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Sensors Get Their Field of Dreams
Source: EETimes
MicroStrain (Williston, Vt.) applies mechanical strain on piezoelectric materials using a
power-management scheme based on charge storage in a capacitor. The wireless circuit is held
in the off state until enough charge accumulates to drive it. The power-management circuit
detects the voltage from the piezoelectric strip and shuts down the transmitter to allow
charge buildup in the capacitor. When a threshold of 9.5 volts is detected on the capacitor,
the wireless sensor node is turned on and transmits data. The transmitter sends a 418-MHz
frequency-shift-keyed encoded data stream at distances of up to one-third of a mile using 12
milliamps at 3 V.
MicroStrain's network uses addressable sensing nodes that incorporate data-logging
capabilities and a bidirectional RF transceiver communications link.
Meanwhile, work is ongoing to develop low-power wireless sensor nets by making each node
self-contained and power-stingy. In a paper submitted to the International Symposium on
Smart Structures & Materials/NDE for Health Monitoring and Diagnostics 2005, to be held
Feb. 26 to March 2 in San Diego, MicroStrain researchers describe work on smart wireless
sensing nodes capable of operation at extremely low power. The systems were designed to be
compatible with energy-harvesting techniques using piezoelectric materials, solar cells
or both. The wireless sensing nodes included a Microchip microprocessor, on-board memory,
sensor signal conditioning, 2.4-GHz IEEE 802.15.4 radio transceiver and rechargeable battery.
Sensing was accomplished via a 1,000-ohm foil strain gauge . The system slept when not
sampling. At 10 Hz, current consumption was 300 microamps at 3 Vdc (900 microW); at 5 Hz,
the system generated 400 microW; and at 1 Hz, it generated 90 microW. When the RF stage was
not used but data was logged to memory, consumption was cut further.
Piezoelectric-strain energy-harvesting systems delivered approximately 2,000 microW under
low-level vibration conditions, the researchers reported. Output power levels were also
measured from two miniature solar cells, which provided a wide range of output power
(from approximately 100 to 1,400 microW), depending on the light type and distance from the
source.
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