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Author Topic: Actual pre-control in LDRPID  (Read 353 times)
prj
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« on: March 20, 2017, 07:49:04 AM »

As an intro - ME7 does not have pre-control.
Yup, that's right - there is no feed-forward.

All MED17 ECU's remedy this. And actually M5.92 and M3.83 have feed-forward.
As does every diesel ECU made starting with MSA-15.

Why did they kill it in ME7? Who knows. But the result is a convoluted piece of cr*p which is stupidly difficult to tune.
Because as you need more DC up top, the PID has to work really hard all the time, and I and P have to be super sensitive, which then causes oscillations in other conditions. They introduced tons of bullshit like LDDIMNN and LDDIMXN, I-part adaptation and so on.

And why?

To somehow make this clusterfuck work.

However, someone realized how bad this is at some point, and for example the RS6 and RS4 are calibrated way differently. By giving them some actual feed-forward and reducing the work the PID must do.
It's a giant hack, but the way it works is like this:


Since in steady state the majority of the boost input comes from I, KFLDIMX is used as a map to basically convert pressure to duty cycle - hence the reason it is perfectly linear.
And then you can view KFLDRL basically as a pressure-dc pre-control map. The axes are now essentially pressure - for example the axis value 60% in KFLDRL becomes 667mbar, and the map is a proper pre-control map, as it actually drives duty cycle off of it, while the PID only has to compensate for the actual drift from the pre-set map. Also, you don't need all that wastegate actuator linearization bullshit, as it's all now baked into one single map.

As for what happens to I - you need to choose the range for KFLDIMX so that it covers your required boost conditions. You will still need to do the linearization runs, but you will have just one map, and it will work much better than the stock clusterfuck.

I need to adjust my spreadsheet for this, but thought I'd post about this, as a few people who have been on here for a while might find the answer to the "why?" question useful.
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KasperH
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« Reply #1 on: March 20, 2017, 08:25:25 AM »

If this really works "that simple" it would be awesome  Grin
It took me forever to get my PID in the range of sane.
And through it all I was always wondering why it was so extremely complicated  Huh
And I was wondering why there was no feed forward, with would have made the PID correction so much more easier to do Smiley
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NOTORIOUS VR
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« Reply #2 on: March 20, 2017, 09:08:54 AM »

Well that's interesting... I"ll have to try that trick
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nyet
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« Reply #3 on: March 20, 2017, 09:52:05 AM »

Thanks for this, prj.
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vwaudiguy
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« Reply #4 on: March 20, 2017, 10:47:43 AM »

Bravo
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TijnCU
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flying brick


« Reply #5 on: March 20, 2017, 10:57:47 AM »

Looks very logical, thanks for sharing! If your spreadsheet works well this will save a lot if calibration time!
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aef
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« Reply #6 on: March 20, 2017, 01:58:53 PM »

Something must be wrong here.
PRJ never started a thread before.  Grin

Thank you for this post!
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prj
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« Reply #7 on: March 20, 2017, 02:30:55 PM »

Something must be wrong here.
PRJ never started a thread before.  Grin

Thank you for this post!

You would be wrong.
I just prefer quality over quantity.
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SB_GLI
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« Reply #8 on: March 20, 2017, 04:15:30 PM »

From a stock BEL allroad.  Same as you describe here?
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prj
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« Reply #9 on: March 20, 2017, 04:16:31 PM »

Yup, exactly.
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contrast
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« Reply #10 on: March 21, 2017, 11:45:47 AM »

Excuse my ignorance, but how are the values of KFLDRL populated? KFLDRAPP?
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nyet
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« Reply #11 on: March 21, 2017, 12:01:42 PM »

But since this would be I riding IMX, it seems that the PID can only REMOVE WG via I, never add, so if you have a persistent underboost, the PID cannot compensate? Or you just let the I adaptation take care of that?
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ME7.1 tuning guide (READ FIRST)
ECUx Plot
ME7Sum checksum checker/corrrector for ME7.x

Please do not ask me for tunes. I'm here to help people make their own.

Do not PM me technical questions! Please, ask all questions on the forums! Doing so will ensure the next person with the same issue gets the opportunity to learn from your experience.
prj
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« Reply #12 on: March 21, 2017, 03:54:21 PM »

But since this would be I riding IMX, it seems that the PID can only REMOVE WG via I, never add, so if you have a persistent underboost, the PID cannot compensate? Or you just let the I adaptation take care of that?
Not correct. That is what LDDIMXN is for.
Generally your PID should never ever ride I-max. There should be always a bit of free room. The only place it should be riding it somewhat would probably be 2nd gear, as in 1st gear it's still in dynamic mode.

I-max is not there to control overshoot whatsoever. It is there to provide a sane limit, which should always be a little higher than what is needed.
Overshoot needs to be controlled using D, so that it lands in steady state and after that P and I will take care of it.

You basically have two controllers in ME7 - a P/D dynamic controller and a P/I static controller with switching between them. This is a good way to do things from the PID perspective and boost control.
I adaptation is fairly useless in aftermarket situations. It is mostly there so it meets the target with loose component tolerances (due to aging) on a stock tune where there is room everywhere.
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Jason
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Breaks everything!


« Reply #13 on: March 21, 2017, 08:07:08 PM »

Can confirm this works well.  I was having all kinds of boost control issues on my Allroad with chinese turbos until I did this.  It's been great for the last year.
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