Determination of geometry errors of large coordinate measuring machines by multilateration

The measurement accuracy of coordinate measuring machines (CMMs) depends essentially on the accuracy of the guideways. For small and medium-sized CMMs an accepted and well-proven method is available to detect geometry errors to their full extent with the aid of two-dimensional reference objects (ball or hole plate). For coordinate measuring machines with at least one axis greater than 2 metre, this method cannot, however, be applied as reference objects of sufficient stability and accuracy are almost impossible to fabricate and hard to handle.

Especially for large CMMs a novel procedure for determining geometry errors was therefore developed at the PTB; similar to the well-known ball or hole plate method., It uses two-dimensional reference objects [Trapet, E.; Wäldele, F., 1991] but produces these "virtually" only in situ by multilateration. In selected planes of the CMM, two-dimensional sets of reference coordinates are determined by interferometric distance measurements. For this purpose, a rotatable interferometric length measuring system (e.g. a conventional laser tracker) is used in at least three or better four arrangements to automatically track an orientable corner reflector fixed to the probe of the CMM and to measure the interferometric distance for discrete positions. From the measured lengths coordinates of the selected measurement point positions are then calculated in situ. The differences between the calculated and the displayed positions of the CMM are analyzed in analogy to the ball or hole plate method to determine geometry errors.

Determination of geometry errors of large CMMs with four laser trackers

The advantages of the method are:

• flexibility of use for measurement volumes of different sizes
• high accuracy due to the interferometric determination of the reference positions
• direct traceability to the SI unit of metre due to length measurement
• compatible with the well-accepted ball plate method

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Literature:

- Trapet, E; Wäldele, F.:
A reference object based method to determine the parametric error components of coordinate measuring machines and machine tools. In: Measurement 9 (1991), pp. 17-22

- Wendt, K., Schwenke, H., Wäldele, F., Krawczyk, M., Kniel, K.:
Error mapping of large CMMs by sequential multi-lateration using a laser tracker

Contact: Dr. K.Wendt