Texas Instruments demonstrates energy-harvesting sensor systems

BOSTON ' Texas Instruments demonstrated a number of handheld microsystems here that draw all their power from the surrounding environment, either from light or vibrational energy. Such devices could serve as platforms for running environmental or other types of sensors in remote locations.

"There are many, many [sensor] applications out there that could [run on] perpetually powered systems," said Adrian Valenzuela, a Texas Instruments product manager, during a presentation at the Embedded Systems Conference yesterday. "All the technology is available today."

Such platforms would rely on ultra-low-power components now coming on the market, along with what Valenzuela called "real-time energy harvesting" ' a new set of technologies that draw power from their surroundings. Such platforms would never need to be recharged or provided with a source of electricity.

The trick to building such systems is to limit the amount of power such sensor systems use, Valenzuela said. Most energy harvesting components, such as solar panels or piezoelectric devices, can produce only a few milliwatts (one-thousandth of a watt) or even microwatts (one-millionth of a watt) every hour. Though minute, such amounts, when banked, can power small systems, thanks to recent improvements in lower-powered microcontrollers and transmitters.

Using such self-powering systems, sensors networks could be set up where it was not formerly feasible to do so, due to the inability to deliver power (by replacing batteries or running a power line) to the sensors. A network of sensors could be embedded within bridges or on board aircraft to measure material stress. Or, a series of sensors could be placed in remote locations to sense environmental conditions, such as the health of trees in a forest.

During his talk, Valenzuela showed off hand-held sensor platforms, both running on the Texas Instruments' low power MSP430 microcontroller (MCU), as well as a Texas Instruments low-powered transceivers.

One of the units demonstrated could transmit up to 400 wireless messages a day, using only energy collected by a solar panel a few centimeters in size affixed to the circuit board. Normal indoor light is sufficient to power this device, Valenzuela noted.

A second unit got its energy from a component that collects energy from being vibrated. Developed by AdaptivEnergy and funded in part by the Central Intelligence Agency's In-Q-Tel investment arm, this module collects energy using the piezoelectric effect, in which a small charge is generated whenever stress is applied to a material.

Valenzuela placed the microsystem on top of a small air pump, which, when operating, vibrated. Though barely audible, the vibrations provided enough energy to build up and maintain a charge between 3 and 4 milli-joules (one-thousandth of a joule) ' enough power for the unit to wirelessly send continuous updates to a laptop. (A joule is one watt flowing per second).

Such platforms would never need to be recharged or depend on an external supply of power, Valenzuela said. A vibration-based, energy-generating module, for instance, could be used in industrial applications to monitor the health of a continuously running engine, which would shake enough to keep the module running. When used within a bridge, such a module could also derive energy from passing traffic.

A small sensor package, such as the ones demonstrated, would be composed of a number of different low-power electronic components. It would include a sensor that would measure some aspect of its environment, such as temperature or humidity. It would include a microcontroller, preferably one that could convert analog input signals into digital form (eliminating the need for an extra power-consuming analog-to-digital converter).

The platform would also require device that would collect ambient energy, as well as a battery or capacitor to store the energy. Finally, it would require a transmitter to communicate with some data collection device.

The system must be designed so that it will only be used for very short periods of time'when it takes a reading, for instance, or transmits its data. Moreover, the controller and transmitter must leak only a very minimal amount of power when in standby mode ' on the order of nanowatts (one millionth of a watt).
The two largest consumers of power on such a system would be the microcontroller and the transmitter, Valenzuela said. Texas Instruments, as well as other chip manufacturers, has been working on slimming down the energy consumption of some of these components. For instance, the MSP 430 draws only 160 microamps for per each megahertz of speed that it cycles (up to 25 Mhz), and draws less than 1 microamp on standby. A Texas Instruments 2.4 Ghz transmitter, the CC2550 draws between 12 and 21 milliamps when active and 400 nanoamps when in sleep mode.

Because these components operate anywhere from 1 volt to 3 volts (An amp is the number of watts divided by the voltage), they can be easily supported by energy-harvesting components ' assuming they spend most of their time asleep.

In addition to the introduction of new energy-harvesting technologies, as well as the lower-powered controllers and transmitters, energy-harvesting systems are also now possible thanks to improvements in battery technology. New form-fitting thin film batteries, long-lasting easy-to-charge "supercap" batteries and even the veritable Lithium Ion batteries can hold charges for considerable lengths of time and can stay operational through multiple cycles, Valenzuela said.

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