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
� 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