Level Control Experiment

Proportional Integral Control Answers

At low values of integral action (reset) time and low proportional bands, the system is prone to significant oscillation. This large amount of oscillation means that the control tends towards on/off control as the valve swings from fully open to fully closed.
As the proportional band is increased the oscillation in the system becomes less and the process is successfully returned to the setpoint (recall with proportional only control, there was an "offset"). However, the process is away from the setpoint for a relatively long period of time.
As the integral action (reset) time is increased, so the system is returned to the setpoint more quickly. However, at high proportional bands and high integral action (reset) times. the system again takes a long time to return (and in fact it may not) to the setpoint as the system is "overdamped"
The main advantage of P+I control is that offset is eliminated. The combination of correct proportional bands and a correct integral action (reset) time produces a quick response to a disturbance in the process that returns the process to the setpoint without offset.