3D Anwendungen

Best Practice G 30

3D Reference Frame

How to build and calibrate an intelligent holding fixture for optical 3D measuring objects

Reference frames stand for secure, robust inspection of objects of small series in the optical 3D metrology, without painfully putting reference points on the surface of each object. They also help measuring objects which are much larger than the actually available measuring volume (measuring field of view).

A. Construction and accessories

Reference frames may vary in their form or size. The relation between frame profile and length of a frame bar, however, should be such that sufficiant stability and visibility are granted. The material used for its construction is flexion and torsion resistant light metal.

Fig 01 Construction of the 3D reference frame

In our example the 3D reference frame has lateral sizes of 80 x 100 x 120 mm, with a profile of 8 x 8 mm. It was manufactured by a CNC tool machine out of a solid block.

Fig 02 Elox coated frame with object and clamping accessories

In order to fix the object, we use M3 thread bars of inoxidable steel, clamps and other accessories like washers and screws.

To pass the thread bars, the frame has Ø3 mm through holes in 20 mm distance.

The reference frame has a black elox coating. It's fixed to the plate of the rotary table of the optical 3D measuring system by using spring clamps. To avoid lateral displacement, its position should be secured by cylindric pins.

B. Calibration

The reference frame is calibrated by using photogrammetry. Before doing this, its surface is applied on all faces with randomly distributed reference points of the adequate size. In our case, we used reference points of Ø0.8 mm size.

Fig 03 Photogrammetric Project Setup (Object: car rear closure inner sheet)

The photogrammetric project is prepared by placing - in addition to the uncoded reference points on the object - single and combined coded markers, on artifacts like crosses, and scale bars.

By this way, the operator is taking 2D pictures around the object, which are oriented one to another in the final 3D calculation. The scale bars serve to scale all recorded distances in high precision.

In our application of the 3D reference frame, each the upper and the lower side of the frame will be recorded as a single photogrammetric measuring series, each one resulting a reference point file in .refxml format. At this stage, both reference point files (point clouds) are still independant one from another.

C. Creation of the calibration file in GOM ATOS

We load both reference point files in a new GOM ATOS project. Each one gets its own measurement series. In the 3D view, it's obvious to see that many of the points are 'double'. Series ‚Scan1‘ has 386 points, ‚Scan2‘ 383.

Fig 04 Two reference point files in GOM ATOS

In the workspace 'Digitizing' it's the third icon from left in the upper tool bar to click on: 'transformation by common reference points'.

Fig 05 Transformation by common reference points

Suddenly, on of the two measuring series gets invisible, which is the later 'reference measuring series'.

We click on those green points in the visible measuring series that we asume to have common points in the other (invisible) series.

Around 5-6 points are sufficiant to go on. A clicked green point changes its color into yellow.

Fig 06 Selected reference points and residual error

A small residual error ( 0.002 mm) points at a rather good preparatory work of the photogrammetry project.

To collect all remaining common reference points for this transformation project of both measuring series, we click on the icon 'identify all reference points automatically'.

Fig 07 Automatic identification of common reference points

277 points (yellow) were automatically identified here. By clicking on OK we confirm this transformation with a residual error (deviation) of only 0.003 mm.

Both series of measurements are now visible again.

Fig. 08 Select all points of the measurement series

The next step is to select all points from the first series of measurements by right-clicking 'select all points of the element'. They change color to red.

In the properties we see 360 selected out of a total of 386 points.

We do the same with the second point cloud.

Fig 09 Point clouds 'merge'

From the menu 'Construct / point cloud / discrete point cloud from selection' we create a common 'merged' point cloud. We assign a name and click OK.

The common 'merged' point cloud appears in the element explorer under the element group 'point clouds discrete'. The merger reduced the number from 769 to 440 points.

Fig 10 Export reference points

Using the command 'File / Export / Geometry / Reference points (XML)', these reference points can finally be exported as a refxml file (calibration file).

By entering the correct expansion coefficient and the calibration temperature during photogrammetry, the new reference point frame with its calibration file can also be used at different working temperatures.

Page updated

Google Sites

Report abuse