Download MScMS (Mobile Spatial coordinate Measuring System)

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Chapter 5
phisticated instrumentation and it is carried out by measuring a standard reference artefact, with a priori known geometry.
Obviously, this calibration procedure is not valid for CMMs because of their different
technology and, in particular, their rigid structure. CMMs calibration can not be accomplished directly by the user, but requires a more complex procedure defined by international standards [ISO 10360, 2001]. In particular, CMMs calibration consists in a
sequence of manual activities that must be carried out once or twice a year, and requires highly qualified operators and complex instruments like laser interferometers.
• Verification. It is defined as: “confirmation through examination of a given item and
provision of objective evidence that it fulfils specified requirements” [ISO, 1993]. Another activity to make MScMS suitable for the measurement is the system verification.
It should be periodically performed to verify and adjust the measuring scale adopted
(for example, the ultrasound speed changes with air temperature and humidity). This
operation is performed by the use of a standard reference artefact [ISO 10360, 2001].
CMMs verification is done using some standard reference artefacts or repeatedly
measuring the same points to evaluate eventual measurements drifts. Different approaches have been proposed in this direction [Franceschini and Galetto, 2007]. Whenever a CMM does not fulfil specified requirements, highly qualified operators have to
intervene.
• System positioning. It is defined as: “operation establishing the initial position of the
constellation devices”. Every time MScMS is installed, a crucial activity is the constellation devices positioning. In order to locate the mobile probe, MScMS has to know
the position of constellation devices. This step needs to be completed before performing measurements and has strong effects on the measurements uncertainty. MScMS
software provides a semi-automatic procedure to achieve the constellation localization,
minimizing the user’s effort. CMMs do not need such procedure, due to the different
system technology.
5.3.4 Metrological performances
Dimensional measurement
The technology employed (in particular, the use of US transceiver) is responsible for
MScMS’s low accuracy compared to CMMs [Franceschini et al., 2008-II]. The use of US