The stepper motor is a discrete motion device that is intrinsically linked to modern digital control technology. In the current domestic digital control system, the application of stepper motors is very extensive. With the advent of all-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, stepping motor or all-digital AC servo motor is mostly used as the execution motor in the motion control system. Although the two are similar in control mode (burst and direction signals), there are large differences in performance and application. Now compare the performance of the two

 

First, the control accuracy is different

 

The two-phase hybrid stepping motor has a step angle of 3.6 ° and 1.8 °, and the five-phase hybrid stepping motor has a step angle of 0.72 ° and 0.36 °. There are also some high performance stepper motors with smaller step angles. For example, a stepping motor for a slow wire-cutting machine produced by Sitong has a step angle of 0.09 °; the three-phase hybrid stepping motor produced by BERGER LAHR can pass the step angle. The DIP switches are set to 1.8 °, 0.9 °, 0.72 °, 0.36 °, 0.18 °, 0.09 °, 0.072 °, 0.036 ° and are compatible with the step angle of two-phase and five-phase hybrid stepping motors.

 

The control accuracy of the AC servo motor is guaranteed by the rotary encoder at the rear 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 inside the driver . For a motor with a 17 -bit encoder, the drive receives one revolution per 217 = 131072 pulse motors, ie 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 motors are prone to low frequency vibration at low speeds. The vibration frequency is related to the load condition and the performance of the driver. It is generally considered that the vibration frequency is half of the take-off frequency of the motor no-load. This low frequency vibration phenomenon, which is determined by the working principle of the stepper motor, is very detrimental to the normal operation of the machine. When the stepper motor works at low speed, damping technology should generally be used to overcome the low frequency vibration phenomenon, such as adding a damper to the motor or using subdivision technology on the drive.

 

The AC servo motor runs very smoothly, and vibration does not occur even at low speeds. The AC servo system has a resonance suppression function that covers the lack of rigidity of the machine, and has a frequency analysis function ( FFT ) inside the system to detect the resonance point of the machine and facilitate system adjustment.

 

Third, the difference in frequency characteristics

 

The output torque of the stepper motor decreases with increasing speed and drops sharply at higher speeds, so the maximum operating speed is typically between 300 and 600 RPM . AC servo motor is a constant torque output, i.e. at its rated speed (3000RPM to 2000RPM or general) or less, can output rated torque, above the rated speed for the constant power output.

 

Fourth, the overload capacity is different

 

Stepper motors generally do not have overload capability. The AC servo motor has a strong overload capability. Take the 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 moment of inertia of the inertia load at the moment of starting. Because there is no such overload capability in the stepping motor, in order to overcome this moment of inertia during the selection, it is often necessary to select a motor with a large torque, and the machine does not need such a large torque during normal operation, and a torque appears. The phenomenon of waste.

 

Five, different operating performance

 

The control of the stepping motor is open-loop control. If the starting frequency is too high or the load is too large, it may be lost or blocked. If the speed is too high during the stop, the overshoot may occur. Therefore, in order to ensure the control accuracy, it should be handled well. The problem of rising and falling speed. The AC servo drive system is closed-loop control. The drive can directly sample the feedback signal of the motor encoder. The internal position loop and speed loop are formed. Generally, the stepless motor is lost or overshooted, and the control performance is more reliable.

 

Sixth, the speed response performance is different

 

It takes 200 to 400 milliseconds for the stepper motor to accelerate from standstill to operating speed (typically a few hundred revolutions per minute) . The AC servo system has better acceleration performance. Taking the Panasonic MSMA 400W AC servo motor as an example, it takes only a few milliseconds to accelerate from standstill to its rated speed of 3000 RPM , which can be used for control applications requiring fast start and stop.

 

In summary, the AC servo system is superior to the stepper motor in many performance aspects. However, in some occasions where the requirements are not high, stepper motors are often used to perform the motor. Therefore, in the design process of the control system, it is necessary to comprehensively consider the control requirements, cost and other factors, and select the appropriate control motor.

 

The person who wrote this post is selling AC servo motor? The content is basically correct, but it is not comprehensive. Some of the parts of the landlord used for comparison, some can be seen from another perspective.

Obviously, the landlord does not know that the stepper motor driver has the concept of " segmentation " . The step angle of a two-phase stepper motor is 1.8 degrees, but now 64- segment drives are also common. Note that at this time, the motor is 200*64=12800 pulses for one revolution. The common AC servo on the market, the encoder is only 2048 or 2500 lines. Of course, there are 17 -bit encoder motors, but the stepper drive also has 256 subdivisions. In terms of resolution, the AC servo is still higher, but it is far less exaggerated by the landlord. Moreover, since it is said that the control accuracy, then, those who have used the servo should know that the dynamic reproducibility of the servo is how many times the resolution. In the case of conventional design, the reproducibility index is multiplied by 5 as the resolution of the servo feedback. In this way, is the servo control accuracy really better than the servo?

When the number of subdivisions of the stepping motor reaches 32 or more, there is basically no problem of low frequency vibration. The servo wants to maintain an accurate and stable low speed. The used person should know how difficult the parameters are (as long as the speed, do not position, do a little better)

For the torque, it is necessary to add a point, the servo itself does not have a holding torque, and the stepping motor has a holding torque. The difference is that the so-called static of the servo motor is actually a dynamic balancing process, and the motor does not really stop at the specified position (so the reproducibility of the AC servo is set to 3-5 times the feedback resolution , and the step is The reproducibility of the motor can be higher than the resolution).

There is nothing to say about this, but there is still some opinion on the theory of wasted torque. Many stepper drives offer a half-flow function that reduces current and reduces torque when full torque output is not required.

Lost step is indeed a fatal flaw in the stepper motor, but can the servo not consider the curve of acceleration and deceleration? You really try a step signal, how much jitter the motor will have. But shaking and shaking, eventually will stop in the right position, which is indeed stronger than the step. If it is positioning control, this jitter does not matter, if it is process control, who would dare to use it?

Because the AC servo can have an instantaneous high torque output, the acceleration performance may be stronger than the stepping, but Panasonic is added to 3000 RPM for a few milliseconds. Would you like to try again before you speak? And when it comes to response, you can't help but say the essential flaw of AC servo - lag. In general motors, the speed loop responds for 2 milliseconds, and the position loop response rarely sees the data, which is generally considered to be 8 milliseconds. When it comes to fast start and stop, the servo always has its response frequency limit, and the stepper motor basically does not need to consider the response time. With the stepper motor, you can easily start and stop 100 times in one second, and move 20 microns each time . You can try it with the servo.

 

Stepping and servo, there is no difference between good and bad, each has its own application. Generally speaking, for large load and high speed applications, do not use stepper motor, but low load, low speed, high subdivision step performance ratio AC servo is better

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