1. Introduction to Micromotors
Micromotors are motors that differ from conventional motors in principle, structure, performance, and function, and are also much smaller in size and output power. Generally, the outer diameter of a micromotor is no greater than 130mm, and its power ranges from several hundred milliwatts to several hundred watts. They are widely used in various modern military and civilian equipment and their control systems, such as artillery control, missile guidance, aircraft autopilot, CNC machine tools, shuttleless loom control, industrial sewing machine control, telemetry and remote control, audio-visual equipment, automated instruments, and computer peripherals.
Today, in practical applications, micromotors have evolved from simple starting control and power provision to precise control of speed, position, and torque. This is particularly evident in industrial automation, office automation, and home automation, where they are almost universally adopted as mechatronic products combining motor technology, microelectronics technology, and power electronics technology. Electrification is an inevitable trend in the development of micromotors.
2 Application areas of micro motors
Modern micromotor technology integrates multiple high technologies such as motors, computers, control theory, and new materials, and is moving from military and industrial applications into everyday life. Therefore, the development of micromotor technology must adapt to the development needs of pillar industries and high-tech industries. Micromotors are mainly used in the following areas:
2.1 Micro-motors for household appliances
To continuously meet user demands and adapt to the needs of the information age, achieving energy efficiency, comfort, networking, intelligence, and even networked appliances (information appliances), the rapid replacement cycle of home appliances places high demands on the motors used in them, requiring high efficiency, low noise, low vibration, low price, adjustable speed, and intelligence. Micromotors used in home appliances account for 8% of the total micromotors market, including those in air conditioners, washing machines, refrigerators, microwave ovens, electric fans, vacuum cleaners, and spin dryers. The global annual demand is 450-500 million units. These motors have relatively low power but come in a wide variety. The development trends of micromotors used in home appliances include:
① Permanent magnet brushless motors will gradually replace single-phase asynchronous motors;
② Optimize the design to improve product quality and efficiency;
③ Adopt new structures and processes to improve production efficiency.
2.2 Micromotors for Information Processing Equipment
Micromotors account for 29% of information processing equipment: these include components for information input, storage, processing, output, and transmission, and also include communication equipment. The world needs 1.5 billion units annually, mainly permanent magnet DC motors, brushless DC motors, stepper motors, and micro synchronous motors. The annual production of microcomputers (PCs) was approximately 100 million units in 2000 and was projected to reach 200 million units in 2005, leading to a large demand for micromotors, a key component for these computers, with increasingly higher requirements. The vast majority of these motors are precision permanent magnet brushless motors and precision stepper motors.
Their characteristics and development direction are:
(1) High-investment products
These types of motors have extremely high requirements for speed stability and shaft runout. Therefore, they are high-tech, high-investment products that combine advanced manufacturing technology and emerging power electronics technology. Internationally, they are generally developed and produced by large companies.
(2) Miniaturization and sheet-like shape
To meet the demands for miniaturization and portability of information products, the corresponding motors are required to be miniaturized and flat.
(3) High speed
As the storage density of computer peripherals continues to increase, the required motor speed is above 8000 r/min.
2.3 Micromotors for Automobiles
Micro motors account for 13% of automotive applications, including starter generators, wiper motors, motors for air conditioning and cooling fans, electric speedometer motors, window regulator motors, and door lock motors. In 2000, the world's automobile production was approximately 54 million vehicles. If each vehicle requires an average of 15 motors, then the global demand would be 810 million motors.
The key development areas for automotive micro-motor technology are:
(1) High efficiency, high output, and energy-saving type
Its operating efficiency is improved by measures such as increasing speed, selecting high-performance magnetic materials, using efficient cooling methods, and improving controller efficiency.
(2) Intelligent
By making the car's motor and controller intelligent, the car can operate in the best condition and achieve the minimum energy consumption.
2.4 Micromotors for audio equipment
Micromotors used in audio equipment account for 18%, including those for record players, tape recorders, VCD players, and DVD players. The global annual demand exceeds 1 billion units. Currently, domestic production accounts for about 60%, mainly consisting of printed winding motors and wound disc motors.
2.5 Micromotors for visual equipment
Micro motors used in visual equipment account for 7%, including those used in cameras and camcorders. The global annual demand is between 350 and 400 million units. These motors are precision motors, which are difficult to manufacture and process. Especially with the advent of digitalization, even more stringent requirements are placed on these motors.
2.6 Micromotors for Industrial Electrical Drives and Control
These micro-motors account for 2% of the total, including those used in CNC machine tools, robotic arms, and robots. They mainly consist of AC servo motors, power stepper motors, wide-speed DC motors, and AC brushless motors. This type of motor has a wide variety of models and high technical requirements, and it is one of the fastest-growing categories of motors in terms of domestic demand.
2.7 Special Purpose Micro Motors
This type of motor accounts for approximately 23%, including those used in aerospace, various aircraft, automated weaponry, and medical equipment. These motors are mostly special or novel types, differing from general electromagnetic principles in their operating principles, structure, and operation. They primarily include low-speed synchronous motors, harmonic motors, finite-angle motors, ultrasonic motors, microwave motors, capacitor motors, and electrostatic motors. Among these, ultrasonic motors, microwave motors, capacitor motors, and electrostatic motors are special motors that differ from general motors in their operating principles, structure, and operation. The emergence and development of these motors are inextricably linked to the development of electronic and control technologies.
3 Micro Motor New Product Technology
With the continuous advancement of science and technology and the emergence of new requirements in practical applications, various micro-motors, different from traditional electromagnetic motors, have appeared. They employ novel design concepts, methods, structures, and principles.
3.1 Permanent Magnet Brushless Motor
Brushless motors represent the future of micro-motors and have already found applications in information technology, home appliances, audio-visual equipment, and transportation. With the rapid development of permanent magnet materials and power electronics technology, performance is continuously improving while prices are decreasing, inevitably leading to further development and increasing demand for brushless motors. Compared to conventional asynchronous motors, new brushless motors consume 30%–35% less power, achieving high efficiency, energy saving, small size, and lightweight design. Although brushless motors are more expensive than asynchronous motors, their lower power consumption, higher efficiency, and reduced operating costs make their widespread adoption an inevitable trend from an energy-saving perspective. Major global companies are already engaged in fierce competition in the brushless motor field. Therefore, with improvements in component and material performance, brushless motor performance will significantly increase, making the competition in technological development even more intense.
3.2 Ultrasonic motor
An ultrasonic motor (USM) utilizes the inverse piezoelectric effect of piezoelectric materials to induce microscopic mechanical vibrations in an elastic body (stator) within the ultrasonic frequency range (vibration frequency above 20kHz). Through friction between the stator and rotor (or mover), the microscopic vibrations of the stator are converted into macroscopic unidirectional rotation (or linear motion) of the rotor (or mover). This breaks away from the traditional concept of obtaining speed and torque through electromagnetic effects, representing another noteworthy new technology in the development of micromotors.
Compared with traditional electric motors, ultrasonic motors have a number of advantages:
(1) It has a simple structure, consisting of two basic components: a vibrating component and a moving component;
(2) It has a large torque per unit volume, which is 10 times that of a traditional electric motor of the same volume;
(3) It has good low-speed performance, and the speed can be adjusted to zero, allowing it to output large torque directly at low speeds;
(4) It has a large braking torque and does not require an additional brake;
(5) It has a small mechanical time constant and good high-speed performance;
(6) There are no magnetic fields or electric fields, no electromagnetic interference or electromagnetic noise, etc.
Currently, ultrasonic motors have been commercially applied in many companies in countries such as Japan. New ultrasonic motor products from companies like Canon, Panasonic, and Hitachi are already used in high-end cameras, camcorders, and optical instruments. The future development direction of ultrasonic motor technology is to further improve efficiency.
Ultrasonic motors employ entirely new principles and structures, eliminating the need for magnets and coils. Instead, they directly generate motion and force (torque) through the inverse piezoelectric effect of piezoelectric materials and ultrasonic vibration. This breaks with the previous concept of motors that generated speed and torque through electromagnetic effects, representing a cutting-edge technology at the forefront of global science. Due to its ultrasonic properties, the motor possesses many characteristics not found in electromagnetic motors. Despite its relatively short history of invention and development (only 20 years), it has already found successful applications in aerospace, robotics, automotive, precision positioning, medical devices, and micromechanical fields.
3.3 High- speed dynamic pressure bearing motor
As information products develop towards high efficiency, high density, and miniaturization, the precision permanent magnet brushless motors used to power them reach speeds of 8000–50000 r/min. The bearings in these high-speed motors will also use hydrodynamic bearings instead of traditional sliding bearings to overcome the numerous technical challenges associated with high speeds.
Compared with ball and sliding bearings, hydrodynamic bearings have many advantages; they can suppress irregular shaft wobble, improve impact resistance, have a longer lifespan, and lower noise. Hydrodynamic bearing motors come in two types: fluid-driven and air-driven. Generally, fluid-driven hydrodynamic bearings are used for lower speeds, while air-driven hydrodynamic bearings are used for higher speeds. Although some technical problems still need to be further solved in hydrodynamic bearing motors, the development direction of high-speed hydrodynamic bearing motors is widely recognized.
3.4 Linear Motor
With the rapid development of automatic control technology, the positioning accuracy requirements for various automatic control systems are becoming increasingly stringent. Traditional linear motion devices composed of a rotary motor and a motion conversion mechanism are far from meeting these accuracy requirements. Direct linear drive is one of the research areas in modern servo drive technology, and linear motors are a key technology within it. Linear motors have a wide range of applications; in devices requiring linear motion, direct-drive linear motors are superior to rotary motors. Because the motion conversion mechanism is eliminated, control accuracy can be improved.
3.5 Micro Motor
Micromotors are a new high-tech field utilizing Micro-Electro-Mechanical Systems (MEMS) technology, which has been developed in the last 20 years. They are characterized by microfabrication techniques based on silicon semiconductor materials, used to manufacture devices with energy conversion and transmission functions ranging in size from millimeters to micrometers. The emergence of MEMS technology has brought about a revolutionary leap in traditional mechanical manufacturing technology. Micromotors can be categorized into electrostatic and electromagnetic types. Electromagnetic micromotors, due to their higher torque, higher conversion efficiency, and longer lifespan compared to electrostatic micromotors, have been applied in many fields such as endoscopes and microrobots. Currently, the United States, Japan, Russia, Germany, and other countries are investing significant human, material, and financial resources in the research and application of this technology, achieving considerable progress, with some reaching practical application. For example, Toshiba Corporation of Japan developed the world's smallest micromotor, weighing 40 mg, rotating at 60–1000 r/min, operating at 1.7V, and with a diameter of only 0.8 mm. In my country, universities such as Shanghai Jiao Tong University are also developing micromotors with a diameter of 1 mm. It is expected that with the development and application of nanotechnology, micro motors will also see significant growth, leading to a wider range of applications.
3.6 molecular motor
With the development of MEMS, nanoelectromechanical systems (NEMS) have emerged, with feature sizes ranging from hundreds to several nanometers. Some of these have important potential applications in the biomedical field. Ricky K. Soong and colleagues at Cornell University integrated a single biomolecular motor with a nanoscale inorganic system to create a hybrid nanomechanical device driven by a molecular motor. This biomolecular motor (less than 8 nm in diameter and 14 nm in length) generates a maximum torque of 80–100 pN·nm by hydrolyzing ATP (adenosine triphosphate) in an active system, which is compatible with the size and mechanical constants of currently achievable nanomechanical structures. This new technology holds promise for applications in vascular cleansing.
4. Development Trends of Micromotors
Since the beginning of the 21st century, the world economy has faced two key challenges for sustainable development: energy and environmental protection. On the one hand, with societal progress, people have increasingly higher demands for quality of life and stronger environmental awareness. Micromotors are widely used not only in industrial and mining enterprises but also in commerce and service industries, and especially in more household products. Therefore, the safety of motors directly endangers personal and property safety; motor vibration, noise, and electromagnetic interference become environmental hazards; and motor efficiency directly affects energy consumption and harmful gas emissions. Consequently, international requirements for these technical indicators are becoming increasingly stringent, attracting the attention of the motor industry both domestically and internationally. Energy-saving research has been conducted on various aspects, including motor structure, processes, materials, electronic components, control circuits, and electromagnetic design. The new round of micromotor products, based on excellent technical performance, will further focus on energy conservation and environmental protection, implementing relevant international standards and promoting related technological advancements. These include research on new motor laminations, winding designs, improved ventilation structures, low-loss high-permeability materials, rare-earth permanent magnet materials, noise reduction and vibration damping technologies, power electronics technology, control technology, and electromagnetic interference reduction technologies.
With the accelerating trend of economic globalization, countries paying more attention to energy conservation and environmental protection, strengthening international technological exchanges and cooperation, and accelerating the pace of technological innovation, the development trend of micro-motor technology is as follows:
(1) Adopting advanced technologies and developing towards electronicization;
(2) High-efficiency, energy-saving and green development;
(3) Development towards higher reliability and electromagnetic compatibility;
(4) Develop towards low noise, low vibration, low cost, and low price;
(5) Develop towards specialization, diversification and intelligence.
Furthermore, micromotors are developing towards modularization, combination, intelligent mechatronics, and brushless, coreless, and permanent magnet designs. Of particular note is the fact that with the expanding application areas and changing environments of micromotors, traditional electromagnetic motors can no longer fully meet the requirements. Developing micromotors with non-electromagnetic principles using new achievements in related disciplines, including new principles and materials, has become an important direction in motor development.
5 Development Recommendations
Although domestic institutions and research institutes have made significant progress in the research of micromotors in recent years, a gap still exists compared with foreign counterparts. Faced with fierce international competition, we must strengthen research and cooperation, fully leverage our innovation capabilities, and narrow the gap with foreign micromotors to remain invincible in this field. Therefore, I believe we should strive in the following aspects:
(1) Many institutions engaged in micro-motor technology research suffer from severe funding shortages and extremely weak research capabilities (human resources and equipment), making it impossible to guarantee basic research conditions. It is recommended that relevant departments significantly increase investment in micro-motor technology research and development in the near future.
(2) It is recommended that relevant departments promptly organize specialized research units to study related micro-motors, such as piezoelectric ceramics, friction materials, rare earth materials, permanent magnet materials, etc., and further strengthen the communication and cooperation between research units and production enterprises.
(3) Strengthen communication and exchange among research units, concentrate efforts, and avoid duplication of work.
(4) At present, the research results of some universities and research institutes cannot be transformed into productivity; it is necessary to form a micro-motor industrialization base that integrates production, learning and research.
(5) Actively carry out research on long life and high reliability design and advanced manufacturing technology of micro motors.
(6) Based on in-depth research on the theory, manufacturing technology and materials of micro motors, a large number of experimental studies are conducted, including: various performance tests, life tests, reliability tests and environmental (high temperature, low temperature, humidity and vacuum) tests of micro motors, etc.
(7) Develop new micro-motor technologies, including new motor motion mechanisms, new modal transformation methods, and the development of ultra-micro motors, linear motors and non-contact motors, etc.
