Build Your Own Combat Robot

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FIGURE 4-4 Motor power changes by doubling the input voltage. FIGURE 4-5 Heat generated by doubling the applied voltage. Chapter 4: Motor Selection and Performance 71 the current, increases the heat by a factor of four! Stalling the motor will cause the motors to overheat and be seriously damaged in a short period of time. Nothing is free in the world of physics.
72 Build Your Own Combat Robot Heat can destroy a motor in several ways. Most lower-cost PMDC motors use ferrite magnets, which can become permanently demagnetized if they are overheated. They can also be demagnetized by the magnetic fields produced when the motor is running at a voltage higher than that at which it is rated. The flexible braided copper leads that feed current to the brushes (called shunts) can melt after just a few seconds of severe over-current demands. The insulation on the heavy copper windings can fail, or the windings can even melt. Depending on the motor brush mounting technique used, the springs used to keep the brushes on the commutator can heat up and lose their strength, thus causing the brushes to press less tightly against the commutator. When this happens, the brushes can arc more, heat up, and finally disintegrate. You don’t want to use that expensive motor as a fuse, so make sure it can handle the heat. Motor heating is proportional to the current 2 × resistance. Our 18-volt motor example has a resistance of 0.174 ohms. If you were to stall it, it would draw 103 amps. If you stalled the same motor at 36 volts, it would draw 207 amps. Since heating is a function of current 2 , the motor would get four times as hot. Pushing 207 amps through a resistance of .174 ohms will generate 7,455 watts of heat, which is five times more than the heating output of a typical home electric space heater. Now imagine all the power of your portable heater multiplied by five and concentrated into a lump of metal that weighs just a few pounds. You can see why survival time is limited. The physical size of the motor that would best fit your robotic needs is in large part determined by the amount of heat that will be generated. Some people find it surprising that a 12-ounce motor can produce exactly the same amount of power as a 5-pound motor. The same formula for motor power is just as true for small motors as it is for large motors. The difference is in how long that power can be produced. The larger motor has a larger thermal mass, and can therefore absorb a lot more heat energy for a given temperature rise. Pushing the Limits Okay, so you would like to use a greater-than-recommended voltage on your motor to get more power out of it, but you are worried about damaging it. What should you do? First, you must realize that you always run the risk of destroying your motor if you choose to boost its performance past the manufacturer’s specifications. Following are some things you can do to minimize the risk. Limit the duty cycle. If you run your motor for, say, 1 minute on and 5 minutes off, it should survive. Cooling is critical for an overdriven motor. One Robot Wars heavyweight (La Machine) cooled its over-volted motors by directing the output of a ducted fan into them. This ducted fan was originally created for use in propulsion in model airplanes because they put out a lot of air. An easier way to accomplish this same effect is to use batteries that are limited in the amount of current that they can produce. The problem here, though, is that you will often be pushing your battery to output levels that will shorten its useful life. Even the sealed lead-acid batteries can sometimes boil and leak under heavy loads.
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Build Your Own Combat Robot Pete Mi
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For more information about this boo
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Contents For more information about
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Current Ratings, 129 How It All Wor
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Object Detector, 290 Sensor Integra
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Acknowledgments We would like to th
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Introduction Some kids spend their
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About the Authors xv About the Auth
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2 ELCOME to the world of combat rob
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Page 80 and 81: chapter Motor Selection and Perform
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chapter Remotely Controlling Your R
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FIGURE 8-1 Wiring and rotational po
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FIGURE 8-2 Futaba’s top of-the-li
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Chapter 8: Remotely Controlling You
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FIGURE 8-4 Typical radio frequency
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FIGURE 8-5 Motor with three capacit
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Innovation First Isaac Robot Contro
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FIGURE 8-6 Block diagram of Isaac o
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chapter 9 Robot Material and Constr
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Metals Aluminum Chapter 9: Robot Ma
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Stainless Steel Chapter 9: Robot Ma
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Titanium used to solder small brass
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Chapter 9: Robot Material and Const
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Welding, Joining, and Fastening We
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place, or some liquid may have ente
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vibration and bounce around the ins
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204 ECAUSE robot combat has evolved
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240 HE robots described in the book
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chapter 12 Robot Brains Copyright 2
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FIGURE 12-1 From top left to bottom
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Chapter 12: Robot Brains 263 space
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FIGURE 12-2 The BoeBot from Paralla
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Handy Board BotBoard Chapter 12: Ro
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FIGURE 12-5 Robo-Goose, a robotic g
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FIGURE 12-8 A fully autonomous robo
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S ummary Chapter 12: Robot Brains 2
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276 HE referee signals, and my hear
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chapter 14 Real-Life Robots: Lesson
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FIGURE 14-1 Chew Toy Chapter 14: Re
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FIGURE 14-2 Chew Toy with protectiv
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FIGURE 14-3 Wheel shown bolted to d
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FIGURE 14-5 Front view with arm dow
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Final Words Chapter 14: Real-Life R
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Chapter 14: Real-Life Robots: Lesso
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design all the parts, and Dave did
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FIGURE 14-12 The 4-inch-tall alumin
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FIGURE 14-15 Internal view of Live
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330 The Future of Robot Combat The
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336 NLESS you’re building an exac
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356 OLLOWING are formulas that you
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358 Index 1BDI, 272 1/4 wave antenn
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sources of robot parts, 35 top ten
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INTERNATIONAL CONTACT INFORMATION A
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