Stepper motors, as an open-loop control system, are fundamentally linked to modern digital control technology. They are widely used in current domestic digital control systems. With the emergence of fully digital AC servo systems, AC servo motors are also increasingly being applied in digital control systems. To adapt to the development trend of digital control, most motion control systems use stepper motors or fully digital AC servo motors as actuators. Although they are similar in control methods (pulse and direction signals), they differ significantly in performance and application scenarios. A comparison of their performance is presented below.
• Basic Structure
I. Different control precision
Two-phase hybrid stepper motors typically have step angles of 1.8° or 0.9°, while five-phase hybrid stepper motors typically have step angles of 0.72° or 0.36°. Some high-performance stepper motors can have even smaller step angles after microstepping. For example, MOONS's two-phase hybrid stepper motors, paired with its SR series stepper drivers, offer 16 microstepping settings via DIP switches: 1.8°, 0.9°, 0.45°, 0.36°, 0.225°, 0.18°, 0.1125°, 0.09°, 0.072°, 0.05625°, 0.045°, 0.036°, 0.028125°, 0.018°, 0.0144°, and 0.014°, making them compatible with both two-phase and five-phase hybrid stepper motors.
The control precision of an AC servo motor is ensured by a rotary encoder at the rear end of the motor shaft. Taking an M2 AC servo motor as an example, for a motor with a 2500-line incremental encoder, due to the use of quadruple frequency technology in the driver, its pulse equivalent is 360°/10000 = 0.036°. For a motor with a 17-bit encoder, the motor rotates once for every 131072 pulses received by the driver, meaning 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°.
II. Different Low-Frequency Characteristics
Stepper motors are prone to low-frequency vibration at low speeds. The vibration frequency is related to the load and driver performance, and is generally considered to be half of the motor's no-load starting frequency. This low-frequency vibration, determined by the working principle of stepper motors, is very detrimental to the normal operation of the machine. When stepper motors operate at low speeds, damping techniques should generally be used to overcome low-frequency vibration, such as adding a damper to the motor or using microstepping technology in the driver.
Anti-resonance
One drawback of stepper systems is the inherent resonance point. The SR series stepper driver automatically calculates the resonance point and adjusts the control algorithm accordingly to suppress resonance, greatly improving mid-frequency stability and enabling greater torque output and superior high-speed performance.
AC servo motors operate very smoothly, without vibration even at low speeds. AC servo systems feature resonance suppression to compensate for insufficient mechanical rigidity, and internal frequency response testing (FFT) can detect mechanical resonance points, facilitating system adjustments.
Vibration Suppression
The vibration suppression function of the M2 servo system includes two parts: resonance suppression and damping vibration reduction.
• Resonance suppression function
Two sets of notch filters are provided to effectively overcome the resonance problem caused by the inherent characteristics of the equipment's mechanical structure.
• Damping and vibration reduction function
The damping characteristics of the entire motion system can be improved by adjusting the damping coefficient provided by the controller, thereby effectively reducing the vibration of the system.
III. Different Moment-Frequency Characteristics
The output torque of a stepper motor decreases as the speed increases, and drops sharply at higher speeds. Therefore, its maximum operating speed is generally between 300 and 600 RPM. AC servo motors provide constant torque output, meaning they can output rated torque up to their rated speed (generally 2000 or 3000 RPM), and provide constant power output above the rated speed.
• Comparison of speed and torque characteristics between servo motors and stepper motors of the same size
IV. Different Overload Capacities
Stepper motors generally lack overload capacity. AC servo motors, on the other hand, have strong overload capacity. Taking the M2 AC servo system as an example, it has both speed and torque overload capabilities. Its maximum torque is two to three times its rated torque, which can be used to overcome the inertial torque of inertial loads at startup. Because stepper motors lack this overload capacity, a motor with a larger torque is often selected to overcome this inertial torque during selection. However, the machine does not require such a large torque during normal operation, resulting in wasted torque.
V. Different operating performance
Stepper motors are controlled in an open-loop manner. Excessive starting frequency or load can easily lead to step loss or stalling, while excessive stopping speed can cause overshoot. Therefore, to ensure control accuracy, the acceleration and deceleration issues must be properly addressed. AC servo drive systems, on the other hand, use closed-loop control. The driver can directly sample the feedback signal from the motor encoder, internally forming position and speed loops. Generally, stepper motors do not experience step loss or overshoot, resulting in more reliable control performance.
VI. Different speed response performance
Stepper motors require 200–400 milliseconds to accelerate from a standstill to their operating speed (typically several hundred revolutions per minute). AC servo systems offer better acceleration performance. For example, the Mingzhi 400W AC servo motor accelerates from a standstill to its rated speed of 3000 RPM in just a few milliseconds, making it suitable for control applications requiring rapid start and stop.
In summary, AC servo systems outperform stepper motors in many aspects. However, stepper motors are often used as actuators in less demanding applications. Therefore, the design of a control system must comprehensively consider factors such as control requirements and cost to select an appropriate control motor.
