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Effective Use of PID Features for Loop Performance and
Optimization
Greg McMillan CDI Process & Industrial
Hector Torres Solutia Inc.
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.
Presenters
Greg McMillan Principal Consultant
Email: [email protected]
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
Email: [email protected]
16 years Solutia Inc.
Six Sigma Black Belt
BS Control Theory
MS industrial Engineering
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
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
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
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)
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)
click on PID tag
and then Tune
Nonlinear Valve Auto Tuning
Nonlinear Valve Adaptive Tuning
Process gain is
approximately
proportional
to flow for
equal percentage
flow characteristic
Nonlinear Valve Model Viewing
Identification
Out Limit that
sets deadzone
should be set
approximately
equal to valve
deadband and
stick-slip near
closed position
Nonlinear Valve Learning Setup
Nonlinear Valve Simulate Tests
Bioreactor Adaptive Control
External-Reset Feedback Gain
E-R is external reset
(e.g. secondary %PVs)
Dynamic Reset Limit
S
%SP
b
derivative
*
*
* * *
Rate
D
D
g
%COp
filter
filter
%PV filter Filter Time =
a * Rate Time
S
filter
Filter Time =
Reset Time
E-R
Positive
Feedback
All signals are % of scale for PID algorithm
Out1
Out2
S
D
P FF
D
Feedforward
filter S
P
FF
D
Filter Time =
Reset Time
Back out positive feedback
of Feedforward (FF) and
ISA Standard Form of
Proportional (P) and Derivative (D) modes
with b and g factors
*
-
+
-
+
D +
-
+ -
P = (b -1) * Gain * %SP
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)
(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
(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
(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
Effect of Structure on SP Response for Self-Regulating Process
Setpoint filter could
have eliminated
overshoot
Structures 2, 3 and 8
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
Structure (8) Two Degrees of Freedom
(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 (Ti > 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 Ki (integrating process gain) is extremely small on most vessels (Ki < 0.000001 %/sec/%).
iic KTK /2*
Typical Batch Temperature
0
10
20
30
40
50
60
70
80
1 51 101 151 201 251 301 351 401
Time (min)
deg
rees C
Setpoint PV CO%
Batch temperature response in a single ended temperature
control. Integral action causes overshoot.
Batch Temperature (new tuning)
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
1 51 101 151 201 251 301 351 401
Time (min)d
eg
rees C
Setpoint PV CO%
Batch temperature response in a single ended temperature
control. PD on error. No I action.
(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 > Kc*Ti 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
Effect of Structure on SP Response for Integrating Process
Setpoint Filter and Feedforward
Process
Measurement
Feedforward
Summer Control Valve
Measurement
Feedback
Controller
AI
AO
SP
PV
OUT
Disturbance
setpoint
feedforward
Filter
setpoint filter time is simply equal to feedback controller reset time
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
Feedforward Applications 1
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
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.
Feedforward Applications 3
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
Feedforward Applications 4
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
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
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 (lf =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
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
Open Loop Backup Configuration
AO SP_Rate_DN and SP_RATE_UP used to insure fast getaway and slow approach
Output Tracking for Protection 2
Output Tracking for Protection 3
Feedback Action
Open Loop Backup
Output Tracking for Protection 4
Mixer
Attenuation
Tank
AY
AT
middle selector
AY
splitter
AT
FT
FT
AT
AY
AT AT AT
AY
AT AT AT
Mixer
AY
FT
Stage 2 Stage 1
middle selector
Waste
middle selector RCAS RCAS
splitter
AY
Filter
AY ROUT
Kicker AC-1 AC-2
MPC-2
MPC-1
RCRA pH Kicker
Optimization of pH filter and kicker increment saved $50K in reagent costs
Evaporator Conductivity Kicker
Conductivity spike
WBL Flow Kicker
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
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
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
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
+
+
+
+
Elapsed
Time
Elapsed
Time
TD
Kc
Kc
TD
http://www2.emersonprocess.com/siteadmincenter/PM%20DeltaV%20Documents/
Whitepapers/WP_DeltaV%20PID%20Enhancements%20for%20Wireless.pdf
Link to PIDPlus White Paper
PIDPlus Features 2
Traditional PID
Sensor PV
Enhanced PID
(PIDPlus)
Sensor PV
PIDPlus Flow Setpoint Response
Traditional PID
Sensor PV
Enhanced PID
(PIDPlus)
Sensor PV
PIDPlus Flow Load Response
Enhanced PID
(PIDPlus)
Sensor PV
Traditional PID
Sensor PV
PIDPlus Flow Failure Response
Enhanced PID
(PIDPlus)
Sensor PV
Traditional PID
Sensor PV
PIDPlus pH Setpoint Response
Traditional PID
Sensor PV
Enhanced PID
(PIDPlus)
Sensor PV
PIDPlus pH Load Response
Traditional PID
Sensor PV
Enhanced PID
(PIDPlus)
Sensor PV
PIDPlus pH Failure Response
PID PV
PID Output
Enhanced PID Traditional PID
Limit Cycles from Valve Stick-Slip
PIDPlus Stops Limit Cycles
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
PIDPlus Benefits Beyond Wireless 1
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.
http://www.modelingandcontrol.com/2010/08/wireless_pid_benefits_extend_t.html
http://www.modelingandcontrol.com/2010/10/enhanced_pid_for_wireless_elim.html
http://www.modelingandcontrol.com/2010/11/a_delay_of_any_sorts.html
Website entries on Enhanced PID (PIDPlus) Benefits
PIDPlus Benefits Beyond Wireless 2
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
VPC for Small and Big Valve
Small valve provides precision and big valve gives rangeability
Examples of Optimization by VPC
Optimization VPC PID PV VPC PID SP VPC PID Out
Minimize Prime
Mover Energy
Reactor Feed
Flow PID Out
Max Throttle Position Compressor or Pump
Pressure SP
Minimize Boiler
Fuel Cost
Steam Flow PID Out Max Throttle Position Boiler
Pressure SP
Minimize Boiler
Fuel Cost
Equipment Temperature
PID Out
Max Throttle Position Boiler
Pressure SP
Minimize Chiller
or CTW Energy
Equipment Temperature
PID Out
Max Throttle Position Chiller or CTW
Temperature SP
Minimize Purchased
Reagent or Fuel Cost
Purchased Reagent or
Fuel Flow PID Out
Min Throttle Position
Waste Reagent
Or Fuel Flow SP
Minimize Total Reagent
Use
Final Neutralization
Stage pH PID Out
Min Throttle Position
First Neutralization
Stage pH PID SP
Maximize Reactor
Production Rate
Reactor or Condenser
Temperature PID Out
Max Throttle Position Feed Flow or Reaction
Temperature SP
Maximize Reactor
Production Rate
Reactor Vent
Pressure PID Out
Max Throttle Position Feed Flow or Reaction
Temperature SP
Maximize Column
Production Rate
Reboiler or Condenser
Flow PID Out
Max Throttle Position Feed Flow or Column
Pressure SP
Maximize Ratio or
Feedforward Accuracy
Process Feedback
Correction PID Out
50%
(Zero Correction)
Flow Ratio or
Feedforward Gain
54
Liquid Reactants (Jacket CTW) Liquid Product Optimization
TT
1-4
TC
1-3
TC
1-4
AT 1-6
LY 1-8
FY 1-6
FT
1-2
FC
1-2
reactant A
reactant B
CAS
residence
time calc
CAS
ratio
calc
AC
1-6
makeup
return
LY 1-8
FY 1-6
reactant A
reactant B
residence
time calc LT
1-8
TT
1-3
LC
1-8
CAS
ratio
calc
product
vent FT
1-1
FC
1-1 FC 1-1
CAS
ZC1-4
OUT
ZC
1-4
FC
1-7
FT 1-7
PT
1-5
PC
1-5
FT 1-5
CTW
ZC1-4 is an enhanced PID VPC
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.
55
Liquid Reactants (Jacket CTW) Gas & Liquid Products Optimization
TT
1-4
TC
1-4
AT 1-6
LY 1-8
FY 1-6
FT
1-2
FC
1-2
reactant A
reactant B
residence
time calc
CAS
ratio
calc
AC
1-6
makeup
return
LY 1-8
FY 1-6
reactant A
reactant B
CAS
residence
time calc LT
1-8
LC
1-8
CAS
ratio
calc
product
FT
1-1
FC
1-1
FC
1-7
FT 1-7
CAS
TC
1-3 TT
1-3
product
PC
1-5
FT 1-5
W
PT
1-5
TT
1-10 TC
1-10
ZC
1-4
ZC
1-10
ZC
1-5
ZY 1-1
FC1-1
CAS
ZY1-1
OUT
low signal
selector
ZC-10
OUT
ZC-4
OUT
ZC-5
OUT
ZY-1
IN1
ZY-1
IN2
ZY-1
IN3
CTW
ZC1-4, ZC-5, & ZC-10 are enhanced PID VPC
56 56
Innovative PID System to Optimize Ethanol Yield and Carbon Footprint
AT
1- 4
AC
1- 4 DC
2- 4
SC
1-4
FT
1- 5
FC
1- 5
AY
1- 4
Corn
NIR-T
Production Rate
Enhanced PID
DT
2- 4
Slurry Solids
Enhanced PID
DX
2- 4
Feedforward
FT
1- 6
FC
1- 6
Backset Recycle
Dilution Water
XC
1- 4
XY
1- 4
Average Fermentation Time
Enhanced PID
Fermentable Starch
Correction
Slurry
Tank 1
Slurry
Tank 2
Coriolis
Meter
setpoint
DY
2- 4
Lag and Delay
RCAS
Predicted Fermentable Starch
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
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?
Where To Get More Information
Greg McMillan, “What is the Key PID Feature for Basic
and Advanced Control”, Control Talk Blog, 4-26-2012 – http://community.controlglobal.com/content/what-key-pid-feature-basic-and-advanced-control
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