As the demand for humanoid robots grows, so does the importance of robot actuator design.
In this guide, we’ll look at actuators, walking-related stresses, gear ratio, heat dissipation, controls, sensors, and the key considerations for optimal performance.
What is a Robot Actuator?
Robot actuators are the mechanism responsible for causing motion in a robot. We use actuators to convert forms of energy like electricity into mechanical energy, which moves the joints, wheels, or any other movable part of the robot. They function as the muscles of a robot.
Types of Humanoid Robot Actuators
Here are the two types of humanoid robot actuators:
Rotary Actuators: The Rotating Joints
We use rotary actuators for joints requiring continuous rotation, such as the shoulder, hip, elbow, and wrist joints. These actuators comprise BLDC motors along with the gear reducer, usually a harmonic drive with a ratio of 50:1–160:1. This combination offers high torque in a compact design. They offer great accuracy but are more susceptible to shock loading.
Linear Actuators: The Thrusting Joints
We utilize linear actuators for joints that experience high loads, such as the knee and ankle joints. With the aid of a screw mechanism, like a planetary roller screw, these actuators convert the rotation of the motor into thrust and deliver forces in excess of 10,000 N.
The Walking Problem: Why Humanoid Break Actuators
While working on actuators for humanoid robots, walking becomes one of the most complicated problems to be solved. Contrary to industrial machines that work in defined load conditions, humanoid actuators are subjected to thousands of impacts daily.
When a robot performs about 5,000 steps hourly, it gets more than 40,000 load cycles during 8 hours of its work, which means that every step is followed by shock loads twice or even three times higher than the robot’s weight. Repeated impacts soon show their negative effect on mechanisms of gears, bearings, and constructions.
In our practice, the problem was not the generation of sufficient force or torque but high performance with low weight. Every extra kilogram makes every step harder.
The Actuator Count: How Many Does It Take?
It depends on the targeted capabilities of the robot; however, for a full-scale humanoid robot, we would need 40 to 55 actuators. Robots with extremely dexterous hands can have more than 70 actuators. However, in our opinion, what really matters is placing the proper actuator in the proper place.
Legs
In our designs, we give the legs 12 to 14 actuators, with six to seven per leg. Three actuators operate the hip, one actuator operates the knee, and two to three actuators operate the ankle for balancing and foot movements. These joints are the most loaded; therefore, the actuator selection here has great importance for walking and durability.
Arms
For the arms, we give 14 to 16 actuators or 7 to 8 actuators per arm. Three actuators operate the shoulder, one or two actuators operate the elbow, and two or three actuators operate the wrist. As a general rule, we prefer to use rotary actuators for manipulating tasks.
Hands
In the hands, the actuators have the maximum density, with 10-20 actuators being common. However, even more actuators are used by some advanced humanoids. For example, Optimus Gen 3 from Tesla uses 25 actuators to realize 22 degrees of freedom with tendon drive in the hands.
Torso and Neck
The actuator requirement for the torso and neck is in the range of 3 to 5 actuators. Though the number of actuators used is low, the joint is critical for balance and coordination of movements.
The Pros and Cons of Gear Reduction
Gear reduction has a greater impact on actuator performance than engineers may realize at first. It affects not only the torque for each joint, but also its efficiency, speed, and reliability under real-world loads.
Pros of Gear Reduction
- Increases torque output (for example, 100:1 = 100x more torque)
- Increases motor speed 3,000–8,000 RPM down to joint speed 20–60 RPM
- Allows for the creation of actuators with 100–300+ Nm of output torque
- Enhances torque retention and precision
- Increases the resolution of the output encoder
- Allows for reducing the size of the motor without changing output torque
- Permits compact and high torque
Cons of Gear Reduction
- Reflected inertia rises as N² (for example, 100:1 ratio = 10,000 times increase)
- Decreases back drivability and force transmission
- Lessens mechanical efficiency to 70–95%
- Increases gear friction heat generation
- Restricts joint speed and response
- Increases shock loads in case of collision
- Increases gearbox mass by 0.5–3 kg for high torque joints
- Creates backlash and increases gear wear
Managing Heat in Humanoid Robot Actuators
Based on our experiences, heat dissipation is among the major issues impacting the performance and reliability of humanoid robot actuators.
Air-cooled actuators can provide about 25 to 30 percent of their peak torque capacity in steady-state operation because they generate significant I2R heat when holding any weight, even when the motor is not moving. We can improve steady-state torque capacity to 50 to 70 percent of peak torque through thermal management.
How Humanoid Robot Actuators Are Controlled
Humanoid robot actuators are controlled through torque control, Field Oriented Control (FOC), impedance control, and real-time feedback. While industrial robots are designed mostly to reach a certain position, humanoid robots require dynamic force control to stay balanced and respond to unexpected environmental conditions.
FOC ensures precise current control inside the BLDC motor at high frequencies of 10,000-40,000 Hz, thus providing torque control for each actuator. In this way, each joint can make immediate changes depending on load, movement, and external forces.
Sensors and Feedback Systems
Sensing and feedback are what make the robot walk steadily, precisely, and confidently. Based on our observations, the actuator performance can only be as good as the feedback that supports it.
We use the proprioceptive sensing of joint positions, orientation of the body, and applied forces by means of encoders, IMUs, and torque sensors together with the exteroceptive sensing from foot contact and tactile sensors that provide the environmental information required for stable walking and manipulation.
Since no sensor can measure anything exactly, we fuse several measurements into one to eliminate noise, drifts, and latencies. We have discovered that the fast sensors like encoders and IMUs are necessary for control loops related to balance and motion control and that the slow vision system is more suitable for navigation and recognition tasks.
Key Design Requirements for Humanoid Robot Actuators
These are the key requirements we focus on:
- Torque Density: >15 Nm/kg
The lightweight design of an actuator is crucial since each additional kilogram influences the whole robot. At least 15 Nm/kg peak torque is required, with some advanced actuator concepts achieving over 25 Nm/kg.
- Backdrivability: <1 Nm
It is necessary to let the external forces affect the motor to achieve better control of forces and provide force sensing.
- Peak-to-Continuous Torque Ratio: 3:1+
Walking, running, and jumping require a burst of torque and simultaneous maintenance of continuous loading. Good heat dissipation makes it possible to achieve it.
- Response Speed: 50-100 Hz+ Bandwidth
Fast reactions to disturbances are required from a humanoid to remain stable. It can be reached through a fast control loop, low backlash, and fast communication between the actuator and controller.
- Low Reflexive Inertia: <0.1 kg·m²
A lower reflexive inertia allows for more efficient movement and safer operation. Both the motor and gearbox should work together in optimization for quick movement.
- High Impact Resistance
The actuator should be able to withstand impacts from walking as well as accidental impacts. The design goal is 50g+ impact resistance for high cycle life.
- Smart Joint Design
In today’s advanced humanoid actuators, both the motor, driver, sensor, and thermal measurement systems have been integrated into one unit.
Conclusion
Engineering a reliable actuator for a humanoid robot demands striking a balance between torque, efficiency, heat management, sensing, and durability. Each engineering choice impacts system performance over millions of operations.
With the continuous evolution of humanoid robotics, precision engineering and manufacturing will remain critical. Learn how RobotCNCParts can help you with your next robotics project through CNC manufacturing. Contact us today.
The machined body of the module is covered under actuator housing.
Read the servo actuator basics.
