For the first time a complete humanoid actuator set is public, part numbers included, and part numbers lead to datasheets. Menlo Research published the full bill of materials for Asimov 1, its open source humanoid, in August 2026: 25 powered joints, five actuator part numbers, with reduction ratios, torque constants, joint limits, velocity ceilings and friction parameters for every joint [1][2]. Humanoid actuator selections are normally a torque figure and a photograph. We mapped this one and cross checked every number against the manufacturer's datasheets [3]. This is the notebook version of our post of 13 August 2026 [5].
The machine is 1.20 m and 35 kg, with 25 powered joints and two passive toe hinges: six degrees of freedom per leg, five per arm, one waist yaw and two in the neck [1]. Motion control runs on a Radxa CM5 across six CAN branches [2].
Five part numbers cover 25 joints
Every actuator is an ENCOS unit with reducer, dual encoders and a CAN driver inside one sealed housing [1][3].
| Part number | Qty | Reducer | Ratio | Rated | Peak | Speed | Mass | Where |
|---|---|---|---|---|---|---|---|---|
| EC-A4310-P2-36 | 10 | Planetary | 36:1 | 12 Nm | 36 Nm | 75 rpm | 382 g | ankles, elbows, wrists, neck |
| EC-A4315-P2-36 | 4 | Planetary | 36:1 | 25 Nm | 75 Nm | 109 rpm | 485 g | knees, shoulder roll |
| EC-A6416-P2-25 | 3 | Planetary | 25:1 | 40 Nm | 120 Nm | 107 rpm | 805 g | hip pitch, waist yaw |
| EC-A5013-H17-100 | 4 | Harmonic | 100:1 | 30 Nm | 90 Nm | 33 rpm | 630 g | hip roll, shoulder pitch |
| EC-A3814-H14-107 | 4 | Harmonic | 107:1 | 20 Nm | 60 Nm | 47 rpm | 434 g | hip yaw, shoulder yaw |
The digits after the A are stator diameter and stack height in millimetres, P2 is a two stage planetary, H14 and H17 are harmonic frame sizes, and the trailing number is the ratio [3].
Two reducer families, two jobs
The planetary units run at 75 to 109 rpm and deliver 94 to 448 W, rated torque times rated speed from the table [3]. They sit on the joints that move fast and do work: hip pitch, knee, ankles, shoulder roll, elbow, wrist and neck. The harmonic units run at 33 to 47 rpm and deliver roughly 100 W on the same arithmetic [3]. They sit on hip roll, hip yaw, shoulder pitch and shoulder yaw, the joints that hold a posture against gravity and place a limb without backlash.
Hip roll and shoulder pitch share a part number, hip yaw and shoulder yaw share one, knee and shoulder roll share one. Menlo describe a universal motor mount that lets modules interchange [1], and the actuator selection follows the same logic.
The ankle is the best engineering in the machine
Ankle pitch tops the whole robot at 145.4 Nm, ahead of the hips [1]. Each ankle is driven by two A4310 units through a parallel RSU linkage, both motors high on the shank driving the foot through pushrods [1]. A single A4310 peaks at 36 Nm [3], so two together give 72 Nm before the linkage and the mechanism buys a mechanical advantage of roughly 2.02 in pitch; in roll the motors oppose each other and the published 57.6 Nm implies roughly 0.8 [1]. Both factors are our derivation. The published inertia confirms the doubling: every other A4310 joint carries an armature of 0.0282528, both ankle axes carry 0.0565056, exactly twice [1]. Moving 764 g of actuator off the foot also cuts the leg's swing inertia.
Actuators are 35.5 percent of body mass
Everything else, structure, battery, compute, sensors, wiring and shell, fits in 22.57 kg [1][2]. The sum of rated mechanical power across all 25 joints, derived from the table, is 4,237 W, which never happens at once [3]. The battery is a 13S4P pack at 10.5 Ah and 46.8 V [2], roughly 491 Wh by our arithmetic. Against that ceiling you get about seven minutes, so runtime is entirely a function of duty cycle and how efficiently the machine holds a posture.
What the drive has to deliver
Because the torque constants are published on the output side, current demand at each joint drops straight out of the table: 10.5 A at the harmonic yaw joints, 43.6 A at hip pitch and waist yaw, our derivation from the published Kt and peak torque [1][3].
The pattern is the reduction ratio talking. A 107:1 harmonic hip yaw makes 60 Nm from 10.5 A; a 25:1 planetary hip pitch needs 43.6 A for 120 Nm. This robot buys most of its torque with gearing. The derivation cross checks against the manufacturer: the A4315 datasheet quotes 30 A peak phase current, and 75 Nm over a Kt of 2.53 Nm/A gives 29.6 A [3].
One structural detail follows. Because every unit ships with its driver inside the housing, there is no motor drive line item anywhere in the BOM. The entire electronics budget of 818 dollars covers four custom boards, two compute modules, a camera and a speaker [2], and the motion control board, four layers with 1 oz outer copper, is in our reading an aggregator, not an inverter [2]. For a GaN drive supplier that is the point to sit with: in this architecture the inverter lives inside the actuator, and the question is who builds it.
What the same 25 actuators cost in the West
Menlo put actuators at roughly 7,000 dollars of a 16,000 dollar build [2], about 280 dollars each across 25 units, our derivation from an aggregate since Menlo are not permitted to disclose ENCOS pricing [2]. Every one of these part numbers is also available through western distribution. We priced the exact set at AIFITLAB list in August 2026 [4].
Total: 24,275 dollars, or 971 dollars per joint against the implied 280, a 3.5x spread on identical part numbers [4]. Hold everything else constant at 8,648 dollars and the build goes from 15,648 to 32,923 dollars [2][4]. The 15,000 dollar kit costs less than its actuators alone at western list price.
The harmonic units carry the spread: 32 percent of the units but 47 percent of the spend [4]. And 24,275 dollars is a floor: shipping is calculated at checkout, the USB-CAN and EtherCAT adapters are sold separately, duty is not included [4]. None of this is a criticism of Menlo, who were straightforward about the constraint. Publishing the part numbers makes the design legible. It does not make the supply chain accessible, and that gap, roughly 17,000 dollars on one 35 kg platform, shows up in no spec sheet.
Why we trust the data
Every velocity limit in the joint table equals the ENCOS datasheet peak speed on all 25 rows [1][3], the BOM counts match the joint table part for part [1][2], and the armature values are consistent by part number throughout [1]. Four things we could not verify: ENCOS direct pricing, the exact RSU link geometry, a discrepancy between rated and peak lever ratios at the ankle, and payload ratings, which Menlo mark as unavailable [2]. What would you have specified differently?
References
- Menlo Research, Asimov 1 joint design and actuation reference, asimov 1 repository, 2026.
- Menlo Research, Asimov 1 bill of materials, asimov 1 repository, 2026.
- ENCOS, datasheets for EC-A4310-P2-36, EC-A4315-P2-36, EC-A6416-P2-25, EC-A5013-H17-100 and EC-A3814-H14-107.
- AIFITLAB, product pages for the five ENCOS part numbers, list prices as read in August 2026.
- aleph-b, Every joint in the Asimov humanoid, mapped, LinkedIn, 13 August 2026.