Geometrical measurement

# Headline Geometrical measurement

Geometrical measurement is a way to collect data about the geometrical appearance (“shape”) of a given part. Parts with a simple geometry can be measured using simple tools like rulers and calipers.

More complex parts on the other hand require more sophisticated methods of measuring. Therefore, “Metrology” which is the science of measurement, brought various measuring technologies to life, each with its own benefits and drawbacks which ultimately define its spectrum of application. While not being a subject of research at the CTC in itself, the field of geometrical measurement is a crucial component in the process of quality control. Therefore, it is an essential part of most research and manufacturing projects conducted at the CTC.

Technologies

The field of geometrical measurement can be roughly subdivided into in tactile measuring principles and contact-less measuring. Tactile measuring involves a probe, which is brought into contact with a specific point of the part of which the specific position in 3D space has to be measured. By measuring multiple contact points on specific features of a part, geometrical information like e.g. flatness, roundness or concentricity can be acquired. Contact-less measurement usually involves measuring methods like laser triangulation, structured light, stereoscopy or x-ray tomography. Those measuring methods usually acquire larger areas of a part at once, resulting in much higher densities of measured points. Therefore, those techniques are more suitable for analyzing complex shapes, e.g. in respect to local deformation or deviation from an originally desired shape (e.g. CAD model).

Equipment

Measuring Arm
The measuring arm “Absolute Arm 8725” by “Hexagon” allows measuring small to medium sized parts. It is manually guided and can be equipped with tactile measuring tips as well as with a laser scanner (“Hexagon AS1 Scanner”). The measuring device, which is attached to the arm, is measuring point positions on the surface of a part relative to the measuring device. Since the measuring device is attached to the arm and all angles of its joints are recorded simultaneously with the measured points, the points' relative positions are transformed into absolute positions within the arms global coordinate system. By this procedure, complex shaped parts can be precisely digitized while using a manual measuring process.

Tracker + Scanner
The tracker/scanner system “Absolute Tracker AT 960” by “Hexagon” is a system comprising a fixed base station (the “tracker”) and a handheld measuring device. The measuring device can be equipped with tactile probes as well as a laser scanner (“Hexagon AS1 Scanner”). The measuring devices position and orientation are tracked by the tracker, which by doing so is a functional equivalent to a measuring arm (described above). The system is especially suited for measuring on big parts, as there is no physical bond to an arm structure with a fixed base required. On the other hand, a line of sight between the tracker and the measurement device must be ensured at all times while taking measurements.

Metrology Software
For recording, processing and analyzing measured data, the software “Polyworks Inspector” by “InnovMetric Software Inc.” is used. It allows the acquisition and visualization of measured data, as well as the combination of measurement data, which has been created after repositioning the measured part, e.g for better accessibility. The data can then be analyzed for specific information, such as local wall thicknesses or geometric deviations relative to an ideal CAD model. Exporting the aquired data (e.g. as a mesh model) is also possible.

Current research projects

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