Abstract: Apparatus for effecting relative positioning between two mechanical members in accordance with digital (or analog) commands includes a standing wave supporting, electrical wave propagating structure terminated other than by its iterative impedance. The wave structure is mechanically fixed to one mechanical member, and a probe is coupled to the wave structure and fixed for translation with the other mechanical member.A voltage controlled oscillator excites the wave structure developing a standing electrical wave therein -- the wave pattern node positioning being determined by the input digital command. Feedback servomechanism apparatus then responds to the potential induced in the probe (or, by superposition, that on the wave structure) for positioning the probe at the requisite standing wave null point.In accordance with one aspect of the present invention, the wave propagating structure is sealed, and the probe disposed external thereto, to preserve measuring accuracy.