I have been searching for a while trying to figure out what are the units for ZIEGLER-NICHOLS kc, ti, and td are.
The equation for Kc= Ku/1.7
Ti=pu/2
Td=pu/8
Sam,
You questioned the meaning of the variables. Here is what I know (not much).
Remove Derivative completely from the loop before trying this. Also take the Integral out to the longest time allowed (I usually use 500)
Pn(Pu) = Natural period of the loop or system. Expressed in minutes. This is best determined by increasing the gain and upsetting the loop. Keep increasing the gain, upsetting the loop, until the output rings uncontrollably or without decay (yea, get permission to do this on an online system). Measure the period (P to P) of this "ring", and you have Pn in minutes.
Kc = Gain = Gain is an expressionless unit. Think of it as a ratio of input to output. (1 = for every unit of change in the input, the output will change by 1 unit).
Kn(Ku) = the gain necessary to create a "natural" ring of the system.
Ti = Time of integration (integral)= Usually expressed in minutes. The equation used in your question is used to determing a starting point for initial tuning.
Td = Time of derivitave (derivitave) = Usually expressed in minutes. The equation used in your question is used to determing a starting point for initial tuning.
Here are the "rule of thumb" calcs I use. This would be considered a fairly agressive "tune". I usually do not use Td:
Pn = Natural Period
Kc = Kn/2
Ti = Pn * 1.2
Td = Pn/8
Hope this helps
David