Difference between Form and Postion and Centroid

[[Aa...]]

I am a ATOS Professional 2019 user inspecting airfoils. I need to explain to my customer the difference between the position measurements of "Profile Form and Position" versus "Profile Centroid". When I measure a sample of airfoil blades using the two methods, I get significantly different results. The Form and Position measurement is what we historically used to measure airfoil displacement, but a new part we're developing specifies centroid.

I believe that I understand Centroid better than Form and Position. Centroid uses the actual section to calculate 4 quadrants of equal area, and puts the centroid at the intersection of the quadrants. Is that correct?

How does Form and Position measure X-displacement and Y-displacement? With Centroid, at least I can visualize the actual point that it found for the centroid. However, there is no such option with Form and Position.

[[Se...]]

Hi,

The "Create Centroid From Seciton" function allows to create the centroid of the nominal and actual section. Then the centroid position can be checked for displacement.

The Form and Position requires a local coordinate system as input, and produces a curve check (with the form analysis information) and an actual coordinate system (with the best fit information) as a result. To measure the X and Y displacement, and the Z rotation, the corresponding checks can be created on the coordinate system.
More information can be found on the tech guide - ZEISS Quality Tech Guide 

Note that the nominal coordinate system must lie on the section plane. The origin can be defined as required: nominal stacking point, nominal centroid, or any other provided definition.

Hope this helps!

[[Se...]]

In short:

- With the centroid approach, the geometric centroid of the section is computed on nominal and actual separately. Then the position between both  is inspected.

- With the form and position approach, the position of the coordinate systems is inspected. This is nothing else than the resulting transformation of the best fit.