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Sensors and Sensor Networks
- What are sensors and what are they for?
- Various sensors and their features
- What are sensor networks & their interconnections?
- Sensor network technologies & applications
Sensor and Actuator
A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. (Wikipedia)
An actuator is a device for moving/controlling a mechanism/system, or generate some output, e.g., motor, LED, buzzer, speaker, etc.
- Types of Sensors
- Acoustic, sound, vibration
- Microphone, geophone, seismometer, accelerometer,
- Automotive, transportation
- Speedometer, map sensor, water sensor, parking sensor,
- Chemical
- Sensing carbon, gas, hydrogen, oxygen, smoke, etc.
- Electric, magnetic, radio
- Hall effect, magnetometer, metal detector, telescope,
- Environment, weather, moisture, humidity
- Leaf sensor, rain/snow gauge, pyranometer,
- Flow, fluid velocity
- Air flow meter, flow sensor, water meter,
- Navigation instruments
- Air speed indicator, depth gauge, gyroscope, turn coordinate,
- Position, angle, displacement, distance, speed, acceleration
- Accelerometer, position sensor, tilt sensor, ultrasonic sensor,
- Optical, light, imaging
- Colorimeter, electro-optical sensor, infra-red sensor, photodiode,
- Pressure
- Barometer, boost gauge, pressure gauge, tactile sensor,
- Force, density, level
- Force gauge, level sensor, load cell, hydrometer,
- Thermal, heat, temperature
- Heat sensor, radiometer, thermometer, thermistor,
- Proximity, presence
- Motion detector, occupancy sensor, touch switch,
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Sensor Performance
- Range - maximum and minimum values that can be measured
- Resolution or discrimination - smallest discernible change in the measured value
- Linearity - maximum deviation from a ‘straight-line’ response
- Sensitivity - a measure of the change produced at the output for a given change
- Error - Accuracy/Precision - difference between measured & actual values -> Random/System Errors
- Temperature Sensors
- Resistive Thermometers
- typical devices use platinum wire, linear but has poor sensitivity
- Thermistors
- use materials with a high thermal coefficient, sensitive, highly non-linear
Examples of temperature Sensors
Motion Sensors
Motion sensors measure quantities such as velocity and acceleration can be obtained by differentiating displacement, however differentiation tends to amplify high-frequency noise
Alternatively, can be measured directly some sensors give velocity directly e.g. measuring frequency of pulses in the counting techniques described earlier gives speed rather than position some sensors give acceleration directly e.g. accelerometers usually measure the force on a mass.
Sound Sensors
Sound sensors are used to capture sounds for instance in microphones available in a number of forms e.g. carbon (resistive), capacitive, piezoelectric and moving-coil microphones moving-coil devices use a magnet and a coil.
Ultrasonic Sensors
Ultrasonic sensors are used for position measurements and sound waves emitted are in the range of 2-13 MHz e.g. Sound Navigation And Ranging (SONAR)and Radio Detection And Ranging
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Sensor Networks
A sensor network (SN) consists of multiple interconnected sensors and a wireless sensor network (WSN) consists of spatially distributed autonomous sensors (called sensor nodes) to cooperatively monitor physical or environmental conditions. Together they form sensors and wireless Networks>
Sensor Nodes
Also called a mote in North America, is a WSN node that is capable of performing some processing, gathering sensory information and communicating with other connected nodes in the WSN.
- General Features of Sensor Node are:
- Small Size : few mm to a few inches
- Limited processing and communication
- MHz clock, MB flash, KB RAM, 100’s Kbps (wireless) bandwidth
- Limited power (MICA: 7-10 days at full blast)
- Failure prone nodes and links (due to deployment, fab, wireless medium, etc.)
- But easy to manufacture and deploy in large numbers.
Sensor-node Operating System
- Issues
- Size of code and run-time memory footprint
- Embedded System OS’s inapplicable: > 100KB ROM
- Workload characteristics
- Continuous ? Bursty ?
- Application diversity
- Want to reuse sensor nodes
- Tasks and processes
- Scheduling, hard and soft real-time
- Power consumption
- Communication
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Sensor-node Operating System characteristics
- Event-driven execution (reactive mote)
- Use a variant of language C called NesC
- WSN Characteristics
- Limited power they can harvest or store
- Ability to withstand harsh environmental conditions
- Ability to cope with node failures
- Mobility of nodes
- Dynamic network topology
- Communication failures
- Heterogeneity of nodes
- Large scale of deployment
- Unattended operation
- Nodes are scalable, only limited by bandwidth of gateway node.
- Sensor Data Management
- Interested in phenomena with certain tolerance
- Accuracy, freshness, delay
- Sensors sample the phenomena as Sensor Data Management include determining spatio-temporal sampling schedule but difficult to determine locally e.g. Data aggregation such as interaction with routing
- Network/Resource limitations such as congestion management, load balancing, QoS/Realtime scheduling.