Tag Archives: weapon

Coercing More Effectively

Coercion’s blade needs a lot of work – I think the video of Ripto bending it 30 or so degrees proved that. The question is how to change it. I know Coercion is a good bot, but I have noticed some continual flaws. First of all, the blade is too heavy. It spins up too slowly and it takes way too much work to get up to speed (thanks to losses all over the place), so the weapon motor runs hot and pulls something in the 30A neighborhood. I thought I had more or less fixed the problem before Bot Blast, but running my robot on 7.4V made it far more sluggish than I anticipated and I still wasn’t getting ideal blade performance. The blade is also so heavy that it tends to flip over its nose, which provided a loss at Boy Blast. I have decided to apply some physics to make Coercion more hard-hitting and hopefully more efficient than ever.

Erotational = Iw^2/2

Where omega (henceforth referred to as w) is angular velocity, E is kinetic energy, and I is moment of inertia.

Awesome, so based on that, we can do one of two things to increase the kinetic energy of the weapon:

  1. Increase the angular velocity (how fast the weapon is spinning). This will be the more effective of the two methods because it is a squared term, so if we double the angular velocity, we quadruple the kinetic energy.
  2. Increase the moment of inertia. MOI is the sum of the masses of the particles that make up an object multiplied by the radii from the center of rotation squared. In other words, the heavier something is, the greater its MOI, and the greater something’s diameter is, the greater its MOI.

So there we have it. To make Coercion’s blade work better, we make it heavier and spin it faster. Great physics lesson, Einstein. The problem here is that to make weight for better wheel guards, a better weapon motor, new batteries and the other improvements I have planned for Coercion, I need to make the blade lighter. Remember that MOI is also based on the distance the particles of an object are from its center of rotation. Thus, if I can concentrate the mass as far away from the center as possible, I can “cheat” (not really) a little bit: the blade will hit harder while not weighing in any more.

My target weight of the blade was 11ish ounces, which is about a big reduction over the old blade. I had to make the inside as skinny as possible while making the outside part as wide as possible. I kept the same blade dimensions – 11.75″x1.50″x0.19″ – and I plan to have this blade waterjetted out of S7 and then hardened (I learned my lesson at Bot Blast!).

New Coercion Blade

10.35oz, far lighter than the old, nearly-one-pound blade.

The plan is to spin this blade at 10,000rpm with a 3S LiPoly and a new weapon motor in the 1500-1900kv range (and a new, 2:1 weapon gear ratio). If I recall correctly, Coercion’s blade was spinning at about 7000rpm at Bot Blast, and according to the MOI differences I saw in Solidworks (the new one has about a 22% lower MOI than the old one), the new blade will actually store about 50% more energy. Not bad for saving three or four ounces, huh?