Questions and Answers about Combat Robotics from Team Run Amok.

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7851 Questions and Answers about Combat Robotics
from Team Run Amok
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A Rare Exception
Q: i am making a 15kg bot in which i have decided the specs for the bot should be:
  1. GenX 22.2V 6S 3300mAh 40C / 80C Premium Lipo Lithium Polymer Battery
  2. Rhino 12V DC 100RPM 40Kgcm IG32 Heavy Duty Planetary Geared Motor
  3. Rhino MDD 20Amp 6V-30V Dual DC Motor Driver (2 Channels)
  4. FlySky CT6B 6ch 2.4GHz transmitter & receiver 1Km range
  5. Astro Heavy Duty 75mm TPR Rubber HIgh Endurance Wheels (1"Wide)
  6. 300W 20A DC-DC Step Down Converter Module High Power Buck Voltage Regulator 6V-40V to 1.25V-36V
  7. TecoKart Battery Cut Off Switch 12V/24V
  8. 24V DC Motor 2500rpm
i have decided to make a spinner weapon bot where the 24v battery would drive the spinner and the 12v are for wheels the buck converter would step down the voltage from 24v to 12v to drive the motor which would be connected to motor driver. Now i cannot find a compatible esc for 24v motor which would have braking functions. i can find the motor speed controller but they change the speed by potentiometer of spinner weapon but i want it with pwm compatible. What other alternative could i try for the spinner weapon and any other suggestion would be helpful. [Maharashtra, Bharat]

A: Mark J. 'Ask Aaron' has a general rule that we do not answer questions from competitors on the Indian subcontinent ( why not? ) - but I'm making a rare exception this time because:

  • You're building a fairly small robot;
  • You're not asking about spinner weapon design; and
  • You're going to have a very disappointing robot if you go ahead with those components.
Maybe I'm getting soft in my old age, but let's have a look at your parts list:
  1. GenX 6S 3300mAh 40C/80C Lithium Polymer Battery -- You've told me too little about your spinner weapon for me to calculate the actual battery capacity requirement. The chosen drive motors will use very little power so this may be larger than needed. Assuming that you have a proper LiPo charger this should be OK.
  2. Rhino 12V DC 100 RPM 40Kgcm IG32 Planetary Geared Motor -- Find larger motors This is a small RS-385 brushed motor with a peak output of about 15 watts. Rule of thumb has the ABSOLUTE MINIMUM drive power needed for a 15 Kg robot at 120 watts (8 watts per Kg) and a pair of these motors provide only 30 watts. With the 100 RPM output gearbox and 75mm wheels, the top speed of your robot will be a VERY slow 1.3 KPH (0.8 MPH).

    I also worry about the durability of these gearmotors. The output shaft is only 6mm diameter, which is considered marginal for direct drive to a wheel in the 3-pound combat class -- it is far too small to survive in 15Kg combat.

  3. Rhino MDD 20Amp 6V-30V Dual DC Motor Driver -- I don't know of any combat robots using this driver. Appears to be adequate to control the RS-385 motors, but not much more. You'll need an upgrade when you go to larger drive motors.
  4. FlySky CT6B 6ch 2.4GHz transmitter & receiver -- Upgrade to the FS-i6 The CT6B is a very basic and difficult to adjust R/C system. For very little more you can purchase the FlySky FS-i6 system which is much easier to program, has many more control options, and does not require the added expense of a programing cable.
  5. Astro Heavy Duty 75mm TPR Rubber Wheels -- These wheels will not attach to your motor output shafts without modification. They are designed to spin freely on an unpowerd shaft; the bearing must be removed and an adaptor hub fabricated to mate to your very small 6mm gearmotor shaft. That's a fair amount of work.
  6. 300W 20A DC-DC Step Down Voltage Regulator -- This is not needed A proper transmitter (like the FS-i6) can be programmed to provide reduced power to the drive motors via the motor controller. As your motor driver is rated for your full battery voltage you can use this trick. See this archived post for specific instructions using the FS-i6: you'll decrease the CH2 mix setting from 100% to 50% to get half-voltage.
  7. TecoKart Battery Cut Off Switch 12V/24V -- WAY bigger and heavier than you need, but it will work.
  8. 24V DC Motor 2500rpm -- You have spec'd all the other components, but all I get here is a voltage and an RPM. I can make no comment.
Do you require "braking functions" on your 24 volt brushed motor controller to meet some weapon spin-down time requirement? I have never heard of such a requirement in Bharat. As you have provided no motor specs I can make no specific ESC recommendations, but I will mention that if you use a bi-directional ESC you can GENTLY apply a small reverse current to create a braking force and decrease the spin-down time.
All Fall Down
Q: I am making a 150 g grabber robot that clamps onto other robots and pushes them into a pit, and since our competition mostly consist of zippy vertical spinners, i am using two 1200 rpm motors to catch up with them. To prevent the robot from tipping while driving fast, I have placed the battery near the front of the robot. However [I'm concerned that] when I drive the other robots into the pit, I also fall in due to the far-forward center of mass (I think). How can I optimize my robot for not falling into the pit? (My robot consists of a bent metal box with wheels and two metal forks.) [San Diego, California]

A: Mark J. The description of your robot is quite sparse. You say it is a grabber, but only mention two metal forks, a metal box, and an unspecified number of wheels. Is there a servo powered clamp? I'm imagining a design something like 'Jawbreaker' from 'Robotica' (pictured below). A few general comments:

  • You probably do not want or need to mount the battery in the front of the robot. See Where Does it Go?" two posts down this page for guidance on weight distribution to prevent nose lifting under acceleration.
  • If two robots go into a pit together, most competitions award a win to the robot that was in control when they went in. A two-wheeled clampbot will most often follow its victim into a pit.
  • A longer four-wheeled clampbot can have a center of mass far enough back to allow it to safely approach a pit and drop in an opponent without tipping itself in as well -- if that's what you actually need.
I'll need more information about your 'bot to give more specific advice.

Reply: Thank you for your feedback! Here is a sketch of my robot:

A: The render helps. Your design has very little weight on your drive wheels. When you add a good chunk of your opponent's weight onto the front forks you're going to have very little traction to push your opponent around and real difficulty in executing a turn. I would suggest smaller diameter wheels moved well forward so that an opponent clamped above them could add weight and improve traction. You might need to extend the forks upward to keep your trapped opponent away from the wheels if the new wheels extend too high. That should give you enough maneuvering control to set them right on the pit edge and drop them in.

I'd also add a rubber tip to the "grabber". Your motor is unlikely to have enough power to get that sharp point to dig in and hold.

Reply: According to SPARC, if the robots are stuck together when entering a pit they will be separated and taken back to the arena. If I am still clamping the robot on my dustpan will the count as us being stuck together?

A: There are two paragraphs in the SPARC Match Rules covering this situation. The first says whoever goes in first loses:

If the arena is equipped with a Death Zone/Pit/Push-out or similar hazard a robot entering this area in a one-on-one match will result in the end of the match and a loss for the robot that first entered the area.
It's just like a sumo match: whoever first touches outside the line loses and the match is immediately over. That is the common rule in robot combat and it makes universal sense. However, two paragraphs farther down in the rules is a paragraph that confuses the issue:
In the event that both robots enter the death zone simultaneously they will be returned to the combat area and the match will resume. A robot that places its opponent in the death zone must be able to do so without also becoming stuck itself. If it is not able to separate from the other robot this will be treated as simultaneous entry.
Different event organizers interpret this paragraph in different ways. Some interpret this rule as only covering cases where the 'bots are literally stuck together. Others will reset anytime both robots end up in the pit. It varies from one event to another so it pays to clarify this with the organizer of your specific event.
Precious Little Distance
Q: For those flipped xl belt designs now prevalent in the modern beetleweight clsss (w/the smooth side wrapped directly around the outrunner can as a slip clutch) how am I supposed to gauge the correct distance to get the belt to not slip off and also not be vulnerable to snapping? Like how undersized should the belt be? Say for example I'm in the 50T range, do I want my weapon to require a theoretical 53T belt distance and then buy a 50T belt so it stays on? Or a 51T belt distance?

Should the motor be mounted on oval screw holes such that it can be slid back and forth to determine the perfect tension? [Hidden in an iCloud] A: Mark J. I'm not a big fan of this approach to weapon belt drive.

Timing belts have effectively zero 'stretch' and brushless outrunner motors are not designed for lateral loading on their cans. This combination leaves precious little distance between "too loose" and "can deformation destroys motor". Adding or subtracting a single tooth changes the circumference of an XL belt by 0.20", which is far too coarse an adjustment for a friction-only belt drive.

The preferred method of getting the tension right is to gather the weapon components (hub/motor/belt) and mock up the system on your workbench. Snug it up, measure the distance between hub center and motor shaft center, and transfer that distance to your weapon mount CAD.

Yes, do use elongated holes for motor mounting to get your final adjustment after testing. I add hot-glue to fill in the elongated holes once the screw positions are set to prevent unexpected tension failure if the screws loosen.

Where Does it Go?
Q: You've said that a two wheeled robot should have 65% of the weight on the drive wheels. My CAD program gives me the location of the center of mass but where do I place the mass center to put the right amount of weight on the wheels? [West Sacramento, California]

A: Mark J. For best traction you want as much weight on the drive wheels as possible, but leaving too little weight on the front of your 'bot will allow it lift up off the arena floor under acceleration and make it vulnerable to attack. Designing for 65% of the robot weight on the drive wheels gives good traction and leaves enough front weight in most cases.

To place 65% of the weight on the rear wheels:
  • Measure the distance from the weight-bearing point of contact at the front of the robot back to the axles of the drive motors;
  • Move components as needed to place the mass center at a point 65% of the way rearward along that measured line -- see illustration above.
Note 1 - Robots with unusual layouts or odd dimensions may require different axle weights. Page 46 of the RioBotz Combat Tutorial has a discussion on calculating optimum placement of the center of mass based on traction and the height of the mass center.

Note 2 - Magnet downforce may be used to correct traction and lift problems in a steel-floored arena. Combined gravity and magnetic weight of 65% on the drive wheels will still be a good starting point.

The Tools You Know
Q: I'd like to get started in combat robots by building an antweight. I've asked about getting started on [social media site] and people there tell me to use a 3D printer. I don't have access to a 3D printer so is there another way to build a simple robot? [Southern Germany]

A: Mark J. Builders who regularly use a 3D printer get that method stuck in their head.

"If the only tool you have is a hammer, every problem looks like a nail."

 - Abraham Maslow, Psychologist

The above saying describes the tendency to rely too heavily on a familiar tool or method, even when it's not appropriate for the situation. It highlights a cognitive bias where people apply a single, known approach to various situations, potentially overlooking more effective alternatives.

3D printers have a large up-front cost in terms of both skills and expense. Once you have paid that price the printer becomes a quick and effective tool. But if you have not paid that price it is more effective to use techniques and skills with which you are comfortable. You can build a simple robot simply by gathering compatible components, bolting them down to a stiff baseplate, and adding protective structures and offensive capability as you see fit.

I'll also point out that there are combat robot kits -- with or without a chassis -- that allow you to quickly construct a functional combat robot you may later modify as you see fit.

Horizontals Stay Flat
Q: Hi - I'm building my first UK antweight robot, which is a giant 50g bar overhead spinner, with inspiration from robots icewave, moros and bloodsport. It's 2WD, with the brushless motor in between the two drive motors, and a screw in the back to put the blade at an angle. My problem is so: When testing the drive, it's completely fine, when testing the weapon, it's also fine. When I drive and spin the weapon, the robot is incredibly unbalanced, and pings around my test box like an air hockey game.

Could you give me some tips on how to make it balance better? (Also the weapon bar itself is balanced.)

Cheers! [Eaton, England] A: Mark J. That's a very pretty 'bot with a lovely spinner bar, but you've changed a critical design element from the robots you credit as inspirations. That change is causing your problem.

The spinner bars for 'Icewave', 'Moros', and 'Bloodsport' are all precisely horizontal -- the rotational axis of the weapon remains pointing straight up when the robot turns. You have elected to place "...a screw in the back to put the blade at an angle." That angle causes the direction the weapon axis points to change when the robot turns. Combined with the huge rotational inertia of your weapon this causes a strong gyroscopic reaction, which raises one side of the 'bot, which then causes additional changes in the weapon axis, which then... you get the idea. This is why you don't see angled horizontal spinners.

There is no fix other than tilting the spinner axis back upright and making sure it stays like that. 'Bloodsport' and 'Icewave' are four-wheeled chassis to help keep their blades 'flat' while 'Moros' is two-wheeled but has the center of mass well back toward the trailing rear skid for stability.

Yes, setting your bar fully horizontal raises the weapon up too high to be terribly useful. Call it a rookie mistake. Do not be tempted to 'droop' the impactor end of the bar -- that causes a different type of instability.

I Still See It Clearly
Q: Does any footage of your insectweight bots like 'Rat Amok' and 'Zpatula' exist at all? [Havertown, Pennsylvania]

A: Mark J. Our insects all fought before video was easy. The only footage runs in my head, and you don't want to go in there.

Until Its Hat Floats
Q: 'Ramfire 100' won [the heavyweight class at the] 1994 US Robot Wars, then fell off Earth as far as I know. Do you know if ANYTHING else happened to it? [Lehi, Utah]

A: Mark J. 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: Aaron's Madlibs.

Q: What about 'Chrome Fly'?

A: Oh, that's completely different! Embarrassed to learn that its direct-drive spinner weapons (that work pretty well in 1-pound robots) instantly self-destruct in the 250-pound class, 'Chrome Fly' heads home from the 2016 BattleBots tournament but makes a left turn off I-15 at Provo and drives west until its hat floats. It sank into the deepest part of the Great Salt Lake, which isn't all that deep. The end.

Resetting the Rat
Q: Has anyone tried to make a robot like 'Rat Amok' with a 'Snail Cam' (second of four designs on our Spring Flipper Designs page)? A rat trap with a near infinite number of resets sounds very effective.

 - sincerely, Iceywave [West of San Antionio]

A: Mark J. I'm quite proud of 'Rat Amok' -- she is a one-of-a-kind antweight that won her first tournament and was later victorious at the "King of Robotica" match between myself and season two champion Mike Konshak with a clean OOTA ejection. Her one-shot weapon struck fear into her opponents, and the suspense of "when will it fire?" added much to the drama of her matches.

A 'snail cam' mechanism is quite bulky due to the large spiral cam. It would be difficult to translate the linear motion of the cam follower into a 180 degree rat trap reset. I suspect this is why no one has attempted a snail cam version of 'Rat Amok' -- but your question got me thinking...

I just now took Rat Amok off the shelf and measured the torque required for a reset against the torsion springs. A full 180 degree reset requires an initial torque at the axle of 12 kg-cm that rises to 29 kg-cm at the end. That is well within the capacity of high-torque R/C servos that weigh about 2 ounces and have up to 270 degrees of motion.

Rather than a snail cam, I believe that the 'Servo Latch' (fourth of four designs on our Spring Flipper Designs page) would be more easily modified to reset the trap, latch it, and trigger the release. Might be fun!

Are Smaller Bots Weaker?
Q: I saw a discussion on [social media] where somebody said that multibots don't work out well because pound-for-pound the smaller robots don't have as much weapon power as larger robots. They said they didn't know the math but that's how it works out. I figure you know the math. Is there an explanation for this? [iCloud Private Relay]

A: Mark J. I'll guess that 'the math' referred to here is the the Square-Cube Law which points out that directly upscaling an optimized design requires disproportionally greater mass to be allotted to structural components, which leaves less mass available for things like weapons. But I think the poster has it backwards -- smaller robots require less proportional mass for structure which leaves more mass available for weaponry.

The chart below is from our Spinner Weapon FAQ. It shows typical spinner weapon motor weight as a percentage of robot weight for 1-pound thru 220-pound robots. A typical 60-pound lightweight weapon motor weighs about 30% greater per pound of robot than does a typical heavyweight weapon motor. However: If you're going to toss four 60-pound robots into an arena with a 250-pound robot, you'd best build those lightweights with thicker armor than they would need when facing robots of their own weight. I figure the added armor mass will just about negate the weapon weight bonus from the square-cube law.

There are good reasons multi-bots aren't generally effective, but a reduction in the total available weapon power of a multibot swarm compared to a single heavier robot is not one of them.

I Know a Trick
Q: I'm building a combat robot for a local competition, but it is very difficult for me to get suitable motors and electronics in my country. The only high currrent brushed motor controllers available to me are the BTS7960 H-bridge motor drivers. The BTS7950 accepts pulse width modulated signals for variable speed cotrol but I cannot figure out how to set up my FlySky FS-i6 radio to get both forward and reverse response from the motors. Can you help? [Location Withheld]

A: Mark J. The BTS7960 motor drivers are inexpensive and claim a continuous bi-directional output of 43 amps at up to 27 volts. They are a little bulky, but appealing for controlling brushed motors in intermediate weight combat robots.

The problem is that the BTS7960 is designed to be controlled by an Arduino microcontroller board rather than directly by R/C receiver output. There are seperate input pins for forward and reverse motor rotation, and each pin expects a full 0-100% duty cycle PWM signal. The FS-i6 just can't manage that and still provide single-stick control -- but I know a trick.

It just happens that the circuit board in an analog servo takes standard receiver output and splits it into the two PWM signals the BTS7960 is looking for. You'll need a couple old servos and a 5 volt power source:

  • Take apart an old servo and strip out the control board.
  • Connect the control board leads that went to the servo motor to input pins 1 and 2 of the BTS7960.
  • Provide +5 volts to pins 3, 4, and 7, with a ground connection to pin 8.
  • Plug the servo lead into the appropriate receiver port and your BTS7960 will respond just like a regular ESC.
Here is a video that shows the whole process: Brushed ESC Using Servo and BTS7960.

Below is my circuit diagram with an inset showing the details of the input pin wiring. An explanation of why this trick works gets technical. If you want the details write back -- if not just be glad it works.

Bench Warmer
Q: AYOH! It’s ya boi, CRAIGYBOT!!! I’ve been playing lots more baseball since we chatted. I scored 97 poggers and 58 assists! The coach was so impressed his face turned red and he pulverized his hat into the ground. I've now been promoted to "bench warmer"!! Robots use radiators, right? I figured I'd build one to warm the benches during the offseason.

Twelve Hours Later...

Great news!!! I've just got drafted to a team in Wisconsin! They don't have bench warmers so I won't be needing that radiator anymore.

- sorry not sorry, CRAIGYBOT [Not Wisconsin]

Top of the Charts
Q: Your site has dozens of combat robot webpages with a wide range of topics. Which are the most frequently viewed? Are the technical pages more popular or do the fan pages get the traffic? [Tampa, Florida]

A: Mark J. Our most popular pages vary a bit over time, but in a typical month these are the top five runamok.tech combat robot pages:

  1. Ask Aaron: Questions and Answers about Combat Robots
  2. FlySky FS-i6 Transmitter Programming for Combat Robots
  3. Ask Aaron: Combat Robot Design Tools
  4. Ask Aaron: Combat Robot Spinning Weapon FAQ
  5. Ask Aaron: Frequently Asked Questions
On any given day one of the fan-oriented pages like The Historic Tournament Trees or The Combat Robot Hall of Fame may get a mention on social media and rocket briefly to the top of the list, but the technical pages are reliably found at the top.
Two Versus One
Q: How is a control bot supposed to deal with a multibot?

My local competition recently added a multibot bonus mirroring the one found in NHRL, and I plan on competing with a control bot. Assuming competently designed, built and driven 'Bots on both sides, I can't think of a way to deal with them whether or not they are themselves control bots or have some sort of weapon. [Shore of the Mediterranean]

A: Mark J. I had no idea that there were combat robot events in your country. I found photos from the April event that showed a nice arena, good attendance, and some interesting competitors. Congratulations.

It kinda bites to be a control 'bot under the current rulesets. Not all design challenges have solutions:

  • How do you beat Paper with Rock?
  • How do you beat Rock with Scissors?
  • How do you beat Scissors with Paper?
I'm not a fan of the multibot weight bonus, but you fight under the rules as written. You could build twin control bots, or add a small wedgebot to cause chaos. You could pick up one bot and use it to beat up the other 'bots -- I once did that with a control 'bot in a hobbyweight rumble (full story). Be creative and have fun -- fun is as good as winning.
Cardboard and Imagination
Q: I work at a California Public Library and I want to create a program introducing children to combat robotics. I want to put the kids into teams and have them build and then fight their robots. I want to keep things safe for kids and cost effective with out compromising the coolness of the bots. How would you recommend card board vs PLA plastic for the frame and chasis? [Murrieta, California]

A: Mark J. I like the sound of this!

I'll assume we're talking about thin sheets of PLA to be cut/folded/assembled. While this could certainly work I'm personally very fond of cardboard as a construction material.

  • Easy to cut/trim with scissors or rotary cutters;
  • Accepts pen/pencil markings without smudging;
  • Bonds well and rapidly with non-toxic adhesives;
  • Quickly laminates for increased strength and stiffness; and
  • Can be recycled at the end of the day.
Throw in some popsicle sticks and a few colorful markers/stickers/googly-eyes and you have a great activity. Send me some photos? You might get a few ideas from this video: Build a Cereal Box Combat Robot!
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.

 
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.


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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