Performance Comparison of Servo Motors and Stepper Motors
Stepper motors, as an open-loop control system, have a fundamental connection with modern digital control technology. In domestic digital control systems, stepper motors are widely used. With the emergence of full-digital AC servo systems, AC servo motors are increasingly applied in digital control systems. To adapt to the development trend of digital control, most motion control systems adopt stepper motors or full-digital AC servo motors as the driving motors. Although they have similar control methods (pulse strings and direction signals), there are significant differences in their performance and application scenarios. Now, let’s compare their performance.
1. Control Accuracy is Different
The step angle of two-phase hybrid stepper motors is generally 1.8° or 0.9°, while that of five-phase hybrid stepper motors is generally 0.72° or 0.36°. Some high-performance stepper motors have smaller step angles after subdivision. For example, the two-phase hybrid stepper motor produced by SANYO DENKI can have its step angle set to 1.8°, 0.9°, 0.72°, 0.36°, 0.18°, 0.09°, 0.072°, or 0.036° through a code switch, which is compatible with the step angles of both two-phase and five-phase hybrid stepper motors.
The control accuracy of AC servo motors is guaranteed by the rotary encoder at the rear end of the motor shaft. Taking the SANYO full-digital AC servo motor as an example, for motors with a standard 2000-line encoder, due to the internal four-fold frequency technology of the driver, the pulse equivalent is 360°/8000 = 0.045°. For motors with a 17-bit encoder, the driver receives 131072 pulses for one revolution of the motor, which means its pulse equivalent is 360°/131072 = 0.0027466°, which is 1/655 of the pulse equivalent of a stepper motor with a step angle of 1.8°.
2. Low-Frequency Characteristics are Different
Stepper motors are prone to low-frequency vibration at low speeds. The vibration frequency is related to the load condition and the performance of the driver. Generally, the vibration frequency is half of the motor’s no-load startup frequency. This low-frequency vibration phenomenon determined by the working principle of stepper motors is very unfavorable for the normal operation of the machine. When stepper motors operate at low speeds, damping technology should be adopted to overcome the low-frequency vibration phenomenon, such as adding a damper to the motor or using subdivision technology in the driver.
AC servo motors operate very smoothly and do not exhibit vibration phenomena even at low speeds. The AC servo system has a resonance suppression function, which can cover the insufficient rigidity of the machinery and has a frequency analysis function (FFT) within the system, which can detect the resonance points of the machinery, facilitating system adjustment.
3. Torque-Frequency Characteristics are Different
The output torque of stepper motors decreases as the speed increases and drops sharply at higher speeds, so their maximum operating speed is generally between 300 and 600 RPM. AC servo motors provide constant torque output, that is, within their rated speed (generally 2000 RPM or 3000 RPM), they can output the rated torque, and above the rated speed, it is constant power output.
4. Overload Capacity is Different
Stepper motors generally do not have overload capacity. AC servo motors have strong overload capacity. Taking 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 inertia torque of the inertia load during the start-up moment. Stepper motors do not have this overload capacity, so when selecting the motor, to overcome this inertia torque, a motor with a larger torque is often required, while the machine does not need such a large torque during normal operation, resulting in a phenomenon of torque waste.
5. Operating Performance is Different The control of stepper motors is of open-loop type. If the starting frequency is too high or the load is too heavy, stepping loss or stall may occur. When stopping, if the speed is too high, overshoot may happen. Therefore, to ensure the control accuracy, the problems of speed increase and decrease should be handled properly. The AC servo drive system is of closed-loop control. The driver can directly sample the feedback signal from the motor encoder, and form a position loop and a speed loop internally. Generally, stepping motor stepping loss or overshoot phenomenon will not occur. The control performance is more reliable.
Six. Different speed response performance
It takes 200 to 400 milliseconds for a stepper motor to accelerate from rest to the working speed (generally several hundred revolutions per minute). The acceleration performance of the AC servo system is better. Taking the Shanyang 400W AC servo motor as an example, it only takes a few milliseconds to accelerate from rest to its rated speed of 3000 RPM, which can be used in control scenarios requiring rapid start and stop.
In conclusion, the AC servo system outperforms the stepper motor in many aspects. However, in some less demanding scenarios, stepper motors are often used as the executive motor. Therefore, in the design process of the control system, various factors such as control requirements and cost should be comprehensively considered, and an appropriate control motor should be selected.