The difference between servo motors and stepper motors

Performance comparison between stepper motor and AC servo motor
A stepper motor is a discrete motion device that is fundamentally related to modern digital control technology. In the current digital control systems in China, stepper motors are widely used. With the emergence of fully digital AC servo systems, AC servo motors are increasingly being used in digital control systems. In order to adapt to the development trend of digital control, stepper motors or fully digital AC servo motors are mostly used as executive motors in motion control systems. Although the two have similar control methods (pulse trains and directional signals), there are significant differences in performance and application scenarios. Let's compare the performance of the two. 
1、 Different control accuracy
The step angles of two-phase hybrid stepper motors are generally 3.6 ° and 1.8 °, while those of five-phase hybrid stepper motors are generally 0.72 ° and 0.36 °. There are also some high-performance stepper motors with smaller step angles. For example, a stepper motor produced by Sitong Company for slow threading machine tools has a step angle of 0.09 °; The step angle of three-phase hybrid stepping motor produced by BERGER LAHR can be set to 1.8 °, 0.9 °, 0.72 °, 0.36 °, 0.18 °, 0.09 °, 0.072 ° and 0.036 ° through the Dip switch, which is compatible with the step angle of two-phase and five phase hybrid stepping motors. 
The control accuracy of the AC servo motor is ensured by the rotary encoder at the rear end of the motor shaft. Taking Panasonic's fully digital AC servo motor as an example, for a motor with a standard 2500 line encoder, due to the use of quadruple frequency technology inside the driver, its pulse equivalent is 360 °/10000=0.036 °. For motors with a 17 bit encoder, for every 217=131072 pulses received by the driver, the motor rotates once, which means its pulse equivalent is 360 °/131072=9.89 seconds. It is 1/655 of the pulse equivalent of a stepper motor with a step angle of 1.8 °. 
2、 Different low-frequency characteristics
The stepper motor is prone to low-frequency vibration at low speeds. The vibration frequency is related to the load situation and driver performance, and is generally considered to be half of the no-load takeoff frequency of the motor. This low-frequency vibration phenomenon determined by the working principle of a stepper motor is very detrimental to the normal operation of the machine. When stepper motors operate at low speeds, damping technology should generally be used to overcome low-frequency vibration phenomena, such as adding dampers to the motor or using subdivision technology on the driver. 
The AC servo motor runs very smoothly, and even at low speeds, there is no vibration phenomenon. The AC servo system has resonance suppression function, which can cover the insufficient rigidity of the machinery, and the system has frequency analysis function (fft) internally, which can detect the resonance points of the machinery and facilitate system adjustment. 
3、 Different torque frequency characteristics
The output torque of a stepper motor decreases as the speed increases, and it sharply decreases at higher speeds, so its higher operating speed is generally between 300 and 600 RPM. The AC servo motor has a constant torque output, which means it can output rated torque within its rated speed (usually 2000RPM or 3000RPM), and constant power output above the rated speed. 
4、 Different overload capacity
Stepper motors generally do not have overload capacity. AC servo motors have strong overload capacity. Taking the Panasonic AC servo system as an example, it has speed overload and torque overload capabilities. Its larger torque is three times the rated torque and can be used to overcome the inertia torque of the inertia load at the moment of starting. Due to the lack of such overload capacity, stepper motors often need to choose motors with larger torque to overcome this inertia torque during selection. However, the machine does not require such a large torque during normal operation, resulting in torque waste. 
5、 Different operational performance
The control of the stepper motor is open-loop control. If the starting frequency is too high or the load is too large, it is easy to lose step or block the rotor. If the speed is too high when stopping, it is easy to cause overshoot. Therefore, to ensure its control accuracy, the problem of speed increase and decrease should be handled well. The AC servo drive system adopts closed-loop control, and the driver can directly sample the feedback signal of the motor encoder. The internal structure consists of a position loop and a speed loop, which generally does not cause step loss or overshoot of the stepper motor, and the control performance is more reliable. 
6、 Different speed response performance
It takes 200-400 milliseconds for a stepper motor to accelerate from stationary to operating speed (usually several hundred revolutions per minute). The acceleration performance of the AC servo system is good. Taking the Panasonic MSMA 400W AC servo motor as an example, it only takes a few milliseconds to accelerate from static to its rated speed of 3000RPM, and can be used in control situations that require fast start stop. 
In summary, AC servo systems perform better than stepper motors in many aspects. But in some low demand situations, stepper motors are often used as execution motors. Therefore, in the design process of the control system, multiple factors such as control requirements and costs should be comprehensively considered, and appropriate control motors should be selected.