Evil Robotics

Battery Type

Batteries come in many different types and shapes.  The challenge is to have enough power for the duration of the match and not have so much that it is overly heavy or too much capacity for the match run time.  

The batteries must also be accepted in the competition rules.  Check the specific contest.  I started out using gel-cells but they were too big and heavy.  A better choice is to use Odyssey absorbed-glass-mat (AGM) batteries from Hawker Energy.  Another choice would be to solder up your own NiCd bundles.  This may allow you some adjustability as to size and shape.  

Some builders even use nickel metal hydride (NiMH).  NiMH batteries offer more power for the weight without the hassle of discharging memory.  However there are some drawbacks such as price and special charging requirements.  Both NiCd and NiMH batteries are 1.2 volts per cell.  Basic or alkaline batteries are 1.5 volts per cell.

To calculate if you have enough battery power you need to calculate amp hours.  The ampere-hour (Ah) rating defines the rated storage capacity of the battery before the voltage drops to a standard  discharged value such as 10 volts for a 12 volt battery.  For example the Hawker PC 680 (one of the best batteries available for robotics) is rated at 16 Ah  for 10 hours.  That means it can supply 1.6 amps for 10 hours.  

1.6 amps x 10 hours = 16Ah.  

This same battery is only rated at 7.5 Ah at 5 minutes.  That means it can supply 90 amps for .0833 hours. (5 minutes = .0833 hours)

90 amps x .0833 hours = 7.5 Ah

I know 90 amps sound like a lot, but it may not be enough if you are trying to run a 
4.5 horsepower weapons motor (such as a 3" diameter MagMotor) continuously under load.  For a  MagMotor detailed discussion see robotbooks.com.

For a detailed battery comparison see Battery recommendations by Team Tornado or Battery types by Paul Hills

Horse Power

    Here you want the most horsepower you can get with the least amount of weight.  Another approach to solving the how much power is enough question is to look at what other builders use and figure out ways to pack in more horsepower.  Also don't forget that with the higher voltage motors you also have to include the battery weight in any horsepower per weight comparison.  Now on to an example.

Question; How many Hp will it take to move a 220 lb mass from 0 to 10.0 mph (14.66 ft/s) in 0.5 seconds?

m = 220 lb,    v = 14.66 ft/s,    t = 0.5 s

Answer;  Since Hp is an English unit I did not show any metric units in the solution.

This makes a big assumption that the motor gear ratios are set properly for the maximum velocity.  Lower gear ratios will increase the torque to the wheels (better in a pushing match) and shorten the time to reach the maximum velocity, but the maximum velocity will be lower.  Higher gear ratios will increase the top end velocity but sacrifice acceleration time and pushing power.

To convert these calculations to include rotation such as a spinning shell you need to use torque and moment of inertia (MOI) in the equations.  For a detailed discussion see Paul Hill's section on spinning weapons.

Materials

The real challenge is to apply that destruction power to your opponent without destroying yourself.  That means strong light materials used in a design that maximizes the strength and durability.  

Mild steel is really to heavy and usually not strong enough.  Chrome-Molly (4140 steel) would be a better choice because it is about 50% stronger than mild steel with basically the same density. 

Stainless Steel is also about 50% stronger than mild steel but some types (300 series) can work harden.  That means that it can heat up during milling or drilling and once it has been work hardened you will smoke your cutting tools.  This can be a good thing in battle but a bad thing during fabrication of the robot.  One nice advantage of stainless steel is that it is very rust resistant.

Aluminum is far lighter than mild steel, about a third the density, but generally more difficult to weld.  6061 aluminum is readily available and weldable by a skilled welder.  Many types of aluminum are not weldable. Check with your Lincoln Welder representative as to weld ability.  One advantage to aluminum is that cutting it doesn't throw a shower of sparks.  In battle that could help you in a close judges decision.   

Titanium is the robot builders preferred material.  Titanium is very light and strong. Think of titanium as 50% heavier than 6061 Aluminum and significantly stronger than steel, but may be out of your price range.  In my quest for titanium I have paid from $10 to $30 a pound.  One disadvantage of using titanium tubing is that it is not readily available in square or rectangular hollow shapes.  

See Materials by Paul Hills chart for some data that may help you in your design.  

Drive Speed

I'll handle this with an example calculation.

If a 3” diameter wheel is rotated 400 rpm assuming no wheel slippage, what is the top end speed?

Common  Sense

Durability can also be viewed as ease of maintenance.  A well thought out design allows access to all fasteners, interchangeable parts, quick battery replacement and easy access to battery charging terminals.  Don't design so that many components need to be removed just to access one buried component.  In a 20 minute maintenance pit stop you don't have time for extra work.  Do like I say, not as I do.  I have learned this one the hard way.

To survive the harsh shock of battle, use hot glue to hold your radio wire connections to the receiver and to the speed controllers.  Hot glue should also be used to hold the radio frequency crystals in place.  Another tip is to coat all exposed electrical contacts with liquid electrical tape.   Both of these tips were ideas from the very helpful makers of Ankle Biter (Odin Robotics).

Other

I think all good designers have their eyes and ears open.  What I am saying is be open to new ideas.  Notice what other builders are doing.  Look at how nature has provided similar solutions.  Let me give some examples.  The helicopter was developed after extensive study into the mechanics of a fly.  Velcro was an industrial attempt to simulate a cock-a-burr.  Some of the best inventions were due to mistakes.  Plastics were originally a chemical mix up.  Lead was first added to gasoline because of it's red color.  To be truly innovative you have to try something new.

To see more design tips from other sites go to the cool links design information page.