From a friend:
holy crap? So, it knows the tooth spacing, but you have to calculate it all by hand??Originally Posted by Zaphod
Hi Abe,
I just loaded the newest beta of the MS2 firmware and don't seem to get steady rpm readings anymore. I think it's due to a noise filter they added.
Could you have a look at this thread and give me a hint on what kind of values I would have to enter in the noise filter options?
http://www.msextra.com/viewtopic.php?f=91&t=30352
It's a bit hard for me to understand in english...
Thanks a lot!
Sven
P.S: How is your board coming along?
Now, first off, not just saying "25% filtering" and letting the computer do it either in real time, or once on boot up... Is inane. All this does is introduce a place for a user to make an error doing math. I just had to type two pages re-explaining this all.Primary and secondary refer to the tach inputs to Megasquirt. You only have a primary trigger, so the secondary options will do nothing.
The percentage and time are in relation to the period during which input signals are ignored. (The concept of these numbers comes from MS2 B&G code.)
A typical percentage is 30%. The time period is then added to this. Be careful on a multi-tooth wheel as the tooth times are quite small.
'tooth time' in milliseconds = 60000/rpm / no. teeth per rev
On your engine, at 1000rpm you will be getting a tach signal every ...
60000/1000 / 3 = 20ms
So setting say 30% plus 1ms would be ok.
You can use this formula to calculate noise filter numbers. They are in microseconds so X 1000.
If you picked 30% of a tooth as a safe threshold..
500rpm -> 12000
1000 -> 6000
3000 -> 2000
6000 -> 1000
etc.
A 60-2 uses very different numbers.
The tooth time at 1000rpm = 1ms.
e.g.
500rpm -> 600
1000 -> 300
3000 -> 100
6000 -> 50
etc.
Note the tooth time - be careful with the tach masking time. I would suggest sticking with 0.1ms and using the percentage alone.
hope that helps
James
And what about irregular teeth? The miata uses tooth spacing of 70*, 110*, 70*, and 110*. Which means your 4.7 ms for 6000* rpm, after a 30% margin, becomes a 70/110 * .7 = 0.445 or a 55% duty cycle where the filter is off.
Yes, it's nice to have a percentage you type in - I would expect the cam to need more margin. But the computer already knows when the next tooth is coming, so why be a douchefag and make people do something by hand just to give them the opportunity to screw up as well as make it less applicable for various wheels (the more missing teeth your wheel has the worse it would get).
Certainly on the miata cam, which has a singlet on one side, and a doublet on the other, the "filter" would only filter 5 degrees of cam rotation (space between the doublette), and be useless for the 80 degree gap before the next tooth.
My advice: With new features from MS, turn them off until you've figured them out, or you can't drive.
