Ask Aaron: questions and answers about combat robots

7111 Questions and Answers about Combat Robotics from Team Run Amok

Team Run Amok receives a lot of email about designing and building combat robots. In 2003 my son and team member Aaron Joerger (then 12 years old) asked for a question and answer page to document our responses.

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.

Click to browse thousands of previously answered questions by category, or search for specific topics. Includes

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.

Tweakeroo
I'm having an issue with my beetleweight. I am running 4 silverspark motors with fingertech ESCs on 4s. I know silversparks aren't great in a beetle, but it's an overhead spinner and I couldn't find another option that was flat enough to work.
When I try to turn while at a complete stop only two motors, one on each side diagonally, will spin. All 4 motors spin just fine going forward and backward, and I can turn just fine when I am already moving forward or backward. My guess is that it is an issue with the load on the drive motors, but I'm hoping it's not cause there's no way to change those with this design.
Have you come across this or do you have any suggestions on what to try to improve it? [Social Media]
A: [Mark J.] Your chassis is tweeked. It's warped enough to put most of the weight on two diagonal wheels and very little weight on the other pair of wheels. All the wheels are actually spinning, but one pair is spinning so much faster that the other pair looks like they're just dragging.

The lightly loaded wheels are breaking traction and spinning freely in a stationary turn while contributing little to the turning motion.
The heavily weighed wheels are getting much better traction, doing all the real work, and cannot break free to spin as rapidly as the other pair.
Un-tweek the chassis to get equal weight on all wheels and you'll be fine.
If the chassis itself can't be de-tweeked you can shave down the tire diameter a bit on the wheels that have too much weight on them to shift weight onto the other wheels.

A Robot Walks into a Bar...
Q: What’s your best robot combat dad joke? [Belmont, California]
A: [Mark J.] One of several from Aaron's Greatest Hits:
Q: Can you tell me how to make a robot walk?
A: Take away its bus pass.

Nothing More to Prove
Q: If Robotica had more challenges rather than just 'The Speedway', 'The Maze/Labyrinth', 'The Gauntlet' and 'The Fight to the Finish' (coz i think there were some games that were planned but scrapped) what events do you think they would put in? for me, remember there was meant to be a 'Storm The Castle' event, well, if there was one It would rather be called 'The Fort' or 'Storm The Fort' or something and yes the robots would break through barriers enter a fort and take on a guardian bot and i wonder what was the guardian bot please don't say the Robotica Rats. [Inchinnan, Scotland]
A: [Mark J.] There were a number of challenges were considered and abandoned in the development of 'Robotica' but none of them moved past the early planning stage: no blueprints, no guardian robot design, no artwork.
For season 1 the show developers settled on three preliminary challenges that tested two robots at a time for speed, agility, and power:

The Speedway A test of speed and stability that perhaps drew inspiration from the Season 1 UK Robot Wars trial "Stock Car" figure-eight race, but with competitors running in opposite directions!
The Maze/Labyrinth A showcase for agility and control that combined elements requiring tight maneuvers and delicate coordination between machine and operator.
The Gauntlet In spite of sharing a name with the early UK Robot Wars stage, Robotica's challenge was a unique arrangement of progressively more difficult barriers to test applied power and the ability to deal with the resulting debris.
Given the elements tested by these challenges I'm not certain what an additional challenge might add that would be unique. What new capability might be proven in a 'Storm the Castle' challenge, and how do you assure that a 'Guardian Bot' is an equal challenge for two competitors at the same time?
The developers did a fine job of selecting a set of challenges that fully tested the abilities of the competitors, and "The Fight to the Finish" was the perfect concluding element.

Keeping it Simple
Q: If Robotica the TV Series was rebooted what ideas would you think would be good, for me i would say:
Bring the speedway back shaped like the hot wheels criss cross crash set where all 4 robots come to play each turn with 4 obstacle 1.sheet of metal, 2.metal cans, 3.red bricks and 4.concrete cement pillars, and elevated bridges to the centre platform with red and blue glass panes and a fifth and final ramp with an orange pane of glass, a medium sized pit, bring back the labyrinth with almost all the obstacles with new ones including pulverizers, rapid spinning turntables, flame pits and towers one with a 90 degree turn the other a full circle turn with 4 robotica rats, cyan (which is a light blue), orange, purple and green, have a new event called the arena (similar to the fight to the finish but unelevated with the walls being higher) where the robots fight in a minileague and score 50 points for the winner by Knockout and 0 points for the loser and in the event of a judges decision on Style, Control, Damage and Aggression, 30 points for the winner and 20 points for the loser, and then bring back the fight to the finish itself with the platform elevated and larger giving the bots more room to drive in with the same way the fight to the finish from previous series works with rails falling over after 1 minute.
And i don't mind TLC bringing it back but maybe Discovery has the chainsaws flamethrowers and axes. [Inchinnan, Scotland]
A: [Mark J.] Quite detailed and imaginative. I personally like to keep things a bit simpler. Antbotica works well and has ample opportunity for combat in every stage. I would scale it up to heavyweights. Q: Now this is something that always get me, in episodes 3 4 5 6 and 7 of robotica season 1, it would usually be red team vs blue team first, then silver team vs gold team and purple team vs orange team (only in the championship) BUT... in the speedway and maze of episode 1 and the gauntlet of episode 2 it was silver team vs gold team first, then red team vs blue team. why's that? (i wonder what the speedway would look like and be called if it was in seasons 2 & 3)
A: The order of events in the televised version of the show had little relation to the order in which those events were filmed. For example: the two shows that would eventually become episodes 1 and 5 were filmed on day two of the four filming days.
The order of teams in specific events within each episode was edited together in whatever order the series producer believed would be most entertaining for the television audience. I specifically remember in filming episode 1 that 'Run Amok' (red) versus 'Spring Breaker' (blue) was filmed before 'Killer B' (silver) versus 'Mini Inferno' (gold). See our Robotica Competition Journal for more filming details.

It's All About Torque
Q: How powerful are the brushed Nerf motors for beetleweights? Are they as strong as the brushless outrunners? Are there any tricks to getting them to work well in a beetle? [Reno, Nevada]
A: [Mark J.] It's difficult to directly compare brushed and brushless motors, but brushed has certain advantages:

The first nice thing about brushed motors is that they produce much greater torque in the lowest RPM range than do comparable brushless motors.
The second nice thing about brushed motors is that there is no need to experiment with the motor controller firmware to get reliable start-up performance.
I've mentioned before that you don't need either brushless or Nerf brushed motor performance to be competitive in the beetlewight class. Stock Kitbots motors are plenty. I've recently added a few Nerf motors to the Tentacle Drivetrain Calculator so that you might compare the performance of these high-performance motors to the Kitbots motors:

DartBox Viper Gearmotor
DartBox Dragon Gearmotor
Nerf: Fang ReVAMPed
Nerf: MTB Neo Rhino
The drivetrain calculator does not include brushless motors because their performance is so dependent on the set-up of the brushless ESC used with them, but if I were building a beetle and wanted stupid performance levels I would design the 'bot around Nerf brushed motors for their ease of use.
Like any brushed motor, it is important to 'run them in' at low-speed/no-load to allow the brushes to wear a bit and match the curve of the commutator. Failure to do this may result in electrical arcing that can damage the brushes and commutator.

Put a small drop of lubricating oil on the motor bushings/bearings.
Secure the motor and apply about 1/4 to 1/3 its rated voltage.
Allow the motor to run unloaded for 10 to 15 minutes. Monitor for heat build-up and pause for cooling if needed.
The motor speed should be constant and stable at the end of this period - if not, continue until it is.
Reverse the polarity and run in the opposite direction for 10 to 15 minutes, as above.

A properly run-in motor will run cooler, produce more power, and be more reliable. It's well worth the time and effort.

As Much Art as Science
Q: Hello, I'm the guy that had the 1lb vert that wouldn't spin. The ESC you recommended cleared up a lot of headaches, so many thanks.
I recently competed in my first competition with my plastic 1lb Drum spinner, 'Rupture'. While getting some good hits and damage with the weapon, the drum had a tendency to chip off large chunks of the drum after a few hits, rendering the drum useless due to unbalance. It seemed to consistently shear off in roughly the same spots on each end of the drum, but I'm not 100% sure why. Each drum was 80% infill PETG, but consistently shattered on impact against robots even without active weapons, even when spinning at lower speeds. Any ideas on how to fix this? [Redmond, Washington]

A: [Mark J.] 3D printing is sometimes as much an art as a science. PTEG has some printing requirements that - if not met - can result in a brittle print that fractures in just the way your drum is failing. There are print tweaks to correct the problem but it may take some experimentation to find the right combination. Some of the possible actions likely to improve your print:

Dry your PETG filament in an oven at 60°c for a minimum of 6 hours prior to printing;
Decrease cooling fan speed or turn it off entirely;
Slow printing speed to 55mm/s or less;
Increase printing temperature;
Calibrate extruder steps;
Increase wall thickness.
More information: Eleven Ways to Fix Brittle PETG Printing

It Isn't Worth the Effort
Q: should i order pizza [Cardiff, Wales]
A: [Mark J.] I've had pizza in Wales. Don't bother. Go out for crempogau.

Why Would I Change?
Q: If you had your druthers, would you have preferred to run the S1 version of the Robotica gauntlet or the S2-3 version? [Cambridge, Massachusetts]
A: [Mark J.] We designed 'Run Amok' to meet the frequently changing specs for the Season 1 challenges. Those specs did not "finalize" until 3-weeks prior to the shipping date for our 'bot, and a few unexpected changes were waiting for us when we arrived on set. We believed that many of the competitors would have too narrow a design focus and encounter trouble in whatever the final versions of 'The Maze' and 'The Gauntlet' turned out to be, so our design mandate was to make 'Run Amok' capable of completing variations on each of those challenges. Our belief turned out to be correct. Although I think 'Run Amok' would be capable of scrambling thru the Season 2-3 version of The Gauntlet obstacle walls (with our wedge removed), the added challenge of the 'glass forest' at the end would have put us at a serious disadvantage to more conventional tank-steered robots. We would have designed a very different robot to compete in the later Robotica seasons.
On a 'just for fun' basis I would have liked to have had a shot at the Season 2-3 challenges, but 'Run Amok' was designed for Season 1 and that worked out just fine.

Four Years Later...
Comment: Hey, I saw you mention my use of JMT R/C Car motor controllers in 'Melanistic Leopard'. I just wanted to note that I used them for a short time in 'Shadow Leopard' after that but have since stopped, because their lack of drag brake became too annoying to put up with. I will note that since the post I made on Reddit back in 2018, I encountered a weird issue with them where they only actually work on 2S or 4S, and attempting to run them on 3S actually causes them to catch fire. They also tend to lose internal power to the logic side of the controller and the BEC when run on 4S, so the limiting factor to their voltage seems to be the voltage regulator.
I'm still willing to recommend them for people who are just messing around with fun builds, but I wouldn't put them in anything intended to be competitive. [InquisitorWarth]
Reply: [Mark J.] Thanks for the update. If you've only got $7 and you need a pair of brushed motor controllers for your beetle, JMTs may be your best answer. I know there are builders who love to save a few bucks but as your notes point out: cheap comes with weird -- particularly if you pay little attention to voltage ratings. I'm glad you were able to get some use from them and I think your current advice is sound.

A Cool Dry Basement is ideal
Q: I’ve been fighting combat robots for a while now and have learned a lot thanks to the awesome community and resources out there. I first want to extend a huge thank you to you and everyone else. Unfortunately, life is starting to get in the way and I will have less time for robot fighting. What’s the best way to store a combat robot long term? In particular, any tips on safely storing LiPo batteries? Thank you. [Beaverton, Oregon]
A: [Mark J.] The combat robot community is most certainly awesome and giving, but it's sometimes difficult to seperate the factual from the apocryphal. Pick your sources carefully. Yes Reddit, I'm looking at you.
Combat robots come in all sizes and are constructed from a wide selection of materials and components. Fortunately the common materials require no special storage conditions. Storage at room temperature and humidity is just fine, with a few precautions:

Bleed off any pneumatic system pressure and leave all valves open.
Any foam rubber tires should be removed and stored in sealed plastic bags to limit degradation of the rubber.
The LiPo battery should be removed and examined for signs of damage. A swollen 'puffy' battery should be disposed of immediately. See the Ask Aaron LiPoly FAQ for disposal instructions.
If healthy, balance charge or discharge the battery as needed to 3.8 volts per cell. Your charger may have a "Storage Charge" setting to achieve this voltage level, or you may charge/discharge manually.
Place the battery in a LiPo charging bag or a vented fireproof container. Store at room temperature away from flamable materials and limit temperature fluctuations.
Some sources recommend storing LiPo batteries in your refrigerator; DO NOT do this. Condensation forming on and in the pack when removed from cold storage can ruin the pack.

Returning the LiPo to service after prolonged storage may take a couple of charge/discharge cycles to restore the pack to full capacity.

Open the Pod Bay Doors, HAL
Q: Are there any combat robot podcasts you might recommend? [Reno, Nevada]
A: [Mark J.] The Robot Report keeps a list of botcasts that represent a wide range of styles and focus. Some are labeled Family Friendly and some Mature. You should be able to find something there that suits your interests.
I'm personally quite fond of Rise of the Robots -- a good-natured look back at televised combat robot shows from all eras and networks. More than 170 episodes available and a new cast each week. Their Facebook page says they're releasing a super special episode on Friday, September 2nd... I wonder what that might be?

Maybe Something in the Middle?
Q: Apologies if this is a stupid question and exposes me as a relative newcomer to the sport, but I’ve recently and unexpectedly gotten my hands on a Taranis X9D+, which is a significant step up in complexity and customizability from the Flysky FS-CT6B that I’ve been using. However, I’m running into some trouble choosing a receiver to use with it in my beetleweight. Due to its price, form factor, and availability from online retailers, I’d been considering using an XM+, but that then raised the issue of dealing with it only outputting SBUS rather than straight PWM, something I’m not familiar with.
As a beginner, do you think the added features of the X9D+ are worth the headache of upgrading, given that I currently have a functional transmitter/receiver pair? Is there an obvious choice of receiver that I’m overlooking? Any input would be greatly appreciated! [Biddeford, Maine]
A: [Mark J.] Many people were kind enough to assist me when I started out in combat robotics. I appreciate the opportunity to pass on that help. Thoughtful newcomers are always welcomed here at Ask Aaron.
You have two radio systems out at the extreme ends of the performance spectrum, and I'm not fond of either for most combat robot applications.
Your FS-CT6B is a very basic radio just barely adequate for combat robotics.

Of the eleven evaluation points in my Combat R/C Functions Evaluation List it does six awkwardly and fails at the other five:

LCD Screen: No, the transmitter must be plugged into a computer to change settings. Quick adjustments are a problem.
Fail Safe: No options. You have to hope that your receiver and ESCs work it out correctly.
The Manual: A marginal Chinese translation ("This function offer the great help to beginner") and few combat users to ask for help.
Transmitter Channel Mixing: None. For single-stick robot control you'll need an on-board mixing module with limited adjustments.
Exponential Rate Response: Not on any channel. This limits your bag of tools to maintain fine control in combat.
New drivers typically don't appreciate how important these missing functions can be in improving their ability to achieve smooth and effective control of the 'bot. A well set-up full-featured radio system can make the driving task fluid and natural. You will benefit from stepping your transmitter game up a bit.

It's a VERY big step up in function and capabilityto the Taranis X9D+.

I cannot think of a single combat robot control challenge that the X9D+ cannot handle -- but all that adaptability and flexibility comes with the price of a complex user interface that requires the user to build functions from scratch rather than select them from a system of menus. Functions that are very simple to implement on a menu-driven transmitter may require a good bit of programming and de-bugging on the Taranis OpenTX firmware, but functions that are impossible on a standard transmitter can be relatively easy to implement.
Take a look at my Combat Guide for the Taranis Q X7 Transmitter. The Q X7 runs a slightly different version of the OpenTX firmware, but the process of creating functions is the same. The guide should help you decide if you are willing to take on the process required to set up these transmitters. I consider OpenTX radios to be massive overkill for most combat robot users.
About SBUS ReceiversCombat robot components in general do not play well with SBUS output so, unless you construct your 'bot around a drone flight controller module, an SBUS-only receiver like the FrSky XM+ drone receiver is right out. If you're going to stick with your Taranis ACCESS protocol transmitter you'll want a receiver with PWM outputs: perhaps the Archer R4 or R6.
My Suggestion: Sell your X9D+ and obtain a full-featured menu-driven transmitter. Take a look at my Combat Guide for the FlySky FS-i6. The FS-i6 is likely the most commonly used transmitter in combat robotics. It checks off all eleven of the points in my Combat R/C Functions Evaluation List, sells for about $50, and has a wide selection of compatible AFHDS 2A receivers. It will not work with the FS-R6B receiver you probably use with your FS-CT6B, but the tiny FS2A receiver would be a fine choice for your beetle.

My Conscience Won't Let Me
Q: weapon was around 2.8kg
Propdrive 50-60 380kv
Red brick 200 amps
This was our previous weapon config.
The redbrick burnt out every battle.
How to solve this issue [Karnataka, India]
A: [Mark J.] A poem:
Harry built a spinner bar,
It weighed three thousand grams.
He spun it with an outrunner,
The ESC went BLAM!
He fights in southern India
In arenas far too flimsy.
Won't help him fix his weapon, 'cause
My conscience just won't let me.
See also: Why I don't answer questions from builders fighting in India

Too Much to Ask
Q: I'm playing with my little FS-i6x transmitter and I don't see a way to mix 4 wheel strafing. Am I correct that the FS-i6x cannot mix mecanum or X-drive, but that the Taranis QX7 can?
Seems like the kind of thing that could be solved with a firmware hack? Or do I just buy my way out? [Reddit]
A: [Mark J.] You're asking a lot from a very inexpensive R/C transmitter. A Mecanum drive transmitter mix requires four custom mixes each involving three channel inputs. The FS-i6x has only three custom mixes and each mix can only involve two inputs. No firmware hack is going to help.
You could buy an on-board Mecanum mixing module for your 'bot from Robologic or a few other suppliers. They run $50 or $60. I would recommend puting that money toward a more capable transmitter.
It is very simple to program a Mecanum mix on an OpenTX transmitter like the Taranis Q X7: Mecanum wheel omni-drive mix in OpenTX.

Better to reign in hell...
Q: I don't understand the NHRL weight bonus for multibots. Why are they giving a weight bonus? How much is the bonus for a beetleweight? Are any other events doing this? [A Cry in the Darkness]
A: [Mark J.] The organizers at the Norwalk Havoc Robot League seem to enjoy experimenting with robot design and construction rules. As long as they run safe events quirky rules are fine with me, but some of their rules violate the Do's and Don'ts of Writing Combat Robot Rulesets. A few head scratchers:
3lb and 12lb Sportsman bots must fit into a 30 x 30 x 24 inch cube at the start of the fight. 12lb and 30lb bots must fit into a 36 x 36 x 36 inch cube at the start of the fight.

What are they trying to prevent here? Why is there no size limit on beetles? The usual rule is: "If it fits thru the arena door it can fight." A few words of explanation would reduce builder frustration.

Weapons that primarily act by obstructing visibility (e.g. smoke grenade or fog machine) are not permitted. Weapons that produce smoke or fog as a by-product of their attack (e.g. rocket motors) are allowed.

OK, we can all agree that it's bad if nobody can see what's going on because you set off a smoke grenade. But if nobody can see what's going on because you set off a rocket motor that's fine?

Batteries that are charged inside the bot are permitted if the bot measures less than 11 x 8 x 6 inches.

Why does the rule address arbitrary robot dimensions instead of the type and capacity of the battery? This seems less like a safety issue than a whim. Explanation?

Traditional (usually wheeled) motion systems are generally defined as a locomotion system that operates primarily through angular momentum of the component touching the ground, but also includes any system that uses unpowered rotating objects (wheels, drums, rollers, ball bearings, etc.) as a means of friction reduction. Any motion system that does not meet this definition qualifies for the weight bonus.

I have no idea what might qualify under these criteria. What does 'angular momentum' have to do with this? Any rotating object has angular momentum -- did they simply mean 'rotation' but wanted something that sounded more technical???

Back to your question: NHRL weight bonuses become complicated because the non-wheeled and multibot weight bonuses may be combined under some circumstances, but the 12-pound classes have an absolute weight cap below the total of the two possible bonuses. Why are 12-pounders treated differently? No explanation is given. Excerpted from the NHRL rule set:
Multibot Bonus
An entrant consisting of multiple independent bots qualifies for a weight bonus. Every bot in a multibot must have active control and be able to influence the fight. In 12 lb and 30 lb entrants, the maximum weight of any single bot cannot exceed 110% of the normal bot weight. Only the heaviest bot needs an active weapon.

Weight Bonuses
Weight Class Non-Wheeled Multibot Absolute Maximum
3 lb +2 lbs +1 lbs 6 lbs total
12 lb (including Sportsmen) +6 lbs +3 lbs 19 lbs total*
30 lb +15 lbs +8 lbs 53 lbs total

* 12 lb bots cannot fully take advantage of both weight bonuses at the same time.
Good rules are clear and objective. Unfortunately,
Every bot in a multibot must... be able to influence the fight.
...is not objective. Two reasonable people might disagree on what a 'bot must be capable of doing in order to 'influence the fight'.
I assume that the clause is there to keep a builder from entering a 3-pound 14-ounce uber-beetle and a useless 2-ounce fleaweight, but an objective cap on the maximum weight of any single 'bot in a multibot entry (as the 12 and 30 pound classes already have in this ruleset) would (IMHO) be preferable to leaving the decision up to the whim of the organizer.
As written it is possible that any of the following combinations - and more - could fight as a beetle multibot:

a pair of 2-pound wheeled bots
a 3-pound beetle and a 1-pound antweight
a 3-pound 8-ounce overweight beetle and an 8-ounce minibot
a swarm of twelve 150-gram fairyweights

And because for multibots only the heaviest bot needs to be non-wheeled to get the bonus:

a 5-pound non-wheeled bot and a 1-pound wheeled antweight
a 3-pound non-wheeled beetle and a 3-pound wheeled beetle
a 1-pound non-wheeled antweight and fifteen 150-gram fairyweights!

To the best of my knowledge, no other events are running the NHRL ruleset.

Can't Stop Shakin'
Q: Hi, I'm 11 years old and new to combat robot building. I just built a fairy weight bot with a hammer that's operated by a servo and everything is operated by a Malenki Nano ESC/receiver. I have a mode 2 Flysky i6 transmitter.
The transmitter's right stick is operating the drive and that all works ok. I have the left stick operating the hammer and am having issues with this. When I try to use the hammer it doesn't just move forward and backwards but also shakes a lot when I'm not touching the stick. Do you know why this might be happening and is there a way to fix it? [Westlake Village, California]
A: [Mark J.] That shaking is called 'servo jitter' and is often caused by electrical interference or voltage fluctuation. It would help to know what servo you are using, but I can offer some general guidance:

If your servo is wired as shown in the diagram above it MUST be rated for operation at 8.4 volts or more. Most servos are rated for lower voltages and will behave erratically or fail if powered directly from a 2S LiPo battery.
If your servo is not rated for 8.4 volts you may use a Battery Eliminator Circuit (BEC) between the battery and your servo. The BEC must supply a voltage within the operating range of your servo with enough current to support full stall torque -- at least one amp for a micro servo.
The servo signal wire (white or orange/yellow) may be too close to specific components on the Malenki ESC or the drive motors. Try moving the signal wire away from sources of possible interference.
Some inexpensive servos are very sensitive to jitter. If everything appears to be in order and you are still getting uncontroled servo motion you may want to upgrade your servo.

Write back and tell me more about your servo if you need help selecting and installing a BEC.
Q: First off, thanks so much for answering my question! I have a TIAN KONG RC MG90 micro-servo. It looks like it's only rated for 4.8 volts. Is there a certain BEC I would use with this? This is a picture of my bot and what the inside looks like so I don't have much room. For the amount of space I have, would it be better to get a good higher rated servo and are there any you would suggest that are not too expensive?
A: Nice little 'bot! I'm always happy to support new builders.
You need a BEC that supplies enough current for full-power operation of your micro-servo (about one amp) at a voltage it can handle. I checked the manufacturer's specs and your Tian Kong servo is rated up to 6 volts, so a small BEC like the ShareGoo 5 volt 3 amp BEC would do nicely. It weighs about 1/4 ounce (7 grams), costs about $8, provides more than enough current for your servo, and it looks like you have room for it. You can cut the wires shorter to save space. It installs like so: If you'd rather swap out your servo I have a couple of low-price options:

The distributor for the Malenki ESC in England recommends the ALZRC DS452PM Micro Servo for direct use with 2S LiPo voltage -- but I can find no US sources for this product. You can buy one on eBay for $12 but it takes about a month to ship direct from China.
The EcoPower 827 Metal Gear Micro Servo is available from multiple US sources for about $15. It is rated for 8.4 volts and provides good performance for its price class.
Using a BEC to drop the voltage to your servo will reduce the speed and power of your hammer by a bit, while switching to a high-voltage servo will keep your weapon performance close to its current level.
Comment: Thank you so much for all your help! Your website is really cool. Can't wait to get my bot working

Pork Low Mein
Q: Can you tell me the best kind of battery for a beetle weight robot as I would love to start making one soon and obviously I'll need some sort of battery to power it? I would truly appreciate the help and advice. [Stratffordshire, England]
A: [Mark J.] It's not good design practice to start with a battery and select a robot design based on the restrictions imposed by that battery. That's like picking a restaurant before you know what type of food you want; you may end up eating a burger at a Chinese cafe.
Now, you CAN simply select a 3-cell Lithium Polymer battery with 850 mAh capacity and a 35C discharge rate. Such a battery is almost certainly adequate for a first-time beetleweight build, but is likely bulkier and heavier than required. Decide on a design and suitable components for your robot, then we can select a battery correctly sized to your task.

How TV Does Things
Spoiler Alert if you haven't watched Robotica.
Q: So I've been watching "Robotica" through the Internet Archive link you posted and I was wondering: were the team colors indicative of an internal seeding system? I noticed that every single Red team managed to make it to their show's 'Fight to the Finish', and there was a 50/50 split over whether they faced the Silver team or the Gold team in the arena; plus, 2/3 of the Red teams made it to the finals (alongside one Gold and one Silver).
I did re-read your Robotica journal and saw that you mentioned getting first seed, so I figure such a system existed. [Lake Oswego, Oregon]
A: [Mark J.] It was never officially discussed during the filming, but yes the team patch colors for the preliminary rounds did represent a seeding system based on qualifying performance -- at least for season 1. The filming schedule complicated things a bit...
The six preliminary shows were filmed over three days. Each morning there was a qualifying session open to any and all teams on-site. The top eight qualifiers would compete in the two complete shows to be filmed that day. Failed qualifiers could try again the next day. Details of the qualifying process are in our Robotica Journal.

The first day competitions became episodes four and six;
The second day competitions became episodes one and five;
The third day competitions became episodes two and three;
Our flight got us to the studio too late for day one qualifying, but we were fully ready for our shot on day two. My somewhat fallible memory has the top eight qualifiers for that session finishing in this order:

Robot Assigned Color
1) Run Amok Episode 1 Red
2) JuggerBot Episode 5 Red
3) Mini Inferno Episode 1 Gold
4) Krypler Episode 5 Gold
5) Killer Bee Episode 1 Silver
6) Hamerschlag Episode 5 Silver
7) Spring Breaker Episode 1 Blue
8) Noll Episode 5 Blue
Each episode paired a Red team with a Blue team, giving Red teams the advantage of competing against a much weaker qualifier. The Gold/Silver matches paired competitors of similar strength and were generally more closely fought.
The episode 7 finals were filmed on day four and required two new team patch colors (Purple and Orange) to accommodate the field of six competitors. Assignment of colors for the finals appears to have been arbitrary, pairing the winners from episodes 1-2, 3-4, and 5-6.
Robotica Seasons 2 and 3 were filmed together in a single studio session and featured re-designed challenges for the robots. There were also no extra robots to weed out; all of the pre-selected competitors got to compete. There were no qualifying sessions.

No single team color assignment dominated in Season 2.
Season 3 featured returning teams from Season 1, and although five of six alumnae made it to the finals, not all of the alumnae shared a single team color.
I asked Team Juggerbot lead Mike Morrow what he could recall about the team color assignments and match pairings at the filming of Season 3:
We were an alumnus [from Season 1] and each alumnus was assigned a different episode so they had to fight their way thru new competitors to the finals. I'm not sure how they decided the early pairings but I really think everything was simply manipulated by [the producer] to attempt to maximize screen appeal.
I've re-drawn the tournament trees for Robotica season 1, Robotica season 2, and Robotica season 3 to show the team patch colors and assign episode numbers to the matches.

Effectively Incalculable
Q: Hey, Mark! I know there are a lot of shell spinners that use hollow shafts for their weapons. I was wondering if there was some sort of rule of thumb when it came to determining the thickness of the shaft’s walls? I’m in the process of designing a shaft myself, which is for a 1lb robot and has a 1/8in hole. The shaft itself is currently planned to be machined from 7075 and attached to my robot with bolts and a mount built into one end of the shaft. I know a lot of robots in the 1lb class use 1/4in 18-8 stainless steel shoulder bolts and they hold up well, as such initial plans were to have a 1/4in OD on the shaft. However, I was worried with a wall thickness of only 1/16in if that would cause the shaft to easily bend or snap under load, especially while handling a weapon that’ll be 8oz or more. The image I provided is a mockup of what I’m working on, designed with a 3/8in OD shaft… yes, this is for a full body eggbeater/drum [See this archived post for an example]. It’s something I’ve wanted to build for a long time not out of the desire to win, but because I’ve always loved the idea. To compensate for this more showboat-y design I have a few competitive designs in the works that’ll hopefully be up and running for an MRCA event next year! [Oak Lawn, Illinois]
A: [Mark J.] Unfortunately, the number of design variables makes this type of engineering decision a great deal more complex than adopting a "Rule of Thumb". Where 'I' is the moment of inertia of the cross sectional area of the shaft, 'σ_b' is bending stress, and 'y' is the distance from support to farthest loading -- the bending moment 'M' to be survived by the shaft is: Click the formula pic for a video description. Attempting to figure all this out for the unpredictable loading a combat robot weapon encounters is futile. I like the look of your 3/8" hollow stub axle - it passes the eyeball test. Give it a try.

Plug In a Module
Q: I'm considering an upgrade from my FlySky FS-i6 transmitter to the Taranis Q X7. Will I have to replace all my FlySky AFHDS 2a protocol receivers? [Dallas, Texas] A: [Mark J.] Out of the box the Q X7 supports only the FrSky ACCESS protocol, but the transmitter has a "Module Bay" on the back that accepts JR-style plug-in transmitter modules to allow use of various receiver protocols. Add a single or multiple protocol module supporting the AFHDS 2a protocol and you're good to go.

What To Do When It Stops
Q: Hello Mark, Even though its been years, I would still like to express my condolences for the passing away of your son Aaron.
Me and my son are newbies to robot building and I have a very rudimentary question regarding weapon motor. I have always wondered how does the motor continue running even when the weapon attached to it has stopped spinning or rotating.
What mechanism should I use to prevent the motor from burning?
Thanks in advance. [Long Island, New York]
A: [Mark J.] Thank you for your condolences, Long Island. The passage of years means little; I think of Aaron every day. Keeping 'Ask Aaron' going is the best therapy I have and I appreciate the on-going support of the community.
Your concern about stalling electric motors is well placed -- particularly for high-performance motors pushed close to their physical limits. A motor receiving power but unable to spin can draw enough current to damage itself as well as its controller and battery. The measures required to protect your weapon motor system will vary a great deal with weight class and weapon type:

A small hobby brushless motor powering a spinning weapon in an antweight robot may require no stall protection at all. When such a motor stops spinning the brushless ESC will electrically sense the stall and will limit current as it attempts to re-start rotation.
With increasing size and weight come additional challenges. Larger motors shed heat less efficiently and are also susceptible to damage caused by physical shock from sudden deceleration from weapon impact. It's common to find slightly slippy belt and pulley drives between motor and spinner weapon that can help with both stall and shock load issues. Large robots may have mechanical slip-clutches built into the weapon drive for more predictable slippage control than a loose belt, but with mechanical complexity come penalties in weight and reliability.
Motors powering non-spinning weapons like hammers, axes, and clamps have different needs in stall prevention. These weapons often use 'sensored' brushless or brushed designs that have greater stall current and reduced forgiveness before damage sets in. A surprising number of builders simply rely on their own quick reflexes to power-down such weapons when stalled: see this previous post. Some builders will also use "current limiting" motor controllers that can provide a few extra seconds of grace before things get out of hand.

There is a whole lot of trial and error in designing combat robots. You can expect to destroy a large pile of components getting the hang of this undertaking -- often in embarassing ways and with a crowd watching. I'll help where I can, but I'll need specific details of the design you're considering. Let me know when you get to that stage.

Just Stop It
Q: Is there any way to make a regular motor behave like a servo, that is, only pivot to a defined endpoint in each direction? Seems like you could have a good axe weapon if that was possible.[Cambridge, Massachusetts]
A: [Mark J.] A servo is a gearmotor plus an ESC and a sensor that tells the ESC the position of the output shaft. Adding a position sensor with an interface to the ESC effectively turns a gearmotor into a servo, and a web search for "convert motor to servo" will supply instructions and videos for this type of conversion.
But you don't need a servo for your purpose...
You're just looking to cut power to the motor when the output shaft reaches an end point in either direction. There are a few brushed motor controllers that have inputs for limit switches that are placed to cut power when the end of desired motion is reached but still allow power in the other direction to let the motor to return to the start point.
But you don't need limit switches either...
Take a look at this previous post from the Ask Aaron Weapons archive:

Q: Looking at robots like 'Sawblaze' and 'Skorpios', how are they controlling the end points of their arm movement? I know they aren't using servos at this scale, and I don't think they are simply using hard stops and letting the motor stall. Do they use limit switches? Are they building their own servo control system into their larger mechanism? Thanks. [Kansas City, Missouri]
A: [Mark J.] You may be surprised, Kansas City. I know multiple veteran teams that DO use hard endstops and DO manually cut power at the ends of travel. More parts equals more failure points, and a failed limit switch can disable a weapon just as effectively as having your opponent rip it off. Simple is good.
I asked the teams you mentioned for details on their weapon arms and got these responses:
Jameson Go writes: "[Sawblaze has] hard end points with rubber bump stops. [I have the ESC] current limit enough to do the job and give me time to stop commands in a stall situation."
Zachary Lytle writes: "[Skorpios] is far less complicated than you might expect. We believe the fewer things you have in the robot to break the better. So the arm is just clutched and it's Diana's job to turn the motor off before it fries."

Mea Culpa
Q: I was looking through the Combat Robot Hall of Fame and noticed that Killerhurtz was missing. Why is that? Is it an error, did someone hack into the website, or was it removed for some reason?
Also, I had never seen the robot Beauty on the list and then one day it just appeared out of nowhere saying it was inducted in 2009! What's up with that?
I'd really appreciate if you could answer these questions. [Direct Email]
A: [Mark J.] Someone has been paying attention! I did a reformat of the Combat Robot Hall of Fame a few months ago to make the page look more 'modern' and two unrelated things happened:

I simply lost KillerHurtz in converting the CRHoF to the new format. I also lost Panic Attack in this same process. My error. Thanks for reporting this! I have properly restored the entries for 'KillerHurtz' and 'Panic Attack' to the webpage.
Implementing the new format made me curious about changes in the number of votes cast in each ballot year. I went back thru the spreadsheets I use to tally the votes to gather this data and stumbled across an error in the 2009 balloting. I had incorrectly inserted a new row for a robot that had received its first votes in 2009 and threw off the vote count formula for robots in one section of the sheet. I corrected this error and found that the ballot count for two robots had been changed enough to effect their status:

Beauty had originally been given 'Honorable Mention' status in the 2009 ballot. Correcting the spreadsheet gave Beauty sufficient additional votes to move them into full membership. I created an entry for 'Beauty' that reflects their belated 2009 induction.
Envy had originally been awarded full CRHoF membership for 2009, but correcting the spreadsheet removed a handfull of votes and dropped them down to 'Honorable Mention' status. I removed the membership entry for 'Envy' and created an Honorable Mention entry for them under the 2009 ballot year.

I had intended to make a note in the CRHoF explaining the transposition of Beauty and Envy but got distracted by something and entirely forgot about it. I've now placed a note below Beauty's listing that explains what happened.

View, Cast, or Download
Q: I'm a big fan of the TLC Robotica series, but I'm having trouble finding some of the season 2 and 3 episodes. Would you consider uploading these seasons from your library to one of the video services? [Tempe, Arizona]
A: [Mark J.] We won't upload copyrighted video to the internet services -- see FAQ #13. However, I've recently learned that the Internet Archive has a complete set of Robotica episodes from all three seasons available to view, cast, or download:

Season 1 Complete
Seasons 2 and 3 Complete

Consider clicking on the Internet Archive's "donate" button and chipping in a couple bucks to support their work.

They Don't Get It
Q: What do your combat robots think of the current COVID-19 pandemic? [Kansas City, Missouri]
A: [Mark J.] My robots don't care. My robots don't spread, suffer from, or die from Covid-19 -- but you can. Don't be selfish. Follow the science. Stay safe.

Remembering Aaron Joerger, 1991 - 2013

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

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.

Aaron's Greatest Hits! More of Aaron's Poems Aaron at Nickelodeon Robot Wars Aaron's Minecraft High Dive Video Aaron's World of Warcraft Player Guide

We welcome combat robot questions. Please search the Ask Aaron Archives first to see if we've already answered your question. Recent Q&A are posted above.

Type your question into the box.
Click 'Choose Files' to attach - if needed.
Click 'Send'.

or
Why is there a 'Cheerleader' button?

Thousands more robot combat questions and answers in the searchable Ask Aaron archives.

how to design and build a battlebot

Questions and Answers about Combat Robotics from Team Run Amok.