A COMPARATIVE STUDY OF THE IMPULSIVE NOISE REDUCTION ALGORITHMS IN ULTRASONIC B-SCANS Ramon Miralles, Raquel Molina Instituto de Telecomunicaciones y Aplicaciones Multimedia (iTEAM) Universidad Polit´ ecnica de Valencia, Camino de Vera S/N,46022, Valencia, Spain phone: +3496 3879737, fax: +3496 3877309, email: rmiralle@dcom.upv.es, web: www.gts.upv.es ABSTRACT As the technology becomes cheaper and industries are con- scious of the quality control importance, ultrasonic inspec- tion becomes more popular. However, automatic inspection systems instead of manual ones should be designed if a suc- cessful implantation is desired. Signal processing algorithms play an important role on the designing of these systems. Unfriendly environment found in production lines cause that these systems should make intensive use of noise reduction algorithms. This work deals with studying the best algorithm for impulsive noise reduction in an industrial environment. A review of the typical impulsive noise reduction systems is done and particularized for the described situation. Finally, computational aspects are analyzed and some solutions are proposed. 1. INTRODUCTION In industrial applications of non-destructive testing using ul- trasounds is very frequent to find alternating current (AC) motors. These AC motors are commonly running in the vicinity of non-destructive testing equipment and are used for example for operating transport belts. The rotational speed of this engines is controlled by the Variable Frequency Drive (VFD). The VFD is a nonlinear device that introduces a large amount of noise that is picked up by the ultrasonic receiver electronics difficulting the flaw detection. In some situations the noise presence could be minimized with a good hard- ware and shielding design. However there are other situ- ations where it is not possible to make such a design, for instance impossibility to use relatively short cables between the transducer and the ultrasonic pulser/ receiver. In this sit- uations, signal processing algorithms can help to remove the impulsive noise and facilitate the design of the defect detec- tor. Additionally, it has to be taken into account the need of real time algorithms in industrial systems. Production lines work fast and the distance from product to product is in most of the situations a few centimeters, so that fast algorithms should be employed. In this work different algorithms for re- ducing the impulsive noise in the ultrasonic B-scans will be analyzed. The work will be structured as follows. In the next section we will study and model the noise. Later in section 3 different signal processing algorithms for reducing this noise will be proposed. In section 4 the presented algorithms will be tested in a simulated scenario (subsection 4.1) and finally in subsection 4.2 real examples will be shown. The work finishes with some conclusions. This work was supported by the national R+D program under grant TEC2008-02975 (Spain) and FEDER programme. 2. ON THE MODELLING OF IMPULSIVE NOISE INA TRAVELLING BELT SCENARIO Lets assume that we are doing an ultrasonic inspection of a material that is travelling on a transport belt of a production line. The ultrasonic transducer somehow fixed to the produc- tion line. We will assume that the coupling of the transducer to the inspected material is somewhat solved. We will as- sume also that material geometry allows doing this kind of inspection. If the ultrasonic transducer works in pulse-echo mode, B-scans are obtained. B-scans are defined as a suc- cessive number of captured A-scans (ultrasonic traces) for different positions. The following experiment was done to measure and model the effect of the ultrasonic noise in the B-scans as a consequence of the alternating current engine. A polyethylene rectangular box filled with distilled water was placed on the transport belt of the production line. Dif- ferent B-scans were acquired for different rotational speeds of the engines. The rotational speed was controlled vary- ing the frequency of the VFD. The VFD generates Electro Magnetic Interferences (EMI) that couple into the receiver electronics contaminating the B-scan in form of impulsive noise. The ultrasonic B-scans acquired with this experiment resemble a noisy background with a unique ultrasonic echo due to the reflection on the top of the water tank (see figure 1). This experiment was repeated for different frequencies of the VFD in the range f VFD = {0Hz - 50Hz}. This analysis allows to study the impulsive noise statistics. The figure 2 shows the measured probability density function of the cou- pled noise at the different speeds. The results obtained are similar to what some other authors have measured [1]. Time (samples) Position (centimeters) 500 1000 1500 2000 2500 3000 1 2 3 4 5 6 Figure 1: An acquired ultrasonic B-scan with impulsive noise (only the backwall echo should be visible). 17th European Signal Processing Conference (EUSIPCO 2009) Glasgow, Scotland, August 24-28, 2009 © EURASIP, 2009 1577