Ultrasound imaging uses high-frequency sound waves and their echoes to get a real-time view of materials. An ultrasound sensor transmits sound pulses or sound waves into the area of interest inside the volume to be imaged. The high-frequency sound waves of 1-5 megahertz travel into the area until they hit an interface between materials with different speeds of sound. The waves are scattered or reflected, depending on the angle between the incoming waves and the interface. Some of the sound waves travel further, reach another interface and are reflected. The reflected waves travel back to the sensing device, then acting as a receiver. The time of each echo’s return is logged and the distances from the transmitter/receiver to the different interfaces are calculated, considering the speed of sound within the material. Data are processed by inversion algorithms and the distances as well as the intensities of these echoes are displayed to present an image of two, three or four (movie) dimensions. The best results are obtained in applications where the balance of transmission and reflection along with the generation of echoes is facilitated, i.e. where the various materials have a comparable speed of sound.
The pulse-echo technique is useful when producing detailed images of a surface thanks to its high resolution (about 1 mm pixel width) and short capture time (less than 0.1 second per frame). Typically, it is used in navigation (SONAR), geology and in the medical context to produce images of internal organs or tissues. Other applications include leak detection, predictive maintenance and applications in the field of technical waste treatment). In the W2Plastics project, this type of imaging is used in quality assessment of materials and for separating different plastics. A four-dimensional imaging of interfaces between solids and liquids is performed in order to separate the plastics floating in a magnetic liquid.
In the project, a medical imaging system was tested to indicate whether it would be sufficient for monitoring the separation of plastics using Magnetic Density Separation (MDS) or not, including tests for real-time viewing of plastics in a moving ferro-fluid, quantitative through-put analysis (waste mass or volume) and for identifying materials (quality inspection). The hypothesis was that the medical imaging system might be an appropriate tool for these purposes since the produced image clearly shows internal interfaces between solids and liquids.
The testing indicated that the medical imaging system was suitable for all these uses except for materials identification (quality inspection). However, the potential for ultrasound technology as such in quality inspection was realized through these trials. In order to achieve the necessary responsiveness for detection of the acoustically distinctive material properties of the different plastics, the method of data scanning must be changed from reflection- oriented to transmission- oriented.
S.A. Sanaee and M: Bakker (2009): Ultrasound for Monitoring and Quality Inspection In MDS Plastics Recycling. ISWA-APESB, Lisbon, Portugal, 2009
F. Di Maio, P. Rem, B. Hu, S. Serranti and G. Bonifazi(2010): The W2Plastics project: Exploring the Limits of Polymer Separation. The Open Waste Management Journal no.3 2010, pages 90-98