<%@ Page Language="VB" ContentType="text/html" ResponseEncoding="iso-8859-1" %> PID Tuning Tool - Level


Level control is mostly accomplished by the use of Proportional action with a little Integral action and no Derivative action. Most level controllers can be tuned by calculation based on vessel dimensions, control valve capacities and tuning objective. The details are found in tuning guidlines for each type of tuning. But it is first necessary to decide which of the two tuning objectives described below is appropriate:
Tight Control
This tuning objective attempts to maintain the level close to the required setpoint irrespective of the affect on the controller output. Thus the controller output (to a control valve or variable-speed pump) may change rapidly in response to a process change. The result may be severe disturbance to upstream or downstream processes. 
Averaging Control
This tuning objective allows the level to drift in the short term, and gradually bring the measured level back to the setpoint. The rate and magnitude of changes to the controller output are therefore reduced, so causing the minimum disturbance to upstream or downstream processes. The majority of level controllers in the industry should be tuned for averaging control.
Gap Control      
Where a level is subject to known and regular disturbances it may be appropriate to implement Gap Control. In gap control, a band is defined either side of the setpoint within which no (or reduced) control action is taken. When gap control is selected, then either of the two tuning objectives and associated calculation methods can be applied to the control action.