The commonly used servo motor is an AC servo motor, and an optical encoder is mounted on the shaft end of the motor, and the angle of the rotor is used for frequency conversion control. From the lowest speed to the highest speed, the servo motor can run smoothly and the torque ripple is small. The servo motor has a long overload capacity, a small moment of inertia and a large stall torque. The servo motor has a very small starting frequency and can quickly accelerate from the lowest speed to the rated speed.
The AC servo motor is used as the driving device, which can be the same as a DC servo motor to form a high-precision, high-performance semi-closed-loop or closed-loop control system. Because the AC servo motor has a brushless structure, it needs little maintenance, and its volume is relatively small, which is beneficial to the increase of speed and power. At present, DC servo motors have been replaced in a wide range. After the emergence of a fully digital AC servo system using high-speed microprocessors and dedicated digital signal processors (DSPs), the original hardware servo control becomes software servo control, and some advanced algorithms in modern control theory have been implemented, which has greatly improved Because of the performance of the servo system, the servo unit can greatly improve machining efficiency and machining accuracy, but the price of the servo drive unit is also high. With the gradual improvement of the servo control technology, the servo drive unit is gradually becoming the main force of the drive unit, and the price of the servo drive unit is gradually reduced.
There are two servo drives. An impulse control method is adopted. Such a driver and the motor are closed-loop, but they are not fed back to the numerical control system. Such a driver can be called open-loop servo control to some extent. Another kind adopts the voltage control way, carries on the electrical machinery rotational speed control through the high and low voltage, the electrical machinery feedback signal passes the driver feedback to the numerical control system to carry on the position control.
The drive unit includes the drive device and the motor. The purchase of the drive unit is mainly based on the selection of the drive device. Because the motor is a common component, the performance difference exists only in different manufacturers and models.
Drive motors can be divided into three major categories: reactive stepper drive motors, hybrid (also known as permanent magnet reaction) stepper drive motors and servo drive motors.
Reactive stepper drive motor rotor without windings, by the excitation of the stator windings generate reaction torque to achieve step-by-step operation. The permanent magnet for the rotor of the hybrid stepper motor is stepped by the electromagnetic torque generated by excitation and permanent magnets. The stepper motor is controlled by pulses. The direction of rotation of the motor can be changed by changing the order of energization. Changing the frequency of the pulse can change the rotation speed of the motor. The stepper motor has a certain step accuracy and there is no cumulative error. However, the efficiency of the stepping motor is low, the ability to drag the load is not large, the pulse equivalent can not be too large, and the speed range is not large. At present, the stepping motor can be divided into two phases, three phases, and five equal ones, and a five-phase stepping motor is commonly used. For a long time in the past, stepper motors accounted for a large market, but are currently being replaced by servo motors.
When selecting the drive unit, the proportion of the drive unit's price in the entire CNC machine tool should also be considered. The lower price of the entire CNC machine tool generally chooses the stepping drive unit, while the higher-priced machine tool selects the servo drive unit. However, when selecting the drive unit, the matching between the drive unit and the numerical control system must also be considered. When selecting the closed-loop control system, the closed-loop servo drive unit must be selected. AC servo systems are superior to stepper motors in many performance aspects. However, stepper motors are often used to perform the motor in some less demanding applications. Therefore, in the design process of the control system, various factors such as control requirements and costs should be comprehensively considered, and appropriate control motors should be selected.
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