a lower time-averaged power during at least a portion of the pre-heat period than during the actuation period.

33. The shape memory alloy haptic assembly of claim 32, wherein, The drive current has a lower time-averaged power during at least 50% of the pre-heat period than during the actuation period.

34. The shape memory alloy haptic assembly of claim 33, wherein, The drive current has a lower time-averaged power during the entire pre-heat period than during the actuation period.

35. The shape memory alloy haptic assembly of any of claims 32-34, wherein, During the pre-heating period, the temperature of the shape memory alloy is raised to a transition temperature at which the shape memory alloy starts to phase change or to a temperature close to the transition temperature.

36. The shape memory alloy haptic assembly of any of claims 32-34, wherein, During the actuation period, the temperature of the shape memory alloy is raised to a temperature above a transition temperature at which the shape memory alloy starts to phase change.

37. The shape memory alloy haptic assembly of any of claims 32-34, wherein, The pre-heating period is started in response to a demand signal for a haptic signal.

38. The shape memory alloy haptic assembly of claim 37, wherein, The pre-heating period has a length of at most 80 ms.

39. The shape memory alloy haptic assembly of any one of claims 32-34 and 38, wherein, The pre-heating period is started before a demand signal for a haptic signal and the actuation period is started in response to the demand signal.

40. The shape memory alloy haptic assembly of claim 39, wherein, The pre-heating period is started in response to an occurrence of a precursor event.

41. The shape memory alloy haptic assembly of any of claims 32-34, 38, and 40, wherein, The pre-heating period comprises an initial heating period in which the temperature of the shape memory alloy is raised and a holding period in which the temperature of the shape memory alloy is kept within a predetermined temperature range.

42. The shape memory alloy haptic assembly of any of claims 32-34, 38, and 40, wherein, The drive circuit comprises a current source and is arranged to provide a drive current from the current source to the actuator during the pre-heating period and during the actuation period.

43. The shape memory alloy haptic assembly of claim 42, wherein, The drive circuit is arranged to provide a pulse width modulated drive current to the actuator during the pre-heating period and during the actuation period with a modulation having a different profile in the pre-heating period and in the actuation period.

44. The shape memory alloy haptic assembly of claim 42, wherein, The drive circuit is arranged to provide a drive current to the actuator during the pre-heating period and during the actuation period with a different voltage in the pre-heating period and in the actuation period.

45. The shape memory alloy haptic assembly of claim 43, wherein, The drive circuit is arranged to provide a drive current to the actuator during the pre-heating period and during the actuation period with a different voltage in the pre-heating period and in the actuation period.

46. The shape memory alloy haptic assembly of any one of claims 32-34, 38, and 40, wherein, The drive circuit comprises a current source and a capacitive element and is arranged to provide the drive current from the current source to the actuator during the pre-heating period and at least partly from the capacitive element to the actuator during the actuation period.

47. The shape memory alloy haptic assembly of claim 46, wherein, The drive circuit is arranged such that the current source charges the capacitive element during the pre-heating period.

48. The shape memory alloy haptic assembly of any one of claims 32-34, 38, 40, 43-45, and 47, further comprising a resistance measurement circuit arranged to detect a measured value of a resistance of the actuator, and wherein, The drive circuit is arranged to control the provision of drive current during the pre-heating period based on a detected measure of electrical resistance.

49. The shape memory alloy haptic assembly of claim 48, wherein, The control of the provision of drive current during the pre-heating period is a control of the length of the pre-heating period.

50. The shape memory alloy haptic assembly of claim 49, wherein, The drive circuit is arranged to control the length of the actuation period based on the length of the pre-heating period.

51. The shape memory alloy haptic assembly of any of claims 49-50, wherein, The control of the provision of drive current during the pre-heating period is a control of the power of the drive current during the pre-heating period.

52. The shape memory alloy haptic assembly of any one of claims 32-34, 38, 40, 43-45, 47, and 49-50, further comprising a temperature sensor arranged to sense an ambient temperature, and wherein, The drive circuit is arranged to control the provision of drive current during the pre-heating period and / or during the actuation period based on a sensed ambient temperature.

53. The shape memory alloy haptic assembly of claim 52, wherein, The control of the provision of drive current during the pre-heating period and / or during the actuation period is a control of the length of the pre-heating period and / or of the length of the actuation period.

54. The shape memory alloy haptic assembly of claim 52, wherein, The control of the provision of drive current during the pre-heat period and / or during the actuation period is a control of the power of the drive current during the pre-heat period and / or a control of the power of the drive current during the actuation period.

55. The shape memory alloy haptic assembly of claim 53, wherein, The control of the provision of drive current during the pre-heat period and / or during the actuation period is a control of the power of the drive current during the pre-heat period and / or a control of the power of the drive current during the actuation period.

56. The shape memory alloy haptic assembly of any of claims 32-34, 38, 40, 43-45, 47, 49-50, and 53-55, wherein, The length of the actuation period is at most 10 ms.

57. The shape memory alloy haptic of claim 38, wherein, The length of the pre-heat period is at most 60 ms.

58. The shape memory alloy haptic assembly of claim 38, wherein, The length of the pre-heat period is at most 50 ms.

59. The shape memory alloy haptic of claim 38, wherein, The length of the pre-heat period is at most 40 ms.

60. The shape memory alloy haptic assembly of claim 38, wherein, The length of the pre-heat period is at most 20 ms.

61. The shape memory alloy haptic of claim 56, wherein, The length of the actuation period is at most 5 ms.

62. The shape memory alloy haptic assembly of claim 56, wherein, The length of the actuation period is at most 2 ms.

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