Drivetrain

No matter what anybody says, the most important part of your bot is your drivetrain, or the way your robot moves. If your robot doesn't move it's not particurally effective now is it? Motion is usually achieved in the Battlebox via electric motors and wheels, though several other approaches have been taken (hydraulic rotary actuators, walkers, gas engines, etc) but for now we'll focus on the electric motor and the common circular machine that we all know and love as the wheel.

The electric motor is a neat beast that can pack a considerably large amount of energy in a small space, can be purchased for fairly cheap and are quite reliable. They run on batteries which can be recharged. Not too shabby eh? Thus this is the most popular road taken. Since combat robotics is not a major industry you generally have to purchase motors from sources not intended for robotic combat. These motors usually come from electric hand drills, car fans, wheelchairs, large RC airplane propeller motors, and other oddball applications. Check the links section for places to find them. As a general rule of thumb, all motors must be DC current so that standard off-the-shelf electronic speed controllers can be used with them. AC motors are more difficult to work with and generally not good for Battlebots or similar competitions.

And now some handy calculations for Permanent Magnet DC motors!

It produces it's max torque at 0 rpm.
It produces no torque at it's max RPM.
The peak horsepower is reached at 50% of it's maximum RPM.
At it's peak horsepower it is no more than 50% efficient

Ohm's Law: V=I*R (Voltage = current * resistance). I for some reason means current, measured in Amps. If rearranged, we get I=V/R, so the amount of current that a motor can draw is equal to the number of volts divided by the number of ohms. Thus a motor with a resistance of 1 ohm at 24 volts can draw 24 amps of current. Handy, eh?

General Electric Power Equation: Volts x Amps = Watts. There are 746 watts in one horsepower, but remember your motor will not be 100% efficient no matter how great it is.
Now put the two above equations together and you'll notice something. When you double the voltage, you also double the amount of current a motor can draw and thus the magic happens and the motor becomes 4 times as powerful. At the same time the speed doubles and the torque doubles with it. Thus a 12 volt 1/4 horsepower motor could be 1 horsepower at 24 volts!

How powerful is my motor?
This might get a bit complicated. The calculation to find a motor's horsepower is as such:

Torque(in inchpounds)*rpm
63,025

Now to make sense of all that we'll use a popular motor for the higher weight classes, the Bosch GPA 750. According to it's datasheet, it produces 2.2 newtonmeters of torque at 3300 RPM. Now what does that mean? Well 1 newtonmeter is roughly equal to 8.86 inchpounds, so 2.2 * 8.86 gives us 19.492. It produces this torque at 3300 rpm, so we multiply 19.492 times 3300 and get 64323.6. Divide by 63025 and we get 1.02 horsepower at 3300 rpm. Nice and powerful! This of course is to calculate the horsepower at a given speed. To get a "ballpark" figure for how powerful your motor is it goes something like this:

Stall torque (inchpounds) * No load RPM
250,000


How efficient is my motor?
For this problem we add one more step to the above. This is all done by HP out/HP in. Now let's get exact. Sticking to the above specs, we get exactly 1.020 horsepower out. At that power, the motor draws 40 amps at 24 volts. 40 * 24 is 960. There are 746 watts in one horsepower so it draws 960/746 horsepower, or 1.286 horsepower. 1.020 / 1.286 gives us 0.79 * 100% is 79% efficient. Very good! Any motor over 60% is generally a good buy.

How fast will this thing go?
Good question! Now we have to do a bit of circular math, so grab yourself a slice of the pi and we'll be on our way. We'll keep with the Bosch GPA 750 for simplicity. With no load the Bosch GPA 750 spins at 4100 rpm, which is far too fast for practical use, plus it doesn't put out enough torque so it must be geared down. The general rule of thumb for the GPA is to reduce by the number of inches tall your wheel is, so if you have a 10 inch wheel, you should use a 10:1 reduction. 4100 with a 10:1 reduction gives us a wheel that spins at 410 rpm. Now we have to figure out all the scary stuff with the wheel. The circumference (distance around) of a circle is defined as D*pi, or diameter x pi. The diameter of our wheel is 10 inches and pi is usuall expressed as 3.14. 10*3.14 is 31.4 inches around. Now convert to feet with 31.4/12 because there are 12 inches in a foot. This gives us 2.62 roughly. Now multiply by the RPM (410) which gives us 1074.2 feet per minute. Now figure feet per hour. There are 60 minutes in an hour, so 1074.2 * 60 gives us 64452 feet per hour. There are 5280 feet in a mile, so divide by 5280. 64452/5280= 12.2 mph, quite zippy. Now that's a lot, so here's it all together:
RPM*(((D*3.14)/12)*60)/5280)=MPH. To be fair, it's much easier to do (RPM x Diameter)/336. This is much easier and as accurate as you're going to need it.

How much can I push?
Note, previous information here was proven to be false. here is an updated version
The amount of raw linear force you can exert is defined as axle torque/wheel radius. If you use an EV warrior (50 inchpounds of torque at the motor) geared down 6:1 that gives you 6*50 inchpounds, which is 300 inchpounds of torque at your axle. Assuming a 6 inch wheel, that's a radius of 3 inches. 300inchpounds/3 inches = 100 pounds of linear force per motor. 2 motors makes 200 pounds of linear force. An object can only exert at ABSOLUTE MAXIMUM its weight in linear force before the wheels spin. The amount of actual linear force that can be exerted by any given object is defined by the downward force on the wheels x coefficient of friction. In the battlebox you will likely not see a Coefficient of Friction higher than 0.8, so a 120 pound robot can only exert about 96 pounds of linear force on any given object. This can be gotten around by having magnets that increase the downward force on the wheels (as seen by lightweight General Gau) but that is tricky to implement. It is also generally accepted that your motors should be able to exert a total of 1.5-2x your robots weight in linear force, so 200 pounds of linear force is about right on a 120 pound robot. This ensures adequate acceleration under the load of another robot. Most robots have well over their weight in theorhetical linear force so to be a successful pushbot you need to have as much of your weight as possible on powered wheels. That means four wheel drive. You also need as high of a friction coefficient as possible so you need to get the grippiest tires you can find. Team Whyachi makes some of the grippiest tires you can buy. They are a bit pricey but from what I've seen of them they are worth every penny. If all else fails you can always use Colsons, which are fairly grippy.

Handy, eh?

But how does this all work into your robot physically? How can a reduction be DONE? Well look no farther than this handy dandy chart you see below you now. Yeeeha! Note: To calculate multiple gear ratios, figure each ratio by itself then multiply the to together. So if you have a 3:1 reduction that goes to a 5:1 reduction, total you get a 15:1 reduction. By definition a reduction means that the input must rotate x amount of times for the ouput to rotate y. 15:1 means the motor rotates 15 times for the output to rotate once. Speed and force are inversely proportional so as the RPM goes down, torque goes up proportionately.
Method Advantages Disadvantages Additional notes
Chain and sprockets Generally easy to implement, does not slip easily, sprockets don't strip easily, widely available, scalable, chain can span long distances. Difficult to get high reductions in a small space, needs to have proper tension on the chain, chain may fall off sprocket if misaligned or impacted. This is generally the best place to start for a gear reduction. it's easy, it's scalable, and it's available at decent prices from many different manufacturers and distributors. Gear ratio = output teeth / input teeth.
Spur Gears High reductions and multiple stages can be implemented in a comparitively small space, can be very light and compact, highly efficient, many different kinds of gears available, strong. High precision needed to properly align gears, can be very expensive. This is what most people think of when they think of "gears." Two circular gears mesh with teeth around the circumferences. The shafts on all gears are parallel. Gear ratio = output teeth / input teeth.
Bevel/miter gears Allows for a reduction or power transmission to occur at a 90 degree angle. It also allows many low-profile motors to lay flat in a low robot and their mechanical power directed upwards for a spinning blade (as done by Hazard). Require EXTREMELY high precision, quite noisy at high speeds, very expensive, does not lend itself well to multi-stage reduction. This is a good way to fit a narrow, long motor into a small space: drive the wheel at a 90-degree angle to the motor. Also good for weapons. Gear ratio = output teeth / input teeth.
Worm Gears Allows for a very high gear reduction to be done in a small space, output at a right-angle to input. Requires two different kinds of gears for a single reduction, expensive, comparitively inefficient at high loads. A quick and easy way to do a large reduction in a small space for drive. Gear ratio = output teeth #, so a 10 tooth gear with a worm gear on it is a 10:1 reduction. Easy peasy.
V-belts and pulleys Dirt cheap and easily available. A v-belt pulley can even be made with a simple lathe. Cannot transfer very high torque loads without slipping, require frequent adjustments, belt length cannot be adjusted like chain can. V-belts are best suited for powering spinning weapons because it will act like a clutch and softening impacts to a spinning motor and may prevent the motor from stallin.g
Timing Pulleys Can transfer more torque than a v-belt, lighter than chain and sprockets, soft belt won't strip pulleys so they can be of low-grade aluminum, can handle some misalignment. Wider than a chain-sprocket setup, can slip in very high torque applications, more expensive than chain. This can be used for drive or for a weapon setup. Cool stuff and is often accepted as superior to chains. Gear ratio = Output teeth / input teeth.