Using Wireless Measurements in Control Applications

Using Wireless Measurements
in Control Applications

Standards

Terry Blevins
Mark Nixon
Marty Zielinski

Certification
Education & Training
Publishing
Conferences & Exhibits
Copyright 2013 ISA. All Rights Reserved. www.isa.org
Presented at ISA Automation Week 2013
Nashville, Tennessee •Renaissance Nashville, USA, 4-7 November 2013

Presenters

Marty Zielinski
Terry Blevins


2

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 oversample 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

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

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
Lambda Tuning ʎ = 1.0
Communication Resolution = 1%
Communication Refresh = 10sec
Setpoint

PIDPlus

Control
Measurement

PID
PIDPlus
Control Output
PID
Unmeasured
Disturbance


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

Setpoint

PIDPlus
Control
Measurement
PID
PIDPlus

Control Output
PID

Communication Loss


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
Wired Measurement
Used in Control

Wireless Measurement
Used in 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

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
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•
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•

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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.5363, http://classes.engineering.wustl.edu/2009/spring/che433/2009LAB/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%20S
uccessfully%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. ISA. All Rights Reserved. www.isa.org
Copyright 2013

19

Using Wireless Measurements in Control Applications

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