The Complete Guide to Magnets for Humanoid Robot Actuators

Q: What grade of NdFeB magnet is best for humanoid robot joint motors?

N48SH is the working standard for most humanoid robot joint motors — 45–48 MGOe BHmax, 150°C continuous rating, HcJ ≥20 kOe. N45SH is the cost-down alternative for less torque-dense designs. N45UH at 180°C steps up for high-duty hip and knee joints. Always specify GBD processing — it cuts heavy rare earth content by 50–70% at equal performance.

Q: How tight should magnet ring tolerances be for humanoid actuators?

ID tolerance ±0.02mm, OD tolerance ±0.05mm, concentricity ≤0.02mm, perpendicularity ≤0.03mm. Air-gap sensitivity in joint motors is extreme — a 0.1mm variation in effective air gap causes 5–10% torque output variation. Surface flux variance must stay under 1% across a production lot. 100% dimensional inspection on incoming batches is standard practice for serious production programs.

Q: What is the difference between Kt and Ke for an actuator magnet?

Both are determined by the magnet flux. Torque constant Kt (N·m/A) describes how much torque the motor produces per amp of current. Back-EMF constant Ke (V·s/rad) describes the voltage generated per unit angular velocity. In SI units they are numerically equal for an ideal motor — Kt = Ke. The magnet's remanence (Br) and the air-gap geometry are the dominant inputs to both, which is why magnet flux consistency directly drives motor consistency across a production fleet.

Q: Should I use radial multi-pole rings or segmented arc magnets in humanoid joint motors?

Radial multi-pole rings for any joint where torque smoothness matters — which is virtually every humanoid joint. Sinusoidal surface flux distribution drives 2–5% torque ripple versus 8–15% for segmented arc assemblies. The torque smoothness translates directly into control stability and the fluid motion humans expect from human-interactive robots. Above 500 units/month the cost-per-unit converges with segmented as assembly labor is eliminated.

Q: How many magnets does a typical humanoid robot use?

20–50 actuator motors per humanoid depending on degrees of freedom, with one magnet ring or assembly per motor. A bipedal platform like Agility Digit has around 16 actuated joints. Full humanoids like Figure 03 and Tesla Optimus Gen 3 have over 50 actuators per hand alone, putting the total well above 100 magnetic motors per unit. Total NdFeB content per humanoid runs 1.5–3.5 kg.

Q: Can I source humanoid robot magnets directly from a Chinese factory under MOFCOM export controls?

Yes, with the right supplier. Commercial robotics applications are routinely approved under China's April 2025 MOFCOM licensing regime for SH and UH grades — license timelines run 6–10 weeks under normal conditions. The key is choosing a manufacturer that operates its own MOFCOM licensing pipeline rather than routing through a trading company. See China rare earth export controls in 2026 and How US and EU Tier 1 suppliers qualify Chinese magnet manufacturers for the full process.

Q: What incoming inspection should I run on humanoid actuator magnets?

100% dimensional inspection on the first production batch (ID, OD, height, concentricity), then AQL sampling thereafter. 100% surface flux measurement with a gaussmeter at defined reference positions. Tape test (ASTM D3359) for coating adhesion on sample pieces from each batch. Weight check on every ring — weight variance correlates with density and flux consistency. Reconcile supplier Certificate of Conformance data against your own measurements before releasing the lot to motor assembly.

01Actuator Architecture Overview
02Step 1: Determine Your Torque and Speed Requirements
03Step 2: Select the Magnet Grade
04Step 3: Choose the Magnet Architecture
05Step 4: Define Critical Dimensions and Tolerances
06Step 5: Specify Magnetic Properties
07Step 6: Incoming Inspection Protocol
FAQFrequently Asked Questions

Key Takeaways

◆A typical humanoid actuator is a PMSM motor + harmonic or cycloidal reducer + sensor stack — the magnet is the energy source that determines torque constant Kt and back-EMF Ke.
◆Grade selection follows operating temperature: N48H below 120°C, N48SH for 120–150°C (the working standard), N45UH above 150°C — always add a 30°C safety margin.
◆Radial multi-pole sintered rings are the correct architecture for any joint where torque smoothness matters — 2–5% torque ripple vs 8–15% for segmented assemblies.
◆Critical tolerances for servo-grade joint motors: ID ±0.02mm, OD ±0.05mm, concentricity ≤0.02mm, perpendicularity ≤0.03mm — air gap variation of 0.1mm causes 5–10% torque variation.
◆Specify GBD-processed grades by default in 2026 — the cost premium is small or negative once heavy rare earth savings are counted, and Dy/Tb supply exposure is reduced.
◆Demand 100% flux mapping with serial-number traceability and at-temperature BH-curve data, not nominal spec-sheet values — production-grade qualification requires production-grade documentation.

Actuator Architecture Overview

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.

Step 1: Determine Your Torque and Speed Requirements

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

Step 2: Select the Magnet Grade

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 joint 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

Key Insight: We see many robotics companies start with N52 (highest flux) and then have demagnetization problems in the field. N48SH with GBD processing is almost always the better choice — it gives you thermal safety margin with only a 5% reduction in flux compared to N52. See the N45SH vs N48SH comparison for the practical SH-grade trade-off.

Step 3: Choose the Magnet Architecture

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. See our radial multi-pole vs arc segment comparison for the side-by-side trade-off.

•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

Step 4: Define Critical Dimensions and Tolerances

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)

Step 5: Specify Magnetic Properties

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

Step 6: Incoming Inspection Protocol

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

Key Insight: Tip: Ask your magnet supplier to ship individual flux data for each ring, keyed to a serial number. This lets you trace any motor performance issue back to a specific magnet — invaluable for root cause analysis. For the full Tier 1 qualification process see How US and EU Tier 1 suppliers qualify Chinese NdFeB manufacturers.

FAQ

Frequently Asked Questions

What grade of NdFeB magnet is best for humanoid robot joint motors?

N48SH is the working standard for most humanoid robot joint motors — 45–48 MGOe BHmax, 150°C continuous rating, HcJ ≥20 kOe. N45SH is the cost-down alternative for less torque-dense designs. N45UH at 180°C steps up for high-duty hip and knee joints. Always specify GBD processing — it cuts heavy rare earth content by 50–70% at equal performance.

How tight should magnet ring tolerances be for humanoid actuators?

ID tolerance ±0.02mm, OD tolerance ±0.05mm, concentricity ≤0.02mm, perpendicularity ≤0.03mm. Air-gap sensitivity in joint motors is extreme — a 0.1mm variation in effective air gap causes 5–10% torque output variation. Surface flux variance must stay under 1% across a production lot. 100% dimensional inspection on incoming batches is standard practice for serious production programs.

What is the difference between Kt and Ke for an actuator magnet?