QMT Features: May 2008
Multisensor metrology
With no ISO standards yet released, how do you establish measurement uncertainty and the capability of the measurement process for optical and multisensor coordinate metrology systems?


Every measurement of dimensions such as size, angle, radius, form and position on workpieces is subject to a certain measuring uncertainty. The entire measuring process including the machine technology, the attributes of the part, the geometry of the features measured, the environment and the operator all influence the magnitude of this measuring uncertainty.

The geometry of the features has an especially strong influence on the results of real measurements.Thus, using identical machine technology, the radius of a sector, for example, can be measured much less accurately than that of a full circle. When measuring angles or axis directions, the length of the sides are calculated directly into the measuring uncertainty (Fig. 1). Other part attributes such as form, roughness and contamination exert additional influence.

For multisensor coordinate measuring machines, the parameters of the sensors are especially important for the attainable measuring uncertainty and must be added to the other machine attributes. Classified according to five important sensor types, Table 1 summarizes which parameters influence the measuring uncertainty of the machine and of the entire process.

Various methods can be used to determine the measuring uncertainty. If only measures of length are used, the maximum permissible error MPEE can be used for assessment purposes. However, this value does not actually constitute the uncertainty and is only used to evaluate a particular case. Improvements of results (for example, such as those achieved by measuring a large number of points or through mathematical best fit) and the negative influence of attributes of the workpiece are not taken into consideration. According to the “Guide to the Expression of Uncertainty in Measurement” (also known as the “GUM”), the measuring uncertainty should be determined by mathematically superimposing the individually evaluated error components (error budget). The procedure described below is based on this principle.

The measuring uncertainty can be assessed for a subdomain of tactile coordinate metrology by means of mathematical simulation (a virtual coordinate measuring machine). This process is described in the standard ISO 15530-4. This standard is not yet available for optical or multisensor coordinate measuring machines, since reliable error simulation has not yet been mastered for optical sensor systems.
The standard ISO 15530-3 contains a process for determining the measuring uncertainty by measuring calibrated workpieces. This technique can also be used to determine correction values (substitution method) which can be used to substantially reduce the systematic commonly used in the measurement of gages and shafts, for example.

This method does not take into account the influence of changing workpiece surface attributes such as the position of ghost lines, colour, and radiant reflectivity. Testing of real parts is still the most reliable method. This method has often been used to assess the overall measuring uncertainty. It is described in numerous company standards and has been introduced under the term “Measurement System Capability”.

Through representative measurements, both the repeatability and the traceability of the measurement to individual externally calibrated components are checked. The repeatability of the measurement is checked by continuously measuring different parts of the same type (representatives of a typical manufacturing process) and jointly evaluating them.

Ambient influences, influences of the workpiece itself (surface, colour) and influences caused by the operator (clamping and unclamping) can all be examined in conjunction with random errors caused by the measuring machine. However, in order to obtain the total measuring uncertainty, influencing parameters not taken into account during the test phase (i.e., long-term temperature fluctuations) must also be assessed.

With multisensor coordinate measuring machines, it is also possible to alternatively perform measurements with precision sensors (such as the Werth Fibre Probe) or calibrate parts on the same coordinate measuring machine. Systematic errors of dimension in optical measurements can thus be checked.
Capability of the Measuring Process
The capability of the measuring process is examined based on a comparison of the attainable (feature-dependent) measuring uncertainty, including all related influences, and the equally feature-related tolerance. A similar procedure is described in the company standards mentioned above.

General procedures for determining the measuring uncertainty and assessing the capability of the measuring process are provided in the German VDA Directive 5. Another VDI/VDE regulation containing similar information especially for coordinate measuring machines is in preparation. The capability of the measuring process can be ensured only if the measuring uncertainty is substantially lower than the corresponding dimensional tolerance. A ratio of 1:10 is generally required to ensure the capability of the measuring process. However, in cases involving dimensions with extremely close tolerances, it may occasionally be necessary to make concessions.

In any case, it should be ensured that the process tolerance is less than the drawing tolerance by an amount equal to that of the measuring uncertainty (ISO 14253-1). In the end, the lower the quality of the measuring technology, the higher the requirements regarding the stability and accuracy of the manufacturing processes. Any additional manufacturing costs thus incurred may be much higher than the extra costs for purchasing a modern, multisensor coordinate measuring machine.
The measuring uncertainty of the corresponding measuring processes plays an especially important role with respect to the interface between the supplier and the customer. The supplier has tolerance limits which he must maintain and guarantee in order to reduce the measuring uncertainty of his coordinate measuring machines (process tolerance). On the other hand, the customer must extend the tolerance of the measuring machines installed in his incoming inspection department to include the amount of the measuring uncertainty (Fig. 2).
Since he cannot make the supplier accountable for his own measuring uncertainty, the customer can only file claims if these extended limits are violated. This is especially likely to lead to a contradiction if the customer’s own measuring uncertainty is excessively high. The customer must instruct his own quality assurance department to refrain from using any parts for which a certain tolerance deviation has been measured. This applies especially in cases where the measuring machine used in the incoming inspection department has a measuring uncertainty equal to or lower than the one used by the supplier. To prevent this from happening, the customer should prescribe a closer tolerance to the supplier. The term “contractual tolerance” can be used to describe this situation. The following tolerance chain results:
Contractual tolerance = specified tolerance
 – measuring uncertainty of customer

Process tolerance = contractual tolerance
 – measuring uncertainty of supplier

Under these conditions, it can be ensured that the terms of the contract are clearly defined and verifiable. One example of this case is shown in Figure 3. Here, the tolerance for the supplier has been reduced to the contractual tolerance. This means that the customer can accept all parts approved by the supplier as within tolerance without having to make any concessions with respect to quality assurance. Since this procedure could result in higher costs, it creates a demand for sufficiently accurate coordinate measuring technology both in the supplier’s and in the customer’s inspection department.
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