1. Main differences between step drive and servo drive.

2. Several practical problems in the specific selection process.

Summary:
Stepper motors are mainly classified according to the number of phases, among which two-phase and five-phase stepper motors are widely used in the market. The two-phase stepper motor can be divided into 400 equal parts per revolution and the five-phase stepper motor can be divided into 1000 equal parts. Therefore, the characteristics of the five-phase stepper motor are better, the acceleration and deceleration time is shorter, and the dynamic inertia is lower.

With the emergence of the all-digital ac servo system, the ac servo motor is more and more used in the digital control system. To adapt to the development trend of digital control, stepper motor or all-digital ac servo motor is used as the executive motor in most motion control systems. Although they are similar in control mode (pulse train and direction signal), they are quite different in performance and application.

The performance of the two is compared.

One, the control precision is different

The stepping Angle of the two-phase hybrid stepping motor is generally 3.6 degrees and 1.8 degrees, while the stepping Angle of the five-phase hybrid stepping motor is generally 0.72 degrees and 0.36 degrees. There are also some high-performance stepping motors with smaller step angles. For example, a stepping motor for a slow wire machine produced by the exiting company has a step angle of 0.09 degrees. The step Angle of the 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, 0.036 through the dial switch, which is compatible with the step Angle of the two-phase and five-phase hybrid stepping motor.

The control precision of the ac servo motor is guaranteed by the rotary encoder at the rear end of the motor shaft. Taking Panasonic all-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 quad-frequency technology used in the drive. For a motor with a 17-bit encoder, the driver rotates once for every 217=131072 pulse motors received, i.e. its pulse equivalent is 360 ° /131072=9.89 seconds. It is 1/655 of the pulse equivalent of a stepping motor with a step angle of 1.8 degrees.

Two: low-frequency characteristics are different

The stepper motor is prone to low-frequency vibration at low speed. The vibration frequency is related to the load condition and the performance of the driver. The low-frequency vibration phenomenon determined by the working principle of the stepping motor is very unfavorable to the normal operation of the machine. When the stepper motor works at low speed, damping technology should be generally used to overcome the low-frequency vibration phenomenon, such as adding a damper to the motor, or subdivision technology on the driver.

The ac servo motor runs very smoothly and does not vibrate even at low speed. The ac servo system has the function of resonance suppression, which can cover the mechanical rigidity deficiency, and the system has the frequency analysis function (FFT), which can detect the mechanical vibration point, which is convenient for the system adjustment.

Three: the moment frequency characteristic is different

The output torque of the stepping motor decreases with the increase of the speed, and it will decrease sharply at a higher speed, so its maximum operating speed is generally 300 ~ 600rpm.

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.

Four: overload capacity is different

Stepping motors generally do not have overload capacity. Ac servo motor has a strong overload capacity. Take the Panasonic ac servo system, for example, it has speed overload and torque overload capacity. The maximum torque is three times the rated torque and can be used to overcome the moment of inertia at the start of the inertial load. Because the stepping motor does not have this overload capacity, to overcome this kind of inertia torque in the model selection, often need to choose the motor with large torque, and the machine in the normal working period and do not need so much torque, the phenomenon of wasted torque.

Five: different operating performance

The stepper motor is controlled by open-loop control. If the starting frequency is too high or the load is too large, it is easy to lose step or block rotation. If the speed is too high, it is easy to overshoot when stopping. Ac servo drive system is closed-loop control, the driver can directly sample the feedback signal of the motor encoder, the internal structure of the position ring and speed ring, generally, there is no step loss or overshoot of the stepper motor, the control performance is more reliable.

Six: speed effect performance is different

It takes 200 to 400 milliseconds for a stepper motor to accelerate from rest to operating speed (typically several hundred revolutions per minute). Ac servo system has good acceleration performance, take Panasonic msma400w ac servo motor as an example, from the static acceleration to its rated speed of 3000rpm only a few milliseconds, it can be used in the control occasions requiring rapid start and stop.