Questions about one long tooth crank trigger

[Andy_Stprbeck]

This is kind of a continuation of my questions in the "how many teeth needed for crank trigger" post but I couldn't figure out how to edit the subject.

I have a one cylinder engine with a stock VR crank trigger that reads a single "long" tooth. The tooth goes from about 75 deg btdc to 10 deg btdc.

I'd really like to use this single tooth if at all possible because putting anything else on here will be very difficult. The other option I'm considering is to put another sensor on the primary gear (gear that is on the crank and meshes with a gear on the clutch as is common on bikes) and using this stock sensor as a sync. Either way I'd have to use this stock sensor/tooth one way or another.

My concern is that due to the long length of the tooth the signal from the vr sensor will have a short rise as the tooth starts, back to zero, then a short fall as the end of the tooth passes, then back to zero. So there will never really be a good well defined zero crossing point. I attatched a very crude picture of what I mean.

one long tooth.JPG

(11.88 KiB) Downloaded 31 times

Do I understand this correctly? Is there any way to deal with this? Can the signal come from some point other than zero crossing with a VR sensor? The Hall sensor configuration I think doesn't use zero crossing but can't handle the high voltage of a VR sensor?

Thanks
Andy

[SQLGUY]

You should still get zero-crossings at the leading and trailing edges of the "tooth". It's the transition from metal to nothing, or nothing to metal, as the reluctor moves past the pickup that creates pulses. My stock reluctor was similar and it was able to trigger uS.



As I mentioned before, you should be able to trigger the VR input with the stock reluctor, it's just that you'll have a decent amount of timing inaccuracy at lower RPM's.

[Andy_Stprbeck]

SQLguy,

Yes you already pointed that out, thank you for your help. I'm just trying to further understand this.

I understand it is the transition from nothing to something and then something back to nothing that causes the pulse, but is it a pulse up when you go from nothing to something, then a pulse down when you go from something to nothing? If this is true you would need it to go up then back down quickly so that the up pulse and the down pulse blend into eachother to get a nice well defined zero crossing. If the two pulses don't blend into eachother, you get a poorly defined zero crossing like in my picture.

The various diagrams that are in the megamanual are leading me to believe this is true.

Also, I know the peak voltage of the pulse varies with how fast the tooth passes the VR sensor, does this mean that the pulse also gets longer in duration? If so it might be possible that the pulse is short at low rpms, so that the two pulses don't blend into each other, then widens at high rpms so that they do. It would be an explanation for poor accuracy at low rpms and good at high rpms.

However it looks like you were using a home made narrow tooth when you were having low rpm accuracy problems, so that must have not been the case.

Just trying to understand the situation a little better, as soon as I get a scope I will check to see for sure what the stock signal is, but for now I was hoping somebody would know. It really would be very difficult for me to add more teeth, so I want to make sure I understand this before doing anything drastic.

SQLguy what was the reason you made the narrow tooth rather than using the stock wide tooth?

Thanks
Andy

[SQLGUY]

I went to the single narrow tooth because I was using dual tachs and couldn't easily move the pickups to change where the physical tach pulse occurred. By making my own trigger wheel I could move the tooth as needed. For single tach this is not an issue, because dwell doesn't have to occur immediately after the tach pulse... it can skip a pulse.

The low RPM accuracy is real; it is caused by the crank's angular velocity actually changing as the piston moves through compression, power, etc. uS will try to estimate the next TDC based on the previous time to make one revolution, but this can be off by quite a bit. A trigger wheel with more teeth allows uS to estimate much more accurately when TDC will happen.

[myk777]

Andy,

Here's some scope pics from my VR sensor output on the YFZ450 motor, same engine as your WR450. The first is at idle ~1700rpm and the second is ~5600rpms, the square wave is the output of my LM1815 circuit. As you know I'm only running fuel with this setup.

YFZ Idle VR_In and CPU_Pin14.jpg

(115.07 KiB) Downloaded 52 times

YFZ Pin14 and VR 5800rpm MS1 closeup.jpg

(121.06 KiB) Downloaded 39 times

Mike