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The last operation after the above described rectifications is
necessarily, as in all photogrammetrie instruments, the testing of the
precision of the whole instrument carried out with calibrated grids.
Before beginning the test readings it is necessary to check the
perpendicularity between the axes X and Y, to adjust the backlash of
the screw nuts and finally to check the eventual plays in the moving
parts. The grid plate used to check the perpendicularity between the
axes of the co-ordinatometers is always the same and it is severely
checked with a Genevoise laboratory equipment.
The coordinates of the grid plate points are known with a Root Mean
Square Error smaller than 1 micron. In order to measure the error for
the orthogonality, readings of 9 points are made first, then, after
having introduced the eventual correction in the way already mentioned,
the coordinates of the 25 points of the grid plate are read.
The residual perpendicularity error is not higher that 3" to 4".
The elimination of the plays is made repeating several collimations
of points suitably distributed on the grid plate, and making the
collimation by approaching the measuring mark from opposite directions.
This operation is particularly delicate; it is possible to eliminate
almost completely the play of the screw-nut considering the particular
type of screw-nut used. The residual error (checked on a test bench
also) is most of the times in the field of 1 micron. In order to
eliminate the plays in the kinematic system, it is necessary to operate
on a satellite gear housed in the gear-box connecting the servomotor and
the Quantizer to the measuring screw.
When the adjustment is made, the total backlash must not be higher
than 3 to 4 microns.
As in all instruments of the Stereocomparator type, even on the
AP/C the planimetric readings carried out on a grid, must be corrected
by the k rotation with an appropriate calculation. It is also possible
furthermore to introduce the correction due to errors in the length of
the screws. On this matter, it is wise to remember that all measuring
screws for Analytical Plotters are submitted to rigorous and severe
testing and therefore to a sorting for the couplings on the instrument.
In fact the electronic Computer can bring about corrections on the
systematic errors in length of the measuring screws both for the two A'
coordinates and for the two Y. coordinates.
For the calibration of the lead screws used in the Analytical
Plotters a special equipment has been built. It mainly consists in an
AP/C Co-ordinatometer, a rule calibrated with the precision of the micron,
and a microscope.