InputTrigger/RunOut (2006-03-16 04:43:08)

Runout

Some factors can impact VR trigger quality.

The amplitude of tooth pulses is not equal

This is a common error with homemade trigger wheels that have a long deep gap at the missing tooth (this results in a higher amplitude around the missing tooth). Professional wheels have a continuous depth-change at the missing tooth, full depth is only reached at the middle. Examine a 60-2 Bosch wheel.

triggerwheel is not centered on the shaft

Sine wave will modulate the VR signal amplitude, so the arming threshold becomes important, especially at high RPM.

If in doubt, record VR signal with scope or your notebook soundcard.

Variation in individual tooth-height just makes the problem worse. Mik was the first to experience such problem when he patched his original (quite exotic) 18-1 + 18-1 crankwheel with a surgery for normal 36-1 tooth. The added tooth had smaller amplitude than the original teeth.


Soundcard can be used to record VR signal

WhatEverYouDoYouDoItAtYourOwnRisk !!! (take care of your soundcard)

Since the VR signal amplitude can be quite high (especially at high-RPM, often >30V), the line-in input should be preferred to the mic-input. With mic-level input a much higher divider can be used, eg. 100k and 1k.


LM1815 arms at about 0.8 * peak

Note that the 20% safety margin in the VR chip isn't shown. The margin should eliminate any runout problems on a multitooth triggerwheel, but if the teeth for some reason result in a 20% amplitude difference there will be trigger problems.


This is a simulation of a four tooth triggerwheel at 3k (V8). The trigger occur when the VR signal fall down past 0v. The trigger is armed for the next trigg after going higher then the blue trace.

TriggerV8@3k.jpg

60 tooth at 3000rpm

Trigger%2060tooth@3k.jpg

The same triggerwheel, with the same runout at 6000rpm. As you can see the margin between the ARM threshold and the VR signal has decreased.

Trigger%2060tooth@6k.jpg


Notes

If someone want to waste some time on simulating this better:

http://www.vems.hu/files/JorgenKarlsson/triggersim.zip


Proposed solution


See also