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This comes up often enough to be worth writing down in one place.
Logix has two separate PID instructions — not two flavours of one. PIDE from the process instruction set (function block) and the classic PID from the special instruction set (usually ladder). In the dependent form their parameters are identical: Kc unitless, Ti in minutes/repeat, Td in minutes. In the independent form they are not.
PIDE, independent:
PGain — unitless
IGain — 1/minutes
DGain — minutes
Classic PID, independent:
.KP — unitless
.KI — 1/seconds
.KD — seconds
So for Kc = 2.5, Ti = 1.5 min/repeat, Td = 0.25 min:
P: PIDE 2.5, classic PID 2.5
I: PIDE 1.6667 (1/min), classic PID 0.0278 (1/s)
D: PIDE 0.625 (min), classic PID 37.5 (s)
Same controller, same tuning, and the I and D numbers differ by exactly 60.
Paste PIDE gains into a classic PID block and the integral runs 60x faster than you intended while the derivative is 60x weaker. Proportional is untouched, which is what makes it nasty — the loop looks roughly right at first and then walks away from you. And 1.6667 is a perfectly plausible number in either unit, so it survives an eyeball review. Nothing surfaces until commissioning.
Two things worth adding:
Rockwell documents Kd = Kc · Td · 60 for the classic PID directly. The symmetric integral relation Ki = Kc / (Ti · 60) follows from the stated units, but in the pages I found it is rendered as an image rather than text — if anyone has it in citable text form, I'd like the reference.
Separate trap inside the dependent form: the tag is called IGain, but it holds a time (minutes/repeat). A larger IGain there means a slower integral — the opposite of the independent form. The name says gain, the units say time, and only one of them is right.
Sources:
PIDE algorithm, both forms: Rockwell, Perform Common Process Loop Control Algorithms Using the PIDE Instruction, logix-wp008_-en-p, pp. 2–3
PIDE tag units: Studio 5000 Logix Designer instruction reference, Enhanced PID (PIDE)
Classic PID units: Studio 5000 Logix Designer instruction reference, Proportional Integral Derivative (PID)
Kd = Kc · Td · 60: FactoryTalk Design Studio instruction reference, Using PID instructions
Has anyone here actually hit the 60x version in the field — platform migration, or tuning copied off an old drawing? Curious whether it shows up more in conversions or in new work.
Logix has two separate PID instructions — not two flavours of one. PIDE from the process instruction set (function block) and the classic PID from the special instruction set (usually ladder). In the dependent form their parameters are identical: Kc unitless, Ti in minutes/repeat, Td in minutes. In the independent form they are not.
PIDE, independent:
PGain — unitless
IGain — 1/minutes
DGain — minutes
Classic PID, independent:
.KP — unitless
.KI — 1/seconds
.KD — seconds
So for Kc = 2.5, Ti = 1.5 min/repeat, Td = 0.25 min:
P: PIDE 2.5, classic PID 2.5
I: PIDE 1.6667 (1/min), classic PID 0.0278 (1/s)
D: PIDE 0.625 (min), classic PID 37.5 (s)
Same controller, same tuning, and the I and D numbers differ by exactly 60.
Paste PIDE gains into a classic PID block and the integral runs 60x faster than you intended while the derivative is 60x weaker. Proportional is untouched, which is what makes it nasty — the loop looks roughly right at first and then walks away from you. And 1.6667 is a perfectly plausible number in either unit, so it survives an eyeball review. Nothing surfaces until commissioning.
Two things worth adding:
Rockwell documents Kd = Kc · Td · 60 for the classic PID directly. The symmetric integral relation Ki = Kc / (Ti · 60) follows from the stated units, but in the pages I found it is rendered as an image rather than text — if anyone has it in citable text form, I'd like the reference.
Separate trap inside the dependent form: the tag is called IGain, but it holds a time (minutes/repeat). A larger IGain there means a slower integral — the opposite of the independent form. The name says gain, the units say time, and only one of them is right.
Sources:
PIDE algorithm, both forms: Rockwell, Perform Common Process Loop Control Algorithms Using the PIDE Instruction, logix-wp008_-en-p, pp. 2–3
PIDE tag units: Studio 5000 Logix Designer instruction reference, Enhanced PID (PIDE)
Classic PID units: Studio 5000 Logix Designer instruction reference, Proportional Integral Derivative (PID)
Kd = Kc · Td · 60: FactoryTalk Design Studio instruction reference, Using PID instructions
Has anyone here actually hit the 60x version in the field — platform migration, or tuning copied off an old drawing? Curious whether it shows up more in conversions or in new work.
