Intermittent Idle Instability / DBW Throttle Control

[Hodgdon Extreme]

Turbo 2J with Bosch 68mm DBW throttle. Have been using closed loop idle control and closed loop ignition control. Overall it works great, but I've noticed from time to time the engine/ECU will dither ±150 rpm chasing a target idle speed. It'll do this for a few seconds or a minute; but eventually settle out.

Over last day or two I've worked on nailing down the idle control. I turned off closed loop idle, set my spark timing table (in the entire start/idle region) to 15º; which is equal to my idle ignition target. Did a cold start and populated idle base position values to achieve target idle speeds for all ECT as the engine warmed up.

Once fully warmed, I incremented idle base position value 0.2% at a time - basically to characterize how much RPM every extra 0.2% of TPS creates - assuming 15º spark.

This all worked great except for  2.4% TPS. The instant idle base position was set to 2.4% (12:03 in the datalog), the idle began to dither, and it appears my DBW throttle is having a hard time hitting that specific opening percentage. As you can see in datalog, the throttle has no issue achieving any value under or over 2.4%.

I'm pretty sure this is causing my intermittent idle instability.

PID values for the throttle are the ones published by Link:

4, 0.080, 50

Aside from this issue, the throttle tracks along with APS great.

Suggestions?

Cal File: https://drive.google.com/file/d/19D9WQcWioeabMbIaeRM7Sy1h-9xWDE2i/view?usp=drive_link

Log File: https://drive.google.com/file/d/1wTYGXOtWcjItN-udFZkkoYLSEuT698nf/view?usp=drive_link

[Adamw]

Yes I have seen this before just recently, it appears to be stiction/friction related where the TP is just sitting a little off target, but the throttle motor doesnt respond to a small change in DC.  The integral continues to wind in more and more DC until all of a sudden it has enough torque to overcome the stiction so the TP does a big jump past target.  Then the exact same happens again in the reverse direction trying to get back to target.   

Good PID control generally relies on the process variable (TPS) to respond reasonably linearly to the control variable (motor DC), so you can see in your log pic below this is where the issue is coming from.  Where my cursor is the TPS is 0.5% below target, motor DC is initially at -36.5%, the ecu increases DC all the way to about -17% with no TP movement at all, then suddenly with just the last couple of percent more DC, the TP jumps 1%, now above target.    

I noticed this happening on one of our dyno engines a couple of weeks ago when I was at head office doing some testing so I played around with tuning and a few firmware strategies trying to solve it but run out of time without getting to the bottom of it.  I initially thought it was the return spring "neutral position" that was causing it so got the firmware guys to code in a feed forward table etc with no improvement.  More investigation showed it was actually well away from the neutral position.  I was starting to suspect possibly there was some damage to the internal gears even though moving the blade by hand it felt reasonably smooth.  It wasn't dismantlable so I haven't ruled that out yet.  I have a few other ideas in the back of my mind but it adds tuning complication so I really need to rule out the mechanical possibilities then get a scope connected and spend a lot more time on it when I get the chance.   Unfortunately that's not going to be for a while as I have a fair bit on at the minute.  

I dont have a lot of good advice except try to get your normal warm idle outside of the TP range where this occurs. Change ignition timing and mixture etc to see if you can get it to idle with the TP half a % above or below this problem spot.  

[Hodgdon Extreme]

Appreciate the response!

Hopefully you/Link can get to the bottom of this anomaly. 

In the meantime, I completely understand your workaround suggestion.

[Ryan Heagney]

Hey Adam 

we are seeing this same issue on a Bosch 82mm DBW throttle on a fury X 

the throttle is new possibly only a meeting or two old I have turned off all closed loop control and this helps but often it will still get jammed particularly in the - DC areas 

doesnt ever give trouble except for idling  we are 1.7% for idle seems 1.9 % it is worse 

when is does move it’ll jump to 3-4% then quickly settle back to the 1.7 

on blade closing it will sometimes go back to 1.5 and take a while to get back to 1.7 and it seems to change with engine temp. 
 

can obviously change the PID on the e throttle and “dull” the issue out a little but still present 

any ideas to sort this? Possibly a faulty throttle body? 
thanks 

[Adamw]

We added a deadband, a feedforward table and increased derivative range since this post.  Since your issue isn't around the natural spring point I would say you will probably get away without adjusting feedforward, but set the deadband to 0.1% and the derivative up around 65-80 usually solves the jerky movement due to  high stiction.

[mapper]

I saw the same problem a few times. Slip/stick effect is a well known phenomen in control loop science.

I also agree that I often sit at 65% derivativ.

When is the new firware available?

[Adamw]

The changes I mentioned above are already in the current firmware and have been for some time.