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ROBOT BUILDING TIPS
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INTRODUCTION
I constantly receive e-mail messages asking me whether I have any general tips and/or suggestions for people who are interested in designing and entering robots in future robotic combat competitions. However, I always find this question difficult to answer. Everybody who builds a robot has a different philosophy. Some people design their robots to have a lot of speed and torque (e.g., La Machine), others attempt to create robots with effective weapons (e.g., The Master), others design their robots with lifting mechanisms (e.g., Biohazard), and still others merely enter a robot with a humorous theme, even though they have little chance of winning. There is clearly no "right" way to build a robot. Having said that, listed below are some general suggestions, observations, and resources that I think may be helpful for those of you are interested in building combat robots.
ROBOTS IN THE WORKSHOP
OTHER WEB PAGES WITH TIPS FOR ROBOT WARRIORS
The first thing you should do before attempting to design or build a combat robot is read as much as you can on the subject and look at what other builders have to say. The links below should answer most of your basic questions and give you a good head start on building your first robot.
Tony and Dan's Top Ten ListCarlo Bertocchini's Building Tips The BattleBots How To Compete Page Robot Science & Technology Magazine Information Page
Note: there are a lot of other good robotic combat pages on the web. You should read as many as you can before building your own bot. Jim Smentowkiw has a great links page where many of these pages can be found.
COST
After reading what others have to say on the subject, you will quickly realize that entering a robotic combat event can be a very expensive venture. Where you may plan on spending $600, by the time you have finished you may well have invested over $1,500 in parts alone! Similarly, tools and spare parts wil lset you back even more. The following is the materials cost break down for a battery drill based robot:
$600
- Vantec RDFR 36E speed controller
Together these parts total $1,350. This total does not include the frame, shell, or suspension materials, and does not account for the spare parts you will need or the vast quantities of wood, metal, and plastic that may be discarded during the building process after such materials either failed, proved to be ineffective, or were improperly machined. Lastly, the tools to build a robot can be very costly. You are likely to need a drill press, tool chest, and numerous drill bits and other miscellaneous shop equipment. While there are ways to reduce these costs, it is difficult to do so and remain competitive, where some builders run machine shops.
TIME
In addition to money, a robot warrior usually spends countless hours building his or her robot. This factor is often overlooked by those people who are thinking about entering a robotic combat competition. Some contestants claim to have spent over 1000 hours building their robots. While these may be extreme cases, they nevertheless illustrate how complex a robot can be. Obviously, a simpler design, such as a wedge, will take less time. However, to make any robot competitive, the creator will still need to spend a lot of time designing, testing, and building it. Each of these phases is very important and cannot be overlooked.
DESIGNING AND BUILDING YOUR ROBOT
So, you still want to enter a robotic combat event. O.K. Well the first thing you need to do is decide what type of robot you want to build (e.g., bot with weapons, lifter, wedge, cartoon figure, etc.), and how much money you want to spend. I cannot emphasize enough how much these robots cost. Robots in the heavier weight classes can cost as much as $30,000! Generally, the radio control, speed controller(s), battery(s), battery charger(s), and drive motor(s) are the most expensive components of a robot.
After deciding what type of robot you want to build, you should determine how you will acquire all the components you will need. A hobby store is usually the place to purchase the radio control. I would recommend an FM radio for those who are on a budget, and a PCM radio for those who can afford them. I would stay away from AM radios since they tend to experience the most control problems.
As for drive motors, use something other than hobby motors. They put out measly amounts of torque, and are only designed to handle 7-9 volts. Instead, take a look in the catalogs of electronic surplus stores. The motors you find there are much stronger. I would also look in those stores and in hobby stores for batteries and battery chargers. The following surplus providers have a lot of good materials for building robots. Call them to get a free catalog:
MECI
SERVO
SYSTEMS
HERBACK
& RADEMAN
C&H
SALES COMPANY
ALLTRONICS
SURPLUS
CENTER
ALL
ELECTRONICS
With regard to speed controllers, don't try to build your own, unless you are a real electronics wiz. Many of the contestants that enter robots controlled by home built speed controllers either brake down or do not operate properly. Personally, after spending all that time building a robot, I would be devastated if my speed controller didn't work. So, I would recommend using either an electronic speed controller bought at a hobby store, a Vantec RDFR speed controller purchased from Vantec, a 4QD speed controller purchased from 4QD, or an Innovation First speed controller purchsed from Innovation First
As for the frame and shell of the robot, look in hardware stores, industrial supply catalogs, and scrap metal yards. The Wedge of Doom 1996 was built entirely out of wood and aluminum purchased at my local ACE hardware.
Lastly, I would recommend getting an early start. I was amazed at the number of contestants who were putting the final touches on their robots the day before the competition. By completing your robot early, you will have time to practice driving and time to work out the numerous glitches and bugs that your robot is sure to have.
TOP TWENTY COMBAT ROBOT FAILURES
One thing that is helpful to think about when designing a robot are the possible ways the robot can fail during combat. Indeed, one of the primary reasons robots lose matches is not that their opponents beat them -- it's because some part of the robot fails. Listed below is my top twenty list of robot failures. Prior to any competition you should test your robot thoroughly to ensure that none of these failures will occur. By designing your robot to avoid such failures, you will have a much improved chance of success.
1.
Battery connections and other wires coming loose ELECTRIC MOTORS
Since most combat robots use electric motors, listed below is some information that is useful in determining what motor will be most appropriate for your robot. The basics: : An electric motor converts electric energy into twisting movement, measured in torque. When it turns, the motor draws a certain amount of current from its power source. As the motor is required to do more work (i.e., put out more torque) the current draw will increase. In direct current (D.C.) motors, speed is generally proportional to voltage. Increasing the voltage will increase the motor speed.
The following units are often used when discussing the characteristics of electric motors: Voltage is measured in volts (V). Current is measured in amperes (A). Wattage is measured in watts (W). These measures are interrelated so that W = V * A.
T = torque or twisting force. In the United States torque is commonly measured in Lb-ft (foot pounds), Lb-in (inch pounds), or Oz-in (ounce inches). For example, in theory, an inch pound is a force of one pound acting at the end of a lever (i.e., a wrench) only one inch long. Another unit is the foot-pound, which is the force in pounds along a one foot long lever.
To
convert Oz-in into Lb-in, divide Oz-in by 16.
Another unit of measure is the N-m (Newton-meter) which is a force of one Newton on a meter long lever.
Note: To measure torque, affix a pulley to the shaft to be driven. Secure one end of a cord to the outer surface of the pulley and wrap the cord around it a few times. Tie the other end of the cord to a spring scale (like those used to weigh food in the supermarket). Pull on the scale until the shaft turns. The force, in pounds indicated on the scale, multiplied by the radius of the pulley (in inches) gives the torque in Lb-in (if the scale is read in ounces the result will be Oz-in).
Electric motor horsepower (HP): Just like gasoline engines, electric motors are rated in HP. To measure the HP a motor actually puts out, use the following formula:
T
(Lb-in) * Motor RPM
Thus, a motor spinning at 2000 rpm while exerting 31.5 Lb-in (or 504 Oz-in) of torque is putting out 1 HP.
To measure the HP a motor consumes, use the following formula:
Watts
Input (V*A)
Electric motor efficiency(HP): One of the most important characteristics of an electric motor is how efficiently it converts electrical energy into twisting force. The efficiency of an electric motor is typically obtianed by the following formula:
Motor efficiency = HP Out / HP In * 100
Thus, a motor that consumes 746 watts, but only puts out .5 HP, is only 50% efficient. No motor -- even the most expensive and technologically advanced -- is 100% efficient. For example, as noted below, the Astroflight Cobalt 90 with a 2.7/1 gearbox can take a 40 V power source and exert a torque of 810 Oz-in while spinning at 3,100 RPM. At this voltage and speed the motor consumes 60 A. In this case the Motor HP In = 3.2 (2400W(40V*60A)/746) and the Motor HP Out = 2.5 ([(50.6 Lb-in(810 Oz-in /16)*3,100]/63,025). Thus, at this speed the motor efficiency = 78% (2.5(HP In)/3.2(HP Out))* 100
The efficiency of a motor is significant because the more efficient the motor, the fewer batteries needed to obtain the desired HP. For example, a 50% efficient 24volt/1 HP motor would require 1492 watts of power (or 24v*62amps) to put out 1HP. A 75% efficient 24volt/1HP motor on the other hand would only require 1119 watts of power (or 24v*47amps) to put out 1HP. That is a savings of 15amps, which translates into fewer batteries and more weight to use for other components of your robot.
Motor gearing: Because electric motors deliver torque, gearing is important. A 2:1 gear reduction will result in a 50% decrease in speed, but a 200% increase in torque. Choosing Your Electric Motor: One of the best ways to choose an electric motor is to look at what others use. Listed below are some approximate specifications for a few of the most popular electric motors among combat robot builders.
BATTERIES
If you are going to use an electric motor, you are going to need a battery, or more likely a number of batteries, to power your robot. Listed below is some information that I have found helpful in determining what batteries to use in my robots.
Battery capacity in amp hours: The amp hour (Ah) or milli-amp (mAh) hour is the unit of storage capacity for a battery. A battery pack with a 1 Ah or 1000 mAh rating theoretically should provide one amp of current for one hour. However, the AH rating on a battery can be deceiving. Manufacturers usually state their batteries' Ah rating at a 15-20 hour discharge rate. In English: a 20Ah battery will not supply 20A for one hour. However, it *will* supply one amp for 20 hours, and 0.5A for 40 hours. Thus, the shorter your discharge time, the fewer total amp-hours you will get. By extending the discharge time, the battery cells have a chance to undergo chemical processes on the plates, and are then able to deliver the full current capacity. Most manufacturers can provide discharge current graphs for their batteries. For example, the specifications for Hawker's G13EP battery (a 13 A-hr battery, measured at a 10 hour discharge time) are as follows:
0.70
amps for 20 hours = 14.0 amp-hours
As you can see, the a Hawker 13 amp-hour battery loses over half its effective capacity simply by being discharged in 5 minutes instead of 10 hours.
Lead acid batteries: The lead acid battery is the most common power source for robotic combat. Automotive starter batteries, also known as "wet-cells" (because of the liquid acid inside) are a member of the lead acid family. However, wet cells are generally prohibited in robotic combat. See BattleBot Rules. So, competitors use other lead acid variations, such as the "gel cell" or sealed lead acid battery. Such batteries are basically the same chemistry as wet cells. The difference is that a gelling agent is added to the electrolyte to reduce or eliminate movement inside the battery case. These batteries may be used in any position and are exceptionally leak resistant.
Such batteries are typically sold in 6 and 12 volt models, and have Amp hour ratings from 4Ah to 65Ah. Their large capacity is what makes lead acid batteries so popular in the heavier weight categories. The main disadvantaged of lead acid batteries is their higher internal resistance. This resistance means that lead acid batteries are unable to deliver and receive current as efficiently when compared to other power sources, such as Nickel-Cadmium batteries (discussed below). This effect is especially apparent at higher amperage levels. Accordingly, competitors are forced to use larger batteries to supply the requisite current.
A well known maker of sealed lead acid batteries is Hawker Energy. Listed below are some approximate specifications of Hawker batteries that are suitable for use in robotic combat.
Hawker has a web page describing its batteries in detail, as well as providing lots of useful battery information. Click here to go to Hawker's web page.
NiCad (nickel-cadmium) batteries: Another popular battery in robotic combat is the NiCad. NiCad batteries are usually assembled in packs consisting of a number of individual "SC" (sub-c) sized NiCad cells wired in series. Each SC cell supplies about 1.2 volts of electricity. In these packs, the number of cells determines the voltage supplied to your motor. For example, each cell provides 1.2 volts, so a 6 cell pack provides 7.2 volts (1.2 volts * 6 cells) and a 10 cell pack provides 12.0 volts (1.2 vots * 10 cells). NiCads have a number of advantages over lead acid batteries: (1) they are lighter (approximately 10 ounces per six cell pack); (2) can be recharged much quicker; and (3) can provide higher currents in proportion to their total capacity. The main disadvantage of NiCad batteries is their high cost. An inexpensive 6-cell 1500mAh NiCad pack will cost $15-20, a 1700mAh pack will cost $30-40, and a 2000mAh pack will cost $40-50. High end six-cell 2000mAh packs cost as much as $120! As a result, NiCads tend to be used in the lower weight classes.
NiCad vs. Lead Acid: People mistakenly think that because lead acid batteries have high Ah ratings that they are necessarily better suited for robotic combat than NiCads are. Not so. The key is looking at the 5-minute discharge rate, which is all you really need for the major robotic combat events. For example, the 42EP Hawker lead acid battery is a 12 volt battery that is rated at 42Ah and will put out approximately 11.5 volts and 212 Amps for five minutes, and weighs about 33 lbs (note: although the battery is rated for 12 volts, its initial voltage when fully charged is greater than 13 volts and its voltage when discharged is approximately 10 volts. Thus, the average voltage output is approximately 11.5 volts at a 212 amp discharge rate). See http://www.hepi.com/products/genesis/genperf.htm This translates to a total power output of 2438 watts (11.5 volts * 212 amps) over five minutes.
Compare this with a battery comprised of 18 - 7.2 volt Sanyo SCR 1700 NiCad packs (like Tower Hobbies sells http://www.towerhobbies.com/listings/listbatt.html) (each pack will put out an average of approximately 6.6 volts (1.1 volts * 6 cells) and 25 amps for five minutes, but only weigh about 11.5 Oz each)(note: although each NiCad cell is rated at 1.2 volts, at a 25 amp discharge rate the actual voltage a cell puts out will drop, thus a 6-cell pack puts out an average of 6.6 volts -- not 7.2 volts). If you connect those 18 packs in pairs you will have 9 packs that will each put out approximately 13.2 volts (1.1 volts * 12 cells) and 25 amps for five minutes, and if you connect those 9 packs in parallel you will wind up with a single battery that will put out 225 amps (9 packs * 25 amps/pack) for five minutes at 13.2 volts. This translates to 2970 watts (13.2 volts * 225amps) over five minutes (532 watts greater than the Hawker battery). Most importantly, those 18 NiCad packs would only weigh 207 Oz (18 * 11.5 Oz/pack) or 12.9 lbs (207 Oz/16). Thus the 18 NiCad packs will put out 532 watts more power and weigh 20 lbs. less than the Hawker battery! Note: this performance advantage continues to increase with higher capacity cells, like the Sanyo 2000 NiCad and Panasonic 3000 NiMh cells, some of which will put out over 30 amps for six minutes!
Of course this peformance difference comes at a significant price. The Hawker battery can be purchased for $150 - $200. Each 1700 SCR pack, on the other hand, will cost $25 - $30. Thus, a battery comprised of 18 * 1700 NiCad packs will cost you as much as $540 -- that's over $300 more than the Hawker. NiCads do initially cost more, but for the builder with enough money, their performance advantages are well worth the investment. Also, NiCads, if properly cared for (this is a key operative qualification!) can be recharged 3 to 5 times as many times as lead acid batteries before they wear out. Thus, overall, NiCad batteries are at least as inexpensive, and probably actually somewhat less expensive a source of power than lead acid batteries if you are using them frequently, over the course of their total life. The following link has additional information regarding the differences between Lead Acid and NiCad batteries http://www.sheldonbrown.com/marty_sla-nicad.html.
NiMH (nickel-metal hydride) batteries: Untill 2000, NiMH batteries had been confined to use in cellular phones and portable computers due to their high internal resistance, which made such batteries poor choices for high current applications. However, this year Panasonic and Sanyo released high current NiMH rechargeable batteries that are sub-c sized and are rated at 3000mAh. They can be purchased at any good hobbie store. These batteries have made their way around the RC car circuit and are getting good reviews.
Listed below are some approximate run times (in seconds) for NiCad and NiMH batteries at 20, 25, and 30 Amp discharge rates.
Battery charging: In order to determine how long it will take to charge your battery, use the following formula: Charging current (amps) = Capacity (Ah) / Time to Charge (h). For example, to charge a 1 Ah battery in 30 minutes would require a current setting of 2 amps.
If you want to learn more about batteries, C. L. "Red" Scholefield has an excellent web page that you should check out. Click here to go to Red's page.
COPPER WIRE
Another important factor in building a robot is choosing proper wire to transmit the large amounts of current drawn by the robot's drive and weapons motors. Although there is some debate over the type of wire to use, most competitors agree that the most important factor in selecting wire is to use the proper size. Wire is generally rated in size by American Wire Gauge, abbreviated AWG, or commonly just "gauge". The larger the AWG, the smaller the wire, and vice versa. For example, while 100W lamps, which draw approximately 1 amp, are often connected with 18AWG wire, automobile jumper cables, which often must transmit 300-500 amps, may use 2AWG. For a AWG selection chart and other great technical information, check out the Alpha Wire web site at: http://www.alphawire.com/pages/tech_368.htm
METAL
SHEET METAL:
Sheet metal is used on many robots. While it is very strong and durable, it is also very heavy. U.S. gauge, the standard measurement for sheet metal, is based on the weight of the metal, not on the thickness. Smaller numbers refer to greater thickness. There is no formula for converting gauge to thickness or weight. Here are some approximate values for galvanized steel sheet:
Here are some approximate values for other metals frequently used in robotic combat: Stainless
Steel = 4.48 ounces per cubic inch METAL SUPPLY SOURCES:
Here are some online sources of various types of metal: Here are some Southern California metal supply sources: Industrial
Metal Supply Co. Burbank
Metals Apex
Electronics
PLASTIC
Plastic is another material that is frequently used to build robots. Of the various plastics, LEXAN is probably the most popular. It is very light (.043 lbs. per cubic inch), shatterproof, and comes in a variety of colors. Here are some approximate values for LEXAN sheet that is widely available from local plastic suppliers:
Note: a common misconception is that Lexas cannot be cracked. Well, on our robot HAZARD we learned that this is not the case. In fact, we cracked virtually every piece of Lexan on the robot -- including the 3/8 and 1/2 inch thick pieces we used for the sides and internal supports!
LOS ANGELES PLASTIC SUPPLY SOURCES:
Here are some places in Los Angeles where you can buy various types of plastic: Hastings
Plastics Solter
Plastics, Inc. Plastic
Mart Inc. Note: a good way to find plastic stores in your area is to look in the Phone book/Yellow Pages under "Plastics."
Building a BattleBot can be quite a project. Sometimes sheer will and determination are not enough to get the job done. You need help from a veteran builder. Tips from the Pros aims to provide this help by showcasing tips, tricks and tutorials from robot builders like yourself.
DO
NOT CONSTRUCT OR OPERATE A ROBOT OR BATTLEBOT UNLESS
SUPERVISED BY A QUALIFIED ADULT.
In order to compete in BattleBots,
contestants must comply with the BattleBots Official
Rules and Regulations in their entirety. The information
below contains the necessary guidelines for responsible
building and a safe competition. Common sense and strict
adherence to safety protocols are required in all areas
of robot construction and operation.
New
BattleBots Rules:
ROBOTICS, CONTROL & ELECTRONICS TECHNOLOGY
News • OOPic • Accessories • Brainstem • Beam • Programmable • Speech Rec • BasicX • Examples
PLEASE REMEMBER: The BattleBots Safety & Technical Inspection Officials have the final say on whether or not a robot is safe and eligible to compete.
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