The servo mainly depends on the pulse to position, basically can be understood in this way, the servo motor receives a pulse, it will rotate the corresponding Angle of a pulse, so as to achieve displacement, because the servo motor itself has the function of pulse, so each rotation Angle of the servo motor, will emit the corresponding number of pulses, so that the pulse accepted by the servo motor echoes. Or closed loop, so that the system will know how many pulses are sent to the servo motor, and how many pulses are received back, so that the rotation of the motor can be controlled very accurately, so as to achieve accurate positioning, which can reach 0.001mm.

Stepper motor is a discrete motion device, which is intrinsically related to modern digital control technology. In the current domestic digital control system, the stepper motor is widely used. With the emergence of full 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, most of the motion control systems use stepper motor or full digital AC servo motor as the execution motor. Although the two are similar in the control mode (pulse train and direction signal), there are big differences in the performance and application. The performance of the two is compared.

First, the control accuracy is different

The step Angle of the two-phase hybrid stepping motor is generally 3.6°, 1.8°, and the step Angle of the five-phase hybrid stepping motor is generally 0.72 °, 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 wire machine tools has a step Angle of 0.09°; The step Angle of the three-phase hybrid stepper motor produced by BERGER LAHR can be set to 1.8°, 0.9°, 0.72°, 0.36°, 0.18°, 0.09°, 0.072°, 0.036° through the dip switch, which is compatible with the step Angle of the two-phase and five-phase hybrid stepper motor. The control accuracy of the AC servo motor is guaranteed by the rotary encoder at the rear end of the motor shaft. Taking Panasonic's all-digital AC servo motor as an example, for a motor with a standard 2500 line encoder, the pulse equivalent is 360°/10000=0.036° due to the quadruple frequency technology used inside the driver. For a motor with a 17-bit encoder, the driver receives 217=131072 pulse motor for one revolution, that is, 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°.

Second, the low frequency characteristics are different

Stepper motor is prone to low frequency vibration at low speed. The vibration frequency is related to the load and the performance of the driver, and 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 stepper motor is very unfavorable to the normal operation of the machine. When the stepper motor works at a low speed, damping technology should generally be used to overcome low-frequency vibration, such as adding a damper to the motor, or using subdivision technology on the driver. Ac servo motor runs very smoothly, even at low speeds there is no vibration phenomenon. The AC servo system has resonance suppression function, which can cover the lack of rigidity of the machine, and the frequency resolution function (FFT) in the system can detect the resonance point of the machine, which is easy to adjust the system.

Third, the moment frequency characteristics are different

The output torque of the stepper motor decreases with the increase of speed, and will drop sharply at higher speed, so its maximum working speed is generally 300 ~ 600RPM. Ac servo motor is a constant torque output, that is, within its rated speed (generally 2000RPM or 3000RPM), it can output rated torque, and above the rated speed is a constant power output.

Fourth, overload capacity is different

Stepper motors generally do not have overload capability. Ac servo motor has strong overload capability. Take Panasonic AC servo system as an example, it has speed overload and torque overload capability. Its maximum torque is three times the rated torque and can be used to overcome the inertial torque of the inertial load at the moment of start. Because the stepper motor does not have this overload capacity, in order to overcome this inertia moment in the selection, it is often necessary to choose a motor with a larger torque, and the machine does not need so much torque during normal operation, and the phenomenon of torque waste appears.

Five, the operating performance is different

The control of the stepper motor is open-loop control, the starting frequency is too high or the load is too large, and the stopping speed is too high and the overshoot phenomenon is easy, so in order to ensure its control accuracy, the problem of raising and lowering speed should be handled well. Ac servo drive system is closed-loop control, the driver can directly sample the feedback signal of the motor encoder, and the internal position ring and speed ring are formed, generally there will be no step loss or overkill phenomenon of the stepper motor, and the control performance is more reliable.

Six, speed response performance is different

It takes 200 to 400 milliseconds for the stepper motor to accelerate from rest to working speed (generally several hundred revolutions per minute). The acceleration performance of the AC servo system is better, taking the Panasonic MSMA 400W AC servo motor as an example, from the static acceleration to its rated speed of 3000RPM only a few milliseconds, can be used for control occasions requiring fast start and stop %%%%% In summary, the AC servo system is better than the stepper motor in many aspects of performance. However, in some situations where the requirements are not high, stepper motors are often used as execution motors. Therefore, in the design process of the control system, it is necessary to comprehensively consider the control requirements, costs and other factors, and choose the appropriate control motor.