In the evolution of humanoid robot technology, the industry is shifting from prototype validation to large-scale engineering applications. As the degrees of freedom increase and task complexity grows, selecting the right drive system has become a critical factor.

Thanks to their unique performance advantages, coreless DC motors and frameless torque motors are being increasingly adopted in various joints of humanoid robots.

However, these two types of motors differ significantly in their physical characteristics, making them suitable for different application scenarios.

Without a thorough evaluation of joint-specific functional requirements, system designers may face unnecessary trade-offs between performance and efficiency.

Rather than defining a “one-size-fits-all” solution, this article explores the integration logic of both motor types from four perspectives—structure, load, control, and manufacturing—to serve as a reference for engineers and designers.

Technical Comparison Between the Two Motor Types

The coreless DC motor features an ironless rotor design, providing low inertia, fast response, and zero cogging effect for smooth and precise torque output. These characteristics make it highly suitable for joints that require frequent start-stop motion and fine force control—such as the fingers, eyes, or neck.

However, due to its limited continuous torque capability, challenges such as rapid temperature rise and thrust fluctuation may occur in joints that handle higher loads, such as the wrist or shoulder. Adding a reduction mechanism to compensate for these issues could diminish its high dynamic responsiveness and increase system complexity.

In contrast, the frameless torque motor features a separated stator and rotor design, allowing for direct integration into robotic joints. This enables a compact layout and high-torque direct drive capability, making it ideal for large-load joints such as the shoulder, hip, and knee.

However, when applied to smaller end joints such as the fingers, its size and weight can become limiting factors. The added mass at the extremities propagates upstream, increasing the inertia of the wrist and even the entire arm. As a result, the system’s dynamic response slows down, and the naturalness of motion may degrade. In other words, improving local performance does not necessarily lead to overall system optimization.

In humanoid robots, multiple motors often operate simultaneously—for example, during a five-finger grasping motion.

Even slight variations in thrust constant or response speed between motors can result in uneven grip force or reduced control accuracy.

These inconsistencies typically do not originate from the design itself but rather from subtle variations during manufacturing, such as unstable winding tension, inconsistent turn count, or deviations in assembly air gaps.

In high-degree-of-freedom systems, minor discrepancies in a single motor can be amplified through the control loop, potentially affecting the robot’s overall stability and reliability.

Therefore, choosing the appropriate motor type depends on how well it matches the joint’s load characteristics, motion frequency, spatial constraints, and control requirements.

Coreless DC motors are generally more advantageous for high-frequency, lightweight, and precision movements, while frameless torque motors tend to perform better in joints that require high torque and operate at medium to low speeds.

Ultimately, the optimal configuration is a careful balance between performance, mechanical structure, control complexity, and manufacturability.

Honest Automation doesn't design motors, nor do we define the overall architecture of humanoid robots.

However, we specialize in manufacturing fields where motor performance consistency is of utmost importance. We understand that even the most precise motor design must be supported by a stable and controlled production process to achieve its intended performance.

From winding tension and turn accuracy to assembly air gaps, even the slightest variation in process parameters can affect a motor’s thrust output, thermal behavior, and dynamic response.

That’s why Honest Automation is dedicated to the research and production of intelligent motor manufacturing equipment—helping customers achieve high-precision, high-stability, and high-consistency assembly processes.

If you are exploring drive solutions for humanoid robots, or evaluating the manufacturability and mass-production feasibility of high-performance motors, we invite you to visit us or schedule a technical consultation.

Let’s start with one essential question: How can every motor be made truly reliable?