High precision contour tracking with a joystick sensor

High performance contour tracking is achievable with simple sensing and control. We describe a system that tracks unknown shapes with a joystick sensor mounted on an Adept SCARA robot. The joystick's limited force sensing is combined with the Adept's high positional accuracy to yield precise contact measurements. Force control is carried out with a simple feedback loop to avoid unnecessary oscillations. Due to contact friction and random sensor noise, prediction of the tangential motion from force measurements alone is unreliable. A position control strategy is devised to update the tracking direction based on a quadratic t to local turning of the contour. Fitting yields more accurate shape approximation while the use of local data keeps the computational cost low for constant updates. The result is fast tracking with hardly any loss of shape accuracy.