First, the control accuracy is different
The step angle of the two-phase hybrid stepper motor is generally 3.6 ° and 1.8 °, and the step angle of the five-phase hybrid stepper motor is 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 Sitong company for slow wire machine tool, its step angle is 0.09 °; The step angle of the three-phase hybrid stepper motor produced by the Germany company BERGER LAHR can be set to 1.8°, 0.9°, 0.72°, 0.36°, 0.18°, 0.09°, 0.072°, 0.036° by dip switch, which is 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 a rotary encoder at the rear end of the motor shaft. In the case of Panasonic's all-digital AC servo motor, for a motor with a standard 2500-wire encoder, the pulse equivalent is 360°/10000=0.036° due to the quadruple technology used inside the driver. For a motor with a 17-bit encoder, the driver receives 217 = 131072 pulses per motor revolution, i.e. 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, the low-frequency characteristics are different
Stepper motors are prone to low-frequency vibration at low speeds. The vibration frequency is related to the load situation and the performance of the drive, and it is generally considered that the vibration frequency is half of the no-load take-off frequency of the motor. This low-frequency vibration phenomenon, which is determined by the working principle of the stepper motor, is very detrimental 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.
The AC servo motor runs very smoothly and does not vibrate even at low speeds. The AC servo system has a resonance suppression function to 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, the moment frequency characteristics are different
The output torque of the stepper motor decreases with the increase of speed, and will drop sharply at higher speed, so its maximum working speed is generally 300-600RPM. The AC servo motor is a constant torque output, that is, within its rated speed (generally 2000RPM or 3000RPM), it can output the rated torque, and it is a constant power output above the rated speed.
Fourth, the overload capacity is different
Stepper motors generally do not have an overload capacity. The AC servo motor has a strong overload capacity. Take the Panasonic AC servo system as an example, it has speed overload and torque overload capabilities. It has a maximum torque of three times the rated torque and can be used to overcome the moment of inertia of an inertial load at the moment of start-up. Because the stepper motor does not have this overload capacity, in order to overcome this inertia torque during the selection, 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 there is a phenomenon of torque waste.
Fifth, the operation performance is different
The control of the stepper motor is open-loop control, the starting frequency is too high or the load is too large, it is easy to lose the step or stall the phenomenon, and the speed is too high when stopping, and it is easy to overshoot, so in order to ensure its control accuracy, the problem of rising and decreasing speed should be dealt with. The AC servo drive system is closed-loop control, the driver can directly sample the feedback signal of the motor encoder, and the internal position ring and speed loop are formed, and there will generally be no loss of step or overshoot of the stepper motor, and the control performance is more reliable.
Sixth, the speed response performance is different
It takes 200-400 milliseconds for a stepper motor to accelerate from a standstill to a working speed (typically a few hundred revolutions per minute). Taking Panasonic MSMA 400W AC servo motor as an example, it only takes a few milliseconds to accelerate from a standstill to its rated speed of 3000RPM, which can be used in control occasions that require fast start and stop.
To sum up, the AC servo system is superior to stepper motors in many performance aspects. However, in some undemanding occasions, stepper motors are often used as actuator motors. Therefore, in the design process of the control system, it is necessary to comprehensively consider the control requirements, cost and other factors, and select the appropriate control motor.
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