AC permanent magnet servo motors are modern high-tech products that integrate the latest advancements in microelectronics, sensing, and motor manufacturing technologies.
It is mainly used in CNC machine tools , industrial robots and other CNC devices. With the continuous development of CNC technology, servo motors, as one of the actuators, will be widely used. Servo motors have a three-phase permanent magnet rotor structure, use high energy product permanent magnet materials, and have the characteristics of high power-to-weight ratio, fast dynamic response speed, and high machining precision requirements.
Therefore, the processing technology of servo motors is a technical problem that must be solved. Only by developing advanced and reasonable processing technology can we provide a reliable technical guarantee for the mass production of servo motors.
Structure and characteristics of servo motors
The motor features a fully enclosed, self-cooled, brushless structure, and the rotor is made of neodymium iron boron, a high-energy permanent magnet material, which makes it small in size and light in weight.
The stator has no outer casing, allowing for direct cooling and minimizing heat transfer to the machinery. A brushless decomposer and pulse encoder can be mounted on the rear shaft of the motor, enabling high-precision speed and position control via a servo system driver.
In addition, servo motors also have the following characteristics:
1. It must be able to continuously and frequently start, stop, brake, reverse, and operate at low speeds. Due to the extremely harsh operating conditions, it must have sufficient mechanical strength and take into full consideration heat resistance and cooling conditions.
2. High sensitivity is required.
3. The ineffective time must be short, the mechanical friction of the structure must be small, and both the magnetic and electrical aspects must be uniform.
4. The motor itself must have good stability, its speed-torque characteristic must be downward sloping, and there must be braking torque when the control signal is zero. In addition, the forward and reverse rotation characteristics must be consistent.
5. The control performance must be good. The ratio of the generated torque (speed) to the control signal should be large and stable. The output should not be saturated when a large instantaneous input signal is applied. Sufficient margin is essential.
6. High reliability is required.
7. It must be able to fully adapt to various installation locations and usage environments.
Because the structure and characteristics of servo motors make their manufacturing process much more complex than that of ordinary motors, this study explores the manufacturing processes of three key components: rotor, stator, and stator-rotor assembly.
Rotor machining process
As shown in Figure 2, the rotor is composed of a rotor core, magnets, magnet pads, cladding, adhesive, and screws.
The processing steps are as follows: rotor core—magnetization of magnets—bonding of magnets—adhesive coating—dynamic balancing. As can be seen from the process, the magnets need to be magnetized before bonding, which presents significant challenges to the bonding process. The magnets are 1/4 bearing-shaped, with the magnetization direction being radial, and the material is neodymium iron boron.
The bonding method of the magnets is shown in Figure 3.
As shown in Figure 3, each layer of magnets has four pieces. The magnetization directions of two symmetrical pieces are the same, and the magnetization directions of two axially adjacent magnets are also the same. When bonding the magnets, the four magnets in the first layer are relatively easy to bond. However, when bonding the second layer, because the adjacent magnets have the same polarity and the magnetization direction is the same, they repel each other, making it difficult to push the magnets to the predetermined position by hand. Therefore, a universal magnet bonding fixture as shown in Figure 3 was designed, and a common drilling machine was used to bond the magnets. The method of using the fixture is as follows: First, fix the fixture on the radial drilling machine, and align the pressure sleeve of the drill bit fixed by the drill with the center of the upper pressure sleeve of the fixture (the positioning sleeve, bushing, and pressure ring in Figure 4 are made according to the size specifications of the rotor and can be interchanged, so the fixture is universal). Select the positioning sleeve, bushing, and pressure ring according to the specifications of the rotor. Place the rotor on the positioning sleeve. There is a one-way thrust bearing under the positioning sleeve. After the rotor is placed on it, it can rotate freely. At this time, the magnets can be bonded.
First, evenly apply GJ301 adhesive to the surfaces of the magnetized magnets and rotor core. Then, bond the first layer of magnets. Since the rotor can rotate, rotating the rotor will allow bonding to be performed in a fixed working position. During bonding, ensure that the polarities of any two adjacent radially adjacent magnets are opposite.
After gluing, use soft copper strips and screws to fix it in place. Then, place a magnetic pad on each magnet for axial positioning of the two magnets, and then you can proceed with the second layer of magnet bonding.
As shown in the diagram, the two axial magnets have the same polarity and repel each other. Therefore, they cannot be immediately in place after bonding. The four magnets must be bonded quickly. Then, the upper pressure ring is pressed down through the lower, middle, and upper pressure sleeves. Rotating the rocker arm drill handle applies pressure to the upper pressure sleeve, causing the four magnets to be pressed down parallel to each other and into place simultaneously. After the magnets are in place, they are secured with soft copper tape. The second layer of magnets is then bonded. Subsequent layers are bonded similarly.
After bonding, apply GJ301 adhesive to the surface, then wrap the rotor surface with alkali-free glass fiber tape, thus completing the rotor processing. Experiments have shown that this process can produce qualified rotors.
Stator machining process
As shown in Figure 8, the stator is shell-less. After lamination, the stator end rings and stator laminations are first connected together with bolts. Then, welding is performed on the four grooves on the stator core to weld the stator end rings and stator laminations together as a single unit. The main processing techniques for the stator are: slotting the stator laminations, laminating the stator core, and welding the stator core. Lamination is done using a high-speed slotting machine. Using a slotting machine saves on mold processing costs and production cycle, requiring only a single-slot mold. Therefore, the application of slotting machines is particularly suitable for the development of diverse and serialized servo motors. After lamination, stator lamination is required. Lamination fixtures are used during lamination, and ensuring lamination accuracy is crucial. Uneven lamination will directly affect the winding windings and the appearance quality of the motor. During lamination, precise positioning using center holes and stator slots ensures lamination accuracy. The welding quality of the stator core directly affects the strength of the stator core and the appearance of the stator. Therefore, it is required that the weld joints be smooth and uniform. During the process design, a stator argon arc welding machine was modified so that the stator core could achieve uniform speed movement through mechanical transmission, thus ensuring the smoothness and uniformity of the weld joints.
Stator and rotor assembly process
As shown in the figure, the rotor has a large magnetism after it is processed. The key problem to be solved is how to install the rotor into the stator. At first, it was assembled by hand. Because the rotor is magnetic, the stator and rotor immediately attract each other during assembly. By pushing inward by hand, problems such as tearing the rotor coating, deformation of the stator, and crushing of workers' fingers occurred. It was also difficult to guarantee the quality of the product.
Based on the process requirements and the problems encountered in manual assembly, a set of universal fixtures for stator and rotor assembly was designed. See Figure 5. This fixture was designed and used using a 63 lathe.
Its operating principle is as follows: one end of the rotor is supported by the front support sleeve, which is fixed on the lathe chuck, and the other end of the rotor is supported by the rear support sleeve, with the tail of the rear support sleeve matched with the taper of the tailstock tip.
Then push the tailstock to its maximum position to support the rotor. At the same time, fix the fixture base on the tool holder. The base has a fixed panel and an adjustable panel (the adjustable panel and support sleeve are determined according to the stator size specifications). Then fix the stator between the two panels (see figure). Shake the tool holder to smoothly and non-contactly install the rotor into the stator. The stator and rotor assembly is then complete. This saves effort and ensures product quality.
in conclusion
1. The magnetic steel clamp is reasonable and practical, and can fully meet the product quality requirements.
2. The stator is processed accurately and reliably, and the argon arc welding method is economical and applicable, which can meet the needs of mass production.
3. The stator and rotor assembly process is reasonable and applicable, and fully meets the product quality requirements.
