Minimum pulse width on DL1/DL2 RPM input?

[David Ferguson]

We have been trying to use a wheel speed sensor to sense a magnet on the flywheel of a Formula Ford 1600 engine. We are getting good RPM reading at low RPM, but at higher RPM it just stops altogether. Is there a minimum low-going pulse width that the RPM signal must have in order to be detected properly by the DL1/DL2?

Thanks,

David Ferguson
Veracity Racing Data

[osborni]

How many teeth are on the flywheel?

If you are getting a signal at low RPMs, but not high, then it's probably because you are exceeding the frequency capability of the unit, and not a pulse width issue. If it where, you shouldn't be getting any signal at all.

You can also pick up RPM from the injectors or the spark system. (EFI or carbed? Coil on plug, or is it a distributor?)

[David Ferguson]

There is only one magnet on the flywheel, and it's about 7" from the centerline. That's why I think the pulse width may be too small the faster the engine turns (less time for the magnet to pass under the sensor).

The engine is a carb'ed Ford 1600 engine, so there is no ECU or fancy ignition system. Just points and a coil and a plain old distributor.

We've successfully used pickups like this before, but typically we put them on the front pulley, so the radius is much smaller (like 1"). This customer chose to mount the magnet on the flywheel -- and we're seeing a problem.

At 4000 RPM, there are 66 pulses/sec. So one pulse every 15 mS. If we assume the magnet was only picked up for 5 degrees of the 360 (a conservative guesstimate), then the pulse width of the low-going pulse would be somewhere around 209 microSeconds. I have not confirmed this with an oscilloscope, but we could probably do that.

If we take 209 microseconds as 1/2 the period (it's only half the waveform), then that could yield a frequency during just that half period of 2391 hz -- so that is pretty close to the max frequency a DL1/DL2 can handle. Maybe it's just that simple.

[osborni]

The specs on most RPM outputs is a 50% duty cycle. That may be it. How about moving the sensor closer to the shaft centerline? Not sure what the max frequency is, but more bolt heads might work too.

[faraday]

When you say magnet, I think of an inductive pickup, which will generate an increasing voltage when the revs rise. I would expect this would stay in whatever the range might be, but perhaps...

I recall seeing a specification somewhere in R-T literature...

My experience with sensors/triggers other than proprietory ignition system components is mostly with the (originally Honeywell) GT101.
Like many non-OEM ECU and DAQ makers, R-T use this as their "speed" sensor. I've had no problems. It is quite tolerant of poorly formed and irregular "teeth".

[s2klotus7]

We have been trying to use a wheel speed sensor to sense a magnet on the flywheel of a Formula Ford 1600 engine. We are getting good RPM reading at low RPM, but at higher RPM it just stops altogether. Is there a minimum low-going pulse width that the RPM signal must have in order to be detected properly by the DL1/DL2?

Thanks,

David Ferguson
Veracity Racing Data

Same problem here. I found out my 1 bolt sensor way too small to be sensored in a hi-rpm. Low RPM works. I made the head bolt bigger...try that.