Study On A Pneumatic System Controlled By On-Off Valve Based On The Choking Characteristics

Pneumatic technology has many advantages such as high performance, low cost and no pollution. It has been widely applied in industrial automation fields like automobile, package and semiconductor. The application of pneumatic technology can improve production efficiency and reduce cost, at the same time, its environmental friendly. However, the compressibility of the gas, low stiffness and cylinder friction make the pneumatic system hardly get the ideal control precision and response time. People are constantly studying pneumatic technology and trying to improve the performance of all kinds of pneumatic products. In order to simplify the pneumatic system model which has a small control volume and improve its stability and adaptability to load change, this paper researches the choking phenomenon in pneumatic flow characteristics, and this phenomenon was used in on-off valve controlled system. The main research contents are as follows:As the main characteristic of the pneumatic system which was researched in this paper, choking principle and the related definitions are expounded. According to the energy equation, continuity equation and state equation, the characteristic of the choked flow are deduced. By the experimental method of pneumatic flow characteristic GB/T14513, a kind of small-hole throttle valve’s flow characteristic was obtained and confirmed that this valve can create choked flow.Based on the analysis of choking principle and pneumatic flow characteristic, research the on-off valve controlled system which adds the small hole valve. Through setting certain parameters the pneumatic system can produce choked flow which drive cylinder move. The structure and principle of choking pneumatic system was introduced, and then the system model was established which includes the model of on-off valve, the mass flow equation, the pressure equation and fiction equation of cylinder and the kinematics equation of piston. According to the system model, this paper researched the driving characteristic and position control of the system.The mass flow of small-hole throttle valve under choking condition is constant. It’s utilized to research the driving characteristic of the system. This paper gets the result of cylinder displacement, velocity and pressure under different parameters. The influence of the change of parameters also was analyzed. Due to the flow stability, the system model can be implied and the cylinder speed can be expressed by pressure and equivalent in-let area. So based on the requirement of the cylinder stroke and time, we can directly deduce the system pressure and equivalent in-let area. Besides, this paper gives two design parameters for choking pneumatic system with two different types of load, distance and time. Through simulation, the driving result is very close to the goal.Except the research of driving characteristic, this paper also researcher the position control of the cylinder. The choking pneumatic system is controlled by on-off valve under PWM. Through simulation, the influence of source pressure, small-hole area, position, and control precision was analyzed. A comparison between chocking pneumatic system and normal pneumatic system was done under the condition of two different types of load. To both systems, the result shows that different load leads different stability and precision. When the load is inertia, chocking pneumatic system is better in stability and precision. While the load is constant, chocking pneumatic system is not as better as normal pneumatic system in precision.This project researched the on-off valve controlled pneumatic system with a special direction, and got some basic research results. It is still a long way for this project whether in research and application.