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Towards the Development of a MEMS-Based Health Monitoring System for Lightweight Structures
* 1 , 2 , 2 , 3
1  Politecnico di Milano, Dipartimento di Chimica, Materiali e Ingegneria Chimica "Giulio Natta", Piazza L. da Vinci 32, 20133 Milano, Italy
2  Politecnico di Milano, Dipartimento di Ingegneria Civile e Ambientale, Piazza L. da Vinci 32, 20133 Milano, Italy
3  STMicroelectronics, AMS Product Division, Via Tolomeo 1, 20010 Cornaredo (ITALY)

Abstract: We recently proposed a surface-mounted structural health monitoring (SHM) scheme based on commercial, low-cost inertial MEMS sensors. While such commercial-off-the-shelf sensors are not very accurate, their low cost and negligible weight allow them to be deployed in dense arrays, possibly overcoming inaccuracy through redundancy. Taking the sensor characteristics into account, the development of a MEMS-based SHM method for lightweight structures like thin plates, was tackled from two different viewpoints: sensor accuracy verification, and optimal sensor placement. To assess the accuracy, a preliminary investigation was run on standard composite specimens for delamination testing, adopting a single MEMS three-axis accelerometer. A theoretical interpretation of the results, based on beam bending theory, showed the ability of the system to provide a one-to-one relationship between the crack length and the sensor output. Concerning the placement of the sensors, an approach for their optimal deployment over thin structures was developed, using a topology optimization-like formulation. Such formulation is able to search for the optimal layout of the network, by maximizing the sensitivity of the overall sensors output to a damage possibly located anywhere. In this work, accounting for the characteristic sizes of a structural element and of the MEMS package, which might differ by orders of magnitude, we also introduce a multi-scale (actually, two-scale) approach to sensor deployment. It is shown that, no matter what the location, size, and shape of the damaged area are, a trivial array of evenly spaced sensors does not represent the optimal solution to monitor the structural health.
Keywords: structural health monitoring, composites, MEMS
Comments on this paper
Dirk Lehmhus
Weight of Sensor, weight of Board
Dear authors,
thanks for a very interesting and promising contribution, on which I am happy to start a discussion - looking at your presentation, I was wondering, you mention that the weight of the MEMS accelerometer is 0.2 g, but if I understand correctly, you need to attach signal and data processing electronics as situated on the PCB you show in one of the images to the structure, too. Is this correct, or is it a misconception on my side? And if it is correct, what does this add in terms of weight, say, per accelerometer, and what kind of solution would you then foresee for a real application of the system you suggest?

I am very much looking forward to your reply.

Kind regards,

Dirk Lehmhus
Francesco Caimmi
Dear Dr. Lehmhus,
thank you for your comment. The weight of the PCB we are using at the moment is 6 grams, which is actually a significant portion of the test specimen weight that is about 60 grams (DCB, load blocks included).
While at the moment this is not an issue, given the load variation rate and the overall test setup, we do agree that for a real application a better solution would be needed.
At the moment we are developing a sensor network based on a flexible PCBs with spots where MEMS sensors could be soldered. Each sensor shares the same set of tracks providing power supply and  allowing data acquisition.

We are now working to get a (almost) synchronous acquisition, which is a bit tricky,and on the firmware  that will operate the sensor network. Such a firmware could be placed  off the structure, and possibly connected only on demand.

Best regards,
Francesco Caimmi