Six key differences between servo motors and stepper motors:
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 also have even smaller step angles. For example, a stepper motor produced by Sitong Company for use in slow wire EDM machines has a step angle of 0.09 °; a three-phase hybrid stepper motor produced by the German company BERGERLAHR has a step angle that can be set to 1.8 °, 0.9 °, 0.72 °, 0.36° , 0.18 °, 0.09 °, 0.072 °, and 0.036 ° via a DIP switch, making it 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 a Panasonic all-digital AC servo motor as an example, for a motor with a standard 2500-line encoder, due to the use of quadruple frequency conversion technology within the driver, its pulse equivalent is 360°/10000 = 0.036 °. 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 dampers to the motor or using microstepping technology in the driver. AC servo motors operate very smoothly and do not exhibit vibration even at low speeds. AC servo systems have resonance suppression capabilities, which can cover insufficient mechanical rigidity, and the system has an internal frequency analysis function (FFT) that can 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. For example, Panasonic's AC servo system features 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 operation. 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 heavy load can easily lead to missed steps or stalling. Similarly, excessive stopping speed can cause overshoot. Therefore, to ensure control accuracy, the acceleration and deceleration issues must be carefully addressed. AC servo drive systems, on the other hand, use closed-loop control. The driver can directly sample the encoder feedback signal, internally forming position and speed loops. Generally, they do not exhibit 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 Panasonic MSMA400W 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 still frequently used as actuators in less demanding applications. Therefore, during the design of a control system, factors such as control requirements and cost must be comprehensively considered to select an appropriate control motor.
