PTB News 2/2002 (149 kB) PTB News 2/2002, English edition, Issue August 2002Large coordinate measuring machines are used for quality assurance in the automobile and aircraft industry. To monitor and calibrate these machines, a procedure, based on the multilateration principle known from geodesy, has been developed. It meets the increased accuracy requirements and, in particular, does not require reference objects such as, for example, ball plates.
Small and medium-sized coordinate measuring machines with measurement volumes below 1 m3 can be well monitored and calibrated by the ball plate method which measures calibrated ball plates at different positions in the measurement volume of the coordinate measuring machine. A comparison between measured and calibrated positions of the balls allows to calculate all error parameters.
For larger coordinate measuring machines with axial lengths of two meters and more, the size of the ball plates become too large to handle. The procedure now developed by PTB makes use of the principle of the ball plate method without needing the large plates physically. The test is carried out by creating a “virtual plate” realized on the coordinate measuring machine through a large number of interferometric distance measurements.
In the course of the test, a special optical reflector unit is mounted on the coordinate measuring machine instead of the probe head. It is moved to predefined positions on a plane. An automatically tracking laser interferometer (laser tracker) successively measures the distance to the reflector unit from four different positions (therefore: multilateration procedure). Processing the four groups of length measurements, a special software calculates a two-dimensional reference grid similar to a very large ball plate. The deviations of the coordinate measuring machine can be determined by comparison with the indicated positions. Very high accuracy can be achieved as the evaluation principle only makes use of the interferometric distance signal of the laser tracker. Under good ambient conditions, deviations for coordinate measuring machines with a measurement volume of 5 m x 2 m x 2 m were determined with uncertainties below 10 µm.
The patented procedure was successfully tested in cooperation with a German car manufacturer and further developed for industrial use by a company engaged in measurement technology in Braunschweig.