jrowe4
Member
Hello! I have a question that I can't seem to find a solution to here on the forums. I am working on an evaporator system where there are several PID loops that are being used as a feed forward bias to other PID Loops.
One arrangement in particular has me scratching my head. I have a "Loop 4" that tries to control the level inside the evaporator based on a level transmitter. The loop is direct controlling (SP-PV) which to me makes sense. "If I want more level, then open the valve!" The CV of this loop is being written to N10:167.
I have another loop, "loop 9” that tries to control the inlet feed to the evaporator based off a flow transmitter. This loop is also direct controlling (SP-PV). Loop 9's control block is N10:161, meaning that the CV from Loop 4 is sent to Loop 9 as a feed forward bias.
In a way, I understand what the programmer was trying to do. We want to maintain flow, but for some reason if the level is off then kick the flow loop to get the level back into spec. However, I only see output from the level PID loop 4 when transmitted level is below setpoint.
According to the manual, the Feed Forward Bias parameter has the range 0-4095 for integer PID blocks. Does this mean that the feed forward bias cannot send negative bias? I think it’s odd in this setup that I can only account for Level below SP scenarios and not above. It’s almost a one sided feed forward. Does anyone have any information on the PLC5 FF? I can't seem to find anything out there. Thanks, much appreciated!
One arrangement in particular has me scratching my head. I have a "Loop 4" that tries to control the level inside the evaporator based on a level transmitter. The loop is direct controlling (SP-PV) which to me makes sense. "If I want more level, then open the valve!" The CV of this loop is being written to N10:167.
I have another loop, "loop 9” that tries to control the inlet feed to the evaporator based off a flow transmitter. This loop is also direct controlling (SP-PV). Loop 9's control block is N10:161, meaning that the CV from Loop 4 is sent to Loop 9 as a feed forward bias.
In a way, I understand what the programmer was trying to do. We want to maintain flow, but for some reason if the level is off then kick the flow loop to get the level back into spec. However, I only see output from the level PID loop 4 when transmitted level is below setpoint.
According to the manual, the Feed Forward Bias parameter has the range 0-4095 for integer PID blocks. Does this mean that the feed forward bias cannot send negative bias? I think it’s odd in this setup that I can only account for Level below SP scenarios and not above. It’s almost a one sided feed forward. Does anyone have any information on the PLC5 FF? I can't seem to find anything out there. Thanks, much appreciated!