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Examining the operations of piezo wireless switch

Posted: 08 Oct 2012     Print Version  Bookmark and Share

Keywords:piezoelectric  AC/DC converter  vibrational harvesters 

As early as the 1970s, the principles behind using piezoelectric materials for energy harvesting have been well known. 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.

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.

Figure 1: Expanded view of the Dynapic technology showing the multi-layer laminate construction with the piezoelectric disc.

As early as the 1990s, 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 a solution where a discshaped piezoelectric material is bent within a controlled range from 100 to 300µm through the use of a multi-layered sandwich construction (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 another development, Dynapic Wireless, 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 optimised layer for a Dynapic concept.

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 can be integrated into existing or new designer switches. The switch is noise-free and has a low force of activation of approximately 5N.

Dedicated power mgmt for piezo switches
The primary challenge involved in such a piezoelectric switch was to develop suitable power management devices. There are several energy management ICs and voltage converters on the market but most are aimed at battery-based applications, where the voltage from a battery is reduced or boosted. What's more, most of these power management devices are developed for energy harvesters that work on continuous vibrational modes.

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