substantially identical shapes but to be phase displaced by 90 degrees the command signal on command signal line 40 may be applied to multipliers 80 and 90 for controlling the force generated by linear motor 10 and for controlling the direction of motion of movable element 18 that is multipliers 80 and 90 preferably include variable gain which can be controlled by the command signal this permits varying the amplitudes of the phase a and phase b signals according to the desired force in addition multipliers 80 and 90 may include conventional circuits for inverting their output signals under control of the command signal on command signal line 40 this has the effect of reversing the direction of motion of movable element 18 the desired control signals on command signal line 40 are produced by conventional devices and it is believed that further discussion of equipment for generating such control signals would not contribute to the ability of one skilled in the art to make and use the invention in addition apparatus for controlling the gain and performing signal inversions in multipliers 80 and 90 are conventional and their construction and function are fully within the knowledge of one skilled in the art thus a further description of the contents of multipliers 80 and 90 is considered redundant to the present disclosure electro optical sensing device 44 may be mounted external to u shaped channel 12 without departing from the spirit of.