A stepper motor is a device for discrete motion, inherently linked to modern digital control technology. At present, stepper motors are widely applied in domestic digital control systems. With the emergence of fully digital AC servo systems, AC servo motors are also increasingly adopted within digital control systems. To keep up with the development trend of digital control, most motion control systems employ either stepper motors or fully digital AC servo motors as actuators. Although both share similar control schemes (pulse trains and direction signals), they differ significantly in operational performance and applicable scenarios. A comparison of their performance characteristics is presented below.

Difference 1: Control Accuracy

Two-phase hybrid stepper motors generally have a step angle of 3.6° or 1.8°, while five-phase hybrid stepper motors normally feature a step angle of 0.72° or 0.36°. Certain high-performance stepper motors achieve even smaller step angles. For instance, a stepper motor manufactured by Sitong Company for wire electrical discharge machines boasts a step angle of 0.09°. The three-phase hybrid stepper motor produced by BERGER LAHR (Germany) allows step angle selection via DIP switches: 1.8°, 0.9°, 0.72°, 0.36°, 0.18°, 0.09°, 0.072°, and 0.036°, covering all step angles of two-phase and five-phase hybrid stepper motors.

The control accuracy of an AC servo motor is guaranteed by the rotary encoder mounted at the rear of the motor shaft. Taking Panasonic fully digital AC servo motors as an example: motors equipped with a standard 2500-line encoder adopt quadruple frequency technology inside the driver, yielding a pulse equivalent of 360°/10000 = 0.036°. For motors fitted with a 17-bit encoder, the motor completes one full rotation after the driver receives 2¹⁷ = 131,072 pulses. Its pulse equivalent equals 360°/131072 = 9.89 arcseconds, which is 1/655 of the pulse equivalent of a 1.8° stepper motor.

Difference 2: Low-Frequency Characteristics

Stepper motors are prone to low-frequency vibration at low speeds. The vibration frequency depends on load conditions and driver performance, and is generally regarded as half the unloaded pull-in frequency of the motor. This low-frequency vibration, determined by the operating principle of stepper motors, severely hinders stable machine operation. When stepper motors run at low speeds, damping techniques are usually adopted to suppress low-frequency vibration, such as installing dampers on motors or enabling microstepping technology in drivers.

AC servo motors operate extremely smoothly with no vibration even at low rotational speeds. AC servo systems feature resonance suppression functions to compensate for insufficient mechanical rigidity. Furthermore, the built-in frequency analysis function (FFT) can detect mechanical resonance points to facilitate system tuning.

Difference 3: Torque-Speed Characteristics

The output torque of a stepper motor declines as rotational speed rises and drops sharply at relatively high speeds. Therefore, its maximum operating speed generally ranges from 300 to 600 RPM. AC servo motors deliver constant torque output; within their rated speed (typically 2000 RPM or 3000 RPM), they can continuously output rated torque, and operate under constant power output above rated speed.

Difference 4: Overload Capacity

Stepper motors generally have no overload capability. AC servo motors possess strong overload capacity. For example, Panasonic AC servo systems support speed overload and torque overload. Their maximum torque reaches three times the rated torque, which can overcome inertial torque from inertial loads at startup. Since stepper motors lack such overload capability, designers often select motors with excessive torque to handle startup inertial torque during model selection. However, the machine does not require such high torque during regular operation, resulting in torque waste.

Difference 5: Operating Performance

Stepper motors adopt open-loop control. Excessively high startup frequency or heavy loads easily cause step loss or stalling; excessive speed during stopping may trigger overshoot. Accordingly, proper acceleration and deceleration ramps must be implemented to guarantee control accuracy.

AC servo drive systems utilize closed-loop control. The driver directly samples feedback signals from the motor encoder, forming an internal position loop and speed loop. Step loss and overshoot commonly seen with stepper motors rarely occur, delivering more reliable control performance.

Difference 6: Speed Response Performance

It takes 200–400 ms for a stepper motor to accelerate from standstill to operating speed (typically several hundred revolutions per minute). AC servo systems offer superior acceleration performance. As demonstrated by the Panasonic MSMA 400W AC servo motor, acceleration from standstill to its rated speed of 3000 RPM only requires several milliseconds, making it suitable for control applications demanding rapid start and stop.

In summary, AC servo systems outperform stepper motors in many performance aspects. Nevertheless, stepper motors are frequently used as actuators for low-demand applications. Hence, comprehensive factors including control requirements and costs should be evaluated during control system design to select the appropriate drive motor.

A stepper motor is an actuator that converts electrical pulses into angular displacement. Simply put: every time the stepper driver receives a pulse signal, it drives the stepper motor to rotate by a fixed angle (the step angle) in the designated direction.

You can control angular displacement by regulating the number of pulses to achieve precise positioning. Meanwhile, motor rotational speed and acceleration can be adjusted by controlling pulse frequency to realize speed regulation.

Stepper motors fall into three categories: Permanent Magnet (PM), Variable Reluctance (VR), and Hybrid (HB).

• Permanent Magnet (PM) stepper motors are mostly two-phase types with relatively low torque and compact dimensions. Typical step angles are 7.5° or 15°.

• Variable Reluctance (VR) stepper motors are generally three-phase designs capable of high torque output, with a standard step angle of 1.5°, yet accompanied by significant noise and vibration. This type was phased out in developed countries such as Europe and the United States back in the 1980s.

• Hybrid (HB) stepper motors combine the merits of PM and VR types. They are further divided into two-phase and five-phase versions: two-phase models normally have a step angle of 1.8°, while five-phase models feature 0.72°. Hybrid stepper motors are the most widely adopted type.