Tag Archives: CAD

Breaking Ground on the Cannon

My father’s health has improved a lot and I have a three-day weekend: it’s high time I started getting the new version of Cannon built. I have spent the last few days machining away and making small changes to my drawings. With the exception of the first side rail (on which I carelessly used a 1/4″ end mill instead of the 1/8″ end mill I told my GCode generator I would use), everything has gone surprisingly smoothly. There have been no bumps in the road.

Just kidding, this is combat robotics — where the probability of fire is about equal to the probability of success. Things actually have gone relatively smoothly. The biggest issue I have had during my time standing in my machine shop was that the power went out while I was making the top. Luckily, I was able to finish cutting the piece out with an X-Acto knife. X-Acto knives always seem to save the day.

As of writing (I am taking a break from working), I have a very nice looking pile of parts. I just need to cut the back out and drill 20 holes before I can wire up Cannon and take it for a drive. Before any kind of spinning takes place, I will need to create some pulleys on my lathe and make a belt. It looks like everything will somehow get done before Motorama, which is a big relief.

Here are some pictures of the build process so far:

This pile of parts has about a month to becoming a functioning, radio-controlled killer.

This pile of parts has about a month to becoming a functioning, radio-controlled killer.

The beginnings of a 3/8"-thick side rail for Cannon.

The beginnings of a 3/8″-thick side rail for Cannon.

The mill putting finishing touches on one of Cannon's rails.

The mill putting finishing touches on one of Cannon’s rails.

A finished side rail. It needs to be cleaned up a little more and get mounting holes drilled in it, but it is more or less complete.

A finished side rail. It needs to be cleaned up a little more and get mounting holes drilled in it, but it is more or less complete.

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?