Ask Aaron: questions and answers about combat robots

It would help to know a bit about the robot you're building. Different designs, different builders, different shop tools, and differing experience levels all call for differing timetables -- but I can give some general advice.

Delays in receiving parts or receiving faulty parts can scuttle your schedule. Order your parts as soon as possible and test them on arrival.
It's a good idea to have one or more experienced builders take a look at a sketch of your design and suggest changes based on their experience. There are several online forums suited for this purpose -- as well as 'Ask Aaron'.
If this is a school project the CAD may be a requirement, but many combat robots have been and continue to be fabricated without Computer Aided Design. You can gather your parts and use "Cardboard Aided Design" to construct a mockup of a robot and make sure everything fits before commiting to chassis dimensions and layout. See FAQ #7 and the photo of a cardboard chassis mockup below.
Simple robots win. A chassis made from blocks of UHMW polyethyene held together with long wood screws is simple, durable, and easy to construct. Add top and bottom covers and you have a viable chassis. Below is a photo of "Cloud of Suspicion" with the top cover removed.
Schedule time between completion of the robot and the event to become familiar with operating the robot and finding things that need to be changed. About a week is not too much time for this. Spend some of that time adjusting the settings on your R/C transmitter for improved control. See: Transmitter Tweeks for Better Driving Control

A: Mark J. Yes, it is entirely true. The girl in the mouse suit had little to do with it, but the yam (it may have been a sweet potato) and the nails were critically important.

I prefer to not put the story in print as it is best told with a great many gestures, sound effects, and exagerated facial expressions. As I said in the original post, "...you'll need to buy me a beer to hear that story."

I have been pondering the usefulness of tangential drive for antweight (1 lb) applications, but have concerns regarding its compatibility with foam wheels. In particular, the Repeat Robotics Repeat Tangent motors seem to be effective on the likes of robots like "Super Space Turtle", but I have only seen them used with tires made via custom-molded polyurethane/rubber. Could it be that foam wheels are too "squishy" to create any meaningful contact points with the shaft or simply have a poor friction coefficient (if the latter, could this be mitigated using liquid latex/some other traction coating)? This seemed like a "too good to be true" lightweight 4WD solution. Any feedback is greatly appreciated. [Zanesville, Ohio]

A: Mark J. You are entirely correct to worry about this, Zanesville. Tangential drive (YouTube video) and squishy foam tires do not mix. The transfer of power from the drive shaft to the tire is the product of the coefficient of friction and the force by which the shaft is pressed against the tire. Power in excess of this power limit results in slippage, and getting enough force against a foam tire will depress the shaft so deeply into the tire that drive force vectors would no longer be 'tangential' and power transfer efficiency plummets -- with or without a traction coating on the tire.

I was thinking n20s/30s (but I could totally be wrong) but all the ones I can find that actually have specs have a mere 8.6mm long shaft. Is there a good way to use those short shafts in a wheel that is nearly double the width? I see people all over using them with little to no issues but every time I have tried to it has not gone well because the shaft was wobbly because it was just not long enough. [Purdue, University]

A: Mark J. Good quality N20 gearmotors would have ample power for your control ant, but the gearboxes are potato chip fragile. In the 150 gram class they're fine, but even with wheel protection they don't survive well in full combat one-pound ants. For full combat I can't recommend anything without a stout planetary gearbox -- something like the Repeat Robotics Brushed Mk2. They are larger and heavier than N20s, but two would be enough.

If you're building for the plastic ant class you could certainly give N20s a shot. If you do a web search for N20 long shaft you'll find multiple choices for both threaded and smooth shafts in assorted lengths. The pictured example (fingers for scale) from eBay has a 23mm long shaft. Cheap eBay N20s typically have low-power motors, but you can swap in the high-power carbon brush robot motors if needed.

I must note that it is unusual to first pick your wheels and then design the drivetrain around them.

A: Mark J. I'm curious about the weight class of your robot. At 6.56 ounces the BadAss 2825 1360 Kv motor is too small for a hobbyweight, but if you're building a beetle you'd do better with a smaller motor and a heavier beater.

The Spinner Weapon Kinetic Energy Calculator lets you assemble and model a symmetric design from the geometric shapes available in the calculator, but an asymmetric design requires specialized center of mass and rotational inertia functions that the calculator cannot perform -- you'll need a CAD program for that. When you have your asymmetric weapon design, here's how to enter it into the calculator for modeling:

The CAD program you use to design your asymmetric beater bar will be able to provide the weapon Moment of Inertia .
Enter that value into the Asymmetric: MOI field in the Weapon Elements section of the calculator and leave the other fields blank.
In the Brushless Weapon Motor section enter your 'BadAss' motor data:

Kv: 1360
Ri: 13 mΩ
Volts: 21.6
Reduction: 2 to 1

Click Calculate .

A: Mark J. Yea, and I want a deep dish pizza -- but I won't write up a synopsis of the eleventh and twelfth weeks of Mechanical Engineering 230 to get one. I don't suppose you'd like to research by hand as well?

The problem with simply handing out equations is that you need background in the subject to understand when and how to apply them. Then you'll find out that you require additional equations to supply the inputs to the first set of equations, and then if your weapon is a complex shape you throw out the equations and integrate r²dm over the entire object. How good is your calculus?

Moment of Inertia for your weapon plugs into the kinetic energy equation below. Here is a list of equations to obtain the the moment of inertia for basic geometric shapes. If your weapon can be represented as a combination of these shapes, calculate the MoI for each part and add them up: ~~Moment of Inertia Formulas~~. Wait, those formulas leave out mass -- This link is better.
RPM is the speed constant for your motor (K_v) times the applied voltage divided by the reduction ratio of your weapon drive. This approximation does not consider aerodynamic drag, which increases with the cube of speed. Super high RPM will simply not be attained regardless of what this approximation says.
Kinetic Energy = 1/2 × I × ω² where "I" is the Moment of Inertia of the weapon and "ω" is the weapon's angular velocity expressed in radians per second. Radians per second = RPM × 2π/60.
Spin-up Time is a calculus thing that I'm not gonna try to walk you thru here -- but there is a handy shortcut that relies on the relationship between joules and average watt-seconds of motor output. I've got a whole webpage that outlines that process with plenty of equations: Estimating Weapon Spin-up Time.

A: Mark J. It's very common (and often advantageous) for the base of a 4-bar lifter to set in a plane angled relative to the plane of the top bar. The image below shows the layout of the 4-bar lifter for our beetleweight 'Zpatula' as modeled by the Team Run Amok 4-Bar Lifter Spreadsheet. Note that the rear pivot on the base frame sets well above the front pivot; it is typically the rear pivot that is elevated.

I suggest that you download the lifter spreadsheet (requires Microsoft Excel) and use it to evaluate designs with differing element lengths and pivot positions. It takes a while to find a design that combines the desired lifter motion with a consistent motor torque requirement. Have fun.

Questions and Answers about Combat Robotics from Team Run Amok.