@anne-francoise.pele; I appreciate the follow up and the links (& the larger versions of figures in the article!).
The need for cost-effective energy harvesting sensor nodes can not be overstated and is critical to deploying sensor networks for infrastructure monitoring.
Click on the link below to check out the collection of the Design Articles, Case Studies, Product How-To articles, Teardowns, etc... related to energy scavenging that have been published on Smart Energy Designline.
Click here: http://www.eetimes.com/design/smart-energy-design/4372778/Energy-harvesting---Design-archive
Check back frequently. The list will be updated as new articles arrive.
FYI, Stephane Boisseau and Ghislain Despesse at the CEA-Leti (France) also contributed the article, entitled: "Energy harvesting, wireless sensor networks & opportunities for industrial applications".
The link to the article is: http://www.eetimes.com/design/smart-energy-design/4237022/Energy-harvesting--wireless-sensor-networks---opportunities-for-industrial-applications
I wish figures were hyperlinked to their larger size versions; figure 2 which is an important one to understand is unreadable; same problems with figure 5, and 7.
I did like the authors' tripartite approach the energy harvesting and utilization problem.
Surely, the approach can be extended to multiple spring mass systems; such an ensemble has the advantage of not needing fine tuning to optimize a system as long as the bounds of the dominant resonant frequencies are known.