Over the past several weeks, ModelingAndControl.com’s Greg McMillan has shared a three-part series on common control myths with his readers. In the initial post in the series, Common Control Myths – Part 1, Greg offers five myths, which he then shoots down, one by one.
Three of his five involve disturbances in the process. On unmeasured disturbances, Greg offers the following myth and rebuttal:
Unmeasured disturbances are a side issue – if there were no unmeasured disturbances, control would be a non issue because you could home in on the controller output that corresponds to the desired set point for a process variable. You would just need to run some data fitting algorithm one time and the loop would be set for the life of the process. In reality, there are always unmeasured disturbances.
Often the best-laid designs on newly commissioned loops require adjustments over time as the control engineer learns about the unmeasured disturbances impacting the loop.
In the second post in the series, Greg gets deep into the equations to show the scan time effects on peak and integrated errors. Since I’ve forgotten more than I’ve remembered, I’ll trust him that the math checks out… J
In the final post of the trilogy, Greg debunks control myths six through ten. These include control valve performance, pH sensors, and thermocouples versus RTDs. For example, on pH sensors, he writes:
The most accurate type of pH sensors are used most often – the most popular sensors are the ones that require the least amount of maintenance, such as references with solid electrolytes, even though these may require more time to equilibrate and have a more variable junction potential. The flowing liquid junction reference for the right materials of construction and electrolyte is generally the most accurate but the least used type of pH electrode in industry because of the need to pressurize and refill the reservoir.
Update: I was a bit too hasty calling it a trilogy! Greg has unleashed Part 4 of his common control myths.