Research On Novel Piezoelectric On-off Pressure-regulation Type Pneumatic Digital Proportional Pressure Valve

Pneumatic proportional pressure valve is the key component of continuous control and the integration of pneumatic, electronic and mechanical. Directly controlled by the computer without D/A converter, with the advantages of simple structure, low cost and high reliability and so on, the digital control of pneumatic valve has gained more and more popularity and become the focus of recent research. However, the traditional on-off type digital electro-pneumatic proportional pressure valve driven by the electro-mechanical converter of electromagnet has the disadvantages of slow response and poor control precision etc, due to the driving component made of electromagnet, cannot meet the certain requirements of high frequency response and high precision, which are commonly required in the rapid gas injection system and the precision pneumatic positioning system, etc. Therefore, it is an urgent need to develop a new type of high-frequency and high-precision digital proportional pressure valve. To achieve this goal, a thorough theoretical and experimental research on the interrelated problem is performed in this thesis.First of all, based on the deep analysis of the working cycle of the traditional on-off type electro-pneumatic digital proportional pressure valve driven by electromagnet actuator, it is found that the most fundamental way to improve the valve response speed and control precision is to improve the actuator dynamic performance. Therefore, the conception of novel piezoelectric on-off pressure-regulation type electro-pneumatic digital proportional pressure valve (PDPPV for short) with rapid response and high precision driven by the piezoelectric actuator is proposed. Based on its functional analysis, the key technologies of the prestage of piezoelectric rapid driven and displacement amplification, the structure of swing-switch flapper type pilot valve and diaphragm-balance type main valve are researched. Furthermore, the overall structure of the novel PDPPV is presented and designed by using modular design method. Its prestage is the piezoelectric stack actuator whose output displacement is amplified by the pilot switch flapper amplification mechanism based on flexure hinge and its pilot stage is the two-position three-way swing-switch flapper type valve with the high speed on-off function. And its power stage is diaphragm balanced main valve to enlarge the output flow. Moreover, the pilot switch flapper amplification mechanism is designed and the finite element analysis of its static, dynamic and fatigue life characteristics is correspondingly conducted.In order to analyze the static and dynamic response characteristics of the PDPPV and the effect of main structural parameters and control parameters on its performance, as well as to provide the theoretical basis for the appropriate design of its structure parameters and control parameters, the PDPPV dynamic characteristics mathematical model is established and the relevant simulation research is finished. By the comparison between simulation and experiment results, the built mathematical model is proved to be accurate. Through simulation studies, the static and dynamic characteristics on working process of the valve are understood and the effects of pilot switch flapper displacement, the flow passage diameter of pilot valve and main valve, the chamber volume of the valve, PWM carrier frequency and PID control on the PDPPV main performance are also deeply clarified, which will provide theoretical guidance to further deep research.Based on the previous work, the experimental prototype of the PDPPV is developed and the corresponding test and control system for the function of high speed real time collection and digital proportional control is built. The static and dynamic tests on the prcstage piezoelectric driven-amplification mechanism, the pilot stage (swing-switch flapper type valve) and the PDPPV are conducted. These tests indicate that:(1) the step rise-time of the pilot switch flapper is about 0.57ms and the step-fall time is about 0.3ms, with faster response speed and wider working bandwidth; (2) it is practical to digitally control the export flow of the swing-switch flapper type valve by regulating the PWM duty cycle, which makes it applicable to the application situation of small flow and low demand of control accuracy; (3) the unloaded flow and outlet pressure of the developed PDPPV have a certain degree of linearity with the PWM duty cycle and the PDPPV has large output flow.Finally, in order to improve the dynamic performance and steady-state accuracy of the PDPPV and decrease its pressure fluctuations, its control algorithms are researched. The Bang-Bang control algorithm and the compound control algorithm of P with dead zone plus PWM can basically achieve the digital proportional control of the PDPPV, but its steady-state control performance is poor, with the disadvantages of some steady-state error and pressure fluctuations, especially under the flow load conditions. The experimental research found that the pressure response delay of the PDPPV is the major influence factor of the valve performance. According to this reason and the work characteristics of the PDPPV, based on PWM control algorithm, an adjusted-shift PWM algorithm (AS-PWM) is presented. Moreover, the compound control algorithm using Bang-Bang Control, P with dead zone and AS-PWM is proposed to control the PDPPV. Experimental results show that with this controller, the PDPPV control performance can be effectively improved, especially under flow load conditions, which the steady-state error reduces from 2kPa to 1kPa and pressure fluctuations reduces from 15kPa to 5kPa.The performance comparison experiments of the developed PDPPV experimental prototype and the traditional electro-pneumatic proportion pressure valve is conducted. Experiments indicate that under the same experimental conditions, compared with the traditional electro-pneumatic proportion pressure valve, the PDPPV has better performance in response speed and steady-state accuracy, which the response speed improves by about 66% and the steady-state accuracy improves by about 50%. The PDPPV has great dynamic characteristics and high-speed, high-precision characteristics, with good industrial application prospect.