Using Wireless Measurements in Control Applications

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Using Wireless Measurements in Control Applications Terry Blevins Mark Nixon Marty Zielinski

Copyright 2013 ISA. All Rights Reserved. www.isa.org Presented at ISA Automation Week 2013

Nashville, Tennessee •Renaissance Nashville, USA, 4-7 November 2013

2

Presenters

Marty Zielinski Terry Blevins



Agenda

• Wireless Impact on Control • Modified PID, PIDPlus, for Wireless Measurements • Performance Comparison to Wired Transmitter • Test results – Field Installations • Conclusion



Challenge – Control Using Wireless

• It is desirable to minimize how often a measurement value is communicated to reduce transmitter power consumption,.

• Most multi-loop controller in use today are designed to over-sample the measurement by a factor of 2-10X to avoid the restrictions of synchronizing the measurement value with the control,

• Also, to minimize control variation, the typical rule of thumb is that feedback control should be executed 4X to 10X times faster that the process response time, process time constant plus process delay.

• The conventional PID design (based on difference equation, z-transform) assumes that a new measurement value is available each execution and that control is executed on a periodic basis.



Conventional Approach – Over Sampling of Measurement



Wireless Communication

Two communication techniques best fit control applications and minimize the power consumption by the wireless device transmitting the measurement value.

• Continuous – The device wakes up at a configured update period, senses the measurement and then communicates the value.

• Window – The device wakes up at a configured update period, senses the measurement and then communicates the measurement if the specified trigger value is exceeded.

Window communications is the preferred method since for the same update period less power is required.

6 Copyright 2013 ISA. All Rights Reserved. www.isa.org

Presented at ISA Automation Week 2013 Nashville, Tennessee •Renaissance Nashville, USA, 4-7 November 2013

Conventional PID - Impact of Wireless

• The underlying assumption in traditional control design is that the PID is executed on a periodic basis.

• When the measurement is not updated on a periodic basis, then the calculated reset action may not be appropriate.

• If control is only executed when a new measurement is communicated, then this could delay control response to setpoint changes and feedforward action on measured disturbances.

Conventional PID Design Copyright 2013 ISA. All Rights Reserved. www.isa.org


PIDPlus for Wireless Communications

Automatic compensation for setpoint change, measurement update rate. No need to modify tuning as sample rate changes



PIDPlus Using Wireless Transmitter vs. Conventional PID and Wired Transmitter

Control Measurement

Control Output

Unmeasured Disturbance

Setpoint PIDPlus

PIDPlus

PID

PID

Lambda Tuning ʎ = 1.0 Communication Resolution = 1%

Communication Refresh = 10sec



CONTROL PERFORMANCE DIFFERENCE

• Window communications reduce the number of transmissions by over 96 %.

• The impact of non-periodic measurement updates on control performance as measured by Integral of Absolute Error (IAE) is minimized through the use of the PIDPlus for wireless communication.



PID Performance for Lost Communications

• The Conventional PID provides poor dynamic response when wireless communications are lost.

• The PIDPlus improves the dynamic response under these conditions



Wireless Communication Loss – During Setpoint Change

Communication Loss

PID

PIDPlus

PIDPlus

PID

Control Measurement

Control Output

Setpoint



Field Trail Site - University of Texas at Austin

• The Separations Research Program was established at the J.J. Pickle Research Campus in 1984

• This cooperative industry/university program performs fundamental research of interest to chemical, biotechnological, petroleum refining, gas processing, pharmaceutical, and food companies.

• CO2 removal from stack gas is a focus project for which WirelessHART transmitters were installed for pressure and steam flow control



Steam Flow To Stripper Heater



Column Pressure Control



PC215 On-line Column Pressure Control • The same

dynamic control response was observed for SP changes

• Original plant PID tuning was used for both wired and wireless control

GAIN=2.5 RESET=4 RATE=1

Wired Measurement Used in Control

Wireless Measurement Used in Control



Control Performance – Wired vs Wireless

• Comparable control as measured by IAE was achieved using WirelessHART Measurements and PIDPlus vs. control with wired measurements and PID.

• The number of measurement samples with WirelessHART vs Wired transmitter was reduced by a factor of 10X for flow control and 6X for pressure control – accounting for differences in test duration.



Summary

• Wireless measurements may be used in closed loop control applications. – Window communications minimize power consumption

• The performance of PIDPlus in a wireless control network is comparable to PID with wired inputs – PIDPlus handles lost communications and recovery after loss of

communications.

• PIDPlus tuning depends only upon process dynamics, not on wireless update rate



References • K.J. Åstrӧm, and T. Hägglund, in Advanced PID Control, ISA, 2006., pp. 85-86 • K.J. Åstrӧm. “Event based control”, in Analysis and Design of Nonlinear Control

Systems, Springer Verlag, 2007, pp.127-147 • T. Blevins, and M. Nixon, in Control Loop Foundation – Batch and Continuous

Processes, ISA. pp. 266, 270, 393 • T. Blevins, (2012) “PID Advances in Industrial Control”, IFAC Conference on Advances

in PID Control PID'12, 2012, http://pid12.ing.unibs.it/sp_blevins.html • S. Han, X. Zhu, K.M. Aloysius, M. Nixon, T. Blevins, D. Chen, “Control over

WirelessHART Network”, 36th Annual Conference of the IEEE Industrial Electronics Society, 2010, http:// www.cs.utexas.edu/~shan/paper/slides-iecon10.pptx

• F. G., Shinskey, “The Power of External Reset Feedback”, Control, ,May, 2006, pp.53-63, http://classes.engineering.wustl.edu/2009/spring/che433/2009-LAB/Experiment%206/external-reset.pdf

• F. Siebert, and T. Blevins, “WirelessHART Successfully Handles Control”, Chemical Process, January, 2011 http://www2.emersonprocess.com/siteadmincenter/PM%20Articles/WirelessHART%20Successfully%20Handles%20Control.pdf

• M. Rabi and K. H. Johansson. “Event-triggered strategies for industrial control over wireless networks”, In Proceedings of 4th Annual International Conference on Wireless Internet, Maui, Hawaii, USA, 2008.

19 Copyright 2013 ISA. All Rights Reserved. www.isa.org


http://pid12.ing.unibs.it/sp_blevins.html

http://www.cs.utexas.edu/~shan/paper/slides-iecon10.pptx

http://classes.engineering.wustl.edu/2009/spring/che433/2009-LAB/Experiment 6/external-reset.pdf

http://classes.engineering.wustl.edu/2009/spring/che433/2009-LAB/Experiment 6/external-reset.pdf

http://www2.emersonprocess.com/siteadmincenter/PM Articles/WirelessHART Successfully Handles Control.pdf

http://www2.emersonprocess.com/siteadmincenter/PM Articles/WirelessHART Successfully Handles Control.pdf

Using Wireless Measurements in Control Applications

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