corresponding memory address (Figure 1.10). An ON (1) signal to a particular output terminal causes the wired output field device to be energized or turned ON. An OFF (0) signal to the output terminal de-energizes the output field device (Figure 1.11).
Figure 1.10: PLC memory address for output module
Figure 1.11: Signals from PLC to output module
1.4.3PLC Program
The PLC program is the road map of the operation. In most PLC systems, PLC programs are written in ladder diagram format. A ladder diagram consists of two rails and several rungs (Figure 1.12). Two rails, arranged vertically, represent the power lines. Program instructions are arranged along the horizontal rungs. The number of rungs increases as the program becomes larger and complicated. Each input and output instruction is assigned a memory address. A PLC program is implemented in cycles. Each cycle involves three steps:
Figure 1.12: A PLC program
1.Reading the status of input field devices in input modules and writing these signal data to their memory address
2.Scanning the program to update the status of input instructions and placing the output results to their address
3.Sending the control signals to the output modules
1.4.4PLC Scanning
The PLC processor scans a program in cyclic manner. The scanning cycle starts from left to right at the top rung and proceeds to the second rung until reaching the bottom rung to complete a cycle. It then returns back to the top rung to continue the next cycle (Figure 1.13). During scanning, the processor simultaneously updates the status of input and output instructions, in both the program and the memory, as well as interacting with the input and output modules.
Figure 1.13: PLC scanning cycle
1.5 PLC Applications
1.5.1PLCs versus Hard-Wired Relay Systems
PLCs initially were intended to replace hard-wired relay systems. They offer many advantages over their hard-wired counterparts. Table 1.2 compares these two types of control systems.
Table 1.2: Comparison between PLC control and relay control
| Feature | Hard-Wired Relays | PLCs |
|---|---|---|
| Functions | Limited to relay types of control. | Extended and advanced control functions available. |
| Feasibility | Complex systems require many relays to control complicated tasks. | Capable of controlling any complex systems. |
| Flexibility | Need to change wiring when applications change. | Programmable. Simply replace a program for a new application. |
| Reliability | Reliable, but susceptive to poor contact and limited life. | Highly reliable due to semi-conductor devices. |
| Expandability | Hard to expand. | Can be expanded to sizable memory for control program. |
| Maintainability | Requires regular maintenance and inspection. Need to replace components in their due life. | Easy to maintain. Replace single module if needed. |
| Required technical knowledge | Many people know relay logic. | Do not need to understand the hardware. |
| Space requirement | Relatively large. | Relatively small. |
| Required time for design and implementation | Takes time to prepare drawings, installation and testing. | Relatively short. |
| Cost | Cost-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 collection | Cannot 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