Tag Archives: design

A New Cannon

I’ve said it before and I’ll say it again: I was very pleased with Cannon’s second-place performance at Motorama. Still, its three fights against the eventual champion, Low Blow, highlighted some severe design flaws:

  • Exposed wheels. This is probably the single biggest flaw a robot can have.
  • The ball bearings pushing against the sides on big hits expand the beater holes.
  • There was no firewall; parts could be ejected from inside the robot into the blade.
  • Slow drive motors. 22.2:1s would have been a much better choice.

The scale was hardly Cannon’s friend, so figuring out how to add more material to the robot without adding any weight will be a challenge in and of itself. However, the first (and easiest) place to look is the wheel hubs. Cannon used aluminum “Dave’s Hubs” to hold its wheels on. Much lighter alternatives exist, such as the FingerTech Robotics Lite Hubs. Switching to the Lite Hubs will save over half an ounce off the next version of Cannon. That is more than enough to get started.

The next place I will look for weight will be the back of the robot. The old back weighed about 0.6oz and was made of 3/32″-thick garolite. Although stiff, garolite is about twice as dense as UHMW and is very brittle. The next Cannon will use a piece of (likely 1/16″-thick) UHMW that wraps around the back and protects the wheels. Kitbot creator Pete Smith gave me some UHMW bending tips at Motorama that should mean the clearance between the wheels and the UHMW is better on Cannon than it is on Coercion. Actually, I’m not sure Coercion actually has any clearance!

I put together a quick sketch of my thoughts for the next version of Cannon:

I am not a very talented artist.

I am not a very talented artist.

The new firewall will be bolted to the side rails, which will help to make the entire robot stiffer. I will make a groove in the sides so that the pieces fit together relatively snugly.

I also sketched the side rails (remarkably well). I think I might apply to art school if the whole top-ten school in the world thing doesn’t work out.

Note that the pockets are bigger and I have added holes for the firewall. Also note that the hole that will hold in the weapon bearing has been enlarged. This will allow me to put in a custom bushing, which will increase the surface area inside the hole and keep it from enlarging.

Note that the pockets are bigger and I have added holes for the firewall. Also note that the hole that will hold in the weapon bearing has been enlarged. This will allow me to put in a custom bushing, which will increase the surface area inside the hole and keep it from enlarging.

The next step will be to design the firewall and do more weight calculations. I’ll then need to order parts. Unfortunately (well…fortunately), I will be in Washington, D.C. from March 9th to the 16th, which isn’t conducive to robot design work, and over the next week, I’ll be busy getting ahead on schoolwork to get ready for my week off: not much will get done. Still, it feels good to finally put some of the ideas I have come up with on my blog.

Keep checking back–Bot Blast registration is up on the Builders’ Database. I’ll be taking Cannon, lolcat, maybe Coercion, and maybe Amatol if one of my friends wants to drive it.

Introducing Cannon

Continuing my alliterating-c tradition of robot names is Cannon, an antweight version of Lolcat. On paper (and in CAD), Cannon looks like it’s going to be a lot of fun.

Cannon CAD Drawing

Very rough, yes, but it still lets me visualize everything and make sure the parts fit.

Here are the specs I have planned out:

  • Weapon motor: Turnigy Aerodrive SK3 2826-1240. It is rated at 150W and will provide more than enough weapon power. The old “Beloved Shardy” undercutter I built simply did not have a powerful enough weapon motor, and this one will be sure to suffice!
  • Weapon ESC: Thunderbird-18. It’s tried-and-true, and the weapon motor is rated at 16A continuous so a fire seems unlikely right now. Unlikely.
  • Battery: Turnigy Nano-tech 3S 460mAh LiPoly. I love the idea of three cells, and in theory, a 460mAh can supply about 14 amps for an entire two-minute fight (throwing out any drops/inefficiencies). I think I will have bigger problems than the battery if the robot requires 14A continuous for two minutes!
  • Drive motors: Fingertech Silver Spark 33.3:1 gearmotors.
  • Drive ESCs: Fingertech tinyESC v2.1, the same kind used in Lolcat. I can’t say I’m wild about these controllers; I killed five or six of them before I learned that I’d discovered a hardware issue. They now seem to work okay, and I do have to say that Fingertech customer service is phenomenal!
  • Weapon: 3.0ish ounce 2.5-inch diameter titanium eggbeater spinning at about 9200rpm. It will be supported by ball bearings and connected to the weapon motor by a urethane belt with a gear ratio of 3:2. The weapon shaft will be hardened S7.

I already ordered some of the parts, and I was actually very impressed by how much higher-quality everything appears. Just look at the packaging.

Turnigy Product Packaging

Look at those beautiful graphics. It’s almost worth the huge premium over normal Turnigy products for the SK3…

Actually, just look at the motor and battery!

Aerodrive SK3 Motor

These motors are actually a little bigger than I expected but feel very solidly built.

460mAh Nano-tech Battery

Speaking of big, these batteries are HUGE for their capacity. Maybe I’m just used to dealing with Lolcat parts.

I do have some small quips with these parts.

  • Both the battery and motor are enormous. It’s a good thing I have minuscule drive ESCs because I may have a hard time getting everything to fit, and the body I designed for Cannon is fairly large.
  • The brushless magnets don’t feel very strong, though the motors feel very precise when you turn them. My feeling is that I am mistaken and when I actually wire the motor up, I’ll be in for a pleasant surprise.
  • The motor wires are small and flimsy.
  • The battery came with a JST connector on it. The battery is rated for 25C (11.5A) continuous, 40C (18.4A) burst. JST connectors are rated for 5A. That’s a waste of plastic.

Still, despite my minor complaints, I am happy with my purchase so far. I can’t wait to see that motor spin.

The motor isn’t the only part of Cannon I am excited about. I learned a lot about building eggbeaters well with Lolcat, and I believe most of it will apply in the antweight class as well. For one, protecting your belt is very important. Lolcat’s belt almost got sliced twice at Bot Blast, so I’ll try to get the side rails to cover up the pully. However, the bottom of the side rails is far more important. Tracked Terror kept getting underneath Lolcat and flipping it over. I worry that wedges could do the same with Cannon, but I have devised a fix of sorts.

Cannon Side View

Side view of the above image.

Notice that the sides are swept back. The beater extends way forward of the front of the supports. I am concerned about what effect this will have on stability. Since the beater, likely the heaviest individual part of the robot, will be centered in front of the support, the entire robot will act even more front-heavy. It could lead to some flipping over on hits; only time will tell. I am also concerned by how small the bottoms of the supports are, but they will be 3/8″-thick UHMW so antweight weapons shouldn’t be able to completely wreck them.

The only way we’ll see how this plays out is in the arena. I will continue to post progress updates on this blog as Cannon comes together.

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?