Make sure you check out S.L.A.M's robotics links for lots of part suppliers and stuff.
Operators used in explanations are..
+ means plus
- means minus
* means times
/ means divide
Pi = 3.14 (This is Our good friend Pi)
Wheel Diameter * Pi /12 = feet per Revolution
FtRev * RPM = FtMin
FtMin * 60 min = FtHr
FtHr / 5280 ft = Miles per hour.
Wheel diameter 10.5" * Pi = 32.97 / 12 = 2.7475 FtRev
Rpm = 350 * FtRev = FtMin = 961.625
FtMin * 60 = FtHr = 57697.5
FtHr divided by 5280 = 10.92 Miles per Hr.
The quick way to calculate the robots speed in miles
Tire diameter * Axle shaft speed / 336
Tire diameter = 10.5 inches.
Axle shaft speed = 350 RPM's
10.5 * 350 = 3675 / 336 = 10.9375 MPH
Power(WATTS) = Current(AMPS) * Voltage(VOLTS)
Power(HP) = Power(WATTS) / 746
Take Watts times .6 (60% efficiency) before you convert it to Horsepower.
1. You must decide on what type of battery that
you will be using. Sealed lead acid (SLA), Nickel cadmium or Metal nickel
Each type of battery have different weights per Amp Hr. rating so this can effect your choices. Sealed lead acid types are the least efficient and the heavyset of the three types. They are also the cheapest in cost. Nickel Cadmium are a lot lighter than Sealed Lead Acid and are a lot more efficient and they can be recharged several times faster. Of course the are a lot more expensive. Metal Nickel Hydride battery are even more efficient than Nickel Cadmium and of course a LOT more expensive. For weight saving reason plus quick recharge capability. The Nickel Cadmium are becoming quite popular. Especially in the smaller weight classes.
2. You need to know the TOTAL current requirement
for your drive system and weapon systems.
You will have to know the Stall current and the Nominal current draw of ALL of you motors.
For the most part your motors will not be drawing their Stall current for an entire match. Yet they may see brief periods at that high current draw. So you have to make a guess at what the current draw will be by taking an average current draw some where between the Stall current and the nominal value.
3. Battery's are rated by a scale called Amp Hour rating. In theory this rating is the maximum current the battery can supply for one hour in a perfect world. Unfortunately battery's have internal resistance which reduces their efficiency. So different types of battery's are more efficient than others this also varies between the same type of battery's from different manufactures. So of course this complicates your battery choice's a little.
Any ways here goes the math part. I'll try and keep it
Motors used 2 Bosch 750 Watt. 24 VDC
Single motor nominal draw 60 amps. Stall current draw 180 amps.
Total of both motors current draw at Stall 360 Amps
Guestimate of average current draw 90 amps per motor total of 180 amps.
Fortunately we only have to run for a maximum of 5 minutes in a rumble and 3 minutes in a normal match.
So we take the 180 amp draw for 60 minutes and divide it by 12 to find the battery size required.
for a 5 minute rumble and divide by 20 for a 3 minute match.
180/12 = 15 amp hrs for a 5 minute rumble
180/20 = 9 amp hrs for a 3 minute match
Now we have to figure in the battery's efficiency
For Sealed Lead Acid type multiply by .4 to .6 they are only about 40-60 percent efficient
For Hawker battery's multiply by .6 they are about 60 percent efficient.
For Nicad batterys multiply by .9 they are about 90 percent efficient
Lets take a standard sealed lead acid battery with a rating of 17 amp hours
17 Amp hr * .6 = 10.2 amp hr actual capacity this will be fine for a 3 min match and pretty close for a rumble under a light load. Your mileage and power will vary due to gearing but you should have good power for the first 3 mins and then it will drop off.
Lets take a Nicad battery with a 17 amp hr rating
17 amp hr * .9 = 15 amp hr actual capasity as you can see you will have full power for an entire rumble.
And of course you would need two 12 volt battery's with at least this Amp Hr. rating in series to produce 24 VDC.
By the way Sealed Lead Acid battery's weigh around .9 to 1 lb per Amp Hr. that they are rated in. So a 14 Amp Hr. battery could weigh around 14 lbs.
Another thing to consider when choosing you battery size is how much load you will apply on your motors. This is why it is a good idea to use as low of a drive axle gear ratio as possible because lower gearing reduces the time that the motors will be at stall. If you use high gearing for more speed you will have to use a higher capacity set of battery's.
To convert ft-lbs to in-lbs multiply by 12.
To convert in-lbs to oz-in multiply by 16.
1 ft-lb = 12 in-lbs = 192 oz-in.
So to get motor torque in ft/lbs divide oz/in rating by 192.
Some more info on calculating drive train power requirements from good old Team K.I.SS.
Wheels and drive trains.
Kart World. A great source for Wheels and Tires, Sprockets, Keyed Axles, Chains Engines and Centrifugal Clutches and Torque converters. You should order their Catalog. The Catalog also has lots of good tips for gearing and chain requirements. They also sell gokart racing chain which is the strong stuff to use. Order Line: 1- 440-357-5569
Tool a Good source of low cost hydraulics, valves,
pumps, cylinders. They also sell GoKart parts/ Engines and are great source
for wheels and Keyed axle shafts.
You can build a robot using this catalog. A beefy one.
Order Line 1-800-533-5545
GraingerA great one stop source for many of your drive train needs. Chains sprockets, gears , bearings ect..
Kaman industrial technologies. Supplier of most of your motion control components. Belts pulleys chains sprockets and much much more.
Turk Bros. Racing, Inc.
450 South Taft
Mason City, IA 50401 U.S.A.
A great source for high performance go kart tires wheels and IC engines. Get their catalog..
Choosing an axle shaft size.
Your drive train is the most important part of your robot. Regardless of the weapon that you plan to use if your robot can't move well and also survive battle damage You LOSE.
So basically your choice in axle shaft size and the type of bearings that support it can be one of the most critical choices that you will have to make.
The way I think bigger is better. But then again using too large of an axle shaft causes the drive train to be very heavy. In my current Super heavyweight design I'm using four 1" axle shafts to support the wheels. Both sides of the wheels are supported with a pillow block bearing. So they are not light. In my opinion I think that the following sizes are correct for a 4 axle robot in each weight class.
1. Lightweight 1/2" steel shaft.
2. Middleweight 1/2"-5/8" steel shaft.
3. Heavyweight 3/4"-1" steel shaft.
4. Super heavyweight 1"-1 1/4" steel shaft.
Now of course if you have over hung or exposed unprotected wheels. You should use a larger shaft to protect against impacts.
Contactors and Solenoids
Somtimes it is neccessary to control a motor or other electronic device that draws a lot of current. They most reliable way to do this is by using a Large solenoid or contactor. Heres a few brands that have been used successfully by many robotic teams.
of large contactors in many voltage and current ranges.The 586 series is
very popular with many builders. These puppys can be purchased at Graingers
Curtis Albrite Manufacture of contactors and solenoids in many voltage and current ranges. The duel contactors they produce make building a HIGH current H-Bridge an easy task.
Wood: I've seen wood use for bumpers or ablative armor and it actually has some good properties. It absorbs impacts well and its light. Never under estimate the power of a well placed 2"x 4"
Steel: is very strong it can be cut easily with a torch and its easy to weld but it is very heavy. For the strength its probably the cheapest metal you could use.
Case hardened Steel: This is steel that has been surfaced hardened it has good property's for axle shafts and clevis pins for hammer pivots and stuff like that. The hardness doesn't go all the way through the metal so it is not as brittle as say a high carbon steel shaft.
Carbide Steel: This is usually a surface coating applied to things like saw blades and drill bits. Even though it's very hard it can be very brittle.
Hard facing welding rod: This is a cool way to make a hard surface on a piece of steel say for blade or spike type weapons. This is a very common technique that's used on heavy equipment on like the bucket on back hoes and the blades on bull dozers to prevent wear and strengthen a cutting edge.
Aluminum: Is about half as strong as a similar piece of steel but weighs half as much. It is sorta easily cut with a band saw or jigsaw It usually costs more than steel and requires special welding equipment such as Heli-arc.
T6 Aluminum: This is an alloy that is much stronger and has a harder surface than (mild) Aluminum. Its a little bit harder to cut but it weighs the same and welds the same. This is some cool stuff.
Poor mans Aluminum test. This takes a little practice
but it works for me. I test Aluminum in the salvage yard with a piece of
Steel by doing a scratch test. You can compare different grades of aluminum
by scratching them and watching how deep the scratch penetrates the metal.
Pure untempered Aluminum is easy to scratch deeply. While hardened Aluminum like T6 wont scratch as deeply or with the same amount of pressure. You have to try this on different grades to get a feel for it. But it does work for me.
Another poor mans test I like to perform is the ball peen hammer test. If you can hit say a piece of 3/16" thick Aluminum (HARD) with the ball side of the hammer and not dent it much. That is the piece I would buy. Other than this simple test. You should look for the MIL spec number that's sometimes printed on the metal. But I do recommend scratching and beating on metal to test its actual physical properties. Of course you should only do this in scrap yards. If you try this with new metal you might have to buy it first.
Titanium: This metal is amazing it's probably the strongest metal around. It is lighter than Steel. It's also very expensive very hard to cut, weld, and drill. But boy is it tuff stuff.
Polymers: You know that plastic stuff A lot of people have discovered the amazing power of Lexan It has great properties it is very impact resistant and strong and yet light.
The Macrogalleria - a cyberwonderland of polymer fun.
Concrete: I haven't seen this stuff used yet but I'll bet it would be hard on metal cutting saw blades. Hmmm?
Composites: From what I've seen some of it applies sorta like fiber glass. Using a matting that may contain Kevlar or carbon fiber and a resin over a form or a honey comb webbing. You have to put it in a vacuum to remove all of the air bubbles while the resin is curing. Team Mortis made there body from it and it took a lot of beating and came back for more. I have heard it is extremely expensive to work with.
One of the Main problems you will
probably run into when building a battlebot is that the robot always ends
up too heavy.
Matweb If you need to know the properties of materials look here!
Here's a link to a cool little
program that will calculate Metal weights.
Another great site for a Metal weight calculator , Metal properties and online Metric conversion is. Online metals and yes they also sell lots of metals.
One of the Books that we have
found to to be very useful is the Pocket
Ref It has tons of information and fits in your pocket.
1. Lots of power in a small lightweight package Especially in two stroke engines.
2. It only takes seconds to recharge them between rounds. Just pour in more fuel.
3. There are millions of them available. Unlike high horsepower DC electric motors. Which can be hard to find.
4. They have very good throttle response and don't drain your batteries. Except for starting with electric motors.
There are disadvantages to gasoline power also...
1. We currently have a 10-22 oz fuel limit. So that limits the size of the engine you can use. Because of fuel consumption.
2. Of course the fuel can catch on fire. That can be really hard on the robots wiring and other components.
3. You really must come up with R/C electric start. Small two stroke engines don't usually come this way.
4. If you use gasoline engines for the main drive power
directly. You will need a reverse gear or transmission.
Like we used in the robot K.I.S.S.
So you have to stop moving and idle down to shift your transmission FWD/REV
which takes time. Unlike electric drive motors which can change direction
almost instantly. Another part you will have to use is a centrifugal
clutch. You should check out NORAM.
You can also use a torque converter type variable transmission check out.
Comet torque converters.
Futaba GV-1 R/C Engine speed governor. This engine governor can control the maximum speed of you engine to help reduce your driving task by preventing engine over speed
5. Engines can require a lot of tuning and tweaking to get them to run right (Especially two strokes). Due to changes in altitude,air temperature, humidity and engine temperature.
6. They are very messy when running (Especially two strokes because of the oil in the exhaust gasses) not to mention the soot produced by the burning fuel.
Good things to know...
The rules require that you use braided fuel lines that are safety wired at both ends.
The engine must return to idle or shut off if you lose radio reception. A throttle return spring will take care of this. Or a Radio with Fail-safe mode or both.
The ignition system produces lots of radio interference
so you need to use resistor spark plugs.
You should always try to mount your radio antenna and servo wires as far from the spark plug wire as you can to reduce radio interference.
It takes two to three radio channels to control an engine.
1. Start switch.
2. Kill switch.
3. Throttle control servo . A good thing to consider before buying a radio transmitter.
A lot of small 4 stroke engines come with engine governors so they adjust for engine load. This is useful for driving say a hydraulic pump. That way the engine wont have to run full throttle except when needed.
Engines need to have clean cool air in and free flowing exhaust out. Consider this when enclosing them inside the robot body.
Engines create a lot of heat. If you enclose them within the body they will need lots of airflow to stay cool. Make sure you leave some vents in the body. You also might have to add a fan.
Always try to warm up your engines right before your
match you wont be given time once you are in the arena!!!
Air compressor: I've yet to see a robot with a gas powered or electric powered compressor but with the super heavyweights it could be possible.
Air tank: Holds the gas to operate the weapon. Some people have used liquid co2 you convert it to a gas before using it. The nice thing about liquid co2 is you get a lot more shots from the same size tank. Nitrogen gas is an option. Do Not use pure oxygen like from welding bottles unless you like fire and large explosions.
Pressure regulator: You would use this to adjust the working pressure of the system if it is less than the pressure in the air tank
Control valve: (These can be
operated either mechanically or electrically)
Drawing of servo controlled valve
Air lines: Make sure you use good
lines that are rated at your operating pressure.
(Aeroquip are the standard in industry) I have seen robots using plastic air tubing usually 1/8" to 3/16".
Fittings: These come in all shapes and size's and types they are usually made of brass or aluminum for low pressure systems. High pressure systems require steel fittings.
Pneumatic Cylinder: These come in almost any diameter or length that you could want. Normally the bigger the diameter you make the piston. The more power you will get out of a cylinder at the same working pressure.
PneuCalcDownload this pneumatic calculator and just fill in the blanks.
Pneumatics online A great place to start. Also has an online pneumatic calculator.
Caution Hydraulic pressures are extremely high if you notice a leak in a line do not touch it. The high pressure can inject fluid into your body and it could take several weeks in the hospital to get it out. Also a pencil thin stream of oil can remove your body parts.
Hydraulic fluid A special 10 wt oil that has good lubricating qualities and the ability to handle temperatures up to about 250 deg. Normal operating temperatures shouldn't exceed 170-180 deg.
Hydraulic Reservoirs Most of the
time these are just used as no pressure storage tanks for the surplus hydraulic
fluid used by the system. They are also where the fluid return line and
the pumps suction line connects to.
Positive displacement gear type: Most common, high pressure as their RPM's increase so does their fluid flow.
Vane type pumps: A little higher end they can turn faster than gear type pumps.
Slipper Vane type pumps: probably the most expensive type.
Multi stage: These pumps are
cool they can move a lot of fluid at low pressure quickly. Until
a load is applied. Then a valve inside of them switches. Then they
turn into high pressure low flow Robot killing monsters.
These a commonly used for log splitters.
There are lots of different kinds of valves they can be operated
either mechanically or electrically. They have very heavy castings and
are extremely high precision.
Drawing of servo controlled valve
Hydraulic lines: The lines must be rated for at least the system working pressure to prevent rupture. Hydraulic lines are constructed with rubber hose surrounded by steel mesh then another outside covering is added and they use special high pressure fittings. You can have your lines made up at a hydraulic shop There's a special machine for this purpose.
Fittings: All of the fittings used in hydraulic are special use parts they are usually made from steel and are rated for specific working pressures.
Hydraulic Cylinder: These come in lots of lengths and diameters. There are also some that are telescopic. The bigger the diameter of the cylinder. The more force it will apply at the same working pressure.
Hydraulic motors: These are motors that can be driven with hydraulic fluid this is called hydrostatic drive. The robot called World Peace uses this system. Use of a proportional valve to control the motor can act like a speed controller.
Clevis and Pins these are the eyes and pins used to connect the ram to your machine. You have to use pins that's ratings exceed the force applied from your ram.
BAUM HYDRAULICS online hydraulics calculator (This is cool)!! Just plug in the numbers. Also gear , chain and belt calculators.
W.M. Berg, Inc. - FREE Download able Engineering Tools - Sprocket Spec, Gear Spec, Dimension Calculator, Caliper Dimension Calculator.
Hydraulic Ram Capacity Calculator Online calculator.