How physics works

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“Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed thereon.” — Isaac Newton

Simply put, if you want something to move, you’ll need another force to act on it.  If you want something to stop moving, you’ll need another force to act against it.

In something like a bicycle this is easy. Legs create a linear force by pushing downward on the pedals using the gravitational force of the rider’s body which is then transferred by chain to the wheels. The wheels grip the road (friction caused by gravity acting on the spandex road troll) and the bicycle moves forward.

A car is slightly more complex. We replace the leg with a piston and gravitational force previously attributed to the body with a controlled explosion in the engine cylinders. That linear movement is translated to rotational motion (crankshaft) and geared properly before being transferred to the wheels. The wheels grip the road (friction caused by gravity acting on the vehicle) and the car moves forward.

In both of these oversimplified examples, there are only a few forces that really matter at the scale we’re talking about (BB-8 scale that is). We can throw wind speed and aero dynamics out the window since he’s not winning any derbies.  We really only need to keep track of a few forces. Namely gravity, friction and inertia. Since we’ll be discussing going from one state of INERTIA to another, we only need to add GRAVITY and FRICTION (and this lamp). As a point of order, I reserve the right to make up additional forces as we go.  That’s the law according to the rules.

Onward to Spherical Stability:

Weebles demonstrate part of this discussion very practically.  For simple spherical stability, you need a low center of gravity (I’m purposely ignoring gyroscopic stability since it would not be considered simple).  We’ll refer to this weight as a ballast. As you adjust the center of mass to the outside of the sphere (lower the center of gravity) you increase stability. Weebles (and weight driven spheres) have very low centers of gravity. You can increase stability even further by increasing the weight ratio of ballast to total weight.

Spherical motion, in many ways is far simpler than the bike or vehicle explanations. You can apply motion to the sphere by moving the ballast to another location in the sphere. Done. By moving the ballast, you change the sphere’s center of gravity. Then the sphere rolls to orient the ballast to it’s lowest point. How you move that ballast is up to the designer. The point is, you are always applying force somewhere in order to change the position of the ballast.

Hamster in a ball

The literal hamster in a literal ball works because the hamsters weight offsets the sphere center of gravity. When the hamster walks (or runs, trips, slides, rolls) ‘up’ one of the sides, its weight changes the spheres center of gravity and the balls rolls. Replace the hamster with a small robot and you have Hamster drive.

Axel drive

If you connect two opposing poles of the sphere via a shaft (or axel) and hang a ballast from it, you have effectively also lowered the center of gravity. By attaching motors to the shaft supporting the ballast, you can ‘move’ the center of gravity and thus move the sphere.

Turning can be achieved by tilting the ballast left and right perpendicular to the rotation force, parallel to the axel. Then the sphere will lean. You can spin the sphere by quickly rotating the ballast. Since the ballast outweighs the sphere, the sphere inertia is overcome before the ballasts inertia.

Omni Vector Drive

My solution (for now) is to attach the ballast to the sphere via omni wheels. The weight of the ballast gives the wheels enough friction to reposition the ballast and thus drive the sphere.

I’ll leave you with two pictures and a gif. I’ll highlight more of my design in another post.

Omni vector bot inside sphere
The ballast under the bot is 10 lbs

Here you can see the wheels rotating to correct the ballast position. This is me rotating the sphere and watching the wheels correct, but in reality the reverse will be happening.

Unpowered Movement


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