multiplies your programming options.
Table 1.9: Omron PLCs
| PLC Model | Feature |
|---|---|
| Compact CP1 series | Designed for compact machines, the CP1is the advanced high-speed all-in-one compact PLC. Four high-speed counters and four pulse outputs are ideal for multi-axis positioning control. The CP1H can be expanded with different I/O. |
| Modular CJ1 | The smallest members of the CJ1 family are fully upward-compatible to the CJ1G/H and CS1 series regarding instruction set, communication commands, and memory organization. Even small machines can be built in a consistently modular way, reducing the cost to enhance, expand or customize machines without a complete redesign of the control system. |
| CJ2 series | The CJ2H series is ideal for advanced machine automation needs, such as those required in image processing inspection of electrical components and high speed sorting on conveyors. It has special instructions that provide direct data access to high-speed analog I/O units and serial communication units. Position control units can be synchronized for coordinated control of up to 20 axes. |
| Rack CS1 | CS1 is Omron’s most extensive PLC family, with a maximum capacity of 5120 local digital I/Os. Up to 7 expansion racks can be connected to a single CPU rack, bringing the maximum number of I/O units to 80. Any combination from over 200 models of digital I/Os, analog I/Os, control units, and communication units can be mounted in any order. |
Review Questions
1. What is a PLC?
2. 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,