US 7,974,025 B2Granted

Shape memory alloy actuation apparatus

AssigneeCambridge Mechatronics Limited

InventorRichard Topliss

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Abstract

A miniature camera lens actuation apparatus employs an SMA actuator comprising SMA wire to move a camera lens element. To provide autofocus, the SMA actuator is heated across its range of contraction and the resistance at which the focus is at an acceptable level is stored. To combat hysteresis, there is performed a flyback in which the SMA actuator is cooled, before heating the SMA actuator to the stored resistance. The stored resistance is adjusted to combat creep caused by non-linear heating of the SMA actuator. The SMA wire has conductive material extending along on a portion of the SMA wire which extends from a member connected to the SMA wire and being in electrical connection with the SMA wire in order to short out that portion of the SMA wire to reduce creep. Additional material is applied over the SMA wire and the member to reduce fatigue.

Claims20

Dependent claims

A method of controlling an SMA actuator apparatus comprising an SMA actuator arranged on contraction due to heating to drive movement of a movable element, in which method heating of the SMA actuator is performed by passing a current through the SMA actuator, the method comprising:

a first heating step of heating the SMA actuator from a state in which it is not contracted, whilst monitoring the resistance of the SMA actuator, until a first maximum in the resistance of the SMA actuator is detected, the resistance value of the first maximum being stored;
a scan step of further heating the SMA actuator to cause the SMA actuator to contract, whilst monitoring a signal which varies with the position of the movable element, the value of the resistance of the SMA actuator when the signal is at an acceptable level being measured and stored;
a cooling step of cooling the SMA actuator back into a state in which it is not contracted;
a second heating step of heating the SMA actuator, whilst monitoring the resistance of the SMA actuator, until a second maximum in the resistance of the SMA actuator is detected;
deriving a target resistance value equal to the stored value of the resistance of the SMA actuator when the signal is at an acceptable level adjusted by an adjustment which is determined on the basis of the resistance values of the first and second maximums;
and
a seek step of further heating the SMA actuator, whilst monitoring the resistance of the SMA actuator, until the monitored resistance value reaches the target resistance value.

4 more elements

A control system for an SMA actuator apparatus comprising an SMA actuator arranged on contraction due to heating to drive movement of a movable element, the control system comprising:

a current source operable to pass current through the SMA actuator to heat the SMA actuator;
a detector circuit operable to detect the resistance of the SMA actuator;
and
a controller arranged to control the current source, the controller being responsive to the resistance of the SMA actuator detected by the detector circuit, wherein the controller is operative to perform an operation comprising:
a first heating step of controlling the current source to heat the SMA actuator from a state in which it is not contracted, whilst monitoring the resistance of the SMA actuator, until a first maximum in the resistance of the SMA actuator is detected, the resistance value of the first maximum being stored;
a scan step of controlling the current source to further heat the SMA actuator to cause the SMA actuator to contract, whilst monitoring a signal which varies with the position of the movable element, the value of the resistance of the SMA actuator when the signal is at an acceptable level being measured and stored;
a cooling step of controlling the current source to cool the SMA actuator back into a state in which it is not contracted;
a second heating step of controlling the current source to heat the SMA actuator, whilst monitoring the resistance of the SMA actuator, until a second maximum in the resistance of the SMA actuator is detected;
deriving a target resistance value equal to the stored value of the resistance of the SMA actuator when the signal is at an acceptable level adjusted by an adjustment which is determined on the basis of the resistance values of the first and second maximums;
and
a seek step of controlling the current source to further heat the SMA actuator, whilst monitoring the resistance of the SMA actuator, until the monitored resistance value reaches the target resistance value.

8 more elements

Dependencies are inferred from the claim text.

Facts

Application: 12/596,925Filed 22 Apr 2008
Priority: GB 0707788.623 Apr 2007
Published: US 2010/0074608 A125 Mar 2010
Granted: 5 Jul 2011
Term ends: Not estimatedThe source records don’t support an estimate.

Classification4 codes

G03B 3/10
Focusing arrangements of general interest for cameras, projectors or printers: power-operated focusing
G02B 7/08
With mechanism for focusing or varying magnification: adapted to co-operate with a remote control mechanism
G03B
Apparatus or arrangements for taking photographs or for projecting or viewing them
G02B
Optical elements, systems or apparatus