Questions and Answers about Combat Robotics from Team Run Amok.

A: Mark J. Is that pulley your entire weapon hub? It is better to have a wider hub that separates weapon bearings for a more stable base to improve stability and strength. Needle bearings are sensitive to even small static misalignment. Jamming two needle bearings right up against each other will magnify the effect of any installation or bore problems. You might be fine -- you might not be.

I would prefer one of two alternatives:

Fell Off a Bridge

Q: Ramfire 100 won 1994 US Robot Wars, then fell off Earth as far as I know. Do you know if ANYTHING else happened to it? [Sent from my iPad]

A: Mark J. Yes, I think I remember hearing something about 'Ramfire 100'...

Michael Sorenson's 'Ramfire 100' fought two matches at the 1994 US Robot Wars and won them both to become the heavyweight champion. Depressed that there were no more opponents to defeat, 'Ramfire 100' made its way to the center of the Golden Gate Bridge and leapt off - only to land on the deck of a freighter bound for Finland. After hitching a ride into Helsinki, 'Ramfire 100' worked as a forklift in a paper mill for several years and saved every penny until it had enough money to open a small cigar shop. It married a lovely girl named Helmi and had four children: a girl, two boys, and a fax machine. The family is doing well.

See also: Ask Aaron MadLibs.

A: Mark J. Yes, you do need some RPMs for a spinner weapon. See the 'Rotational Speed' section of our Spinning Weapon FAQ for help in figuring out how many.

Q: I am the same person who asked the question

Nothing Practical

[archived here]. My design is based on the quintessential, exposed wheel 2WD bar spinner that you no doubt have seen several hundred times. I'm already aware of the glaringly obvious issues with this weapon, such as the fact that I will need to drive like Gary Gin crossed with Ray Billings to avoid the lightning-fast modern vertical spinners for 3.9 seconds with a massive whirling gyroscope trying to send me into the arena side rails, and the difficulty of squishing a 430g, 10" bar into 3 lbs. The former issue is compounded by the fact that events like NHRL have you start in the corner of the arena, halving my dodging angle from 180 degrees to 90 despite the increased length across the diagonals. I'm basically using the Last Rites design philosophy of "I don't need armor if I knock out the other guy first."

My motor (SunnySky X series V3 2814 1400 kv) has a max output of 1,180W, so using the Vector bar statistics in this post for a rough reference, I should be okay in terms of air resistance. By using these newfangled LiHV cells I found, I was able to push even more power than my planned 14.8V into the weapon for the same weight, giving me 15.2V. The pulley would be 3D printed nylon with a central hub containing a bearing running through the hole in the bar. With two Dartbox Viper motors for drive, I should have more than adequate speed for the weight class. Chassis setup is probably going to be 0.25" UHMW sides with 3mm carbon fiber top and bottom panels/weapon supports.

A nice image of HellaChopper - just to break up the blocks of text.

Is there anything I'm not considering here? Of course, I won't get 2.5 kJ in real life, but even 2,000J is nearly an order of magnitude stronger than your average beetle spinner. Maybe 3.9 seconds is long enough to bog the poor brushless outrunner motor and catch it on fire before I even get the chance to unleash my >2 kilojoule impact? Or is it a safety concern, in that insect arenas may not be equipped to handle the impact of this out-of-control tactical nuke of a robot? With some driving practice to counter the gyro on spinup, I'll take 4 vulnerable seconds (Probably even less, as the spreadsheet has me at 1kJ at 1 second, which is still hugely damaging) for 2:56 of a bar that can probably take out the other bot in one hit, and then half of the arena with it like a modern-day Blendo. (Hyperbolically speaking, of course)

Recoil is a consideration, but it's a 3 lb bot, and beetleweights are known to hockey puck across the arena like crazy without much internal shock damage. Suffice to say, I won't see the catastrophic failures of modern Tombstone. Plus, shallow wedges won't reflect the full force of my weapon back into me, and anything that engages heavily enough to do so will end up obliterated.

There's also bite, but in my opinion, a shallow engagement isn't the end of the world on a horizontal spinner that doesn't need to grab and toss like a vert. Hell, the high engagement that allows verts to toss their opponent 15 feet into the air can even be bad, as it will contribute to sending me flying back in a horizontal that can't push energy into the ground on impact. Just look at Last Rites vs Whoops from Robogames 2017 where Ray was kicking Whoops' ass until he got a big engagement strike that sent him recoiling violently. [Arlington, Virginia - no iCloud this time?]

A: Mark J. When you're atop a determined horse that manages to work its bridle into a position where it can grab the bit with its teeth, you can pull that bridle in any direction you like but the horse will pay you no mind and keep to the path it chooses. I don't think my input will effect your design decisions, but perhaps other builders will benefit.

You may want to stop reading at this point and click this button:

Your image attachment was lost in an internet eddy, but from the numbers you gave I was able to back into some of the numbers you did not supply. It looks like you're planning to use a 1.5:1 pully reduction to spin your weapon bar up to an attempted 14,200 RPM with a 425 MPH tip speed and store 2500 joules. I believe you are minimizing multiple issues. Here are a couple:

Reply: "Shades of Hellachopper" here -- dropping to 2:1 reduction. Thanks for the reality check.

Comment: That's dangerously close to rational, Arlington. A change from a 1.5:1 reduction to a 2:1 reduction:

A: Mark J. No. ESCs operate with internal frequency generators to produce the current pulses used to control motor speed. The pulse generators for two ESCs -- even from identical sources and receiving identical receiver signals -- will not be 'synched'. At times the full current of the motor will be passed thru one unit, then the other. This negates their ability to "split the current load" and will very likely smoke both ESCs. Equally bad, while they survive you will get irregular pulsating speed variances rendering consistent control impossible.

Don't do it.

Before someone writes in to tell me that 'back in the day' some builders used two ESCs to "split the current load" on high-current Magmotors and Bosch GPAs, let me clarify. Those motors had a system of four brushes set 90 degrees from each other, and each of the ESCs were wired to a different pair of those brushes. This was a special application for specialised motors and the outputs of the ESCs were not "spliced together".

See also this previous post in the Ask Aaron Motors and Controllers archive.

A: Mark J. There is a similar situation that involves operating a lifter servo weapon at 2S when it is plugged into a receiver that can take no more than the voltage from a Battery Eliminator Circuit (BEC). The two situations have similar solutions.

With your Malenki, it is possible to use a battery with more than 2S to provide power to your brushless ESC by tapping into the balance plug of the same battery to obtain 2S power for the ESC/receiver and your drive motors. A diagram for a 3S battery is provided below.

There are a few drawbacks to this solution:

A: Mark J. This is a job for the Google A.I.: How does a vertical spinner weapon on a robot work?. The A.I. is much more patient in answering this type of broad question than I am. Each time you ask it generates a slightly different answer, but the answers I've seen for this particular question have all been reasonably complete and accurate.

If you want more detailed technical information, see the Ask Aaron Spinner Weapon FAQ.

If you have specific questions I'm glad to help.

Someone Made the Calls

Q: If i remember correct: in the Robotica Labyrinth match up between Jawbreaker's Revenge Vs Buzz Bomb Dan Danknick says "The Judges said they'll be Jawbreaker's Points." DID ROBOTICA HAVE ANY JUDGES?! [Erskine, Scotland ☆]

A: Mark J. Someone had to count the laps. Someone had to keep the clock. Someone had to decide who left the platform first. Someone had to determine if a goal in 'The Maze' had been blocked. Someone had to award me a penalty for going out-of-bounds in 'The Speedway'.

For Season 1 that 'someone' was Terry Huntsberger, the NASA-JPL Roboticist listed in the end credits as "Robot Consultant". I mention Terry a few times as the Robotica 'tech judge' in my Robotica Journal.

Terry was not listed in the credits for seasons 2 and 3 and no one is credited as a robot consultant -- but someone had to make those calls...

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A Lot of VEX Parts

Q: Hello, I am currently designing a 30lb combat robot and have 3 main questions. To start, I have added a small photo of the design so far to give an idea. The goal is to have a vertical spinner design, that has a 4 wheels driven by two motors. I am running an 8s system, with Talon SRX and Redline 775 + 10:1 planetary gearbox for the drive. And a Castle 2028 800 Kv and Mamba Monster X 8s for the Weapon. The Talons are technically only rated for 28V, but have been tested and work at 30V while mixing the output to 40% for the 12V redlines. We do have 8s ESCs but they are almost double in size and would be much more difficult to design the plating around.

I know this is a lot in one post, but I know you're the guy to ask. So let me know your thoughts on my setup. [Mount Prospect, Illinois]

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First off with just the specs I would be curious any advice for the design just based off the electronics and CAD photo.

A: Mark J. I see that you have made some changes to your design since your earlier post. A few observations:

• I'm personally not a fan of overvolting ESCs. A LiPo cell has a nominal potential of 3.7 volts, but when fully charged it sets at a resting 4.2 volts. That means your 8S system can push 33.6 volts thru your Talon SRX. I can hear the capacitors failing already. Still feel confident?
• A Castle 2028 weapon motor is massive overkill for a featherweight -- particularly given that your weapon itself appears to be on the small size. A typical featherweight weapon motor weighs in at about 21 ounces (4.4% of the robot mass), whereas the Castle tips the scale at 44 ounces (9.2% of the robot mass). See: Brushless Motor Selection.
  Spinning weapons are flywheels. They rely on rotational inertia to collect energy from a continuous power source (electric motor, internal combustion engine...) over time and store it as rotational kinetic energy. On impact, the flywheel releases the stored energy in a blow that far exceeds the energy directly available from the continuous source.
  Having a huge motor paired to a weapon too small to store a significant multiple of the motor's output power at a reasonable rotational speed is not a path to victory. Cut the motor weight in half and add the mass to the weapon. See: Ask Aaron Spinner Weapon FAQ.
• Your design makes VERY little effort to 'feed' your opponent up and into the rotating weapon. Your plow is far too steep and elevated to 'win the ground game'. The current spinner design 'meta' employs floating forks to get under the opponent and lift their leading edge up into the weapon path where the weapon will get excellent 'bite' and deliver a massive hit. Without forks your weapon is set up to strike a glancing and ineffective blow on some smooth surface of your opponent.
• Can I assume there is a chain or belt drive running from the rear wheels to the fronts to provide four-wheel drive? If not, the rear wheels by themselves have very little weight on them which will result in poor pushing power, reduced acceleration, and impaired maneuverability
• I wonder what events you plan to enter with this 'bot. You are using a lot of VEX parts that are common in non-combat robotics in preference to components in use by successful combat competitors. This seems a little odd.

I have additional comments, but they fit well into the answers to your other questions below.

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Q: Getting into my questions though, my first is about the drive. In another post you said that Featherweights should be around a 20:1 drive system. My first question is, if I have an arena that is utilizing a steel floor, and I add magnets for extra down force, would 10:1 be suitable? Or does the extra downforce not affect the torque?

A: It is not a matter of "Featherweights should be around a 20:1 drive system" -- different drive motors will require differing gear reductions for any given combination of robot weight, wheel diameter, and arena size. See: Optimizing Drivetrains.

You have the relationship between traction and gearing reversed. Adding magnetic downforce to a robot creates a need for greater torque from greater gear reduction -- not the other way around. The Team Tentacle Drivetrain Calculator includes a field to enter magnetic downforce and can calculate adjustments to the drivetrain requirements based on that added traction.

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Q: Another comment about the drive system, is when running the redlines and gearbox at 12v, they seem to get hot very fast. Besides adding a cooling fan, is there an easy way to dissipate the heat? Would 3D printing a TPU heatsink be beneficial? Or maybe it isn't a concern since the matches are only 3 mins max.

A: As noted in my answer to your earlier post, AndyMark Redlines are quite commonly run at 6S voltage in combat robots . If run with a reasonable gear reduction they can handle the heat. If run with insufficient gear reduction they will melt. Wrapping them in a TPU heatsink would be worse than doing nothing as TPU is a much better insulator than heat conductor.

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Q: Another question I have is about the weapon system. I am trying to decide whether it would be better to have the bearings spin on the inner or outer ring. The main difference in setup would be spinning the Weapon + Shaft, and attaching the bearings to a side plate. Or attaching the bearings inside the weapon and spinning it around the stationary shaft. I am not sure which would protect the bearing most from impact, as well as produce the most RPM/Inertia.

A: There are essentially no differences in bearing protection. speed, or rotational inertia between a Live Shaft that spins with the weapon and a Dead Shaft that is fixed relative to the chassis. The primary difference is that a 'dead shaft' can be a structural member of the chassis that adds to both the strength and rigidity of the robot.

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Q: Along with that, I've done a lot of research on bearings and have heard a lot of good things about angular contact bearings. But did not know the best type for high RPM yet high force.

A: Angular contact bearings (expensive) are used where a single bearing must absorb forces from all directions. This is not the case for your weapon. A pair of tapered roller bearings (much less expensive - and stronger) will do very nicely.

Some builders prefer oilite bushings for their weapons. These bushings can absorb huge impact loads and are inexpensive -- at the cost of a bit greater friction.

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Q: My third question [You and I count differently!] is about bolts/tapping plates connection. I currently have 1/16", 1/8", 1/4" and 1/2" aluminum to work with for plating. I also have M3, M4, M5, M6 bolts to work with. In my design I wanted to countersink my bolts so that I didn't leave an exposed bolt to be sheared off by the enemy weapon. But my main confusion is the best combination of these. Because say if we use a 1/2" plate, the bigger the bore a tap we make, the less wall material we leave to support the bolt.

A: See Frequently Asked Questions #17. Note also the difference between 'countersink' and 'counter bore'.

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More Than You Need

Q: Could a spring from a typical rat trap be repurposed into an antweight flipper?

  - sincerely, Iceywave [West of San Antonio ✪]

A: Mark J. Sincerely? I suspect you're just prompting me to post a pic of Team Run Amok's infamous antweight snapper/crusher/flipper 'Rat Amok'.

In truth a rat trap can store far more energy than is required for a properly designed antweight spring powered flipper -- a mouse trap spring should be adequate. I found an analysis on the energy stored in a typical mouse trap at the Physics Stack Exchange:

I calculated the torsion constant in a Victor Original Mousetrap with a spring arm length of 4.3cm) to be approximately 0.09088 N∗m/Rad by using Hooke's Law and comparing the angle between the spring arm and the wood base when hanging different weights from the arm with the trap upside down.

I then used τ = −kθ (torque applied to the spring arm by the weight is equal to torsional constant times the angle rotated) along with the values from one of the weights to calculate how far the spring is twisted by default [θ = (τ_weight / k) − θ_weight ]. This came out to about 73.63°.

I used this 'starting angle' to calculate how much potential energy (U = 1/2kθ²) the spring would have when totally open (an additional 180° from that last angle we found). The maximum energy that you could get out of this "standard" mousetrap is approximately 0.815 J.

Using the potential energy calculator at OMNI Calculator reveals that 0.815 joules -- if perfectly converted to vertical speed -- could loft a 1-pound object just over 17 feet straight up.

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No Steering Wheel

Q: drive train [Madhya Pradesh, Bharat]

A: Mark J. I've never actually tried, but it shouldn't be very hard. You can only go where the rails go, so just give it a little throttle and toot the whistle once in a while.

 

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They Don't Get It

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

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

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.

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

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