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Modified Lambda Tuning Guide

The modified lambda tuning method is a simple way to tune a controller using only the gain and reset modes. It involves measuring the total time (T98) for the process variable to reach 98% of its final value after a disturbance. The gain is calculated based on the change in output and process variable. The reset time is calculated as T98 divided by 4. This method provides quick tuning without overshoot, which plant operators generally prefer over alternatives with overshoot.

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Uary Buza Regio
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127 views3 pages

Modified Lambda Tuning Guide

The modified lambda tuning method is a simple way to tune a controller using only the gain and reset modes. It involves measuring the total time (T98) for the process variable to reach 98% of its final value after a disturbance. The gain is calculated based on the change in output and process variable. The reset time is calculated as T98 divided by 4. This method provides quick tuning without overshoot, which plant operators generally prefer over alternatives with overshoot.

Uploaded by

Uary Buza Regio
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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Modified Lambda Tuning Method

A very simple way to tune a controller is to use the modified lambda tuning setting. This method is
made easier because it only uses two of the modes, gain and reset, and the reset setting only
requires the user to measure the total time the process variable was in transition. The method
calls this T98 or the time for the change in process variable to reach 98% of its total change. This
happens to be 4 time constants since 1-exp(-4) = 0.9817. For all practical purposes, this can be
assumed to be 100%. It is far easier to note the time when the PV is finished with the disturbance
than it is to calculate slopes. This method works well for many loops and can give you a quick
answer to the settings required.
The controller gain is calculated by:

Output in % 1
Kc = *
PV in %
Where lambda is the term used to increase or decrease the speed of response. Decrease to
speed up and increase to slow down the control response.
The reset setting, Ti in minutes per repeat, is calculated by:

T98 TD
Ti = = + TC
4 4
TD is the process dead time and TC is the time constant.

First order plus deadtime response


1

0.9

0.8

0.7
Normalized PV

0.6

0.5

0.4

0.3

0.2

0.1

0
0 50 100 150 200
Time

In the above curve, assume the normalized 0 PV is 130 degF and the 1.0 PV is 140 degF for an
input range of 0 to 200 degF and the response was created by a step change of 20 percent or
Output = 20%. The PV is (10 DegF /200 DegF)*100% or 5%.
The change in output is 20% Therefore the Gain should be (20/5)*(1/4) or 1.0 for = 4.
The reset time is 180/4 or 45 minutes pre repeat.
This controller setting is tuned not have any overshoot. This is generally accepted as by plant
operators. They frequently think overshoot is the sign of a plant out of control.
The following simulation shows the difference between ZN tuning and a hot lambda setting.
The principal behind this method is to set the reset value equal to the total time the controller
was in transition. It assumes the combination of deadtime and the time constants are just one
large first order time constant and sets the reset value to compensate for that assumed lag. This
is the basic concept used for dryer control systems where the rate of moisture removal is
controlled.

Simulated First Order with Dead Time Lambda Tuning


Simulated First Order with Dead Time ZN Tuning

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