Apparatus for and method of conditioning shape memory alloy wire
An apparatus for and method of conditioning a thermally activated shape memory alloy wire for use in an application, wherein the apparatus includes an adjustable hard-stop and the preferred method includes pre-determining a minimum activating current, allowable strain, and a loading magnitude and form based on the wire configuration and application, and further includes applying a double-exponential model to determine a final recoverable strain over fewer cycles.
1. A method of conditioning a thermally activated shape memory alloy wire so as to achieve steady state performance in an application, said method comprising:
a). applying a load to, so as to produce tension in, the wire;
b). determining a hard-stop location based on a predetermined Austenite free length, maximum allowable strain, and wire fatigue life,
c). setting at least one hard-stop at the location wherein the hard-stop is spaced from the wire and load, and selectively engaging the wire and load, so as to limit strain in and prevent damage to the wire when in the Martensitic phase, or limit strain recovery when the wire transforms back to the Austenitic phase;
d). incrementally increasing an input current and observing the wire, so as to determine a minimum current sufficient to completely transform the wire from a Martensitic phase to an Austenitic phase based on the load, wire type, and wire diameter;
e). repetitively applying the minimum current to the wire over a plurality of cycles, such that the wire heats, so as to fully transform from the Martensitic and to the Austenitic phase, and then cools, so as to fully transform back to the Martensitic phase;
f). plotting a steady-state wire strain when in the Martensitic and Austenitic phases for each cycle; and
g). determining a final recoverable Austenitic strain, based on plotting the steady-state wire strain.
2. The method as claimed in claim 1 , wherein step a) further includes the steps of selecting a load form based on the application.
3. The method as claimed in claim 1 , wherein step d). further includes the steps of externally supporting the load so as to reduce tension in the wire, applying a target current to the wire, removing the external support so as to reproduce tension in the wire, and observing a force-deflection curve of the wire.
4. The method as claimed in claim 1 , wherein step g). includes the steps of pre-determining an allowable strain based on the application.
5. The method as claimed in claim 1 , wherein step g). includes the steps of fitting a curve to the points using a model, and extrapolating the final recoverable strain.
6. The method as claimed in claim 5 , wherein the model employs a double-exponential curve.
7. The method as claimed in claim 6 , wherein the model employs an equation in the form:
ε=− Ae −x/B −Ce −x/D +E
x is the number of cycles,
B and D are decay rate constants with units of cycles, and
A and C describe the amount of strain lost at each decay rate.
8. The method as claimed in claim 1 , further comprising:
g. controlling the number and location of nucleation sites within the wire.
9. The method as claimed in claim 1 , wherein steps a) through d). further include the steps of iteratively determining an optimum load and hard-stop location using a curve-fit model.
10. The method as claimed in claim 1 , wherein step g). includes the steps of fitting a curve to the points, estimating steady state performance parameters, and adjusting or terminating the method based on the estimated parameters, using a model.