Contek Systems Ltd, Aberdeen, UK

Control engineering consultant for the oil and gas industry. Process control, de-bottlenecking, compressors, slugging and control system PID tuning.

Recent press releases and other news are available here.
Contek Systems Ltd,
Shampher Cottage,
Strachan,
AB31 6NN

+44  (0)1330 850598

Try my well-known Process Identification and PID Tuning package U-Tune.

Click here for details and screenshot.

See my FREE tuning calculator.

Download here.

I have written several tutorials on the practicalities of process control.

Click here for links to our tutorials.

Check back from time to time. New articles will be added as time permits.
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U-Tune Tuning Calculator

Calculate proportional-integral tuning parameters for process controllers using the results of an open-loop step response.

The calculator supports the common controller algorithms and parameter types and provides two sets of parameters, one for fast and the other for slow responding loops.

This product is absolutely FREE.

U-Tune Tuning Calculator

To install the calculator, click on the Download button. When the dialog window opens, select Run and follow the instructions. Download
The documentation is included in the installation, but you may also view the PDF here. User Guide
 
Process Identification

The U-Tune Tuning Calculator is designed for calculating proportional-integral 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.

Response of process to a control output step

 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.

Step response in practice

Controller Type
Controller manufacturers have different ways of implementing the proportional-integral 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:

 

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

Series PI algorithm

The U-Tune 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 U-Tune User Manual for further details.