Actuator assemblies
Actuator assemblies and methods of operating actuator assemblies are provided, in particular with the aim of reducing bearing jitter. In one approach an actuator assembly ( 1 ) is provided which has: a support element ( 4 ); a movable element ( 20 ) movable relative to the support element; an SMA component ( 80 ) connected between the movable element and the support element and arranged to cause movement of the movable element relative to the support element; a bearing ( 100 ) arranged to guide the movement of the movable element; and a controller ( 30 ) arranged to control energy supplied to the SMA component to thereby cause contraction and/or relaxation of the SMA component and to control movement of the movable element relative to the support element in a first direction so as to provide an actuation function, wherein the controller is arranged to, at least whilst the movable element starts to move in the first direction, control the movable element to cause it to be moving in a second direction orthogonal to the first direction.
1. An actuator assembly having:
a support element;
a movable element movable relative to the support element;
a shape memory alloy (SMA) component connected between the movable element and the support element and arranged to cause movement of the movable element relative to the support element;
a bearing arranged to guide the movement of the movable element; and
a controller arranged to control energy supplied to the SMA component to thereby cause contraction and/or relaxation of the SMA component and to control movement of the movable element relative to the support element in a first direction so as to provide an actuation function,
wherein the controller is arranged to, at least whilst the movable element starts to move in the first direction, control the movable element to cause it to be moving in a second direction orthogonal to the first direction.
2. The actuator assembly according to claim 1 , wherein the SMA component is part of a plurality of SMA components and the controller is arranged to control the movable element to cause it to move in the second direction by causing contraction and/or relaxation of one or more of said SMA components.
3. The actuator assembly according to claim 1 , wherein the actuator assembly has a principal axis and the first direction is perpendicular to said principal axis and the second direction is rotation about an axis parallel to said principal axis.
4. The actuator assembly according to claim 1 , wherein the bearing is arranged to constrain movement of the movable element to directions in a predetermined plane.
5. The actuator assembly according to claim 1 , wherein the controller is arranged to control the movable element to cause it to oscillate by moving in the second direction and a third direction opposite to the second direction.
6. The actuator assembly according to claim 5 , wherein the frequency of said oscillation is between 10-200 Hz.
7. A method of controlling an actuator assembly, the actuator assembly having:
a support element;
a movable element movable relative to the support element;
a shape memory alloy (SMA) component connected between the movable element and the support element and arranged to cause movement of the movable element relative to the support element; and
a bearing arranged to guide the movement of the movable element,
wherein the method includes the steps of:
supplying energy to the actuator assembly to control movement of the movable element relative to the support element in a first direction so as to provide an actuation function; and,
at least whilst the movable element starts to move in the first direction, driving the movable element such that it is moving in a second direction orthogonal to the first direction.
8. The method according to claim 7 , wherein the actuator assembly has a plurality of SMA components and the step of driving the movable element includes contracting and/or relaxing one or more of said SMA components.
9. The method according to claim 8 , wherein there is a predetermined maximum of total power that can be supplied to said SMA components, and further including the step of controlling the power supplied to the SMA components such that if the power required to cause the movement in both the first and second directions exceeds said maximum, power is allocated preferentially to the component or components used to control movement in the first direction over the component or components used to cause the movable element to move in the second direction.
10. The method according to claim 7 , wherein the actuator assembly has a principal axis and the first direction is perpendicular to said principal axis and the second direction is rotation about an axis parallel to said principal axis.
11. The method according to claim 7 , wherein the step of driving the movable element includes driving it such that it oscillates by moving in the second direction and a third direction opposite to the second direction, and wherein the frequency of said oscillation is between 10-200 Hz.
12. An actuator assembly having:
a support element;
a movable element movable relative to the support element;
a shape memory alloy (SMA) component connected between the movable element and the support element and arranged to cause movement of the movable element relative to the support element on contraction and/or relaxation of the SMA component; and
a bearing arranged to guide the movement of the movable element,
wherein the bearing includes two contact surfaces which are arranged to slide relative to each other when the movable element moves, and the bearing is configured such that its coefficient of static friction is substantially equal to its coefficient of dynamic friction.
13. The actuator assembly according to claim 12 , wherein the material of said contact surfaces is selected so that the static coefficient of friction between said contact surfaces is substantially equal to the dynamic coefficient of friction between said contact surfaces.
14. The actuator assembly according to claim 13 , wherein at least one of the contact surfaces is PTFE, polyoxymethylene (POM), PTFE-loaded POM, phosphor bronze, stainless steel, or stainless steel coated with one of: diamond-like carbon (DLC), CrC-DLC, Si-DLC, Cr-DLC, Ti-DLC, WC/C, TiAIN-WC/C or PTFE.
15. The actuator assembly according to claim 12 , further including a lubricant arranged between said contact surfaces so as to cause said coefficient of static friction to be substantially equal to said coefficient of dynamic friction.