Brushless motor selection for combat robot weapon and drive.

Questions and Answers about Combat Robotics
from Team Run Amok

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Brushless Motor Selection  

Brushless motors are smaller and lighter than comparable brushed motors for combat robots, but there are special considerations in selecting the right brushless motor. Combat robots place very different demands on small brushless motors and controllers than the model aircraft they are designed to power. The advice here should get you pointed in the right direction for your sub-light 'bot.
Weapon Motors
Q: About how large a brushless motor do I need for a beetle spinner weapon? [Cincinnati, Ohio]

A: Specifics will depend on your design, but popular beetleweight spinner weapon motors run about 150 watts per pound of robot and about 6% of the total robot weight. For a beetle, that typically translates to outrunner motors in 28mm or 36mm diameters.

Q: Weapon BLDC motor: 6% of total mass; this rule applicable to FW and LW weight classes? [Paris, France]

A: The guideline (not a rule) was specific for beetleweights, and extrapolating to larger robots is non-linear. The Square-Cube Law requires larger robots to devote a greater percentage of their mass to structural elements, leaving less for other components. As robot weight increases you find proportionally smaller spinner motors and those motors are pumping out less power per unit weight because they are also subject to square-cube issues for structure and heat dissipation. A heavyweight spinner motor might typically produce 50 watts per pound of robot at about 3% of the robot mass. Fitting this to a log curve gives the chart shown -- but it's only a guideline.

Current successful beetle spinners have brushless weapon motors that cluster around 6% of the robot weight -- but you can't run that backwards to say that any motor that is 6% of robot weight will make a good beetle spinner motor. Different BLDC motor versions of the same size and mass will have quite different performance figures. Combined with the '150 watts per pound' guideline it's a quick check to see if you're in the ballpark. If you're considering a beetle weapon motor that weighs 3% or 12% of your total weight allowance you'll want to make sure you have good reason to use it.

Drive Motors

Q: I'm investigating using brushless drive on a 3-lb Meltybrain spinner and I was wondering if you had some advice on picking motors. I've only used brushed in the past, which is a pretty straightforward process with the Tentacle Drivetrain Calculator. However, I can't seem to find any analogous tools or equations for brushless motor torque.

It seems like people have had success with the DYS BE1806 and AX-2810Q, but simply based on the motor characteristics on HobbyKing I don't know what makes them better than others. I know that a lower Kv means higher torque, but how do you know that the Kv is low enough to drive your bot? [Social Media]

A: A brushless version of a drive train tool like the Tentacle Torque Calculator(1) would be difficult to create and possibly misleading to use because brushless performance depends as much on the controller firmware as on the motor. The number of variable combinations is both staggering and largely theoretical. See this archived discussion: Brushed vs. Brushless Drive.

The way brushless drive is done in practice is to go for overkill and not worry all that much about the details:

Rule of Thumb - There's a framework for brushless motor selection circulating out on the builder forums that appears to work well for drive motors. The method says that a set of brushless drive motors together weighing between 2% and 3% of the total weight of the robot will have more than enough power and adequate 'thermal mass' to survive.
By that rule a pair of DYS-BE1806s is about right for a beetle and twin AX-2810Qs is overkill (which might be just right for a Melty). What makes a motor better than others? Somebody used it successfully on their robot and word got out. Brushless meltybrain spinner? You're on your own. Best luck.
(1) In response to builder demand (and against my better judgement) I have created a version of the Drivetrain Calculator specifically for brushless motors. It assumes that you have the controller firmware well sorted out and that you have selected a motor that fits the "Rule of Thumb" discussed above. It's a little sketchy but I think it can provide some guidance: Brushless Drivetrain Calculator.

Q: Is 2x Castle 1406 1900kv @ 20:1 3in wheels drivetrain underpowered for 30-pounder? [Ann Arbor, Michigan]

A: The "rule of thumb" for brushless motors in the post above says that a set of brushless drive motors that together weigh between 2% and 3% of the total weight of the robot will be in the ballpark. Exactly why this works is a topic for a future discussion.

A pair of Castle 1406 1900kv motors weigh in at 2.9% of a 30 pound robot. That puts you on the high end of the golden range. On 4 cells with 3" wheels and 20:1 reduction you'll have about 12 MPH speed. Sounds healthy to me.

Q: I'd like to use brushless drive motors in my next robot (a featherweight) but the process of finding a suitable ESC, "flashing" new controller firmware, and tinkering with the various settings to get reversing and decent startup torque is putting me off. Then I need to find suitable motors and gearboxes and bodge the whole thing together. Are their any short cuts for a first time brushless hipster wannabe? [Altadena, California]

A: The process of selecting and modifying components for brushless drive is daunting, but you're in luck! The Australian combat robot supplier BotBitz has been modifying firmware in brushless ESCs for years. They're best known for converting brushless hobby aircraft brushless ESCs to brushed opperation, but they also modify brushless ESCs for robot drivetrain use. Here's their recommended 'off-the-shelf' setup for novice hipsters:

For insect-class builders looking to join the brushless drive club, BotBitz also has BotBitz 10 Amp brushless drive controllers ready to go with the same firmware modifications found in their larger drive ESC.

Note Prices on the BotBitz website are in Australian dollars. Multiply by 0.7 to get an approximate price in US dollars.

Q: Hold on... BotBitz recommends pairing their 30A Brushless ESC to a Propdrive 42-48 motor that calls for an 80A ESC to operate. Am I missing something here or does the motor not require that much current? [Yuba City, California]

A: Current drawn by an electric motor is proportional to the load placed on the motor. HobbyKing recommends use of an 80 amp ESC with the Propdrive 42-48 - For Aircraft Use - but we aren't building aircraft. The Rule of Thumb noted a couple posts above calls for oversizing brushless drive motors for combat robots to assure enough 'thermal mass' to survive in an environment well outside their intended use. This results in motors operating under lighter loading than they would be in aircraft service -- and lighter loading equals lower current draw.

The BotBitz setup is widely used in Australian featherweights and works quite well.

Q: Are there any 'off-the-shelf' brushless drive kits for insect weight classes? [Rockford, Illinois]

A: Rectified Robotics is currently selling a complete brushless Beetleweight drive kit: two 1806 2300kv brushless outrunner motors mated to 22mm planetary gearboxes, and two pre-programmed ESCs. Solder up the connections and run.

But before you decide to go brushless for a Beetleweight, you should consider the option of high-power brushed NERF motors, or even stock 22mm gearmotors. There is really only so much power that can be used in a insect-size arena.

Inrunner vs. Outrunner

Q: Is there a reason those who use brushless only have used the ones made for airplanes? Why not cars? Something like this: Turnigy 1/8th Scale 4-pole 2100KV. [Hagerstown, Maryland]

A: There are two broad types of brushless motor design:

  • Outrunners have a stationary core (stator) and a magnet-lined 'can' (rotor) that spins around the outside.
  • Inrunners have a stationary outer 'can' like a brushed motor with a spinning magnet rotor inside.
The outrunner-style motors are commonly used in smaller combat robots because they spin more slowly and with greater torque than inrunners. This makes it simpler to get the correct reduction for weapon use, and keeps input RPM low enough for inexpensive drive gearboxes.

As robots get larger you see more inrunner designs in use. Larger inrunner motors spin more slowly than the tiny ones which makes them a workable option for both drive and weapon power. The Turnigy motor you referenced has about the right amount of power for a hobbyweight weapon but spins at better than 38K RPM, which could be difficult for a belt drive to handle -- simpler to use an outrunner. For drive use, a pair of the Turnigy inrunners would have enough power for a featherweight 'bot, and this is probably the lightest class where I would consider using inrunner drive motors.

By the time you get up to heavyweight 'bots, the outrunner motor styles have disappeared and everyone uses inrunners if they're running brushless.

More Info

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