Research On The Pneumatic Synchronization System Based On Separate Control Of Motion Trajectory And Pressure Trajectory

With the rapid development of microelectronic technique, the pneumatic servo system, especially the pneumatic servo position control system, has more and more widespread application in many industrial fields since multiple-point stepless positioning (flexible positioning) and stepless velocity modulation can be implemented by pneumatic servo position control. Moreover, since the conventional manner of using flow control valve and end-position cushioning can be replaced by the manner of pneumatic servo position control system with the merit of continuous velocity modulation, the optimal velocity and cushioning effect can be achieved and the action time of pneumatic cylinder can be significantly decreased, which result in less time cost of working operation and then high productivity. Owing to air compressibility, pneumatic products can realize soft contact and soft action, which can not be achieved by other electromechanical products. The pneumatic synchronization system based on separate control of motion trajectory and pressure trajectory is in fact applying the separate control of meter-in and meter-out orifices through controlling motion and pressure simultaneously to the pneumatic synchronization system. Compared with the traditional pneumatic position servo system controlled by one proportional directional valve(i.e. mechanical linkage between meter-in and meter-out orifices), the pneumatic system based on separate control of meter-in and meter-out orifices has some advantages listed as follows. (1) The flexibility of system is enhanced because there are two control degrees of freedom in one pneumatic cylinder and the area ratio of meter-in to meter-out orifices can be varied. Thus, the control strategy can be modified according to load type and then the optimal control performance can be achieved at all the operation points. (2) The technique of accurate computational flow feedback is utilized to achieve fast and accurate pressure response, which can avoid incorrect switchover of the directional valve due to delay of unpredictable pressure response caused by air compressibility and environmental factor. (3) The fast response can be guaranteed through enlarging orifice areas of inlet valve and outlet valve during acceleration process and the stability of system can be enhanced through controlling counter pressure to avoid the concussion and oscillation of pressure during deceleration and braking process. (4) The mod- eling of friction force is very complex. Especially when the direction of velocity is reverse, the accurate friction force model of rodless pneumatic cylinder is hardly obtained. However, the reversal of velocity during motion process can be avoided by the method of velocity-approach-position-modification in the pneumatic system based on separate control of meter-in and meter-out orifices. Hence, the friction force will slowly change and high precision trajectory tracking will be guaranteed. The pneumatic synchronization system based on separate control of motion trajectory and pressure trajectory can be used in portal framed structure and pneumatic lift platform, which will have promising wide applications in automatic production line, semiconductor processing equipment, aerospace driving device, medical instruments etc..At present, few domestic and foreign researchers have published papers about the pneumatic synchronization system, which the synchronization accuracy and anti-disturbance ability need to be further enhanced in. There exist not only severe nonlin-earity and time-varying parameters in the pneumatic system, but also the contradiction between large friction force of pneumatic cylinder and low stiffness of air. These factors bring a challenge for achieving a high precision synchronization motion of pneumatic servo system. Firstly, the uncertain model and severe nonlinearity of transient mass flow rate, the unknown structure parameters of pneumatic system and unknown non-linearity of pressure dynamics will present adverse influence on control performance. Secondly, the ratio of friction force to driving force is much large and it is very difficult to obtain the accurate friction model of pneumatic cylinder, and furthermore the low stiffness of air and the reversal of friction force direction would restrict the tracking accuracy of pneumatic servo position control system during feedback control. Thirdly, the composite control scheme consisting of an adaptive robust pressure controller, an adaptive robust motion controller and a synchronous controller is proposed to achieve high precision synchronization motion and strong robustness. Specifically, the pressure control will affect the synchronization accuracy during startup of the pneumatic servo system or when subjected to a sudden disturbance, and thus an effective method should be developed to solve the strong coupling effect caused by low response.The subject of this doctoral dissertation is a pneumatic synchronization system based on separate control of motion trajectory and pressure trajectory, and the research aim is to realize high precision trajectory tracking and synchronization motion of two rodless pneumatic cylinders with large friction force. Combining theoretical analysis and experimental measurements, control strategies of synchronization motion in a pneumatic servo system are investigated thoroughly step by step.The innovations in this doctoral dissertation are concluded as follows. First, the energy-saving control strategies of synchronization motion for the pneumatic servo system based on separate control of motion trajectory and pressure trajectory are proposed. Concretely, 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 cylinder's friction force and facilitate the precise modeling of friction force, and an adaptive robust motion controller is utilized to compensate friction force of pneumatic cylinder accurately and improve the motion tracking accuracy through guaranteeing that the direction of velocity is not reversed by means of velocity-approach-position-modification, and moreover a synchronization controller is employed to further improve the synchronization accuracy through feeding the synchronization error into it and adding the output of it into two adaptive robust motion controllers according to a composite principle of equality mode and master-slave mode. Second, according to the physical phenomenon that stable pressure values of port A and port B would be different, the mid-voltage of proportional directional valve could be obtained through minimizing the offset value of two stable pressure ratio curves. According to the physical phenomenon that pressure response curves at the mid-voltage would be different but stable pressure values would be equal when the pneumatic cylinder is fixed at two ends, the dead volume of rodless pneumatic cylinder could be obtained through equal leakage mass flow rate under equal pressure. The improved equation of mass flow rate is obtained by parametrically linearizing flow coefficient and practically correcting critical pressure ratio. Then, the adaptive robust control strategy using the above parameters is integrated with an adaptive Kalman filter to achieve accurate model compensation and effective parameter identification. Third, a new friction model for pneumatic cylinder is proposed, which is able to reflect the phenomena of the breakaway static friction force varying with both dwell time and increasing rate of applied force, the Stribeck effect and the descending viscous friction coefficient. It is found that the break-away static friction force of pneumatic cylinder is related to both dwell time and increasing rate of applied force by designing specific experiments of segregating dwell time from increasing rate of applied force. The slight descending of dynamic friction force is described by an exponential decay function varying slowly with the moving time and moreover it is proved by experiments that the viscous friction coefficient is approximately proportional to the pressure.This doctoral dissertation are divided into the following eight chapters.In Chapter 1, the development survey of pneumatic servo position control system is simply introduced. According to the mechanical structure of pneumatic system consisting of pneumatic components, the pneumatic servo system could be divided into two kinds of cylinder-controlled-by-valve systems both based on mechanical linkage of meter-in and meter-out orifices and based on separate control of meter-in and meter-out orifices, and then according to this division, domestic and foreign research methods of pneumatic servo position control system are concluded. The research on pneumatic synchronization system is divided into two development stages, and moreover the previous research fruits about it are analyzed. Finally, necessity, difficulties and main contents of the pneumatic synchronization system based on separate control of motion trajectory and pressure trajectory are summarized concisely and clearly.In Chapter 2, the synchronization device and loading device of pneumatic synchronization system are designed. According to valid modeling assumptions, the motion dynamics and pressure dynamics of pneumatic cylinder are deduced. The mid-voltage of proportional directional valve, the leakage flow rate model in the deadzone and the dead volume of rodless pneumatic cylinder are all obtained by different parameter identification methods. The improved equation of mass flow rate is obtained by online parameter estimation of parametrically linearized flow coefficient and practical correction of critical pressure ratio.In Chapter 3, an adaptive robust pressure controller is developed to improve the tracking accuracy of pressure trajectory in the chamber when the pneumatic cylinder is moving. In the proposed pressure controller, off-line fitting of the orifice area and online parameter estimation of the flow coefficient are utilized to have an improved model compensation, and meanwhile the robust feedback and Kalman filter are used to have a strong robustness against uncertain nonlinearities, parameter fluctuations and noises and so on. The adjustment method of nonlinear robust feedback term with varying gain is explored. In Chapter 4, an adaptive robust pressure controller is designed 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 cylinder's friction force and facilitate the precise modeling of friction force. It is guaranteed by means of velocity-approach-position-modification that the direction of friction force is not reversed. An adaptive robust motion controller is designed to achieve high precision position tracking for the rodless pneumatic cylinder with large time-varying friction force. The function and choosing method of desired velocity correction term is analyzed. The working conditions and their conversion modes in the process of adjusting errors are summarized.In Chapter 5, two expressions of flow conductance are analyzed and the static model of fluid power system based on separate control of meter-in and meter-out orifices is established according to equivalent flow conductance. On this basis, it is concluded that the flow conductance reversing criterion and energy loss conservation criterion of fluid power system based on separate control of meter-in and meter-out orifices, and meanwhile velocity sensitivity coefficient is defined. The air consumption of pneumatic servo system based on separate control of meter-in and meter-out orifices is obtained and a new approach for computing air consumption is developed. The transfer function of linearized models of this pneumatic servo system is constructed, and then load sensitivity and dynamic stiffness of this pneumatic servo system are analyzed.In Chapter 6, the modeling of presliding displacement, break-away static friction force, Stribeck effect and friction hysteresis are summarized in detail. A new friction model for pneumatic cylinder is proposed, which is able to reflect the phenomena of the break-away static friction force, the Stribeck effect and the descending viscous friction coefficient. It is proved by designing specific experiments of segregating dwell time from increasing rate of applied force that the break-away static friction force of pneumatic cylinder is related to both dwell time and increasing rate of applied force. An exponential decay function varying slowly with the moving time is used to describe the slight descending phenomenon of dynamic friction force.In Chapter 7, six synchronous control modes for the pneumatic synchronization system based on separate control of meter-in and meter-out orifices are developed and tested by experiments. It is verified by open-loop motion control of Mode 1 and Mode 2 that there exist great differences between two driving system hardware and then it is impossible to move synchronically only by the open-loop motion control, and also proved that good uniform motion of one driving system can be realized under a constant back pressure. It is confirmed that the high precision synchronization motion is hard to be achieved only by a single master-slave mode, owing to velocity variation of master pneumatic cylinder. And it is confirmed that the high precision motion trajectory tracking is the precondition of high precision synchronization motion by a single equality mode. The equality mode with double feedback of synchronization error can break through the restriction that synchronization accuracy is guaranteed only by trajectory tracking accuracy. The composite synchronous control mode integrating equality mode with master-slave mode can achieve good results with maximum synchronization error being 3.3mm, average synchronization error being 0.9mm, steady synchronization error being 0.2mm and maximum synchronization error being 1.1% of stroke length.In Chapter 8, main research work, conclusions and innovation points of pneumatic synchronization system based on separate control of motion trajectory and pressure trajectory are summarized. At the same time, future development of pneumatic synchronization system is predicted in order to provide references for the further research on this project.