High precision assembly of large irregular shape workpiece with radian laser tracker

High precision assembly of super large workpieces

In some fields of industrial production, it involves the use of super large workpieces. Super large workpieces often have the characteristics of huge volume and weight, and based on the quality requirements of production, the manufacturing and assembly tolerances of workpieces are strictly limited. Therefore, the overall assembly accuracy of workpieces is difficult to be guaranteed by using traditional methods.

According to the summary of production practice, using Laser Tracker to detect the precision of such workpieces with large volume and weight and strict assembly tolerance requirements is the scheme with the highest efficiency, the best effect and the most prominent economic benefit.

Laser tracker can be used for large-scale and high-precision 3D measurement and detection. The measurement radius can often reach tens of meters, and the comprehensive measurement accuracy is at the micron level( μ m. 1 / 1000mm), and the use is relatively flexible. Just set the instrument at a suitable position next to the workpiece to be measured, use the target ball with built-in prism to touch the position to be measured, the tracker will shoot laser to the center of the target ball, lock, track and read the spatial coordinates of the contact position points and record them in the software. Through the spatial coordinates of these collected points, various lines, surfaces and bodies can be constructed, So as to accurately analyze the corresponding geometric tolerance and achieve the effect of measurement and detection.

Points of concern

Although the laser tracker has high precision and wide coverage, there are still some key points worthy of attention and summary in the assembly of some super large workpieces tens of meters and more than 100 meters, so as to give better play to the characteristics of the laser tracker and further improve the assembly accuracy and quality:

① Unification of accuracy standards

Although the assembly accuracy standard can be realized and reached within the effective working range of the laser tracker, the phenomenon that the near end measurement accuracy is higher than the far end measurement accuracy is based on the working principle and characteristics of the laser tracker.

This phenomenon occurs because there is an angle error in the measurement of the laser tracker. The measurement uncertainty range caused by the same angle deviation at the far end will be much larger than that at the near end.

As shown in Fig. 2, when the laser tracker measures the proximal plane P1 and the distal plane P2, the projection points of the laser on the plane should theoretically be circle center a and circle center B respectively; However, due to the angle error of the laser tracker α The laser may be projected at any point within the area of circle C and circle F; The area range of circle C and Circle F is the measurement uncertainty when the tracker measures the proximal plane P1 and the distal plane P2 respectively; Through the front view at the lower right corner, the area of Circle F is much larger than that of circle C, that is, the measurement uncertainty range at the far end is much larger than that at the near end.

In production, the assembly accuracy requirements of the whole workpiece are unified; Then, when a large workpiece is assembled with a laser tracker in a single station, even if its near and far end accuracy can meet the requirements under normal circumstances, the assembly points in different position segments will still be in different accuracy standards; If it is necessary to keep improving, further improve the assembly accuracy and make each section of the workpiece under the same accuracy standard, it is necessary to control the large measurement error range at the far end of a single tracker.

② Relationship between sites

When measuring the workpiece beyond the measurement range of the laser tracker or measuring the shielding surface of the workpiece, it is often necessary to move the laser tracker to another position to obtain sufficient measurement coverage. Namely, implement "station transfer" operation.

As shown in Figure 3, when transferring stations, it is necessary to set "transfer stations" (usually 4 points) at the positions that can be observed by both stations, use the laser tracker to measure these "transfer stations" at the two stations respectively, and then bring the two stations into the same coordinate system for measurement through the "best fitting" function of the software.

The advantage of using the "transfer station" + "best fitting" function for transfer station measurement is that it is relatively fast. It can realize the full coverage of the measurement range of conventional size workpieces by turning 1-2 stations, and meet the requirements of measurement accuracy at the same time.

However, when assembling super large workpieces, if the best fitting station transfer method is used, each new station will accumulate the error of each station in front, that is, the more stations are set, the greater the measurement error, and continue to cause the inconsistency of accuracy standards at each assembly point.

It can be seen that the assembly of super large workpieces also has strict requirements for the error of the relationship between tracker stations.

API super large workpiece high precision assembly solution

After summary and practice, taking the cases introduced below as a reference, the adjustment measurement method with laser tracker can be used to meet the requirements of users for ultra-high precision assembly of super large workpieces.

Measurement case: high precision assembly of oversized irregular shaped workpieces with a cumulative total length of more than 100m.

Workpiece Characteristics & measurement requirements: as shown in Figure 4, the cumulative total length of each section of the workpiece to be measured exceeds 100m; The maximum allowable error of the whole assembly is required to be 0.3mm.

Instrument selection: according to the measurement requirements and workpiece characteristics, API Radian Pro Laser Tracker is selected as the equipment for operation. The radian Pro laser tracker is integrated with IFM interference laser, and its ranging accuracy reaches 1 / 10000mm, which is higher than the comprehensive accuracy of the laser tracker (1 / 1000mm).
Measurement steps & key points

Step 1: analyze workpiece Characteristics & set up site

According to the characteristics of the workpiece, multiple laser tracker stations are set at the most appropriate position to form a high-quality coverage of the measurement range of the workpiece. When setting the station, it is necessary to consider the near end accuracy advantage characteristics of the laser tracker and the ranging advantage characteristics of the IFM tracker, and set the tracker station on the front workpiece within the best measurement range as far as possible (see Fig. 5).

Step 2: data collection

As shown in Fig. 5, at each station, the tracker measures and collects the data of all points to be measured within the visual range, forms a data network, and comprehensively records it in the software for final adjustment and analysis.

Step 3: data adjustment and analysis

After the data acquisition is completed, it can be set in the software to bundle all data for adjustment analysis, and take the data collected by the tracker of the dominant station as the higher weight to obtain the final overall measurement result under the unified accuracy standard and unified coordinate system.

conclusion

Actual measurement verification: the overall assembly accuracy of oversized irregular shaped workpieces with a total length of more than 100m is controlled within 0.2mm, which fully meets the tolerance requirements of 0.3mm.