Piezo wireless switch: No battery, no cable, no wear
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.
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.
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.
Dedicated power management 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.
Linear Technology has recently introduced the “LTC3588-1”, which is an AC/DC converter for piezo vibrational harvesters. These do perform very well for piezo harvesters based on continuous vibrational modes, optimized for a resonant frequency. However, such components could not be deployed for the Dynapic piezo keys, as the energy comes in voltage busts generated by intermittent finger pressure. The piezo keys deliver high voltages (typically 20-50 V) at very low current with a typical time span of approximately 100ms. Thus, designers must wake up the microcontroller within a short period of time and transmit a simple coded signal to the receiver using the energy generated in the order of 2-20 μJ – see figure 5. This requires custom developed power management electronics that can function at extremely low currents/high impedances.
Figure 5: Voltage monitoring over the storage capacitor.
For this purpose, Algra has developed a power management ASIC that includes an active bridge rectifier with a voltage conditioner drawing less than 50nA typically with a forward bias voltage under 40mV. It is worth mentioning that the wireless protocol used here is proprietary, designed to work with ultra-low power radio modules. We currently achieve a wireless transmission distance of 10 to 30m at a frequency of 2.4 GHz.
Algra is experimenting further with high performance ceramics for the piezoelectric membrane, which could lead together with custom designed ASICs to more flexible solutions based on standard wireless protocols. More harvested energy should allow the switch to send redundant signals for increased reliability.
The Dynapic piezo can also be used as an energy source for small embedded systems where wireless communication is not required. This could be the case for contact (shock) loggers, say in the transportation industry. Besides the Dynapic Wireless, Algra also manufactures flexible piezoelectric polymer composite sensor foils commercially known as Dynasim – see figure 6. The layered construction is very similar to the Dynapic, but instead of a sintered piezoelectric disc, a screen printable piezoelectric lacquer with suitable electrode material is built-up using a roll-to-roll thick film deposition technique. This screen printing technique supports the fast serial production of sensor foils in various forms and shapes.
Dynasim foils are already proven for their reliability and efficient production in large quantities for keyboard-based applications. Currently, Algra is exploring new applications where the generated energy would wake up microcontrollers from a deep sleep state – see figure 7. In Sleep mode, a microcontroller is placed in its lowest current consumption state whereby the device’s oscillator is turned off, so there are no system clocks running. However, the I/O ports maintain the status they had before the SLEEP instruction was executed. In order to minimize the Sleep mode’s power consumption, the output ports should not be sourcing or sinking any current before going into Sleep mode, optimizing battery life. All the unused I/O pins should be configured as inputs and pulled either high (VDD) or low (VSS). Figure 7 shows a graph where the energy (yellow trace) from pressing a Dynasim key generates the energy required to wake up/ provide interrupts to the micro controller (green trace) and set it to a toggle state. In contrast to capacitive keys, the piezo key does not consume any power from the supply at all, allowing for a Sleep mode current that only consists of the microcontroller’s leakage. For this reason such Dynasim piezo foil applications could open new areas of optimization in power management.
About the author:
Dr. Venkatesh Sivasubramaniam is Material Scientist at Algra AG – www.algra.ch
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