A source of motion for actuation of a downhole tool is in a sealed housing to protect it from well fluids. The sealed housing is filled with an incompressible fluid and thermal effects can be compensated preferably with a compensation bellows in communication with the fluid. The source of motion is preferable electrically powered and the longitudinal motion that results presses against a bellows within the sealed housing. This master bellows is in sealed contact with a slave bellows through a rigid housing that is seal welded to the housing wall. Movement of the master bellows in the housing results in an equal movement of the slave bellows outside the housing. Feedback loops or calibration for thermal effects are also contemplated.