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TRANSCRIPT
Interactive Exploration of Process Control Improvement
Jack Ahlers - Process Control Specialist
Greg McMillan - Principle Consultant
Presenters
Jack Ahlers
Greg McMillan
Introduction
An online process control lab is used to provide an interactive
exploration and quantification of process control improvements to
get the most out of your PID
Demos of PID solutions via online process control labs show how
to reduce process variability and improve setpoint response
– Dynamic Reset solution for slow valves and secondary loops
– PIDPlus solution for
• Wireless Measurements
• Failed Measurements
• Analyzers with Sample Systems
• Valve Backlash and Stick-slip
– Smart Bang-Bang Logic solution for batch and startup sequences
– Deadtime Compensation solution for high process deadtime
Online Process Control Labs -Free Access to Virtual Plants
Visit http://www.processcontrollab.com/to Create Valuable New Skills
Free State of the Art Virtual Plant
Not an emulation but a DCS (SimulatePro)
Independent Interactive Study
Structural Changes “On the Fly”
Advanced PID Options and Tuning Tools
Enough variety of valve, measurement, and process dynamics to study 90% of the process industry’s control applications
Learn in 10 minutes rather than 10 years
Online Performance Metrics
Standard Operator Graphics & Historian
Control Room Type Environment
No Modeling Expertise Needed
No Configuration Expertise Needed
Rapid Risk-Free Plant Experimentation
Deeper Understanding of Concepts
Process Control Improvement Demos
Sample Lessons (Recorded Deminars)
The Opportunities Beyond Operator Training Systems
Dynamic simulations offer the opportunity to explore, quantify, demonstrate, detail, and prototype process control improvements (PCI)
However, the investment in software and time to learn and develop simulations typically limits the creation of models to specialists who have significant simulation and DCS expertise.
Process deadtime, measurement dynamics, and valve response is often not modeled (not understood by traditional process simulation software suppliers)
The emulation of the basic and advanced control capability in a DCS by process simulation software is unrealistic
What is needed is a virtual plant that uses the actual DCS with all of its capability and uses dynamics of all parts of the process and automation systems in a friendly control room environment by the use of the DCS operator interface
The virtual plant should be useable by any one who wants to learn the best of the practical control technologies for the process industry and to find, demonstrate, estimate, and convince people of the benefits of process control improvement
– Automation Engineers
– Local Business Partners
– Process Engineers
– Students
– System Integrators
– Suppliers
PCI and OTS Virtual Plants
Virtual
Process
Virtual
Sensors
Virtual
Valves
Virtual
DCS
Virtual
Process
Virtual
Sensors
Virtual
Valves
Virtual
I/OActual
DCS
Traditional
Process
Simulators
MiMiC
PCI
MiMiC
OTS
DeltaV
SimulatePro
DeltaV
ProPlus
Virtual Plant Essentials
Feed 1
Feed 2
Condenser
Cooling water Fcw
Reflux Drum
Lc, Vc_out
Reflux L_R
Distillate product L_D
CW Out
V_DA_VD1
A_Vlv1
Reboiler
A_v
L_B + V_B
V_B
Buttom product L_B
Heating steam
HE condensate
Side withdraw 2
Side withdraw 1
Heavy liquid L_HvLiq
Vnt
V_D1
DeltaV Simulate Product Family
MiMiC Simulation Software
Top Ten Things You Don’t Want to Hear in a Project Definition Meeting
(10) I don’t want any smart instrumentation talking back to me
(9) Let’s study each loop to see if the valve really needs a positioner
(8) Lets slap an actuator on our piping valves and use them for
control valves
(7) We just need to make sure the control valve spec requires the
tightest shutoff
(6) What is the big deal about process control, we just have to set the
flow per the PFD
Top Ten Things You Don’t Want to Hear in a Project Definition Meeting
(5) Cascade control seems awfully complex
(4) The operators can tune the loops
(3) Let’s do the project for half the money in half the time
(2) Let’s go with packaged equipment and let the equipment supplier
select and design the automation system
(1) Let’s go out for bids and have purchasing pick the best deal
Online Process Control Labs - Demo
Objective – Show access to virtual plant
Activities:– Go to http://www.processcontrollab.com/
– Look at Overview (PDF File)
– Look at How to Connect screen
– Show Request Access to a Virtual Plant (VP)
– Make a Remote Desktop connection to the assigned VP
– Click on DeltaV Operate
– Put all of the Process Control Labs in the Run mode
– Click on Trend Chart icon and open up charts for the Labs
– On Main DeltaV Operate screen discuss the Restore button
– In the Process Control Lab website show Help screen
– In the Process Control Lab website show Instructions screen
– Minimize screens and in FlexLock select Logoff Window
Unifying Concepts
“It is all about management of change” – Intentional change (setpoints)
– Unintentional change (disturbances)
– 90% of process control improvements involve the following concepts:• Delay
• Speed
• Gain
• Sensitivity-Resolution
• Backlash-Deadband
• Nonlinearity
• Noise
• Oscillations
• Resonance
• Attenuation
• Optimum
We will cover delay, speed, and gain since these are most prevalent limiting concepts
Checkout Deminar #9 “Process Control Improvement Primer” for other concepts
http://www.modelingandcontrol.com/2010/09/review_of_deminar_9_-_process.html
Delay “Without deadtime I would be out of a job” Fundamentals
– A more descriptive name would be total loop deadtime. The loop deadtime is the amount of time for the start of a change to completely circle the control loop and end up at the point of origin. For example, an unmeasured disturbance cannot be corrected until the change is seen and the correction arrives in the process at the same point as the disturbance.
– While process deadtime offers a continuous train of values whereas digital devices and analyzers offer non continuous data values at discrete intervals, these delays add a phase shift and increase the ultimate period (decrease natural frequency) like process deadtime.
Goals– Minimize delay (the loop cannot do anything until it sees and enacts change)
Sources– Pure delay from deadtimes and discontinuous updates
• Piping, duct, plug flow reactor, conveyor, extruder, spin-line, and sheet transportation delays• Digital devices - scan, update, reporting, and execution times (0.5*DT)• Analyzers - sample processing and analysis cycle time (1.5*DT)• Sensitivity-resolution limits• Backlash-deadband
– Equivalent delay from lags• Mixing • Column trays • Heat transfer surfaces• Thermowells• Electrodes • Transmitter damping and signal filter settings
Speed (Rate of Change)
“Speed kills - (high speed processes and disturbances and low speed control systems can kill performance)”
Fundamentals– The rate of change in 4 deadtime intervals is most important. By the end of 4
deadtimes, the control loop should have completed most of its correction. Thus, the short cut tuning method (Deminar #6) is consistent with performance objectives.
Goals– Make control systems faster and make processes and disturbances slower
Sources– Control system
• PID tuning settings (gain, reset, and rate)
• Slewing rate of control valves and velocity limits of variable speed drives
– Disturbances• Steps - Batch operations, on-off control, manual actions, SIS, startups, and shutdowns
• Oscillations - limit cycles, interactions, and excessively fast PID tuning
• Ramps - reset action in PID
– Process• Mixing in volumes due to agitation, boiling, mass transfer, diffusion, and migration
Gain
“All is lost if nothing is gained” Fundamentals
– Gain is the change in output for a change in input to any part of the control system. Thus there is a gain for the PID, valve, disturbance, process, and measurement. Knowing the disturbance gain (e.g. change in manipulated flow per change in disturbance) is important for sizing valves and feedforward control.
Goals– Maximize control system gains (maximize control system reaction to change) and
minimize process and disturbance gains (minimize process reaction to change).
Sources– PID controller gain – Inferential measurements (e.g. temperature change for composition change in
distillation column) – Slope of control valve or variable speed drive installed characteristic (inherent
characteristic & system loss curve)– Measurement calibration (100% / span). Important where accuracy is % of span– Process design– Attenuation by volumes (can be estimated)– Attenuation by PID (transfer of variability from controlled to manipulated variables)
Time (seconds)
% Controlled Variable (CV)
or
% Controller Output (CO)
DCO
DCV
qo tp2
Kp = DCV / DCO
0.63*DCV
CO
CV
Self-regulating process
open loop
negative feedback time constant
Self-regulating process gain (%/%)
Response to change in controller output with controller in manual
observed
total loop
deadtime
toor
Maximum speed
in 4 deadtimes
is critical speed
Self-Regulating Process Open Loop Response
Time (seconds)qo
Ki = { [ CV2 / Dt2 ] - [ CV1 / Dt1 ] } / DCO
DCO
ramp rate is
DCV1 / Dt1
ramp rate is
DCV2 / Dt2
CO
CV
Integrating process gain (%/sec/%)
Response to change in controller output with controller in manual% Controlled Variable (CV)
or
% Controller Output (CO)
observed
total loop
deadtime
Maximum speed
in 4 deadtimes
is critical speed
Integrating Process Open Loop Response
Response to change in controller output with controller in manual
qo t’p2
Noise Band
Acceleration
DCV
DCO
1.72*DCV
Kp = DCV / DCO
Runaway process gain (%/%)
% Controlled Variable (CV)
or
% Controller Output (CO)
Time (seconds)observed
total loop
deadtimerunaway process
open loop
positive feedback time constant
For safety reasons, tests are
terminated after 4 deadtimes
t’oor
Maximum speed
in 4 deadtimes
is critical speed
Runaway Process Open Loop Response
tp1 qp2 tp2Kpvqp1
tc1 tm2 qm2 tm1 qm1Kcvqctc2
Kc Ti Td
Valve Process
Controller Measurement
Kmvtvqv
KLtLqL
Load Upset
DCV
DCO
DMVDPV
PID
Delay Lag
Delay Delay Delay
Delay
Delay
Delay
Lag Lag Lag
LagLagLag
Lag
Gain
Gain
Gain
Gain
Local
Set Point
DDV
First Order Approximation: qo @ qv + qp1 + qp2 +
qm1 + qm2 + qc + tv + tp1 + tm1 + tm2 + tc1 + tc2
%
%
%
Delay => Dead Time
Lag =>Time Constant
Ki = Kmv * (Kpv / tp2 ) * Kcv
100% / span
Loop Block Diagram(First Order Approximation)
DCO = change in controller output (%) DCV = change in controlled variable (%) DDV = change in disturbance variable (%) Eo = open loop error (%) - step disturbance EL = load (%) - lagged disturbance Kc = controller gain (dimensionless) Ki = integrating process gain (%/sec/% or 1/sec) Kp = process gain (dimensionless) also known as open loop gain Rv = slew rate of control valve (%/sec) MV = manipulated variable (engineering units) PV = process variable (engineering units) Dt = change in time (sec) Dtx = PID execution time (sec) qo = total loop dead time (sec) tf = filter time constant (sec) tm = measurement time constant (sec) tp2 = primary (large) self-regulating process time constant (sec) t’p2 = primary (large) runaway process time constant (sec) tp1 = secondary (small) process time constant (sec) Ti = integral (reset) time setting (sec/repeat) Td = derivative (rate) time setting (sec) To = oscillation period (sec) l = Lambda (closed loop time constant or arrest time) (sec)
Nomenclature
Practical Limit to Loop Performance
o
cp
x EKK
E **+
=)1(
1
o
cp
fxi
i EKK
tTE *
*
+D+=
t
Peak error decreases as the controller gain increases but is essentially the
open loop error for systems when total deadtime >> process time constant
Integrated error decreases as the controller gain increases and reset time decreases
but is essentially the open loop error multiplied by the reset time plus signal
delays and lags for systems when total deadtime >> process time constant
Peak and integrated errors cannot be better than ultimate limit - The errors predicted
by these equations for the PIDPlus and deadtime compensators cannot be better
than the ultimate limit set by the loop deadtime and process time constant
i
c
o
v
T
K
E
R
Valve slew rate (Rv)
limits effective
speed of tuning
Ultimate Limit to Loop Performance
o
po
ox EE *
+=
)( tq
q
o
po
oi EE *
+=
)(
2
tq
q
Peak error is proportional to the ratio of loop deadtime to 63% response time
Integrated error is proportional to the ratio of loop deadtime squared to 63% response time
For a sensor lag (e.g. electrode or thermowell lag) or signal filter that is much larger
than the process time constant, the unfiltered actual process variable error can be
found from the equation for attenuation
The PIDPlus enhancement for measurement delay and the
addition of a DT block for compensation of process deadtime
can achieve the ultimate limits to loop performance !
Peak and Integrated Error Check List
Disturbance Speed and Attenuation
oL EeE Lo *-=-
)1(/tq
f
oof
TAA
t *=
2*
Effect of load disturbance lag (tL) can be estimated by replacing the open loop error
with the exponential response of the disturbance during the loop deadtime
The attenuation of oscillations van be estimated from the expression of the Bode plot
equation for the attenuation of oscillations slower than the break frequency where (tf) is
the filter time constant, electrode or thermowell lag, or a mixed volume residence time
For biological processes the load disturbance lag (tL) can be so slow (e.g. days),
temperature control is a non-issue for a well designed coolant system
Implied Deadtime from Slow Tuning
)(5.0 oi qlq +*=
Slow tuning (large Lambda) creates an implied deadtime where the loop performs
about the same as a loop with fast tuning and an actual deadtime equal to the
implied deadtime (qi)
Money spent on improving measurement and process dynamics
(e.g. reducing measurement delays and process deadtimes)
will be waste if the controller is not tuned faster to take
advantage of the faster dynamics
You can prove most any point you want to make in a comparison
of control system performance, by how you tune the PID.
inventors of special algorithms as alternatives to the PID
naturally tend to tune the PID to prove their case.
“Advanced Control Algorithms; Beware of False Prophecies”
http://www.modelingandcontrol.com/FunnyThing/
Effect of Implied Deadtime onAllowable Digital or Analyzer Delay
In this self-regulating process the original process delay (dead time) was 10 sec.
Lambda was 20 sec and the sample time was set at 0, 5, 10, 20, 30, and 80 sec (Loops 1 - 6)
The loop integrated error increased slightly by 1%*sec for a sample time of 10 sec which corresponded to a
total deadtime (original process deadtime + 1/2 sample time) equal to the implied deadtime of 15 seconds.
http://www.modelingandcontrol.com/repository/AdvancedApplicationNote005.pdf
sample time = 0 sec
sample time = 5 sec
sample time = 10 sec
sample time = 20 sec
sample time = 30 sec
sample time = 80 sec
Fastest Practical PID Tuning Settings(Practical Limit to Loop Performance)
op
p
cK
Kq
t
**=
24.0
oiT q*= 2 1d pT t=
For runaway processes:
For self-regulating processes:
oi
cK
Kq*
*=1
5.0oiT q*= 4 1d pT t=
oi
cK
Kq*
*=1
6.0
oiT q*= 40 1d 2 pT t*=
For integrating processes:
op
p
cK
Kq
t
**=
2'6.0
oi
cK
Kq*
*=1
4.0
Near integrator (tp2 >> qo):
oiT q*= 5.0
Near integrator (t’p2 >> qo):
Deadtime dominant (tp2 << qo):
0d =T
Effect of Tuning Speed Oscillatory Disturbance
1
Ultimate
Period
1
1Faster
Tuning
Log of Ratio of
closed loop amplitude
to open loop amplitude
Log of ratio of
disturbance period
to ultimate period
no attenuation
of disturbances
resonance (amplification)
of disturbances
amplitude ratio is
proportional to ratio of
break frequency lag to
disturbance period
1
no better than manual worse than manual improving control
Fast Valve and Fast Cascade Demo
Objective – Show access to cascade loop lab setup and how to make load upsets to see response for fast valve and fast cascade
Activities:– Show access to Cascade Loop Lab02 user interface
– Show access to PID faceplate and detail
– Show access to “Process History View” trend chart
– Click on secondary PID faceplate and put secondary PID in AUTO mode
– Make load change to show secondary response by putting secondary PIDmomentarily in manual and changing its output (e.g. 50% to 60%)
– Click on any block in block diagram to access Detail for parameters that will be changed in these demos via tabs for PID, process, and valve
– Put secondary PID in CAS mode and click on primary PID faceplate
– Make load change to show cascade response by putting primary PID momentarily in manual and changing its PID output (e.g. 50% to 60%)
Note: AC1-1 is primary PID and AC1-2 is secondary PID
Volume Booster with Integral Bypass(Furnace Pressure and Surge Control)
Signal from
Positioner
Air Supply from
Filter-Regulator
Air Loading
to Actuator
Adjustable Bypass
Needle Valve
Booster and Positioner Setup(Furnace Pressure and Surge Control)
Port A
Port B
Supply
ZZ
ZZ
ZZ
Z
Control Signal
Digital Valve Controller
Must be functionally
tested
before commissioning!
1:1
Bypass
Volume
Booster
Open bypass just
enough to ensure
a non-oscillatory
fast response
Air Supply
High Capacity
Filter Regulator
Increase air line size
Increase connection size
Terminal Box
Slow Valve Demo
Objective – Show response of secondary PID to slow valve for
small and large upsets
Activities:
– First look at Demo for fast valve and fast secondary loop
– Click on any block in block diagram – Click on Control Valve tab
– Change Slew Inc and Slew Dec of valve from 100%/sec to 1%/sec
– Click on secondary PID faceplate and put secondary PID in AUTO mode
– Make small load change to show response by putting secondary PID
momentarily in manual and changing its PID output (e.g. 50% to 52%)
– Make large load change to show response by putting secondary PID
momentarily in manual and changing its PID output (e.g. 50% to 70%)
Ramping Response of Actuator for a Large Step or a Large Actuator
0
5
10
15
20
25
30
35
40
45
50
0 1 2 3 4 5 6 7 8 9 10
Time (sec)
Str
oke
(%
)
Multiply time by 10 for large actuator
without volume booster
Exponential Response of Actuator for a Small Step or a Small Actuator
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0 1 2 3 4 5 6 7 8 9 10
S
tro
ke
(%
)
Time (sec)
Slow Valve DemoDynamic Reset Limit Enabled
Objective – Show response of secondary PID with Dynamic
Reset Limit to slow valve
Activities:
– First look at Demo for slow valve
– Click on any block in block diagram – Click on PID tab
– Enable Dynamic Reset Limit for secondary PID
– Make large load change to show response by putting secondary PID
momentarily in manual and changing its PID output (e.g. 50% to 70%)
Positive Feedback Implementation of Integral Mode with Dynamic Reset Limit
S
*
SP
b
D proportional
derivative
*
Gain
*
** *
Rate
D
D
g
CO
filter
filter
CV filter
Filter Time =
a * Rate Time
S
filter
Filter Time =
Reset Time
ER is external reset
(e.g. secondary PV)
Dynamic Reset Limit
ER
Positive
Feedback
Slow Secondary Loop Demo
Objective – Show response of self-regulating primary PID to slow
secondary loop for small and large upsets
Activities:
– First look at Demo with dynamic reset limit for slow valve
– Click on any block in block diagram – Click on Control Valve tab
– Change valve Slew Inc and Slew Dec from 1%/sec to 100%/sec
– Click on PID tab and disable Dynamic Reset Limit for secondary PID
– Click on Process tab and increase secondary Lag 2 Inc and Lag 2 Dec from
2 to 10 sec
– On secondary PID detail, increase reset time from 2 to 10 sec
– Put secondary PID is in CAS mode
– Click on primary PID faceplate and detail
– Make small load change to show response by putting primary PID
momentarily in manual and changing its PID output (e.g. 50% to 52%)
– Make large load change to show response by putting primary PID
momentarily in manual and changing its PID output (e.g. 50% to 70%)
Cascade Control Benefit (self-regulating process)
qi / qo= 0.6
(ti / to)
qi / qo= 1.0
ti = inner loop process time constant
to = outer loop process time constant
qi = inner loop process deadtime
qo = outer loop process deadtime
Improvement significantly greater
(ratio of peak errors much smaller)
for integrating and runaway
primary processes
Slow Secondary Loop DemoDynamic Reset Limit Enabled
Objective – Show response of self-regulating primary PID with
Dynamic Reset Limit to slow secondary loop
Activities:
– First look at Demo for slow secondary PID
– Click on any block in block diagram – Click on PID tab
– Enable Dynamic Reset Limit for primary PID
– Make large load change to show response by putting primary PID
momentarily in manual and changing its PID output (e.g. 50% to 70%)
PIDPlus Solution - Algorithm
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 sensitivity limit, 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
PIDPlus Benefit DemoFast Secondary (Flow)
Objective – Show how PIDPlus can achieve the ultimate performance limit for a wireless refresh time of 16 sec in a secondary loop (flow)
Activities:– First look at Demo for slow secondary PID with Dynamic Reset Limit enabled
– Put secondary loop in AUTO
– In PID detail, disable Dynamic Reset Limit in secondary and primary PID
– Change secondary setpoint from 50% to 60%
– On Measurement tab Detail set Refresh = 16 sec and Sensitivity = 100% for secondary measurement
– Change secondary setpoint from 60% to 50%
– Wait for oscillations to develop
– In PID detail, Enable PIDPlus for secondary loop
– Wait for oscillations to die out
– Change secondary setpoint from 50% to 60%
http://www2.emersonprocess.com/siteadmincenter/PM%20DeltaV%20Documents/
Whitepapers/WP_DeltaV%20PID%20Enhancements%20for%20Wireless.pdf
Flow Loop Setpoint ResponsePIDPlus versus Traditional PID
Flow Loop Load ResponsePIDPlus versus Traditional PID
Flow Loop Failure ResponsePIDPlus versus Traditional PID
PIDPlus Benefit DemoInline Primary Loop (Static Mixer pH)
Objective – Show how PIDPlus can achieve the ultimate performance limit for wireless refresh time of 60 sec in a primary loop (static mixer pH)
Activities:– First look at Demo for wireless secondary loop with PIDPlus enabled
– Put secondary loop in MAN
– Change secondary loop manual output from 60 to 50%
– Put secondary loop in CAS
– Change primary PID setpoint from 50% to 60%
– Note response with wired primary loop and PIDPlus wireless secondary loop
– In Measurement detail, Set Refresh = 60 sec and Sensitivity = 100% for primary measurement
– Make primary PID setpoint change from 60% to 50%
– Wait for oscillations to develop
– In PID detail, enable PIDPlus for primary loop
– Wait for oscillations to die out
– Change secondary setpoint from 50% to 60%
http://www2.emersonprocess.com/siteadmincenter/PM%20DeltaV%20Documents/
Whitepapers/WP_DeltaV%20PID%20Enhancements%20for%20Wireless.pdf
Static Mixer pH Loop Setpoint ResponsePIDPlus versus Traditional PID
Static Mixer pH Loop Load ResponsePIDPlus versus Traditional PID
Static Mixer pH Loop Failure ResponsePIDPlus versus Traditional PID
PIDPlus Benefit DemoFast Valve Limit Cycle (e.g. Flow)
Objective – Show how PIDPlus can eliminate limit cycles from valve stick-slip
Activities:– First look at Demo for wireless secondary and primary loops with PID
PIDPlus enabled in both the secondary and primary loops
– Put secondary loop in MAN
– Change secondary loop manual output from 60 to 50%
– In Measurement detail, return to wired measurements (set Refresh = 0 sec and Sensitivity = 0% for both the secondary and primary measurements)
– In PID detail, disable PIDPlus in secondary and primary PID
– In Control Valve detail, set Stick-Slip = 4%
– Change secondary loop manual output from 50 to 60%
– Put secondary loop in AUTO
– Wait for oscillations to develop
– In PID detail, enable PIDPlus in secondary PID
– Wait for oscillations to die out
– Change secondary setpoint from 50 to 60%
Smart Bang-Bang Control toReduce Fed-Batch and Startup Time Full Throttle (Smart Bang-Bang) Control - The controller output is stepped
to it output limit to maximize the rate of approach to setpoint and when the projected PV equals the setpoint less a bias, the controller output is repositioned to the final resting value. The output is held at the final resting value for one deadtime. For more details, check out the Control magazine article “Full Throttle Batch and Startup Response.”
http://www.controlglobal.com/articles/2006/096.html– A deadtime (DT) block must be used to compute the rate of change so that new
values of the PV are seen immediately as a change in the rate of approach.
– If the total loop deadtime (qo) is used in the DT block, the projected PV is simply the current PV minus the output of the DT block (DPV) plus the current PV.
• If the PV rate of change (DPV/Dt) is useful for other reasons (e.g. near integrator or true integrating process tuning), then DPV/Dt = DPV/qo can be computed.
– If the process changes during the setpoint response (e.g. reaction or evaporation), the resting value can be captured from the last batch or startup
– If the process changes are negligible during the setpoint response, the resting value can be estimated as:
• PID output just before the setpoint change for an integrating (e.g. batch) process
• PID output just before the setpoint change plus the setpoint change divided by the process gain for a self-regulating (e.g. continuous) process
– For self-regulating processes such as flow with the loop deadtime (qo) approaching or less than the largest process time constant (tp ), the logic is revised to step the PID output immediately to the resting value. The PID output is held at the resting value for the T98 process response time (T98 = qo + 4* tp ).
Smart Bang-Bang Control Demo
Objective – Show how to reduce batch and startup time by a full
throttle setpoint response (bang-bang control)
Activities:
– Go to Main Display and then select Single Loop Lab01
– Click on PID faceplate and then on its Detail icon (faceplate lower left corner)
– Enter tuning settings: Gain = 1.7, Reset = 210 sec, Rate = 2 sec
– Click on any block in block diagram and then on Process tab detail
– Set primary process Delay = 9 sec, Lag 2 Inc & Lag 2 Dec = 100 sec
– Set primary process Type = Integrating
– Enable setpoint response metrics
– Make setpoint change from 50% to 60%
– Wait for setpoint response to complete and note metrics
– In PID detail, set Bang-Bang Bias = 4%
– Make setpoint change from 60% to 50%
– Wait for setpoint response to complete and note metrics
Setpoint Response Results (Deminar #7)
Structure 3
Rise Time = 8.5 min
Settling Time = 8.5 min
Overshoot = 0%
Structure 1
Rise Time = 1.6 min
Settling Time = 7.5 min
Overshoot = 1.7%
Structure 1 + SP FF
Rise Time = 1.2 min
Settling Time = 6.5 min
Overshoot = 1.3%
Structure 1 + Bang-Bang
Rise Time = 0.5 min
Settling Time = 0.5 min
Overshoot = 0.2%
http://www.screencast.com/users/JimCahill/folders/Deminars/media/6074a326-f7c9-4485-b827-a306515c63c9
Deadtime Compensation Demo
Objective – Show how to achieve ultimate limit for loop performance by process deadtime compensation
Activities:– Go to Main Display and then select Single Loop Lab01
– Click on any block in block diagram and then on Process tab detail
– Set primary process type = Self-Regulating
– Set primary process Delay = 9 sec
– Set primary and secondary Lag 2 Inc & Lag 2 Dec = 1 sec
– Put loop in MAN
– Click on PID faceplate and then on its Detail icon (faceplate lower left corner)
– Enter tuning settings: Gain = 0.4, Reset = 5 sec, Rate = 0 sec
– Set primary process type = Self-Regulating
– Put loop in AUTO
– Enable setpoint response metrics
– Make setpoint change from 50% to 60%
– Wait for setpoint response to complete and note metrics
– In PID Detail, enable Dynamic Reset Limit and set PID Deadtime = 10 sec
– Enter tuning settings: Gain = 1.0, Reset = 1 sec, Rate = 0 sec
– Make setpoint change from 60% to 50%
– Wait for setpoint response to complete and note metrics
Deadtime Compensation Configuration
Insert
deadtime
block
Must enable dynamic reset limit !
Business Results Achieved
A faster setpoint response and load rejection for
improved process capacity and efficiency can be
achieved by the use of:
– Dynamic Reset to prevent the primary loop output from
changing faster than the valve or secondary loop can respond
– PIDPlus (DeltaV v11) to prevent ramp of output from
measurement failure, eliminate spikes in output from
measurement restore, achieve ultimate the performance limit
for a large measurement delay, and stop limit cycles from
measurement sensitivity limit and valve stick-slip or backlash
– Smart Bang-Bang for faster startups of slow processes
– Deadtime compensation to achieve ultimate performance
limit for large well known process deadtime
Summary
Use process control labs for “hands on learning”
Enable dynamic reset for big valves with fast readback of actual valve position and slow secondary loops
Use PIDPlus for wireless measurements, analyzers, and valves or dampers with excessive backlash and stick-slip
Use smart bang-bang composite template library module for faster setpoint response for slow integrating and near-integrating processes (startups and batch ops)
Add a deadtime block to PID external reset BK_CAL and enable dynamic reset limit to compensate for large well known process deadtimes
Questions?
The Latest on Smart & Wireless
Royalties are donated to the
University of Texas Research
Campus for Energy and
Environmental Resources
for Development of Wireless
Instrumentation and Control
Where To Get More Information
Modeling and Control Website http://www.modelingandcontrol.com/
Process Control Lab Website http://www.processcontrollab.com/
“DeltaV Version 11 PID Enhancements for Wireless”, Emerson White Paper, July 2010 http://www2.emersonprocess.com/siteadmincenter/PM%20DeltaV%20Documents/Whitepapers/WP_DeltaV%20PID%20Enhancements%20for%20Wireless.pdf
“Wireless – Overcoming Challenges of PID Control & Analyzer Applications”, InTech, July-Aug 2010 http://www.isa.org/InTechTemplate.cfm?template=/ContentManagement/ContentDisplay.cfm&ContentID=83041
“Adaptive Level Control”, Control, Jan 2010 http://www.controlglobal.com/articles/2010/LevelControl1002.html
“Is Wireless Process Control Ready for Prime Time”, Control, May 2009 http://www.modelingandcontrol.com/repository/WirelessPrimeTime.pdf