PID programming in TIA Portal V15.1

Good Afternoon Everyone,
I hope you can help me with a problem. I have been converting a program from a Micrologix 1400 over to a S7-1200 PLC and I have finished most of the program but I am getting hung up w/ the PID conversion. I have been referencing the S7-1200 manual and an application note from 2014 and I find that the literature is not very friendly in Portal. There is a lot of trees spent on utilizing their auto-tune feature but when you have to program the process to accomodate another mode of operation to accomodate the auto tune it is just not worth the effort. I find basic things like is the reset term 1/Ti like in AB or is is Ti, therefore I need to think of the values inversely. Things like the ManualUP and ManualDN parameters just make my head hurt, especially when I have no interest in the feature. Typically in the PID we use the auto/manual bit to engage the PID. WHen the PID is in manual I flush the CV. When in auto I send all the parameters from the HMI to the PID parameters (Kc, Ti, Td, LUT, SP). I was hoping someone out there can help me get to the bottom of this. I am working with the PID compact technology object.

siemens sample code:
https://support.industry.siemens.co...ct-for-simatic-s7-1200-s7-1500?dti=0&lc=en-WW


On old step7 there was Cont_C / FB41 block

Most of the features of the PIDs are optional, from what I recall.



The autotune stuff CAN be programmed, but I think most people use the auto-tune screen in the Technology Object commissioning window.


I think otherwise it can mostly be operated the way you said. It'll definitely get cranky if you try to change the tuning parameters in auto, though.

The problem I have with PIDs is that few people know the units. It took me a while to understand how a Rockwell PID supposed to be used. The Rockwell PID uses the ISA standard where the controller gain is multiplied by time constants. The time constants are in minutes which works well for temperature and other process control but not so much for motion control where a time constant could be in milliseconds. The other thing that I thought was odd or strange is how Rockwell handled the controller gain but it makes sense if you look at the units. The first part is to convert temperature to counts. Then the controller gain converts error counts in to control counts out but basically the scale has units of counts/counts. Then the counts out must be converted to % control.
If you look at it you will see there are three stages. Temperature/counts, counts/counts and finally counts/percent. If you simplify the total gain is temperature error/% control output. Other units could be used instead of temperature.
I haven't looked at the Siemens PID but the key to understanding the PID is understanding the units.
Actually, our motion controller has 3 stages too but we convert transducer or encoder counts to positions immediately, The PID proportional gain has units of % control output / position unit of error. The output% is then converted to DAC counts. The two methods seem backwards from each other but both will work.

I haven't played with the Siemens PID but it you want to convert the gains from one controller to another it is necessary to know how the gains are used and the units for each of the gains or time constants. It should take no more than 5 minutes to do.

About the auto tuning. The problem with auto tuning is that you need to know what is being tuned. The auto tuner must have the right model for the system being tuned. For instance, the Python auto tuning program is good for SOPDT models. It won't work for position control. It may work for velocity control but as I mentioned above, time constants in minutes don't convert well for time constants in milliseconds unless the time constant in minutes is using floating point.