PLC and SCADA – Tools of Automation

plc scadaIndustrial automation is a phrase we often hear, when reducing labor cost and increasing production volume are the goals of all manufacturing companies. But what do we mean by automation? An industrial manufacturing setup involves different processes which have to be controlled as per a particular sequence, and need to be monitored 24/7.These operations can either be done by deploying manpower or can be automated by employing control systems, information technologies, machines, and automation devices like PLC and SCADA. These automation devices can be used for controlling Variable frequency Drives (VFDs), industrial processes, machinery and factory assembly lines. Replacement of human interference and operator may result in substantial cost reduction in the long run, can lead to a steady and fast rate of production, may be good substitutes for humans in hazardous conditions, and more apt for monotonous human work.

PLC and SCADA – What are they?

What is a PLC?

A PLC or a Programmable Logic Controller is a microprocessor into which a program is fed so that it can control several functions in industrial processes. The functionality of the PLC has evolved over the years to include sequential relay control, motion control, process control, distributed control systems and networking. They were initially invented to replace the electromagnetic relays and the cumbersome wirings in a control circuit. The data handling, storage, processing power and communication capabilities of some modern PLCs are approximately equivalent to desktop computers. The logic or sequence of operation for a process is executed as per a control logic program or software. But it differs in some ways from a computer. Unlike desktop computers, PLCs have multiple inputs and outputs, and are designed for the rugged operation under extreme industrial conditions. They operate under extended temperature ranges, have immunity to electrical noise, and have resistance to vibration and impact.

Main Units of a PLC

A PLC consists of the following parts:

  • CPU (Central processing unit)
  • Input modules
  • Output modules
  • Power supply unit or module
  • Programming device

CPU: CPU or the Central Processing Unit is the heart of the PLC system. It executes the control instructions of the control logic program. The CPU reads input data from various sensing devices, executes the user program from memory and sends appropriate output commands to control devices.

Input modules: These are used for interfacing between input devices (such as start and stop push buttons, sensors, limited switch, selector switch) and microprocessor. The input devices which are in the field or remote locations are hard-wired to terminals on the input modules.

Output module:

Output devices which are located in the field or remote locations such as small motor, motor starters, solenoid valve, and indicator lights are hard wired to the output modules.  The output interface modules convert the controller or processor signals into external signals which are used to control the output devices.

Power Supply Module:  This module provides the direct current (DC) power source required to produce low-level voltage used by processor and I/O modules.

Programming device: This is a device, which is usually a personal computer, used to enter instructions or software program into the PLC.

What is a SCADA?

In an industrial scenario, routine supervision of machinery and industrial processes can be an excruciatingly tiresome job. A 24×7 supervision of assembly line equipment, checking their temperature levels, water levels, oil level and performing other checks need lot of time, manpower, and investment. So in the era of automation, engineers have devised the SCADA or Supervisory Control and Data Acquisition system which consist of equipments and sensors that monitor the field equipment from one central location and involve less human intervention for routine checks.

The SCADA systems perform data collection and control at the supervisory level. Some systems are called SCADA though they only perform data acquisition and do not control equipment. SCADA is basically a software tool which helps you to view, monitor, control your process variable data and provides a graphical representation of your plant and processes on a front end display before the operator stationed in a control room.

Elements of SCADA Systems

SCADA monitors, controls and raises alarms from a centralized location. The SCADA system handles communication of information between a SCADA central host computer, and other scattered units and PLCs. For example, in a vacuum degassing plant, the remote units measure the pressure in the system, and report the readings to the central computer located in the control room. In case of any discrepancy, the SCADA system would show the errors on the control screen, raise an alarm, and if required shutdown certain machineries and take other corrective actions. A SCADA system consists of the following elements

  1. SCADA Master Station Computer Systems: It is the repository of the real-time or near real-time reported data collected from the remote terminal units connected to it. The back end SCADA software must be able to collect data values from the RTUs, should have software for their retrieval, storage and processing. The processing may include unit conversion, cataloging into tables etc.
  2. Human-Machine Interface (HMI): The values and data collected and stored are presented to the human operator in the form of understandable and comprehensible HMIs. This includes easy-to-understand screen layouts, pictorial representation, detailed schematics, and animations representing the running states and health of the field equipment and machines.

Conclusion

In the past, Supervisory Control and Data Acquisition (SCADA) functions were primarily performed by dedicated computer-based SCADA systems. Whereas these systems still do exist and are widely used in industry, the SCADA functions can increasingly be performed by TCP/IP/Ethernet-based systems. The advantage of the latter approach is that the system is open; hence hardware and software components from various vendors can be seamlessly and easily integrated to perform control and data acquisition functions. However, stand-alone SCADA systems are still being marketed.  Thus as PLCs and SCADA systems continue to be lucrative automation devices,  several companies like Allen Bradley, Delta, Siemens, and Schneider have their own set of PLC, SCADA, HMI products and their own proprietary software programming language.