A stepper motor is a discrete motion device, fundamentally linked to modern digital control technology. It is widely used in current domestic digital control systems. 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 they are similar in control methods (pulse trains and direction signals), 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 3.6 ° and 1.8 °, while five-phase hybrid stepper motors typically have step angles of 0.72 ° and 0.36 °. Some high-performance stepper motors have even smaller step angles, such as a stepper motor for wire EDM machines produced by Beijing Hollysys Motor Technology Co., Ltd. (formerly Sitong Motor), with a step angle of 0.09 °. Three-phase hybrid stepper motors can have their step angles set via DIP switches to 0.9 °, 0.72 °, 0.36 °, 0.18 °, 0.09 °, 0.072 °, 0.036 °, etc., thus being compatible with the step angles of 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 a certain imported brand motor as an example, a motor with a standard 2500-line encoder has a pulse equivalent of 360°/10000 = 0.036 ° due to the use of quadruple frequency technology in the driver. For a motor with a 17-bit encoder, the motor rotates once for every 2^17 = 131072 pulses received by the driver, meaning its pulse equivalent is 360°/131072 = 9.89 seconds. This 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 Senchuang AC servo system as an example, it has speed overload and torque overload capabilities. Its maximum torque is three times the rated torque, which can be used to overcome the inertial torque of inertial loads at the moment of startup. Because stepper motors lack this overload capacity, when selecting a model to overcome this inertial torque, it is often necessary to choose a motor with a larger torque. However, the machine does not need 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. Generally, the missed steps or overshoot issues of stepper motors are not present, 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, a 400W AC servo motor from a certain brand can accelerate 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.
