3D scanning systems help provide verification of the reflector dish
CGC Technology Ltd, based at Farnham in Surrey, design, manufacture and install tracking antennas for customers throughout the world. CGC employ approximately 30 highly trained staff and have been at the leading edge of satellite tracking for over a decade. They currently produce X/Y antennas and reflectors that range from 1.8m to 10m in diameter. These antennas are ideal for tracking Low Earth Orbiting Satellites for a variety of applications.
CGC were in the final stages of the antenna installation process and as part of the sign off procedure an inspection report was required. CGC decided that optical 3D scanning was the most efficient method available and contracted Physical Digital® for the project. The aim was to inspect the antenna and then depending on the results gained from the inspection, the reflector could be altered using the report information. A second inspection would then be carried out to verify the correct calibration of the antenna.
Objectives of the project
The client wanted to verify:
The perpendicularity of the X, Y axis and Z plane
The location of the receiver relative to calculated signal focus
The deviation of the dish in North/South/Zenith positions
The data capture process
The measurement of the antenna was carried out while on site, fully assembled. This was possible due to the portability of the 3D scanning equipment used and the ability to work in environments that would otherwise be impossible for traditional tactile CMM machines. The system used to capture the necessary data for this project was the GOM TRITOPCMM Photogrammetry system. All the 3D scanning equipment was transported within standard personal baggage allowances, which helped to reduce time and cost for the completion of the project.
The antenna was fully operational during the 3D scanning process
The antenna was fully operational at the time of the measurements; therefore allowances were made to allow the antenna to track its subject for periods during the day. The antenna was on the roof of a 4 storey building which would have been a problem for data capture using other metrology methods.
3D scanning using photogrammetry
The first stage of the verification was to confirm the correct assembly of the dish. This was done by applying markers to the front surface of the antenna, whilst in the zenith position, along with scale bars, and the photogrammetry was carried out. It was discovered that the dish had significant deviation from the intended geometry.
Engineers from CGC used this information to adjust the shape of the dish by moving the eight petals to the correct positions. The photogrammetry was then carried out again and this confirmed the correct assembly of the dish and the next stage of inspection could be completed. This is an iterative process which builds a better understanding of cause and effect of shimming the dish into the nominal position.
The next stage of the verification process was to confirm the perpendicularity of the X and Y pivots. Physical Digital began by placing markers (coded and un-coded) to the rear side of the dish and pivoting mechanism, Scale bars were also placed within the volume to be captured.
Two-dimensional photos were then taken with the antenna in the south facing position, with this data Physical Digital were able to create ‘Secondary Scale Bars’ within the software. These were created on a selection of points in positions that were stable enough to remain the same length over the series of measurements. This data was then saved. The ‘secondary scale bars’ were imported into the new project and the antenna was positioned in the Zenith position. Measurements were taken again to create a second set of data, this was saved for later inspection.
Markers were applied to the front surface of the dish, supports and receivers along with the scale bars. The photogrammetry was then carried out again in the North and South positions. This data was used to ensure the antenna maintained its geometry when in axial extremes.
Inspection and verification
The different sets of data captured during the project were inspected against each other to verify all criteria as required.
Deviation from CAD
CAD data supplied by CGC was used to align the points. From this geometry, labels were created showing the deviation of the points to the CAD. With this information CGC could then adjust each petal of the dish to calibrate the antenna correctly, however all the points were found to be within design tolerance.
From the data captured and due to the specifically designed positioning of the optical markers, cylinders were created on the pivot mechanism in both X and Y directions. These were used because these components are machine turned and an accurate fitting feature can be applied to the points. These cylinders were then inspected using a ‘Line, Line, Angle’ calculation within the software and the result was 89.97°, this was within the designed tolerances.
Focal Point Verification (relative to CAD)
The focal point of the system can be calculated based on the fit of the data to the CAD model of the dish, using an understanding of the RMS values and the direction of any localised deviation. Using this model the real life direction of the focus can be calculated. With this information the ancillary items such as sub reflectors and signal feed can be correctly positing
Deformation of Dish in North/Zenith/South Positions
The next stage was to evaluate and quantify any deformation that occurs during the tracking cycle of the dish. By repeating the measurement at 22.5° steps and aligning the data to a datum set of points in the centre of the dish the deformation was measured relative to the zenith position. At each position the values of the measurement points were exported into a spreadsheet to enable the calculation of the Root Mean Squared value for the dish (RMS) these values can be used to work out the theoretical focus of the dish and where this moves to throughout the cycle. The findings enables CGC to understand the ‘As manufactured’ results which directly influenced the design decisions that can aid future dish development.
Results and Achievements
A complete set of data was supplied to CGC, including:
Original scan data
- GOM INSPECT (Software for further analysis)
The project undertaken by Physical Digital® achieved:
Quick and accurate capture of all surfaces
Reduction in time, compared to traditional methods
Increases in accuracy, compared to alternative methods
Final product inspection using Computer Aided Verification
For further information or to discuss your inspection and verification project with one of our expert team, call 01483 750200 or contact us.