In current domestic digital control systems, stepper motors are widely used. With the emergence of fully digital AC servo systems, AC servo motors are also increasingly 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 the two are similar in control methods (pulse train and direction signal), they differ significantly in performance and application scenarios. A comparison of their performance is presented below.
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, the two-phase hybrid stepper motors manufactured by SANYODENKI can have their step angles set to 1.8°, 0.9°, 0.72°, 0.36°, 0.18°, 0.09°, 0.072°, and 0.036° via DIP switches, 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 the KINGSERVO all-digital AC servo motor as an example, for a motor with a standard 2500-line encoder, due to the quadruple frequency technology used 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.
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 analysis (FFT) capabilities to detect mechanical resonance points, facilitating system adjustments.
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.
IV. Different Overload Capacities
Stepper motors generally lack overload capacity. AC servo motors, on the other hand, have strong overload capacity. Taking the KINGSERVO AC servo system as an example, it possesses both speed and torque overload capabilities. Its maximum torque is 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 missed steps or stalling. 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. This eliminates the missed steps or overshoot issues common in stepper motors, 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 KINGSERVO 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.
