Estimating the remaining life of shape memory alloy actuators

A method of alerting a vehicle operator of an impending failure of a spring-biased, shape memory alloy (SMA) element employed as an actuator in an SMA-actuated device in a vehicle, the SMA element having a length and an area, the SMA element repeatedly transitioning between a martensite phase and an austenite phase while cycling between a first and a second length, and, in the course of such cycling, undergoing a progressive loss of utility, the method comprising:

• subjecting the element to a plurality of heating and cooling cycles by:
• applying an electrical current to the SMA element at an initial temperature to heat the SMA element and promote a time-varying change in length and a time-varying change in area;
• during passage of the electric current, repeatedly making simultaneous measurements of the applied electric current and an applied voltage on the SMA element and using these measurements to compute a time-varying resistance, or, after computing the time-varying resistance, using the time-varying resistance in combination with the time-varying element length and the time-varying element area to compute a time-varying resistivity of the SMA element;
• the time varying resistance having at least an initial resistance, a maximum resistance, a minimum resistance and a terminal resistance, and the time varying resistivity having, at least an initial resistivity, a maximum resistivity, a minimum resistivity, and a terminal resistivity;
• and then
• ceasing to apply current to the SMA element, the SMA element cooling to substantially its initial temperature and being deformed by the bias spring to attain substantially its first length;
• and
• during each of the heating segments of the plurality of repeated heating and cooling cycles on the SMA element, computing minimum and terminal resistance and minimum and terminal resistivity, and the difference between the terminal and minimum resistance or the difference between the minimum and terminal resistivity, until a positive and non-zero difference is determined with statistical reliability;
• then
• alerting the vehicle operator of the impending failure of the SMA element.