Mecartex - Detailed Case Study
3-D probe with nanometer resolution
Abstract
With the continued trend in miniaturisation of electro-mechanical and optical systems there is an
increasing demand for accurate geometrical measurements on small parts. In the last years CMMs
(Coordinate Measuring Machines) have become versatile and widespread metrology tools. Today's
CMMs can efficiently perform very complex measurement tasks but up to now, limiting factors for their
application on small objects were the size of the probing element, the probing force and the accuracy
of the CMM-stage.
A new probe for Coordinate Measuring Machines (CMM) now allows measurements with a previously
unattainable accuracy.
Client
Swiss Federal Office of Metrology and
Accreditation (METAS).
In collaboration with: the Laboratory of
Robotics Systems (LSRO) of the ETH Lausanne
(EPFL).
Demand
The manufacturing of such small geometric
dimensions as pieces of consumer electronics
devices (e.g. cell phones, watches and medical
devices) requires highly accurate geometrical
measurements on miniature parts. Therefore, it
was identified the demand to overcome the
limitations of traditional CMMs.
On the initiative of METAS together with the
team of Mecartex and the ETH of Lausanne it
was launched a project with the goal to develop
new traceable 3D measurement capabilities on
small parts with submicron accuracy. The new
instrument should probe objects with very small
spheres and, because of the small contact area,
it should also use very low contact forces.
Solution
An ultraprecision CMM stage was combined with
the new probe head at METAS, leading to an
unprecedented CMM performance.
The team developed a 3D touch probe for
coordinate
measuring
machines
with
exchangeable probes with sphere diameters
from 0.1 mm to 0.3 mm and probing forces
below 0.5 mN. Based on parallelograms and
flexure hinges a new kinematic structure was
designed.
Mecartex SA - © Copyright 2009 - all rights reserved.
This structure leaves the probing sphere exactly
three degrees of freedom. The rotational
movements are blocked and the translational
motion is separated in its xyz-components,
which are each measured by inductive sensors.
Due to the special orientation of the probe head
coordinate system all axes are identical with
respect to gravity which results in an equal
probing force in all directions. The main part of
the structure is manufactured out of a single
piece of aluminium using electro-discharge
machining (EDM) and does not need to be
assembled. The flexure hinges have a thickness
of only 60 µm resulting in a stiffness of 20
mN/mm. Due to the low stiffness the
deformation caused by gravity needs to be
compensated. For this purpose an adjustable
system
with
permanent
magnets
was
developed. The measurement range is ±0.2
mm while the mechanical limits allow a tip
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deflection of ±0.5 mm in all directions. The
effective moving mass is 7 g. Having such a
small moving mass is important to keep the
dynamic contact forces low while maintaining
reasonable approach speeds.
The magnetic holding of the probing element
allows an easy tip replacement and cleaning.
Additionally, it acts also as mechanical fuse in
case of a collision. This highly sensitive device
is therefore quite robust and its handling
remains easy. Initial experiments performed on
a linear measuring machine equipped with a
laser interferometer showed a repeatability of
the probe head in the order of 5 nm.
Application fields
 Services for Industrial Metrology
 MEMS (Micro-electromechanical Systems)
manufacturers and users
 Watch Industry
 Manufacturing of micro-optics and fiber
 Medical Devices
 Automotive Technology
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