Reference pulse, how to extract it?

On a sunny day (Thu, 17 Apr 2008 18:06:48 GMT) it happened "Jacques St-Pierre" wrote in :

I can think of many ways. Perhaps, if the speed does not change extremely fast, you can use a micro to count some reference clock pulses between the teeth. Say you measure 100, 101, 103, 52, 52, 105 Then you know that the first 52 was the extra tooth, you also know the speed, and you know it was speeding up.

That is the way we thought we will do it, but maybe we can find a more simple way, a totally hardware method. The resulting pulse will be use to generate an synchronization interrupt to a CPU.

Bye Jacques

What the heck is a "37e teeth squeeze between two others?"

It look a bit like this:

R = Reference (Zero) = 37e Pulse That the one I need to extract to mark that the next pulse is 1 Note: The time between those pulses change with the engine speed

Bye Jacques

I think that's how "37th" comes out in French: trente-septième.

They do it like this:

Yup. Up/Down Integrate with a Clear.

...Jim Thompson

Any schematics available?

Jacques

Right, my mistake.

Bye Jacques

Would you prefer analog or digital? Looks like John Fields is contemplating a digital (up/down counter) method, which I'm sure he will whip out momentarily ;-)

If you want analog it will be late week-end before I get to it... I'm working a critical project at the moment.

...Jim Thompson

Use two sensors spaced to pickup #35 and #36. The output would be a 1 and a

Slick! Two sensors spaced 1/2 normal spacing... makes it trivial and time/speed independent!

...Jim Thompson

Even easier, two sensors spaced an odd-number of half spacings apart... gets the sensors conveniently spaced apart.

...Jim Thompson

If they wanted to use two sensors they would not have put the index 'tooth' in the middle of the others would they. The electronics or more likely software required to figure it out with one sensor is cheaper and more reliable than an second sensor.

Even easier is to punch a hole thru the side of the gear and sense it without the teeth interfering. I don't see why anyone would want to sense an odd tooth like this, I must be missing something. Two sensors is very common on alot of motors. When spaced 180 degrees apart they can offset any wobble in the shaft and spaced 90 degrees apart you can use them to detect direction. All he needs is an exclusive Or or Nor depending on which output he wants. It don't get any easier than that. bg

In the volumes engines are made the cost of an extra few bytes of ROM if it's done in software or a few hundred transistors if it's done in an ASIC would not even buy a hole for another sensor never mind the sensor and cable and connector and extra unreliability.

Normally I prefer digital circuit, but analog may also be acceptable if it can be simpler or more reliable. I will try to draw some kind of digital circuit myself using your suggestions, I am not very good with analog. I will also try to do it by software, it may be a lot more simple to use a low cost MCU to do the job than design a digital circuit or use any ASIC.

Bye Jacques

We are talking around 500rpm at idle to 4500rpm at maximum. But we can forget that speed is very low when starting the engine.

Bye Jacques