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4647 Questions and Answers about Combat Robotics
from Team Run Amok
Ten Years of Ask Aaron

Team Run Amok receives a lot of email asking about the design and operation of combat robots. In 2003 my son and team member Aaron Joerger (then 12 years old) requested a question and answer page to document our responses.

Got a question? We welcome combat robot questions. Check the Ask Aaron Archives first to see if your question has already been answered, then click the blue button.

The Ask Aaron Archives Click to browse thousands of previously answered questions by category, or search for specific topics. Includes FAQ

Caution   Even small combat robots can be dangerous! Learn proper construction and safety techniques before attempting to build and operate a combat robot. Do not operate combat robots without proper safeguards.
It greatly saddens me to announce that my son, Aaron Joerger, died very suddenly on the afternoon of October 18th, 2013 of an apparent pulmonary embolism. He was 22 years old. Aaron's obituary.

The 'Ask Aaron' project was important to Aaron, and I have decided to continue the site in his memory. Thank you for the many kind messages of sympathy and support that have found their way to me.

- Mark Joerger, Team Run Amok

Welcome to my website Recent Questions

Q: Hi, still i have some doubt in Drum energy storage concept , already i read spread sheet calculation i didn't under stand the concept .. (if the weapon is 14kg and run at 5000 rpm with 5000 joules energy storage means ,what is the size of the drum length and diameter?) [India]

A: [Mark J.] There are a great many posts about spinner weapon design in the Robot Weapons archive -- suggest you spend some time reading there.

The Team Run Amok Excel Spinner Spreadsheet can model spinner weapons and answer the type of question you are asking, although it's unusual to design a weapon by setting speed, mass, and energy targets and 'backing into' the drum size. There are several things to consider:

  • General Rule 1: for a given mass, increasing the diameter of the drum and thinning the wall thickness increases the energy storage.

  • General Rule 2: for a given mass, increasing the length of the drum and thinning the wall thickness has negligible impact on the energy storage.

  • 5000 RPM is too fast to spin a drum this massive. The problems with spinning a weapon too fast have been discussed repeatedly in the archives. Half that speed might be about right.

  • Why have you choosen 5000 joules of energy as your energy target? A good weapon design has other parameters that are of equal or greater importance as stored energy.

  • You'll need to consider the performace of the weapon motor you plan to use. Is it capable of spinning the drum to speed quickly enough to be useful?
Given your original parameters, the Spinner Spreadsheet quickly shows one possible solution: a steel drum 120 mm in diameter, 450 mm in length, with 10 mm wall thickness and 8 mm thick endplates will weigh close to 14 kilos (I left a little mass for impact teeth) and have about 5000 joules energy at 5000 RPM.

Note: there was a transcription error in the original dimensions I published for the drum. I have replaced those dimensions with the correct dimensions above in red.]

By using other materials and drum thicknesses, you can find a great many diameter and length combinations that will give the same energy storage at the same mass.

Q: As here you told 8mm thickness and 4mm thick end plate means the steel 8mm is i think its very small thickness hw can i fit 4mm end plate? and the center hole also big if the 120mm diameter means i fix big size of bearing?

A: The dimensions I published earlier were incorrectly transcribed: see corrected dimensions above -- drum wall thickness is 10 mm, end plate thickness is 8 mm, and drum length is 450 mm. Sorry for the error.

I do not specifically recommend the given dimensions -- they were just the first solution that came from the spreadsheet after a few minutes of playing with it. If you don't like some of the dimensions, change them and use the spreadsheet to tinker with the other dimensions until the energy numbers come back into line.

The endplates may be welded into place, or machine screws may be run in to afix the end plates to the drum. The endplates may also be reduced in diameter a bit, and inserted into the ends of the drum tube before afixing. Look for a solution that suits your construction preferences.

The endplates may be drilled for any size bearing you like. Pick a bearing that will handle the high stress placed on the weapon and the speed of the drum.

Q: read spinner sheet there is one option for motor and drive which motor they asked driven motor (or) weapon motor?

A: This is a weapon spreadsheet. The 'motor' is the weapon motor and the 'drive' is the weapon drivetrain -- typically a belt and pulleys.

Q: if i can use Ampflow A28 400 means what is the gear ratio?

A: I really don't like the idea of running the drum at 5000 RPM. Search for 'bite' in the Robot Weapons archive for discussion on problems with running a spinner weapon too fast. The AmpFlow A28-400 at 24 volts spins very close to 5000 RPM, so if you really insist on spinning the drum that fast, a 1:1 gear ratio would be correct.

Q: For disc column: what is material density Kg/m3? I'm not understand m3.

A: The spreadsheet uses units of kilograms per cubic meter (kg/m3) for material density. The spreadsheet includes a table of densities for common robot materials:

  • Steel is 7800 kg/m3
  • Titanium is 4500 kg/m3
  • Aluminum is 2700 kg/m3, and so on.

Q: Dear Mark, do you know this robot? It has six legs, a weapon like Razer's weapon. This picture may show you what is it like. I ask it in the Tieba (that's a kind of forum) and didn't get a satisfied answer. I saw it in the BattleBots Long Beach '99 DVD. [Guangdong, China]

A: [Mark J.] Yes -- that's 'Scorpion Beta 3' built by the Washburn family. The legs and the weapon were all powered by CO2. An earlier wheeled version of the 'bot fought at the 1996 Robot Wars as 'Red Scorpion' and at the 1997 Robot Wars as 'Scorpion'. An upgraded walker version fought at BattleBots 2.0, again under the 'Red Scorpion' name.

200 pound walking combat robot 'Scoprion Beta 3''

Q: mark lucky again in case of drum's weapon [impactor] which material i should prefer i am thinking about cast iron and ms or i have also option of steel what u say? [India]

A: [Mark J.] NOT CAST IRON! You want a 'tough' material for your impactors -- a material that can take high impact. Cast iron is fairly strong, but it is brittle and can shatter on a high-energy impact sending metal shards in all directions. Various steel alloys cover a wide range of strength and toughness. I don't know what types of steel are available to you, but even mild steel is preferable to cast iron.

Q: hello sir !!
we are now making our 2nd bot, last time we had used 6 Johnson motors (200rpm, 32kgcm @12V) and had overvolted them to 24. they worked well but only for 2-3 tournaments, latter performance of the motor got was not that good... can you suggest me some motor which will be better for a 25kg bot (for drive) [India]

A: [Mark J.] The 'Johnson' motors available in India must be a much different motor than what we call a 'Small Johnson' in the US. A US Small Johnson geared down to 200 RPM would have more than 500 kg-cm stall torque at 12 volts.

In my opinion, 24 volts is too much for a 'throwaway' 12 volt can motor. A 'Small Johnson' is only $6.49 -- you can't expect a lot of unused capability in so cheap a motor. You might want to consider finding out if a 'real' Johnson or RS-550 motor might fit your gearbox. Running six of those motors at 12 volts would give you a much more powerful and reliable drive than the six weak motors you're running at 24 volts.

Q: I'm using an AmpFlow E30-400 motor for my drum weapon and 2 E30-150 motors for my drive. Will they work if I don't use any controller or solenoid and directly supply power to them? What if I supply 30 or 36 volts to them? [Maharashtra, India]

A: [Mark J.] 'Brushed' motors like the AmpFlows do not require an external controller to run -- simply provide suitable DC power across the leads and they will spin. 'Brushless' motors have no mechanical commutation and require a suitable electronic controller to supply power to the correct field windings at the correct time. Without a controller, brushless motors will not spin. See Frequently Asked Questions #23.

For general information on overvolting see Frequently Asked Questions #25. Use caution in overvolting high performance motors like AmpFlows - they are already providing very high output for brushed motors of their size and weight. I recommend evaluating the performance and function of the motors at their rated voltage in the specific application you have for them and increasing voltage ONLY if the motors are showing no signs of stress.

Q: Hello. We've all seen videos of antweight full body spinners (FBS) colliding with another bot and then flying off the walls of the arena (ultimately ending upside down or outside of the arena). This is why generally [Ask Aaron] has advised to avoid building FBS at this weight class (even though it is so fun to build them!).

What do you think of mounting the spinner on a suspension between the spinner and chassis. When the spinner hits another bot it will recoil into this suspension, attenuating the impact over a longer period of time so the bot as a whole will not bounce back as far. I imagine that it will not throw the other bot as far either. The shell of the bot at least is still very rigid. it just has a soft linkage to the chassis. not sure if there is a benefit to doing this, what do you think? [San Diego, California]

A: [Mark J.] Your analysis is correct. Placing suspension between the horizontal spinner shell and the chassis could reduce the recoil to your chassis, but it would also reduce the effective impact of the weapon on your opponent. Effectively, it would help your opponent as much as it would help you! You might just as well reduce the power of the weapon - it would be simpler. I've gotta vote 'no' on this idea.

For maximum weapon impact in a horizontal spinner you want the structural path from the point where the weapon strikes to the center of mass of the robot to be as stiff as possible. Vertical spinners have a strong advantage in this regard, as the recoil path (downward) is eventually supported by the arena floor (and the planet beneath). Section 6.6 of the RioBotz Combat Tutorial discusses the importance of mounting stiffness in maximizing weapon impact -- a good read.

Q: Is a robot a computer? [United Arab Emirates]

A: [Mark J.] Many robots have computer processors that control at least some of their actions, but modern automobiles also have on-board computers that control many of their systems. Like an automobile, a robot not a computer - it is a machine that may be controlled by computers.

Q: mark i have 30 kg-cm torque 500 rpm motors this o/p is available at 12 volt what will be effect on output if i run it at 24 volt spply [Maharashtra, India]

A: [Mark J.] Doubling the voltage will approximately double the RPM, and will double stall torque if adequate current is available from the battery. Doubling the voltage will also shorten motor life.

Q: hey mark lucky here i am thinking to combine my design so that it should be combination of spearbot and drum bot whats your view [Maharashtra, India]

A: [Mark J.] See Frequently Asked Questions #26. My advice is to place your entire weapon weight allowance into a single weapon. Dual weapon robots have a serious drawback: two weak weapons that cannot work together are not as good as a single powerful weapon. Add the weight of that spear to the drum weapon to increase its power.

Q: Hi Mark, Chinese Forum guy again. I noticed during Robot Wars US days there was a "pinninglifting for 10 seconds to win" rule, is that true? Because Biohazard made use of that rule in 96 semi final against Vlad to win. And was this rule abolished during early Battlebots years? If so how could Voltarc hold Biohazard for that long in 1.0 quarter final to win while Sean Salisbury commentated there's a 30 sec limit? I read old post on Battlebots forum by Stephen Felk but still can't quite get it. Thanks [China]

A: [Mark J.] Thru the 1996 Robot Wars event, a robot could win a match by pinning their opponent "against any arena surface" for 30 seconds (not 10 seconds). This rule wa abolished in 1997, and a robot pinned against the arena had to be promptly released. However, a robot that had apparatus to grab or lift an opponent could hold their opponent for up to one minute before releasing.

The rules continued to evolve, and by BattleBots 1.0 the rule read:

"BattleBots may not win by pinning or lifting their opponents. Judges will allow pinning and/or lifting for a maximum of thirty (30) seconds per pin/lift then ask the attacker to release. Matches will be paused to separate BattleBots in the event that they become stuck together."

Full rule sets for major events are available at the Run Amok Who Won webpage.

Q: hey mark i want ampflowE30-400 motor in india how can i get that 1? please help [Maharashtra, India]

A: [Mark J.] The word from Indian builders is that you can order AmpFlow motors direct from the AmpFlow website for shipment to India. Be prepared to pay as much as $150 for shipping, a 41.5% duty (payable in person at the customs office in Mumbai), and a few hundred rupees in assorted courier charges.

The preferred method of obtaining an AmpFlow motor is to find someone travelling from the US willing to purchase your motor and bring it to India. They will pay only a 37% customs tax at the airport.

I can't confirm that info, but it should give you some idea about what is involved.

Q: Hey Mark, I scoured the solenoid archive but I don't think I can find an answer to my particular question...

My question is, where does the power for the coil draw from? The particular solenoid I'm planning to use has a 'coil voltage' of 24V. The 'contacts' are rated up to 48V- In this instance I am powering a 36V motor by means of the contacts. Does that mean the 24V to activate the coil must come from my receiver battery, since that is what my Battleswitch is wired into? That battery is only 6V - am I risking not having enough power to activate the coil? What else can I do?

Thanks for the help. [Philadelphia, Pennsylvania]

A: [Mark J.] Your BattleSwitch gets power for its own electronics from the receiver connection, but it does not pass that power thru to the screw terminals. The power it switches comes from an external battery source. If the coil voltage is the same as the weapon motor voltage, you can use the diagram in the very first question in our Solenoid and Relay Guide to wire your weapon (reprinted at right - your battery will be 36 volts).

My recommendation is that you find a solenoid with a 36 volt coil to match the voltage you will supply to your motor (example). If you absolutely have to use the 24 volt coil solenoid, write back and I'll sketch a diagram of how to 'tap' your battery pack to supply 24 volts to the coil and 36 volts to the motor.

Solenoid wiring diagram

Q: Mark, Philadelphia here again.
I have already purchased the 24v coil solenoid, so I'd like to stick with it if I could.
I have two NiMH batteries with identical capacities- one 24V and one 12V. I was planning on wiring these in series to get my 36V. (I know this isn't recommended with batteries of different capacities, but I should be okay here- right?). In that case, would it be easier to break away power from the 24V battery for the coil? Or, if there are way too many red flags here, maybe it's better to keep everything at 24V and be safe all around...


A: Yes, you can get away with running two identical spec NiMH packs with different voltages in series. Check the diagram on right for the circuit to run 24 volts to your coil and 36 volts to the motor. The current draw for the coil is small (about half an amp) compared to the motor draw, so it shouldn't cause an imbalance problem. I think you'll be OK.

Solenoid powered from battery tap

Q: Hello Mark. After seeing Biohazard being torn up into pieces in Combots Cup 2005 I was wondering a question. Could 4-Bar Lifting arms still be competitive in Heavyweight Class? [Beijing, China]

A: [Mark J.] The high-energy spinner weapons in the US heavyweight class are brutally destructive. With judging leaning so heavilly on damage and no real possibility of boosting an opponent out of a US arena for a 'knockout', I don't think an electric lifter is a competitive design in the heavier weight classes.

Q: Dear Mark,I think Overkill and Toe-Crusher's design is quite different from other thwackbot. Is it has any advantages or disadvantages? [Yangzhou, China]

A: [Mark J.] 'Overkill' and 'Toe Crusher' are not technically 'thwackbots', they're torque-reaction hammers. There is a description of how they function and the drawbacks of the design in the Robot Weapons archive -- search there for 'reaction hammer'. A torque-reaction hammer has far too little power to be a contender in current competition.

Combat robot 'Toecrusher'

Q: Is 'Ripper' use the overlimit gas to fight (video)? This time is so powerful. [Guangdong, China]

A: [Mark J.] I can't translate the caption, but that looks like a fight from the 2013 Robot Wars. The FRA ruleset used for current Robot Wars events allows for a maximum pressure of 1000 psi at any point in a pneumatic system. I doubt very much that the technical crew would allow an overpressure pneumatic system to compete, and I certainly wouldn't accuse an established team of deliberately ignoring safety rules. I suspect the team just made some improvements in their pneumatic system.

Q: Hey Mark, Las Vegas here. I have been thinking on this relentlessly since yesterday - Consider a typical PMDC motor which will be used for the drive unit of the bot. Say it has a peak RPM of about 5000. Now we all know that the peak power comes at roughly around half the peak RPM; so it's safe to assume that the peak power comes at 2500 RPM. After 2500 RPM the power starts decreasing gradually until it reaches almost 0 at peak RPM. So coming to my question, can we somehow limit the peak RPM of motor to 2500 RPM? In that way we can use a much smaller gearbox, which will be not only cheap but also light in weight. Thanks in advance. [India]

A: [Mark J.] Unfortunately, there are a few catches to your plan, Las Vegas...

A PMDC produces its greatest power when the work load upon the motor requires a torque level which restricts its speed to half the unloaded RPM for a given voltage. If factors other than motor loading are restricting the rpm (reduced voltage, artificially induced load), the net motor output is also reduced.

The only valid method for doing what you suggest is by use of a continuously variable (CV) gearbox that would decrease the gear reduction as the robot gained speed to keep the motor loaded to a specific RPM. CV gearboxes are larger, heavier, more expensive and much more complex than a simple single-speed gearbox -- which defeats the gains you were seeking.

Motor performance curves

Q: Hey Mark,

I am building a 1lb ant weight vertical spinner bot. What is the best way to tell if a motor is adequate for a vertical spinner? I know that you can calculate what the kinetic energy in a spinning weapon. However, the motor I have (and I am trying to figure out if it is adequate) is a Rimfire 370 and is rated to 1000kV, weighs 1.4 oz and has a Burst Watts of 165. So basically, is there a way to look at the critical specs of a brushless outrunner (maybe there is a range of kV?) and know if it would be a good motor? I also know that the forces of a weapon are largely dependent on the radius and mass of the weapon itself, but I can assume that I can judge if said motor makes sense before designing the impactor. Does that make sense? If not, let me know.

Thank you very much,
New York

A: [Mark J.] You're on the right track, New York. The load on the motor when spinning up a weapon is dependent on the 'moment of inertia' (MoI) of the spinner, which does depend on the spinner shape, dimensions, and mass distribution. The challenge is in matching the MoI of the weapon to the power of the motor to assure that the motor can spin the weapon up to speed quickly enough to be useful.

If you're looking for a quick estimate of the suitability of the motor for a spinner, you'll want to look at the 'kV' and 'burst wattage' ratings:

  • The 'kV' of a motor tells you how fast the motor will spin for each volt of electricity it receives. Multiply the kV by your battery voltage for the no-load RPM of the motor. This is important because you don't want a motor that has to be 'geared down' a lot to provide a (not too fast) useful speed for your spinner. The Rimfire 370 has a kV of 1000, so with a 7.4 volt battery it will have a no-load speed of about 7400 RPM, which can be made into a reasonable spinner speed with a belt reduction of around 2:1. That's good!

  • The 'burst wattage' is an indication of the power the motor can produce to accelerate the spinning mass up to speed. Twice the wattage will spin a given weapon blade up to speed twice as quickly. The energy storage of the spinning weapon is measured in 'joules', and for an ant about 30 joules is a reasonable amount of stored energy. A joule is equivalent to one watt of power applied over one second of time, so your proposed 165 watt motor could (at peak output) store 30 Joules (watt-seconds) of energy in a spinner weapon in less than 1/5th of a second (30 watt-seconds / 165 watts = 0.18 second). That's huge overkill!
Now, in actual operation the motor has to start from a standstill and generates only a fraction of it's rated output power until well up in the RPM range - but even estimating that the average output of the motor as it accelerates is half the peak output, this motor has WAY MORE than ample power for an ant spinner. The Rimfire 370 would be more than enough motor for a 3 pound 'beetleweight -- for an ant I'd go with a lighter motor and put the saved weight into a heavier spinner bar/disk/drum. Around 50 watts output per pound of 'bot is plenty!

Q: Hello Mark, its Rishabh. Which motor is sufficient enough for robo war(max lim 20 kg) just for crashing and pushing opponent battle bot? [Gujarat, India]

A: [Mark J.] A rambot/pushybot needs more drive power than a robot with an active weapon. For ramming speed and pushing power in a small Indian arena I'd suggest a combined output of least 360 watts from your motors.

I don't know all of the motors available in India, but I understand that a version of the RS-550 motor (also called a 'small Johnson') can be purchased from local suppliers. The versions of this motor available from US robot suppliers are typically in the 250 watt output range. A pair of these motors geared down about 20:1 with 2.5" to 3" wheels would give good performance in a 20 kg 'bot.

The Team Tentacle Torque Calculator is an excellent tool for selecting appropriate motors for combat robots of all weights and configurations.

Q: Which bot is the most flat bot - Biohazard, Panic Attack 24K, Brutality or Original Sin 2? [Guangdong, China]

A: [Mark J.] Tough question.

  • We can rule out 'Panic Attack Gold' (24K). The photo you've probably seen has been 'color filled' and is misleading; the 'bot actually has has a large gap between the apparent front edge of the body and the arena floor. It's at least twice as tall as the photo leads you to believe.

  • 'Biohazard' is just a bit over 3" tall with the lifter retracted. This is verified from photos and text on the official website.

  • Comparing videos of 'Megabyte vs. Biohazard' with 'Megabyte vs. Original Sin 2' shows that OS2 is close to the same height as Biohazard. I've written to builder Gary Gin to see if he can provide a measurement.

  • 'Brutality' has a lower body height than 'Biohazard', but photographic analysis of stills from their fight clearly shows that Brutality's overhead spinner weapon sticks up to a greater height than the top of Biohazard.
That's the best I can do. If Gary Gin writes back with a measurement I'll update this post.

UPDATE: Gary Gin wrote back with some info on 'Original Sin 2'. OS2 was based on a Heavyweight BattleKit chassis. Although the BattleKits were based on the chassis used by BioHazard, they are a bit taller and use 4" diameter wheels. That places the height of OS2 about 1" taller than BioHazard.

The height order - tallest to shortest: Panic Attack Gold, Original Sin 2, Brutality, BioHazard.

Q: hi aaron
is it must for me to use sabretooth controller for operating my E30-400 motor explain please? [Maharashtra, India]

A: [Mark J.] The output of an R/C receiver cannot directly control a motor -- you will require an interface to convert the receiver output signal into current/voltage to supply to your motor.

Depending on the specifics of the use you have for that motor (robot propulsion, spinner weapon, lifter, etc.) you may need simple on/off control, the ability to reverse direction, or full variable speed control.

  • An electronic speed controler ('ESC' - like those made by Sabertooth) can supply variable speed and direction control. The capabilities of the ESC must be matched to the voltage and current demands the motor will place upon it in the application you have in mind.

  • For simple on-/off control a power relay or solenoid can be combined with a small R/C switch. Read thru our solenoid quide for more information on this option.

Q: Hi Mark!
Just a question related to above ESC discussion....
An RC receiver can always be modified using relays or some other things to control the motor...and for a differential drive mechanism..all we need is on/off and direction reverse..
So why is that most combat roboticists prefer to go with an ESC rather than a power relay as you mentioned above? i.e what advantage an ESC gives over a normal power relay on/off salutation?
Thanks for your time.

A: The answer to your question is in the blue box at the very top of the solenoid quide that I suggested you read:

General Advice

Solenoid control can be a reasonable option for simple on/off control of a weapon motor.

For weapon or drive motors that must be able to reverse direction, an electronic speed controller costs very little more than a solenoid system of comparable capacity. An ESC is more reliable, lighter, less bulky, and provides much greater control.

For on/off with direction reversing you'll need two reliable DPST relays with large power handling capacity and a pair of R/C switches for each motor. The relays will weigh more, take up more room, give much less control, be more vulnerable to mechanical failure, and cost very close to as much as an ESC of comparable capacity.

Q: Hi, I wanted to build a combat robot. What all stuff I need to buy to make it wireless? Are there any resources? [Maharashtra, India]

A: [Mark J.] Read our 'Frequently Asked Questions'. Write back if you have additional specific questions.

Q: Hello Mark. I was reading Team Toad's competing experience in Battlebots 4.0 and came up with a question: Is it unclever to exchange weight used for mobility (for example two of the four motors driving Frostbite 1.0 and 2.0) for better weaponry (the powerful blades in front of Frostbite 3.0 and the drivetrain of it)? Worsened mobility let Surgeon General tear its wheels off while the blade sent Surgeon General's disc flying. [Chinese Forum]

A: [Mark J.] Everything about combat robot design is a compromise, and finding the right balance is tough. Worse, the right balance will change depending on your opponent.

I don't believe that Frostbite 3.0 lost much mobility in converting it to an active weapon 'bot. Their four heavy motors were replaced by two lighter but more powerful motors, and a chain drive was added to retain 4-wheel drive. The Team Toad website doesn't seem to blame reduced mobility for their loss, just that they weren't quite quick enough in their turn.
Q: Then another question: Can using electromagnets to let your robot closer to the ground be a good idea? Complete Control used it in 5.0 but it seemed like a failure, making it being very sluggish.

A: Magnets have been tried on many robots over the years. Most commonly, permanent magnets have been used to increase traction and pushing power in arenas with steel floors. There have been many successful magnet-bots, most in the insect weight classes. A good example is beetleweight 'Wollop', which uses axially polarized magnets for wheels. Team Nightmare claims that 'Wollop' can drive upside-down on a steel surface or even straight up a steel wall.

Beetleweight 'Wollop
'Wollop' chassis showing magnet wheels.

Q: Thanks for clarifying magnet question,but what led to Complete Control's failure in using this technology to get lower ground clearence in 5.0? Wrath Jr. still got underneath it easily while itself could hardly move around the box,is it because of the design or control?

A: According to Derek Young's build journal Complete Control Version 4 (BattleBots 5.0) used powerful permanent magnets for "balance assist and wedge hold down." He blames his failure in the match to his lack of preparedness -- he worked on the robot for 36 hours straight before leaving for the event. That left little time for testing, adjustment, and driving practice. I think the magnets might have been useful if enough time had been available to get the set-up right.

Q: Sorry for another question about fights in old days: Did Killerhurtz vs Surgeon General fight in 4.0 prove car style steering is not suitable for arenas like Battlebox? Its anti spinner device in front worked very well in the begining, but it can't steer quick enough to face SG's blade again. then the side armour of it got torn off completely. Same thing happened in 3.0 as well, Hexadecimator could always get a quick turn to get underneath Killerhurtz and flip it over, while Killerhurtz's only successful hit was made after Hexy D thought it couldn't self-right anymore.

A: 'KillerHurtz' had a sophisticated turning system that included servo-controlled front wheels plus a differential steering system for the two drive motors -- so it wasn't really a car-style steering 'bot. It could not, however, pivot as quickly as a standard differential-steer 'bot.

Quite a few car-steer robots had been tried at BattleBots, and none did remotely as well as KillerHurtz. I think it's fair to say that differential steering had been proven to be the more effective method for the BattleBox long before KillerHurtz' loss to Hexy-D.

Q: (1)how the starter motre works ? [Maharashtra, India]

A: [Mark J.] Video: How a Starter Motor Works. It would have been faster for you to look this up yourself.

Q: (2)which kind of moter will use to work(move) on car like suv and how many moter will use?

A: The electric Chevrolet Volt is propelled by a single electric motor with a peak output of 149 hp (111 kW) delivering 273 lb-ft (370 N-m) of torque. That's equivalent to about 33 powerful AmpFlow A28-400 robot motors.

Base Curb Weight of the Volt is 3786 lbs. A similar power to weight ratio will deliver similar performance for an SUV.

Combat robot questions, please.

Q: how to wind belt drives on wheels of robot so that it does not slips? [Maharashtra, India]

A: [Mark J.] You need a belt tensioner -- an adjustable pulley or wheel that presses against the belt between the drive pulleys to increase the belt tension. A trip to an automotive supplier or wrecking yard may provide a suitable tensioner.

Alternately, you may wish to use a toothed timing belt with matching pulleys that require little tension to operate without slippage.

Diagram of a belt tensioner in use on a flat belt

Q: Which relay is compatible with P17/040000(30 rpm, 12V, 17W) for battle bot of 30kg? [Gujarat, India]

A: [Mark J.] The more important question is 'Should I use this motor for my 30kg robot?' I'm not familliar with that specific (wiper?) motor, but if you plan to use it for the robot drive system you should reconsider!

  • Only 30 RPM? That's VERY slow. Even with large 6" wheels the robot would crawl along at barely 0.5 MPH.

  • Total motor power is far too little for a 30 kilo robot. General rule: you need a minimum of 9 watts drive power per kilo of robot. By that rule you would need sixteen of these 17 watt motors to power your 'bot.
Pick a different motor.

If you plan some other use for the motor and require only on/off control, a simple automotive relay will suffice. For more information on using relays and solenoids in combat robots, see our solenoid quide.

Q: hello. I am building a sumo bot. Max weight is 7 lbs and must use 12v motors. Can I use power window motors? [Ontario, Canada]

A: [Mark J.] You can, but window gear motors (like the AME-210) are relatively heavy, bulky, and very slow (about 100 RPM). Even with large 4" wheels, the speed of your 'bot will be barely more than 1 MPH.

Something like the PD27M gear motor weighs half as much, would give about three times the speed, and still has more than enough torque to maximize sumo pushing power.

Q: Hello sir my name is DpD me & my team make a robo for robo war it consist 2 high tork moter & 4 side veg bot so,what is a circuit diagram for robot & Rimot control? [Maharashtra, India]

A: [Mark J.] See Frequently Asked Questions #19. Your 'bot won't have a 'Weapon ESC' or 'Weapon Motor', but everything else is correct.

For R/C transmitter programming help, see the Run Amok R/C Transmitter Programming Guide.

Q: How to make a non weapon simple 4 side wedge robot with non wire less rimot control ?

A: I've never built a non-wireless combat 'bot, and I don't know what the Indian rules allow. Other Indian builders would have experience with wired remotes, so I suggest asking for guidance at the 'Combat Robotics India' Facebook page.

'Combat Robotics India' is a closed group that requires an invitation to join. If you need an invitation, write back to me with your Facebook account name and I'll invite you.

Q: i want to make a drum of 8 kg with length 15cm and dia 13cm, what material should i use for drum and teeth, and what should be the thickness??[Maharashtra, India]

A: [Mark J.] That's a very small drum to weigh 8 kg. A solid aluminum drum that size only weighs 5.5 kilos, so I guess you'd best use a mild steel alloy.

A steel drum 15 cm long and 13 cm in diameter with a 15 mm thickness weighs 6.34 kg. Add two steel end plates 5mm thick and the total weight comes to about 7.4 kg. That leaves you 0.6 kg for your teeth. Energy storage works out to about 1800 joules at 3000 RPM.

Threading large, short bolts into the drum and leaving the heads exposed works reasonably well for teeth. They are easy to replace when damaged and give you flexibility in placement and size. If you have more conventional teeth in mind, a hardened tool-grade steel (like S7 alloy) is the preferred material.

I will mention that it is unusual to design a drum weapon starting with a specific size and mass. Changes to the dimensions can greatly impact the energy storage capacity of the drum for a given mass. Some time spent with the Team Run Amok Excel Spinner Spreadsheet may yield a large bonus in performance with small dimension changes. Example: increasing the drum diameter to 15 cm with a 12.5 mm thickness will increase the energy storage by 22% at the same weight.

Q: if i use same size pulley to rotate my 8kg drum from ampflow e30-400 how will be the loss and how much rpm will i get?

A: A V-belt has very high power transmission efficiency: up to 95%. There is very little power loss. Peak RPM will depend largely on the aerodynamic drag of the drum and teeth. As a guess, I'd say perhaps 5200 RPM @ 24 volts. Spin-up time will be about 4 seconds and energy storage close to 4000 joules.

Q: Hey, i am from india. I have two wolfpack lipo's 8000mah 35c 3s... yesterday during their usage the voltage dropped slightly below 9v (went to 8.9v)... the batteries developed a slight puff... i use a charger with output current of 700ma... when im charging them they dont go beyond 11.1v overall... any comments on their state what could it be ? should i use them anyway? [Maharashtra, India]

A: [Mark J.] Do NOT attempt to use a 'puffy' lithium battery. Do NOT even attempt to recharge a 'puffy' lithium battery. It's dangerous!

Lithium batteries are succeptable to damage from excessive current draw or excessive discharge. When damaged, the battery releases hydrogen gas that inflates the battery covering and causes it to puff outward. A puffy lithium battery has reduced capacity and is in danger of bursting into flame with continued use. It should be properly disposed of immediately.

How to dispose of a damaged lithium battery:
  • In a plastic container, dissolve 1/2 cup of salt in 1 gallon of cold water.
  • Drop the battery into the salt water and leave it there for at least two weeks.
  • Remove the battery, wrap it in paper, and throw it in the trash.

A good tech inspection at any combat event should always examine lithium batteries for signs of damage and should immediately refuse entry of a 'bot using such a battery -- no exceptions. Why be so cautious about damaged lithium batteries? Here's what happens when a LiPoly ignites.

Q: Hey mark. One question i want to ask. How are brushless motor for 132lb bot? Its an eggbeater with weight 20kg. I was thinkng to power it by two f-series ampflow. I heard that touro uses brushless scorpion hk 5035 500kv. [India]

A: [Mark J.] Note: it's heavyweight 'Tauro Maximus' that uses the Scorpion brushless motors for propulsion, not middleweight 'Tauro'.

Few robots use brushless motors for propulsion. I consider brushless propulsion to be experimental at his time -- you'd better be ready for many trials and many failures. Amongst the challenges:

  • Most hobby brushless motors are 'sensorless'. Sensorless motors rely on the controller to read the electrical properties of the stator windings and logically infer the position of the rotor. This is difficult to do when the motor is stationary or under heavy load, and the controller may have to 'guess' rotor position at startup. This process may result in poor starting torque and erratic low-speed performance.

  • Finding a reversing brushless ESC to match your motor is difficult! Most brushless motor applications are uni-directional, and R/C car brushless ESCs may have limits on the number of motor poles they can control.
The selection of a brushless ESC is much more critical than selection of a brushed ESC. The brushless ESC must not only control the amount of power going to the motor, it must also control the timing of power pulses to the motor -- something done by the commutator of a brushed motor. You may have to try several different ESCs to find one that matches the properties of the specific brushless drive motor you choose. This will be largely trial and error unless you find someone with a working motor/ESC combination that you can copy.

My advice is to leave the expense and frustration of brushless drive motor experimentation to other builders. Use the proven AmpFlow motors for propulsion.

Q: Hello! I was wondering what gear reduction is? My science teacher has assigned us to make a mousetrap car, and I really want to get a good grade! :D So, could you please explain it to me? Thank you bunches! -Lizzy [Mount Union, Iowa]

Typical mousetrap car A: [Mark J.] I love mousetrap cars! You can get some good help on design at the Doc Fizzix Mousetrap Car site.

Gear reduction is a method of changing the output characteristics of a mechanical power source. Your power source is the spring-loaded 'snapper bar' of the mouse trap. If you pull that snapper bar over and let it go it moves very forcefully back to its original position, but it only moves about half a revolution. You will want the wheels on your mousetrap car to revolve many revolutions, and you will want to reduce the force (torque) of the snapper bar to keep your wheels from spinning and wasting power. Gear reduction is used here to reduce the force (torque) of the bar and increase the number of revolutions the wheels will revolve under power.

You won't be using any actual 'gears' in your mousetrap car, but a string attached to the snapper bar and wrapped around the axle of the drive wheels. When the bar is released it will pull the string and cause the axle to revolve many times while the bar only moves thru half a revolution.

I hope this helps -- best luck with the project!

Remembering Aaron...


Q: how can robots help us deal better with hurricanes and why? [Ontario, California]

A: [Aaron] Few people in Nebraska are threatened by hurricanes, so send a swarm of killer robots into low Atlantic and gulf coastal areas to drive the puny human inhabitants toward Nebraska. Problem solved.

Robot haiku:
That's obviously
A question from your homework.
Do your own research.

Killer Robot drawing by Garrett Shikuma

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