Factors to consider for precise thickness measurement using laser displacement sensors

01 April, 2016

Thermal expansion of mounting materials is often overlooked as a source for large error 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 just a 5degC change in ambient temperature can move the sensors by >80µm! In contrast, using a standard grade Invar mounting frame with a thermal expansion coefficient of typically 1.2ppm/K reduces this to 6µm. Specialist Invar grades can reduce this error by half again.

Measuring rates of laser 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.

Therefore, selecting as low a laser measurement speed as possible to meet the spatial resolution or cycle time required will enable the laser sensor to receive as much reflected light as possible, which will result in more precise measurements.

Sensor compactness

For thickness measurement tasks where available space to mount the sensors is limited or restricted, the size of the sensor is critical. Non-contact laser displacement sensors are now available with very compact dimensions despite having a fully integrated controller.

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