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6022 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) 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 'Got Question?'
The Ask Aaron Archives Click to browse thousands of previously answered questions by category, or search for specific topics. Includes FAQ
In Memoriam: 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   
 
Caution
Even small combat robots can be dangerous! Learn proper construction and safety techniques before attempting to build and operate a combat robot.

Q: Hi Mark. For a 15 lb robot that will be a square ram-bot driven offensively I was interested in an armor comparison I could not find in the archives. Section 2.6 of the Riobotz Tutorial talks about the difference in types of armor. Considering the weight class I figure a robot of said design will be beat to hell without shock absorption. Between pieces of ar400 would ceramic tile or soft rubber or foam be a better medium for absorbing shock? Would this be considered ablative armor and should it be securely, tightly fastened or loose?

Congratulations on breaking through 6000 questions, the community warmly thanks you from here in western Pennsylvania. [New Castle, PA]

A: [Mark J.] Thank you, New Castle. I hope 'Ask Aaron' continues as a useful resource for you and the other builders on the Allegheny Plateau.

Shock absorbtion is a too-frequently overlooked aspect of armor mounting. In combat robotics, layered armor has proven inferior to single-piece armor with rubber shock mounts. The photos at right show the front armor pods and rubber shock mounts used on BattleBots competitor 'Overhaul'. Properly sized mounts will be quite firm -- they will barely flex under the best pressure you can exert on the panel by hand.

A web search for 'vibration isolators' will find items similar to those used by 'Overhaul' in a wide range of sizes. This is reactive armor, not ablative armor. Torque them down snug, without play or slop.

If you really want to try a layered armor, a sheet of solid 'Buna-N' rubber between metal layers would be a good choice. Try about 3/16" rubber thickness for a 15 lb robot.


Q: Why do saws have less bite than discs with less teeth? And why do hyperspeed weapons not get bite? Yes I did read the spinner weapon FAQ. It said the reason was longer insertion time. Over speed weapons should do the same thing as mid-speed weapons except faster. Bots like Saifu, Weta, Algos, and DDT all use high speed weapons and they have no problems. [Arlington, Virginia]

A: [Mark J.] I believe you misunderstand what is ment by weapon 'bite'. A faster weapon, or one with more teeth, allows less time for the opponent to be inserted into the arc of the spinning weapon before the next tooth rotates around to strike. The Spinner Weapon FAQ points readers to Section 6.3 in the RioBotz Combat Tutorial for an explanation of the relationship between weapon speed and bite, as well as the formulas for calculating bite depth. If the explanation in the FAQ is unclear to you, I suggest reading the RioBotz tutorial.

The robots you list do have effective weapons - it is possible to have an effective spinner that has poor bite if you compensate with other design elements.

Examples:

  • 'Algos' uses a wedge to lift the opponent up and into the drum, exposing a sharp angle on the underside of the opponent that requres very little bite to grab and throw.
  • 'DDT' does not direct-drive the big weapon blade -- the large diameter give a high tip speed but allows a slower rotational speed for better bite.
  • 'Weta' has a reduction belt drive to keep the RPM reasonable, plus offset impactors for adequate bite.
  • 'Saifu' is a direct-drive high-speed drum, but you don't need or want full RPM against all types of opponents -- you can and should throttle back for hard to hit opponents.
Opponents with sharp edges to grab do not require good bite, but better bite allows effective attacks on flat or rounded surfaces.

Q: This is the bite guy and I have a question. Does bite apply to hammers?

A: 'Bite' as decribed here applies only to continuously rotating 'spinner' weapons. The concept of 'bite' does not apply to 'strike on demand' axe or hammer weapons.


Q: Hello, Mark! Back with some questions: this time, these questions are regarding a heavyweight build. The robot would be heavily based off of Electric Boogaloo:
  1. My original idea was to use a spinner similar in shape to Deathroll's spinner (I even asked them if they could sell me a spare! Of course, the dimensions would probably change), but around 12" tall. However, I may be better off with using a 12" bar seeing as it will be easier to put together and get machined, especially since this would be my first heavyweight bot. Which would you recommend and why? Also, which is better for self-righting, like with Aftershock? I'd like to be able to do so with this design.

  2. I was thinking of mounting the bar or disc with two motors. Why? First, if one motor gives out, I got a spare. Second, I like symmetrical designs or designs with singular motors centered (motors to the side that make the bot look asymmetrical always bugged me for some reason lol). Of course, this would mean extra batteries and more money (which I probably would have if I used a bar rather than a custom disc). What do you think? Should I use two motors or one? What would the major perks of this, and what would be the biggest flaws or concerns be?

  3. Depending on the parts I use and disc's dimensions (as well as personal taste), I may be able to split the chassis entirely in half (EB had a small but reinforced portion in the back if I recall correctly, so it wasn't split in half). Only one problem: this adds onto the whole "expense" and "complexity" issues of having two motors (When I say "expense" and "complexity", I'd prefer not to spend over $7500 on the bot). I'd probably need two receivers, and coding would probably be a PITA. More importantly, it would be hard to keep it in one piece if it's being held together by one small piece of aluminum. What do you recommend as solutions?
As with all of your responses, your response and help will be greatly appreciated! [Champaign, Illinois]

A: [Mark J.] Having conquered the lower weight classes you have decided to move up to the heavyweight class?

  • How many combat robots have you built?
  • What prior experience have you in mechanical design and construction?
  • In how many tournaments have you fought?
  • How many battles have you won?
  • The last set of questions you asked had you attempting to turn metal salvaged from the scrapped drinking fountain at your school into an antweight -- how did that go?
Very seriously: You have no business building a heavyweight combat robot. If you're actually planning a heavyweight build you should reconsider. If this is simply an exercise in fantasy design, you should stop wasting my time. I will be pleased to assist you in building an insect or sub-light robot, and when you have learned the painful but relatively inexpensive lessons to be overcome in those weight classes I will assist you in moving up to a suitably heavier robot.

Your questions are well thought out and nicely presented. I'm not taking this position to be cruel, I'm attempting to do you a favor by giving you the best advice I can provide.


Update I've reflected on my reply to your questions. I still believe I gave you my best advice, but it does not sit well with me to leave your specific questions unanswered. What follows is as much for my benefit as yours:
  • Keep your design as simple as possible -- simple 'bots win. As you noted, a bar spinner is simpler and less expensive than an oddball custom single-toothed cutaway disk. It's also more durable.
  • A bar is not good for self-righting. Your best choice there is a full-circle disk like 'Aftershock'. An inverted circular disk in contact with the arena floor is still easy to spin, even if slowed by the impact that flipped the 'bot. When the tooth strikes the floor it has a chance of popping the 'bot back upright (if you're lucky).
  • One motor, please. Let it bug you. It's more efficient, less complex, less expensive, and - in spite of your hopes - more reliable. If one motor quits you probably took enough damage to take the whole weapon out.
  • A split chassis is a weak chassis. Dual receivers would be the least of your problems. It's very embarrassing to have your robot torn in half. One... Solid... Strong... Box... Chassis.
Still, do not build a heavyweight!

Q: Iím the admin of the Facebook group for combat robotics. I've got this guy who posts stuff that doesn't agree with the way I think about things. Iíve tried replying with snide personal attacks, but he persists. What can I do? [Edison, New Jersey]

A: [Mark J.] As a Facebook administrator you have access to a pull-down menu in the upper-right corner of a post that includes the option to 'Delete Post and Block User'. Historically it has worked out really well to just get rid of people with dissenting views. Why not give it a try?


Twenty minutes later: Hey, how come I can't read the combat robot posts on Facebook?
Q: Hey Mark, what did you do to get banned from the combat robotics group? [Facebook Messenger]

A: [Mark J.] The group owner/admin didn't like my comment to a post from a builder who was looking for info on his idea to build an updated 'Buddy Lee' robot: a manequin in a ride-on toy -- now improved with a flamethrower in its mouth. I believe you were one of the commentors on that post.

I made three points [from memory -- I cannot access the actual post]:

  1. Showing up with the proposed robot would be like showing up to play tennis at Wimbledon wearing a tutu, swim fins, and a football helmet.
  2. Entering a joke robot mocks builders who are taking the competition seriously and building to the best of their ability, IMHO.
  3. If the combat robot community does not take our own sport seriously, no one else will. #ForgetSeason3
This set off a storm of discussion, which I did not enter until the moderator commented that I had insufficient standing within the community to express such opinions. I replied with the full text of the fictional Q&A that appears above. For the first time ever he took my advice on something.

The moderator and I have had similar run-ins previously, and this was the second personally derisive reply he had made to me that day. I will not request reinstatement. Combat robot builders who want my opinion or advice know where to find me.


Q: I have a custom pulley i wanted to press fit to a 5mm shaft, the bore came back from the cnc shop at 5.3mm. How do i fit it without getting another cut? i've heard of loctite and shimming? [Ile-de-France]

A: [Mark J.] First, don't use that machine shop again. A just-passable machine shop will hit a critical bore to Ī 0.03mm, and your guys are off by ten times that much. I'd reject it and ask them to try again, but that may not be an option for you. Solutions:

  • Loctite There are many different formulations of Loctite intended for differing purposes. Using the wrong type to fill so large a gap will most certainly result in failure. Loctite 680 retaining compound is intended to join cylindrical fitted parts with gaps as large as 0.38 mm and is rated for torque carrying assemblies like pulleys. It isn't cheap, can be difficult to find, has a 24 hour cure time, and does not provide a perfectly concentric fit. I'm not a fan of gluing a pulley to a shaft but should you wish to try, 680 is the correct formulation.
  • Shimming I think this is your best option. An aluminum beverage can is a handy source of shim material. The sides of a typical can are about 0.1 mm -- too thin for your purpose -- but the top and bottom of the can are enough thicker that you may be in luck. Roll your shim stock into a tube with a little gap, slide it into the pulley bore with enough sticking out the end to pre-fit over the shaft, and press away!
  • Knurling A non-hardened shaft can be knurl rolled to expand it's diameter. Given that your machine shop botched the bore I don't think I'd trust them to properly knurl the shaft, but it is an option.
  • "Speed Knurling" I mention this only because some very poor craftsman may recommend it. You place the shaft flat on a hard anvil surface and strike it with a hammer to deform it and make it a bit larger in one direction. Please don't do this.
A properly shimmed shaft won't need any additional help, but some builders like a drop of standard blue Loctite on everything. It won't do any harm.
Q: How are D2 kits like 'Just a Wedge' or 'Mugga Junk' getting these wedge killer forks that look like the wedge mounts? I want to have my D2 beat other wedges too. [Arlington, Virginia]

A: [Mark J.] I believe the drop forks on the bots you mention are prototype parts from BotKits. Botkits will very soon offer a production version 'Wolverine' steel fork kit for the D2. The tool steel forks are off for heat treatment right now. Stay tuned for updates.

UPDATE July 28, 2017 -- The D2 Wolverine steel fork kit is now available.


Q: I would like to build a beetleweight version of Razer, and I was wondering if I could use a high torque motor for the crusher? Or do I have to use hydraulics? If I have to use hydraulics could you please tell me how to use hydraulics? [Livonia, MI]

A: [Mark J.] There are several posts about insect crushers in the Ants, Beetles and Fairies archive. Search there for 'crusher'.

A quick summary of these posts:

  • Electric servos, actuators, and gearmotors cannot directly generate the force needed;
  • There are no off-the-shelf miniature hydraulic components small enough to be practical;
  • It is difficult to build a chassis strong enough to survive the forces a crusher will generate;
  • Lots of people have tried to build small crushers -- none have had success.
A full guide on the details of hydraulic crushers in combat robots would fill a small book. If you're interested in the general principles, take a look at this video.
Q: If Robotica ever gets rebooted, would you create a new version of Run Amok and compete in honor of Aaron? [Syracuse, New York]

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: My antweight is a direct drive drum. When I tested it,
  1. I need to push the joystick slow or the motor stops and sputters.
  2. On big impact the motor stops, and I need to push the joystick slow again or as mentioned earlier the motor stops and sputters.
Why is this happening? [Arlington, Virginia]

A: [Mark J.] The more info you can provide about your 'bot, the better chance I have to give a direct and useful answer. Given the symptoms, I'm gonna guess that the motor is a brushless outrunner, since brushless is 'in' and an 'inrunner' style motor would be a horrible mistake for this purpose.

Possible cause #1

Brushless motors rely on the brushless ESC to provide power to the correct motor windings at precisely the correct time. With the non-sensored brushless motors commonly used in insect-class combat robots, the ESC is not given data on the position of the rotor relative to the motor coils -- it has to take a guess based on small changes to the electrical properties of the motor as it rotates. At slow speeds and heavy loading, the ESC may guess incorrectly and send power to the motor windings at the wrong times. What you call 'sputtering' is in this case more correctly called 'cogging'.

I suspect that the ESC you have chosen for your weapon is a poor match to the motor. There are so many different motors and ESCs that it is very difficult to track their compatability. I suggest that you join the Combat Robotics group on Facebook and tell them:

  1. the exact make and model of your weapon motor;
  2. the exact make and model of your weapon ESC;
  3. the voltage and mAh capacity of your battery;
  4. that you're using them for a direct drive ant weapon;
  5. the diameter and weight of the drum;
  6. that your motor is cogging badly; and
  7. that you need some help.
Someone in the group may have experience with that motor and/or ESC and may offer an alternative ESC that is more compatible with your motor and useage. But, before you do that...

Possible cause #2

It's also entirely possible that your battery is not fully charged and/or does not have enough capacity to deliver full voltage under the heavy load of your unspecified weapon motor starting up from a full stop. The drop in voltage can 'brown out' your electronics and cause a sputtering of the type you describe.

LiPoly batteries are shipped with only a partial charge, so before you go off and ask the Facebook group for help give your battery a FULL CHARGE and try another test.

Five Days Later...

I haven't heard back from Arlington, nor have I seen any posts on this topic in the forums. I'm guessing that a battery charge did the trick.


Q: For a heavyweight horizontal spinner weapon system, is the perm PMG 132 a better choice than the E-tek Motoenergy ME0909 PMDC motor? [Portsmouth, Ohio]

A: [Mark J.] The preferred brushed heavyweight weapon motor - for a balance of power and durability - is the Motoenergy ME0708.


Q: I am planning a heavyweight in the future and I need a 72 volt battery but can't find any good ones so maybe puttting a lot of small ones in would work. But how do you bundle up a lot of (relatively) smaller batteries in a pack to make 72 volts?

P.S don't tell me anything about amperage just voltage. [Arlington, Virginia]

A: [Mark J.] I'm concerned that you're considering a heavyweight build but that you do not understand how to connect a group of batteries in a series circuit.

I'm also concerned that you don't want to discuss amperage.

See this post in the Radio and Electrical archive about connecting batteries in series and parallel circuits, and 'mouseover' the image at left.


Q: Here is a hypothetical question. If you used a 40 volt battery to drive a 20 volt maximum motor how would you get the motor to run without it breaking? I hope this does not lead to the hamburger is bad. [Portsmouth, Ohio]

A: [Mark J.] A couple of more-or-less practical methods:

  1. If running a LiPoly battery, you can 'tap' into the balance plug to pull power from only half the cells for your 20 volt max motor. This can also work with a multiple-cell pack you make for yourself, as shown in the illustration at right.

    Disadvantage - some of the individual cells will be drained more than others which may unbalance charging. The whole pack must have excess capacity for this to work.

  2. Go ahead and wire the ESC for the 20 volt motor right into the 40 volt supply, but restrict the R/C channel controlling the ESC to 50% of full 'throw' or 'volume' at the transmitter. See the 'Adjustable Travel Volume' section of Programming R/C Transmitters for Combat Robotics. Technically this restricts current rather than voltage, but in practice the motor will never 'see' more than 20 volts.

    Disadvantage - it is VERY hard on the ESC to spend equal time flowing and restricting current flow under all motor load conditions. You may blow the ESC, but it will save the motor. Make sure the ESC can handle the full 40 volts.

Do not try to add fixed resistance to the motor circuit to control voltage! The effective resistance of the motor circuit changes with load and speed -- you will either allow too much current thru the motor or massively reduce its performance.
Q: Hello! I was wondering if you could help me out. I'm a bit new to the whole Robot Combat thing, with hopes of making some amazing machines (hopefully soon!).

First, I would like to know what some of the cheapest, but EFFICIENT, motors, batteries, power supplies, tires, etc, are for an antweight. I'm hoping to have my first ant to be a drum spinner, but that'll have to depend on how much money I have. [Champaign, Illinois]

A: [Mark J.] We all hope to make amazing machines, but 'first combat robot' and 'drum spinner' do not go together. You may be surprised to learn what weapon types are most successful. See also this post farther down the page.

I'm not willing to provide a shopping list -- see Frequently Asked Questions #4 and #16.

Q: Second, I'd like to know if a particular metal would work or not for an antweight: it was a thin panel from the school's old fountain, but I managed to save it from being scrapped. I don't know exactly what it is, but I can provide you with a pic if need be.

A: I don't do psychic metal identification. There are hundreds of metal alloys and useful/useless ones look the same in photos. Common metal identification methods.

Q: Third, what kinds of printers can be used to make 3D printed robots? I know a lot of people in the antweight category 3D print, but I didn't know if my school's 3D printer would be able to print parts off: I'm assuming the 3D printers used by roboteers either prints things off in HDPE or UHMW, which is why I was asking if you knew any printers that would be best for this.

A: [Mark J.] Robot parts are rarely printed in HDPE or UHMW. I suggest that you join the Facebook 'Combat Robotics' group.

  • There have been several recent group posts about 3D printing. Read thru those posts to get some background and flavor for the group; then
  • Tell the group the make and model of your school printer and ask for tips on plastic type and set-up from builders who have used that specific printer.
Warning - the Facebook group does not tolerate overly general questions from beginners. If you ask, "How do I print a robot" you will get a great many abusive responses. Be very specific with your questions and recognize the need to sort the useful answers from the crap.

Q: Also, while on the topic of plastic, if one were to make a 250-pound robot with 100% plastic armor, how thick must said person make it so that it can be as strong as the armor of, say, Minotaur's, and is it really worth the time, money, and weight?

A: By the time you are ready to build a heavyweight robot you will know the answers to those questions. See Frequently Asked Questions #17.

Q: ALSO also, and this is off topic, do you happen to know where I can find a picture of two of 'The Bishop' from the Robot Wars that was unaired in 1997?

A: No photos. A web search for -- "Robot Wars" "The Bishop" -- will turn up a very poor video of the only match Stephen Gaudio ever won with the 'bot. I believe the robot was taken from the event and thrown off the Oakland Bay bridge. See Frequently Asked Questions #32.

Note - none of the US Robot Wars (1994 to 1997) were 'aired'.

Q: Alright, I have a pic of the metal (and also a pic of another part of the fountain I found as well!) I asked about in my previous question. The one on the left was the one originally in question. I was unable to do any of the main tests in the helpful link provided (due to the lack of tools with my only tools being a soldering iron and some Allen Wrenches), but I was able to find out the following:

  1. The metal on the left is very flimsy, and I can bend it easily with my hand by curling it.
  2. It's magnetic
  3. The panel is light
As for the right fountain part, I found out:
  1. It's very sturdy and rigid
  2. It's not magnetic
  3. It's not very thick, but it's thicker and significantly heavier than the other
I apologize if this isn't enough information for you, and for seeming to break so many rules. Maybe I should've read them first before I got excited and posted some of those senseless questions: It would've saved you a lot of time and energy.

A: I understand how enthusiasm leads new builders to plunge past multiple prompts in the 'submit a question' process to 'Read the FAQ'. Don't worry -- if I didn't want to answer your questions I simply wouldn't do it. Nobody has a gun to my head.

About those metals: the link to the metal identification tests was there largely to convince you of the difficulty of the process. I guess you didn't beleive me when I said that I really can't identify metal from a picture. Although that is generally true, given the history of the pieces and their apparent use I can take a guess.

  • The thin flat magnetic sheet is likely A366/1008 steel. Commonly used for appliance panels, this particular alloy is easy to bend and shape and has much lower strength than more exotic steels. Not a good material for combat robots as it deforms (dents, bends, tears) easily.

  • The non-magnetic fountain basin is most likely 304 stainless steel widely and commonly used in kitchen utensils, sinks, and food preparation surfaces. Stainless steels have greater corrosion resistance than regular steel (they don't rust) but are still intended to be easily bent to shape. Forming the material into a basin shape is what gave it greater rigidity than a flat sheet. A better material than the steel sheet, but still too easily deformed for combat use.

Summary - there are lighter, tougher, harder, and stronger materials available for combat robots. Your opponents will be built from them.


Q: What is your least favorite bot design? Okay, got it?

Now the real question: how would you go about building something similar to it anyway without feeling bad the entire time, and trying to win some fights? [Buffalo, New York]

A: [Mark J.] This really doesn't fit in with the Ask Aaron mission statement.

There are two broad groups of combat robot builders: those who build to win tournaments, and those who build to impress other builders with their cool designs and machine skills. I have no issue with 'cool bot' builders, but Team Run Amok builds to win. If you wanna be cool you're gonna have do it without our help.

Short answers:

A) Anything that would be accepted for ABC Battlebots.

B) I'd rather shoot myself.


Q: Hello, I have 2 questions. Bot info:
  • A 12-Pound Tombstone clone
  • Two wheels: 2.375" diameter, 0.5" width
  • Two RS395 Banebots motors
  • 4:1 P60 Banebots gearboxes
  • Hobby King ACK-5312CP 330KV weapon motor

1) I'm very new to this, could you double check my gearbox choice (or even motor choice if necessary)?

2) I will be using A123 LiFe batteries (3.2v, 1100 mah 30 amp discharge). If my calculations are correct (which I doubt) 1 of these batteries should be able to power the weapon and drive for 25.714 minutes.

How many batteries do you think I need? Thanks for the help. [Hicksville, New York]

A: [Mark J.] I can't see all your input fields in the image you sent from the Team Tentacle Torque Calculator , but it's clear that you have not entered the correct values for the RS-395 motor. Let's start over with the correct values:

The 20:1 ratio P60 gearboxes with the 2.375" wheels give the best performance from the RS-395 motors in a moderate size arena: around 5.5 MPH, reaching that speed in about 6.5 feet. That's ample speed and power to push around a big spinner weapon. The 26:1 gearboxes would be a little easier on the motors, but the 20:1 should be fine.

The RS-395 motors run well between 12 and 15 volts -- you'll need four LiFe cells wired in series to provide 12.8 volts. The drivetrain will use about half of the capacity of the 1100 mah cells in a 5 minute match.

Your weapon motor is a powerful choice for a hobbyweight. It operates in the 22 to 30 volt range, so you will want a separate battery to operate your weapon. That battery will require between 7 and 9 LiFe cells wired in series. You have not given me enough information about your weapon to calculate the battery capacity (mah) needed. Heavier, longer blades will use more battery power to spin up, and the reduction ratio between your weapon motor and blade will also impact current usage. Example, courtesy of the Team Run Amok Excel Spinner Spreadsheet:

A steel bar 15" long by 2" wide by 0.5" thick running a 2:1 reduction from the ACK-5312CP motor at 28.8 volts (9 LiFe cells) would spin to about 1300 joules energy storage in roughly three seconds. Assuming six spin-ups from a dead stop in a 5 minute match, total weapon current consumption would be about 520 mah.

So, you'd need one four-cell LiFe battery to run your drivetrain and one 7 to 9 cell LiFe battery to power your weapon. Combined, they should run your 'bot in combat for about 10 minutes on a full charge.

Are you sure you wouldn't like to build a nice wedge robot instead?

Q: Thanks for the quick response. My weapon's current plan is a 5 pound 12 inch diameter 1/2 inch thick S7 steel asymmetrical "disc", shaped somewhat like a teardrop. The thickness is to avoid vertical spinners from breaking the blade. None of this is set in stone but the 12 inch diameter, and somewhat the weight.

My fights will only be 3 minutes long. The bot does strategically not need to be fast - in fact it should be as slow as reasonable to save weight and power consumption. The 26:1 gearboxes look best to me to that effect.

I hope to have as little power left over after a fight as reasonable, perhaps only enough to last 4 minutes in total. That being said, I still think I can squeeze 12 LiFe A123-18650 batteries into my bot, in 2 groups of 4 and (with slight design alterations) 2 groups of 2. But of course the fewer needed, the better.

Funny you should mention wedgebots, as I eventually hope to build a bot to take down Original Sin! Anyway, do you have any blade alteration or battery number suggestions? Thank you.

A: A 12" diameter, 1/2" thick steel disk weighs more than 16 pounds, so your 'teardrop' design must have a LOT of material cut away from the full disk. A simple steel blade 12" long, 1/2" thick, and 2.875" wide weighs 5 pounds. A 5-pound weapon is heavy for a hobbyweight, so be careful with your weight calculations.

The number of cells I'm suggesting for your batteries are not there for current capacity, but simply to obtain the voltage needed for your two motor systems running at differing voltages. You can't run your drive motors at 30 volts, and running your weapon motor at 15 volts would drop it's power output from 1000 watts to 250 watts. 'Tombstone' has this same problem and solves it the same way that I suggest you do: two battery packs with differing voltages.

I'm a bit concerned about the ability of the cells you have selected to provide the start-up amperage needed by your monster weapon motor. The motor is rated 40 amps continuous, but can briefly draw as much as 140 amps under starting load and will draw over 70 amps for as much as two seconds while staining to pull that heavy weapon up to speed. Given that your cells are rated for 30 amps continuous draw, pulling more than twice that current may damage the weapon battery. You may want to reconsider your LiFe cell choice.

Q: Thanks for the advice. I'm happy to lock in 4 1100 mAh batteries (the yellow ones) for the drive power with 26:1 gearboxes, if you think that's not too many.

A: My earlier explanation was not sufficiently clear. A battery pack for a specific application must supply three things:

1) Adequate capacity to power the device for the required time (mAh).
2) The current needed by the device -- without damage to the battery (amps); and
3) A suitable voltage for the device (volts);
We have calculated that the drivetrain for your robot will use about 330 mAh of total current in a three-minute match, so your 1100 mAh cells have that well covered.

We have also calculated that the peak current draw of the drivetrain is about 10 amps, so your 30 amp continuous output cells also have that well covered.

That leaves voltage. To get adequate performance from your chosen drive motors you need a battery pack that will supply 12 to 15 volts. Each of your cells produces 3.2 volts:

One cell = 3.2 volts;
Two cells wired in series = 6.4 volts;
Three cells wired in series = 9.6 volts;
Four cells wired in series = 12.8 volts.
You require four cells wired in series to supply adequate voltage to your drive motors. Fewer will not do. You could get by with smaller cells, but you need four of them.

Q: Now for the weapon power. Apparently the bigger A123s (green) are back in stock. They have a very slightly lower voltage (3.2V) but a higher amperage(50A), plus a maximum impulse discharge of 120A.

However, as I understand it, the same number (7-9) of these green batteries would be needed for the weapon motor's voltage as of the yellow batteries. Since the greens weigh more and are slightly bigger, this is less ideal. LiPos are not allowed at my upcoming event. Is there another battery you recommend instead?

Not using 100% of my motor's capability seems ok to me, as it might help prevent breakage (I think). Obviously I want to use as much as possible, but I'm okay with using less. Perhaps not filling up the voltage requirement completely but satisfying the corresponding Amp requirement would do the trick. The green A123s might be better at that. I currently have 1-1.5 pounds to spare for batteries, and the fewer cells I can reasonably use the better. What do you think?

1 yellow = 0.088125 pounds
4 yellows = 0.3525 pounds
8 yellows = 0.705 pounds
12 yellows = 1.0575 pounds
1 green = 0.18125 pounds
4 greens = 0.725 pounds
8 greens = 1.45 pounds
I plan on ordering the parts this week so I can experiment with a prototype before I finalize the bot.

A: Your problem is that your weapon motor is, as I mentioned before, a powerful choice for a hobbyweight. It requires both high voltage and a high peak current capacity. Having a big weapon motor and not running it at full voltage is a serious waste of power. Power varies with the square of voltage, so dropping a couple of cells cuts your power by almost half:

9 cells = 920 watts
8 cells = 728 watts
7 cells = 558 watts
6 cells = 410 watts
With each drop in power your spinup time increases and your weapon power storage drops. That big weapon motor is causing you more problems than it solves. If you want to run at a lower voltage you'd be much better off with a smaller motor designed to run at that voltage.

If you ask around the on-line forums, somone is going to suggest that you build a 9-cell battery pack to power the weapon and 'tap' the pack at 4-cells to power the drivetrain. This is possible, but I cannot recommend it. You'll be drawing more mower from some of the cells than others, and the draw from that big weapon motor may 'brown out' the drive and electronics. Proceed on that path at your own risk.

I think I'd go find a more reasonably sized weapon motor that would run well at four or five cells, downsize the weapon rotor, and run the whole bot off a single battery pack.


Q: what is the scale factor in combat robot? and what is its importance? [Al-Jizah, Egypt]

A: [Mark J.] There have been several questions about 'scale factor' and the 'square-cube law' recently. Take a look at this post from 'Urbana' in our Weapons archive and this post from Buzzards Bay in our Design archive.

Given this interest, I've summarized the earlier posts and added it to Frequently Asked Questions #17.


Q: Can you make a permanent page for those series of questions about crazy weapon designs? I think you'd be doing a great service to dissuade new builders from doing something too crazy. [Waltham, Massachusetts]

A: [Mark J.] I generally assume that builders writing in to 'Ask Aaron' are interested in combat robots that will win matches, but an increasing segment of builders are interested in impressing audiences and other builders with 'show-off' designs. While our focus remains on supporting simple and effective designs, I also recognize that one man's crazy is another man's awesome.

The recent long series' of questions about... unusual... weaponry and design has found a place in the Ask Aaron Design and Construction archive and I'll keep trying to set reasonable expectations for any design thrown at me.


Q: Is a 1/4 pound hamburger good [Arlington, Virginia]

A: [Mark J.] I suppose. Does it have a bun?

Q: I told you, it's a 1/4 pound hamburger. It has a bun.

A: That makes it easier. You can use my JavaScript Burger Goodness Calculator.

Fill in the blue boxes and click on 'Calculate':

Burger Goodness Calculator
Patty Weight: ounces Price: dollars
Patties: # Goodness: %
Sesame Seeds: on bun

Q: The burger calculator was bad. You should have told me it was bad. You suck.

A: Have a nice day.


Q: Why do wedges leading up to mini discs do so well such as Algos or Project Darkness do so well? [Arlington, Virginia]

A: [Mark J.] It's a common error for new builders to assume that a given combat robot performs well largely because of the design. The robots you mention are successful for many reasons. They are:

  • Built by experienced and skilled teams;
  • Driven well with carefully set-up R/C systems;
  • Made from properly selected materials and components;
  • Given careful and repeated attention to even the smallest details;
  • Improved over time based on lessons learned rather than abandoned to chase new ideas;
  • ...and yes, based on deceptively simple and rugged designs.
Simple designs do well in combat, but it takes the 'whole package' to do as well as Near Chaos Robotics or Team Slaughterhouse. An average builder copying either robot will have average results.
Q: In the weapon FAQ or something it says discs are better than bars. Why do bots like tombstone or last rites use bars if discs with carvings such as in Nightmare or DDT are better?

A: [Mark J.] A spinning disk stores more energy at a given RPM than does a spinning bar of the same mass and swept diameter. This is because the disk has more of its mass located farther from the axis of rotation. The Ask Aaron Spinner Weapon FAQ features a comparison of the energy storage capacity of four spinning weapon designs with the same mass and swept diameter: bar, disk, eggbeater, and hollow drum.

Better energy storage is a good thing, but it is not the only factor to consider when designing a spinning weapon.

  • Simple designs like bars and solid disks are much easier to construct than eggbeaters and drum weapons.
  • At large sizes and very high energy storage levels the durability of thin disks, eggbeaters, and hollow drums becomes a serious concern.
  • Some consideration must be given to the vulnerability of the weapon to attack by other weapon types. For example, the large edge area of a horizontal disk is vulnerable to a fast-spinning vertical drum weapon.

Q: And if vertical spinners supposedly generate more kinetic energy then why do people use horizontals? [Baltimore, Maryland]

A: Who said anything about verticals storing more energy? You didn't find that here. The orientation of the spinner has no effect on energy storage.


Q: Hello! I'm here this time on behalf of another builder. He is currently designing a full body, horizontal clampbot. You can check out his design process on his YouTube Channel, AltaPowerDog. I come here asking this question: What are your concerns or opinions regarding what Alta has made thus far?

As with every response, your time is greatly appreciated! [Champaign, Illinois]

A: [Mark J.]  Strike 1 I don't accept third-party questions on un-built designs. The builder may not want me to comment; if he does he can ask me directly.

Strike 2 You should know quite well how I feel about complex combat robots. I've made that clear in answering your prior questions.

Strike 3 There is no YouTube channel for 'AltaPowerDog'. Please be more careful with your spelling.

Q: I would like to chime in the horizontal clamper question. There IS a channel called AltaPowederDog (he won a competition with a cardboard bot).

Anyways,I came across a video of Japanese sumo and those bot are fast! Can you explain how they are so fast? [Perth, Australia]

A: [Mark J.] I've heard of 'Australian Rules Football', so I suppose there may be an 'Australian Rules Baseball' as well. Perhaps three strikes aren't enough in that league?

  Strike 4 There is no YouTube channel for 'AltaPowederDog' either. Spellin' it wrong a second way don't help, ya drongo.

Here are a few more YouTube channels that do not exist:

AltoPowderDog - AltaPowderHog - AlpoDogPowder - AlphaPoodleDog - AllboyPlutoCog - AlamoPowwowSmog - SaltyFlowerPog

If you're asking a favor of someone that requires them to reference a specific internet location, proper 'net etiquette calls for you to include a working link. Failing that, you should at very least make certain that you have spelled the obscure reference correctly so that the person from whom you are requesting said favor does not spend an inordinate amount of time searching the vast internet for something that is not there and who is then forced to back into the YouTube channel by Googling 'horizontal clamper robot video'. Yes, I eventually did find 'AltaPowderDog', but I will make no comment on the design for reasons expressed in 'Strike 1' above.

Japanese Sumo: We have discussed the incredible quickness of autonomous sumo bots multiple times here at Ask Aaron. We've posted videos, we've discussed suitable motors, and we've calculated required downforce levels. You ignored the plea appearing directly above the box into which you typed your question:

Please search the Ask Aaron Archives first to see if we've already answered your question.

I'm going to provide you with a correctly spelled and formatted link that will transport you directly to one of several archived Ask Aaron posts which discusses how sumo bots achieve their quickness. By clicking on the link you agree to be nice to some total stranger who asks you for something they could do on their own but who would rather inconvenience you. Here you go: Sumo Magnetic Downforce.

P.S. - A cardboard bot? Big deal. Team Run Amok once won a competition with a yam, four nails, and a girl in a mouse suit -- but you'll need to buy me a beer to hear that story.


Q: How do I reprogram brushless motor controllers to make them useable for brushless drive motors? Is it easy? [Space Coast, Florida]

A: [Mark J.] Builder Robert Cowan has a nice video that covers the entire process of 'flashing' the reversing SimonK firmware onto brushless ESCs. I wouldn't call it either simple or easy: SimonK Firmware Flashing Tutorial.


Q: Is there a rule of thumb for how much battery capacity to have for a spinning weapon? [Manassas, Virginia]

A: [Mark J.] Too many variables for a 'rule of thumb' -- how about a calculated solution? The Team Run Amok Spinner Excel Spreadsheet calculates the mass, moment of inertia, stored kinetic energy, tip speed, spin-up time, AND the approximate battery capacity requirement for your spinner weapon. You'll need Microsoft Excel to run the spreadsheet.

In general, the capacity needed for your weapon will be much less than the capacity needed for your drivetrain.


Q: Is there any advantage in hooking 2 brushless motors to a single spinning disk weapon?

A: [Mark J.] Two motors = twice the power -- but because each brushless motor relies on it's controller for commutation as well as current, each motor requires its own ESC. It's generally preferable to use a single larger motor with twice the power.

Q: Also, can I put a custom aluminium pulley around an outrunner brushless motor retained by a couple of set screws with plenty of loctite in a hobbyweight? [Quebec, Canada]

A: Set screws are best avoided for this purpose: they apply large point-pressure that may deform the rotor and are failure-prone even with a threadlocker. If you're making a custom pulley you should size the bore for an interference fit and press the rotor into place.

A word about threadlockers: some builders recommend applying Loctite to shafts and other sliding assemblies to secure them, but that's well outside the intended use of the standard blue or red Loctite. There are special formulations of green Loctite designed for holding loose-fitting components to shafts, but I can't recommend them for torque carrying applications in combat.


Q: I saw some of your old answers about 'Beta' and I thought I would investigate how much better 'Beta' is than just using regular gears, but I couldn't figure out if I was doing it right.

So anyways, what I was doing was putting some motor statistics into your hammer spreadsheet and figuring out the best gear ratio for maximum joules as a starting point. Then I doubled the gear reduction and wrote down the number of degrees to reach 60% speed and what the speed was. Then I reduced the ratio a little, and took the speed from the previous step as a starting point, and found how many more degrees it would take to reach 60% again from that point and repeated this out to 180 degrees.

I ended up with almost the same amount of power as without all the fancy gearing! Did I do something wrong? The only thing I could think of was maybe it is only worth it if the hammer is really heavy and the motor would really struggle or something. [Quebec, Canada]

A: [Mark J.] For readers unfamiliar with John Reid's 'beta', the heavyweight British hammerbot uses an intricate chain 'snail cam' (technical term: 'fusee') that decreases the motor reduction ratio as the hammer accelerates thru its arc in order to keep the motor close to peak output power. This previous post in the Ask Aaron Design and Construction archive discusses both the design theory and how to use the Team Run Amok Excel Hammer Spreadsheet to find a snail cam design solution.

You aren't doing anything wrong in your calculations; the approach you used is quite similar to the one I outline in the above referenced post. You are also correct that the snail cam results in only a small increase in weapon strike force -- I make it about 20%. There are, however, other benefits to the snail cam. In particular the lower starting load on the weapon motor increases motor efficiency and reduces current consumption, making life easier on the motor, battery, and motor controller. This is very important when your expensive motor is stressed very close to its limits! Think of it as an investment in longevity.



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.

Not all the questions we receive at Ask Aaron are serious. Some are odd, some misdirected, and a few are incomprehensible. Aaron enjoyed dealing with these questions, and I've collected some of his best responses in a special page:
Aaron's Greatest Hits



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