Test Stand Setup?

I've put together a test stand for my 450 class quadcopter running APM 2.6/Arducopter 3.2. After a recent crash where my quad suddenly fell out of the sky, I want to run some tests on the stand, keeping most variables the same as if it were in flight. I'm pretty sure the crash was due to battery cell failure but I want to run some tests on the stand using the same battery (which still balance charges okay).

My quad is zip tied solidly to the stand, i.e. it cannot move. This presents an apparent problem because the motors won't all run at the same speed in Stabilize flight mode. How can I achieve this kind of test? Do I need to attach the ship to a rod on a ball joint? I've seen a couple of videos on YouTube where people did this, but I would rather have my ship solidly mounted. Have any of you done this?


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  • This graph is from the log of the previous test video. Can someone confirm this is the output of the flight controller to the 4 motors? If so, it confirms my suspicion that the motors are not being powered the same, especially at the end...


    • MR60

      got to be.  while not actual power going to the motors, the longer (larger number) the pwm square sign wave length, the more power those motors are being commanded to take, thus a true reflection of what the FMU is commanding the ESC to do.

  • Here's a test I did tonight. I'm using the same battery I was using when I crashed. I think the results of this test are interesting, however, I can't rule out a bad ESC or something else because I can't be sure the flight controller is driving all of the motors equally. For this test, I used ACRO mode with ACRO_TRAINER set to zero. My logic was that acro mode doesn't do any self leveling.

    Also, I was hoping there was some log I could turn on that would show individual motor output. I can't seem to find that. Does it exist?

    • MR60

      so i might be wrong on this, because i don't have much experience with acro:

      - it self levels & keeps yaw stable?

      - it eliminates the maximum roll and pitch angles to allow for flips

      - the stick inputs become different controlling acceleration, not rates?

        ... in stable, the stick at 0 means 0 rotation (the rate is corrected back to zero)

        ... in acro, the stick at 0 means no more acceleration (so forward engages acceleration, and bringing the stick back to zero would keep the end rate or rotation the same so a flipping ship would continue to flip unless countered with a reverse stick move)

      anyway, that's my guess.  so someone who knows, please correct the above.

  • What you want is called a "string cage". I use a big (20l) paint tin with a rim-clamp, and short lengths of light cord. Fasten one cord to each arm near the motor. Hold the quad above the can lid, a bit less than one arm-length above it, and trap the other ends of the cord around the rim. Then put the quad on top of the can and set it up.

    If you do it right, you create a small envelope where the quad can fly relatively normally (the string cage), but it can't flip or fly away or even strike a prop on anything.

    My big heavy paint tin is just right. The rim-clamp means setup/string calibration takes a few seconds, a single action. I also think putting the string cage on top of a small pillar probably minimises ground effect type iterations, closer to normal flight (unconfirmed, just a theory). But nails in a board could be fine too.
    • This sounds interesting. Is this similar to what Mark Whitton posted? Do you have any pictures?

  • MR60

    what i do is tie the ship to a tripod with a universal ball joint that you might all ready have on one of your camera mounts.  in that way, the ship will "fly" itself and stay level. 

    • As always, Forrest always has great ideas!! Sometimes its right in front of me and I miss it. Of course I HAVE all kinds of tripods and ball mounts- I'm a photographer. I have a big, heavy video tripod and sandbags. I'll use a ball mount and try to do a test tomorrow.

      I just need to figure the best way to attach the ball mount near the CG of the frame. That's right where my power distribution board is- on the underside of the frame.

    • MR60

      i actually cut a piece of wood that mounts to the ship and has the tripod connection shape (or bolt) on the bottom to lock into the tripod.  the sand bags on the legs are critical and the ship has the power to pull the tripod over if it goes a little nutzo.

      jim's idea of the pool noodle is good because it offers a little more freedom of movement, but cut power and it "lands" where you want it.  a noodle that just barely holds the ship up would give the most freedom of motion

    • Years ago with RC copters it was whiffle balls on the end of long two fiberglass poles tied in the middle to make an "X".

      The effects of attached objects make it hard to tune and create distorted thrust lines, that why hanging from the ceiling has worked so well.  We do all the PID tuning this way.

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