1 EE 43/100 Smart Dust Lab: Theory 1. Objectives The purpose of this experiment is to introduce you to a new sophisticated wireless sensor system that can be used to make a wide variety of measurements. Along the way, you’ll also experiment with individual sensors of temperature and illumination like those on the wireless “motes” -- so-called because they are only about an inch across now and can ultimately be made much smaller. The wireless sensor system that we’ll explore, which communicates via high-frequency radio waves, was named Smart Dust by its inventor, Kris Pister, a Berkeley EECS prof. 2. Introducing Smart Dust A Smart Dust mote is an electronic package composed of: an integrated-circuit radio transmitter and receiver (the combination is called a “transceiver”); a microcontroller that controls the operation of the mote; a random-access memory (RAM) like the one(s) in your computer; a “flash” memory like the one that stores pictures in a digital camera; some standard sensors – a resistive temperature sensor and a semiconductor illumination (light) sensor that produces a current when it is illuminated; an analog-to-digital converter (ADC) that converts the analog temperature and illumination sensor outputs to digital form for transmission elsewhere; a power source for the mote (typically a battery); and an antenna used both for transmitting and receiving signals. The motes we’ll use – called “Mica2dot” motes (don’t ask) – also have three light-emitting diodes (LEDs) on them. The red LED indicates when the mote is turned on; the yellow and green LEDs flash when mote-to-mote communication is occurring. The Experiment Guide describes this more fully. A computer software operating system – TinyOS – was developed in Berkeley’s Computer Science Division to control mote operation. When wireless motes are first dispersed in an area – a room, a hallway, a building, or in a meadow – they autonomously attempt to set up a network along which they can send information from one mote to another. Somewhere, one mote is plugged into a special printed circuit board (PCB) that is connected to the serial port of a computer, such as a laptop. This board is known as a “base station”; its function is to collect the data provided to it by the assembly of motes. The topology of a typical Smart Dust mote network is shown in Fig. 1 below. Fig. 2 shows a magnified view of one of the Smart Dust wireless sensor motes. Several of its components are identified; beneath the printed circuit board that you see is an integrated circuit like those used in portable telephones to drive an antenna connected to the mote for transmitting the mote identification and measured sensor data to the base station. These motes are made commercially by the Xbow Company (“cross-bow”), which you can reach at www.xbow.com .2 Fig. 1. Topology of a typical Smart Dust mote network.3 Fig. 2. Magnified view of the Smart Dust mote. The wireless telephone chip is beneath this printed circuit board. The mote connects to a short antenna for transmitting and receiving radio waves. MICA2DOT Wireless MoteDigital Temperature SensorDigital Light Sensor3.6 Volt Lithium Battery ON-OFF Switch General Purpose LEDsProgramming Connector4 Each mote in the network has a unique identifier that it uses to preface each of its transmissions. Every mote transmits any sensor data that it has taken, along with information from other motes sent to it for passing along to the base station. If the proper software has been installed in the base station’s computer, the network topology can be displayed on the base station’s monitor. One should thus be able to observe when motes are added to the network, or when they leave it. The sensor data taken by each mote could also be displayed, processed and stored by that computer. 3. Uses The possible uses are limited only by the available sensors and by the range that these motes can achieve – tens to hundreds of meters with radio communication, up to miles in certain circumstances when the motes are outfitted for optical communication using lasers. The long list of sensed quantities that have been demonstrated includes: temperature, illumination, relative humidity, toxic gases, magnetic field, sound, acceleration, and rotation rate. As examples of some of their uses, motes bearing on-board accelerometers are being installed on the Golden Gate bridge to measure the accelerations produced by wind and traffic, and burrowing birds on an Eastern seaboard island are being monitored unobtrusively in their nests by motes that measure temperature and illumination, which are surrogate indications of burrow occupancy and activity. A key feature of the motes is their low cost. The motes that you will use cost approximately $100 each in a quantity order of a few hundreds, and it is predicted that within a year or so the price might be as low as a few dollars. An important practical requirement is that the power required by any mote be kept low to prolong battery life; this means that the mote radios must be efficient, and that any sensors used must require very little power. Present motes require 10-20 mW for radio transmission and reception, and a number of sensors are available that are either passive (require no power source, an example being a photodiode for measuring illumination) or have low power requirements. 3. References Here are two web sites you can check out: robotics.eecs.berkeley.edu/~pister/SmartDust/ http://webs.cs.Berkeley.edu/tos/5 EE 43 Smart Dust Lab: Experiment Guide Smart Dust Motes The motes that you’ll use are contained in translucent plastic boxes that measure 1.5 x 2.5 x 0.6 cubic inches. There is an insulated antenna (inside the plastic tubing – a soda straw) attached to the box. You can turn on a mote by moving the black slide switch (visible through a hole in the box) with a small key or a pencil point; move the switch handle in the direction of the attachment point of the antenna to turn it on. Be sure to turn the motes off when you’re finished with them to prevent battery drain. Each pair of students will have available two motes in which stored TinyOS programs enable them to transmit and receive signals from similar motes, but be immune to transmissions from other motes. The boxes are marked S (send) or R (receive) to identify the transmitter and
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