using wireless measurements in control applications
DESCRIPTION
Source: International Society of Automation (ISA) Copyright © 2013. Reprinted by permission. All rights reserved. Presented at ISA Automation Week by Emerson Process Management's Terry Blevins, Mark Nixon, and Marty ZielinskiTRANSCRIPT
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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
Copyright 2013 ISA. All Rights Reserved. www.isa.org Presented at ISA Automation Week 2013
Nashville, Tennessee •Renaissance Nashville, USA, 4-7 November 2013
Agenda
• Wireless Impact on Control • Modified PID, PIDPlus, for Wireless Measurements • Performance Comparison to Wired Transmitter • Test results – Field Installations • Conclusion
Copyright 2013 ISA. All Rights Reserved. www.isa.org Presented at ISA Automation Week 2013
Nashville, Tennessee •Renaissance Nashville, USA, 4-7 November 2013
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.
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 Approach – Over Sampling of Measurement
Copyright 2013 ISA. All Rights Reserved. www.isa.org Presented at ISA Automation Week 2013
Nashville, Tennessee •Renaissance Nashville, USA, 4-7 November 2013
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
Presented at ISA Automation Week 2013 Nashville, Tennessee •Renaissance Nashville, USA, 4-7 November 2013
PIDPlus for Wireless Communications
Automatic compensation for setpoint change, measurement update rate. No need to modify tuning as sample rate changes
Copyright 2013 ISA. All Rights Reserved. www.isa.org Presented at ISA Automation Week 2013
Nashville, Tennessee •Renaissance Nashville, USA, 4-7 November 2013
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
Copyright 2013 ISA. All Rights Reserved. www.isa.org Presented at ISA Automation Week 2013
Nashville, Tennessee •Renaissance Nashville, USA, 4-7 November 2013
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.
Copyright 2013 ISA. All Rights Reserved. www.isa.org Presented at ISA Automation Week 2013
Nashville, Tennessee •Renaissance Nashville, USA, 4-7 November 2013
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
Copyright 2013 ISA. All Rights Reserved. www.isa.org Presented at ISA Automation Week 2013
Nashville, Tennessee •Renaissance Nashville, USA, 4-7 November 2013
Wireless Communication Loss – During Setpoint Change
Communication Loss
PID
PIDPlus
PIDPlus
PID
Control Measurement
Control Output
Setpoint
Copyright 2013 ISA. All Rights Reserved. www.isa.org Presented at ISA Automation Week 2013
Nashville, Tennessee •Renaissance Nashville, USA, 4-7 November 2013
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
Copyright 2013 ISA. All Rights Reserved. www.isa.org Presented at ISA Automation Week 2013
Nashville, Tennessee •Renaissance Nashville, USA, 4-7 November 2013
Steam Flow To Stripper Heater
Copyright 2013 ISA. All Rights Reserved. www.isa.org Presented at ISA Automation Week 2013
Nashville, Tennessee •Renaissance Nashville, USA, 4-7 November 2013
Column Pressure Control
Copyright 2013 ISA. All Rights Reserved. www.isa.org Presented at ISA Automation Week 2013
Nashville, Tennessee •Renaissance Nashville, USA, 4-7 November 2013
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
Copyright 2013 ISA. All Rights Reserved. www.isa.org Presented at ISA Automation Week 2013
Nashville, Tennessee •Renaissance Nashville, USA, 4-7 November 2013
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.
Copyright 2013 ISA. All Rights Reserved. www.isa.org Presented at ISA Automation Week 2013
Nashville, Tennessee •Renaissance Nashville, USA, 4-7 November 2013
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
Copyright 2013 ISA. All Rights Reserved. www.isa.org Presented at ISA Automation Week 2013
Nashville, Tennessee •Renaissance Nashville, USA, 4-7 November 2013
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
Presented at ISA Automation Week 2013 Nashville, Tennessee •Renaissance Nashville, USA, 4-7 November 2013