Selecting the right actuator for each humanoid robot joint is the most consequential engineering decision in robot design. Get it wrong, and you compromise the entire kinematic chain—too heavy means poor dynamics; too weak means instability. This guide walks through every selection criterion in order: torque, weight, bandwidth, backlash, communication, and thermal limits—with concrete numbers for each joint.
1. The Five Selection Dimensions
Every humanoid robot joint actuator must be evaluated across five coupled dimensions. Missing any one leads to system-level failure:
2. Torque Requirements by Joint
The required joint torque depends on distal mass, link length, and motion profile. The table below provides practical minimum values for a 60-80 kg humanoid robot at walking speed (1.5 m/s):
| Joint | Min Rated Torque | Min Torque Density | Recommended Model |
|---|---|---|---|
| Hip (pitch/roll) | 60-100 Nm | ≥25 Nm/kg | ZHR-H25 (91 Nm) |
| Knee | 50-80 Nm | ≥25 Nm/kg | ZHR-H20 (64 Nm) |
| Ankle | 30-40 Nm | ≥28 Nm/kg | ZHR-H17 (43 Nm) |
| Shoulder | 20-30 Nm | ≥30 Nm/kg | ZHR-H17 (43 Nm) |
| Elbow | 10-20 Nm | ≥30 Nm/kg | ZHR-H14 (14 Nm) |
| Wrist / Hand | 3-10 Nm | ≥35 Nm/kg | CyberGear (12 Nm) |
3. Choosing the Reducer Type
For humanoid robots, two reducer technologies dominate. The choice hinges on the joint's primary load profile:
Strain Wave Gear (Harmonic Reducer)
- ? Torque density: 30-40 Nm/kg
- ? Backlash: <1 arcmin (near-zero)
- ? Compact pancake form factor
- ? Lower shock resistance (2-3± rated)
- ? Higher unit cost
Best for: arm joints, wrists, precision tasks requiring force control
View ZHR-H Harmonic Series →Planetary Gearbox
- ? Shock tolerance: 3± rated torque
- ? High efficiency: 90%
- ? Wide speed range
- ? Backlash: 3-10 arcmin
- ? Lower torque density (10-20 Nm/kg)
Best for: hip/knee joints, high-impact locomotion, AGV applications
View ZHR-P Planetary Series →Quick rule: If the joint needs force control at <3 arcmin backlash harmonic (ZHR-H). If the joint sustains impact loads >3± rated torque planetary (ZHR-P).
4. Motor Constant Km: The Hidden Efficiency Metric
The motor constant Km (also written Km) is defined as:
Km = Kt / Rm
Kt = torque constant (Nm/A) | Rm = winding resistance ()
A higher Km means the motor produces more torque per unit of heat generated. For humanoid robots where thermal budget is tight, Km is the most important motor-level metric. Typical values:
| Application | Min Km (Nm/W) |
|---|---|
| Hobbyist / Educational | > 0.05 |
| Industrial Cobot | > 0.10 |
| Humanoid Robot (recommended) | > 0.15 |
5. Control Bandwidth Requirements
Walking stability at 1.5 m/s requires limb joints to respond to disturbances in <50 ms—which means the torque control loop must run at 500 Hz and the communication bus must support 1 kHz update rates.
ZHR-P series with EtherCAT supports 2,000 Hz real-time control loops. ZHR-H series with CAN/RS485 supports 500-1,000 Hz, sufficient for most arm joints.
6. Form Factor & Integration Constraints
Humanoid joint geometry imposes strict envelope constraints. Integrated joint modules (motor + reducer + encoder + driver in one housing) eliminate 20-30% of total mass versus component assembly.
| Model | Diameter | Mass (kg) | Rated Torque (Nm) | Torque Density |
|---|---|---|---|---|
| ZHR-H14 | 70 mm | 0.52 | 5-14 | 65.7 Nm/kg |
| ZHR-H17 | 80 mm | 1.30 | 13-43 | 33.1 Nm/kg |
| ZHR-H20 | 90 mm | 2.10 | 20-64 | 30.5 Nm/kg |
| ZHR-H25 | 110 mm | 3.00 | 27-91 | 30.3 Nm/kg |
| CyberGear | 98 mm | 0.317 | 12 | 37.9 Nm/kg |
7. Selection Checklist Summary
- 1Calculate required rated torque per joint using gravity + inertia + friction formula
- 2Apply 1.5± safety factor (sustained loads) or 2.0± (shock/impact loads)
- 3Verify torque density joint requirement (hip 5, shoulder 0, wrist 5 Nm/kg)
- 4Choose reducer type: harmonic if backlash <3 arcmin required; planetary if shock >3± rated
- 5Confirm Km ≥0.15 Nm/W for humanoid-grade thermal budget
- 6Verify control bandwidth: EtherCAT for legs (>1 kHz), CAN/RS485 for arms (500 Hz)
- 7Check form factor fits the joint envelope (diameter, stack height, hollow shaft needed)
Not sure which series is right? Try our interactive Product Selector Guide.
Related Reading
- → Robotic Backlash Explained: Measurement & Mitigation Guide
- → Torque Density Formula: τ/m Calculation Guide (Nm/kg)
- → Harmonic vs Planetary Gearboxes: Actuators in Robots like Tesla Optimus
- → Definitive Buyer's Guide: Planetary Robot Actuators & Integrated Joint Modules [2026]
- → Integrated Robot Joint Module Guide: Selection & Specification
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