Hägglund, Tore

Theorin, Alfred

English

Lund University Publications

LUND UNIVERSITYPO Box 117221 00 Lund+46 46-222 00 00Derivative Backoff: A Process Value Saturation Problem for PID ControllersTheorin, Alfred; Hägglund, Tore2015Link to publicationCitation for published version (APA):Theorin, A., & Hägglund, T. (2015). Derivative Backoff: A Process Value Saturation Problem for PID Controllers.1-2. Abstract from 19th Nordic Process Control Workshop (NPCW'2015), Norway.Total number of authors:2General rightsUnless other specific re-use rights are stated the following general rights apply:Copyright and moral rights for the publications made accessible in the public portal are retained by the authorsand/or other copyright owners and it is a condition of accessing publications that users recognise and abide by thelegal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private studyor research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portalRead more about Creative commons licenses: https://creativecommons.org/licenses/Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will removeaccess to the work immediately and investigate your claim.Derivative Backoff:A Process Value Saturation Problem for PID ControllersAlfred Theorin (alfred.theorin@control.lth.se), Tore HägglundThe PID controller is by far the most commonly used controller. To ensuregood behavior under all circumstances, a PID implementation must considermany aspects. In particular, physical limits of signals must be considered. Ifphysical limits for the control signal are not considered, there is integratorwindup when the control signal saturates. An analogous issue has been discov-ered and named derivative backoff. If physical limits for the process value arenot considered, there is derivative backoff when the process value saturates.The derivative part of PID controllers predicts future process values byestimating the process value derivative. With a saturated process value thereis no information about the process value derivative and thus the derivativepart breaks down. At saturation, the derivative of the measured process valueis 0 and, consequently, there is no derivative action. This makes sense as thereis no information about future process values. However, the behavior whenthe process value saturates makes less sense, see Figure 1. Here, the process iscontrolled close to a physical limit (100) when a large load disturbance occurs.Just before the process value saturates, the process value derivative is positive,which means that the derivative action is negative. Shortly after the processvalue saturates, the derivative part is 0, that is, there is no derivative action.Together, this means that when the process value reaches its maximum, thenegative derivative action backs off and thus the control signal increases. Thisis not a desirable behavior.In Figure 1, the process P = 1(s+1)4 and its corresponding MIGO de-sign method parameters have been used. Note that MIGO produces a robustcontroller with optimal load disturbance handling. The example shows whatderivative backoff looks like and demonstrates that it is an issue for industriallyrelevant processes with well tuned PID controllers.To implement anti-backoff, that is, to avoid derivative backoff, the derivativeaction needs to be kept when the process value saturates. Different anti-backoffapproaches have been evaluated and the proposed solution is to disable andimmediately re-enable the derivative part bumplessly when the process valuesaturates. This requires that the process value limits are known, which is truefor most control loops. The PID controller must also have integral action, sinceit is otherwise not possible to bumplessly disable the derivative part. This isalso true for most PID controllers with derivative action. Further investigationis needed to find a good solution for controllers without integral action.120 5 10 15 20 259095100105110Process Valuet0 5 10 15 20 2520406080Control Signalt0 5 10 15 20 25−20020406080100Control Signal PartstReal process valueMeasured process valueReference valueControl signalP partI partD partFigure 1: A large load disturbance occurs. When the process value saturates,the derivative part backs off and thus the control signal increases. The processvalue is thus pushed further away from the measurement range.

Derivative Backoff: A Process Value Saturation Problem for PID Controllers

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Derivative Backoff: A Process Value Saturation Problem for PID Controllers

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