But the ripple can be negligible when the attached battery is fully charged, I would think some clever pre-processing/AGC would be required to extract a reliable signal from just a few mV, while rejecting noise from other sources. (Lead-acid battery = very large low impedance capacitor!)The alternator output won't be true DC, but three phase rectified AC, which has probably got enough ripple on it to be able to AC couple it to a comparator.
Nope, the rotor is PWM'd to control the output, by varying the magnetic field in the rotor, there is no switching at the high current field windings at all. The field windings just go directly to six diodes, normally, with no switching or smoothing. As already mentioned, some diesel alternators have an additional single phase tacho winding, which can be used, but this one doesn't seem to have that.The electronic part is easy to get the signal to the picaxe and only requires a opto coupler to isolate the AC, but where the real problem is with most auto alternators is the field windings are switched on and off by the voltage regulator, to maintain a regulated DC output.
This will mess with your pulse count something shocking, and all you will get is garbage for your RPM reading.
I think some diesel alternators might include a small permanent magnet to induce a weak AC voltage into the windings or include a separate winding for the taco.
How i have setup tacos on diesel motors (excavators, bobcats, tractors etc, ) is to use a proximity sensor and detect a passing nut or bolt on a rotating pulley or shaft somewhere on the machine, dead simple and you know exactly what you have to work with.
But the field windings aren't normally switched at all, the three windings on the field stator are hard wired directly to the six rectifier diodes. The single armature (rotor) winding is switched, and you're right, the PWM is crude, just an on-off whenever the voltage threshold is passed. The current through the armature is modest, though, usually just a few amps at most, which is why switching it to regulate voltage is a lot simpler than trying to switch the high current field windings.Jeremy,
Yes i basically agree, but from every alt. voltage reg i have ever checked out there is no fancy PWM, its a simple circuit of........ above 14.2 +/- volts = alternator OFF ( no excitation to the field windings) below 13.8+/- volts alternator ON (field windings excited) and thats it, although this will give a form of PWM, when you consider engine rpm and current load etc, but the ripple could cause false readings, but i also susspect the regulator PWM is far higher than the 3 phase frequency so it might not be a problem.
I have built many tacos for permanent magnet alternators which is a given as the field is alway there as long as there is rotation, and even they can have false ripple due to increasing/decreasing rpm across the coils.
Its the once in a blue moon spike that throws the entire system out of whack.
I also agree there is enough ripple on the alternator output to extract a reading from, all you need do is place a AC current tong meter on the alt, cable to the battery and you will read the current in the cable. (proof of AC ripple)
The diagram shows Alternator with FOUR wires plus an earth; looks like B R L and WBut see part wire diagram of this which might help.
I would use a 10kohm resistor and a 0.1uF across the Zener to form a lowpass filter to help filter out interfering signals. Then connect a 1kohm resistor from Zener to PICAXE input to act as a current limiter for any spikes that may drive the Zener above 4.7 volt.. . . . . Can it be done with a resistor and a zenor diode of say 4.7V? . . . .
Make up the zener-based input limiter, and attach to W. Write a Picaxe test program to monitor what you get for PULSIN out of that. I'd be expecting pulses in the 2-20mS league. You'll want to check at idle and increasing RPM to see what values you get. You'll want to check that it remains consistent with heavy electrical loads applied (that could alter the W output) at any given actual RPM.
We/you are assuming that the original circuit only used RPM for display - no fancy RPM limiting system or whatever.