Let me talk about the working principle first: A stepper motor is an actuator that converts electrical pulses into angular displacement.
When the stepper driver receives a pulse signal, it drives the stepper motor to rotate a fixed angle (called "step angle") in the set direction, and its rotation runs step by step at a fixed angle. The angular displacement can be controlled by controlling the number of pulses, so as to achieve the purpose of accurate positioning; at the same time, the speed and acceleration of the motor rotation can be controlled by controlling the pulse frequency, so as to achieve the purpose of speed regulation.
The rotor inside the servo motor is a permanent magnet. The U/V/W three-phase electricity controlled by the driver forms an electromagnetic field. The rotor rotates under the action of this magnetic field. Compared with the target value, adjust the angle of rotation of the rotor. The accuracy of the servo motor depends on the accuracy (number of lines) of the encoder.
Difference 1: Different control methods The stepper motor controls the rotation angle by controlling the number of pulses, and one pulse corresponds to one step angle. The servo motor controls the rotation angle by controlling the length of the pulse time.
Difference 2: The required working equipment and working process are different. The power supply required by the stepper motor (the required voltage is given by the driver parameters), a pulse generator (mostly use a plate now), a stepper motor, and a driver (The driver sets the step angle angle, such as setting the step angle to 0.45°, at this time, give a pulse, and the motor moves 0.45°); the working process generally requires two pulses for the stepping motor: signal pulse and direction pulse.
The power supply required by the servo motor is a switch (relay switch or relay board), a servo motor; its working process is a power connection switch, and then connect the servo motor.
Difference 3: Different low-frequency characteristics Stepping motors are prone to low-frequency vibrations at low speeds. The vibration frequency is related to the load condition and the performance of the driver. It is generally believed that the vibration frequency is half of the no-load take-off frequency of the motor. This low-frequency vibration phenomenon determined by the working principle of the stepping motor is very unfavorable to the normal operation of the machine.
When the stepper motor works at low speed, damping technology should generally be used to overcome the low-frequency vibration phenomenon, such as adding a damper to the motor, or using subdivision technology on the driver, etc.
The AC servo motor runs very smoothly and does not vibrate even at low speeds. The AC servo system has a resonance suppression function, which can cover the lack of rigidity of the machine, and the system has a frequency analysis function (FFT) inside the system, which can detect the resonance point of the machine and facilitate system adjustment.
Difference 4: Different torque-frequency characteristics The output torque of the stepper motor decreases with the increase of the speed, and it will drop sharply at higher speeds, so the maximum working speed is generally 300-600r/min. The AC servo motor has a constant torque output, that is, it can output a rated torque within its rated speed (generally 2000 or 3000 r/min), and it is a constant power output above the rated speed.
Difference 5: Different overload capacity Stepper motors generally do not have overload capacity. AC servo motor has strong overload capacity. Take the Panasonic AC servo system as an example, it has speed overload and torque overload capabilities. Its maximum torque is 3 times of the rated torque, which can be used to overcome the moment of inertia of the inertial load at the moment of starting. (Because the stepper motor does not have this kind of overload capacity, in order to overcome this moment of inertia when selecting a model, it is often necessary to select a motor with a larger torque, and the machine does not need such a large torque during normal operation, so it appears phenomenon of wasted torque)
Difference 6: Different speed response performance It takes 200-400ms for a stepping motor to accelerate from a standstill to a working speed (generally several hundred revolutions per minute). The acceleration performance of the AC servo system is better. Taking the Panasonic MSMA400W AC servo motor as an example, it accelerates from static to its rated speed of 3000 r/min. It only takes a few ms and can be used in control occasions requiring fast start and stop.
