Xiaocong Zhu

Adaptive robust posture control of a parallel manipulator driven by pneumatic muscles

Rather severe parametric uncertainties and uncertain nonlinearities exist in the dynamic modeling of a parallel manipulator driven by pneumatic muscles. Those uncertainties not only come from the time-varying friction forces and the static force modeling errors of pneumatic muscles but also from the inherent complex nonlinearities and unknown disturbances of the parallel manipulator. In this paper, a discontinuous projection-based adaptive robust control strategy is adopted to compensate for both the parametric uncertainties and uncertain nonlinearities of a three-pneumatic-muscles-driven parallel manipulator to achieve precise posture trajectory tracking control. The resulting controller effectively handles the effects of various parameter variations and the hard-to-model nonlinearities such as the friction forces of the pneumatic muscles. Simulation and experimental results are obtained to illustrate the effectiveness of the proposed adaptive robust controller.

Adaptive Robust Posture Control of Parallel Manipulator Driven by Pneumatic Muscles With Redundancy

This paper presents an adaptive robust posture controller for a parallel manipulator driven by pneumatic muscles (PMDPM) with a redundant DOF. Rather severe parametric uncertainties and uncertain nonlinearities exist in the dynamics of the PMDPM. To deal with these uncertainties effectively, the recently developed adaptive robust control strategy is applied. Furthermore, the developed control strategy explicitly takes into account the particular physical properties of the system studied. Specifically, the symmetric geometric structure of the parallel manipulator driven by identical pneumatic muscles and no external moments around symmetry-axis direction of the parallel manipulator make the rotation angle of the parallel manipulator around its symmetry axis direction negligible and a nonfactor during normal operations. As such, the axial rotation angle is not measured and controlled when the PMDPM are used in practice, leading to a single-DOF redundancy in synthesizing the precise posture controller for rotation angles around other axes. To make full use of this redundancy, an equivalent average-stiffness-like desired constraint is introduced in the development of the adaptive robust posture controller to achieve precise posture tracking while reducing control input chattering caused by measurement noise. Experimental results are obtained to verify the validity of the proposed controller for the redundant PMDPM.

Integrated Direct/Indirect Adaptive Robust Posture Trajectory Tracking Control of a Parallel Manipulator Driven by Pneumatic Muscles

An integrated direct/indirect adaptive robust controller (DIARC) is proposed to further improve the achievable posture trajectory tracking control performance of a parallel manipulator driven by pneumatic muscles. Due to the model errors of the static forces and friction forces of pneumatic muscles, the simplified average flow rate characteristics of valves, and the unknown disturbances of entire system, there exist large extent of parametric uncertainties and rather severe uncertain nonlinearities in the modeling of the parallel manipulator. To address these problems, in this paper, an indirect type parameter estimation is used to obtain reliable estimates of effective model parameters for reducing the parametric uncertainties while an integrated direct/indirect ARC with dynamic compensation type fast adaptation is utilized to further attenuate the influences of uncertain nonlinearities for better tracking performance. Considering that the conventional parameter estimation algorithm based on single error minimizing criterion normally fails to provide reliable parameter estimation for the parallel manipulator with symmetric structure due to the difficulty in satisfying the persistent exciting conditions all the time-the theoretical requirement for the convergence of online parameter estimation, additional practical constraints are imposed to further condition the parameter estimation process and a new parameter estimation algorithm based on composite error minimizing criterion in task-space is developed. Experimental results demonstrate that the parallel manipulator under the control of the proposed integrated DIARC has strong self-adaptability and robustness with the steady-state posture tracking error being less than 0.01deg, average tracking error less than 0.1deg, and maximum tracking error less than 0.3deg, which are significantly better than those of the direct ARC.

Time Optimal Contouring Control of Industrial Biaxial Gantry: A Highly Efficient Analytical Solution of Trajectory Planning

Contouring control is an important issue in industrial applications. For better product quality and higher productivity, achieving higher contour tracking performance with feed rate as large as possible is essential. However, most existing research has focused on improving contour tracking accuracy only. The potential instability issue due to control input saturation under high feed-rate operations has been largely ignored. In this paper, the minimum time trajectory planning problem is formulated as a step toward solving this practically important problem. A novel back and forward check algorithm is developed to take into account physical constraints of the system when generating the desired trajectory to be followed. Compared with traditional numerical searching methods, the proposed algorithm is very computationally efficient because the optimal solution at each node can be obtained by several analytical equations directly. Moreover, the algorithm can incorporate some velocity-dependent constraints easily. Planning a Lissajous curve is used as the case study to verify the optimality and computational efficiency of the proposed algorithm. Experiments are conducted on an industrial biaxial gantry. Several implementation issues on tracking complicated contours are discussed. Experimental results indicate that the proposed trajectory planning approach enables simultaneous achievement of high contouring tracking accuracy and higher feed rate or shorter operation time.

Adaptive Robust Posture Control of a Pneumatic Muscles Driven Parallel Manipulator with Redundancy

This paper presents an adaptive robust posture controller for a pneumatic muscles driven parallel manipulator with a redundant degree-of-freedom (DOF). The symmetric geometric structure of the parallel manipulator driven by identical pneumatic muscles studied in this paper makes the rotation angle of the manipulator along its axial direction negligible and a non-factor in using the manipulator. As such, the axial rotation angle is normally not measured and controlled when these types of manipulator are used in practice, leading to a single DOF redundancy in synthesizing the precise posture controller for rotation angles along other axes. To make full use of this redundancy as well as effectively tackle severe uncertainties in the system dynamics, an equivalent average-stiffness-like desired constraint is introduced in the development of adaptive robust posture controller to achieve precise posture tracking while reducing control chattering due to measurement noise. Experimental results are obtained to verify the validity of the proposed controller for the pneumatic muscles driven redundant parallel manipulator.

Adaptive robust synchronous control with dynamic thrust allocation of dual drive gantry stage

H-type dual drive gantry structure is usually set up with dual mechanically coupled linear motors to provide higher power density. It has the potential to yield high speed motion and positioning accuracy at the same time. This paper addresses the dynamics and synchronous control of a H-type gantry stage. The mechanical coupling between the dual drives is analyzed. Adaptive robust control (ARC) algorithm is developed to deal with the effect of parametric uncertainties and uncertain nonlinearities of the linear motor system. A synchronous control scheme with thrust allocation is proposed to regulate the internal force at the same time. The proposed dynamic thrust allocation is also able to handle the significant change of load distribution due to the head motion. Experiments are carried out on a dual drive industrial gantry stage. The results suggest that, with the proposed control scheme, the motion performance is guaranteed and the two drives are appropriately coordinated and synchronized as well, validating the effectiveness and the performance improvement of the proposed dynamic thrust allocation approach.

Dual drive system modeling and analysis for synchronous control of an H-type gantry

In this paper, the dynamics and synchronous control issues of dual drive system for gantry stages will be addressed. A physical dynamics model considering the mechanical coupling structure is presented. By carefully studying the dynamics we also try to find the main factors that cause interaction between different axes. Identification experiments are carried out in frequency domain to validate the proposed model, and to reveal the presence of high frequency resonance modes caused by the guide bearings and the rotation of the crossbeam. Synchronous control scheme with cross-coupled compensation are applied to achieve good synchronization between the two motors. Experiments are carried out on a base of an H-type gantry stage. The results verify the analysis, and provide a guideline for future research on synchronous control of this system when making trade-off between maximizing the achievable control performance and not exciting high-order dynamics in practice.

Adaptive Robust Cascade Force Control of 1-DOF Hydraulic Exoskeleton for Human Performance Augmentation

Hydraulic exoskeleton with human–robot interaction becomes an important solution for those heavy load carrying applications. Good human motion intent inference and accurate human trajectory tracking are two challenging issues for the control of these systems, especially for hydraulically actuated exoskeleton where the nonlinear dynamics is quite complicated and various uncertainties exist. However, robust performance to model uncertainties has been ignored in most of the existing research. To regulate these control problems, an adaptive robust cascade force control strategy is proposed for 1-DOF hydraulically actuated exoskeleton, which is namely grouped into two control levels. In the high level, the integral of human–machine interaction force is minimized to generate the desired position (which can also be seen as the human motion intent). And in the low level, the accurate motion tracking of the generated human motion intent is developed. The nonlinear high-order dynamics with unknown parameters and modeling uncertainties are built, and adaptive robust control algorithms are designed in both control levels to deal with the complicated nonlinear dynamics and the effect of parametric and modeling uncertainties. Comparative simulation and experimental results indicate that the proposed approach can achieve smaller human–machine interaction force and good robust performance to various uncertainties.

Synchronization strategy research of pneumatic servo system based on separate control of meter-in and meter-out

In the pneumatic synchronization system based on separate control of meter-in and meter-out, both motion trajectory and pressure trajectory could be tracked in a single pneumatic cylinder and then the cylinder could be controlled completely without internal dynamics. In this paper, an adaptive robust pressure controller is used to keep the pressure level in chamber of cylinder on an even keel when the pneumatic cylinder is moving, which will result in small variation of cylinders friction force and facilitate the precise modeling of friction force, and an adaptive robust motion controller is designed to improve the motion tracking accuracy of pneumatic cylinder, and on-line parameter estimation of the flow coefficient is utilized to have improved model compensation, and moreover a synchronization controller is added to further reduce the synchronization error. Experimental results demonstrate that this synchronization strategy could not only make two cylinders synchronized moving accurately, but also obtain very smooth control inputs which indicate the effectiveness of model compensation.

ADAPTIVE ROBUST POSTURE CONTROL OF A PNEUMATIC MUSCLES DRIVEN PARALLEL MANIPULATOR1

Abstract Considering rather severe parametric uncertainties and nonlinear uncertainties exist in the dynamic model of pneumatic muscles driven parallel manipulator, a discontinuous projection-based adaptive robust control strategy (ARC) is adopted to effectively handle the effect of various parameter variations of the system and hard-to-model nonlinearities such as the friction forces of the pneumatic muscles and external disturbances of the entire pneumatic system to achieve remarkably precise posture trajectory control. Experimental results are obtained to illustrate the effectiveness of the proposed adaptive robust controller.