Electric motors are the fundamental building blocks of modern robotics and drones, yet their optimal design varies dramatically based on application. A drone motor, robotic arm actuator, and legged robot joint each require distinct characteristics for peak performance. This post explores these differences and provides insights into application-specific motor design.

Understanding Operating Conditions: The Key to Optimal Design

The first step in motor design is understanding how the actuator will be used. Different applications exhibit distinct load-speed duty cycles - patterns of operation that fundamentally influence design choices.

Let's examine three key applications:

1. Drone Propulsion

• Motors operate predominantly at high speed and high load

• Efficiency matters most at operational power levels

• Weight directly impacts vehicle payload capacity

• Core losses at high speed are acceptable if overall efficiency is maintained

2. Robotic Arms/Manipulators

• Experience varying loads at different speeds

• Often hold static positions

• Limited range of motion (typically less than one full rotation)

• High-speed operation is intermittent

• Thermal management during static holds is crucial

3. Legged Robots

• Face dynamic loads and impacts

• Operate in cycles of high and low power — this influence thermal management

• Require excellent torque control

• Must handle peak loads many times their continuous rating

Loss Mechanisms and Their Impact

Motor losses come in two main forms:

1. Copper Losses: Proportional to the square of current (I²R)

• Dominant during high-torque, low-speed operation

• Critical for static holding in manipulators

• Primary concern for thermal management

2. Core Losses: Increase with speed

• More significant in high-speed operation

• Less impactful in robotics due to limited speed range

• Can be significant in drone motors at operational speeds

Understanding these losses helps inform design choices. For instance, drone motors can tolerate higher core losses if they achieve better power density, as they primarily operate at a single high-speed point. In contrast, robotic actuators need to manage copper losses effectively for static holding and low-speed operation.

Design Trade-offs and Solutions

Pole Count Considerations

The number of magnetic poles in a motor significantly impacts its characteristics:

High Pole Count Benefits:

• Improved torque density

• Reduced end-turn length

• Lower rotor inertia

• Reduced armature reaction (beneficial for impact tolerance)

High Pole Count Challenges:

• Increased electrical frequency

• Higher core losses

• More complex control requirements

• Potential for increased leakage flux

For robotics applications, particularly those requiring backdrivability and impact tolerance, motors with higher pole counts coupled with moderate reduction ratios (10:1 to 20:1) can offer excellent performance. This approach provides good torque density while maintaining controllability and shock tolerance.

Winding Configurations

Double-layer concentrated windings offer several advantages for robotics and drone applications:

• Higher power density

• Better thermal performance

• Simpler manufacturing

• Shorter end turns

• Higher slot fill factor possible

The trade-off is slightly lower torque production compared to distributed windings, but this is often outweighed by the practical benefits.

Application-Specific Design Guidelines

Drone Motors

• Use as high of a pole count in the design as possible to maximize power density and minimize weight

  • The bottle neck here will be space for fitting the copper windings

• Design for shallow stator teeth to reduce core losses at high speeds

• Accept higher slot current density since operation is primarily at a single operating point

• Optimize winding design for efficiency at operational speed (typically >70% max speed)

• Keep rotor mass minimal as it affects vehicle dynamics

• Use concentrated windings for better thermal performance and simpler manufacturing

• Core losses up to 50% of total loss at rated power are acceptable if achieving high power density

Robotic Arm Actuators

• Use moderate pole counts to balance torque density and control quality

• Design deeper stator teeth for better thermal management during static holds

• Optimize tooth width and area ratio (30-40% of total area) for maximum torque density

• Keep air gap minimal (0.5-1mm) to maximize torque production

• Use concentrated windings for better thermal performance

• Design for continuous operation at 20-30% of peak torque

• Keep core losses under 20% of total losses to maintain static efficiency

• Consider sensor integration and through-hole designs for cabling

  • Packaging is one of the most challenging areas for design trade-offs

Legged Robot Actuators

• Use higher pole counts to reduce armature reaction and improve impact tolerance

• Design moderate tooth depths to balance torque density and dynamic performance

• Maintain good backdrivability with gear ratios typically under 20:1

• Size for peak torques 3-4x continuous rating for impact conditions

• Keep rotor inertia low for faster dynamic response

• Use concentrated windings to maximize slot fill factor and torque density

• Design thermal management for cyclical loading patterns vs. constant load

• Ensure adequate air gap (0.8-1.2mm) for mechanical robustness

  • Under high impact loading, material deformation can cause operational problems

• Consider integrated position/torque sensing

  • Generally this is critical for advance locomotion control strategies

Performance Metrics

When evaluating motor designs, consider these key metrics:

1. Torque Density (Nm/kg)

• Critical for robotics applications

• Define at a specific copper loss for fair comparison

• Consider active mass (magnets, copper, steel)

2. Power Density (W/kg)

• More relevant for drone applications

• Consider continuous operation point

• Factor in cooling capability

3. Efficiency

• Consider application-specific operating points

• Balance copper and core losses

• Account for thermal constraints

Practical Implementation Tips

1. For Robotic Applications:

• Consider using higher pole count motors with moderate reduction ratios

• Focus on thermal management for continuous operation

• Design for good controllability at low speeds

• Consider backdrivability requirements

2. For Drone Applications:

• Optimize for operational point efficiency

• Use high pole counts for better power density

• Focus on weight reduction

• Consider integrated cooling solutions

Wrapping It All Up!

Successful actuator design requires a deep understanding of application-specific requirements and constraints. The key is matching motor characteristics to application needs while considering practical limitations:

• Drone motors can sacrifice low-speed performance for high-speed efficiency and power density

• Robotic arm actuators need to balance control quality with torque density

• Legged robot actuators must handle impacts while maintaining good dynamic response

Remember that theoretical optimal design must be balanced against practical constraints like cost, manufacturability, and reliability. The best design is often one that satisfies the critical requirements while maintaining reasonable trade-offs in other areas.

Need Help with Designing and Manufacturing?

Ethereal specializes in the design and manufacturing of low-cost, reliable, and high-volume electric actuators to serve robotics, drone, and industrial automation companies.

We would love to learn more about your use case, and if we can partner with you on your development and production process.

If you are interested in learning more, please contact us!