22-03-2011, 03:39 PM
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INTRODUCTION
The current ultramodern technologies are focusing on automation and miniaturization. The decreasing computing device size, increased connectivity and enhanced interaction with the physical world have characterized computing’s history. Recently, the popularity of small computing devices, such as hand held computers and cell phones; rapidly flourishing internet group and the diminishing size and cost of sensors and especially transistors have accelerated these strengths. The emergence ofsmall computing elements, with sporadic connectivity and increased interaction with the environment, provides enriched opportunities to reshape interactions between people and computers and spur ubiquitous computing researches. is tiny electronic devices designed to capture mountains of information about their surroundings while literally floating on air. Nowadays, sensors, computers and communicators are shrinking down to ridiculously small sizes. If all of these are packed into a single tiny device, it can open up new dimensions in the field of communications. The idea behind ‘smart dust’ is to pack sophisticated sensors, tiny computers and wireless communicators in to a cubic-millimeter mote to form the basis of integrated, massively distributed sensor networks. They will be light enough to remain suspended in air for hours. As the motes drift on wind, they can monitor the environment for light, sound, temperature, chemical composition and a wide range of other information, and beam that data back to the base station, miles away.
1 MAJOR COMPONENTS AND REQUIREMENTS OF
Smart Dust requires both evolutionary and revolutionary advances in miniaturization, integration, and energy management. Designers can use microelectromechanical systems to build small sensors, optical communication components, and power supplies, whereas microelectronics provides increasing functionality in smaller areas, with lower energy consumption. The power system consists of a thick-film battery, a solar cell with a charge-integrating capacitor for periods of darkness, or both. Depending on its objective, the design integrates various sensors, including light, temperature, vibration, magnetic field, acoustic, and wind shear, onto the mote. An integrated circuit provides sensor-signal processing, communication, control, data storage, and energy management. A photodiode allows optical data reception. There are presently two transmission schemes: passive transmission using a corner-cube retro reflector, and active transmission using a laser diode and steerable mirrors. The mote’s minuscule size makes energy management a key component. The integrated circuit will contain sensor signal conditioning circuits, a temperature sensor, and A/D converter, microprocessor, SRAM, communications circuits, and power control circuits. The IC, together with the sensors, will operate from a power source integrated with the platform. The MEMS industry has major markets in automotive pressure sensors and accelerometers, medical sensors, and process control sensors. Recent advances in technology have put many of these sensor processes on 2
exponentially decreasing size, power, and cost curves. In addition, variations of MEMS sensor technology are used to build micro motors. Figure 1: Components of
DESCRIPTION OF WORKING OF
The smart dust mote is run by a microcontroller that not only determines the task performed by the mote, but consists of the power to the various components of the system to conserve energy. Periodically the micro controller 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, process the data, and store it in 3
memory. It also turns on optical receiver to see if anyone is trying to communicate with it. This communication may include new programs ormessages from other motes. In response to a message or upon its own initiative, the microcontroller will use the corner cube retro reflector or laserto 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. Another timer controls the receiver. When that timer expires, the receiver powers up and look 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 4
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 timerexpiring, the microcontroller will assemble a packet containing sensor data or a message and transmit it using either the corner cube retro reflector orthe laser diode, depending on which it has. The corner cube retro reflectortransmits 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 othermotes.
COMPUTING AT THE MILLIMETER SCALE
Computing in an autonomous cubic-millimeter package must focus on minimizing a given task’s energy consumption. Smaller, faster transistors have reduced parasitic capacitance, thereby resulting in diminished dynamic power consumption. Constant electric field scaling has reduced supply voltages, producing dramatic power reductions for both high-performance and low-energy computing because dynamic power has a quadratic dependence on supply voltage. However, constant electric field scaling also calls for a reduction in the threshold voltage. This will result in larger leakage currents, which are already a concern in the high-performance processors to be released in 2001 that will leak amps ofcurrent. The process engineers need to keep leakage currents low, which will also benefit low-energy designers. In millimeter-scale computing, the shrinking transistor’s size lets designer’s compact significant computing power into this small area 5
Low-energy computation
Besides advanced micro fabrication technology processes, using other techniques at every level achieves low-energy computation. First, because we use a high-performance process but operate at low speeds, we can drop the supply voltage to the minimum level at which the devices still function; theoretically this is 0.1 volt, 6 but for 0.5- to 0.2-micron processes it is more realistically 0.2 to 0.3 volt. To minimize current leakage, which can cause significant power consumption at the low clock rates and duty cycles that these low-energy architectures use, we can increase the channel-to-source junction’s reverse bias, thus increasing the threshold voltage. Initially, adding two extra supply voltages in this package may seem onerous; however, if the mote scavenges solar power, placing two small photodiodes on the integrated circuit provides the few atto-amps per device necessary to bias these junctions. The Smart Dust mote’s tasks closely relate to the physical realm, where the fastest sampling is 10 to 20 kHz forvibration and acoustic sensors so the amount of data is small enough that we can use low data transmition rates. Therefore we can use clock rates in the 1 to 180 kHz range to decrease dynamic power consumption. Despite these low clock rates, the circuits perform all their transitions during a small portion of the cycle; then they remain idle. Thus, powering down blocks foreven a few clock cycles saves energy.