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March 2005

In this issue

puce About us
puce Classes and seminars
puce Trick for this month
puce To contact us

See our previous issues :
http://www.topcontrol.com/techinfo/news.php

About us

See our 2005 calendar, Classes & Seminars :

See at : http://www.topcontrol.com/training/tcalendar.php

Classes & seminars

Mexico, March 15-17
SE-100-3 - Seminario en métodos de control modernos
http://www.topcontrol.com/training/printable.php?id=90&type=cours

Montreal, March 24-25
SE-120-2 - Seminar on performance assessment techniques using PlantTriage
http://www.topcontrol.com/training/printable.php?id=29&type=cours

New-York, March 27, Easter, March 29-31
SE-130-3 - Seminar on advanced tools in process control
http://www.topcontrol.com/training/printable.php?id=28&type=cours

Montreal, April 19-21
EX-501-3 - Detailed Loop Analysis with ExperTune
http://www.topcontrol.com/training/printable.php?id=13&type=cours



Trick for this month : Kp was reduced and oscillations appeared!

It is well known that if one reduces proportional gain (Kp) in a PID controller:

  • loop will be more stable,
  • response time will be longer,
  • oscillation will be reduced,
  • overshoot will be reduced,
  • robustness (process changes sensitivity) will be increased,
  • etc.

Sometimes, it is not true!
If the PID controller has a parallel structure, reducing the proportional gain will change the balance between proportional action and Integral action. In most controllers (non parallel, Ideal/ISA, Series) the output from the Proportional (Kp X Error) feeds other modes (Integral and Derivative) therefore, reducing Kp will reduce Integral and Derivative at the same time. In a parallel structure, all modes are fed by the error.

For example, both controllers below are equivalent:

Parallel PID Ideal PID
Kp = 0.25 Kp = 0.25
Kl = 6 r.p.m. (Tl = 10s) Kl = 24 r.p.m. (Tl = 2.5s)

If parallel controller Kp is reduced to 0.05, then Integral part remains the same and equivalent Ideal controller has now an Integral part stronger; the loop could start to oscillate. These oscillations will have a long period being induced by Integral; at first glance, they could be undetected.

Parallel PID Ideal PID
Kp = 0.05 Kp = 0.05
Kl = 6 r.p.m. (Tl = 10s) Kl = 120 r.p.m. (Tl = 0.5s)

Parallel controllers are popular in PLCs and in some DCSs. Flow loops cycling at long (minutes) period are frequent when parallel controllers are used; numbers seem acceptable but in fact sometimes they are not! Stay alert!

To contact us

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

puce

49 Bel-Air Street, Lévis, QC
G6V 6K9, Canada
Phone: (418) 834-2242
Fax: (418) 834-2651  
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1020 James Drive, Suite A
Hartland, WI 53029
Toll free: (877) 867-6473