Application study of the electrohydraulic system for hot press of wood-based panels

摘要

First, the paper described briefly a traditional control technique of the hot press which accord with traditional technology (See Fig.1) for made in China. The control signals of the hydraulic system come from time relays, pressure gauges with electric terminals, pressure-switches and a pair of stop gauges (stroke dog). The system desn't obtain enough accuracy of position, physical property of products isn't very well. Second, according to new heat pressing technology (See Fig.3), the system must control not only accuracy of positions, must control but also speed of displacement in order to obtain satisfactory products. Since the eighties, the system is improved on some methods. The improvement show mainly as improving of electric equipment, for example, PLC takes the place of relays. Because hydraulic system isn't improved essentially and position is controlled by stop gauge (stroke dog), therefore, the capability of system isn't advanced. It is a conventional Bang-band control system tool. In imported single-opening press line, accuracy of board thickness is controlled by electrohydraulic servo closed-loop control-system (See Fig.4). As a result of this, the error of board thickness can be limited down to +-0.2-0.3 mm. Although the servo-system can obtain higher positioning accuracy, but the environment of the workshop is undesirable for the servo-system. The costing of servo-system is also expensive for consumer. To improving the systems, the paper discussed possiblity which using new Bang-Band control system or proportional closed-loop control-system takes the place of the old Bang-Bang control-system or servo-system in the end.