how to setup and adjust the dynamic compensation of feedforward signals

Interactive Opportunity Interactive Opportunity AssessmentAssessmentInteractive Opportunity Interactive Opportunity AssessmentAssessment

Demo and Seminar (Deminar) Series for Web Labs –

Feedforward Control Feedforward Control Jan 5, 2011

Sponsored by Emerson, Experitec, Monsanto, & MynahCreated by

Greg McMillan and Jack Ahlerswww.processcontrollab.com Website - Charlie Schliesser (csdesignco.com)

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

Welcome Welcome 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, was honored by InTech Magazine in 2003 as one of the most influential innovators in automation, and received the ISA “Life Achievement Award” in 2010. 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 Top Ten Things You Don’t Want to Hear on a Startup


(10) You need the owner to be a little more patient (supplier expert). (9) Don’t bother with a checkout - just light it up! What is the worst that

can happen? (8) We didn’t do any simulation or testing. We decided that would spoil

the adventure. (7) I don’t understand. It fit fine on the drawing. (6) Cool - This is my first time in a real plant (supplier expert). (5) I tried to open the valve and nothing happened. Wait! The same

valve on the other reactor just opened. (4) Should the Variable Frequency Drive smoke like that? (3) I don’t understand. I am sure I left all your tools and radios in a box

right here. (2) The CEO is holding on a phone for you.

And the Number 1 thing:

Source: “Final Word on Instrument Upgrade Projects”, Control Talk, Control, Dec 2010http://www.controlglobal.com/articles/2010/InstrumentProjects1012.html




(1) Boom!!! WHAT was that?!?!


Cascade Loop Block Diagram(First Order Approximation)

Cascade Loop Block Diagram(First Order Approximation)

p1 p2 p2 Kp2p1

m2 m2Km2c2 f2

Kc2 Ti2 Td2

PrimaryProcess

Kvvv

KL2L2L2

Primary Load Upset

CVp

COp

MVPVp2

Delay Lag

Delay Delay Delay

Delay

Delay

Lag Lag Lag

LagLag

Gain

Gain

Gain

Gain

LocalSet Point

DVp2

%

%

%

Delay <=> Dead TimeLag <=>Time ConstantKL1L1L1

Delay Lag Gain

DVp1

Secondary Load Upset

Kc1 Ti1 Td1

COs

SecondaryPID

CascadeSet Point

%

%

Kp1

Gain

CVs

m2 m2Km2

Delay

LagGain

c2 f2

Delay Lag

SecondaryProcess

PrimaryPID

Primary: o2v p1 p2 m2 c2

f2vp1Secondary: o1v p1 m1 c1 f1v

20 sec

Secondary Feedforward

Primary Feedforward


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), a multiplier creates scaling problems for the user, consequently the correction of most feedforward signals is done via a bias.

– The bias correction must have sufficient positive and negative range for worst case.– Model predictive control (MPC) and PID loops get into a severe nonlinearity by creating a

controlled variable that is the ratio. Instead of a ratio as the controlled variable, the independent variable is multiplied by a desired ratio and the result is corrected by a feedback loop with the process variable (composition, conductivity, gage, temperature, or pH) as the controlled variable.

Feedforward gain is the desired ratio for flow (load) upsets. Feedforward gain is the inverse of the process gain for setpoint feedforward.

– Process gain is the open loop gain seen by the PID (product of manipulated variable, process variable, and measurement variable gain) that is dimensionless.

Feedforward action must be in the same direction as feedback action for upset. Feedforward action is the opposite of the control action for setpoint feedforward. Feedforward delay and lag adjusted to match any additional delay and lag,

respectively in path of upset so feedforward correction does not arrive too soon. Feedforward lead is adjusted to compensate for any additional lag in the path of the

manipulated variable 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 controller. This is often best done by the use of a ratio block and a bias/gain block instead of the internal PID feedforward calculation.

Feedforward Implementation


Bias Correction of Ratio ControlBias Correction of Ratio Control

http://www.modelingandcontrol.com/2009/04/what_have_i_learned_-_ratio_co_1.html


Feedforward Demo 1Feedforward Demo 1Feedforward Demo 1Feedforward Demo 1 Objective – Show the upset without feedforward

Activities:– Go to Main Display, select Feedforward Loop Lab03, and click on any block

– In Lab03 Disturbances detail check primary upset size is 10%

– In Lab03 Measurements detail set primary feedforward gain = 0.0

– Change Lab03 desired run time from 600 to 70 seconds

– Change Lab03 mode from Explore to Run

– Click on Trend icon next to faceplate icon and open Lab03 chart


Feedforward Demo 2Feedforward Demo 2Feedforward Demo 2Feedforward Demo 2 Objective – Show effect of a feedforward correction arriving right on time

Activities:– In Lab03 Measurements detail set primary feedforward gain = 1.0

– In Lab03 Measurements detail set primary feedforward delay = 20 sec


– Click on Trend icon next to faceplate icon and look at Lab03 chart


Feedforward Demo 3Feedforward Demo 3Feedforward Demo 3Feedforward Demo 3 Objective – Show effect of a feedforward correction arriving too late

Activities:– In Lab03 Measurements detail set primary feedforward delay = 40 sec




Feedforward Demo 4Feedforward Demo 4Feedforward Demo 4Feedforward Demo 4 Objective – Show effect of a feedforward correction arriving too early

Activities:– In Lab03 Measurements detail set primary feedforward delay = 0 sec




Feedforward Demo 5Feedforward Demo 5Feedforward Demo 5Feedforward Demo 5 Objective – Show effect of a feedforward correction arriving too early with

wireless measurement and traditional PID

Activities:– In Lab03 Measurements detail check primary feedforward delay = 0 sec

– In Lab03 Measurements detail set primary sensitivity = 100% & refresh = 60 sec




Feedforward ApplicationsFeedforward Applications 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 feedback process controller.

– Why? - Flow is the predominant process input that is manipulated to set production rate and to control key process outputs (e.g. composition, level, pressure, and temperature). Exception is power input.

• Blend composition control - additive/feed (flow/flow) ratio

• Column temperature control - distillate/feed, reflux/feed, stm/feed, and 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 line 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 is most effective if the loop deadtime is large, disturbance speed is fast and size is large, feedforward gain is well known, feedforward measurement and dynamic compensation are accurate.

Setpoint feedforward is most effective if the loop deadtime exceeds the process time constant and the process gain is well known.

http://www.controlglobal.com/articles/2008/171.html “Feeding on Feedforward” Control Talk, Control, May 2008


Feedforward Feedforward Assumptions Feedforward Feedforward Assumptions Feedforward gain can be computed from a material or energy balance Ordinary

Differential Equations (ODE) * & 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 variable to independent variable (e.g. feed) but is most often simply based on operating experience.

– * http://www.modelingandcontrol.com/repository/AdvancedApplicationNote004.pdf

– Plots are based on an assumed composition, pressure, temperature, and/or quality

• 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 and 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.

Dynamic compensation is used to insure the feedforward signal arrives at same point at same time in process as upset

– Assumption is delay in the feedforward path is not greater than delay in upset path !


Feedforward Demo 6Feedforward Demo 6Feedforward Demo 6Feedforward Demo 6 Objective – Show effect of a feedforward correction arriving too early with

wireless measurement and enhanced wireless PID (PIDPlus)

Activities:– In Lab03 Measurements detail check primary feedforward delay = 0 sec

– In Lab03 PID detail, Enable Primary PIDPlus


– Wait for process to stabilize with PIDPlus

– In Lab03 Measurements detail set primary refresh = 0 sec

– Wait for process to line out with PIDPlus

– In Lab03 Measurements detail set primary refresh = 60 sec




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Summary of Feedforward Conclusions Summary of Feedforward Conclusions Feedforward correction for load upsets usually involves multiplying a

feed flow or speed by a ratio that is corrected by a bias from a feedback process controller via a bias and gain block

The ratio PV and SP should be displayed and trended from a ratio block An Integral-only loop can slowly correct ratio SP to match the ratio PV

(similar to a valve position control whose reset time is set greater than 10x process feedback controller reset time divided by controller gain)

The feedforward delay, lead, and lag should be adjusted so the feedforward does not arrive too early or too late

Compensation of a delay in the feedforward path greater than delay in upset path is not possible.

A feedforward signal too early creates inverse response. A feedforward signal too late creates a second disturbance. Since inverse response is particularly disruptive, a conservative

approach is to make the feedforward gain slightly less and the timing slightly slower than estimated requirements.

Wireless measurements with a PIDPlus can prevent a feedback controller from reacting to a feedforward timing error if the refresh time (default update rate) is larger than the process response time.


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how to setup and adjust the dynamic compensation of feedforward signals

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