As an open-loop control system, stepping motor has essential connection with modern digital control technology. In the current domestic 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 of the two.
- Different control precision
The step angle of two-phase hybrid stepping motor is 1.8 ° and 0.9 ° 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 after subdivision. For example, the step angle of the two-phase hybrid stepping motor produced by SANYO DENKI can be set as 1.8 °, 0.9 °, 0.72 °, 0.36 °, 0.18 °, 0.09 °, 0.072 ° and 0.036 ° through the dial switch, which is compatible with the step angle of two-phase and five phase hybrid stepping motor.
The control precision of AC servo motor is guaranteed by the rotary encoder at the back end of motor shaft. Taking SANYO full digital AC servo motor as an example, for the motor with standard 2000 line encoder, the pulse equivalent is 360 ° / 8000 = 0.045 ° due to the quadruple frequency technology adopted in the driver. For the motor with 17 bit encoder, every 131072 pulses received by the driver, the motor turns one turn, that is, the pulse equivalent is 360 ° / 131072 = 0.0027466 ° which is 1 / 655 of the pulse equivalent of the stepper motor with 1.8 ° step angle.
- 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.
- Thedifferent moment frequency characteristics
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 is a constant torque output mode, that is, in its rated speed (generally 2000rpm or 3000rpm), it can output rated torque, above the rated speed,it is working in constant power output.
- Different overload capacity
Stepper motor generally does not have overload capacity. AC servo motor has strong overload capacity. Taking SANYO AC servo system as an example, it has the ability of speed overload and torque overload. Its maximum torque is two to 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.
- Different running performance
Stepper motor control is open-loop control, too high starting frequency or load is easy to lose step or locked rotor phenomenon, that speed is too high when stopped is easy to cause overshoot , 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.
- Different speed response performance
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 good. Taking SANYO 400W AC servo motor as an example, it takes only a few ms from static acceleration to its rated speed of 3000RPM, which can be used for control occasions requiring fast start 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.
