Embed Size (px)
DESCRIPTION
Béla Lipták defines 5 levels of process control
Citation preview
8/17/2015 Béla Lipták defines 5 levels of process control
http://www.controlglobal.com/articles/2015/belaliptakdefinesprocesscontrol/ 1/8
LOGIN | REGISTER
Print Email Tweet
A A A A +
Home / Articles / 2015 / Béla Lipták defines 5 levels of process control
Béla Lipták defines 5 levels of process controlManual, feedback, cascade, feedforward and lead/lag all have their place.
By Bela Liptak
May 11, 2015
1 of 2 < 1 | 2 > View on one page
About the AuthorBéla Lipták, PE, control consultant, is also editor of the Instrument Engineers'Handbook and is seeking new coauthors for the for coming new edition of that multivolume work. He can be reached at [email protected].
In process control, there are at least five levels of sophistication that the automationengineer can apply. The advantage of the higher levels is better control, but applying
a higher level also requires better understanding of the process. In the analog age, using higher levelsof sophistication also involved substantially higher costs, but in the digital age, this consideration hasbecome less significant. To illustrate the five levels, I will use the example of controlling the outlettemperature from a steampowered liquid heat exchanger.
Manual Control (Figure 1): Manual feedback control is performed by the operator by watching the hotwater temperature and manually throttling the steam flow to keep this outlet temperature constant.Manual feedforward control is performed when the steam flow is throttled on the basis of thetemperature and flow of the incoming cold water.
Feedback Control (green components in Figure 2) is similar to manual control, except that theoperator is replaced by an automatic controller, which is more accurate and does not get tired ordistracted, and its "personality" is tuned to match that of the controlled process. By "personality" I refer
MENU
Share Share
8/17/2015 Béla Lipták defines 5 levels of process control
http://www.controlglobal.com/articles/2015/belaliptakdefinesprocesscontrol/ 2/8
to the dynamics (gain, time constant, dead time) of the process.
See also "Controlling distributed processes"
Hands onManual feedforward and feedback control depend on the operator to controlthe process.
Figure 2 illustrates the case when the control valve is on the steam side of the heater. If it is on thecondensate side, a smaller valve can be used, but the dynamics of the process will be slower and nonsymmetrical, because lowering the condensate level is much faster than building it up. On the otherhand, this configuration eliminates the minimum condensing pressure problems that can interfere withthe steam trap operation at low loads.
While feedback control is the most commonly used, it has serious limitations. The main one is that itcannot anticipate and, therefore, cannot prevent errors because it can initiate corrective action onlyafter an error has developed. Therefore, in the case of the liquid heater process, if either the propertiesof the manipulated variable (for example, steam pressure) or the load (cold fluid flow or temperature)change, feedback control will wait until these changes upset the outlet temperature before initiatingcorrective action.
Cascade Control (red components in Figure 2) can correct for manipulated variable upsets, such assteam pressure changes, before they could upset the controlled variable (temperature). As shown inFigure 2, the cascade loop consist of two controllers in series, but has only a single setpoint, that of theprimary (master) controller, the TIC. The main value of having a secondary (slave) controller (the FICshown in red) is that it will act as the first line of defense against steam quality disturbances, preventingthese upsets from entering and upsetting the primary controlled variable (temperature).
8/17/2015 Béla Lipták defines 5 levels of process control
http://www.controlglobal.com/articles/2015/belaliptakdefinesprocesscontrol/ 3/8
Let the controller do itMore advanced systems replace theoperating person with an automaticcontroller capable of handling thepersonality of the process.
For the cascade slave loop (FIC) to be effective, it should be more responsive (faster) than the master.Some rules of thumb suggest that the slave time constant should be less than 1/4 to 1/10 that of themaster loop, and the slave’s period of oscillation should be less than 1/2 to 1/3 of the master loop. Incase of the steam heater process, this requirement is usually satisfied.
One should also be careful to prevent "reset windup" in the primary controller (TIC). This can occurbecause the TIC is usually provided with an integral mode, which keeps integrating the error as long asit exists and, therefore, if the FIC setpoint is blocked (such as when it is switched to manual) it keepsintegrating until its output saturates (reaches maximum), and when the FIC is returned to automaticcontrol, a large upset occurs. To guarantee "bumpless transfer," provide "external reset," which is aninternal logic component in the TIC algorithm that deactivates the integral mode if the TIC output doesnot equal the FIC measurement, and reactivates it only when it does.
1 of 2 < 1 | 2 > View on one page
Join the discussionWe welcome your thoughtful comments. Please comply with our Community rules.All comments will display your user name.
Want to participate in the discussion?
REGISTER FOR FREE
Log in for complete access.
CommentsNo one has commented on this page yet.
RSS feed for comments on this page | RSS feed for all comments
8/17/2015 Béla Lipták defines 5 levels of process control
http://www.controlglobal.com/articles/2015/belaliptakdefinesprocesscontrol/ 4/8
Related Content
Don't overlook the virtues of PID when optimizing processesModel predictive control isn't always the best choice
Why automation isn't process controlProducts shaped by the needs of expedient automation may get us bitten by the wild process…
Process Control Improvement Recommendations TipsHere we look at how to make sure the measurement system is able to provide the analysis,…
The power of external-reset feedbackPreventing windup requires reconfiguration of the controller, according to process control…
8/17/2015 Béla Lipták defines 5 levels of process control
http://www.controlglobal.com/articles/2015/belaliptakdefinesprocesscontrol/ 5/8
Past 7 Days Past 30 Days Past 6 Months All Time
Most Popular
Actual control system cyber incidents are numerous and very expensive yet the C-Suite is missingMy database now has more than 725 actual control system cyber incidents.
01When will we see more four-year U.S. degrees in automation engineering?If Americans want to strengthen our manufacturing base, we need more…02How to do wireless for safety systemsISA TR 84.00.08 offers some important guidelines for making it work03Technology trends in process energy efficiencyToday's monitoring and control systems are making it easier to…04Why automation isn't process controlProducts shaped by the needs of expedient automation may get us bitten by…05
8/17/2015 Béla Lipták defines 5 levels of process control
http://www.controlglobal.com/articles/2015/belaliptakdefinesprocesscontrol/ 6/8
About
Contact Us
Advertise
Media Kit
Rates & Spec
Privacy Policy
Legal / T&C
Content
8/17/2015 Béla Lipták defines 5 levels of process control
http://www.controlglobal.com/articles/2015/belaliptakdefinesprocesscontrol/ 7/8
Voices
Blogs
Out of Control Cartoons
News
White Papers
Multimedia
Special Reports
Events
Products
Technology Roundups
Elits
Magazine
Subscribe
Digital Edition
Issue Archive
Reprints
Site Tools
Jobs
Knowledge Center
Microsites
Company Profiles
Site Map
Stay Connected
My Account
Newsletters
Social Media
RSS
Contact Us | Advertise | Privacy Policy | Legal Disclaimers, Terms & Conditions
Copyright © 2004 2015 Control Global. All rights reserved.
P: 6304671300 | 1501 E. Woodfield Road, Suite 400N, Schaumburg, IL 60173
Chemical Processing | Control | Control Design | Food Processing | Industrial Networking | PharmaceuticalManufacturing | Pharma QBD | Plant Services | Wellness Foods
8/17/2015 Béla Lipták defines 5 levels of process control
http://www.controlglobal.com/articles/2015/belaliptakdefinesprocesscontrol/ 8/8
