Basic Sensor StructuresBasic Sensor Structures

Prof. Xingwei (Vivian) WangProf. Xingwei (Vivian) Wang

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Sensor - System

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Sensor - System

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Characteristics?

• Brain Storm

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Characteristics?

• Sensitivity• Linearity• Detection limit• Resolution• Hysteresis/repeatability/interchangeability• Selectivity• Response time

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Characteristics (1)

• Sensitivity: slope• Full-scale output

(FSO)• Nonlinearity: Max.

deviation/FSO• Limit of detection

(LOD)• Resolution: smallest

increment/FSO.

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Characteristics (2)

• Hysteresis: Max. difference (increasing-decreasing)/FSO.

• Repeatability (reproducibility)

• Interchangeability: Max. error

• Zero-measurand output

• Zero shift/drift7

Characteristics (3)

• Response time: 90% of final value• Selectivity• Lifetime

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Integrated and smart sensors

• Sensor array:• Multisensors:• Multifunction sensor:

• Same sensor units• Different sensor units• Single sensor with

different functions under different conditions

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Different level of integration

• Balancing the offset; temperature compensation

• Amplification/signal conversion• Smart sensors: microprocessor/memory

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Biomedical applications

• Diagnosis: measurement• Monitoring: over time• Built-in controlling units: actuators included

• In vivo (inside the human body)• In vitro (outside to analyze a sample)

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Transduction mechanism• Impedance-type

sensor• Sensing

semiconductor devices

• Acoustic waves• Electrochemical

cells• Calorimetric sensors• Fiber optic sensors

• capacitance; resistance; inductance changes

• alter threshold voltage; shift diode characteristics

• shift of resonance freq.• electrode potential; cell

current; cell resistance• excess heat/heat loss• change of light

propagation12

Impedance-type

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Acoustic-wave propagation

• Electromechanical resonance freq.• Piezoelectric resonant sensor (elastic

properties of various piezoelectric materials)• Quartz crystal disk:

– 10-15mm diameter– 0.1-0.2 mm thickness– 6-20 MHz

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Quartz Microbalance (QMB)

• Thin film thickness monitoring• Mass change

• For a 10-MHz crystal, detection limit is 0.1 ng/mm2

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f/f0=- m/m

Video (QCM)

• http://www.youtube.com/watch?v=xDKOUpSR3EQ&list=PL501CB0BE4C320685 (QCM biosensor)

• http://www.youtube.com/watch?v=NyscQUWxGoE (QCM more applications)

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Calorimetric Sensors

• Temperature variation• Chemical reactions

on the sensor surface• Heat generation from

IR (infrared) or other radiation

• Heat transfer from a flowing medium

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Electrochemical Cells

• Ion/gas-molecule concentration

• 2 electrodes + electrolyte

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3 categories

• Based on characteristics of the electrode reaction:– Type A: direct participation of the mobile ions

(originally present in the electrolyte)– Type B: electrolyte is the “solvents” for the

charged products formed by redox reactions from neutral molecules that are present in the electrolyte)

– Type C: competing/several step electrode reactions

Potentiometric

• Measure the potential difference between 2 electrodes

• Membrane electrode (ion-selective)• Liquid junction ion-selective electrode

– Long-term stability (evaporation of water)– Ion-selective polymeric membranes directly deposited onto

electrode surface 20

Amperometric

• Current output is linearly dependent on the concentration of the analyte

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I = constant × c

Disadvantages

• Equilibrium/stationary process stages

• Long lasting electrochemical processes at the electrode

• Response time is long

• May alter the composition of analytes in small volume -> equilibrium stage can never be reached

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Optrode

• Intensity • Spectrum• Intrinsic• Extrinsic

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Optical waveguide

Waveguide

• Carry waves over a wide portion of the electromagnetic spectrum – Optical fiber sensor– Other sensors

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