Portable CMM - Laser Tracker

3D Portable Metrology Instruments - Laser Tracker

The laser tracker, is a 3D contact measuring device able to measure three dimensional coordinate points, based on which dedicated measurement software applications create geometrical entities such as points, lines, planes, spheres, circles or cylinders. The probed coordinates are generally referenced to laser tracker’s local coordinate system.

The 3D measurement is done using a spherical probe (retro-reflective metallic probe) that contain an optical prism whose mirror intersection tip preciselly determines the sphere’s center. The measurement is taken with the probe touching the part to be inspected. The laser tracker determines the probe’s center position relative to the measuring head, and further, by software compensation of the probe’s radius, the measured point on the inspected part is determined.

By combining software options and a range of dedicated tools (probe holders) the dimensional inspector can determine not only discrete points but also surfaces, surface deviations (e.g. flatness deviation, form deviations for a range of geometric primitives), angles, the alignment of parts in an assembly, etc.

From a functional point of view a laser tracker is a polar coordinates measuring system, a 3D coordinate measuring machine able to determine two angles (azimuth and elevation) and a distance. By trigonometrical transformations the measured values are further transformed into carthesian coordinates: X, Y, Z. The two angles are measured by the encoders included in the tracker’s fine mechanical system.

The tracker emits a laser beam to the reflector’s center (CCR, TBR, etc), and the reflected beam is analysed by the distance reading system (ADM or IFM depending on the system) determining the laser head to reflector distance. The laser’s angular position is continuosly monitored by the highly accurate encoders that read the azimuth and the elevation of the measured point. 

By combining the three values (laser to reflector distance and the angles of the azimuth and elevation) the laser tracker determines the precise 3D position of the point measured in the reflector’s center. In order to have the measured point’s values on the part to be inspected, the dedicated 3D inspection software application substracts the reflector’s radius along the part’s normal vector.

Depending on the laser tracker’s type and model, the 3D distances are measured either absolute (ADM- Absolute Distance Meter) or incremental (IFM – Interferometer).

The incremental 3D measurement – IFM (Interferometer)

The IFM distance determination description: The reflector is placed in the initial position (Birdbath – a position whose 3D coordinates are known by the laser) and the read distance is analysed and corrected by the interferometer based difference between teh actual reading and the known position of the birdbath. Once the reflector moved, the laser tracker continuously reads the the coordinates values for azimuth, elevation and distance reported by the interferometer. The 3D probing can be continued as long as the laser tracker’s beam is not interrupted. Once interrupted by any obstacle interfering between the laser tracker and the reflector, an error message anounces that the reading is terminated and the reflector should be initiated in the birdbath.

Laser tracker ADM based distance measurement

ADM can measure the distances even after breaking the beam without the need of re-calibrating in the birdbath. With ADM the reflector’s position is constantly read, and the moment the beam is lost the system records the last read position before the beam breaking event occured. Once the reflector is re-positioned where it lost the beam, the laser tracker’s reading is engaged and the measurement can continue.

There are mulltiple advantages in using ADM, the main advantage being the fact that it cuts down the need of re-positioning the reflector in the birthbath every time the beam is broken. Another major advantage is the fact that the laser beam can be directed via software to certain coordinates where the reflector is positioned and ready to read the coordinates. Once the reflector is detected by the laser tracker, the 3D measurement can begin.

ADM also enables dynamic real time 3D coordinate readings during assembly alignments for example, where the software constantly displays the deviations enabling the 3D inspection operator to align the parts in place accuratelly.

The Advantages of Laser Tracker Systems

• High precission and accuracy of the 3D acquired data for large scale parts; 
• The mobility of the laser tracker instruments facilitates on site 3D inspections, dynamic analisys of the processes on the clients’ workshop or wherever they need precission 3D dimensional control applications;  
• The laser tracker systems use laser beams to precisely determine certain 3D coordinates for large scale parts (0-70meters, depending on the tracker’s model);
• Laser tracker instruments can be used in almost any environmental conditions;  
• Due to the fact that trackers can get 3D coordinates on a large spherical volume, they can measure from almost any 3D perspective;
• Precise 3D measurements – the coordination of the vertical and the horizontal encoders with the distance measurement system (ADM or IFM) allows for a very precise determination of the 3D measured points;
• Laser trackers are very well suited to align and position machine tools and industrial machinery;
• Laser trackers can be succesfully used during machining of large parts for real time inspections;
• Laser trackers can assist in aligning in poitiong assembly parts;
• During the alignment or positioning, laser trackers show the real-time position of the parts, supporting informed decisions;  
• Laser trackers can also be succesfully used in the alignment or positioning of automated processes, real time communication with the industrial robots or machine tools;

Typical applications for laser trackers

• 3D inspection of components and systems;
• Gravity measurement and positioning of parts, form fault, flatness inspection;
• 3D inspection of large scale parts and assemblies;
• Hi precission 3D measurement of mechanical jigs and fixtures;
• 3D inspection of windmill blades and turbines;
• Tri dimensional Reverse Engineering;
• 3D alignment and positioning of industrial machinery;
• 3D shaft and bore alignments;
• 3D inspection of repeatability, working position and kinematics;
• 3D thermal deformation monitoring;
• 3D measurement of antennas and sattelites;