jrowe4
Member
Hey PLCtalk! Wall of text comming in 3.2.1.. I'm a summer intern trying to learn how to work with PLCs and tuning processes. I've been given the task with trying to tune a counter flow drying system that uses cascade control. In our process, the slave loop controls a gas valve that regulates hot inlet air, and a master loop that controls the outlet temperature of our material. The inlet air is used to heat up the material causing calcination in our product. The idea of this cascade setup I'm guessing is that the slave loop is "supposed" to help against air flow problems in our process before the material temperature is affected by them. As this is my first attempt at tuning loops (hopefully I can make a living out of this one day, I love reading about control theory and plcs!) I have tried some attempts to solving this.
First thing, we are using a plc-5 using the INDEPENDANT PID equation. Also, I have come to understand that the numbers I enter for kp,ki,kd are scaled by .01, .001, .01 respectivly. I sort of understand what P,I,D do after reading rons website, but I don't have that operators intuitive feeling for it yet. I've attached a couple pictures in a zip giving you a better idea of my tuning problem.
The daily trend picture is what we get on a normal day basis. The top line is the material temperature, and the bottom line is the inlet air. It's almost as if they are "fighting" each other. There is also a lot of deadtime it seems before a change is seen in the material temperature relating to a change in the inlet air. Currently I have a slave loop (gas valve) tuned using the lambda rules at kp=6, kp =.017 r/s, and kd=0. I have the master loop tuned to kp=7, kp=.002 r/2 and kd=0. The master loop tuning is pretty much unchanged from when I started.For the slave, I used a step test to tune, which actually works pretty well now. We found out that the valve linkage was shot because we were not able to get repeatable actions during manual tests. Now when we dial in a temperature on manual and step down or up we get good results.
I also posted a picture of the pid block used for the master loop found in rslogix 5. This is just to give you guys a better idea of what I'm working with. Some of the numbers like feedforward and deadtime were there when I got there. I also included a picture of what I believe to be a pid trigger on the logic rung. I have to still convince the higher ups that the pid block is not unconditional and is triggerd by this pid timmer. I still need to check to see if the trigger matches the loop update time.
So my question is, does anybody have any tips or advice for my next step? I feel that there is a problem with the master loop tuning. Maybe it is too agressive? When I tried to do a step test by placing the master in manual and leaving the slave in auto, I had to estimate my process gain and deadtime as I was not able to get very good results. The lambda formulas suggested a kp = 1 and a ki = .001 r/s. The cohen coon method I also tried gave similar results. When I put a low gain into the master loop, the temperature overshoots way too much. This could be because the system needs time to recover from the fast acting controls. I am not to sure.
In all, if anyone has any suggestions, tips, advice or a similar situation that can be lent to a beginner, I would extremly appreciate it. Thanks so much everybody, I really love this website as it is the only means of teaching my self these complex topics. There are some very intelligent people here. Thanks all, have a good night.
First thing, we are using a plc-5 using the INDEPENDANT PID equation. Also, I have come to understand that the numbers I enter for kp,ki,kd are scaled by .01, .001, .01 respectivly. I sort of understand what P,I,D do after reading rons website, but I don't have that operators intuitive feeling for it yet. I've attached a couple pictures in a zip giving you a better idea of my tuning problem.
The daily trend picture is what we get on a normal day basis. The top line is the material temperature, and the bottom line is the inlet air. It's almost as if they are "fighting" each other. There is also a lot of deadtime it seems before a change is seen in the material temperature relating to a change in the inlet air. Currently I have a slave loop (gas valve) tuned using the lambda rules at kp=6, kp =.017 r/s, and kd=0. I have the master loop tuned to kp=7, kp=.002 r/2 and kd=0. The master loop tuning is pretty much unchanged from when I started.For the slave, I used a step test to tune, which actually works pretty well now. We found out that the valve linkage was shot because we were not able to get repeatable actions during manual tests. Now when we dial in a temperature on manual and step down or up we get good results.
I also posted a picture of the pid block used for the master loop found in rslogix 5. This is just to give you guys a better idea of what I'm working with. Some of the numbers like feedforward and deadtime were there when I got there. I also included a picture of what I believe to be a pid trigger on the logic rung. I have to still convince the higher ups that the pid block is not unconditional and is triggerd by this pid timmer. I still need to check to see if the trigger matches the loop update time.
So my question is, does anybody have any tips or advice for my next step? I feel that there is a problem with the master loop tuning. Maybe it is too agressive? When I tried to do a step test by placing the master in manual and leaving the slave in auto, I had to estimate my process gain and deadtime as I was not able to get very good results. The lambda formulas suggested a kp = 1 and a ki = .001 r/s. The cohen coon method I also tried gave similar results. When I put a low gain into the master loop, the temperature overshoots way too much. This could be because the system needs time to recover from the fast acting controls. I am not to sure.
In all, if anyone has any suggestions, tips, advice or a similar situation that can be lent to a beginner, I would extremly appreciate it. Thanks so much everybody, I really love this website as it is the only means of teaching my self these complex topics. There are some very intelligent people here. Thanks all, have a good night.