Φ20mm × 20mm Micro Joint Actuator: How Small Size Delivers 450 mNm Torque

As robots become smaller and more integrated, the space available for each motion component continues to shrink. This creates a practical engineering problem: how can a robot generate sufficient joint torque when the actuator has to fit into a very limited mechanical envelope?

For compact robotic joints, reducing the size of a motor is only part of the solution. The actuator also needs an efficient electromagnetic structure, suitable reduction mechanism, position feedback, and enough thermal and mechanical capability for the intended duty cycle.

The Φ20mm Micro Joint Actuator addresses this challenge with a compact Φ20mm × 20mm form factor and an axial flux motor architecture. The flagship X20 can deliver up to 450 mNm of continuous torque and up to 800 mNm of initial torque while maintaining a weight of approximately 37.2–41.6 g.

Why 20 mm Is a Significant Size for a Joint Module

A Φ20mm × 20mm envelope leaves very little room for the components normally required in a complete joint actuator.

A practical joint module may need to accommodate a motor, reduction mechanism, encoder, PCB or winding structure, bearings, housing, electrical connections, and other mechanical elements. Increasing the size of any one component can affect the complete joint architecture.

This becomes especially important in multi-axis robots. If several actuators are installed close together, the space occupied by each joint directly affects the size of the fingers, arms, or other articulated structures.

The X20 is therefore designed as an integrated joint module rather than simply a small standalone motor. Its compact architecture combines the drive system and reduction mechanism within a Φ20mm × 20mm footprint.

The objective is to provide useful output torque without requiring a much larger joint housing.

How Axial Flux Motor Architecture Supports Torque Density

One of the key technologies behind the X20 is its axial flux motor architecture.

In a conventional motor design, the magnetic flux and active components are arranged according to a radial geometry. An axial flux motor uses a different electromagnetic configuration, which can provide a more compact form factor for certain applications.

For a miniature joint actuator, the advantage is not simply the motor's physical shape. The motor architecture needs to work together with the transmission and mechanical housing to make efficient use of the available volume.

The X20 is designed to deliver more than 30% higher torque density than conventional designs while maintaining an ultra-compact footprint.

Torque density is particularly relevant when the actuator has a strict size limit. Instead of asking only how much torque a motor can generate, engineers need to consider how much torque can be generated within a specific volume or weight.

For compact robots, this can be a more useful design parameter than absolute motor output alone.

How 450 mNm Continuous Torque Fits Into a Compact Package

The X20 provides continuous torque of up to 450 mNm.

For a Φ20mm-class joint module, this output needs to be considered together with the reduction ratio, operating voltage, duty cycle, and mechanical load.

Continuous torque is especially important when the joint needs to maintain a load or perform repeated movements rather than simply produce a short burst of force.

For example, a robotic joint may need to repeatedly position a mechanism, hold an object, or move against resistance. In such cases, the continuous torque specification provides a more useful reference than a short-duration peak value.

The X20 also provides initial torque of up to 800 mNm. This higher initial output can be relevant to movement conditions that require additional torque at the beginning of an operation, depending on the application and control strategy.

Engineers should still evaluate the actual load profile rather than treating the maximum torque values as a universal operating condition.

The Role of the Miniature Reduction Mechanism

The motor itself is only one part of the torque-generation system.

The X20 uses a miniature reduction mechanism to convert motor output into the required joint movement. The available gear ratios are 15:1, 30:1, and 50:1.

Different ratios produce different combinations of output speed and torque. A lower ratio can be suitable when higher output speed is required, while a higher ratio can provide greater mechanical advantage and more controlled output movement.

This makes gear ratio selection an important part of actuator integration.

A robotic hand, compact robotic arm, or other articulated mechanism may have different requirements at each joint. Designers should consider whether a particular joint needs faster movement, higher output torque, or a balance between the two.

The three available gear ratio options give system designers more flexibility when configuring the Φ20mm Micro Joint Actuator for different mechanical requirements.

12V, 24V, and 48V Options for Different Systems

The X20 supports 12V, 24V, and 48V configurations.

Voltage selection should be based on the electrical architecture of the complete robot rather than the actuator alone.

For a multi-joint system, the designer needs to consider the available power supply, motor driver, wiring, controller compatibility, and the number of actuators operating simultaneously.

A system already designed around a 24V power architecture may benefit from using a compatible actuator configuration rather than introducing a separate voltage level. In other applications, 12V or 48V may be more appropriate depending on the system design.

Checking voltage compatibility early can prevent unnecessary changes to the power and control architecture during later stages of robot development.

Weight Matters When Multiple Joints Are Used

The X20 weighs approximately 37.2–41.6 g.

At first glance, a difference of several grams may appear small. However, the effect becomes more noticeable when a robot contains multiple actuated joints.

For example, a robotic hand may use several joint modules across multiple fingers. A multi-axis mechanism may also place several actuators along the same mechanical structure.

The total actuator mass then becomes part of the moving load.

Lower joint weight can help reduce the load on upstream mechanisms and may allow designers to build more compact supporting structures. This is one reason why torque-to-weight and torque-density considerations are important when selecting actuators for lightweight robotic systems.

Absolute Magnetic Encoder for Joint Position Feedback

The X20 integrates an absolute magnetic encoder.

An encoder provides position feedback to the control system, allowing the controller to monitor the angular position of the joint. In a compact robotic mechanism, reliable position information is essential because several joints may need to coordinate their movement.

An absolute encoder can provide direct positional information rather than relying solely on tracking incremental movement from a known reference point.

For applications such as robotic hands, compact manipulators, and other multi-axis systems, this type of feedback can support more controlled joint operation.

The encoder also needs to fit within the same compact mechanical envelope. Integrating sensing into a small actuator therefore requires both electrical and mechanical design to be considered from the beginning.

SPI Communication in a Compact Joint System

The actuator uses SPI communication for digital control and data exchange.

For a multi-joint robot, the communication interface needs to be compatible with the controller and overall electronic architecture. Designers should consider how individual actuators will be connected, how data will be exchanged, and how the control system will coordinate multiple joints.

SPI can be integrated into compact embedded control systems, making it suitable for applications where the actuator needs to communicate directly with a local controller or other electronic architecture.

Before selecting an actuator, engineers should confirm communication compatibility alongside voltage, mechanical dimensions, and encoder requirements.

What the 20 mm Form Factor Can Enable

A compact joint module can be useful in applications where conventional motor-and-reducer combinations occupy too much space.

Potential applications include dexterous robotic hands, compact robotic arms, lightweight articulated mechanisms, medical equipment, precision instruments, intelligent equipment, and other highly integrated machines.

The Φ20mm × 20mm size can be particularly relevant when several joints must be placed close together. Instead of designing a large external housing around a separate motor and gearbox, engineers can work with a more integrated joint architecture.

This can simplify mechanical packaging and make it easier to allocate space for multiple motion axes.

The Φ20mm Micro Joint Actuator is therefore not defined only by its 20 mm diameter. Its value comes from combining compact dimensions with torque output, reduction, feedback, communication, and integrated mechanical design.

VAXOR's Approach to Compact Motion Systems

VAXOR was founded in 2024 and is headquartered in Suzhou, China. The company develops micro joint actuators and ultra-micro coreless motors for robotics, medical devices, precision instruments, and intelligent equipment.

Its technology roadmap focuses on compact size, high performance, and scalable production. The company combines axial flux motors, PCB/FPCB winding, optimized electromagnetic design, and miniature reduction mechanisms to develop micro-drive solutions with high power density and fast response.

VAXOR's multidisciplinary engineering team covers motor design, precision manufacturing, structural design, robotics control, and intelligent manufacturing. Its development process extends from prototype validation and pilot production to batch delivery.

This engineering approach is important for customers who need to move beyond laboratory prototypes and integrate miniature actuators into repeatable robotic products.

How Engineers Should Evaluate a Small High-Torque Actuator

A small actuator should not be selected based on diameter alone. Engineers should evaluate the complete operating requirements of the joint.

Important factors include:

• Available installation space

• Required continuous and initial torque

• Desired gear ratio

• Joint speed requirements

• Operating voltage

• Encoder type

• Communication interface

• Actuator weight

• Duty cycle

• Thermal conditions

• Mechanical mounting requirements

For example, choosing a 50:1 reduction ratio may provide a different output characteristic from a 15:1 configuration. Similarly, selecting a 48V actuator configuration may require a different power and driver architecture from a 12V system.

The best configuration therefore depends on how the actuator will be used rather than on a single specification.

Balancing Size, Torque, and Integration

The challenge of miniature robotics is not simply making every component smaller. A useful joint actuator needs to maintain sufficient output capability while fitting into a restricted mechanical space.

The X20 approaches this problem through an axial flux motor architecture, miniature reduction mechanism, integrated absolute magnetic encoder, and compact mechanical packaging.

With a Φ20mm × 20mm footprint, up to 450 mNm continuous torque, up to 800 mNm initial torque, three gear ratio options, three voltage configurations, and a weight of 37.2–41.6 g, the Φ20mm Micro Joint Actuator provides a combination of dimensions and output characteristics aimed at highly integrated motion systems.

For engineers developing compact robotic joints, the key question is not simply whether a motor can fit into 20 mm. It is whether the complete actuator can provide the required torque, feedback, transmission, and control functions within that same limited space. That is where an integrated Φ20mm Micro Joint Actuator can become a practical option for compact robotic designs.