High Precision Synchronized Motion Trajectory Tracking Control Of Multiple Pneumatic Cylinders

Pneumatic cylinders are clean, easy to work with, and low cost. In addition, they have a high power-to-weight ratio and an excellent heat dissipation performance. These properties make them favorable for servo applications. The need for motion synchronization in multiple linear pneumatic cylinders can be found in many applications, such as industrial automation, robotics and medical equipment. However, the dynamics of pneumatic systems are highly nonlinear. Furthermore, there normally exist rather severe parametric uncertainties and uncertain nonlinearities in the modelling of pneumatic systems. The precision motion trajectory tracking control of a pneumatic cylinder is still a big challenge, not to mention the motion synchronization of multiple ones. In this dis-sertation, a two-cylinder pneumatic servo system is considered. The two pneumatic cylinders are controlled by two individual proportional directional control valve.To achieve a high precision motion trajectory tracking control of pneumatic cylinders, an adaptive robust controller with LuGre model-based dynamic friction compensation and valve dead-zone compensation is constructed. The proposed controller employs on-line recursive least squares estimation (RLSE) to reduce the extent of parametric uncertainties, and utilizes the sliding mod-e control method to attenuate the effects of parameter estimation errors, unmodelled dynamics and disturbances. In addition, in order to realize LuGre model-based friction compensation, the modified dual-observer structure for estimating immeasurable friction internal state is developed. Therefore, a prescribed motion tracking transient performance and final tracking accuracy can be guaranteed. Since the system model uncertainties are unmatched, the recursive backstepping de-sign technology is applied. In order to solve the conflicts between the sliding mode control design and the adaptive control design, the projection mapping is used to condition the RLSE algorith-m so that the parameter estimates are kept within a known bounded convex set. Since the valve dead-zone parameters are estimated, an accurate dead-zone inverse function can be constructed and perfect asymptotic dead zone compensation is theoretically achieved. The same high tracking performance can be obtained without any controller retuning when a different valve is used.A new control approach to motion synchronization of multiple pneumatic cylinders is devel-oped by incorporating the cross-coupling technology into the adaptive robust architecture. The proposed synchronization controller, designed by utilizing the feedback of both the position er-rors and the differential position errors amongst cylinders (defined as the synchronization errors), guarantees asymptotic convergence to zero of both position tracking and synchronization errors simultaneously.This doctoral dissertation consists of six chapters.In chapter1, the literature related to pneumatic position servo system and motion synchroniza-tion of multiple pneumatic cylinders is reviewed, and the objective of this dissertation is illustrated.In chapter2, the hardware of the two-cylinder pneumatic servo system is described, and a detailed model for a rodless pneumatic cylinder controlled by a proportional directional control valve is developed. The dynamics of the valve spool was firstly investigated and an equation was introduced to describe the mass flow through the valve’s variable orifice. The thermodynamics in cylinder chambers was carefully considered and the heat transfer coefficient between the air in the chamber and the inside of the barrel was identified experimentally. The LuGre dynamic friction model was introduced to fulfill the requirement of a good friction model of pneumatic cylinder-s in applications with high precision positioning and with low velocity tracking. An experiment setup used for the friction force measurements was developed and the static and dynamic param-eters associated with the model were estimated separately. The static parameters were estimated by construction of the friction-velocity map measured during constant velocity motions. And, the second-order description of the linearized LuGre model in the stiction regime was used to perform a frequency domain identification of the dynamic parameters. Furthermore, the influence of the chamber pressures on the friction in pneumatic cylinders was investigated. The Coulomb friction level, the level of the stiction force and the coefficient of the viscous friction increased with cham-ber pressure and pressure differential across the piston, while the Stribeck velocity and dynamic parameters may be considered as independent of pressure variation in chambers. Experimental re-sults showed that the estimated model parameters and the proposed model can predict the friction force accurately.In chapter3, two typical nonlinear controllers-tuning function based adaptive backstepping controller and deterministic robust controller based on backstepping design-are constructed firstly. Considering that they each have benefits and limitations, an adaptive robust controller (ARC) is proposed. The ARC is a combination of the first two controllers, it employs on-line recursive least squares estimation (RLSE) to reduce the extent of parametric uncertainties, and utilizes the sliding mode control method to attenuate the effects of parameter estimation errors, unmodelled dynamics and disturbances. In order to solve the conflicts between the sliding mode control design and the parameter adaption law design, the projection mapping is used to condition the RLSE algorithm so that the parameter estimates are kept within a known bounded convex set. Theoretically, ARC possesses the advantages of the adaptive control and the robust control, and thus an even better tracking performance can be expected. Extensive comparative experimental results are presented to illustrate the achievable performance of the three proposed controllers and their performance robustness to the parameter variations and sudden disturbance.In chapter4, in order to achieve better transient performance, an integrated direct/indirect adaptive robust controller (DIARC) is proposed by introducing dynamic compensation type fast adaptation into the ARC. Extensive comparative experimental results demonstrate that the achiev-able performance of the DIARC controller is excellent and is much better than most other studies in literature. Furthermore, an adaptive dead-zone compensation strategy is introduced. The proposed approach makes full use of the fact that the unknown valve dead-zone nonlinearities can be linearly parameterized beyond dead-zone region, and employs least square type indirect parameter estima-tion algorithm with on-line condition monitoring to estimate the dead-zone parameters. Therefore, an accurate dead-zone inverse function can be constructed and perfect asymptotic dead zone com-pensation is theoretically achieved. Finally, a DIARC controller with LuGre model-based dynamic friction compensation is proposed to achieve high precision motion trajectory tracking control of pneumatic cylinders especially at low speed movement. A modified dual-observer, whose digital implementation will not become unstable if the piston velocity exceeds a critical value, is proposed to estimate the immeasurable friction internal state z. For tracking a0.5Hz sinusoidal trajectory, the maximum final tracking error is1.32mm, the average final tracking error is0.68mm and the transient tracking error is1.61mm. Especially when a low-speed sinusoidal trajectory is tracked, the maximum final tracking error is0.59mm and the average final tracking error is0.21mm.In chapter5, an adaptive robust synchronization controller for multi-cylinder pneumatic servo system is developed by incorporating the cross-coupling technology into the DIARC architecture. The proposed controller is synthesized through feedback of position and synchronization errors to ensure that each cylinder tracks its desired motion trajectory while synchronizing its position with other cylinders’positions. Asymptotic convergence is achieved for both trajectory tracking and motion synchronization. Experiments conducted on a two-cylinder pneumatic servo system demonstrate the effectiveness of the controller. For tracking a0.5Hz sinusoidal trajectory, the maximum final synchronization error is1.25mm and the average final synchronization error is 0.67mm.In chapter6, the conclusions of the thesis are presented. The main contributions of this research are summarized and the recommendations for future study are described.