The difference between stepper motor and servo motor

Stepper motor and AC servo motor performance comparison
Stepper motors are discrete motion devices that have an essential connection to 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 is different
The step angle of the two-phase hybrid stepping motor is generally 3.6° and 1.8°. The step angle of the five-phase hybrid stepping motor is generally 0.72° and 0.36°. There are also some high performance stepper motors with smaller step angles. For example, a step motor manufactured by Stone Company for use in a slow-moving wire machine has a step angle of 0.09°. The step angle of the three-phase hybrid stepper motor produced by BERGER LAHR in Germany can be passed. DIP switches are set to 1.8°, 0.9°, 0.72°, 0.36°, 0.18°, 0.09°, 0.072°, 0.036° and are compatible with step angles of two-phase and five-phase hybrid stepper motors.
AC servo motor control accuracy is guaranteed by a rotary encoder at the rear end of the motor shaft. Taking Panasonic's all-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, the driver receives one revolution of 217=131,072 pulses per pulse, ie, its pulse equivalent is 360°/131072=9.89 seconds. It is 1/655 of the pulse equivalent of a stepping motor with a step angle of 1.8°.
Second, different low-frequency characteristics
Stepper motors are prone to low frequency vibration at low speeds. 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 analysis function (FFT), which can detect the mechanical resonance point and facilitate system adjustment.
Third, different characteristics of the moment frequency
The output torque of the stepping motor decreases with the increase of the rotational speed, and drops sharply at higher rotational speeds, so the maximum operating rotational speed is generally 300-600 RPM. The AC servo motor is constant torque output, that is, it can output rated torque within its rated speed (generally 2000RPM or 3000RPM), and it is constant power output above the rated speed.
Fourth, different overload capacity
Stepper motors generally do not have overload capabilities. AC servo motor has strong overload capability. Take Panasonic AC servo system as an example, it has speed overload and torque overload capability. Its maximum torque is 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 relatively large torque, and when the machine does not require such large torque during normal operation, a torque occurs. Wasted phenomenon.
Fifth, different operating performance
The control of stepping motor is open-loop control. If the starting frequency is too high or the load is too large, it is easy to lose the steps or stall. When the speed is too high, the overshoot may occur. Therefore, in order to ensure the control accuracy, it should be handled well. Increase or decrease speed. 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.
Sixth, different speed response performance
It takes 200-400 milliseconds for a stepper motor to accelerate from rest to operating speed (typically several hundred revolutions per minute). Acceleration performance of AC servo system is better. Taking Panasonic MSMA 400W AC servo motor as an example, it only takes a few milliseconds to accelerate from standstill to its rated speed of 3000RPM. It can be used in control applications requiring fast start-stop.
In summary, AC servo systems are superior to stepper motors in many aspects of performance. 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.