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What is meant by reset time?

The I component of the controller output signal has the effect of continuously altering the manipulating variable, until the process value has reached the setpoint.
As long as the control deviation is present, the manipulating variable is integrated upwards or downwards. The longer the control deviation continues to be present in a controller, the larger the integral effect on the manipulating variable. The larger the control deviation and the smaller the reset time, the more pronounced (faster) the effect of the I component.
The I component ensures stabilization of the control loop without permanent control deviation. The reset time is a measure of the effect the control deviation duration has on the control action. A larger reset time means that the I component is less effective and vice versa. Within the specified time Tn (in sec.), the change in the manipulating variable that is produced by the P component (xp or pb), is added once more. Accordingly, there is a fixed relationship between the P and the I component. A change in the P component (xp) also means a changed time response, at a constant value for Tn.