DuPont
& EnTech Use VisSim/Real Time for Dynamic Process Control Simulation
Authors: Extracted from a paper by Hank Graeser (DuPont) and Andy Waite
(EnTech)
In a joint effort, engineers from E. I. DuPont de Nemours and EnTech Control
Engineering used Visual Solutions' VisSim
control system design software, and its real-time option VisSim/Real-Time,
to develop a high-fidelity dynamic simulation model of DuPont's non-woven
sheet manufacturing facility in Richmond, Virginia.
The model consists of approximately 31,500 blocks and 250 differential
equations and simulates roughly a half dozen interrelated processes. It
is used by DuPont's control and design engineers to verify process dynamics
during product transitions; develop and tune control strategies; and explore
possible design changes to enhance control performance. In addition, system
operators train on the model to maintain proficiency and learn new procedures
without impacting plant operations.
"High Fidelity modeling of a large scale project can be done with VisSim.
From a financial standpoint, itís extremely worthwhile to the process
control community. There are tremendous cost savings in reduced downtime
due to offline tuning and control design as well as operator training."
According to Hank Graeser, senior engineer at DuPont,"VisSim is a highly
intuitive environment for developing large scale high-fidelity process
models. The DuPont Spruance model, developed in VisSim, has saved the
company an estimated one million dollars to date.We developed the model
in a third of the time it would normally take using conventional methods.
VisSim's block diagram interface made it easy to document and maintain
the model.Every time we use the model for control design and off-line
tuning, DuPont saves significant dollars as plant down time is reduced.
We also train our operators using the VisSim model."
Aerial view of the DuPont non-woven sheet manufacturing
facility in Richmond, Virginia.
DuPont paper machine simulation in VisSim, showing
Reel scanner trends. Reel scanner position, reel ash, dry weight, and
moisture scan averages are displayed.
The Ideal Simulation Software As DuPont
engineers drew up the specifications for the model, EnTech engineers were
tasked with finding the best simulation software with which to build it.
The sheer scope of the model, which included the
entire DuPont facility 15 tanks; 20 sets of pumps, lines, and valves;
refiners; headbox and drainage table; vacuum devices, dryer cylinders,
and scanning sensors; and other minutiaewarranted a simulation software
package capable of modeling and simulating large, multivariable dynamic
processes with a high degree of fidelity.
The software had to be interactive and graphically
oriented so that dynamic information could be presented in an intuitive
manner. In addition, the block set had to include a complete selection
of continuous, discrete, transfer function, Boolean, arithmetic, and I/O
blocks.
Other key requirements included the capability
to run in simulated time, real time, and continuous time; drive real-time
analog and digital I/O; stop and continue simulations; initialize all
state variables; and extend the block set with custom blocks written in
C, for enhanced speed and additional functionality.
Because system operators would also use the simulation
model for training purposes, the ability to create realistic control panels
with controller faceplates, dynamic tank levels, and built-in alarms was
also important.
Based on these requirements, the simulation software
that best met EnTech's needs was VisSim and the VisSim/Real-Time companion
software.
Model Design
According
to DuPont and EnTech engineers, the DuPont model simulates the outputs of
80 sensors and transmitters and accepts input from 50 controller outputs.
In addition, the model provides high integrity dynamics as "seen" through
the eyes of the actual sensors and transmitters, with a time constant in
the range of about 3 seconds. This means that the truly fast dynamics, such
as that of incompressible fluid flow, which typically have time constants
of 20 milliseconds, do not have to be solved rigorously. Instead, the equations
associated with the pump curves, fluid flow, and control valve characteristic
curves can be approximated by solving only the nonlinear algebraic equations.
These "algebraic loops" involve the on-line
iteration from the last known flow and are solved using algebraic loop
time constants of typically 1 second, which provides an adequate safety
margin compared to the high-fidelity specification of 3 seconds.
In the resulting simulation, the time constant
spread ranges from a fast value of 1 second to that of the mixing time
constant for some tanks of 20 minutes. This time constant spread, even
though quite large, means that the simulation avoids some of the pitfalls
of "stiff systems of differential equations" which are very difficult
to solve numerically.
The final simulation model is organized in a multi-layer
format in which detailed simulation subelements collapse into "compound
blocks." There are 900 compound blocks, in about six layers, organized
in an easy-to-follow, process-oriented layout.
Model Verification
In the testing phase, close to 200 real-time I/O channels were used to validate
the model and control hardware. The simulated process runs ten times faster
than the real process on a Pentium 100 MHz personal computer, at a simulation
step size of 0.5 seconds.
Predicting The Future
The DuPont model is an excellent example of how a dynamic multivariable
process control model can be developed and utilized using VisSim and VisSim/Real-Time.
Graeser and other DuPont engineers have observed a "close match" between
model and actual plant data. Based on these results, they are confident
that the DuPont model can be used as a "life-cycle" tool to faithfully
predict the effects of future design decisions before modifications are
actually made to the plant.
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