Dynamic Controller

Frustrated by Conventional Advanced Control Techniques, Charlie Cutler Had to Find a Better Way

By Charlie Cutler

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If it wasn't for a football injury, this year's third Hall of Famer might have been running for the Green Bay Packers in the first Super Bowl in 1967, not rewriting model predictive control.

But a second fracture in his right leg made Charles Cutler realize football was ephemeral, and chemical engineering would at least be less crippling. After a second try at studies at Lamar University, Cutler graduated in 1961 and went to work for Shell Oil Co. There he would conceive and implement the concept of a dynamic matrix control (DMC) algorithm, which would save the petrochemical industry millions of dollars.

"Clearly, the problems that DMC solved and the benefit it has had on the welfare of the process industry allows it to be rated as a revolution in technology," says R. Russell Rhinehart, head of the School of Chemical Engineering at Oklahoma State University. "It was Charlie's particular innovative ability, interpersonal skills, persistence, and leadership that generated the worldwide acceptance of DMC."

Wrong Hall of Fame?

On a football scholarship to Lamar University, Charles Cutler was the leading ground gainer in his conference until he broke his leg in a game. He had an offer to try out for the Green Bay Packers.

"I was in the right place at the right time," Cutler, now 66, says of his arrival at Shell. "I was only there a year when they decided to try out computerized process control. We would take a linear process, perturb it, take the derivative, and repeat the whole process, using a successive linear process."

In 1963, Shell transferred him from the Houston refinery to a research group that was assigned to optimize and control a fluid catalytic cracking unit. While the real-time optimization was worked out, "the optimization used up all its degrees of freedom at some combination of constraints, which meant the PID controllers had to operate at multiple constraints. The optimum operation couldn't be reached because the controller would cycle around the setpoint in coming to steady state."

Even after working on that problem for several years with other engineers at Shell, the dilemma vexed him. It was one of the reasons he entered graduate school--"to see if I could find a way to control at multiple constraints. While taking a course in Z transforms, it occurred to me that if you truncate the infinite series of Z transforms and evaluate each term in the series, the process dynamics could be represented by a set of linear equations that are a function of time. These equations could in turn be put into a linear program that could handle constraints on the dynamics."

This was the budding theory of dynamic matrix control (DMC). It was the subject of his dissertation in 1967 and was demonstrated on a fluid catalytic cracking unit six years later in Shell's New Orleans refinery.

Cutler was implementing a computerized control system based on his DMC theory when the plant's union went on strike. Shell decided to run the refinery with non-union staff. "The computer was optimizing and controlling the unit at the time we took over. I decided to see how well the controller would perform with no intervention from the operators. It ran for five weeks with zero intervention."

Five weeks into the strike and locked in the refinery, Cutler decided to write a program to demonstrate the DMC algorithm. "The program did not consider constraints. It was written to use an error minimization calculation to hold the process at the setpoint and reject disturbances. The basic control was adjusting the fuel to the preheat furnace on the FCC to control the outlet temperature, while looking at disturbances in the fuel gas quality, changes in the stack damper, feed rate, and feed inlet temperature. The controller was a great success." At least under normal operating conditions.

Then the cracker lost a feedpump. "The inlet temperature dropped 75* [DEG] in 10 minutes, which would have really torn up the cracker. The controller saw the disturbance coming and adjusted the fuel in such a way that resulted in a 2-3* [DEG] drop in the furnace outlet temperature. The normal drop with a PID controller would have been 25-30* [DEG]."

Read Also: Cascade, Scan Time, PID Tuning

This experience led to the publication of Cutler's seminal paper with Brian Ramaker, "Dynamic Matrix Control-A Computer Control Algorithm," presented at the national meeting of the American Institute of Chemical Engineers in 1979.

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