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![Fifteen Years of Ask Aaron [Click Me]](pic2/Ten_2.gif)
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.
It Still Won't Spin
Q: Hey, it's me again with the 1-lb vert with spin up issues, I've tried multiple KV motors of different brands, multiple different batteries, a different ESC (all of the ones I have used are Turnigy Multistar ones found on HobbyKing), and I've checked all my connections and joints, but the same scenario keeps popping up: The drivetrain works just fine, but the weapon motor continues to refuse to go past 10% power with any load on the motor. Do you think the problem is the ESCs or the Lemon-RX board? I'm really not too sure what it is at this point. One more thing to note is sometimes when I have the weapon motor trying to spin the weapon and I give it a little boost, it will speed up to full speed like it's supposed to and will have consistent speed with the stick on the controller. It seems like it's biggest problem is getting past the 10% throttle mark and then it works fine.
Thanks in advance, what you do for the community is greatly appreciated. [Redmond, Washington]
A: [Mark J.] My answer remains the same as it was the first time you asked this question. This type of brushless startup problem is typically traced to ESC firmware settings that are incompatible with the motor. MultiStar ESCs have their firmware settings optimized for drone applications that use a different size and style of motor than the weapon motor you have choosen to use.
User changes to the MultiStar ESC firmware without the MultiStar programming card (no longer available) are very limited, so I think it would be best to switch to an ESC designed for use with fixed-wing aircraft - like the AeroStar or Turnigy Plush. These would be likely to come out of the box with firmware settings that match the needs of your largish ant-sized brushless outrunner weapon motor.
It Says in the Manual...
Q: Fingertech tinymixer is not working when I turn on my transmitter. When I turn on my bot the led lights up and blinks but as soon as I turn on the transmitter the led goes off and it doesn’t do anything. I have three receivers one tgy-ia6 and two fs-ia6 with a turnigy evolution pro transmitter. Without the tinymixer the bot drives but I’m having to move the stick diagonal to move forward and reverse. That’s why I got the tinymixer to fix the problem. I have two tinymixers both are doing the same thing. My bot is the fingertech viper kit.
A: [Mark J.] FingerTech Viper manual, page 5:
Try starting your 'bot with the transmitter already on and write back if this does not solve your problem.
Holding in UHMW
Q: I'm designing a UHMW panel frame beetle and I wonder which is better, Plastite screws or heat set inserts with machine screws?
A: [Mark J.] For panels you will be removing regularly, inserts work best as they do not lose strength with each assembly/disassembly.
For panels you do not plan to take back apart, Plastite screws into correctly sized pilot holes are great.
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 dont need limit switches either...
Take a look at this previous post from the Ask Aaron Weapons archive:
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:
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."
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.
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:
A: [Mark J.] The RioBotz combat robotics tutorial is a fine piece of work. Their warning on shortening the leads on sensorless brushless motors is not outdated, but saying that "you should never shorten brushless motor wires" [RBCRT, page 239] is too harsh.
Hobby sensorless brushless motor controllers rely on monotoring the changes in induction between the electrical paths thru the motor windings as the motor spins. If you haphazardly clip and re-crimp motor leads it is theoretically possible to cause an imbalance in the induction between a couple of these paths that might confuse the controller.
Keep the new lengths of the motor leads equal and pay careful attention to all electrical connections you re-do. You'll be fine.
The weapon power to stick position is consistent until reaching a certain point on the controller, where it will not spin any faster above that, even while under no load and just spinning freely. Sometimes the motor will only stutter, and on a rare occasion it will spin to normal speed and the motor speed to stick position will be consistent like it should be. I've determined that the transmitter is not the issue here. Is there any way to tell what could be the problem? Thanks [Redmond, Washington]
A: [Mark J.] Your symptoms point to a problem with the firmware settings in your Turnigy ESC. You didn't mention which Turnigy ESC series you have, but I'm going to guess that you have the Turnigy MultiStar that's commonly used in drones. Low kv motors like your D2830-850 require different timing-advance and soft-start settings than do higher kv drone motors.
You can break out the Turnigy manual and figure out how to change the programming parameters:
A: [Mark J.] Yes... and no.
YES - Houston Area Combat Robotics has acquired rights and will use the Robotica name for their upcoming event at the Comicpalooza convention in Houston, Texas on July 16-17, 2022. Multiple vintage heavyweights are rumored to be 'interested'.
NO - The event will not follow the Robotica competition format. Fairy, beetle, feather, and 'sportsman' heavyweights (no destructive spinners) will compete in conventional 3-minute combat matches. The event will not be televised.
The organizers did contact me to ask if I was interested, but Team Run Amok has retired from robot combat.
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:
Consider clicking on the Internet Archive's "donate" button and chipping in a couple bucks to support their work.
A: [Mark J.] The 'rule of thumb' for best pushing power without lifting the nose of a two-wheeled robot is to place about 65% of the weight of the 'bot on the wheels with the rest on the front conact point with the arena. Measure the weight on the front of your 'bot and move components forward to get ~1/3 of the weight on the nose. Test to confirm that the front stays down on acceleration.
A more complete engineering explanation involving the height of the robot's center of gravity and the coefficient of friction between the tires and the arena surface can be found on page 46 of the RioBotz Combat Tutorial.
A: [Mark J.] Loctite blue 242 threadlocker is for metal/metal fasteners only. Geting a strong adhesive bond between UHMW and anything is effectively impossible due to its 'slick' molecular structure and flexibility. There are some special UHMW surface primers available for bonding UHMW with cyanoacrylate, but the resulting bond is not adequate for combat purposes.
You aren't giving me any details about the structure you're atttempting to bond, but UHMW is typically held with mechanical fasteners. I like Plastite screws for this purpose, but wood screws are adequate and easy to find.
Q: Hi! It's the guy with the Loctite question. I guess I should have been more clear; I am using plastite screws. I was just unsure if Loctite worked for a metal screw on a plastic thread or if there were any special threadlockers for that purpose that people are using.
A: Builders sometimes use Loctite for press-fit applications. I was concerned that you might be fitting something like a UHMW ring around an outrunner motor shell.
Proper screws in UHMW won't vibrate loose. The plastic compresses against the threads and holds them securely in place, just like a Nyloc nut. If you've correctly sized the pilot hole you'll need no additional security.
A: [Mark J.] The RobotMatter website is entirely useless. You can try to contact them at robotmatterllc@gmail.com, but the last I heard of RobotMatter was about a year and a half ago when builders were wondering where their orders were. The website does not appear to have been updated since that time. Proceed with caution -- I would consider other gearbox sources.
I bought the FingerTech's "Viper Vertical Spinner Add-On" as a first weapon to use for an ant (rest of bot won't be viper kit though). My first thought is - what if I ordered custom teeth from a site like sendcutsend that were a little longer or thicker to add weight and weapon diameter, and possibly at different sizes (but same weight) that could provide an asymmetrical design? Is there anything I should keep in mind before playing around with this idea, or any way you would approach the exercise?
Best, Neil [Tukwila. Washington]
A: [Mark J.] Thicker or (reasonably) longer teeth will not significantly increase the moment of inertia of this weapon system -- but it will significantly increase the stress on the tooth and the tooth mounting site. Increasing weapon diameter only adds energy storage to the extent the you move mass away from the center outward to occupy the increased span. Don't expect to gain much weapon performance from small changes to the teeth.
Going asymmetric has the potential to increase weapon "bite", which is a good thing -- but the process is a bit trickier than you might think...
This is like placing two kids on a teeter-totter but seating one kid 10% farther from the fulcrum point: they don't balance. To get the system back in balance you need the shorter/thicker tooth to weigh 10% more than the longer/thinner tooth to make up for being closer to the center of rotation.
See Also: What Weapons Win.
A: [Mark J.] An Acoustically Coupled Fast Fourier Transform Tachometer can actually give you RPM from the ambient sound of a spinning object. From that and data specific to the weapon in question you can calculate a tip speed. It sounds very fancy but you can download a simple acoustic tachometer for your smart-phone.
I've seen this tech used to measure tool speeds in otherwise quiet workshops, but accuaracy may be questionable in a combat arena. I suspect this might be more of a warning to builders than an accurate prevention measure.
Related from the archives:
A: [Mark J.] A few, but I can't vouch for any of them...
If you actually want to know how fast your weapon really spins you can buy a digital laser tachometer online for less than $20.
A: [Mark J.] The FS-BS6 receiver is designed to pair with the FlySky FS-GT5 transmitter -- your FS-i6x is not equipped to adjust or turn off the receiver's internal gyro. However... the receiver gyro only affects the output on receiver port CH1. If you leave your Elevon drive mix as it is and mirror CH1 to another port (CH3, CH4...) you can plug the ESC lead that was in CH1 into the mirrored port and eliminate all gyro influence.
To mirror CH1 to the CH4 port: Go to the MIX page in the FUNCTIONS menu on your FS-i6x transmitter and set up Mix #1 like this:
Press and hold the 'CANCEL' key to save, then move your ESC plug from the CH1 port to the CH4 port and you'll have no gyro action. For help in navigating the FlySky menus see: Programming the FlySky FS-i6 Transmitter for Combat Robotics.
For example, I’m considering upgrading my plastic ant horizontal from 6000rpm to 10000rpm. If the 10k rpm weapon can reliably survive its own power on impacts with a static object, and the 6k rpm weapon could survive impacts from an opponent’s vertical spinner, would getting hit vertically while using the 10k weapon be more likely to cause breakage than the 6k one would?
My intuition says no, since spinning the weapon faster horizontally doesn’t significantly affect the load a weapon would take in the vertical axis on an opposing hit. To my understanding, in horizontal vs vertical impacts, it's either one or the other imparting their energy, not both at once. But I’d want a sanity check on that one first before going taking the rpm upgrade to competition. [Philadelphia, Pennsylvania]
A: [Mark J.] You're overlooking something.
A: [Mark J.] The Melenki-Nano has on-board mixing; don't complicate things by turning on transmitter mixing as well.
Running thse symptoms thru the Run Amok Mixer Fixer gives this result:
A: [Mark J.] The part you're pointing to in your picture is the R/C radio receiver. The two 3-wire connectors are correctly positioned and appear to be well inserted. These connectors are quite reliable and seldom if ever give trouble. You should remove the plugs and verify that the metal pins in the receiver are straight and engage correctly when the plugs are re-inserted -- but I think your connection problem is elsewhere.
Pulling on one wire will move other wires within the system and make it difficult to trace the source of the trouble. I notice in the large photo you provided that you are using large yellow 'Wire Nuts' to connect the power leads instead of the 'Mini Terminal Blocks' that come with the kit. The small gauge wires used in the Viper kit will not make a reliable connection in a connector made for lager gauge wires -- I believe that is where your problem resides.
Those terminal blocks cost more than wire nuts and are included in the kit for good reason. Find them, use them, and your problem will disappear.
A: [Mark J.] That's a common problem with a simple fix:
A: [Mark J.] A problem of this type is difficult to diagnose from a distance. With hands-on I'm sure the problem could be found and fixed in five minutes. I wanted to quicky post this reply to ease your anxiety.
Assuming that is not the problem:
I note that you have abandoned the 'Mini Terminal Blocks' that come with the kit and now have power wires taped together with blue masking tape. That is not a reliable connection method. While that may or may not be the cause of your non-responsive motor it will cause trouble at some point. I STRONGLY suggest that you use the terminal blocks.
Reply: Thank you for your response. It was exactly what we needed to hear. We’re going tomorrow with our almost working bot! Thanks again.
A: [Mark J.] Given that Robot Wars has hosted six Tag Team tournaments I think it is safe to say that yes, BattleBots has considered that option. Here's video of one of the Robot Wars events we fought in: Robot Wars Extreme Warriors Season 1: Tag Team Terror.
A: [Mark J.] BattleBots is a TV show made to look like a combat robot tournament. They cater to an audience that likes to see stuff destroyed. They are not interested in control robots and they do not reward such designs. Norwalk Havoc has adopted a similar approach in an attempt to build the on-line audience for their well produced video offerings.
Take a look at the BattleBots Judges' Guide to see what qualifies as 'damage'. Note that BattleBots judges not only score damage for more points than 'control' and 'aggression', but their definition of 'control' is likely very different from what you think it might be:
If flow rate over an orifice is dependent on the pressure on both sides, why is flow rate in all the calculators and spreadsheets a single value and not a function with respect to pressure during piston extension? It seems to me that in a system with a proper buffer tank, as the pressure in the cylinder increases the flow rate should decrease, not remain constant. Can you help me understand this, and, additionally understand how I should go about calculating this single value?
My expectation is that I'm misunderstanding how pressure dissipates in such a system, or perhaps misunderstanding how a buffer tank truly works, or trying to calculate something that doesn't actually matter because these cylinders extend so fast when built properly, but I'm interested to hear what you have to say on this. Our team is planning on using a buffer tank so this shouldn't be one of those cases of running a piston directly from the regulator's relatively low flow rate, and we're making sure to consult manufacturer websites to get an adequate flow coefficient (Cv) value on everything before buying parts. Thank you so much for your help and for maintaining so many resources, this has been an incalculable wealth of knowledge for me as I've gotten into this sport. [Corvallis, Oregon]
A: [Mark J.] Thank you for your kind complement on the various Team Run Amok resources.
Combat robotics is chaotic by nature. Broad engineering analysis has some value in a chaotic system, but the deeper you go into detail the farther down the rabbit hole you go.
A: [Mark J.] The Hamburger is Bad. There are a good dozen reasons why the wheels might be spinning by themselves; I need much more information before I can offer a useful opinion.
A: [Mark J.] Section 3.4 of the SPARC Robot Construction Specifications provides expectations for weapon locking devices:
Small robots may use other locking methods depending on weapon design. FingerTech sells a small but strong Safety Clamp for ant/beetle robots that you may find suitable -- see photo.
A: [Mark J.] Eager to help, but many things could be wrong and you have given me very little information. The Hamburger is Bad.
I'll assume for the moment that you have the controllers wired up correctly, but I need details about your components and how they are behaving:
A: [Mark J.] First steps are confusing -- I can help.
Q: Is there any advantage to ordering the T6A radio with the Viper kit?
A: Yes, but there are also some serious disadvantages:
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.
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:
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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. 
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?
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]
Q: How can I minimize the "torque reaction" created by my robot? I have a 2 wheel wedge and during acceleration, the front end always lifts up. I have a lot of my components relatively close to the drive axles because I thought I would help with pushing power; should I try moving them further forward? [Chico, California]
A: [Mark J.] The servo included with the Viper lifter can provide about two pounds of lift out at the tip of the lifter arm. Why do you want more lifting power in the one-pound antweight class?
A: [Mark J.] The diagrammed circuit will work just fine... until the link is removed. With the link gone the higher-voltage 6S battery will back-drive current thru the 4S battery to find a new path to ground:
This will result in reverse current thru the drive ESC and forward current applied to the weapon ESC. Reverse current applied to the ESC will typically destroy the device, and back-driving a LiPo battery can be destructive as well. If you really want to run two batteries at different voltages you will need two power links.
Q: Does Loctite Blue work with UHMW or does it have to be a metal on metal thread? If not, is there an alternative out there that people use? [Parker, Colorado]
Q: Hello,
2k RPM
Q: I know increasing weapon speed also increases risk of that weapon breaking under its own power on impacts. But does that also increase risk of breaking on off-axis hits from an opponent?
Q: My fairyweight has a Malenki-Nano ESC/receiver driving two N20 gearmotors and I'm having weird drive issues that I'm hoping to fix by mixing my Flysky FSi6:
A: [Mark J.] The Taranis Q-X7 uses OpenTX 2.2 firmware that is a remarkably flexible, but the user interface is very different from standard transmitters. I just finished a full revision of the Team Run Amok "
Q: My son is 10 and new to combat robotics. I am trying to help him with his FingerTech Viper kit, but have zero background in this hobby/field. He has his first battle next weekend, and suddenly the wiring for one of the parts doesn't work. He says that the wires are really sensitive and he can't get them just right so that they stay connected. I've included a picture of the part he's talking about. Is there something he's supposed to do? Thank you so much for your help. I tried searching the archives but wasn't sure what terms to use. Thanks again. 
Q: Has battle bots ever considered tag teams [North of Knoxville, Tennessee]
Q: I’ve been following various combat robot events such as Battlebots and Norwalk Havoc. It seems like the judging systems used are quite skewed toward damage. In particular, I keep hearing Kenny and Chris say “damage with a primary weapon”. But how are control bots supposed to score damage points given their weapon is meant for control rather than damage? Does using the wall to jam up a spinner count? Or using a robot’s kinetic energy against it? As a new control bot driver myself, it’s something that has struck my curiosity. [Concord, California]
Q: Hi there! I'm a student working on a 12 pound pneumatic flipper as a part of a university club. We're all trying to do our engineering the right way so we're investigating the relevant equations to solve for the throw and max height of a theoretical bot we launch. Initially I, for reasons I can't quite explain besides "it sounded fun" and "I wanted the challenge," intended to essentially rebuild the flipper calculator spreadsheet on my own to build a better understanding of the concepts at work, but I ran into this problem enough to kill some of that naivete. I've looked through the various calculators and spreadsheets, and I've searched the archives as well as read the wonderful understanding pneumatics page many times over, but something I can't quite get my head around came up from the air flow rate equation I was using.
Q: I recently completed my first active weapon bot. It is an antweight lifter. How would I go about building a weapon lock for it? All the resources and videos I've seen focus on spinner safety which is understandable but I don't want to take chances with a lifter either. Any safety tips? Can I just wrap a bungee cord around it? [San Francisco]
Q: What do your combat robots think of the current COVID-19 pandemic? [Kansas City, Missouri]
