Micro gears are applied in an increasing quantity in many applications. Therefore, precise measurements are of growing
importance to ensure their quality. This contribution describes the measurement of gears of a micro planetary gear set
with a tactile probe, a tactile-optical probe, an optical sensor, and computed tomography (CT).
For the tactile measurements, a high precision piezoresistive microprobe was used. A so-called fiber probe was applied
for tactile-optical measurements. This probe applies image processing to determine the position of the tactile probing
element. For all tactile and tactile-optical measurements, single point probing was used. The optical measurements were
carried out with an imaging sensor based on focus variation. Due to limited accessibility, on some gears not all regions
could be measured by the optical sensor and the tactile-optical probe. In contrast to this, with CT the whole part could be
measured with high point density. We used a micro-CT system and carried out measurements with Synchrotron-CT.
All the sensors used deliver measurement data in Cartesian coordinates. It is a challenge to transfer these data into
coordinates in which gear parameters are defined. For this, special attention must be paid to the determination of the gear
axis and to the orientation of the teeth.
The applied procedures are detailed for different micro gears. The comparison between data of different measurements
was carried out successfully. The deviations between the CT data and the tactile or tactile-optical data lie in the range of
only a few micrometers.
The tactile-optical probe (so-called fiber probe) is a well-known probe in micro-coordinate metrology. It consists of an
optical fiber with a probing element at its end. This probing element is adjusted in the imaging plane of the optical
system of an optical coordinate measuring machine (CMM). It can be illuminated through the fiber by a LED. The
position of the probe is directly detected by image processing algorithms available in every modern optical CMM and
not by deflections at the fixation of the probing shaft. Therefore, the probing shaft can be very thin and flexible. This
facilitates the measurement with very small probing forces and the realization of very small probing elements (diameter:
down to 10 μm).
A limitation of this method is that at present the probe does not have full 3D measurement capability.
At the Physikalisch-Technische Bundesanstalt (PTB), several arrangements and measurement principles for a full 3D
tactile-optical probe have been implemented and tested successfully in cooperation with Werth-Messtechnik, Giessen,
Germany. This contribution provides an overview of the results of these activities.
Optical sensors are gaining increasing importance in the field of coordinate metrology. Especially for micro range
measurements, different optical sensor principles (e.g. white-light interferometers, autofocus sensors, and confocal
microscopes) are used. Micro measurement covers the detection and evaluation of measurands for length, size and form
of geometrical structures in the range between 1 μm and 1 mm. These reduced dimensions lead to increased
requirements for the applied measuring technique and the verification of the measurement systems. The Physikalisch-
Technische Bundesanstalt (PTB) works intensively on the development of suitable measurement standards and test
procedures to make a broader industrial use of CMMs with optical sensors possible. The test procedures are analogue to
the well-established tests for classical coordinate measuring machines (CMMs). For this purpose, adequate and
miniaturized reference standards were manufactured, calibrated and tested considering the specific characteristics of
optical sensors. This paper gives a summary of this work. Advice on future developments is given.
Optical measurements are performed in coordinate metrology in a wide range of applications, ranging from large-scale
workpieces with dimensions of several meters down to microstructures with &mgr;m-dimensions, using a large number of
different measurement devices and sensors. These different devices and sensors must be tested to check their compliance
with the specifications, to trace back the measurement results and to compare different devices. Thus, appropriate
procedures and measurement standards (artefacts) are necessary. We present test procedures and artefacts for optical
measurement systems similar to the well-known test procedures for classical tactile coordinate measuring machines
(CMMs) according to the international standard ISO 10360. It is important that the surface characteristic of the
artefacts is cooperative to the sensor. Appropriate artefacts were realized and tested. The main focus of this paper is on
micro-artefacts. Furthermore, task-related standards to trace back specific measurement tasks were developed. By means
of micro-contour artefacts, the measurement characteristics on critical geometries can be evaluated and compared. By
means of a micro-gear standard, the measurement uncertainty on micro-planetary gear can be evaluated. The
measurement deviations of measurement systems in the automotive industry can be determined with the special artefacts
to test sensors for inline measurements.
Optical microsensors are used to carry out a great variety of coordinate metrology tasks on micro-parts. For the testing of
such sensors calibrated artefacts are needed. The existing micro-artefacts have smooth surfaces and can therefore only be
used for white-light interferometry and tactile probing. For sensors based on triangulation (structured light, autofocus,
confocal...), artefacts with optically rough surfaces are needed. Consequently artefact surfaces with a small mechanical
roughness but diffuse optical scattering (high optical roughness) are required. For this purpose, different production
techniques to roughen smooth surfaces and to form parts having rough surfaces are tested successfully at the
Physikalisch-Technische Bundesanstalt (PTB). The roughness Ra is about 0.3 &mgr;m. A suitable artefact set is currently
being developed in compliance with the existing standards. A first micro-artefact (micro-contour artefact) is already
commercially available. By means of the developed artefacts it also becomes possible to analyze for different optical
sensors the dependence between the uncertainty and the measured surface as well as the surface slope.
The frequency response of force-measuring microlaser sensors (Nd:YAG) is tested in a special opto-mechatronic test setup. In this setup a piezo translator/frame configuration generates sinusoidal forces in the range from DC up to 100 kHz and higher. The force amplitude covers a range of approximately eight decades (1 N - 10 nN). Amplitude and phase response of the test setup including the laser sensor under test are measured. Because of the frequency-analogue output signal of the laser sensor, dynamic frequency measurements based on frequency/period counting, Bessel spectrum analyzing and FM-demodulation have to be performed. To conclude on the dynamic response of the microlaser itself the mechanical part of the test setup and the laser sensor is modeled mathematically. The theoretical response of the test setup is in good agreement with its measured frequency response, which means that modeling of the force-to-frequency conversion by the laser sensor is realistic. Based on static and dynamic measurement data, we conclude an excellent proportional response of the laser sensor for modulation frequencies up to 100 kHz. In this frequency range, the characteristics of the force-to-frequency conversion are strictly linear over approximately nine decades.
Surface measurement is an important tool for quality control In our contribution we describe a novel application of reflection ellipsometry to profile measurement and material detection. The ellipsometric parameters (Delta) and (Psi) are measured by using a PSA-ellipsometer arrangement and a 4- zone-intensity-ellipsometric algorithm. To achieve a high lateral resolution we focus the laser beam on the surface through a microscope objective with high numerical aperture. The spot diameter is in the order to 1 micrometers . The focus adjustment is carried out by an integrated autofocus system. From the ellipsometric parameters we conclude to the refractive index of the local surface material and the local surface gradients. The height profile is calculated by an integrating and filtering algorithm. The material is ascertained form the refractive index. The feasibility of our novel microellipsometric measurement system is demonstrated by several tests. Measurements of material transitions and the height profile of a glass surface standard are presented.
We are reporting about a novel ellipsometric measurement procedure which enables us to detect forces and force- related sizes at high resolution by use of the laserinternal photoelastic effect. The measurement procedure is based on the intracavity transmission ellipsometry developed by us. Relative retardation (Delta) and orientation of the main axis p of the component under test can be concluded from the polarization state and from the beat frequency of the orthogonally polarized modes of an active laser. Our measurements of weight forces show a very good linearity in a measurement range of seven decades. The Nd:YAG-laser technology enables us to develop very small sensors at high accuracy.
In our contribution we describe a novel application of reflection ellipsometry to surface measurement. Not only the refractive index of the surface material but also the deterministic surface structure is concluded from the ellipsometric parameters (Delta) , (Psi) , and p. To achieve this, we here proposed two novel procedures: (1) the ellipsometric differential topometry and (2) the ellipsometric reference film method. The required measurement setup is described. To measure the ellipsometric parameters we developed a novel measurement procedure. Results of preliminary experiments are presented.