In Studio 5000 Logix Designer, I've noticed some odd behavior of the ladder PID instruction.
Using the .MINO tag, you can set a minimum output for the PID (or you can set it directly in the instruction setup window). If you then set the PID to software manual (.SWM bit) and set the output (.SO) BELOW this minimum, the output will ignore the minimum output setting and go to your setpoint. That part works fine.
When you switch the PID out of software manual and back to automatic, the PID output jumps back up to the minimum output. That also makes sense.
However, the PID appears to continue its output calculations from the manual setpoint below the minimum, which means if the output should be rising based on the calculations, there will be no increase in output until the calculated output rises ABOVE the minimum output setting, which makes the PID appear to be stuck at the minimum for that amount of time. This is similar to an integral wind-up effect on cheap PID controllers.
I did find that once the output does rise above the minimum, the integral calculations are locked such that they can never get "trapped" below the minimum again, which is good.
Has anyone else noticed this and found a workaround? Does the PIDE instruction not have this issue? I've been hesitant to switch to PIDE because most of our customers don't have the function block diagram package for Studio 5000, but if this is going to be a common issue, I may take the plunge.
Using the .MINO tag, you can set a minimum output for the PID (or you can set it directly in the instruction setup window). If you then set the PID to software manual (.SWM bit) and set the output (.SO) BELOW this minimum, the output will ignore the minimum output setting and go to your setpoint. That part works fine.
When you switch the PID out of software manual and back to automatic, the PID output jumps back up to the minimum output. That also makes sense.
However, the PID appears to continue its output calculations from the manual setpoint below the minimum, which means if the output should be rising based on the calculations, there will be no increase in output until the calculated output rises ABOVE the minimum output setting, which makes the PID appear to be stuck at the minimum for that amount of time. This is similar to an integral wind-up effect on cheap PID controllers.
I did find that once the output does rise above the minimum, the integral calculations are locked such that they can never get "trapped" below the minimum again, which is good.
Has anyone else noticed this and found a workaround? Does the PIDE instruction not have this issue? I've been hesitant to switch to PIDE because most of our customers don't have the function block diagram package for Studio 5000, but if this is going to be a common issue, I may take the plunge.