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August 21, 2003

3 Min Read
Higher film-quality demands put pressure on gauge makers

As end-user requirements and demandsfor film thickness consistency increase, a number of new developments in thickness gauges have-or are about to-come onto the market.

One of the most interesting is the patent-pending Isosint LESS (low emission solid state), a high-resolution system for detecting ionization radiation produced by beta-, gamma-, or x-ray sources. These retrofitable units are manufactured by Electronic Systems (Momo, Italy).

Sensors are heated or cooled to a constant temperature and are located on a thermo-regulated aluminum base, shielded from light. The 16 diodes provide one measurement every 100 ms, giving a band pass of 1 msec. This is up to 100 times faster than existing commercial gauges, which provide readings between 50 to 100 msec.

"There were a number of reasons for introducing this new device. It reduces measurement response time of variations in thickness and weight of the films being measured," says Piergiorgio Mora, Electronic Systems technical director. Sensitivity of the measurement is increased, which means less radioisotope activity is needed for the same quality of signal detection as in existing units, he says. Signal to noise ratio (SNR) is improved for faster measurements, and streak resolution in both cross and machine direction are improved.

Signals received from each of the diodes are amplified and sampled by a dedicated local microprocessor that processes and combines them via encoders in a data structure with other measurements such as gap temperatures and shift of web position. The information is sent to the main control computer via a 12 MB/sec Profibus communication line.

A comparison of the 16 signals in each position, with and without film, is made. The two matrices are evaluated at the same cross position but at different scan times. The difference in the signal distribution is compared, evaluating the shift between the emitter and detector, in order to adjust the zero profile. Mora says this feature is particularly important when the gauge is used on very wide scanners (>7m). Here, mechanical-structure deformations, due to environmental temperature changes on the shop floor, can affect measurement accuracy. Fast, accurate measurement can significantly improve a conventional, automatic cross control of the die bolts.

The first Isosint LESS prototype appeared in 2002 and has been tested at a processing installation originally equipped with a conventional sensor. The units should be available commercially by Q4 2003.

Based on magnetic induction, ElektroPhysik (Cologne, Germany) has a thickness measuring system which is said to be unaffected by humidity, density, or color. Unlike an air gauge, this sensor is unaffected by vibrations in the surrounding field. It can accommodate line speeds of up to 100 m/min and measures thickness from 10 µm to 6 mm with an accuracy of ±1 percent. EGS Gauging, formerly Eurotherm (Billerica, MA), introduced its Solution 21 system for film processors who need high-end measurement or minimal, automatic process control where a large investment in gauging and controls is not feasible.

NCD Infrared Engineering (Irwindale, CA) has come out with its 312 x-ray transmission sensor to substitute for beta gauging where radioactive isotopes are unacceptable. It provides sharper streak detection than beta gauges, has lower statistical noise, and is less sensitive to film flutter.

Thermo Radiometrie (Erlangen, Germany) has two thickness gauges which use either beta particles (RM200EK) or x-rays (RM210EK) linked to automatic dies with up to 1024 heated bolts for closed loop automatic control.

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