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6604 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

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: Hey there Mark, I was hoping to ask you about two seperate but semi-related questions concerning some bots I'm building (beetle and feather), both using HDPE.

1) All of my bots have been made of HDPE from 5 to 20mm thick, cut with a mixture of a handsaw, a jigsaw, a chop saw, and a circular/skil saw. I notice however with making curved cuts, the resulting cut tends not to be right-angled, getting more and more drastic the further you go up in thickness. The jigsaw gives cleaner finishes but bends more often, and the hacksaw gives squarer finishes but tends not to curve as well and gives a rougher finish. Do you have any tips for getting cleaner curved cuts with plastics like HDPE/UMHW that don't involve machining?

A: [Mark J.] Given your selection of tools, I think the jigsaw is your best option. Several tips:

  • Buy a brand new blade for your jigsaw and use it only for soft plastic.
  • Several manufacturers offer special blades for soft plastics that reduce heating and make for smoother cuts -- example.
  • Blade tooth count: about 10 TPI works well for HDPE.
  • For tight curves: rough cut about a centimeter from your final cut line, then come back with a second cut to trim away the thin edge.
  • A little wax on the blade helps reduce friction and heating.
  • Try lower blade speeds, and keep the material moving.
  • Square up edges with a sanding block.
2) My second question concerns a snail cam design for a spring flipper. I'm aware that you've been asked questions concerning this topic multiple times and have read through all relevant posts, but for my design I've attempted to adjust the parabolic spiral to allow for a continuous length for the final 1/4 of the rotation to allow for leeway when winding and to prevent misfiring. The motor still seems to struggle to wind spring loads it should be perfectly capable of, and while there may be other issues in the design that could cause that, I believe the cam contributes to the problem.

I've included a render of the cam in question, winding down springs 15mm from their original position. The tolerances in the drawing (eg. the cam not making contact with the spring when the flipper is fired) is to prevent shock damage to key components when the weapon is fired. The cam was also drawn by hand essentially, with me trying to compact the parabolic spiral over 3/4's of the circle. Would you know of an equation that would allow me to draw a more accurate parabolic spiral to fit these tolerances, and would a cam with it's last 1/8th being even put less stress on the motor than the current design?

A: I've been trying to avoid banging out equations for parabolic snail cams, but I suppose I've put it off long enough. Let me find a pad of scratch paper and have a go at it...

One hour passes

OK, I've got it. The general equation for a parabolic sprial cam is... wait... %#!@*&%$!!!

Another hour passes

Right! It's important to get the shape of the parabolic spiral correct to even out the torque load on the motor. The spring becomes harder to compress as it compresses farther, so the cam has to provide a lot of compression at the start of rotation (half the total is in the first 25% of rotation) and taper to less compression as the load increases (the last 12.5% takes the final 25% of rotation).

The formula for the cam in polar coordinates is: Radius = k · θ 0.5 + minimum cam radius where 'k' is a growth variable that defines the lift of the cam; a 'k' of 0.0527 will give one unit of lift over 360 degrees of rotation. Here's an example for a small snail cam with a 5mm minimum radius and 15mm total lift:

Radius = (15 * 0.0527) * θ 0.5 + 5

0 degrees rotation
(15 * 0.0527) * 0 0.5 + 5 = (0.79 * 0.0) + 5 = 5.0mm

90 degrees rotation
(15 * 0.0527) * 90 0.5 + 5 = (0.79 * 9.5) + 5 = 12.5mm

180 degrees rotation
(15 * 0.0527) * 180 0.5 + 5 = (0.79 * 13.4) + 5 = 15.6mm

270 degrees rotation
(15 * 0.0527) * 270 0.5 + 5 = (0.79 * 16.4) + 5 = 18.0mm

360 degrees rotation
(15 * 0.0527) * 360 0.5 + 5 = (0.79 * 18.9) + 5 = 20.0mm

Note The mathematically correct spiral (below left) has far too steep a rise at the start of its lift to allow a cam follower to drop in and correctly follow the profile. The exact needs of that 'landing area' will depend on the details of your lifter mechanism, but it will require some modification (below right) to be mechanically functional.

I've put together a 'Google Sheets' spreadsheet that will produce a snail cam profile similar to the above. The link requires you to sign-in to your Google account then offers you a copy of the sheet: Snail Cam Sheet

Mark gets himself a celebration beer

So far, so good. Now, the torque calculations I've given in previous snail cam posts assumes that spring compression occurs evenly throughout the full 360 degree rotation of the cam. You've modified that in two ways:

  1. Compression reaches maximum at 270 degrees of rotation, and
  2. There is no compression during the first ~60 degrees of rotation due to the cam follower being held away from the cam.
The combination requires the motor to fully compress the spring in about 210 degrees of rotation, which requires ( 360 / 210 ) - 1 = 71% more torque from the motor. No wonder the poor thing is struggling. Yes, reducing the no-lift zones on the cam will reduce the motor torque requirement and result in smoother operation.

How to get a proper cam profile that includes a no-lift 'flat sector' at the end of rotation? Increase the growth variable until you get the full lift you need at the degrees of rotation you want your flat sector to start and freeze the radius at that point. The smaller the flat sector, the less torque your motor will need to put out to wind the spring.

3) Finally, for a question that ties everything together, what would be the best way of cutting a cam like this out of 10mm HDPE be? I believe my first attempt was too rough, as it seems the cam gets caught on the smallest of differences during the winding. I'd love to try to make a good cam myself without having to resort to machining.

Thanks for reading. [Galway, Ireland]

A: I have concerns about 10mm HDPE being too thin for the purpose, even for a beetle. A wider cam will spread the loading over a larger area and prevent the cam follower from deforming the surface and increasing turning resistance -- that may be one of the reasons your motor is struggling. Take a look at the width of the snail cam on 450-gram antweight 'Jännä' (video) for comparison. You might also consider a larger 'flat' cam follower to provide greater surface area that would be less affected by surface irregularities.

I think you can make an entirely satisfactory cam by following the jigsaw tips I gave above. A little work with a sanding block can remove any troublesome irregularities, and a flat cam follower will be less sensitive to catching. Best luck!

Comment: Hey again Mark. Cheers for the equations, that's a great deal of useful information! I've attempted a new updated design for the cam based on the recommendations made, with a better gradual increase, less of a loss of torque, and a thicker cam with a recessed hub to compensate for the size increase (space is at a premium within the mechanism). Comparing the old cam design to the new one, I noticed that where the difference between the two was at its greatest was where the gears in my motor sheared, so your theories on load seem bang on! The fact that it's not a direct pull down is still an issue, but that would require more adjustments to equations, and the old mechanism was able to wind that part fine with a similar profile, so I should be alright there. I'll also keep your tips for HDPE in mind when working on the new cam.

Cheers again, if I'm ever in the area I reckon I'd owe you another celebratory beer!


Q: Why did the superheavyweight class die out? Was the cost of building such robots too high, or did the audience and builders just lose interest? [Nobleboro, Maine]

A: [Mark J.] The increasing costs of building and transporting a 340-pound superheavyweight class 'bot had certainly damped enthusiasm, but more important was the expense of reinforcing existing combat arenas to keep up with raising energy levels in superheavy weaponry. By 2010 there was only one event still willing to host the extra-big 'bots: RoboGames. The self-proclaimed "World's Largest Robot Competition" was a three-day event with more than fifty different robot events (combat robots, fire-fighters, LEGO bots, hockey bots, walking humanoids, soccer bots, sumo bots...) and the schedule was very tightly packed.

The RoboGames combat tournament had already eliminated the builder-popular 12-pound class and was considering elimination of the well-attended 30-pound featherweight class just to gain a little schedule room. When only two superheavys showed up at the 2010 event their fate was sealed. There was no superheavy class at the 2011 RoboGames and the cost of an arena to safely contain a 2019-era superheavy is prohibitive. They're gone and they aren't coming back.


Q: I'm designing a front-hinged pneumatic flipper and I can't figure out how to calculate the initial force at the flipper (see sketch). What is the formula to calculate the red force vector? [Facebook]

A: [Mark J.] The force will change as the flipper extends and the angle of intersection changes, but the initial angle that your pneumatic cylinder intersects the flipper is 180 - 145 = 35 degrees which makes the formula for the initial force at the flipper: sin(35) * 300 = 0.5736 * 300 = 172 pounds

Q: So, the remaining force (300 - 172 = 128 pounds) goes into compressive load in the direction of the flipper hinge?

A: No. The compressive load on the flipper hinge is: cos(35) * 300 = 0.8192 * 300 = 246 pounds

Q: Wait, 172 pounds plus 246 pounds equals 418 pounds of force. How do you get 418 pounds of force from a pneumatic ram with only 300 pounds of output force?

A: We're summing 'force vectors' that have both magnitudes and directions, and you can't sum just the magnitudes. Your ram is producing 300 pounds of force in one direction and 300 pounds of force in the opposite direction as well. In a closed system like this lifter, the full set of force vectors will sum to zero, which isn't of much use in answering your question. Fortunately, trig functions are based on right triangles, and Pythagoras figured out that in a right triangle the sum of the squares of the lengths of the legs is equal to the square of the length of the hypotenuse. We can use that as a shortcut to verify that we are calculating the specific set of forces we're interested in correctly:

172.12 + 245.72 = 29,610 + 60,390 = 90,000

90,000 = 300 pounds at the actuator


Q: This is me again from Anacortes! The Beetleweight wedge I've been building finally has it's electronics! I took your advice and am not going brushless right away, but I have plans to do so in the future. Upon the arrival of the final part, I excitedly botched together a test chassis using some spare 3d printed parts, cardboard, and a lot of hot glue. Everything works after testing using at 2s lipo, but I have run into two problems.

First, the green wire ports on the Scorpion Mini [ESC] I am using are nothing but trouble. I had a lot of trouble getting the wires to stay in as I positioned the motors and drove around, no matter how tight I screwed in the retaining screws. I really don't want to use any solder, as I do not plan to keep this esc in my robot, and want it to be easy to sell or reuse for another project once I switch to brushless. Is there a way to better secure the wires to the esc in a less permanent fashion?

My second problem is that I forgot to solder the capacitors onto the Kitbots 1000 rpm gearmotors that I bought. How exactly should they be soldered on? Is there a wrong and right way? I'll admit that I'm not exactly sure how they work, only that you've said that they "make thing easier for your escs." Anyway thank you so much for your time! [Anacortes, Washington]

A: [Mark J.] I'm always happy to get a progress report and I really love the cardboard test chassis! I think I can help you with your problems.

The Scorpion Mini ESC has screw terminals to accept power input and twin motor output wires. A wire slides into the terminal and tightening a screw clamps the wire in place. The problem you're having comes from the stranded wire shifting and loostening after it is clamped in place. You can prevent the wires from loosening by twisting the wire ends, applying a dab of soldering flux, and 'tinning' the wire with a touch of hot solder. Where you have two wires in a single port, solder them together and insert them as a single wire. This will prevent the wires from compressing and shifting when clamped.

The motor capacitors are important! Solder one leg of the capacitor to each of the motor terminals, as shown in the photo. Polarity does not matter - either leg to either terminal. You may want to slide a length of wire insulation over the bare capacitor leads to prevent accidental shorts. An explanation of the need for the capacitors plus some tips on properly 'breaking in' your motors are found in this archived post.


Q: I've noticed that the Combat Robot Hall of Fame has a lot more heavyweight members than any of the other classes. There are a lot of really great sublight and insect robots. What do you have against them? [Tacoma, Washington]

A: [Mark J.] I feel your pain, but I don't pick the 'bots that get into the Combat Robot Hall of Fame. I get one ballot just like everybody else and I can assure you that my ballot is packed full of sub-lightweight and insect class 'bots. I suspect that the international television exposure of heavyweight 'bots simply makes them better known and better appreciated than the lighter robots.

I'm considering action for the upcoming 2019 ballot to focus voter attention on lighter robots, but I don't want to tip my hand just yet. The Combat Robot Hall of Fame opens for balloting in August of odd-numbered years. At that time the voting details are released to combat robot forums in the US, Europe, and Australia.

Keep your eye on the forums -- your next chance to vote will come in about: .


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

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

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

  • If only part of the weight of your opponent is pushing down on your robot, that weight is no longer on their drive wheels and their pushing force is proportionally reduced which makes it easier to push them.
  • If your opponent is entirely on top of your robot it cannot exert ANY lateral pushing force on your robot -- you won't have to 'push hard' against anything.
Carrying extra weight across a level surface is orders of magnitude easier than pushing against that weight while it is actively pushing against you. The only time stalling under this type of condition would be a problem would be if your opponent was entirely on top of your robot and you decided to push full-throttle against an immoveable object like the arena wall. Why would you do that?
Q: Hey guys! Its a little specific and I'm really sorry if its not the kind of question you want to receive :/

I run a 3lbs Beetleweight hammer bot and for some reason my brushless drive weapon system always has like a quarter to a half second of delay on it! For the first version of the hammer I used basically the same weapon setup as an earlier hammer bot "Dain" did. I ran a Fingertech 2838 22 max amp, 300W 22 max amp, 300W 2970kv motor hooked up to the next generation R/C car Trackstar ESC that Dain had. I powered it all by a 850mah 3S battery, a Fingertech receiver and 20AWG wires (whoops). The entire time the hammer had this bad input lag. Despite being hooked up to the same drive receiver as a lag-less drive system

Since the first iteration I've swapped to a bigger motor (650W, 50 something-ish max amps) gotten and programmed a bigger 60A Trackstar ESC, have bought a higher 80C rating battery and I have even changed the radio and receiver and upped the wires to 16AWG and yet I still swing with 0.25 to 0.5 seconds of lag when compared to Dain throughout the entire match! It's maddening!

I've never been great with electronics and I'm honestly out of ideas as to what causes this. It was the exact same problem with the 30A ESC as the 60A, the same with 2 entirely different radio and the same with a bigger C rating and lower gauge wiring!! I really feel like I've cycled out all of the electrical components and still have the same issue. I would love your opinion on this.

Again genuinely sorry if this isn't the kinda question you're looking for on this forum. I've tried to include as much info to avoid the bad hamburger. Have a good one! [Ontario, Canada]

A: [Mark J.] Your question is exactly the type of question we do like to see, Ontario. You've got a problem that has you stumped and it's a problem that other builders might run into as well.

The bad news is that you've put a lot of time and effort trying to fix the electronics when the electronics aren't causing the delayed response. The good news is that there's a little secret about brushless motors that I'm willing to tip. Follow along...

Brushed and brushless motors are very different:

  • Brushed motors are simple. Hook a brushed motor directly to a battery and it spins right up, producing huge chunks of start-up torque.
  • Brushless motors are not simple. Hook one directly to a battery and it will twitch and quickly melt; they require an intelligent external motor controller to supply each of their three input leads with power of the correct polarity at just the right instant and then change everything an instant later as the motor rotates.
Getting a 'sensorless' style brushless motor (like yours) to start rotating is a tricky problem. The correct current polarity to apply to each of the leads to spin the motor in the right direction depends on the relative positions of the magnets and coils in the motor, but with the motor at rest the sensorless controller does not know what that position is. The controller has to 'guess' and send a small current pulse thru one pair of the motor leads, then it waits to detect and analyse the electrical feedback from the motor when the motor starts to rotate in response to that current pulse.

This start sequence works well if the motor has a light load like an airplane propeller, but if the motor has a heavy load (like an overhead hammer) the small current pulse may not cause the motor to move enough to provide feedback to the controller. The controller will wait a few milliseconds and try again... and again... and again until the motor eventually twitches enough to allow the controller to figure out the correct polarity sequence and provide more current to properly spin the motor.

If there is enough free play ('slop') in the gearbox and/or belt drive between the motor and the hammer so that the motor can spin freely for about one full revolution before it takes up the load of lifting the hammer the whole start-up process takes only an instant. However, if the drive train beween the motor and load is very 'tight' and there is not enough free play to allow the motor to start its spin-up without load you're going to get the type of delay you describe.

You've given me a complete description of your hammer electronics but you have provided no details about the mechanics of the hammer drive train. Twisted Sick Robotics' beetleweight 'Dain' has the weapon motor mated to a 14:1 gearbox that then goes to a 3:1 belt drive for a combined 42:1 reduction. It's a good bet that the combined play in the two-stage gearbox along with a fairly loose timing belt allow the weapon motor enough no-load slop to get thru the startup sequence muy pronto. Given your description of the problem it's also a good bet that your unspecified weapon drive train does not have enough slop. Add some.

Reply: Hi Mark! I appreciate the in-depth tips. I run the exact same gearbox and additional reduction as 'Dain' and - credit where credit is due - WOW it works well! The only difference is that I run the additional reduction using specially cut gears, not belts. I'll be looking into a sensored weapon motor setup or swapping to a belt drive system now. Thanks for all your help! Cheers!

A: Very pretty, Ontario! It would be a pity to remove those bespoke gears; a sensored motor and controller will cure your response delay. May your opponents all have fragile top armor.


Q: I'm designing a single-tooth drum but I'm having some trouble. I started with an Archimedean spiral drum cross-section for correct bite clearance, then I shifted the rotation axis to the CAD-calculated center of gravity to balance it. What I got was a drum with a big lump sticking out opposite the tooth that will interfere with the tooth 'bite'. Is there a simple way to reduce the size of that lump and keep the drum in balance like the RioBotz 'snail drum'? [On-line Forum]

A: [Mark J.] You started out well -- the ideal profile will fit snugly inside an Archimedean spiral with nothing sticking out to interfere with maximum tooth 'bite'. Unfortunately, simply starting with a spiral and shifting the spin axis to the CG leaves that lump you described outside the ideal spiral (red spiral line in the diagrams below). You need to modify the mass distribution to pull the profile back inside the spiral.

The RioBotz 'snail drum' is optimized for that purpose, but it is highly challenging to reproduce. Fortunately, there is another Brazilian solution that's a little easier to design and make. Uai!rrior's 'Federal MT' and 'General' have interesting single-tooth disc profiles that could be used to make an effective drum with properties comparable to the RioBotz 'snail drum'.

Start your design with a basic Archimedian spiral cross section, then take slices off the bottom until the CAD center of mass stops getting closer to the spiral axis. A few additional design tweeks will bring the CG right onto the spiral axis.


Q: Do you know of any good power switches for antweights? FingerTech seems to be out of stock for their mini power switches at the moment [Marysville, California]

A: [Mark J.] 'Good' depends on you design and current requirements.

  • If you really need 40 amps continuous current the FingerTech Mini Switch is a fine choice -- if it was 'in stock'.
  • If your design has room and allows access, my preferred method for a high current switch in a small and light package is to make a 'removable link' from a suitable wire connector -- see the diagram. I use Deans connectors for this purpose.
  • If you need a direct substitute for the Mini Switch and your 'bot pulls no more than 6 amps continuous (15 amp burst) you can use the FingerTech 3.5mm Switch/Charge Jack. Insert the plug and the robot is off; remove the plug and the robot is on.
The 3.5mm switches are used by the FingerTech Viper antweight kits, and they are both reliable and 'in stock'.

Q: Hey I'm building a 30kg/60lb vertical spinner robot and i wanted to know... (deleted) [Goa, India]
Although I very much wish to support the technical aspects of robot construction in the energetic and expanding Indian subcontinent, I am also greatly worried that I may be contributing to an extremely dangerous situation for builders and spectators. This has brought me to a painful decision:

The 'Ask Aaron' website is closed to questions from builders competing in India.

The best enclosed arenas in India would be considered inadequate for 30 pound robots in Europe or the US but are hosting events for much heavier 'bots. Aaron certainly wouldn't approve of the reckless endangerment of life and limb, and I will not contribute to the development of more powerful combat robot weaponry in any region where the arenas are not universally able to safely contain them.


Q: Do you have any recommendations for in-depth combat robot making guides that are up to date? Back then there was the Riobotz Combot Tutorial but it is from 2009 and the world has changed a lot since then.

Thanks. [Jakarta, Indonesia]

A: [Mark J.] Some things have changed since 2009, and some have not. Are you planning to build a 'bot with a 3D printed chassis and brushless drive motors? The RioBotz tutorial may not provide adequate guidance to build a 250 pound class heavyweight to compete at Discovery BattleBots, but it should do very nicely for an Indonesian combat event.

There is no comprehensive guide more current than the RioBotz tutorial, but for clear and detailed information on a variety of combat robot topics I can recommend the 'Robert Cowan YouTube Channel' as mentioned on my Team Run Amok page:

Robert Cowan YouTube Channel

There once were a great many combat robot build logs out on the 'net. A new builder could learn from the experiences of other builders and find answers to questions they didn't even know they had. Now there are very few builders that even maintain websites, and fewer still who are willing to share their build secrets.

Fortunately, builder Robert Cowan has taken on the task of providing well produced videos of the intimate details of building a combat robot, as well as other tech projects he undertakes. His YouTube channel page is here. You might want to start with his video series on his antweight robot 'Sgt. Cuddles'.

I'll also point out that you're sitting on top of an archived collection of nearly 6600 questions and answers about combat robots in the 'Ask Aaron' archives, plus a selection of combat robot design tools unmatched anywhere on the planet. If you can't find the answers to your robot design and construction questions here you probably don't need the answer. Just sayin'.
Clickbait month at Ask Aaron is over.

Did you miss 'clickbait' month? Do you want to relive the excitement? Check the archived version here.

Q: I just got my first disk weapon back from the metal shop and I think it has some problems. Builders are telling me that the sharp internal angles on the spokes are places that will start fractures. Worse, the water jet cutter overshot and left some little notches in the corners. I think I'm in trouble.

Is there anything I can do about this or do I have to start over? [Walsall, England]

A: [Mark J.] Those sharp corners concentrate and focus stress on a small area, and if that area is aleready weakened by a flaw caused by poor cutting technique you're begging for a failure. Yes, you're in trouble -- but it's not hopeless.

You need to smooth out those corners. The preferred way to avoid a 'stress riser' sharp angle is to carve a smooth, rounded 'fillet' curve when designing the disk -- but that ship has sailed. You can actually strengthen the problem region by removing a little material to create a smooth radius curve. Don't go crazy -- even a small radius relief cut will improve survivability.


Q: I've got a big wheelchair motor with a magnet that popped loose from the motor case. What is the recommended adhesive to stick it back into place? [Pittsburgh, Pennsylvania]

A: [Mark J.] There are specialty adhesives made for this purpose, but they are expensive and difficult to find. I've been very satisfied with 'Original J-B Weld' epoxy which is widely available at hardware and auto parts stores. I prefer it to other 'off the rack' epoxies for its ability to survive high temperatures; not all epoxies are stable when hot, and motor magnets do get hot.

Chip away any residue of the previous adhesive and give the surfaces a quick solvent wipe before applying an even coat of new epoxy. Tip the magnet carefully back into position and let the epoxy cure overnight before completing motor re-assembly.

For tips on 'battle hardening' magnets in smaller motors see this post in the Ask Aaron archives.


Q: Hi Mark. I had a design that I thought was a bit outlandish and would be a waste of time, but the cheerleader button told me it was a great idea and to go for it.

For bristlebots, is there a preferred orientation for which way the offset weight spins? The small hexbug toys have the weight spinning perpendicular to the direction of travel, while 'Clean Sweeper' has the weights spinning parallel to the direction of travel. Does it matter or will it be terrible regardless? [Manchester, England]

A: [Mark J.] I'm happy to hear that 'the cheerleader' is doing her job... kinda.

There are so many variables in the design of a bristlebot that it becomes difficult to sort out the best vibrator motor axis. I've seen working bristlebots with every possible orientation of the vibratory axis, and with adjustments to the bristle design they all worked pretty well. A pair of Italian scientists have even claimed to have worked out vibration frequencies that will get your bristlebot to reliably back up -- in theory.

If you plan to use your rotary weapon array as the vibration source it makes sense to orient the weapon axis in a useable direction -- as 'Clean Sweeper' has chosen with their dual vertical bar spinners. If you're using dedicated vibration motors you may have other considerations:

  • Although single-motor bristlebots with the rotation axis pointing straight up (Z-axis) do work, a steerable two-motor bristlebot would likely be inefficient with twin Z-axis vibrators as they would 'fight' each other.
  • Positioning the imbalanced mass with either X-axis (Hexbug) or Y-axis ('Clean Sweeper') orientation directly over a brush may provide greater efficiency than placing them farther away.
  • Build a full scale mock-up of the brushes and vibratory motors to tinker with before committing to a final design. Experiment with bush length, brush angle, and the amount if imbalance to find a workable combination for your particular size and layout.
At this time bristlebot design is more of an art form than a science. About all you can be sure of is that power and control is going to be inferior to a simple wheeled design. Approach it as a learning experience and you'll have the right attitude.

Q: Hey Mark,

I'm putting together an antweight with a somewhat experimental weapon. The weapon would be a slightly different take on the overhead saw. Instead of a single abrasive disk trying to cut its way inside the opponent, it would be 4 or 5 abrasive disks put together to create a single thick disk. The idea would be that the thick abrasive disk wouldn't necessarily cut through, but rather take large scoops out of the opponents top armor. Admittedly, it wouldn't be the most efficient way to get to the insides, but in theory it would cause quite a bit of visible damage on the opponents' topside and hopefully create a terrific spark show...if done right.

So let's jump in on some questions about how to "do it right". For this scenario we'll assume the weapon consists of four combined abrasive cutoff wheels with 3" diameter and the motor driving the weapon is a V-spec 2205 2350kv motor.

Abrasive cutoff wheels are pretty light. I do not know the exact weight, but I'd wager that a 2205 brushless motor would have no problem spinning them up on 1:1 gearing. I'd imagine that most overhead saw bots try to find a sweet spot with their gearing where the weapon has good speed, but at the same time, enough torque to keep spinning while making contact with the opponent.

With 4 abrasive disks, that presents 4x the width of the cutting edge, and presumably, 4x the amount of friction that will attempt to slow the weapon down as it makes contact. Would I be correct in assuming that this would also require 4x the amount of torque (4:1 gearing) to pull off than a saw bot only running a single disk on 1:1 with the same specs?

Now, as I mentioned before, many saw bots try to find the happy medium of sufficient torque and good speed. With the inevitable gearing down that would have to happen, the speed would take a dive, but the torque would remain sufficient. At this rate, we could probably see a steady decline in effectiveness because we have to compensate for the extra friction that multiple abrasive disks create. Of course, we're not even getting into the amount of pressure that the overhead arm would be trying to put on the opponent as it comes down! Let's just assume that in this case it is simply "a decent amount".

Do you think the thick abrasive cutting disk would still be effective with such a sharp decline in speed, or would I be better off approaching this by using motors that tend to be a step up in terms of power than what we normally see in antweights?

Thanks, David R. [Livermore, CA]

A: [Mark J.] You've spent some time thinking about this, David. Let me see if I can redirect your thinking just a bit...

  • Consider the relationship between tires and the arena surface. Doubling the width of the tire does not appreciably increase the traction (friction) between the tire and the surface, but doubling the downward force (weight) on the tire will double the traction.
  • A similar relationship exists between a grinding wheel and the top armor of your opponent: increasing the width of the grinding wheel will not increase the friction attempting to slow the wheel, but increasing the downward force applied will proportionally increase that friction.
  • The rate of material removal is a function of the friction (applied downward force) and the rotational speed. For a given downward force and speed, a change in abrasion wheel width will give the same material removal over a larger area -- and the same number of sparks.
I think judges would be more impressed by you cutting a narrow slit thru the armor than grinding a wide but shallow divot across it.

Q: I watched the season 1 'Robotica' finals on YouTube last night. Is there any special way you celebrate the anniversary of your win? [Saint Charles, Illinois]

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

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



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   

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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