Control Systems

Table of Contents

What is a Control System?

A control system can be defined as a system of devices that command, regulate, and manage other devices in order to produce a desired outcome. This can be represented as a simple diagram shown in Figure 1 where the control system is represented as a block. The control system receives the input through messages relayed through devices such as sensors where they are then processed using software or other means and produces the desired output. [1]

Figure 1. Control System Diagram [1]

Image showing a simple block diagram of a control system.



Types of Control Systems

Open-Loop

In an open-loop control system, the output is not used as a control variable for the system. The output is determined by the present state input meaning that the input will return to zero before the output returns to zero. This means that the system cannot correct any transition errors that occur in the system making it more prone to errors.

An example of an open-loop control system is a washing machine. Washing machines are set to run for an amount of time determined by the user. Regardless of how clean the clothes is, it will stop after the set time has passed. [3]

AdvantagesDisadvantages
  • Simple and easy to design
  • Cheap
  • Low cost of maintenance
  • Convenient to use
  • Stable to some extent
  • Has less bandwidth
  • Non feedback system does not facilitate automation of process
  • Inaccurate in nature
  • Cannot correct output deviations
  • Since output can be affected by external system disturbances, they are unreliable

Figure 2. Open-Loop Control System [3]

Image showing a block diagram of an open loop control system.



Closed-Loop

In a closed-loop control system, a feedback loop is used to attain a consistent and stable system with a desired output. A feedback loop means that a part of the output is connected to the input so that it can automatically correct any errors that may have occurred, and maintain stability of the control system automatically. It compares the generated output with the actual condition. If it has deviated from the desired output, the system will generate an error signal which is fed into the input and automatically corrected. Closed systems are less prone to external system disturbances.

An example of a closed-loop control system is a water level controller. A water level sensor in a reservoir outputs the level of the water which is fed into the control system. If it has deviated from the desired water level, it will add or remove water so that it maintains that level. [3]

AdvantagesDisadvantages
  • Accurate and less error prone
  • Automatically correct errors
  • Bandwidth is very large
  • Supports automation
  • Less affected by noise due to high noise margin
  • Very complex and difficult to design
  • Expensive
  • High maintenance required
  • Feedback signal may cause the system to oscillate

Figure 3. Closed-Loop Control System [3]

Image showing a block diagram of a closed loop control system.



Positive Feedback System

In a positive feedback control system, the set point (target value) and output values are added together as the feedback is "in-phase" with the input. The effect of a positive feedback loop is that it increases the systems gain such that the overall gain with positive feedback is greater than the gain without feedback.

An example of a positive feedback loop is an electronic amplifier based on an operational amplifier shown in Figure 4. Positive feedback control is achieved by feeding the output, Vout, back to the positive input terminal of the op-amp through the feedback resistor, Rf. If the input voltage, Vin, is positive/negative, the op-amp will first amplify this and output it. Since some of the output is fed back, the input becomes larger thus making Vout even larger until the positive or negative supply rail is saturated.

Figure 4. Positive Feedback Loop [4]

Image showing an example of a positive feedback system.



Negative Feedback System

In a negative feedback control system, the set point and output values are subtracted from each other as the feedback is "out-of-phase" with the input. The effect of a negative feedback loop is that it decreases the systems gain.

Negative feedback produces stable circuit responses, improves stability, and increases the operating bandwidth of a system, making it the most common feedback system used in control systems.

An example of a negative feedback loop is an electronic attenuator shown in Figure 5. Negative feedback control is achieved by feeding part of Vout back into the inverting input terminal through the feedback resistor, Rf. If the input voltage, Vin, is positive, the op-amp amplifies, but makes the output negative causing the input to become smaller and will continue to decrease until it settles at a value determined by the gain ratio of Rf/Rin. The opposite occurs if the voltage input is negative, the op-amp makes it positive and decreases the magnitude of the input until it stabilizes.

In general, negative feedback in control systems sees far more use as they are more stable than positive feedback systems, and they do not oscillate at any frequency other than a specific circuit condition.

Figure 5. Negative Feedback Loop [4]

Image showing an example of a negative feedback system.




What are Programmable Logic Controllers?

Programmable logic controllers or PLCs are the devices that allow logic to be added to control systems. In industry they are used to control different electro-mechanical processes for various automation environments. PLCs are used very often as they are fast, easy to operate, and can be programmed easily. Modern PLCs can be programmed in several different ways, from ladder logic to adapted programming languages based off BASIC and C.

A PLC’s key functions can be categorized as the following: inputs, outputs, and the CPU. PLCs accept data by monitoring inputs from connected devices and sensors. Next, the CPU applies the programmed logic to the data. Then the CPU executes the program logic which sends commands or outputs data to machines or devices connected to it. And since PLCs operate in cycles, this process continues to repeat [5].


On-Off Control Systems

This is a type of control system made for control elements (motors, pumps, etc.) that only operate at either an on or off state. In other words, these control elements do not operate at intermediate positions. For this control system, when a certain input variable changes and crosses a preset level, the output of the system is suddenly changed to be fully on.

Typically for on-off control systems, the result of the output being changed causes the input variable to changing back in the reverse direction. And again, once the input variable falls below the preset level, the output of the system is suddenly changed to off. This creates a cyclical system that continues while this on-off control system is operating [6].


Linear Control Systems

Linear control systems are defined to be systems that have the two properties of homogeneity and additivity.

Homogeneity: A system is homogenous if, multiplying the input by some constant A, results in the output also being multiplied by the same constant A.

Additivity: Say first for a particular input a1, we get the corresponding output of b1. Then for a particular input a2, we get the corresponding output of b2. Now suppose we give the system an input of the summation of the previous inputs (a1 + a2) and the output is the summation of the previous outputs (b1 + b2), we can then say that that system has the property of additivity.

Therefore, if a system has both of these properties, by definition it is a linear control system.

Systems that do not have these two properties, are defined as non-linear control systems [7].


References

[1] “Control Systems - Introduction,” Tutorialspoint. [Online]. Available: https://www.tutorialspoint.com/control_systems/control_systems_introduction.htm. [Accessed: 26-Jan-2021].

[2] Electrical4U, “What is a Control System? (Open Loop & Closed Loop Control Systems Explained),” Electrical4U, 27-Dec-2020. [Online]. Available: https://www.electrical4u.com/control-system-closed-loop-open-loop-control-system/#What-is-a-Control-System. [Accessed: 26-Jan-2021].

[3]Administrator, C. H. says, and C. Home, “Open Loop System,” Electronics Hub, 24-Dec-2017. [Online]. Available: https://www.electronicshub.org/open-loop-system/. [Accessed: 03-Feb-2021].

[4] “Feedback Systems and Feedback Control Systems,” Basic Electronics Tutorials, 04-Mar-2018. [Online]. Available: https://www.electronics-tutorials.ws/systems/feedback-systems.html. [Accessed: 16-Mar-2021].

[5] Inductive Automation, “What is a PLC,” Inductive Automation. https://www.inductiveautomation.com/resources/article/what-is-a-PLC [Accessed Mar. 16, 2021].

[6] Electrical4U, “On Off Control Controller: What is it? (Working Principle),” Electrical4U. https://www.electrical4u.com/on-off-control-theory-controller/ [Accessed Mar. 17, 2021].

[7] Electrical4U, “Types of Control Systems | Linear and Non Linear Control System,” Electrical4U. https://www.electrical4u.com/types-of-systems-linear-and-non-linear-system/ [Accessed Mar. 17, 2021].


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