Friction Compensation for High Precision Mechanical Bearing Turntable
|School||Harbin Institute of Technology|
|Course||Control Science and Engineering|
|Keywords||mechanical bearing turntable DC motor friction compensation acceleration feedback control adaptive control robust control|
High precision test turntable is an important device for model calibration of inertial navigation systems and inertial instruments. The quality of test turntable determines the reliability and accuracy of the test and is the foundation on which the precision and performance of aeronautics and astronautics products and weapon systems is ensured. With the rapid progress of aeronautics and astronautics technologies, the performance and accuracy of navigation and guidance equipments need to be further improved, which in turn demands the improvement of the accuracy of test turntable. However, the improvement of the turntable accuracy is limited by the disturbing moments which are mainly composed of friction torque. And the reduction of friction torque is limited by technological level and experimental funds. Therefore, designing control laws to eliminate friction from the viewpoint of control and thereby achieving high precision control is of great theoretical significance and application value to the development of turntables with simple structure and high quality.As a main participant, the author took part in the development of Harbin Institue of Technology Subject Construction Project Sponsored by 211 entitled“SWST-1 Two-axis Position and Rate Turntable”. Based on that, several control methods were studied in this dissertation to compensate for nonlinear friction existing in mechanical bearing turntable system. The destination was to eliminate the influence of friction on the turntable control system and improve the position tracking performance.First, the control system structure of SWST-1 two-axis turntable was introduced. According to the required performance specifications, the model parameters of each module except for the controller were given. Based on that, the PID controller of the position control system was designed and the digital logic part of the system was also designed and integrated using CPLD-based techniques. A series of experiments were conducted to test each module of the developed turntable control system. Moreover, the closed loop frequency characteristic and step response of the control system were tested. Experimental results verified the correctness and effectiveness of the system design.Based on the above developed turntable control system and taking into account that the actual friction model and friction parameters in turntable are difficult to obtain, a friction compensation approach based on acceleration feedback control was proposed. Above all, the disturbance suppression principle of acceleration feedback control was analyzed for the turntable control system. Based on that, regarding the friction torque as a disturbance, the classical PID controller was enhanced to suppress the nonlinear friction in the turntable system by incorporating an inner acceleration feedback loop. The acceleration and position loop controllers were designed according to performance specifications of the system and in compliance with corresponding design criteria. Simulation study and experimental verification were both performed for the proposed scheme, and simulation as well as experimental results showed that acceleration feedback control suppressed the nonlinear friction effectively and improved the position tracking performance.Adaptive friction compensation strategies in three cases were studied for the turntable control system with nonlinear dynamic friction and possible system parametric uncertainties. Firstly, the dynamic friction was assumed to vary uniformly due to variation in temperature, etc, and the system parameters were known. The observer/filter structure based adaptive control method was utilized to compensate for the parametric uncertainty in friction. In the second case, the dynamic friction parameters were assumed to vary non-uniformly and the system parameters were known. An observer/filter structure based adaptive control method, for which adjustable gains were introduced into the dual filters, was proposed to compensate for the non-uniform parametric uncertainties in friction. And the dual filters could be tuned to improve the position tracking performance further. Finally, the system parametric uncertainties and the non-uniformly varying friction were both considered in the third case. The observer/filter structure based adaptive control method was utilized to compensate for both parametric uncertainties. In each case, the convergence of parameter estimates and global asymptotic stability of the closed-loop system was proved via a Lyapunov-like argument.Besides the parametric uncertainties associated with plant model and dynamic friction, there exist modeling errors and external disturbances in the turntable control system. The modeling errors and external disturbances have a detrimental influence on the effect of adaptive friction compensation schemes, which results in the degradation of position tracking performance and even makes the closed-loop system unstable. Therefore, two robust adaptive friction compensation schemes were presented. Firstly, the uncertain nonlinearities, which could be a combination of modeling errors and external disturbances, were assumed to be bounded by a known constant, while the system parameters and dynamic friction parameters were assumed to be unknown. To handle this problem, a robust adaptive friction compensation scheme based on an adaptive sliding mode controller was employed to compensate for the parametric uncertainties in plant model and dynamic friction as well as the uncertain nonlinearities. Furthermore, in order to improve the robust performance of the system, a smooth projection algorithm was introduced to modify the adaptive laws and friction state observers. Next, considering that the bound of the uncertain nonlinearities was unknown, a smooth robust adaptive friction compensation scheme was developed. To handle the bounded nonlinearities, a chattering free sliding mode control term was utilized. The unknown friction parameters and the constant that bounds the nonlinearities were updated according to the modified adaptive laws based on theσmodification scheme. And the smooth projection algorithm was exploited to modify the friction state observers. In both cases, the convergence of parameter estimates and stability of the closed-loop system was proved theoretically.