An automatic seam tracking system that uses an ultrasonic-based sensor has been developed and interfaced to a P-50 Process Robot in an effort to achieve online seam tracking of joints, without the use of workpiece geometrical models or subsequent 'teaching' routines. The transducer is used to inspect the workpiece ahead of the torch and to measure joint orientation as well as lateral deviation caused by curvature or discontinuities in the joint path. Measurements of echo pulse amplitude and transit time are combined with a particular torch-sensor arrangement and transducer positioning strategy to eliminate ambiguities resulting from symmetry in the acoustic signal and from environmental factors such as joint geometry and roughness. Data pertaining to the joint orientation and lateral deviation are acquired periodically by sampling equispaced points along the joint as the torch advances. With each sample, data are transferred to the microcomputer, where a trajectory-generating algorithm calculates the (x, y, theta ) coordinates of the torch tip trajectory needed to meet the tracking requirements. The actual tracking task is accomplished online by executing a series of shifts in these coordinates.>