7.2 Tuning

In tuning, the PID parameters of the controller are matched to the process being controlled in order to obtain good control. Good control means:

 

Stable, ‘straight-line’ control of the PV at setpoint without fluctuation

 

No overshoot, or undershoot, of the PV setpoint

 

Quick response to deviations from the setpoint caused by external disturbances, thereby rapidly restoring the PV to the setpoint value.

 

Tuning involves setting the following parameters in a PID controller:-

 

Proportional Band ‘PB’, Integral Time ‘Ti’, Derivative Time ‘TD’, Cutback High ‘CB.Hi’, Cutback Low ‘CB.Lo’, and Relative Cool Gain ‘r2G’ (applicable to heat/cool systems only). Loop Break Time ‘Lbt’ is also set by the self tune algorithm provided that the integral time ‘ti’ is not set to OFF.

 

The controller is shipped with these parameters set to default values (section 4.6.2 ‘Level 2 Parameters’). In many cases the default values will give adequate stable straight line control, however, the response of the loop may not be optimised. Because the process characteristics are fixed by the design of the process it is necessary to adjust the control parameters in the controller to achieve best match. To determine the optimum values for any particular loop or process it is necessary to carry out a procedure called loop tuning. If significant changes are later made to the process which affect the way in which it responds it may be necessary to retune the loop.

 

Users have the choice of tuning the loop automatically or manually. Both procedures require the loop to oscillate and both are described in the following sections.