Optimum Design of a Pan-Tilt Drive for Parallel Robots

The need to drive a serial array of two robotic axes by means of two motors fixedly mounted on a common base is the motivation behind the work reported here. An innovative twodegree- of-freedom drive is proposed, which is based on a serial array of an epicyclic gear train and a five-bar linkage. The ring and sun gears are driven by the two motors, its two outputs being the angular velocity of the planet-carrier and the relative angular velocity of the planet gears w.r.t. their carrier. The former is the pan rate, the latter the input to the five-bar linkage whose output is the tilt rate. Optimization is used to determine the proportions of the various dimensions involved so as to obtain a 2 × 2 Jacobian, mapping the motor rates into the pan and tilt rates, whose condition number, at a prescribed drive posture, is a minimum. Moreover, to improve the force-torque transmission characteristics, the transmission defect of the mechanism, as introduced elsewhere, is also minimized. To this end, a sequential-quadratic programming method using an orthogonal decomposition of the space of design variables is implemented; the dimension ratios thus resulting lead to a mechanism whose transmission defect is a minimum, while the condition number of its Jacobian matrix is a minimum as well, at the posture where the transmission angle is 90◦.