23 September, 2017

Factors to consider for precise thickness measurement using laser displacement sensors

01 April, 2016

If measurements from two non-contact laser displacement sensors are evaluated together, the thickness of an object can be determined. However, the accuracy of thickness measurements is determined by a number of important factors and not just linearity, writes Chris Jones, managing director at Micro-Epsilon UK.


Non-contact laser triangulation displacement sensors can be used to measure distance, movement and dimensions. If the measured values from two separate laser displacement sensors are evaluated together, the thickness of an object can be determined from this. Often, the thickness measurement accuracy expected by the user is based solely on the datasheet sensor linearity and resolution statements. However, many more factors not normally considered can result in thickness measurement errors much greater than these values, often an order of magnitude greater. Therefore, in order to achieve an accurate, precise statement of the target thickness, these factors need to be considered.

Alignment of the sensors

Special attention must be paid to the alignment of the two sensors that are installed opposite one another. No misalignment, tilting or inclination of the sensors relative to the target object is permissible. For example, for a misalignment of 1mm and an inclination of 2 degrees, there will be a thickness measurement error of 35µm. In the case of a 10mm target thickness, this error increases to 41µm.

Synchronisation

In order to avoid interference 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.

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.

Arrangement of laser sensors

Non-contact laser displacement sensors should not be installed until the running direction of the target object has been specified. In this way, higher thickness measurement accuracy and smaller deviations caused by the target surface will be achieved. Shadowing of the laser beam path can also occur if the sensor is incorrectly installed.

Mounting of the sensors

In addition to the above requirements for sensor positioning and alignment, a mechanically and thermally stable sensor mounting frame should be used for the laser sensors. 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.




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