[Explainer] Humanoid Actuators: Motors, Gearboxes, Torque and DoF—Atlas, Figure and Optimus Compared

Actuators provide the physical effort that lets a humanoid lift a box and walk. Understanding performance means looking beyond the motor: how does its effort reach the joint, and how long can the complete system control it?

Imagine holding a pack of water bottles against your chest, then extending your arms. The load has not changed, but the second posture is much harder. Robots face the same problem. A payload in kilograms cannot, by itself, determine the required motor or reducer.

1. What turns a motor into an actuator?

An actuator converts energy into motion. A typical electric humanoid joint combines a motor, a transmission such as a reducer or screw, load-bearing bearings, position encoders and a current-controlling drive. Torque sensors, brakes and cooling may also be included. These elements need not all share one housing.

As a limited analogy, the motor supplies muscle, the transmission acts like tendons and leverage, and the encoder reports posture. The hardware is not anatomically identical to a person. The point is that changing the motor alone may not deliver the joint performance you need.

Component	Role	Specifications to check
Motor	Creates rotational effort	Shaft torque, speed, current and temperature
Reducer, screw or cable	Changes effort and motion	Ratio, efficiency, backlash, friction and life
Bearings and structure	Carry external loads	Axial/radial loads and bending moments
Encoders and torque sensors	Measure motion and load	Accuracy and measurement range
Drive and controller	Regulate current	Voltage, continuous/peak current, control rate and communication

Tesla’s published actuator patent illustrates several of these elements. A patent embodiment is evidence of a disclosed design, not a confirmed bill of materials for a current product. Tesla · WO2024072984A1, §0112–0115 (2024-04-04)

2. Brushed DC versus BLDC: what actually changes?

The useful comparison is brushed DC versus brushless DC. BLDC already means Brushless Direct Current. Treating “DC motor” and “BLDC motor” as completely separate categories creates confusion.

A brushed motor uses brushes and a commutator to switch current as it rotates. A BLDC motor does that electronically. It avoids brush wear but requires electronic commutation. Its bearings and gears can still wear out.

Question	Brushed DC	BLDC
How is current switched?	Mechanical brush contacts	Electronic switching
Potential advantage	Simple drive arrangements	No brush wear; capable speed and precision control
Main trade-off	Brush wear and electrical noise	Drive electronics and control design
Can it drive a hand?	Yes, depending on packaging and life requirements	Yes, depending on packaging, heat and control requirements

PMSM means permanent-magnet synchronous motor. It shares permanent-magnet and electronic-drive principles with common BLDC designs, while conventional terminology distinguishes back-EMF waveforms and drive approaches. The label alone does not rank performance; inspect the implemented current control. MPS · Brushless vs Brushed DC Motors

“Frameless” describes packaging: the rotor and stator are integrated into the robot structure without a separate conventional motor housing. “Coreless” describes the winding/core construction. Neither is the same classification as BLDC, and a coreless motor is not necessarily brushless.

3. Rotary, linear and tendon-driven motion

A rotary actuator turns a joint through its output shaft. An electric linear actuator uses a screw to convert motor rotation into push-pull motion, then moves a linkage to bend the joint. A syringe plunger is a useful analogy for straight-line movement, but an electric linear actuator creates its force with a motor and screw.

A tendon drive pulls cables to move fingers or other links. Motors can sit in the palm or forearm, potentially reducing distal mass. Cable stretch, friction and tension management become design concerns. Whether this beats placing motors inside fingers depends on the task and packaging.

A planetary gearbox is not a planetary roller screw. The former transmits rotation to rotation. The latter converts rotation to linear motion. The shared word “planetary” does not make them interchangeable components.

4. Where do strain-wave and planetary reducers fit?

A reducer lets the motor turn faster while the output turns more slowly with greater torque. Strain-wave gearing, often called harmonic gearing, deforms a thin metal component elliptically to move the engagement region. Planetary gearing shares the load across gears arranged around a central sun gear.

Approach	Design strengths	Possible applications	What to check
Strain wave	Compact high reduction, very low backlash	Space-constrained precise arms and wrists	Friction, elastic deflection, temperature, shock limits and life
Planetary	Load sharing; low-ratio or multistage options	Dynamic leg/arm joints and small hand modules	Backlash, stage losses and precision grade
Linear screw	Push-pull force and linkage packaging	Knee, hip or elbow linkage drives	Stroke, efficiency, external loads and changing moment arms

These are selection considerations, not fixed assignments. There is no rule that every arm uses harmonic gearing and every leg uses planetary gearing. Harmonic Drive itself offers both technologies. Harmonic Drive · Strain Wave / Planetary Principles

Higher reduction makes it easier to obtain large output torque from a smaller motor, at the expense of speed and potentially greater reflected motor inertia. Lower-ratio quasi-direct drives can support responsive interaction, but require more motor torque and current for a given output. A low ratio is not a safety guarantee.

5. What does N·m mean, and how much motor torque is typical?

N·m, pronounced newton-metre, measures torque: the tendency of a force to rotate something. A perpendicular 1N force at a 1m lever arm produces 1 N·m. On Earth, a 1kg mass weighs approximately 9.8N, so holding it statically at a horizontal 1m lever arm requires about 9.8 N·m.

Joint output torque ≈ motor-shaft torque × reduction ratio × transmission efficiency. A 1 N·m motor with 50:1 reduction and 80% efficiency produces about 40 N·m at the output. It does not create energy: output speed falls to about one-fiftieth, while losses become heat.

A useful answer to “how many newton-metres does a humanoid motor need?” starts with the joint and transmission. The following are illustrative calculations, not industry averages or specifications of named robots. Even a 100 N·m output can require motor torque differing by a factor of ten.

Assumed output	Assumed ratio and efficiency	Required shaft torque
Small finger joint: 0.5 N·m	50:1; 70%	About 0.014 N·m = 14 mN·m
Body joint: 40 N·m	50:1; 80%	1 N·m
Body joint: 100 N·m	100:1; 80%	1.25 N·m
Body joint: 100 N·m	10:1; 80%	12.5 N·m

The finger example assumes a simple geared drive to that joint. A tendon-driven hand instead needs calculations using pulley radius, tension and joint coupling. One newton-metre equals 1,000 millinewton-metres. Confusing mN·m with N·m introduces a thousandfold error.

A 10N fingertip force acting at a perpendicular 3cm lever arm corresponds to 0.3 N·m at that joint. This is a calculation for one finger posture, not the complete hand’s grip force or payload.

A linear actuator normally quotes output force in newtons. A 2,000N force acting with a perpendicular 5cm moment arm produces 100 N·m. As the linkage bends, the effective moment arm changes, so available joint torque changes with posture.

6. What do real public torque specifications look like?

Rather than invent detailed torque values for the three headline robots, consider other humanoids with explicit manufacturer data. These are maximum or peak joint-output examples, not continuous bare-motor ratings or results from a common comparative test.

Product and location	Manufacturer torque figure	Interpretation
Unitree G1 / G1 EDU knee	Maximum 90 / 120 N·m	Different model ratings
Unitree H1-2 wrist	About 30 N·m peak	Not finger torque
Unitree H1-2 shoulder and elbow	About 120 N·m peak each	Not every arm axis has the same rating
Unitree H1-2 hip and knee	About 220 / 360 N·m	Ultimate/maximum specification context

These examples show tens of newton-metres at wrists and hundreds at large leg joints. They do not establish a universal humanoid specification range. G1 and H1-2 differ in size, structure and loads. Unitree G1 · Specifications · Unitree H1 / H1-2 · Specifications

Motor selection also requires at least six checks: speed at the requested torque, temperature during continuous operation, permitted peak duration, complete joint mass, gear/bearing life, and voltage/current requirements. A fast control update rate does not imply equally fast mechanical response.

At standstill, mechanical power—torque multiplied by angular speed—is zero, yet an energized motor can still generate heat. At 100 N·m and 2 radians per second, mechanical output is 200W, with additional electrical input needed to cover losses. Peak lifting strength is different from repeated work over an entire shift.

7. How many body and hand degrees of freedom are needed?

Degrees of freedom count independent coordinates of motion. A hinged door has one. A shoulder that independently swings forward, sideways and twists has three axes. DoF is not simply the number of anatomical joint names or fingers.

Actual body counts vary. The basic G1 lists six axes per leg, five per arm and one waist axis: 23 in total. H1-2 lists seven per arm, six per leg and a total of 27 DoF, with hand options requiring separate inspection. Unitree G1 · Specifications · Unitree H1 / H1-2 · Specifications

Illustrative body layout	Axes
Two arms: shoulder 3 + elbow 1 + wrist 3	7 × 2 = 14
Two legs: hip 3 + knee 1 + ankle 2	6 × 2 = 12
Waist 3 + neck 2	5
Total, excluding fingers	31 DoF

The 31-axis layout is an educational design example, not a teardown of Atlas, Figure or Optimus. Published compact and adult-sized examples show bodies with twenty-something or thirty-something axes, but that is not a standard. Adding hands can substantially change the total.

Grippers and articulated hands also differ. Unitree’s optional Dex3-1 has three active thumb axes and two on each of its other two fingers: seven active DoF per hand. Research designs such as Tactile SoftHand-A move five fingers with just two actuators. Always separate joint count, independently controlled axes and motor count. Unitree G1 · Specifications · Li et al. · Tactile SoftHand-A (2024)

8. Atlas, Figure and Optimus: what can actually be compared?

Sources were checked on September 20, 2026. The table separates current product specifications, earlier control research and patent embodiments. “Not established in the reviewed sources” does not mean a component or capability is absent.

Item	Electric product Atlas	Figure 03	Optimus
Actuation evidence	Official electric platform	Electric, internally designed actuators	Tesla-designed actuators and sensors; rotary/linear patent embodiments
Total DoF	Officially 56	Not stated in reviewed product/launch material	Equivalent current-product specification not established
Body excluding hands / one hand	No breakdown in the cited sheet	No verified Figure 03 body/hand breakdown	No verified current-generation breakdown
Number easily misread	Do not add hands again to 56	Helix 35 refers to the 2025 upper-body action space	28 in Tesla’s patent counts embodiment actuators
Reducers/transmissions	Joint-by-joint bill of materials not verified	Joint-by-joint bill of materials not verified	Patent examples: strain-wave rotary and roller-screw linear
Joint continuous/peak torque	Not in reviewed specification sheet	Not in reviewed product/launch sources	Current joint-by-joint table not established

Atlas: Its specification sheet states 56 DoF without a per-hand breakdown. A sum such as 30 body axes plus 13 per hand may add up, but arithmetic alone cannot turn it into an official specification. Boston Dynamics · Atlas specification sheet · Boston Dynamics · Atlas

Figure: The 2025 Helix source describes a 35-dimensional upper-body action space, including fingers, wrists, head and torso—not the whole robot including legs. Figure 03’s launch claims twice the actuator speed and improved torque density, but does not provide joint-by-joint torque, rotational speed and continuous-duty conditions. “Twice the speed” must not become “twice the torque.” Figure · Helix (2025-02-20) · Figure · Introducing Figure 03 (2025-10-09)

A historical hand specification is available: in August 2024, Figure introduced the Figure 02 fourth-generation hand with 16 DoF. This describes that hand generation; it should not be carried over to Figure 03 without confirmation. Figure · Figure 02 announcement (2024-08-06)

Optimus: Tesla’s patent published in 2024 describes a 28-actuator embodiment in §0112 and strain-wave rotary and roller-screw linear embodiments in §0114–0115. These establish disclosed design directions. Adding hand figures from another date would not establish the total DoF of a 2026 production robot. The official Gen 2 video description confirms Tesla-designed actuators and sensors; hand counts that could not be directly cross-checked are excluded as confirmed values here. Tesla · WO2024072984A1, §0112–0115 (2024-04-04) · Tesla · Optimus Gen 2 (2023-12-13)

The reviewed sources do not establish a complete joint-by-joint motor subtype or transmission supplier list for all three companies. Appearance alone cannot prove that a knee uses harmonic gearing or a hand uses coreless brushed motors. Hand comparisons require matching generation, wrist inclusion and active versus passive counting.

9. Is the best actuator simply the strongest one?

For putting boxes on a shelf, peak torque is only the first question. Can the robot do it at full reach? How quickly can it repeat? Does it overheat after hours of work? Can it detect contact and stop or yield when a box catches on the shelf?

ITTimes focuses on the ability to repeat the required task, in the required posture, at the required speed, for the required duration. Bodies must support loads and balance; hands must provide dexterity and adapt to contact. Reducing both to motor count or peak torque misses practical usefulness.

The same 10kg box requires about 24.5 N·m at a 25cm horizontal lever arm and 49 N·m at 50cm, before adding arm weight and acceleration. Understanding that calculation makes posture, continuous ratings and temperature conditions more useful questions than the largest headline specification.

Related reading

Impedance control: a gentle touch · World models: predicting before acting

Sources reviewed

MPS · Brushless vs Brushed DC Motors

Harmonic Drive · Strain Wave / Planetary Principles

Unitree G1 · Specifications

Unitree H1 / H1-2 · Specifications

Boston Dynamics · Atlas specification sheet

Boston Dynamics · Atlas

Figure · Introducing Figure 03 (2025-10-09)

Figure · Helix (2025-02-20)

Tesla · WO2024072984A1, §0112–0115 (2024-04-04)

Tesla · Optimus Gen 2 (2023-12-13)

Li et al. · Tactile SoftHand-A (2024)

Figure · Figure 02 announcement (2024-08-06)

Sources checked on September 20, 2026.

Sean Woo

I have spent more than 15 years shaping robotics technology and business direction. Drawing on public technical documents, research papers, and company announcements, I explain changes in robotics and AI in accessible terms. The interpretations published here do not represent the official position of any company or organization.