Dynamic analysis of the SCARA type direct-drive robot and trajectory tracking using robust controller

Abstract

Robot has been used mainly for dangerous, tedious work in unpleasant environment. Applications of industrial robot were expanded, various types of robot were developed to fulfill specific requirements. Because an electrical motor is inexpensive, compact and easy to control, most of conventional robots choose the electrical motor as an actuator nowadays. In general, electrical motor can not generate large torque in low speed, so appropriate gear reducer is necessary for amplifying small torque of the motor. However, a gear reducer causes backlash, large friction and produces vibration in low frequency. To overcome these shortcomings, a direct-drive robot, in which no gear reducer was used, has been developed. Because the rotor of an electrical motor is directly coupled to the robot link, backlash is completely removed and friction is reduced significantly. Also, the dynamic response is more predictable because the simple structure of the direct-drive system can be modeled accurately. These desirable characteristics make it easy and effective to apply advanced control schemes to a robot system. If the dynamic model of a robot is accurate, a model-based control scheme such as the computed torque method is known to be capable of giving good results. however, the performance of the model-based control is degraded by model uncertainty and external disturbance. Moreover, the coupling force between robot links and the external disturbance have to be compensated for precise manipulation of the direct-drive robot because these terms are directly transmitted to the actuator. In conventional robot with a gear reduceer, these coupling terms are attenuated remarkably by the gear reducer so that these can be ignored compared with the inertia of the motor. Therefore, the independent PD joint controller can yield acceptable performance in positioning accuracy. In this paper, we present the combined controller for a direct-drive robot. The proposed controller consists of two por...