In the field of humanoid robots, the selection of joint drive systems is a crucial factor determining motion dexterity, energy efficiency, and mass production feasibility. The rise of Quasi-Direct Drive (QDD) technology marks a paradigm shift in the industry from “pursuing ultimate rigidity” to “prioritizing intelligent force control.” It is not a simple superposition of motors and reducers, but rather uses a high-torque-density permanent magnet synchronous motor as its core, coupled with an extremely low-ratio harmonic reduction mechanism. Through an open-loop force feedback mechanism , it achieves millimeter-level dynamic response and sub-Newton-level precision control of contact forces without relying on external torque sensors.
This architecture achieves a breakthrough balance in energy recovery efficiency, shock resistance, and system lightweighting, enabling robots to exhibit the flexibility and robustness of biological joints in unstructured scenarios such as walking, pushing, and pulling, and human-robot collaboration. Compared to all-direct drive solutions, it reduces system costs by approximately 40%, and compared to traditional rigid drives, it increases force control bandwidth by more than 3 times. QDD is becoming the preferred drive architecture for mainstream humanoid robot platforms such as Tesla Optimus, Figure 01, and UBTECH Walker X.
The Essential Definition of a Quasi-Direct Drive Joint
The quasi-direct drive joint is a drive system that adopts a “high torque density frameless motor + low transmission ratio reducer” architecture. Its core feature is the use of a planetary reducer with a transmission ratio of less than 10:1, which is different from the high transmission ratio design of traditional industrial robots, which often ranges from 50:1 to 120:1.
From a mechanical perspective, the design philosophy of quasi-direct drive joints is to have the motor handle the main torque output task, while the reducer only provides limited torque amplification. This design gives the joint force control characteristics close to those of a direct drive system, while maintaining the structural compactness of the reduction system.
Taking CubeMars’ AKE80-8 product as an example, this actuator uses an 8:1 planetary reduction ratio to achieve a torque density of 52 Nm/kg and a backlash accuracy of 9 arc minutes. This combination of parameters provides the optimal balance between force control sensitivity and output capability in humanoid robot applications.
Technical Comparison: Engineering Trade-offs of Three Drive Solutions
Traditional Rigid Actuator
Traditional industrial robots commonly use harmonic reducers or RV reducers, with transmission ratios typically between 50:1 and 120:1. This design aims for extreme torque density, reaching over 600 Nm/kg. Early versions of the Tesla Optimus adopted this approach, using a 120:1 harmonic reducer in the hip joint, with peak torque exceeding 200 Nm.
However, the problems brought about by a high gear ratio are also obvious:
- The force control bandwidth is limited by the mechanical damping of the reducer, typically only 200 Hz.
- Backlash and friction cause a nonlinear response at low speeds
- Closed-loop force control requires expensive force sensors, with a single sensor costing thousands of yuan.
Pure Direct Drive Solution
The direct-drive system eliminates all reduction gears, with the motor directly driving the load. This design is widely used in academic research, offering force control bandwidth exceeding 2 kHz and excellent dynamic response.
However, pure direct drive faces severe engineering challenges:
- To obtain sufficient torque, a large-diameter external rotor motor is required, resulting in excessively large joint dimensions.
- Torque density is typically only around 150 Nm/kg, which is insufficient to meet the power requirements of humanoid robots.
- Motors are expensive and difficult to industrialize.
The Technical Positioning of Quasi-Direct Drive
The quasi-direct drive joint found the optimal engineering solution between two extreme approaches:
- The torque density reaches 300 Nm/kg, which is twice that of pure direct drive.
- The force control bandwidth is 1-2 kHz, which is 5-10 times that of traditional solutions.
- Sensorless force control is achieved through a current loop, reducing costs by 30-50%.
This combination of performance characteristics perfectly suits the application scenarios of humanoid robots: they require frequent dynamic interactions, but do not demand the absolute load-bearing capacity of industrial robots.
Force Control Principle: The Technical Advantages of Quasi-Direct Drive
The core technological advantage of quasi-direct drive joints lies in their excellent force control characteristics. This capability stems from the increased “transparency” brought about by the low transmission ratio.
The physical nature of mechanical transparency
In a transmission system, the external force felt at the output end needs to be transmitted back to the motor side via a reducer. The square effect of the transmission ratio significantly amplifies the effects of friction and inertia.
- Reverse driving torque = External force / Transmission ratio
- Equivalent frictional torque = Actual friction × Transmission ratio²
For a 120:1 harmonic reducer, an external force of 1 Nm at the output end can only generate a feedback of about 8 mNm on the motor side, while the frictional torque is amplified by 14,400 times. This makes it difficult for traditional joints to sense the mechanical information of the external environment.
The 8:1 transmission ratio of the quasi-direct drive joint reduces this “isolation effect” by an order of magnitude. The motor can more directly sense the mechanical state at the output end, achieving high-bandwidth force control.
Engineering Implementation of Sensorless Force Control
The core of Quasi-Direct Drive (QDD) joints lies in the deep synergy between mechanical transparency and current-torque linear mapping: the output torque is directly sensed and controlled by the current loop of a high-precision permanent magnet synchronous motor, achieving sub-Newton level force control accuracy and 1–2 kHz dynamic response bandwidth without the need for external force sensors.
The engineering value of this solution is reflected in three main dimensions:
System simplification: Completely eliminates force/torque sensors and their associated signal conditioning circuits, reducing BOM costs by approximately 30%, decreasing mechanical assembly links, and improving system reliability.
Control robustness: Avoids the risks of sensor zero drift, temperature drift, and calibration failure, maintaining stable force feedback in industrial environments such as humidity, vibration, and frequent shocks.
Real-time compliance: Millisecond-level force response based on current closed-loop enables robots to achieve muscle-like passive compliant behavior in sudden contact situations (such as fall cushioning and human hand traction).
The Zhiyuan Expedition A1 is a prime example of this architecture—its 28 QDD joints are controlled via a distributed current loop, constructing a full-stack sensorless force control network. This enables force control errors down to 0.1 N·m in scenarios such as fall protection and interaction with unstructured environments.
Analysis of Technical Solutions from Mainstream Manufacturers
Tesla Optimus: The Evolution from Rigid to Flexible
Tesla underwent significant changes to its drivetrain during the development of the Optimus. Early versions used a traditional harmonic reducer, achieving hip joint torque of over 200 Nm, but performed poorly in terms of human-machine interaction and compliant control.
The latest Gen 2 version introduces a low gear ratio design, with reduction ratios in some joints dropping below 20:1. Combined with customized force sensors, this results in a significant improvement in force control bandwidth. This shift reflects Tesla’s profound understanding of the interactive characteristics of humanoid robots.
Zhiyuan Expedition A1: A Benchmark Application of Quasi-Direct Drive
As a benchmark product of domestic quasi-direct drive technology, the Zhiyuan Yuanzheng A1 uses a high torque density frameless motor as the core of its joint actuator, matched with an 8:1 planetary reducer , to achieve an industry-leading torque density of 52 Nm/kg , redefining the energy efficiency boundary of humanoid robot drive systems.
Its technical architecture features three major engineering breakthroughs:
Integrated package: The motor, planetary gearbox, and high-resolution encoder are deeply integrated into a single package, reducing volume by 40%, increasing structural rigidity, and significantly reducing cumulative assembly tolerance errors.
1ms Ultra-Low Delay Current Loop Force Control: Based on real-time feedback of motor phase current, a sensorless force closed loop is constructed, compressing the control cycle to 1 millisecond, achieving sub-millisecond force response and 0.1 N·m steady-state accuracy , supporting high-dynamic interactive scenarios such as fall buffering and human-machine collaboration.
High-efficiency thermal management design: Utilizing a multi-layer composite heat dissipation structure and an intelligent temperature control algorithm, the continuous output power remains stable at 150W, with temperature rise controlled within ±2°C under continuous operating conditions, ensuring long-term reliability.
The entire fuselage of 28 joints adopts a unified and standardized QDD architecture, enabling modular production and rapid iteration of the drive unit, shortening the R&D cycle by 35% and increasing the manufacturing yield to over 98%, laying a core foundation for large-scale deployment.
Fourier GR-1: Engineering Practice for Medical-Grade Precision
Fourier Intelligence’s GR-1 humanoid robot is designed for rehabilitation and medical applications, requiring extremely high joint precision and flexibility. Its collimated drive joints employ a variable transmission ratio design ranging from 6:1 to 10:1, optimizing performance based on joint position and load requirements.
Technical features include:
- Peak torque 230 Nm, continuous torque 120 Nm
- Positioning accuracy reaches 0.1 degrees, meeting the requirements for medical operations.
- Multi-layered safety redundancy design, including hardware limits and torque monitoring.
The successful application of GR-1 proves the feasibility of collimation drive technology in high-precision scenarios.
CubeMars AKE Series: A Key Driver of Commercialization
The launch of the CubeMars AKE series actuators marks a key turning point for Quasi-Direct Drive (QDD) technology, moving it from laboratory prototypes to large-scale applications. This series, centered on standardized mechanical interfaces and open control protocols, significantly reduces the technical barriers and development cycles for robot developers in drive system integration.
The AKE series, through its modular design and out-of-the-box control ecosystem, enables small and medium-sized robotics teams to quickly build fully compliant, bio-like joint systems without developing their own drive hardware. Its commercialization not only validates the engineering feasibility of the QDD architecture but also accelerates the comprehensive penetration of collimator technology in humanoid robots, service robots, and collaborative robots through its competitive pricing and open ecosystem.
Engineering Significance of Key Technical Parameters
Torque density: Balancing performance and size
Torque density (Nm/kg) is a core indicator for evaluating joint actuators. The joints of humanoid robots need to provide sufficient driving force within a limited space, and torque density directly affects the robot’s load capacity and motion performance.
Currently, the torque density of collimated direct drive joints is generally between 200-400 Nm/kg, with a typical value of about 300 Nm/kg. Although this level is lower than that of traditional harmonic drive schemes, it is sufficient for most humanoid robot applications.
For example, a 70 kg humanoid robot requires a peak torque of approximately 150-200 Nm at the hip joint. Using a 2.5 kg collimated drive actuator (torque density 300 Nm/kg) can provide sufficient power reserve while maintaining reasonable joint dimensions.
Transmission stiffness affects the frequency response and vibration suppression capability of joints. Planetary reducers typically have higher torsional stiffness than harmonic reducers, which is beneficial for achieving high dynamic response.
Technical Challenges in Engineering Implementation
Motor Design: The Pursuit of High Torque Density
The quasi-direct drive solution requires the motor to provide higher output torque, which places stringent demands on electromagnetic design. Traditional framed motors, limited by their structural dimensions, struggle to achieve the required torque density.
The current mainstream solution uses frameless torque motors, which increase torque density by increasing the air gap diameter and optimizing the magnetic circuit design. However, this also brings new problems:
- A large rotor inertia affects the dynamic response.
- Higher copper and iron losses increase thermal management stress.
- High manufacturing precision requirements and difficult assembly
Some manufacturers have begun to experiment with new magnetic circuit structures, such as Halbach arrays, to further improve torque density.
Gearbox Selection: Balancing Precision and Lifespan
The design of low-ratio planetary reducers is essentially a delicate balance between transmission chain stiffness, manufacturing precision, and long-term durability. When the transmission ratio drops to the 6:1–10:1 range, the number of teeth in a single-stage gear decreases sharply, leading to increased stress concentration at the tooth root and a narrower contact spot distribution. This places almost stringent requirements on tooth profile error and surface integrity—making it difficult for traditional roughing processes to meet the demanding requirements of robot joints for dynamic response and zero backlash.
Mainstream architectures generally use two- or three-stage planetary transmission chains , reducing the load on a single stage through multi-stage load sharing, while achieving a high reduction ratio within a limited space. Its core technology system includes:
Ultra-precision gear manufacturing: After precision grinding, the surface roughness is controlled to Ra ≤ 0.4 μm , combined with nanoscale profile detection to ensure a meshing contact rate > 95%.
Nonlinear tooth surface modification: Based on contact stress simulation, optimize the micro-convexity of the tooth tip/root to suppress vibration harmonics under impact loads and reduce noise by more than 15 dB.
High-strength alloy material system: using carburized and quenched 18CrNiMo7-6 or low-carbon martensitic steel, increasing the contact fatigue limit to over 2100 MPa and extending the service life to over 10,000 hours.
Preloaded planetary carrier structure: Zero backlash preload is achieved for the gear pair through elastic washers or axial springs, and the dynamic backlash is stabilized at ≤9 arc minutes , meeting the requirements of high-precision trajectory tracking.
In high-end products, the harmonic gear-planetary gear composite transmission structure is becoming a new paradigm for breaking through performance bottlenecks: the harmonic reducer provides sub-arc graded accuracy and zero backlash characteristics, while the planetary structure undertakes the main torque transmission and shock resistance functions. When the two are connected in series, they have the triple advantages of high precision, high rigidity, and high torque density , and have been successfully implemented in industrial-grade actuators such as the CubeMars AKE series.
Technological Development Direction
The key development areas for quasi-direct drive joint technology in the next 3-5 years include:
Increased torque density: Through novel magnetic circuit design and high-performance materials, the torque density is increased to 80-100 Nm/kg, further narrowing the gap with traditional solutions.
Cost control: Through standardized design and large-scale production, the cost per joint is expected to decrease by 30-40%, accelerating the industrialization process.
Intelligent upgrade: Integrating edge computing capabilities to achieve joint-level intelligent perception and decision-making, thereby enhancing the system’s autonomy.
Modularization and standardization: Establish unified mechanical interfaces and communication protocols to improve the interoperability of products from different manufacturers.
Industrial Chain Layout
The supply chain for collimated direct drive joints is rapidly improving.
Upstream: Key components such as permanent magnet materials, high-performance bearings, and precision gears
Midstream: Joint actuator integrators, such as CubeMars, Calcium, and Unitree.
Downstream: Humanoid robot manufacturers, including Tesla, UBTECH, Fourier, etc.
Local companies have developed strong competitiveness in the midstream and downstream sectors, but they still need to strengthen their technological accumulation in upstream fields such as high-performance permanent magnet materials and precision bearings.
