Let's talk about the working principle first: Stepper motor is a kind of curator that concerts electrical pulses into angular displacement. When the stepper drivers receive a pulse signal, it will drive the stepper motor to rotate a fixed angle, which is also called as "step angle", in the set direction, and its rotation is performed step by step with a fixed angle. To achieve the purpose of accurate positioning, we can control the angular displacement by controlling the number of pulses. Meanwhile, we can also control the speed and accelerated speed of the motor's rotation by controlling the impulse frequency. By doing so, we can achieve the purpose of speed regulation. However, with regard to the servo motor, its inner rotor is a permanent magnet. The driver of servo motors normally controls the UVW three-phase electricity to form an electromagnetic field, and the rotor of servo motors usually rotates under the action of the magnetic field. Moreover, as for the signal transmission of servo motors, it is motor's own encoder that feeds back signal to the driver, and the driver will then compare the feedback value with the target value and adjust the angle of the rotor's rotation. Thus, the precision of the servo motor actually depends on the precision of its encoder.
Difference 1: The Difference in Methods of Controlling
CNC stepper motor controls the angle of rotation by controlling the number of pulses, and normally one pulse corresponds to one step angle.
Servo motor controls the angle of rotation by controlling the duration of pulses.
Difference 2: The Difference in Working Equipment and Working Processes
To perform perfectly, the stepper motor needs a power supply (the required voltage is given by the driver parameters, a pulse generator (now plates are mostly used), and a stepper driver. Typically, the angle of a stepper motor is determined by its driver. For instance, when the step angle is set as 0.45°, if there is a pulse, the motor will then go 0.45°. The working flow for the stepper motor generally needs two pulses: signal pulse and directional pulse.
The power supply that servo motors need is a switch (relay switch or relay board). For servo motors, its working flow is that a power is first connected to the switch, and then it will connect to the servo motor.
Difference 3: The Difference in Low-frequency Characteristics
Stepper motor is prone to vibrate with a low frequency at low speed. The frequency of vibration and the condition of load relates to the performance of drivers, and the frequency of vibration is generally half of the motor's no-load takeoff frequency.
This low-frequency vibration determined by the working principle of the stepper motor is very disadvantageous to the normal operation of the machine. When the stepper motor works at a low speed, we generally use damping technology to overcome the low-frequency vibration, such as adding a damper to the motor, using subdivision technology on the driver, and so on.
AC servo motor normally operates stably, and it doesn't have a low-frequency vibration even if the speed is low. The system of servo motors has the function of resonance suppression, which can cover the insufficient rigidity of machines. Then, the inner system has a frequency resolution function (FFT) that can test the point of resonance of machines, which is easy to adjust the system.
Difference 4: The Difference in Torque-frequency Characteristics
The output torque of stepper motors decreases with the increase of rotation speed, and it will drop dramatically at a higher rotation speed, so its highest speed of working rotation is generally 300 - 600r/min. In contrast, AC servo motor has a constant output torque, which means that within its rated speed (which is generally around 2,000 or 3,000r/min), it can always output the rated torque, and the constant power output is above the rated speed.
Difference 5: The Difference in Overload Capacity
In general, stepper motors don't have the overload capacity, while AC servo motors have a strong overload capacity.
Because stepper motors don't have this overload capacity which can help you to overcome the inertia torque, when you choose stepper motor models, you usually need to choose the motors with large torque. However, machines don't need such large torque during normal working hours. Thus, as for the purcahse of stepper motors, there can occur situations where the torque is wasted.
Difference 6: The Difference in the Performance of Velocity Response
While stepper motor needs 200 - 400ms so as to move from the static acceleration to the working speed, AC servo system has excellent accelerating ability. For example, Panasonic MSMA400W AC servo motor costs only several seconds to accelerate from the static acceleration to its rated speed of 3,000r/min. Therefore, servo motors can be widely used for the control occasions that require quick start and stop.