FEATURE TEST & MEASUREMENT
has the same thickness. The combined error of non-linearity from both sensors must therefore be compensated for. As a supplier of non- contact inline thickness measurement systems, Micro-Epsilon has developed its own methods of precise sensor alignment (which took three years to develop) and patented algorithms for this process.
ERRORS DUE TO NON- SYNCHRONISED DATA In order to avoid thickness calculation error due to movement of the target, both sensors must be perfectly synchronised so that they perform the measurement at the same time, at the exact opposite point of the target. If synchronisation does not occur, inaccurate measurement data is produced. For example, if measurements are taken at different time intervals, micro-vibrations of the target or of the sensor mechanics will result in a thickness measurement error. For example, for a time-delayed measurement of 1ms, a deviation of 125µm is produced (assuming 1mm vibration at 20Hz).
process or machine vibration as best as possible. Mounting with an O-frame is more stable than using a C-frame. Thermal expansion of mounting
materials is often overlooked as a source of large errors in precise thickness measurement. Therefore, selecting materials with as low a thermal expansion coefficient as possible is very important. For example, mounting sensors on a typical aluminium or stainless steel extruded profile, with a thermal expansion coefficient of ~16ppm/K, experimental testing has shown that just a 5°C change in ambient temperature can move the sensors by >80µm. In contrast, using a standard grade Invar mounting frame with a thermal
Thickness measurement of metal strip using Micro-Epsilon's thicknessCONTROL C- frame system
expansion coefficient of typically 1.2ppm/K reduces this to 6µm. Specialist Invar grades can reduce this error by half again. Of course, some suppliers of inline
thickness measurement systems provide automatic calibration features built into their system as standard, which can continuously compensate for thermal expansion errors. This feature also means the operator does not have to spend time calibrating and checking the system.
Special attention must be given to the alignment of sensors
ERRORS DUE TO NON-ALIGNMENT OF THE SENSORS Special attention must be paid to the alignment of the sensors, which are typically installed opposite one another. No misalignment, tilting or inclination of the sensors relative to the target object is permissible in order to ensure the sensor spots are measuring at the same point all the way through the measurement range. For example, for a misalignment of 1mm and an inclination of 2°, there will be a thickness measurement error of 35µm. In the case of a 10mm target thickness, this error increases to 41µm. Combined linearity errors of up to 8µm, for example, can be seen with a vertical target movement of just 200µm of the target in the measurement field even if the target
36 NOVEMBER 2017 | INSTRUMENTATION
POSITIONING OF THE SENSORS/MEASURING RANGE Position, measuring range, thickness deviation and vibrations must be taken into account when the sensors are installed. For correct thickness measurements, the target must always be located within the ‘measuring range’ of the sensors. If the target moves outside the measuring range at any time, this can lead to inaccurate measurements. In particular, any special operating conditions such as start, stop or speed changes must be carefully considered when positioning the sensors.
MEASURING RATES OF THE SENSORS Most thickness measurement applications are either in a process/quality control environment where the object to be measured is fast moving, or in a ‘part inspection’ machine where the object is static and the laser sensors are scanning the part in as short a cycle time as possible. In both cases, it is important to match the laser measurement speed to the spatial resolution or cycle time required. Faster measuring rates are not always best as the laser sensor accuracy can suffer on difficult-to-measure surfaces when using high speed measurement rates.
Micro-Epsilon
www.micro-epsilon.co.uk
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