2024/08/28 13:04
The core of humanoid robot motion control: planetary roller screw
2024/08/28 13:04
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The purpose of this paper is to discuss the technical characteristics, market status and future prospects of lead screws through the study of their technical characteristics.
This article is 7819 words, about 11.2 minutes
Author | Rongzhong research
Source | Jellyfish research
With the rapid development of science and technology and the continuous growth of the field of artificial intelligence, humanoid robots have gradually become the latest bright spot in the industry with their wide application in many fields such as medical care and services. In order to further promote the development of the industry, local governments have introduced policies to increase support for humanoid robots and their key components. In humanoid robots, planetary roller screws are its important precision transmission components, such as Tesla Optimus humanoid robot linear joints with a total of 14 planetary roller screws. The purpose of this paper is to discuss the technical characteristics, market status and future prospects of lead screws through the study of their technical characteristics.
Pictured: Each Tesla Optimus contains 14 lead screws
Source: Tesla AI day
Lead screws and their technical principles
The lead screw is an ideal product for converting rotary motion into linear motion, or converting linear motion into rotary motion, and is the most commonly used transmission element in tool machinery and precision machinery. The main structure of the lead screw is composed of a screw and a nut, and its main function is to convert rotary motion into linear motion, or torque into axial repetitive force.
The working principle of the lead screw: when the screw is used as an active rotating body, the nut will convert the rotating motion of the screw into linear motion according to the lead of the corresponding specification with the rotation angle of the screw, and the workpiece involved in the work can be connected with the nut to achieve the corresponding straight motion. The angle of rotation of the screw is proportional to the linear movement of the nut, and the proportional relationship is determined by the lead of the screw (the pitch of the thread). Conversely, if the nut moves as a moving member, the screw can also be rotated, so that the linear motion is converted into a rotary motion.
Figure: Classification and working principle of lead screws
Data source: Rongzhong Research
Classification of lead screws: According to their friction characteristics, lead screws can be divided into three categories: sliding lead screws, rolling lead screws and hydrostatic lead screws.
1) Sliding screw is the most common screw machinery, and the tooth type is mostly trapezoidal, also known as trapezoidal threaded screw. It has no rolling element, relies on sliding friction to transmit thrust, simple structure, easy to manufacture, self-locking, and is widely used in machine tools. It has severe heat in continuous work and low transmission efficiency (30-50%).
2) According to the different rolling elements, the rolling screw can be further divided into two categories: ball screw and planetary roller screw. At present, Tesla's humanoid robot mainly uses planetary roller screws, so this article will focus on rolling lead screws.
3) The hydrostatic screw needs to add a set of oil/gas supply system, which realizes contactless or near-contactless movement by forming a fluid film between the nut and the screw, which greatly reduces friction and wear, and can achieve extremely high positioning accuracy, but its process is complex, the processing is very difficult, the cost is high, and the application is less.
Planetary roller screws have become an important component in humanoid robots due to their unique advantages in efficiency and volume, and this article will also focus on planetary roller screws. Planetary roller screws originated in 1942 when a Sweden inventor, Carl Bruno Strandgren, patented the first recirculating roller screw, ·· Strandgren. However, due to the complex structure, difficult processing and high cost, the initial stage has not been widely used. Since the 70s, with the development of weapons and equipment technology and the increase in the demand for high thrust, high precision, high efficiency and long life of petroleum, chemical industry and CNC machine tools, planetary roller screws have begun to develop rapidly.
The structure and working principle of the planetary roller screw: the planetary roller screw relies on the rotation of the screw rod and the roller to drive the nut to do linear motion. The structure of the planetary roller screw is similar to that of the ball screw, including the lead screw, the planetary roller, the cage, the internal gear and the nut, etc., in which the rolling element is the planetary roller instead of the ball in the ball screw. The roller is evenly distributed between the lead screw and the nut through the cage, in order to prevent the phenomenon that the roller is axially channeled relative to the nut, the axial direction between the cage and the nut is relatively fixed, and the two sides of the roller are processed with spur gears, so that the purpose is to avoid the sliding of the roller relative to the nut and produce the phenomenon of part deflection. When the lead screw rotates, the roller will move in a straight line along the axis of the lead screw through the thread meshing between the screw and the lead screw on the one hand, and planetary motion between the screw and the nut on the other hand, and the nut will have no relative axial displacement between the nut and the roller through the cage, so the nut and the roller maintain the same axial moving speed.
Compared with the ball screws used for precision transmission in the past, under the premise of small loss of transmission efficiency, it also has the characteristics of high speed, high load, high stiffness, high range lead, smaller volume, lower noise, and more convenient maintenance and disassembly.
Figure: Structural diagram of planetary roller screws
Data source: Application Status of Planetary Roller Screw Electric Cylinder
Figure: Illustration of the working principle of planetary roller screws
Data source: Rongzhong Research
Compared with ball screws, roller screws have the advantages of strong bearing capacity, small size and long life.
Strong bearing capacity: The bearing capacity mainly depends on the radius of curvature of the contact surface between the rolling elements, the number of contact points, and the hardness and finish of the contact surface. Under the conditions of the same contact surface hardness, finish and precision, the roller screw has more contact points than the ball screw, and the radius of curvature of the envelope of the roller contact surface is much greater than the radius of curvature of the ball screw steel ball, which determines that the roller screw has a higher bearing capacity than the ordinary ball screw. In general, the axial stiffness of the planetary roller screw pair is about 50% higher than that of the ball screw pair.
Small size: Planetary roller screws are smaller in size while providing the same load, helping to reduce the overall size of the machine and making the design more compact. Under the same high load, the volume of the planetary roller screw is 1/3 smaller than that of the ball screw.
Long life: According to Hertz's pressure law, the life of planetary roller screws can reach 15 times that of ball screws, which means that under the same operating conditions, planetary roller screws have longer maintenance intervals and higher reliability.
The specific classification of planetary roller screws: according to their different structural composition and motion relationship, they can be divided into 5 types, including standard planetary roller screws, reverse planetary roller screws, circulating planetary roller screws, differential planetary roller screws, and bearing ring planetary roller screws, which are suitable for different occasions with their unique structural characteristics and advantages. The Tesla Optimus humanoid robot uses a reverse planetary roller screw.
The technical barriers of planetary roller screw pairs are mainly reflected in structural design, workpiece materials, surface heat treatment, processing technology, etc.
(1) Structural design: The roller part of the planetary roller screw is composed of two parts: thread and end gear, and there is thread meshing and gear meshing at the same time during operation, so the design and modeling of the two parameters are more complicated. In addition, the thread profile parameters have a great influence on the load distribution of the planetary roller screw, and the thread profile parameters need to meet the thread strength criterion to prevent the fracture of the thread tooth root. It is necessary to meet the thread contact yield criterion to ensure that the contact of the thread teeth is in the elastic stage and will not undergo irreversible plastic deformation, so as to improve the fatigue life of the planetary roller screw. Therefore, it is necessary to pay attention to the design of the threaded tooth profile of the planetary roller screw.
(2) Workpiece material: When the planetary roller screw pair is working, the threaded raceway of the lead screw, roller and nut will be subjected to continuous, periodic and frequent vibration, impact and friction, resulting in the temperature increase of the lead screw pair, and with the increase of speed and load, the temperature rise will increase significantly, which will eventually lead to raceway wear, creep relaxation, thermal deformation, fatigue, yield, etc. Therefore, materials with high hardness, strong wear resistance, strong fatigue resistance, high bearing capacity, good temperature adaptability, good cutting performance and economy should be selected.
The material of foreign planetary roller screws is mainly alloy structural steel. According to Ewellix's official website, its standard lead screw shaft is mainly made of pre-treated 50CrMo4 (or 42CrMo4), which is case-hardened by induction; The nuts and rollers are made of fully hardened 100Cr6 bearing steel. Alloy structural steel has excellent hardenability and strength, high fatigue limit and multiple impact resistance after quenching and tempering, and good low-temperature impact toughness.
The materials of domestic planetary roller screws are mainly martensitic stainless steel and austenitic stainless steel. Martensitic stainless steel has good quenchability, which can be strengthened by heat treatment processes such as quenching and tempering, so as to obtain higher hardness, strength and wear resistance, but there are generally shortcomings in toughness and corrosion resistance. Austenitic stainless steel has good plasticity, good corrosion resistance, oxidation resistance and excellent welding performance, but its strength is worse than that of other steels.
(3) Surface heat treatment: It is divided into two categories: preparatory heat treatment and final heat treatment. Preparatory heat treatment can improve the cutting performance of the lead screw and eliminate residual stress; The final heat treatment can improve the surface hardness and wear resistance of the thread. Due to the backward technical equipment, improper selection of heat treatment process parameters and other reasons, resulting in poor heat treatment quality and large deformation after heat treatment, the main damage form of planetary roller screw pair is contact fatigue wear, and improper heat treatment process will reduce its life.
(4) Grinding process: planetary roller screw relies on the thread meshing between parts to achieve transmission function, and the machining accuracy of the thread will directly affect the transmission accuracy, service life, and stability of the system, so optimizing the processing technology of the thread is the key to improving the overall performance of the planetary roller screw. At present, the commonly used screw threading processing methods are: rolling, hard turning, whirlwind milling and grinding, among which the grinding processing accuracy is the highest, and cold rolling, hard turning and whirlwind milling are mostly used for rough machining, therefore, grinding is the main processing method of high-precision screw thread.
Planetary roller screw industry chain
The planetary roller screw industry chain consists of three major links: upstream raw material and parts supply, professional manufacturing of midstream planetary roller screws, and downstream diversified application fields.
In the upstream link, the industrial chain focuses on the supply of high-quality raw materials and the precision manufacturing of key components. The lead screw is mostly made of alloy structural steel, while the nut and roller are preferably high-carbon chromium bearing steel. The processing technology of core components such as nuts requires highly specialized technology and equipment support.
The midstream of the industrial chain is the core of the planetary roller screw industry chain, and through advanced production technology and strict quality control system, the upstream raw materials and parts are transformed into high-precision and high-performance planetary roller screw products, providing key power transmission and positioning solutions for downstream applications.
Planetary roller screws have become an indispensable key component in many industries with their excellent transmission performance and stability, from traditional automobile manufacturing and oil and gas extraction to cutting-edge medical equipment and optical instruments, to the booming construction machinery, robots, automated production lines and high-end machine tools.
Planetary roller screws are an important component of linear actuators in humanoid robots. Structurally, humanoid robots can generally be divided into actuation systems, perception systems, and others. Among them, the execution system consists of three parts: linear actuator, rotary actuator, and dexterous hand. Linear actuators move objects in a straight line and are mainly used in the joints of humanoid robots such as wrists, elbows, knees, and ankles. The linear actuator of the Tesla humanoid robot is mainly composed of frameless torque motors, planetary roller screws, force sensors, encoders, drives, etc.
Pictured: Tesla Optimus Straight Joint Teardown
Source: Tesla Robotics 2022 AI Day
Tesla Optimus Linear Joints use reverse planetary roller screws to meet the needs of high load and small size. Under the same lead screw diameter, the reverse planetary roller screw has higher load bearing and higher accuracy than the ball screw; Compared with the standard planetary roller screw, the reverse planetary roller screw is smaller in size. Currently, Optimus uses a total of 14 reverse planetary roller screws for elbows, wrists, hips, knees, and ankles. There is no official data on the proportion of value, but according to our dismantling and comparison of various parts, the planetary roller screw accounts for about 8%-10% of the entire humanoid robot, which is the key cost reduction link of the humanoid robot at present.
Planetary roller screws market size
The global planetary roller screw market is in the billions of dollars market. ACCORDING TO DATAINTELO DATA, THE GLOBAL PLANETARY ROLLER SCREW MARKET SIZE IN 2022 WILL BE 1.27 BILLION USD DOLLARS, AND THE GLOBAL MARKET SIZE OF PLANETARY ROLLER SCREWS IN 2022 WILL BE 8.78 BILLION YUAN BASED ON THE EXCHANGE RATE OF USD/RMB 6.91 AT THE END OF 2022. At the same time, the compound growth rate of the market size from 2018 to 2022 is 4.5%. This paper argues that planetary roller screws have competitive advantages in terms of load, stiffness, precision, and miniaturization, and are superimposed on the current demand for industrial automation and high-end equipment manufacturing, and the industry market size can maintain a compound growth rate of 4.5% from 2023 to 2030 without considering the increment brought by humanoid robots.
Chart: Planetary roller screw market size and forecast (100 million yuan) (Note: Humanoid robot increment is not considered)
DATA SOURCE: DATAINTELO, COMPILED BY RONGZHONG RESEARCH
To calculate the increment that the field of humanoid robots can bring to planetary roller screws in the future, this paper considers the following assumptions:
(1) At present, the rapid development of artificial intelligence has solved the two core difficulties of robots being unintelligent and having no scenes, and the hardware end of humanoid robots is also iterating rapidly. This paper believes that the growth of the humanoid robot market will be divided into three stages, the first stage: 2024-2026, mainly driven by policy and capital, it is expected that enterprises will gradually enter the mass production period of humanoid robots, and the first three years of commercial application scenarios are expected to meet the unstructured needs of the industrial market and realize the supplement to the traditional industrial production line. At this stage, it is expected that the compound growth rate of humanoid robot sales is about 50%; The second stage: from 2027 to 2030, the supply chain will continue to reduce costs and increase efficiency, and technological breakthroughs will continue to make breakthroughs, humanoid robots will gradually spread and popularize to potential household and service markets, and the potential of scenario applications will continue to be tapped, and the compound growth rate of humanoid robot sales is expected to be about 100% at this stage; The third stage: After 2030, the demand for pension, emotional, companionship, and military scenarios will become the main driving force for the growth of humanoid robots, and the market will show a long-term fluctuating upward trend, and the compound growth rate of humanoid robot sales is expected to be about 20% at this stage.
(2) From the perspective of price, the gradual increase in the volume of humanoid robots can open the ceiling of the planetary roller screw market, and the increase in usage will reduce the manufacturing cost, which may also bring about an increase in future industry entrants and further reduce product prices. Assuming that the current sales volume of humanoid robots is 10,000 units, the price of a single planetary roller screw is about 2,000 yuan; It is expected that when the sales of humanoid robots reach 180,000 units in 2029, the price of a single planetary roller screw will drop to about 1,500 yuan; When the sales volume of humanoid robots reaches 500,000 units in 2032, the price of a single planetary roller screw will drop to about 1,200 yuan.
(3) Suppose that the amount of planetary roller screws used in a single humanoid robot is the same as now, that is, 14.
Judging from the calculation results, the market size increase of planetary roller screws brought by the field of humanoid robots will reach one billion yuan from 2027; Starting from 2030, the incremental market size of planetary roller screws brought by the field of humanoid robots will exceed more than 60% of the combined market size of other fields.
Figure: Market size and estimation of planetary roller screws after considering the increment of humanoid robots (100 million yuan)
Data source: Rongzhong Research
Planetary roller screw competitive landscape
In the planetary roller screw market, which is difficult to process, foreign brands are in a dominant position. GSA, Ewellix, Rollvis, Rexroth and other foreign companies represent the first-class level of roller screw manufacturing, GSA and Rollvis each occupy 26% of the market share, followed by Ewellix and Rexroth occupy 14% and 12% of the market share respectively, and CR4 is 78%. Nanjing Technology and Bote Fine Work each occupy 8% of the market share for domestic roller screw enterprises.
Figure: Planetary roller screw competition pattern in China
Data source: Research on the Marketing Strategy of Roller Screw Products of Company E
Introduction of key foreign players:
GSA, Switzerland: GSA was founded in 1982 and is based on decades of experience in threading at GZ (GZ was founded in 1932) and is dedicated to the production of planetary ball screws. The product model is complete, including roller non-circulation, roller circulation, and reversing planetary roller screw. Lead accuracy classes from KL5 to KL10 (G1-G3). GSA itself is an excellent supplier and holding company of Schaeffler Group in Germany, and in 2016, GSA led the complete acquisition of Switzerland rollvis in 2016, so GSA has the ROLLVIS brand, and its agent LTK, United States 3, United Kingdom 2 agents cooperation rights. Its products are mainly sold to aerospace, military equipment and iron and steel metallurgy industries, and it is also trying to develop applications in general industrial fields.
Switzerland ROLLVIS: Founded in 1970 and headquartered in Geneva, Switzerland, focusing on planetary lead screws; The products include four series of roller screws, RV, RVR, RVI, RVD, with complete models; The company can provide almost a full range of grinding-grade high-precision products with diameters ranging from 5mm to 245mm and leads ranging from 0.1mm to 50mm. Rollvis is an aviation confidentiality certification unit, and its products are mainly used in aerospace, military equipment, iron and steel metallurgy and other fields, and in recent years, it has also provided mass supply for general industrial applications such as electric cylinders. In 2016, it was acquired by GSA of Switzerland.
Ewellix Sweden: Founded more than 50 years ago as SKF Motion Technology, the linear technology division of SKF, Ewellix is a linear solutions company and a global innovator and manufacturer of linear motion and drive solutions for assembly automation, medical applications and mobile machinery. EWELLIX has 16 sales offices and 250 factories around the world. Net external sales amounted to around 125 million euros. In January 2023, the Schaeffler Group of Germany completed the acquisition of Ewellix.
Rexroth, Germany: The company and brand are part of the Bosch Rexroth Group, a large industrial company that produces hydraulic systems, gear units, electronic control technology and mechatronic products. In the field of linear drive technology, Rexroth provides products such as linear guides, ball and roller screws, electric cylinders and linear motion modules, and its roller screw products are new products launched in the last 10 years, using rolling forming technology to meet the needs of customers who are looking for higher economy, but cannot be used in scenarios where high precision and long life are required, and the products are mainly sold to the general industrial field, especially in applications such as injection molding machines and servo pressure machinery.
Key players in the country:
Nanjing Technology: Founded in 1952, Nanjing Technology is a holding subsidiary of Nanjing Xingong Investment Group Co., Ltd., a key high-tech enterprise of the National Torch Program, and a single champion cultivation enterprise in the manufacturing industry awarded by the Ministry of Industry and Information Technology. The company's main products are rolling functional components represented by ball screw pairs and rolling guide pairs, which are widely used in high-end CNC machine tools, artificial intelligence, 3C automation, national defense and military industry, etc., and are the core components of intelligent manufacturing of high-end CNC equipment. At present, the hardness of the planetary roller screw pair of Nanjing process can reach more than HRC58 after quenching, and it is mainly used in servo-electric cylinders.
Bote Seiko: Shandong Bote Seiko Co., Ltd., formerly known as Shandong Jining Screw Factory, was founded in 1966, a national high-tech enterprise, and a domestic backbone specialized production plant of rolling functional components. For more than 50 years, the company has been committed to the research and production of rolling functional components, and is the main drafting unit of China's rolling functional components standards (ball screw pairs, linear guide pairs), the main drafting unit of national high-speed spindle standards, the leader unit of the national ball screw deputy standard working group, and the vice chairman unit of China Machine Tool Industry Association. At the beginning of the 21st century, the company has successfully developed new products such as planetary roller screw pairs, heavy-duty ball screw pairs, high-speed mechanical spindles, and electric spindles.
Pictured: Bot Seiko BT-GZXS planetary roller screw
Data source: Bot Seiko official website
Xinjian Transmission: The main business of Xinjian Transmission is to provide worm gear, worm transmission components and precision parts for semiconductor communications, computers, electronics, automobiles, construction machinery, human-machine collaboration and service robots and other industries. The company has formed strategic partnerships with a number of research institutions at home and abroad, and established a R&D center and a joint R&D laboratory branch of Xinjian Transmission (Shanghai), with the research and development direction of mechanical transmission technology and artificial intelligence application integration technology. At present, the company's customers in the automotive field of lead screw products include Geely, Ideal, Huawei, FAW, etc.; Robot partners include Xiaomi, Xpeng, Zhiyuan Robotics, Ecovacs Robotics, etc.
The company's planetary roller screw production line was mass-produced in March 2022, and currently has a mass production capacity of 3.5 million sets. In 2022, the sales revenue of planetary roller screw electric cylinder production line series products will be 23.678 million yuan, a year-on-year increase of 100%, and the gross profit margin will reach 33.5%. At present, the company's ball screw and planetary roller screw products have been successfully applied to automotive chassis EMB braking systems and humanoid robot linear drives.
Best: Best is deeply engaged in the research and development, production and sales of precision parts and intelligent equipment and tooling products, and has the technical advantages formed in the field of precision parts processing and the flexible processing ability of automated production lines. The company gives full play to the core competitive advantage of the industrial linkage development of "precision machining as a specialty, foundry industry as the support, and intelligent equipment as the drive", and comprehensively lays out the linear rolling functional components and introduces the new track of "industrial machine tool". The company has the ability to turn and manufacture large-scale production of micron-level tolerance parts, and has a deep precipitation in high-precision machining.
In 2022, Yuhua Precision Machinery, a subsidiary, was established to carry the core rolling functional components of high-end equipment, mainly including the manufacture of high-precision ball screw pairs, high-precision rolling guide pairs and other products. Yuhua Precision Machinery's rolling functional parts products have completed sample delivery in the first half of 2023. At present, the project has been supported by national special funds, with a total investment of 1.2 billion yuan, using 45,000 square meters of factory buildings, the introduction of 279 sets of production and testing equipment (including 70 sets of R&D equipment), 29 sets of supporting software and systems, and 5 sets of public and auxiliary facilities. After reaching production, it has an annual production capacity of 150,000 sets of high-precision ball screw pairs and 248,000 meters of high-precision rolling guide rail pairs, with an annual output value of 1.5 billion yuan. As of April 2023, some of the imported equipment has arrived in the workshop and is being installed and commissioned, and the rest of the equipment is being procured one after another. In the first half of 2023, Yuhua Precision Machinery has successfully completed the first set of high-precision ball screw pairs, linear guide pairs and other products, and has sent samples of some products to strategic customers for verification.
From the comprehensive comparison of various products, the product models and specifications of domestic manufacturers are very complete, and they are in a leading position in terms of performance.
Many well-known domestic institutions have also entered the game and participated in the investment in the planetary roller screw sector. For example, Shanghai Hengsheng Investment is the third largest shareholder of Nanjing Technology, accounting for 6.5% of the shares; Mindray's investment company participated in the private placement of Dingzhi Technology, and Mindray is also one of the top 10 shareholders of Dingzhi Technology. China Development Bank Development Fund and Shandong Modern Industry Investment account for nearly 20% of the shares in Bote Seiko; Zhejiang Venture Capital Group owns 2% of the shares of Xinjian Transmission, and its subsidiary Zhejiang Qichen invests in 4% of the shares of Xinjian Transmission; Wuxi Xinshi Investment and the National Social Security Fund are also the top ten shareholders of Best. With the pull of market demand, the domestic planetary roller screw industry is expected to usher in significant growth, and many domestic investment institutions will also benefit.
With the maturity of humanoid robot products in the future, the empowerment of end-to-end AI large models, and the balance of functionality and practicability, humanoid robots are expected to become "disruptive products after computers, smart phones, and new energy vehicles". Planetary roller screws are an important part of the humanoid robot industry chain, and the growth space will also be greatly expanded. Due to the high manufacturing barriers of planetary roller screws, the current domestic market is mainly occupied by foreign companies, but domestic enterprises are catching up, the gap between product performance and foreign countries is gradually decreasing, and the prospect of domestic substitution is promising. In the foreseeable future, this paper believes that planetary roller screws will become an important subdivision investment track in the humanoid robot track.
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