The difference between stepper motor and servo motor in linear module applicatio

1. Usually, we need to choose according to the application requirements. Stepper motors are suitable for low-speed occasions; servo motors are suitable for high-precision and high-speed occasions. In addition, it is necessary to comprehensively consider the matching with the output signal of the controller, the cost and other aspects.

2. The performance difference between stepper motor and AC servo motor

Stepper motor is a discrete motion device, which has an essential connection with modern digital control technology. In the current domestic digital control system, the application of stepper motor is very extensive. With the emergence of all-digital AC servo systems, AC servo motors are increasingly used in digital control systems. In order to adapt to the development trend of digital control, most of the motion control systems use stepper motors or all-digital AC servo motors as executive motors. Although the two are similar in control mode (pulse train and direction signal), there are big differences in performance and application. Now compare the performance of the two.

1) Different control accuracy

The step angle of the two-phase hybrid stepping motor is generally 3.6° and 1.8°, and the step angle of the five-phase hybrid stepping motor is generally 0.72° and 0.36°. There are also some high-performance stepper motors with smaller step angles. For example, a stepping motor used for a slow wire-feeding machine tool has a step angle of 0.09°; the step angle of a three-phase hybrid stepping motor produced by BERGER LAHR can be set to 1.8 through a DIP switch. °, 0.9°, 0.72°, 0.36°, 0.18°, 0.09°, 0.072°, 0.036°, 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 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, every time the driver receives 217=131072 pulses, the motor rotates 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°.

2) Different low frequency characteristics

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 drive, and it is generally considered 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 stepping 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 driver.

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

4) Different overload capacity

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

5) Different operating performance

The control of the stepper motor is open-loop control. If the starting frequency is too high or the load is too large, it is easy to lose steps or stall. When the speed is too high, it is easy to cause overshoot. Therefore, in order to ensure its control accuracy, it should be handled properly. Ascending and decelerating problems. The AC servo drive system is closed-loop control. The drive can directly sample the feedback signal of the motor encoder, and the internal position loop and speed loop are formed. Generally, the phenomenon of step loss or overshoot of the stepping motor will not occur, and the control performance is more reliable.

6) Different speed response performance

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

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