Electro-Pneumatic Control System with Hydraulically Positioning Actuator Motor

Abstract

A compressibility of the actuating fluid of a pneumatic drive (compressed air) leads to significant landing of the pneumatic cylinder piston at the time of stop and hold of the load, a constant component of which can fluctuate significantly for the holding period. There are a lot of factors, which have a significant impact on the landing value of piston. Those are: an initial position of the piston at its stop, which determines the volume of the an active area of the piston, a value of the constant load component at the time of stop and its variation for the holding period, a transfer coefficient of the position component of the load, an active area of the pneumatic cylinder piston, as well as reduction in atmospheric pressure, which can significantly affect the operation of the control systems of small aircrafts flying at high altitudes. To reduce the landing value of piston due to changing value of the constant load component for its holding period, it is proposed to use a hydraulic positioner, which comprises a hydraulic cylinder the rod of which is rigidly connected to the rod of the pneumatic cylinder through the traverse, a cross-feed valve of the hydro-cylinder cavities with discrete electro-magnetic control, and adjustable chokes. A programmable logic controller provides the hydraulic positioner control. At the moment the piston stops and the load is held the cross-feed valve overlaps the hydro-cylinder cavities thereby locking the pneumatic cylinder piston and preventing its landing. With available pneumatic cylinder-controlled signal the cross-feed valve connects the piston and rod cavities of the positioner hydro-cylinder, the pneumatic cylinder piston is released and becomes capable of moving. A numerical estimate of landing of the pneumatic cylinder piston and its positioning quality is of essential interest. For this purpose, a technique to calculate the landing of piston has been developed taking into consideration that different factors, which determine operation conditions of the pneumatic cylinder, have the impact on it. Experimental investigation of piston landing was conducted using the pneumatic cylinder with piston and rod diameters of 32 and 16 mm, respectively, at operating pressure of 0.8 MPa. With a vertical arrangement of the pneumatic cylinder axis, the control weights of a dead-weight pressure-gauge tester created a constant component of the load and its variation, and an indicator with resolution of 0.05 mm estimated a landing value. The article shows the calculated data of piston landing, which are in good compliance with the experimental results. To substantially reduce the piston landing for the holding period of the load varying in value and sign was designed a hydraulic positioner of pneumatic cylinder piston, and its operation quality is studied on the physical layout of electro-pneumatic control system operating in the discrete and tracking modes. For control system service, work programs of the programmable logic controller integrated in the control circuit of the hydraulic positioner in discrete mode, and, as a digital discriminator, in tracking mode are developed. In conclusion as a result of the research activities the article estimates the impact of factors on the quality of the load position at the time of its hold and notes a high efficiency of the hydraulic positioner of pneumatic cylinder piston both in discrete mode and in tracking one.