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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
6949 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.

A Flash in the Pan
This question from Anacortes came in two years ago. I happened to re-read it today and relaized that my answer had been less than clear. For the record, I've re-posted the Q&A below and updated the copy in the archives.
Q: I'm having trouble calibrating the invert function in my tinyMixer. I plug the INV wire into a Ground pin on my receiver then power on the tinyMixer, and it flashes, but when I turn on the transmitter, it starts flashing faster instead of showing a solid light. Is there anything I could be doing wrong? [Anacortes, Washington]

A: [Mark J.] I think you're fine, Anacortes. The calibration instructions read:

  • To calibrate, plug the INV wire into a Ground pin on your receiver then power on the tinyMixer. It will begin to flash.

  • Center your transmitter trims and sticks and power it on.

  • Remove the INV wire from the ground pin. The LED will go solid and save the center position. Cycle the robot power to reboot.
When the fast flashing starts you may remove the INV wire from the ground pin. You should see a solid light after the INV wire is removed. Cycle power, insert the INV wire onto the SIGNAL pin on the channel you want to use for inverted function, and you're good!
When Up Ain't Up
Q: I've recently finished my first antweight, a FingerTech Viper with the lifter kit and the usual HobbyKing HK-T6A radio. Everything is running smoothly except for one thing: left and right controls on my joystick are correct, but it moves forward when the stick is pulled down and and backwards when the stick is pushed up. I'd like 'up' to be forward.

Is this configuration normal, or did I wire something backwards? I tried reversing CH1 and CH2 with the programming cable, but that flipped left and right too. [A Little to the Left]

A: [Mark J.] The T6A transmitter comes from FingerTech pre-programmed for single-stick control. Two steps will set everything right:

  • Reverse CH1 and CH2 as you have aleady done. This will correct forward/reverse (good) and will also reverse the steering response (bad).
  • Unplug the ESC connectors from the receiver posts and replug the connector that was in CH1 into CH2 and vice versa. This will put the steering back in order.
See also: Elevon Mixing Setup for Combat Robots
Aircraft Specs Mislead
Q: I've choosen a Brushless Turnigy D2830-11 1000kv for my weapon motor and I'm trying to find how much current it draws so I can choose a battery. There's no stall current given, but they list a Max Motor Current of 21 amps. Is that the same as stall current? If not, what current value do I need to consider when picking my battery? [Social Media]

A: [Mark J.] 'Max current' for a hobby brushless motor is not the same as 'stall current', and neither specification will be much help in battery selection.

  • Stall current is not the highest current level in small hobby brushless motors. Current supplied to brushless motors at low speeds is limited by firmware in the Electronic Speed Controller to provide smooth operation for spinning model aircraft propellers -- a very different task from spinning up a heavy robot weapon. Many different types of firmware are available, and user settings in each type effect the current profile. Typically, full current is not available to the motor until it is running at about 25% of full no-load speed, and the motor will pass thru that current peak very quickly.
  • Max motor current is entirely different, and its name is misleading. Small hobby brushless motors are designed to spin aircraft propellers at about 85% of the motor's no-load speed, pulling a fairly constant current that depends on the design of the propeller. Greater load on the motor causes greater current flow, which causes greater heat build-up in the motor. 'Max motor current' is the highest level of continuous current the motor can use without failing from heat build-up. Note that this figure assumes high cooling airflow and operation near the 'peak efficiency' motor speed.
Robot builders place demands on hobby brushless motors that are very different than their designed use. Designing from motor specifications provided for aircraft use can give unsatisfactory results. I suggest finding successful robots running similar weapon systems and using their components as a model for your own weapon. Assuming that you're building a beetleweight with typical weaponry and drive, a 1000mAh 3S 60C LiPo battery will be about right.

See also: Combat robot brushless motor selection.


Unpredictable Impacts
Q: What's a good method to calculate the impact loads on a spinning weapon? My initial approach was to calculate force at the blade tip from angular deceleration, but I can't find anywhere that gives such information. Do you know what the ballpark deceleration force a drum spinner is subject to, or an alternate method for calculating the loads? Thanks! [A Series of Tubes]

A: [Mark J.] The force vectors encountered by a combat robot weapon are unpredictable. Maybe you hit your opponent, maybe an immoveable arena structure, maybe their weapon strikes your weapon with force outside your control -- all at unpredictable angles.

Learn what you can from the designs of successful robots with comparable weaponry. Build something that makes sense. If it breaks, make it stronger.

"Damage is weakness leaving your robot."

- Team Juggerbot

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!


Critter or Terminator?
Q: I'm confused about the first ever robot combat event. The Robot Battles website claims that the first 'Critter Crunch' robot combat event took place in 1987, but you claim that the 'Terminator Tournament' in 1988 was first. Doesn't the Critter Crunch count? [The Upper Penisula]

A: [Mark J.] I've seen several sources claiming that the first 'Critter Crunch took place in 1987, but I've never seen a photo, film, or document to support that claim -- not even a mention in the program from the 1987 'MileHiCon' science fiction/fantasy convention where the event was supposed to have taken place.

A 2011 article in Wired magazine titled Critter Crunch, Mother of All Robot Battles sets the date of the first 'Critter Crunch' at Oct. 28, 1989 and documents that date with photos and a written mailing from event organizer Bill Llewellin. The article does mention a 1988 non-combat event at MileHiCon called the 'Critter Crawl' which was, "a sort of beauty pageant for windup toys and remote-control gizmos" with no judging or winner. This makes a strong case for 1989 as the first Critter Crunch.

The 'Terminator Tournament' has documented support for their event in September, 1988. In addition to video footage of the event, the YouTube page has the following statement:

September 1988. Long before Robot Wars, BattleBots or the band called "Dangerous Toys", Los Angeles toymakers created the ultimate battlefield. State-of-the-art toy vehicles were designed to battle to the death using man's oldest, most destructive weapon: FIRE. The rules were simple: build onto a specified toy car chassis using tether control to avoid R/C interference, use minimum plastic armor, and limit combustibles to road flares and charcoal lighter fluid. "Two man enter, one man leave." This original VHS video was the first of many covert contests, and led to two appearances of the "Dangerous Toys Team" on Late Night with David Letterman in Dec 1988 and Dec 1989.
Pending the production of documentation for the claimed 1987 Critter Crunch event, I believe the 'Terminator Tournament' to be the first organized robot combat event. Further, these tournaments continued for many years. Here's video from the 2009 Terminator Tournament.
Flashback: an archived post from 2019
Q: Hey Mark, I want to try building a sensored brushless motor powered lifter. I'm looking at a worm-drive gearbox with an additional belt-drive stage so I can just "park" the lifter with the motor unpowered and not worry about it slipping back down.

The motor I'm considering gives a "30-second Max Current" rating but does not provide a torque constant (Kt). Is there a way to calculate how much torque I can get from the motor at that 30-second current level? [The Panhandle]

A: [Mark J.] You're very brave to try a brushless lifter, but your plan sounds workable. You don't have the motor Kt, but I'm sure you have the speed constant Kv, so there is an equation that will give you an approximate torque level at a given current draw. I'd dial back the number it gives by about 20% to adjust for 'real world' conditions:

Torque (oz-in) = (1352 × Amps) ÷ Kv
Example Your motor has a Kv of 1800 RPM/volt and you want to hold your current draw to 20 amps. The equation is:
(1352 × 20 Amps) ÷ 1800 = 15 oz-in torque
Gear for 80% of that number (12 oz-in) at maximum lifter load and you should be OK.

An Exponential Increase
Q: I couldn't decide on a weapon blade for my new 4wd vert 1-pound ant, so thanks to the magic of [online metal service] I made five. All of these blades have a 72mm cut diameter, are 0.250" AR500, and press fit into a .750" aluminum hex hub. Arranged from left to right, heaviest to lightest.
Interesting how the "Asym" blade has almost the same MoI as the "Buckle" despite weighing 25% more. Losing the weight off the back killed the MoI, but of course it is able to spin twice as fast to make up for it without losing bite.

The "S-Hook" is meant for countering horizontals; less material to get caught by their blade. I think the real standout here is the "Reaper" assuming it's strong enough to survive, but the big MoI number for the "Pendulum" is hard to ignore.

Thoughts? Feedback? [Social Media]

A: [Mark J.] I think you're underestimating the impact of spinning the asymetric blade "twice as fast to make up for it". Rotational energy storage increases with the square of speed so twice as fast equals four times the energy storage -- that's an exponential increase in weapon power. The Run Amok Spinner Weapon Kinetic Energy Calculator gives these kinetic energy storage figures for each of your blades spinning at 8000 RPM:

NameEnergy @ 8KJoules/Gram
Asym13 joules0.16
Pendulum16 joules0.22
Buckle14 joules0.21
S-Hook10 joules0.18
Reaper12 joules0.21

In this comparison 'Asym' doesn't look like a great option, but taking advantage of the added bite 'Asym' has (What's bite?) and spinning it faster has dramatic results:

  • Spinning 'Asym' 50% faster to 12,000 RPM increases energy storage to 30 joules and yields 33% better bite than the dual impactor bars have at 8000 RPM. That's 0.36 joules per gram of bar weight.
  • Spinning 'Asym' 100% faster to 16,000 RPM increases energy storage to 53 joules and maintains the same bite the dual impactor bars have at 8000 RPM. That's 0.64 joules per gram of bar weight.
This huge increase in energy storage while retaining good 'bite' is why single-tooth spinner weapons are popular. Don't handicap your 'bot with a symetric weapon bar.

Something is Missing
Q: Hey, sorry in advance if any of this is super naive, but I'm in the process of building my first beetleweight, and I'm all but a beginner with electronics. I'm just messing around with it tonight, trying to make sure I understand something, but I can't seem to get it to work.

What I'm trying to do is just to connect one motor and ESC to my receiver, and to run that one motor. I'm using a Vex Motor Controller 29, and I made the alterations to it like it says in Jameson Go's blog post. I'm running this to a Servocity 730rpm planetary gear motor. The battery is a 4s 850 mAh Turnigy LiPo. [Photo at right]

So what I have is the two wires coming off the ESC onto the motor, those are all good. Then on the other end of the ESC, I have the white signal wire and the a black ground wire going to my receiver, as well as an orange positive current wire and another black ground wire going to the battery, but when I plug them in, nothing happens.

Is there something I'm missing in the wiring system, or must it be an issue with my soldering or something else I've done? Any help much appreciated. [The Usual Places]

A: [Mark J.] You appear to have correctly modified the VEX 29 ESC for high voltage operation. You have also properly connected the ESC to the battery, motor, and receiver. Unfortunately you're missing a vital component.

  • A conventional ESC obtains power directly from the battery and uses an internal battery eliminator circuit (BEC) to send constant 5 volt power to the receiver via the 3-wire receiver cable.
  • The VEX 29 is designed to operate by obtaining power from the receiver and passing that power on to the motor. This requires that the receiver be connected directly to the battery and eliminates the need for a BEC -- the VEX has none. The receiver and ESC both operate at full battery voltage.
  • The alterations you made to the ESC allow it to operate at a higher voltage than your receiver could directly tolerate. In the new circuit the VEX 29 connects directly to the battery and motor, and the only connections to the receiver are the signal and ground wires. No power source for the receiver is included in your circuit.
Your test circuit needs a 6V nominal (3.5-7.4V) power source to directly power the Element-6 receiver. This could be a stand-alone BEC connected between your battery and receiver, a small battery pack, or even another ESC that has its own BEC.
Don't Blame Roger
Q: Hi there.

Just a quick one, on your "Who won?" page for series 10 of Robot Wars (Heat 3) it says that Track-tion withdrew but it was actually Vulture that withdrew.

Regards. [University of Manchester]

A: [Mark J.] I'd like to blame Roger the Web Gerbil for this, but I drew up that tournament tree myself. It must have been a late night; not only did I swap 'Trac-tion' and 'Vulture' but I put in an extra copy of Heat 4. Roger has corrected the tree: Robot Wars series 10.


Like Frozen Pizza
Q: Isn't there anything we can do about those stupid kit bots? They're ruining the insect weight classes! I really hate to see some guy show up at a tournament with a kit they put together in an afternoon and beat up real robots that real builders took a lot of real time designing and fabricating. Can't they just compete in a class for kit bots only? [Everybody on Reddit]

A: [Mark J.] Complaining about kit bots is like complaining that frozen pizza is ruining dinner because it tastes better than what you cook. If you can't beat a kit bot you need to up your game.


An Engineering Perspective
Q: I'm learning how to design bots and I'm starting out with the RioBotz Combat Robot tutorial from 2009. I'm guessing the design fundamentals and calculations will all hold up, but are the sections about electronics, motors, materials, batteries and such still relevant? If not, are there any good comprehensive resources on currently used components? [Social Media]

A: [Mark J.] The RBCRT is a fine document to study after you've built a couple of robots and want to invoke engineering principles to refine weak elements in your designs. Unless you have a strong engineering bent it is not where you should start. Read some build logs, view Robert Cowan's YouTube videos, drop in on some events for the weight class you plan to build and talk with successful builders.

Response: Thanks! This is really helpful advice.


It Pays for Itself
Q: In your FlySky FS-i6 Transmitter Combat Guide you recommend installing a complete replacement stick assembly if spring centering is needed for all axes on the left transmitter stick. Why not just install the cheaper self centering throttle conversion kit? [Columbus, Ohio]

A: [Mark J.] The conversion kit from Banggood is about $6, and the complete stick assembly is about $10.

  • Installation of the full stick assembly is a cinch -- four screws hold it in place and a single electrical plug handles the wiring.
  • Installation of the throttle conversion kit is tight, exacting work with small parts that can get lost and a tiny spring that hooks over hard-to-reach posts while under tension.
For me, the time savings and reduced frustration is worth more than the difference in price.
Ask Aaron is Live!
Q: I saw a link to Ask Aaron on a robot site in New Zealand that says you're not answering any new questions. They claim that everything here is old posts from years ago. Is that true? [Tacoma, Washington]

A: [Mark J.] Entirely not true. I'm here 'live' and answering as many questions as ever. I don't generally time stamp posts but I'll make an exception here: Wed 04-21-2021 T 19:32 UTC.

If I do re-post from the Ask Aaron Archives I will label the post to make that clear -- like this:


Flashback: an archived post from 2019
Q: Hi! I was working with the Team Tentacle Drivetrain Calculator to figure out an appropriate gear ratio for my new 'bot and I had a doubt. Shouldn't I double the 'Actual Robot Weight' input field in case my opponent ends up on top of my 'bot and weighs it down? I don't want to risk stalling the drive motors. [Combat Robot Forum]

A: [Mark J.] The Tentacle Drivetrain Calculator is a tremendously useful combat robot design tool, but you need to understand what it's actually telling you about the drive train design. Among other things, the calculator estimates the amount of torque needed to 'break traction' and spin the drive wheels of the robot based upon the weight pressing down on the wheels. If the drive train cannot provide at least that much torque the drive motors may 'stall' under heavy pushing loads, which will both reduce pushing power and risk damage to the motors. The calculator will warn you of this condition.

For calculation of pushing force and stall conditions you do not need to worry about your opponent weighing you down.

  • If only part of the weight of your opponent is pushing down on your robot, that weight is no longer on their drive wheels and their pushing force is proportionally reduced which makes it easier to push them.
  • If your opponent is entirely on top of your robot it cannot exert ANY lateral pushing force on your robot -- you won't have to 'push hard' against anything.
Carrying extra weight across a level surface is orders of magnitude easier than pushing against that weight while it is actively pushing against you. The only time stalling under this type of condition would be a problem would be if your opponent was entirely on top of your robot and you decided to push full-throttle against an immovable object like the arena wall. Why would you do that?

That's Not How it Works
Q: I saw BattleBots competitor get split in half, but each half was still able to respond to radio commands. On social media the team said their drive pods and the center weapon pod each had their own receivers, and that all three receivers responded to one transmitter. What type of transmitter can send independent signals to three receivers? Since a six-channel transmitter uses six radio frequencies, does this use eighteen frequencies? [Social Media]

A: [Mark J.] Any R/C transmitter can send a signal to multiple receivers -- only one radio frequency is used and the signal received by each receiver is identical. Here's how it works:

  • Modern 2.4 GHz R/C systems require that a receiver be 'bound' to a specific transmitter. A receiver 'bound' to a specific transmitter will ignore any signals from other transmitters. Each receiver can be 'bound' to only one transmitter at a time, but as many receivers as needed may be 'bound' to a single transmitter.
  • Dozens of times each second the transmitter sends out a 'frame' of Pulse Position Modulated information derived from the positions of each of the active sticks/knobs/switches on the transmitter. Each frame is a series of short pulses with the time between the start of one pulse and the start of the next providing the position information. A six-channel PPM signal frame is shown below.
  • This signal is sent on a single radio frequency and accepted by all receivers bound to the transmitter. Each receiver deciphers this signal and routes the position information for each channel to an assigned output port on the receiver as a Servo Control Signal. Servos, speed controllers, and R/C switches all understand and respond to this signal, where the position (or speed) is represented by the length of the pulse.
  • For a standard single-stick control mix the left side drive motor is controlled by the signal from receiver port 1, so the left pod will have the Electronic Speed Controller (ESC) for its drive motor plugged into port 1 of its receiver -- the other ports on this receiver are not used. Likewise, the right drive pod will plug the ESC for its drive motor into port 2 of of its receiver, and the center pod will plug its the ESC for its weapon motor into whichever port has been selected for its control.
So, each receiver has access to all of the transmitter control signals, but each 'pod' only makes use of the Servo Control Signals that apply to components in that pod. It's not uncommon for a large combat robot to make use of multiple receivers. It helps avoid long signal wire runs and offers some redundancy in case of unusual situations, such as you describe.

Bulky, Complex, and Expensive
Q: I've been watching some old Comedy Central series BattleBots episodes on YouTube and something caught my eye. Some teams were using what looked like Logitech flight sim joysticks meant for PC gaming to control their bots. Do you know why these were used as opposed to more standard controls? Why you don't you see them anymore? [Social Media]

A: [Mark J.] Those joysticks were part of the IFI Robot Controller System, as originally developed for the F.I.R.S.T. Robotics competitions in the late 1990s. Battlebots encouraged teams to use these 900 mhz controllers because they were less susceptible to interference than the 75 mhz hobby radios available at the time. The IFI system was bulky, complex, expensive, and had very limited control options. They were made obsolete by the frequency-hopping 2.4 ghz radios now in use.


Ask Me Again...
Q: Here’s a question I have: I’ve been trying to find bots that had planned to compete at the "US Robot Wars 1998 / Robotica 1998" event. That was why I asked the earlier question. Now, I’ve been going through the bots on this link:

Team Nightmare list of US Robot Combat Links

But it’s hard to tell for certain which had planned to complete there. I’m particularly interested in bots that planned to compete there or debut there but would never attempt another event after they were canceled. I’m also interested in those that would later compete, but had planned to debut there. [Fremont, Nebraska]

A: [Mark J.] I don't believe there is anything in the Team Nightmare Links that will help you fill in your list of probable entrants for the 1998 Robotica event. The list was made 20 years ago and most of the links are broken -- but you can use the trick in Frequently Asked Questions #30 to find an archived copy of many of the old web pages.

Brad Stone's book "Gearheads: The Turbulent Rise of Robotic Sports" mentions one robot that planned to debut at the cancelled event: Combat Robot Hall of Fame member 'Voltarc'. If you have an interest in the history of combat robotics and the early US Robot Wars events, "Gearheads" is well worth a read.

As I noted in my reply to your earlier question, I doubt that there ever was an official registration list for the 1998 Robotica event. The few robots I listed in my reply as competing in the 1998 'San Leandro Parking Lot' event and the 1999 'Underground Robot Street Fight' are the best candidates for your probable entrants list. Beyond that, the list is all speculation.


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.


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.

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.

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


It's a mystery!
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