What functions does a PLC perform?
3. Explain the main problems with those hardwired relay-based systems.
4. Describe the initial specifications of PLCs proposed by GM’s Hydromatic Division.
5. List the five generations of PLC development and briefly describe each of their contributions.
6. What are the four major components of a PLC system? Briefly describe each.
7. What are the three parts of a processor unit?
8. Describe the functions of a CPU.
9. What are the two types of memory used in PLCs? Describe each type.
10. Explain the purpose of input modules?
11. Use a drawing to help describe the functions of an input module in a PLC project.
12. Use a drawing to help explain the functions of an output module in a PLC project.
13. What are the three types of PLC programming devices? Briefly describe each.
14. List the two types of interface wiring.
15. Use a drawing to help describe the input wiring.
16. Use a drawing to help describe the output wiring.
17. Explain a ladder diagram.
18. Describe the three steps of a PLC implementation cycle.
19. Use a drawing to help describe the PLC scanning process.
20. Compare PLCs and hard-wired relay control systems.
21. List at least five benefits of using PLCs.
22. List five typical applications of PLCs.
23. List the six PLC categories.
Electrical Control Fundamentals
PLCs are a powerful means for implementing electrical control in various processes and control systems. To take full advantage of PLCs, we need to understand the basic electrical control elements, circuits, and systems. Electrical control is extensively used in every aspect of industry, from manufacturing to chemical process to transportation. Electrical control is implemented by placing three types of control elements in a ladder form of control scheme. Ladder diagrams are popular graphic representations of control circuits. They have two vertical rungs representing power lines and several horizontal rungs with control elements. The status of continuity in each rung controls the output element being energized or de-energized.
There are three types of control elements: input, logic, and output. Input devices provide a switching function for generating an ON/OFF signal or connection to the control rung. Logic elements implement logic decisions for the output based on the input signals. Output devices are normally the last element in the control rung. The output device is energized when its control rung has continuity. It is de-energized when the continuity of its control rung is broken.
In some circumstances, we may not have any documentation for an existing electrical control circuit that we need to service or we may have no circuit diagram of an existing circuit with which to work. We need to know how to analyze an existing electrical control system step-by-step, and finally describe what the systems are performing and their operation steps. This chapter concludes with a discussion of how to analyze electrical control circuits.
2.1 Electrical Control Systems
Many control components in mechanical systems and processes are controlled electrically. An electrical control system is the most effective means of operating over long distances and is efficient when low power levels are involved. Electrical circuits often illustrate the control schemes of the electrical control systems. The most common way to present the control scheme of electrical circuits is the ladder diagram. Electrical control circuits arrange three types of control elements in an appropriate form for accomplishing certain control schemes:
•input (information) devices such as push buttons, limit switches, pressure switches, temperature switches, photodetectors, and liquid level switches
•logic (decision) elements such as relays, timers, and counters
•output (action) devices such as solenoids, contactors, motor starters, lights, and alarms
2.2 Ladder Diagrams
In electrical control circuits, symbols are used to represent the control components. The ladder diagram is the most often used circuit diagram for illustrating the control functions. A ladder diagram uses control component symbols and a plan of their connections to show in simple form the scheme of control. It contains vertical rails and horizontal rungs, much like a ladder. The vertical rails represent the source voltage lines whereas the horizontal rungs depict the devices and circuits connected across the line. In ladder diagrams, the source of electrical power energy will always be two vertical lines (or side of the ladder), as shown in Figure 2.1. The power runs vertically from up to down. A horizontal format can also be used (Figure 2.2).
Figure 2.1: Vertical ladder diagram
