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1、外文原文Programmable Logic Controllers versus Personal Computers for Process ControlIndustrial Engineering DepartmentUniversity of Puerto RicoP.O. Box 5000Mayaguez, Puerto Rico 00681ABSTRACT It is proposed that Personal Computers (PCs) can be used effectively for the same industrial applications as Prog
2、rammable Logic Controllers (PLCs). The basic concepts related to the operation of a PLC to emulate the behavior of a relay panel are explained. This is used to build a simple model for using standard PCs in the same applications as PLCs. This is demonstrated using a simple example of an automated pr
3、ocess, Sample code in a standard high level language is presented that can be used as a template for future developments. Finally, the potential advantages and enhancements that can result in using PCs for process control are presented.Keywords: Programmable Logic Controller, Personal Computer, Auto
4、mation, Ladder Logic, Relay PanelsINTRODUCTION Personal Computers (PCs) can be used effectively for the same applications as Programmable Logic Controllers (PLCs). A modem PLC is a computer-based device designed to control a process. It relates information coming from sensors that monitor the state
5、of a process, with the status of some actuators that are capable of changing it. This relationship is established in most cases in Boolean logic. Typical sensors used in industrial applications include limit switches, proximity sensors, and other binary sensors. Actuators may include solenoids, moto
6、r starters, and other similar devices. Figure 1shows a simplified model of an industrial application where a PLC might be used. Fig. 1: Model of Simple ProcessRELAY PANELS PLCs wore designed to replace relay panels. These are custom made controllers dedicated to a particular application. They can be
7、 expensive for complex systems, cannot be easily reconfigured, are difficult to troubleshoot, consume lots of energy, have a relatively moderate speed of operation, and have low reliability. Relay panels are not the most suitable alternative for a moderate to complex industrial application where fle
8、xibility, ease of maintenance and troubleshooting are very important. On the other hand, they are relatively low-tech and are easy to understand by electricians and non-engineering personnel.The electrical control circuits of relay panels are generally drawn using so-called electric ladder diagrams.
9、 They differ slightly from conventional wiring diagrams in that they do not show the physical arrangement of the components, but emphasize the function of each circuit. They are a set of parallel circuits that in essence represent a hardwired program that controls the sequence of operations in a giv
10、en process. Being electrical circuits in parallel gives the advantage of solving all the control logic simultaneously and practically instantaneously. Figure 2 represents an electric ladder diagram for a relay panel that can be used to control the process presented in fig. 1. Fig. 2: Electric Ladder
11、 Diagram PLCs are typically computer-based, solid-state, single-processor devices that emulate the behavior of an electric ladder diagram. Since they are sequential machines, to emulate the workings of parallel circuits that respond instantaneously, PLCs use an input/output image table and a scannin
12、g cycle. An input/output image table is a memory structure that stores all the relevant information about the current scanning cycle. It can be subdivided in three basic parts: the input table, the output table, and the internal relay table, The current state of the inputs (sensors) is kept in the i
13、nput table; the desired state of the outputs (actuators) is kept in the output table; and the state of the virtual control relays is kept in the internal relay table. When a program is being nm in a PLC it is continuously executing a scanning cycle (fig. 3). The scanning cycle has two major parts (i
14、n an actual PLC it has other parts but two are relevant here for the sake of the discussion): the input/output scan, and the program scan. In the input/output scan the current state of the inputs is read from the input points and stored in the input table, and the desired state of theFig. 3: PLC Sca
15、nning Cycleoutputs (from the output table) is sent to the output points. The program scan solves the Boolean logic that relates the information in the input table, with that in the output and internal relay tables. Also, the information in the output and internal relay tables is updated during the p
16、rogram scan. In a PLC this Boolean logic is typically represented in a graphical language that looks very much like the electrical circuit that it emulates. This language is known as ladder logic. In fig. 4 there is a ladder logic program that can be used to control the process in fig. 1. Note the similarities with the electric ladder diagram in fig. 2. The advantage of the PLC scanning cycle scheme is
