Performance comparison between servo motor and stepper motor

Performance comparison between servo motor and stepper motor

As an open-loop control system, stepper motor has an essential relationship with modern digital control technology. Stepper motor and servo motor are similar in control mode (pulse train and direction signal), but there are great differences in use performance and application occasions.

1. Different control accuracy

The step angle of two-phase hybrid stepping motor is generally 1.8 ° and 0.9 °, and that of five-phase hybrid stepping motor is generally 0.72 ° and 0.36 °. There are also some high-performance stepping motors with smaller step angle after subdivision. The step angle of the two-phase hybrid stepping motor produced by SANYO DENKI can be set to 1.8 °, 0.9 °, 0.72 °, 0.36 °, 0.18 °, 0.09 °, 0.072 ° and 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 accuracy of the AC servo motor is guaranteed by the rotary encoder at the rear end of the motor shaft. Taking Yamayo all-digital AC servo motor as an example, for the motor with standard 2000 line encoder, the pulse equivalent is 360 °/8000=0.045 ° due to the use of quadruple frequency technology inside the driver. For the motor with 17-bit encoder, the driver receives 131072 pulse motors for one revolution, that is, its pulse equivalent is 360 °/131072=0.0027466 °, which is 1/655 of the pulse equivalent of the step 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 driver. 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 unfavorable 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 damper on the motor or adopting subdivision technology on the driver.

The AC servo motor runs very smoothly, and it will not vibrate even at low speed. The AC servo system has the resonance suppression function, which can cover the mechanical rigidity deficiency, and the system has the frequency analysis function (FFT) inside, which can detect the mechanical resonance point for the system adjustment.

3 Different torque-frequency characteristics

The output torque of the stepping motor decreases with the increase of the speed, and will drop sharply at higher speed, so its maximum operating speed is generally 300~600RPM. AC servo motor is constant torque output, that is, it can output rated torque within its rated speed (generally 2000RPM or 3000RPM), and constant power output above the rated speed.

4 Different overload capacity

Stepper motors generally do not have overload capacity. AC servo motor has strong overload capacity. Take Shanyang AC servo system as an example, it has the ability of speed overload and torque overload. Its maximum torque is two to three times the rated torque, which can be used to overcome the inertia moment of inertia load at the moment of starting. Because the stepping motor has no such overload capacity, in order to overcome this inertia torque, it is often necessary to select a motor with a larger torque when selecting the type, and the machine does not need such a large torque during normal operation, which leads to the phenomenon of torque waste.

5. Different operation 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 the step or lock the rotor. If the speed is too high when stopping, it is easy to overshoot. Therefore, in order to ensure its control accuracy, the problem of raising and lowering the speed should be properly handled. The AC servo drive system is closed loop control. The driver can directly sample the feedback signal of the motor encoder. The internal structure is a position loop and a speed loop. Generally, there is no step loss or overshoot of the stepping motor, and the control performance is more reliable.

6 Different speed response performance

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

So AC servo system is superior to stepping motor in many aspects of performance. But in some places where the requirements are not high, the stepping motor is often used as the executive motor. Therefore, in the design process of the control system, control requirements, costs and other factors should be comprehensively considered, and appropriate control motors should be selected.

Performance comparison between servo motor and single asynchronous motor

1 No rotation

As long as the control voltage is lost, the normal running servo motor will stop running immediately. When the servo motor loses its control voltage, it is in single-phase operation. Due to the large rotor resistance, the two rotating magnetic fields in the opposite direction in the stator and the two torque characteristics (T1-S1 and T2-S2 curves) and the resultant torque characteristics (T-S curve) generated by the rotor action

The output power of servo motor is generally 0.1-100W. When the power frequency is 50Hz, the voltage is 36V, 110V, 220V and 380V; When the power frequency is 400Hz, the voltage is 20V, 26V, 36V, 115V, etc.

2. Wide operation range

3. Large starting torque

Due to the large rotor resistance, its torque characteristic curve is shown in curve 1 in Figure 3, which is significantly different from the torque characteristic curve 2 of ordinary asynchronous motor. It can make the critical slip S0>1, which not only makes the torque characteristics (mechanical characteristics) closer to linear, but also has a large starting torque. Therefore, as soon as the stator has control voltage, the rotor rotates immediately, which is characterized by fast starting and high sensitivity.