effects of macrocycle time and sampling rates on control loop performance

Effects of Macrocycle Time and Sampling Rates on

Control Loop Performance

Effects of Macrocycle Time and Sampling Rates on

Control Loop Performance

Dan Daugherty – Sr. Engineer – Product Engineering

Ferrill Ford – Sr. Engineer – Product Engineering

Mark Coughran – Sr. Industry Consultant – Industry Solutions Group

PresentersPresenters

Dan Daugherty

Mark Coughran

Ferrill Ford

WhyWhy

FOUNDATION Fieldbus perceived as slow by some Control Response specifications by end user or

process licensor Lack of actual field data Questionable recommendation for oversampling

(module execution = 2x macrocycle)

WhatWhat

Safe place to do a controlled test on a real process Availability of both FOUNDATION Fieldbus and 4-20 mA

loops Ability to test

– Control Response period– load frequency response– setpoint step response

Lab setup for hydraulic pressure controlLab setup for hydraulic pressure control

– Fluid process dynamics are negligible

– Significant dynamics are in the sensor/transmitter, control valve, controller, communications

– Control valve first with DVC6010f, then DVC6010

– PT FF, 4-20 were Rosemount 3051C

– PT FAST were Toolkit, 100 Hz

– All signals recorded with Emerson’s EnTech™ Toolkit

OtherStaticLoads

PT FF

DeltaV

PT FAST

PT4-20

OtherStaticLoads

PT FF

DeltaV

PT FAST

PT4-20

PT FAST

TestValveTestValve

LoadValveLoadValve

PV

PT

Disturbance

EnTech

Toolkit

3rd Loop – Marshalltown Flow Lab3rd Loop – Marshalltown Flow Lab

PIDD/A

Conversion

DVC

4-20/HART

Pneumatic

Actuator

DVC dead

time and

time

constant

Load

Valve

Motion

Hydraulic

Pressure

(Process)

Change

3051

4-20/HART

output

3051C

Dead Time

and Time

Constant

A/D

Conversion

Timing – 4-20mATiming – 4-20mA

FF PID FF AO Pneumatic

Actuator

DVC dead

time and

timeconstant

Load

Valve

Motion

Hydraulic

Pressure

(Process)

Change

3051

FF AI

3051C

Dead Time

and Time

Constant

FF

Compel Data

Timing – FF CIFTiming – FF CIF

0.05 sec

Control Response Period by subtraction4-20 mA / HARTControl Response Period by subtraction4-20 mA / HART

Load

Valve

Motion

Hydraulic

Pressure

(Process)

3051C

Dead Time

and Time

Constant

3051C

4-20

output

PIDA/D DVC

4-20

input

D/A

DVC6000

Dead Time

and Time

Constant

Pneumatic

Actuator

Fast

Reference

Pressure

Sensor

0-750 psig

Fast

Reference

Pressure

Sensor

0-50 psig

Control Response Period

Typical Customer Spec.

0.07 sec

Measured Loop Dead Time

In Load Step Test

0.10 sec

Control Response Period by subtractionFoundation Fieldbus Control-In-the-Field (CIF)Control Response Period by subtractionFoundation Fieldbus Control-In-the-Field (CIF)

Load

Valve

Motion

Hydraulic

Pressure

(Process)

3051

Dead Time

and Time

Constant

3051

FF AI

FF PIDFF Compel

Data

FF

AO

DVC6000f

Dead Time

and Time

Constant

Pneumatic

Actuator

Fast

Reference

Pressure

Sensor

0-750 psig

Fast

Reference

Pressure

Sensor

0-50 psig

Control Response Period

Typical Customer Spec.

0.07 sec

Measured Loop Dead Time

In Load Step Test

Load step tests for Control Response PeriodLoad step tests for Control Response Period Step the output to the load valve The PID control loop approximates proportional-only

action– Gain = 0.5– Reset = 100000

Fit the responses in Emerson’s EnTech™ Toolkit Only the “dead time” part of the measurement is

significant Subtract the response times of transmitter and

control valve that are not defined as part of Control Response Period

Average the results from at least 10 measurements

Sample Control Response Period measurementCIC, module execution = 1.0, macrocycle = 0.5Sample Control Response Period measurementCIC, module execution = 1.0, macrocycle = 0.5

1.37 – 0.10 – 0.07 = 1.20 seconds

Sample Control Response Period measurement4-20 mA, module execution = 0.2Sample Control Response Period measurement4-20 mA, module execution = 0.2

0.30 – 0.05 – 0.07 = 0.18 seconds

0

2

4

6

8

10

12

14

16

18

20

1.20 1.25 1.30 1.35 1.40 1.45 1.50 1.55 1.60

Bin Intervals (seconds)

# o

f c

ou

nts

Sample histogram from 21 measurementsCIC, module execution = 1.0, macrocycle = 0.5Sample histogram from 21 measurementsCIC, module execution = 1.0, macrocycle = 0.5

Mean value of raw dead time

= 1.39 seconds

Corrected value

(Control Response Period)

= 1.22 seconds

Control Response Period results overviewControl Response Period results overview

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0 0.5 1.0

Macrocycle for Fieldbus or Module Execution for 4-20 mA (seconds)

Co

ntr

ol

Res

po

nse

Per

iod

(se

con

ds)

4-20 mA, DeltaV

Control in DVC (CIF)

Control in DeltaV (CIC) 2:1



Ratio for Fieldbus Control in DeltaV is

Module Execution : Macrocycle

Lambda Tuning for self-regulating processesLambda Tuning for self-regulating processes

Closed Loop (Auto)– No oscillation is the closed-loop time

constant– Choose the speed

SETPOINT

PV

63%

Open loop (Manual)

is the open-loop time constant

63%

PV

OUT

Lambda Tuning for self-regulating processsample Manual step 5% on controller outputLambda Tuning for self-regulating processsample Manual step 5% on controller output

Average process dynamics and recommended tuningAverage process dynamics and recommended tuning

Controller tuning philosophyController tuning philosophy

Only needed for sine wave load disturbance and setpoint response tests– Does not apply to a Control Response Period specification

Lambda = 1.5 seconds is fast relative to typical tuning of flow and pressure loops in the field

Is based on fast controller execution In principle, this should be changed (detuned) as we

increase either module execution time or macrocycle In practice, we didn’t have time to customize tuning

for each combination of communication method, module execution, and macrocycle

Theoretical setpoint step responseTheoretical setpoint step response

Theoretical load frequency responseTheoretical load frequency response

Load Frequency Response Tests—Introduction and NotationLoad Frequency Response Tests—Introduction and Notation

Sinusoidal output to the load valve Most tests used disturbance period = 100 seconds

– This period gives the feedback loop a chance to attenuate a significant amount of the variability

Same PID tuning for all: Gain = 0.35, Reset = 0.48 CIC ≡ Fieldbus, DeltaV, DVC6000f CIF ≡ Fieldbus, DVC6000f Analog ≡ 4-20 mA, DeltaV, DVC6000 AR ≡Amplitude Ratio

– Auto Amplitude / Manual Amplitude

Load Frequency Response, period 100,CIC, module execution = 1.0, macrocycle = 1.0Load Frequency Response, period 100,CIC, module execution = 1.0, macrocycle = 1.0

Var 01 Analog3051C.PV FFF030EC Manual.dat3051C HART 07/09/2008 13:59:53

Time Series

0.0 100.0 200.0 300.0 400.0Sec

30.00

35.00

40.00

45.00

50.00psia

Mean=39.2588 2Sig=11.56 (29.4%)


Power Spectrum PeaksDe-Trend=No, Win=None, Seg=0

Lower Threshold: 0.083463, Change Threshold: 0.10016

Total Variance: 34.187% Total P-P 2 Sigma

Peak Freq. Period Shape Variance Amplit. Remain.

1 0.010173 98.304 1 99.41 16.488 0.90073

Var 02 Analog3051C.PV FFF030EC Auto.dat3051C HART 07/09/2008 13:59:53

Time Series

0.0 100.0 200.0 300.0 400.0Sec

30.00

35.00

40.00

45.00

50.00psia

Mean=40.1802 2Sig=4.715 (11.7%)






1 0.010150 98.524 -2 95.55 6.4219 0.98010

2 0.020345 49.152 1 3.290 1.1916 4.5685

3 0.040690 24.576 1 0.2706 0.34176 4.6392

AR = 0.41

Var 01 Analog3051C.PV FFF039AC Manual.dat3051C HART 07/09/2008 15:14:45

Time Series

0.0 100.0 200.0 300.0 400.0Sec

30.00

35.00

40.00

45.00

50.00psia

Mean=39.607 2Sig=11.7 (29.6%)






1 0.010173 98.304 1 99.45 16.600 0.86959

Var 02 Analog3051C.PV FFF039AC Auto.dat3051C HART 07/09/2008 15:14:45

Time Series

0.0 100.0 200.0 300.0 400.0Sec

30.00

35.00

40.00

45.00

50.00psia

Mean=40.0829 2Sig=3.018 (7.53%)

Var 02 Analog3051C.PV FFF039AC Auto.dat3051C HART 07/09/2008 15:14:45


Lower Threshold: 2.807E-3, Change Threshold: 3.368E-3



1 0.010188 98.152 6 96.00 4.2023 0.60662

2 0.020345 49.152 1 2.372 0.66051 2.9965

3 0.040690 24.576 1 0.1721 0.17792 3.0301

4 0.061035 16.384 1 0.1223 0.15000 3.0309


AR = 0.26


Time Series

0.0 100.0 200.0 300.0 400.0Sec

30.00

35.00

40.00

45.00

50.00psia

Mean=38.7695 2Sig=11.77 (30.4%)






1 0.010173 98.304 1 99.54 16.604 0.79930


Time Series

0.0 100.0 200.0 300.0 400.0Sec

30.00

35.00

40.00

45.00

50.00psia

Mean=39.9527 2Sig=4.464 (11.2%)






1 0.010186 98.176 2 97.20 6.2818 0.75383

2 0.020345 49.152 1 1.943 0.88823 4.4615


AR = 0.38

Load Frequency Response, period 100,CIF, macrocycle = 0.15Load Frequency Response, period 100,CIF, macrocycle = 0.15


Time Series

0.0 100.0 200.0 300.0 400.0Sec

30.00

35.00

40.00

45.00

50.00psia

Mean=38.8851 2Sig=11.68 (30%)






1 0.010173 98.304 1 99.66 16.592 0.68533


Time Series

0.0 100.0 200.0 300.0 400.0Sec

30.00

35.00

40.00

45.00

50.00psia

Mean=39.9814 2Sig=2.13 (5.33%)



Lower Threshold: 2.753E-3, Change Threshold: 3.303E-3



1 0.010173 98.304 1 94.76 2.9237 0.48602

2 0.020345 49.152 1 2.669 0.49065 2.0952

3 0.030518 32.768 1 0.2957 0.16332 2.1206

4 0.040690 24.576 1 0.2773 0.15816 2.1208

AR = 0.18

What if 8 loops on the FF segment?CIC (DeltaV) theoreticalWhat if 8 loops on the FF segment?CIC (DeltaV) theoretical

What if 8 loops on the FF segment?CIF (DVC) theoreticalWhat if 8 loops on the FF segment?CIF (DVC) theoretical

Conclusions with more loops on the segmentConclusions with more loops on the segment

Shows even more reason to use CIF CIF should be fast enough for nearly all loops in the

plant Exceptional loops already have dedicated controllers;

e.g. surge control, compressor lube oil– Even these applications can be handled in some cases with

CIF; see Rezabek and Peluso, EGUE2008

Business Results AchievedBusiness Results Achieved

Density on Fieldbus segments Identifying latency ‘opportunities’ Avoid slow responses

AcknowledgementsAcknowledgements

In the Marshalltown lab, thanks to– Rick Osborne– Mike Himes– Kyle Hokanson– Others

Other Emerson sponsors– Advanced Applied Technologies in PS&S

SummarySummary

Foundation Fieldbus Control-In-the-Field– proved Control Response Period equal to macrocycle– Can get 0.18 seconds, adequate for almost all loops

Foundation Fieldbus Control-In-the-Controller/Host– Control Response Period can be much greater than

expected– C-I-C not recommended to get full benefit from Fieldbus– Oversampling (Module Execution>Macrocycle) did not show

any benefit

Your comments and questions are welcome

Where To Get More InformationWhere To Get More Information

[email protected] [email protected] [email protected] John Rezabek in Control Magazine

(www.controlglobal.com); July 2008 “Ready for Control in the Field?”; November 2007 “Load ‘Em Up!”

John Rezabek and Marcos Peluso, EGUE2008, “Field- based control for compressor anti-surge”

Pang et al., “Analysis of control interval for foundation fieldbus-based control systems”, ISA Transactions, Volume 45, Number 3, July 2006, pages 447-458.

Appendix—Setpoint Step ResponseAppendix—Setpoint Step Response

Setpoint Step Tests—Introduction and NotationSetpoint Step Tests—Introduction and Notation

Timing of the setpoint steps was not automated Same PID tuning for all: Gain = 0.35, Reset = 0.48 CIC ≡ Fieldbus, DeltaV, DVC6000f CIF ≡ Fieldbus, DVC6000f Analog ≡ 4-20 mA, DeltaV, DVC6000 AST ≡ Average Settling Time Settling time ≡ dead time plus four time constants

from first-order curve fit

Setpoint step test sample dataSetpoint step test sample data

30

35

40

45

50

0 50 100 150 200

Time (seconds)

Pro

cess

Pre

ssu

re (

psi

a)

0

1

2

3

4

5

6

7

8

Po

siti

on

er O

utp

ut

(psi

g)

AUTO

CIC, Module = 1.0, Macrocycle = 1.0AST = 13 seconds


30

35

40

45

50

0 50 100 150 200

Time (seconds)

Pro

cess

Pre

ssu

re (

psi

a)

0

1

2

3

4

5

6

7

8

Po

siti

on

er O

utp

ut

(psi

g)

AUTO

CIF, Macrocycle = 0.15AST = 10 seconds


30

35

40

45

50

0 50 100 150 200

Time (seconds)

Pro

cess

Pre

ssu

re (

psi

a)

25

35

45

55

65

Co

ntr

olle

r O

utp

ut

(%)

AUTO

Analog, Module = 0.2AST = 11 seconds

Setpoint step test conclusionsSetpoint step test conclusions

Did not attempt to optimize PID tuning for each case All SP responses were stable and quick, with settling

time on the order of 5* as per theory Settling times generally faster with smaller module

execution time and/or macrocycle The limit cycle caused by control valve nonlinearity

makes it difficult to measure or compare the responses

effects of macrocycle time and sampling rates on control loop performance

Technology

value control response

deltav control

module execution time

pid control loop

communications control

closedloop time constant

effects of macrocycle

dvc cif control