Laser Trackers: Defining Accuracy (Part 2)

Distance Measurement (Ranging) Accuracy

As mentioned previously, the distance measurement systems typically found in a laser tracker are an IFM and ADM or even ADM only. Independent upon whether IFM or ADM systems are being used, the ability of these systems to detect and measure displacement is well known and documented.

Interferometers were typically used to measure the displacement between two points and therefore the product would be a distance between the two points. It follows that in order for a laser tracker instrument to measure distance optimally, it needs to be positioned in line with the points to be measured. In this case there is no influence from the angle measurement system.

Angular (Transverse) Measurement Accuracy
The angular accuracy of a laser tracker describes how well the instrument discerns angle measurements from its angular measurement encoders prior to processing them together with the distance or ranging element in the form of a coordinate.

Angular Measurement Accuracy versus Distance
With some laser trackers specified out at ranges of 50 meters or more, the pure ability for the unit to accurately measure angles is very important for respectable performance in the field. This becomes more or less important depending upon the volume of the object and where it is practical and economical to position the instrument in order to measure the points of interest.

Volumetric Accuracy
Volumetric accuracy is often used as a term to describe how accurate the instrument is for a particular measured volume. Where they exist, illustrated concepts of volumetric accuracy from manufacturers have to be aligned with the ability of the user to practically position the instrument in the real-life situation for their particular measured object.

Where measurement volumes are large it is sometimes more efficient for the user to reposition the instrument in strategic and practical positions to enable the viewing of all the required points of interest. The use of more instrument stations will have the effect of reducing the distance from the instrument to the points of interest which will also tend to weight the contribution of the angle measurement. This is especially true if there are practical limitations for the positioning of the instrument, placing more emphasis on the angular measurement capability in the quest to achieve an accurate set of coordinates.

In this scenario the following is relevant:
• The laser trackers have limited room to maneuver in the Z direction
• All points of interest cannot be seen from a single position
• Multiple laser tracker positions are required to achieve the required accuracy
• The highest possible accuracy is required
• The measurement volume dictates that the laser tracker angle measurement capability is exercised, especially in the Y (vertical) direction
• Measurement distances have been cut, but laser tracker angles are exercised more severely

To be continued…