Micrologix 1400 PID Questions

hollinsheadja

Member
Join Date
Jan 2019
Location
Denver
Posts
8
HI - I have a tough process and have been struggling with controlling my PV using a Micrologix 1400. I uploaded the file, changed .rss to .txt

My biggest issue is that the PV will not track SP. Small adjustments to gain help, however I can not find the right tuning values. My biggest problem is scaling the output to that i do not over treat my process.

Process: Adding a polymer (floc) to a clarifier to settle solids. Could someone take a look at my current program to confirm the PID is setup correctly?
 
also - during startup the PID CV does not ramp to full scale - it takes 5-6 cycles for the CV to increase towards maximum, but even then i can not get the output to reach full scale incrementally. I am trying to ramp my output based on error from setpoint at a controllable rate to prevent over/under treating.

PV: 0-100%, 100% dirty water
CV: VFD Pump, 4-20mA output, pump is 0-60hz
SP: ~50%
 
one more note, when i turn the gain up, i will reach the max CV. However, I would like the PID CV to slowly approach maximum. This is not the case, the PV (photoeye) will suddenly increase rapidly (20 and increase toward 100) and I'd like the CV to ramp slowly to max output. Increasing gain, the pump ramps to quickly and i can not catch the loop.

ALso - the plant feed is very inconsistent, and i do not have a feed forward signal for feed rate. All I have to detect is error from SP. My thought is that I can incrementally change the gain over time (say 4 cycles, cycles average around 2 minuets each) so that the PID eventually finds setpoint. Essentially an adaptive gain - any help would be much appreciated.
 
I would be able to pay an hourly rate for support. the logic is close, just need help dialing in the PID constant. I have a feeling i have a Flag set improperly.
 
Post a pic of the PID setup screen.

Are you familiar with the standard dependent PID equation? Note that when you adjust the P gain, you are automatically affecting the I gain. If you double your P gain, you must double the I gain number too or else you will double both. Many people don't realize that and end up chasing their tail.
 

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