Differences between Stepper Motors and Servo Motors

Both stepper motors and servo motors are servo units for motion control purposes, which can precisely control the location and speed of motion mechanisms.To adapt to the development trend of digital control, stepper motors or all-digital motors are mostly hired as executive motors in motion control systems, which are widely used in the era of labor saving. Despite the similarity in position controlling, namely pulse train and direction signal, both two motors greatly differ in performance and application.

1.Different Control Precision

The step angle of the two-phase hybrid stepper motor is generally 1.8°, and the step angle of the three-phase hybrid stepper motor is 1.2°.There are also some high-performance stepper motors with smaller step angles. After using subdivision drive technology, the control precision get enhanced so that the number of steps the two-phase hybrid stepper motor can distinguish is about 51200 steps/rev. There are also some motors with higher precision, but their cost will be more.

The control precision of AC servo motors is guaranteed by the rotary encoder at the rear end of the motor shaft. For a servo motor with a standard 2500 line encoder, its pulse equivalent should be 360°/10000=0.036°due to the adoption of quadruple frequency technology.

It is worth noting that a considerable number of people think highly of the precision of AC servo motors, which is actually a misinterpretation. Even with 100 times electronic gear or 30000 line encoder, it will make no difference if the servo motor cannot distinguish that small step. The feedback of encoder is nothing more than making the servo driver align it teeth repeatedly.

Compare with the vast majority of users, the stepper servo motors possess the highest physical precision in terms of the mechanical transmission precision or positioning precision of photoelectric sensor. It is unnecessary to pursue the highest precision in single aspect.

2.Different Overload Capacity

Stepper motors are not able to address overload, while AC servo motors own abilities to address overload and torque overload at 2 to 3 times, which can be used to overcome the inertial torque when starting the inertial overload. Without this ability, the torque of overload and movement characteristics should be taken into consideration when you select the type of motors.

3.Different Operating Performance

The stepper motors control in a open-loop way, where situations including step loss or stalling will easily occur when its starting frequency is too high or its load is too large. Therefore, acceleration and deceleration control program need to be handled in order to ensure its control precision.However, the AC servo motors adopts a closed-loop way. The driver can directly sample the feedback signal of the motor encoder and form position loop and speed loop inside, which makes its control more reliable. Generally, there will be no step loss or overshooting.Besides, the servo driver can automatically rectify lost pulse and give feedback on time to the controller when it is blocked. However, the stepper motor must use sufficient torque margin to avoid stalling.

4.Different Speed Response Performance

It takes 100-2000 milliseconds for a stepping motor to accelerate from a standstill to a working speed (generally several hundred revolutions per minute). With better acceleration performance, it only takes a few milliseconds for the AC servo motors to accelerate from a standstill to its rated speed of 3000RPM, which can be applied in occasions requiring fast start and stop.

In one word, the AC servo motor is superior to stepper motor in terms of some aspects. However the latter will not be replaced wholly due to it unique characteristics.

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