
Process
Identification

The
UTune Tuning Calculator is designed for calculating
proportionalintegral tuning parameters for processes with an
open loop step response of the form shown below. That is, when
the controller is operated in manual mode and its output is
stepped to a new value, the process variable (PV) does not
respond initially but then rises to settle onto a new value.


The
critical parameters defining the step response are:

The step size of the controller output (CO).

The change in value of the PV.

The time delay between the output step and commencement of
the PV response.

The time lag of the PV response i.e. the time from the end
of the delay until the PV reaches 63% of its total change in
value.
The
easiest way to make these measurements is to read them from a
DCS trend display or pen recording of the process variable and
the controller output. If these are not available, try recording
the data every few seconds and sketching the response curve.
Just enter the results into the calculator, select the
controller type and press the 'Calc' button.


Identification in Practice

Of course, step tests don't
often produce the perfect results shown above, but the
calculator can still give excellent results even with noisy
signals as long as care is taken in measuring the response
characteristics. 

Controller Type

Controller manufacturers
have different ways of implementing the proportionalintegral
algorithm. Two of the most common are the parallel and the ideal
form.

In the parallel form,
the gain and integral terms are completely independent:



In the more common ideal form, the gain term also multiplies the
integral term: 


The UTune Calculator supports
both these forms. You should check the manufacturer's literature
to determine the exact algorithm used
by your controller.
There are several
variants on these algorithms e.g. the integral is
measured in repeats/second, seconds/repeat, repeats/minute etc.
and the gain may be defined as proportional band. All these
options are
provided by the calculator. Refer to the UTune User Manual
for further details.
