One of the two internal state variables of a SMA actuator,temperature or electric resistance, is fed back. However, the
temperature feedback method is not practical because the SMA must be isolated from the thermal sensor and it is very difficult to precisely measure the temperature of the small SMA body. Even if these problems are overcome, temperature-strainlstress hysteresis still appears in phase transformation, meaning that the internal state cannot be exactly known by measuring the temperature. Therefore, the temperature feedback method is not practical, particularly in microlminiature SMA actuators.