PID Control of Valve Sticktion and Backlash - Greg McMillan Deminar Series

Interactive Opportunity Interactive Opportunity AssessmentAssessmentInteractive Opportunity Interactive Opportunity AssessmentAssessment

Demo and Seminar (Deminar) Series for Web Labs –

PID Control of Valve Sticktion and Backlash

April 21, 2010Sponsored by Emerson, Experitec, and Mynah

Created byGreg McMillan and Jack Ahlers

[File Name or Event]Emerson Confidential27-Jun-01, Slide 2 Slide 2

WelcomeWelcome WelcomeWelcome Gregory K. McMillan

– Greg is a retired Senior Fellow from Solutia/Monsanto and an ISA Fellow. Presently, Greg contracts as a consultant in DeltaV R&D via CDI Process & Industrial. Greg received the ISA “Kermit Fischer Environmental” Award for pH control in 1991, the Control Magazine “Engineer of the Year” Award for the Process Industry in 1994, was inducted into the Control “Process Automation Hall of Fame” in 2001, and was honored by InTech Magazine in 2003 as one of the most influential innovators in automation. Greg is the author of numerous books on process control, his most recent being Essentials of Modern Measurements and Final Elements for the Process Industry. Greg has been the monthly “Control Talk” columnist for Control magazine since 2002. Greg’s expertise is available on the web site: http://www.modelingandcontrol.com/


The Latest on Valve Response The Latest on Valve Response and Smart Positionersand Smart Positioners

The Latest on Valve Response The Latest on Valve Response and Smart Positionersand Smart Positioners

Royalties are donated to theUniversity of Texas Research Campus for Energy and Environmental Resources for Development of WirelessInstrumentation and Control


Why?Why?Why?Why?

My personal calling is to develop and communicate a deeper understanding of instrumentation, valves, and process control. My 20 books, 50+ articles and papers, and 100+ “Control Talk” Columns are a reflection of this quest.

Hopefully the unconventional cartoons, “Top Ten” lists, and musings provide some insights and breaks down barriers, and opens up minds as only humor can do.

However all of these literary means are static. I am ready to take the web site, which is an attempt to provide a more dynamic sharing of knowledge, to the next level.

My dream is that users would have worldwide access to an interactive learning tool that offers the control room experience to discover, innovate, and generate (DIG) process control improvements (PCI) to make plants more efficient and productive.

The interactive tool would be virtual plant labs of generic loops and common unit operations available anywhere anytime over the web featuring:

– A non system specific interface by means of control room type operator graphics

– The power of the DCS and in particular the PID controller

– Performance metrics (e.g. integrated and peak errors for generic loops, number of communications for wireless measurements, accumulated travel for valves, and COGS and profit for specific unit operations)


Top Ten Benefits of Membership in Top Ten Benefits of Membership in Adorable Automator’s AssociationAdorable Automator’s AssociationTop Ten Benefits of Membership in Top Ten Benefits of Membership in Adorable Automator’s AssociationAdorable Automator’s Association

(10) Free admission to the Adorable Automator’s Hall of Fame - the hottest ticket in town

(9) Discounted admission to the Cinemax Film “Famous Control Scenes” - 3D trend charts

(8) The automator’s alternative to “Fruit of the Loom” underwear – “Geek in the Room”

(7) “Fuzzy is Cute” buttons - fuzzy logic memorabilia (6) Dating service - “let us automate your pleasures” (5) Simulation services - “almost as good as the real thing” (4) Calendar - “Adorable Automators of the Month” (3) Souvenir T-Shirts - “My boss went to the ISA executive meeting

and all I got was this shirt” (2) Hardhats that double as extreme bull riding helmets

And the Number 1 Benefit:


Top Ten Benefits of Membership in Top Ten Benefits of Membership in Adorable Automator’s AssociationAdorable Automator’s AssociationTop Ten Benefits of Membership in Top Ten Benefits of Membership in Adorable Automator’s AssociationAdorable Automator’s Association

(1) Travel discounts to exotic plants - ride elephants in Thailand

Sheldon Lloyd – Developer of I/P and Positioner - Retired VP of Technology for Fisher ControlsSee Control Magazine May 2007 Control Talk “The Best of the Best – Part 5

http://www.controlglobal.com/articles/2007/127.html


Loop Lab02 Demo 1Loop Lab02 Demo 1Loop Lab02 Demo 1Loop Lab02 Demo 1 Objective – Show access to cascade loop lab setup and

how to make load upsets to see PID response Activities:

– Show access to Cascade Loop Lab02 user interface

– Show access to PID faceplate and detail

– Show access to “Process History View” trend chart

– Make load change to show cascade response by putting primary PID momentarily in manual and changing its PID output (e.g. 50% to 60%)

– Show access to valve, process, and measurement parameters for sensitivity, noise, and dynamics

– Put secondary PID in AUTO mode

– Make load change to show secondary response by putting secondary PID momentarily in manual and changing its output (e.g. 50% to 60%)


QuestionQuestionQuestionQuestion What oscillation cannot be eliminated by PID controller tuning? What is the most common cause of a sustained oscillation? What response problems are not seen in open loop tuning tests? What creates a deadtime inversely proportional to controller gain? What problem became rampant in the 1980s? What is the biggest hidden menace to loop performance?

Answer : ?Answer : ?


QuestionQuestionQuestionQuestion What oscillation cannot be eliminated by PID controller tuning? What is the most common cause of a sustained oscillation? What response problems are not seen in open loop tuning tests? What creates a deadtime inversely proportional to controller gain? What problem became rampant in the 1980s? What is the biggest hidden menace to loop performance?

Answer : Valve Backlash and SticktionAnswer : Valve Backlash and Sticktion


Loop Lab02 Demo 2Loop Lab02 Demo 2Loop Lab02 Demo 2Loop Lab02 Demo 2 Objective – Compare base case to scenario with 5%

stick-slip on response of Secondary PID in AUTO mode Activities:

– First look at load response of PID for base case

– Keep the secondary PID in AUTO mode

– Increase valve stick-slip to 5%

– Make load change to show secondary response by putting secondary PID momentarily in manual and changing its output (e.g. 50% to 60%)


Valve Backlash (Deadband) Valve Backlash (Deadband) and Sticktion (Stick-Slip)and Sticktion (Stick-Slip)

Valve Backlash (Deadband) Valve Backlash (Deadband) and Sticktion (Stick-Slip)and Sticktion (Stick-Slip)

dead band

Deadband

Stick-Slip is worse near closed position

Signal (%)

0

Stroke (%) Digital positioner

will force valve shut at 0% signal

Pneumatic positionerrequires a negative % signal to close valve

Valve deadband and stick-slip is greatest near the closed position

Deadband is 5% - 50%without a positioner !

Deadband is the change in signal to reverse direction of stroke -principal sources are links and mechanisms for rotary valve actuation

Stick-slip is the smallest possible change in valve stroke – principal sources are friction in packing and friction in closure element (trim) seal

(can be seen as shaft windup and breakaway in rotary valves)

Sensitivity limits associated with piston actuator and pneumatic positionerscreate a staircase response similar to stick-slip


Loop Lab02 Demo 3Loop Lab02 Demo 3Loop Lab02 Demo 3Loop Lab02 Demo 3 Objective – Compare slow versus fast tuning effect on

Secondary PID AUTO Mode Response to 5% Stick-Slip Activities:

– First look at limit cycle in secondary PID for 5% stick-slip for fast tuning

– Increase secondary PID reset time (e.g. 2 to 10 sec) to show the effect of slow tuning


Direct Connection of Piston Direct Connection of Piston Actuator to Rotary ValveActuator to Rotary Valve

Direct Connection of Piston Direct Connection of Piston Actuator to Rotary ValveActuator to Rotary Valve

Less backlash but wear of piston O-ring seal from piston pitch is concern

See “Best Practices for Valve Performance” slidefor best solution to minimize backlash and stick-slip


Link-Arm Connection of Piston Link-Arm Connection of Piston Actuator to Rotary ValveActuator to Rotary Valve

Link-Arm Connection of Piston Link-Arm Connection of Piston Actuator to Rotary ValveActuator to Rotary Valve

Significant backlash from link pin points 1 and 2



Rack and Pinion Connection of Piston Rack and Pinion Connection of Piston Actuator to Rotary ValveActuator to Rotary Valve

Rack and Pinion Connection of Piston Rack and Pinion Connection of Piston Actuator to Rotary ValveActuator to Rotary Valve

Stick-slip from rack and gear teeth - particularly bad for worn teeth



Scotch-Yoke Connection of Piston Scotch-Yoke Connection of Piston Actuator to Rotary ValveActuator to Rotary Valve

Scotch-Yoke Connection of Piston Scotch-Yoke Connection of Piston Actuator to Rotary ValveActuator to Rotary Valve

Lots of backlash from slot



Loop Lab02 Demo 4Loop Lab02 Demo 4Loop Lab02 Demo 4Loop Lab02 Demo 4 Objective – See the effect of secondary PID I-deadband

on limit cycle in PID AUTO mode for 5% stick-slip Activities:

– First look at limit cycle for 5% stick-slip for slow tuning

– Return secondary PID to original reset time (e.g. 10 to 2 sec)

– Set secondary PID I-deadband = 4%

– Discuss function and estimation of I-deadband setting


Installed Characteristic (Linear)Installed Characteristic (Linear)Installed Characteristic (Linear)Installed Characteristic (Linear)

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Inherent Characteristic

Installed Characteristic 1




Valve pressure drop ratio (PR)for installed characteristic:

Characteristic 1: PR 0.5 Characteristic 2: PR 0.25Characteristic 3: PR 0.125

Characteristic 4: PR 0.0625


Installed Characteristic (Equal Percentage)Installed Characteristic (Equal Percentage)Installed Characteristic (Equal Percentage)Installed Characteristic (Equal Percentage)

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Inherent Characteristic





Valve pressure drop ratio (PR)for installed characteristic:

Characteristic 1: PR 0.5 Characteristic 2: PR 0.25

Characteristic 3: PR 0.125Characteristic 4: PR 0.0625


Loop Lab02 Demo 5Loop Lab02 Demo 5Loop Lab02 Demo 5Loop Lab02 Demo 5 Objective – See the effect of secondary PID I-Deadband

on limit cycle in PID CAS mode for 5% stick-slip Activities:

– First look at elimination of limit cycle by secondary PID I-deadband for 5% stick-slip for AUTO mode

– Put secondary PID in CAS Mode


Real Valve Rangeability Real Valve Rangeability Real Valve Rangeability Real Valve Rangeability

Minimum fractional flow coefficient for an equal percentage trim and stick-slip:

Minimum controllable fractional flow for installed characteristic and stick-slip:

Cxmin minimum flow coefficient expressed as a fraction of maximum (dimensionless)Pr valve pressure drop ratio (dimensionless) Qxmin minimum flow expressed as a fraction of the maximum (dimensionless)Rv rangeability of control valve (dimensionless) R range of the equal percentage characteristic (e.g. 50 - 200)Xvmin maximum valve stroke (%)Sv stick-slip near closed position (%)

maxmin

v

vx X

SC

]1[

minmax

v

vv

X

S

x RC

2min

minmin

)1( xRR

xx

CPP

CQ

min

1

xv QR


Loop Lab02 Demo 6Loop Lab02 Demo 6Loop Lab02 Demo 6Loop Lab02 Demo 6 Objective – See effect of I-deadband in both primary and

secondary on PID CAS Mode for 5% stick-slip Activities:

– First look at creation of limit cycle in secondary PID for 5% stick-slip for CAS mode

– Set primary PID I-deadband = 4%


Effect of Tuning on Limit Cycle from Effect of Tuning on Limit Cycle from Stick-Slip in Self-Regulating ProcessesStick-Slip in Self-Regulating ProcessesEffect of Tuning on Limit Cycle from Effect of Tuning on Limit Cycle from

Stick-Slip in Self-Regulating ProcessesStick-Slip in Self-Regulating Processes

Ao = SsKo

Ko =KmvKpvKcv

To = 4 Ti [1( Ko Kc) 1]

Ao = oscillation amplitude (%)

Ss = valve stick-slip (%)

Ko = open loop gain (%/%) (more commonly known as process gain)

Kmv = manipulated variable gain (valve gain) (e.g. kg/sec per %)

Kpv = process variable gain (unit operation gain) (e.g. oC per kg/sec)

Kcv = controlled variable gain (measurement gain) (e.g. % per oC)

Ti = controller integral time (sec)

To = oscillation period (%) http://www.controldesign.com/articles/2003/164.html

“What’s Your Flow Control Valve Telling You?”, Control Design, May 2003


Loop Lab02 Demo 7Loop Lab02 Demo 7Loop Lab02 Demo 7Loop Lab02 Demo 7 Objective – See effect of valve deadband on CAS Mode

with no I-deadband in either PID for 5% valve deadband Activities:

– First look at cascade loop response with 5% valve stick-slip with existing I-deadband in both PID for CAS Mode

– Set valve stick-slip = 0% and valve deadband = 5%

– Set I-Deadband = 0% in both PID


Effect of Step Size on Effect of Step Size on Small Valve ResponseSmall Valve ResponseEffect of Step Size on Effect of Step Size on Small Valve ResponseSmall Valve Response

Results show sensitivity limitation of piston actuators and pneumatic positioners


Loop Lab02 Demo 8Loop Lab02 Demo 8Loop Lab02 Demo 8Loop Lab02 Demo 8 Objective – See effect of valve deadband on AUTO

Mode Response with no I-deadband in either PID Activities:

– First look at cascade loop response with 5% valve deadband with the I-deadband = 0% in both PID for CAS Mode

– Put secondary PID in AUTO mode


Best Practices for Valve PerformanceBest Practices for Valve PerformanceBest Practices for Valve PerformanceBest Practices for Valve Performance Actuator, valve, and positioner package from a control valve manufacturer Digital positioner tuned for valve package and application Diaphragm actuators where application permits (large valves and high

pressure drops may require piston actuators) Sliding stem (globe) valves where size and fluid permit (large flows and

slurries may require rotary valves) Best rotary is low seal friction segmented V-ball or contoured butterfly with short splined

shaft connection and rotary positioner feedback and actuation (original valve design is for throttling (process control) rather than on-off or isolation service (discrete control)

Low packing, sealing, seating, and actuator friction Booster(s) on positioner output(s) for large valves on fast loops (e.g.,

compressor anti-surge control) Valve sizing for a throttle range that provides good linearity [4]:

o 5% to 75% (sliding stem globe)

o 10o to 60o (segmented V-ball)

o 25o to 45o (conventional butterfly)

o 5o to 65o (contoured and toothed butterfly)

Online diagnostics and step response tests for small changes in signal Dynamic reset limiting (FRSPID_OPTS) using digital positioner feedback


Loop Lab02 Demo 9Loop Lab02 Demo 9Loop Lab02 Demo 9Loop Lab02 Demo 9 Objective – See effect of valve deadband on AUTO mode

response of high gain PID for an integrating process Activities:

– First look at single loop response with 5% valve deadband with the I-deadband = 0% in both PID for AUTO Mode

– Change to secondary process type to integrating. To prevent instability, the order of following steps is important:

• Check secondary PID is in AUTO mode

• Set the Secondary Process Lag 2 = 20 sec

• Set the Secondary PID reset time = 100 sec

• Set the Secondary PID gain = 5

• Set the Secondary Process Type = Integrating


Top Ten Signs Your Control Valve Top Ten Signs Your Control Valve is an On-Off Valve in Disguiseis an On-Off Valve in Disguise

Top Ten Signs Your Control Valve Top Ten Signs Your Control Valve is an On-Off Valve in Disguiseis an On-Off Valve in Disguise

(10) Valve body looks suspiciously like the block valve next to it (9) Actuator looks suspiciously like the one on the SIS valve (8) Process engineer is seen going to lunch with piping valve supplier (7) The valve deal is a steal (6) Your flow is on-off (5) Positioner measures actuator shaft instead of ball or disk stem

position for feedback (4) Oscillation amplitude exceeds largest historian compression setting (3) 360o feedback in your loop becomes 3600 feedback in your

performance review (2) The valve package is nicknamed “Sloppy Joe” (1) No valve leakage even with a controller output of 40%


Loop Lab02 Demo 10Loop Lab02 Demo 10Loop Lab02 Demo 10Loop Lab02 Demo 10 Objective – See effect of valve deadband on AUTO mode

response of low gain PID for an integrating process Activities:

– First look at single loop response with 5% valve deadband of high gain PID for an integrating process

– Change secondary PID gain from 5 to 1


Effect of Tuning on Limit Cycle from Effect of Tuning on Limit Cycle from Valve Deadband in Integrating ProcessesValve Deadband in Integrating Processes

Effect of Tuning on Limit Cycle from Effect of Tuning on Limit Cycle from Valve Deadband in Integrating ProcessesValve Deadband in Integrating Processes

Ao = DB Kc

To = 5 Ti [ 1 2 (Kc)0.5 ]

Ao = oscillation amplitude (%)

DB = valve deadband (%)Kc = controller gain

Ti = controller integral time (sec)

To = oscillation period (%)http://www.controldesign.com/articles/2003/164.html

“What’s Your Flow Control Valve Telling You?”, Control Design, May 2003


SummarySummarySummarySummary Valve Stick-Slip:

– Adds deadtime inversely proportional to PID output rate of change

– Creates a limit cycle (persistent constant amplitude oscillation) in any PID with integral action

– Slower tuning increases period but has no affect on amplitude

– I-Deadband must be added to both secondary and primary PID to eliminate limit cycle in cascade control system

Valve Deadband:– Adds deadtime inversely proportional to PID output rate of change

– No limit cycle for a single loop on a self-regulating process

– Creates a limit cycle in cascade loops

– Creates a limit cycle for integrating process (e.g. level and batch temperature) and runaway process (e.g. Exothermic reactor)

– Limit cycle amplitude & period are inversely proportional to PID gain


PID SolutionsPID SolutionsPID SolutionsPID Solutions For self-regulating processes

– I-deadband eliminates limit cycle but creates offset

For all types of processes – Deadband and stick-slip compensator or lead-lag on

PID output can reduce deadtime and increase frequency of limit cycle but increases valve travel (wears out packing and seals)

Best solution is control valve selected per “Best Practices for Valve Performance”


Summary of DemosSummary of DemosSummary of DemosSummary of DemosDemo Sec

PIDMode

SecPID

Tuning

SecPID

I-deadband

PrimPID

I-deadband

ValveStick-Slip

ValveDead-band

ProcessType

1 CAS FAST 0% 0% 0% 0% Self-Reg

2 AUTO FAST 0% 0% 5% 0% Self-Reg

3 AUTO SLOW 0% 0% 5% 0% Self-Reg






9 AUTO HI GAIN 0% 0% 0% 5% Integ

10 AUTO LO GAIN 0% 0% 0% 5% Integ


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PID Control of Slow Valves and PID Control of Slow Valves and Secondary LoopsSecondary Loops

Look for a recording of Today’s Deminar Look for a recording of Today’s Deminar later this week at:later this week at:www.ModelingAndControl.com

www.EmersonProcessXperts.com


QUESTIONS? QUESTIONS? QUESTIONS? QUESTIONS?

PID Control of Valve Sticktion and Backlash - Greg McMillan Deminar Series

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