Anatomy of a Mote
While the particular size, type, and configuration of motes that form a network are mostly determined by the intended application, all of the devices face the same overarching design constraint. A mote is only as effective as its ability to conserve power. Ideally, each mote should be able to survive on its own for at least a year on a pair of AA batteries. Yet each reading a mote takes and every bit of data it transmits brings the device a moment closer to death. To that end, motes must be on a strict power diet.
At its core, this diet is based on enabling the motes to run at extremely low duty cycles. The mote is active as little as one percent of the time. It “wakes up” only to take scheduled readings or to transmit or receive data from neighboring devices. Every one of the mote’s hardware and software components is designed to support low duty cycles.
As semiconducting circuits become smaller, they consume less power. Simple microcontrollers like those that function as a mote’s brain can operate with just a milliwatt of power when active, or 1-10 microwatts in standby mode. A mote’s memory must also be limited due to the energy constraints. Each mote typically has less than 10 kilobytes of RAM, one hundred kilobytes of software, and a megabyte of data storage. All told, that’s approximately 10,000 times less data storage than a desktop PC.
The low power approach is continued through a mote’s sensing system. For example, commercially-available macroscale sensors such as thermistors and fog detectors show a change in voltage as, respectively, they get warmer or wetter. Analogto-digital converters (ADCs) translate that voltage into a zero or one that the microprocessor can understand. The development of extremely efficient ADCs keep the power profile of a mote’s sensing system similar to that of the processor.
Meanwhile, MEMS provide the motes with a much broader array of low-power sensory inputs. The simplest example of a MEMS device resembles a diving board with a mass mounted on the end. Gravitational forces or acceleration cause the mass to spring up and down, forces that can easily be converted into a digital signal. These devices, called accelerometers, are commonly used in automobiles to trigger the release of airbags. A growing variety of MEMS sensors are available to detect myriad factors, from the body heat of a bird in its burrow to the presence of environmental contaminants in the air. Intel Research is also developing biochips, devices that can sense biological materials and organic chemistry.
While commercial sensors are already present in such everyday products as automobiles and washing machines, motes boast one essential capability that sets them apart from their predecessors: wireless networking using radio. Low-power transceivers enable the motes to transmit their sensor readings throughout the network. Like MEMS sensors, these lowpower radios can now be inexpensively produced using conventional silicon processing techniques. This new class of RF (radio frequency) devices is one of the key enabling technologies behind 802.11 (WiFi) networks, Internet-enabled PDAs, ever-smaller mobile phones, and sensor networks.
Currently, consumer AA or “coin” batteries can keep motes alive for six months to one year. Other energy scavenging power sources are also being developed. Ambient lighting or sunlight could provide enough solar energy in applications where the motes are exposed. At an earlier stage of development are MEMS devices demonstrated at UC Berkeley that convert the ambient vibration of structural components like air-conditioning ducts and windows into enough electricity to keep the motes operational indefinitely.
|Last edited November 21, 2004
Return to WelcomeVisitors