Industrial Automation using SCADA, HMI & PLC

Vandana R Mesta is an Electronics and Communication Engineer from A. I. T., Chikmagaluru. She has worked as Lecturer at R.N.Shetty Rural Polytechnic, Murdeswar for 5 years

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With the introduction of personal computers (PC) in 1980s, industrial automation was revolutionised. Industrial Automation is the use of automated control devices such as Programmable logic Controller (PLC), Computer Numerical Control (CNC), Remote Terminal Unit (RTU) etc to control industrial processes and machinery in place of labour intervention and dangerous assembly operations with automated ones.

There are various process variables such as temperature, flow, pressure, distance and liquid levels that can be sensed simultaneously, processed and controlled by complex microprocessor systems or personal computer (PC) based data processing controllers. They are designed in such a way that they can withstand temperature, pressure and other extreme environmental conditions yet be accurate, efficient, giving timely response and economical.

Functional Elements of Industrial Automation

An Industrial Automation system consists of various elements in synchronisation with each other performing functions like sensing, controlling, supervision and monitoring of industrial processes. The following figure shows various levels of functional elements of industrial automation system.

Different functional levels of industrial automation
Fig. 1: Different functional levels of industrial automation
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Functionally, an industrial automation system consists of Field level (Sensors & Actuators), Control level, Supervisory, production control level (SCADA) and Information & enterprise level (MES & ERP)

Field level

At the lowest level we find sensors and actuators which are installed at the field. Sensors are used to collect physical process signals and measurements such as flow, pressure, temperature etc. and convert them into electrical or pneumatic form. For example, LDR, thermocouple, strainguage etc. The signals from these sensors are used for processing, analysing and decision making to produce the control output. Control Systems like Programmable Logic controllers(PLCs), RTUs etc are connected to these sensors to produce the required output by processing the signals in accordance with the program and set values. Then the controllers produce the computed output and are applied as electrical or pneumatic signal inputs to the actuating elements. Actuators convert the electrical or pneumatic signals into the physical process variables such as controlling valves, relays, pneumatic actuators, DC motors etc.

Control Level

This level consists of control elements, microprocessor based electronic controllers or simply industrial computers that accept the signals from various sensors as well as command signals from human operators. These controllers can be analog or digital systems depending on the structure of control nature. Signals from sensors are processed by controller producing the control output to various actuating devices depending on the control structure. Out of available modern control devices, PLCs are most widely used robust controllers in industrial automation system which can deliver automatic control functions based on input from sensors.

A programmable logic controller (PLC) is a special form of microprocessor-based controller that uses programmable memory to store instructions and to implement functions such as logical, sequencing, timing, counting and arithmetic to control machine and processes. PLCs have rugged CPU, digital I/O, analog I/O and communication modules such that they can operate at industry environment conditions to control the various process parameters using the program written in accordance with the industrial process. They are programmed using application software on personal computers. The software allows users to create, edit, document, store and troubleshoot programs. The most commonly used programming language for PLC is ladder programming, which is easier for any operator to understand, operate, write and debug.

Example of PLC - BoschRexrothX5-w
Fig. 2: Example of PLC – BoschRexrothX5-w
Implementation of PLCs in an industry
Fig. 3: Implementation of PLCs in an industry

Programmer writes, debugs and installs the program into PLC. Then PLC performs a repetitive cycle of operations called program scan. Wherein it scans the input devices and updates a memory table indicating their status. Next, the PLC executes its ladder program and updates a memory table which indicates whether output devices should be on or off. Depending on this output table, PLC changes the condition of the output devices.

Examples of PLCs are Allen Bradley PLC 5, Allen Bradley SLC500, SIMATIC PLC from Siemens, Omron Plc, ABB plc etc.

PLCs come with advantages like flexibility, easy implementation of changes and correcting errors, enormous quantities of contacts, low cost, pilot running, visual observation, easy programming method, reliability and maintainability and documentation. Only disadvantage in using PLC is that it makes use of fixed programs for applications and in some cases fail safe operations can be possible only through wired control.

Supervisory control and production control level

Next level of control is supervisory control, which controls the smaller subsystems. It consists mainly of process computers and Human Machine Interfaces(HMIs). These personal computers perform elevated level control operations like set point computations, performance monitoring, diagnostics, start-up, shut down and other emergency operations. Most of the system contains Distribution Control System (DCS) or Supervisory Control and Data Acquisition (SCADA) and Human Machine Interface (HMI) in this level.

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