Pergamon Computers ind. Engng Vol. 33, Nos 1-2, pp. 421-424, 1997 © 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0360-8352/97 $17.00 + 0.00 PII: S0360-8352(97)00127-7 Programmable Logic Controllers versus Personal Computers for Process Control Agust/n Rullhn, Ph.D. Industrial Engineering Department University of Puerto Rico P.O. Box 5000 Mayagiiez, Puerto Rico 00681 ABSTRACT It is proposed that Personal Computers (PCs) can be used effectively for the same industrial applications as Programmable 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 process. 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. © 1997 Elsevier Science Ltd Keywords: Programmable Logic Controller, Personal Computer, Automation, Ladder Logic, Relay Panels INTRODUCTION 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 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, motor starters, and other similar devices. Figure 1 shows a simplified model of an industrial application where a PLC might be used. RELAY PANELS PLCs were designed to replace relay panels Is1. These are custom made controllers dedicated to a particular application. They can be expensive for complex systems, cannot be easily reconfigured, are oo,,o,. ' ,u. ta Fig. 1: Model of Simple Process 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 flexibility, ease of maintenance and troubleshooting are very important. On the other hand, they are relatively Iow-tech and are easy to understand by electrieian_s and non-engineering personnel. The electrical control circuits of relay panels are generally drawn using so-called electric ladder diagrams. 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 given precess. 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. 421422 21st International Conference on Computers and Industrial Engineering PROGRAMMABLE LOGIC CONTROLLERS 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 scanning 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 input 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 run in a PLC it is continuously executing a scanning cycle (fig. 3). The scanning cycle has two major parts (in 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 the I-I START-P3 I.SI-CR LZ -.1-- L$2 o- ~nt O-- L$1--I~ O- Fig. 2: Electric Ladder Diagram outputs (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 ¢ Fig. 3: PLC Scanning Cycle information in the output and internal relay tables is updated during the program 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 that it allows multiple processes to be controlled concurrently as in a relay panel. PLCs overcome all of the relay panel shortcomings and currently are the most widely used industrial automation controllers. At the time of their introduction they were very successful because their language, ladder logic, was based on electric ladder diagrams which engineers and electricians of the time were already familiar with[5]. Nevertheless, PLCs have some shortcomings of their own that are evident when compared with other available technology. Some of the most limiting ones include: there is no industry-standard hardware or software platform, they may limit the programmer as to the control actions and manipulations that can be made, and offer relatively low computing power for the money. PERSONAL COMPUTERS Another technology that could be used for the same purpose is the standard personal computer
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