Power management for energy harvesting wireless sensors

The objective of this work was to demonstrate smart wireless sensing nodes capable of operation at extremely low power levels. These systems were designed to be compatible with energy harvesting systems using piezoelectric materials and/or solar cells. The wireless sensing nodes included a microprocessor, on-board memory, sensing means (1000 ohm foil strain gauge), sensor signal conditioning, 2.4 GHz IEEE 802.15.4 radio transceiver, and rechargeable battery. Extremely low power consumption sleep currents combined with periodic, timed wake-up was used to minimize the average power consumption. Furthermore, we deployed pulsed sensor excitation and microprocessor power control of the signal conditioning elements to minimize the sensors' average contribution to power draw. By sleeping in between samples, we were able to demonstrate extremely low average power consumption. At 10 Hz, current consumption was 300 microamps at 3 VDC (900 microwatts); at 5 Hz: 400 microwatts, at I Hz: 90 microwatts. When the RF stage was not used, but data were logged to memory, consumption was further reduced. Piezoelectric strain energy harvesting systems delivered similar to 2000 microwatts under low level vibration conditions. Output power levels were also measured from two miniature solar cells; which provided a wide range of output power (similar to 100 to 1400 microwatts), depending on the light type & distance from the source. In summary, system power consumption may be reduced by: 1) removing the load from the energy harvesting & storage elements while charging, 2) by using sleep modes in between samples, 3) pulsing excitation to the sensing and signal conditioning elements in between samples, and 4) by recording and/or averaging, rather than frequently transmitting, sensor data.