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Jan
28

Fifths Rule

I tend to be descriptive instead of proscriptive in my discussion of process control and improvement. This may be due to my interest in the scientific method and concepts rather than details that may only be good for specific sets of conditions. However, there are certain rules of thumb based on solid principles that serve as guidance for good practices. It is interesting how the factor 1/5 turns up over and over again. So here is a list where “fifths” rule for best performance. For my drinking buddies, this is not about best bottles.

(1) loop dead time < 1/5 x (process time constant + load time constant)*

(2) measurement filter time < 1/5 x (process time constant + load time constant)*

(3) module execution time < 1/5 x (loop dead time)

(4) module execution time < 1/5 x (process time constant)

(5) measurement scan time < 1/5 x (module execution time)

(6) standard deviation < 1/5 x (allowable control error)

(7) noise in controller output < 1/5 x (required change in controller output)

(8) stick-slip of valve < 1/5 x (required changed in controller output)

(9) half dead band of valve < 1/5 x (required changed in controller output)

(10) secondary loop time constant < 1/5 x (primary loop time constant)

(11) secondary loop dead time < 1/5 x (primary loop dead time)

(12) controller gain < 1/5 x (ultimate gain)

(13) controller rate setting < 1/5 x (process time constant)

(14) historian update time < 1/5 x (oscillation period)

* – your main hope for dead time processes is to slow down the load disturbance.

My final recommendation is the 3-D U2 concert film. I have been in front row seats for BB King, BTO, and Joan Jett but this was even better for seeing and feeling how musicians get into their music. Bono and Edge were 7 feet tall playing right above you. The positive energy from the lyrics and instrumentation was incredible.