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Gobble Squabble: Selecting Motors

Gobble Squabble: Selecting Motors

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Motors are a key ingredient in your own customized design for a combat robot. Therefore, it’s recommended that you select your motors very early in your design process. Motors provide the drive for the wheels to move your robot around, and for the weapon (if your robot has one) to control and/or attack the opponent’s robot. Selecting motors requires attention to voltage rating, size, speed, power, shaft size, mounting options, durability, weight and cost. Here’s information to help you work through those factors to a good choice for your design goals, including some useful recommendations.

Drive Motors

These are the motors that move the robot around the arena floor. The robot will have 2 to 4 of these, one for each wheel that contacts the floor. Almost universally these are brushed DC gearmotors, which you must factor into your selection of the motor speed controllers (aka “ESC”) you will also use. The gears are internal to the motor and determine the theoretical top rotation speed of the output shaft, but the quality of the gears also determine the durability and reliability of the motor. You should plan to lose one drive motor for every 4-5 combat matches due to internal mechanical breakage unless you invest in highly-durable motors or try to protect your wheels (at additional weight); these are important trade-offs for your design and budget. (Hint: in your design, make it as easy as possible to replace the robot motors, while holding them securely in place in the robot while in operation.)

Key Performance Specs

  • Voltage: 12 volts or 6 volts, but even ratings as low “3-6 volts” can work. (Hint: these motors can be operated at higher than rated voltages, resulting in higher speed and power, but also more heat and mechanical wear. If possible, you should use motors that are rated for at or near the supply battery voltage. Running a 6 volt motor at 12 volts will double the speed of the motor and quadruple the power it must dissipate, most of which goes to heat.)
  • Speed: rated speeds in the range of 250 to 1000 rpm tend to work well. (Other key related factors are the size and traction of the wheels, and the drag of the robot body on the arena floor, which is the result of contact surface size and shape, and the slipperiness of the robot body material.)
    • The lower the rpm the greater the tractive power and controlability of the robot.
    • Note that some motors are specified based on the gear ratio: a higher gear ratio means a lower rpm, so ratios of 11:1 to 18:1 are similar to 1000 rpm, and 33:1 to 50:1 are comparable to 250 rpm.
    • If you plan to run a gearmotor at a different voltage than the spec, factor that in too: a 3 volt 150 rpm motor running at 6 volts will actually rotate in the 250 – 300 rpm speed range.
  • Size: the size of the motor, usually measured by gearbox diameter, is exponentially related to motor power and weight. The most common practice in Antweight robotics is to use gearmotors no larger than 16 mm as a balance of size, power and weight. Multiple smaller motors (e.g., 12 mm “micro gear motors”) have been used in some top-performing robots.
  • Reliability: motor motor failures in combat robotics are due to mechanical issues and breakage — usually of the internal gearbox. The electrical components rarely fail. Therefore, a primary focus for competitive roboticists is the quality and durability of the internal motor gears, and to either protect to wheels from impact or make the motors quickly replaceable. However, experience has shown that cheap motors with plastic gears are quite reliable too and may be considered to reduce the cost and weight of the robot motors.
Overview of gearmotors for Antweight robots

Available Sources

These are sources of drive motors of useful sizes for your Antweight combat robot. In the past they have provided products of useful quality and price, but there may be additional sources you should consider. If you tell us of other good quality sources we will update this listing.

  • Servo City: a great source for all kinds of robot parts, especially small mechanical parts useful for weapons. Here are some motor options:
    • 12 mm micro gear motors
    • 16 mm gear motors: Servo City doesn’t carry this size
    • 20 mm gear motors: these are probably too heavy for Antweight robots, but if you’re willing to work aggressively on trade-offs you might get them to fit in a defensive robot design.
    • Plastic gear motors: hint — run these at higher voltages to get more speed. These are the highest quality plastic gear motors you can get.
  • Fingertech: the “granddaddy” of Antweight (and Beetleweight) robotics, with a wide range of supplies and options. You can buy everything for your project here, if you chose. They generally sell high-quality items at good prices, so you’ll see them referenced throughout these pages. But you may find better options for your specific design and sometimes lower prices elsewhere. They are located in Canada, so expect 1 to 2 weeks of shipping time.
    • 16 mm gear motors: the “Silver Spark” is the very best 16 mm gear motor available. But often out of stock because of popularity.
    • 16 mm gear motors, option 2: the “Gold Spark” motors are also very good, and cost a little less than Silver Sparks.
  • Pololu: a reliable large supplier of good-quality core components, that not enough roboticists know about. Offers the best price and quality on a few key components.
    • 12 mm “high power” micro gear motors; for more options, review this summary page
    • 20 mm gear motors: these are probably too heavy for Antweight robots, but if you’re willing to work aggressively on trade-offs you might get them to fit in a defensive robot design.
    • Plastic gear motors:
      • Pololu brand: there are some really interesting options here, including very-small size and weight, and options to significantly increase the power of your plastic motor. A great place to research and innovate!
      • Solarbotics brand: many different motor configurations to give you design flexibility. Pololu delivers Solarbotics motors faster in the US than Solarbotics can.
  • RobotShop: is a reseller, where you can find many of the other products listed here, plus additional options, sometimes at better prices. Robotshop allows you to bundle up purchases from many different sources into one shipment from one store, which may be convenient and reduce shipping costs. These listings are just a sample of what you can buy through RobotShop.
  • Solarbotics: another reliable source with a good reputation. They offer many unique options, including 14 mm gear motors. They cater to hobby robotics rather than combat robotics, so their gear ratios tend to be too high for combat. They are based in Canada so expect up to 2 weeks for delivery.
  • LEGO: if you’re a fan of LEGO and you have some M motors, shaft pieces and wheels around, all those could be useful in your Antweight robot (although the motors are on the heavy side). These listings are for M motors, which will require modification of the electrical leads to work with robot motor speed controllers (check here for more info — break-out and connect to the C1 and C2 wires). Assume the motors are rated for 6 volts, but probably will work OK at 12 volts.
  • VEX is an outstanding supplier of robot components, but to our knowledge their current lineup of motors is just too large and heavy for use in Antweight robots. However, they do make an amazing and useful motor controller/ESC that can be used in this scale of robot.
  • eBay: 10 years ago, eBay was the only source for many of these components, but other suppliers now offer equal or better quality, almost as many options, and (much) faster shipping. Be aware that most eBay suppliers in this space are located in China, so shipping times can be multiple weeks. The listings provided here were current in early October 2019, and are just examples — NOT recommendations.
  • Amazon: has jumped into this space fully, with a wide-range of products available — you can purchase all the robot parts you need from Amazon, if desired. Most of the products are actually provided by 3rd-party sellers, so the quality may be variable. Amazon Prime’s one-day shipping is hard to beat when you’re faced with a deadline, but many 3rd-party products are actually shipped from China so carefully read the shipping details if time is an issue. These listings are just examples of what you can find on Amazon, not recommendations:

Weapon Motors

These days rotating weapons on Antweight combat robots are almost always driven by brushless DC motors, with any gearing (if used at all) external to the motor. Direct coupling of the motor to the weapon is common, although mechanical isolation through belt drive can also be implemented within the weight limit (this protects the motor to some degree, but decreases the weight of the weapon). For weapon drives, brushless motors with low KV ratings (i.e., low rpm per drive voltage) are recommended, so drive power is most effectively turned into hitting power. Experience has shown that 22 mm brushless motors (size 22xx) and 28 mm (size 28xx) with KV ratings from 600 to 2200 work well for this application.

Available Sources

Servos

Servos can be very useful in Antweight combat robots as the weapon drives for lifters and arms. Standard-size hobby servos can fit into robots of this size and weight with a bit of good design. HiTec 3xx series servos are a good choice for maximum power, cost and quality. Micro (“9g”) servos are another good option that might provide enough power in a smaller size and weight. Note that servos might not be able to tolerate direct connection to 12 volt DC power; they may become jittery and overheat, even burn up; to prevent this, a lower-voltage regulated supply should be provided.

Continuous-rotation servos can be used as the robot drive motors: they’re simple to use and connect directly to the control receiver (no motor controller required), but they tend to be slow and provide less power that the drive motors described above.

Available Sources for Weapon-Drive Servos

Available Sources for Continuous-Rotation Servos

This information is original work by Techno Chaos and is published under the terms of Creative Common license mode Attribution-NonCommercial-ShareAlike (CC BY-NC-SA).