Ortenburg, Germany -A common method for measuring the thickness of plastic film is to use radioactive isotopes or X-rays. The extent to which the material attenuates the emitted radiation depends on the thickness of the film. Health hazards associated with these techniques, however, call for alternative methods of accurate thickness measurement. Example? The use of two sensors-one to measure the position of the roller, the other to determine the film's top surface. The difference equals thickness.

Micro-Epsilon Messtechnik has developed this technique for various types of film and manufacturing methods. With single-layer, homogeneous film where dielectric constant does not vary, a combination of capacitive and eddy current sensors applies. Integrated into one, the two sensors form a concentric design. The capacitive sensor supplies the thickness values; the eddy current sensor measures the position of the roller surface supporting the film to compensate for roller eccentricity and thermal expansion. This gives resolution and linearity better than one micrometer.

"Unfortunately," points out Johann Salzberger, Micro-Epsilon's director of marketing & sales, "you can't use this technique where the film consists of a number of co-extruded layers." Instead, Salzberger recommends a laser micrometer/eddy current sensor combination. Again, the eddy current sensor measures the position of the supporting roller. Here though, a through-beam laser measures the position of the top of the film. The laser transmitter produces a curtain of light that is detected at the receiver by an array of charge-coupled devices. The two measurements then pass to analytical software that calculates film thickness. This method, Salzberger says, produces a resolution better than two micrometers, and a linearity better than five micrometers.

If the film material has air inclusions or the laser light passes through its surface, Micro-Epsilon applies a contacting method using a precision ceramic ball. Employing a slight contact pressure, the ball follows the film surface, moving up and down as film thickness varies. A laser triangulation sensor, in turn, measures the top of the ball while an eddy current sensor detects the top of the roller on which the film is supported. Evaluation software takes the two measurements and calculates film thickness. Linearity performance is maintained at five micrometers and the resolution figure clocks up 1.5 micrometers.

Applying sensors in this way demands its own expertise. Salzberger again: "Combining sensors is a powerful technique for problems like this. It does mean, though, that you must have a wide range of sensors up your sleeve and know how to use them!"

Additional details

Contact Micro-Epsilon;3200 Glenn Royal, Suite 100, Raleigh, NC 27617; www.micro-epsilon.com