> Sensor utilizing laser triangulation
 


For the thickness measurement of non-homogeneous materials

Introduction

This sensor is for measuring the thickness of non-homogeneous density materials such as foams or complex structures consisting of different layers.

There is a wide selection of this type of sensor on the market. Most of them operate on the same principle: A laser beam is projected onto the material and the position of the point of impact is measured by a linear sensor (PSD). The position of the laser spot can be found by measuring the voltage at both ends of the sensor, (the position relative to the center is given by (Va-Vb) / (Va + Vb) where Va and Vb represents the voltage measured at the ends). In general, the measurement result is given as a voltage 0-10 V or current 4-20 mA.

The PSD principle systems work well for materials with smooth and well defined surfaces, but they become unusable on more complex surfaces, as in the case of transparent materials, foams or non-wovens. Indeed the linear sensor only calculates the centroid of the laser image.

There are more sophisticated sensors that utilize CCD cameras which provide complete image information, but their implementation is often difficult because they are generally not designed for industrial use.

This is why Scantech has developed its own laser triangulation technology for thickness measurement.





Principe

The principle is similar to that explained in the introduction, except that the linear sensor is replaced by a CCD camera.
When the upper surface of the measured material is in position A, the laser beam illuminates point A', which is seen by the camera at position A'' on the CCD
Likewise, if the material is further from the sensor at position B, the laser illuminates point B' which is seen by the camera in B''. If the laser diode is positioned so that the laser beam is perpendicular to the optical axis of the camera, points A' and B' are in the same focal plane. In this case, the system is linear, (i.e. the distance A''-B'' is directly proportional to the distance A'-B 'and also the distance AB.)



Using a laser beam spot shape is not reliable. Indeed, if the material has a rough surface, the cavities can block the reflection and in this case there is no image formed on the camera as shown in Figure 2.

Generally the measurement reading can undergo very large variations if the surface is not smooth because the reflected beam can take any direction.



Pour éviter cette limitation, Scantech utilise un faisceau laser en forme de ligne comme le montre la figure n°3.

Multiplying the number of points ensures good statistics and reliable measurement regardless of the surface.

The position of the laser pencil beam in the CCD camera is obtained by projecting the image on the horizontal axis (Fig. 4).

A mathematical algorithm allows the location of the material to be determined with an accuracy that is much better than the camera pixel size (Fig. 5).


Measuring the thickness of transparent materials.
A single sensor is sufficient to measure the thickness of a transparent product since reflections on A and B are simultaneously obtained (two peaks in the spectrum of light distribution).

Measuring the thickness of opaque materials.
When the product is opaque, one sensor measures only the distance that separates it from the material. It is sufficient to have a sensor on each side to determine its thickness as given by:


D = D0 – D1 - D2

It is important that D0 remains constant. Scantech has developed compensation to correct for variations of distances when the sensors are installed on an O-frame type scanner. But the measurement is always better when the sensor is installed on a C frame type scanner since D0 actually never changes.

Measurement of surface state.
The light spectrum dispersion is related to the material surface. Therefore this type of sensor actually provides a measure of roughness.

Characteristics

  • Nature of the material: The nature of the material, its density, and its internal composition does not affect the measurement.
  • Measurement Range: This sensor can accurately measure materials from a few hundred microns to several centimeters thick.
  • Good accuracy:In low thickness range the accuracy is typically 1% and is even getting better as the thickness increases to reach 0.1%.
  • Color: This sensor is unaffected by variations in color. It works as well on white or black material.
  • Brightness: It works as well on surfaces that are dull or highly reflective (eg stainless steel).
  • Measurement update rate: the sensor delivers one measurement every 20msec.
  • Good streak resolution: It is a few millimeters and depends on the speed of scanning.
  • Very easy to use: This sensor is very easy to use because no calibration is required for non-compressible materials. An offset is necessary in order to match the measurement with that obtained in the laboratory when the material is compressible
  • No administrative or legal formality required: No permit is required to operate this equipment since the laser power is very low.

Design

Laser Triangulation is a widely used technique and therefore appears quite simple to integrate into a scanner. Therefore, sensors that are available on the market are often used. In fact it is a technology that is far more complex than it seems and available commercial sensors are absolutely not suitable for online measurement.

Scantech developed its first laser triangulation system in 1997 and really propose the technology adapted to online measurement constraints:

  • No drift over time regardless of variations in temperature, pressure, humidity.
  • No influences due to the material or air temperature.
  • Air gap of several centimeters.
  • No influence of variations in ambient light or even spurious reflections.
  • Sensor that works on semi opaque materials.
  • No influence due to contrast changes on the material surface.