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

Presenters
Dan Daugherty
Mark Coughran
Ferrill Ford

Why
F OUNDATION 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)

What
Safe place to do a controlled test on a real process
Availability of both F OUNDATION Fieldbus and 4-20 mA loops
Ability to test
Control Response period
load frequency response
setpoint step response

Lab 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

3 rd Loop – Marshalltown Flow Lab PV PT Disturbance EnTech Toolkit

Timing – 4-20mA PID D/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 – FF CIF FF PID FF AO Pneumatic Actuator DVC dead time and time constant Load Valve Motion Hydraulic Pressure (Process) Change 3051 FF AI 3051C Dead Time and Time Constant FF Compel Data

Control Response Period by subtraction 4-20 mA / HART 0.05 sec Load Valve Motion Hydraulic Pressure (Process) 3051C Dead Time and Time Constant 3051C 4-20 output PID A/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

Control Response Period by subtraction Foundation Fieldbus Control-In-the-Field (CIF) 0.10 sec Load Valve Motion Hydraulic Pressure (Process) 3051 Dead Time and Time Constant 3051 FF AI FF PID FF 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 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 measurement CIC, module execution = 1.0, macrocycle = 0.5 1.37 – 0.10 – 0.07 = 1.20 seconds

Sample Control Response Period measurement 4-20 mA, module execution = 0.2 0.30 – 0.05 – 0.07 = 0.18 seconds

Sample histogram from 21 measurements CIC, 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 overview 4-20 mA, DeltaV Control in DVC (CIF) Control in DeltaV (CIC) 2:1 Control in DeltaV (CIC) 4:1 Control in DeltaV (CIC) 1:1 Ratio for Fieldbus Control in DeltaV is Module Execution : Macrocycle

Lambda Tuning for self-regulating processes
Closed Loop (Auto)
No oscillation
 is the closed-loop time constant
Choose the speed
Open loop (Manual)
 is the open-loop time constant
SETPOINT PV  63% 63% PV OUT 

Lambda Tuning for self-regulating process sample Manual step 5% on controller output

Average process dynamics and recommended tuning

Controller 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 response

Theoretical load frequency response

Load 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 = 1.0 AR = 0.41

Load Frequency Response, period 100, CIF, macrocycle = 0.15 AR = 0.18

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

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

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

Acknowledgements
In the Marshalltown lab, thanks to
Rick Osborne
Mike Himes
Kyle Hokanson
Others
Other Emerson sponsors
Advanced Applied Technologies in PS&S

Summary
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 Information
[email_address]
Ferrill. [email_address]
[email_address]
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 Response

Setpoint 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 data

Setpoint 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

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Effects of Macrocycle Time and Sampling Rates on Control Loop Performance

by Jim Cahill on Jan 13, 2010

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Effects of Macrocycle Time and Sampling Rates on Control Loop Performance — Presentation Transcript