1) Here we explore the tuning of a PI controller by trial and error when disturbance rejection is the
control objective. The process will be an ideal linear simulation designed using Custom Process.
Start by clicking the Custom Process button on Loop Pro’s main screen and then choose Single
Loop Process from the list.
When the simulation starts, notice that the graphic to the right of the scrolling plots is comprised of
a Process button, Disturb(ance) button and Controller button (the C in the white circle).
To create the simulation, click the Process button on the graphic. This opens a “Construct Process
and Disturbance Models” form. First specify the controller output to measured process variable
dynamic behavior. Click the Process Model tab (it should already be active if you have not done
any exploring) and select from the list of models available, the:
– Overdamped Linear Model
– Self Regulating (Stable) Process.
Enter the process gain, three time constants and dead time that define the controller output to
measured process variable dynamic behavior of this overdamped linear third order plus dead time
Process Gain, KP = 1.0
First Time Constant, τP1 = 10.0
Second Time Constant, τP2 = 1.0
Third Time Constant, τP3 = 1.0
Lead Time, τPL = 0
Dead Time, θP = 1.0
Next specify the behavior of the disturbance to measured process variable dynamics. Click the
Disturbance tab at the top of the form, ensure that Overdamped Linear Model is selected, and
Disturbance Gain, KD = 1.2
First Time Constant, τD1 = 12.0
Second Time Constant, τD2 = 1.2
Third Time Constant, τD3 = 1.2
Lead Time, τDL = 0
Dead Time, θD = 1.2
2) Controller output and disturbance variable changes differ in their impact on the measured process
variable because the dynamic models defined above are different. Verify this by stepping the
controller output from its default value of 50% up to 60% and when the response is substantially
complete, step it back to 50%. Next, step the disturbance variable from its default value of 50% up
to 60% and when the response is substantially complete, step it back to 50%.
Compare the two responses side by side (you may need to use the Change Strip Chart History icon
on the toolbar to see both responses). Does a step in the disturbance variable show a larger but
slightly slower response associated with a larger gain and longer overall time constant?
3) Click the Controller icon on the graphic and select PID from the controller design menu. Specify a
very conservative (sluggish) PI controller by entering the tuning values:
Set Point = 50
Controller Gain = 1.0
Reset Time = 50
Be sure the Integral Action is On, and to ensure you are using PI control, set the Derivative Action
to Off. Click Done to put the controller in automatic.
4) Trial and error tuning can use up significant production time and create expensive off-spec
product. To track how inefficient a trial and error approach can be, zero the simulation clock by
clicking on the Clock icon (upper left of screen on tool bar) and then Reset Simulation Clock to
Zero. From this point forward, do not reset the clock or pause process execution until instructed to
5) Start tuning for disturbance rejection by considering the current performance of the controller.
Click the white “Disturbance, D” box on the graphic, step the disturbance variable from 50% up to
60% and after the response, step it back to 50%.
Now try and improve controller performance by adjusting KC and/or τI on the controller design
menu. The PI controller tuning map in Chapter 8 of the Practical Process Control book is for set
point tracking but may assist your thought process as you search for your best tuning values. Test
your new values by again stepping the disturbance from 50% up to 60% and back again. Repeat
this procedure as necessary to arrive at “best” tuning values for disturbance rejection. Use your
own experience and opinion to define “best.”
When you have determined best tuning values for disturbance rejection, pause the simulation and
record them below. Also record the elapsed time displayed on the process clock at the lower left of
KC = τI = Elapsed Time = minutes
6) Repeat the above exercise with the objective of controller tuning for set point tracking. Start with
your best disturbance rejection tuning values above and determine by trial and error whether they
are best when the control objective is tracking set point steps from 50% up to 60% and back again.
Record your results:
KC = τI = Elapsed Time = minutes
Is your best tuning for set point tracking the same as your best tuning for disturbance rejection?
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