Control Using Wireless Measurements PIDPlus vs an Observer with PID
Jan 28, 2015
Control Using Wireless Measurements
PIDPlus vs an Observer with PID
Presenters
Terry Blevins
Mark Nixon
Introduction
In this workshop we show how an observer may be modified for use with a wireless measurement and compare control performance to that achieve using DeltaV PIDPlus.
Background on observers and development of PIDPlus
Modification of Kalman filter and Smith Predictor for use with a Wireless Measurement in Control applications
Test Results –PIDPlus, Modified Kalman Filter and Smith Predictor
Summary
Where To Get More Information
Traditional Approach in Control
Controllers are commonly designed to over-sample the measurement by a factor of 2-10X.
To minimize control variation feedback control is executed 4X to 10X times faster than the process response time i.e. process time constant plus process delay.
However, the underlying assumption in the control design (using z transform, difference equations) and digital implementation of the PID is that the algorithm is executed on a periodic basis.
Control Data Sample Rate
Control Using Wireless Measurements
To minimize the power consumed in communicating new measurement values
the transmitter may use Window communication according to the following
rules:
The transmitter will periodically sample the measurement 4-10x faster than
the process response time.
If the magnitude of the difference between the new measurement value and
the last communicated measurement value is greater than a specified
resolution or if the time since the last communication exceeds a refresh time
the new value is communicated.
When the measurement is not updated, the calculated reset and derivative
action may not be appropriate.
PIDPLUS for Wireless Control
The PID is restructured to reflect the reset contribution for the expected process response since the last measurement update.
The rate contribution is recomputed and updated only when a new measurement is received - using the elapsed time since the last new measurement.
Use of Observers in Wireless Control
The DeltaV Future architecture team has investigated whether an observer can be used for PID control with a wireless measurement. Two types of observers were considered:
Kalman Filter
Smith Predictor
It was necessary to modify these observers to work correctly in control using a PID and wireless measurement
Kalman Filter - Modificatons for Noise with Non-zero Mean
Composite is available
through Application exchange
for use with a continuous
measurement
See workshop 124381,
Emerson Exchange 2013,
“Addressing Control in the
Presence of Process and
Measurement Noise”
Kalman Filter - Modificatons for Wireless Measurement and Noise with Non-zero Mean
The calculation of
the residual must be
modified to account
for the slow, non-
periodic update of
the process
measurement
Application of Smith Predictor With PID
Smith Predictor is a
standard module in the
DeltaV library.
Most commonly used
in continuous process
control applications that
are dominated by large
deadtime.
Smith Predictor – Modification for Wireless Measurement
The calculation of the
residual must be
modified to account
for the slow, non-
periodic update of the
process measurement
Comparing Performance - Test Conditions
A first order plus deadtime process was used to compare the performance of
PIDPlus to PID with observers modified for wireless control.
Process Gain = 1
Process Time Constant = 6 sec
Process Deadtime = 2 sec
The PI control was tuned for a lambda factor of 1.
GAIN = 1/Process Gain
RESET = Process Time Constant + Process Deadtime
The process input and output were scaled 0-100%. The simulated wireless
transmitter is configured for 1% change and 10 sec default period using
Windowed communications. The module execution rate was set at 0.5 sec.
Test Environment – Wireless Control Using Kalman Filter vs wired PID
Modified Kalman vs. Wired PID
Test Environment – Wireless Control Using Smith Predictor vs Wired PID
Modified Smith Predictor vs Wired PID
Test Environment – Wireless Control Using PIDPlus vs Wired PID
PIDPlus vs Wired PID
Wireless Control Performance
Test were performed under
ideal conditions where the
model used in the Kalman
filter and Smith predictor
are based on the process
gain and response.
All methods provide good
control. As measured by
IAE, the PIDPlus did
significantly better than the
modified Smith Predictor
and slight worse that the
modified Kalman filter.
PIDPlus does not require
that the user entry process
model parameters.
Conclusion
A method for dealing with non-periodic measurement updates is a requirement when closed loop control is implemented using wireless transmitters.
The DeltaV PIDPlus is designed to use non-periodic, slow measurement updates. Alternative approach based on using PID with a modifed Kalman filter or a Smith Predictor may also be used but requires setup of the process model.
The PIDPlus is preferred for wireless control since it is more robust for changes in process gain, dynamics, and noise level.
Where To Get More Information
Workshop 12-4381, Emerson Exchange 2013, Wojsznis and Blevins, “Addressing
Control in the Presence of Process and Measurement Noise” (Using Kalman Filter)
E. Cheever. “Introduction to Kalman Filter”, http://
www.swarthmore.edu/NatSci/echeeve1/Ref/Kalman/ScalarKalman.html
Application Exchange, “Kalman Filtering in DeltaV – Control in the Presence of
Noise”, http://
www2.emersonprocess.com/en-US/brands/deltav/interactive/Pages/Interactive.aspx
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
F. Siebert, and T. Blevins, “WirelessHART Successfully Handles Control”,
Chemical Process, January, 2011
Thank You for Attending!
Enjoy the rest of the conference.