Terminology for Accuracy Tests of High-Performance Rotary Stages

Motivation

In this post, we present terminology related to the test and metrology of highly accurate rotary stages (spindles) commonly used in the precision motion industry. The aim is to help customers and users from different industries speak the same “language” as our engineers. 

Introduction of Terms and Concepts

Below are the terms that are used to define the performance of a spindle. The often requested “repeatability of eccentricity” spec by customers is equivalent to AEM (Asynchronous Error Motion) in the language of our metrology engineers.

ACE

Centering Error of the Artifact

AEM

Asynchronous Error Motion

AIE

Inherent Error from the Imperfection of the Artifact

NRRO

Non-Repeatable Run Out

It’s the non-repeatable portion of the error motion and it is also called as Asynchronous Error Motion (AEM). In the customer’s terms, this is the repeatability of the eccentricity. The NRRO also contains the noise of the measurement devices and the test setup, so very careful data processing is required to derive the true error motion coming from the spindle. For this reason, the AEM data of any spindle is not published yet.

NRRO (AEM) = TIR – RRO

NRRO = NRRO (spindle) + Measurement Noise

RRO

Repeatable Run Out

On top of the true spindle error, there are also errors coming from the test setup including the centering error of the artifact (ACE) and the inherent error from the imperfection of the artifact (AIE). The ACE component needs to be taken out during post processing of the data. The AIE can be also taken out during the post processing using rather sophisticated algorithms (such as the Donaldson Reversal Method) but the error is relatively small (often less than 25 nm) and can be ignored. Please note that ACE and the AIE are not the common terms but are used to express in a simple equation as below.

RRO = SEM + ACE (+ AIE)

SEM 

Synchronous Error Motion

The SEM is the repeatable error coming directly from the performance of the spindle and it can be derived by taking the ACE component out of the RRO.

SEM = RRO – ACE (+ AIE)

TIR

Total Indicated Reading

This is the raw data taken with any measurement device. TIR can be categorized as a repeatable portion (RRO) and a non-repeatable portion (NRRO) of the motion error.

TIR = RRO + NRRO

Example of Test Results

The asynchronous error motion (AEM) is derived by taking the average error motion out of the Synchronous Error Motion (SEM) plot (Green Line minus Blue Line, Orange Line minus Blue Line). This is the Non-Repeatable Run Out (NRRO) of the spindle and, as stated above, it also contains the measurement noise coming from the test setup and the devices used, which could contribute significantly in an uncontrolled test environment.

In general, the performance of a rotary stage depends on the bearing type and diameter. Larger diameter stages provide better performance than smaller units, air bearings provide better precision than mechanical bearings. A high performance controller can also improve performance by smoothing out torque ripples of the motor.

Conclusion

To address customer questions regarding the repeatability of eccentricity, it is essential to provide contextual information about rotary stage tests. By doing so, we can gain a deeper understanding of the value associated with eccentricity and the various factors that contribute to it. It is important to note that there are both controllable aspects related to design and assembly, as well as inherent environmental factors that influence the overall error.

Note: For those interested in the actual ANSI/ASME Standard, please refer to ASME B89-3-4 2010. Rev2019. Axes of Rotation: Methods for Specifying and Testing


Product Overview


More Reading

Subscribe to Tech Blog

Sign up to receive an email of new blog posts.

Please read the following and confirm your consent to the processing of your data.

By submitting this form, you consent to the PI Group contacting you through the communication channels you specify. Additionally, you acknowledge that the PI Group may store, process, and analyze the necessary data.

For further information, see our >> Privacy Policy.

Each email includes an unsubscribe link that allows you to withdraw your consent.

Privacy Policy*