HANCOCK, N.H. Wireless sensor networks are becoming an attractive solution in a wide variety of applications, but one aspect of the approach how to wirelessly generate power tends to cancel out their advantages. Either batteries need to be changed, a high-maintenance job, or the sensors need a wired power source.
Engineers at Microstrain Inc. think they have found an answer in an "energy-harvesting" component that can power wireless nodes directly from ambient energy in the environment. The Williston, Vt., company recently received a $700,000 Small Business Innovative Research Grant from the Defense Department to develop the technology.
"The combination of improved energy-harvesting methods, smart energy-management strategies and accepted standards for wireless sensors will greatly expand the market potential of wireless sensing networks," said Steven Arms, president of Microstrain. "We envision maintenance-free, infinite-life-span wireless sensing networks combined with Wi-Fi and cellular phone networks to enable the efficient and timely delivery of critical information."
To realize that vision, some means of reliably extracting energy from the environment and converting it into microwatt electrical energy needs to be devised. Light, heat and mechanical energy in the form of vibration and strain are the most likely sources, but only mechanical energy is abundant and reliable inside buildings, where the majority of wireless-sensor networks will be installed. Microstrain's product-development team identified mechanical strain as the best source due to rapid advances in the performance of piezoelectric materials. These materials change their physical volume when placed in an electrical field or, conversely, generate an electrical field when subjected to mechanical strain. Not only is strain a commonly available force in buildings and machines, recent advances in piezoelectric materials have made high-efficiency fibers commercially available.
By creating single-crystal fibers, the energy-conversion efficiency of piezoelectric materials has increased from around 60 percent to over 90 percent. These new fibers are being used to damp vibrations in sports equipment such as tennis rackets and baseball bats.
To extract enough electrical power from a strip of piezoelectric material bonded to a beam under variable stress, the Microstrain design team devised 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 prototype system was put together with off-the-shelf components such as a low-power comparator from Linear Technology Corp. (Milpitas, Calif.), which was used for voltage sensing.
When voltage from the piezoelectric strip is detected, the power-management system shuts down the transmitter to allow charge buildup in the capacitor. When a threshold voltage of 9.5 V 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 only 12 milliamps at 3 V.
The time taken to charge up the capacitor to the threshold level was used as a figure of merit to examine the feasibility of the system. Taking typical strain values from structures along with reasonable strain cycle times, the experimental setup revealed charge times between 20 and 80 seconds.
The size of the piezoelectric material was 17 square centimeters. With integration of the RF and power circuits, a wireless node could be embedded in aircraft structures, tires or structural members in buildings to watch for impending failure.
Strategies for power reduction in the control and RF circuits, as well as in the wireless network itself, are critical to making the energy-harvesting technique workable.
"Our wireless sensing systems employ data-logging transceivers, which represent much greater flexibility in terms of how sensors are powered and how the sensed data is locally managed," said Arms. "For example, we provide our customers with on-demand capability of high sensor update rates. We also recognize that our customers will require increasing control over the processing algorithms that the wireless sensing nodes are running."
Another area for power-saving strategies is the wireless network itself. An ad hoc network was designed to enable a large number of sensors deployed throughout a factory or building to communicate with a central receiver hooked to an Ethernet node. The network is already being used with Microstrain's battery-powered wireless sensors.
Communications are controlled by a time-division multiple-access protocol, which allows the nodes to be in power-down mode except for short intervals when bursts of data are sent.
This scheme has made the battery-powered systems far easier to maintain. For example, five thermocouples can be maintained for five years on a single AA battery while transmitting data at 30-minute intervals.
"For low-power-consumption, periodic wireless sensing applications, batteries can be a good solution," Arms said. "Applications such as temperature and humidity, or soil moisture sensing, can be well served by batteries, since update rates of 30 minutes are acceptable." So there will always be a place for battery-powered wireless networks, he said, and the energy-harvesting approach will be used when it becomes essential.
"There are many applications where battery changes are not practical, such as corrosion sensors embedded in concrete or strain sensors placed on an airframe, or strain sensors scattered about on remote areas of a large civil structure, such as a bridge," Arms said. In the case of security monitoring, the possibility of batteries dying without warning excludes them from consideration.