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.
Fig. 6: The Dynasim screenprinted piezoelectric polymer composite foils are manufactured using fast and efficient roll-to-roll screen printing technique.