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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.
Tossing the Caber
Q: I am designing an antweight (1lb) brushless lifter that I would like to be able to throw opponents (not into the ceiling or anything). I am planning to use a Flash Hobby 1204 2500Kv motor, connected to an 135:1 planetary gearbox. The lifting arm is 6.22in long, and the robot will be powered by an 11.4V LiHV battery. At 211.11 rpm, do you think that this would be too fast? Would it provide enough torque to not be too slow under load? (The manufacturer does not give stall torque.) Also, I know that brushless motor torque depends on the ESC, so I am planning to use AM32. Would you say that this is necessary? Thanks!
A: [Mark J.] The unsensored brushless motors and ESCs we use to build small robots are borrowed from the model aircraft hobby industry. They are designed to spin-up lightweight propellers to about 80% of their free RPM and run at that speed for several minutes. That task is far different from being bogged down against a heavy load right at zero RPM and being asked to develop enough torque to accelerate that load upward against gravity fast enough to toss it into the air. It's like asking a chicken to toss the caber -- they aren't made for it.
Add to this handicap your choice of a brushless motor that weighs 6 grams 0.21 ounce and puts out about 50 peak watts of power maybe 10 watts at low RPM on a 4S lipo and you're running 3S. A typical antweight brushless spinner weapon motor that has a couple seconds to come up to speed against its load weighs perhaps 30 grams and pumps out more than 200 watts of power (see Brushless Motor Selection). With that tiny motor you're not going to be tossing opponents into the air, and the strain of moderately quick lifts may drive the motor into thermal failure.
As noted in FAQ #17 Ask Aaron is not a free engineering service - I'm not going to model the performance of your weapon for you. However, we do supply design tools like the Team Run Amok Electric Hammer Spreadsheet that can be used to model flipper designs. A lifter/flipper is effectively an overhead hammer with a very heavy hammer head (your opponent's 'bot) that traverses a small arc.
Walking Distance
Q: Will you be attending any competitions in the next few months? (Of interest specifically to me is Robogames.) And if you've been to Robogames in the past whether you have any recommendations of places to check out while in San Jose. [Ohio-ish]
A: [Mark J.] There are no events on our calendar, but Team Run Amok has spent some time in San Jose.
Mathematically...
Q: mathematically (not realistically, I do not want to know if it is realistic or not, just want to know your take) what is the best combat robot design for a combat robot, with a combat robot being 1, 100, or 250 lbs, and best is the greatest advantage of the following:
A: [Mark J.] Mathematically:
The Center Isn't Off
Q: I am trying to make a combat robot for the first time, and with the right joystick in the center resting position it reads as half power and turns the motor on. I have looked across the internet on how to make the resting position read as off and if I go down the motor reverses and forward it goes forward. I’m using the fs-i6s transmitter and a TOYTENSI 20A esc. [Social Media]
A: [Mark J.] Your radio is not the problem - the problem is your ESC.
All the TOYTENSI ESCs I can find are for model airplanes. An airplane ESC is uni-directional with the zero-throttle stick position all the way down. Some aircraft ESCs can be switched to forward/reverse response with center off, but the functions available on the TOYTENSI programming card do not include this option. You need a different ESC.
To keep things simple search for an ESC that specifies 'bidirectional' or 'forward/reverse', like the Repeat Robotics Brushless Drive ESC. ESCs that say 'thrust reversing' are different and will not give you the stick response you're seeking.
Just Point the Front Wheels
Q: With car-steering being obsolete in combat robotics for the past two decades, would you say there are any advantages to using it (as opposed to tank steering) in modern times?
While we're on the subject, was Run Away! the last combat robot to use car-steering?
A: [Mark J.] Realtively few combat robots have used automotive-style 'pivot steering', but many fans are surprised to hear that Combat Robot Hall of Fame members 'KillerHurtz' and 'The Judge v1.0' had steered front wheels. Stability in a straight-line and the ability to hold a smooth turn radius remain good reasons to consider the design, although the development of solid-state gyro controllers has brought similar control to skid-steer 'bots.
Anywhere But There
Q: I am part of a team that is brand new to combat robotics. We just completed and tested our very first robot and are ready to fight in our first tournament. Got any tips on how to find the right competition for our skill level? Thanks.
A: [Mark J.] The typical order of events to get into combat robotics is:
The IP address attached to your question submittal has ties to servers in Maryland, California, Florida, New Jersey, and maybe other states -- so I can't provide regional advice on beginner friendly events. You aren't likely to find more than a couple of events within reasonable traveling distance, so your list of options will not be long. There are two websites that list upcoming events in North America:
Plastic Traction
Q: dont know if you have any experience in the plastic ant category, but two things
first, what would be the best infill for high-impact hits with PLA plus secondly, what would be your thoughts on our current design, and issues that you can spot (yes I know it needs a top-plate, and there are no screws) Ive had problems in the past with traction, so that's mainly what I'm looking for. [Close to Raleigh, NC]
A: [Mark J.] It's a very pretty design, isn't it.
First question: I claim no specific knowledge in 3D printing PLA, but I do know that more infill equals more strength. How much strength you need depends on your design and expected impact forces. There are two groups on Facebook that might provide more useful responses:
Take a look at successful two-wheeled insect spinnerbots. They have the motors as far forward in the chassis as reasonably possible, with battery and electronics hanging out behind to shift the 'bot center of gravity closer to the wheels. Unfortunately, unless you switch to much larger wheels you don't have room for much rear overhang. This is a serious flaw.
The rule-of thumb for two-wheel 'bots is to put ~65% of the robot weight on the wheels. More than that may let the 'bot raise the nose off the floor under acceleration -- see Section 2.2.7 of the RioBotz Combot Tutorial. Given the wheel placement on your design I'm guessing that less than 50% of the weight is on the wheels, which will give both poor traction and poor turning response. No amount of latex or silicon rubber coating on the tires is gonna help.
Q: this was our earlier design, and it had the most abysmal center of gravity possible, which was over the forks, we are currently using 2.25 inch wheels, what do you thing would be the best size option?
A: You're not trying to tell me that the little circle marker on the current design render is the calculated center of gravity, are you? I'm entirely not buying that. Nope. No way.
From a practical standpoint, I'm really not sure your design is salvageable. That very large and heavy spinner hangs so far out in front that it's going to take a lot of mass out behind the wheels to make the drive work:
As I said at the start, it's a pretty design -- I just don't think it's practical.
Q: for the forks, would having what basically is a plastic popsicle stick going perpendicular to the forks help us with that issue [of being stuck on its side]?
A: I'm sure you have an image in your mind as to where that stick would be placed and how long it might be, but my psychic abilities are quite limited these days.
Hitting a Wedgeless Brick
Q: My team just competed at NHRL over the weekend and we have a question about the results of our hits. In the attached clip we hit 'Beast' twice with the big beater bar on 'ARES' only to spin them backwards. What’s the most probable reason for ARES recoiling backwards instead of launching Beast? Could this be a result of rake angle (0 degrees), too much bite, or something else? [Cambridge, Massachusetts]
A: [Mark J.] It is my considered opinion that this is a clear case of something else. Here's the full 3-second clip: ARES and Beast at NHRL.
Both of the hits in the clip have your beater bar impacting hard vertical surfaces on the front corners of the wedgeless brick Beast. Nothing to grab, and no downward impact vector to press down onto your opponent as a multiplier for the poor steel-on-steel coefficient of friction. The hits are just gonna slide up the steel wall and tap the brick and ARES apart.
Things that would help:
Worth the Trouble?
Q: I was desoldering a FingerTech Silver Spark and the little metal tab that I solder to the wire to broke off. I tried adding some solder to the remaining visible metal but its not sticking well. Is there anything I can do to save it or is the motor just screwed?
A: [Mark J.] The photo shows what the inside of the Silver Spark motor 'endbell' looks like. The nylon that the metal tab disappears into goes down a fair distance and becomes the support for a motor brush. If you're REALLY determined to save the motor, you can carve away some of the nylon to reveal more of the brass tab, then use a good quality soldering flux (I use Rubyfluid Soldering Paste) to tin the exposed brass. It will then be easy to attach the power wire.
If your soldering skills are not good, I'd suggest buying a new Silver Spark and saving the gearbox as a spare.
Fifty-Six Megabytes
Q: I had a video planned to celebrate Run Amok's championship anniversary, but it said "only 8 megabytes may be sent at a time". The video file is 56 megabytes, and I can’t decrease the file size to the desired amount without a severe decrease in quality that I’m not willing to sacrifice. Hopefully I can post it to YouTube sometime.
Happy Anniversary! -sincerely, Iceywave ☆ A: [Mark J.] Thanks, Icey! We can't let a few megabytes come between friends. Let's try this: There, that wasn't so hard. Let's see what we've got here... Whoa, thanks, Icey! I've embedded your video recreation of the Robotica 'Fight to the Finish' below, and tossed a copy onto my YouTube channel.
Reply: I'm happy to hear you enjoyed my video! Turns out the file I sent was actually 59 megabytes. The original recording was 56 mbs, but I had to put it through some editing software to add fade in/fade out on the video. The models of Juggerbot and Ram Force were built by myself, in case you were wondering. I also have a big surprise coming April 1st, so be prepared! Bang Bang Bang Thud - Again
Q: Hi there, I've been playing around with the hammer calculator v2.2 spreadsheet and believe I've found two errors in the methodology that throw off the results by a very large amount:
Since this is a Q/A, any errors in my math/methodology? Cheers, Joseph Duchesne
A: [Mark J.] I appreciate your effort to "keep me honest" by digging through the code in my Hammer Spreadsheet, Joseph. Few people make that effort.
Given that I authored the spreadsheet eight years ago, my working memory of the calculation details is not terribly good. I would appreciate corrections to my comments below should your recent review prove them to be in error.
Flavor #1: Multiply the weight of of an object in newtons by the height the object is lifted to get the unit of work: a newton‑meter (N‑m). In this context 1 N‑m is a measure of energy equal to 1 joule.
Flavor #2: When measuring torque, a force of 1 newton applied at a radial distance of 1 meter equals a newton-meter (N‑m). In this context a N‑m is a measure of force rather than energy. Equating this flavor of N‑m to joules is fallacious.
We could resolve any confusion on this issue by building a miniature "Test Your Strength" carnival machine for a robot hammer weapon to strike. The height reached by the known weight could be quickly translated into a measured value for the hammer energy. I'm betting on my calculations.
Now please pardon me, I must take my medication and lay down for a bit. Q: Thanks for pointing me in the right direction with regard to torque vs. energy. I got tripped up since most times when the units check out, the physics does too. The missing part was that the actual unit of torque is best described as Joules/radian, but since radians are unitless, this misleadingly appears to just be Joules. A: It's a trap. Using the same units for two different measures will cause reasonable minds to make the same assumption you made. The usage really should be clarified and/or replaced. Q: Intuitively, I still feel like there would be a slightly harder hit if the motor torque is applied beyond impact, vs. becoming disconnected the instant before impact, but this is likely a very small fraction of the additional system energy I had been hoping for.
A: Yes, small.
A: You're killing me, Joseph. Why would I have written that nice column of equations and then failed to spread them across the full swing? Now I have to dig thru the code of an eight-year-old spreadsheet to look for the place where I dropped that thread...
I woke up this morning with a clear head, determined to sort out the evolution of my Hammer Spreadsheet.
The link to download the most current version of the Hammer Spreadsheet is always on the Ask Aaron Combat Robot Design Tools page -- but I'll save you a page jump:
Unspecified
Q: I know what I said about not being back until April, but I just remembered something. I heard a rumor that a specified team was asked to leave a specified event because they committed a specified offense. Is that true?
-sincerely, Iceywave☆
A: [Mark J.] I have removed the names of the team, event, and offense from your question. If I can neither confirm or quash a rumor of this nature from personal knowledge I don't care to spread it. Here is what I can tell you, based on reliable sources:
Just Keep Fighting
Q: Hi, when building battle bots which temperature sensors/ impact sensors do you recommend using? [Parts Unknown]
A: [Mark J.] It is the nature of combat robotics to push components well beyond their design limits.
Crank Up the Noise!
Q: Hi Mark, it’s Iceywave here. I wanted to do something interesting this time around, so here’s the very first ever Ask Aaron audio question!
-see you April 1st, Iceywave☆ A: [Mark J.] For those who prefer to read:
Wow, our first audio question and our first 1.4 megabyte commercial for somebody's YouTube channel! It seems a shame to answer your early April Fool question in regular text, so I went with Morse code:
Gotta Be Tight
Q: Hello, I am building a 1lb undercutter bot based on Jameson Go's 'Silent Spring/DDT' build, but am not sure how to securely mount the weapon to the bottom. I am currently planning on using a pulley driven disc on a dead shaft, with bearings in a printed hub/pulley attached to the disc. Do you think a long nut/coupling hex rod in the chassis above the blade with a nylock nut on the end would support the weapon shaft? or what would be a more robust way to make sure the weapon shaft does not go anywhere? I don't want the blade to bend upwards and chop my weapon belt. Attached is a mock up of the basic structure. Thanks! I wouldn't trust a printed hub holding both bearings to take the loads this style of weapon will put on it. 'Silent Spring' has the top bearing in an aluminum pulley and the lower bearing in the weapon disk itself. How are you mounting the weapon motor? Your mock-up appears to have the spinning can pressed into the chassis plates and the motor base sticking out unsupported. Can I assume that's just a rendering error?
Q: On the topic of the latest post of a Silent Spring inspired bot, I can say that it can work out well even with a printed pulley since that is exactly what I did for my 1lb Shock! kits [GrabCad]. The big thing I did was adding a pair of concentric washers between the bearings to take the compressive loads and help axially retain the pulley and disk in place. [Newark, Delaware]
A: [Mark J.] Thank you, Ryan. Your 'Shock!' weapon design is well thought out with the lower bearing in the weapon disk itself and the upper bearing in the printed pulley. In their question, Redmond spoke of a design with "bearings in a printed hub/pulley attached to the disc" which led to my comment that I wouldn't trust a printed hub holding both bearings. I think you might agree that placing both bearings in the printed pulley/hub would be risky.
Q: Hello, Redmond undercutter guy again. Good call on the bearing spacing, I will implement one into the disc and one into the pulley rather than both in the pulley. The weapon motor will be mounted similar to 'Silent spring', but I have not designed a housing for it yet. However, a few more questions about the shaft mounting:
1. Do you know how he mounts the shaft so securely to the frame? It looks like just a hole in the print with a nut on top, but wouldn't the print deform/break in big hits? Ive been looking around at various undercutter designs but can't seem to find exactly how they keep the shaft from moving or bending at all.
A: The mount is exactly as it appears. Take a look at the assembly instructions for the Shock! kit. Steps 9 and 10 cover weapon assembly. When assembled and torqued down the shoulder bolt turns into a structural element that locks the two chassis plates together. The Shock! chassis plates are cut from high-strength carbon fiber composite material.
'Silent Spring' has a thick, single-plate chassis printed from NylonG (early versions) or NinjaTek Cheetah TPU. Printed chassis plates do flex - it helps to absorb the energy of the hit - but they snap back into place (you hope). Design note: wider spaced chassis plates make a stronger base for an undercutter.
2. How thick of a bolt do you think is safe enough to assume won't break? Thanks! A: The Shock! kit used this 1/4" shoulder bolt. Please note that a shoulder bolt (about $8) is hardened alloy steel and is MUCH STRONGER than a nasty hardware department bolt made to mount the horn on your bicycle handlebars. Do not go cheap on this critical part. El Escarabajo Vibrante
Pregunta: ¡Hola!Mi girador vertical de 3 libras tiembla violentamente cuando el arma gira. Ya leí "¡La hamburguesa es mala!", asì que aquì te dejo una foto para ilustrar mejor mi problema. ¡Àdios!
Respuesta: [Mark J.] He examinado cuidadosamente tu fotografìa y no veo nada malo. Quita las ruedas y la Liga Nacional de Robots Havoc te darà una bonificaciòn de peso.
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:
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