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Title: Sensors in Wireless Sensor Networks


1
Sensors in Wireless Sensor Networks
MentorProf. Dr. Veljko Milutinovic
vm_at_etf.bg.ac.yu
Goran Rakocevic goxy_83_at_yahoo.com

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Agenda
  • Introduction
  • Microsensing principles
  • Off the shelf (available sensors)?
  • In research (not yet available, but in
    preparation)?
  • On the wish list (for someone to make)?

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Classification of sensors
  • Based on the function
  • that the sensor performs
  • Mechanical
  • Thermal
  • Electrical
  • Magnetic
  • Radiant
  • Chemical (including bio-chemical)?

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Trends in sensor technology
  • Miniaturization
  • Integration (sensor, signal processing and
    actuator)?
  • sensor with signal processing circuits for
    linearising sensor output, etc.
  • sensor with built-in actuator for automatic
    calibration, change of sensitivity
  • etc.
  • Sensor arrays
  • one-function units (to improve reliability)?
  • multiple-function units

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Microsensors
  • Microsensor A miniature electronic device that
    detects information about a specific variable
  • Why microsensors?
  • lower manufacturing cost (mass-production, less
    materials)?
  • wider exploitation of IC technology
    (integration)?
  • wider applicability to sensor arrays
  • lower weight (greater portability)?

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Two Families of Microsensors
  • Those which were Born with semiconductors
  • Photodetectors UV, Vis, IR,
  • Hall effect
  • Optical
  • Magnetic
  • Those which are feasible without
    microtechnologies
  • Pressure, accelero, gyro
  • Gas, Ph, Ions
  • DNA analyses
  • Mechanical
  • Chemical
  • Biological
  • (MEMS)?

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Pressure sensors
  • First microsensors developed by the industry
  • Low production costs, high sensitivity
  • Piezoresistive
  • Membrane sensors
  • deflection of the membrane
  • change in the resonance frequency
  • Planar comb structures
  • Optical methods 33(Mach-Zehnder interferometer)?

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Piezoresistive pressure sensor
  • Piezoresistors integrated in the membrane
  • Pressure deflects the membrane
  • Resistance changes proportional to deflection and
    thus to pressure
  • Resistance change measured with Wheatstone bridge

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Capacitive membrane pressure sensor
  • Membrane deflects when pressure is applied
  • Distance between the electrodes changes
  • Capacitance changes
  • Capacitive sensors have
  • no hysteresis
  • better long-term stability and
  • higher sensitivity
  • but higher production costs

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Capacitive pressure sensor, based on comb
structure
  • Utilizes parallel comb structure
  • Force is applied parallel to the sensor surface
  • Force is transformed into displacementgt change
    in capacitance
  • On one side capacitance increases and on the
    other side decreases gt higher linearity and
    sensitivity

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Mach-Zehnder interferometer
  • Laser light brought into the sensor by optical
    fiber
  • Light is split to two beams
  • One light beam crosses a micromembrane which is
    deformed by pressure
  • The deformation changes light properties
  • The beams are combined and brought a photodiode
  • Different propagation speeds result in phase
    shift

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Position and speed microsensors
  • The most significant are
  • Contact-free optical
  • magnetic methods are

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Magnetic sensor to measure angular displacement
  • Hall sensor based measurement of angular
    displacement
  • Rotor with a row of teeth
  • Stator contains Hall sensors
  • Permanent magnet located under the sensors
  • Teeth passing by a Hall sensor change magnetic
    field

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Capacitive angular speed sensor
  • The fork arrangement is used as a resonator
  • The resonator starts to oscillatewhen magnetic
    field and alternating current are
    applied(Lorentz force)?
  • The amplitude of the swing angleis detected by
    the capacitance change between movable and fixed
    electrodes

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Acceleration microsensors
  • Usually detected with capacitive and
    piezoresistive methods
  • An elastic cantilever where a mass is attached is
    mostly used
  • Under acceleration mass displaces the cantilever
  • Deflection of the cantilever is detected
  • By increasing the mass sensitivity can be
    increased

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Capacitive cantilever microsensor
  • Sensor consists of cantilevers acting as one
    electrode, an electrode strip and a contact
    strip
  • Sawtooth voltage applied to gradually increase
    the electrostatic force
  • Finally cantilever touches the contact strip
  • Acceleration affects the magnitude of the voltage
    that is required for contact

Cantilever length 120 - 500 µm Sensitivity 0.6
- 100 mV/g Fabrication dry etching
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Piezoresistive microsensorwith oil damping
  • Sensor consists of cantilever beams, a seismic
    mass and oil.
  • Oil dampens the resonance of the suspended mass

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Temperature sensors
  • Thermoresistive sensors
  • Resistive Temperature Devices (RTD)?
  • Thermistors
  • Thermoelectric sensors
  • The Seebeck effect
  • The Peltier effect
  • Thermocouples
  • The p-n junction sensors

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Applications of temperature sensors
  • Important role in monitoring systems
  • process industry
  • environmental protection
  • medicine
  • body temperature (oral, nasal, rectal,
    surface,catheter and tympanic, ventilator
    airway, and needle temperature probes)?
  • Heating and air conditioning systems
  • Indirect measurement of other parameters, e.g.
    in flow sensors
  • Error compensation for temperature dependent
    sensors and actuators

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Thermoresistive sensors
  • Based on materials whose resistance changes in
    accordance with temperature
  • Resistance Temperature Detectors (RTDs)?
  • The material is a metal
  • Platinum, Nickel, Copper are typically used
  • Thermistors (thermally sensitive resistor)?
  • The material is a semiconductor
  • A composite of a ceramic and a metallic oxide
    (Mn, Co, Cu or Fe)?
  • Typically have negative temperature coefficients
    (NTC thermistors)?

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Thermoelectric sensors
  • The Seebeck effect
  • When a pair of dissimilar metals are joined at
    one end, and there is a temperature difference
    between the joined ends and the open ends,
    thermal emf is generated, which can be measured
    in the open ends
  • The Peltier effect
  • When a current passes through the junction of two
    different conductors, heat can be either
    absorbed or released depending on the direction
    of current flow
  • Thermocouples
  • Based on the Seebeck effect
  • Open ends must be kept at a constant reference
    temperature TREF
  • A number of standard TCs are used
  • These are denominated with different letter
    codes T, J, K, S, R
  • i.e, type J (the most popular) is made of Iron
    and Constantan (Cu/Ni alloy 57/43)?

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Chemical sensors
  • Detect presence or concentrationof a chemical
    substance
  • Applications
  • medical diagnostics
  • nutritional science
  • environmental protection
  • automobile industry
  • About 60 of chemical sensorsare gas sensors

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Chemical sensors
  • Conventional measurement methods are often very
    complicated and expensive, require laboratory
    conditions, etc.
  • Objectives of microsensors
  • small and inexpensive
  • mass-produced
  • accurate and robust
  • use only small amount of reagents
  • short response times

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Chemical sensors
  • Research trends (in addition to the development
    of sensor units)
  • integration of sensors into measurement systems
  • integration of several types of sensors
  • microsystems with several identical sensors
    (local analysis of a substance,distribution of
    a parameter over a certain domain)?
  • Sensor principles
  • potentiometer principle in connection with FET
  • acoustic sensors
  • optical sensors

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Structure of a chemical sensor system
  • A sensitive layer is in contact with the
    substance
  • Chemical reaction occurs on the sensitive layer
  • Due to the reaction physical, optical, acoustic
    or dielectric properties are changed
  • Transducer transforms the signal into electrical
    form

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Interdigital transducer sensors
  • Interdigital transducers using capacitive
    measurement are often used in chemical sensors
  • The capacitance can be adjusted by changing the
    dielectric properties of the sensitive layer
  • E.g. resistance of SnO2 sensitive layer changes
    when it interacts with certain substances

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Optical sensor principle
  • Optical sensors are inexpensive,easy to
    sterilize, can handle small samples and are
    highly sensitive
  • Coupling grid detector
  • substance to be analyzed is in directcontact
    with the waveguide
  • depending on the concentration of the substance
    its index of refraction variesgt amount of light
    striking the sensor depends on the
    concentration

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Ion sensitive FET sensor
  • For continuous measurement ofpH value and gases
    in blood (O2, CO2)?
  • A device for external use to makeon-line
    diagnosis of a patient
  • Consists of a sensor, a bloodsampling and
    processing part
  • Uses ion sensitive FET gatepotential is
    proportional to gas concentration

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Conductivity sensors
  • Absorption of gases modifies the conductivity of
    sensing layer
  • Sensing layer types
  • Metal Oxide
  • Typically SnO2 doped with Pt or Pd
  • Operate at high temperatures (300-5000C)?
  • Particularly suitable for combustible gases
  • Conducting Polymers
  • Based on pyrrole, aniline or thiophene
  • Operate at room temperature
  • CPs vs MOXs
  • CP advantages
  • Large number of polymers available with various
    selectivities
  • Sensitivity to wide number of VOCs
  • Low power consumption
  • Faster response and recovery times
  • CP Limitations
  • Cross-sensitivity to humidity
  • Lower sensitivity than MOXs

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Piezo-electric chemical sensors
  • Piezo-electric effect
  • The generation of an electric charge by a
    crystalline material upon subjecting it to
    stress (or the opposite)?
  • A typical piezo-electric material is Quartz
    (SiO2)?
  • Piezo-electric sensors
  • Thin, rubbery polymer layer on a piezo-electric
    substrate
  • Sensing principle mass and viscosity changes in
    the sensing membrane with sorption of VOCs
  • Surface Acoustic Wave (SAW)?
  • AC signal (30-300MHz) applied to interdigitated
    input electrode generates a surface (Rayleigh)
    wave
  • Propagation delays to output electrode are
    affected by changes in the surface properties
  • Phase shifts of the output electrode signal are
    used as a response
  • Quartz Crystal Microbalance (QMB)?
  • Also known as Bulk Acoustic Wave (BAW) devices
  • Device is operated in an oscillator circuit
  • Changes in the sensing membrane affect the
    resonant frequency (5-20MHz) of the device

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Biosensors
  • Measurement principle is similar as with chemical
    sensors
  • Sensitive layer is biologically sensitive,
    containing e.g. enzymes or antibodies
  • Interaction between the molecules of the
    bioelementand the molecules of the substance
    changes a physical or chemical parameter
  • Parameter change is converted into electrical
    signal
  • Signal represents concentration to be measured

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Applications of biosensors
  • Biological and nutritional research
  • to detect e.g. heavy metals or allergens
  • Medical applications
  • patient data recording for correct and quick
    diagnosis during surgery
  • Integration of biosensors with microfluidic
    componentsgt very small analyzers
  • Difficulties
  • immobilization of proteins
  • proteins are not stable for a very long time

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Metabolism sensors
  • Use bio-sensitive enzymes to catalyze a chemical
    reaction
  • Phosphate measurement
  • enzyme NP detects phosphate and triggers
    chemical reaction
  • one product of the reaction HX is transformed
    into XO in another chemical reaction after
    consuming oxygen
  • amount of oxygen can be measured using a
    chemical sensor
  • phosphate concentration is proportionalto the
    amount of consumed oxygen

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Immuno-sensors
  • Antibody is a bio-sensitive element
  • Immobilized antibody moleculesbond with antigen
    molecules in the substance (lock and key)?
  • The concentration of antigens can be measured
    using for example interferometric method (light
    intensity changes)?

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Pulse oximetry
  • Based on the red and infrared light absorption
    characteristics of oxygenated and deoxygenated
    hemoglobin
  • Oxygenated hemoglobin absorbs more infrared light
    and allows more red light to pass through
  • Deoxygenated (or reduced) hemoglobin absorbs more
    red light and allows more infrared light to pass
    through
  • The emitter and photodetector are opposite of
    each other with the measuring site in-between
  • After the transmitted red (R) and infrared (IR)
    signals pass through the measuring site and are
    received at the photodetector, the R/IR ratio is
    calculated
  • The R/IR is compared to a "look-up" tables that
    convert the ratio to an SpO2 value

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Pulse oximetry
  • A major advancement from the oximeters of the
    '70s was the inclusion of arterial pulsation to
    differentiate the light absorption in the
    measuring site due to skin, tissue, and venous
    blood from that of arterial blood
  • Constant light absorbers at the measuring site
    (skin, tissue, venous blood, arterial blood)?
  • With each heart beat there is a surge of arterial
    blood, which momentarily increase arterial blood
    volume
  • Result (when received signals are looked at 'as a
    waveform') peaks with each heartbeat and
    troughs between heartbeats
  • The light absorption at the trough is subtracted
    from the light absorption at the peakthe
    resultants are the absorption characteristics due
    to added volume of blood only which is arterial
  • The advent of "Next Generation" pulse oximetry
    technology has demonstrated significant
    improvements in the ability to read through
    motion and low perfusion

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Pulse oximetry- limitatons
  • Not a complete measure of respiratory
    sufficiency. A patient suffering from
    hypoventilation given 100 oxygen can have
    excellent blood oxygen levels while still
    suffering from respiratory acidosis due to
    excessive carbon dioxide
  • Not a complete measure of circulatory
    sufficiency. If there is insufficient bloodflow
    or insufficient hemoglobin in the blood, tissues
    can suffer hypoxia despite high oxygen saturation
    in the blood.
  • A higher level of methemoglobin will tend to
    cause a pulse oximeter to read closer to 85
    regardless of the true level of oxygen
    saturation.
  • Carbon monoxide concentrations in human blood,
    are normally insignificant, but significant
    levels will follow smoke inhalation.Carboxyhemogl
    obin prevents oxygen binding to hemoglobin, yet
    being bright red, causes over reading of oxygen
    saturationPulse oximetry should be avoided where
    significant amounts of carbon monoxide have been
    inhaled
  • A CO-oximeter measures absorption at additional
    wavelengths to distinguish CO from O2 and
    determines the blood oxygen saturation more
    reliably.

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Continuous blood glucose monitors
  • A continuous blood glucose monitor determines
    blood glucose levels on a continuous basis
  • A disposable glucose sensor placed just under the
    skin is worn for a few days until replacement.
  • A link from the sensor to a non-implanted
    transmitter communicates to a radio receiver.
  • An electronic receiver is worn like a pager that
    displays blood glucose levels, as well as
    monitors rising and falling trends in glycemic
    excursions
  • Continuous blood glucose monitors measure the
    glucose level of interstitial fluid.
  • Disadvantages
  • continuous systems must be calibrated with a
    traditional blood glucose measurement and do not
    yet fully replace "fingerstick" measurements.
  • glucose levels in interstitial fluid lag
    temporally (aprox. 5 minutes )behind blood
    glucose values.
  • Patients therefore require traditional
    fingerstick measurements for calibration
    (typically twice per day) and are often advised
    to use fingerstick measurements to confirm hypo-
    or hyperglycemia before taking corrective action.

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Wireless sensor networks
  • A wireless network consisting of spatially
    distributed autonomous devices using sensors to
    cooperatively monitor physical or environmental
    conditions, such as temperature, sound,
    vibration, pressure, pollutants, etc.
  • Sensors in wireless sensor networks
  • usually microsenzors
  • must not require too much power

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Off the shelf
  • Wireless sensor systems are commercially
    available from
  • Crossbow Berkeley
  • Microstrain
  • A number of sensors has been developed
  • for these systems
  • some are integrated in the basic versions
  • others come as add-ons

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Crossbow Berkeley
  • 51-pin sensor boards for MICA2, MICAz and IRIS
    modules
  • MTS series Sensor Boards
  • MDA Data Acquisition Boards
  • ITS Sensor Board

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Crossbow Berkeley - MDA
MDA300CA is an extremely versatile data
acquisition board that also includes an onboard
temperature/ humidity sensor 7 single-ended or
3 differential ADC channels 4 precise
differential ADC channels 6 digital I/O
channels with event detection interrupt
  • MDA320CA is a high-performance data acquisition
    board with up to 8 channels of 16-bit analog
    input
  • 8 single-ended 0-2.5V inputs, or 4
    differential 0-2.5V ADC channels
  • 8 digital 0-2.5V I/O channels with event
    detection interrupt

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Microstrain
  • V-LINK
  • SG-LIN
  • G-LINK
  • TC-LINK
  • EmbedSense Wireless Sensor

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Microstrain
  • V-LINK, SG-LIN, G-LINK, TC-LINK
  • Come with integrated temperature sensors
  • compatible with a wide range of analog sensors,
    including -strain gauges, -displacement
    sensors, -load cells, -torque transducers,
    -pressure sensors, -accelerometers,
    -geophones, -temperature sensors,
    -inclinometers, -etc.

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Microstrain FAS-G gyro enhanced Inclinometer
  • Inclination Angle Range 360 degrees full scale
    (FS)?
  • Angular Velocity Range /- 300 degrees/second
    (max.)?
  • Dynamic Compensation Closed loop digital control
    (0 to 50 Hz)(angle resolution specs. taken at
    most aggressive filter setting)?
  • Temperature Drift Single axis 0.025 /deg.C
  • Output Data Rate 150 Hz (digital analog)?
  • Inclination Resolution lt 0.1 degrees
  • Nonlinearity 0.23 full scale (static)?
  • A/D Resolution 12 bits
  • D/A Resolution 12 bits

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Microstrain 3DMsolid state pitch, roll, and yaw
module
  • Range yaw 180 degrees
  • Angle resolution Pitch lt 0.1 degrees
  • Update rate (angle mode) 30 Hz/ 3 channels
  • (raw mode) 70 Hz/ 6 channels
  • A/D resolution 12 bits
  • Roll lt 0.1 degrees
  • Yaw lt 0.1 degrees
  • pitch 180 degrees
  • roll 70 degrees

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EmbedSense Wireless Sensor
  • Batteries are completely eliminated
  • uses an inductive link to receive power from an
    external coil
  • Sensor types
  • Piezoresistive bonded foil
  • semiconductor strain gauges,
  • pressure/load/torque transducers,
  • thermocouples

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  • Microsensors that are available,
  • but not yet implemented in wireless sensor nodes
  • Chemical sensors
  • Biosensors
  • Radiation sensors
  • Both the microsensor and wireless node
    manufacturers
  • offer custom-building,
  • so many of these sensors might be specially
    ordered

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Chemical sensors
  • Oxygen Sensors
  • Sensitivity 0 - 150 mmHg
  • Time Constant or Response Rate (0 - 63 of
    response) 4 sec
  • Bias Voltage -0.7 V
  • Current Response for ambient pO2 18 nA
  • Thermal Drift 4 /C
  • pH Sensors
  • Response Time 10 sec
  • Temperature Range -4?C - 75?C
  • pH Range 0 14
  • Resolution better 0,01 pH
  • Accuracy 0,02 pH

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Chemical sensors
  • CO, CO2 Sensors
  • Response Time (T1/e) 20 sec diffusion time
  • Measurement Range 0 - 5 000 ppmvol.
  • Sensitivity 20 ppm 1 of measured value
  • Accuracy 30 ppm 5 of measured value
  • Pressure Dependence 1.6 reading per kPa
    deviation from normal pressure, 100 kPa
  • NOx Sensors
  • Measurement Range 0 to 100 PPM
  • Operating Temperature -20 to 50o C
  • Response Time lt 40s typical at 20o C
  • Estimated Service Life 24 months Depends on
    application
  • Lower Detection Limit 0.2 PPM (depends on
    circuitry)
  • Resolution 0.2 PPM (depends on circuitry)?

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Chemical sensors
  • H2S Sensors
  • Measurement Range 0 to 200 PPM
  • Response Time lt 60 s typical at 20o C
  • Estimated Service Life 24 month (Depends on
    application)?
  • Lower Detection Limit 2 PPM depends on circuitry
  • Resolution 2 PPM depends on circuitry
  • And others sucha as
  • Methane Sensors
  • Conductivity Sensors
  • Water Quality Sensors
  • Air Quality Sensors

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Example POCKET CO
  • CO Digital Dosimeter Detector
  • Digital carbon monoxide dosimeter detector
  • Alerts users of CO exposure from 5 to 1000 PPM
  • Dosimetry variables include TWA time weighted
    average,
  • TE total exposure, MAX maximum ppm, TIME when max
    occurred
  • 5 minute inspection mode, up to 8 hr collection
    mode
  • 2 yr limited instrument, sensor warranty
  • 85 decibel alarm at 2 feet

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Biosensors
  • Detection of (bio-)molecules
  • Proteins
  • Allergens
  • nucleic acids
  • Carbohydrates
  • viral particles
  • Glucose monitors
  • Detection of pathogens
  • Example
  • HiPerChip HIGH PERFORMANCE CHIP

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Radiation microsensors
  • Nuclear Radiation Microsensorsfor detecting a,
    and ß particles, ?-rays, x-rays
  • residual radiation (0.001 cGy/hr to 999 cGy/hr)?
  • prompt radiation
  • personal dosimeters
  • Infrared Radiation Microsensors
  • responsivity1,2 gt800 V/W
  • noise1 lt370 nV/vHz
  • specific detectivity1,2 gt2.5108 cm vHz / W
  • operating temperature 20 to 70 C

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Radiation
  • UV Radiation Microsensors
  • Broad band UVA(315-400 nm), UVB(280-315 nm), and
    UVC(100-280 nm) photodiodes
  • UVI type sensor (for accurate sun-UV dosimetry)?
  • precision up to /- 0.5UVI
  • operating temperature 20 to 70 C
  • Visible and NIR Radiation Microsensors
  • Illumination detectors (400-700nm)?
  • Blue light detectors measure the blue light
    radiation hazard to eyes
  • Flash detectors (detect flashes with a width from
    0.1ms to 100ms,measure peak irradiance and total
    effective energy)?
  • Micro Cameras (resolutions up to 628 X 582)?
  • Thermal radiation Microsensors
  • Responsivity 90.0 V/W
  • Time Constant 6ms
  • Operating Temperature -50-85 C
  • Up to 32X32 resolution

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Pulse oximetry
  • Oxygen Saturation Range (Sp02) 0 to 100
  • Pulse Rate Range 18 to 300 pulses per minute
  • Accuracy
  • Blood Oxygen Saturation 70 - 100 2 digits
  • Pulse Indicator 3
  • Operating temperature 0 C to 50 C
  • Also available (but devices somewhat larger in
    size and more battery-consuming)
  • Carboxyhemoglobin (SpCO)?
  • Methemoglobin (SpMet)?
  • Perfusion Index

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Continuous blood glucose monitors
  • REAL-Time display , 24 hours a day
  • Accuracy (up to, depending on the system)
  • Consensus Error Grid 98.9 AB
  • MARD (Mean) - 19.7
  • MARD (Median) - 15.6
  • Sensor life FDA approved for up to 7 days
  • Start-up Initialization Time 2 hours
  • Waterproof transmitters, up to 8 feet for 30
    minutes
  • Calibration usually every 12 hours
  • Also available
  • A system with an integrated insulin pump
  • The glucose sensor transmits data directly to
    the insulin pump
  • Helps keep blood glucose levels within
    personalized target range byrecommending
    necessary correction doses

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Pollen sensors
  • Much smaller than conventional methods
  • Pollen discrimination and count, by "Degree of
    Polarization
  • Built-in Suction Fan
  • Pollen count information can be made available
    without delay by taking advantage of the real
    time analysis
  • Compact, Light and Low Cost

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Contact Microphones
  • Broad bandwidth (8Hz 2.2 KHz)?
  • High sensitivity (40 V/mm)?
  • Excelelent impact resistance
  • Operating Temperature 5 60 C
  • Light Weight
  • Low Cost
  • Ideal for detecting body sounds.

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In research
  • Advanced gas sensors
  • Micro-scale cell analysis device
  • Sensors that detect chemical warfare agents,such
    as sarin, tabun, sulfur, and mustard
  • Sensors that detect Explosives such as TNB, TNT,
    and 2,4-DNT
  • Sensors that detect pesticides and insecticides
  • Improvements on the existing microsensors(better
    precision, less power consumption, smaller in
    size )?

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In research
  • Glucose sensing bio-implants
  • longer term solutions to continuous monitoring
    under development use a long-lasting
    bio-implants
  • a minor surgical implantation of the sensor
  • should last from one year to more than five years
  • Non-Invasive glucose sensing
  • new technologies to monitor blood glucose levels
    will not require access to blood
  • near IR detection
  • ultrasound
  • dielectric spectroscopy

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On the wish list
  • Detection of bio-currents
  • Micro-sized sensor nodes with integrated
    short-distance communication equipment

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Sensors in Wireless Sensor Networks
  • Goran Rakocevic goxy_83_at_yahoo.com
  • Zoran Babovic zbabovic_at_verat.net
  • Mentor
  • Prof. Dr. Veljko Milutinovic vm_at_etf.bg.ac.yu

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