Smart Dust: Communicating with a Cubic- Millimeter Computer
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COVER FEATURE Smart Dust: Communicating with a Cubic- Millimeter Computer The Smart Dust project is probing microfabrication technology’s limitations to determine whether an autonomous sensing, computing, and communication system can be packed into a cubic-millimeter mote to form the basis of integrated, massively distributed sensor networks. Brett ecreasing computing device size, increased • monitoring environmental conditions that affect Warneke connectivity, and enhanced interaction with crops and livestock; the physical world have characterized com- • building virtual keyboards; Matt Last puting’s history. Recently, the popularity of • managing inventory control; Brian Dsmall computing devices, such as handheld • monitoring product quality; Liebowitz computers and cell phones, burgeoning Internet • constructing smart-office spaces; and growth, and the diminishing size and cost of sensors— • providing interfaces for the disabled. Kristofer S.J. especially transistors—have accelerated these trends. Pister The emergence of small computing elements, with SMART DUST REQUIREMENTS University of sporadic connectivity and increased interaction with Smart Dust requires both evolutionary and revolu- California, the environment, provides enriched opportunities to tionary advances in miniaturization, integration, and Berkeley reshape interactions between people and computers energy management. Designers can use microelectro- and spur ubiquitous computing research.1 mechanical systems (MEMS) to build small sensors, The Smart Dust project2 is exploring whether an optical communication components, and power sup- autonomous sensing, computing, and communication plies, whereas microelectronics provides increasing system can be packed into a cubic-millimeter mote (a functionality in smaller areas, with lower energy con- small particle or speck) to form the basis of integrated, sumption. Figure 1 shows the conceptual diagram of massively distributed sensor networks. Although we’ve a Smart Dust mote. The power system consists of a chosen a somewhat arbitrary size for our sensor sys- thick-film battery, a solar cell with a charge-integrat- tems, exploring microfabrication technology’s limita- ing capacitor for periods of darkness, or both. tions is our fundamental goal. Because of its discrete Depending on its objective, the design integrates var- size, substantial functionality, connectivity, and antic- ious sensors, including light, temperature, vibration, ipated low cost, Smart Dust will facilitate innovative magnetic field, acoustic, and wind shear, onto the methods of interacting with the environment, provid- mote. An integrated circuit provides sensor-signal pro- ing more information from more places less intrusively. cessing, communication, control, data storage, and We use Smart Dust to pursue projects such as energy management. A photodiode allows optical data reception. We are presently exploring two transmis- • deploying defense networks rapidly by unmanned sion schemes: passive transmission using a corner-cube aerial vehicles or artillery; retroreflector, and active transmission using a laser • monitoring rotating-compression-blade high- diode and steerable mirrors. cycle fatigue; The mote’s minuscule size makes energy manage- • tracking the movements of birds, small animals, ment a key component. Current battery and capacitor and insects; technology stores approximately 1 joule per cubic mm 44 Computer 0018-9162/01/$10.00 © 2001 IEEE Active transmitter with Passive transmitter with laser diode and beam steering corner-cube retroreflector Receiver with photodetector Sensors Analog I/O, DSP, control Power capacitor Solar cell Thick-film battery 1-2 mm Figure 1. Conceptual diagram showing a Smart Dust mote’s major components: a power system, sensors, an optical transceiver, and an integrated circuit. and 10 millijoules per cubic mm, respectively, whereas Sensors and motors solar cells provide 1 joule per day per square mm in The multibillion-dollar MEMS industry has been sunlight and 1 to 10 millijoules per day per square mm growing for several decades, with major markets in indoors. Our optical receiver consumes approximately automotive pressure sensors and accelerometers, med- 0.1 nanojoule per bit, and the transmitter uses 1 nano- ical sensors, and process control sensors. Recent joule per bit. We expect our analog-to-digital converter advances in technology have put many of these sensor to require 1 nanojoule per sample and computations to processes on exponentially decreasing size/power/cost consume less than 1 picojoule per instruction, in con- curves. In addition, variations of MEMS sensor tech- trast to present processors such as the CoolRisc 813 nology are used to build micromotors; millions of core, which uses 22 picojoules per instruction, and the these micromotors are used in commercially available StrongARM SA1100, which consumes approximately projection display systems, such as the Texas 1 nanojoule per instruction. These estimates demon- Instruments Digital Micromirror Device. Micro- strate that for every sensor sample or transmission, we motors, combined with Smart Dust, raise the inter- can perform about 1,000 8-bit operations, so it is esting possibility of making synthetic insects (see the advantageous to exchange extra calculations for fewer “Microrobotics” sidebar). samples or transmitted bits. Further, given our 1 milli- joule per day of energy from indoor lighting, each sec- COMPUTING AT THE MILLIMETER SCALE ond we can sample a sensor, think about the result, Traditional computer architecture design has focused and transmit some data. on decreasing a given task’s execution time.5 To accom- To determine our research baseline and quickly plish this goal, engineers have improved semiconduc- develop hardware for testing networking algorithms, tor processing exponentially, increasing the transistors’ we used commercial off-the-shelf hardware to build a speed while decreasing their size, thus allowing more series of wireless sensor nodes. We used either optical complex architectures that use increased parallelism on or radio-frequency communication models to produce a single die. In contrast, computing in an autonomous one-cubic-inch devices. Furthermore, other research cubic-millimeter package must focus on minimizing a groups have used these motes to develop a tiny oper- given task’s energy consumption. Smaller, faster tran- ating system4 and deploy a 100-node network. sistors have reduced parasitic capacitance, thereby January 2001 45 resulting in diminished dynamic power consumption. achieves low-energy computation. First, because we Constant electric-field scaling has reduced supply volt- use a high-performance process but operate at low ages, producing dramatic power reductions for both speeds, we can drop the supply voltage to the mini- high-performance and low-energy computing because mum level at which the devices still function; theoret- dynamic power has a quadratic dependence on supply ically this is 0.1 volt,6 but for 0.5- to 0.2-micron voltage. However, constant electric-field scaling also processes it is more realistically 0.2 to 0.3 volt. To min- calls for a reduction in the threshold voltage. This will imize current leakage, which can cause significant result in larger leakage currents, which are already a power consumption at the low clock rates and duty concern in the high-performance processors to be cycles that these low-energy architectures use, we can released in 2001 that will leak amps of current. increase the channel-to-source junction’s reverse bias, Therefore, process engineers need to keep leakage cur- thus increasing the threshold voltage. Initially, adding rents low, which will also benefit low-energy designers. two extra supply voltages in this package may seem In millimeter-scale computing, the shrinking transis- onerous; however, if the mote scavenges solar power, tor’s size lets designers compact significant computing placing two small photodiodes on the integrated cir- power into this small area. For example, the Intel 8088 cuit provides the few atto-amps per device necessary core, originally fabricated in a 3-micron process, would to bias these junctions. Various low-power layout, cir- only require 0.12-square millimeter after shrinking lith- cuit, and logic level techniques have been published.7 ographically into a current 0.18-micron process, with Figure 2 shows a consequence of using these tech- a corresponding 100× decrease in energy/instruction. niques—the worst-case energy consumption of an 8- bit adder in a 0.25-micron process. Low-energy computation The Smart Dust mote’s tasks closely relate to the Besides advanced microfabrication technology physical realm, where the fastest sampling is 10 to 20 processes, using other techniques at every level kHz for vibration and acoustic sensors so the amount Microrobotics Richard Yeh University of California, Berkeley Add legs or wings to Smart Dust and you get microrobots. Like Smart Dust, these synthetic insects will sense, think, and communicate. In addition, they will have the ability to move about and inter- act physically with their environment. We can use micromachining to build microac- tuators and micromechanisms, forming legs and wings, which are integrated with other Smart Dust components. Figure A. Model of a crawling microrobot devel- Figure B. A flying microrobot model capable of The crawling microrobot shown in oped by University of California researchers. autonomous flight. To create the wings and Figure A consumes only