Can't commissioning PID Temperature Multi Loop Controller

Dear experts, 

I'm having a problem to commissioning my heater system using 2 PID_Temp in cascade mode.

I'm following this PDF guide: https://support.industry.siemens.com/cs/us/en/view/103526819

In my case, I have only 2 PID (actuator and a steel block, each one with your own temperature sensor).

At item 5.3 of guide (and attachment), it says that I need to do an "Pretuning Heat", after set some setpoint. My problem is, when I start pretuning, the temperature reach the setpoint so fast that it can't finish pretuning heat.

I receive the following error in the middle of the process:

Error starting pretuning. The process value is too close to the setpoint or greater than the setpoint. Start fine tuning.

There is some way or setting to start heat with low percentage output?

Thanks in advanced!

PS - My HW info:

PLC 1215C DC/DC/DC (AG40)

TIA Portal V15.1

Hi,

it sound like your PID controller is in automatic mode before you try to start the pretuning.

Is this the case?

For fast processes it is difficult to start pretuning from automatic mode by using the commissioning editor, because as soon as you send the necessary setpoint change, the PID algorithm will raise the output value and the process value approaches setpoint. If this happens fast, then process value might already be too close to the setpoint when you want to click the start tuning button.

There are several options to solve this:

1. You could switch to manual mode before you change the setpoint and start pretuning. By doing so your output value and process value remains constant until you start the pretuning.
   If you have a look at point 5. from the picture that you attached from the guide, you can see that manual mode was also active there.
   
   
2. Alternatively you could also change the setpoint and start pretuning in the same step by using a watchtable or your user program (this is not possible by using the commissioning editor).
   You have to modify the following tags in one step:
   - Setpoint or SubstituteSetpoint = your tuning setpoint (if SubstituteSetpoint is used SubstituteSetpointOn must be TRUE)
   - Mode = 1
   - ModeActivate = FALSE -> TRUE edge
   - Heat.EnableTuning and Cool.EnableTuning depending on your desired tuning

Regards

0_Einstein

Set the ti to a very long time, and then set the max rate (slope) the control output can change.

If you have the PID loop configured for bumpless control, set the control output to 0% before starting (enabling) the PID.

Review the PID documentation as well as the DB parameter comments.  Hopefully this is enough to get your control to slow down.

If you need it to be slower, you can also change the scaling of your output so it does not go from 0-100%, but instead goes 0-80% (which will prolong heater life quite a bit).

I set the TI up 60 times the old value (5.xxx seconds to 300 seconds), and it's a bit slower, but not enough. I didn't found the max rate to change it.

Changing my output scaling in range of 0-30% keep system still fast. My resistances sensor is a type J termopar (just for knowing).

Mach11

Hi,

it sound like your PID controller is in automatic mode before you try to start the pretuning.

Is this the case?

For fast processes it is difficult to start pretuning from automatic mode by using the commissioning editor, because as soon as you send the necessary setpoint change, the PID algorithm will raise the output value and the process value approaches setpoint. If this happens fast, then process value might already be too close to the setpoint when you want to click the start tuning button.

There are several options to solve this:

1. You could switch to manual mode before you change the setpoint and start pretuning. By doing so your output value and process value remains constant until you start the pretuning.
   If you have a look at point 5. from the picture that you attached from the guide, you can see that manual mode was also active there.
   
   
2. Alternatively you could also change the setpoint and start pretuning in the same step by using a watchtable or your user program (this is not possible by using the commissioning editor).
   You have to modify the following tags in one step:
   - Setpoint or SubstituteSetpoint = your tuning setpoint (if SubstituteSetpoint is used SubstituteSetpointOn must be TRUE)
   - Mode = 1
   - ModeActivate = FALSE -> TRUE edge
   - Heat.EnableTuning and Cool.EnableTuning depending on your desired tuning

Regards

In attachment, there is a document showing my PID settings. 

My PID is in automatic mode, but my resistances are cutt-of until I start pretuning, so I just start to heat my system right after start pretuning. 

I tried to do your second option too, but my problem it's in the use  comissioning editor without this error.

Hi,

I am not sure, if I understood everything correct. But I see one problem with keeping the PID controller in automatic mode before pretuning and only disconnecting its output: The output value is still calculated based on the current input and setpoint then and this means in your case it is permanently increasing.

When you start pretuning then, it starts with the phase PIDSelfTune.SUT.State = 100 where the standard deviation is determined. In this phase the output remains constant (on the value that ramped up during automatic mode before) and influences the process value accordingly.

At the end of this phase the difference between setpoint and process value is checked, that must meet the following requirements (described e.g. in https://support.industry.siemens.com/cs/ww/en/view/108210036/82267750027):

|Setpoint - Input| > 0.3 * |Config.InputUpperLimit - Config.InputLowerLimit| and

|Setpoint - Input| > 0.5 * |Setpoint|

If your configuration pdf a setpoint = 160 can be seen. If this is used (?), then the input value range rule is more restrictive and you need a difference of setpoint and input that is greater than 105.

If the difference is less than this, the error you observed is displayed.

Beside workarounds like temporarily using a smaller input value range, I would generally recommend to use inactive mode or manual mode with manual value = 0 before pretuning is started. This should work better than using automatic mode and disconnecting the output, because pretuning then starts from output = 0 and with a process value that is at a low level and quite stable (instead of increasing).

If this does not work, maybe a screenshot of a measurement (with scaling of setpoint and process value visible) during pretuning would help to understand the problem better.

Regards

Hi,

that sounds perfectly fine.

Good to hear, that everything works now and thank you for the feedback!

Regards

For information purposes only:

The manual guide about Multi Loop Controller, on commissioning step, says to perform the fine tuning just at the inner PID. At the center and outer PID it's saying to perform "Pretuning Heat" only.

In my case, the pretuning of outer controller wasn't enough to correctly determine the PID parameters. I don't know why this recomendation in the manual guide, but I also performed fine tuning on the outer controller and got a better result (It took almost 4 hours to make, but the system works very well).