OPT FOR SECURITY AND PERFORMANCE
Improve the security and performance of your plant with alarm management, performance supervision, asset supervision, web-based data visualisation, optimisation, advanced control, and tuning with the new TaiJi tool.
Rethink your working methods and obtain immediate results thanks to our services, trainings, and consulting expertise. Manage changes intelligently with our proven solutions.
certified service providerApril 2005
In this issue
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Classes and seminars |
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About us |
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Trick for this month |
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To contact us |
See our previous issues :
http://www.topcontrol.com/techinfo/news.php
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Classes & seminars
Montreal, QC, April 19-21
VA-900-3 - Control Valve Selection
Moncton, NB, May 3-5
ME-200-3 - Analytical methods for troubleshooting and Control loop tuning
Milwaukee, WI, June 6-10
EX-501-3 - Detailed Loop Analysis with ExperTune and
EX-502-3 - ExperTune Advanced Tools
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See our 2005 calendar, Classes & Seminars :
See at : http://www.topcontrol.com/training/tcalendar.php
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Trick for this month : Varying Dead Time
It is well known that when dead time increases, performances decrease. This is particularly true for a loop where the tuning parameters of a controller are selected to reject disturbances
Dead time increases for example if :
- fouling occurs in a heat exchanger,
- flow is reduced in a pipe where consistency, concentration or pH is controlled,
- or velocity of material is reduced (conveyer, pipe, duct, …),
- etc.
For example, in a loop tuned to reject disturbances, without overshoot and without oscillation, if dead time increases, stability and robustness will be reduced, overshoot will appear and cycling may be induced. To maintain stability and robustness, proportional gain has to be reduced when integral and derivative time have to be increased.
If dead time can be calculated for example by measuring flow in a concentration loop,
tuning parameters should be adjusted accordingly to this flow by programming exact formulas; often
the following simple relations are used:
| Kp1 | = | Kp0 | * | (td0/td1) |
| TI1 | = | TI0 | * | (td1/td0) |
| TD1 | = | TD0 | * | (td1/td0) |
Where Kp is proportional gain, TI is integral time, TD is derivative time.0 subscript is used for original values; 1 is for new value.
In our example, td could be replaced by measured flow (programmers should verify to avoid division by zero).
In a loop tuned to follow SP changes, since tuning parameters depend mostly on time constant, formulas should be used to compensate dead time changes.
Stay alert, dead time is the worst enemy in control!
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Comments, suggestions and requests are welcomed. Please e-mail us at: info@topcontrol.com
You are welcomed to visit our web site :http://www.topcontrol.com
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49 Bel-Air Street, Lévis, QC
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1020 James Drive, Suite A
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