beautiful world, wonderful life, precious knowledge, awesome me: CHAPTER 12

Introduction - PLC Hardware Components

In automation industry, PLCs have changed the way we work and run several tasks. Everything is made easier with the use of PLCs. So what should we know about PLCs? PLC is known as a control device as it takes information from the inputs and then makes decisions to do some tasks. All the decisions are made based on outputs and inputs. In PLCs, ladder logic program is known as the most common programming method used in PLCs. Before going further, let’s learn more about PLC hardware components.

PLC Hardware Components and Functions
Talk about PLC hardware components, we should know that a PLC consists of some components such as:

•Memory

•Central processing unit

•Power supply

•Input modules

•Output modules

Each component in a PLC has different functions just like other computerized devices. The main component which controls the whole system is known as central processing unit. Here are some functions from CPU:

•It performs arithmetic and logic operations

•It will update outputs and inputs

•It communicates with other component known as memory

•It will scan the application programs

•Communicating with a programming terminal

The next component is memory which will be responsible to store information, programs and data in a PLC. ROM and RAM are the most common types of memory used in PLCs. In a PLC, the process requires both programming software and a programming terminal for the operation. Aside from memory and CPU, there are also PLC hardware components we should understand.

PLC Hardware Components for the Operation
It is important to understand the function of PLC hardware components. As we may already know, there are several things we should learn when it comes to discussing about PLCs. The other components we should know are input modules, output modules, and power supply. There are some common input devices that will be used in a PLC such as relay contacts, limit switches, proximity switches, photo sensors, and temperature sensors. A PLC can be used to control other devices such as fans, lights, alarms, relays, and motor starters. CPU, input modules, memory, output modules and power supply are some major PLC hardware components we should know first.

Basic PLC Operation

How simple can process control be? Consider a common household space heater.

The heater's components are enclosed inside one container, which makes system communications easy. Expanding on this concept is a household forced-air heater with a remote thermostat. Here the communication paths are just a few meters and a voltage control is typically utilized.
Think now beyond a small, relatively simple process-control system. What controls and configuration are necessary in a factory?
The resistance of long wires, EMI, and RFI make voltage-mode control impractical. Instead, a current loop is a simple, but elegant solution. In this design wire resistance is removed from the equation because Kirchhoff's law tells us that the current anywhere in the loop is equal to all other points in the loop. Because the loop impedance and bandwidth are low (a few hundred ohms and < 100Hz), EMI and RFI spurious pickup issues are minimized. A PLC system is useful for properly controlling such a factory system.