relays to control complicated tasks.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.
