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.