Special attention must be paid to the alignment of the sensors , which are typically installed opposite one another .
ALIGNMENT & TARGET MOVEMENT
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 35 µ m . In the case of a 10mm target thickness , this error increases to 41 µ m .
Combined linearity errors of up to 8 µ m , for example , can be seen with a vertical target movement of just 200 µ m of the target in the measurement field even if the target 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 and patented algorithms for this process .
SYNCHRONISATION
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 125 µ m is produced ( assuming 1mm vibration at 20Hz ).
INDIVIDUAL SENSOR ACCURACY / LINEARITY
Each measurement sensor has its own measurement uncertainty , often referred to as ‘ linearity ’. This means that at any given point in the measuring range , the actual reading from a sensor can vary by a percentage of its measuring range . The challenge is that no sensor is the same , so errors caused by non-linearity can add or subtract in the final result . Micro-Epsilon has overcome this challenge by performing a combined , patented calibration of both sensors to create a new significantly improved thickness measurement linearity .
THERMAL BEHAVIOUR OF SENSORS AND / OR MOUNTING FRAME
It is extremely important to design a mechanically and thermally stable sensor mounting frame . The mounting mechanism should be isolated from 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 > 80 µ m ! In contrast , using a standard grade Invar mounting frame with a thermal expansion coefficient of typically 1.2ppm / K reduces this to 6 µ m . Specialist Invar grades can reduce this error by half again , but can result in a very expensive solution and does not solve the problem when the manufacturing tolerances can be single microns , as is often the case with battery production . Micro-Epsilon has solved this issue with integrated automatic thermal compensation to eliminate measurement errors due to thermal expansion .
In addition , Micro-Epsilon inline thickness measurement systems provide automatic calibration features built into their system as standard , which provide gauge capability results at the touch of a button . This also means the operator does not have to spend time calibrating and checking the system .
For more information , please call the Micro-Epsilon sales department on + 44 ( 0 ) 151 355 6070 or email info @ micro-epsilon . co . uk
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