RPM Sensing

[Helmutt]

Hi. Thanks again for the help so far.

Ok so the next problem in my project. How to sense the RPM using the pulses, obtained from a piece of copper around a plug wire, Soldered to a wire that goes to ????.

Has anyone done this or know a simple way? When I've tried the comparator or Count functions I get very poor results due to the high noise levels. I can reduce the noise down to what would seem to be acceptable, but also reduces the voltage of the signal. If use the comparator the range is to small it seems. The secondary plug fires cause a pulse of about 2.3-2.5v, with primary plug fires causing a Pulse of 2.5-2.9v. Currently the best signal I can obtain is using a 15kR, in line with the signal, then a 1k to ground, followed by a 4.9v 1/4 watt zener Diode. That is where I am getting the above voltages. The issue lies in that both coils sit right next to each other, and Routing of the spark plug wires, are in line. I am trying to pick it up, as Close as I can to the plug its self as to cut down on the other plug firing.

I tired various Schmitt triggers and Op Amps to little success. Maybe Im doing it wrong? But The pulse is so short it does not appear to need "Squaring."

Then the second question if you have any ideas, Would it be best to use the count command, and calculate RPM from there? Or The PULSIN and Measure the time of the low pulse. (I get nothing with this the signal has to much low level noise to be Logic 0, I think.)

I am using a 16F690. So I do at least have a few more options.

Any ideas would be greatly appreciated.

[Archangel]

Hi Helmut,
There are easier ways than picking up from a high tension spark plug wire, your copper will work like a capacitor plate and pick up a signal, or should I say signals, very ugly waveform, then you will need to condition it etc etc . . . Better I think to use some other method, like optical pickup or magnetic pickup. I did not recommend picking up from the primary circuit for similar reasons as the secondary, namely too much voltage, which you have to turn into a TTL level signal. I would have the tach count more than 1 pulse per revolution so as to promote stability at low RPM, 4 seems like a good number. Then RPM = (counts per second / 4) * 60

[Helmutt]

If I could think of a way I would. It is an older RGV Gamma Motor, 500cc Two stroke. The signal coming off the "Crank sensor" is the nastiest thing I have ever seen. I scoped it, then it was so weird I pulled the case off to look at the wheel, Its a recessed tooth cut, with a recess at 180 and 180, then recessing 1/2 depth and then back, So 0-Cut, Down slope, 90, Up Slope, 180 Cut. Repeat, So Nice Pulse drop to lowering to crap distorted interference signal. So short of rebuilding a new wheel... Im getting close. Im kinda at a loss.

[Acetronics2]

Hi, Helmutt

There's also the "Diesel " way ... ( LOL for a RG 500 ... keep cool : mine is a '77 H1 F ... )

take the signal from the Alternator, before being rectified : its freq is strictly proportionnal to Your engine speed ...

This is a C Written project, but, I'm sure you will understand How it works ...

But ... if it is for a stroboscope ... just tell me !!!


Alain

[Acetronics2]

Hi Helmutt

Could THIS help you ???

Alain

[Helmutt]

Thanks for the awesome replies,

I did some quick experimenting with dropping the resistor to ground and replacing it with a cap as in the "Diesel" method, it seems to clean up the signal a whole bunch. I really don't understand why. It goes against my basic understandings. But hey if it works it works.

Just for the interest Ace. The whole reason for this is to allow me to move the motor back 1.3 inches. The RG 500, is actually just a RG motor in a 93' RGV 250 Frame. Its been a wonderful project so far, but sadly the RG motor is so wide it sits to far forward in the frame due to the rear Power valve connector. (It sticks out the motor about 1.75 inches) Even with the frame notched as it is now. So I have built a new bearing to replace the connection and gained a whole inch (WOOT. ><) to a RC servo. So If I can get the electronics to work, I can move it back and roll it forward about 10 degrees. Restoring very very close to 50/50 weight distribution. As it is now its 64/36. It exhibits some very strange high speed head shake. On Power/Braking or visa versa transitions. Tired 3 different suspension re valves, and am just accepted its to far front bias to not have it. So Time to tear it down and move it. At least once the electronics work.....

Note. I have no idea where you got that Ignition schematic... But thank you, I have been looking for that thing for days.

Thanks for the help very very much.

Keep riding.