VEMS: the unstoppable pulsing idle

[A80Avant]

i use very small values for PID
D is something like 16-18 if i remember correctly

[ranz]

i use very small values for PID
D is something like 16-18 if i remember correctly

Same here, I had it also wery low. But now when I increased it to 150, the idle does not drop down anymore, try it out also, and let me/us know.

[A80Avant]

what do you use for the rest of the values?
what is your P and I, and what the HZ?

also what is the pulsewidth curve?

[ranz]

what do you use for the rest of the values?
what is your P and I, and what the HZ?

also what is the pulsewidth curve?

P - 30
I - 2
D - 160

PWM freq is 250Hz (diode at IAC valve because I don´t use motronic vems ecu, older motronic vems ecus also do not have the diode).

The curve Hz with hot engine is 38Hz and going higher wiht cold engine to 50Hz @-40

[A80Avant]

ok
i use different settings on the hz and pwm curve, but i will try making the D higher as you suggest

[loxxrider]

That is a rather high D value. Too much D will make your response slow and if you don't have enough I to compensate, you might not even reach the correct value. If the PID is causing the oscillations, I can give some advice. Just gotta make sure thats whats causing it as its a lot of typing to do to explain PID tuning

[A80Avant]

i have read a few things about PID tuning and found that those values were very good from my megatune setup, and fairly good for my current. I too believe this is too much D, but no hurt trying.

[ranz]

That is a rather high D value. Too much D will make your response slow and if you don't have enough I to compensate, you might not even reach the correct value.

I don´t understand it? My car worked correctly with low D also (P and I where the same) but had some problems catching correct idle RPM when revs where dropping down. Higher D corrected this problem and everything else stayed the same, nothing changed really, but the problem was gone

[loxxrider]

So you were having problems with the system not acting quickly enough. I could try and draw some graphs, but I'll try and explain instead. So you want your idle to be at a stable value BUT the idle is coming down too quickly. Your response is approaching the target value too quickly so you would want to increase D such that it will have more effect with a higher slope. (D works on the derivative of the response...not enough D will allow the response to overshoot the target, and too much will stop the response from reaching the target).

Does that make any sense at all?

[ranz]

I´m not 100% that I understand it, because I don´t understand all the english words, but I think I do
But: "and too much will stop the response from reaching the target" but it reaches the target... so it should be OK?
Or do I need change something else to get it catch the idle correctly.

[A80Avant]

did you end up using that number gradually? or did you make the jump from 30 to 160?

[loxxrider]

I´m not 100% that I understand it, because I don´t understand all the english words, but I think I do
But: "and too much will stop the response from reaching the target" but it reaches the target... so it should be OK?
Or do I need change something else to get it catch the idle correctly.

Just because it reaches the target doesn't mean its at its optimal value. If its too high, putting a lower value in for D might make the response faster.

[ranz]

did you end up using that number gradually? or did you make the jump from 30 to 160?

At first jumped from 30 to 160 and tested how it works, after that tryed lower numbers but then it got back to same situation, not so bad, but still dropped below idle target at first.

[loxxrider]

I think maybe your D is masking problems from your P or I values. Too much P will make the idle oscillate as will too much I. Too much D will help with those oscillations.

Basically you should start with P. If you have some steady-state error (not reaching the desired idle RPM), then you add I. If there is oscillation or stability problems, then you can add D in the system.

I can post my PID values if you'd like BUT I haven't tested them in a while because my IAC valve isn't connected at the moment.

[ranz]

Basically you should start with P. If you have some steady-state error (not reaching the desired idle RPM), then you add I. If there is oscillation or stability problems, then you can add D in the system.

Thanks, I´ll try it out today evnening.