Control Systems

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]

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]

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]

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.

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.