Introduction
A robot dexterous hand needs to replicate the force, speed, and precision of human fingers in a package small enough to fit inside a palm-sized enclosure. The motor driving each finger joint is the critical limiting factor - and for most serious dexterous hand designs, the answer is a frameless brushless motor: a stator and rotor kit without housing, bearings, or shaft, integrated directly into the finger structure.
This guide covers what makes frameless motors the right choice for dexterous hand applications, how to select the NdFeB magnet grade and silicon steel lamination for this specific duty cycle, and what engineers need to specify when sourcing these components.
1. Why Frameless Motors Are the Standard for Dexterous Hand Actuation
Conventional housed motors - even miniature ones - introduce three problems in dexterous hand design:
- Volume penalty: The housing, end caps, and output shaft add length and diameter that cannot be recovered. A 28 mm housed motor may require 45 mm of finger joint space; the equivalent frameless stator fits in 12 mm of axial depth.
- Transmission loss: Output through a shaft requires a gearbox or cable transmission, each adding backlash, friction, and compliance that reduce finger positional accuracy.
- Thermal isolation: The housing acts as a thermal barrier. Heat generated in the windings must travel through the housing to reach the structure - increasing winding temperature and reducing continuous torque.
A frameless motor kit eliminates all three. The stator is press-fit or adhesive-bonded directly into the finger knuckle structure; the rotor is mounted on the joint bearing. Heat conducts directly from the winding to the structural aluminum. There is no shaft, no gearbox, and no housing volume to account for.
The result: torque density 2–3× higher than an equivalent housed motor in the same joint envelope, with lower backlash and better thermal management.
2. Key Design Parameters for Dexterous Hand Frameless Motors
Dexterous hand duty cycles are unusual compared to standard servo applications. The motor must deliver:
| Parameter | Typical Range | Design Implication |
|---|---|---|
| Outer diameter | 20–45 mm | Constrained by finger width; drives pole count |
| Axial length | 8–20 mm | Thin pancake geometry; high pole count preferred |
| Peak torque | 0.1–1.5 N·m | Scales with air-gap flux density × winding current × radius |
| Continuous torque | 30–50% of peak | Dexterous hands operate in intermittent bursts, not continuous duty |
| Speed | 500–4,000 RPM at joint | Low speed, high torque - maximizes magnet utilization |
| Back-EMF constant Ke | Application-specific | Must match driver bus voltage; high pole count raises Ke |
The combination of small diameter and low speed pushes the design toward high pole count - typically 12 to 20 poles (6 to 10 pole pairs) - to maximize torque per ampere without increasing winding current beyond the driver's capability.
3. NdFeB Magnet Selection for Frameless Dexterous Hand Motors
The rotor of a frameless dexterous hand motor carries arc-segment or tile NdFeB magnets bonded to a thin steel back-iron ring. Magnet selection involves four decisions:
3.1 Grade and Energy Product
For most dexterous hand applications, the operating temperature at the magnet surface stays below 80°C under normal duty cycles. This allows the use of standard or M-suffix grades without elevated coercivity:
- N42 to N48: Standard choice for dexterous hand motors operating below 80°C magnet temperature. Maximum energy product 42–48 MGOe. Cost-effective, widely available in thin arc segments.
- N42H to N45H: Specified when the motor operates near a heat source or when thermal analysis shows magnet temperature may approach 100°C.
- N42SH: For high-cycle industrial manipulation robots operating in warm environments. Coercive force sufficient for sustained 150°C magnet temperature.
3.2 Arc Segment Geometry and Magnetization Direction
Frameless motor rotors use radial magnetization as the standard for most dexterous hand designs. Each arc segment is magnetized radially outward or inward, with a pole arc ratio of 0.80–0.95. Halbach-influenced parallel magnetization is used in high-performance designs to concentrate flux at the air gap and reduce back-iron thickness, lowering rotor inertia at the cost of tighter manufacturing requirements.
3.3 Thickness and Bonding
Arc segment magnets in frameless dexterous hand motors are typically 2–4 mm thick radially. Bonding uses structural epoxy with segments held in fixture during cure to maintain angular position within ±0.1°. Segment-to-back-iron concentricity must be verified before winding to avoid uneven air-gap flux that creates cogging torque and positional error at the finger joint.
4. Silicon Steel Lamination Selection for the Stator Core
Dexterous hand frameless motors operate at electrical frequencies of 270–1,000 Hz. At these frequencies, lamination thickness is the dominant variable in stator iron loss - and the correct choice is unambiguous:
- Thickness: 0.20 mm - reduces eddy current loss by ~67% versus 0.35 mm. Mandatory for electrical frequencies above 400 Hz.
- Grade: 20W1200H or 20W1500H - grain non-oriented, high-silicon for low hysteresis loss at high frequency.
- Stacking factor: ≥ 0.97 - verify with compressed stack measurement on first article.
- Slot geometry tolerance: ±0.03 mm - required to maintain consistent winding fill factor in a thin stator stack.
- Lamination flatness: ≤ 0.05 mm bow per sheet - critical in short axial stacks where cumulative error affects air-gap uniformity.
5. The Integrated Supply Advantage
The performance of a frameless dexterous hand motor is determined by the interaction between the stator lamination and the rotor magnet. These two components are not independent - sourcing them separately without a coordinated design review is one of the most common causes of under-performance in custom motor development.
Common mismatches when magnets and laminations are sourced from different suppliers:
- Magnet remanence specified at 20°C but stator iron loss causes the magnet to operate at 95°C - actual Br drops ~4%, reducing torque constant by the same amount
- Stator slot geometry designed for 0.35 mm laminations but switched to 0.20 mm at procurement - different stacking factor changes effective stack length and alters Ke
- Pole arc ratio optimized for one lamination permeability curve, producing excess cogging torque when a different grade is substituted
HIMAGNET supplies matched stator lamination stacks and rotor magnet kits for frameless dexterous hand motors, produced in-house from silicon steel and NdFeB material to the same magnetic circuit specification. Engineering support covers air-gap flux calculation, thermal modeling, and first-article inspection - from 20 mm finger joint actuators to 80 mm wrist torque motors.
6. Procurement Checklist
- ☐ Confirm outer diameter and axial length constraints from finger joint CAD
- ☐ Specify peak torque, continuous torque, and maximum joint speed
- ☐ Confirm driver bus voltage and maximum phase current to determine Ke target
- ☐ Specify magnet grade: N42–N48 standard; N42H–N45H for elevated temperature
- ☐ Specify lamination thickness: 0.20 mm for electrical frequencies above 400 Hz
- ☐ Request stacking factor report and flatness check on first-article lamination stack
- ☐ Confirm arc segment bonding method and positional tolerance (±0.1° typical)
- ☐ Request matched magnet + lamination package with air-gap flux calculation
Contact HIMAGNET's engineering team to discuss your frameless motor specification - from prototype to production.
