Abstract: An electro-thermal actuator has linear displacement of its actuator rod accurately controlled and monitored. The actuator includes a sealed chamber containing an expansible and contractible working fluid. The working fluid is heated by way of pulse width modulated current applied to a heater in thermal contact with the fluid. As heat is applied to the working fluid, it undergoes a phase change to a gas. The resulting gas increases the pressure within the chamber, thereby exerting force on a piston and linearly displacing an actuator rod attached to the piston against the bias of a spring. The actuator includes a temperature sensor mounted within the chamber and in thermal contact with the working fluid to thereby monitor the temperature of the working fluid. The working fluid temperature is provided as an input to a feedback circuit which controls the width of the current pulses applied to the heater used to heat the working fluid.