AXF-1, printable axial-flux actuator
A fully-printable joint actuator: yokeless axial-flux PMSM with printed Fe-Si armature poles, Cu-LPBF concentrated coils, bonded-NdFeB Halbach rotors, and an 8:1 printed cycloidal reduction. Every rating below is derived from the sizing physics shown in §REL, a bureau quotes the dimensioned geometry and per-part process; the numbers are design intent to be confirmed by EM sim and coupon test.
Electromagnetic & geometry
quote these dimensions| Parameter | Value | Basis / note |
|---|---|---|
| Topology | Yokeless axial-flux PMSM (YASA-type) | Segmented poles, no stator yoke → short 3D flux path. The one geometry where isotropic printed iron beats laminations. |
| Slot / pole count | 12s / 14p | Fractional-slot concentrated winding, kw ≈ 0.933. Cogging order LCM(12,14)=84 → inherently smooth. |
| Outer diameter Do | 110 mm | Sets the torque via Ro³ (§REL). Scale Do to buy torque back from the bonded-magnet penalty. |
| Inner / outer ratio | Ri/Ro = 0.577 → ID ≈ 63.5 mm | 1/√3 maximizes torque for a given Do, the classic axial-flux optimum. |
| Axial length (motor) | ≈ 30 mm | Pancake. Envelope only; active-material stack is thinner. |
| Airgap (×2) | 0.6 mm target · <0.5 mm hard | Print + assembly + thermal-growth + bearing runout limited. Bonded machines are less gap-sensitive than sintered (lower Br, longer magnetic length). |
| Gap flux Bg,rms (fundamental) | ≈ 0.40 T (sintered ≈ 0.85 T) | The crux number. Follows from bonded Br + Halbach one-sided boost. Confirm by coupon (MEAS-1). |
| Electric loading Arms | 30 kA/m cont | Enabled by Cu-LPBF fill. σ = Bg·A ≈ 12 kPa continuous. |
| Electrical frequency | ≈ 350 Hz @ 3000 rpm | 7 pole-pairs. Drives the core-loss requirement on printed Fe-Si → SMC-class resistivity (MEAS-2). |
| Reduction | 8:1 printed cycloidal, η ≥ 0.85 | Quasi-direct-drive. Backlash + η set control bandwidth (MEAS-5). |
Materials & per-part process
one line = one printed part| Part | Material | Process | Key spec / caveat |
|---|---|---|---|
| Armature poles ×12 | Fe-6.5%Si / SMC | metal LPBF | Isotropic, high resistivity for 350 Hz. Target ≥ 1.5 T at knee. Bound-metal FFF+sinter is the low-cost fallback. |
| Concentrated coils ×12 | Cu (≥87% IACS) | Cu-LPBF green-λ | Fill ≥ 65% (goal 79%). Turn insulation is NOT printable → conformal dielectric coat (parylene / ceramic e-coat) + hi-pot post-print. |
| Rotor magnets ×2 | bonded NdFeB, Halbach | FFF / binder-jet | Net-shape segmented Halbach, then magnetize in fixture. Br target 0.60–0.65 T. Self-shielding → no back-iron. |
| Rotor discs ×2 | PA-CF or Al | MJF / FDM / LPBF | Structural only. Halbach removes the flux-return duty → thin discs, lower rotor inertia. |
| Cycloidal set (8:1) | PA-CF / PPA-CF | MJF / SLS | Isotropic powder part; disc + pins + eccentric. η ≥ 0.85, backlash to be characterized. |
| Bearings / gap control | 316L flexure or x-roller | LPBF + post-machine | Sets airgap concentricity: target ≤ 50 µm runout. The airgap is a bearing-precision problem. |
| Frame / pole carrier | PA-CF | MJF / FDM | Holds the 12-pole ring, both gaps, and stator reaction torque. |
| ↳ consolidation | Fe-Si + CuCrZr | Aerosint SPD | Poles + coils co-printed in one multi-metal LPBF build, collapses the two hardest parts into a single job. Phase-3 target. |
A bonded-magnet machine loses on raw torque density. Torque scales linearly with gap flux, and bonded NdFeB delivers roughly half the Bg of sintered, so at equal size and current this motor lands near 0.5–0.6× a sintered QDD (Unitree M8010 ≈ 45 N·m/kg peak). Worse, lower flux means lower Kt, so it draws more current per newton-metre → more copper loss → the efficiency penalty compounds the torque penalty.
The single lever that fights both is copper fill: Cu-LPBF reaches 65–79% vs ~45% for round wire, which lowers phase resistance, raises Km, and reclaims thermal headroom (Additive Drives: 65% fill → +45% output). So the targets that matter here are Km, joint torque after reduction, and joules per N·m·s, plus the things sintered can't buy: full printability and a CC0 prior-art release. Density is the tax you knowingly pay for sovereignty.
Measure first: ranked by leverage
coupons before the CAD is frozenBr of the printed + magnetized bonded-NdFeB coupon
Everything scales off this. T ∝ Bg ∝ Br, linear. If Br comes in at 0.50 T instead of 0.65 T, the whole torque envelope drops ~23%. Print a coupon, magnetize, measure on a B-H loop before committing geometry.
Core loss of printed Fe-Si / SMC at 200–500 Hz
Sets efficiency, the thermal ceiling, and max usable speed. Printed iron loses more than laminations at frequency, verify the resistivity is high enough that 350 Hz is comfortable, not marginal.
Achieved Cu fill · IACS · insulation integrity
Directly sets Km, phase resistance, and thermal margin. Measure the real fill fraction, conductivity vs annealed copper, and hi-pot the coated coil turn-to-turn.
Assembled airgap after thermal soak
Magnet CTE + bearing runout + disc deflection under magnetic pull. Confirm the 0.6 mm gap survives a hot rotor and doesn't close to a rub.
Cycloidal efficiency & backlash of the printed gearset
Sets joint efficiency and the achievable control bandwidth. η and lost motion in a printed cycloidal are the two numbers that decide whether the joint is stiff enough to walk on.