Using approximately synchronised accelerometers to identify mode shapes: A case study

With advances in technology approaches for capturing acceleration data for a modal test on civil engineering structures is changing rapidly. This paper presents a study carried out to examine the feasibility of using a number of independent acceleration recorders to capture the mode shapes of a tied arch highway bridge. The acceleration recorders used comprise a MEMS accelerometer mounted on a circuit board, where data from the accelerometer is read to a memory card on the circuit board. These devices are intended to measure acceleration however, to calculate mode shapes the individual recorders need to be synchronised. For conventional sensing system this is easily achieved as data from all sensors is being recorded by a central data logger. For individual recorders this is not the case, i.e. each recorder is logging the data directly to its own memory. Where a unique identifier (i.e. clapper board type event) can be implemented simultaneously at the start of all the recorder signals this will allow the signals be lined up in post processing. However, there is no guarantee that the signals won't drift from one another subsequently. While the potential loss in synchronisation is a problem, gathering the necessary acceleration data using independent recorders is vastly easier than using a wired system. This paper shows for the tied arch highway bridge used in the study the independent recorders provided a very good estimation of the bridge mode shapes.