1. In a method of controlling a robot positioning system to follow a joint between workpieces by projecting a structured light pattern, viewing the scene with a vision sensor and producing images that are transferred to a controller, executing move vectors that cause said positioning system to move by increments to approximately follow said joint and are computed from sets of navigation points extracted from said images indicating the location and width of said joint and from positioning system feedback, a future move vector being extracted and computed while a current move vector is executed; the improvement to track 90.degree. corners and small radius turns wherein said structured light pattern is two concentric arc-shaped stripes and which comprises: (a) every time said move vector is computed, determining two straight lines between the respective sets of navigation points at the intersections of said arc-shaped stripes and joint edges; (b) determining whether said lines are parallel within a prespecified tolerance band; (c) when the parallelism is outside said band, using the previously computed move vector until parallelism is re-established and currently computed move vectors are again executed.