Coded Excitation Strategies to Ensure the Validity of Ultrasonic 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 can fail and an incorrect preload is calculated. This paper describes several coded excitation approaches to ensure the validity of signals for ultrasound-guided bolt tightening. 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. The technology, complexity and computational effort required to implement the methods are also discussed.