Dropping a Rover from Hexacopter - Part III

As a follow-up to the rover drops #1 and #2 I spent a day with Assistant Professor Nagatani-san of Tohoku university in Japan (and his students) and Izu-san of EnRoute (a company that specialises in Industrial use multicopters and hobby use RC vehicles here in Japan and China) at Karuizawa's Mt Asama attempting to autonomously drop a 2.5kg rover from a large (4kg+) EnRoute "ZionPro" hexacopter.

The flight was completely autonomous including activating the servo release which held the rover to the hexacopter (we used the camera shutter release).  After being released the rover was "lowered" on a 30m wire wound around a brushless motor which was meant to slow it's descent (with mixed results).  If you've never heard of this mechanism before, it seems if you connect the bullet connectors of a brushless motor together it resists being turned.  If you attach a resistor between the bullet connectors it will resist less strongly.  In this way we could somewhat adjust the speed at which the rover descended at.

3689544037?profile=original3689544065?profile=originalA couple of things that we learned from this test in case you try something similar:

  • the altitude reported by a Ublox GPS (i.e. APM/PX4) vs a hand held GPS can vary by 10m.
  • the altitude reported in google maps (and thus the mission planner) can vary by 20m from reality because they only provide the average altitude for the area.  I'm not sure how big that area is but we found that google maps altitudes could be high or low from reality depending upon where you were on the slope.
  • trying to drop from a wire is tough!  we need a more reliable system for the next test.  Maybe use a range finder to get the copter closer to the surface without hitting it or perhaps measure the motor output to determine when the dangling rover has reached the ground.
  • the hexacopter and battery were more than sufficient to carry the rover the 600m covered in this test.  At least twice that distance would have been possible.
  • AC3.0.1 is very capable of the accuracy required for this mission.  It was a thing of beauty.

There will be one more attempt within the next 2 months which should be pretty much the same except the distance will be further and the dropping mechanism will be improved!

Thanks and all comments, input welcome!

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  • i am surprised after the first wire break you guys did not immediately scrap the wire and use a different material like fishing line which is far far stronger

  • Developer

    Just to clarify one thing: MP is using the SRTM data-set.

  • How about a very small fishing reel?  They usually have adjustable drag.  

  • Perhaps it is better to give the rover a simple and weak multicoptersetup as well (quad/tricopter). I just think of weak and light motors and a simple flightcontrol. Just enough to level it and brake the fall to an sustainable impactspeed. Just a crazy idea..

  • Can we not use any of the free topographic mapping data available from various sources? Wouldn't this be much more accurate than the GE data?  I have topographic maps available for my area, and I think it's much much finer than a 90m grid. 

  • The google maps DEM (Digital Elevation Model), is collected for most of the world at 3arc/sec, that means 90x90m grid and the data between the grids is interpolated to provide a smooth result. The interpolation seems to be really well done (I did compare google Dem data with an survey D-GPS on an wide hilly terrain) and in most of the sample points the deviation was between +-1.5m. For some country you can have access to 1arc/sec data, that means 30x30m grid, to see if your country is covered check on NASA website.
    Said all that, the drone I use to do magneto metric surveys use a laser altimeter to asses the altitude from the terrain, none of the D-GPS RTK + DEM data did work that well.
    I will suggest you to buy a sonar or if you budget is not tight a laser altimeter.

  • Yup, really cool project.  And a it was very neat to see the copter flying autonomously through the mist.  I can just imagine a system like this being used... say a volcano is erupting, and scientists would like to deposit a seismometer, or get a camera view of the caldera, but poisonous gasses make it impossible for a man to do it.  

    I think it makes more sense to have the winch system on the rover, so that it after deployment, the weight of the winch isn't on the copter.  And the copter also would have the cable release at it's end.  This would allow it to free itself if anything goes wrong.  Especially important when the copter is more valuable than the payload.

  • What a fascinating field test! While probably not as accurate for positioning the rover to collect data in a specific zone, I wonder if it could be dropped with a parachute, with the parachute automatically jettisoned upon impact. Maybe you've already tried that approach.

  • The sonar module will give more accurate readings when nearing ground level.

  • really great! your work is awesome! 

    I am interested in the ways a quadcopter reacts if it loose significant wheight.

    my congratulations!

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