Dual motor control for backlash reduction

Within the EU FP-6 project SMErobotTM a new type of high-performance robots has been developed by ABB Robotics, the Robotics Lab at Lund University and Güdel AG, Switzerland. The new design is based on a parallel configuration of the robot's joints (parallel robots). The main novelty of that concept is its completely new parallel kinematic structure, which allows to exploit all the advantages in terms of performance and cost of parallel kinematics, e.g. having only axial forces in the arm links. Consequently, this new type of robot can be used in many applications, such as laser, water and plasma jet cutting, gluing, assembly and machining. However, the actuator and drive-line of the robot are based on the 'Rack-and-pinion principle'. This leads to some difficulties concerning the positioning accuracy and the stiffness, as flexibility and backlash will occur in both the ordinary gearbox and in the mechanical connection to the rail. To fulfill the requirements on the robot, these effects need to be eliminated. The approach of the present work is to reduce these effects by using two motors for each cart instead of the conventional use of only one. Furthermore also position measurements along the rails are used in addition to the motor angle positions. For this configuration, a nonlinear model is derived and the effects of the additional motor on the system performance and stability are analyzed. Furthermore, several nonlinear, smooth switching control laws for the operation of the system are presented. The theoretical results are furthermore verified with an experimental setup representing a simplified implementation of the dual motor driven robot