spacer

6559 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.

Got a question? We welcome combat robot questions. Check the Ask Aaron Archives first to see if your question has already been answered, then click the blue button.

The Ask Aaron Archives Click to browse thousands of previously answered questions by category, or search for specific topics. Includes FAQ

Caution   Even small combat robots can be dangerous! Learn proper construction and safety techniques before attempting to build and operate a combat robot. Do not operate combat robots without proper safeguards.
 
Q: I loved BotsIQ in high school and wanted to continue with the weight class after I graduated. I look in the Builders Database but cannot seem to find an event. I'm looking for a 15lb competition or something very close. [Monroeville, Pennsylvania]

A: [Mark J.] The 15 pound class is almost exclusively limited to school-based competitions. The only open competitions I know of for this 'dogeweight' class are associated with the 'Battle Beach' tournaments in Florida.

You'll have more luck shaving off some weight and finding a tournament with a 12-pound class. The big annual NERC Motorama event in Harrisberg Pennsylvania has 12-pound 'hobbyweight' and 30-pound 'featherweight' classes. The event is coming up in February -- registration closes soon.

Q: I also want to make a truly big bot as I am starting my own machine shop. I was thinking a 60lb or larger, but also am having trouble finding an event. Do you know of any for that as well?

A: The larger the 'bot the harder it is to find a competition. Robogames in northern California is the only open US tournament fighting 'bots larger than 30 pounds, and they have cancelled their 2019 tournament. Keep an eye on Robot Combat Events as well as The Builders Database.

Comment: Hey, no question, just writing in on the JMT speed controllers [see archived post] (was suggested them by Melanistic Leopard's builder as well). My experience with them has been frankly less than excellent. I switched to them after having a poor experience with the VEX29s, but I've blown around six of the JMT controllers doing little more than just wheels-up tests and I've had several come in that had a "tap brake" despite specifically ordering them without it. The BEC seems to be the main source of problems (even had one catch fire on me), and a controller being used to power the receiver seems to get alarmingly hot while just idling. They also don't seem to have much if any protection from motor noise. I personally use them in my beetleweight only to try and figure out just how many different ways I can get them to fail, and in some starter robots for young builders (where low cost is really the only important element).

I agree on your stance that you're better off paying for a more trusted and reliable speed controller. [Alberta, Canada]

A: [Mark J.] Everyone likes a bargain, and once in a while you can find a batch of Chinese ESCs that have some 'better than adequate' chips soldered to the board just because they were the parts available that day. The next batch of Chinese ESCs that looks kinda the same ain't gonna be as awesome. The JMT is one of those 'next batch' items.

The JMT controller's 8.4 volt max rating is likely based on BEC voltage regulator limits. Cliping the red power wires on the JMT receiver cables and powering the receiver from the weapon BEC (if you have one) will take all the load off the voltage regulator and cut down on flames. I suspect that's how 'Melanistic Leopard' was set up.

I understand the temptation; you can find the JMT 10 amp controller priced under $4. Just don't do it.

Q: I've just read your new FlySky FS-i6 Transmitter Combat Guide. Well done on making this excellent resource, I learnt heaps, I have one question. Is there any way to use one of the transmitter switches to quickly reconfigure elevon control settings when the bot is running inverted? [New Zealand]

A: [Mark J.] I'm glad to hear you're enjoying the FlySky combat guide, New Zealand.

The bad news is that you'd need to spend a fair bit more than the cost of the FlySky FS-i6 to get a transmitter you can program to perform an invert mix fix with a switch. The programing you want requires at least four custom mixes that can be assigned to a transmitter switch: instructions are given on the Team Cosmos website. Your FS-i6 has only three custom mixes and none of them can turned 'on/off' with a transmitter switch.

The good news is that you can add a bit of hardware that will give you a switchable invert solution. Several on-board mixers like the FingerTech tinyMixer have two inputs for the channels to me mixed plus a third input cable that plugs into an auxilliary channel (5 or 6) on your receiver. Assign that auxilliary channel to one of the transmitter switches on the FS-i6 with the 'Aux Channels' function and a flip of that switch will change the mixer output to inverted operation.

Q: Following with interest. I've been trying to figure out an 'invert' switch on my Taranis Q X7 and I can't seem to do it properly. Either left/right inputs is inverted or forward/back is. Am I simply trying to do something beyond the transmitter's capabilities? [Reddit comment]

A: Taranis transmitters use the highly configurable 'OpenTX' firmware. There is very little that an OpenTX transmitter cannot do, but figuring out how to implement even simple functions can be a challenge.

Here's a video showing how to setup an inverted airplane 'flight mode' on a Taranis transmitter. The same method works for inverted robot control -- don't reverse the rudder or aileron channels, just the elevator.

Q: How to adjust the motor shaft to colson tires [Guayaquil, Ecuador]

A: [Mark J.] Web search "colson hubs". Example: BaneBots Colson Hubs.

Q: I'm in the process of building my first ever beetleweight robot. I have a Hobbyking 6ch radio set featuring an HK-T6A V2 transmitter and an HK-TR6A V2 receiver. I'm using a Sabertooth 2x5 dual ESC to send power from a 2200mAh 3S lipo battery (I know, it sounds like overkill, but it's my first build and I'm still learning.) to a couple of cheap gearmotors.

I'm having a bit of trouble trying to setup the radio gear. I followed all the instructions on binding the system:

  • I connected one of the ESC's plugs into the receiver the correct way up (with the ground wire away from the label)
  • I connected the bind plug into BAT on the receiver
  • I connected the battery to the ESC, causing the red light in the receiver to blink consistantly
  • I held down the bind button on the transmitter while turning it on, causing the receiver's light to then shine solid
  • I turned off the transmitter, then disconnected the battery from the ESC
  • I removed the bind plug
  • I turned on the transmitter, then reconnected the battery to the ESC.
By this stage, the light in the receiver should be shining solid, but instead, it's flickering like it's having trouble. I tried operating a servo but it's not responding very well.

Any idea what's causing this problem? Is it because of the battery, the receiver, the ESC, or is it something else? [Mandurah, Western Australia]

A: [Mark J.] You've done your homework and followed the receiver binding process correctly. There are a couple of 'first ever' builder traps that are possible sources for your problem:

  1. LiPo batteries ship with a partial charge for safety and storage longevity. Give the battery a full charge and try again.
  2. You mention a servo. The Battery Eliminator Circuit (BEC) on your Sabertooth that provides 5 volts to the receiver can provide a maximum of 0.1 amp of current, decreasing to as little as 0.01 amp as the supply voltage increases. That's enough to run the receiver but adding a servo can cause the voltage to sag below the required level. That sounds very much like what's happening to your receiver (flickering light). If you're going to power servos or other devices thru the receiver ports you're going to need a BEC with greater current capacity.
Here's a link to Dimension Engineering's BEC FAQ.

Q: Hi. It's the one asking about the Hobbyking radio gear. Thanks for diagnosing the problem. It turns out the radio gear does work fine with just the Sabertooth ESC, so fitting the servo was the problem, as you pointed out. Also, based on your recommendation, I have just recently placed an order for a BEC rated at 3A. Will this be enough so I can operate the servo as well?

Also, I don't know whether or not you have already posted a wiring diagram of how this should be fitted together? I really don't want to mess this up.

A: You're welcome, Mandurah.

Different servos will draw different levels of current, but three amps should be ample unless it's some monster servo. A 'standard' servo draws about 0.5 amp under heavy load.

The diagram should give you confidence:

  • Most BECs come with a pair of power input wires that connect to the main battery supply and a three-wire cable that plugs into any free receiver port. If your's is different, write back and send a picture.
  • It's best to have only one power source powering the receiver. The center red leads on the cables from the Sabertooth to the receiver carry power from the on-board BEC to the receiver. If you examine the connectors on these cables you will find the little barbs that hold the connector pins into the plastic holder. With a small probe you can unclip that barb. Pull the center pin backward out of the holder and tape it back out of the way (video). Do that for each of the Sabertooth cables.
  • Alternately, you can simply clip the red center wire on each of the Sabertooth receiver cables.

Comment: Hobbyking radio and Sabertooth ESC guy here. Just wanted to let you know the BEC has arrived and has been installed based on the diagram. Everything now works perfectly.

Thank you so much for all your help and knowledge. You are a credit to the whole robot combat community.

A: Aw, shucks, 'tweren't nothin'.

Q: I have a problem my drive Esc are gettin hot. It is a brushed esc witg brake. And I used a 6v motor 280 rpm and 65 mm diameter of the wheels. The chasis are on the attached file which is made from aluminum with a weight of 1kg. How to lessen the temp of esc [Quezon, Philippines]

A: [Mark J.] I'll assume your ESCs are the "RC ESC 20A Brush Motor Speed Controller w/ Brake" widely available on eBay, and from the photo of your chassis you attached it appears that the motors are the "6V DC Small Speed Reduction Gear Motor Metal Gearbox with 65mm Wheel Kit" also found on eBay. These are both fairly standard low-budget components for insect-class robots and should work for you.

It's not unusual for small ESCs to run hot -- hot enough to burn your skin if you touch them. The solution is to not touch them.

A few suggestions:

  • These motors are electrically 'noisy'. Soldering a 1uF 200v ceramic capacitor across the motor terminals will make things easier for your ESCs.
  • Clip or remove the red wire on one of the ESC leads to the receiver. The ESC has a built-in BEC that powers the receiver thru that red wire. Having two BECS trying to power the receiver can cause problems, so remove/clip one of the red leads. If your weapon ESC has a BEC you can clip it's red receiver lead also.
  • A drop of light oil on the output bushing of each gearmotor can keep the output shaft free spinning. Make sure the wheels aren't rubbing on anything and that the gearboxes are turning freely.
Q: I was researching brushless outrunners and I understand that the four digits that they are described with refer to the diameter and length [in millimeters], but do these values mean anything in terms of rpm or torque? For example, would a motor with a larger diameter always have more torque than a similar motor with a smaller diameter? [Anacortes, Washington]

A: [Mark J.] The simple answer is 'no'. Larger diameter outrunner motors tend to spin slower and deliver more torque than smaller diameter outrunner, and longer motors tend to produce more torque than shorter motors -- but there are multiple elements in the internal design of the motor that have larger effects than do external measurements:

Q: How to build armor for combat robots? [Ho Chi Minh City, Vietnam]

A: [Mark J.] There is a great deal of information on combat robot armor in the RioBotz Combat Tutorial. Start with Section 3.9.6: 'Minimum Weight Traditional Armor'.

Q: I was about to replace my FingerTech tinyMixer (accident, don't ask) when someone told me my FlySky i6 radio could do mixing and I didn't need a stand-alone mixer. Is that true? My radio didn't come with a manual in the box. [The Aether]

A: [Mark J.] Yes, the FS-i6 transmitter has perfectly good mixing. Unfortunately, the FlySky-i6 manual is so poor that even if you had a copy you'd still be a long way from setting mixing up correctly. I strongly recommend that you download a manual for any radio system you're considering before you buy it -- if it doesn't make sense to you, pick another radio.

I'm working on a full guide for combat robotics use of the FS-i6, but in the meantime I can give you enough help to get mixing enabled. Note that a lot of this makes very little sense -- like pressing and holding the 'Cancel' button to save your new settings. Chinese radios...

General FS-i6 Menu Navigation

To enter the main menu, long press the 'OK' key. Use the 'Up' and 'Down' keys to select the desired section and press 'OK'. Then use the 'Up' and 'Down' keys to select the desired section ('System' or 'Functions') and press 'OK'.

Navigation Within a Section:

  • Use the 'OK' key to select a parameter to modify.
  • Use the 'Up' and 'Down' keys to modify the value of the selected parameter.
  • Long press 'Cancel' to exit and save the new parameters, or short press 'Cancel' to exit without saving.
Set-up Elevon Mixing:
  • Open the Main Menu and select the "Functions' section.
  • Scroll down to 'Elevon'.
  • Turn elevon mixing on and set Channel 1 to 50%, Channel 2 to 100%.
  • Long press 'Cancel' to exit and save the new parameters.
Plug your left drive ESC into receiver port #1, right drive ESC into receiver port #2 -- just like you did with your FingerTech mixer. If the robot does not respond correctly to stick inputs, follow this troubleshooting checklist.

If you think the turn rate of your robot is too slow, go back into the 'Elevon' menu and increase the Channel 1 control rate upwards from the 50% rate entered as default. Do not tinker with the 100% control rate on Channel 2.

Come back in a couple weeks and I'll have the full combat guide for the FS-i6 ready.

Update The full Team Run Amok FlySky FS-i6 Transmitter Combat Robotics Guide is now live. Take a look and tell me what you think.
Q: When I was looking through pictures and build reports of insect weight robots, and I noticed that some builders use these orange belts that apparently can be cut to size and melted into a loop. In the picture I attached, you can see in use both on the weapon and drivetrain. Where can I find a belt like this, and what sort of applications do you recommend it for? [Oak Harbor, Washington]

A: [Mark J.] You can find this material with a search for "urethane round belt". Industrial supply dealers like Grainger have a range of sizes and properties in stock. There are multiple videos on how to cut and weld the urethane into a belt -- here's a start: Making Urethane Belts.

Unlike V-belts or timing belts that have very little 'stretch' because of internal fabric reinforcement, urethane round belts stretch a lot and must be cut short to have enough tension to transmit power. Think 'big rubber bands'. They will lose elasticity over time and should be monitored to assure adequate tension for power transmission.

Pulleys for round belts are difficult to find is sizes and bores useful to insect-class robots. Most builders using round belts machine their own pulleys.

The elasticity and ability to 'slip' make round belts a good choice for spinner weapon drives. The elasticity offers some isolation of the weapon motor from impact shock, and the slippage can smooth out current spikes at start-up. For drivetrain use I prefer timing belts for greater slip-free torque transmission.

Q: Hello, Mark! I was thinking about the question I had a while back about how people turned mixing bowls into full-body spinners, and remembered that both Ziggo and Blendo were made from wok pots (IIRC). As with that question, I am simply wondering: how on earth do you turn a wok pot into a spinning shell of amassed destruction? I'd assume their cast-iron structures would gave them more problems than it'd be worth. [Jacksonville, Illinois]

A: [Mark J.] Not cast iron! Cast iron shatters. Iron woks exist but traditional woks are made of a sheet of carbon steel, hand hammered into a deep bowl shape. 'Ziggo', 'Blendo' and other 'wok-based' FBS use the modern equvalent of that style, with one or multiple layers of thin steel stamped to shape.

The design is the same as for the insect-class spinners using small steel mixing bowls you asked about in your earlier question; the addition of a thick bottom ring and reinforcing plates under any blades/impactors on the wok itself spread impact load over a large area of the thin material. 'Ziggo' adds a square steel cross-tube that sticks out either side of the spinning wok.

Q: Hello there! It's the German guy with his strange ideas (the hybrid drum-disc thing). I did in fact listen to your advices given on this page and build some solid wedges and focused on drive trains, solid builds and no fancy ideas at all. I improved a lot on metal working skills, cast my own wheels and by now I even solder my own beetleweight ESC's for myself. The normal way of a good novice. But I got a bit weary overtime. The shapes of all the robots resemble each other so much. Watching other bots on YouTube and listening to your advice it is quite evident why that's the case. Because it works. Of course I couldn't except that completely. After watching the championship of BattleBots it finally struck me.
  • Bots with huge and dangerous weapons mostly are their own worst enemy. Mostly spinners. Dealing damage to themselves while hitting others. Making most builders of such bots seemingly reckless because others try to evade them in combat but most of the time fail. Even though heavy weapons decrease the turn radius (i.e. Bronco vs. Whiplash or gyro effect) and make them vulnerable.
  • The drive train used in 99% of the fights has a weak spot. Its side. Pushed from the side a Bot is helpless.
  • The drive train can not keep the weapon facing towards the opponent bot all the time. So an evading move, trying to sneak around, against i.e. a horizontal spinner could end the fight with a simple turnaround of the opponent and your scrambled.
What would be the solution? Many go for Titanium armor and I can't afford that or the tools. Well not now after all. So the drive train perhaps? Could I get an edge above my super armored, expensive killer bot opponents? Omnidrives with Mecanum wheels have way less torque and to me seem more or less awkward and useless. I have been searching the web for some time but all the community seems to come up with are omniwheels or angular drive trains. Not very battle worthy at all. Since i don't have the money to build a scrapheap for the joy but couldn't give up on the idea i sketched out the plans for a omnidrive system.

First I will explain the idea then I hope for some enlightenment from your side and maybe a pointing finger towards some builders who already did this and obviously better. The idea is to have a bot capable of strafing around the opponent with accuracy. Hitting the weak spots like wheels with accuracy. Pushing from the side and avoiding deadly spinners. The trade off would be less armor. Obviously this system would require a second smaller motor, rotating the Hub of the drive. It would also require more A/h on the battery side and would contain many small parts and if build wrong it would scatter as the above mentioned scrapheap across the arena. The drive has to be located inside the bot.

In the picture (not to scale or anything) we see the main Hub. The motor (blue) is fixed to a circle; it drives the wheels (fixed to the circle of the Hub) with a timer belt. Above the motor is a second circle (the top of the hub) with a shaft for the wires and pressed into the ball bearings of the upper enclosure. On one side of the lower circle is half of a gear. This lets the Hub turn 180°. With the right adjustment of the controller software this would amount in 360° of turn radius and would make the build way more easy. Also there are two blocks on the lower circle (left hand picture) they will prevent the Hub from over turning from i.e. impact. Also a Sensor measures the position of the Hub in total for readjustment.

On the Lower enclosure there will be steel Balls (a poor-mans ball bearing because a BB of this size is expensive and heavy) with the lower circle of the Hub resting on top. Distance rolls on the sides keep the Hub from hitting the wall of the enclosure and turning smoothly. Outside of the enclosure is a smaller motor driving the Hub through a hole. It has, just like a servo, some sensor/potentiometer attached. It's all in an enclosure so I can swap the whole thing fast if damage because it's too complicated for repairs inside the bot. That’s about it. I'm sorry if my bad English got into the way of understanding the idea. I hope the sketch helps out a bit. I plan to build a Nylon 3D-printed version of this for a beetleweight but before I start this dubious task I rather hear a educated opinion on this matter first.

Thanks for all you do for the community and I always enjoy learning from you. The German guy [Niedersachsen, Germany]

A: [Mark J.] Good to hear back from you, Niedersachsen. I enjoy your ideas and I very much love your sketches! Please don't apologize for your English -- es ist viel besser als mein Deutsch.

Your concept is very well thought out, and I will assume that you enjoyed the exercise. I am pleased to report that the concept is entirely workable and that I can point an enlightening finger to design resources. All you need are the magic key words -- Google: swerve drive.

Swerve drive is widely used in the non-combat First Robotics Competition (FRC) but has seen very little exposure in combat. Here are a few links to get you started:

Packing all this into a beetleweight will be challenging, but the potential is huge! Keep me updated on your progress.

Reply: Thank you Mark! What a simple word can do. Swerve would've never crossed my mind. This opened up a whole new world of info. Thanks! The bevel gear option was on the table in an older version but I expected it to be a weak spot when hit. On the other hand I made a trade off with the cable which gets stressed out by turning. So I only did a 180° turn radius and planned for some stopping blocks. If I would try the 360° option I maybe have to add some brake based on relays or something like that. Just to protect the drive from impact.

It's really obvious now why this drive hasn't been done so often in combat robotics. But I will start the modeling in CAD now. You gave my Odyssey a good push forward. Thanks again from Lower Saxony!

Response: I've got another search term for you. You might find some worthwhile information in a search for crab drive. 'Crab' uses the same drive modules as 'swerve' but simplifies the control requirements by linking directional control of some or all modules together. Linking all four isn't very interesting -- but linking the front modules as one unit and the back modules as a second unit gives a lot of maneuverability and still allows use of standard R/C gear for control.

Q: I'm trying to set up a Fly Sky FS-GT3B pistol grip transmitter with a Scorpion Mini ESC. I got the receiver to bind, but controls are kinda 'haywire'. When I power on, the motors just start running. Any help would be greatly appreciated, thanks! [Robot Forum]

A: [Mark J.] Robot controllers like the 'Scorpion Mini' expect the spring-center 'neutral' signal from the radio to be in the middle of the signal range. Your Fly Sky and many other pistol grip transmitters have the spring mechanism for the throttle trigger set off-center to give more control range in the forward direction. That works fine for R/C cars that don't much care about precision in reverse, but your Scorpion ESC interprets that offset transmitter signal as: back up at 1/3 throttle.

Fortunately, we can correct that interpretation by recalibrating the ESC to recognize the new 'center point'. You may want to disconnect the motors from the Scorpion Mini during calibration to prevent run-away:

  • Turn on transmitter, remove any transmitter trims and leave all controls spring 'centered'.
  • Power up the Scorpion Mini and receiver.
  • Press the ESC 'calibrate' button. When the 'Status' LED goes out, release the button.
  • Move the transmitter throttle and steering controls thru their full range (forward/back and left/right) several times.
  • Return controls to spring-center.
  • Press and release the 'calibrate' button again. The 'Status' LED will light up.
  • Verify calibration: both motor LEDs on the controller board should be off with the transmitter controls at rest.
  • Power everything down -- your ESC will retain these settings in memory.
Next we'll set up the channel mixing needed to get correct throttle and steering response from the pistol-grip transmitter. The setup is different than 'transmitter-side' channel mixing with a full-featured twin-stick radio. The FlySky FS-GT3B has no mixing capability, so we'll 'make do' with the Scorpion Mini's limited on-board mixing:
  • The Scorpion 'steering' lead plugs into the Fly Sky receiver channel port #1.
  • The Scorpion 'throttle' lead plugs into Fly Sky receiver channel port #2.

Throttle Test Power up the transmitter and robot, then pull the transmitter throttle trigger back a little -- a 'forward throttle' command.

  • If the robot moves forward: good! Skip down to the Turning Test.
  • If the robot moves backward: select REV from the transmitter setup screen and reverse channel 2.
  • If the robot spins to the left: reverse the motor lead connections for the left motor.
  • If the robot spins to the right: reverse the motor lead connections for the right motor.
Note If your 'bot has multiple motors on each side of the 'bot and not all of those motors are turning in the same direction, reverse the motor leads to the ESC for any motor(s) spinning the wrong direction.
Turning Test Turn the transmitter wheel a little to the right -- a 'spin right' command.
  • If the robot spins right: you're done!
  • If the robot spins left: select REV from the transmitter setup screen and reverse channel 1.
That should do it. There are additional tweaks available from the transmitter's Dual Rate (D/R), Adjustable Travel Volume (E. POINT), and Exponential Response (EXP) adjustments that may make your robot easier to operate. You can learn about them here, then check your Fly Sky manual for instructions on implementing them on your transmitter.

Q: Followed your instructions and, success! I can't thank you enough! This, paired with a 4s battery gives me a lot more power and control than before.

A: You're very welcome. Now get out there and beat up some robots!

Q: Hello again! I was looking at the designs of Gyrobot and Wrecks, and I was wondering if there was an advantage or disadvantage in one design compared to the other. Mainly, I didn't know if the placement of the innards of Wrecks 2 had any advantage over the placement of the innards in Gyrobot, among any other advantages / disadvantages I don't know about (if there are any). [Jacksonville, Illinois]

A: [Mark J.] The layouts of precessional walkers 'Wrecks 2' (heavyweight) and 'Gyrobot' (antweight) are essentially identical. The differences are more a function of their relative sizes than an engineering choice. For example, Gyrobot can get away with a direct-drive weapon because of its small scale, whereas the larger Wrecks must use a chain drive. Placement of batteries and electronics in the base versus alongside the weapon are also scale related. The choice of design would depend on how large a robot you are building.

Q: With Robogames over, are there any non-televised competitions that you’ll be making tournament trees for? [Newark, New Jersey]

A: [Mark J.] Why do you believe that RoboGames is 'over'? The official statement that "There will not be a RoboGames 2019 in California" clearly refers only to next year's event. RoboGames took a year off in 2014 but was back the following year. I fully expect the event to return.

I have compiled tournament trees in a standard format for more than 40 major robot combat events at the Run Amok Combat Robotics Tournament Winners page". I'm considering adding the recent big Chinese events '(Fighting My Bots', 'King of Bots', 'Kentucky Fried Bots') if I can find the necessary data.

Q: I'd like get into robot combat and I thought I'd start with an antweight. I've got a drawer full of parts from flying drones and I thought I'd modify some servos to power my first 'bot. What are the advantages and disadvantages of servo drive? [The Aether]

A: [Mark J.] The 1-pound class robots got their start back in the late 1990's when R/C gear came with servos included. Combat robot builders all had a bunch of unused servos in their parts bins, and when they figured out how to modify those analog servos for continuous rotation tiny robots started wandering around the pits. It didn't take long for the first fight to break out.

Twenty years later there are much better options for insect class propulsion.

A 'standard' R/C servo modified for use as a gearmotor is neither fast nor powerful. Servos are designed to respond to standard R/C receiver output signals by moving the output shaft to a specific angle relative to the servo body. They are not designed to operate as gearmotors and they are certainly not made to survive the abusive loads suffered by combat robot drivetrains. However, most analog servos can be modified to function as a gearmotor.

There are very few advantages to using a modified servo:

  • They have a small speed controller built in which simplifies wiring and reduces cost.
  • They just plug into a receiver output port to draw power and a control signal.
  • Their rectangular shape is generally simple to mount to the chassis.
The disadvantages are many:
  • Standard servos use fragile plastic gears and cases.
  • Tougher metal gear/case servos cost more than real gearmotors/controllers.
  • Servos require modification to convert them to continuous rotation gearmotors.
  • The power output of standard servos is very low compared to common gearmotors.
  • Servos require further modification to operate above receiver voltage level.
  • Even with modification, standard servos should not be run above 9 volts.
The table below compares the performance of 'standard' Futaba S148 servos modified for antweight drive to the FingerTech 'Silver Spark' gearmotor commonly used in antweights. You can see that the performance of the modified servos fall far below the true gearmotors. Overall, servo power drive just isn't worth the trouble in an antweight.
Antweight: 1" wheels Futaba S148 Servo @ 6.0V Silver Spark 11:1
@ 11.1V
Full Rotation Speed Mod
Dimensions: 1.6 x 0.8 x 1.4" 0.63 x 0.63 x 1.5"
Weight: 1.6 oz 1.0 oz
Output Power: 1.1 watts 6.7 watts
Stall Current: 0.75 Amp 2.4 Amps
Stall torque: 38 oz·in 2.2 oz·in 10.8 oz·in
Shaft speed: 50 RPM 875 RPM 1573 RPM
Robot top speed: 0.14 MPH 2.6 MPH 4.7 MPH
8-foot sprint: 27.6 sec 3.2 sec 1.4 sec
Torque note: Ants with 1" wheels may stall below 3 oz·in
Q: The performance table mentions a servo 'Speed Mod'. What's that and how is it done?

A: The basic servo gear layout is a four-stage reduction that includes two gears that rotate independently on a central shaft: the blue and red gears in the diagram below. In the 'standard' Futaba servos typically hacked in the early days of combat the total gear reduction adds up to 278:1, which gives an output speed around 50 RPM. To get any type of speed at all out of that slow shaft rotation requires very large diameter wheels -- awkward and bulky. The 'speed mod' involves removing one of the four compound gears (the yellow 'Top Hat' gear in the diagram) and mechanically fusing the two gears (red and blue) that share the central gear shaft so that they will spin together as a single compound gear. This eliminates two of the reduction stages, leaving a total reduction of about 16:1 which spins the output shaft 17.5 times as fast.

Unfortunately, the speed mod also reduces the output torque by 17.5 times. Even with the smallest possible wheels the servos will stall under heavy pushing loads -- a highly undesirable outcome. The tiny servo motor simply does not provide enough power to make the 'speed mod' more than a novelty.


    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's Minecraft High Dive Video Aaron's World of Warcraft New Player Guide

We welcome combat robot questions. Please search the Ask Aaron Archives first to see if we've already answered your question. Recent Q&A are posted above. Type your question in the box, attach files if needed, then click: 'Send'

or
Attach: Why is there a 'Cheerleader' button?

Thousands more robot combat questions and answers in the searchable Ask Aaron archives.



Copyright 2009, 2018 by Mark Joerger
All rights reserved.
how to design and build a battlebot