Smart Dust

What Is A Smart Dust?

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 on end.

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.

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 retro reflector 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.

 

 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-millimeter radios.

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 levels.

3. They require modulation, band pass filtering and demodulation circuitry.

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 diagonal).If one of the mirrors is misaligned , this retro reflection property is spoiled. The micro fabricated CCR contains an electrostatic actuator that can deflect one of the mirrors at kilohertz rate. It has been demonstrated that a CCR illuminated by an external light source can transmit back a modulated signal at kilobits per second. Since the dust mote itself does not emit light , passive transmitter consumes little power. Using a microfabricated CCR, data transmission at a bit rate upto 1 kilobit per second and upto a range of 150 mts ,using a 5 milliwattt illuminating laser is possible.

It should be emphasized that CCR based passive optical links require an uninterrupted line of sight. The CCR based transmitter is highly directional. A CCR can transmit to the BTS only when the CCR body diagonal happens to point directly towards the BTS, within a few tens of degrees. A passive transmitter can be made more omnidirectional by employing several CCRs, oriented in different directions, at the expense of increased dust mote size. 

Core Functionality Specification

Choose the case of military base monitoring wherein on the order of a thousand Smart Dust motes are deployed outside a base by a micro air vehicle to monitor vehicle movement. The motes can be used to determine when vehicles were moving, what type of vehicle it was, and possibly how fast it was travelling. The motes may contain sensors for vibration, sound, light, IR, temperature, and magnetization. CCRs will be used for transmission, so communication will only be between a base station and the motes, not between motes. A typical operation for this scenario would be to acquire data, store it for a day or two, then upload the data after being interrogated with a laser. However, to really see what functionality the architecture needed to provide and how much reconfigurability would be necessary, an exhaustive list of the potential activities in this scenario was made. The operations that the mote must perform can be broken down into two categories: those that provoke an immediate action and those that reconfigure the mote to affect future behavior.

Summary

Smart dust is made up of thousands of sand-grain-sized sensors that can measure ambient light and temperature. The sensors -- each one is called a "mote" -- have wireless communications devices attached to them, and if you put a bunch of them near each other, they'll network themselves automatically.