| Pneumatic technology has been favored in various industries with its simple structure, low cost, no pollution, strong anti-jamming capability, easy maintenance characteristic. Because of low stiffness, small damping ratio and significant nonlinear characteristic in low pressure environment, pneumatic servo system is limited in practical applications. Especially pneumatic position servo system can not reach the requirements in the low-speed stage. Under the cylinder friction's influence, there will be a crawling phenomenon in pneumatic position servo system. The crawling phenomenon must be avoid in industrial applications for its harmful influence. In order to expand pneumatic position servo system in low-speed applications, it needs to know the impact of cylinder friction factor and the related solutions, through appropriate correction programs to improve the characteristic of pneumatic position servo system.In this paper, the general mathematical model of pneumatic position servo system is bulit firstly, and the linear transfer function of the system is established after the reasonable linearization. Natural frequency of cylinder and related state parameters are defined. These conclusions are prepared for system characteristic analysis and time domain simulation model of the system in the Simulink environment.In the analysis which is based on the Stribeck friction model of the cylinder friction, the effects of proportional valve dead zone is overcome in order to creat a relatively ideal model for the cylinder friction's simulation at first. Use the"define two sectors and discuss another"dissection-type method to analyze the three elements that constitute the cylinder friction influence in the pneumatic servo system. Figure out that the most influential factor in pneumatic position servo system at low-speed stage is the difference between static and dynamic friction. This conclusion provides a standard to judge the effectiveness of the suppression strategy to the cylinder friction.In this paper, the feedforward compensation of friction which is based on friction state observer is elaborated in detail and deeply analyzed. Based on the effect mechanism of the friction elements which have a bad impact on the position servo system, this article discussed the reason for the limited effect of the feedforward compensation of friction, and supported by the simulation results.The paper presents the friction suppression method-the dither signal compensation method. This program is based on the linear transfer function of the system, using the amplitude-frequency characteristic curve to define the relationship between the dither frequency and the system natural frequency. This paper establishs the linear theory for the dither signal compensation. Based on this theory, the dither signal compensation method is supported by the simulation results and experimental data. The effectiveness of the method has been proved by the simulation and experiment.In order to improve the system performance, acceleration feedback compensation method is proposed. In the theoretical analysis, this method could increase the damping ratio of the system and enhance the stability of the system. Given the feedback ratio and supported by the simulation results. |
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