What is a Control System?
Controlling the systems is one of the most, if not the most, basic and fundamentals desires of human beings. According to the requirements, changing and moulding things by controlling certain parameters remained to prevail throughout world history. A system of controlling events and output of a system to get a specific system response is called “Control System”.
In the modern world, controlling the parameters of complex processes to get the desired form of a product is no longer an impossibility. In industry, we repeatedly need to control the parameters of a process according to demand and industry requirement. Most of the time parameters are not in working range and accorded with what required for getting desired output e,g. temperature, pressure etc. are controlled and kept in range for manufacturing of a certain product. So, continuous monitoring and control of process parameters are highly needed.
Basic Terminologies & Concepts
Control systems provide required response using control loops. So, there are some terminologies related to control loops needed to be understood well in order to grasp the concept of a control system:
A flow of activities or steps performed in a given order or sequence to control material for its conversion into a final product.
Physical quantities or parameters related to the process used to indicate its status e.g., temperature, pressure, level, flow etc.
Output variables to be monitored in a process are known as measured variables. These need to be measured accurately with sensors and controlled precisely to keep the process within set limits.
Variables are varied or manipulated by the controller to control the measured variables and keep them in the given range.
Reference Value/Set Point
Value of measured variable or output variable required or desired in a process, also known as a set point.
Any device used for measuring the amplitude of the controlled/output variable. It can sense or detect a physical quantity and gives an output which is some function of the amplitude of physical quantity.
Transducers convert output parameter or sensor output into a form that can be easily used for further conditioning and monitoring. For example, controlled parameters are often converted into electrical signals by transducers.
Measured variables after sensing and converting into another form need to be transmitted to controllers using transmitters. A transmitter is employed to transmits a signal from the sensor to the controller.
Difference of value of Measured variable by sensor and reference value. In most cases, this error signal acts as an input to the controller.
Based on the signal from the sensor/transmitter, the Device decides to keep process parameters within a specified limit or close to the setpoints. It provides signals to actuators.
A device that based on a signal from the controller manipulates a variable in response to a signal from the controller.
Process Control Loop
Control of output variable by comparing its amplitude measured through the sensor with setpoint and using this difference/error as an input to the controller to vary the input/manipulated variables in order to get desired value of output variable is called process control/control loop. The figure below shows different components and parameters of the control system:
Types of Control Systems
There are two types of control Systems used in process control systems:
Open Loop Control System
In open-loop control systems, input to the controller is independent of the output/measured variable and hence decisions taken by the controller to get output close to the setpoint value are independent of the measured variable. Because of its simplicity due to fewer components involved, these are preferred where fine-tuning of output parameters is not required. The figure below shows a typical block diagram of open-loop control systems:
Closed Loop Control System
In closed loop control systems, a complete loop from output to input is present and input to the controller and control signals generated by controllers to manipulate input variables are based on the measured value of output variables. A fine-tuning, at the cost of simplicity, is very much possible. The figure below shows a typical block diagram of closed loop control systems:
Control System Working (An Example)
In industries, we often need to control a Demineralised water level in vessels or tanks for process requirements as shown in the figure below.
De-mineralized water is fed to the tank through an inlet control valve. The opening and closing of the control valve are regulated or controlled through the controller which takes a decision based on the current level of water in the tank and set-point provided to the controller. The water level in the tank is maintained at a reference value determined by the process and continuously monitored. So, the level of water in the tank is a output/Measured variable. This level is maintained by controlling the flow of fed water to the tank. In this case, the inflow of water is Manipulated variable and the control valve is an actuator.
In normal or steady-state conditions, the in-flow of water is equal to the out-flow and the level of water in the tank remains at a prescribed level. The level is being sensed by the sensor and transmitted to the controller using a level transmitter (LT) in the form of voltage or current signals. Upon receiving a signal, the level Controller (LC) compares it with the set-point and provides a signal to the control valve which opens or closes. Direction (whether to open or close) is determined by the polarity of the error signal. If the measured value is less than the set-point, a valve is opened and if the measured value is greater than the setpoint, the valve is closed.
As the requirement of water which is to be delivered to the boiler increases; outflow of water becomes greater than inflow resulting in the reduced level of water in the tank. This change is measured by the sensor and compared by the controller with a reference value. Then error signal is provided to the control valve and the valve is opened proportional to the amplitude of the error signal. The greater the difference of the current value of level and set-point, the greater the opening of the valve and the greater the flow will be.
As requirement decreases; outflow becomes less than the set value and the valve is subjected to close to reduce the in-flow of water to keep the level of water equal to the set point.
Sometimes requirement to boiler remains constant; however; due to some other process requirement, the set-point is changed, resulting in opening or closing the valve to keep the level in the tank even to the set-point. Again, the direction and percentage opening or closing of the control valve are dictated by the polarity of the error signal and amplitude of the error signal respectively.
Control systems have always been needed at times to get the desired end product in a process. The Control system consists of process control loops. Sensors, transmitters and controllers are essential components of control systems. The Control system controls the output parameters by manipulating the input parameters. Control is based on comparing the output signal value with the reference value and making decisions based by controllers based on this error signal. Open-loop control and closed-loop control are two types of control systems and both of them have their advantages associated with them.
I hope you’ve liked this article on the introduction of Control Systems. You may also like our detailed article on the working and construction of a power system and Load flow analysis in ETAP software.
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