Coded excitation strategies to ensure the validity of ultrasound controlled tightening of bolted joints

Ultrasonic time-of-flight measurement is a well-established technique for accurately determining stress in materials. However, this method can be fragile and lacks robustness to interference in demanding applications such as process control for bolt and screw assembly. Depending on the test object, material, external influence, geometry, or other disturbances, the evaluation results in an incorrect preload and the process control fails. This paper describes several coded excitation approaches to ensure the validity of signals for ultrasound-guided tightening of bolted joints. The methods presented range from the use of a binary coded sequence (pure frequency modulation) for excitation, to inverse calculation (combination of amplitude and frequency modulation), to On–Off keying (pure amplitude modulation). The different approaches and the state-of-the-art sinusoidal excitation are evaluated in terms of side lobe distance and noise immunity by reducing the excitation voltage. We show that Coded stimulation allows excitation of ultrasonic echoes with high side lobe distance in their correlation function, even during the tightening process and based on challenging screws. The methods vary in technological complexity, computational effort and stability which need to be optimized according to the use case for translation to application.