Difference between servo motor and stepper motor

Update:19 Jan 2021

Servo motor mainly relies on pulse to locate. Basically, it can be understood that when the servo motor receives one pulse, it will rotate the corresponding angle of one pulse to realize displacement. Because the servo motor itself has the function of sending out pulse, so every rotation angle of the servo motor will send out the corresponding number of pulses. In this way, the displacement is formed with the pulse received by the servo motor Echo, or closed-loop, in this way, the system will know how many pulses are sent to the servo motor, and how many pulses are received at the same time. In this way, it can control the rotation of the motor very accurately, so as to achieve accurate positioning, which can reach 0.001mm.
Stepping motor is a kind of discrete motion device, which has essential connection with modern digital control technology. In the current digital control system, stepping motor is widely used. With the emergence of all digital AC servo system, AC servo motor is more and more used in digital control system. In order to adapt to the development trend of digital control, stepping motor or full digital AC servo motor is mostly used as the executive motor in motion control system. Although they are similar in control mode (pulse train and direction signal), they are quite different in performance and application. This paper compares the performance and difference between servo motor and stepping motor.

1、 The control accuracy is different
The step angle of two-phase hybrid stepping motor is 3.6 ° and 1.8 ° in general, and that of five phase hybrid stepping motor is 0.72 ° and 0.36 ° in general. There are also some high-performance stepper motors with smaller step angle. For example, the step angle of a stepper motor used in a slow walking machine tool is 0.09 °, and the step angle of a three-phase hybrid stepper motor can be set to 1.8 °, 0.9 °, 0.72 °, 0.36 °, 0.18 °, 0.09 °, 0.072 ° and 0.036 ° through a dial switch, which is compatible with the step angle of two-phase and five phase hybrid stepper motors.
The control precision of AC servo motor is guaranteed by the rotary encoder at the back end of motor shaft. Taking the full digital AC servo motor as an example, for the motor with standard 2500 line encoder, the pulse equivalent is 360 ° / 10000 = 0.036 ° due to the quadruple frequency technology used in the driver. For the motor with 17 bit encoder, every 217 = 131072 pulses received by the driver, the motor turns one revolution, that is, the pulse equivalent is 360 ° / 131072 = 9.89 seconds. It is 1 / 655 of pulse equivalent of stepper motor with step angle of 1.8 degrees.

2、 Different low frequency characteristics
Stepping motor is prone to low frequency vibration at low speed. The vibration frequency is related to the load and the performance of the driver. It is generally considered that the vibration frequency is half of the no-load take-off frequency of the motor. This kind of low frequency vibration phenomenon determined by the working principle of stepping motor is very unfavorable for the normal operation of the machine. When the stepper motor works at low speed, damping technology should be used to overcome the low frequency vibration phenomenon, such as adding damper on the motor or using subdivision technology on the driver.
AC servo motor runs very smoothly, even at low speed, there will be no vibration phenomenon. The AC servo system has the function of resonance suppression, which can cover the lack of mechanical rigidity, and has the function of frequency analysis (FFT) in the system, which can detect the mechanical resonance point and facilitate the system adjustment.

3、 The moment frequency characteristics are different
The output torque of stepper motor decreases with the increase of speed, and it will drop sharply at higher speed, so its maximum working speed is generally 300 ~ 600 rpm. AC servo motor for constant torque output, that is, in its rated speed (generally 2000rpm or 3000rpm), can output rated torque, above the rated speed for constant power output.
4、 The overload capacity is different
Stepper motor generally does not have overload capacity. AC servo motor has strong overload capacity. Taking Panasonic AC servo system as an example, it has the ability of speed overload and torque overload. Its maximum torque is three times of the rated torque, which can be used to overcome the inertia torque of inertia load at the moment of starting. Stepping motor has no such overload capacity. In order to overcome the inertia torque, it is often necessary to select the motor with larger torque when selecting the type. However, the machine does not need such a large torque during normal operation, which leads to torque waste.

5、 Different operation performance
Stepper motor control is open-loop control, too high starting frequency or load is easy to lose step or locked rotor phenomenon, stop too high speed is easy to appear overshoot phenomenon, so in order to ensure its control accuracy, we should deal with the problem of speed up and down. AC servo drive system is a closed-loop control, the driver can directly sample the feedback signal of motor encoder, and the internal structure is position loop and speed loop. Generally, there is no step loss or overshoot phenomenon of stepper motor, and the control performance is more reliable.

6、 The speed response performance is different
It takes 200-400 MS for stepper motor to accelerate from rest to working speed (generally several hundred revolutions per minute). The acceleration performance of AC servo system is better. Taking 400W AC servo motor as an example, it takes only a few milliseconds to accelerate from static speed to its rated speed of 3000rpm, which can be used in the control occasions requiring fast start-up and stop.
To sum up, AC servo system is better than stepping motor in many aspects. But in some occasions, stepping motor is often used as executive motor. Therefore, in the design process of the control system, we should comprehensively consider the control requirements, cost and other factors, and select the appropriate control motor.

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