Humanoid robotics represents one of the most challenging applications for electric motor systems, requiring an optimal balance of power density, precision control, and energy efficiency. This guide provides a comprehensive framework for selecting motors based on joint-specific requirements and overall system integration considerations.
1.1 Torque Density Requirements
Lower limbs: 5-10 Nm/kg (stance phase demands)
Upper limbs: 2-5 Nm/kg (manipulation tasks)
Axial loading capacity: Minimum 3× body weight for impact absorption
1.2 Dynamic Response Specifications
Bandwidth: >50Hz for balance control
Settling time: <50ms for step adjustment
Acceleration: >100 rad/s² for dynamic motions
1.3 Efficiency Targets
Peak efficiency: >92% for BLDC/PMSM
Continuous operation efficiency: >85% at 30% load
Regenerative braking capability for energy recovery
2.1 High-Performance Options
Custom wound BLDC: 12-15 Nm/kg (MIT Cheetah derivatives)
Slotless PMSM: <1% torque ripple (surgical-grade precision)
Magnetic gear motors: Backlash-free torque amplification
2.2 Emerging Solutions
Dual-stator axial flux motors: 40% volume reduction
Liquid-cooled integrated modules: 20% higher continuous torque
Hybrid stepper-servo systems: Cost-effective precision
3.1 Optimal Transmission Selection
Strain wave gears: 80-120:1 ratio, zero-backlash
Magnetic gearboxes: Maintenance-free operation
Direct drive: Bearingless designs for compact joints
3.2 Thermal Management Strategies
Phase-change materials for peak loads
Microchannel cooling in stator windings
Thermally conductive potting compounds
4.1 Bipedal Locomotion Systems
Boston Dynamics Atlas: Hydraulic-electric hybrid
Tesla Optimus: 28 DoF all-electric actuation
Honda ASIMO: Distributed drive architecture
4.2 Manipulator Subsystems
Shadow Hand: Series elastic actuation
DLR Hand Arm System: Torque-controlled fingers
OpenAI robotic hand: Low-cost modular design
5.1 Decision Matrix
Performance (40% weighting)
Reliability (30%)
Integration complexity (20%)
Cost (10%)
5.2 Verification Process
FEM analysis for structural integrity
Thermal modeling for continuous operation
Dynamic simulation in MATLAB/Simulink
Conclusion
The motor selection process for humanoid robots requires multi-disciplinary optimization across electrical, mechanical, and control domains. Future developments in wide-bandgap semiconductors and advanced magnetic materials promise further improvements in power-to-weight ratios and energy efficiency.
