What is a Motor with Good Stopping Accuracy?

What's the matter, Vex? You look so confused.

A customer asked me, "I'm looking for a motor with good stopping accuracy, but I want to know how much difference are there between a stepper motor and a servo motor"...

What do you think, Vex?

The servo motor NX Series has a 20-bit encoder, so I believe it has fine resolution and good stopping accuracy.

It seems that you need to review the difference between resolution and stopping accuracy! Resolution is the "number of divisions per revolution," and in stepper motors, it is also called as step angle. Stopping accuracy is the "difference between the theoretical stopping position and the actual stop positions," which are completely different.

◆Resolution
Stepper Motor: Maximum 125,000 divisions
(The minimum step angle of Oriental Motor's stepper motors is 0.00288˚.)
Servo motor: Maximum 100,000 divisions
(The minimum resolution of Oriental Motor's servo motors is 0.0036˚.)
◆Stopping accuracy
Stepper Motor: ±0.05˚ (Stop position accuracy)
Servo Motor: Within ±1 pulse (encoder resolution)

I see! The resolution is necessary when "considering the extent of the fine positioning required," and the stopping accuracy is "the error of the stop position relative to the target position."
This means that servo motor with highly accurate encoder will have better stopping accuracy!!

That's a little different. In the past, "the stopping accuracy of a servo motor = encoder resolution ±1 pulse or less" was true, but since the encoder used in recent servo motor have a very fine resolution of 20 bits (1,048,576 divisions), so now the assembly precision have become a major influence to the encoder error. Therefore, the way of thinking about stopping accuracy has changed a little.
In fact, there is data that actually measures the stopping accuracy of a single revolution of a stepper motor and a servo motor, so take a look.

Huh?! The stopping accuracy of stepper motor and servo motor is almost the same, around ±0.02˚~0.03˚!!

Yes, they have the same ability in terms of stopping accuracy.
Moreover, the accuracy of a stepper motor is determined by the mechanical accuracy of the motor parts, so if the stop position can be set every 7.2˚, from the motor structure the same rotor small teeth is used for positioning all the times, which will be possible to improve the stopping accuracy.

I guess I was a little mistaken. So stepper motor is also capable for high accurate positioning.

However, you have to keep in mind that the displacement angle of a stepper motor depends on the load torque value, and the hunting range of a servo motor may increase in response to gain adjustment, depending on the mechanical conditions.

I understand well.
I will introduce to the customer right away!!

Click here for details about the servo motor NX Series.