PPM Encoder updated.

So today, I have got the updated PPM encoder s/ware working to do RPM measurement on one input channel.




For an RPM range of about 300 - 2000, the output pulse to the AP CPU is ranging from 2mS to 0.5mS.  The ACM code might need modding to accept this, without thinking its a sync pulse, but thats the next task.  All this will need further calibration when I test it with the motor running.


The RPM sensor is an IR LED and IR photodiode, glued into holes drilled into the frame either side of the heli.  One of the holes was used to mark the rotor shaft, which I then drilled through (offset about 2mm from center).  This gives a very clean pulse, hopefully long enough to be detected at full RPM.


Above you can see the wires leading to the photodiode, just above the main gear.

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  • What - microphone or RPM sensor ?  The microphone I guess could sense engine RPM if the signal was processed right.  The RPM sensor I need to keep the engine / rotor running at constant revs.  With collective pitch, you want your lift controlled by blade pitch, and not variations in RPM.  I told that electric helis have this feature built into the ESC, but with nitro motors, you have to match the engine performance with the rotor performance.  So if the autopilot can sense RPM, it can adjust the throttle to compensate for any RPM variations.

    Hope I answered your question.

  • What would this be used for?
  • belive me microphone sux. about magnet. you need very sensetive hall effect sensor. to manage it. event motor will effect result. you need lots of test but when you are done its better then optic sensors for me.
  • Yeah - thanks guys.  I tried a hall effect sensor I had in my collection, which was from an old tape drive motor, but it had some circuit inside and was setup for a specific magnetic field strength.  I had the sensor mounted next to the bolt that connects the main gear to the shaft, but I couldn't get the bolt to retain a strong enough field.

    In the end, I figured that a strong enough magnet will play with the magnetometer so I went optical instead.

    I looked at a hold through the main gear too - but it has to be outside the tail drive cog diameter, which limited mounting locations.  The other method I thought about for a while was with an electret microphone mounted on or near the muffler / motor.  But optical seemed much simpler.

  • Moderator

    Very nice work.

    Good call on hall effect sensor Ersin. I'd go for a tiny magnet embedded on the main gear over drilling my main mast- ingenious none the less, Hardy. ;)  I like the compact nature of your hardware mod- I could see myself popping a hole in the main gear and using your same sensor setup though.

    As for the software- are you posting your mods on that, too? :)

  • We used same method on volkswagen T2(1972) minibus' engine. but in dirty places hall effect sensor & magnet rox. you should try it too :)
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