Wednesday, January 18, 2012

Precision SMALL Prop Balancer

The biggest problem I've had with my PCB quadrotor has been mechanical vibration disturbing the rate gyros and causing the angle estimate to drift or be unstable. While the small and flexible frame exaggerates the vibrations, their source is imbalance of the propellers and, to a lesser extent, the motors themselves. So far, I've been balancing the propellers by spinning them on a small allen wrench and watching which way they prefer to settle, but this is obvious not ideal. The problem is, commercial prop balancers are useless for props this size:

Here's a commercial prop balancer from Hobby King, which has "very low friction bearings" on which a shaft with conical plugs rides. There are other types that use magnets to hold the shaft. They work pretty well for large props. This one can probably be useful for 8" or larger, and the magnetic one might be able to do 5" props. Smaller props weigh less and the friction in the bearings or at the shaft/magnet contact cannot be overcome by the slight imbalance of a 4" prop. My main beef with them is pretty simple:

For small props, they work better if you ditch the bearings altogether and use them as horizontal rails. (You could even argue that the same is true for any size prop...) Why not minimize the number of rolling contacts by using the shaft directly? There is much less friction this way. It's like somebody Google'd "prop balancer" and built something that looked like it should work without thinking about a simpler way to make the same thing. There are some commercial balancers that use simple rails, and no surprise, they tend to be for boat props and RC car wheels. If they work for such small things, they must work better for large props.

I know what you're thinking: How do you know the rails are level? The answer is the key to my new precision small prop balancer:

Those are two nice-but-cheap bubble levels from McMaster, P/N 2151A65. They are aluminum-and-plastic levels for $8.51 each. Add to that one precision-ground tool steel shaft, P/N 2900A222, and you have a precision small prop balancer for about $20. I use the prop adapters that come with these small 1.5mm-shaft motors to sandwich a prop on the shaft:

Then after finding a level surface, I place the shaft on the flat aluminum edges of the levels and let gravity do the rest:

It's almost that simple. There are a few subtleties:
  1. Make sure the level edges are clean and free of dust/dirt.

  2. Make sure the set screws are tightened evenly and facing perpendicular to the prop blades, so they don't contribute to the spanwise imbalance. (They will contribute to chordwise way around that.)

  3. Depending on the chordwise balance, you might get to a state where the prop seems bistable, so that it will settle on either side but never stay horizontal. If so, rotate 180º.
That's pretty much it. 4pcb's props were already pretty well-balanced, but after a bit of tweaking with the more precise balancer, it became even easier to fly. It takes off and hovers more smoothly and remains in one place for longer with no command inputs. Because of this, I took some time to learn a new trick: hand launching!

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