Frameless BLDC Motors for Joint Actuation in Elbows, Knees, and Wrists

Robotics engineers are continually pushing the boundaries of what machines can do in compact spaces. One area of innovation is joint actuation – powering the elbow bends, knee swings, and wrist rotations of robots to be as fluid and strong as their biological counterparts. In this article, we’ll explore how frameless BLDC motors are revolutionizing robotic joints, discuss their benefits (like high efficiency, flexible integration, and customization), compare model specifications, and answer common FAQs for engineers considering these motors.

Frameless BLDC motors use only the rotor and stator, so engineers can integrate the motor directly into the robot joint instead of packaging a full housed motor. That helps reduce mass, free up internal space, and keep the actuator concentric with the joint axis. 

Key Takeaways

• Frameless BLDC motors are often used in elbow, knee, and wrist joint actuation because they deliver high torque density in compact robotic and prosthetic designs.
• These motors help engineers reduce size and weight while keeping precise motion control, which matters in articulated systems that need smooth, responsive movement.
• Joint design usually depends on more than motor size alone, it also depends on torque requirements, thermal limits, control strategy, and packaging constraints.
• Elbow, knee, and wrist applications have different load profiles and motion demands, so the right frameless BLDC motor choice depends on the joint’s exact use case.
• Early motor selection works better when the article clearly connects each joint application to performance criteria such as torque, speed, efficiency, and integration needs.

Frameless BLDC Motors offered through www.pibsales.com

Frameless BLDC motors consist of just a rotor and stator, allowing seamless integration into a robot’s joint. This frameless design eliminates the bulky housing of a traditional motor, saving space and weight. As a result, even small joints can pack a powerful punch, achieving human-like movement without oversized actuators. Robotics engineers can embed the motor’s stator and rotor directly into an elbow, knee, or wrist assembly, which means the joint’s structure becomes the motor housing. This design flexibility leads to a lighter, more compact joint that still delivers the torque and precision required for smooth, controlled motion.

Why Are Frameless BLDC Motors Used in Robotic Joints?

Frameless BLDC motors are used in robotic joints because they deliver high torque density and precise control in a compact form factor that can be built directly into the joint structure. Kollmorgen states that frameless motors are often chosen when the application requires a compact form together with high power and torque density, plus a smaller footprint and better system bandwidth.

That combination matters in elbows, knees, and wrists because these joints usually have limited volume but still need accurate torque control, smooth motion, and low total mass. The current PIB draft already says the stator and rotor can be embedded directly into the joint so the structure becomes the motor housing, which is exactly the kind of integration benefit engineers look for in compact robot joints.

What Makes Frameless BLDC Motors a Good Fit for Joint Actuation?

Frameless BLDC motors fit joint actuation well because they improve packaging efficiency, support precise motion control, and help keep actuator mass close to the joint axis.

High torque density

Torque density is one of the strongest reasons to choose a frameless motor for a robotic joint. maxon lists high torque density and high torques due to external, multipole rotor as core frameless-motor traits, while Kollmorgen highlights the need for high power and torque density in compact robotics designs.

This matters most in joints like knees and elbows, where the actuator must generate useful joint torque without adding unnecessary bulk or distal mass. The current PIB article already frames this correctly, but the claim becomes more citable when tied to named manufacturers instead of general marketing phrasing.

Precise and smooth motion

Frameless BLDC motors are also chosen for smooth and precise motion, especially when paired with encoders and suitable controllers. maxon notes that frameless motors support Hall-sensor-based speed and position control and that, with encoder-based sinusoidal commutation, the result is theoretically very smooth, precise running and high-quality control.

That is especially relevant in wrists and fine-manipulation joints, where low jerk, repeatable positioning, and stable control matter as much as raw torque.

Better packaging and integration

Frameless motors improve joint packaging because there is no external motor housing, and the center bore can often be used for cable or line routing. maxon explicitly lists space-saving integration and a hollow shaft for transferring cables, vacuum lines, light, and more.

This is particularly useful in wrists and upper-limb joints, where routing sensors, wires, or pneumatic lines through the joint axis can simplify the overall mechanical design.

Thermal advantages in embedded joints

Frameless motors can also help thermal design when the surrounding structure is used as part of the heat path. maxon cites good heat dissipation and high overload capacity as characteristics of its frameless motors.

That makes the current PIB statement about the surrounding structure acting as a heat sink more credible when supported by a manufacturer source rather than left as a general assertion.

How Do Elbow, Knee, and Wrist Motor Requirements Differ?

Elbow, knee, and wrist joints do not need the same actuator balance, even when all three use frameless BLDC motors. Knees usually prioritize torque and load support, elbows balance torque and control, and wrists usually prioritize compactness, low inertia, and fine motion control. This is an engineering inference from the joint roles described on the current page and from robotics/exoskeleton literature emphasizing accurate torque control, compact packaging, and low impedance in upper-limb joints.

Comparison: joint priorities for frameless BLDC motor selection

This kind of comparison block is missing from the current page, and it would make the article much easier for AI tools to summarize accurately.

Benefits of Frameless BLDC Motors in Robotic Joints

High Torque Density in a Compact Package

Frameless BLDC motors are renowned for their high torque-to-size ratio. By eliminating the external case and unnecessary bulk, all that remains are the core components that generate torque. This means more of the motor’s volume is dedicated to producing power. In practice, a frameless motor can deliver the torque of a much larger traditional motor while fitting inside a tight joint cavity. For example, a robotic knee or shoulder joint can integrate a frameless motor to lift significant loads without an oversized actuator. Every gram and millimeter saved is critical in robotics – especially in humanoid or mobile robots – making torque-dense frameless motors invaluable for elbows, knees, wrists, and other joints where space is at a premium.

Smooth and Precise Motion

A key advantage of frameless BLDC motors is their ability to provide buttery-smooth rotation and precise control. These motors are designed with low cogging torque, meaning there is minimal internal torque ripple as they turn. Low cogging ensures that even at slow speeds or when making micro-adjustments, the motor’s motion is steady and jerk-free. For robotic joints, this translates to movements that appear fluid and natural rather than choppy. Whether a robot is performing delicate surgical maneuvers or smoothly lifting an arm to pick up an object, frameless BLDC motors help achieve highly precise positioning (often with control accuracy to fractions of a degree) and repeatable motion. Combined with advanced electronic controllers, they give robotics engineers fine-grained command over speed, position, and torque – crucial for tasks like balancing bipedal robots or coordinating multi-axis robotic arms.

High Efficiency and Reliability

Efficiency is a defining feature of brushless DC technology, and frameless BLDC motors exemplify this with excellent performance-to-power ratios. High efficiency (often in the range of 85–92% in these models) means that more of the electrical energy goes into useful mechanical work (torque and speed), and less is wasted as heat. For battery-powered robots, this translates to longer operation per charge and less heat build-up in joints. The frameless design can also aid cooling – since the motor is integrated into the robot’s structure, the surrounding assembly can serve as a heat sink, helping dissipate heat quickly during intensive operation.

In terms of reliability, frameless BLDC motors have no brushes or commutators, eliminating the primary wear components of traditional DC motors. Fewer moving parts and no brush friction lead to lower maintenance requirements and a longer lifespan. This is especially important in robots that operate continuously or in hard-to-service locations (like an industrial robot in a sealed environment or a rover crawling through tight spaces). Engineers can count on these motors to perform consistently over time, sustaining joint performance without frequent intervention. The robust design also handles high-duty cycles (even 100% duty) without compromising on output, ensuring that critical joints like knees and elbows remain operational under continuous loads.

Flexible Integration into Joint Designs

One of the most compelling features of frameless motors is flexible integration. Because the motor comes as a kit of just rotor and stator, you have the freedom to incorporate these pieces directly into your mechanical design. Imagine mounting the stator inside the joint housing and attaching the rotor to the joint’s moving link – the motor essentially becomes part of the joint. This eliminates the need for bulky motor mounts and allows you to place the motor exactly where it’s most effective for your design’s kinematics and balance. For instance, in an elbow joint, the frameless motor can be concentric with the joint axis, keeping the mass centered and reducing offset loads. Likewise, a frameless motor in a robotic wrist can be tucked within the forearm structure, freeing up space for tools or sensors at the end effector.

Integration is also aided by the hollow center many frameless BLDC motors offer. With no output shaft running through the middle, the rotor often has a large inner bore. This hollow shaft design lets engineers run cables, tubing, or even other mechanisms through the joint axis. In complex robots, that means you can route sensor wires or pneumatic lines through a shoulder or wrist motor itself, simplifying the overall system layout.

Customization and Turn-Key Solutions

Every robot has unique requirements, and frameless BLDC motors cater to this need for customization. At the basic level, these motors can be tuned through different windings to operate at various voltage levels or speed-torque characteristics. Need a higher speed for a wrist spin or more torque for a knee lift? Manufacturers can often provide custom windings or magnet configurations to optimize the motor’s performance for your specific use case. Additionally, frameless motors come in a variety of diameters and stack lengths. Engineers can select a motor size that best fits the joint dimensions and torque requirements, or even stack multiple rotor/stator sets for higher output in the same footprint.

A frameless BLDC motor can be the core of a complete joint solution when combined with other components. For example, a single joint assembly might incorporate:

• Precision gearbox reducers – to multiply torque for heavy loads (while the motor’s high efficiency keeps the system responsive).
• Torque sensors – for feedback and control, enabling advanced features like force control or collision detection in a robotic arm.
• Safety brakes – to hold position when power is off or in emergency stops, crucial for vertical joints like knees that bear loads.
• Magnetic or optical encoders – for high-resolution position sensing, ensuring the joint knows its exact angle and moves with accuracy.
• Custom housings and bearings – designed to support the frameless motor and the robot’s structural needs, often using the robot’s own joint casing as the motor frame.
• Integrated drive electronics – compact driver boards that can fit inside the joint, directly adjacent to the motor, minimizing wiring and latency.
  

Frameless BLDC Motor Model Comparison

To illustrate the capabilities of these motors, the table below compares a few example frameless BLDC motor models commonly used in robotic joints. Each model is characterized by its outer diameter (size), power output, torque, efficiency, and typical voltage. This gives an idea of how scaling the motor size affects performance:

Table: Key specifications for three frameless BLDC motor kits (model names correspond to approximate diameters). The torque and power values indicate the range from continuous to peak output.

Frequently Asked Questions About Frameless BLDC Motors for Joint Actuation

What is a frameless BLDC motor?

A frameless BLDC motor is a motor built without an outer housing, so the rotor and stator can be integrated directly into a larger mechanical assembly. That makes it a strong fit for compact robotic joints, prosthetics, and exoskeleton systems where space and weight matter.

Why are frameless BLDC motors used in elbows, knees, and wrists?

They are used because joint actuation systems need compact size, precise control, and strong torque output. Frameless designs also give engineers more flexibility when packaging the motor into custom joint assemblies.

Which joint places the highest demands on the motor?

The knee often handles higher loads than the wrist and usually requires more torque, while the wrist tends to prioritize compact packaging and fine control. The exact requirement depends on the device, range of motion, and intended use.

How do you choose the right frameless BLDC motor for a robotic joint?

Start with the joint’s torque, speed, duty cycle, thermal limits, and available space. Then match those requirements to a motor that fits the mechanical design and the control system.

Are frameless BLDC motors only used in medical devices?

No, they are also used in robotics, industrial automation, exoskeletons, and other precision motion systems. Any application that needs compact, built-in actuation can be a good fit.

Contact sales staff at [email protected] or call us at 1-800-228-8895 for more detailed information.