Actuators & Motors: The Muscles of China's Robots

Motor Types for Humanoid Robots

A full-size humanoid robot like the Unitree H2 or AgiBot Yuanzheng A2 contains 25–35 powered joints, each requiring a motor suited to its specific load, speed, and precision requirements. Four motor types dominate the current generation of Chinese humanoid robots: frameless torque motors (for high-torque joints like hips and knees), coreless cup motors (for lightweight, fast-response joints in hands and wrists), hollow cup motors, and servo motors for simpler positional joints.

The physical and electrical characteristics of each motor type determine where it fits within the robot's body — and each choice involves tradeoffs in cost, weight, and performance. Frameless torque motors alone account for approximately 16% of total robot component value — making them the second most expensive category after lead screws.

Frameless Torque Motors

Used primarily in the large joints of the lower body — hips, knees, and sometimes ankles — frameless torque motors deliver high rotational force in a compact, lightweight package. They lack an external housing (hence "frameless"), which allows robot designers to integrate them directly into the joint structure, saving both space and weight. Leading foreign suppliers include Kollmorgen (US) and Maxon (Switzerland), while Chinese challengers such as Inovance (汇川技术), Step Electric (步科股份), and Dongjie Zhikong (动界智控, a joint venture between Shanghai Electric and Johnson Electric) are ramping capacity rapidly. Typical torque-to-weight ratios for humanoid-grade frameless motors range from 2–5 Nm/kg, with the best Chinese units now approaching foreign benchmarks at 30–50% lower cost.

Coreless Cup Motors

Coreless motors, which lack the iron core of a conventional motor, are lighter, faster, and more efficient — making them ideal for the fingers and wrists of dexterous hands. The cost revolution in this category has been dramatic: Aoyi Technology (傲意科技) developed coreless motors at just ¥100 per unit, compared to ¥4,000+ for imported German equivalents from Maxon and Faulhaber. Moons' Industries (鸣志电器) offers self-wound coreless motors at ¥1,200–2,300 — a 40–70% discount to foreign alternatives — while maintaining comparable response times (sub-10ms) and energy efficiency (85%+). This domestic price disruption is enabling the sub-¥10,000 dexterous hands that are transforming the market.

Harmonic Reducers Market

Harmonic reducers (also called strain wave gears) are precision gearboxes that convert high-speed, low-torque motor output into low-speed, high-torque joint movement. They are essential for the smooth, backdrivable motion that defines high-quality humanoid robots. A typical bipedal humanoid uses 12–17 harmonic reducers, with an average of about 14.5 per robot.

The global market has historically been dominated by Japan's Harmonic Drive Systems (capacity: ~1.98 million units/year). Chinese challengers Leaderdrive (绿的谐波) and Laifu (来福谐波) are expanding capacity rapidly — Leaderdrive grew from 330,000 units in 2022 to a projected 790,000 in 2025. Quality gaps remain at the top performance tier, but for mid-range specifications — the sweet spot for most humanoid robot joints — Chinese suppliers now offer comparable performance at significantly lower cost, and leading robot makers like Unitree and AgiBot have already qualified domestic harmonic reducers for production use.

The Two Competing Electric Drive Architectures

According to CAICT (China Academy of Information and Communications Technology), the industry has fully converged on electric drive — Boston Dynamics' 2024 switch from hydraulic to electric marked the definitive end of the hydraulic era. Within electric drive, two architectures are now competing:

• High reduction ratio (TSA/SEA): High-speed, low-torque motor + harmonic reducer (reduction ratio 50–300). Delivers high output torque and precision, widely used in lower-body joints. Downside: the reducer adds bulk and cost, and force control requires additional torque sensors.
• Quasi-direct drive (PA): High-torque motor + low-ratio planetary reducer. The planetary reducer's rigid transmission allows back-calculation of torque — eliminating the need for expensive torque sensors. Planetary reducers cost roughly 1/5 of harmonic reducers, making this the more economical option within its load range. Downside: high-torque motors are bulky and expensive, and scaling torque density requires physically larger motors. This approach also demands better thermal management.

The choice between these architectures depends on joint requirements — legs and hips favor the harmonic approach for precision, while arms and lighter joints increasingly use quasi-direct drive for cost and simplicity.

Planetary Reducers

Planetary reducers — named for the "planet" gears that orbit a central "sun" gear — are used alongside harmonic reducers in humanoid joints, particularly in joints that require higher stiffness and load capacity. A typical bipedal humanoid uses 12–15 planetary reducers, averaging about 14 per robot. They are generally less expensive than harmonic reducers but offer different torque and stiffness characteristics.

Robot designers choose planetary reducers when they need higher stiffness and load capacity than harmonic reducers can provide — particularly in joints that experience heavy impact loads or require rigid torque transmission. Leading foreign suppliers include Nabtesco (Japan) and Neugart (Germany), while Chinese companies like GPM Intelligent Technology (嘉鹏精密), Talls Intelligent Technology (陶世智能), and Guohua Intelligent Equipment (国华智能) are rapidly developing competitive planetary products for humanoid applications. The emerging category of miniaturized planetary reducers — optimized specifically for robot finger joints — is a particularly active area of Chinese innovation.

Reducer Usage per Robot — A Breakdown

A typical full-size humanoid robot uses approximately 14.5 harmonic reducers and 14 planetary reducers, allocated by joint requirements. The lower body (hips, knees, ankles) relies heavily on harmonic reducers for their smooth, backdrivable motion — critical for natural-looking locomotion. The upper body (shoulders, elbows) increasingly uses planetary reducers for their stiffness and cost advantages. Finger and wrist joints use miniaturized versions of both types, though this segment remains supply-constrained. Understanding this joint-by-joint allocation is essential context: the total reducer opportunity per robot is roughly 28 units, and as production scales toward tens of thousands of units per year, even small cost reductions per reducer translate to millions in savings.

Chinese Challengers vs Foreign Leaders

Japan has dominated the harmonic reducer market for decades, and European companies lead in high-performance servo drives. But China's humanoid robot boom is creating strong incentives for domestic substitution — and Chinese suppliers are responding with rapid capacity expansion and aggressive pricing. The competitive landscape varies significantly across motor and reducer categories:

• Frameless torque motors: Chinese suppliers are ramping fast; quality remains a key concern for precision applications
• Coreless cup motors: Strong domestic progress — Moons' Industries and Aoyi Technology have driven prices down dramatically
• Harmonic reducers: Leaderdrive and Laifu are credible alternatives for mid-range specs; Japan still leads at top performance tier
• Planetary reducers: Domestic supply is relatively strong; less of a gap than harmonic reducers