Calculate Flywheel/Counterweight Weight

[Khurtana]

Hello People

I am looking for some assistance for a robotics project I am working on with the BB-8 builders club. We are working to replicate the BB-8 droid from the new Star Wars film (geeky I know...). If this is not the place for this post, please let me know and/or point me in the right direction. Thanks.

We are looking for a reliable method of calculating the counterweight inside the BB-8 droid. Currently we use trial and error to get a weight for the counterweight which is not conducive to accuracy or stability. 100% accuracy is probably not possible, however a starting value that we could work with would be very useful.

The BB-8 droid is an axel-driven robot with a counterweight providing stability and gyro-driven turning. See picture attached for the mechanism.

For given values, what would an equation look like to derive the weight?

Diameter of sphere - 500mm
Speed of roll - maximum of 10km/hr (2.8m/s approx)
Weight of sphere, including axle but excluding counterweight/flywheel - 8-15kgs (variation depends on sphere design, cage vs Polycarbonate sphere).
Length of counterweight arm - 150-200mm

The flywheel is also used to rotate the sphere to allow turning, however the speed of the motor (and torque chosen) is tuned so we are not looking for help there.

If the counterweight is too light, the reduction in motor power (to slow and change direction) results in the entire mechanism doing loops inside the ball as it starts to slow down. If it is too heavy, the sphere takes time to start and stop and consumes battery power very quickly.

My thoughts as it stands is that the counterweight should be equal to the weight of the sphere and components, although I would like some science behind it.

Thanks

Khurtana.

 

[3DDave]

You should post on the BB-8 forum - just kidding. Seems like a good enough place to ask.

The main thing is to get the weight as low as possible.

I suggest casting a set of them in lead with different thicknesses and see which one gets the best handling, then compare the weight on either side to see which direction might provide even better performance. Or build a box fill it with lead shot and then start taking shot out until performance degrades.

The problem with the current description is that it really depends on the weight distribution, the moment of inertia, of the sphere and parts.

 

[Khurtana]

Thanks for your thoughts.

The thing is, we have many, many BB-8s in progress and other than the common values above, they are all different to a degree (some use internal 3D printed skeletons which weigh 3kgs and some use balls that weigh 6 kgs and are made of polycarbonate. Some use plywood framing, some use sheet metal framing etc).

What we are after ideally is an equation, or failing that thoughts on minimum weights estimated by more experienced mechanical engineers than I to give our members a starting weight.

To answer some of your questions, weight distribution of the sphere with the axle fitted is along the linear axis. Naturally the batteries and weights would lower the CoG, however that can be factored as part of the weight equation.

 

[MintJulep]

This thread would probably be best in forum1528

 

[chicopee]

Buy one and reverse engineering. That should remove a lot of guess work. Of course, there could be some patent infringement but if you are in China, that should not be a problem for local markets.