Is it better to use a stepper motor or a servo motor for the motor in the CNC machine tool?

The servo mainly relies on pulses for positioning. Basically, it can be understood that when the servo motor receives a pulse, it will rotate the angle corresponding to the pulse to achieve displacement. Because the servo motor itself has the function of sending pulses, so Every time the servo motor rotates an angle, it will send a corresponding number of pulses. In this way, it forms an echo with the pulses received by the servo motor, or it is called a closed loop. In this way, the system will know how many pulses are sent to the servo motor, and at the same time receive. How many pulses come back, in this way, the rotation of the motor can be precisely controlled, so as to achieve precise positioning, which can reach 0.001mm.
Stepper motor is a discrete motion device, which has an essential connection with modern digital control technology. In the current domestic digital control system, stepper motors are widely used. With the emergence of all-digital AC servo systems, AC servo motors are increasingly used in digital control systems. In order to adapt to the development trend of digital control, stepper motors or all-digital AC servo motors are mostly used as executive motors in motion control systems. Although both are similar in control mode (pulse train and direction signal), there are big differences in performance and application occasions. Now compare the performance of the two.
1. Different control accuracy
The step angles of two-phase hybrid stepping motors are generally 3.6° and 1.8°, and the step angles of five-phase hybrid stepping motors are generally 0.72° and 0.36°. There are also some high-performance stepper motors with smaller step angles. For example, a stepper motor produced by Stone Company for slow wire machine tools has a step angle of 0.09°; the step angle of the three-phase hybrid stepper motor produced by BERGER LAHR can be passed The DIP switch is set to 1.8°, 0.9°, 0.72°, 0.36°, 0.18°, 0.09°, 0.072°, 0.036°, which is compatible with the step angle of two-phase and five-phase hybrid stepping motors.
The control accuracy of the AC servo motor is guaranteed by the rotary encoder at the rear end of the motor shaft. Taking the Panasonic full digital AC servo motor as an example, for a motor with a standard 2500-line encoder, the pulse equivalent is 360°/10000=0.036° due to the quadruple frequency technology inside the driver. For a motor with a 17-bit encoder, every time the driver receives 217=131072 pulses, the motor makes one revolution, that is, its pulse equivalent is 360°/131072=9.89 seconds. It is 1/655 of the pulse equivalent of a stepper motor with a step angle of 1.8°.
Second, different low-frequency characteristics
Stepper motors are prone to low-frequency vibrations at low speeds. The vibration frequency is related to the load condition and the performance of the driver. It is generally believed that the vibration frequency is half of the no-load take-off frequency of the motor. This low-frequency vibration phenomenon determined by the working principle of the stepping motor is very unfavorable to the normal operation of the machine. When the stepper motor works at low speed, damping technology should generally be usedTo overcome the low-frequency vibration phenomenon, such as adding a damper to the motor, or using subdivision technology on the driver, etc.
The AC servo motor runs very smoothly, and there will be no vibration even at low speeds. The AC servo system has a resonance suppression function, which can cover the lack of rigidity of the machine, and the system has a frequency analysis function (FFT) inside the system, which can detect the resonance point of the machine and facilitate system adjustment.
Third, different torque-frequency characteristics
The output torque of the stepper motor decreases with the increase of the speed, and it will drop sharply at a higher speed, so its maximum working speed is generally 300-600RPM. The AC servo motor has a constant torque output, that is, it can output a rated torque within its rated speed (generally 2000RPM or 3000RPM), and it is a constant power output above the rated speed.
Fourth, the overload capacity is different
stepper motors generally do not have overload capacity. AC servo motor has strong overload capacity. Take the Panasonic AC servo system as an example, it has speed overload and torque overload capabilities. Its maximum torque is three times of the rated torque, which can be used to overcome the moment of inertia of the inertial load at the moment of starting. Because the stepper motor does not have this kind of overload capacity, in order to overcome this moment of inertia when selecting a model, it is often necessary to select a motor with a larger torque, and the machine does not need such a large torque during normal operation, so the torque appears. The phenomenon of waste.
5. Different operating performance
The control of the stepping motor is an open-loop control. If the starting frequency is too high or the load is too large, it is easy to lose steps or stall, and when the speed is too high, it is easy to overshoot. , so in order to ensure its control accuracy, the problem of speed up and speed down should be handled well. The AC servo drive system is closed-loop control. The drive can directly sample the feedback signal of the motor encoder, and the internal position loop and speed loop are formed. Generally, there will be no step loss or overshoot of the stepping motor, and the control performance is more reliable.
Six, different speed response performance
It takes 200-400 milliseconds for a stepper motor to accelerate from a standstill to a working speed (generally several hundred revolutions per minute). AC servo system has better acceleration performance. Taking Panasonic MSMA 400W AC servo motor as an example, it only takes a few milliseconds to accelerate from static to its rated speed of 3000RPM, which can be used in control occasions requiring fast start and stop %%%%% In summary , AC servo systems are superior to stepper motors in many aspects. But in some less demanding occasions, stepper motors are often used as executive motors. Therefore, in the design process of the control system, various factors such as control requirements and cost should be considered comprehensively, and an appropriate control motor should be selected.