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 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.

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