State the principle of ON-OFF control action. Write its standard equation & define neutral zone.

State the principle of ON-OFF control action. Write its standard equation & define neutral zone.
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Answer :

This is one of the most common & simplest mode of controller. * It has to control two positions of control element, either on or off hence this mode is called as ON OFF controller, it is the cheapest controller & often used if its limitations are well within the tolerance. * This controller mode has two possible output states namely 0 % & 100%. * Mathematically this can be expressed as

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Hence if the error rises above a certain critical value, the output changes from 0% to 100%. If the error decreases below certain critical value, the output falls from 100% to 0%. 

Neutral Zone-

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* In practical implementation of the two-position controller, there is an overlap as e(t) increases through zero or decreases through zero. In this span, no change in the controller output occurs. * Until an increasing error changes by Δe(t) above zero, the controller output will not change state. In decreasing, it must fall Δe(t) below zero before the controller changes to 0%. * The range 2Δe(t) is referred to as neutral zone or differential gap. * Two-position controllers are purposely designed with neutral zone to prevent excessive cycling. * The existence of such a neutral zone is an example of desirable hysteresis in a system.

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