Effective Use of PID Features for Loop Performance and Optimization. Greg McMillan CDI Process & Industrial Hector Torres Solutia Inc.

Transcription

1 Effective Use of PID Features for Loop Performance and Optimization Greg McMillan CDI Process & Industrial Hector Torres Solutia Inc.

2 Photography & Video Recording Policy Photography and audio/video recording is not permitted in any sessions or in the exhibition areas without press credentials or written permission from the Emerson Exchange Board of Directors. Inquiries should be directed to: Thank you.

3 Presenters Greg McMillan Principal Consultant 33 years Monsanto-Solutia Fellow 2 years WU Adjunct Professor 8 years DeltaV R&D Contractor BS Engineering Physics MS Control Theory Hector Torres Senior Process, Control Engineer Specialist hhtorr@solutia.com 16 years Solutia Inc. Six Sigma Black Belt BS Control Theory MS industrial Engineering

4 Key Benefits DeltaV PID options and parameters can: Provide maximum disturbance rejection Minimize setpoint overshoot and rise time Eliminate limit cycles Reduce valve maintenance Coordinate loops for consistency and minimal interactions Increase process efficiency Increase feed rates Protect equipment Enable wireless control Enable analyzer control

5 PID Features Covered PID options and parameters: Anti-Reset Windup (ARW) and Output Limits Auto Tuner and Adaptive Controller Dynamic Reset Limit Structure Integral Deadband Nonlinear Gain Feedforward Output Tracking Setpoint Filter Setpoint Rate Limits PIDPlus

6 Application Examples Given PID Features exemplified by following applications: Fast generic continuous loop with nonlinear valve Slow bioreactor batch loop with integrating response Fast generic batch loop with integrating response Valve with backlash and stick-slip Surge valve open loop backup Conductivity and ph kicker Valve position control for prime movers, chillers, & reactors Wireless control Analyzer control

7 ARW and Output Limits ARW limits set equal output limits for precise valves For Digital Valve Controllers (DVC) & Fisher valves ARW & Out Lo Lim = 0%, ARW & Out Hi Lim = 100% For pneumatic positioners & on-off heritage valves Lo Lim = -5%, Hi Lim = 105% ARW set inside output limits to get thru zone of ineffective valve response (stick-slip, shaft windup, & poor sensitivity)

8 Auto Tuner and Adaptive Control Set PV filter before tuning PV filter time large enough to keep PID output fluctuations form noise within control valve deadband (e.g. < 0.25%) PV filter time < 0.1x current reset time (sec) Choose output step size to see process response Output step size > 0.2x PID Gain Output step size > 4x backlash and stick-slip Use Structure with Proportional Action on Error Use auto tuner for initial settings at operating point Check Integrating Processes and 3 cycles for slow processes Setup adaptive tuner regions Use Out as state parameter for nonlinear valve Use PV as state parameter for nonlinear process (e.g. ph)

9 Nonlinear Valve Auto Tuning click on PID tag and then Tune

10 Nonlinear Valve Adaptive Tuning

11 Nonlinear Valve Model Viewing Process gain is approximately proportional to flow for equal percentage flow characteristic

12 Nonlinear Valve Learning Setup Identification Out Limit that sets deadzone should be set approximately equal to valve deadband and stick-slip near closed position

13 Nonlinear Valve Simulate Tests

14 Bioreactor Adaptive Control

15 External-Reset Feedback Gain * + - D * P = (b -1) * Gain * %SP %SP b All signals are % of scale for PID algorithm Feedforward filter + - D * Out1 Positive Feedback S Out2 Filter Time = Reset Time S P D S FF %CO p g Rate - D + filter %PV * D * * filter + - filter Back out positive feedback Filter Time = of Feedforward (FF) and a * Rate Time ISA Standard Form of Proportional (P) and Derivative (D) modes with b and g factors derivative Filter Time = Reset Time filter S P D E-R E-R is external reset (e.g. secondary %PV s ) Dynamic Reset Limit FF

16 PID Structure Options (1) PID action on error (b = 1 and g = 1) (2) PI action on error, D action on PV (b = 1 and g = 0) (3) I action on error, PD action on PV (b = 0 and g = 0) (4) PD action on error, no I action (b = 1 and g = 1) (5) P action on error, D action on PV, no I action (b = 1 and g = 0) (6) ID action on error, no P action (g = 1) (7) I action on error, D action on PV, no P action (g = 0) (8) Two degrees of freedom controller (b and g adjustable 0 to 1)

17 (1) PID action on error Fastest response to rapid (e.g. step) SP change by Step in output from proportional mode Spike in output from derivative mode can be made more like a bump by decreasing gamma factor (g <1) Zero deadtime from deadband, resolution limit, & stiction Burst of flow may affect other uses of fluid Operations do not like sudden changes in output Fast approach to SP more likely to cause overshoot Setpoint filter & rate limits eliminate step & overshoot

18 (2) PI action on error, D action on PV Slightly slower SP response than structure (1) Still have step from proportional mode Spike or bump from derivative mode eliminated Decrease in SP response speed is negligible if Output hits output limit due to large SP change or PID gain Rate time is less than total loop deadtime Alpha factor is increased (a > 0.125) (rate filter increased) Setpoint filter & rate limits eliminate step & overshoot Most popular structure choice

19 (3) I action on error, PD action on PV Provides gradual change in output for SP change Slows down SP response dramatically Eliminates overshoot for SP changes Used for bioreactor temperature and ph SP changes (overshoot is much more important than cycle time) Used for temperature startup to warm up equipment Generally not recommended for secondary loops

20 Effect of Structure on SP Response for Self-Regulating Process Setpoint filter could have eliminated overshoot Structures 2, 3 and 8 Structure (8) Two Degrees of Freedom b and g SP weighting factors are adjusted to balance fast approach & minimal overshoot for SP response Simpler method is setting SP filter time = reset time

21 degrees C degrees C (4-5) No Integral action Used if integral action adversely affects process Used if batch response is only in one direction Must set bias (output when PV = SP) Highly exothermic reactors use structure 4 because integral action and overshoot can cause a runaway 10x reset time (T i > 40x deadtime) to prevent runaway Traditionally used on Total Dissolved Solids (TDS) drum and surge tank level control because of slow integrating response and permissibility of SP offset. Low controller gain (Kc) cause slow rolling oscillations due to violation of inequality for integrating process. The inequality is commonly violated since K i (integrating process gain) is extremely small on most vessels (K i < %/sec/%). Batch temperature response in a single ended temperature control. Integral action Typical causes Batch Temperature overshoot Time (min) Setpoint PV CO% Batch temperature response in a single ended temperature control. PD on Batch error. Temperature No I action. (new tuning) Time (min) Setpoint PV CO% K * T 2 / c i K i

22 (6-7) No Proportional Action Predominantly used for valve position control (VPC) Parallel valve control (VPC SP & PV are small valve desired & actual position, respectively, & VPC out positions large valve) Optimization (VPC SP & PV are limiting valve desired & actual position, respectively, & VPC out optimizes process PID SP) VPC reset time > 10x residence time to reduce interaction VPC reset time > K c *T i of process PID to reduce interaction VPC tuning is difficult & too slow for fast & large disturbances Better solution is dynamic reset limit & SP rate limits Valve position control increases precision and rangeability

23 Effect of Structure on SP Response for Integrating Process

24 Setpoint Filter and Feedforward Disturbance SP Filter Measurement setpoint feedforward Feedback Controller Feedforward Summer OUT AO Control Valve Process PV setpoint filter time is simply equal to feedback controller reset time AI Measurement

25 Feedforward Applications 1 Feedforward is the most common advanced control technique used. Often the feedforward signal is a flow or speed for ratio control that is corrected by a process PID (e.g. temperature, ph, or composition) Blend composition control - additive/feed (flow/flow) ratio Column temperature control - distillate/feed, reflux/feed, stm/feed, & bttms/feed (flow/flow) ratio Combustion temperature control - air/fuel (flow/flow) ratio Drum level control - feedwater/steam (flow/flow) ratio Extruder quality control - extruder/mixer (power/power) ratio Heat exchanger temperature control - coolant/feed (flow/flow) ratio Neutralizer ph control - reagent/feed (flow/flow) ratio Reactor reaction rate control - catalyst/reactant (speed/flow) ratio Reactor composition control - reactant/reactant (flow/flow) ratio Sheet, web, and film machine direction (MD) gage control - roller/pump (speed/speed) ratio Slaker conductivity control - lime/liquor (speed/flow) ratio Spin line fiber diameter gage control - winder/pump (speed/speed) ratio

26 Feedforward Applications 2 Feedforward is most effective if the loop deadtime is large, disturbance speed is fast & size is large, feedforward gain is known, feedforward measurement & dynamic compensation are accurate Dynamic compensation is used so the feedforward signal arrives at same point at same time in process as upset Compensation of feedforward delay > feedback delay is not possible Feedback correction is essential in industrial processes While technically, the correction should be a multiplier for a change in slope and a bias for a change in the intercept in a plot of the manipulated variable versus independent variable (independent from this loop but possibly set by another PID or MPC), a multiplier creates scaling problems for the user, a multiplier introduces a nonlinearity in vessels and columns (non plug flow equipment), and bias errors are bigger than span errors in measurements. For these and other reasons the correction of most feedforward signals is done via a bias Correction must have enough positive & negative range for worst case

27 Feedforward Applications 3 Feedforward gain can be computed from a material or energy balance & explored for different setpoints and conditions from a plot of the controlled variable (e.g. composition, conductivity, ph, temperature, or gage) vs. ratio of manipulated to independent variable (e.g. feed) but is often based on operating experience For concentration and ph control, the flow/flow ratio is valid if the changes in the composition of both the manipulated and feed flow are negligible. For column and reactor temperature control, the flow/flow ratio is valid if the changes in the composition and temperature of both the manipulated and feed flow are negligible. For reactor reaction rate control, the speed/flow is valid if changes in catalyst quality and void fraction and reactant composition are negligible. For heat exchanger control, the flow/flow ratio is valid if changes in temperatures of coolant and feed flow are negligible. For reactor temperature control, the flow/flow ratio is valid if changes in temperatures of coolant and feed flow are negligible. For slaker conductivity (effective alkali) control, the speed/flow ratio is valid if changes in lime quality, void fraction, and liquor composition are negligible. For spin or sheet line gage control, the speed/speed ratio is valid only if changes in the pump pressure and the polymer melt quality are negligible.

28 Feedforward Applications 4 Feedforward gain is a ratio for most load upsets Feedforward gain is inverse of open loop gain for SP feedforward open loop gain is dimensionless product of manipulated variable gain, process variable gain, and measurement variable gain Feedforward action is in same direction as feedback action for upset but is in opposite of control action for SP feedforward Feedforward delay & lag adjusted to match delay & lag, respectively in upset path so feedforward correction does not arrive too soon Feedforward lead is adjusted to compensate for lag in manipulated variable path so the feedforward correction does not arrive too late The actual and desired feedforward ratio should be displayed along with the bias correction by the process PID

29 Integral Deadband Will stop limit cycles from deadband and backlash Reduces valve packing, trim, & seal wear and piston o-ring wear IDEADBAND setting must be greater than largest limit cycle PV excursion on either side of setpoint For integrating process or stick-slip or resolution limit Integral deadband must be set in every PID Integral deadband must be set in DVC when integral enabled For self-regulating process & backlash (deadband) Integral deadband does not have to be set in single loop PID if DVC has integral deadband when integral enabled Integral deadband must be set in primary PID for cascade control Limit cycle amplitude is highly variable Backlash & stick-slip varies with position and age Process gain varies with output, PV, and time Better solution is PIDPlus with threshold sensitivity limit Threshold sensitivity limit screens out noise as not valid PV update

30 Nonlinear Gain Used to reduce cycling around SP from hi process gain (7 ph) Used to ignore noise at SP Used to reduce interactions Used in surge tank level control Better solution is adaptive tuner and signal characterizer

31 Output Tracking for SP Response Head-Start logic for startup & batch SP changes: For SP change PID tracks best/last startup or batch final settling value for best/last rise time less total loop deadtime Closed loop time constant is open loop time constant (l f =1) Not as fast as Bang-Bang (PID OUT is not at output limit) Bang-Bang logic for startup & batch SP changes: For SP change PID tracks output limit until the predicted PV one deadtime into future gets within a deadband of setpoint, the output is then set at best/last startup or batch final settling value for one deadtime Implementation uses simple DT block (loop deadtime) to create an old PV subtracted from the new PV to give a delta PV that is added to old PV to create a PV one deadtime into future Works best on slow batch and integrating processes

32 Output Tracking for Protection 1 Open Loop Backup to prevent compressor surge: Once a compressor gets into surge, cycles are so fast & large that feedback control can not get compressor out of surge When compressor flow drops below surge SP or a precipitous drop occurs in flow, PID tracks an output that provides a flow large enough to compensate for the loss in downstream flow for a time larger than the loop deadtime plus the surge period. Open Loop Backup to prevent RCRA violation: An excursion < 2 ph or > 12 ph for even a few sec can be a recordable RCRA violation regardless of downstream volume When an inline ph system PV approaches the RCRA ph limit the PID tracks an incremental output (e.g. 0.25% per sec) opening the reagent valve until the ph sufficiently backs away Open Loop Backup for evaporator conductivity

33 Output Tracking for Protection 2 AO SP_Rate_DN and SP_RATE_UP used to insure fast getaway and slow approach Open Loop Backup Configuration

34 Output Tracking for Protection 3 Feedback Action

35 Output Tracking for Protection 4 Open Loop Backup

36 RCRA ph Kicker Optimization of ph filter and kicker increment saved $50K in reagent costs MPC-1 MPC-2 RCAS AC-1 Waste RCAS AC-2 ROUT AY Kicker middle selector AY splitter AY FT Stage 1 middle selector AY AT AT AT AY FT splitter middle selector AY Stage 2 AT AT AT AY Filter Attenuation Tank AT AT AT Mixer Mixer FT

37 Evaporator Conductivity Kicker Conductivity spike WBL Flow Kicker

38 Setpoint Filter PID SP filter reduces overshoot enabling fast tuning Setpoint filter time set equal reset time PID SP filter coordinates timing of flow ratio control Simultaneous changes in feeds for blending and reactions Consistent closed loop response for model predictive control PID SP filter sets closed loop time constant PID SP filter in secondary loop slows down cascade control system rejection of primary loop disturbances Secondary loop must be > 4x faster than primary loop Primary PID must have dynamic reset limit enabled

39 Setpoint Rate Limits AO & PID SP rate limits minimize disruption while protecting equipment and optimizing processes surge valve fast opening and slow closing VPC fast recovery for upset and slow approach to optimum AO SP rate limits minimize interaction between loops Less important loops are made 10x slower than critical loops PID driving AO SP or secondary PID SP rate limit must have dynamic reset limit enabled

40 PIDPlus Features 1 Positive feedback implementation of reset with external-reset feedback (dynamic reset limit) Immediate response to a setpoint change or feedforward signal or mode change Suspension of integral action until change in PV Integral action is the exponential response of the positive feedback filter to the change in controller output for the time interval since last update Derivative action is the PV or error change divided by the time interval since the last update multiplied by the gain and rate time

41 PIDPlus Features 2 K c Elapsed Time K c Elapsed Time + + T D T D + + Link to PIDPlus White Paper Whitepapers/WP_DeltaV%20PID%20Enhancements%20for%20Wireless.pdf PID integral mode is restructured to provide integral action to match the process response in the elapsed time (reset time set equal to process time constant) PID derivative mode is modified to compute a rate of change over the elapsed time from the last new measurement value PID reset and rate action are only computed when there is a new value If transmitter damping is set to make noise amplitude less than communication trigger level, valve packing and battery life is dramatically improved Enhancement compensates for measurement sample time suppressing oscillations and enabling a smooth recovery from a loss in communications further extending packing -battery life

42 PIDPlus Flow Setpoint Response Enhanced PID (PIDPlus) Sensor PV Traditional PID Sensor PV

43 PIDPlus Flow Load Response Enhanced PID (PIDPlus) Sensor PV Traditional PID Sensor PV

44 PIDPlus Flow Failure Response Enhanced PID (PIDPlus) Sensor PV Traditional PID Sensor PV

45 PIDPlus ph Setpoint Response Enhanced PID (PIDPlus) Sensor PV Traditional PID Sensor PV

46 PIDPlus ph Load Response Enhanced PID (PIDPlus) Sensor PV Traditional PID Sensor PV

47 PIDPlus ph Failure Response Enhanced PID (PIDPlus) Sensor PV Traditional PID Sensor PV

48 PIDPlus Stops Limit Cycles Traditional PID Enhanced PID PID PV PID Output Limit Cycles from Valve Stick-Slip

49 PIDPlus Benefits Beyond Wireless 1 The PID enhancement for wireless (PIDPlus) offers an improvement wherever there is an update time in the loop. In the broadest sense, an update time can range from seconds (wireless updates and valve or measurement sensitivity limits) to hours (failures in communication, valve, or measurement). Some of the sources of update time are: Wireless update time for periodic reporting (default update rate) Wireless measurement trigger level for exception reporting (trigger level) Wireless communication failure Broken ph electrode glass or lead wires (failure point is about 7 ph) Valve with backlash (deadband) and stick-slip (resolution) Operating at split range point (no response & abrupt response discontinuity) Valve with solids, high temperature, or sticky fluid (plugging and seizing) Plugged impulse lines Analyzer sample, analysis cycle, and multiplex time Analyzer resolution and threshold sensitivity limit To completely stop a valve limit cycle from backlash or stick-slip, measurement updates must not occur due to noise

50 PIDPlus Benefits Beyond Wireless 2 Enhanced PID executes for a change in setpoint, feedforward, or remote output to provide an immediate reaction based on PID structure The improvement in control by the enhanced PID is most noticeable as the update time becomes much larger than the 63% process response time (defined in the white paper as the sum of the process deadtime and time constant). When the update time becomes 4 times larger than this 63% process response time ( 98% response time frequently cited in the literature), the feedforward and controller gains can be set to provide a complete correction for changes in the measurement and setpoint. Helps ignore inverse response and errors in feedforward timing Helps ignore discontinuity (e.g. steam shock) at split range point Helps extend packing life by reducing oscillations and hence valve travel Since enhanced PID can be set to execute only upon a significant change in user valve position, this PID as a valve position controller offers less interaction and cycling for optimization of unit operations by increasing reactor feed, column feed or increasing refrigeration unit temperature, or decreasing compressor pressure till feed, vent, coolant, and/or steam, valves are at maximum good throttle position. Website entries on Enhanced PID (PIDPlus) Benefits

51 Key Features for VPC Feature Function Advantage 1 Advantage 2 Direction Velocity Limits Limit VPC Action Speed Based on Direction Prevent Running Out of Valve Minimize Disruption to Process Dynamic Reset Limit Limit VPC Action Speed to Process Response Direction Velocity Limits Prevent Burst of Oscillations Adaptive Tuning Automatically Identify and Schedule Tuning Eliminate Manual Tuning Compensation of Nonlinearity Feedforward Preemptively Set VPC Out for Upset Prevent Running Out of Valve Minimize Disruption Enhanced PID (PIDPlus) Suspend Integral Action until PV Update Eliminate Limit Cycles from Stiction & Backlash Minimize Oscillations from Interaction & Delay

52 VPC for Small and Big Valve Small valve provides precision and big valve gives rangeability

53 Examples of Optimization by VPC Optimization VPC PID PV VPC PID SP VPC PID Out Minimize Prime Mover Energy Minimize Boiler Fuel Cost Minimize Boiler Fuel Cost Minimize Chiller or CTW Energy Minimize Purchased Reagent or Fuel Cost Minimize Total Reagent Use Maximize Reactor Production Rate Maximize Reactor Production Rate Maximize Column Production Rate Maximize Ratio or Feedforward Accuracy Reactor Feed Flow PID Out Max Throttle Position Compressor or Pump Pressure SP Steam Flow PID Out Max Throttle Position Boiler Pressure SP Equipment Temperature PID Out Equipment Temperature PID Out Purchased Reagent or Fuel Flow PID Out Final Neutralization Stage ph PID Out Reactor or Condenser Temperature PID Out Reactor Vent Pressure PID Out Reboiler or Condenser Flow PID Out Process Feedback Correction PID Out Max Throttle Position Max Throttle Position Min Throttle Position Min Throttle Position Max Throttle Position Max Throttle Position Max Throttle Position 50% (Zero Correction) Boiler Pressure SP Chiller or CTW Temperature SP Waste Reagent Or Fuel Flow SP First Neutralization Stage ph PID SP Feed Flow or Reaction Temperature SP Feed Flow or Reaction Temperature SP Feed Flow or Column Pressure SP Flow Ratio or Feedforward Gain

54 Liquid Reactants (Jacket CTW) Liquid Product Optimization ratio calc FY 1-6 LY 1-8 ZC1-4 OUT reactant A residence time calc CAS FC 1-1 FT 1-1 FC 1-2 CAS LC 1-8 LT 1-8 PT 1-5 TT 1-3 FT 1-5 ZC1-4 is an enhanced PID VPC PC 1-5 FC 1-1 CAS vent TC 1-3 ZC 1-4 reactant B FT 1-2 TT 1-4 TC 1-4 return Valve position controller (VPC) setpoint is the maximum throttle position. The VPC should turn off integral action to prevent interaction and limit cycles. The correction for a valve position less than setpoint should be slow to provide a slow approach to optimum. The correction for a valve position greater than setpoint must be fast to provide a fast getaway from the point of loss of control. Directional velocity limits in AO with dynamic reset limit in an enhanced PID that tempers integral action can achieve these optimization objectives. AT 1-6 FC 1-7 FT 1-7 AC 1-6 makeup CTW product 54

55 Liquid Reactants (Jacket CTW) Gas & Liquid Products Optimization ratio calc FY 1-6 LY 1-8 ZY1-1 OUT reactant A residence time calc CAS FC 1-1 FT 1-1 FC 1-2 CAS LC 1-8 LT 1-8 TT 1-10 PT 1-5 W TT 1-3 FT 1-5 PC 1-5 product TC 1-10 TC 1-3 ZC 1-5 ZY-1 IN1 ZC 1-10 reactant B FT 1-2 TT 1-4 TC 1-4 return ZY-1 IN2 FC1-1 CAS ZC-5 OUT low signal selector ZY ZC OUT ZC-4 OUT CAS AT 1-6 FC 1-7 AC 1-6 makeup CTW ZC 1-4 ZY-1 IN3 ZC1-4, ZC-5, & ZC-10 are enhanced PID VPC FT 1-7 product 55

56 Innovative PID System to Optimize Ethanol Yield and Carbon Footprint Corn Production Rate Enhanced PID setpoint AC 1-4 SC 1-4 AY 1-4 AT 1-4 XY 1-4 NIR-T Fermentable Starch Correction Average Fermentation Time Enhanced PID XC 1-4 Feedforward DX 2-4 Slurry Solids Enhanced PID DC 2-4 RCAS FC 1-5 Dilution Water FT 1-5 FC 1-6 Backset Recycle FT 1-6 DT 2-4 Coriolis Meter Slurry Tank 1 Slurry Tank 2 Lag and Delay DY 2-4 Predicted Fermentable Starch 56

57 Business Results Achieved Batch cycle time & startup time reduction PID structures 1 & 2, SP feedforward, & output tracking for Bang-Bang logic to speed-up SP response Valve life cycle cost reduction Integral deadband & PIDPlus to reduce valve dither Equipment and environmental protection Dynamic reset limit & AO SP rate limits to ensure slow approach to normal operating point & fast getaway for abnormal conditions Output tracking for open loop backup & kicker for fast recovery Process variability reduction Dynamic reset limit & PID SP filter & rate limits for max upset rejection with min SP overshoot & for consistent blending & parallel train response Dynamic reset limit & AO SP rate limits for interaction reduction PIDPlus for smooth analyzer & wireless control & for failure recovery PIDPlus for robustness in feedforward timing correction Process efficiency and capacity improvement PIDPlus for more effective valve position control PIDPlus for more effective analyzer & batch end point control Dynamic reset limit & SP rate limits to ensure slow approach to optimum operating point & fast getaway for abnormal conditions

58 Summary The role of the PID is expanding from basic control into advanced regulatory control with the ability to provide quick optimization solutions by innovative use of key PID features, such as dynamic reset limit & PIDPlus Tuning is simplified in that the same tuning used for disturbance rejection can be used for SP response, coordination of loops, optimization of loops, and loops where significant measurement update delay has been introduced by wireless devices & analyzers Feedback? Questions?

59 Where To Get More Information Greg McMillan, What is the Key PID Feature for Basic and Advanced Control, Control Talk Blog, Greg McMillan and Hector Torres, Effective use of Key PID features, ISA Automation Week 2012 Greg McMillan and Hunter Vegas, 101 Tips for a Successful Automation Career, ISA, 2012