Control of SMA haptic assembly
There is provided a method of driving a shape memory alloy haptic assembly comprising an actuator comprising shape memory alloy that is arranged on actuation to provide a haptic effect, the method comprising supplying drive current to the actuator successively 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. A shape memory alloy haptic assembly is also provided.
1. A method of driving a shape memory alloy haptic assembly comprising an actuator comprising shape memory alloy that is arranged on actuation to provide a haptic effect,
the method comprising supplying drive current to the actuator successively 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 part of the pre-heating period than during the actuation period.
2. The method according to 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 according to claim 1 , 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 transform or to a temperature close to the transition temperature and 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 transform.
4. The method according to claim 1 , wherein the pre-heating period is commenced in response to a demand signal for a haptic effect.
5. The method according to claim 4 , wherein the pre-heating period has a length of at most 80 ms.
6. The method according to claim 1 , wherein the pre-heating period is commenced in advance of a demand signal for a haptic effect and the actuation period is commenced in response to said demand signal.
7. The method according to claim 6 , wherein the pre-heating period is commenced in response to occurrence of a precursor event.
8. The method according to claim 1 , 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.
9. The method according to claim 1 , wherein the drive current is supplied to the actuator during the pre-heating period and the actuation period from the same current source.
10. The method according to claim 9 , wherein the drive current is supplied to the actuator during the pre-heating period and the actuation period from the same current source that is pulse width modulated by modulations having a different profile in the pre-heating period and the actuation period or from the same current source with different voltages in the pre-heating period and the actuation period.
11. The method according to claim 1 , wherein the drive current is supplied to the actuator during the pre-heating period from a current source and during the actuation period at least partially from a capacitive element.
12. The method according to claim 11 , further comprising charging the capacitive element during the pre-heating period.
13. The method according to claim 1 , further comprising detecting a measure of resistance of the actuator, and controlling the supply of drive current during the pre-heating period based on the detected measure of resistance.
14. The method according to claim 13 , wherein said control of the supply of drive current during the pre-heating period is control of the length of the pre-heating period or control of the power of the drive current during the pre-heating period.
15. The method according to claim 14 , further comprising controlling the length of the actuation period based on the length of the pre-heating period.
16. The method according to claim 1 , further comprising sensing the ambient temperature with a temperature sensor, and controlling the supply of drive current during the pre-heating period and/or the actuation period based on sensed ambient temperature.
17. The method according to claim 16 , wherein said control of the supply of drive current during the pre-heating period and/or the actuation period is control of the length of the pre-heating period and/or the length of the actuation period or control of the power of the drive current during the pre-heating period and/or control of the power of the drive current during the actuation period.
18. The method according to claim 1 , wherein the actuation period has a length of at most 10 ms.
19. A shape memory alloy haptic assembly comprising:
an actuator comprising shape memory alloy that is arranged on actuation to provide a haptic effect; and
a drive circuit arranged to supply drive current to the actuator successively 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 part of the pre-heating period than during the actuation period.
20. The method according to claim 1 , wherein the drive current has a lower time-averaged power during the entire pre-heating period than during the actuation period.