Choosing the most suitable humanoid robot joint motor is crucial as it influences movement accuracy, energy efficiency, lifespan, and overall performance of the robot. So there must be a balance of factors like precision, torque, weight, power density, and lots more. In this guide, we explain all you need to know to help make the right choice.
What is a Humanoid Robot Joint Motor?
This is an actuator system that powers the robot’s joint, converting electrical energy into regulated mechanical motion.
It combines the encoder, controller, and gearbox, which helps it to provide the high torque and force needed for the joint to rotate, lift, and respond to the environment.
Components of a Humanoid Robot Joint Motor
Here are the 4 basic components you should know.
Reducer
Either planetary, cycloidal, or harmonic, this mechanical part helps reduce speed and increase the torque necessary for moving limbs and carrying loads.
Encoder
This is a sensor that provides data or feedback regarding position, speed, acceleration, and so on. Due to high precision ability, dual encoders are usually used. In a humanoid robot, encoder resolution varies from 17-bit to 24-bit.
Motor
A brushless DC motor is preferred because of its high power-to-weight ratio, efficiency, and longevity. The component is vital for converting electrical energy into mechanical energy.
Controller
The controller is the main component that helps in receiving data from the humanoid robot central processor, analyzing it, and managing action flow. It regulates the speed, torque, and position, and it utilizes advanced communication systems like EtherCAT and CANopen.
Key Factors When Selecting a Humanoid Robot Joint Motor
Some of the critical factors you must check are:
Torque Requirements
Torque is the rotational force needed for a joint to move. When a motor lacks sufficient torque, then it becomes difficult for the robot to lift its limbs or maintain balance.
As such, you must consider the torque requirement for every joint. For the wrist, 1-10Nm applies, 10-40Nm for the elbow, and 30-150Nm for the shoulder.
Speed Requirements
The speed rate must align with the limbs’ required rate of motion. Shoulders usually require moderate speed with high torque, while the wrist needs faster movement. Too much speed, however, can increase power consumption and reduce stability.
Precision and Encoder Resolution
Several robot activities such as assembly operations and tool handling demand precise positioning. With that, you must use high-resolution encoders from 12 bits.
Power Density and Weight
Power density is the amount of power generated in relation to the motor weight. In a human-robot joint, you should prioritize high power density and low weight for efficient energy use and mobility.
Thermal Performance
You also need to look out for motors with high-performance cooling channels, heat dissipation flow, and temperature control
Gear Ratio
This influences the relationship between the torque and speed. High gear ratios give more torque but reduce output speed, and vice versa.
Why Choose Motors by Joint Type?
We select different motors by joint type because:
- Each joint, including the elbow, wrist, and shoulder, carries out varying functions, which makes them require specific motors. Shoulder joints need high torque for lifting, wrist requires accurate precision, and for the elbow, strength and speed must be optimized.
- Also, selecting an inappropriate motor or using the same for all joints can cause excessive heating, risk of motion imbalance, and low performance.
How to Select a Humanoid Robot Shoulder Joint Motor
The shoulder carries the weight of the entire arm and torque output. It supports carrying loads, pushing and lifting objects, and so on. So, it must facilitate precise movement, stability, and load handling.
Selection criteria thus include high torque, ability to withstand high weight, low speed, large reduction ratio, low backlash, high encoder resolution.
How to Choose a Humanoid Robot Elbow Joint Motor
While the wrist joint requires precision and the shoulder needs power, the elbow joint motor demands a balance between torque, speed, and control. It is required for posture alignment, lifting, and dynamic movement regulation.
When choosing, consider a torque range of 40-80Nm and torque density within 25Nm/Kg or greater, and a harmonic drive reducer and dual encoders.
It should also be moderate in size with a brake for safety.
How to Select a Humanoid Robot Wrist Joint Motor
In a humanoid robot joint, the wrist helps in hand posture adjustment, grasping, and manipulation. As such, when you are making a selection, flexibility and lightness must be considered.
Also, the motor ideally should have a low torque, small size, high precision, and be perfect for fine movements. While the lightweight helps reduce inertia and makes it very responsive for quick movement, the compact nature ensures other components like sensors fit in the limited space.
Humanoid Robot Joint Motor Recommendations
For the best motor recommendation for each joint, we need to consider the ideal characteristics requirement for the specific joint. This table explains that.
| Joint | Ideal Characteristics | Motor Recommendation |
| Shoulder | High torque, harmonic reducer, 20-bit encoder | Large BLDC motor with harmonic drive |
| Elbow | Medium torque, lightweight gearbox | Medium BLDC motor with planetary/harmonic gearbox |
| Wrist | Compact actuator, high precision | Frameless or direct-drive with precision encoder |
| Knee | Shock-resistant, high torque | Heavy-duty actuator with reinforced gearbox |
| Ankle | Fast response, high stability | Lightweight actuator for balance control |
How to Calculate Torque for Humanoid Robot Motors
When making humanoid robot joint design selection, torque calculation is crucial.
The main formula is t = F x r
Where t = torque
F = Force
r = radius
Gravity Torque
tg = m x g x l
Where
M = load mass (Kg)
G = gravitational acceleration (9.81m/s²)
L = Distance from center of mass of joint (m)
Dynamic Torque
td = I x α
Where
I= Moment of Inertia
a= Angular acceleration
Final Tips for Humanoid Robot Joint Motor Selection
- Check out the torque and make sure the motor can handle the required load before considering speed and precision.
- Ensure the motor aligns with the controller and sensor and communication system
- Look out for the maintenance requirements and expected service life
- Check out for the full system. Encoders, gearboxes, and mechanical parts influence performance
- You would also need to make sure the controller and battery can handle current changes for sudden movements.
- Dual encoders are key. One for the motor shaft and the other for the output shaft
Conclusion
Selecting the right humanoid robot joint motor involves balancing a number of factors from torque, speed, and precision to thermal performance and gear ratio. Also, you need to consider the specific function of the joint.
Whether you’re looking to develop a motor for your robot joint or you require other robotics components, RobotCNCPart has the required expertise to bring your project to life. We deliver precise, durable, and quality components all through the world. With our advanced CNC processes, Injection molding, 3D printing, and so on, and wide material knowledge, we ensure rapid lead times. Contact Us Today.
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