December 25, 2019 by Sam Holland
Automated process control is an integral part of modern industry. For example, automation allows integrated circuit manufacturers to both minimise errors during the fabrication process and improve production efficiency—ultimately enhancing the quality of the final products.
To achieve automated process control, industrial processes must carry on in a precise, predictable, and repeatable manner. In practice, this requires a controller that monitors a process variable and compares it to a setpoint or reference value.
The difference between the value of the process variable and the setpoint (i.e. the ideal condition of the process variable) is defined as an ‘error signal’. This sends some feedback to the controller to apply an action to return the process value to the ideal setpoint.
Fundamentally, control and automation engineering (or C&E) is an electrical engineering subfield that deals with the design, development, and overall operation of hardware and software elements for industrial process control.
An engineer amongst control automation machinery. Image Credit: Fagor Automation via Flickr.
A control system is a network of electrical and/or electromechanical devices used to regulate the behaviour of dynamic process systems via control loops. There are two main types of control action: open-loop control and closed-loop control.
In an open-loop control system, the ‘corrective’ action of the controller is not dependent on the value of the process variable. For example, a timer used to switch a heating element on or off is open-loop, given that the timer will switch the device to ‘OFF’ at a specified time, regardless of its temperature.
Conversely, for a closed-loop control system, the controller action relies on the feedback signal from the process.
The main control loop types utilised in industries include supervisory control and data acquisition (SCADA) systems, industrial control systems (ICS), or distributed control systems (DCS).
The next three sections cover more on control loops.
SCADA refers to computer applications designed for remote process control. Although SCADA systems run on proprietary software, they may have hardware components. The hardware component gathers information on a given process and transmits it to a computer that is running the SCADA software in question.
An engineer working in the control room of a power plant. Image Credit: Pixabay.
Such a program logs all events into a local memory file and continuously monitors the system. When an abnormal condition occurs, it is made to alert the engineer in the control room (example pictured above) so that they may fix the problem manually.
Industrial Control SystemAn ICS is the integration of hardware and software devices via wired or wireless networks to control critical industrial processes. Notable applications for ICS include power, electronic component manufacturing, and the industrial IoT. Most ICSs utilise proprietary control software with integrated network security protocols.
DCSs are computerised control systems for plants or large facilities: they comprise several control loops with distributed, dedicated controllers (rather than a central control system).
DCSs are utilised in industrial applications, such as oil and gas refineries, heating ventilation and air conditioning (or HVAC), food processing plants, sewage treatment plants, and more. Like SCADA systems, distributed control systems have both software and hardware components.
A control board for power automation. Image Credit: Pixabay.
Depending on the application, many kinds of equipment are utilised for process control. Six examples of such equipment are covered in the next five sections:
PLCs are portable and ruggedised industrial computers programmed to carry out a specific task. They consist of a CPU, input and output assemblies, computer software, power supply, and a rack assembly.
PID technologies are industrial control devices whose controllers utilise proportional, integral, and derivative controls to adjust a process variable to the desired setpoint.
PIDs are used to regulate such variables as temperature, flow rate, and pressure in closed-loop applications (for instance, automobile assembly plants and wastewater treatment facilities).
Industrial sensors are devices that detect physical stimuli, such as light, heat, motion, mechanical pressure, before responding accordingly (e.g., by displaying a value on an LCD screen, actuating a switch, etc.).
Control systems utilise a variety of sensors to regulate industrial processes by linking them to control devices. For example, a temperature sensor can detect an abnormal process temperature and actuate a relay to shut down what may otherwise become overheated machinery.
An electromechanical relay. Image Credit: Pixabay.
IEDs are microprocessor-based devices that are widely utilised in power automation systems. Such devices typically have power monitoring, control, metering, and communications features and comprise both software and hardware components.
Common applications for IEDs include smart grids and power distribution stations.
RTUs (aka telecontrol units) are also, like IEDs, microprocessor-based devices. They interface between various hardware elements of ICS or SCADA. Similarly to PLCs, they contain a dedicated processor, memory, and power supply and can act as independent computers.
RTUs allow for both the remote control and monitoring of devices and equipment for automation via sensors and digital input systems, before transmitting the resultant data to a central monitoring system.
In view of all of the above technologies, it’s clear that—in all kinds of industries that utilise automation—control systems are critical to ensure that dynamic processes carry on in keeping with the required condition mentioned earlier: a precise, predictable, and repeatable state.
Such technology is vital to ensure that control systems engineers can design, test, and ultimately operate the electrical and electromechanical equipment utilised for automated process control.
