A challenge to some claims re-recent AAC article.

Thread Starter

MaxHeadRoom

Joined Jul 18, 2013
28,534
In a recently received article from AAC on the different merits between Servo's and Stepper motors in positioning control. " https://control.com/technical-articles/servo-motor-vs-stepper-motor-understanding-the-differences/ "I had a few issues with some of the claims.
I spent a good part of my career retro-fitting Industrial CNC machinery, and have never really come across any major CNC manuf. that uses steppers for motion control, it is usually left to the much smaller, less demanding systems manuf. or the DIY builder for small home use CNC tables etc.
I would refute the idea that Servo's cannot position to the same accuracy.
Servo's have higher RPM's, higher power, positioning precision and accuracy/resolution.
They maintain a high torque from zero RPM up to their maximum rated speed.
There are a few engineering papers out there that support the same view.
An extraction from a recent example :- .

"Servo systems provide the highest possible level of performance for precise control of position, velocity, and/or torque.
Compared to lower cost stepper motor systems, servo systems provide more torque at higher speeds, up to 5,000 rpm.
With stepper motors, the maximum torque is at zero speed, but servomotors have maximum torque at higher speeds.
Typical servo systems for machine control also provide a broader range of power, up to 3 kW or more, than stepper motors.

Perhaps the most notable difference between steppers and servomotors is that servomotors improve positioning with closed loop control.
Although some stepper packages take advantage of closed loop control, accurate and high-speed motion profiles without the motor stalling
and the related position error is a common advantage of servomotors.
Closed-loop position control, higher torque and higher speeds of the servomotor all confer benefits in high-accuracy applications."
 

nsaspook

Joined Aug 27, 2009
12,859
That articles is IMO obviously misleading with numbers about random stop accuracy,

Maybe a little more accurate article.
https://blog.orientalmotor.com/the-choice-between-servo-motors-and-stepper-motors
For positioning applications, one of the primary requirements we have to satisfy is the motor's stop accuracy. Both a stepper motor and a servo motor can stop accurately.

A stepper motor's stop accuracy depends on the manufacturing quality of the windings (electrical) and teeth construction (mechanical), while a servo motor's accuracy depends on the assembly accuracy, encoder resolution, and algorithm.
...
Notice that they both offer a stop accuracy of about +/-0.02°, which is under a stepper motor's typical repetitive stop accuracy specification of 3 arc minutes, or +/-0.05°. While a servo motor can increase its stop accuracy by increasing its encoder resolution, a stepper motor offers better repeatability at 7.2° increments or almost perfect repeatability at 360° increments.

The stop accuracy of a stepper motor is highly dependent on its winding characteristics, rotor construction accuracy, as well as the number of teeth/poles in its rotor. The stop accuracy of a servo motor is dependent on assembly accuracy, encoder resolution, and operating algorithm. In a way, you can say that a stepper motor is "mechanically designed" for positioning applications, and servo motors are "electrically designed" for positioning applications.
There are much more accurate servo motor systems on the market than these specifications.
 

Papabravo

Joined Feb 24, 2006
21,030
Maybe when editors are desperate for articles to publish, they get lax on their standards for accuracy and correctness. This would not be the first site to become the victim of disinformation. I'm here for the forum, I could care less about the rest of the site.
 

Papabravo

Joined Feb 24, 2006
21,030
I worked for a major industrial robotics manufacturer. We never used stepper motors. We used servo motors.
I have noticed some deficient reporting in the AAC technical articles but choose not to comment. I take it all with a grain of salt.
There is a distinct difference between a moderator and an editor. I don't think the failings of the editorial staff reflect on you at all.
 
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