Up to this point, we have defined the term “accuracy” in how it relates to laser tracker measurement systems.
We reviewed angular versus distance accuracy, as well as volumetric accuracy.
In this final section, we will continue our discussion on volumetric accuracy, as well as cover some consequences of sub-optimal angular accuracy.
In order to illustrate how much angular errors dominate for a measured object, consider Figures 1 and 2.
Figure 1 – Points on measured object in line with the laser tracker
Figure 2 – Line between two points on the object perpendicular to the laser tracker
If the laser tracker in Figures 1 and 2 is deemed to be 2 meters from the measured object, the following typical MPE performance is attained from a measurement of the 2.3 meter length.
Assume use of IFM and a Typical MPE
Figure 1 = 3 micrometers
Figure 2 = 33 micrometers
Note the relatively large difference - this is because in Figure 2 the laser tracker’s relative position to the points which require to be measured dictate that the angular or transverse measurement system errors dominate.
Objects with features which require measurement are rarely offered up as depicted in Figure 1. Objects tend to be more irregular in shape and size, and very often not all points can be viewed from an in-line position. Typical examples might be large assembly tools for the aerospace industry.
Figure 2 orientation is more common with the scenario mentioned previously (in Part 2 of our discussion) coming into play if not all points can be seen from one position. The moment that multiple laser tracker positions come into play coupled with objects positioned to the laser tracker as depicted in Figure 2, means that the angular measurement system errors dominate.
It can therefore be concluded that the specifications and actual performance of the angular or transverse measurement systems onboard the typical laser tracker play a very important role in its day-to-day performance for the average user. It’s very easy to forget this fact when confronted with the specifications and by the outstanding performance of modern IFM and ADM distance measurement systems.
Selected Consequences of Sub-Optimal Angular Accuracy
Examples of sub-optimal angular accuracy are apparent for large assembly tooling within the aerospace industry. If angular accuracy is sub-optimal, the following could ensue:
• Poor initial reference system leading to a lack of accuracy and repeatability when setting and certifying the tool
• Poorly fitting parts leading to cost issues downstream of the assembly process
• Costly rework based on a poor signal from the measurement system (costs include labor and time tying up a tool which is on critical path)
If the angular accuracy is optimized, it is more likely that the tool will not have to be reworked during its first recertification due to measurement variation at least. Reworking tools such as these described can cost several thousand dollars.
Conclusion
Understanding accuracy terms is an important aspect of selecting the best instrument for a particular application. In the case of laser trackers, distance accuracy specifications are often not achievable in the end user’s application due to the limited measurement conditions that would be required. The majority of the time the laser tracker user is most interested in measuring points and dimensions that require movement of the encoders and place the application into the volumetric accuracy case. Thus it is the volumetric performance of the instrument that is most critical when considering a laser tracker for the majority of applications.
Click here to read the complete white paper
We reviewed angular versus distance accuracy, as well as volumetric accuracy.
In this final section, we will continue our discussion on volumetric accuracy, as well as cover some consequences of sub-optimal angular accuracy.
In order to illustrate how much angular errors dominate for a measured object, consider Figures 1 and 2.
Figure 1 – Points on measured object in line with the laser tracker
Figure 2 – Line between two points on the object perpendicular to the laser tracker
If the laser tracker in Figures 1 and 2 is deemed to be 2 meters from the measured object, the following typical MPE performance is attained from a measurement of the 2.3 meter length.
Assume use of IFM and a Typical MPE
Figure 1 = 3 micrometers
Figure 2 = 33 micrometers
Note the relatively large difference - this is because in Figure 2 the laser tracker’s relative position to the points which require to be measured dictate that the angular or transverse measurement system errors dominate.
Objects with features which require measurement are rarely offered up as depicted in Figure 1. Objects tend to be more irregular in shape and size, and very often not all points can be viewed from an in-line position. Typical examples might be large assembly tools for the aerospace industry.
Figure 2 orientation is more common with the scenario mentioned previously (in Part 2 of our discussion) coming into play if not all points can be seen from one position. The moment that multiple laser tracker positions come into play coupled with objects positioned to the laser tracker as depicted in Figure 2, means that the angular measurement system errors dominate.
It can therefore be concluded that the specifications and actual performance of the angular or transverse measurement systems onboard the typical laser tracker play a very important role in its day-to-day performance for the average user. It’s very easy to forget this fact when confronted with the specifications and by the outstanding performance of modern IFM and ADM distance measurement systems.
Selected Consequences of Sub-Optimal Angular Accuracy
Examples of sub-optimal angular accuracy are apparent for large assembly tooling within the aerospace industry. If angular accuracy is sub-optimal, the following could ensue:
• Poor initial reference system leading to a lack of accuracy and repeatability when setting and certifying the tool
• Poorly fitting parts leading to cost issues downstream of the assembly process
• Costly rework based on a poor signal from the measurement system (costs include labor and time tying up a tool which is on critical path)
If the angular accuracy is optimized, it is more likely that the tool will not have to be reworked during its first recertification due to measurement variation at least. Reworking tools such as these described can cost several thousand dollars.
Conclusion
Understanding accuracy terms is an important aspect of selecting the best instrument for a particular application. In the case of laser trackers, distance accuracy specifications are often not achievable in the end user’s application due to the limited measurement conditions that would be required. The majority of the time the laser tracker user is most interested in measuring points and dimensions that require movement of the encoders and place the application into the volumetric accuracy case. Thus it is the volumetric performance of the instrument that is most critical when considering a laser tracker for the majority of applications.
Click here to read the complete white paper
It is more complicated and expensive to measure accuracy, but it must be done for you to know the data of accuracy.
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Can a Laser Tracker be used for Alignment of a Ships Engine, Gearbox, Ships Stern Tube with the P Brackets and AFT A Brackets? If Yes, then with what accuracy over a distance of 60 meters?
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