27-11-2012, 01:28 PM
Motivation for a Network of Wireless Sensor Nodes
1Motivation for a Network.ppt (Size: 1,017 KB / Downloads: 34)
Sensing and Sensors
Sensing: technique to gather information about physical objects or areas
Sensor (transducer): object performing a sensing task; converting one form of energy in the physical world into electrical energy
Examples of sensors from biology: the human body
eyes: capture optical information (light)
ears: capture acoustic information (sound)
nose: captures olfactory information (smell)
skin: captures tactile information (shape, texture)
Sensing (Data Acquisition)
Sensors capture phenomena in the physical world (process, system, plant)
Signal conditioning prepare captured signals for further use (amplification, attenuation, filtering of unwanted frequencies, etc.)
Analog-to-digital conversion (ADC) translates analog signal into digital signal
Digital signal is processed and output is often given (via digital-analog converter and signal conditioner) to an actuator (device able to control the physical world)
Other Classifications
Power supply:
active sensors require external power, i.e., they emit energy (microwaves, light, sound) to trigger response or detect change in energy of transmitted signal (e.g., electromagnetic proximity sensor)
passive sensors detect energy in the environment and derive their power from this energy input (e.g., passive infrared sensor)
Electrical phenomenon:
resistive sensors use changes in electrical resistivity (ρ) based on physical properties such as temperature (resistance R = ρ*l/A)
capacitive sensors use changes in capacitor dimensions or permittivity (ε) based on physical properties (capacitance C = ε*A/d)
inductive sensors rely on the principle of inductance (electromagnetic force is induced by fluctuating current)
piezoelectric sensors rely on materials (crystals, ceramics) that generate a displacement of charges in response to mechanical deformation
WSN Communication
Characteristics of typical WSN:
low data rates (comparable to dial-up modems)
energy-constrained sensors
IEEE 802.11 family of standards
most widely used WLAN protocols for wireless communications in general
can be found in early sensor networks or sensors networks without stringent energy constraints
IEEE 802.15.4 is an example for a protocol that has been designed specifically for short-range communications in WSNs
low data rates
low power consumption
widely used in academic and commercial WSN solutions
Single-Hop versus Multi-Hop
Star topology:
every sensor communicates directly (single-hop) with the base station
may require large transmit powers and may be infeasible in large geographic areas
Mesh topology
sensors serve as relays (forwarders) for other sensor nodes (multi-hop)
may reduce power consumption and allows for larger coverage
introduces the problem of routing
Challenges in WSNs: Energy
Sensors typically powered through batteries
replace battery when depleted
recharge battery, e.g., using solar power
discard sensor node when battery depleted
For batteries that cannot be recharged, sensor node should be able to operate during its entire mission time or until battery can be replaced
Energy efficiency is affected by various aspects of sensor node/network design