Tuesday, May 20, 2014

Mobius Brushless Gimbal

 Here's some notes on making muellerr_ch's brushless gimbal (BLG) for the Mobius.  He has a parts list of motors and other required parts on his page.
 The parts printed out nicely on our Makerbot 2.  There's a version 2 of arm 2 that has a deeper recess for the sensor. Use that one, it holds the standard Martinez-style sensor perfectly.

From front to back, the three pieces are Arm 2 (holds camera), Arm 1 (holds Arm 2), and Arm 3 (attaches gimbal to airframe).
 I made all  the motor attachments using only two bolts per motor.  It seems to make a nice sturdy connection.  I used M3 x 10 bolts.

Start by attaching a motor to Arm 3.  Note that Arm 3 is extending away from the gimbal.  This was for convenience in experimentation, and I'll flip it around so that Arm 3 extends under the gimbal when I've got everything figured out.
 Atach Arm 1 to the motor attached to Arm 3.  This motor controls the roll by rotating Arm 3.
 Put the ball bearing into the 8mm hole in Arm 2.  I screwed up ordering my ball bearings from ebay.  I ordered 3x8x4, when I should have ordered (I think!) 4x8x3.  Note how it stands proud by 1mm.  That's not a problem.  I had to replace the M4x10 machine screw lower down with an M3x10.
 Attach the other motor to arm 2.  Make sure the wire points towards the rear.  If you get it wrong, just remove the motor and flip 180 degrees.
 Oops, before you attach the motor attach the M4x10 machine screw.  It's easier to screw in if you do it before you attach the motor.
 Here's what Arm 2 looks like when it's completed.
 Be sure your ball bearing is in Arm 1, and wiggle Arm 2 into Arm 1.  Attach the motor.  Arm 2 should rotate smoothly in Arm 1.
 I had to replace the M4x10 machine screw with an M3X10 machine screw in order to fit in the ball bearing.  The extra mm of ball bearing width did not affect the fit.  I had to be extra careful tightening the M3 machine screw to make sure it was centered, since the hole was sized for an M4 machine screw.  So far it doesn't seem to have had a bad effect, but I'll get the proper sized ball bearing and swap things out if need be.
 Here's the entire unit assembled and hooked up.

(photo question: all these pictures were taken by the same iphone.  Is there a way I can force the white balance to be consistent?)
  I'm not sure why, but the roll is super-unbalanced.  I drilled a hole and screwed on some random pieces from the junk box to make it balance.  It would be great to figure out how to eliminate these weights.
 The sensor board fits nicely in the new revision of Arm 2.  I attached it with two M2x8 machine screws and locknuts.  I was concerned that there wasn't enough vibration protection because of the direct attachment, but I haven't noticed a problem yet.  If I do notice excess vibration, I'll try using some gyro tape instead of the screws.

I drilled a hole on Arm 3 to attach a zip tie for strain relief.  Move arms 1 and to into their farthest positions to see how much wire slack is necessary.

I downloaded and installed the latest BLG software for the Martinez board.  Be sure and unclick the "reverse Z axis" option, since the sensor is sitting component-side-up.  I'll follow up later with some notes on the latest BLG software.

Here's two videos showing the progress so far.  I've done a bit of tuning.  I think the tilt axis is good, but still need some work on the roll axis.  I'll get the RC tilt/roll control going as well.  More to come!

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