Thickness measurement method
Thickness measurement processes play a major role in production and material testing, especially when it comes to non-destructive processes.
There are offline thickness measurement methods that are applied manually and not on the production line. The thickness measurement of a coating, a wire , a foil or a tape can be done offline with contact, but non-destructively by tactile measurement ( micrometer , caliper , ultrasound) or destructively by examining a cross section with a measuring microscope ( object micrometer or measuring eyepiece).
Industrial thickness measurement processes, i.e. automatic measurement directly in the production line, are referred to as inline measurement processes.
A wire thickness measurement can be carried out optically (shading a fan of light on a CCD line ), microscopically or mechanically (caliper).
It is often necessary to determine the layer thickness on a wire (e.g. insulating layer on enamelled copper wire ), it can be done capacitively or by determining the amount of charge carried away by the moving wire .
Process for inline measurement of sheets and strips
In industrial production, deviations from the nominal thickness should be sorted out and corrected by means of thickness measurement. Criteria are the thickness in the middle of the strip in the longitudinal direction and also the transverse profile.
In the production of metal strip ( coils ), a distinction is made between the process lines cut-to-length line (the strip is cut across into sheet metal) and length-to-length line (the strip is cut lengthways into strips).
In the case of slitting lines, the cross profiles of the strips are also measured in front of the slitting shears for reasons of space and strip calming. To check all strip strips, the sheet thickness must be measured in the complete cross profile. The measured values can be assigned to the later strips with different widths. The cross profile thickness measurement covers strip widths of up to four meters across the material flow. While the sensor system is usually held in the measuring location with a C-frame in the case of a strip center thickness measurement, the movable sensor system in a traversing measuring track is e.g. B. for reasons of space carried by a static O-frame. The thickness profiles along and across the slit strips are determined and documented individually for each coil . Thus, the longitudinal position of detected tolerance violations is also known for each strip. This seamless quality documentation enables the products to be selected according to order and customer.
The motivation for 100% control of the strip thickness is to meet today's quality requirements of automotive end customers and a measurement system analysis (MSA) according to ISO / TS 16949 .
Tactile thickness measurement
The tactile thickness measurement consists of two measuring probes , which scan the material to be measured vertically and collinearly from the top and bottom . The distance between the two buttons is determined beforehand with a referencing.
The measurement process is traceable .
The disadvantages include the influence on sensitive material to be measured and wear on the probe heads. This led to the development of non-contact measurement methods.
Radiometric thickness measurement
The radiometric thickness measurement takes place continuously and without contact. A distinction is made between measuring methods with radioactive isotope emitters or X-ray emitters as the source. Furthermore, a distinction is made between radiometric thickness measurement using the transmission method (source and detector on opposite sides of the material to be measured) and backscattering method (source and detector on the same side of the material to be measured).
The measurement methods have been established and widespread in industry at the latest since the availability of new isotope sources in the 1950s. They are insensitive to external temperature changes. However, changes in the material or alloy affect the measured value in a similar way to changes in thickness. In the case of β and γ emitters , the absorption also deviates significantly from the exponential law when there are large changes in thickness. A calibration is therefore required specifically for the target to be measured (alloy, thickness). The measurement accuracies achieved with such systems are less than 1 micrometer deviation with a stable alloy.
The use of radiometric thickness measurement places increased demands on radiation protection.
Optical thickness measurement
The optical thickness measurement is carried out according to the tactile thickness measurement with two distance sensors, which measure the distance to the strip material from the top and bottom - arranged collinearly . With referencing, the distance between the two sensors is determined iteratively (e.g. at the beginning of a measurement). The industrially used distance sensors often measure without contact using the principle of laser triangulation .
The measurement takes place discretely in time with measurement rates of typically a few 10 kHz.
Other physical processes such as confocal sensor technology come among others. a. Due to increasing requirements (reflective surfaces, higher accuracies, etc.), they are increasingly used.
The triangulation sensors used for thickness measurement are designed with a laser point, optically extended laser point or discrete laser line (with many laser measuring points simultaneously), depending on the main application. The discrete laser line also enables the measurement of tilt angles in the direction of the line and their complete compensation.
Common to all triangulation is a strictly defined range for the upper and the lower sensor and the measuring range proportional scaling uncertainty . There are also triangulation sensors (laser line) which, through selective deactivation of pixel lines in the image sensor, contain not only a maximum measuring range but also smaller measuring ranges in a triangulation sensor. The delimitation of the image sensor area used allows a higher measuring frequency, which is naturally lower with laser line technology than with laser point sensors with typically only one pixel line used.
The measurement errors of both triangulation sensors, which are independent of each other, are included in the measurement accuracy of a thickness measurement system. The accuracy or linearity of a laser triangulation sensor on simple (matt) surfaces is in practice no better than 0.01% of the measuring range for point lasers, and no better than 0.1% for line lasers (e.g. for ± 2 mm measuring range: ± 2 µm at 2 sigma). However, with line laser technology, more than 1000 measuring points are obtained at the same time. It is therefore less dependent on optically difficult (shiny) surfaces or tilts that are recognized, measured and compensated for.
A high precision of the thickness values is achieved through sliding averaging in the material transport direction (typically a few millimeters) . The resolution given by many manufacturers, on the other hand, says practically nothing about the measurement accuracy .
The thickness measurement using laser triangulation is traceable . The operator is always able to carry out a measurement equipment capability verification (audit).
Compared to radiometric thickness measurement, optical thickness measurement has a high spatial resolution and, in relation to tactile thickness measurement, a quick reaction to changes in thickness is possible. With these advantages, optical thickness measurement enables the approach to special measuring tasks that cannot be carried out with tactile measuring systems, isotopes and radiometers. This includes B. the dynamic roller control, the measurement of corrugated sheets (thickness of the base sheet and the corrugation height) or the thickness of "sandwich sheets" made of inhomogeneous material (structures made of aluminum with air content, structure similar to corrugated cardboard).
The optical thickness measurement works unaffected by the material alloy.
The maintenance effort of the optical thickness measurement is low. There is practically no wear with the measuring principle. The service life of the laser diodes used with outputs in the lower milliwatt range is typically> 40,000 h.
With the highest accuracy requirements, temperature fluctuations in the measurement technology must be compensated or prevented. One approach to compensate for errors (e.g. due to temperature changes) is regular (preferably automatic) referencing, both before the start and iteratively during the measurements.
Other physical methods for non-contact thickness measurement
In general, capacitive sensors are also possible (high accuracy, but sensitive to emulsion or oil). Here, however, the physical measuring gap is only somewhat as large as the measuring range.
Eddy current sensors can be used for static measurements, but not on ferritic, moving material. These sensors are more stable against environmental influences such as oil or emulsion.
literature
- Walter Wegener, Heinz Bechlenberg: Comparative investigations on measuring devices for the continuous determination of the material unevenness. Springer Fachmedien Wiesbaden GmbH, Wiesbaden 1970.
- Siegfried Methfessel: Thin layers, their production and measurement. W. Knapp, 1953.
See also
Web links
- Thickness measurement method patent specification, publication number EP0324896 B1 (accessed on October 15, 2015)
- Vapor deposition in a high vacuum and layer thickness measurement (accessed on October 15, 2015)
Individual evidence
- ↑ Werner Stolz: Radioactivity - Springer . doi : 10.1007 / 978-3-663-01497-3 ( springer.com [accessed September 5, 2016]).
- ↑ LG Erwall, HG Forsberg, K. Ljunggren: Radioactive isotopes in the art - Springer . doi : 10.1007 / 978-3-663-02872-7 ( springer.com [accessed September 5, 2016]).
- ↑ Radiation Protection Ordinance - StrlSchV
- ^ Bela G. Liptak: Instrument Engineers' Handbook Process Measurement and Analysis . 4th ed.CRC Press, 2003, ISBN 978-0-8493-1083-6 , pp. 1049 .
- ↑ Dirk Jansen: Optoelectronics - Springer . doi : 10.1007 / 978-3-663-05975-2 ( springer.com [accessed September 5, 2016]).