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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.
A: Mark J. Our most popular pages vary a bit over time, but in a typical month these are the top five runamok.tech combat robot pages:
My local competition recently added a multibot bonus mirroring the one found in NHRL, and I plan on competing with a control bot. Assuming competently designed, built and driven 'Bots on both sides, I can't think of a way to deal with them whether or not they are themselves control bots or have some sort of weapon.
A: Mark J. I had no idea that there were combat robot events in your country. I found photos from the April event that showed a nice arena, good attendance, and some interesting competitors. Congratulations.
It kinda bites to be a control 'bot under the current rulesets. Not all design challenges have solutions:
A: Mark J. I like the sound of this!
I'll assume we're talking about thin sheets of PLA to be cut/folded/assembled. While this could certainly work I'm personally very fond of cardboard as a construction material.
A: Mark J. I reviewed the fight between 'Atomic Bomb' and 'Princess of Wales' (Clash Bots Videos Playlist) to take a look at the flipper cord you mention.
For some reason I suddenly want a bucket of KFC.
The simplest pneumatic flipper systems apply force to extend the ram cylinder, but must rely on gravity to return the flipper to its retracted position. It is common practice with such systems to add an elastic bungee cord to retract the flipper should the robot be in an awkward position where gravity will not suffice.
The cord does not strengthen the flipping action - it actually creates a relatively small force counter to the primary motion. The benefit is that the bungee may be able to reset the flipper to allow self-righting where the 'bot might otherwise be stuck.
Are there any alternative drive methods that might work in such a small space? [Social Media]
A: Mark J. I dug thru the Ask Aaron Archives and found a post from 2016 that described a couple unconventional combat robot drivetrains that require no gears, pulleys, or sprockets:
Tiny wheels If direct-drive for a normal sized wheel isn't practical, how about a really tiny wheel? Mount a very small wheel to the motor shaft and mount the motor at an angle to put the wheel in contact with the floor. Unconventional, but a successful antweight full-body spinner used this method as a space and weight-saving measure. The 'wheels' could be something as simple as a small plumbing washer, a short length of small diameter rubber tubing, or even a few layers of electrical tape wound around the shaft. They won't last long, but they're easy to replace.
A: Mark J.
You didn't hear quite right. It wasn't BattleBots, but a combat robots show on The Science Channel did piece together a fake tournament by cherry-picking fights from an actual tournament.
Here is what Aaron wrote about it at the time:
The tournament tree shown repeatedly during the "Killer Robots" show was a fabrication that stitched together matches from the preliminary round, the losers bracket, and the main bracket of the double-elimination tournament to make up a fictional tournament that greatly distorted the actual event. Why did they lie to the audience?
As a competitor in the 2011 robogames, I understood why they would choose to portray the matches as single elimination. The producers explained to us that they thought the double elimination system was too hard to explain to the public. I agree: it was hard enough for us competitors to figure out who we would be fighting next. I see why you are frustrated, but sometimes profit and popularity are more important than accuracy.
A: Mark J. here: I sympathize with the desire to avoid the complexity of presenting a double-elimination tournament, but I don't believe that putting profit or popularity in front of truth is an acceptable option.
Watch the Full 'Killer Robots: Robogames 2011' video.
Compare the Killer Robots and the Robogames Tournament Trees.
Q: The 'Sports Should Be Credible' post reminds me of something. There was a rumor that the 'USA vs the World' special from BattleBots 2018 was rigged. The rumor originated from The Robot Combat Iceberg Version 3 [on Reddit, 2021]. One commenter [u/murdock129] claimed that the Kraken vs Red Devil fight was just an exhibition match completely unrelated to the event. They also said that there were unaired fights [that together] would've changed the result to 3-4 in favor of 'The World'. They didn’t leave any links backing up their claims, so I was wondering if you had more info.
- sincerely, Iceywave
A: The comment at the end of my exchange with the RoboGames competitor where I say, "BattleBots never re-arranged a sequence of fights to deliberately distort the audience perception of the tournament" was true at the time I wrote it in 2011. The original Comedy Central version of Battlebots was an actual open-to-anyone combat robot tournament with hundreds of robots in multiple weight classes. That has changed.
See my archived editorial post: BattleBots is Not a Televised Sports Tournament.
I had been powering the receiver from the BEC on the Repeat Dual ESC but I've been experimenting with using the balance lead of my 6S LIHV battery to power the receiver with a 2S voltage, and I am currently using wires 1 and 3 (marked with an A). Could I switch to using two wires in the middle, such as the ones marked with a B?
I had hoped to have the problems fixed by now, but the next event is NHRL and I can't reliably replicate any of these issues at home to test possible solutions. I am trying to add as many layers of protection as possible to try to fix the possible causes of the issues, such as:
I'm trying to do everything possible to solve these issues now since I don't have any more chances to test changes individually.
A: Mark J. That must be a HUGE current draw To cause such trouble.
Going with only a signal wire from the receiver to the ESC prevents the ground loop -- but the ESCs expect to see a receiver signal that ranges from about ~6 volts (high) to 0 volts (low) compared to the ESC ground. If the receiver is setting on a circuit with a ground three cells above the ESC ground, the ESCs are going to see a signal ranging from about 18 volts (high) to 12 volts (low). ESCs aren't likely to respond well to that signal.
The general rule is:
To your main question: Pulling power from the center cells of the battery will not offer significant protection from voltage fluctuation. Current is drawn equally across all cells in the battery and any voltage fluctuations will effect all cells equally as well. Even if you could avoid the ground loop and signal voltage problems the tactic will not solve your voltage issues.
Hard core R/C airplane guys often use a voltage stabilizer on their receivers that "helps to eliminate brownouts and prevents your receiver from rebooting." It's just a good sized electrolytic capacitor that plugs into an unused receiver port to offer a little buffering. But it sounds like the voltage fluctuations you're seeing are way too large to be solved by a capacitor, ferrite rings, or a larger BEC.
You've run out of time for experimentation and you need something absolutely guaranteed to eliminate receiver power fluctuations: stop trying to clean up your existing receiver power source and simply replace it. Your ER4 receiver is happy with anything from 4.5 to 8.4 volts input, so add a dedicated 2S 150mAh LiPo to power the receiver directly. Weight is only about 10 grams and a full charge is good for well better than an hour of operation. You will likely want to include a ground wire from one receiver port to one of the ESCs to provide a reliable signal return path.
- couldn' t resist, Iceywave : )
A: Mark J. A meta-question deserves a meta-answer:
'Ask Aaron' has a long history of receiving requests for very specific recommendations on materials or components that provide far too little information for us to give a useful answer. We needed a name for this type of question and a way to explain our problem in answering. In 2010 we published a webpage titled "The Hamburger is Bad" that uses a fictional exchange of questions and answers about whether a 1/4 pound hamburger is good to depict the problem. The meme caught on in the combat robot community.
So, "The Hamburger is Bad" is not so much a rule as a shorthand request to provide adequate information when asking a question.
- sincerely, Iceywave
This one was on me -- the link is now correct.
A: Mark J. "Best" depends on many factors which you have not shared in your question. Additionally, brushless motors of this size require substanital expertise in adjusting their controllers to provide acceptable performance; they are not "plug-and-play". The "best" motors for your particular situation may not be brushless at all.
Further, I have seen videos of recent Tunisian robot combat (see image below). Wire mesh is wildly inadequate for the safety of the competitors and spectators. Combat robots of this size and power require a much higher standard to protect human life and limb from potentially deadly shrapenel. My conscience does not allow me to assist in creating faster or more powerful robots for use under such conditions.
A: Mark J. I hesitate to answer this question because I have to guess at how you define "a considerable amount" and "really long". If your robot is "SMEEEEEEEEEEEEEEEEEE" wide you might need special techniques -- may I assume your 'bot is not that expansive?
A: Mark J. Read the "Don't Do That!!!" post immediately below for a link to a wiring diagram for skid steer with a dual ESC -- plus several reasons why a novice builder should not construct a robot with a spinner weapon.
Then read the Ask Aaron FAQ to answer the next few questions you'll want to ask.
Q: On a skid steer with a duel esc should I power each wheel individually or is it better to create some form of a linkage between the two. If its best to individually power them do I wire two motors to each input on the esc or do I need two duel ESCs
A: You have not mentioned how many wheels your robot has. Most beetleweight horizontal spinners are two-wheeled. From your question I will guess that you have a four-wheel design with each wheel powered by its own motor. It helps to tell me things like this because my psychic abilities are weak.
Consider dropping out of the competition and revising your design to something more appropriate for a new builder. I'll write you an excuse slip if you like.
A: Mark J. A common size for beetleweight competition is 8' by 6'. One-pound antweight arenas are often 4' by 4'.
Here is a YouTube video covering the design and building of a 4' by 4' antweight arena with a link to a parts list and design drawings.
Looking for some problem solving tips. [Social Media]
A: Mark J. OK -- it isn't the motor, it isn't the ESC, and it isn't the battery. Tell me about your radio system.
Comment: Ooo, good call. I'll switch to another Rx and Tx.
A: Mark J. I've never actually tried, but it shouldn't be very hard. You can only go where the rails go, so just give it a little throttle and toot the whistle once in a while.
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
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:
On any given day one of the fan-oriented pages like The Historic Tournament Trees or The Combat Robot Hall of Fame may get a mention on social media and rocket briefly to the top of the list, but the technical pages are reliably found at the top.
You might get a few ideas from this video: Build a Cereal Box Combat Robot!
Q: My design for an antweight full body spinner is very tight on space. I don't have room for regular gearmotors, even the little N10 size, and I can't find any right angle gearboxes small enough and fast enough to work.
The producers of 'Killer Robots' went out of their way to mislead the viewers. Examples from their fictional tournament tree:
Q: My robot is a beetleweight with an unusually high current draw from the weapon, and I have switched from a Neutron ESC to the Iflight Blitz E80 (the Neutrons kept dying), but now there are issues with drive sides turning off in the middle of a fight, with the robot losing connection for a few seconds, and with the ER4 receivers going bad.
My main question is: Would powering the receiver from the balance lead reduce voltage spikes and voltage sag since the positive wire is now separated by multiple cells from the main power wire?
About your port tap: The balance port tap you propose creates a "floating ground" within the wiring circuit. The receiver ground is now at a higher voltage potential than the ground used by the ESCs. If you include the ground lead from the receiver to the ESC there will be a dead short across the first two cells of your battery -- a condition known as a "ground loop". See this archived post for a more complete discussion of ground loops.
Whose Limericks went to line six.
They started OK
But then went astray
When he found himself in a fix
Toward the end.
A: Mark J. You came to the right place.
Mark J. Rodger the Web Gerbil's new contract gives him the entire month of May off. Rodger used to negotiate his own contract, but this year a hard-nosed red tree squirrel claiming to be his agent showed up. The squirrel turned out to be a skilled negotiator. In addition to a full month off I had to throw in two pounds of pecans, a gym membership, and bottled water for his water bottle.
As shown in the wiring diagram above, it is typical to wire both motors on one side of the robot to a single output on the dual ESC. The diagram makes a couple assumptions:
If you require specific recommendations I will need to know the specific components selected for your robot: drive motors, drive ESC, wheel diameters, battery voltage, and battery capacity (mAh). It may also be handy to know what radio brand and model you have selected to control your robot.
A: Mark J. What happened is that you plugged two random wires from your brushless weapon motor directly into the main power plug on your 6S LiPo battery. Brushless motors are 'dumb' -- they require a motor controller to route current thru each of the three winding circuits in the proper sequence and timing in order to operate. Plugging a pair of brushless motor wires into a battery creates essentially a dead short which pumps hundreds of amps thru one motor loop.
Q: What is the usual size of an 'insect sized' arena? [Lagos, Nigeria]
Q: Build: Repeat Robotics 16mm Brushed Planetary Motors MK2 and a Scorpion Nano 2.1 brushed ESC. When starting moving one motor stops. After a restart, the motor works again. I've switched out the Scorpion Nano, the motor, and the fully charged 3S battery. The issue remains.
Remembering Aaron Joerger, 1991 - 2013
That's obviously
A question from your homework.
Do your own research.
how to design and build a battlebot
Copyright 2009, 2025 by Mark Joerger
All rights reserved.
Q: What do your combat robots think of the current COVID-19 pandemic? [Kansas City, Missouri]