This paper introduces the principle and design of an ultrasonic transducer system with auto-alignment mechanism. The proposed system is used for detecting foreign bodies in beverage containers. Variation in reflection amplitude is analysed as a function of the ultrasound beam incident angle to beverage container surface. It is concluded that there exists a quadratic relationship between the strength of the reflected signal and the incident angle. Furthermore, a calculation for effective angular increment for searching the normal of a curved surface is introduced. Experiments conducted using the prototype demonstrated that foreign bodies are detectable in various juices. Experiments also observed some false echo signals that occur due to curved container surface. Threshold in confined time region is therefore set to ensure no fail of detection in low signal
