Control Systems

For improving productivity and the quality of end products, most of the industries are equipped with advanced control systems by adopting complex control strategies for both critical and non-critical applications.

These modern industrial control systems are usually made to produce one or more control actions depending on the application implemented. This article describes the basic control strategies used in industrial control system and its types with a brief explanation.



Introduction to Industrial Control Systems

Industrial process control system consists of four basic elements:

  1. A measurement to know the status of the condition of a process
  2. A controller to take action by considering the set value and measured value
  3. An output signal to manipulate the process that results from the controller
  4. The process itself that reacts to the signal (Input or output)

The two most important signals considered considered in industrial control systems are

  • Process Variable (PV)
  • Manipulating Variable (MV)

Process variable (PV) is an input signal to the automated controller, which is measured by field devices. Depending on this value, the controller action is decided by either the operator manually or through the automatic controller.

The manipulating variable (MV) is a variable to be manipulated in order to have control over the process variable. Suppose, for a particular flow control operation, the flow control valve position is the manipulated variable to control the flow rate (process variable PV).

The nature of the industrial control system is shown in the above figure wherein the process is controlled through sensory, controller and actuator systems. The controller gets the input signals from the process measurement devices like sensors. These values are compared with the set point values in the controller and the resultant is used to control the actuator in order to produce control input to control the process as shown in the figure.

Basic Control Strategies Used in Industrial Control System

How well the process parameters are controlled depends on the control strategy implemented for that process. Basic control strategies used in most of the industries are

  • ON – OFF Control
  • Open-Loop Control
  • Feed- Forward Control
  • Closed-Loop control

ON – OFF Control:

It is also called as two- position control system that has only two states fully on and fully off. It is the oldest type of control strategy that gives simple on- off mechanism for the set limits. When the process variable (PV) or measurement is below the set limit, then the controller is switched ON and the manipulated variable (MV) changes to maximum value. Similarly, if the PV is above the set limit, then the controller gets turned OFF and the MV changes to minimum value.

But, practically ideal on-off controller is not recommended since it is subject to process disturbances and other electrical interferences. To prevent this, on –off hysteresis or differential is added to the control function.

Open-Loop Control:

In this control structure, control action is not a function of process variable or any load changes. This is also not a self-correcting one. This control scheme independently calculates control output needed to the actuator to produce the desired response. But this system never knows the status of actuator output or process condition and also controller doesn’t consider any disturbance inputs in the process. That’s why this is limited for most of the applications.

Feed-Forward Control:

This control scheme is implemented to reduce the effect of input disturbance on process. A sensor in this system detects the disturbances or any load changes in the process, and then controller calculates the necessary signal to the actuator. This allows the controller to take corrective action before the disturbance effect the process. But this control scheme is more expensive and complicated and requires a better understanding of the process to implement control logic.

 Closed-Loop Control:

Closed-loop control scheme is widely implemented in many of the industrial control applications. It uses the output measurement as feedback signal; and, it is compared with desired or set-point value. The difference between these actual and desired output is called error. Thus, the controller gives the control input to the actuator till the error gets minimized so that the output is maintained at the desired value as shown in  the figure. Most of the closed loop control systems are implemented with PID controller for accurate and reliable control.

Types of Industrial Control systems

Depending on the functionality and complexity of the control action, industrial control systems are differentiated into several types, but most commonly and widely used control systems are of three types:

  1. Programmable Logic Controllers  (PLCs)
  2. Distributed Control Systems (DCS)
  3. Supervisory control and Data Acquisition (SCADA)

Programmable Logic Controllers (PLCs)

As an industrial computer, Programmable Logic controller (PLC) is capable of being programmed to perform various industrial control applications. PLC is built with modules like CPU, power supply, I/O modules (both digital and analog) and communication modules. These PLCs can be modular or integrated types. A modular type PLC is fixed and compact one with limited I/O functionalities whereas an integrated PLC is capable to extend I/O modules based on the required I/O functionalities.

The sensors are connected to the input module and output devices or actuators to the output module of the PLC. The CPU of the PLC continuously reads the inputs from the sensors and according to the program, it produces the outputs to operate the actuators.

Distributed Control Systems (DCS)

Distributed Control System is a specially designed industrial control system which is used to control complex and geographically distributed applications. As the name implies, these controllers are distributed in the entire plant area. The discrete field devices such as actuators and sensors are connected to these controllers and also maintains the continuous communications with operating PCs through a high speed communication network or bus.

The basic elements in the DCS control systems is an engineering PC controls all the distributed controllers, which controls the all the field devices connected to it. The HMI operating station used to monitor the field parameters graphically and to log the data and communication media to establish data transfer between the controllers and operating stations. DCS facilitates the human machine interface (HMI), trend display and face plates for the effective monitoring of industrial processes.

Supervisory Control and Data Acquisition (SCADA)

A SCADA system is a type of industrial process automation system that collects data from various instruments which are located in plant area or even at remote sites. This data is further processed at a central location for monitoring and controlling purpose. Based on the information from various remote stations, automated supervisory commands are sent back to these remote stations to control the field devices. Thus the remote control operation of equipments is possible with this SCADA.

SCADA gathers data using both hardware and software systems. It consists of Remote Terminal Unit (RTU) and Communication Terminal Unit (CTU) or Master Terminal Unit as hardware devices. The RTU is nothing but a programmable logic controller which collects the data from various field instruments and sends it to the CTU to transfer to remote location as shown in the figure. At receiving side, CTU with SCADA HMI software displays all these parameters statuses and allows the user to send control signals to plant area. Apart from the industrial applications, SCADA is also popularly used in power transmission systems.

Hence the industrial control system is a multi-discipline system, which deals with disciplines like control systems, communication, instrumentation, electronics and electrical systems. Apart from the discussed control strategies some more advanced-control strategies are also used like cascade and ratio-control schemes. For any further discussions on this topic, you can leave a comment in the comment section.