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Smart Dust 1 ABSTRACT Advances in hardware technology has enabled very compact autonomous and mobile nodes each having one or more sensors computation and communication capabilities and a power supply The Smart Dust project is exploring whether an autonomous sensing computing and communication system can be packed into a cubic millimeter mote to form the basis of integrated massively
Smart Dust,WHAT IS A SMART DUST,THE MEMS TECHNOLOGY IN SMART DUST. SMART DUST TECHNOLOGY,OPERATION OF THE MOTE,COMMUNICATING WITH A SMART DUST. OPTICAL COMMUNICATIONS,LISTENING TO A DUST FIELD, CORE FUNCTIONALITY SPECIFICATION PERFORMING A TASK. MAJOR CHALLENGES,APPLICATIONS,HOW FAR THEY HAVE BEEN IMPLEMENTED. REFERENCES,Smart Dust,WHAT IS A SMART DUST, Autonomous sensing and communication in a cubic millimeter. Berkeley s Smart Dust project led by Professors Pister and Kahn. explores the limits on size and power consumption in autonomous sensor. nodes Size reduction is paramount to make the nodes as inexpensive and. easy to deploy as possible The research team is confident that they can. incorporate the requisite sensing communication and computing hardware. along with a power supply in a volume no more than a few cubic. millimeters while still achieving impressive performance in terms of sensor. functionality and communications capability These millimeter scale nodes. are called Smart Dust It is certainly within the realm of possibility that. future prototypes of Smart Dust could be small enough to remain suspended. in air buoyed by air currents sensing and communicating for hours or days. Smart dust sensor laden networked computer nodes that are. just cubic millimetres in volume The smart dust project envisions a. complete sensor network node including power supply processor sensor. and communications mechanisms in a single cubic millimetre Smart dust. motes could run for years given that a cubic millimetre battery can store 1J. and could be backed up with a solar cell or vibrational energy source. The goal of the Smart Dust project is to build a millimeter scale. sensing and communication platform for a massively distributed sensor. network This device will be around the size of a grain of sand and will. contain sensors computational ability bi directional wireless. communications and a power supply Smart dust consists of series of circuit. and micro electro mechanical systems MEMS designs to cast those. functions into custom silicon Microelectromechanical systems MEMS. consist of extremely tiny mechanical elements often integrated together with. electronic circuitry,Smart Dust,THE MEMS TECHNOLOGY IN SMART DUST. Smart dust requires mainly revolutionary advances in. miniaturization integration energy management Hence designers have. used MEMS technology to build small sensors optical communication. components and power supplies Microelectro mechanical systems consists. of extremely tiny mechanical elements often integrated together with. electronic circuitory They are measured in micrometers that is millions of a. meter They are made in a similar fashion as computer chips The advantage. of this manufacturing process is not simply that small structures can be. achieved but also that thousands or even millions of system elements can be. fabricated simultaneously This allows systems to be both highly complex. and extremely low cost, Micro Electro Mechanical Systems MEMS is the integration of. mechanical elements sensors actuators and electronics on a common. silicon substrate through microfabrication technology While the electronics. are fabricated using integrated circuit IC process sequences e g CMOS. Bipolar processes the micromechanical components are fabricated using. compatible micromachining processes that selectively etch away parts of. the silicon wafer or add new structural layers to form the mechanical and. electromechanical devices MEMS realizes a complete System On chip. technology, Microelectronic integrated circuits can be thought of as the brains. of a system and allow microsystems to sense and control the environment. Sensors gather information from the environment through measuring. mechanical thermal biological chemical optical and magnetic phenomena. The electronics then process the information derived from the sensors and. Smart Dust, through some decision making capability direct the actuators to respond by. moving positioning regulating and filtering thereby controlling the. environment for some desired purpose Because MEMS devices are. manufactured using batch fabrication techniques similar to those used for. integrated circuits unprecedented levels of functionality reliability and. sophistication can be placed on a small silicon chip at a relatively low cost. The deep insight of MEMS is as a new manufacturing technology a way of. making complex electromechanical systems using batch fabrication. techniques similar to those used for integrated circuits and uniting these. electromechanical elements together with electronics Historically sensors. and actuators are the most costly and unreliable part of a sensor actuator. electronics system MEMS technology allows these complex. electromechanical systems to be manufactured using batch fabrication. techniques increasing the reliability of the sensors and actuators to equal that. of integrated circuits The performance of MEMS devices and systems is. expected to be superior to macroscale components and systems the price is. predicted to be much lower,Smart Dust,Smart Dust,SMART DUST TECHNOLOGY. Integrated into a single package are,1 MEMS sensors. 2 MEMS beam steering mirror for active optical transmission. 3 MEMS corner cube retroreflector for passive optical transmission. 4 An optical receiver,5 Signal processing and control circuitory. 6 A power source based on thick film batteries and solar cells. This remarkable package has the ability to sense and communicate and is. self powered A major challenge is to incorporate all these functions while. maintaining very low power consumption, Sensors collect information from the environment such as light sound. temperature chemical composition etc, Smart dust employs 2 types of transmission schemes passive transmission. using corner cube retroreflector to transmit to base stations and active. transmission using a laser diode steerable mirrors for mote to mote. communication,The photo diode allows optical data reception. Signal processing control circuitory consists of analog I O DSPs to. control process the incoming data, The power system consists of a thick film battery a solar cell with a charge. integrating capacitor for a period of darkness,Smart Dust. OPERATION OF THE MOTE, The Smart Dust mote is run by a microcontroller that not only. determines the tasks performed by the mote but controls power to the. various components of the system to conserve energy Periodically the. microcontroller gets a reading from one of the sensors which measure one of. a number of physical or chemical stimuli such as temperature ambient light. vibration acceleration or air pressure processes the data and stores it in. memory It also occasionally turns on the optical receiver to see if anyone is. trying to communicate with it This communication may include new. programs or messages from other motes In response to a message or upon its. own initiative the microcontroller will use the corner cube retroreflector or. laser to transmit sensor data or a message to a base station or another mote. The primary constraint in the design of the Smart Dust motes is volume. which in turn puts a severe constraint on energy since we do not have much. room for batteries or large solar cells Thus the motes must operate. efficiently and conserve energy whenever possible Most of the time the. majority of the mote is powered off with only a clock and a few timers. running When a timer expires it powers up a part of the mote to carry out a. job then powers off A few of the timers control the sensors that measure. one of a number of physical or chemical stimuli such as temperature. ambient light vibration acceleration or air pressure When one of these. timers expires it powers up the corresponding sensor takes a sample and. converts it to a digital word If the data is interesting it may either be stored. directly in the SRAM or the microcontroller is powered up to perform more. complex operations with it When this task is complete everything is again. powered down and the timer begins counting again,Smart Dust. Another timer controls the receiver When that timer expires the. receiver powers up and looks for an incoming packet If it doesn t see one. after a certain length of time it is powered down again The mote can receive. several types of packets including ones that are new program code that is. stored in the program memory This allows the user to change the behavior. of the mote remotely Packets may also include messages from the base. station or other motes When one of these is received the microcontroller is. powered up and used to interpret the contents of the message The message. may tell the mote to do something in particular or it may be a message that. is just being passed from one mote to another on its way to a particular. destination In response to a message or to another timer expiring the. microcontroller will assemble a packet containing sensor data or a message. and transmit it using either the corner cube retroreflector or the laser diode. depending on which it has The laser diode contains the onboard laser which. sends signals to the base station by blinking on and off The corner cube. retroreflector transmits information just by moving a mirror and thus. changing the reflection of a laser beam from the base station. This technique is substantially more energy efficient than actually. generating some radiation With the laser diode and a set of beam scanning. mirrors we can transmit data in any direction desired allowing the mote to. communicate with other Smart Dust motes,Smart Dust. COMMUNICATING WITH A SMART DUST,COMMUNICATING FROM A GRAIN OF SAND. Smart Dust s full potential can only be attained when the sensor. nodes communicate with one another or with a central base station Wireless. communication facilitates simultaneous data collection from thousands of. sensors There are several options for communicating to and from a cubic. millimeter computer, Radio frequency and optical communications each have their. strengths and weaknesses Radio frequency communication is well under. stood but currently requires minimum power levels in the multiple milliwatt. range due to analog mixers filters and oscillators If whisker thin antennas. of centimeter length can be accepted as a part of a dust mote then reasonably. efficient antennas can be made for radio frequency communication While. the smallest complete radios are still on the order of a few hundred cubic. millimeters there is active work in the industry to produce cubic millmeter. Moreover RF techniques cannot be used because of the following. disadvantages, 1 Dust motes offer very limited space for antennas thereby demanding. extremely short wavelength high frequency transmission. Communication in this regime is not currently compatible with low. power operation of the smart dust, 2 Furthermore radio transceivers are relatively complex circuits making it. difficult to reduce their power consumption to required microwatt. 3 They require modulation band pass filtering and demodulation. circuitory,Smart Dust, So an attractive alternative is to employ free space optical. transmission Studies have shown that when a line of sight path is available. well defined free space optical links require significantly lower energy per. bit than their RF counterpaths, There are several reasons for power advantage of optical links. 1 Optical transceivers require only simple baseband analog and digital. circuitory, 2 No modulators active band pass filters or demodulators are needed. 3 The short wavelength of visible or near infra red light of the order of 1. micron makes it possible for a millimeter scale device to emit a. narrow beam ie high antenna gain can be achieved, As another consequence of this short wavelength a Base Station. Transceiver BTS equipped with a compact imaging receiver can decode the. simultaneous transmissions from a large number of dust motes from different. locations within the receiver field of view which is a form of space division. multiplexing Successful decoding of these simultaneous transmissions. requires that dust motes not block one another s line of sight to the BTS. Such blockage is unlikely in view of dust mote s small size. Semiconductor lasers and diode receivers are intrinsically small. and the corresponding transmission and detection circuitry for on off keyed. optical communication is more amenable to low power operation than most. radio schema Perhaps most important optical power can be collimated in. tight beams even from small apertures Diffraction enforces a fundamental. limit on the divergence of a beam whether it comes from an antenna or a. lens Laser pointers are cheap examples of milliradian collimation from a. millimeter aperture To get similar collimation for a 1 GHz radio frequency. Smart Dust, signal would require an antenna 100 meters across due to the difference in. wavelength of the two transmissions As a result optical transmitters of. millimeter size can get antenna gains of one million or more while similarly. sized radio frequency antennas are doomed by physics to be mostly. Collimated optical communication has two major drawbacks Line. of sight is required for all but the shortest distances and narrow beams imply. the need for accurate pointing Of these the pointing accuracy can be solved. by MEMS technology and clever algorithms but an optical transmitter under. a leaf or in a shirt pocket is of little use to anyone We have chosen to. explore optical communication in some depth due to the potential for. extreme low power communication,Smart Dust,OPTICAL COMMUNICATIONS. We have explored two approaches to optical communications passive. reflective systems and active steered laser systems In a passive. communication system the dust mote does not require an onboard light. source Instead a special configuration of mirrors can either reflect or not. reflect light to a remote source,Passive reflective systems. The passive reflective communication is obtained by a special. device called CCR Corner cube retro reflector consists of three mutually. orthogonal mirrors Light enters the CCR bounces off each of the three. mirrors and is reflected back parallel to the direction it entered In the. MEMS version the device has one mirror mounted on a spring at an angle. slightly askew from perpendicularity to the other mirrors. In this position because the light entering the CCR does not return. along the same entry path little light returns to the source a digital 0. Applying voltage between this mirror and an electrode beneath it causes the. mirror to shift to a position perpendicular to other mirrors thus causing the. light entering the CCR to return to its source a digital 1 The mirror s low. mass allows the CCR to switch between these two states up to a thousand. times per second using less than a nanojoule per 0 1 transition A 1I0. transition on the other hand is practically free because dumping the charge. stored on the electrode to the ground requires almost no energy Our latest. Smart Dust device is a 63 mm3 autonomous bidirectional communication. mote that receives an optical signal generates a pseudorandom sequence. Smart Dust, based on this signal to emulate sensor data and then optically transmits the. result The system contains a micromachined corner cube reflector a 0 078. mm3 CMOS chip that draws 17 microwatts and a hearing aid battery In. addition to a battery based operation we have also powered the device using. a 2 mm2 solar cell This mote demonstrates Smart Dust s essential concepts. such as optical data transmission data processing energy management. miniaturization and system integration, A passive communication system suffers several limitations Unable. to communicate with each other motes rely on a central station equipped. with a light source to send and receive data from other motes If a given mote. does have a clear line of sight to the central station that mote will be isolated. from the network Also because the CCR reflects only a small fraction of the. light emitted from the base station this system s range cannot easily extend. beyond 1 kilometer To circumvent these limitations dust motes must be. active and have their own onboard light source,Active steered laser systems. For mote to mote communication an active steered laser. communication system uses an onboard light source to send a tightly. collimated light beam toward an intended receiver Steered laser. communication has the advantage of high power density for example a 1. milliwatt laser radiating into 1 milliradian 3 4 arcseconds has a density of. approximately 318 kilowatts per steradian there are 4 steradians in a. sphere as opposed to a 100 watt lightbulb that radiates 8 watts per steradian. isotropically A Smart Dust mote s emitted beam would have a divergence of. approximately 1 milliradian permitting communication over enormous. distances using milliwatts of power Each mote must carefully weigh the. needs to sense compute communicate and evaluate its energy reserve status. Smart Dust, before allocating precious nanojoules of energy to turn on its transmitter or. receiver Because these motes spend most of their time sleeping with their. receivers turned off scheduling a common awake time across the network is. difficult If motes don t wake up in a synchronized manner a highly dynamic. network topology and large packet latency result Using burstmode. communication in which the laser operates at up to several tens of megabits. per second for a few milliseconds provides the most energy efficient way to. schedule this network This procedure minimizes the mote s duty cycle and. better utilizes its energy reserves The steered agile laser transmitter consists. of a semiconductor diode laser coupled with a collimating lens and MEMS. beam steering optics based on a two degree of freedom silicon micromirror. This system integrates all optical components into an active 8 mm3 volume. as the figure shows,CORNER CUBE RETROREFLECTOR, These MEMS structure makes it possible for dust motes to use. passive optical transmission techniques ie to transmit modulated optical. signals without supplying any optical power It comprises of three mutually. perpendicular mirrors of gold coated polysilicon The CCR has the property. that any incident ray of light is reflected back to the source provided that it. is incident within a certain range of angles centered about the cube s body.