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6347 Questions and Answers about Combat Robotics
from Team Run Amok

Team Run Amok receives a lot of email asking about the design and operation of 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.

    In Memoriam: 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   
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 'Got Question?'
The Ask Aaron Archives Click to browse thousands of previously answered questions by category, or search for specific topics. Includes FAQ
 
Caution
Even small combat robots can be dangerous! Learn proper construction and safety techniques before attempting to build and operate a combat robot.

Calculator Update

A builder in one of the combat robot groups asked how long it had been since the motor and battery lists for our version of the Team Tentacle Torque/Amp Calculator had been updated. I couldn't recall offhand, so I set aside some time for updates.

I've added popular new motors, patched in the latest upgraded motor performance figures, and caught one minor error. Listing all the possible combinations of 'can' motors and their matching BaneBots gearbox options was redundant and cluttering, so I added a new option to popup a window displaying currently available BaneBots P-60 gearbox ratios. Now you select your favorite BaneBots motor and fill in the gear ratio from the list. Much cleaner.

I also updated the 'battery chooser' by porting the extensive 'SPARC Tools' data into our calculator. There are several minor format and function tweeks as well. Let me know if you find something I broke with the update.


Q: I'm very curious as to what actually goes into hits as far as transferring energy into the other team's chassis without self destructing yourself. I have been playing with vertical spinners for the last few years and I still have alot to learn with this.

When I first joined [the team] our director essentially told us that a higher moment of inertia is the key to the kingdom as far as making a weapon that hits hard. In the past few years I've played with making high moment of inertia weapons to try to hit hard but never quite getting the results I want. I only really seem to hit hard and 'out' and send way too much shock back into my own chassis and end up bending my weapon shaft and chassis considerably. I've seen several bots with tiny weapons like 'Knockout' and 'Disko' hit ridiculously hard and almost directly vertical. As far as throwing 'up' goes, they tend to roof the other teams quite often and never seem to wreck themselves too much.

Is there something simple I'm missing? Is there a way to graph bite, RPM, and moment of inertia to find the optimal setup? Is there any help with this in general? [Warren, Pennsylvania]

A: [Mark J.] We've discussed the elements of your questions previously, but they're scattered broadly around the 'Design and Construction' and 'Robot Weapons' archives. Let's see if I can pull those prior posts together to get your answer in one place.

Newton's third law of motion states that for every action, there is an equal and opposite reaction. This means that when your kinetic energy weapon strikes your opponent the impact on your weapon and its support structure is as great as the impact on your opponent. There isn't any escaping this 'kickback' force: your weapon has to be strong enough to withstand this impact. The advantage you have is that you know the place and direction of this force, while your opponent may have to take this force pretty much anywhere. If you're bending your weapon and shaft they simply aren't strong enough.

Hitting 'up' versus 'out' is a function of where in the weapon arc it strikes your opponent. A large diameter vertical weapon will tend to strike the opponent low in the upsweep zone and will hit more 'out' than 'up. This is why you see so many vertical spinners with a wedge/fork ramp leading up to the weapon. By elevating your opponent as they approach your weapon you will get more of an 'up' angle on your hit and as a bonus will often get greatly improved 'bite' by exposing a beautiful sharp edge at the underside edge of your opponent.

There is no magic equation to balance weapon speed, bite, and MoI because the optimal balance of RPM and MoI is entirely situational.

  • Against an opponent with nice sharp edges for your weapon to dig into you will want high weapon speed to build maximum energy storage because the importance of 'bite' and MoI are greatly reduced.
  • Conversely, facing a carefully smoothed opponent that presents only gently curved surfaces (like any of Russ Barrows' "Dark" series of robots) will require maximum 'bite' and a high MoI to compensate for the reduced weapon speed.
If you aren't getting good hits you'll need to examine what it is about your opponents that is preventing your attack from being successful. Adjust your weapon and attack strategy to adapt to their design. Remember, a faster closing rate on your opponent increases 'bite', so don't be timid in your charge toward them.

If you haven't read thru our Spinner Weapon FAQ you will be interested in doing so. There are some more detailed expansions on the topics I've mentioned above. You may also wish to read thru section 6.5.2 of the RioBotz Combat Tutorial.


Q: So I was wondering how they make flames shoot out huge bursts of flame like Shamen did but there would not be a lot of room for lots of flame? ty! [Lynn, Massachusetts]

A: [Mark J.] See Frequently Asked Questions #28.


Q: How quick is a featherweight running 720rpm motors with 5 inch wheels? I'm scared that the front end is gonna lift off the ground when I accelerate. How do I balance it out? Is it better to run 4 inch wheels? [On-line Forum]

A: [Mark J.] Torque determines acceleration but you've provided only RPM; I can't tell you 'how quick' your robot might be. Smaller wheels will increase acceleration, not reduce.

Placement of the center of gravity relative to the drive wheels determines how much acceleration you can have without lifting the nose of the 'bot. There is a complete explanation of acceleration and lift - with an illustration - on page 46 of the RioBotz Combat Tutorial.


Q: I was watching the new season of Battlebots and I noticed something when I stumbled across an internal image of the competitor "DUCK!": It seems that it uses universal joints instead of a standard axle? Why?! What would these joints allow that a standard ol' stiff axle doesn't? Aren't there a number of drawbacks with these joints? [Champaign, Illinois]

A: [Mark J.] When you see a Hal Rucker robot with some unusual component that you don't understand, the correct question is not "Why is he doing that?" The correct question is 'Why am I not doing that?"

The metal cylinders with olive-green inserts that sit between the gearboxes and axle shafts on 'DUCK!' are not universal joints. They are flexible plumb shaft couplers, consisting of pairs of multi-jawed hubs with a squishy synthetic rubber 'spider' sandwiched between them. The couplers serve multiple functions:

  • Joining relatively small gearbox shafts to much larger axle shafts;
  • Accomodating minor axial, radial and angular misalignment; and
  • Absorbing shock loads before they are transmitted to the gearbox.
Hal has each of the large diameter drive axles fully supported by dual bearings and the gearbox firmly mounted in the chassis. Any minor tweek in the chassis would place huge side-loading on both the axle and gearbox shaft unless some accomodation was present in the drive line. The isolation of the gearbox from transmitted shock loading has more obvious benefits.

'DUCK!' had a rear wheel sheared off by a devistating weapon strike in its BattleBots debut, but I'm willing to bet that the associated gearbox lived to fight another day. Hal Rucker knows what he's doing -- pay close attention.


Q: Looking at the previous questions and answers, what are your thoughts on bots like Brutality in the modern era Mark? Recently just watched a match of it with Last Rites back in the late 2000s, the wedge seemed to have made it under Ray quite easily. [San Jose, California]

A: [Mark J.] Bot's like 'Brutality'... meaning top bar spinners? Top spinners (like tradional bar spinners and full-body spinners) are a design that is very effective if you get everything right. People remember the successes of specific examples of these designs ('Hazard', 'Last Rites', 'Ziggo'...) but overlook the myriad copies that failed miserably. None of these designs are as simple to get right as they appear.

More specifically to your question, I think the 2007 ComBots Cup fight between 'Last Rites' and 'Brutality' marked a turning point in weapon design. It became obvious that long and relatively thin weapon blades like Brutality's were subject to breakage from the 'modern era' hyper-energy thick and robust weaponry; compact fast-spinning single-toothed drums/drumettes like 'Touro' and big bars like Last Rites/Tombstone had a strong advantage.

If you specifically want to beat 'Last Rites/Tombstone' you would do well to take note of the design and tactics of the robot with the best record against Ray's big spinners: 'Original Sin' has won 6 of its last 7 matches vs. 'Last Rites' and is 12 of 19 overall against the big bar spinner. Simple robots win.


Q: So I saw that some robots have tank tracks so I was wondering what were the pros and cons of tracks vs wheels? Tysm! [M] [Lynn, Massachusetts]

A: [Mark J.] That's simple...

Pros They look cool.

Cons Everything else.

They're heavy, vulnerable to attack, complex, unreliable, reduce turning agility, and most importantly have no traction advantage on the smooth hard surfaces where combat robots fight. 'Bite Force' had experimental magnetic tracks at the 2015 ABC BattleBots tournament where the 'look' was important in getting accepted to compete on the show, but they were more trouble than they were worth and were replaced with wheels for the 2016 season. If you need a robot to fight on sand or in mud, consider treads -- otherwise avoid them.


Q: Is there any robotic design that has failed execution-wise that you would ever want to see come back because of it's concept? If so, what would it be? [Isenburg, Germany]

A: [Mark J.] It's been nearly 24 years since the first Robot Wars, and some of the most creative minds on the planet have spent that time designing, evaluating, and refining the state of the art in robotic combat. It would be incredibly egotistical of me to suggest that they missed something, or neglected to recognize a promising solution that was tried and failed because of execution. Yep, that would be boastful vanity...


Q: Hello Mark.

Our team is currently designing a heavyweight bot with a Hazard/Brutality style overhead bar spinner. The bars will likely be between 75-90 lbs ranging from different materials depending on matches.

Right now we are looking to drive the weapon with four 3" long mags, or four 3" AmpFlow a28-400 motors, with Whyachi M3R2 right-angle gearboxes joining the two motors and two belt/chain drives to join the two pairs. This was designed so we could keep our bot chassis rather low to the ground for an overhead spinner, hopefully keeping most components in a 4-5" height constraint excluding the bar itself, instead of using the big and bulky ETEKs.

Are there any disadvantages or advantages to this setup other than parts costs? We have heard from builders that describe Ampflow motors as inconsistent and unreliable.

Are there any better alternatives to this? Two or three LEM-130s perhaps?

And are there any sufficient ways to synchronize the motors for these kind of setups? [New York, New York]

A: [Mark J.] Have you noticed that Team Whyachi isn't using their own gearboxes to power their weapons? Take a look at this recent post in the Ask Aaron weapons archive about the 2016 version of 'Son of Whyachi'. Their weapon drive solution meets your design goals with much less weight, complexity, and expense. Team Whyachi has been running a common spur gear weapon drive across multiple design revisions with good results.

AmpFlows/Magmotors become unreliable when loaded heavily for prolonged periods or overvolted on robot drive trains or lifter weapons. Their brushes are not up to sustained operation at very high amperage. They are quite reliable when running at nominal voltage with reasonable gear reduction in a spinner weapon.

All the 'cool kids' are running brushless weapon motors, but that's a high risk power solution. I consider brushed motors to be a very viable weapons drive option in heavy robots. If SoW can spin their huge weapon with eight Mini Mags I suspect that you could get away nicely with a bit less power -- like a couple LEM-130s.

I know it's a bit counterintuitive, but you really don't need to worry about motor synchronization with multiple similar motors driving a common load. The math is more than I want to write out on a nice spring day like this, but buy me a beer the next time I'm in New York and I'll explain. In the mean time, don't sweat it.


Comment: We can always count on you, even if we don't want to hear the truth because it might prove our ludicrous designs are just that! [Internet Forum]

Reply:


Q: if team hurtz made a featherweight would john ried have to wait twise as long or half as long for a good hit [Southampton, England]

A: [Mark J.] Previously answered. See this archived post about knowing when to eat lunch. If you take cranberries and stew them like applesauce they taste much more like prunes than rhubarb does.

Q: 24 hours later why are wedge flippers like Eruption not popular in the US but so popular in the UK

A: Previously answered. See this archived post for a clean explanation.

Q: 4 hours later in your opinion who is the best active robot driver/driving team

A: I have too many friends (and enemies) in the sport to start an international flame war with a direct answer to your question. I'm going to side-step...

If the object of combat robotics is winning, then whoever won the most recent major competition is the best active team.

Q: What happened of the whyachi-killer [power switch]? [Quebec, Canada]

A: [Mark J.] I checked with Will Bales about this. He says...

'Whyachi Killer' fought a marketing battle against Team Whyachi and lost due to manufacturing problems and a possible rules violation. Then the CAD design files were lost. Depressed, 'Whyachi Killer' made its way to the center of the Pensacola Bay Bridge and leapt off - only to land on the deck of a trawler bound for the Keys. After paddling a board into Islamorada, 'Whyachi Killer' found a job controlling a ceiling fan in a Conch Bar and saved every penny until it had enough money to open a small Margarita stand on the beach. It married a lovely girl named Mandi and had four children: a girl, two boys, and a rotary dimmer. The family is doing well.

Q: Can I Use A Nuclear Powered battery in a 12 pound combat robot? If so what do you recommend. I'm looking to get MAXIMUM POWER out of my vertical spinner. Thanks :) [Mount Laurel, New Jersey]

A: [Mark J.] I'd recommend that you work on keeping the head on your flaming hammer, Astronaut.

#leastpopularbattlebotstoy

Q: why do small diameter wedges commonly called ‘forks’ or wedgelets get under big plow blades & drum spinners? [Roseville, California]

A: [Mark J.] Combat arenas are not very smooth surfaces. A wide blade will set up on top of the highest part of the arena floor it rests on. Think of a straight edge resting on an irregular surface -- there will be gaps underneath most of the length of the edge.

A hinged narrow wedgelete approaching that broad wedge will stand a good chance of riding in one of those low gap areas and sliding under. At worst it can be no higher.


Q: Hey, Mark! Just had a quick question: I noticed a lot of eggbeaters don't have leading wedges and instead use flat fronts, such as 'Bigly' and 'Conker 3'. Is there an advantage to having a flat front on an eggbeater? Are there any drawbacks? [Champaign, Illinois]

A: [Mark J.] Take a closer look. Those 'flat-front' beaters actually slope back underneath -- kind of a 'negative wedge'. It is deliberate, and it's done to take advantage of the huge 'bite' available from a large diameter beater bar.

  • A fast-spinning mini-disc with small 'bite' has to play the 'lowest wedge' game to get under their opponent and find an edge to grab.
  • A beater can store the same level of energy as a disc of the same mass while spinning much slower. That gives it enough bite to be effective against flat surfaces.
The attack plan is to ride up the opponent's wedge and beat the stuffing out of whatever it finds up there. If there is no wedge, just hit whatever you run into. It often works quite well.

Disadvantage Going weapon-to-weapon against a smaller diameter drum with a faster tip speed is pretty much instant death.


Q: So I heard "Wrecks" uses his weapon to power his legs. How does this work? Ty! :) [M] [Lynn. Massachusetts]

A: [Mark J.] You don't have that quite right. 'Wrecks' is a 'precessional walker' -- its legs aren't powered at all, but the weapon still makes it walk. See this post in the Ask Aaron Design archive for a description of the principle.

Using the Ask Aaron search box would have given you a quicker answer than writing in and waiting for my reply.


Q: Hi once again. It´s the antweight 'Geronimo' guy once again with a few questions. I have purchased the lifter kit, and I am considering also getting the uppercutter spinner kit. To prevent the motor shaft from braking, I will be running a saw on top of the wedge that sticks out just enough to make contact when pinning robots to act as a nibbler rather than a kinetic energy spinner. What advice can you give me on making these weapons easy, and fast to swap out? [Boxborough, Massachusetts]

A: [Mark J.] The weapon kits from FingerTech are not designed for quick swap-out and would be difficult to convert. Ideally, interchangeable weapons should be assembled as complete modules that slide into slots or channels in the chassis and are then secured by a minimal number of fasteners. See the modular weapons used by Chaos Corps' Bombshell as an excellent example.

Q: Taking your advice about not having an active weapon, I´m considering only using these weapons in non-competition matches. Either way, the weapons will just be regarded as optional extras, and all of my focus will go into robot with the weapons being lower of a priority. I plan to have a variety of configurations to best suit the opponent that I am fighting. I have an anti-spinner configuration with all aluminum armour, and with a wedge similar to the D2 kit wedge/Algos wedge.

A: If you're going to upgrade your wedge I'd suggest upgrading the material as well as the design. Even the aircraft grades of aluminum are too 'soft' to fully qualify as 'anti-spinner'. The 'D2' and 'Algos' wedges are made from thick titanium alloy for very good reasons. It's a better purchase than another active weapon.

Q: In addition, the lifter servo is giving me problems with its range of motion being too big. The servo tries to push the lifter against the top of the wedge. Could a servo saver be useful in allowing room for error?

A: Servo savers protect against momentary shock loads, which is not what you have here. Prolonged straining against the servo saver spring will pull a lot of current and shorten servo life. Be nice to your servo and fix the problem correctly. Two options:

  • Mechanical The arm on the servo that drives the lifter is on a splined shaft. Remove the screw holding it in place, wiggle the arm free, and rotate it one spline farther 'up' before replacing. Limited precision, but it may be just what's needed if the problem is all with the 'down' location.
  • Electronic Your R/C Transmitter has a function to adjust the range of servo motion that can be set to precisely position the servo at each end of travel. If you're using the FS-CT6A radio from Fingertech, see Section 14 of the radio manual for instructions on setting the servo 'Endpoint'. Servo travel adjustment instructions for other radios may be listed in their manuals under 'Adjustable Throttle Volume' or 'Travel Adjust'.
Adjusting the function settings on the CT6A transmitter is awkward; it requires a special cable to connect the transmitter to a PC running software downloaded from the FingerTech website. More advanced radios have built-in display panels and function menus that make performance adjustments quick and easy without a cable or PC. Next time spend an extra $15 for a better transmitter.

For more info on transmitter functions and their use in combat robots see Radio Control Systems for Combat Robots.


Q: What weapon esc would be appropriate for a a28 150 ampflow motor running an ar400 8 inch diameter 2 inch thick disc at 24 volts with a 2:1 chain reduction? [Philadelphia, Pennsylvania]

A: [Mark J.] Calculating ESC requirements for a spinner weapon is much different than finding a suitable drive train ESC. There is a strong surge of current at startup that falls away to a low maintenence current level. Attempts to use current limiting ESCs or adding in a 'servo slower' to the signal line can severely reduce spin-up performance, so you should seek out an ESC with a high surge capacity.

The first step is to model the performance of the weapon system to find out how quickly the start-up current surge lasts. The A28-150 AmpFlow has a stated armature resistance of 64 mΩ, which would give a stall current of 375 amps -- but there is additional resistance in the battery/wiring/ESC. Adding in 20 mΩ (my default 'guess' factor) for those other circuit elements brings the real-world 'stall' current for the A28-150 to about 285 amps @ 24 volts.

Plugging 84 mΩ into the Run Amok Excel Spinner Spreadsheet along with the other motor and weapon details (thanks for providing those) gives a chart of spin-up performance and the current draw. It looks like you've done your homework and you have a good balance of energy storage and spin-up time. Current draw drops from 285 amps at hard start to about 100 amps at the one-second mark.

There's a good ESC match for this level of current and voltage. The Robot Power Vyper has a 120 amp continuous rating and a 'greater than' 250 amp peak current rating. It's good to 36 volts, and has over temp protection. Size is 3.125" x 2.875" x 1.375" with a 185 gram weight. I think that's your winner.

The only potential problem I see is possible stalling of your weapon. Replacing your chain drive with a very slightly slippy v-belt might prevent a full-scale meltdown if your weapon stalls in combat and you're too busy to tend to that immediately.

There are other options for control of your weapon motor. I mention them here for completeness:

  • I mentioned current limiting and adding a 'servo slower' to the receiver signal line to bring the peak current down for a mid-range ESC, but either of these options lengthen spin-up time and waste the big torque of the AmpFlow. You might as well just swap in a less powerful motor.
  • You could go overkill on the ESC with a Vex Pro Victor BB. That will certainly do the job, but your pocketbook will ache.
  • You might consider a power solenoid/contactor for simple on/off weapon control. 'Spinner god' Ray Billings doesn't like solenoids with high-performance AmpFlows, but your quick spin-up time gets you out of huge amp draw quickly, so the motor should survive the big power dump - particularly if you switch to a v-belt with a little slip.
  • And then there's the old trick with four-brush AmpFlows: split the wiring for the two brush pairs and power each set with a separate ESC. Yes, it's a little Mickey Mouse and you can fry the ESCs if you don't know what you're doing. You might just get away with spliting the brushes and running two VEX pro Talon SRX ESCs. The Vyper is a better option, but some builders like to be crazy/different.

Q: If the amperage rating on the ESC isn't 'real' then how do you find the 'true' amperage rating?

A: There are too many design, component, and construction variables to get a 'true' number from a physical examination of an ESC, and you certainly can't rely on any given manufacturer's advertized ratings due to huge inconsistencies in how that number is determined. Your choices are:

  • Precision Buy one, instrument it, and test to failure.
  • Reputation Consult with builders who have used that ESC in applications similar to the one you have planned.
  • YOLO Stuff one in and see what happens.
The last two options are the most popular. The 'Vyper' is widely used in combat - particularly in the UK - and has a good reputation. I would not hesitate to use it in your application.

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Q: So you said that simple robots win but I was wondering how Tazbot did so well even though he was kinda complicated. Ty! [Lynn, Massachusetts]

A: [Mark J.] 'Tazbot' was very successful... at getting a good share of the air time on the original BattleBots TV series. Only a small percentage of the actual tournament fights made it onto the televised shows, but Taz was fun to watch so it got a lot of TV time. However, it was only an average combatant. Official record: 11 wins / 10 losses / zero championships.

Like 'Vlad the Impaler' - which you asked about in your last post - Taz last fought more than fifteen years ago. Combat robots have changed a great deal since then. Even the most successful 'bots of that era would be poor models for success in current competition. Pick newer 'bots to wonder about.


Q: So I was wondering how vlad did so well. I mean most lifters don't do that well without another weapon. Is it the power of the lifter or the way the driver drives or Something else? (No offense to lifters.) Thanks! [M] [Lynn, Massachusetts]

A: [Mark J.] Two things:

  1. Watch some videos of 'Vlad the Impaler' and you'll see that it was not as much a 'lifterbot' as it is a powerful 'rambot' that happened to have a small lifter.
  2. Whaddya mean 'lifters don't do that well'? At the time Vlad was fighting (its last fight was in 2001) lifters were very successful. Ever hear of 'BioHazard'?

Q: So I saw overdrive S1 had some technology in it's wheels. What does this do? Thanks! [M]

A: Series 1 'OverDrive' was powered by direct-drive electric bicycle brushless hub motors, which place the entire driveline into the wheel. This greatly simplifies robot design and speeds construction, but also limits the ability of the builder to optimize the drive system.

Although the motors were placed in the smallest practical wheels, the drive provided considerably less torque than conventional combat robot drive systems. For series 2, a completely new version of 'Overdrive' wisely reverted to conventional drive motors.


Q: Do amp flow motors only work with amp flow escs? [Arlington, Virginia]

A: [Mark J.] AmpFlow motors may be controlled with any brushed motor ESC of suitable voltage rating and current capacity, and AmpFlow ESCs will operate any standard brushed motor within their current and voltage rating.

Q: Can you make a calculator for how much current a given motor will draw at a given load?

A: Permanent magnet direct current brushed motors have a linear relation between torque output and current. If you know the torque output required, the formula is very simple:

Current Required = Motor Stall Current * (Torque Required / Motor Stall Torque)
If you're building a combat robot drive train and don't know the torque output needed, the Tentacle Torque Calculator will evaluate the motor, drivetrain, and chassis elements for you and will model robot performance -- including maximum current draw. An example drive train analysis using the Tentacle Torque Calculator is available here.

Q: Would the ampflow a28 400 running a small disc be appropriate for a lightweight? If so, would the RageBridge 2 be a good weapon ESC?

A: Several comments:

  • I cannot make a weapon motor recommendation based on a weight class and a general description of the weapon type. We offer a collection of design tools so that builders may perform their own evaluations of combat robot system. These tools include two different programs that will model the performance of specific spinner weapon systems. You will do well to learn to use these tools.
  • You will also benefit from reading our Spinner Weapon FAQ to gain an understanding of the principles of spinner weapon design and performance.
  • The RageBridge 2 is a dual-channel brushed controller designed to control drive motors on opposite sides if a tank-steer robot. The special features it offers are intended to support drive train applications. Using it to run a single weapon motor would be a waste of its features.
  • The A28-400 AmpFlow motor is a big, heavy brushed motor -- less than ideal for a 'small disc' on a lightweight robot. Current weapon design favors much lighter brushless motors that are well suited to spinner weapon applications.
So... I'm gonna say 'no' and 'no'. Stop guessing and do your design homework.

Q: What makes the RageBridge 2 so good for lightweights? It’s amperage rating is so tiny.

A: Previously answered. See this post in the Ask Aaron Motors & Controllers archive


Q: Would a sabertooth 2 x 60 driving 2 amp flow e30 150 motors be adequate for a lightweight? [Dallas, Texas]

A: [Mark J.] Motor current draw depends on voltage, wheel diameter and gear reduction ratio as well as motor type and robot weight. The Tentacle Torque Calculator can evaluate these factors and provide an estimate of maximum current draw as an aid in motor controller selection. It will also model the speed, acceleration, and battery requirements for a robot of a specific drive train design. Learn to use this valuable tool.

The Sabertooth ESCs have a poor reputation in combat applications. Builders using them have reported sudden and unexplained complete failure of the controllers following a high-energy impact. Although they work well in general robotics I no longer recommend their use in combat robots. In this power range I recommend the RageBridge 2.


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Q: Hello. Glad to see the site is still going strong.

I am in the process of building a beetleweight robot, and I decided to try making a rubber band powered flipper. I have been insipred by a mechanism used by a UK antweight named 'Militant', as well as footage of a couple of Lego robots that weigh nearly as much as a beetleweight.

Essentially, I'm aiming to build a beetleweight replica of Robot Wars series 10 champion, 'Eruption'.

I have a servo which has 33kg/cm of torque, which I'm hoping will be enough to crank the flipper into position. The same servo will also be linked to a release mechanism which will fire the flipper.

I have a 2kg (4.4lbs) box of dumbbells ready as my practice opponent, thinking that if it can flip this, it can flip other beetles easily. (I know it sounds excessive, but I couldn't find anything weighing exactly 3lbs that would be suitable for the job.)

My questions are:

  1. How many rubber bands will be required for the flipper?
  2. Do I have the sufficient amount of torque required to crank the flipper in place?
  3. If rubber band power isn't strong enough, will the servo itself be enough to be used as a quick lifter instead?
[Perth, Australia]

A: [Mark J.] G'day, Perth. I'm sorry, but 'The Hamburger is Bad'. The answers to each of your questions depend on details of your design that you have not given: layout and dimensions. I think I can give enough guidance for you to evaluate your progress as your design develops. I'll address your questions in reverse order, as the discussion of the later questions helps to explain the earlier questions.

  1. Assuming that you have a simple lever arm attached to your servo, the lifting capacity decreases with the length of the arm measured from the servo axis. Quick torque calulations show that a servo capable of 33 kg-cm torque at stall could lift a 3-pound load out at the end of a 4.75" arm while maintaining half of its no-load speed. A 3-pound load on the end of a 9.5" arm would stall the servo and you would get no lift.

  2. The force required to stretch a rubber band increases linearly as the band is stretched -- up to a point. Assuming a design like 'Militant' where the servo is coaxial with the flipper pivot, the torque required by your servo will be the force supplied by the bands at the 'latch' point of the flipper times the distance from the servo axis to the band attachment point on the flipper arm.

    Example - If your rubber bands exert a 10 kg force when the flipper arm is latched, and the distance from the flipper pivot to the rubber band attachment is 2.5 cm, the torque required by the servo to latch the arm will be 10 kg * 2.5 cm = 25 kg-cm. That's far enough below your servo's 33 kg-cm stall point to achieve reasonable reset speed and keep the servo healthy.

  3. Rubber bands come in all sorts of sizes; width/length/thickness. Different sizes will give different force profiles. The better question is, "How much elastic force is needed?" With a flipper arm as shown in the photo, the force out at the end of the flipper arm is:
    Elastic Force * (Pivot to Band Attachment / Pivot to Flipper Tip)
    Example - Bands exert 10 kg force at latch; the band arm is 2.5cm, the flipper arm is 10cm. Force at the end of the flipper arm is 10 kg * (2.5 / 10) = 2.5 kg.
Is that enough to deliver a good flip? There are variables I left out of the calculations (like elastic pre-load) to keep this from becoming a graduate level thesis, so my best advice is to build a simple mock-up of the flipper arm and play around with critical lengths and forces to see what it takes to deliver a good flip, then run the numbers to see if you can reset it with your servo.

Note - Don't copy 'Militant' too closely; it has far too little weight on the drive wheels.


Q: Your post about a rubber band powered flipper looks interesting, but I don't understand how the servo release mechanism works. Can you walk me thru that? [Some guy that cornered me in a alley]

A: [Mark J.] I made an animation that should help. The flipper arm pivots on an axle that is seperate from but coaxial with the servo output. The green line is the elastic band (or spring). The red rectangle is the servo arm. The servo itself is not shown so you can better see the release/reset action.

  • To trigger the flip - the servo arm moves downward from its rest position to contact a metal pin extending from the catch arm.
  • The plastic catch arm deflects downward, releasing the flipper.
  • To reset the flipper - the servo arm moves upward to contact a metal pin extending from the flipper arm.
  • The servo continues upward, pulling against the elastic/spring and rotating the flipper arm back to the latched position.
  • The servo arm then returns to the rest position, ready to trigger another flip.
The mechanism is simpler than a snail cam, but the force placed on the servo increases greatly as it approaches the latched position. Moving the servo off-axis and adding a curve to the servo arm could help correct this drawback and reduce the required servo force.

Remembering Aaron... 

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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