What are the differences between the performance of servo motors and stepper motors?
As an open-loop control system, stepper motors have an essential connection with modern digital control technology. In the current domestic digital control system, stepper motors are widely used. With the advent of all-digital AC servo systems, AC servo motors are also increasingly used in digital control systems. In order to adapt to the development trend of digital control, most of the motion control systems use stepper motors or all-digital AC servo motors as the execution motors. Although the two are similar in control mode (burst and direction signals), there is a big difference in performance and application. Now compare the performance of the two.
First, the control accuracy of two-phase hybrid stepper motor step angle is generally 1.8 °, 0.9 °, five-phase hybrid stepping motor step angle is generally 0.72 °, 0.36 °. There are also some high performance stepper motors with smaller step angles after subdivision. For example, a two-phase hybrid stepper motor manufactured by SANYO DENKI can be set to 1.8°, 0.9°, 0.72°, 0.36°, 0.18°, 0.09°, 0.072°, and 0.036° by the dial switch. It is compatible with the step angle of two-phase and five-phase hybrid stepping motors.
AC servo motor control accuracy is guaranteed by a rotary encoder at the rear end of the motor shaft. Taking Shanyang's all-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 inside the driver. For a motor with a 17-bit encoder, the driver receives one revolution of 131,072 impulse motors per 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 of different stepper motors at low speeds are prone to low-frequency vibration phenomenon. The vibration frequency is related to load conditions and driver performance. It is generally believed that the vibration frequency is half of the motor's no-load take-off frequency. This kind of low frequency vibration, which is determined by the working principle of the stepper motor, is very unfavorable to the normal operation of the machine. When the stepper motor operates at a low speed, damping techniques are generally used to overcome the low-frequency vibration phenomena, such as adding a damper to the motor, or using a subdivision technique on the driver.
The AC servo motor operates very smoothly and does not vibrate even at low speeds. The AC servo system has a resonance suppression function that can cover the lack of mechanical rigidity, and the system has a frequency resolution function (FFT), which can detect the mechanical resonance point and facilitate system adjustment.
Third, the torque characteristics of different stepper motor output torque decreases with the increase of speed, and at high speeds will be sharply reduced, so the maximum operating speed is generally 300 ~ 600RPM. AC servo motor is a constant torque output, that is, it can output rated torque within its rated speed (generally 2000RPM or 3000RPM), and it is a constant power output above the rated speed.
Fourth, overload capacity Different stepper motors generally do not have overload capability. AC servo motor has strong overload capability. Take Shanyang AC servo system as an example, it has speed overload and torque overload capability. Its maximum torque is two to three times the rated torque and can be used to overcome the moment of inertia of the inertial load at the moment of starting. Because there is no such overload ability for the stepping motor, in order to overcome this kind of moment of inertia during selection, it is often necessary to select a motor with a large torque, and when the machine does not require such a large torque during normal operation, a torque occurs. Wasted phenomenon.
Fifth, the performance of different stepper motor control for the open-loop control, the starting frequency is too high or the load is too large prone to lost or locked-rotor phenomenon, stop overspeed is too high overshoot phenomenon, so to ensure its control Accuracy should deal with problems of rising and falling speeds. The AC servo drive system is closed-loop control, and the drive can directly sample the feedback signal of the motor encoder. The position loop and the speed loop are internally formed. Generally, there is no phenomenon of lost or overshoot of the stepping motor, and the control performance is more reliable.
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