517 Questions and Answers about Combat Robotics from Team Run Amok
Ask Aaron!

Team Run Amok receives a lot of email asking about the design and operation of combat robots. My son and team member Aaron Joerger (age 16) requested a question and answer page to document our responses. Aaron answers most of the questions, but I'll step in if it gets very technical.

Aaron Joerger holding trophy at TLC Discovery Robotica, 2001
Aaron at Robotica - 2001
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TopCategory: Recent Questions

Q: I'm on a really tight budget. What speed controller would be good for a middleweight 'bot?

A: Mark J. here: A speed controller is one of the last components to consider when building a 'bot -- not the first. Your speed controller specs will be determined by the amperage requirements of the drivetrain. The process goes like this:

  1. Decide what you want your 'bot to do in the way of fighting strategy.
  2. Outline a design that will accomplish that strategy.
  3. Find components that will meet the design requirements.
If you're on a tight budget, pick a weight class that will allow you to build a good 'bot with the available funding. I'd much rather see you build a great featherweight than a poor middleweight.
Q: You told me to search for 'composite armor' in the archive, but I can't find a search box. Am I missing something?

A: Use the search function in your browser: open the Ask Aaron Archive and hit CTRL-F to bring up a search box.

Q: What motor specs would you recommend for a middleweight 'bot using 2 drive motors? I'm planning on a wheel diameter of about 8 inches, tops.

A: How much drive power a combat robot needs depends on the design and attack strategy of the 'bot. A wedge or rammer will need a lot more drive power than a full-body spinner. Several top-ranked middleweight wedges run a pair of S28 "mini" Magmotors -- that's 6 horsepower worth of go-power! Even for a middleweight 'bot with a large active weapon, you'd still want a couple horsepower to keep the 'bot in the fight. One horsepower equals approximately 750 watts.

Wheel diameter isn't a factor in motor selection. For larger wheels, just increase the gear reduction to keep the torque high enough to avoid stalling the drivetrain.

Q: Why is the IBC Dual Motor+2AUX Speed controller so expensive? Would it be too small for a basic middleweight 'bot?

A: Mark J. here: Reliable robot motor speed controllers are expensive, but at around $260 the Robowars IBC controller isn't more expensive than comparable dual-channel controllers like the Vantec RDFR23, or Robot Solutions RS80D. Don't confuse the power ratings of robot controllers with the advertised ratings for R/C car controllers. The R/C car controller ratings are entirely hypothetical -- try to pull the full rated amperage thru an R/C car controller and you'll very quickly get an expensive puff of smoke. Remember: the most expensive speed controllers are the ones you overload and blow up!

The Robowars IBC controller is popular for hobbyweight and featherweight robots, but with a 50 amp-per-channel power rating I wouldn't try to build a middleweight around it.

Q: Who would win a BattleBots fight between 'Slam Job' and 'Son of Whyachi'?

A: If it were five years ago I'd care.

Under current rules, Son of Whyachi would be a superheavyweight 'cause shufflebots no longer get a weight bonus. SOW won all of it's fights as a heavyweight and won the BB 3.0 heavyweight tournament, but had only a 1 win/2 loss record as a superheavy.

Team Blackroot has kept active in the sport and has continued to upgrade 'Slam Job', now renamed 'SJ'. They won the 2004 RFL National Championships, and hold the current #4 and historic #3 heavyweight rankings at botrank.com.

Q: Has 'Ziggo' ever lost a battle?

A: BattleBots uber-spinner and Combat Robot Hall of Fame member Ziggo has a great record. I count 17 head-to-head wins (16 by knockout), one rumble win, three lightweight championships, and four losses. The losses were to:

  • 'Defiant' - Long Beach '99 (Ziggo came back to defeat Defiant in double-elimination)
  • 'Das Bot' - BB 1.0
  • 'Sallad' - BB 3.0
  • 'Code Black' - BB 5.0

Q: Should I use 7075, 6061, or 2024 aluminum for my armor? It will have an outer layer of Lexan on it for shock absorbtion, as well as being bolted to my bot using shockmounts. That should stop those spinners!

A: Mark J. here: Multi-layer armor like you suggest is tricky and often less effective than single-layer armor of the same weight. Lexan is effective at energy absorbtion only if it is allowed to flex. By backing Lexan with rigid aluminum you will greatly reduce the desireable energy absorbing properties and impact will be transmitted directly to your relatively thin underlayer. You can vary the hardness of armor in layers, but the layer stiffness should remain fairly constant to allow the load to be spread evenly. Search the Ask Aaron Archive for 'composite armor' for more comments.

I'm not a big fan of aluminum armor, but T6 tempered 7075 is generally the best aluminum alloy for that purpose. It's harder and has higher shear and yield strength than 2024 or 6061. It's price is comparable to 2024, and about twice that of 6061. Note that both 7075 and 2024 are very difficult to weld.

P.S. Don't you think Aaron was a bit harsh on the guy asking about how to support his spinning blade when he said, "Sorry -- my mind reading is a little weak"?

A: Not at all. Aaron provided a complete answer with a photo link in response to the initial question. The questioner responded that Aaron's answer wasn't useful because the questioner was thinking about a different design type. Aaron answered the revised question with a revised explanation and an original diagram, which I think was generous. His apologetic 'mind reading' preface simply pointed out that he couldn't know what the questioner had been thinking, only what was written in the question.

Questions we receive from the 'Ask Aaron' website all come from the same webmail server address. If they aren't signed we can only guess from whom they come and we may not know that a question is linked to an earlier discussion. Our psychic abilities really are weak, so please give us enough information to at least have a shot at answering your questions.

Q: How do I make my robot faster?

A: Try pointing it downhill. If that doesn't work, write back and tell me enough about your robot that I can make a reasonable suggestion.

Q: What 'bot is your favorite? I'm a fan of Hypno-disc!

A: Hypno-disc rocks, but my vote goes to Carlo Bertocchini's Biohazard. Twenty-seven wins and three losses is just impossible to beat. My beetleweight Zpatula is based on his electric lifter design, and Carlo was kind enough to autograph Zpatula for me at the 2004 RFL nationals.

Of the Robot Wars 'bots, I always liked Firestorm. Hypno-Disc beat Firestorm 3 in the 5th Wars, but it was a great fight!

I thought biohazard had five losses at: BBLV'99, BB1.0, BB3.0, and Combots cup 2005 (x2)?

I was counting the record at BattleBots, including melee wins. If you want to count the 2005 losses to Megabyte and Brutality, count the 2005 win over Jawbreaker and throw in the undefeated record at the early Robot Wars for an overall head-to-head record of 35 wins and 5 losses. That remains the best win percentage of any heavyweight with more than 10 fights.

Q: I was wondering, what is the difference betweeen brushless and brushed motors? Also, can brushless speed controls be used with brushed motors and visa versa? Thanks!

A: Mark J. here: A brushed permanent magnet direct current (PMDC) motor has a rotating 'armature' of wire coils, typically wound around iron pole cores. The armature is positioned in a field generated by stationary permanent magnets. Electrical power is transferred to and correctly switched between the armature windings as they rotate by stationary brushes pressing against a set of contacts on the armature (the 'commutator'). There is a good diagram at Wikipedia: DC Motors. The brushes sliding across the commutator create friction, wear, and inefficiency.

A brushless PMDC motor has stationary wire coils and rotating permanent magnets. Since the coils are stationary, sliding brushes and commutator are not required -- however, the switching of electrical power to the correct coil windings must be handled by an 'intelligent' motor controller that senses the position of the rotating permanent magnet field. If the rotating magnets are outside the wire coils, the motor is an 'outrunner' or 'rotating can' design. If the rotating magnets are surrounded by the wire coils, it is called an 'inrunner'.

Speed controllers for brushed and brushless motors are very different in design and cannot be interchanged.

Q: I'm entering a robot sumo competition and I was wondering: what's the most effective design to win it? Weight is an issue and I do have two very powerful and somewhat heavy motors. Would a basic wedge work with such powerful motors, or should I consider something else? Also, how necessary are dropskirts on the side/rear of my robot?

A: Robot sumo competitions have highly variable rules, and careful study of the rules is needed to figure out what designs will work well. Are you entering the autonomous or R/C category? What weight/size class? What material is the arena surface made of? Do the rules allow magnet traction aids? Do the rules allow vacuum traction aids? Are there rules about how 'sharp' the edge of your wedge can be? Are 'sticky' tires allowed, or do tires have to pass the 'paper drop' test?

Write back with details about your competition, and tell me more about your motors. In the mean time, try to find a copy of Robot Sumo: The Official Guide by Pete Miles in your local library.

Q: I am entering a R/C category. The maximum weight is 2.5 kg and the robot must fit within a 50cm X 50cm box with no height restriction. The arena floor is made of vinyl. The rules do not specify anthing about traction aids (nor do I know what they are). There are no rules about how sharp the wedge can be. And, tires may not leave residue on the playing field. Thanks!

A: It's details that win R/C sumo competitions. A wedge is a winner only if it's better than your opponent's wedge. You need to make sure that it sets perfectly level all the way across it's width and that the leading edge is sharp and flush with the arena surface. Read thru those rules again -- there's usually some restriction on dangerous sharp edges. If not, make it razor sharp, but fit a cover to it between matches for safety. Side and rear drop-skirts are uncommon in sumo competitions and may be more trouble than they are worth.

Tires are another critical detail area. Super-tacky reusable lint rollers are very effective as sumo tires, and they leave no residue. See Dave Chu's Sumo Project for an example of their use. Clean and dry them before each match. Other tire compounds (polyurethane, silicone) are useable if you're not willing to custom make your hubs and tires. See Pete Miles' book referenced above for details.

Traction aids are devices that make the apparent weight of the 'bot greater in order to get more traction. If the arena has a steel base, magnets can be used to pull the 'bot toward the surface with great force. A magnet-bot can climb right up the vertical face of a steel cabinet and even run upside-down on a metal ceiling! A similar effect can be achieved on non-magnetic surfaces with a vacuum fan system and sliding surface seals.

I'd suggest sticking with a conventional design for your first build. Finish it early enough to get plenty of driving practice. Keep things simple and sweat the details.

Punjar Q: Is 788 in-lbs enough torque for a middleweight similar in design to 'Punjar'? It will run 20 MPH, so I have some speed to spare if it's not.

A: Mark J. here: Is that total stall torque from all motors at the axles, or stall torque available per driven wheel? I also need to know the wheel radius, the number of driven wheels, and the weight on the drive wheels to run the calculations.

'Punjar' was a scoop-fronted rambot with a lot of speed and pushing power. To duplicate that performance, you'll want to design for thrust available at each driven wheel to be about two times the weight normally on that wheel. Much more than that just smokes the tires and wastes weight (and money) on heavy drivetrains and batteries.

thrust = axle torque / wheel radius

I'll guess that 788 inch-pounds is the total torque available per drive wheel for a two-wheel drive 'bot. Assuming a 3" wheel radius, that gives you (788/3) = 263 pounds of thrust and no more than 50 pounds of weight on the wheel. If my assumptions right, that's more than five times the weight in thrust -- way overkill. If that 788 in-pounds is the total of all your drive axles, you're pretty close to right.

Q: How do I attach a heavy, high-energy horizontal spinning weapon to my robot so it won't tear away on impact? I need something really, really strong!

A: Take a look at Team Boomer's Fright Knight for a good example. Note that their weapon shaft is supported both above and below the plane of the disc. The supports form a triangular shape and are anchored directly and firmly to the chassis. Top-mounted spinner weapon supportThe shaft anchor point is heavilly gusseted with flat metal plate to spread the load to the supports. The support arms are also no longer than they need to be.

Q: Well, that type of design works well for 'Fright Knight', but I was talking about a 48 inch bar similar to 'Hazard' in design. I needed to know: how do keep it from simply pulling out of the bearing at the base of the robot?

A: Sorry -- my mind reading is a little weak. You'll still want to support the shaft at two locations, above and below the drive pulley. If the pulley is pinned to the weapon shaft and tubular spacers are inserted as needed, the pulley will locate the shaft.

Q: Do you have any ideas on where to get a good lifter motor? I planned on using the banebots motor 42mm 48:1 motor and gearbox, with the output shaft machined to go into the 27:1 gearbox. This would give me about 285 lbs of lift at the end of my 20 inch-long arm. That seemed good, but I heard that efficiency can decrease through extremely high gearing, so the inefficiency might reduce the force.

A: Mark J. here: the efficiency of gearboxes depends on their design, but forget about the gearing losses -- trying to put 476 foot-pounds of torque thru a Banebots gearbox would instantly turn it into scrap. You'll need a gearbox rated for that type of torque -- search industrial suppliers like McMaster-Carr or Grainger, or look at truck/ATV winches.

Q: How much torque at the axle of that 20 inch long arm do you think I should have, since it will be a middle weight and the lifting attachment will weigh a lot?

A: Your torque calculations are about correct -- you'll want to aim for about twice as much lift as the load you expect to have on your lifter. Does your lifter arm really have to be that long? The problem is getting a gearbox heavy enough to handle that amount of torque. Why don't you write to Jim Smentowski and see if he'll tell you what gearboxes he uses on Breaker Box?

Q: OK. I talked to Jim and I think I should use an NPC motor because of their low rpm and high torque. McMaster-Carr also has worm gear reducers up to 60:1, but I still need to know how the gearbox efficiency will impact my torque output. How do I calculate the real output of my motor through a gearbox?

A: You will lose some power with any type of gear reduction. The general equation is: output power = input power * gearbox efficiency

Gearbox efficiency depends on many factors:

  • the type of gears used (spur, bevel, crown, worm);
  • the number of gear meshes under load;
  • the number and type of internal bearings;
  • the gear clearances in the design;
  • the lubricant used; and
  • the design and construction of the shaft seals.
Further, efficiency varies with the load that is placed on it and with individual manufacturing differences. Calculating exact gearbox efficiency is impractical -- it needs to be measured.

As a very broad rule, spur gears are more efficient than bevel, bevel are more efficient than crown, and crown are more efficient than worm. For a very rough approximation, you can figure that each stage on a spur gear reduction loses 3% of the power that goes thru it. A planetary gearbox has many meshing surfaces and is less efficient than a simple multi-stage spur gear reduction. Each right-angle bevel gear stage loses about 15%. Worm gear losses are very high and depend on the reduction ratio: a 10:1 reduction will lose about 20%, and a 60:1 reduction can lose 60%.

This is a good time to remind builders that the best way to figure out if something works is to look at similar designs built by other builders. If you're building something entirely new, you're kinda on your own. Best luck!

Q: Another question about my middleweight spinner killer. Steel is to heavy, and I just can't afford titanium -- it would be thousands of dollars for the scoop alone. So what about half-inch thick 6061 aluminum?

A: Yes, titanium is expensive -- but thousands of dollars?? Check with Titanium Joe. Aluminum is not a good match against the tool-steel teeth of a serious spinner. With scoring based so heavily on visual damage, you can't afford big gashes in your scoop. I'd rather see you go with a thinner, hardened or tempered steel scoop with reinforcing ribs welded across the back as needed.

Q: If I build a 4 wheel drive robot that is 18 by 18 inches with the wheels in each corner, will it be able to spin in place fast enough to make sure all impacts hit one certain part of my robot?

A: It's simple to build a 'bot with a high spin rate, but it's very difficult to get it to spin and stop just where you want it. Your skill as a driver will be the determining factor. You also have to plan for unexpected situations where you aren't free to maneuver. Most builders armor-up all the way around.

Q: How do I get anti-wedge fenders on my robot?

A: I think you're asking about drop-skirts. They are armor panels hinged along the top edge that drop down at an angle and slide along the arena floor. Small 'bots can use strong tape to make the hinge, but larger 'bots need full-length mechanical hinges.

Q: How do I calculate how high a pneumatic flipper will be able to throw an opponent weighing X pounds and with Y amount of force at a given point of contact?

A: Mark J. here: The questions are getting tougher. A full answer to your question would make a good chapter in an advanced physics text.

Simplest case - where the flipper is positioned directly below the opponent's center of mass, and the flipper force perfectly vertical and uniform for the entire stroke length:

Calculate the net upward acceleration force available:net force = flipper force - weight of opponent
Calculate the net acceleration of the opponent:acceleration = ( net force / weight of opponent ) * g
Calculate the time over which the acceleration will be applied:seconds = square root ( 2 * flipper stroke / acceleration )
Calculate the speed of your opponent as it leaves the flipper:launch speed = acceleration * seconds
Calculate the peak altitude the opponent will achieve:height = launch speed / ( 2 * g )
For english units (pounds, feet, feet/second): g = 32.15 ft/sec2
For metric units (newtons, meters, meters/second): g = 9.8 m/sec2

Complex cases - if you want to calculate the height achieved from off-center flipper hits, non-constant flipper force, and non-linear flipper vectors, break out your calculus text. You'll need to calculate - amongst other things - the moment of inertia for a specific opponent and axis of rotation. That ain't easy, and life's too short. For more information on Newtonian mechanics, visit the Hyperphysics website.

Q: When they say titanium has an ultimate tensile strength of 150,000, how thick of a piece of titanium is that measured on? Obviously 1/8" thick titanium isn't as strong as 1" thick titanium.

A: Mark J. here: Tensile strength is measured by placing an increasing linear pulling force on a material sample until it breaks. The unit of measure is 'pounds per square inch', and is calculated by dividing the maximum resistence provided by the material sample by the cross-sectional area of the sample in square inches.

Tensile strength is not a very useful measure in evaluating the suitability of a material for most combat robot uses. More interesting for applications such as armor are measures of impact resistence, toughness, elasticity, and hardness.

Q: A couple of times I've drained my 3-cell lipoly battery down until the weapon ESC starts cutting cutting off at 9 volts. Am I harming my battery? Will I get less cycles out of the pack? Will this effect the battery's capacity and discharge rate? What is the average number of cycles for a lithium polymer battery?

A: Mark J. here: a three-cell lithium polymer battery can be safely drained down to 8.4 volts, so running into the 9 volt cutoff is fine. The usual cause of damage to Lipoly batteries is overheating caused by too high a discharge rate. Temperatures over 140 degrees can damage or destroy the pack. Leave some space around the battery for cooling air circulation and keep the discharge rate within the manufacturer's spec. A well cared for Lipoly battery can exceed 1000 charge/discharge cycles. Check the Electrifly Lipoly Manual for more tips on care and disposal.

Q: How can I make a BattleBot and fight it in competitions?

A: See the FAQ section of the Ask Aaron archive.

Q: How do I calculate the spin-up time and kinetic energy for my spinning weapon design?

A: Mark J. here: you'll need to know the stall torque and top speed of your electric motor, the shape and dimensions of the spinning weapon, the type of material you'll use to make the weapon, and the gear reduction between the motor and the weapon. The math gets a little sticky, but I wrote an Excel spreadsheet that will do the calculations for you:

Team Run Amok Spinning Weapon Spreadsheet

As a rule of thumb, you'll want at least 14 joules of energy per pound of weight class, and you'll want to spin-up to 7 joules per pound within 2 seconds. A high-energy spinner can have more than twice that energy! You might have a little more spin-up time in a big arena, or if your 'bot is nimble enough to protect the weapon from premature impact.

Q: Hi Aaron: I am building a small robot. I plan to purchase the Sabertooth 2x10rc ESC with built-in mixing for the two drive motors. I know the bot will move forward and reverse in a straight line and turn left and right, but will the 'bot turn left and right in reverse?

Thank you for your answer.

A: All variable-speed dual-channel robot motor controllers on a robot with differential steering will allow you to:

  • move straight forward and backward;
  • rotate in place clockwise and counterclockwise;
  • move forward while turning left or right; and
  • move backward while turning left or right.
Some dual-channel ESCs (Sabertooth included) have a plug for a third control channel that allows you to reverse the direction of the steering response if the 'bot is flipped. Very handy in an invertable 'bot.
Q: I want to build a middleweight spinner killer and need to know more about scoops. Got any info on best angles? Best length to hight ratio? About how thick is should be? How to calculate total surface area on the front?

A: Spinner killers are all about the scoop. For a great example, take a look at Jim Smentowski's middleweight actuated-scoop spinner killer Breaker Box. The scoop must start out at a very shallow angle - nearly horizontal - and curve up to around 60 degrees. Length and height are about equal, with a gentle and uniform curve. Jim's using 3/8" titanium for the scoop, with very heavy support arms. His scoop makes up almost half the weight of the 'bot! The simplest way to figure the surface area of a curved surface is to make a thin cardboard mock-up, then flatten it out and measure.

Another question about my spinner killer: do you think I should go with .25" S7 tool steel or 3/8" titanium 6Al-4V? I thought titanium would be better, but S7 tool steel is what impact teeth are made of, so it should be able to stop the spinners I guess. The tool steel would weigh about 3 pounds more for the size of my scoop, but would cost much, much less. What do you think?

A: Mark J. here: S7 tool steel is very hard and unyielding, which makes it perfect for transmitting energy from a rotary weapon to the target. It is also somewhat brittle and will break before it bends. You'll have to anneal the metal before bending it into your scoop shape, and heat treat it to restore it's properties after the scoop is assembled. This will add to your costs. You will still run the risk of a big weapon hit shattering the relatively thin scoop like glass! If you want to use steel, consider a spring steel that can flex on impact.

6Al-4V titanium is extremely tough and resilient. It will take enormous impacts and come back for more. It's perfect for absorbing spinner attacks. It is expensive, but the experienced builders that use it wouldn't be spending their money if it weren't worth it.

Q: I need to be able to spin my 'bot in place to keep the heavy front scoop pointed toward my opponent. Will a controller that mixes channels like Vantec be capable of spinning one wheel one way and the other wheel the other way, or do I need to drive 'tank style' with one stick assigned to each controller to get that?

A: You can mix channels with a computerized transmitter, a plug-in electronic mixer, or a twin-channel controller with built-in mixing. All of them produce the same control style: one control channel (stick axis or throttle trigger) moves the 'bot forward and back, while a second channel (stick axis or wheel) controls turning. If you activate only the turning channel, the 'bot will spin in place without moving forward or back -- just like throwing the control sticks in opposite directions when working tank-style.

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