Abstract: The invention combines a plurality of ultrasonic ranging transducers (11) arrayed proximate the end (12) of a movable robot arm (13), all transducers (11) being mounted to a common base so as to be pivotally movable both horizontally and vertically relative to the robot arm (13) by stepping motors (17 and 20). By controlling the pivotal movements of the transducer array (10) by a microcomputer (22) so as to look in the direction of intended travel of the robot arm end (13), and as to the robot arm end (13) moves, unexpected objects in the path of the robot (14) or aberrant motion by the robot (14) become easily detected. Upon the sensing of an obstruction to the direction of motion of the robot arm (13), or aberrant motion thereof, a signal from the microcomputer (22) to the robot (14) controls will halt further operation until the obstruction is removed or the problem corrected.