Analysis of Pneumatic Servo Load System in Souble-sided Polishing Machine

摘要

The precise double-sided polishing process is one of the main methods to get the ultra-smooth surface of workpiece. In double-sided polishing machine, a loading system is required to be able to precisely control the load superimposed on the workpiece, while the polishing is being undertaken. A pneumatic servo loading system is proposed, which is composed of a low-friction-cylinder, load cell, personal computer, digital servo valve and signal processing and adapting units. The load exerted on the workpiece is measured by the load cell and fed back to the personal computer to undertake closed-loop control through the digital servo valve. In the pneumatic servo system, the servo valve, acting as role of both the electrical to mechanical converter and the power amplifier, plays a key role in the performance of whole servo system. For this reason the pneumatic digital servo valve applied in the pneumatic servo system for eliminating the influence of nonlinearities, such as electromagnetic saturation and hysteresis. In this way, the load exerted on the workpiece is controlled accurately by the application of direct pneumatic digital control technology during the polishing process. The construction of the pneumatic servo loading system in double-sided polishing machine and control strategy associated with the digital servo valve are first addressed. The mathematical model of the system established and the fundamental static and dynamic characteristics of the system is then analyzed though linear system theory. Finally, the experiments are carried out by measuring the practical load on the workpiece and the quality of the surface finish. It is demonstrated that the error rate of load is less than 5% and a super-smooth surface of silicon wafer with roughness Ra 0.401 nm can be obtained.