Abstract: A positioning mechanism positions a transducer along a linear axis relative to a support structure and includes a movable mount for the transducer, a rotatable cam element with camming structure that is maintained in contact with the movable mount and a rotatable drive to rotate the cam element. The cam element is preferably a wheel with angularly spaced openings with spherical balls of different diameters disposed in respective ones of the openings to define the camming structure. Alternatively, the camming structure may be a continuous or a stepped annular ramp around the surface of a disc-shaped member. The movable mount is a shaft and a shuttle that support the transducer with the shaft slideably received between two opposed pieces of a yoke-shaped support arm. The shaft is spring biased against the cam element. The drive is a servo motor, a stepper motor or a solenoid, and an encoder operates to sense the rotational orientation of the cam element.