Why Use PLC?

The softwiring advantage provided by programmable controllers is tremendous (Figure 5 shows a practical industrial example). In fact, it is one of the most important features of PLCs. Softwiring makes changes in the control system easy and cheap. If you want a device in a PLC system to behave differently or to control a different process element, all you have to do is change the control program. In a traditional system, making this type of change would involve physically changing the wiring between the devices, a costly and time-consuming endeavor.
Example:
Let’s say that two push buttons, PB1 and PB2, are connected to a PLC. Two pilot lights, PL1 and PL2, are also connected to the PLC. The way these devices are connected now is pressing push button PB1 turns on pilot light PL1 and pressing push button PB2 turns on pilot light PL2 as shown in Figure 3. Let’s say that you want to change this around so that PB1 controls PL2 and PB2 controls PL1. In a traditional system, you would have to rewire the circuit so that the wiring from the first push button goes to the second pilot light and vice versa. However, because these devices are connected to a PLC as in Figure 4, making this change is as simple as making a small change in the control program.

Figure 3 Hardwired system

Figure 3 Hardwired system

Figure 4 Softwired system

Figure 4 Softwired system

PLCs offer other advantages over traditional control systems. These advantages include:

  • Increased Reliability. Once a program has been written and tested, it can be easily downloaded to other PLCs. The program takes the place of much of the external wiring that would normally be required for control of a process. Hardwiring required to connect field devices is less intensive. PLCs also offer the reliability associated with solidstate components.
  • More Flexibility. It is easier to create and change a program in a PLC than to wire and rewire a circuit. With a PLC the relationships between the inputs and outputs are determined by the user program (Figure 6).
  • Lower Cost. PLCs were originally designed to replace relay control logic, and the cost savings have been so significant that relay control is becoming obsolete except for power applications. Generally, if an application has more than about a half-dozen control relays, it will probably be less expensive to install a PLC.
Figure 5 Relay logic vs PLC

Figure 5 Relay logic vs PLC

  • Communications Capability. A PLC can communicate with other controllers or computer equipment to perform such functions as supervisory control, data gathering, monitoring devices and process parameters, and download and upload of programs.
  • Faster Response Time. PLCs are designed for high-speed and real-time applications (Figure 7). The programmable controller operates in real time, which means that an event taking place in the field will result in the execution of an operation or output. Machines that process thousands of items per second and objects that spend only a fraction of a second in front of a sensor require the PLC’s quick-response capability.
  • Easier to Troubleshoot. PLCs have diagnostics and override functions that allow users to easily trace and correct software and hardware problems. To find and fix problems, users can display the control program on a monitor and watch it in real time as it executes.
Figure 6 Input and output relation is determined by user program.

Figure 6 Input and output relation is determined by user program.

Figure 7 High speed counting

Figure 7 High speed counting

 

 

 

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