A Low-Cost Excitation System for Operational Modal Analysis (OMA)

We introduce a low-cost excitation system for application in operational modal analysis (OMA) working with vibration motors as they can be found in mobile phones. In contrast to experimental modal analysis (EMA), OMA makes an impulse hammer with a force s

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A Low-Cost Excitation System for Operational Modal Analysis (OMA) Max Gille, Johannes Maierhofer, and Daniel J. Rixen

Abstract We introduce a low-cost excitation system for application in operational modal analysis (OMA) working with vibration motors as they can be found in mobile phones. In contrast to experimental modal analysis (EMA), OMA makes an impulse hammer with a force sensor obsolete. If, furthermore, the structural response is measured with a camera instead of multiple accelerometers, this setup does not require an expensive data acquisition system, because no signals need to be recorded synchronously. The excitation of the structure is carried out through vibration motors from mobile phones that can be freely distributed across the structure and which are controlled by a microcontroller. The excitation through many distributed light-weight motors, in contrast to usual methods like manually tapping on the structure, enables long measurements without interfering with the optical measurement. Camera based modal analysis makes very high spatial resolution possible without the need for many accelerometers or a scanning vibrometer. The setup is tested on two example structures and its potentials are evaluated. Keywords OMA · Modal analysis · Gradient-based optical flow · Non-contact displacement measurement · Photogrammetry · Random excitation

13.1 Introduction The use of a digital camera as a displacement sensor for dynamic structural mechanics has been a new development over the past few years. In contrast to the widely-used piezo-electric accelerometers, camera-based displacement identification offers the possibility of full-field measurements instead of having to choose few measurement points. There exist different algorithms to determine the displacements of a captured structure between two frames of a video [1], one very prominent technique is Digital Image Correlation (DIC) [2]. Computationally less demanding algorithms that are applicable for very small displacements are based on the optical flow equation [3]. These optical flow techniques rely on a continuous bright source of light. Operational modal analysis (OMA) is a way to determine the modal parameters of a structure without the measurement of the excitation forces. Historically, the method originated in the field of civil engineering, where large structures (buildings, bridges) can not be moved to a lab and excited with a force hammer. Instead, the forces acting on these structures due to wind or traffic are considered to be broad band and random. Under this assumption, eigenfrequencies, mode shapes and modal damping can still be extracted from a measurement of displacements, velocities or accelerations on the structure [4]. Operational measurements are also a useful tool in the lab for quick troubleshooting or to investigate non-linearities that are activated by putting much energy into structures. Furthermore, the investigation of different OMA techniques is best performed in a lab to enable reproducible basic conditions. Applying OMA in a