PORTLAND, Ore. ó Ultralow-power accelerometers, which draw only nanoamps while holding idle electronics in zero-consumption standby mode, can extend the battery life of freestanding sensors for up to a decade. Analog Devices Inc. says its newest three-axis accelerometer can extend battery life for months even in always-on devices. The part is being tested for use in the Blast Gauge, which the Defense Department has deployed to monitor soldiers 24/7 for exposure to the concussive forces of munitions.
"We wanted to build the industry's lowest-power accelerometer," said Bill Murphy, product line director for the MEMS/Sensor Technology Group at Analog Devices. "The ADXL362 can make infrastructure sensor nets virtually maintenance free, and even for applications where the sensor must always be on, battery life can be extended for months."
The ADXL362 consumes 1.8 microamps while making 12-bit motion measurements that allow it to detect whether a device like the Blast Gauge is being used. If it determines that the soldier wearing the Blast Gauge is idle, then it goes into a 300-nanoamp fast-wakeup mode. For applications with long periods of idleness, it goes into a 10-nA sleep mode.
Blackbox Biometrics uses ADI's accelerometer to extend battery life for months. (Source: Blackbox Biometrics)
"The Blast Gauge today has a battery lifetime of about 30 to 60 days," said Dave Borkholder, CTO of Blackbox Biometrics. "We are still testing the ADXL362, but we hope it will greatly extend the battery life for the Blast Gauge." Borkholder, a professor of electrical engineering at the Rochester Institute of Technology, developed the Blast Gauge under a Defense Advanced Research Projects Agency contract, and then founded Blackbox Biometrics to commercialize it.
The Blast Gauge does not have an on-off switch, because it needs to be constantly ready to monitor soldiers for exposure to the harmful concussive forces of mortars and improvised explosive devices (IEDs). Soldiers in Afghanistan wear three Blast Gauges: on the helmet, chest and shoulder. Inside each unit, a MEMS pressure sensor measures the concussive force of any nearby explosions and at the push of a button turns on a light to indicate whether the blast was within acceptable limits (green), exceeded safe levels (yellow) or was strong enough to require that the wearer receive immediate medical attention (red).
Medics then remove the Blast Gauges from the injured soldier's helmet and uniform, plug them into a USB port and download the pressure sensor data, which is stored in a large first-in/first-out (FIFO) unit after each concussive event.
The MEMS accelerometer in the Blast Gauge monitors a soldier's activity level to determine whether the electronics can be safely put into standby mode or should remain in active. Since the battery is not replaceable, the battery life of the device needs to be extended as long as possible. Blast Gauges are discarded when the battery finally runs down.
Beyond military applications, ADI is targeting the ADXL362 at many other applications where battery replacement can be impractical or dangerous to the equipment or operator, such as in infrastructure sensor networks and in systems for remote monitoring of homebound patients or of livestock in the field.
The ADXL362 has a sophisticated activity detection system that prevents false positives. A dedicated "activity status" pin can be used to turn system power on or off, obviating the need for application processors to poll it.