Measurement method and apparatus

#The present invention relates to an improved apparatus and method for correcting errors associated with measurements taken using coordinate positioning apparatus, such as coordinate measuring machines (CMMs) or machine tools.

#Coordinate positioning apparatus, such as coordinate measuring machines (CMMs) and numerically controlled machine tools, are well known and widely used in industrial inspection processes. In particular, it is known to use coordinate positioning apparatus to measure the position of multiple points on the surface of a part (e.g. a workpiece) to ascertain if it has been manufactured to within desired tolerances. Measurements acquired with any coordinate positioning apparatus will always have a certain level of uncertainty and many different calibration techniques have been developed over the years to improve the accuracy with which the position of points on the surface of a part can be measured.

#One type of calibration or correction method that is known for coordinate positioning apparatus comprises the generation of an error map that describes the difference between two sets of measurements of the same object. U.S. Pat. No. 7,079,969, for example, describes creating an error map that describes the difference between calibrated measurements of an artefact and measurements of that artefact taken at high speed using coordinate measuring apparatus. It is also described in U.S. Pat. No. 5,426,861 how an error map may be established that describes the difference between measurements taken using a reference CMM placed in a clean room and measurements taken using a shop floor CMM. Once created, such error maps are used to correct subsequent measurements.

#The invention will now be described, by way of example only, with reference to the accompanying drawings in which;

#FIG. 1 illustrates parallel coordinate positioning apparatus,

#FIG. 2 shows the parallel positioning mechanism of the apparatus of FIG. 1 in more detail,

#Referring to FIG. 1, a parallel coordinate positioning apparatus is shown. The apparatus comprises a bed 2 fixed to an upper or base platform 4 by a plurality of support struts 6. The support struts 6 are sufficiently rigid to ensure the base platform 4 is held in a fixed position relative to the bed 2. The base platform 4 is also attached to a moveable platform 8 by a constrained parallel positioning mechanism 10. For clarity, details concerning the parallel positioning mechanism 10 are omitted from FIG. 1 and the mechanism is shown in detail in FIG. 2. The base platform 4, moveable platform 8 and parallel positioning mechanism 10 thus form a constrained parallel positioning machine that controls translational movement of the moveable platform 8 along three axes (X,Y,Z).

#The moveable platform 8 carries a measurement probe 18 having a deflectable stylus 20. A part 24 to be measured by the measurement probe 18 is also shown placed on the bed 2 of the apparatus. A computer controller 22 is provided for controlling operation of the apparatus, in particular for controlling motion of the moveable platform 8 and for receiving measurement data from the measurement probe 18.

#The measurement probe 18 may be an SP25 probe as sold by Renishaw plc, Wotton-under-Edge, Gloucestershire, UK. The SP25 measurement probe is a so-called scanning probe or analogue probe that outputs measurements of stylus tip deflection in its local coordinate system. The probe 18 is moved (i.e. by movement of the moveable platform 8) so that the stylus tip traces a path on the surface of the part 24. The controller 22 receives stylus tip deflection data from the measurement probe 18 and data from the parallel coordinate positioning apparatus relating to the position of the measurement probe. These are combined to enable the position of multiple points on the surface of the part to be found in the machine coordinate system (i.e. relative to a fixed point or origin of the machine).