As an open-loop control system, stepper motor has essential relationship with modern digital control technology. In the present domestic digital control system, the stepper motor is widely used. With the appearance of full digital AC servo system, AC servo motor is more and more applied in digital control system. In order to adapt to the development trend of digital control, most of the motion control systems adopt stepper motor or full digital AC servo motor as the executive motor. Although they are similar in control mode (pulse train and directional signal), they are quite different in performance and application. The performance of the two is compared.
First, Different control accuracy
The stepping Angle of the two-phase hybrid stepping motor is generally 1.8° and 0.9°, and the stepping Angle of the five-phase hybrid stepping motor is generally 0.72 ° and 0.36°. There are also some high-performance stepper motors by subdividing the rear step Angle to be smaller. For example, the step Angle of the two-phase hybrid stepping motor produced by NEWKYE can be set to 1.8°, 0.9°, 0.72°, 0.36°, 0.18°, 0.09°, 0.072° and 0.036° by dial code switch, which is compatible with the step Angle of the two-phase and five-phase hybrid stepping motor.
The control precision of ac servo motor is guaranteed by the rotary encoder at the back end of the motor shaft. Taking NEWKYE full digital AC servo motor as an example, for the motor with standard 2500 line encoder, the pulse equivalent is 360°/8000=0.045° due to the use of quadruple frequency technology inside the driver. For a motor with a 17-bit encoder, the driver receives 131072 pulse motors for one turn, that is, its pulse equivalent is 360°/131072=0.0027466°, which is 1/655 of the pulse equivalent of a stepping motor with a step Angle of 1.8°.
Second, the characteristics of low frequency are different
At low speed, the stepper motor is prone to low-frequency vibration. The vibration frequency is related to the load condition and driver performance. It is generally considered that the vibration frequency is half of the no-load take-off frequency of the motor. The low-frequency vibration phenomenon determined by the working principle of stepper motor is very unfavorable to the normal operation of the machine. When the stepper motor works at low speed, damping technology should be generally used to overcome the phenomenon of low frequency vibration, such as adding a damper on the motor, or driver on the use of subdivision technology.
The AC servo motor runs very smoothly and does not vibrate even at low speeds. Ac servo system with resonance suppression function, can cover the lack of mechanical rigidity, and the system has a frequency analysis function (FFT), can detect the mechanical point of vibration, easy to adjust the system.
Third, the moment frequency characteristic is different
The output torque of stepper motor decreases with the increase of speed, and will drop sharply at a higher speed, so its maximum working speed is generally 300~600RPM. Ac servo motor is constant torque output, that is, it can output rated torque within its rated speed (generally 2000RPM or 3000RPM), and constant power output above the rated speed.
Fourth, overload capacity is different
Stepper motor generally does not have overload capacity. Ac servo motor has strong overload capacity. Taking Sanyo AC servo system as an example, it has the capability of speed overload and torque overload. The maximum torque is two to three times of the rated torque and can be used to overcome the inertial torque of the inertial load at the start. Because the stepping motor does not have such overload capacity, in order to overcome this inertia moment in the selection, it is often necessary to select the motor with a large torque, and the machine does not need such a large torque during normal operation, so the phenomenon of torque waste occurs.
Fifth , Different operation performance
The stepper motor is controlled by open-loop control. If the starting frequency is too high or the load is too large, it is easy to lose step or stall; if the speed is too high, it is easy to overshoot when stopping. Therefore, in order to ensure the control accuracy, the problem of speed rise and speed fall should be handled well. Ac servo drive system is closed-loop control. The driver can directly sample the feedback signals of the motor encoder. The inner part consists of position ring and speed ring.
Sixth, Different speed response performance
It takes 200~400 milliseconds for a stepper motor to accelerate from rest to working speed (generally hundreds of revolutions per minute). The acceleration performance of AC servo system is good. Taking NEWKYE 400W AC servo motor as an example, it only takes a few milliseconds to accelerate from rest to its rated speed of 3000RPM, which can be used in control occasions requiring quick start and stop.
To sum up, AC servo system is superior to stepper motor in many performance aspects. However, stepper motor is often used to perform the motor in some less demanding occasions. Therefore, in the design process of the control system to consider the control requirements, cost and other factors, choose the appropriate control motor.
Post time: Dec-02-2020