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.
First, the control accuracy is different
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.
The control accuracy of the AC servo motor is guaranteed by the rotary encoder at the rear end of the motor shaft. For the motor with the 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 stepping 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 a resonance suppression function, 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 system adjustment.
3. Different moment-frequency characteristics
The output torque of the stepper motor decreases with the increase of the speed, and will drop sharply at a higher speed, so its maximum working speed is generally 300-600RPM. The AC servo motor has a constant torque output, that is, within its rated speed (generally 2000RPM or 3000RPM), it can output the rated torque, and above the rated speed, it is a constant power output.
4. Different overload capacity
Stepper motors generally do not have overload capability. AC servo motors have strong overload capacity. The maximum torque of our servo is three times the rated torque, which can be used to overcome the inertial moment of inertial load at the moment of startup. 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. But our closed loop stepping can be overloaded by a factor of 1.5.
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 stepping 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, and 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.
To sum up, AC servos are superior to stepper motors in many performance aspects. However, in some occasions with low requirements, stepper motors are often used as executive motors. Therefore, in the design process of the control system, various factors such as control requirements and costs should be comprehensively considered, and an appropriate motor should be selected.