Effects of Macrocycle Time and Sampling Rates on Control Loop

Performance

Effects of Macrocycle Time and Sampling Rates on Control Loop

PerformancePerformancePerformance

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 � 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– 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

OtherStaticLoads

OtherStaticLoads

PT FAST

TestValveTestValve

LoadValveLoadValve – PT FF, 4-20 were Rosemount 3051C

– PT FAST were Toolkit, 100 Hz

– All signals recorded with Emerson’s

EnTech™ Toolkit

PT FF

DeltaV

PT FAST

PT4-20

PT FF

DeltaV

PT FAST

PT4-20

PV

PT

Disturbance

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

EnTech

Toolkit

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 PIDFF 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

Fast

Reference

Pressure

Control Response Period

Typical Customer Spec.

0.07 sec

Pressure

Sensor

0-750 psig

Pressure

Sensor

0-50 psig

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 AIFF PID

FF Compel

Data

FF

AO

DVC6000f

Dead Time

and Time

Constant

Pneumatic

Actuator

Fast

Reference

Pressure

Fast

Reference

Pressure

Control Response Period

Typical Customer Spec.

0.07 sec

Pressure

Sensor

0-750 psig

Pressure

Sensor

0-50 psig

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 � 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 seconds1.37 – 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

10

12

14

16

18

20# of counts

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

0

2

4

6

8

10

1.20 1.25 1.30 1.35 1.40 1.45 1.50 1.55 1.60

Bin Intervals (seconds)

# of counts

Control Response Period results overviewControl Response Period results overview

1.5

2.0

2.5

3.0Control Response Period (seconds)

Control in DeltaV (CIC) 4:1

Control in DeltaV (CIC) 1:1

Ratio for Fieldbus Control in DeltaV is

Module Execution : Macrocycle

0.0

0.5

1.0

0.0 0.5 1.0

Macrocycle for Fieldbus or Module Execution for 4-20 mA

(seconds)

Control Response Period (seconds)

4-20 mA, DeltaV

Control in DVC (CIF)

Control in DeltaV (CIC) 2:1

Control in DeltaV (CIC) 4:1

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

Closed Loop (Auto)

– No oscillation

– λ is the closed-loop time constant

– Choose the speed

Open loop (Manual)

– τ is the open-loop time

constant

OUTSETPOINT

PV

λλλλ

63%

63%

PV

OUT

ττττ

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.dat

3051C HART 07/09/2008 13:59:53

Time Series

0.0 100.0 200.0 300.0 400.0

Sec

30.00

35.00

40.00

45.00

50.00

psia

Mean=39.2588 2Sig=11.56 (29.4%)

Var 01 Analog3051C.PV FFF030EC Manual.dat

3051C HART 07/09/2008 13:59:53

Power Spectrum Peaks

De-Trend=No, Win=None, Seg=0

Lower Threshold: 0.083463, Change Threshold: 0.10016

Total Variance: 34.187% Total P-P

Peak Freq. Period Shape Variance Amplit.

1 0.010173 98.304 1 99.41 16.488

AR = 0.41Var 02 Analog3051C.PV FFF030EC Auto.dat

3051C HART 07/09/2008 13:59:53

Time Series

0.0 100.0 200.0 300.0 400.0

Sec

30.00

35.00

40.00

45.00

50.00

psia

Mean=40.1802 2Sig=4.715 (11.7%)

Var 02 Analog3051C.PV FFF030EC Auto.dat

3051C HART 07/09/2008 13:59:53

Power Spectrum Peaks

De-Trend=No, Win=None, Seg=0

Lower Threshold: 0.013172, Change Threshold: 0.015806

Total Variance: 5.3952% Total P-P

Peak Freq. Period Shape Variance Amplit.

1 0.010150 98.524 -2 95.55 6.4219

2 0.020345 49.152 1 3.290 1.1916

3 0.040690 24.576 1 0.2706 0.34176

AR = 0.41

Var 01 Analog3051C.PV FFF039AC Manual.dat

3051C HART 07/09/2008 15:14:45

Time Series

0.0 100.0 200.0 300.0 400.0

Sec

30.00

35.00

40.00

45.00

50.00

psia

Mean=39.607 2Sig=11.7 (29.6%)

Var 01 Analog3051C.PV FFF039AC Manual.dat

3051C HART 07/09/2008 15:14:45

Power Spectrum Peaks

De-Trend=No, Win=None, Seg=0

Lower Threshold: 0.084557, Change Threshold: 0.10147

Total Variance: 34.635% Total P-P 2 Sigma

Peak Freq. Period Shape Variance Amplit. Remain.

1 0.010173 98.304 1 99.45 16.600 0.86959

Load Frequency Response, period 100,CIC, module execution = 0.5, macrocycle = 0.5Load Frequency Response, period 100,CIC, module execution = 0.5, macrocycle = 0.5

AR = 0.26Mean=39.607 2Sig=11.7 (29.6%)

Var 02 Analog3051C.PV FFF039AC Auto.dat

3051C HART 07/09/2008 15:14:45

Time Series

0.0 100.0 200.0 300.0 400.0

Sec

30.00

35.00

40.00

45.00

50.00

psia

Mean=40.0829 2Sig=3.018 (7.53%)

Var 02 Analog3051C.PV FFF039AC Auto.dat

3051C HART 07/09/2008 15:14:45

Power Spectrum Peaks

De-Trend=No, Win=None, Seg=0

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

Total Variance: 2.2994% Total P-P 2 Sigma

Peak Freq. Period Shape Variance Amplit. Remain.

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

Var 01 Analog3051C.PV FFF033EC Manual.dat

3051C HART 07/09/2008 14:21:59

Time Series

0.0 100.0 200.0 300.0 400.0

Sec

30.00

35.00

40.00

45.00

50.00

psia

Mean=38.7695 2Sig=11.77 (30.4%)

Var 01 Analog3051C.PV FFF033EC Manual.dat

3051C HART 07/09/2008 14:21:59

Power Spectrum Peaks

De-Trend=No, Win=None, Seg=0

Lower Threshold: 0.084522, Change Threshold: 0.10143

Total Variance: 34.620% Total P-P 2 Sigma

Peak Freq. Period Shape Variance Amplit. Remain.

1 0.010173 98.304 1 99.54 16.604 0.79930

Load Frequency Response, period 100,CIC, module execution = 1.0, macrocycle = 0.5Load Frequency Response, period 100,CIC, module execution = 1.0, macrocycle = 0.5

AR = 0.38Mean=38.7695 2Sig=11.77 (30.4%)

Var 02 Analog3051C.PV FFF033EC Auto.dat

3051C HART 07/09/2008 14:21:59

Time Series

0.0 100.0 200.0 300.0 400.0

Sec

30.00

35.00

40.00

45.00

50.00

psia

Mean=39.9527 2Sig=4.464 (11.2%)

Var 02 Analog3051C.PV FFF033EC Auto.dat

3051C HART 07/09/2008 14:21:59

Power Spectrum Peaks

De-Trend=No, Win=None, Seg=0

Lower Threshold: 0.012390, Change Threshold: 0.014867

Total Variance: 5.0748% Total P-P 2 Sigma

Peak Freq. Period Shape Variance Amplit. Remain.

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 = 0.15Var 01 Analog3051C.PV FFF025AC Manual.dat

3051C HART 07/09/2008 10:05:05

Time Series

0.0 100.0 200.0 300.0 400.0

Sec

30.00

35.00

40.00

45.00

50.00

psia

Mean=38.8851 2Sig=11.68 (30%)

Var 01 Analog3051C.PV FFF025AC Manual.dat

3051C HART 07/09/2008 10:05:05

Power Spectrum Peaks

De-Trend=No, Win=None, Seg=0

Lower Threshold: 0.084296, Change Threshold: 0.10116

Total Variance: 34.528% Total P-P

Peak Freq. Period Shape Variance Amplit.

1 0.010173 98.304 1 99.66 16.592

AR = 0.18SecMean=38.8851 2Sig=11.68 (30%)

Var 02 Analog3051C.PV FFF026EC Auto.dat

3051C HART 07/09/2008 10:31:19

Time Series

0.0 100.0 200.0 300.0 400.0

Sec

30.00

35.00

40.00

45.00

50.00

psia

Mean=39.9814 2Sig=2.13 (5.33%)

Var 02 Analog3051C.PV FFF026EC Auto.dat

3051C HART 07/09/2008 10:31:19

Power Spectrum Peaks

De-Trend=No, Win=None, Seg=0

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

Total Variance: 1.1275% Total P-P

Peak Freq. Period Shape Variance Amplit.

1 0.010173 98.304 1 94.76 2.9237

2 0.020345 49.152 1 2.669 0.49065

3 0.030518 32.768 1 0.2957 0.16332

4 0.040690 24.576 1 0.2773 0.15816

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 oile.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� 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

expectedexpected

– 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

� Dan.Daugherty@Emerson.com

� Ferrill.Ford@Emerson.com

� Mark.Coughran@Emerson.com

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

40

45

50

Process Pressure (psia)

4

5

6

7

8

Positioner Output (psig)

AUTO

30

35

0 50 100 150 200

Time (seconds)

Process Pressure (psia)

0

1

2

3

Positioner Output (psig)

CIC, Module = 1.0, Macrocycle = 1.0

AST = 13 seconds

Setpoint step test sample dataSetpoint step test sample data

40

45

50

Process Pressure (psia)

4

5

6

7

8

Positioner Output (psig)

AUTO

30

35

0 50 100 150 200

Time (seconds)

Process Pressure (psia)

0

1

2

3

Positioner Output (psig)

CIF, Macrocycle = 0.15

AST = 10 seconds

Setpoint step test sample dataSetpoint step test sample data

40

45

50

Process Pressure (psia)

45

55

65

Controller Output (%)

AUTO

30

35

0 50 100 150 200

Time (seconds)

Process Pressure (psia)

25

35

Controller Output (%)

Analog, Module = 0.2

AST = 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*λ as per theory

� Settling times generally faster with smaller module

execution time and/or macrocycleexecution time and/or macrocycle

� The limit cycle caused by control valve nonlinearity

makes it difficult to measure or compare the

responses