Programmable Logic Controller Basics

Programmable Logic Controllers (PLCs) are widely used in the industrial applications for control the process systems  They are planted at the field terminals of the process. Programmable Logic Controllers (PLCs) are micro-controller devices to control the process by performing certain desired functions such as closing or opening valves, starting and stopping motors. PLCs typically uses programmable memory to store the commands or instructions which will be fed by the programmer to carryout desired operation and implements logical, arithmetical and other  functions to control the process.

The term 'Logic' in PLC is used because the programming implemented in PLC is Logic based so as to carryout certain control functions like switching on or off, closing the valves.

Input devices connecting the PLCs are sensor devices such as switches, relays and corresponding output devices will be valves, motors. These inputs and output devices are connected to the PLCs. Programmer inputs sequence of commands and programs in to the memory of the PLC. PLCs controller monitor the inputs and accordingly takes output control functions as per the instructions and programs  provided in the PLC

Computers and Programmable Logic Controllers are almost similar but computers are meant for calculations and display functions on the other hand PLCs are dedicated to carryout logical operations and execute control functions in Industries.

SCADA Hardware System Industries

SCADA HARDWARE:

SCADA is both Software and Hardware. SCADA hardware includes all the hardware equipment required  in the SCADA system for Data Acquisition, communication of the data between the master station and field devices, Master control station hardware and other display units and printing devices for Human Machine Interface (HMI).

Some of the Hardware components employed in the SCADA systems are listed below:

Remote Terminal Units (RTUs): Remote Terminal Units (RTUs) are the micro-controllers present at the remote side or field of the process system. They acts as an interface between the field analog and digital sensors located at the remote end.

Communication System: Communication system plays an important role in connecting the field devices and the control station. Communication system can be of fiber optic cable, wire, radio or even the satellite connecting the master station and the remote equipment 

Master Stations: These Master Stations gather the data from the field devices (RTUs or PLCs) and process the data and display the processed data to human operator. This include data processing computer systems.

Hardware components in SCADA is detailed as

• Field devices and transducers
• RTUs or PLCs
• Communication system between the Master Station and Remote devices
• Central Control Station
• Data Processing computers.

SCADA systems Fundamentals Industries

SCADA systems:

SCADA in automation world refers to Supervisory Control and Data Acquisition. As the name suggests SCADA role is to acquire the data from the wide area of process system and perform the control function if required. In simple SCADA master station receives the data from the remote terminals which exists in the field and takes control action if required, it also display data in the form of plots, tables, and other display modes to the operator to permits human to take the control actions. 

In modern day industrial growth process system of plant or industry spreads in wide area. There is every need to connect the process systems exists in the remote location to main control center for better control of the system and increase the reliability. Oil pipe lines laid over hundreds of kilometers are to be monitored constantly and takes control actions if required (eg: Closing or opening of valves of the pipes). Hence telemetry is often required to connect the equipment and the process systems which are separated by far distance.  Telemetry helps in receiving the information from the field locations, sends the required commands and programs. 

Hence SCADA is a combination of telemetry and data acquisition. It collects the data from the field and stores in the RTUs (Remote Terminal Units). Transfers the data from RTUs to master station on request. This received data is processed and displays to the operator on screens allowing the operator to take control actions. The required control instructions are transmitted back to the field to carryout control functions (eg: opening or closing the values)

In earlier days hard wired relay logic is used for control functions. With the advent of digital computers and electronic devices hard wired relay logics are replaced by digital electronics  incorporated in to the relay logic devices. Hence Programmable Logic Controllers are being employed in modern day SCADA