Why Is A Stepper Motor Superior To A Servo Motor On A 3D Printer?

- Nov 16, 2018-

Why is a stepper motor superior to a servo motor on a 3D printer?

The motor is a very important power component on the 3D printer. Its accuracy is related to the quality of the 3D printing. Generally, the stepper motor is used for 3D printing. Stepper motor is a kind of discrete motion device. It is different from ordinary AC/DC motor. Ordinary motor power is turned on, but stepper motor is not. Stepper motor performs one step when receiving a command. The servo motor is an engine that controls the operation of mechanical components in the servo system. It can control the speed and position accuracy very accurately. It can convert the voltage signal into torque and speed to drive the control object. Although the two are similar in control mode (burst and direction signals), there are large differences in performance and application. Now, Xiaobian will take you to see the specific differences in performance between the two aspects.

Different control accuracy

The two-phase hybrid stepping motor step angle is generally 3.6 °, 1.8 °, and the five-phase hybrid stepping motor step angle is generally 0.72 °, 0.36 °. There are also some high performance stepper motors with smaller step angles. For example, a stepping motor for a slow wire-feeding machine produced by Sitong has a step angle of 0.09°.

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 of 217=131072 pulse motors, ie its pulse equivalent is 360°/131072=9.89 seconds, which is a stepper motor with a step angle of 1.8°. 1/655 of the pulse equivalent.

Different low frequency characteristics

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.

3D printing motor

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, which can cover the rigidity of the machine, and has a frequency analysis function inside the system, which can detect the resonance point of the machine and facilitate system adjustment.

Different moment frequency characteristics

The output torque of the stepping motor decreases as the speed increases, and it drops sharply at higher speeds, so the maximum operating speed is generally 300-600 RPM.

The AC servo motor is a constant torque output, that is, within its rated speed (generally 2000RPM or 3000RPM), it can output the rated torque, and the constant power output is above the rated speed.

Different overload capabilities

Stepper motors generally do not have overload capability, and AC servo motors have strong overload capabilities.

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.

Different running 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.

Different speed response performance

It takes 200 to 400 milliseconds for the stepper motor to accelerate from standstill to the operating speed (typically several hundred revolutions per minute).

The AC servo system has better acceleration performance. Taking the Panasonic MSMA400W 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.

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