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David Zimdars, Irl Duling, Greg Fichter, and Jeffrey White
Picometrix LLC, 2925 Boardwalk Dr., Ann Arbor, MI 48104 USA
ABSTRACT. The results of both a laboratory and factory trial of a time-domain terahertz (TD-THz) multi-layer gauge for on-line process monitoring are presented. The TD-THz gauge is demonstrated on a two layer laminated plastic insulation material. The TD-THz gauge simultaneously measured the total and the individual layer thicknesses. Measurements were made while transversely scanning across a 12 foot wide sheet extruded at high speed in a factory environment. The results were analyzed for precision, accuracy, and repeatability; and demonstrated that the TD-THz gauge performed in an equivalent or superior manner to existing ionizing radiation gauges (which measure only one layer). Many dielectric materials (e.g., plastic, rubber, paper, paint) are transparent to THz pulses, and the measurement of a wide range of samples is possible.
Keywords: Terahertz, NDE
PACS: 81.70.Fy
INTRODUCTION
The use of time-domain terahertz (TD-THz) non destructive evaluation (NDE) imaging has been well established as a useful sub-surface inspection method for aerospace materials such as non-conductive polymer matrix composites; and polymer, ceramic, and silica thermal protection materials[1-4]. In the current work, we expand the application of TD-THz methods and instrumentation to on-line factory process monitoring of thickness. We present the results of both a laboratory and factory trial of a reflection TD-THz multi-layer thickness gauge which measured both the total and individual layer thicknesses of a multi-layer plastic laminate material used in commercial building construction. Data was collected both in the laboratory and in a factory demonstration continuously over several days, showing the precision, accuracy, and long term stability of the instrument.
The multi-layer plastic laminate material measured in this demonstration is widely used in commercial building construction. The material is a two layer sandwich of thermo-plastic olefin sheets, with a fabric material separating the middle layer; which is pressed and laminated in large sheets. The layers must be thick enough to meet specifications but not so thick as to waste material. While the demonstration was made on a specific type of multi-layer sheet product, the TD-THz gauge is widely suitable for many other products. In general, measurements may be made on most dielectric materials (e.g., plastic, rubber, paper, paint).
The legacy measurement technology currently employed by the manufacturers of the multi-layer plastic construction material is generally based on ionizing radiation. A single ionizing radiation gauge (gamma, beta or x-ray gauge) can measure only the total material thickness. If a multi-layer product is being manufactured, then more than one ionizing
The TD-THz gauge concept can measure the thickness of both layers of the construction material simultaneously, replacing one or more ionizing radiation gauges with a single TD-THz gauge. Furthermore, in contrast to the ionizing radiation of x-ray, gamma, and beta gauges, THz energy presents no hazard to operators or bystanders. There is no need for ionizing radiation safety precautions, security to prevent theft of radioactive material, government mandated record keeping, or special costly end of life disposal of radioactive material.
The TD-THz instrumentation employed utilizes miniature fiber optic interconnected remote THz transmitter and receiver modules. These modules are connected to a rack mounted TD-THz control and analysis unit by flexible cables up to 30 m long. This allows the construction of a small, solid-state TD-THz gauge sensor head which can be mounted in existing ionizing radiation gauge mounts. The software developed is similar in user interface and simplicity to the existing ionizing radiation gauge software, but with the additional multi- layer measurement functionality. This approach will allow users of the gauge to easily replace existing ionizing radiation gauges with TD-THz gauges, and in addition receive the added capability of multiple layer measurements.
TD-THz Reflection Measurement of Multi-Layer Thickness
TD-THz instrumentation makes measurements with ultra-short near single cycle pulses of electromagnetic radiation (approximately 0.5 to 1 ps width). This corresponds to a very wide bandwidth from (0.1 to >3 THz). The instrument records a high fidelity trace of the measured electric field vs. time repeated in a waveform window, similar to a very high speed sampling oscilloscope.
The method of TD-THz measurement of multiple layer materials is shown schematically in Fig. 1 for heuristic purposes. A pitch-catch configuration of transmitter and receiver is shown on the left. The transmitter sends a TD-THz pulse into the two layer sample. Index of refraction discontinuities at each layer boundary will cause Fresnel reflections of the input pulse.
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