RM Hub Motor V2

This post covers the development, production, testing, and reflection of my 2nd generation combat robotics hub motor. The motor is based off of the RM Hub Motor v1, improving upons its exceptional durability and power density. The V2 features a stator attatched to a removable post, allowing the stator to to be rapidly replaced in the event of a failure. Overall, the motor was a complete success, demonstrating exceptional reliability, power density, and servicablility as compared to competiting products. All trial units were immediately sold, demonstrating a positive reception and high demand.

Combat robotics is an emerging sport in which two opposing robots attempt to disable one another. Over time, dominant competitive strategies have led to the prevalence of several distinct archetypes. Among these, robots equipped with rotary weapons have become the most consistently effective. These systems, commonly referred to as spinners, provide a comparatively simple and efficient means of storing and releasing kinetic energy for damaging an opponent.

Although the underlying principles of spinner weapons are relatively simple, the extreme forces involved require consideration. A weapon "blade" must be spun to high speeds, typically in excess of 30k rpm, and withstand the reaction forces of coming to a complete stop in an instant on impact. In addition, weapons are often used in a defensive manner, resisting off-plane impacts from opposing robots. To meet these constraints, I previously designed a custom reinforced outrunner motor that was capable of spinning over 35,000 rpm and withstanding repeated impact. This design was a resounding success, winning multiple competitions and drawing considerable attention from the combat robotics builder community. Despite the performance, there were a few notable drawbacks to the design. The design was made to be easily machined on basic tooling, resulting in an unrefined form that carried excess weight. In addition, the stator was adhered to central shaft, making bearing maintainence difficult. As such, I endeavored to design a V2 which addressed these shortcomings and could be mass-produced and sold.

For the design to be an improvement over the previous generation, it had to adhere to certain design criterion:

• Durability- Fail under no resonable circumstance

• Servicability- Ability to replace bearings, stators, and rotors with basic hand tools

• Performance- Maintain 200W continuous output for 3 minutes

• Weight- Lighter than competitors (<70g)

• Size- 20mm face to face, smaller than all competitors

• Cost- Competitive pricing to current market competitors ~70$

Core

Outer

I redesigned the outer shell to remove excess weight while maintaining full strength, resulting in a 38% weight reduction. Due to the live tooling process, chamfers were applied on the outer perimeter to reduce air resistance, deflect impact, and improve aesthetics. 2024 Alu was selected for its high strength and fracture resistance. Extra material was added around bolt heads to mitigate damage. The bearing pockets were reinforced to better contend with out-of-plane impacts. Lastly, tolerances were refined to target a h7 slip fit to improve serviceability in the field. 

Electronics

The core of the motor is a 2205 2300kv stator. Each stator was potted in a high-temperature silicone conformal coat to protect against foreign debris and internal shorting. The conformal coat was selected for its high thermal conductivity to maximize heat dissipation. The rotor uses n52 magnets to increase field strength, and are potted in a shock resistance epoxy to prevent breakage on impact. 

With a final manufacturing cost of $55, weight of 62g, and small size, the motor meets the preliminary design specifications. To assess its performance, I used it in my personal robots and distributed beta samples. 

Bots utelizing my design amassed over 15 podium finishes over the subsequent 6 months, consistently beating competing hub motor designs. Throughout over a hundred fights, not a single motor has experienced a failure, even when fighting robots in a larger weight class. The performance of the motor secured me a spot in the world championships where I announced the sale of my motor and subsequently sold all available units.