Stepper motors are widely used in open-loop control systems and have a strong connection with modern digital control technology. In today's domestic digital control systems, they are applied extensively. With the rise of all-digital AC servo systems, AC servo motors are becoming more popular in such applications. To keep up with the development of digital control, stepping motors or all-digital AC servo motors are commonly used as the actuating components in motion control systems. Although both share similar control methods (pulse and direction signals), there are significant differences in performance and application scenarios. Let’s compare their key characteristics.
First, control precision varies significantly. A two-phase hybrid stepper motor typically has a step angle of 1.8° or 0.9°, while a five-phase one may offer 0.72° or 0.36°. Some high-performance steppers can achieve even smaller step angles through subdivision techniques. For example, SANYODENKI’s two-phase hybrid motor can be set to 1.8°, 0.9°, 0.72°, 0.36°, 0.18°, 0.09°, 0.072°, or 0.036° via a DIP switch, making it compatible with both two-phase and five-phase configurations.
AC servo motors, on the other hand, rely on encoders at the motor shaft for precise control. For instance, an AC servo motor from Sanyo with a 2000-line encoder achieves a pulse equivalent of 0.045° due to quadrature frequency technology. A 17-bit encoder provides a pulse equivalent of just 0.0027466°, which is about 1/655th of a standard 1.8° stepper motor’s step. This level of accuracy makes AC servos ideal for high-precision applications.
Second, low-speed performance differs. Stepper motors tend to vibrate at low speeds, a phenomenon caused by their operating principle. This vibration can be problematic for machine operation, especially under varying loads. To reduce this, damping mechanisms or subdivision drives are often used. In contrast, AC servo motors operate smoothly even at low speeds, thanks to built-in resonance suppression and frequency analysis (FFT) features that help detect and adjust for system resonances.
Third, torque behavior varies. Stepper motors experience a drop in output torque as speed increases, limiting their maximum operational speed to around 300–600 RPM. AC servo motors, however, provide constant torque up to their rated speed (often 2000–3000 RPM), and then transition to constant power mode beyond that. This makes them more suitable for high-speed applications.
Fourth, overload capability is another key difference. Stepper motors generally lack overload capacity, whereas AC servo motors can handle higher torque peaks—up to two to three times the rated torque. This helps in overcoming inertial loads during startup, avoiding the need for oversized motors that waste energy during normal operation.
Fifth, control reliability is better in AC servo systems. Stepper motors use open-loop control, which can lead to missed steps or over-shooting if the load is too heavy or the speed is too high. AC servos, being closed-loop systems, use feedback from the motor’s encoder to ensure accurate positioning and prevent errors, resulting in more reliable performance.
Sixth, response time is faster with AC servos. While a stepper motor might take 200–400 milliseconds to accelerate from rest to its working speed, an AC servo motor like the Shanyang 400W model can reach its rated speed of 3000 RPM in just a few milliseconds. This makes them ideal for applications requiring quick start and stop cycles.
In summary, AC servo systems outperform stepper motors in many areas, including precision, stability, and responsiveness. However, in less demanding applications, stepper motors are still widely used due to their lower cost and simplicity. When designing a control system, it’s essential to evaluate the specific requirements, budget, and performance needs to choose the most suitable motor type.
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