Actuators Five Key Factors When Selecting Motors for Humanoid Robots

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FAULHABER Drive Systems

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Wuxi InfiMotion Technology Co., Ltd.

Actuators are crucial for executing human-like movements. However, their use in humanoid robots poses specific system requirements. Five factors are decisive for motor selection.

Humanoid robots, with their imitation of humans, represent the next evolutionary stage in robotics and exert a special fascination. The race for the first series-production-ready humanoid robot is one of the most exciting trending topics in the tech world. A market analysis by the management consultancy Horváth from March 2024 predicts that the first human-like robots could be mass-produced for industrial use as early as 2025.

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When it comes to hardware, so-called actuators are crucial for executing human-like movements. They function as the equivalent of human joints and muscles: drive technology components that enable movements in a system—whether rotational or linear.

Actuators consist of a combination of gears, motors, sensors, ball bearings, and encoders. The more degrees of freedom required, the more actuators are needed. Currently, humanoid robots under development are capable of realizing between 16 and 60 degrees of freedom.

Humanoid movements can be controlled by electric, hydraulic, or pneumatic drive systems. The prevailing practice is to use a specific actuator in each joint, consisting of a gearbox, torque motor, encoder, and motor control. Torque motors are highly pole, electric servo motors with high torque and relatively low speeds. Below are the system requirements that are crucial for motor selection in a humanoid robot.

1. It's All About Torque Density

The torque density of an electric motor is a measure of how much torque the motor can generate per unit volume or weight. It is a crucial factor for the performance and compactness of a motor, particularly important for applications where weight is critical—such as in robotics. The total weight of a humanoid robot is significantly determined by the weight of its joints. The lighter the motors in the joints, the lower the total weight. This, in turn, results in a longer battery life, which allows for a higher payload and thus an overall higher dynamics of the robot.

TQ motors are particularly torque-dense compared to other motors, meaning they can achieve double the torque at the same size or the same torque at half the size. TQ achieves this through a unique winding technology that maximizes the copper fill factor compared to conventionally wound electric motors. TQ is currently the only motor developer on the market that fully exploits the physical possibilities thanks to certain manufacturing steps: it is not possible to fit more copper in the respective motor size. Thus, TQ motors have the highest copper fill factor on the market.

The compact design of the drive enables efficient space utilization and weight reduction, lowers the center of gravity of the robot, and improves stability. Robots with a compact design navigate tight spaces and maintain balance when maneuvering through crowds. An additional advantage of the motors is their large hollow shaft, which is very important for routing cable harnesses internally and achieving a clean robot design.

2. The Precision of the Motors Determines the Correct Positioning of the Robot

For the robot to perform controlled, smooth, and versatile movements, the precision of the motor is extremely important. The more precise the drive, the more direct the connection between the robot's movement and the "visual process," consisting of sensor and camera technology. Each joint point is defined by three-dimensional vectors. For the highest possible precision, it is very important that the motors, especially in the sum of several joint points, always assume the "correct" position. Otherwise, even minimal deviations in individual joints, such as the hip, knee, and ankle, may accumulate into significant misalignments. The precision of an electric motor increases with the so-called number of pole pairs, which is a crucial factor for the motor's behavior, as it has a direct impact on control, positioning, and regulation of the motor.

In designing their frameless servo motors, the TQ Group places special emphasis on achieving a high number of pole pairs. TQ electric motors offer very precise position and torque control over a wide range of conditions, from low speed and high torque to high speed and low torque. This is crucial for the natural, fluid movements expected from humanoid robots.

3. Reaction Time and Dynamics Make the Difference in Flexibility

The motor must be able to react or be controlled accordingly to any change in the environment (e.g., a robot steps into an unforeseen hole or the ground gives way). For a humanoid robot to respond flexibly and quickly to changes in the environment like a human, excellent dynamic control and drivability as well as a fast response time are crucial.

How does an electric actuator achieve this high reaction time and dynamics? To react in real-time, very high torques are needed for a short period to, for example, quickly anticipate and adjust the foot movement to the hole in the ground. This torque is then multiplied for a brief moment. This is referred to as the overload capacity of a servo motor and as peak torque, meaning the maximum torque a motor can generate in the short term. The peak torque of TQ motors is approximately three times higher than their nominal torque, or the constant value that can be reached in the long term. Specifically, TQ motors achieve peak torque in the double-digit Newton meter range (Nm) and thus have a market-leading overload capability.

4. Efficiency and Energy Consumption Determine the Operating Duration

Efficiency, that is, how much power is lost over the battery life, determines how long a battery-operated humanoid robot can work. The principle is: the highest efficiency, due to low copper losses, extends battery life. Copper losses describe the losses caused by the electrical resistance of the windings in a motor. They are released in the form of heat and are one of the main sources of loss in electrical machines. Motors with high power loss consume a lot of electricity, which shortens battery life and thus limits working time.

This means that the efficiency factor plays a much more important role in a mobile, humanoid robot than in a collaborative robot (Cobot) that is connected to the power grid. For the practical use of humanoid robots, high efficiency is crucial for their continuous operation. The TQ torque motors have an efficiency of 90% or higher and achieve particularly low copper losses in watts.

5. Robustness and Reliability Contribute to Functionality

The robustness and reliability of the motors are also important factors for applications in humanoid robotics. Especially during the testing phase, falls occur frequently. A robust, maintenance-free design ensures that the joints remain intact and functional despite the learning curve. Drives face the greatest challenges in space. Even with temperature fluctuations from -40°F to 257°F, they must function reliably here. On the ISS (International Space Station), an ILM motor from TQ was used, among other things, in the arm of the robot Rokviss and performed precise tasks in zero gravity—over five years and hundreds of tests, always reliably and with the highest performance.

In industrial application scenarios as well, such as in a production or warehouse hall, it can happen that a humanoid robot, which has to lift heavy loads, drops objects or is subjected to sudden impacts. The robustness of the robot's joints protects it from damage and ensures continuous and reliable operation even under demanding conditions.

Standard Motors or Customized Solution?

Robot manufacturers also face the question of whether to choose a motor manufacturer that supplies standard components or to opt for a custom solution. At TQ, customers receive a selection of standard sizes that can be tailored to the specific application if desired. The advantage of TQ's frameless servo kits is that they are modifiable in diameter and their so-called stack length, allowing their performance and dimensions to be tailored to the specific application. Especially for quantities of more than a few hundred units per year, a custom solution can be the best option. It allows for complete control over design, materials, and the manufacturing process.

Hardware is Hard

The saying "Hardware is hard" aptly applies to the development and production of robot joints and the necessary components. This is because the core competence of some humanoid robotics manufacturers is more likely to be found in the areas of artificial intelligence and software, while experience in the industrialization of robots and development of mechatronics is sometimes lacking. Additionally, the development and production of such a complex system, like a robot joint, are often underestimated. TQ is one of the few providers in Germany that can industrialize complete robots and has already proven it.

Against this background, it can make sense to rely on the technology leaders in their respective fields. TQ often develops entire motor-gear units for mobile applications. This is a decisive advantage that helps save valuable time in the current race for the first mass-produced humanoid robot.