Types of Robot Joint Motors: Model Selection, Advantages and Disadvantages

Robot joint motors can be classified by their driving methods into electric drive, hydraulic drive, pneumatic drive, and some less common drive methods such as shape memory alloy actuators that have not yet reached mass production.

Hydraulic drive has been used less and less because of its large size and the risk of oil leakage. Boston Dynamics used hydraulic systems in some of its earlier robots, but newer humanoid robots have also moved toward electric drive systems. Pneumatic drive is also not widely adopted because of its relatively difficult control, and it is mainly used in systems designed by research institutions.

In this article, I will explain how to select an electric-drive robot joint motorand introduce the main types currently used in robotic systems.

Electric-drive joint modules can be classified according to their reduction methods into harmonic drive robot joint motor, planetary reducer robot joint motor, cycloidal robot joint motor, ball screw robot joint motor, and direct drive robot joint motor.

1. Harmonic Drive Robot Joint Motor

The harmonic drive robot joint motor integrates a harmonic drive reducer into the joint motor design, giving it the advantages of high precision, high reduction ratio, high torque density, a compact coaxial structure, and low backlash.

HONPINE harmonic drive robot joint motors can also be integrated with dual encoders, torque sensors, and other components, enabling a safer, lighter, and highly integrated joint design.

The main disadvantage of a harmonic drive robot joint motor is that its cost is slightly higher compared with a planetary reducer robot joint motor. In applications involving high impact loads and frequent collisions, the peak torque and overload capacity of the reducer need to be considered carefully. Its transmission efficiency is also generally lower than that of a direct drive motor.

2. Planetary Reducer Robot Joint Motor

The planetary reducer robot joint motor generally has a slightly lower cost than a harmonic drive robot joint motor. It offers high transmission efficiency and strong impact resistance, and is commonly used in industrial robots, heavy-load robots, AGVs/AMRs, mobile robots, construction robots, and other joints that require high efficiency and load capacity.

Planetary reducer robot joint motors can also cover a relatively wide range of reduction ratios and torque requirements.

The disadvantage is that planetary reducers generally have a certain amount of backlash. Therefore, they may not be suitable for applications that require extremely high positioning accuracy and very low backlash.

3. Cycloidal Robot Joint Motor

The biggest advantage of a cycloidal robot joint motor is its excellent impact resistance. It also features high structural rigidity, while the output side can withstand relatively large radial and axial loads.

Cycloidal robot joint motors are commonly used in heavy-load robots, industrial robots, construction machinery, heavy-duty robotic arms, and other robotic applications that require high impact resistance.

The disadvantage is that, compared with harmonic drive and planetary reducer solutions, cycloidal robot joint motors are currently used less frequently in humanoid robots. Further optimization of materials and mechanical structures is still required to better meet the requirements of humanoid robot applications.

In addition, the manufacturing process and machining accuracy requirements of cycloidal reducers can result in relatively high production costs.

4. Ball Screw Robot Joint Motor

A ball screw robot joint motor, also known as a linear joint module, is a solution that can simultaneously meet four important requirements: compact size, high force output, high precision, and long service life.

It is particularly valuable in robotic mechanisms that require high thrust, such as robot legs and knee joints.

The disadvantage is that the mechanical structure is relatively complex, making the overall design more difficult. In addition, the cost of this type of joint module is relatively high.

5. Direct Drive Robot Joint Motor

The direct drive robot joint motor does not contain a reducer inside the motor. Therefore, it does not have the backlash introduced by a reduction mechanism. It provides fast response during reverse motion and high mechanical efficiency.

Direct drive robot joint motors are suitable for high-speed dynamic robots, collaborative robots, force-controlled robots, gimbals, precision rotary platforms, and low-speed high-response joints.

The disadvantages are that the motor is generally larger and heavier, and heat dissipation can be a major challenge. The overall cost is also relatively high.

Conclusion

With the development of robot joint motor technology, the classification of joint motors is becoming increasingly diverse. Some semi-enclosed joint modules have also appeared on the market, as well as miniaturized joint modules designed for robotic hands and other compact mechanisms. However, some of these technologies are still relatively immature and have not yet reached widespread mass production.

When selecting a robot joint motor, it is important to choose the most suitable solution according to the specific application, rather than simply choosing the most advanced or most expensive technology.

If you have questions about robot joint motor selection, contact HONPINE. Our technical team can assist you in selecting a suitable joint motor solution according to your application requirements.