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Which Has Higher Stopping Accuracy - Stepper Motor or Servo Motor?

Customer Inquiry: Looking for a motor with good stopping accuracy. How much of a difference is there between stepper motors and servo motors?

Assumption: The AC servo motor NX Series (compared to integrated servo motor) is equipped with a 20-bit encoder, thus it should have a fine resolution and good stopping accuracy.

First, it is necessary to clarify the difference between resolution and stopping accuracy: Resolution is the number of steps per revolution and it is also called a step angle for stepper motors. It is needed when considering how precise the required positioning needs to be. Stopping accuracy is the difference between the actual stop position and theoretical stop position.

Does this mean that the AC servo motor equipped with a high accuracy encoder has better stopping accuracy than stepper motors?

Not quite. In the past there was no issue with the concept of "stopping accuracy of servo motors is equal to encoder resolution within ±1 pulse." However, recent servo motors are equipped with the encoder of 20 bits (1,048,576 steps) which has a very fine resolution. Because of this, errors due to the encoder installation accuracy have a huge effect on stopping accuracy. There- fore, the concept of stopping accuracy has started to change slightly.

According to the comparison charts, stopping accuracy between stepper motors and AC servo motors is almost the same (±0.02°~ 0.03°). Performance of stopping accuracy is the same between the two. Furthermore, accuracy depends on the mechanical precision of the motor for stepper motors, thus if stop position can be done per 7.2°, positioning is done by the same small teeth on the rotor at all times, according to the motor structure. This makes it possible to further improve stopping accuracy.

However, stepping motors may generate displacement angle depending on the load torque value. Also, depending on the mechanism condition, AC servo motors may have wider hunting width as a response to gain adjustments. For these reasons, some caution is required.

Newly Released 0.72° Stepper Motor and Driver Package RK II Series: Compared with the conventional RK Series, the new RK II Series not only achieves lower power consumption and extends functions by implementing the high efficiency motor, but also reduces the size of the driver.