The Complete Guide to Magnets for Humanoid Robot Actuators

A humanoid robot actuator is a self-contained motion unit consisting of a motor, a gear reducer (usually harmonic drive or cycloidal), position sensors, a motor driver, and a housing. The motor is almost universally a permanent magnet synchronous motor (PMSM) using NdFeB magnets. The magnet assembly is the energy source of the motor - it determines the motor's torque constant (Kt), back-EMF constant (Ke), and ultimately the actuator's force and speed capabilities. Getting the magnet right is non-negotiable.

Start with the actuator's output requirements and work backward through the gear ratio to find the motor's operating point. For each joint in the robot, define the peak torque, continuous torque, max speed, and duty cycle. Then determine the motor torque requirement: Motor Torque = Actuator Torque / Gear Ratio / Efficiency. This motor torque requirement, combined with the motor's target dimensions, determines the required magnet flux density - and therefore the magnet grade.

• •Knee/hip actuators: Highest torque, typically 200–360 N·m at the joint, requiring the strongest magnets
• •Shoulder/elbow: Medium torque, 50–120 N·m at the joint
• •Wrist/ankle: Lower torque but highest precision requirements
• •Finger actuators: Miniature magnets with extreme precision requirements

Grade selection balances three factors: flux density (torque), temperature stability (reliability), and cost. For humanoid robots, we recommend the following decision framework:

• •If max motor temp ≤ 80°C: N48 or N50 (rare - most motors run hotter)
• •If max motor temp ≤ 120°C: N48H or N50H - good balance of performance and cost
• •If max motor temp ≤ 150°C: N48SH - the most popular grade for robotics (recommended)
• •If max motor temp ≤ 180°C: N45UH - for extreme-duty applications
• •Always add a 30°C safety margin above your expected max temperature

For humanoid robot joint motors, the two practical options are radial multi-pole rings and segmented arc assemblies. We strongly recommend radial multi-pole rings for any application where torque smoothness, vibration, and motor efficiency are important - which includes virtually all humanoid robot joints.

• •Radial multi-pole ring: Best performance, best consistency, simplest motor assembly
• •Segmented arcs: Lower tooling cost, more geometry flexibility, adequate for less demanding joints
• •Bonded magnets: Only for very low torque applications - insufficient flux density for joint motors
• •Hybrid approach: Some designs use radial rings for legs and segmented arcs for arms to optimize cost

The critical dimensions for a motor magnet ring are: Outer Diameter (OD), Inner Diameter (ID), Height (H), and for multi-pole rings, the pole angle accuracy. For humanoid robot actuators, tight tolerances on ID are essential because the air gap between the magnet and the stator determines motor performance. A 0.1mm change in air gap can cause a 5–10% change in torque output.

• •ID tolerance: ±0.02mm (servo-grade) - this is the most critical dimension
• •OD tolerance: ±0.05mm (typically less critical due to housing interference fit)
• •Height tolerance: ±0.05mm
• •Concentricity: ≤0.05mm (alignment of ID to OD centers)
• •Perpendicularity: ≤0.03mm (critical for bearing alignment)

Beyond the grade designation, specify the actual magnetic property ranges you need. The grade name is a nominal designation - actual production values vary. A reputable supplier will provide guaranteed minimum values.

• •Br (Remanence): The magnetic field strength. Specify a minimum value, e.g., Br ≥ 13.8 kGs
• •Hcj (Intrinsic Coercivity): Resistance to demagnetization. Specify minimum, e.g., Hcj ≥ 20 kOe for SH grades
• •BH Max: Energy product. Specify range, e.g., 46–49 MGOe for N48SH
• •Flux consistency: Specify ring-to-ring variance limit, e.g., <1% peak surface flux variance
• •Request demagnetization curves at your operating temperature - not just room temperature data

When magnets arrive, you need a systematic incoming inspection to catch problems before they reach your motor assembly line. Minimum inspection protocol for robotics-grade magnets:

• •Dimensional check: 100% inspection of ID, OD, Height on first batch; AQL sampling thereafter
• •Flux measurement: Gaussmeter check of surface flux at defined positions - compare to supplier data
• •Visual inspection: Check for chips, cracks, coating defects, and contamination
• •Coating adhesion: Tape test (ASTM D3359) on sample pieces from each batch
• •Weight check: Weigh each ring - weight variance correlates with density and magnetic consistency
• •Certificate review: Compare supplier COC (Certificate of Conformance) data to your incoming measurements