(控制系统技术概论47到67页原文及翻译.docxVIP

Chap. 2 Types of Controlmaining operations are all initiated and terminated by a timer. These include the wash opera- j tion，the drain operation, the rinse operation, and the spin-dry operation. Most batch process control systems are time-driven sequential systems. Time-driven systems are described by schematic diagrams and timing diagrams. Schematic diagrams show the physical configuration, and timing diagrams define the sequential operations. The timing diagram of an automatic washing machine is shown in Figure 2.7.　A traffic counter is a simple example of an event-driven system. The counter is placed at the side of the road，and the sensor, which is a long rubber tube, is stretched across the road. Each time a vehicle axle passes over the tubular sensor, the counter increases its count by 1. Thus an event (an axle passing over the sensor) drives the counter. Manufacturing industries are principal users of event-driven sequential controllers. Before 1970，large relay panels were used to control event-driven operations. In 1968，the Hydramatic Division of General Motors Corporation specified the design criteria for the first programmable logic controller (PLC). The purpose was to replace inflexible relay panels with a computer-controlled solid- state system. The project succeeded beyond anyone’s dreams. Programmable logic controllers have gone beyond replacement of relay panels to include PID modules for process control and communications interfaces that make it possible to link programmable controllers into an integrated manufacturing operation.　Event-driven systems are described by ladder diagrams and Boolean equations. The symbols of components used in ladder diagrams are included in Chapter 9. The components most frequently used include switches, contacts，relays, contactors, motor starters, time-delay relays, pneumatic solenoid valves, pneumatic cylinders, hydraulic solenoid valves, and hydraulic cylinders. The pneumatic cylinder in Figure 2.8 is an example of an