Modeling and Control

Professor Tom Edgar of the University of Texas and Joseph Alford from Eli Lilly and Co. have been collecting data and opinions regarding the current syllabus of the typical undergraduate Chemical Engineering Process Control Course and its relevance to the skills and knowledge needed in today's industrial process control environment. An upcoming issue of Intech magazine will have an article by Edgar on how process control education in the universities should be changed. There will be replies from a key group of professors and practitioners. My contribution is the first 250 words of the following:

Terry Tolliver and I have taught a course on dynamic modeling and control at Washington University (WU) in Saint Louis since 2002 that is a requirement for a degree in chemical engineering. The course uses an industrial virtual plant and the ISA book Advanced Control Unleashed. The students are very computer literate and pick up on the use of industrial software from just a few screen prints put into the laboratory exercises. The knowledge gained is generally applicable since the function blocks are based on Foundation Fieldbus used in millions of devices and by over a hundred manufacturers. The configuration environment is also consistent with the international standard IEC 61804. The students learn how to intelligently discuss and use an industrial process simulation, DCS, and data historian that form a virtual plant on their desk. There is companion course taught by Bob Heider where an actual hardware version of the same DCS is used to control the temperature, pressure, and level of vessels in a hardware lab. The three professors have a total of more than 100 years experience in industry.

Most of the chapters in Advanced Control Unleashed start with an introductory section on “Practice,” continues with sections on “Opportunity Assessment” and “Application” and concludes with “Theory”. The strategy is to provide the relevance and practical considerations before getting into the theory that offers a deeper understanding. For example, in Chapter 2 - “Setting the Foundation”, the student gets an overview and perspective, list of opportunities, examples, application detail, and rules of thumb before getting into the theory where the focus turns to the set up of the differential equations for the material and energy balances to enable the student to learn the source of process time constants and gains in terms of process parameters. The students are not asked to solve or integrate these equations. Instead, the students graphically create a dynamic simulation of processes for unit operations commonly encountered on the job. Blocks for filters, dead times, noise, periodic disturbances, and backlash and sticktion are added to make the challenge of process control more realistic. Additionally the students configure an actual control system that can be downloaded into a real DCS. The students apply industrial embedded tools for auto tuning, statistical analysis, and model predictive control (MPC). The course centers on time response because this is what they see on the trend charts in the control room but there is a session to show how to go from the time domain to the frequency domain.

When I recently went back to WU and gave a guest lecture on the use of PID and MPC for fed-batch control of a fermenter, a student asked “what is a batch?” I knew that students were taught to think in terms of a steady state and the material and energy balances on a Process Flow Diagram (PFD) for continuous operations but I didn’t fully realize the implications until the question.

I have had chemical engineers in industry ask, why do you need a PID or MPC when you can just set the flow shown on the PFD? In fact, the batch sequences defined by process engineers today often try to set a predetermined step sequence of flows instead of using feedback control to sort it out. I have also have had experienced instrument engineers ask why do you need a Coriolis density measurement when the composition is constant as shown on the PFD? I also see ads for pressure and temperature compensated differential pressure orifice meters that claim to offer an unqualified mass flow measurement. If only the composition in all the pipelines were constant. This would sure make life easy. Product quality would be a non issue. Obviously the importance of dynamics and disturbances for process measurement and control is often missing in action.

In a batch process, the product concentration follows a profile. In some case there is also a temperature profile and in almost every case where a PID or MPC is used, the transfer of variability for a constant set point means there is a profile in the controller output. This understanding is lacking when chemical engineers are taught to think steady state. The lessons from batch would also be useful for the automated startups and grade transitions in continuous operations.

To add a bit of levity, I offer the following Top Ten List:

Top Ten Reasons Why an ISA Book on Control is not a University Text

10. Costs less than $100
9. The authors spent too much time in industry
8. Contains top ten lists and cartoons
7. Shows flow sensors upstream instead of downstream of the control valve
6. Discusses stick-slip and backlash
5. Shows unmeasured load upsets as inputs to the process
4. Includes field implementation considerations
3. Estimates tuning settings to just two significant digits
2. Doesn’t use tensor analysis for flow loops
1. Depicts signal lines as electronic instead of the pneumatic

Well, you may not become rich and famous in the conventional sense. You may not make it into the best seller list or your favorite book club but you can impress your friends, relatives, and associates and increase the marketability of your skills. More important is the sense of accomplishment in adding your expertise to the common body of knowledge. Here is the chance to share what you have learned and insure it is not lost.

I didn’t start out as a particularly a good writer and I am still learning. You are not trying to win any literary awards. You would be taking way the job of the ISA copy editor if you were to write perfect sentences. Like anything, writing becomes much easier with practice. A side benefit is it seems to have made me more articulate in public speaking.

I listen to music while writing but I have to admit I prefer peace and quiet when I first get started on a new subject. Once I get a flow going then I can rock. The inspiration and feeling I get from my favorite artists has helped make the writing process more fun and creative. The following is a summary of what has worked for me.

• Think that you are trying to say what is most important to a good friend
• Just get started and get a stream of ideas out
• Do figures last so it doesn’t interfere with the flow
• Start each paragraph with introductory sentence
• Keep sentences short
• Explain each point in detail
• Define nomenclature and engineering units for equations
• Give assumptions and offer examples
• Don’t obsess about wording – this is the ISA copy editors job
• Get associates in your field and related field to proof read everything
• Double check equations and figures

I am a terrible proof reader of my own stuff, because I read what I want to say and not what I actually have in print. Before you send a rough draft to ISA, it is wise to get several of your associates to proof read the text, equations, and figures. All technical comments are useful because even the ones that are off-base reflect a misunderstanding that should be addressed. Note that ISA for the last 3 years requires that you submit drafts of chapters as they are completed for technical review. This process should start about a year before the next ISA Expo. A schedule showing the planned submission date of each chapter is submitted with the outline and brief description of the book’s uniqueness and audience in the proposal. Below is an example of a proposal for my most recent book. The schedule shown was too tight.

Book Proposal Example

I have found it useful to put key concepts (insights) in italics for emphasis and easy reference. So often we get lost in the details and lose track of the underlying principles and governing generalities that are so important for dealing with new situations.

The following presentation at ISA Expo 2006 in Houston in the “Automation Connection Series” on Oct 18 offers a Smorgasbord of books, advantages of being an author, and the steps. Contact me if you want to get started on the road to fame and fortune.

ISA Expo 2006 Presentation

Since we tend to learn by examples and we can all relate to the challenges of parenting, I came up with the following discussion of the relative advantages of PID and model predictive control (MPC) of a teenager for my Control Talk column in the September issue of Control Magazine.

Would Proportional-Integral-Derivative (PID) or Model Predict Control (MPC) be better as a parental advisory control algorithm? The PID excels at handling the unknown upsets, non-stationary behavior, and unpredictable nature of a teenager’s response and derivative action provides some preemptive action. However, the abrupt action by PID could amplify noise in a teenager’s behavior. But then there is the dead band from backlash and the resolution limit from friction. Maybe a lot of dither from the PID could keep the teenager off balance and minimize inaction if it doesn’t wear you out. There is inverse response to contend with where the teenager starts out doing the opposite of what was requested. Theoretically, MPC could build this behavior into its knowledge of the teenager’s future trajectory. Also, the patience of a MPC is inline with modern parenting ideas. But I not sure your can do enough pseudo random binary sequences (PRBS) or know the time to steady state of a teenager so my initial thought is proportional plus derivative (PD) with an adjustable bias to deal with the potentially unstable response. But then what do I really know anymore? I am an empty nester heading out for one of many vacations without children. All I need to know now is how to enjoy the antics of grandchildren and quickly return them to their rightful owner for process control.

The students at Washington University in Saint Louis are ahead of the curve by virtue of the efforts of affiliate professors Terry Tolliver and Robert Heider who have a combined total of more than 65 years of industrial experience at Monsanto and Solutia.

Terry Tolliver teaches a process control course for junior and senior chemical engineers. The students have access on their desk to a virtual plant with embedded high fidelity process simulations and industrial control modules, trends, and operator graphics. The following file shows the university, the virtual plant class room, and text book.

WU Virtual Plant

Robert Heider teaches a computer control lab for chemical and systems engineers that uses an industrial automation system for the control of actual process equipment, such as vessels, heat exchangers and dryers for blending, level, temperature, and moisture control. The equipment, piping, instrumentation and valves are assembled on a cart with quick connects for utilities and Fieldbus signals to make each lab portable. The following file shows one of the lab experiments.

WU Hardware Lab

After some concise instructions with screen prints, the students have had no difficulty in accessing and using the DeltaV DCS system. The only people who seem to have trouble are the other professors who are not accustomed to seeing an industrial control system, which is probably more of a justification than a prohibition for taking this approach.

Modern DCS systems use Fieldbus standards for control module configuration and parameters. Also, most operator graphics and industrial historians have a similar feel that is distinctly different or entirely missing from academic software. Statements that industrial systems are specific are valid if it is meant specific to industry and not a particular manufacturer. Even the 2% of the students who are going on to an advanced degree in control and a future life in academia are better equipped for working with industrial consortia by understanding industrial systems and terminology.

Washington University graduates understand standard Fieldbus terminology (CAS, RCAS, and ROUT modes) and even such far out stuff as the units of reset time (e.g. sec/repeat). They can act more intelligently when they first venture into the control room. Even if they don’t pursue a career in process control, since the DCS is the window into the process and method of affecting the process, WU students are better able to hit the ground running on their first job. After a few labs, a light comes on with chemical engineers. They understand the significance of this approach. With systems engineers it may not happen because they are hoping to end up at an aerospace firm rather than in a chemical plant.

I would ask any skeptics of the validity in using an industrial system in university labs to first speak to some of Terry’s students before passing judgment.

Around 1984, there was a breakthrough in use of simulation for checkout and training. Software packages, such as MIMIC and its predecessor SIMVOX, automatically generated tieback simulations from the configuration and the input and output (I/O) cards and emulated the serial communication between the simulation and the DCS. These packages enabled the simulation to read all of the DCS outputs and send back all of the corresponding DCS inputs. Besides inherently providing a test of the I/O channel assignments, the simulation was separated from the DCS and expanded to cover the entire plant. The tieback simulation sent back the proper motor run contacts for the valve limit switch positions for discrete I/O that was particularly critical for batch operations. For control loops, the process variables was the PID controller output multiplied by a process gain and possibly delayed and filtered to simulate process dynamics. For indicators, fixed values were entered. A method was developed to switch these fixed values and to zero out loop process variables based on whether a flow path was established. A 1 or 0 status of each pump and valve in the piping path were multiplied together to determined the status. Ramps triggered by path status were added to simulate batch and startup responses. Batch operations could be run 100 times faster than real time, and be reset. Failures could be introduced. In Monsanto, these customized tieback simulations were credited with reducing the time to checkout and startup a DCS by 60% or more. By 1986 all Monsanto projects used the software package and its associated methodology and by 1994 nearly all of Monsanto’s 100 operating units were controlled by a DCS. The rapid deployment of the DCS had immediate benefits in terms of safer and more efficient operation plus provided a basis for a program of process control improvement over the next 6 years that lead to 4% further reduction in the cost of goods.

The tieback simulations with pathway logic and custom ramps achieved rapid education of the operators on how to effectively use displays and configuration. To develop better process understanding, the tieback simulations were in some cases enhanced by first principle process models. While the lack of a standard methodology resulted in custom process models of limited scope that were difficult to keep updated, the concept of a process model being connected to an actual DCS forever changed the landscape of process simulation. Up until this time process simulations for operator training used very expensive emulations of the control system at a cost of 200 thousand to 2 million dollars. Most nuclear power plants and some chemical plants and refineries went this route. However, it was not practical to include the detailed features of the control loops (e.g. structure, form, modes, and feedforward), sequences, batch executives, and the operator interface (e.g. displays and historian). Attempts to match and maintain were costly and prone to over simplification. The use of the actual DCS allowed the dynamic simulation to focus on the modeling of the process. The development of packages such as DeltaV Simulate Pro provided the ability to download the actual configuration and displays to a personal computer creating a virtual plant eliminating the need for the DCS console and controllers without any emulation or translation of the control system for training.

Plants are losing experienced operators and engineers so there is an even great potential benefit from periodic operator training. How can we provide training systems that wow decision makers when there may be no one left in the plant to support or even appreciate process simulations for operator training? I don’t have all the answers but here are some key aspects based on my experience:

(1) Live demos of virtual plants for key processes
(2) Online process metrics
(3) Expansion of audience beyond operations to process, control, and maintenance
(4) Modular and generic framework
(5) Ability to run slow processes much faster than real time
(6) Focus on process dynamics and interactions
(7) Readily increasing levels of fidelity for flexible cost and performance

For a perspective of the importance of the operator and some possibilities of online process metrics, check out the Dec 28 entry in the “Tuning and Control System Performance” Category. Next week we will look at some approaches to make the first principle process model more flexible in terms of cost and performance.

About every year since 1985, I wrote a book whether I needed to or not. Some of the books were written to entertain my self and hopefully others by adding a humorous view point to what is normally a very a serious profession. In 2006 my contribution to this unique endeavor was The Life and Times of an Automation Professional – An Illustrated Guide. While I wrote most of the material and generated most of the cartoon concepts, I choose to list the authors in reverse alphabetical order to be different and give the cartoonist Ted Williams some long overdue recognition. Right after this book was published I expanded my horizons to an even more fertile ground for humor – retirement. Since all of my friends were partly or fully retired, I had the experience base, but how could I make it different?

Trillions of atoms in my brain went amok to break free of the 4D dilemma or at least make reading about engineers and retirement less boring. The result was a break in the space and time continuum and a book on retirement like no other. For conservation of spin, I am sure there is a book somewhere in the far reaches of the universe with humor moving in the opposite direction. There is also the matter of possibly anti-matter in a parallel universe or Anti Funnier Side of Retirement book. However, these books are light years from being available although radio waves indicate a Starbucks on alpha centurion is offering extra hot lattes.

So if you are ready to laugh about the impending or depending retirement of people you know and love including yourself, check out The Funnier Side of Retirement for Engineers and Other People of the Technical Persuasion published in 2007 by ISA. What will I do for 2008?

Front and back covers:
Funnier Side of Retirement

Link to publisher:
http://www.isa.org/retirement