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RPM from alternator ripple?

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Azagon

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Hi guys. I understand it is possible to derive engine speed (RPM) from the AC ripple present on the DC line of the vehicle (the source is the alternator ripple). I have searched high and low but cannot find a circuit to condition this signal (which I understand to be very low) before being able to take it on the uC IO. Any help greatly appreciated. Thanks!
 
You mean like an AC blocking capacitor in series with the output of the alternator (to block the DC) and a zero cross detection circuit on the other side?

I don't know if it's practical because I don't know what other noise might be present on the line to distort the ripple produced by the alternator.
 
Yes it is possible

Sure, it is possible. See this nice device:
**broken link removed**
But.... it is WAY..... too expensive!
So I am looking to understand the basics of it and build it myself. Once I have a square wave signal getting in to my uC I am OK, but I don't know how to build the conditioning circuit to detect the ripple (filter it, amplify, etc etc).
 
Hi guys. I understand it is possible to derive engine speed (RPM) from the AC ripple present on the DC line of the vehicle (the source is the alternator ripple). I have searched high and low but cannot find a circuit to condition this signal (which I understand to be very low) before being able to take it on the uC IO. Any help greatly appreciated. Thanks!

Most tacho circuits I have seen, use the IGN line to the ignition coil.
 
Hi guys. I understand it is possible to derive engine speed (RPM) from the AC ripple present on the DC line of the vehicle (the source is the alternator ripple). I have searched high and low but cannot find a circuit to condition this signal (which I understand to be very low) before being able to take it on the uC IO. Any help greatly appreciated. Thanks!

Interesting idea. Of course most altenators are 3 phase and of course the ripple frequency Vs RPM would have to factor that in as well as the pulley to crankshaft ratio, making each installation a custor affair. It would probably be worthwhile first to use a scope at the alternator output, using a blocking capacitor, to see if the 'signature' ripple voltage looked to have enough usable information to work with.

Lefty
 
Yes it is possible, I've done it on a commercial product so I can't give you too many details but ......

Yes you need a blocking capacitor and small load after the capacitor before hooking it up to the scope to see your waveform
Yes there is a hell of a lot of noise on the signal
The signal amplitude varies depending on the condition of your battery (it acts as a capacitor) and the load on the electrical system - turning on and off the headlights varies the amplitude quite significantly.

But its possible and actually in use on a few commercial products out there.
 
Yes it is possible, I've done it on a commercial product so I can't give you too many details but ......

Yes you need a blocking capacitor and small load after the capacitor before hooking it up to the scope to see your waveform
Yes there is a hell of a lot of noise on the signal
The signal amplitude varies depending on the condition of your battery (it acts as a capacitor) and the load on the electrical system - turning on and off the headlights varies the amplitude quite significantly.

But its possible and actually in use on a few commercial products out there.

Thanks PICBITS for your reply. I do understand you will (probably :rolleyes: ) not be able to post your circuits here.... However your generous feedback is much appreciated!

I have done a lot of digging/learning and this is what I have come up with so far: you need to have a bandpass filter to filter out unwanted frequencies, and then amplify the signal (because the ripple is a few tens of mV or so). Then, you need/can use a pair of peak/valley detectors to detect the actual ripple and fashion a signal out of it. Now as the voltage of the battery (alternator?) will fluctuate based on various things, the baseline of this comparison needs to be adaptive. The final result if done correctly should be a square wave whos frequency is proportional to the RPM (of course need to take the pully ratios in mind).

Now if I can only design such circuits....

What say you?
 
Use the ignition cable signal like ericgibbs suggested. It's directly linked to engine speed - no slipping belts or fractional-integer calculations - and has a better signal to noise ratio.
 
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Use the ignition cable signal like ericgibbs suggested. It's directly linked to engine speed - no slipping belts or fractional-integer calculations - and has a better signal to noise ratio.

Yes, very true, but not on a diesel engine..... (guess I should have mentioned that bit in the first place!)

;)
 
Oh. Yeah, that WOULD make a difference!
 
Yes, very true, but not on a diesel engine..... (guess I should have mentioned that bit in the first place!)

;)
It would also help a great deal if we knew what engine/vehicle this is for. Lots of diesels use the alternator for their tach signal. Some alternators even have their own terminal for a tach output (I'm guessing this output might be the pure AC before it gets rectified which would be a pretty good signal vs the low level ripple in the DC output).

Unless you've got a really odd engine it should be easy to come up with something.
 
It would also help a great deal if we knew what engine/vehicle this is for. Lots of diesels use the alternator for their tach signal. Some alternators even have their own terminal for a tach output (I'm guessing this output might be the pure AC before it gets rectified which would be a pretty good signal vs the low level ripple in the DC output).

Unless you've got a really odd engine it should be easy to come up with something.

Google 'alternator w terminal' for tacho pulse
 
You'll have to factor in a correction because the alternator pulley is usually about half the size of the crankshaft pulley, and the crankshaft pulley will be the true engine rpm.
 
You mean like an AC blocking capacitor in series with the output of the alternator (to block the DC) and a zero cross detection circuit on the other side?
Don't you mean an AC pass/DC blocking capacitor?

I don't know if it's practical because I don't know what other noise might be present on the line to distort the ripple produced by the alternator.
You'll need a low pass filter.

The cut-off frequency needs to be just above the maximum frequency you want to measure.

Do most alternators have the same number of poles?

What about the ration between engine revs to alternator speed? Does it vary from car to car?
 
Most tacho circuits I have seen, use the IGN line to the ignition coil.

Finding the ignition line is difficult on diesels.

It is really quite easy to get to the AC output before the rectifiers on most alternators. Some alternators already have one or three connections to the AC outputs.

The windings are big, as are the main diodes. That is why adding an rpm connections is quite easy. The frequency is 1/6th of the ripple frequency and the signal to noise ratio is much better. There will be about 16 V ac whenever the engine is running.
 
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alternative to alternator ac

Depending on your application. Most (nowadays all) vehicles have Canbus. There are many interfaces that will read the canbus info and translate this into an RPM pulse, speed pulse, reverse, lights etc. I'm doing some telematics work on London buses and have been using such a device to get engine and road speed. Also useful for satnav systems that require speed pulses and reverse indication.

**broken link removed**

Where Canbus is not available RPM has been hardwired to the alternators W terminal (pure pre-rectifier ac) and reading the frequency of the pulses (ac peaks) at the vehicles known factory engine idle speed. Then a matter of calculating either pulses per rpm or Hz per rpm. Probably not the most accurate way of doing it but near enough for most uses.
 
You can also tell the condition of the battery.
Old battery=higher internal resistance = higher p-p ripple on the battery terminals.

For RPM you can use an AM radio with the earplug output going to one of these freq. counter DVMs, but you still need a formula, and watch out for "dead sparks" (sparks on the exhaust stroke).
 
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