机电一体化专业英语教学课件 作者 黄星 夏玉波 10.pptVIP

Part I Technical and Practical Reading Passage A Programmable Logic Controller (PLC) This section on programmable logic controllers illustrates just a small sample of their capabilities. As computers, PLCs can perform timing functions (for the equivalent of time-delay relays), drum sequencing, and other advanced functions with far greater accuracy and reliability than what is possible to use electromechanical logic devices. Most PLCs have the capacity for far more than six inputs and six outputs. The following photograph shows several input and output modules of a single Allen-Bradley PLC (Fig. 1). With each module having sixteen points of either input or output, this PLC has the ability to monitor and control dozens of devices. Fit into a control cabinet, a PLC takes up little room, especially considering the equivalent space that would be needed by electromechanical relays to perform the same functions (Fig. 2). One advantage of PLCs that simply cannot be duplicated by electromechanical relays is remote monitoring and control via digital computer networks. Because a PLC is nothing more than a special-purpose digital computer, it has the ability to communicate with other computers rather easily. The following photograph (Fig. 3) shows a personal computer displaying a graphic image of a real liquid-level process (a pumping station for a municipal wastewater treatment system) controlled by a PLC. The actual pumping station is located miles away from the personal computer display. The characteristic structure of programmable logic controllers is illustrated in Fig. 4. Provision is made to accept information from various input switching devices such as push-buttons, limit switches, and relay contacts. Also provided are terminals to which output devices such as solenoids, relay coils, and indicator lights can be connected. There are no direct-wired connections between the inputs and outputs. Instead, the switched conditions of the inputs are c