The core component track that humanoid robot giants are betting on

In today's rapidly developing humanoid robot industry, the most important incremental market in the industry chain cannot be separated from planetary roller screws.

The planetary roller screw was first invented in the 1940s and was initially applied in fields such as aerospace. This precision transmission component achieves high load and high-precision transmission effect through the combination of planetary transmission and threaded rollers.

However, due to its high processing difficulty and cost, the market size has been relatively small for a long time. It was not until the explosion of the humanoid robot industry in the past two years that planetary roller screws gradually received more widespread attention.

So, what role does planetary roller screw play in humanoid robots? What challenges are faced during the development process? Zhang Minliang, the hardware R&D director of Kepler Robotics, gave a wonderful presentation during a recent technology live broadcast.

According to Zhang Minliang, planetary roller screw is a precision transmission component that converts rotational motion into linear motion. It achieves high load and high-precision transmission through the composite principle of "planetary transmission+thread engagement", and has the characteristics of multi-body, multi pair, and multi-point motion. It can generally be divided into 5 categories, namely standard, reverse, cyclic, bearing, and differential.

Through compehensive analysis of the advantages and disadvantages of various types of planetary roller screws in the industry, it has been found that planetary roller screws perform well in terms of size, load-bearing capacity, lifespan, noise, and other aspects. The reverse planetary roller screw is highly compatible with the joint motion of humanoid robots.

Taking Tesla's humanoid robot as an example, Optimus uses a total of 14 planetary roller screws, mainly distributed in the robot's upper arm (2), lower arm (4), thighs (4), and calves (4). By optimizing the design and manufacturing process of the planetary roller screw, Optimus has shown outstanding performance in load capacity, motion accuracy, and energy efficiency.

Besides Tesla, Kepler is the first domestic enterprise to independently develop and mass apply roller screw actuators to humanoid robots.

Kepler started early in the field of humanoid robots. In November 2023, Kepler officially released the Pioneer series of humanoid robots equipped with its self-developed planetary roller screw actuators and rotary actuators, becoming the second company in the world to successfully develop multiple types of planetary roller screw actuators and apply them to robot bodies.

Since the establishment of the company in 2023, after multiple iterations, Kepler released the fifth generation mass-produced humanoid robot K2 "Bumblebee" in the fourth quarter of last year, and is currently preparing for improvements and mass production around this product.

It is reported that the Kepler K2 "Bumblebee" is 175 centimeters tall, weighs 75 kilograms, and has a total of 52 degrees of freedom with its agile hands, emphasizing the concept of biomimicry. The Kepler K2 "Bumblebee" uses a self-developed planetary roller screw actuator, which has unique motion characteristics that can achieve dynamic operation to improve energy efficiency, and static power consumption is almost zero, providing a guarantee for the robot's endurance.

Through actual testing, K2 "Bumblebee" combines its innovative "series parallel structure" design with self-developed roller screw actuators, with a single arm load of 15 kilograms and a dual arm cooperative load of 30 kilograms. It can charge for 1 hour and dry continuously for 8 hours, covering almost all manual handling needs in industrial scenarios.

In addition, Kepler has also independently developed a rotary actuator with a peak torque of 220N. m, which provides powerful driving support, persistent output, and more efficient and stable operation for humanoid robots through a high torque motor combined with intelligent algorithms. Its repetitive positioning accuracy can reach 0.01 degrees.

At the previous Zhangjiang Tailored Intelligence Developer Conference, the Kepler K2 "Bumblebee" won the championship in the "Humanoid Robot Industrial Application Scenario Challenge" with a load record of 20kg, demonstrating the enormous potential of planetary roller screws in humanoid robots.

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High load, high precision, long battery life,

Creating a humanoid robot with a 'steel body'

Why are planetary roller screws favored by Tesla and Kepler? Kepler Hardware Director Zhang Minliang stated that the advantages of planetary roller screws in humanoid robots can be summarized into three main points: high load, high precision, and long endurance.

Firstly, the heavy load. The planetary roller screw adopts a linear contact method, which can withstand higher static and dynamic loads compared to traditional ball screws (point contact). Its load-bearing capacity is 3-6 times higher than that of ball screws of the same specification, and can exceed 10 times at most.

Secondly, high precision. The thread structure design of the planetary roller screw adopts a small lead angle non-circular thread, which can achieve extremely high transmission accuracy and achieve a high level of lead accuracy. The planetary roller screw generates linear contact rolling friction through the engagement of rollers. Compared with the point contact of ball screws, the linear contact method can transmit force and motion more evenly during the transmission process, reduce local errors and uneven force caused by fewer contact points, and thus improve the accuracy and stability of operation.

Also, long battery life. During the operation of robots, the proportion of working hours under small loads is high, and the efficiency fluctuation of linear actuators under different load conditions is small. However, the transmission efficiency of harmonic actuators is greatly affected by loads, and the energy consumption of harmonic actuators is relatively high under small load conditions; Linear actuators have better "self-locking" performance due to the planetary roller screw when the robot joints remain stationary or are subjected to reverse drive loads, reducing motor energy consumption. However, rotary actuators consume more energy when subjected to reverse drive loads, resulting in lower overall energy consumption of linear actuators.

For example, the Kepler K2 "Bumblebee" fully utilizes the characteristics of linear actuators for reverse braking, optimizes its motion control strategy reasonably, and can achieve "charging for 1 hour and continuous drying for 8 hours". Through the linear motion of the planetary roller screw and the design of the linkage mechanism, K2 "Bumblebee" can achieve a stretching and contracting motion similar to human muscles, thus simulating human posture and movements more flexibly.

Overcoming technical difficulties,

Exploring the infinite possibilities in the field of humanoid robots

Due to the high selling price of planetary roller screws, the overall market size is still relatively small, especially in the domestic market where it has just started. Data shows that the global market size of planetary roller screws will be approximately 1.27 billion US dollars in 2022, mainly used in industries such as automobiles, CNC machine tools, and construction machinery. It is expected to exceed 1.5 billion US dollars by 2024. The market size of planetary roller screws in China is only around 450 million yuan, accounting for less than 5% of the global scale, and the market size is negligible.

In practical applications, the planetary roller screw scheme also faces some challenges. For example:

High processing difficulty: The nut of the reverse planetary roller screw is relatively long, and the thread processing accuracy is extremely high, requiring up to 120 processes to complete. The pitch error needs to be controlled within ± 3 microns, and the consistency error of the helix angle needs to be less than 10 arcseconds, which traditional processing equipment cannot meet.

High material performance requirements: Planetary roller screws work under harsh conditions such as high loads and high temperatures, requiring materials with high hardness, high wear resistance, and good thermal stability. The hardness of commonly used bearing steel decreases at high temperatures, leading to increased wear.

Assembly and testing are complex: The assembly of planetary roller screw requires ensuring the synchronous meshing of multiple rollers with the screw and nut, and the assembly accuracy is extremely high. In terms of detection, due to its complex thread structure, traditional detection methods are inefficient, and the detection environment requires strict requirements.

High cost investment: The processing equipment for planetary roller screws is expensive, the production cycle is long, and the scrap and rework rates are high, resulting in high costs. At present, the unit price of its processing equipment is high, and there are numerous processing procedures, with a production cycle of several weeks.

It is worth mentioning that Kepler has gradually overcome the difficulties in the design, processing, testing, and cost of planetary roller screws through independent research and innovation.

The parameter matching technology of high-precision planetary roller screw is the key. The transmission accuracy of actuators directly affects the motion accuracy and load-bearing capacity of humanoid robots. Kepler uses parameter matching design schemes to guide the production process, improving the consistency of multi-point, multi-body, and multi pair motion of planetary roller screw pairs. While solving the problem of transmission accuracy, it also enhances the consistency and load-bearing capacity of robot motion.

In terms of materials and design, in order to balance the needs of high speed and high load, Kepler optimized the structure of the planetary roller screw. By using new materials and innovative structural forms, the overall weight of the robot is reduced while ensuring high load capacity, improving its flexibility and response speed. Kepler also explored more stable materials to enhance the performance of planetary roller screws in high-temperature environments.

In terms of cost, the current production yield and efficiency of planetary roller screws are low, and the unit price is high, which limits their large-scale application. Kepler noticed the application prospects of planetary roller screws in humanoid robots in 2022, and took the path of self-developed and self-made technology. It cooperated with domestic equipment manufacturers to break through the existing common process methods, actively explored new manufacturing processes, and improved the cost-effectiveness of planetary roller screws by solving the problems of inconsistent redundant motion and low production efficiency of screw pairs. This also laid a solid foundation for the large-scale commercial production of Kepler humanoid robots.

In addition, the company has established comprehensive testing standards and specifications to ensure consistency in product quality. As the core component of Kepler humanoid robots, planetary roller screws not only provide powerful power support and precise motion control for the robots, but also bring broad application prospects for Kepler humanoid robots in industries, special industries, scientific research and education through their unique advantages. Despite facing many challenges, Kepler robots are gradually overcoming these difficulties with their technological strength and innovation capabilities, bringing infinite possibilities for the development of the humanoid robot industry.