Humanoid robots have become a shining pearl in the field of artificial intelligence.
In recent years, humanoid robots have become a shining pearl in the field of artificial intelligence with their wide application in many fields such as medical care and service. 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 the humanoid robot industry chain, the hollow cup motor plays an important role in the motion control system of the humanoid robot, such as the core component of the Tesla humanoid robot dexterous hand is the hollow cup motor, a single robot assembly 12 (6 each right hand). This paper aims to discuss the technical characteristics, market status and future prospects of hollow cup motor through the research.
What is hollow cup motor
1. Concept and classification of motor
An electric motor is a device that converts electrical energy into mechanical energy. It uses an energized coil (that is, the stator winding) to generate a rotating magnetic field and is used for the rotor (such as a squirrel-cage closed aluminum frame) to form a magnetoelectric rotational torque, which is to convert the force generated by the current flow in the magnetic field into a rotating action. The principle is to use the magnetic field to force the current to make the motor rotate.
The basic principle of the rotation of the motor: around the permanent magnet with a rotating axis, 1 rotate the magnet (so that the rotating magnetic field is generated), 2 according to the principle of the N pole and the S pole heteropole attraction, the same pole repulsion, 3 the magnet with a rotating axis will rotate.
In a motor, it is actually the current flowing through the wire that creates a rotating magnetic field (magnetic force) around it that causes the magnet to rotate. When the wire is wound into a coil, the magnetic force is synthesized to form a large magnetic field flux (magnetic flux), resulting in the N and S poles. By inserting an iron core into a coil of wire, the magnetic field lines become easier to pass through and can produce a stronger magnetic force.
The structure of the motor is mainly composed of two parts: stator and rotor.
Stator: the stationary part of the motor, the main structure of which includes the magnetic pole, winding and bracket. The magnetic pole is the part of the motor that generates the magnetic field, which is usually composed of an iron core and coils. The winding is the coil in the stator, usually composed of conductors and insulation, whose role is to generate a magnetic field when an electric current passes through it. The bracket is the support structure of the stator, usually made of aluminum alloy and other materials, with good corrosion resistance and strength.
Rotor: The rotating part of a motor, the main structure of which includes armature, bearings and end caps. The armature is the coil in the rotor, usually composed of conductors and insulation, whose role is to generate a magnetic field when an electric current passes through it. Bearings are the support structure of the rotor, usually made of steel or ceramic, with good wear and corrosion resistance. The end cover is the end structure of the motor, usually made of aluminum alloy and other materials, with good sealing and strength.
2, hollow cup motor definition and classification
In 1958, Dr.F. F. aulhaber developed the inclined winding coil technology and obtained the relevant patent for the hollow cup motor in 1965, marking the advent of the hollow cup motor, and its creative structural design allows the motor to be both smaller size and greater efficiency. The hollow cup motor belongs to the DC permanent magnet servo motor, the motor structure is shown in the following figure, mainly composed of stator and rotor. The stator is composed of silicon steel sheet and coil winding, and the silicon steel sheet without tooth groove structure can avoid the tooth groove effect and reduce the iron loss and eddy current loss. The rotor is composed of a permanent magnet, a rotating shaft and its fixed parts, and the motor uses a ring permanent magnet, which is easy to process and install.
Compared with ordinary motors, the biggest feature of the rotor is that it breaks through the rotor structure of the traditional motor in structure, and uses a no-core rotor, also known as a hollow cup rotor. The rotor is a hollow cup-shaped structure surrounded by windings and magnets. In ordinary motors, the role of the iron core is mainly: 1) concentrate and guide the magnetic field: the iron core is made of a material with high magnetic permeability (such as silicon steel sheet), which can concentrate and guide the magnetic flux, thereby improving the magnetic field strength and efficiency of the motor; 2) Support winding: The iron core provides a strong support structure for the winding, ensuring that the winding maintains a stable shape and position during the operation of the motor. In the hollow cup motor, the thin-walled hollow cylinder is used as the rotor, and the hollow cylinder is wound directly inside the winding without additional core support. Advantages of coreless design: 1) Elimination of eddy current and hysteresis losses: The iron core in a common motor will produce eddy current and hysteresis losses in an alternating magnetic field, which will reduce the efficiency of the motor. The hollow cup motor uses a coreless rotor, which completely eliminates these losses, thereby improving the energy conversion efficiency of the motor. 2) Reduce weight and moment of inertia: the core-free design significantly reduces the weight of the rotor, making the entire motor lighter. At the same time, the reduction of moment of inertia allows the motor to have a faster response speed and higher acceleration, which is very beneficial for application scenarios that require fast start and stop.
At the same time, the precision design of the hollow cylinder structure and winding layout can optimize the magnetic field distribution inside the hollow cup motor, reduce the magnetic leakage and energy loss, and further improve the efficiency and performance of the motor.
The hollow cup motor can be divided into two kinds according to its commutation mode: one is the hollow cup brush motor, which adopts the mechanical carbon brush commutation mode; The other is the hollow cup brushless motor, which replaces the brush commutation with electronic commutation, avoiding the electric spark and toner particles generated during the operation of the brush motor, reducing the noise and increasing the service life of the motor. From the comparison of different products of Mingzhi electrical appliances in the following figure, it can be seen that there is no need for a brush in the brushless hollow cup motor, but the Hall sensor detects the rotor magnetic field signal, turns the mechanical reversal into an electronic signal reversal, and further simplifies the physical structure of the hollow cup motor.
3, hollow cup motor advantages
The hollow cup motor breaks through the rotor structure of the traditional motor in structure, reduces the power loss caused by the formation of eddy current in the iron core, and its mass and moment of inertia are greatly reduced, thereby reducing the mechanical energy loss of the rotor itself. In summary, the hollow cup motor has the advantages of high power density, long service life, fast response, high peak torque, good heat dissipation and so on.
High power density: The power density of the hollow cup motor is the ratio of the output power to the weight or volume. In terms of weight, the non-core rotor is lighter than the ordinary core rotor; In terms of efficiency, the coreless rotor eliminates eddy current and hysteresis loss generated by the coreless rotor, improves the efficiency of the micromotor, and ensures high output torque and output power. The maximum efficiency of most hollow cup motors is more than 80%, while the maximum efficiency of most brush DC motors is generally around 50%. Lower weight and higher efficiency allow hollow cup motors to achieve higher power density. Therefore, the hollow cup motor is particularly suitable for battery-powered applications that require long periods of operation, such as portable air sampling pumps, humanoid robots, bionic hands, hand-held power tools and other applications.
High torque density: the coreless design reduces the weight of the rotor and the moment of inertia, and the low moment of inertia means that the motor can accelerate and decelerate faster, thus being able to generate more torque in a short time; At the same time, the absence of an iron core makes the hollow cup motor more compact, smaller, and able to provide higher torque output in a limited space.
Long service life: The number of reversing pieces of the hollow cup motor makes the current fluctuation and the inductance of the motor smaller when reversing, greatly reducing the electrical corrosion of the reversing system during the reversing process, so as to have a longer life. According to the data in the "Application Research of Customized Management of hollow cup motors", the life of brushed DC motors is generally only a few hundred hours, and the life expectancy of hollow cup motors is usually between 1000 and 3000 hours, which can provide longer reliable operation.
Fast response speed: the traditional motor has a relatively large moment of inertia due to the existence of the iron core, while the hollow cup motor is compact, and the rotor is a cup-shaped self-supporting coil, so the weight is lighter, and its smaller moment of inertia also makes the hollow cup motor have sensitive start-stop adjustment characteristics. According to the "Research Progress of hollow cup micro motor and coil", the mechanical time constant of the general core motor is about 100ms, while the mechanical time constant of the hollow cup motor is less than 28ms, and some products are even less than 10ms.
High peak torque: The ratio of peak torque and continuous torque of the hollow cup motor is very large, because the process of the current rising to the peak torque constant is unchanged, and the linear relationship between the current and torque can make the micromotor produce a large peak torque. After the ordinary core DC motor reaches saturation, no matter the current is increased, the torque of the DC motor will not increase.
Good heat dissipation: the surface of the hollow cup rotor has air flow, better than the heat dissipation performance of the core rotor, the enameled wire of the core rotor is embedded in the silicon steel sheet groove, the coil surface airflow is less, the temperature rise is larger, under the same power output conditions, the temperature rise of the hollow cup DC motor is smaller.
4, the technical path of hollow cup motor
The key step in the production of hollow cup motor is the production of coil, so coil design and winding process become its core barriers. The diameter, number of turns and linearity of the wire directly affect the core parameters of the motor. The core barrier of coil winding is directly reflected in coil design, because different winding types have differences in automation rate and copper consumption. On the other hand, it is also reflected in the winding equipment and winding method, and the filling rate of the hollow cup groove wound by different winding machinery is different, which leads to different sparse, directly affecting the motor loss, heat dissipation, power and so on.
Coil design Angle: The winding design of hollow cup motor can be divided into straight winding type, oblique winding type and saddle type.
Straight winding: The wire of the coil is parallel to the axis of the motor, forming a concentrated winding structure. The design idea of the straight-wound coil is to first wind the ordinary circular enameled wire on the winding die according to the requirement of the number of turns, and then connect the winding on the core shaft of the wire, and then use the binder at both ends to cure and form. Relatively speaking, the end of the straight winding produces no torque, and increases the armature weight and armature resistance.
Oblique winding: also known as honeycomb winding, the honeycomb winding method is used, leaving taps in the middle, in order to be able to continuously wind, it is necessary to make the effective side of the element and the armature axis into a certain tilt Angle. The end size of this winding method is small, but because the oblique winding continuous winding requires a certain line Angle, the enameled wire overlaps, and the slot filling rate is low. Compared with the straight wound type, the inclined winding armature has no end winding, reducing the armature weight, and has the advantages of small moment of inertia, small time constant, good drag characteristics and large output torque. Faulhaber in Germany and Portescap in Switzerland mostly use inclined winding.
Saddle type: also known as concentric or rhomboid winding, the method of winding a shaped coil and then wiring is used, that is, the self-adhesive enamelled wire is wound on a special forming winding die, and the armature cup is made of multiple shaping arrangements. When winding, the two layers of coils are arranged neatly and shaped, which is convenient to control the size of the armature cup after reshaping and improve the slot filling rate. At the same time, this method has high production efficiency and is suitable for mass production. The saddle winding armature end has fewer overlapping layers, small air gap and high utilization rate of permanent magnet, which improves the power density of the motor. Some products of Maxon in Switzerland use saddle-type winding.
Winding process point of view: From the production technology point of view, according to the forming method of the coil is mainly divided into three categories: manual winding, winding and one-time forming production.
1) Manual winding. Through a series of complex processes, including pin insertion, manual winding, manual wiring and other steps to produce. It is suitable for products requiring a high degree of customization, but production efficiency and product stability are limited.
2) Winding production technology. The winding production technology is semi-automatic production, the enameled wire is first sequentially wound to the main shaft with a diamond-shaped cross-section, and it is removed after reaching the required length, and then flattened into a wire plate, and finally the wire plate is wound into a cup-shaped coil. According to the "winding hollow cup armature production process and equipment" winding process, the next winding machine can be configured with 4 workers to achieve an annual output of 30,000 units, but the limitation of winding is that it is more suitable for 20-30mm hollow cup diameter, it is difficult to wind smaller coils with tap spacing less than 7mm, that is, products with a diameter of less than 10~12mm. Overall, the production efficiency of the winding process is relatively high, and it can meet the requirements of medium-scale production. However, its high manual participation rate leads to the consistency of the finished product may not be as good as automated production, and it is difficult to meet the smaller size of the hollow cup coil winding.
3) One molding production technology. Winding machine through automation equipment will be a enameled wire according to the rule of a spindle, coil winding into a cup after removal, one molding, no need to roll and flatten multiple processes, high degree of automation, so the production efficiency and finished product consistency are better; But the corresponding upfront equipment investment will be higher.
Overseas winding process developed early, the degree of automation is higher than domestic. The domestic mainly adopts winding production, the process is more complicated, the labor intensity of workers is large, can not complete the coil with thicker wire diameter, and the scrap rate is high. Foreign countries mainly use one-time wound production technology, high degree of automation, high production efficiency, coil diameter range, good coil quality, tight arrangement, motor types, good performance.
Industrial chain links and downstream applications
The upstream of the hollow cup motor is raw materials and parts, raw materials include copper, steel, magnetic steel, plastic, etc., parts include bearings, brushes, commutators, etc. The middle reaches of the industrial chain are motor manufacturers. The downstream of the industrial chain is the application end, and the hollow cup motor has the characteristics of high sensitivity, stable operation and strong control, which meets the strict requirements of the high-end field of electric drive, so it is mainly used in aerospace, medical equipment, industrial automation and robotics and other high-end fields. At the same time, the hollow cup motor is also gradually applied in the civil field, such as office automation, power tools and so on.
A promising hollow cup motor
Hollow cup motor with its unique design without iron core, showing high speed, high efficiency, high dynamic response and other significant advantages, widely used in aerospace, medical equipment and other fields, in the humanoid robot hand flexibility also has a significant impact. Although overseas enterprises such as Maxon and Faulhaber have the first-mover advantage at present, with the continuous improvement of the technical level of domestic manufacturers and the rapid development of the humanoid robot market, domestic hollow cup motors will usher in new development opportunities.
