The Position Control Of The PHCC Servo System

The pneumatic servo system is used in automation and military industry wildly as the transmission medium has the advantages including noncombustibility, no pollution, strong ability of anti-interference to electromagnetic radiation, fitness for bad work environment. Because of its simplicity, low cost and easy operation, it has been paid more and more attention to. But it is gas medium with compressibility and low viscosity that leads to low natural frequency, small damping ratio, low positional precision and rigidity, bad low speed characteristics, which limit the application of pneumatic servo technique.Aiming at the essential defect of pneumatic medium, pneumatic hydraulic combination control (PHCC) servo system is formed, in which the liquid medium with low compressibility and high viscosity is introduced into a normal pneumatic servo system and is closed-looped controlled simultaneously.Firstly, the research and development situation of pneumatic servo system in recent years is summed up. Meanwhile, the development history and present situation of interrelated control theory is reviewed.Considering the cost and the capability of control of PHCC servo system, high-speed on-off valves are used as control valves and controlled with pulse width modulation (PWM) in this system. In this paper, the on-off procedure and drive mode of high-speed on-off valves are introduced, the principle of PWM most in use recently is introduced .Based on the forming method of PWM signal with software, the conclusion is drawn that the occupancy of PWM signal is only in relation to the control value calculated and the mean flowrate is linear to the control value when the carrier wave (cycle and amplitude) is chosen. Therefore, it is proved that the control with high-speed on-off valves can realize the servo control function that be carried out with analog control whose control flowrate is proportional to is value.Secondly, the paper builds up the mathematics model of the normal pneumatic servo system based on the symmetry structure, and the main characteristic parameters are analyzed and then introduce the theories to increase...