Performance comparison between servo motor and stepper motor
As an open-loop control system, the stepper motor has an essential connection with modern digital control technology. In domestic digital control systems, stepper motors are widely used. With the emergence of all-digital AC servo systems, AC servo motors are increasingly used in digital control systems.
As an open-loop control system, the stepper motor has an essential connection with modern digital control technology. In domestic digital control systems, 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 precision
The step angles of two-phase hybrid stepper motors are generally 1.8° and 0.9°, and the step angles of five-phase hybrid stepper motors are generally 0.72° and 0.36°. There are also some high-performance stepper motors with smaller step angles after subdivision. For example, the step angle of the two-phase hybrid stepping motor produced by Sanyo Company (SANYO DENKI) can be set to 1.8°, 0.9°, 0.72°, 0.36°, 0.18°, 0.09°, 0.072°, 0.036° through the dial switch , Compatible with the step angle of two-phase and five-phase hybrid stepper motors.
The control accuracy of the AC servo motor is guaranteed by the rotary encoder at the rear end of the motor shaft. Take Sanyo full-digital AC servo motor as an example. For a motor with a standard 2000-line encoder, the pulse equivalent is 360°/8000=0.045° due to the quadruple frequency technology used inside the driver. For a motor with a 17-bit encoder, every time the driver receives 131072 pulses, the motor makes one revolution, that is, its pulse equivalent is 360°/131072=0.0027466°, which is the pulse equivalent of a stepper motor with a step angle of 1.8° 1/655.
Second, the low frequency characteristics are different
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 used to 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 does not vibrate 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.
3. The moment-frequency characteristics are different
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.
4. Different overload capacity
Stepper motors generally do not have overload capability. AC servo motor has strong overload capacity. Take the Sanyo AC servo system as an example, it has speed overload and torque overload capabilities. Its maximum torque is two to three times 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.
stepper motor,open loop control