Programmable Logic Controllers. Su Chen Jonathon Lin

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СКАЧАТЬ relays to control complicated tasks.Capable of controlling any complex systems.FlexibilityNeed to change wiring when applications change.Programmable. Simply replace a program for a new application.ReliabilityReliable, but susceptive to poor contact and limited life.Highly reliable due to semi-conductor devices.ExpandabilityHard to expand.Can be expanded to sizable memory for control program.MaintainabilityRequires regular maintenance and inspection. Need to replace components in their due life.Easy to maintain. Replace single module if needed.Required technical knowledgeMany people know relay logic.Do not need to understand the hardware.Space requirementRelatively large.Relatively small.Required time for design and implementationTakes time to prepare drawings, installation and testing.Relatively short.CostCost-effective if the number of relays used in the system is less than ten.Cost-effective if the number of relays used in the system is more than ten.Data collectionCannot store the data gathered in the control system for further analysis.Data gathered from the system can be stored in the memory for further use and analysis.

       1.5.2Benefits of Using PLCs

      The architecture of PLCs is modular and flexible in nature, which permits hardware and software elements to be integrated in any combination. They can be uniquely tailored by adding or removing some elements to meet a specific application. The benefits of using PLCs can be summarized in the following five items:

      •Flexible and programmable

      •Ease of installation and implementation

      •Reliable

      •Ease of maintenance and troubleshooting

      •Cost saving

       Flexible and Programmable

      PLCs allow the control systems to be modularly configured to meet specific needs whether they are big or small, simple or complicated, long-term or short-term use. They are programmable so that changes in a control program results in a different application. It is also easy to make any change to the control program without involving much effort in programming and hard-wiring.

       Ease of Installation and Implementation

      PLCs are relatively small size compared to their hard-wired relay counterparts. It takes less than half the space required by its equivalent relay control panel. The amount of wiring is significantly reduced due to the elimination of hard-wired relays, counters, timers, etc. Any changeover can be made readily by connecting the input and output devices to the terminal strips.

       Reliable

      PLC systems are highly reliable because they use solid-state elements that have no mechanical wear, low component failure, and low space and power consumption. They use standard devices and standardized wiring diagrams that eliminate customized interfaces. All of these contribute to them being more reliable systems than their relay hard-wired counterparts.

       Ease of Maintenance and Troubleshooting

      Most system components are solid-state type. Problems with mechanical wear, short-circuiting, and unexpected accidents from wiring and operation mistakes are significantly reduced. Because most system components are solid-state and modularized, maintenance is essentially reduced to replacing plug-in components if needed.

      PLC components normally come with fault detection circuits and LED indicators. They detect any malfunction of the components and give prompt identification of component failures. Modern PLC systems are loaded with diagnosis programs and online monitor systems to show the actual status of each control element. All of these facilitate the troubleshooting of the system when they go wrong.

       Cost Saving

      Generally speaking, when the number of relays used in the system is more than 10, the use of a PLC becomes cost-effective. Today there are many micro PLCs that cost less than two hundred dollars. It is cost-effective using these low cost PLCs to implement those small control systems.

       1.5.3PLC Applications

      Since its inception, the functionality of PLCs has gone beyond simple relay replacement. With their added advanced functions, PLCs have been widely used in almost every sector of industry. Typical applications include:

      •Discrete logic controls

      •Monitoring

      •Continuous control

      •Analog measurement and control

      •Diagnostic information gathering

      •Data logging

      •Production reports generation

      •Communication network

      Table1.3 tabulates some applications in various industries.

      Table 1.3: PLC applications by industry

Industry	Applications
Chemical	Batch processing, blending, off shore drilling, material and product handling, pipeline control, etc.
Food and Beverage	Baking, mixing, blending, brewing, distilling, filling, material and product handling, sorting conveyor control, warehouse storage and retrieval, palletizing and wrapping, etc.
Glass and Film	Cullet weighting, finishing, forming, material handling, packaging, palletizing, etc.
Lumber, Pulp, and Paper	Bark burning, batch digesters, chip handling, coating, cutting, pulp batch blending, wrapping and stamping, etc.
Manufacturing	Conveyor systems, assembly machines, plastic injection molds, test machines, machine tool control, work cell control, etc.
Power	Burner control, coal handling, fuel control, sorting, process control, etc.
Snack Foods	Oven control, batching systems, material handling, cooker systems, extrusion and cutting systems, slurry mixing and distribution, etc.
Fiberglass	Furnace control, batching systems, material handling, forming systems, cutting and slitting systems, binder mixing and distribution, etc.
Material Conveying	Material handling, batching systems, variable speed drives and systems, motor control center systems, slurry mixing and distribution, valve sequencing control systems, and plugged line countermeasure controls, etc.
Extrusion Systems	Forming barrel control systems, variable speed drives and systems, cutting control systems, and batching systems, etc.
Pharmaceuticals	Autoclave control systems, batching operations, vial capper machines, validation reports, R&D operations, vial washing, vial labeling, etc.
Electronic Board Manufacturing	Material handling, material positioning, inventory control system, variable speed drive systems, product conveying systems, machine control systems, etc.
Steel Industry	Material positioning, inventory control system, product conveying systems, variable speed drive systems, sequencer control, pump control systems, valve control systems, temperature control, pressure control, etc.

1.6 PLC Categories

      PLCs are made in various sizes and configurations. They can be classified in six categories according to I/O count, memory size, processor type, function, and application. The various sizes are nano PLCs, micro PLCs, small PLCs, medium PLCs, large PLCs, and very large PLCs. Table 1.4 summarizes these sizes and their PLC categories.

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