Shape memory alloy actuation apparatus
A miniature camera lens actuation apparatus comprises a support structure, a camera lens element supported on the support structure by a suspension system; and an SMA actuator connected between the support structure and the movable element to drive movement of the camera lens element. The control circuit may include a drive circuit and a sensor circuit which have separate electrical connections to the SMA actuator to reduce the impact of the resistance of the electrical connections on the sensing. The control circuit may vary the drive signal in response to a temperature signal indicative of the ambient temperature. An endstop limits movement to prevent extension of the SMA actuator in its unheated state beyond a maximum length which is at or below the length corresponding the local maximum resistance of the resistance-length curve. Control of position is effected using resistance of the SMA actuator as a measure of position. The control employs a ‘ratcheting’ method to prevent a failure condition and an initial calibration step to derive a range of target resistance values. In manufacture, the position of a lens holder is adjusted relative to a carrier to provide focussing on the image sensor of an image of an object at a distance in the range from infinity to the hyperfocal distance when the SMA actuator is heated to a predetermined temperature greater than ambient temperature.
1. A shape memory alloy actuation apparatus comprising:
a support structure;
a movable element supported on the support structure by a suspension system which guides movement of the movable element;
a shape memory alloy actuator connected between the support structure and the movable element such that contraction of the shape memory alloy actuator on heating drives said movement of the movable element relative to the support structure, the shape memory alloy actuator having a property that resistance varies with length along a curve on which the resistance decreases from a local maximum resistance to a local minimum resistance during contraction; and
an endstop arranged to limit movement of the movable element to prevent extension of the shape memory alloy actuator in its unheated state beyond a maximum length which is at or below the length corresponding on said curve to a resistance below the resistance of the local maximum resistance by a predetermined amount of the difference between the resistance of the local maximum resistance and the resistance of the local minimum resistance.
2. A shape memory alloy actuation apparatus according to claim 1 , wherein said predetermined amount of the difference between the resistance of the local maximum resistance and the resistance of the local minimum resistance is 5% of the difference between the resistance of the local maximum resistance and the resistance of the local minimum resistance.
3. A shape memory alloy actuation apparatus according to claim 1 or 2 , wherein the maximum length is at or above the length corresponding on said curve to a resistance below the resistance of the local maximum resistance by an amount of 40% of the difference between the resistance of the local maximum resistance and the resistance of the local minimum resistance.
4. The shape memory alloy actuation apparatus according to claim 1 , wherein the movable element is a camera lens element, the suspension system guiding movement of the camera lens element relative to the support structure along the optical axis of the camera lens element.
5. The shape memory alloy actuation apparatus according to claim 4 , wherein the camera lens element includes one or more lenses having a diameter of at most 10 mm.
6. The shape memory alloy actuation apparatus according to claim 1 , further comprising a biasing arrangement arranged to provide biasing of the movable element relative to the support structure in an opposite direction from the movement driven by contraction of the shape memory alloy actuator.
7. The shape memory alloy actuation apparatus according to claim 1 , wherein the suspension system is arranged to provide biasing of the movable clement relative to the support structure in an opposite direction from the movement driven by contraction of the shape memory alloy actuator.
8. The shape memory alloy actuation apparatus according to claim 7 , wherein the suspension system comprises a plurality of resilient flexures coupled between the movable element and the support structure, the flexures being flexed to provide said biasing.
9. The shape memory alloy actuation apparatus according to claim 1 , wherein the shape memory alloy actuator comprises at least one length of shape memory alloy wire connected in tension between the support structure and the movable element.