> Low energy X-ray transmission sensor
 


The best choice when the cross profile must be finely controlled

Principle

This sensor is designed for the measurement of basis weight or thickness (when the density is constant) of relatively light or very light materials It is based on the transmission principle.

An X-ray tube located in the emitting head (source) produces a beam of X-rays which is detected and whose total energy (I0) is measured by the receiving head. When a material is placed in the space between the two heads (air-gap), part of the beam is absorbed, and only the transmitted fraction (I) is detected (fig. 1).



The transmitted beam is more or less attenuated according to the quantity of matter present in the gap. Figure 2 shows the relation between the thickness (constant density) and the remaining transmitted signal. The signal value is 100% when there is no material between the heads.

Knowing precisely this relation, and measuring, for a given material, the percentage of transmitted signal, one can determine the basis weight or thickness of homogeneous materials.

X-ray tube

An X-ray tube is an electric device which generates X-rays.
A filament (in red on figure 3) is heated by a current. It emits electrons around its surface. The target (in blue), is set to a positive voltage (accelerating voltage). The electrons are therefore accelerated and hit the target. The collisions cause the emission of X-rays. Obviously X-rays are emitted over 4π but are stopped by the tube wall except for those emitted through the window (in light blue).
The window material is selected to minimize X-ray absorption and an almost cylindrical beam of X-rays is formed (with a low taper angle of emission).

There are two principal advantages for using such a tube:

  • No emissions when the supply voltage is off. It is therefore very safe for maintenance interventions.
  • Selection of the beam energy which allows the measuring range to be optimized by adjustment of the accelerating voltage.


Detectors

Scantech develops and manufactures its own detectors. The detector of course is very important and to a large extent, it defines the performance level of the sensor. The control of detection technologies is one of the reasons for the high level of performance reached by Scantech X-ray sensors.

For proprietary reasons, Scantech does not disclose technical details about its detector technologies.


Characteristics

Scantech sensors have remarkable properties. It is the best solution for applications where a fast measurement and a precise profile are necessary, as in the case of controlling a die (e.g. cast and oriented films). They outperform by far infra-red or beta technologies.

Indeed, it is the only sensor to combine the following properties:

  • Astonishing range of measurement: For example with an energy level of less than 5kev (accelerating voltage lower than 5kVolts), polypropylene film from 2µm up to 2000µm can be precisely measured! This sensor reproduces very well the film edge shape of biaxially-oriented films (fig. 4 depicts a left edge).
    With a slightly higher energy level, the range of measurement is significantly extended. With 7kVolts one can reach 6 to 7kg/m2.
  • Excellent precision: At low energy levels, the X-ray absorption interaction with the measured matter is binary. An X-ray is either completely stopped, or it passes through the matter without any energy loss and with no deviation from its trajectory (as if there were no matter). Because of these only two possibilities one obtains the best possible statistics (binominal) and therefore the best accuracy. Figure 5 depicts the typical level of accuracy that can be obtained on a finished film with the measuring system controlling the die. It is a 6.6µm thin BOPP film (capacitor film). The finished film (5.5m wide) has a 0.026µm thickness standard deviation.
  • Very short response time: The scanner's excellent accuracy allows response times lower than 10 msec. Even at high scanning speeds, which are necessary to reduce the influence of the thickness variations in the machine direction, the profile is read correctly by the first scan. Thickness variations are seen with their real amplitude and are not smoothed at all. This is extremely important in order to have fast and effective die control.
  • The best streak resolution: The streak resolution is the best amongst all available technologies. For certain applications, Scantech has achieved resolutions of less than 0.2mm in the cross direction! The very straight X-ray beam, without any diffusion, makes such good streak resolution possible. The following figure depicts a profile zoom from a mesh application measurement, where the material pitch is 3mm. In order to obtain such a level of details the pencil beam width in the cross direction is collimated to 0.2mm!
  • Very easy to use: The sensor is very easy to use and no calibration is necessary for:
    • Any change in thickness or basis weight across the complete range.
    • Color changes.
    • Additive changes.
    Re-calibration can be performed in minutes when there is a change in filler proportion and then the new calibration coefficients are stored in a recipe.
  • Safe maintenance. There is no emission of X-rays when the sensor is not powered. Therefore there is no safety risk when performing maintenance.
  • In most cases, no administrative authorization is required. For a large majority of applications a voltage of less than 5kVolts is sufficient. In this case, there is no emission around the heads of the sensor. No prior approval, authorization, or regular annual control is required in whatever countries.

Design

Although this Scantech sensor is extremely simple to use, it is very difficult to design and requires a high level of expertise.

Scantech was the first company to introduce the x-ray technology into the market for the measurement of light materials (plastic, non-woven, etc.) even though the technology has been used with much higher energies for 40 years in metallurgical applications.

Scantech owns patents for the low energy technology sensors (less than 5keV) in principal countries, although the technology has been largely copied in recent years..

Nevertheless, Scantech is 15 years ahead in terms of technological developments in this field, and its sensors have reached an unequaled level of maturity:

  • There is no drift with time no matter what the variations are in temperature, pressure or moisture. There is no electronic drift (with a film of 20µm, the standard deviation on a 10 hours period is lower than 0.005µm).
  • No influences related to the temperature of the measured material. The sensor withstands material temperature changes without any variation of the measuring signal up to 40°C over the full web width at line speeds up to 550m/min.
  • The influence of relative misalignments of the sensor head during scanning is not significant. The measurement signal variation is less than 1/10000 for head misalignments up to 2mm!
  • Negligible influence of distance variations between the two sensor heads.