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EETE SEPTEMBER 2012

DESIGN & PRODUCTS ENERGY HARVESTING in self-powered ultra-low power radio wireless switching modules. We currently applications. The achieve a wireless energy generated by transmission distance a single key stroke of 10 to 30m at a fre- is sufficient to wake quency of 2.4 GHz. Fig. 2: A) Inhomogenous stress distribution for a clamped piezoelectric troller and transmit ing further with high Fig. 5: Voltage monitoring over the-Algra is experiment-up a microcon cantilever. B) Stress optimized layer for a coded signal to performance ceram- storage capacitor. a simple Dynapic concept. a remote receiver. ics for the piezoelectric membrane, which could lead together The wireless signal itself could be deployed for simple ON/OFF with custom designed ASICs to more flexible solutions based operation. These switches could be deployed in household de- on standard wireless protocols. More harvested energy should vices such as lights or window blinds but would also be suitable allow the switch to send redundant signals for increased reli- for industrial automation. ability. Because there is almost no mechanical movement, the The Dynapic piezo can also be used as an energy source for Dynapic Wireless switch comes as a compact and robust mod- small embedded systems where wireless communication is not ule easy to integrate into existing or new designer switches. The required. This could be the case for contact (shock) loggers, switch is noise-free and has a low force of activation of approxi- say in the transportation industry. Besides the Dynapic Wire- mately 5n. less, Algra also manufactures flexible piezoelectric polymer Dedicated power management composite sensor foils com- for piezo switches mercially known as Dynasim The primary challenge involved in such a piezoelectric switch – see figure 6. The layered was to develop suitable power management devices. There are construction is very similar to several energy management ics and voltage converters on the the Dynapic, but instead of a market but most are aimed at battery-based applications, where sintered piezoelectric disc, a screen printable piezoelectric lacquer with suitable electrode material is built-up using a Fig. 6: The Dynasim screen- roll-to-roll thick film deposi- printed piezoelectric tion technique. This screen polymer composite foils are Fig. 3: Block diagram of the Dynapic Wireless switch. printing technique supports manufactured using fast and the fast serial production of efficient roll-to-roll screen the voltage from a battery is reduced or boosted. What’s more, sensor foils in various forms most of these power management devices are developed for and shapes. printing technique. energy harvesters that work on continuous vibrational modes. Dynasim foils are already proven for their reliability and linear Technology has recently introduced the “lTc3588-1”, efficient production in large quantities for keyboard-based ap- which is an Ac/Dc converter for piezo vibrational harvest- plications. Currently Algra is exploring new applications where ers. These do perform very well for piezo harvesters based on the generated energy would wake up microcontrollers from a continuous vibrational modes, optimized for a resonant fre- deep sleep state - see figure 7. In Sleep mode, a microcontroller quency. however such components could not be deployed for is placed in its lowest current consumption state whereby the the Dynapic piezo keys, as the energy comes in voltage busts device’s oscillator is turned off, so there are no system clocks generated by intermittent finger pressure. The piezo keys deliver running. however, the i/O ports maintain the status they had be- high voltages (typically 20-50 V) at very low current with a typi- fore the SLEEP instruction was executed. In order to minimize cal time span of approximately 100ms. Thus, designers must the Sleep mode’s power consumption, the output ports should wake up the microcontroller within a short period of time and not be sourcing or sinking any current before going into Sleep transmit a simple mode, optimising battery life. All the unused i/O pins should be coded signal configured as inputs and pulled either high (VDD) or low (VSS). to the receiver Figure 7 shows a graph where the energy (yellow trace) from using the energy pressing a Dynasim key generates the energy required to wake generated in the up/ provide interrupts to the micro controller (green trace) and order of 2-20 µJ – set it to a toggle state. in contrast to capacitive keys, the piezo see figure 5. This key does not consume requires custom any power from the developed power supply at all, allowing Fig. 4: The compact form factor of management for a Sleep mode cur- Dynapic Wireless allows various design electronics that rent that only consists possibilities. can function at of the microcontroller’s extremely low currents/high impedances. leakage. For this reason For this purpose, Algra has developed a power manage- such Dynasim piezo ment ASIC that includes an active bridge rectifier with a voltage foil applications could conditioner drawing less than 50nA typically with a forward open new areas of bias voltage under 40mV. it is worth mentioning that the wire- optimization in power Fig. 7: The Dynasim key used to less protocol used here is proprietary, designed to work with management. wake up a microcontroller. 24 Electronic Engineering Times Europe September 2012 www.electronics-eetimes.com


EETE SEPTEMBER 2012
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