4996 Questions and Answers about Combat Robotics
from Team Run Amok

Team Run Amok receives a lot of email asking about the design and operation of combat robots. In 2003 my son and team member Aaron Joerger (then 12 years old) requested 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.
In Memorium: 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   

Welcome to my website Recent Questions

Search the Archives!' Q: Hi Sir..I am completely new to a combat robot making.....I wanna have flipper as a weapon in it...Would you please tell me how can we transmit power from cylinder to function the weapon?And also,what can be the effective orientation of cylinder [with respect to] weapon? Thanks in advance !!

A: [Mark J.] There are DOZENS of posts about pneumatic flipper weapons in the Ask Aaron Weapons archive. Search there for 'pneumatic'. A few notes:

  • The Team Da Vinci Robotics Understanding Pneumatics page has several animated examples of pneumatic weapon designs, and a thorough discussion of pneumatic components.

  • Many Indian robot competitions do not allow pneumatic weapons. Check with the specific events you plan to enter before you build your flipper robot to make certain that you will be allowed to compete.

  • Pneumatic flipper weapons place great loads on the robot chassis and can be extremely dangerous if improper components or techniques are used. I can not recommend that anyone 'completely new' to combat robots attempt to construct and implement a pneumatic weapon. Pick something much simpler for your first combat robot.

Search the Archives!' Q: hey mark .. any idea about botblitz esc? the 85amp version . and why do these esc look like brushless ones? are these combat suitable ? [India]

A: [Mark J.] Your questions about the 85 amp BotBitz ESCs have been previously answered here at Ask Aaron. Search the Ask Aaron Motors & Controllers archive for 'BotBitz' to find multiple posts.

BotBitz takes inexpensive Chinese brushless ESCs and installs new firmware for use with brushed motors. The '85amp' version is popular in Australia for featherweight (30 pound) combat robots, but these are very basic 'no frills' motor controllers. There seem to be some quality control issues with BotBitz controllers that may trace back to the original Chinese manufacturer.

You should consider the Talon SRX ESC as an alternative. The Talons have been well tested in recent US competition and have proven themselves reliable in the lightweight (60 pound) combat class.

Q: Dear Mark,

By now, I have read at least 85% of your forum, which has become like a bible to me. Thank you for this, and for keeping Aaron's dream alive. I'm sure it hurts in an unimaginable way, no parent should ever experience it and it is so easy to give up, but you did not.

What's cool about what you guys have done, combat robotics and robotics in general is the cumulative nature of the learning experience, the more you learn, the more there is to learn and your site is like a never ending stream of fresh info.

I built a 2 wheeler bot with 2 CIM motors with 16:1 P80 gearboxes and 12" wheels, 2 Victor SP speed controllers. This will eventually become a self balancing ride-able scooter for my kids (after I figure out my IMU / Kalman filter issues), but for now I added some casters (kids call them 'training wheels' lol) and it is an R/C experimentation platform to figure out how things work in combat robotics. My kids want a battlebot and I decided to do it, but I'm taking steps and experimenting with some smaller scale stuff, like this 2 wheeler and a 4 wheel hockey bot that's in the works (thanks to the Tentacle calculator, which is an invaluable tool).

I have 2 questions:

  1. For my 2 wheeler with Victor SPs, I tried using the IMX-1 mixer (from Robot Logic). I like it a lot, it is very simple (just plug and play) the steering feels good and the reversing channel works like a charm. I use a Spektrum DX6i, just because that's what I have. The issue I'm having is the fact that I have no idea how to fine tune this system. Basically what I'm noticing is that one of my motors is a bit slower. I also notice that when I push the Elevator stick all the way up, the LEDs on the Victor on the faster motor light steady, which tells me it is at its max, while on the slower Victor, the LEDs are rapidly blinking and never reach the steady max state. I tested the Victors separately (without the IMX-1) and I can reach full speed at full stick throw without issues, the motors have no binding, everything feels pretty equal. When I try to calibrate them with the IMX-1 board, I get a consistent result, one motor is always slower. I know I can probably offset the Victors separately “by eye” then plug the mixer and at some point I might achieve similar speeds, but this is very annoying. Is there anything I'm missing, any other trivial way to trim things with this system? I'm thinking in the heat of the battle, it's slightly awkward to not being able to make fine adjustments. Maybe something I can do within the DX6i? I can't wrap my brain around it because any adjustment I try in the TX, makes a mess after it gets mixed through the board.

  2. Kind of related to 1., I read most (if not all) posts related to transmitters and I noticed you have a preference for the Futaba T6J as an entry level (~$200 and under) TX for combat. Right now, this is what I do to get mixing done through my DX6i [when not using the IMX-1 mixer]:

    ELEVON > ACT (makes 50% throws)

    Then I apply the D/R & EXPO settings as you recommend in your posts.

    For the most part it is very usable, but I can't figure out a way to add flipping/reversing (for driving inverted or with the rear) and there's that diamond shaped throw pattern best described in the IMX-1 mixer's manual:

    "Unlike our mixer, some RC transmitters and mixers use an “add and clamp” algorithm that reduces the controllability of your robot. This "simple" way to do tank/elevon mixing will cause your motor outputs to clamp to their maximum everywhere outside an inscribed "diamond" in your joystick's range of motion. This means that when your stick is anywhere within a triangle in each corner of its range, your robot is trying to spin as fast as it can. Our algorithm gives you smooth proportional control all the way to the corners of your stick's travel. The only time your robot spins at its maximum speed is when you jam the stick all the way to the corner."

    I drove with and without the IMX-1 and there is a noticeable difference, the IMX-1 algorithm is indeed nicer. But I don’t like the lack of fine tuning options, I think doing all the mixing and flipping inside the TX is much more elegant and simple (and less points of failure in combat).

    My question is: Will the Futaba T6J allow more control over the mixing compared to the DX6i? Will it allow me to assign a button/switch for inverted mix (inverted/backwards driving)? I looked all over and could not find any info on how to program the 6J for combat. Does your guide for the 6XAP (or the older Battlebots-era guides for the 9C series) apply to the 6J at all?

Sorry for the many words, so many questions to ask... Thank you in advance for your time and for your great service to all the noobs.

New York, USA

A: [Mark J.] Thank you for your kind comments about 'Ask Aaron'.

Question #1 has a fairly simple solution, once we find the right component to adjust:

  • The IMX-1 mixer - A very nice unit, and as you note it's completely 'plug and play'. Its simplicity is a strength, but it also prevents any fine tuning of the outputs. Nothing to be done there.

  • The DX6i transmitter - It has lots of tweeks, but nothing that can balance the throttle output after it's already gone thru the mixer. That leaves us with...

  • The Victor SP speed controllers - Take a look at page 6 of the Victor SP Quick Start Guide and follow the callibration procedure for each of the two speed controllers with the mixer in place. After callibration both of the Victors should give full throttle in both forward and reverse.

Question #2 [a cluster of three questions] is a little trickier to answer. As you've noticed, the Elevon mixing in the Spectrum transmitters is not optimized for robot use. You've implemented the standard work-around in order to get full-range PWM output. Unfortunately, this fix takes up not only the mix option but also both of the programmable mixes!

Among the reasons I prefer Futaba radio systems is their superior mixing implementation. With a Futaba transmitter you do not need a 'work-around' to get full PWM output, and their mixing does not use the “add and clamp” algorithm -- you have full response all the way out to the far corners of the stick travel, just as you do with the IMX-1 mixer. The 6J transmitter also has 'sub-trims' which allow trim adjustments on individual receiver outputs rather than just on the transmitter stick inputs. This is very handy for adjustments to individual channel outputs after transmitter mixing has taken place: if one motor is a little slow, or if it 'creeps' at neutral.

More good news: programming the Futaba 6J for robot use is nearly identical to programming the Futaba 6XAPs. The menu layout is a little different, and a couple of the function abreviations have changed, but the Team Run Amok Futaba Programming Guide should walk you thru the 6J with very little trouble.

Now the bad news -- I've never found a method to assign an invert function to a switch on any of the commonly used R/C transmiiters. Full-function hobby R/C systems are built for model aircraft, and there isn't an instance where an aircraft needs to reverse the response direction of a channel in flight. Even using the programmable mixes won't get the job done.

My advice: re-calibrate the Victor SPs to work properly with your IMX-1 mixer and continue to use its invert function. You'll need an on-board mixer anyway when you get around to experimenting with peizo gyros.

Q: Am I cool [Pennsylvania]

A: [Mark J.] If you have to ask me, no.

Q: Hey Mark, just a silly thing, do stickers get caught on spinners? My wedge has a Fingertech sticker on the front (a big one), and I'm slightly worried that a drum might catch on to it and launch me. [Pacific Northwest]

A: [Mark J.] Maybe not so silly. An anti-spinner wedge should be smooth, hard, and stiff. A proper spinner can and will dig into anything soft and get at least a little purchase. I can't say that the amount of drag will be enough to launch your 'bot, but I wouldn't take that chance with an insect class robot. I don't like to even paint a wedge! If nothing else, a torn up sticker might influence a judge into thinking that a spinner did at least some damage to your 'bot.

Q: What is the actual stall current of the AmpFLow A28-400? Robot Marketplace says 390 amps, but I calculate it at 571 amps (Current = Voltage / Resistance = 24/0.042 = 571 amps). Which is correct? [India]

A: [Mark J.] Your calculation of 571 amps is correct for a 'theoretical' stall current of the AmpFlow A28-400 motor at 24 volts, but the 'real life' stall current will be lower due to the added resistance of other elements in the motor circuit (controller, connectors, wire, battery...). I don't know exactly how those other elements are accounted for in the Robot Marketplace figure, but 390 amps is likely close to the true stall current of the motor in a typical combat robot application.

Q: Hi. I have questions about calculating a drive train chain reduction ratio.

Drum weapons are common in India, and I need to stop their attack. I'm building a 35 Kg wedgebot with a pair of AmpFlow E30-150-G gearmotors for power. Many builders suggest that I use 4-wheel drive for better traction.

Output speed of the gearmotors is 650rpm @ 24v. I need 500 RPM after making the reduction for 4 wheels. What is the correct chain reduction ratio? What problems do I face in making this chain drive? [India]

A: [Mark J.] Why 500 RPM? I get many questions from Indian robot builders seeking a 500 RPM drivetrain output speed. I don't know how that number became the standard in Indian robotics, but there isn't a valid engineering reason for it. You've spent good money for your AmpFlow gearmotors and it would be a pity to handicap the performance of your robot by picking an arbitrary output RPM, particularly since it requires a needlessly complex chain reduction system to implement.

The Team Tentacle Drivetrain Calculator shows that a 35 Kg robot with two E30-150-G gearmotors and 4" diameter wheels would have excellent performance without additional gear reduction:

  • Acceleration to 7 MPH in less than 5 feet (0.7 second); and
  • About 22 amps per motor current consumption before the wheels slip at maximum pushing power.

That seems ideal for a quick wedgebot in a small Indian combat arena.

A 4-wheel drive system to improve traction is a good plan. However, adding a chain reduction to drop the output speed to 500 RPM gives no increase in pushing power, costs weight, and adds new failure points to the design. Keep it simple! Mount a wheel and sprocket on the strong output shaft of the AmpFlow gearbox, mount a matching sprocket and wheel on a bearing supported live shaft, and run a short chain between them (see top photo at right).

With this design, even if all the chains fail you will still have partial drive power. Align the sprockets carefully and select the correct distance between axles to keep excess slack out of the chain. Simple is good!

If you're still interested in selecting the correct reduction ratio to drop your speed to 500 RPM, the question has been recently answered -- see the third question down from the top in the Design and Construction Archive. The formula for calculating gear reduction to obtain a desired output RPM is:

Gear Reduction = (Motor RPM / Desired RPM) to 1

Your chain and sprocket reduction would be: (650 / 500) to 1 = 1.3 to 1. You could achieve this reduction with 20 tooth sprockets on the gearmotor output driving 26 tooth sprockets on the drive wheels -- as one example.

Simple chain drives with offset gearmotors
Chain and sprocket 4-wheel drive
with offset gearmotors

Gearmotor with chain reduction 4-wheel drive

Indirect 4-wheel drive
with chain reduction

Q: How to operate a wireless Bluetooth control using dpdt switch??? Not joystick [India]

A: [Mark J.] Assuming:

  1. you're replacing a potentiometer-based joystick; and
  2. you want low-medium-high resistance to signal motor reverse-off-forward;

a Double Pole Double Throw (DPDT) switch has the wrong logic for this 'sequential switching' -- it cannot be used.

What you need is a Single Pole Triple Throw (SP3T) switch, also known as an "on-on-on" switch. The upper diagram on the right shows the circuit to replace a 10K ohm potentiometer for simple reverse-off-forward control.

SP3T switches are difficult to find, particularly in a 'momentary contact' style that returns to center when released. A single-axis joystick controller would be much easier to source, and probably cheaper.

Alternately, you can convert your controller to two momentary Single Pole Single Throw (SPST) push button switches. The lower diagram on the right shows the circuit for twin push buttons. The upper button (normally closed) gives a motor forward command when pressed; the lower button (normally open) gives a motor reverse command when pressed.

Extra credit quiz: what signal is sent if both push buttons are pressed?

Progressive switching with SP3T switch

Progressive switching with two SPST push button switches

Q: are you going to watch robogames 2015 !!!!!!!??????? [India]

A: [Mark J.] No - not without Aaron.

[Mark J.] 'Ask Aaron' has received a number of questions recently about power requirements for different robotic tasks. I'm not certain that the questions are worded correctly to get the desired answer, and I haven't been entirely satisfied with the answers I've given.

I've looked back at those recent questions and asked myself three new questions that I think may remove some of the confusion about motor power requirements and pushing force:

Q: How much motor power does it take to move my 60 pound robot?

A: It takes very little power to overcome the initial rolling resistance of a wheeled vehicle on a smooth and level surface, so the real question is how quickly you want to accelerate and to how great a speed. The Tentacle Drivetrain Calculator will model the acceleration, pushing power, and speed of a robot of a given mass given the specific motors used, the number of motors, the gear reduction, the weight supported by the driven wheels, and the wheel diameter.

Move a 60 pound robot

Q: How much motor power does my robot need to push with 60 pounds of force?

A: The maximum potential pushing force of a robot can be calculated from the weight bearing down on the drive wheels and the coefficient of friction between the tire material and the arena surface:

Max Pushing force = weight on driven wheels * coefficient of friction

The effective coefficient of friction for a grippy rubber tire in a somewhat dirty robot combat arena is around 0.8, so unless your robot has at least [60/0.8 =] 75 pounds of force bearing down on the drive wheels, you aren't going to get 60 pounds of pushing force regardless of your motor power.

If you are willing to push very slowly, a very small amount of motor power can be geared down to provide the torque needed to maximize the pushing potential of the robot weight and traction conditions. More motor power will allow greater speed while maintaining the required drive torque, but will not increase your pushing force.

Push with 60 pounds of force

Q: How much motor power does my robot need to push a 60 pound object?

A: The answer depends on the characteristics of the object you want to push:

  • A dead mass of 60kg resting on a smooth and level arena floor will require that you exceed the static friction between the object and floor before the object will move, and you must contunue to apply a force greater than the kinetic (sliding) friction between the object and floor to keep the object in motion. The surface friction depends on the two materials in contact, their smoothness, and any contaminants on the surfaces.

  • A mass of 60kg on freely turning wheels on a smooth and level arena floor only requires a small amount of force to overcome the initial rolling resitance of the object. See 'How much motor power does it take to move a 60kg robot?' above.

  • An actively resisting powered mass of 60kg (such as another robot 'head-to-head') will require that the pushing force of your robot exceed the pushing force of the resisting mass.

As described above, the maximum pushing force of a robot depends on the weight bearing down on the drive wheels and the coefficient of friction between the tire material and the arena surface. Once there is sufficient motor torque applied to achieve this maximum pushing force, additional motor power will only spin the wheels without providing additional pushing force.

Push a 60 pound weight

For general info and advice on motor selection for your combat robot, see: Frequently Asked Questions #21.

Q: Hi Mark, Sage here. Hope you're doing well.

I have bought the iMAX Quattro B6 charger. Although the site states 60W per port, instead it's 50W per port. Soon I'm gonna buy 6S 5000mah LiPos. (Although I'd realistically need about 2000mah worth of battery capacity in a match, I'll just double it to be on the safe side. I read it in the Riobotz Combots Tutorial. Plus 25% reserve for unexpected situations.)

What bothers me is that my charger might take too long to charge my batteries. The manual states: "For 50W charge power, current is regulated accordingly. Eg. 11.1v battery, charge current approx. 4.5A. Eg. 22.2v battery, charge current approx. 2.2A." The current is user selectable but I kinda have a bad feeling that it won't allow me to increase it past 2.2A. That's gonna take >2hrs to fully charge my battery!

Is the charger too puny for my application or will I be fine with it? If the charger is puny, I thought of some alternate ways to make it work:

  • First, I can use 2 small battery packs instead of a big one. Like a pair of 3S 5000mah batteries in series or 6S 2500mah batteries in parallel. I'll also buy another charger. In this way I have 8 charging ports and 10 batteries in total, 2 in my bot and 8 on charge. And so I'm effectively doubling the power pumped into the batteries from 50 to 100 watts as I'm utilizing 2 ports at once, which halves the charging time to just above an hour for one set (2 batteries).

  • Second, instead of five 6S 5000mah batteries, I can buy 7 or 8 and just hope I don't run out of charged packs and stick with just a single charger.
What do u think Mark? Which way should I go? [India]

A: [Mark J.] I know you're asking about your battery charger, but there are a couple of assumptions hidden in your question that I think I'd better address before I get to the charger...

  • Mounting LiPoly batteries properly is tricky -- they're a little 'squishy'. If you just strap the pack to the chassis with a couple narrow zip-ties, a hard impact will cause a lot of localized pressure that can crush the battery enough to cause an internal short. Shorted LiPolys burn! Ideally, the battery pack should be padded and securely enclosed within a rigid container.

  • Good LiPoly mounting does not make for quick and easy battery swaps. If you're planning on swapping packs between matches you'd better place a lot of thought into making those swaps quick and foolproof. If your robot took a lot of damage and you need to scramble to get it ready for the next match you don't need any extra trouble when swapping out the battery.
Now, back to the charger:

You're worrying way too much about battery power. Your calculations are telling you that you only need 2000 mAh for a full match, you added 25% to that for good measure, and then doubled the whole thing! If you only pull down the charge on a battery pack by 2000 mAh in a match, your charger can top that off in an hour. If you get in a rush, you can easilly run two matches back-to-back on a fully charged 5000 mAh pack. You've got enough battery power to run 10 full matches without any charging at all!

Your current charger is fine, and I think five battery packs is overkill. Given that there's a lot of time between matches in the early rounds of a tournament, your current charger with just three 5000 mAh packs should get you thru a tournament just fine.

Q: Hello,for 1hp 900rpm DC motor to push 60 kg of weight, how many 12v 7.5Ah sealed batteries are require? [India]

A: [Mark J.] Depends on how far and how fast you want to push it, and how hard it's pushing back. I can't summarize a full kinematics course here, but I can simplify your question and provide what may be a useful real-world answer.

The Tentacle Drivetrain Calculator can estimate the pushing force and current drain for a combat robot under heavy pushing conditions based on:

  • Operating voltage
  • Stall amperage
  • Motor voltage constant
  • Motor torque constant
  • Robot weight
  • Number of motors
  • Gear ratio
  • Wheel diameter
  • Traction conditions
  • Length of match

Since you haven't given me most of that information and since you probably don't have good motor specifications, I'm going to have to make a series of blind guesses that may or may not yield a figure of any use to you.


  • Two motors similar to the 1 horsepower AmpFlow E30-150 - for which I have specifications
  • Operating at 24 volts
  • Geared down 6.3:1 to give 900 RPM output
  • 60 Kg robot weight
  • Four-inch diameter drive wheels
  • Standard traction conditions -- smooth, hard, clean surface and grippy rubber compound
  • Five minute long match

Assuming that the robot is pushing at maximum capacity 70% of the match, the total current draw is around 6.175 Amp-Hours, so you'd need two of the batteries in series to provide 7.5 Amp-Hours of current at 24 volts.

If your motors are rated 1 horsepower at 12 volts, current drain doubles to 12.35 Amp-Hours (half the voltage requires double the current to provide the same wattage output), so you'd still need two batteries, but wired in parallel to provide 15 Amp-Hours of current at 12 volts.

If ANY of my guesses are wrong, the entire calculation is flawed and you're on your own.

Note: the amp-hour ratings for lead-acid batteries are based on a slow drain period (10 hours). They must be de-rated for rapid discharge, but the degree of de-rating depends on the specific battery type and manufacturer. Given that it's unlikely that your robot will be pushing at full-throttle for 70% of the match length, I think you'll be OK with two of the batteries.

Q: Hi Mark Sir......can you please tell me which motor is good to rotate a 13kg drum and 22kg single tooth spinner ,ampflo A28-400 or A28-150? [Mumbai, India]

A: [Mark J.] I can't recommend a motor based on just the mass of the weapon. I suggest that you read the Ask Aaron Spinner Weapon FAQ for guidance in spinner weapon design and motor selection.

Search the Archives!' Q: Just wanted to know the Mechanism SKF Warrior by Team Whyachi !!!
what kind of mechanism is that ?
They say it stores energy when it spins, and then uses it to flip the opponent. How is this energy stored, spring ?
Or is it something else ? [Mumbai, India]

A: [Mark J.] Previously answered.

Search the Weapons Archive for multiple posts on 'Warrior SKF' and its Spin Kinetic Force (SKF) weapon.

Hi Mark, hope you are doing well. Today, I visited the website after quite a long long time! Reading so much questions from my fellow Indians put a smile on my face. I must admit, the number of questions from Indians nowadays are quite large compared to back in those days (I assume, we were the only ones asking from India back then). Although we are not in the combat robotics scene anymore, someday we hope to start again from where we stopped.

Anyways, I just wanted to let you know Mark, that your website is one of the most valuable resources available to Indian Combat Roboteers. I highly respect and admire your decision to continue this website even though you were devastated because of Aaron's demise. I'm sure that's what Aaron wanted, and he's smiling from the Heavens to see "Ask Aaron" growing bigger and popular day by day. Regards, Las Vegas. [Niedersachsen, Germany]

[Mark J.] Thank you, Las Vegas. There were people kind enough to answer some of our questions when we got our start in combat robotics, and we put 'Ask Aaron' together to pay that favor forward. I'm very pleased that you and others have found our information and advice to be helpful.

Note: 'Las Vegas' earned his nickname here because even though he was in India his internet service went thru a server in Nevada. For quite a while we labeled his location as 'Las Vegas' until he cleared up the situation for us. Now his emails are being routed thru Germany. He claims to actually be in Niedersachsen, but I've given up on trying to figure out where he really is.

A few reflections on 'Ask Aaron' as we approach our 5000th question.

  • The first archived copy of 'Ask Aaron' from 2003 was a very simple page with only seven questions plus Aaron's first robot haiku. Today the site consists of twenty interlinked 'Ask Aaron' pages with a custom keyword search and more than 450 graphics.

  • In the last week 'Ask Aaron' has hosted visitors from Argentina, Australia, Austria, Bangladesh, Brazil, Bulgaria, Canada, Chile, China, Denmark, France, Germany, Greece, Hungary, India, Indonesia, Iran, Ireland, Italy, Kazakhstan, Korea, Malaysia, Mexico, the Netherlands, New Zealand, Pakistan, the Phillippines, Romania, Saudi Arabia, Singapore, Slovakia, Slovenia, South Africa, Spain, Sri Lanka, Sudan, Syria, Thailand, the United Arab Emirates, the United Kingdom, Vietnam, and at least 37 of the United States (Map).

  • I've been running 'Ask Aaron' by myself for almost a year and a half now. It's not as much fun as it was when Aaron and I worked thru the questions together. Sometimes it's comforting; sometimes it's frustrating; sometimes it hurts. I'd considered calling it quits at 5000, but Aaron wouldn't approve of throwing in the towel. I've made a few cosmetic changes to the site to prepare for the start of our next thousand questions. Onward!

Q: hi mark ... this might be a dumb question , but i would still ask . Is there any substitute to ampflow motors ? like motors having same characteristics weight and power bt cheaper than ampflows ? If not same weight , the same power for a little bit of more weight . but cheaper ? expected power is about 2 hp at 4000-6000 rpm . [India]

A: [Mark J.] Carlo Bertocchini went to considerable effort to design, and produce a series of PMDC motors ideally suited to the demands of combat robots. AmpFlow motors are so widely used in combat robotics because there is no cheap substitute.

You can't get a documented 2 hp in that RPM range with anything close to the reliability, efficiency, and ease of use of the AmpFlow in a 'bargain' motor. If you could, you wouldn't need to ask me what motor that was 'cause a great many robot builders would be using them.

Q: I am going to make an combot robot...weight 30 kg.......I want to make drum will be hollw and of 4 to 5 kg.....I only want to know which motor will be best in our budget bcoz we can not afford we want other cheap substitutes....please help us.....I want to rotate drum at 2000 rpm [Haryana, India]

A: [Mark J.] I can't tell you what motor will be best in your budget because:

  1. You didn't tell me what your budget is;

  2. I don't know what motors are available to you; and

  3. Many of the weapon motors used in India (automotive starter motors, e-bike pancake motors...) have no reliable performance figures to use in calculating their acceptability for a specific weapon application.

'Cheap substitutes' are cheap for a reason. Without performance specs you're just guessing, and you're likely to throw your money away on an entirely unsuitable motor.

I suggest that you read the Ask Aaron Spinner Weapon FAQ for guidance in weapon design and motor selection.

Q: Hey Mark, something is off with one of my tinyESC V2s. The problem is that once power and signal are applied, the motor starts spinning at max speed with no control. No matter what I do on my radio or what channel it's on, the esc won't respond, unless I shut it off and then the failsafe kicks in. It's not a problem with the radio, my receiver, my battery, or my motors. The esc responds the same way every time. The red LED is also stuck on, with the green one not even turning on, if that helps. Any clue whats wrong with it, and if its even possible to fix? [Aurora, Oregon]

A: [Mark J.] It doesn't sound good -- but if you're lucky the ESC may have just lost its calibration settings. Try resetting to the default: tinyESC calibration jumper

  1. Plug your tinyESC into the unpowered radio receiver.
  2. Jumper the two tinyESC 'calibrate' pins with a piece of wire.
  3. Power up the receiver - leave the transmitter off.
  4. Remove the jumper.

If that doesn't help, email Kurt at FingerTech Robotics:

Q: hi aaron! after a long time whether roboclaw 60A speed controller(from orinion robotics) will be good to drive a pair of e30-150 motors and any one them have specially used that, comparing with sabertooth 60A driver can u speicify the positives and negatives of both.because sabretooth resets on its impact and results in low voltage of rx [Tamil Nadu, India]

A: [Mark J.] I'm very sorry, but Aaron is not available to answer your question. I'll do the best I can.

I can't recommend a speed controller based solely on the motors that a robot will use -- see Frequently Asked Questions #21. The speed controller amperage capacity requirement for your motors will depend on design factors such as:

  • robot weight;
  • wheel diameter;
  • drivetrain gear reduction;
  • battery voltage and current capacity; and
  • percentage of total weight supported by the drive axle(s).

The merits and drawbacks of the Roboclaw and Sabertooth controlers have been discussed in previous posts on Ask Aaron. I suggest that you search this page and the Motors and Controllers Archive for those posts.

Very briefly:

    Search the Archives!'
  • There is evidence that the Sabertooth controllers have a serious problem with impact resistance. I cannot recommend their use in combat robots at this time.

  • The Roboclaw controllers are designed for general robotics applications -- not specifically for combat robots. They are difficult to set-up and lack key features useful in robot combat.

  • There are other controllers you should consider that may better meet your needs, such as the Talon SRX, Ragebridge, and BotBitz 85 amp. These have each been discussed previously here at Ask Aaron.

Do your homework and find out how much amperage your motors are likely to use, then you will be better equipped to select the proper controller for your purpose.

Q: Who was the composer for Robotica? [United States]

A: [Mark J.] The end credits for TLC's 'Robotica' series list veteran Hollywood composers Danny Lux and John Carta for music.

Q: thinking of building a bot with horizontal sppiner fr 25kg catg. compitations.. whic ampflow shud i use considering the cost factor also [India]

A: [Mark J.] It's generally not a great plan to start by picking a motor and then 'back into' a weapon design to suit that motor. You'll have greater success if you start with a weapon design (type, dimensions, material) that is capable of storing the energy you require at a reasonable speed -- then select a motor that can spin up the weapon to the desired energy level quickly enough to meet your needs.

I used the Run Amok Spinner Excel Spreadsheet to generate this simple example:

  • A steel bar 400 mm long, 90 mm wide, and 25mm thick weighs 7 kilos. That's a reasonable weight for a 25 Kg class spinner weapon.

  • Spinning like a lawnmower blade, the bar will store about 1000 joules of energy at 1400 RPM. That's an acceptable amount of energy for a 25 Kg class weapon and a reasonable speed.

Now that we have a practical weapon design, we can look at weapon performance when powered by a specific motor.

  • An economical AmpFlow E30-150 motor running on 24 volts with a 4:1 reduction belt drive to the bar weapon will spin the bar to about 1400 RPM in less than 2.5 seconds. That should be adequate performance in even a small arena.

That's only one possible weapon that may or may not suit your overall design. It does show that it's possible to design a 25 Kg class weapon powered by an Ampflow E30-150 motor. A more powerful motor could spin a weapon with greater destructive potential -- the choice is yours.

I suggest that you read the Ask Aaron Spinner Weapon FAQ for additional guidance in weapon design and motor selection.

Q: hey mark, can you tell me that how much energy (J)/kg would be sufficient for a drum weapon to attack a bot of 15 kg?- sam [New Delhi, India]

A: [Mark J.] According to Stroker McGurk's Law:

"If some is good, more is better, and too much is just enough."

I suggest that you read the Ask Aaron Spinner Weapon FAQ for a more precise answer to your question, Sam. The first three questions there are:

  1. How much energy should my spinning weapon store?

  2. How do I calculate the kinetic energy storage capacity of a spinning weapon?

  3. How quickly should my weapon spin-up to speed?

Read the whole thing while you're there -- it might save you from asking a few more questions in the future.

Q: Can a Gyro sensor could be used as a substitute by attaching it to the robot's wheel so that the sensor spins around as the wheel turns? [Houston, Texas]

A: [Mark J.] A subtitute for what, exactly?

There is some general confusion about the operation and uses of R/C peizo gyros. Our Gyro Guide answers many of the questions about how and why to use a peizo gyro in a combat robot.

The gyro is mounted on the robot chassis and is connected to the R/C receiver. It monitors the turning motion of the robot and compares it to the turning signal output from the receiver. If the turning motion differs from the turning signal from the receiver, the gyro adjusts the turning signal to correct for the difference before passing it on to the motor controllers.

Mounting the gyro to a wheel would only let it monitor the rate at which the wheel was rotating, not if the robot was turning at the rate the radio was commanding. It would also make it hard to wire the gyro to the receiver and motor controllers!

Q: which type of dc motor suits to rotate 7kg horizontal drum (outer diameter 160mm, thick 40mm , made by MS) ?
it is advisable to use E-bike brushless dc motor having capacity of 200w , 12v , 20-18 A , 2000rmp (torque = p*30/3.14*2000)=0.9Nm) ?
or any other? [Gujarat, India]

A: [Mark J.] First, you've misapplied the formula used to calculate the torque constant (Kt) of a motor in an attempt to calculate the actual torque your motor supplies, and you've incorrectly entered the actual RPM of the motor into that formula instead of the motor's speed constant (Kv). The correct formula for estimating the stall torque of a brushless motor is given in the instructions page of the Run Amok Excel Spinner Spreadsheet. While you have the spreadsheet open, spend some time learning how to use it and you can do your own evalution of motors for powering up a spinning weapon of any size, shape, and material.

You haven't given me enough info about your E-bike motor to model its performance in spinning up your weapon, but I can tell you that in general a 200 watt motor isn't going to be enough for the weapon you describe. The AmpFlow E30-150 motor provides 750 watts of output power, and would be a marginal motor for your purpose. If I correctly understand your description of the weapon, the E30-150 is capable of spinning it to about 1300 RPM and 1500 joules of energy in about 4.5 seconds (3:1 belt reduction). That would make an adequate weapon for a 40 kg robot.

Suggest you read the Ask Aaron Spinner Weapon FAQ for additional information in designing your weapon and selecting a weapon motor.

Q: Hi Mark... bang bang guy here. I have read the transmitter requirements in the archives, and I have become more confused as to which remote to buy. I have seen many transmitters on the net but I am terribly confused! Can you please suggest some which have been used in combat and are trusted and are also not too expensive?

I would like to use the left vertical axis for controlling both the motors together (like the bot going straight and reverse) and the right stick for steering. I will definitely need a inverting function 'coz my bot will be invertible. I also have a weapon to control.

There is also one more confusion. As in bang control, we can make two types of turns:

  • one is 360 degree turn - the left and right drive wheels running a same speed but opposite direction;
  • the other is a half turn, in which only one side drive moves and other is stationary.

I understand how we can achieve the 360 turn by just operating the steering throttle, and smooth turns by using combination of both the throttle and steering sticks, but how will I be able to make a half turn?

And also, can we get ready-made transmitters with the left vertical control axis already positioned to self-center? If not which transmitter should I buy so that I can get parts easily for that modification? Please help me out as I am buying a transmitter for the first time and I dont want to make mistakes.

Also can you explain the modes, the control which I want comes in which kind of mode? Will I require a mode switching?

Thank you SOOOO much! [India]

A: [Mark J.] Lots of questions -- but that's why I'm here.

Your control preferences are entirely normal for combat robots and can be met by just about any full-function 6 channel radio system.

  • Throttle on the left vertical stick axis and steering on the right horizontal stick axis is my preference as well. It requires 'Elevon Mixing' (sometimes called 'Delta Wing') which is a very common function, and a 'Mode 1' transmitter. Your motor controller inputs will plug into channels 1 and 2 on the receiver.

  • A 'Mode 1' transmitter has the 'Throttle' (channel 3) assigned to the vertical axis on the right stick and the 'Elevator' (channel 2) assigned to the vertical axis on the left stick. A 'Mode 2' transmitter has these reversed. A true Mode 1 transmitter will have the vertical axis on the left stick spring-centered - you won't need to worry about changing it.

  • If you purchase a transmitter with 'Mode Switching' capability either the right or left stick may have the vertical axis spring-centered. It's fairly simple to open the transmitter case and swap the spring centering mechanism from one stick to the other as needed.

  • Avionic RCB6i transmitterI know that the Avionic RCB6i radio is available to Indian builders at a good price. Features include:

    • mode switching;
    • Delta Wing mixing;
    • position-settable failsafes on all channels;
    • 3-position switch on channel 5 to operate a reverseable weapon controller forward/off/reverse;
    • 2-position switch on channel 6 to operate the invert function on the 'RageBridge' controller;
    • all of the most needed combat control functions (servo reverse, dual-rates, digital trims, model memory); and
    • a user manual that makes sense.

  • Another possible radio would be the Orange T-Six transmitter and matching Orange R620 receiver. Inexpensive and widely used in combat robots, the Orange radio has more features than the Avionic but lacks the position-settable failsafe controls and useful 3-way switch on channel 5 that the Avionic offers. For your purpose, I'd buy the Avionic.

Once you get used to proportional control of your robot you won't need to worry about the type of maneuvers you had to resort to with your 'Bang Bang' system. If you really need a 'half turn' it can be accomplished by combining equal amounts of throttle and turn input. The turn input will command one motor forward and one motor backward, and the throttle input will balance out the backward motor command with an equal forward command leaving that motor motionless.

Remembering Aaron... 

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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