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Questions and Answers about Combat Robotics from Team Run Amok.

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Fifteen Years of Ask Aaron [Click Me] Ask Aaron banner image
6933 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.

Ice Ice Baby
Q: Why are CO2 pneumatic systems banned at Battlebots now when it has been legal in pretty much any other competition including the early Battlebots era? [The Aether]

A: [Mark J.] There is a potential safety issue with carbon dioxide. At room temperature, CO2 gas converts to a compact liquid form at about 850 psi and is stored in the pressure tank in that form. As gas is vented from the tank, liquid CO2 boils off to replenish the gas. That phase change from liquid to gas draws a great deal of heat from the system, creating extreme cold temperatures that can clog vent lines with ice plugs created from atmospheric moisture. This can prevent gas pressure from being fully vented from the system at the end of a match. The unvented gas may cause unexpected motion in the weapon system.

See the What a gas! section of Team DaVinci's Understanding Pneumatics for details on CO2 expansion, temperature, and pressure.

Q: How is that different from a BattleBots-legal nitrogen pneumatic system?

A: Nitrogen can be compressed and stored at pressures up to 5000 psi but does not convert to a compact liquid form at room temperature. As a result, nitrogen requires a larger tank to hold the same volume of gas.

A tank of nitrogen @ 5000 psi holds only 63% as much gas as the same tank filled with CO2 @ 850 psi.
However, because there no phase conversion from liquid to gas there is much less temperature drop as the gas is drawn off and passed thru the pneumatic system. The problem with ice in the system is avoided.
Control Yourself!
Q: What are Autonomous combat robots exactly? I saw that in Robot Wars 1996-1997, there was a award for those types of robots. Were there any others at those events other than the winners? [Fremont, Nebraska]

A: [Mark J.] Autonomous combat robots are self-controlled -- no human operator giving commands with a radio transmitter. Think: Roomba vacuum with a buzzsaw. They use sensors and an on-board computer to locate their opponent, plan an attack, and execute the plan.

The US Robot Wars tournaments from 1995 thru 1997 featured an autonomous combat class, but there were only a handfull of competitors and the records of the matches are fragmentary. I can tell you that the technology available in the 1990's did not make for exciting matches. The robots often took most of the match just to find each other. The photo shows the 1997 autonomous champion 'Thumper', one of two autonomous robots at the event.

Jump forward a couple decades and autonomous combat robots have greatly improved! Have a look at at modern autonomous sumo robots.


Team Run Amok
 20 Years in Combat Robotics
This year marks Team Run Amok's 20th anniversary in combat robotics. We dove straight into the deep end of the competition pool, starting our career with a win at the nationally televised 'Robotica' tournament. In the years that followed we won more competitions, travelled to compete in England twice, and finished in the top three at more than half the events we entered.

I have several things planned to observe our anniversary year. For a start I've restored a few of our first-year web pages that have suffered from obsolete formats and outdated code. Check the Team Run Amok webpage for a full list, and check back for updates!


Big Boy Pants
Q: How does High Frequency Injection (HFI) work? I know it's supposed to help Brushless motors spin at low RPM but that's about it. [Yuba City, California]

A: [Mark J.] I'll keep this as user-friendly as possible, but you'd best put on your Big Boy Electrical Engineering Pants because it gets pretty deep pretty fast.

Brushless motor controllers have two tasks to perform:
  1. Speed Control - Decipher the receiver signal to determine the direction and speed the motor is supposed to be turning.
  2. Commutation  - Determine the position of the rotor relative to the field coils to figure out which coils should be energized.
Sensored brushless motors have 'Hall effect' magnetic sensors mounted in the motor that relay rotor position to the brushless controller. That makes commutation simple and effective, even at low motor speed. Unfortunately, the hobby-grade motors and controllers used by combat robots are most commonly sourced from the model aircraft industry, and model aircraft don't care about low speed motor performance.

Unsensored brushless motors from model aircraft have no Hall effect sensors. Their motor controllers must estimate the position of the rotor by monitoring the electrical characteristics of the field coils themselves. The common method for doing this takes advantage of the spinning PMDC motor also acting as a generator, creating a voltage potential opposing the power supplied to the motor. This opposing voltage is called Counter-Electromotive Force or "Back EMF" for short. The brushless controller monitors the polarity and strength of the Back EMF on each of the three motor leads and calculates the rotor position from this data.

The problem is at low motor speeds Back EMF is very weak and difficult to accurately read. At zero speed Back EMF completely disappears. The controller must restrict current to the motor when starting rotation and at low speeds while it guesses at rotor position -- and that means poor low-speed torque.

High Frequency Injection is a relatively new method of determining rotor position by sending brief high-frequency electric pulses thru the motor leads and comparing the electrical impedence found in each of those leads. The impedence will vary with the magnetic flux in the stator cores, and from that information the controller software can accurately estimate the rotor position -- even at zero speed. With more accurate low-speed position sensing the controller firmware can confidently supply greater current to the motor for better low-speed torque. Current versions of the VESC motor controllers incorporate HFI in their firmware.

If you'd like to geek-out on details you can start here.
Hot Listings
Q: Is there any place that keeps a complete and up to date list of combat robot kits? I'm looking for something a little different from the kits I see all the time at tournaments. [Irvine, California]

A: [Mark J.] The new Robot Combat Wiki is just getting started, but it does have a section on kits that currently has links to a dozen full or partial kits. With luck, the list will stay current...


Round and Round
Q: With brushed motors, the output RPM scales roughly proportional to the input voltage. Does this concept apply to brushless motors? If not, then how do they relate? [Sacramento, California]

A: [Mark J.] Both brushed and brushless direct current electric motors have a 'speed constant' represented by the term 'Kv'. This term specifies the maximum no-load speed of the motor in RPM per applied volt.

No-Load Motor RPM = Input Voltage × Kv
A motor with a Kv of 1000 operating from a 11.1 volt battery will have a maximum no-load speed of 1000 × 11.1 = 11,100 RPM. Applying a load to the motor output, such as the aerodynamic drag from spinning a propeller, will reduce the motor speed in proportion to the torque required to overcome that load. If the load exceeds the torque the motor can produce, the motor will stop spinning entirely (stall).

Hobby brushless motors often include the Kv in the motor name, like Propdrive 2836 2200kv.


Obey the Law of Ω
Q: Adding on to my question on brushless motors and input voltage above - does the torque increase with a higher input voltage as well? My basic understanding of electronics tells me that as voltage goes up, current goes down (assuming equal load resistance). But I don't know how that concept applies when dealing with brushless. [Sacramento, California]

Four minutes later...

Q: Disregard my previous question. I read your guide to brushless motors - guess I should have started there huh - and I see the answer isn't going to be as straightforward as I thought. Thanks for your help regardless!

A: [Mark J.] No problem, Sacramento. You are correct that the answer to your question is a bit complex, but I've wanted to take a shot at an understandable answer to this question for some time. I'm not going to pass up this opportunity!

Motor torque is proportional to current: more current equals more torque. Ohm's law states that the current through a fixed resistive load is directly proportional to the applied voltage:
Current = Voltage ÷ Resistance

Amps = Volts ÷ Ω

So for a fixed resistance, current increases with voltage. When stalled (zero RPM) a PMDC motor acts as a simple resistive load, so increasing current by increasing voltage results in increased torque.

When the motor starts to rotate things get more complicated. A spinning motor also acts like a generator that creates a voltage potential acting against the applied voltage. This is called counter-electromotive force or 'Back EMF'. The faster the motor spins, the greater the magnitude of this force -- which has the same effect on current as increased resistance. The result is that the current (and torque) decrease with increasing RPM.

With the above for background, I can answer your question. Since motor torque is proportional to current and current is proportional to voltage, a PMDC motor (brushed or brushless) operating at increased voltage will produce increased torque at any given RPM. The chart at right is for brushed motors -- brushless motors have non-linear torque curves, but the increase in torque with increased voltage is the same.
Also consider: Current drawn by an electric motor is proportional to the load placed on the motor, irrespective of voltage. For a given torque load a non-stalled motor will draw a specific current regardless of the voltage applied. More voltage applied to a motor with a constant torque load will increase RPM, but the current draw from the motor will remain ~contstant.

I Want Names!
Q: Hey just wondering is there a list of combat robotic teams? [South Dakota]

A: [Mark J.] Assuming that you're talking about recently active teams, the most complete information is maintained by The Builders Database -- which is currently reporting its status as down until further notice. No word on why they're off-line or when they might return.

If you're trying to contact teams in your area, I would suggest looking thru the well maintained list of upcoming robot combat events at RobotCombatEvents.com. Clicking on one of the events will open a page with information about the event, and from there you can click on one of the 'Bot Weightclasses' to get pictures of the registered robots and links to the teams.


A Bridge Too Far
Q: I have a problem with my Endbots DESC (that burnt it out). I had to calibrate it so I bridged the two calibration pads (CAL), and it sparked and burnt out. Did I do something wrong? Thanks. [Social Media]


A: [Mark J.] It looks like the soldering job you did on the + power input pad created a solder bridge to one of the surface-mount components next to that pad. I suspect that bypassed the voltage regulator and allowed full battery voltage to leak into the DESC circuits. Too much voltage to the calibration pad: POOF!

Small circuit boards require extra care in soldering. Always examine your completed joints carefully to assure that the connection is solid and that you have not created any unwanted electrical paths with excess solder.


Rename: Whiny Boy
Q: I accidentally dropped a brushless motor from a height of about 4 feet on the floor. It still runs fine, but it makes a different sound than it used to. It sounds more whiny than it did before, but I don't think it's spinning any slower than it did before I dropped it. My guess is that it's off balance.

What do you think went wrong, and is there anyway to fix it? [Louisiana State University]

A: [Mark J.] I've been getting a lot of questions lately that fit the same template:

I accidentally [did something bad] to [a fragile part of my robot]. It still works, but it [does something undesirable] that it didn't do before I [did the bad thing]. My guess is that it's [damaged in some way]. What's wrong, and how do I fix it?
The [something undesirable] is invariably something that would be difficult to diagnose even if I had the [fragile robot part] on the workbench in front of me, and I'm often given only a vague description of the [something undesirable].

So... you dropped an unspecified brushless motor onto the floor and now it's 'whinier' but runs fine? I'd consider this an opportunity! Rename your robot 'Whiny Boy' and embrace the change. Oh, and buy a spare motor to have on hand when the one you dropped fails, cause you probably:

  • Warped the rotor
  • Cracked a magnet
  • Pranged a bearing
...or did something else equally more difficult to fix than replace.

Ultimate Workshop
Q: I'd like to remodel my basement robot workshop. Any ideas? [Fresno, California]

A: [Mark J.] Maybe something like this?


The "Down Under" Solution
Q: The BotBitz website recommends pairing their 30A Brushless ESC to a 4258, a motor that calls for an 80A ESC to operate. Am I missing something here or does the motor not require that much current? [Yuba City, California]

A: [Mark J.] BotBitz recommends an 'off-the-shelf' featherweight combat robot drive solution that includes:

HobbyKing recommends use of an 80 amp ESC with the Propdrive 42-48 for aircraft use - but we aren't building aircraft.

Brushless drive motors in combat robots are oversized to assure their survivability in an environment well outside their intended use; see the 'Drive Motor' section of our Brushless Motor Selection Guide. As a result, the motors are loaded far lighter than they would be in aircraft, and lighter loading equals a lower current requirement.

Current drawn by an electric motor is proportional to the load placed on the motor.
The BotBitz setup is widely used in Australian featherweights and works quite well.
So Many Holes!
Q: I have an Endbots DESC with attached Lemon receiver. The brushless ESC for my weapon has white and black wires that go to the receiver. Where on the Lemon receiver do I solder these? [Social Media]

A: [Mark J.] Assuming that you want weapon control on Channel 3, the white 'signal' wire solders in the hole right under Th -- which stands for Th rottle not 'Three'. The black 'ground' wire isn't needed, but you can solder it in the round hole under GND if you like. Why do I assume you want to use CH3? You didn't tell me about your transmitter or your weapon, but on a US standard twin-stick transmitter the Th rottle is assigned to the 'ratcheted' vertical axis on the left stick. That axis is not spring-centered, so you can set your weapon speed and leave the stick alone. The other channels are possibly less convenient:

  • CH1 El evator and CH2 Ai leron are used by the Endbots DESC -- off limits.
  • CH4 Ru dder is typically a spring-centered left/right stick axis -- awkward for a forward-only weapon ESC.
  • CH5 Ge ar and CH6 Au xilliary are often assigned to toggle switches -- which may or may not meet your control needs.

Sit and Spin
Q: I saw a post on [Social Media] proposing a design for a full body spinner with lateral braces that would let it wedge into a corner of the arena and avoid being thrown around by the forces of its own hit. It could still move to show mobility if needed, but otherwise would just sit in the corner. I see lots of problems with this, but how would you best attack such a braced 'bot? [Riverside, California]

A: [Mark J.] They might call it 'braced', but I'd call it 'trapped'. Two options:

  1. A non-moving 'bot has no control over its weapon 'bite'. You can slowly creep up on it and let their weapon 'grind' against your wedge/armor to dissipate its energy. When it stops, they're vulnerable.
  2. Carefully pick your approach angle to slip a wedgelette under the advancing edge of the FBS shell. They're braced against lateral impact force, but they can still be launched by their own energy running up a ramp.
Like they used to say about stationary thwack bots, Sit and spin, never win.
Where Does the Juice Go?
Q: is it better to wire the battery into the receiver or the esc? [Cupertino, California]

A: [Mark J.] See Frequently Asked Question #19 for a description of basic combat robot wiring. The battery is wired to the ESC. Most ESCs have a Battery Eliminator Circuit (BEC) that will reduce the battery voltage to a safe level and supply power to the receiver via the 3-wire cable(s) that connect it to the ESC.

There are specialty ESCs made for antweight and fairyweight robots that plug only into the receiver and require the receiver to be connected direct to the battery. See this archived post for their wiring diagram and limitations on their use.


An Unpopular Transmitter
Q: Hey, has anyone had any experience with Spektrum DX6e in robot combat? I'm not quite sure how to set up mixing or plug in the weapon into the 4 channel receiver. Any help is good help. Thanks, Phoenix [Perth, Australia]

A: [Mark J.] The DX6e is not popular in robot combat. The programming menu system (non-standard and unique to Spektrum) is designed to configure complex control options for a wide range of aircraft types, but finding and setting the functions needed for a combat robot can be confusing.

I've written combat programming guides for Futaba, FlySky, and Tarranis transmitters but I have no complete guide for Spektrum. Although you haven't told me anything about your robot, your control preferences, or your 4-channel receiver, I think I can walk you thru a basic set-up.

Mixing I'll assume your transmitter is set with US-Standard 'Mode 2' stick assginments with channels 1 and 2 controlled by the right stick. Plug the left ESC into receiver CH1 (elevator) and the right ESC into CH2 (aileron). Follow this step-by-step video that sets up an 'elevon' mix for an airplane -- the same mix will put robot throttle and steering control on the right control stick. If a 'forward' command from the transmittter results in one motor forward and the other motor backing up, look up the 'Reverse' function in the Spektrum manual (page 26 in my copy of the manual) and reverse the motor channel that is responding incorrectly. More troubleshooting help.

Weapon Control I'll guess that you have a 'spinner' weapon. Your drive mix uses channels 1 and 2 on the receiver, so your simplest option is to plug the weapon ESC into receiver CH3 ('throttle' left stick vertical) or CH4 ('rudder' left stick horizontal). If you would prefer assigning weapon operation to one of the transmitter switches it's relatively simple to do:

  1. Go to the 'Channel Assignment' screen (manual page 16) and change Channel 4 to AUX1.
  2. Select 'NEXT' to go to 'Channel Input Configuration'.
  3. Scroll to AUX1 and assign it to whichever switch you like. Switch 'C' would do nicely.
  4. Save, exit, and plug your weapon ESC into receiver CH4.
I don't currently have a Spektrum in my collection to check the programming, but the video and my notes should get you a start. Write back if you get stumped.
What's in a Number?
Q: Hello Mark. For brushed motors there seems to be some sort of vaguely standard numeric nomenclature to the motor sizes such as n10, n20, n30, 130, 380, 550, 555, 775, etc... What is the meaning behind these numbers? I searched for charts that would relate the motor "number" to length, diameter, or something useful; but was unable to find any. [El Jebel, Colorado]

A: [Mark J.] It would be nice if these things made sense, wouldn't it? Unlike the numbers used to describe hobby-grade brushless motors, small brushed motor codes reference only catalog numbers used by the original manufacturer. A '550' motor has specific 'frame' dimensions as a set, but '550' does not refer to any specific motor dimensions. For example: '550' motors from different manufacturers will have the same 'can' dimensions, shaft diameter, and mounting holes. They will fit the same mounts and space, but performance may vary greatly.


Somebody Stop Me!
Q: Looking at robots like 'Sawblaze' and 'Skorpios' (and even 'Whiplash'), 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've sent messages to the teams you mention for their specific responses, but 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'll update this post with the team responses as they arrive.

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."

Comment: Kansas City here again. Thanks for the response on the end point question. I couldn't figure out how they could be making limit switches that reliable, so I guess it shouldn't surprise me too much. Thank Jamison also. I forgot about current limiting as an option.
Who Fought & Who Didn't
Q: Hello! I have a couple of questions:

First, where can I find the full melee for 1997 US Robot Wars? I was able to piece together the 1994-1996 results, but not 1997. I know you guys have the overall winners, but I also want to know the competitors in each melee. [Fremont, Nebraska]

A: [Mark J.] I've never been able to find a list of melee participants for the 1997 US Robot Wars. The US Robot Wars operatons shut down immediately after the event and the official website was never updated with photos or lists from the event. Two sources may help:

  • There is a mashup edited video compilation of the '97 melee fights from which you will be able to identify many of the competitors, but sorting out the individual melees will be a challenge.
  • A brief write-up by Jim Smentowski about his robot 'Hercules' confirms that there were two separate heavyweight melees at the '97 event, and he mentions the competitors in one of the fights. There was at least one additional melee for lighter robots.

Q: Secondly, Also are you aware of if a entry list for Robotica 1998 or Battlebots 6.0 (2002) was ever made? Or perhaps something Like it?

A: You're diving deep, aren't you?

The 1998 'Robotica' event -- unrelated to the later Robotica events televised on TLC -- was cancelled due to legal concerns around the break-up of the US Robot Wars series. Following the cancellation, two separate events were organized by disappointed competitors:

  1. Mike Winter built a wood and wire arena in a San Leandro parking lot and had a very informal, invitation-only event with about six lightweight 'bots. Mike's walker 'BORB' and Jonathan Ridder's 'Ziggo' are the only recorded entrants.
  2. In February, 1999 Trey Roski and Greg Munson put on the informal 'Underground Robot Street Fight' literally under an overpass in Novato, California. Jim Smentowski has photos, video, and a write-up. Entrants:
  • La Machine (Roski/Munson)
  • Ginsu (Roski/Munson)
  • Doo-All (LaValley)
  • Junior (Smentowski)
  • S.L.A.M. (The Nelsons)
  • Voltarc (Felk)
I have never heard of a 'BattleBots 6.0' event in 2002. There was a private BattleBots competition with eight competitors held in Las Vegas for a corporate client in June of 2006 mentioned on Smentowski's 'Breaker Box' page but the full competitor list has never been made public.

Q: Hello again! Thanks for answering my other two questions. To answer your question Battlebots Season 6.0 was meant to be run in November 2002 but was cancelled after Comedy Central pulled the plug.

A: As a rule I don't speculate about events that didn't happen. Word leaked out in September of 2002 that Comedy Central would not pick-up a sixth season of BattleBots. I suspect Greg and Trey knew earlier. Since season 6.0 was not set to film until November I doubt that there ever was an official entrant list. Ditto for the 1998 Robotica. Even if there were sign-up lists there is a big difference between Signs Up and Shows Up.

Q: So for my actual question, in your opinion, what could be some bots that might be future Robot Hall of Famers?

A: Given my position as administrator of the Combat Robot Hall of Fame I'm careful to avoid 'playing favorites'. I keep my ballot to myself and I avoid promoting specific robots for membership. That said:

  • Historically, robots from recently televised tournaments get a lot of votes. Sometimes this is merited, sometimes it is not. Some of these 'bots seem out of place a few years farther on. It takes more than one tournament to justify a Hall of Fame membership.
  • In spite of the great numbers of sub-lightweight robots competing around the globe, only 7% of full members in the Hall weigh less than 60 pounds. The growth in popularity of multiple live-streamed tourament series on the internet has lead to increased awareness of smaller robots, which translated to greater representation in the 2019 ballot. I expect voters will promote a few deserving sub-light robots from the 'Honorable Mention' roll to full membership in 2021.
Hall of Fame balloting opens in August of odd-numbered years. It's not too early to start planning your ballot -- be ready!
Buy or Build?
Q: Alex Hattori of my fav Team Uppercut mentioned building his own electric motors using automotive grade parts. I've heard of folks adding epoxy for strength but does anyone know of any resources for a beginner to even start learning this process of custom motor building? It seems to have been a mega advantage for them. [Social Media]

A: [Mark J.] First, attributing the success of 'Uppercut' entirely to their custom weapon motor is an error. There are many examples of winning robots with ineffective weapons, and many more examples of losing robots with awesome weaponry. A successful combat robot depends on the basic mechanical systems working well and reliably.

What advantage do you hope to gain by building your own motor? The available motors are quite effective and represent hundreds of hours of design, prototyping, testing, and refinement by engineers specializing in the field. Alex Hitori is in the senior year of a mechanical engineering program at MIT with access to the design and contruction facillities at the University. Are you prepared to invest the required time and resources?

A few builders take existing motors, strip out the key components, and reassemble them in more robust housings with larger shafts and improved bearing support. This is a more reasonable approach than starting from scratch. You can find an example of such a reconstruction in this archived Ask Aaron post.


Very Hot, Very Fast
Q: Hi Mark. You're going to see that this is coming from Baton Rouge, LA. You might remember me from a few months ago but just in case you don't let me start out by saying we are NOT in the collegiate hobbyweight competition, this is for a beetleweight.

Anyway in that previous post I asked you to diagnose some of the gremlins we were having with the Scorpion Mini Esc. We ended up sending it back to Robot Power and they supposedly repaired it, but now we have a different issue. None of the lights on the esc will come on when we have it connected to a battery, but when we connect it to the receiver one of the chips gets very hot, very fast.

Is this just a bad board? Or is there something else we might be missing that could help to fix it? I really like the scorpion mini and I'd like to think this is just a bad board among a field of otherwise very robust ESC's.

Thanks! [Baton Rouge, Louisiana]

A: [Mark J.] I remember you, Baton Rouge. Sorry to hear that you're continuing to have trouble with the usually-reliable Scorpion ESC. Maybe you have the exception that proves the rule. It's difficult enough to troubleshoot an electronics problem with the board on the bench in front of me. Diagnosing a problem with a board in an unknown circuit based on limited info is effectively impossible, but I'll take a stab at it.

One weakness of the Scorpion Mini is its battery eliminator circuit (BEC). Does the hot chip look like the photo at right, and is it next to the 'BEC' label on the ESC board? That's the voltage regulator that reduces battery voltage to 5 volts to power the ESC and the receiver. Either the voltage regulator has gone bad or your receiver bus is drawing too much power. Take a look at this post in the Ask Aaron Radio and Electronics Archive that covers some of the symptions that can result from overloading the Scorpion BEC. It sounds like you've cooked the voltage regulator.

  • Are you trying to power a servo via a receiver port?
  • Are you using a receiver with telemetry?
  • Are you running more than a 3s LiPo battery?
If you're convinced that you didn't overload the Scorpion's BEC it could just be a bad one, but if you're drawing too much 5-volt power you'll need to add a stand-alone BEC that can provide more current before you replace the Scorpion or you'll just cook it too. See my advice in the post I linked above.
Special Robotica Anniversary Section
March 4th, 2021 marked the twentieth anniversary of Team Run Amok's victory at the premiere 'Robotica' tournament. I've pulled together a few Q&A from the Ask Aaron 'Team Run Amok and Friends' archive for a look back at that event...
Q: I watched the season 1 'Robotica' finals on YouTube last night. Is there any special way you celebrate the anniversary of your win? [Saint Charles, Illinois]

A: [Mark J.] Oddly enough, I make omelettes for my co-workers.

We had a day off between our preliminary round victory and the Robotica finals. When I woke up that morning I turned on the hotel room TV and 'The Big Cheese' episode of 'Dexter's Laboratory' was on. All thru that day and the next I had "omelette du fromage" running thru my head, so now I make about a dozen cheese omelettes on anniversary morning to banish that phrase from my mind for another year.


2021 My office is closed because of the COVID pandemic, so I was only able to make omelettes for my family this year. They came out well.

Q: I always wondered, where was 'Robotica' filmed? [Syracuse, New York]

A: [Mark J.] RBI Productions filmed 'Robotica' at the historic Prospect Studios just east of Hollywood. The studios are owned and operated by the Walt Disney Company, and were formerly the west coast headquarters for ABC television.

I kept a journal of the filming that you may find interesting: Robotica Journal.

Q: If Robotica ever gets rebooted, would you create a new version of Run Amok and compete in honor of Aaron?

A: [Mark J.] Much though Aaron enjoyed competing, he had a greater love of helping people and sharing his knowledge. I believe that Team Run Amok has had a beneficial impact on combat robotics through our efforts to support combat robot builders, and I believe that continuing with those efforts is the best public way for us to honor Aaron.

I would be pleased to support the production of a Robotica reboot, but our team lost its competitive heart. We will not return to competition.


Q: Was it difficult to make your robot waterproof for the waterfall at Robotica? [Watertown, Massachusetts]

A: [Mark J.] We didn't waterproof 'Run Amok' -- it wasn't necessary. The top armor was a single, solid sheet that shed water to the edges like a roof. The only vulnerable components (steering servo, receiver, and speed controller) were all tucked up against the top armor and had their own cases that would resist any random splash. None of the 'bots at 'Robotica' had any trouble with the water.


Q: Then another question about your team in Robotica. Who would be the bigger threat to 'Run Amok' in the Fight to The Finish: 'Mini Inferno' or 'Killer B'? From your previous answers it looks like Jason's bot would be much less dangerous, I think. [Chinese Forum]

A: [Mark J.] Jason Dante Bardis was building a 'bot from scratch to compete at Robotica, but he ran out of time and decided to substitute a very light 'bot that was essentially the chassis for 'Dr. Inferno Jr.' with a carbon fiber wedge added. Although the drivetrain was very powerful (four DeWalt 18v gearmotors running at 24 volts), the robot weighed weighed only 37 pounds. At the weigh-in, Jason walked onto the scale carrying 'Mini Inferno' and was still under-weight! Powerful though it was, Mini just didn't have enough pushing power to be a threat against any of the robotica finalists in a sumo match.

Jason did finish his original Robotica entry and fought with it at BattleBots seasons 3.0 thru 5.0 with limited success. He remarked on a forum post that after he finished 'Towering Inferno' he realized that it would not have been a successful Robotica competitor.

'Killer B', on the other hand, was a very dangerous sumo competitor. Had they put the blunt end of the robot into use instead of the wedge end, they could have used their powerful drivetrain to simply shove 'Run Amok' off the platform. Lucky for me, they didn't choose that option.


While I was looking for video to answer an 'Ask Aaron' question, I came across this tribute to our luckiest robot, 'Run Amok'. Turn up the volume...

Never had my own video tribute, and an Ahmet/Dweezil Zappa soundtrack is a big plus! Thanks Alexander...


Q: I saw a claim on [social media] that heavyweight 'Gruff' uses a combination of brushed and brushless motors in its drive train. Is that true? What benefit do they get? What motors are they using? How are the motors linked? [The Hinterlands]

A: [Mark J.] Yes, the 2020 version of 'Gruff' does use one brushed and one brushless motor per side for the drivetrain in a system they call "brushed-more-or-less". The team needed to trim some weight to upgrade their already massive torches, so they swapped out the four 'long can' brushed AmpFlow motors for a pair of 'short can' Magmotors plus a pair of '58113' Leopard Hobby brushless inrunners. Drive power went up from about 13 kW to more than 21 kW and they saved a net 12 pounds for the flame torches.

Why mix brushed and brushless? Unsensored brushless motors like the Leopard often have poor low-speed torque and may not respond crisply in low-speed maneuvers. The small brushed motors have have great torque at low speed for predicatable driving response, and the brushless motors supply huge power in the mid-range. It's a clever way to add brushless power without a complete re-design. For simplicity, the (orange) Leopard is mounted next to the (black) Magmotor with a timing belt drive joining their shafts. The Leopard spins about three times as fast as the Mag so the pulley sizes are adjusted to compensate. The Mag remains in its original mount position in the chassis, and its shaft drives the chain and sprocket drivetrain. The team reports that the power from the Leopard frequently strips the teeth from the timing belt connecting it to the Mag, so watch for modifications to that part of the drive.


Q: Hi Mark.

My name is Jessica and i've just been through your combat guide for the FlySky FS-I6 Radio. I do have a question about an issue i'm experiencing, that i couldn't find anything in the guide about unfortunately.

I also couldn't find anything that says: "don't email me your crap or i will kill you!" ^^ so here i am. :-P

If you don't mind ...

I built a speed tank, from scratch (made from steel, aluminum and 3d printed parts), powered by identical sets of brand new Traxxas XL-5 ESC's and brand new Titan 550 12T motors with 2 identical 2S LiPos. I'm using the FS-I6 with it and got your 'Quick and Dirty Combat Setup' done... but for some weird reason, the left throttle runs way faster than the right throttle.

I factory reset the thing, finished the binding and finished the combat setup again and it's still running differently. Motors, ESC's and Batteries are new and haven't been running in anything before. Do you have any ideas on what causes that? I really have no clue what could cause that or how it could be aligned or if it indeed is the transmitter. - Jessie [Direct Email]

Jessie and I swapped emails to pin down a few details about the problem with her 'Speed Tank'. I didn't know they were a thing! Turns out you can download and print something similar from Thingverse.

I suspected that the Titan 550 12T motors had a timing advance that caused them to spin faster in one direction than the other, but Jessie's drivetrain is set up so that both motors spin the same direction to move the tank forward. Something else was wrong...

A: [Mark J.] OK, the right side is slow. I'll assume that you've checked that the tread drive operates smoothly on each side without binding. Top up the charge on the batteries and let's find out what the problem is:
  • R/C Check Swap the two leads from the ESCs to the receiver. If the left side is now slow the problem is the R/C set-up -- write back. Otherwise swap the leads back and continue.
  • Motor Check Disconnect the motors from the drive train so they can spin freely under zero-load. Connect one and then the other motor direct to the battery leads to compare their speed. If the right motor sounds slower, continue to the battery check. Otherwise, jump down to the ESC check.
  • Battery Check Try directly connecting each motor with the battery on the other side. If the right side motor still sounds slow, the motor is the problem. If the left side motor now sounds slower, the right side battery is the problem.
  • ESC Check Swap the speed controllers positions of the left and right side ESCs and reconnect the speed controllers to the batteries/motors. If the left side is now slow, the left side ESC is the problem -- try recalibrating it. If the right side is still slow your tank is infested with gremlins -- write back.
Let's hope you don't need to go thru the whole process to isolate the problem! Let me know what you find...

Response: Your trouble shooting guide is awesome, i do have a bad motor. I guess i will go to Hobby Town and buy a really new motor today. hrhrhrhr

Thank you so much, you're amazing.


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.

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


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