Servo motors are primarily controlled using pulse signals. In simple terms, when a servo motor receives one pulse, it rotates by an angle corresponding to that pulse, enabling precise displacement. Since the servo motor itself can generate pulses, each time it rotates to a specific angle, it emits a corresponding number of pulses. This creates a feedback loop, allowing the system to know exactly how many pulses were sent and received, which ensures accurate control and positioning—capable of reaching up to 0.001 mm precision. Stepper motors, on the other hand, are discrete motion devices closely tied to modern digital control systems. They are widely used in domestic digital systems. With the rise of all-digital AC servo systems, AC servo motors have become more common in such applications. To adapt to digital trends, either stepping motors or fully digital AC servo motors are typically used as execution components in motion control systems. While both use similar control methods (pulse and direction signals), they differ significantly in performance and application. Let’s compare them in detail. First, control precision varies. A two-phase hybrid stepper motor usually has a step angle of 3.6° or 1.8°, while a five-phase hybrid model typically has a step angle of 0.72° or 0.36°. Some high-performance steppers, like those used in slow wire-cutting machines from Sitong, can achieve a step angle of 0.09°. For example, a three-phase hybrid stepper from BERGERLAHR can be set via code switches to 1.8°, 0.9°, 0.72°, 0.36°, 0.18°, 0.09°, 0.072°, or 0.036°, making it compatible with both two-phase and five-phase models. AC servo motors rely on encoders at the motor shaft to ensure accuracy. For instance, a Panasonic all-digital AC servo motor with a standard 2500-line encoder achieves a pulse equivalent of 0.036° due to quadrature frequency technology. A 17-bit encoder provides even finer resolution, with a pulse equivalent of approximately 0.0027°, which is about 1/655th of a 1.8° step motor. This level of precision makes AC servos ideal for high-accuracy applications. Second, low-speed performance differs. Stepper motors tend to vibrate at low speeds, especially under load. This vibration can be problematic and often requires damping or subdivision techniques to mitigate. In contrast, AC servo motors operate smoothly even at low speeds. Many AC servo systems include resonance suppression features and FFT analysis to detect and adjust for machine resonances, improving overall stability. Third, torque characteristics vary. 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, maintain constant torque within their rated speed range (often 2000–3000 RPM) and switch to constant power mode beyond that. Fourth, overload capacity is another key difference. Stepper motors generally lack overload capability, while AC servos can handle significant torque surges. For example, a Panasonic AC servo system can deliver up to three times its rated torque, helping overcome inertia during startup. This means that in some cases, a larger motor may be selected for a stepper system, leading to unnecessary energy consumption during normal operation. Fifth, control performance differs. Stepper motors use open-loop control, which can result in missed steps or overshoots if not properly managed. AC servos use closed-loop control, where the driver samples encoder feedback directly, forming internal position and speed loops. This results in more reliable and precise control. Lastly, speed response is a major factor. Stepper motors take 200–400 milliseconds to accelerate from rest to operating speed, while AC servo systems can reach full speed in just a few milliseconds. This makes AC servos more suitable for applications requiring rapid start-stop cycles. In summary, AC servo systems outperform stepper motors in most aspects, including precision, stability, and response time. However, in less demanding applications, steppers are still commonly used due to their lower cost and simplicity. When designing a control system, it's essential to consider factors like performance requirements, budget, and application needs to choose the best motor type.
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