Basic Concepts of a Servo System
The word "servo" originates from Latin, meaning "a slave working faithfully and quickly according to the master's instructions." A servo system refers to a system that uses closed-loop control to control the position, torque, speed, or acceleration of a mechanical system, thereby enabling the output variable to accurately follow or reproduce the input variable. The servo system determines the accuracy, speed, and stability of the actuator, making it the core of industrial automation equipment. The development of servo systems has progressed from hydraulic to pneumatic to electric. Hydraulic servo systems are prone to oil leaks and are difficult to maintain; pneumatic servo systems, due to the poor viscosity and compressibility of air, cannot be used in applications requiring high positioning accuracy. Overall, electric servo systems are currently the most widely used and technologically mature approach in the field of robotics.
The servo systems mentioned later in this article refer to electric servo systems. A servo system mainly consists of three parts: a servo motor, a servo driver, and a command mechanism. The servo driver, also known as a "servo controller" or "servo amplifier," amplifies, transforms, and regulates power according to control commands, supplying voltage and current to the motor coils. Its function is similar to that of a frequency converter for a regular AC motor. It typically contains three embedded servo loops: a current loop, a speed loop, and a position loop. These loops interact to summarize, analyze, and correct signals, achieving precise motion control. The command mechanism sends pulses or speed signals to coordinate with the driver's normal operation. The servo motor converts voltage signals into torque and speed, precisely controlling the mechanical system's movement according to position, torque, or speed commands. The embedded encoder feeds back the servo motor's motion parameters to the servo driver, completing closed-loop control.
Market Prospects for Two-Servo Systems
With the continuous development of China's mid-to-high-end manufacturing industry, various sectors are increasingly using servo systems to achieve high-quality and high-precision product manufacturing. The downstream application industries of servo systems have shifted from traditional fields such as textiles, packaging, and printing to emerging fields such as electronic equipment manufacturing and industrial robots. According to MIR data, in 2021, electronic manufacturing equipment, battery manufacturing equipment, and industrial robots were the three largest markets, accounting for 18%, 11%, and 10% respectively. The transformation and upgrading of downstream applications of servo systems has driven the gradual expansion of the market size.
According to calculations by the China Business Industry Research Institute, from 2017 to 2021, the Chinese servo system market grew from 9.7 billion yuan to 22.4 billion yuan, with a compound annual growth rate of 23.27%. The market size is expected to reach 28.6 billion yuan in 2022, maintaining a relatively rapid growth rate. In 2021, the Chinese servo motor market size was 16.9 billion yuan, with a compound annual growth rate of 9.86% from 2017 to 2021. Among them, AC and DC servo motors accounted for 65% and 35% of the market share, respectively. This is because AC servo motors have many advantages, such as high maximum speed, smooth speed control, low oscillation and low heat generation, and suitability for dusty and explosive environments, making them more suitable for the motion control requirements of industrial automation. Currently, most industrial robots and CNC machine tools, except for collaborative robots, use AC servo motors, but their larger size and weight limit the expansion of application scenarios to some extent.
With the large-scale application of service robots and medical devices, the requirements of downstream industries for motor technology have undergone tremendous changes, which is expected to drive the rapid growth of DC servo motors. In particular, humanoid robots, in order to achieve high flexibility and precision, require approximately 30-40 motors per unit, which is 6-7 times that of multi-joint robots. Humanoid robots use DC servo motors and micro-cylinder systems. Large joints such as the neck, shoulder, elbow, wrist, waist, knee, and ankle joints generally use standard motor combinations with a power of approximately 100-200 watts; hand joints, with their limited space and higher requirements, can utilize a combination of micro-motor and micro-cylinder systems with a power of approximately 30 watts.
Three technological barriers to servo systems used in robots
(I) Performance Requirements
Compared to general-purpose servos, robot servo systems are high-end products in the servo system category, which place higher demands on speed range, reliability, stability, and many other aspects. (1) Fast response: The robot's actions change quickly, and the time from receiving the command signal to completing the required working state should be as short as possible. Generally, the electromechanical time constant is used to reflect the fast response performance of the servo motor; (2) High load operation: Since large industrial robots have very large loads, the servo motor is required to have a large starting torque, small moment of inertia, and a sufficient starting torque-to-inertia ratio; (3) Small size and weight: To ensure flexibility, robots are becoming lighter and lighter. The servo motor must be small in size, light in weight, and short in axial dimension. It must also be able to withstand harsh working conditions, frequently reverse and accelerate/decelerate, and withstand several times the overload in a short time; (4) Good temperature control: The machine The small size, lightweight and high torque output of the robot joint motor are not conducive to temperature control. The design needs to be improved to avoid problems such as irreversible demagnetization of permanent magnets and damage to insulation materials caused by excessive temperature; (5) Continuity and linearity of control characteristics: The speed of the motor can change continuously with the change of control signal. Sometimes the speed needs to be proportional or approximately proportional to the control signal; (6) Stability of operation: Torque pulsation is the uneven torque generated by the rotor in the motor during rotation. It is caused by the cogging torque caused by the attraction between the permanent magnet installed on the rotor and the steel teeth of the stator lamination. It will produce vibration and noise, affecting the low-speed performance and accuracy of the motor. It is necessary to eliminate the cogging torque with reasonable electromagnetic design.
(II) Gap between domestic and international markets
Although the overall technical barrier of servo motors is lower than that of reducers, the requirements of robots for servo systems are higher. Compared with general servo systems, the gap in technology, product quality, stability and variety of servo systems for robots between domestic and foreign companies is greater. The main shortcomings of domestic servo motors are as follows: (1) Most domestic servo motors are imitations of Japanese servo motor designs, with power mostly below 3kw, and small and medium power servo motors are more common, while medium and large power servo motors of 5.5-15kw are less common. (2) They are generally large in size and have a rough appearance, making them difficult to use in high-end robots with strict requirements on the size of servo motors. (3) One of the core technologies of servo systems is high-precision encoders. Encoders account for 30% of the cost of motors, and their resolution determines the control accuracy. The multi-turn absolute encoders and chips used in robots are heavily dependent on imports, which is the main reason why domestic servo motors cannot completely replace imported products. (4) The performance of domestic servo motors is greatly reduced in complex working environments, which is one of the important reasons that hinders domestic servo motors from entering the high-end market. (5) Insufficient basic research, including absolute encoder technology, industrial manufacturing technology of high-end motors, breakthroughs in production processes, practical verification of performance indicators, and formulation of evaluation standards. (6) Insufficient industrial synergy among different parts of the servo system makes it difficult to achieve good overall performance of servo motors and drive systems.
Four Domestic Market Competition Patterns
European, American, and Japanese brands still dominate my country's mid-to-high-end servo system market, holding the majority of the market share. Japanese products offer high cost-performance, reliable quality, small size, and light weight, but their dynamic response capabilities are weaker and their openness is poor, giving them an advantage in the mid-range market. In 2021, Panasonic, Yaskawa, and Mitsubishi, the three major Japanese brands, accounted for approximately 30% of the domestic servo system market share. European and American brands such as Siemens, Bosch Rexroth, and B&R focus more on product performance, offering high overload capacity, good dynamic response, and highly open drivers, but their prices are high and their size and weight are large, firmly occupying the high-end market. Although the market demand for medium-to-large-sized high-power products is limited, the high value per unit allows companies to maintain high profitability.
China's independent research and development of servo systems started late, but through measures such as introducing and learning from advanced international technologies, the product quality and technical level of servo systems have been continuously improved, forming a certain product series and independent supporting capabilities. Meanwhile, affected by factors such as chip shortages and logistical disruptions caused by the pandemic, delivery times for Japanese, Korean, and European and American companies have been extended, creating a window of opportunity for domestic companies to substitute domestic products. Mid-to-low-end servo systems have achieved large-scale mass production, and many domestic products are technically close to Japanese products, meeting the needs of small and medium-sized and budget-conscious users with their cost-effectiveness, gradually breaking the monopoly of foreign brands. The market share of domestic companies, led by Inovance Technology, Hechuan Technology, and Estun, increased from 27.6% in 2017 to 43.3% in 2021. Inovance Technology, relying on its advantages of industry-customized solutions, high cost-effectiveness, and timely response to customer needs, surpassed foreign brands for the first time in 2021, ranking first with a 16% market share. At the same time, we should also be aware that the increase in the market share of domestic brands is due to the decline in the market share of Japanese brands, while the market share of high-end brands occupied by European and American brands has hardly changed. This indicates that domestic high-end servo systems have not yet gained a competitive advantage and need to continue to increase R&D investment. There is a broad space for domestic substitution in the mid-to-high-end application market.
Five future development directions
Micro-motors, represented by coreless motors, servo electric cylinders, and frameless motors, are expected to become a new trend in the development of the servo system industry. These three types of motors all possess higher power density, align with green environmental protection and low-carbon principles, and will gradually penetrate into fields such as home appliances and robots that consume large amounts of electricity and have high usage frequency, meeting the requirements for high efficiency, energy saving, miniaturization, and intelligence.
According to data from China Business Intelligence Network, the market size of micro-motors in my country grew from 187.2 billion yuan in 2017 to 279.3 billion yuan in 2022, with an average annual compound growth rate of 6.9%, indicating huge growth potential and demand for technological innovation. In recent years, a number of competitive micro-motor manufacturers have emerged in my country, gradually expanding their production scale and market share.
(I) Hollow Cup Motor
Coreless motors are DC, permanent magnet, servo micro motors. In a typical brushed DC motor, the stator is a permanent magnet located on the outer ring, while the rotor is a coil fixed to the iron core on the inner ring. In contrast, the coreless motor's stator is a permanent magnet located on the inner ring, and the rotor is a cup-shaped, self-supporting coil located on the outer ring. The coreless structure eliminates energy loss caused by eddy currents formed in the iron core, increasing energy conversion efficiency to over 90%. It also reduces motor weight and rotational inertia, resulting in excellent acceleration and deceleration performance, small size, high power density, and good heat dissipation. Furthermore, coreless motors have no cogging torque, resulting in low torque fluctuation, stable speed, and high operational stability. They effectively meet the requirements of lightweight, compact structure, and strong gripping force in dexterous hands, representing the future direction of motor development. However, due to the lack of a robust iron core support, the coil thickness is relatively thin, and the connection strength between the coil and the output shaft is limited, thus restricting the size and power output; the maximum power of a typical coreless motor is only a few hundred watts.
The coreless motor market has long been monopolized by foreign companies, presenting significant opportunities for domestic substitution. Coreless motors are challenging to manufacture and mass-produce; ensuring the flatness and consistency of the winding cups is the most crucial and critical technology in their production. Currently, the coreless motor market is dominated by German company FAULHABER and Swiss company Maxon. However, as domestic market awareness of coreless motors increases and domestic manufacturers like Mingzhi Electric make breakthroughs in mass production processes, domestic substitution is expected to be achieved in the future.
(II) Frameless DC Torque Motor
A frameless DC torque motor is a servo motor that removes the shaft, bearings, housing, feedback, or end cover, leaving only a rotor and stator. The core advantages of frameless torque motors are: (1) a very light rotor, allowing for high-speed operation; (2) a compact design, occupying little space, and high torque per unit volume; and (3) fewer mechanical parts, with no easily worn or maintained components, reducing later maintenance costs. Changzhou Jingna, a wholly-owned subsidiary of Buke Technology, produces frameless torque motors with leading comprehensive performance in China, primarily used in collaborative robots.
(III) Servo Electric Cylinder
Servo electric cylinders are modular products integrating servo motors and lead screws, converting the rotational motion of the motor into high-precision linear motion. They offer advantages such as low noise, energy saving, high rigidity, and impact resistance, meeting the motor requirements of humanoid robots. Servo electric cylinders can be divided into linear (direct-drive) and parallel (returning) types. Linear electric cylinders consist of a servo motor, servo driver, high-precision ball screw or planetary ball screw, linear displacement sensor, and cylinder body. The servo motor and drive screw are mounted on a straight line and connected by a coupling, resulting in a compact structure and rapid response. Parallel electric cylinders have the motor and cylinder body mounted parallel to each other, connected to the cylinder's drive screw via a synchronous belt and pulley. They retain the advantages of linear cylinders and, due to their shorter overall length, are more suitable for applications with limited space. Servo electric cylinders are mainly used at the joints of the knee, elbow, and shoulder joints of humanoid robots, serving as auxiliary drives.
V. Summary and Recommendations
With the accelerated commercialization of humanoid robots, servo motors, a core component of the robot industry chain, will see significant market growth. However, existing motors struggle to simultaneously meet requirements such as flexibility, shock resistance, rapid response, and high power density. Breakthroughs in motor technology are crucial for the future commercial application of robots, presenting a vast potential for technological iteration. If domestic companies can seize this opportunity, leveraging technological breakthroughs and cost-effectiveness to enter the OEM supply chain, it will not only enhance the market competitiveness of domestic robot companies but also have strategic significance for the development of the entire Chinese robot industry. Currently, domestic manufacturers mainly focus on the low-to-mid-end market, using cost advantages to boost sales. However, to maintain stable and rapid growth, they must actively expand their product categories and efficiently enter new fields. This involves closely monitoring the latest developments in downstream application areas, targeting and cultivating high-growth downstream industries. Actively collaborating with high-quality companies across various industries globally is essential to truly understand and address consumer needs, developing high-value-added and stable motion control systems for complex scenarios, accumulating interdisciplinary and cross-domain knowledge and experience, and honing motor customization and innovative R&D capabilities. Enjoy the first-mover advantage of achieving breakthroughs in emerging fields with high barriers to entry, and maintain bargaining power and improve profitability by leveraging accumulated product R&D and iteration experience. It is recommended to focus on high-quality domestic servo motor suppliers: (1) Mingzhi Electric, which has strong motor customization capabilities, has taken the lead in laying out the domestic downstream market that is not yet mature but has technological and resource advantages and considerable space, and has overseas sales channels and customer resources; (2) Buke Technology, which has laid out torque motors; (3) Huichuan Technology and Hechuan Technology, the leading domestic general AC servo systems companies; (4) Jiangsu Leili and Leisai Intelligent, leading companies in mid-to-high-end micro motors.
