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In this paper, disturbance rejection of DC motor PID trajectory control systems is enhanced for independent joint control of robot arms. The concept of disturbance observer is invoked to propose a linear auxiliary control that augments existing PID controllers. The design of the auxiliary control is developed using a state space approach rather than transfer function approaches commonly employed in many existing designs derived from the concept of disturbance observer. This provides new insight and leads to a compact design requiring only two design parameters. While many of the existing DC motor trajectory control systems assume the availability of current feedback from a motor coil, the proposed auxiliary control does not. This can highly facilitate its applications in the lacking situation. Realizing that the stability of the resulting control systems could be inconvenient to assert due to increased system dimension resulting from incorporating disturbance observer, compact criteria for asserting robust stability using readily available results is given explicitly. To evaluate the capability of the auxiliary control for disturbance rejection, experimental results on a DC joint motor of an articulated robot arm are given. In presence of smooth and abrupt loading variations due to gravity, it appears that the tracking error of the enhanced system can be approximately 67% of that of the unenhanced system. This result is consistent in all three rounds of experiments.
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