Apparatus for determining when the hand of a robot arm, resiliently mounted on a robot arm body, strikes or makes contact with an object that impedes or limits its movement, employs an optical fiber in place of conventional light focusing optics to produce a compact and ruggedized robot arm member relative movement sensor. The apparatus includes a light source mounted in a particular location on the robot arm hand and a light sensitive device spaced from the light source and mounted in a particular location on the robot body, a device that is capable of continuously generating a hand-to-robot arm body relative position signal in response to the particular location of collimated light from the robot hand mounted light source that passes through a light collimating optical fiber and strikes the light sensitive surface of the light sensitive device. Relative movement between the robot arm body and the robot hand resulting from contact between a motion-impeding object and the robot hand causes the light sensitive device to generate a signal indicative of such object contact and/or extent of relative hand-to-robot arm movement. The use of an optical fiber in the light path in place of a conventional lens makes it possible to employ this relative movement sensing apparatus in a vibratory environment and also makes it possible to shorten the light path length and thereby reduce the size of said relative motion sensing apparatus.