Author Topic: Tachometer for motorcycle enginge (single cylinder four stroke)  (Read 253 times)

Offline kamelryttarn

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A while ago I bought a motorcycle and even though there is not need for it, I thought I would make a small arduino-based "computer" for it to monitor different parameters and one thing that would be neat to have is tachometer to see the current rpm. My plan was to use the ignition pulse before the ignition coil. I took my Hantek 1008 oscilloscope and hooked it up and was very pleased with how the signal looked.  It was way cleaner than I had first thought it would be.

Some time later when I had a plan of how to construct it and also an embryo of arduino code for it I connected everything and things did not work as expected so I hooked up my oscilloscope again and this time things looked a lot different. Even though screenshots suggest the time base is the same it was not. The first time I had the software set too 500ms but then I zoomed in and since the actual bandwidth of the 1008 isn't very good I fooled myself a little bit when I looked at the pictures.

I now realize I need the clean up the signal before I send it to my optocoupler and my question is: What is the best way to filter out the noise/ringing without sacrificing too much of the part of the signal I need?

My idea was to basically construct a "speaker crossover" consisting of two capacitors and an inductor in between creating a third order CLC-filter. The two main components to filter out is the ~2kHz ringing when the coil discharges and a much larger in amplitude ringing or possibly "bounce" happening when the coil is charged and that one looks to be close to 17kHz according to my readings.

Offline petespaco

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Re: Tachometer for motorcycle enginge (single cylinder four stroke)
« Reply #1 on: April 07, 2019, 06:05:31 PM »
How about a Schmitt trigger?  That would clean up the signal.
Google:
schmitt trigger IC sine to square wave
You can get a hex inverting schmitt trigger chip for USD $0.55 from Digikey.

But, if you can find a place farther back in the ignition system, maybe you can find a nice clean  square wave.  Off of the Hall Effect sensor, if it has one.   

I can't really tell from your pictures how high the excursions are.  As you probably know, the Arduino doesn't like voltages higher than 5.

If you really want to get into the whole "ECU" thing, go to:
https://speeduino.com/wiki/index.php/Speeduino

Pete Stanaitis
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Offline kamelryttarn

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Re: Tachometer for motorcycle enginge (single cylinder four stroke)
« Reply #2 on: April 07, 2019, 07:22:42 PM »
Signal peak-to-peak is just over 20V so I need to get it down to TTL logic levels which I planned to do with an optocoupler.

There is a short negative spike that I thought I could just handle with a single diode. But the problem with the rest of the signal is that I can't just make it to square wave with a schmitt trigger because that would still give me something like 3-4 pulses for every falling edge in the ingition system and that would seriously mess up my arduino math (I use hardware interrupts so it will pick up every single hickup on the signal). Hence the idea to use some capacitors and chokes to smoothen out the signal.

Offline dexter

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Re: Tachometer for motorcycle enginge (single cylinder four stroke)
« Reply #3 on: April 07, 2019, 09:52:08 PM »
Use a monostable with the time constant a bit higher than the length of the noise or a low pass RC between 2 Schmitt trigger gates with the time constant a bit higher than the length of the noise

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Re: Tachometer for motorcycle enginge (single cylinder four stroke)
« Reply #3 on: April 07, 2019, 09:52:08 PM »

 


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