Common Control Myths – Part 2

Myths are a fertile topic maybe because of all the fertilizer in process control. You can make almost any point you want, by changing what are often obscure details on process and automation system dynamics. For example, you can show a variable speed drive can do better or worse than a control valve. The results can easily be swayed by VSD settings (e.g. deadband and rate limiting), VSD options (e.g tachometer feedback and vector control), and valve typeaccessories (e.g. throttling sliding stem or rotary isolation valve and digital dual relay positioner versus pneumatic spool positioner). For insights into the relative merits of the VSD versus control valve in terms of control loop performance, check out the February Control Talk column in Control magazine titled “Deal or No Deal.”

I promised to post this week the development of equations that are a myth buster. The equations show there is an implied dead time greater than the actual dead time in most loops because the controller is tuned to be slower than what is shown in academic articles and papers. Control loop performance does not appreciably deteriorate until the actual dead time exceeds the implied dead time. The equations go on to provide an estimation of the peak and integrated absolute errors for the implied and actual dead times for step disturbances. The effect of the slowness of real life load disturbances can be roughly included by adding the load disturbance time constant to the process time constant in the equations for the peak and integrated errors. The first page appeared in a blog and Control Talk column in 2006. This updated document has better explanations

omenclature and adds a second page for the estimation of the peak and integrated errors. When I bounce out of negative free time, I will do an application note to study the accuracy and implications of the equations. Next week we will continue on with mythology.