Current (and Airspeed) for $1.45


This is a Current (And Airspeed) Sensor for $1.45.

I raised this question a while back, and was reminded again by the Post below which asks how to stuff all of this into a small box. (Beautiful Glider)


Small planes should remains a primal focus because they reduce the real and regulatory risks and allow broader participation in more fields/parks, etc...


So I'm offering a size reducing part - add a current sensor in exchange for the whole pitot/airpressure spaghetti.


The Airspeed theory is simple - at a given RPM, power consumption is proportional to airspeed, The faster the plane, the less the motor consumes to turn the prop. One can either detect the rpm, or determine that ESC's are essentially RPM = Throttle. Then a bit of data collecting will show the relationship.



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  • 3D Robotics
    Note: we're going to be supporting the AttoPilot current sensor in APM 2.0, out this week. Beta documentation for the sensor is here. (We may make that easier, so don't solder anything yet!)
  • Darren, One can definitely recapture energy, provided one has an ESC designed to do so. These are rare at best, "Electronic Braking" might be a term used to describe same (might).

    (Caveat, Props are quite in efficient when operated against the camber. If you want recapture energy, you would want an uncambered prop.)


    I think in the end, I much prefer the control twitching idea.


  • generated volts? is the voltage inverse of applied voltage for rpm? that then brings up a question of recapturing energy during a decent for example.
  • Hmm...that wasn't what I meant, but that IS a better idea...
  • Yuan Gao has an idea there with the wind turbine.  Switch the engine off and measure the generated volts? you might have a better chance of measuring windspeed.

  • Calculating airspeed from propeller power is not a straightforward linear function. You will have to take into account efficiency of the prop, which varies based some ration between the prop speed and the airspeed.

    Unfortunately, I wouldn't be able to give you more information about the theory - my knowledge of this comes from wind turbines (the control systems of which I have slaved over for the past two years), whose governing equations would probably only be applicable to aircraft in static thrust situations.


    Theory aside, it is probably easier to experimentally determine the relationship between prop RPM, current and thrust, if you have a way of measuring these values (GPS would probably work for calculating thrust based on acceleration vs drag assuming no wind).


    These links may be of some use:

    11.7 Performance of Propellers
    11.7 Performance of Propellers
  • have you ever even flown a plane?!?!?! the RPM is all over the place, you're better off using GPS speed!
  • In theory you could do the calculations. You would have to account for factors like acceleration, air density, voltage... Could be a quite complicated model in the end.

    And regarding the accuracy of current measurement, I would expect the result to be quite unreliable. Especially when it comes to low speed, where airspeed is most important.

    But anyway, give it a shot and collect data of both pitot and current/RPM and see if its possible. Doing math is nice but often doesn´t get reality :-)

  • So if I place this $1.45 sensor on one leg of the motor, I get Current and RPM. knowing RPM and Current, I can calculate how hard I am pushing the prop to maintain the RPM which is inversely proportional to present airspeed.


    Is there actually something wrong with this system theory?


    @Doug - ESC doesn't matter because I directly sense RPM.

    @bcr - no conversion necessary - RMP sensed directly (interupt counter).

    @Matthew - placing the current on the active side of the ESC yields inductive current and RPM. (diode-RC for current / clamped for RPM)



  • Here is an under $10 AC and DC hall effect current sensor.

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