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Questions and Answers about Combat Robotics from Team Run Amok.


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7081 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.

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 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. [Winston-Salem, North Carolina]

A: [Mark J.] FingerTech Viper manual, page 5:

Remember to always turn the transmitter ON before powering on the robot, and always turn the transmitter OFF after powering off the robot.
Turning the transmitter on first and off last is a good habit to develop for R/C equipment in general. I don't have a Viper in my shop at the moment to test but it's possible that the tinyMixer is looking for a transmitter signal when it powers on and does not boot up properly without one.

Try starting your 'bot with the transmitter already on and write back if this does not solve your problem.

Follow Up: I received an email from Kurtis Wanner at FingerTech who tells me this behavior is definitely not typical for a tinyMixer. He suspects a hardware failure of unknown cause on both of Winston-Salem's tinyMixers. Replacement units are on their way.

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? [Winston-Salem, North Carolina]

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:

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:

  1. 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.
  2. 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 accidentally found 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.

Just Don't Be Sloppy
Q: Hi, I was reading the RioBotz combat robotics tutorial and it said that you could not shorten the wires from a sensorless brushless motor. I was recently watching a video where the builder had shortened the wires of a sensorless brushless motor. The RioBotz tutorial was written a while ago so I figured that it was outdated on this issue but I didn't want to ruin a good motor. [Fort Meade, Maryland]

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.

Spinner Won't Spin
Q: Hello, I am constructing a 1-lb vert running off of an Endbots Lemon-RX receiver hooked to a spektrum transmitter, a D2830 Brushless Motor(850kv), a 40a turnigy esc to run it, and a 2s or a 3s LiPo battery to run everything. my biggest issue right now is that whenever I power up my weapon (pulley driven) the motor is slow to start up and very often refuses to go above ~10% power regardless of stick position on the transmitter.

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:

  • Commutation Timing - start with the lowest setting and move upward as needed.
  • Startup Power - start low and increase until you have best spin-up.
Alternately, you can just swap out the ESC for one intended for use with a fixed-wing outrunner. Ask around to get an ESC recommendation from a builder running a similar weapon motor and design; the Turnigy Plush series might do.
A New Robotica?
Q: I read over at r/battlebots that Robotica is coming back! Is this true? [Columbus, Ohio]

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.

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:

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

Keep the Nose Down
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 '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.

Asking the Wrong Question
Q: I'd like to get more lifting power for my FingerTech Viper lifter add-on. The robot currently has a 300 mAh 2S Turnigy Nano-Tech battery. What battery can I use to increase the power? [Reddit]

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?

  • The Viper lifter servo electronics have a maximum voltage rating of 10 volts, so bumping up to a 3-cell 11.1 volt LiPo battery would be risky and would provide only a small performance increase.
  • You could upgrade the servo to a more powerful unit that could lift faster under load, but if you're trying for enough speed to 'flip' your opponent you'll be disappointed.
If you want a true flipper you will need to replace the entire weapon with a new design. Perhaps something like a spring powered flipper.

An Unexpected Reversal
Q: I would like to run by Featherweight robot off two batteries: a 4S for drive and a 6S for weapon. Would the wiring shown in the attached diagram with a single removable link on the negative ground be OK, or do I need a link on each of the positive battery leads? [On-line Forum]

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.

Nothing Sticks
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]

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 Different Saturn
Q: Do you have any experience with the SaturnBoxes from RobotMatter? There really is nothing on the website as to what kinds of motors they accept or what size pinon gear they require. [Roseville, California]

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.

Update - A builder let me know that these gearboxes are designed to couple to RS-775 brushed motors, which means the motor mounting holes are in a 29mm diameter circle and drilled for M4 screws. Details on motor shaft length requirement and pinion gear specs may be found at AliExpress.
Please note that the RobotMatter SaturnBox is not the same as the well-documented and robust Ranglebox Saturn gearbox (archived). The Ranglebox is unfortunately no longer available.
A Tricky Balance
Q: Hello,

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...

  • You're starting with a balanced spinning mass and removing two identical teeth that are equidistant from the center of rotation -- so far so good.
  • If the new teeth are not identical in shape they may not balance even if they are equal in weight -- you need to calculate not only the weight but also the center-of-mass for the new teeth.
Example: You add two new teeth to opposite sides of an otherwise balanced rotating mass. The new teeth weigh the same, but one tooth is longer and thinner which places it's center of mass 10% farther from the axis of rotation than the shorter and thicker tooth.

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.

In this case the calculation of tooth mass-centers is complicated because the teeth are not mounted radially with respect to rotation. A decent CAD program can sort this type of balance calculation out for you, or you could attempt to balance the assembly with the new teeth by hand or trial-and-error. Given that this is your first weapon I'd recommend that you run the FingerTech drum "as is" and save asymmetric weaponry for a future robot.

See Also: What Weapons Win.

There's a Buzz in the Air
Q: Recently, UK robot combat tournament organisers have announced new measures to ensure robots are adhering to the 250 mph maximum tip speed rule. One measure they mentioned was "frequency monitoring using microphones in the arena". Can you accurately determine the speed of a spinning object by the sound it's giving off? Or is this more aimed at the whine brushless motors make? [Armagh, Northern Ireland]

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:

Q: I'd like to find the actual speed of my spinner weapon but I have no measuring tools and no budget. Are there any oddball tricks? [Facebook]

A: [Mark J.] A few, but I can't vouch for any of them...

  • The eyeball test [Paul Gancitano] -- download one of many free stroboscope apps to your phone and put a daub of white paint on the side of the weapon tip. Spin up in a darkened room and find the fastest strobe speed that 'freezes' the white spot in place. Result may be revolutions per second (Hz) or direct RPM depending on the app. Note: I can't find a working app that is fast enough for our purpose.
  • The earball test [Derek Reihe] -- put the edge of a playing card in the path of the tooth and match the tone it makes to a known source (frequency generator app). Matching the tone (Hz) gives revolutions per second. Divide by two if two teeth hit the card on each revolution.
  • The tongue test [Anonymous] -- spin it up, spin it down, then quickly lick it to see what it tastes like:

  Egg Salad
  2k RPM
  4k RPM
  6k RPM
  8k RPM
10k RPM
12k RPM
14k RPM
Let it spin down some more.  

If you actually want to know how fast your weapon really spins you can buy a digital laser tachometer online for less than $20.

Mirror, Mirror...
Q: I'm having problems with the gyro my receiver messing up the drive. Can I turn off the gyro? Any ideas on how to fix it would be much appreciated. I am using the FS-i6x as my transmitter and the FS-BS6 as my receiver. [Asking Tech Questions on the Wrong Forum]

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:

Mix is
Pos. mix
Neg. mix

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.

The Neglected Force
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?

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.

  • Your intuition is properly accounting for the increase in stored energy that occurs with increasing rotational speed, but...
  • Your intuition is neglecting something that adds material stress with increasing rotational speed: centripetal force.
The material strength needed to keep your spinning weapon from ripping apart increases with the square of speed: an increase from 6K RPM to 10K RPM will increase centripital stress by [1.66 × 1.66 =] 2.78 times. That increase in stress pushes your weapon closer to self-failure and renders it more vulnerable to damage from an impact that would add to that stress vector -- like an off-axis impact.
Malenki Malarky
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:
  • I push the stick forward: the 'bot spins to the right (clockwise)
  • I pull the stick back: the 'bot spins to the left (anti-clockwise)
  • Left on the stick goes all forward
  • Right on the stick goes all back
Can I add a transmitter mix to fix this? [The Aether]

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:

Two steps to correct:
  1. Swap the places of the two drive ESC plugs on your receiver - unplug them and plug them back into each other's ports; and
  2. Use the 'Reverse' function on your transmitter to reverse the response of Elevator 'CH2'.
The suggested fix requires a little translation to adapt it to the Melenki-Nano combined receiver/ESC, but a few minutes with a soldering iron and we'll have you up and running.
  1. The motor leads are misplaced: the leads for the left motor have been soldered into the 'R' ports on the Malenki-Nano, and the leads for the right motor leads have been soldered into the 'L' ports. The left motor is the one on your left with the robot moving forward away from you. Mark the top wire of each pair of motor leads to avoid confusion, unsolder them, and swap the positions of the 'Left' and 'Right' pairs. Solder the left motor leads into the 'L' ports on the Melenki-Nano, but stop before you solder the right motor leads...
  2. The motor on the right side of the 'bot was spinning the wrong direction because its polarity was reversed. Since we already have the leads unsoldered the easy fix for this is to swap the top and bottom leads when you solder them into the 'R' ports on the Malenki-Nano.

Lost in the Firmware
Q: I've upgraded to a Taranis Q X7 radio and I'm having some trouble setting it up for my beetle. The model setup menus don't make much sense to me and all the help sites I've found are for airplanes and drones. Can you help me find a basic robot setup? Just an Elevon mix and a switch controlled weapon channel will do. [Harrisburg, Pennsylvania]

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 "Programming the Taranis Q X7 Transmitter for Combat Robotics" guide. It has complete walk-thrus for:

  • A Single-Stick Drive Mix
  • Output Channel on a Switch
  • Dual Steering Rate Switch
  • A Simple Invert Switch
  • Limiting Motion on a Servo
  • Servo Reversing
  • Mix Troubleshooting, and more.
That will get you up and running.
The Friday Harbor Trilogy
This set of three posts follows the challenges of a parent and son building a FingerTech Viper combat robot kit and hustling to get it ready in time for a tournament a week away. Their problems are entirely representative of the issues new builders are likely to encounter. I've put them back in the order received to make their progress easier to follow.
Part 1: Use the Parts Provided
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. [Friday Harbor, Washington]

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.

Part 2: Reversed but Not Inverted
Q: You helped us with the wiring issue on our Viper kit yesterday. My son changed out those yellow caps and everything works great - thank you! This is the next issue. From my son:
"I can't figure out how to fix the steering. The steering is inverted. If you switch the wires, it will go forward but the steering is inverted. If you switch the wires again, it drives backwards but the steering isn’t inverted."
I did search the archives for "inverted steering" and saw a couple things but wasn't sure if it was the same thing as what he's describing above; it didn't seem like it. Thanks again. [Friday Harbor, Washington]

A: [Mark J.] That's a common problem with a simple fix:

  1. Switch the 'wires' you've been moving so the robot drives forward when given a forward command, then;
  2. Swap the positions of the plugs in the receiver:
  • Take the plug out of the 'CH1' receiver port and plug it into the 'CH2' port;
  • Take the plug that was in the 'CH2' port and plug it into 'CH1'.
Some robot terminology for future reference:
  • "Reversed" means the steering responds in the wrong direction.
  • "Inverted' means the robot is upside-down.
Your Viper will operate when inverted because the wheels are large enough to move it around when upside-down. When inverted the steering will be correct but the forward/backward commands from the transmitter will be reversed. You should practice driving with the robot inverted as it is fairly common to have your robot flipped during a match.

Part 3: Showing Up is Half the Battle
Q: One more question (hopefully) about the Vipertech kit my son is working on, for competition tomorrow. Now the problem is that one of the motors isn't working OR at least one of the wheels is not spinning. I attached a picture of the set up, and another with me pointing to the side that's not working. I looked through the ViperTech manual and tried to make sure his connections looked like the picture? I also searched your archives with the terms "one wheel not spinning" and "motor not working". Thanks again, we have both benefited from your help a lot so far, and are learning a lot. Just trying to get the bot going for our event that starts in less than 24 hours. Eek. [Friday Harbor, Washington]

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.

Do Not Hesitate to show up at the event with a non-working robot. Let the organizer know that you have a problem and ask if they might find someone who could help. The robot combat community will come to your aid with time, expertise, and (if needed) tools and spare parts.
I note that you said, "...one of the motors isn't working OR at least one of the wheels is not spinning." From the photo, it looks as though the wheels may be rubbing hard against the chassis. Loosten them and move them a bit out on the axle to get some clearance between tire and robot. Try that first!

Assuming that is not the problem:


  • The power lights on the motor controllers are lit, indicating that they are receiving power. When the radio commands full forward throttle the lights on both speed controllers should be solid green, and when commanding full reverse the lights should be solid red. If this happens, the radio is not the problem.
  • Remove the wire leads from the non-responsive motor and replace them with the wire leads from the functioning motor.
    • If the motor still does not respond to transmitter commands: the motor is faulty.
    • If the motor does now respond: the problem is related to the speed controller.
As best I can tell from the photos, your wiring is correct. Re-check the connections in the problem area identified in the diagnostic above.

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.

They Must Have
Q: Has battle bots ever considered tag teams [North of Knoxville, Tennessee]

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.

They Aren't All TV Shows
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]

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:

The Control of a BattleBot is about the ability of the Bot's Operators to attack an opponent at its weakest points, use their Bot’s weapons in the most effective way, and avoid being damaged by the opponent or the Arena hazards.
The good news is that your local/regional event is likely to have a more builder-oriented scoring approach. There is a lot of variation. Talk with the organizer of any event you plan to enter to discuss the scoring standards in use and what specific scoring opportunities exist for control robots.
A Bottomless Hole With No Rabbit
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.

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.

"Chaotic systems are distinguished by sensitive dependence on initial conditions and by having evolution through phase space that appears to be quite random."
[Wolfram MathWorld]
  1. Your understanding of flow rate into the cylinder is correct -- up to the point where variables start to build faster than they can be dismissed. If the cylinder had a fixed volume it would be simple to calculate the rising pressure and the diminishing flow rate. However, as pressure in the cylinder builds the piston will eventually start to move and the volume will change.
    • The exact point at which the piston will start to move depends on factors including the friction within the system, the weight bearing down on the flipper pad, and the geometry of the flipper mechanism.
    • How fast the piston moves will depend on the effective mass being accelerated and the afore-mentioned flipper geometry.
    • Additionally, the geometry of the flipper mechanism may change the effective leverage acting on the mass being accelerated as the flipper extends.
    • And then there is the matter of how near to being directly under the center of mass of your opponent your flipper pad has come when the flipper is activated, how much it slips as the flipper extends, and how long the lever arm from the center of your opponent's mass to the far end of your opponent's chassis, around which the center of mass will pivot as it rises.
    • All that's left is to determine the moment of rotational inertia of your opponent in the particular orientation it finds itself in at the moment the flipper engages and calculate the amount of flipper energy that will go into rotation of your opponent as opposed to loft.
  2. A buffer tank will help to maintain pressure in the system, but unless it is of infinite capacity it will not prevent all pressure drop. Actual system pressure will depend on the cylinder volume, the buffer tank volume, the flow rate of the pressure regulator, and that annoying rate of cylinder volume expansion discussed above.
  3. The common method used by successful flipper builders in designing the system is to max out all flow, pressure, and capacity elements and put the flipper together while hoping for the best. There's a lot of 'engineering judgment' in this, and a lot of dumb luck.
You mention the "various calculators and spreadsheets" available to provide a value for "throw and max height of a theoretical bot" launched by a specific flipper design. In general these should be considered as qualitative tools -- not quantitative. Adjusting your design to maximize the values reported by such tools will improve performance of your flipper, but the actual values provided by the tools have very little numeric relation to the performance of the system in the real world.

A Florida Big Mac
Q: Why are the wheels spinning by themselves [Miami, Florida]

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.

  1. How fast are the wheels spinning?
  2. Are the wheels spinning forward, reverse, or one each way?
  3. Can you stop the spinning by adjusting the throttle or steering input on the transmitter?
  4. Are you using a "Pistol Grip" or a "Twin Stick" style transmitter?
  5. What specific transmitter are you using?
  6. What specific receiver are you using?
  7. Are any indicator lights on the receiver lit?
  8. Have you successfully 'bound' the receiver to the transmitter?
  9. What speed controller(s) are you using?
  10. Are any indicator lights lit on the speed controller(s)?
  11. What color are the speed controller indicator lights?
  12. Are the speed controller indicator lights steady or do the blink?
Answer however many of those questions you think might be useful and get back to me.
Stay Safe Out There
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]

A: [Mark J.] Section 3.4 of the SPARC Robot Construction Specifications provides expectations for weapon locking devices:

Moving weapons that can cause damage or injury must have a clearly visible locking device in place at all times when not in the arena. Locking devices must be painted in neon orange or another high visibility color. Locking devices must be clearly capable to stopping, arresting or otherwise preventing harmful motion of the weapon. Weapon locking pins must be in place when weapon power is applied during a robot’s power-on procedure. This includes all powered weapons regardless of the power source or weight class.
I don't believe a bungee cord qualifies as "clearly capable" in the above capacity. Robots will typically use some form of spring lock pin that passes thru a hole in the moving part of the weapon and thru a second hole in the chassis or armor to prevent motion. A spring wire retainer holds the pin in place to prevent it from falling out.

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.

Royale With Cheese
Q: So, my fingertech speed controllers are receiving power and signal but are not giving power to my motors. Please Help! [Wilkes-Barre, Pennsylvania]

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:

  • What are the lights on the tinyESCs doing when you signal forward/reverse with the transmitter?

    • Full forward should give a solid green light;
    • Full reverse should give a solid red light;
    • A slow blinking red light indicates no R/C signal.
  • Many transmitters require controls to be in a specific location at start-up. Are you following that procedure?
  • Is there any kind of response from the motors? Do they move even briefly?
  • What receiver do you have? Has it been bound to the transmitter?
  • What is the capacity (mAh) and voltage of the battery?
  • LiPoly batteries are shipped with only a partial charge: have you charged the battery?
You have to give me enough information to at least take a guess.
Take a Trip Up the Parkway
Q: I have no idea what I’m doing but my 11 year old is interested in combat robots as a hobby. All a know is ant weight is the smallest. Is that the best place to start? Is there a starter kit to get going to see if he likes it? Are there books and magazines to suggest? [Long Branch, New Jersey]

A: [Mark J.] First steps are confusing -- I can help.

  1. You'll want to look for combat robot events in your area to see what weight classes they host. It does little good to build for a weight class that is rarely contested. Robot Combat Events has a well kept listing of upcoming events across the US and world-wide.
  2. It's a good idea to attend a combat event to see how things work and talk to a few builders. As it happens the 2022 Mechanical Mayhem: Spring Event takes place this weekend (March 26th, 2022) in Lafayette, New Jersey -- 86 miles up the Garden State Parkway from your home. They will be hosting antweight and beetleweight classes.
  3. Yes, there are kits. The Robot Combat Wiki keeps an up-to-date list of small combat robot kits from the 150-gram Fairyweight class (even smaller than an Antweight) thru the 3-pound Beetleweight class.
  4. For an eager eleven year-old I'd suggest the FingerTech Viper kit: inexpensive, durable, and easy to assemble and repair. The kit is designed to be upgradeable either by add-on accessories or by builder customization.
    NOTE: If you do purchase the Viper kit I would recommend buying the kit without the T6A radio system offered by FingerTech and sourcing the more capable FlySky FS-i6 R/C system for a little less money. The FS-i6 is likely the most popular radio for combat robotics and will not need to be upgraded should your young builder move on to more complex combat robots.
  5. Read thru the first six Q&A in the Ask Aaron FAQ for pointers to sources of additional information. Feel free to read on as long as it holds your interest.
You'll have additional questions as you go -- I'll be here to help.

Q: Is there any advantage to ordering the T6A radio with the Viper kit?

A: Yes, but there are also some serious disadvantages:

  • If you purchase the Turnigy T6A radio with the Viper kit the transmitter and receiver will arrive fully set-up for use -- just add batteries. However, the T6A is very primative with an awkward interface to your personal computer needed to change any of the settings. You will need to purchase a special cable to connect the transmitter to your PC and download an app from the manufacturer in China. There is no on-line guide for using the T6A with combat robots, but there is a short video on use of the PC interface: T6A Interface.
  • If you buy the FlySky FS-i6 radio you will bind the receiver to the transmitter and program the transmitter using its built-in LED screen and menu system. It can be adjusted on the spot to suit the driver's preferences with no interface required. There is a complete guide for using the FS-i6 with combat robots: FlySky FS-i6 Combat Guide.
See also: Combat Robot Radio Systems - what functions do you actually need?
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.

Two photos of Aaron Joerger 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   
Killer Robot drawing by Garrett Shikuma

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.

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It's a mystery!
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