Flexible Pipe Inspection with Terahertz Technology

In principle, two different terahertz techniques are used. In the TDS method (time-domain spectroscopy), very short terahertz pulses are generated and the thicknesses of individual or multiple layers are determined over their duration. The measuring range extends from 10 µm to a few millimeters, with a reproducibility of better than 1 µm. Up to 1,600 thickness measurements can be carried out in one second. The second method is based on an FMCW radar (Frequency Modulated Continuous Wave) and enables coating thickness measurements in the range from 100 µm to several centimeters. The thickness measurement is also carried out using a time-of-flight measurement. Single layers and multi-layer coatings are tested at measuring frequencies of up to 5,000 measurements per second. Both measurement methods can be integrated into an inline inspection system.

A mobile system is available for testing large, stationary objects or in the field – for example after welding plastic pipes or insulating district heating pipes. Ultrasound cannot be used for district heating pipes, as the insulation is made of foamed materials that attenuate the sound too much. A mobile terahertz system is available for these special measurement tasks.

Application Examples From Pipe Production

In these examples, the wall thickness of single and multi-layer smooth pipes is to be measured directly downstream of the extruder and upstream of the first cooling section in order to keep the process control section as short as possible. Conventional ultrasonic systems cannot be used here due to the increased pipe temperature and the plastic core inside the pipe. However, terahertz technology has proven to be a successful alternative. Both examples involve a foamed pipe, albeit with different wall thicknesses. In the first example, we examined a PP plastic pipe with a foamed core. The signal is evaluated by comparing measurement and simulation in real time to reliably determine the thickness with a low wall thickness.

In the second example, we tested a foamed polyvinyl chloride (PVC) pipe. Here, the individual layers are thicker and, therefore, accessible for both terahertz techniques. The measurement results of both methods agree very well. An additional measurement with the terahertz FMCW hand-held scanner along the circumference records the thickness variation of the individual layers.

Testing Corrugated Pipes in the Process

Due to their surface structure, corrugated pipes cannot be inspected with ultrasound during the production process. The shorter the corrugation – i.e. the more corrugations per section – the more difficult the pipe is to inspect. Terahertz testing, on the other hand, is non-contact and is suitable for measuring corrugated pipes. It should be noted that sufficient signals only reach the detector from pipe positions that are almost perpendicular to the incident measuring beam. The two corrugated pipes examined differ in the number of wave crests per unit length, the wave height (height difference between wave crest and trough), the pipe diameter, the wall thickness, and the feed rate. In both cases, the wall thicknesses were determined in real time. The corrugated pipe shown here has pronounced corrugation with an amplitude of 3 cm at a feed rate of 4 m/min. The wall thicknesses were determined with high precision in real time.

Simultaneous Measurement on Multiple Channels

The terahertz systems have a modular design and the hardware can therefore be expanded accordingly. Four-channel systems are used for TDS systems. We recently demonstrated an eight-channel FMCW test system. In all cases, the measurements and the associated evaluation are performed in parallel on all channels.

We have handed over a newly developed pipe inspection system to a global market leader, marking a milestone in the measurement of pipe thickness. Our inline measuring system enables the company to directly check the pipe wall thickness shortly after extrusion of the plastic used and thus also to directly readjust it in order to optimize the quality and efficiency of production.

Outlook: Versatile Applications for Terahertz Technology

Terahertz technology is an extremely versatile and industry-ready measurement technology that is particularly useful in the plastics processing industry and will become more widespread in the coming years. In addition to its use in pipe production, it is also widely used for measuring the thickness of hoses and coatings on plastic. One successful example of this is paint thickness measurement on painted bumpers.