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Questions and Answers about Combat Robotics from Team Run Amok.


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6879 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.

Q: How to activate flipper with no sensors [Dublin. Ireland]

A: [Mark J.] I'm not sure what you're asking. SOMETHING has to sense the right moment to fire your flipper mechanism and that something is by definition a 'sensor'. The Hamburger is Bad.

Typically it's the driver who watches and taps a transmitter button to activate the flipper. If you're trying to fire automatically without electronics, it's possible to use an entirely mechanical sensor trigger like Dale Heatherington's Flip-O-Matic.

A professional golfer and a blind musician are in a bar. They chat for a while and the topic of golf comes up.

Musician "I find that when my golf swing goes wrong, I need to stop playing for a while. The next time I play it's usually all right."

- Golfer - "You play GOLF? But, you're blind! How can you play golf if you can't see?"

Musician "My caddy to leads me to the ball and tells me how far I am from the hole. When I'm set in the right position he taps me on the shoulder and I swing away."

- Golfer - "That's clever! We should play a round sometime."

Musician "Well, people don't take me seriously so I never play for less than $10,000 a hole. Is that a problem?"

- Golfer - "I can afford that. When would you like to play?"

Musician "How about Tuesday night?"

Q: Adding to the earlier Melty Brain question; do Meltys count as an active weapon? If I'm interpreting the SPARC ruleset correctly, they would classify a Melty Brain as a passive weapon. I'm only asking cause I've been to a couple tournaments before with an active weapon rule and I want to make sure I can still compete. [Yuba City, California]

A: [Mark J.] Although there is no functional difference between a Melty Full-Body Spinner and a conventional FBS, the guidance given by the current SPARC Robot Construction Specifications v1.3 is open to interpretation by Event Organizers. The rule often cited in the SPARC ruleset is in the 'Sportsman Class' section 4.2:

4.2.1. Weapons such as a fixed spike that require the movement of the bot to function do not qualify as active weapons.
  • Some EOs interpret "movement of the bot" to mean that an active weapon must be entirely independent of the robot drive system. This interpretation excludes Meltys as active weapons.
  • Other EOs interpret "movement of the bot" as translational motion across the arena. Because Meltys can translate while spinning but do not require translational motion to function, this interpretation includes Meltys as active weapons.
Bottom Line: You'll need to ask the EOs for specific events at which you plan to compete if they consider Meltys to be 'active' weapons.

Q: Hi Mark, me again! Working on my antweight wedge bot and I had a clarification on some of the the information on the Rio Botz Combat Tutorial. On page 213 it states:
A suggested value for the side angle α’ to launch spinners would be, for a smooth titanium wedge, equal to α’ = αlaunch = 37 degree. And, for a hardened steel wedge, which has a larger coefficient of friction than smooth titanium, a suggested side angle α’ to launch spinners would be Leonardo Da Vinci's αlaunch = 34 degrees.
In addition to the above section, on page 211 it provides a reference of μb ≅ 0.5 for a battle worn titanium wedge while a new wedge would have a μb ≅ 0.3.

For my antweight, I'm planning to use a 6061 or 7075 aluminum (cost/weight considerations) mounted to a 3D Printed nylon base wedge. I'd like to get some clarification on what would be a good starting points for μb for this calculation for 6061/7075? Is there a good way to figure out the μb for a given material without using approximations? On the same note, lapping and polishing the aluminum to say a mirror polish would help reduce μb right? [Arlington, Virginia]

A: [Mark J.] I'm glad to see that you've been studying the RioBotz tutorial, but there are a few odd things about the RBT you should know:

  1. The RioBotz team is university based, and the tutorial is written from an academic/engineering standpoint. Recommended design approaches sometimes drift away from the practical and off the path into the theoretical weeds. The difference of two or three degrees on a wedge angle based of the coefficient of friction of a material that is going to change with each hit it takes isn't worth worrying about.
  2. Coefficients of friction are dependent on BOTH of the materials involved -- not just the wedge material. For example, steel-on-steel has a different coefficient of friction than say steel-on-titanium, and while you can pick the material for your wedge you cannot pick the material that is going to hit it. You can find tables of coefficients of friction for pairs of material on-line (example). However, these tables are of limited use in the wedge calculations you are making because they all apply to smooth surfaces sliding over each other -- not a sharp-edged material attempting to dig into the surface of another material at an angle.
  3. Sometimes the best question to ask when reading the RBT is:

    "Why didn't they mention [such and such] in this section?"

    In this case: "Why didn't they mention aluminum in the wedge section?" They didn't mention aluminum because all of their calculations assume that the surface hardness of the spinner and wedge are somewhat comparable, like steel and titanium. Aluminum - even aircraft grade aluminum - is MUCH softer than steel. A steel spinner weapon will dig down into the surface of the material and grab, making all the calculations moot. The rule they forgot to include is:

    "Your wedge has to be at least as hard as the impactor striking it."

    The RBT does not mention this because as trained engineers they know this so well that they assume everyone knows it -- like the sun rises in the east.

Aluminum wedges suck -- even if you polish them mirror brite.

Q: I have been considering building a Melty Brain spinner for my next design. Before you freak out, I am well capable of managing the electronics, my question is more about the physics.

Does a Melty Brain spinner offer a significant increase in kinetic energy out compared to a more traditional full body spinner? My gut says yes however i've seen robots like this hockey puck much more than shell spinners and it looks like more energy ends up wasted on impact.

A: [Mark J.] For the benefit of readers unfamiliar with the concept of the 'Melty Brain': Wikipedia article on Translational Drift.

It's important to draw a distinction between 'kinetic energy stored' and what you're calling 'kinetic energy out':

  • The entire mass of the robot spins and contributes to kinetic energy storage in a Melty -- so if everything else is equal (mass, rotational speed...) a Melty will have greater stored kinetic energy than the equivalent full body shell-style spinner. How much more energy storage will depend on how successful the design is at moving the mass of the components out to the edges of the spinning radius.
  • The effectiveness of delivering that stored energy in an effective 'hit' is another matter. Weapon 'bite' is a key factor in transfering kinetic energy to your opponent, and an important element of the bite calculation is your closing speed on the opponent: the faster you speed toward your opponent, the more 'bite' your weapon will have. Meltys 'translate' slowly and lack the ability to effectively 'dart' into their opponent. The result is often a glancing blow that is ineffective at energy transfer.
Another factor in the 'hockey puck' behavior you report seeing is that minor imbalance issues can tip a spinning Melty up onto one wheel. This makes for very little resistance to being thrown across the arena from a hit. A few Meltys make use of this type of vertical mass imbalance to operate with only one powered wheel, like 'Hit N Spin' pictured below. One advantage that Meltys have is their mechanical simplicity. There is no separate spinner shell support, no weapon motor, no gearboxes -- just one or two direct-drive motors and the exotic electronics. That's worth something!
Q: Hi Mark, I'm working on my antweight wedge and I didn't find an answer on the archives for this. To give some background, this is a 4WD design that's using gearhead motors with the wheels directly attached to the motors. With the way I have the motor mounts setup, I could angle the two front motor mount in such a way to add a degree or so of toe in/toe out. In cars, toe-in helps improves steering response on a corner exit and stability under acceleration while toe-out will improve steering in corner entry but can cause instability under acceleration. Does it make sense to add 1 degree of toe in for each front wheel on a combat robot like this? Or should I leave it at 0?

I saw camber was discussed in relation to Donald Hutson's designs in the archive, but haven't seen anything on toe in/toe out. [Arlington, Virginia]

A: [Mark J.] A crossover question! I know a bit about automotive suspension design from my involvement with vintage British race cars: my hobby car.

Combat robots do not turn like cars.

  • Cars initiate a turning motion thru an off-axis deflection of the front wheels called 'Ackermann steering'. The steering linkage geometry that controls the wheel deflection is designed to vary the relative angle of the front wheels as they deflect to avoid tire drag and wear from unnecessary side-slip.
  • Combat robot wheels to not change angle when the robot turns. The turning motion is created by a change in the relative speed of the wheels on opposite sides of the robot, called 'differential steering'. This steering method relies on very high tire slippage angles during the turning motion.
There has been some experimentaion with very large front/rear toe-in/toe-out angles in hope of improved control in turning motion at the expense of additional straight-line drag -- see photo of Team Delta's lightweight robot 'Archetype' below. Results were inconclusive. Bottom Line: A small toe adjustment on a differential steer robot will be lost in the huge slip angles imparted by the differential steering process. If you're interested in improved turning response in differential steered robots, see our Beginners Guide to Combat Robot Gyros.
Q: A joule of kinetic energy is equal to a watt-second of electrical output, so if I have a spinner weapon that stores 1200 joules of energy and I power it with a 400 watt brushless outrunner motor it should take 1200 joules / 400 watts = 3 seconds to spin up, right? [Social Media]

A: [Mark J.] I see this overly simplified equation popping up on the 'net more and more often. The short answer is no, because a "400 watt" electric motor does not produce full output power over the entire speed range from zero RPM to maximum RPM as it spins up a rotary weapon.

The full answer is too long for an Ask Aaron post, so I've published a new page that provides a detailed explanation of brushless motor power output and the damands of spinning up a rotary weapon: Estimating Weapon Spin-up Time.

Q: Apologies in advance if the hamburger is bad here but how would I go about mounting a brushless outrunner to a P61? I've only used 300 series motors before and the installation was pretty straightforward. Im not looking for specific help but more so a jumping off point in where to begin, thanks. [Los Angeles, California]

A: [Mark J.] The process is very similar to mounting a brushed motor. Here's a bare-bones description based on info from builder Emmanuel Carrillo:

  • Select a suitable outrunner with 25mm mounting bolt pattern and 5mm shaft. The Team Run Amok Brushless Motor Selection Guide will help. Propdrive outrunner motors are popular for the purpose: 3536 for hobbyweight or 4238 for featherweight.
  • Select a P61 gearbox with the RS-500 backplate and order a 700 series pinon gear. This gives you a 25mm bolt pattern and a 5mm bore pinion to match the Propdrive and other popular brushless motors. The RS-500/700 backplate also works; it has both 25mm and 29mm spaced mounting holes.
  • Cut the brushless shaft with a Dremel to 14mm from the face and press on the pinion. Mount the motor to the gearbox backplate and reassemble the gearbox. Don't forget to grease the gearbox.
Builder Chad New has a video walking you thru the assembly process: How to mate a brushless motor to a BaneBot gearbox.

You might also find Robert Cowan's video on preparing outrunner motors for robot combat duty useful: Battle Hardening Outrunner Motors

Mounting the brushless motor is the easy part. Setting up the brushless ESCs is where it gets tough. I'm sure glad you didn't ask about that...

Q: I've seen screws drilled directly into the UHMW frame instead of being attached via NutStrips. How can I attach screws directly to the frame like the robot in the picture? Do you need a hole tapper? [Arlington, Virginia]
A: [Mark J.] DO NOT tap machine screw threads into UHMW. Soft plastic requires screws with deep and widely spaced flutes to hold well. Many builders use commonly available wood screws for UHMW, but special-purpose thread forming screws for plastic are stronger and hold much better. Just drill the recommended size pilot hole to full depth and screw them into place. Available in flat or round head, they are well worth the effort to obtain.
Q: Hi Mark,
I've been watching combat robotics for about 5 years now and I've finally decided to take the plunge and make my first combat robot.

I was thinking of making a featherweight bar spinner, very cliche but in the same sort of vein as 'Suitcase Nuke' but instead of having a solid bar it would have a dense moveable mass probably using a spring to keep it in at low speed and then the centrifugal force would force it further out at higher speeds, theoretically meaning it can spin up faster but still hit hard at full speed.

Has this sort of thing been done before or do you think that it would be too complicated for what it's worth?

Matt (Different from the other Aussie Matt I've seen on here) [Melbourne, Australia]

A: [Mark J.] Hi Different Aussie Matt. Glad to have a new builder joining the ranks. Ask Aaron was started to answer questions from new builders and help them avoid common mistakes that discourage first-time builders. If your first build does not go well it may be your last build. Three comments:

  1. Your first build should not be a featherweight spinner with an experimental weapon. It's difficult enough to get the basics of 'bot construcion and control sorted out without compounding the issue with unproven design elements. See Frequently Asked Questions #8.
  2. The sliding mass idea has been around for a while -- see this post from 2009 in the older section of the Ask Aaron Weapons archive. Some quick back-of-envelope calculations indicate that if you get everything just right on the timing and rate of mass movement you'll get about a 15% improvement in spin-up time. It would be much simpler to use a weapon motor with 15% more power. Simple is good
  3. Up near the top of the Ask Aaron Weapons archive is Aaron's warning about combat robot weapons:

    Aaron's Wisdom  I've said this often but builders don't want to believe me:

    The weapon may be the least important system on a combat robot.
    If you're not winning matches it isn't because you have a poor weapon.

    Drivetrain, radio set-up, general construction practice, and weapon/chassis balance are all much more important than the type of weapon you choose. There are plenty of examples of winning robots with ineffective weapons, and there are many more examples of losing robots with awesome weaponry. If you get the basics right you're going to have an above average robot no matter what weapon it carries.

See also: Centripetal Acceleration and Conservation of Angular Momentum.
UPDATE I broke out the Team Run Amok Spinner Spreadsheet to get more precise modeling of weapon spin-up time for a weapon with a controlled sliding mass that increases Moment of Inertia (MOI) as rotational speed rises. The best practical result I was able to obtain assumed:
  • The sliding mass was large enough to increase the weapon Moment of Inertia by a factor of four; and
  • The sliding mass would begin and complete motion during the RPM range where the weapon motor operates at no less than 90% of peak power.
The improvement in spin-up time was just under 9%. Scrap the sliding weights and buy a more powerful motor.

Q: Would a bar of 1/2" 6061 work for a Hobbyweight spinner? I would want to use steel however all I have to machine with are power hand tools. [Roseville, California]

A: [Mark J.] That's like asking if a spruce 2X4 to would work to build a house -- the answer depends on how you plan to use it. You're not telling me enough about how you plan to use your bar of 1/2" 6061 aluminum for me to comment on its suitability for your unstated design.

Typically I would direct a question like yours to two resources at Ask Aaron:

  1. Frequently Asked Questions #17 - which discusses limits on engineering analyis available here at Ask Aaron and suggests an alternative; and
  2. The Hamburger is Bad - which depicts the trouble that comes from giving advice based on inadequate information.
In this case, I'll take a risk and add a few specific comments:
  • You can make a bar of 1/2" thick 6061 aluminum into an effective bar spinner, assuming reasonable width, length, and rotational speed.
  • The 6061 aluminum is very soft compared to the armor your opponents are likely to employ. To increase impact effectiveness and decrease rapid damage to your bar I would strongly suggest adding hardened steel impact plates to the portion of the leading edges of the bar that will strike your opponent. Even a few steel bolts threaded into the bar edge near the tips would help a great deal.
  • Consult the Spinner Weapon FAQ for general guidance on your weapon design.
I will also point out that there are multiple on-line machine shop services that can laser-cut a simple weapon bar from your design out of AR500 high-impact steel at reasonable cost. I cannot personally recommend any specific on-line machine service, but I will note that many combat builders mention use of SendCutSend.

Q: The Bristol Bots Builders shop sells an N-20 motor with a built-in ESC. They say it gets rid of a lot of wiring clutter in a 150-gram 'bot, but I don't understand how it wires up. What does the wiring diagram look like for two of these motors? [Reddit]

A: [Mark J.] The BBB N-20 wiring diagram is very simple. The battery connects to the receiver power and ground leads on an unused port, and the three-wire motor leads plug into the appropriate receiver ports for signal and power. Very neat and tidy, but drawing power thru the receiver ports creates a few limitations:
  • Your receiver must operate at the same voltage as the motors, which limits your choice of receivers. For their 150 gram kits BBB uses the Flysky FS2A AFHDS receiver, rated for 10 volts.
  • You'll need to construct a cable with a switch to run from the receiver power + and - pins to your battery connector. Make sure you get the polarity correct!
  • Don't try to power a weapon thru the receiver ports -- the thin circuit board traces cannot carry much current. If you need weapon power split off separate battery leads to the weapon ESC to power it directly.
See the BBB website for additional notes: shop.bristolbotbuilders.com.
NOTE: Yes, you can use the same trick robot builders use to run the VEX 29 ESC at higher voltage than the receiver -- but that would ruin the tidy wiring layout and defeat the whole purpose of the compact motor/ESC package.

Q: Hi Mark, it seems like hydraulics are only used in heavyweights. I am wondering if you know of any lower weight class robots that make use of hydraulics? Maybe something that used parts from LESU/RC construction equipment? [Arlington, Virginia]

A: [Mark J.] Hydraulic robots are uncommon in any weight class because of weight, complexity, and fragility. Hobby grade hydraulic systems of suitable size for sub-light combat robots are designed to make scale model constuction equipment operate in convincing and realistic slow motion (LESU Video). A LESU hydraulic actuator provides a maximum 110 pounds of force: control valves are too small, the pumps and actuators have very limited pressure capacity, and the system is both heavy and unsuited to combat shock loads. Unless you're prepared to design, manufacture, test, and certify your own components -- like Team Whyachi does for heavyweight 'Hydra' -- you are unlikely to piece together an effective hydraulic weapon system.

Hello Mark, I come back once again to, our pneumatic robot is in the test phase but we have a problem with the pressure;

We have a 0.5 litre compressed air cylinder with 4000 psi the cylinder is fitted with a paintball regulator set for 160 psi, but the circuit has only 9 bar of pressure and for only 4 shots (despite this the cylinder remains at 3500 psi) are there more suitable regulators? (air flow and outlet pressure) [Provence-Alpes-Côte d'Azur, France]

A: [Mark J.] Hello, Jean Pierre.

I went into the archives and found your prior questions to refresh my memory on your design. You're building a 20 KG pneumatic flipper, but I know little more about the pneumatic design you've settled on: ram size, valving, buffer tank, etc. You also did not give any information about the paintball regulator you have found to be unsuitable.

The most popular small pneumatic regulators used by US builders are paintball units from Palmer Pursuit. Palmer makes a wide range of high performance regulators adaptable to a variety of fittings and layouts. Leave them a note with your requirements and they will help you select a suitable model.

If low flow rate thru the regulator continues to be a problem, you may want to include a small 'buffer tank' as described on the Team DaVinci Understanding Pneumatics page to accumulate a reserve of low-pressure air.

Q: Thanks Mark for answering my question on NylonX/Onyx filaments and it's use in combat robots, I have requested to join the FB group.

I am continuing to go through your website and others as part of my research and I'd like to get some input from you on an observation I've made. For ant/beetles that make use of brushless motors, it seems like many people are using ESCs and motors designed for quad copters. I have a background in quad copters as well as 1/10 and 1/8 scale RC racing buggies. From my experience, the brushess motors used in RC cars (and boats) are a lot more durable (i.e typically larger shaft bearings, shafts, etc) and and can take a lot of hard running. My question is why aren't people using brushless motors and ESCs designed for RC cars in their combat robots? These certainly do weigh more, but I'd imagine the durability and power makes up for it no? I feel like I'm missing something here. [Arlington, Virginia]

A: [Mark J.] Have a read thru the Ask Aaron Brushless Motor Selection Guide. Briefly:

  • For robot drive there is only so much power that a 'bot can make use of in an insect-sized arena and remain controllable. Brushless motors large enough to survive the stress are in general overpowered for drive and are really only used in ants and beetles to show off. Adding extra weight to get even more power isn't productive.
  • High motor RPM is undesirable. Small aircraft outrunner motors run at lower speeds and with greater torque than equivalent inrunner style motors, which makes gearing for necessary torque simpler. Small planetary drive gearboxes don't hold up well with input speeds around 50,000 RPM produced by small inrunner motors. Weapon belt-drive reduction systems don't like such high RPMs either.
  • Weight is critical. A popular beetle drive motor like the DYS BE1806-13 outrunner pumps out ample power and weighs only 24 grams. A small 1/16th scale R/C car inrunner motor like the GoolRC S2435 weighs more than twice as much. That weight saved by using lighter motors and ESCs can be put to much better use elsewhere on the 'bot.

Q: I’ve never 3D printed a robot part in my life, but I want to start. Is there a buyer's guide for 3D printers specifically for making robot parts? The general public-facing reviews tend not to cover that topic. [Woburn, Massachusetts]

A: [Mark J.] Talk to the printers on the Facebook Robot Combat group. The group is your best source for up-to-date information on 3D printers for use in combat robotics. Factors like price, bed size, and the ability to print specific filament types are common discussion topics. Printers given high marks by the group include, in no particular order:

  • Ender 3 Pro
  • Prusa i3 MK3S+
  • Creality3D CR-10 V3

  • Q: Hi, I'm currently designing a 3D printed ant weight (US antweight mind you, 1LB/~450g). I was planning to use NylonX/NylonG for the chassis, however I recently came across Markforged and their continuous Carbon Fiber printing which claims to be approximately equal to 6061-T6. I have been able to find a local rapid prototyping shop which can print with this material for a small price (more expensive than printing it myself with NylonX though).

    I looked on your website and similar sites to see people's experience with this material in combat robots but I haven't seen any solid details. Most deal with regular chopped CF Nylon Onyx version which is similar to NylonX, not the continuous fiber version. Are you aware of any ant/beetle weight (or even larger) bots that make use of this material? [Arlington, Virginia]

    A: [Mark J.] The Facebook Robot Combat group has an active and opinionated corps of combat chassis printers. Some members there have used continuous fiber reinforcement in printed insect chassis -- but the preferred fiber material is Kevlar over Carbon for it's better combination of strength and impact resistance. One of the members is a Markforged employee who had this to say about reinforcing a Markforged Onyx beetleweight robot chassis with continuous fiber:

    Markforged guy here -- Onyx is mostly Nylon 6 so it's pretty impact resistant in general. The chopped fibers don't add much to strength (they're mostly for printability) so if you're looking to add impact resistance/strength past the base Onyx with continuous fiber I'd suggest looking at reinforcing those prints with our Kevlar filament. It's much more energy absorbing than the Carbon Fiber, while still adding a good deal of strength.

    Short fibers add max 1.5-2x UTS over virgin nylon 6 or maybe 1.25-1.5x vs ABS, whereas by adding continuous fibers you're looking at 13-15x UTS for FG/Kevlar and 23x for Carbon Fiber. At that point though you need to look into Izod impact test results too.

    If you'd care to join the Facebook group, I'm sure the printers there would like nothing more than to discuss fiber, infill, drying nylon, base temperature, and all the other variables to consider in printing the ultimate insect chassis.
    Q: I've been looking into receivers and I'm confused about "supply voltage". My current robot uses a 6CH AFHDS Receiver with a stated supply voltage range of 4v to 6.5v, but I run it on 11.1v and it seems fine.

    For my next robot I'm thinking about using a Flysky FS2A with a stated supply voltage range is 3.3v to 10v, but I want to run it on 14.8v. Is that gonna work, or am I gonna smoke the receiver? Thanks! [Anonymous]

    A: [Mark J.] In most combat robots the receiver is not directly connected to the battery; the battery pack connects only to the Electronic Speed Controllers (ESCs). Most robot ESCs and some aircraft ESCs have a 'Battery Eliminator Circuit' (BEC) that sends a nice, steady 5 volts out to the receiver via the 3-wire receiver leads, while passing the full pack voltage to the motors on demand. I suspect that is how your robot is wired. That 5 volt power feed falls within the 'supply voltage' requirement of all hobby-grade receivers, and that is the voltage your receiver will 'see' as long as your main battery voltage remains in the range accepted by your ESC.

    If none of your ESCs have a built-in BEC you can purchase a 'stand alone' BEC that connects to your battery and plugs into your receiver to provide that same steady 5 volts. Search for 'BEC' in the Ask Aaron Radio and Electrical archive for more information on battery eliminator circuits.

    Q: What's the difference between Banebots' Sport Gearboxes vs their P61's? [Lincoln, California]

    A: [Mark J.] The BaneBots 'Sport' gearboxes are designed for FIRST Robotics Competition (FRC) builders. FIRST robots may weigh as much as 150 pounds, and the gearboxes may be subjected to destructive stalling at high torque loading -- something uncommon in the drivetrains of combat robots.

    SpecificationP61 20:1Sport 20:1
    Weight8 oz15 oz
    Length (box only)1.9"2.115"
    Shaft Length1.5"1.0"
    Shaft C/S0.5" Round0.5" Hex
    Shaft Key0.125"N/A
    Max Torque45 ft-lb110 ft-lb

    Compared to P61 gearboxes of the same reduction ratio, the 'Sport' gearboxes are a little larger, a lot heavier, and can survive much greater output torque loads. The lighter weight and longer shaft of the P61 makes it a better choice for use in combat robot drive trains, but the Sport gearboxes may be useful for combat lifting or clamping weapons.

    Note: Some ratios of the Sport gearbox are available in a "heavy duty" version that increases max torque output from 110 ft-lb to 140 ft-lb -- see the BaneBots website for additional information.

    Q: What components do I need to consider if I want to connect a motor, a drive belt and a wheel. [Brighton, England]

    A: [Mark J.] Another school assignment, Brighton?

    == See Next Post Down ==

    You're welcome to use the Ask Aaron archives for your homework, but I'm not going to write essays for every schoolboy in southern England.

    Something's going on in the UK and I think 'Ask Aaron' is being abused. I'm getting multiple versions of very specific robot design questions about the same layout from the same region of southern England. This sounds to me like a school project assignment, and I'm not interested in doing homework about vaporbots for all of Blighty. See for yourself:
    Q: Hi, i have 2 questions to ask
    1. can a robot have 2 wheels that are directly driven by a motor and the other 2 belt driven by the 2 motors?
    2. If the torque calculated requires a motor thats over my budget, i know i can get a motor with less torque but gearing is needed to step up the torque. My question is, can I still directly drive 2 wheels.
    I hope this makes sense [London, England]

    A: [Mark J.] What's going on? Scan down the page and read the last few questions. I'm seeing a theme here...

    The short answers to your questions are:

    1. Yes, this is common practice; and
    2. Yes, this is also common practice.
    Take a look at the question immediately below for additional comments on the belt drive layout and gearing motors for greater torque.
    Q: I need to find a motor that will generate 0.2Nm of torque per wheel for a fightbot. The back 2 wheels will be direct driven from 2 motors, and the front 2 will be belt driven. I have a £50 budget so i know gearing will be required, could you help me choose a good ratio and also how I can find a suitable motor?

    what is the best type of wheels for a small robot and which material is best? [Brighton, England]

    A: You've given me very little information about your robot. You mention that it is 'small', has two motors, is 4-wheel drive, and requires 0.2 Nm (28 in-oz) of torque per wheel. That is not enough information to answer to your questions. The Hamburger is Bad.

    • A very small motor with high gear reduction ratio can supply that torque, but your 'bot would be very slow.
    • A large motor could supply that torque without gear reduction, but might be too heavy for your weight class.
    • Large diameter wheels will increase the torque requirement, and small diameter wheels will reduce needed torque.
    • A small combat arena requires a gear ratio that will give less speed and more acceleration than needed for a large arena.
    • Motors have differing voltage requirements, and your bot's voltage may be limited by rules, design, or other components.
    I suggest that you read thru our Optimizing Robot Drivetrains page and follow along with the calculations given there. You didn't mention how you determined that your robot requires 0.2 Nm (28 in-oz) of torque per wheel, but the equations given on the page will walk you thru those calculations and provide a process to determine proper gearing.

    Once you have determined your actual torque, speed, and voltage requirements you can look thru gearmotors at on-line robot suppliers in your country -- like 'Robotshop'. A quick search there found a 12V 970RPM Econ Metal Gearmotor that may meet your requirements and budget.

    About wheels and tires: There is no single 'best' wheel/tire type. How 'small' is your robot? What type of surface does the arena floor have? How important is traction versus durability? Will the wheels be exposed to direct weapon impacts or are the wheels protected by armor?

    There are dozens of posts on wheel and tire selection in the Ask Aaron Materials and Components archive, and for very small robots there are additional posts in the Ants, Beetles, and Fairies archive. Search there for guidance on wheel/tire selection.

    Q: i have to produce a combat fighting robot and my part is weapons electrical, and therefore what calculations do i make as my robot consists of two axes on the sides of the robot and a spinner blade in the middle [Edgware, England]

    A: [Mark J.] I can't teach you Mechanical Engineering in a few paragraphs, Edgware -- but I do have a collection of on-line tools and Excel spreadsheets here at runamok.tech that can help with your design calculations:

    • My website tracking software tells me that you've already visited the Ask Aaron Spinner FAQ. The information in that FAQ should get you well on your way with the central blade spinner calculations.
    • It looks like you missed the Team Run Amok Electric Hammer Spreadsheet on the 'Combat Robot Design Tools' page. That downloadable Excel spreadsheet will model the performance of your hammer or axe design and allow you to change design elements to see their 'impact' on performance.
    Now, let me save you some time. You may have noticed that there are very few successful multi-weapon combat robots. A typical combat robot devotes about 30% of the total weight to weaponry. Slicing up that weight allowance to make three separate weapons will give you three weapons that are each too weak to be effective in your robot's weight class. My strong recommendation is that you concentrate on a single weapon -- simple robots win.

    What weapons are most effective in robot combat? The answer may surprise you: What Weapons Win?

    Q: Hi, if i am building a robot that is 3kg, is there a way of choosing the right wheel radius using calculations or do i just make an assumption. [London, England]

    A: [Mark J.] There is no single 'right' wheel radius for a robot of a given weight. For a specific motor and weight, the correct drive train will be a function of wheel radius and gear ratio: larger wheels require greater gear reduction, smaller wheels require less gear reduction. See the post immediately below for links to equations and tools to select the correct combination of motor, gearing, and wheel radius.

    Q: Hi I am doing a project where I am building a fighting robot and was wondering what the best way is to choose a motor with calculations. The only thing I have to work with is that the mass of the robot is 3kg. What equations do I need to consider [Brighton, England]

    A: [Mark J.] I've got a whole webpage on that topic, Brighton: Optimizing Combat Robot Drivetrains.

    Once you have the theory down you can automate the selection of drive motors and gear ratios with the on-line Tentacle Drivetrain Calculator.

    If you have trouble with the Tentacle Calculator, I have a step-by-step Example Drivetrain Analysis.

    Q: Hi! My team is building a 15-lb bot. We are going with a kiwi drive holonomic setup and a drum spinner, all powered by brushless motors. We have calculated that we can use Propdrive 2836 2200kv outrunners with 20:1 gearboxes for our drivetrain, and the same motor with a 2:1 belt drive for the spinner. Since all 4 motors are the same, could we run them off one of the 4 in 1 quadcopter ESCs? (With the requisite firmware flashing, control board, etc.) Something like this. These ESCs are designed for quadcopter use where there is plenty of airflow, but we figure we can manage heat by mounting it to our aluminum chassis with a thermal pad.

    Thanks for your help. [Cambridge Massachusetts]

    A: [Mark J.] Massive wheels...  Tiny drum...  But you only asked about the ESC, so let's talk about that.

    Yes, you can run all the motors from a single 4-in-1 quadcopter ESC, and the motors don't even need to all be alike. I know of several small 'bots running two drive motors and a dissimilar weapon motor from a single compact quad ESC.

    NOTE Quadcopter ESCs in general do not use the common 'PWM' receiver output protocol with one three-wire connector per radio channel. The specific ESC you are considering uses the 'DShot' serial protocol, so you will need a receiver with that type of output. Check the requirements of any quad ESC before you proceed.

    Update - I asked some builders familiar with Quad ESCs about your choice. It seems the APD f-series ESCs may not be programmable for reverse operation. Another builder suggested the Racerstar ReachUP 100A, but I don't have confirmation on usability. Stay tuned.

    Q: I give up. How does it work? [Multiple Requests]

    A: It's called a Killough Platform -- similar to an omni-wheel in action, but different in structure. The two wheels in each of three cradles are connected by gears to each other and to a drive motor which can rotate the wheels while they remain oriented at 90 degrees to each other. One of the pair of wheels is always in contact with the floor as they rotate and 'walk' the platform along. The wheels remain free to spin on their own axles and roll sideways to comply with motion imparted by the other two cradles. You can see the action clearly in this video.

    Q: What do your combat robots think of the current COVID-19 pandemic? [Kansas City, Missouri]

    A: [Mark J.] My robots don't care. My robots don't spread, suffer from, or die from Covid-19 -- but you can. Don't be selfish. Follow the science. Stay safe.

    Two photos of Aaron Joerger Remembering Aaron Joerger, 1991 - 2013

    The 'Ask Aaron' project was important to Aaron, and I continue the site in his memory. Thank you for the many kind messages of sympathy and support that have found their way to me. Aaron's obituary

    - Mark Joerger   
    Killer Robot drawing by Garrett Shikuma

    Q: how can robots help us deal better with hurricanes and why? [Ontario, California]

    A: [Aaron] Few people in Nebraska are threatened by hurricanes, so send a swarm of killer robots into low Atlantic and gulf coastal areas to drive the puny human inhabitants toward Nebraska. Problem solved.

    Robot haiku:

    That's obviously
    A question from your homework.
    Do your own research.

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