Compensating for incomplete reversal in mechanisms incorporating shape memory alloy wire

摘要

Shape memory alloy (SMA) actuating elements are commonly simpler and of lower mass than alternative actuator designs and may find particular application in the transportation industry. Such SMA-powered devices are usually reliable and long-lived but the phase transformations which occur in the SMA alloy and are responsible for its utility are not totally reversible. This irreversibility, a consequence of irrecoverable strain, may progressively degrade the long-term actuator performance as the irrecoverable strain accumulates over many operating cycles. Methods and devices for compensating for these effects and extending the useful cycle life of SMA actuators are described.

主权项

1. An actuator for operating a device with one or more moveable components, the actuator comprising a linear Shape Memory Alloy (SMA) element with an axis and two ends, and a return spring with two ends, the SMA element having an active length and being initially taut when in a lower temperature phase;
a first end of each of the SMA element and the return spring each being attachable to the moveable component of the device, or to an attachment adapted to engage the moveable component of the device, the second end of the return spring being attachable to a fixed support, the SMA element, return spring and moveable component or attachment being arranged so that the return spring tensions the SMA wire; the SMA element being adapted to shorten, and thereby extend the return spring to displace the moveable component, or the attachment, from an initial configuration and operate the device when the SMA element is heated and adopts a higher temperature phase, the SMA element being adapted to be deformed and lengthened by the return spring when the SMA element is cooled and adopts the lower temperature phase; the actuator also comprising a stop, the stop limiting the lengthening of the SMA element by the return spring to enforce a predetermined lengthening of the SMA element when the element is in its lower temperature phase, the predetermined lengthening of the SMA element being selected to return the moveable component, or the attachment, to its initial configuration and ready the device for further operation, the SMA element progressively lengthening under repeated use so that the SMA element develops slack and loses its initial tautness when in its lower temperature phase; the actuator further comprising a tensioner for minimizing developed slack in the SMA element and maintaining the tautness of the SMA element, the tensioner having two inter-engaging parts, a moveable part attached to the second end of the SMA element, the moveable part engaging a tensioning spring which engages a stationary part of the tensioner secured to a support, the stationary part being spaced apart from the moveable part along the axis of the SMA element, the tensioning spring and the tensioner parts being arranged so that the tensioning spring acts to displace the moveable part, and the second end of the SMA element, along the SMA element axis and extend the SMA element to remove slack in the SMA element and restore the SMA element to its initially-taut configuration when the SMA element is in its low temperature phase; and, the stationary and moveable parts of the tensioner each having mutually-engageable features adapted to secure the moveable part to the stationary part and oppose any relative motion of the stationary and moveable parts of the tensioner in a direction which might produce slack in the SMA element while enabling repeated relative motion of the moveable and stationary parts in a direction for removing slack in the SMA element.