The principles behind using piezoelectric materials for energy harvesting have been well known since the early 1970’s. Despite various patents on such energy harvesting implementations, only a few inventions have been crystallized into industrial products.
The primary limitation of deploying a piezoelectric material is related to its poor mechanical reliability while it is often bowed and clamped in a certain manner so as to generate the required voltage by mechanical deformation. In reality, the deflection of most piezoelectric cantilevers easily exceeds a millimeter. [Get a 10% discount on ARM TechCon 2012 conference passes by using promo code EDIT. Click here to learn about the show and register.]
Additionally, the piezoelectric cantilever suffers inhomogeneous stress concentrations due to clamped boundary conditions. A larger deflection with inhomogeneous stress states has a negative influence on the mechanical integrity of the piezoelectric energy harvesting source.
Fig. 1: Expanded view of the Dynapic technology showing the multilayer laminate construction with the piezoelectric disc.
As early as the 1990’s, the Swiss-based company Algra demonstrated that the reliability issues of bending piezoelectric materials can be circumvented with two proprietary technologies known as Dynapic and Dynasim. The Dynapic technology offers an innovative solution where a discshaped piezoelectric material is bent within a controlled range from 100 to 300 μm through the use of a multi-layered sandwich construction - see figure 1
. Figure 2
shows side by side the stress analysis of a typical cantilever bending and the Dynapic piezo key. A homogenous stress distribution ensures the mechanical integrity of the Dynapic piezo keys, enabling such implementations to exceed 10 million switching cycles. Algra’s latest innovation, Dynapic Wireless - as shown in figure 3
, demonstrates that energy from the Dynapic piezo switch alone is sufficient to be used in self-powered wireless switching applications. The energy generated by a single key stroke is sufficient to wake up a microcontroller and transmit a coded signal to a remote receiver.
Figure 2: A) Inhomogenous stress distribution for a clamped piezoelectric cantilever. B) Stress optimized layer for a simple Dynapic concept.Figure 3: Block diagram of the Dynapic Wireless switch.
The wireless signal itself could be deployed for simple ON/OFF operation. These switches could be deployed in household devices such as lights or window blinds but would also be suitable for industrial automation. Because there is almost no mechanical movement, the Dynapic Wireless switch comes as a compact and robust module easy to integrate into existing or new designer switches. The switch is noise-free and has a low force of activation of approximately 5N.
Figure 4: The compact form factor of Dynapic Wireless allows various design possibilities.