Download - Micro Sensors
-
8/7/2019 Micro Sensors
1/35
Microsensors
-
8/7/2019 Micro Sensors
2/35
Outline
s Sensor & microsensor
s Force and pressure microsensors
s
Position and speed microsensorss Acceleration microsensors
s Chemical microsensors
s Biosensors
s Temperature sensors
-
8/7/2019 Micro Sensors
3/35
Sensor
sDevice that converts a non-electrical physical
or chemical quantity into an electrical signal
Sensor Processor DisplayOutput signalMeasurand
-
8/7/2019 Micro Sensors
4/35
Classification of sensors
s In the course bookclassification is based on
the function that the sensor
performs
3 pressure3 position
3 acceleration
3 etc.
-
8/7/2019 Micro Sensors
5/35
Sensor market
s Pressure 40 %
s Temperature 25 %
s
Acceleration 13 %s Flow 9 %
s Force 5 %
-
8/7/2019 Micro Sensors
6/35
Trends in sensor technology
s Miniaturization
s Integration (sensor, signal processing and actuator)3 sensor with signal processing circuits for linearising sensor output, etc.
3 sensor with built-in actuator for automatic calibration, change of
sensitivity etc.
s Sensor arrays
3 one-function units (to improve reliability)3multiple-function units
-
8/7/2019 Micro Sensors
7/35
Microsensors
s 1995 global sensor market 6 billion $US, 25 % from
MEMS based devices
s Annual increase in the market volume 20%
s Why microsensors3 lower manufacturing cost (mass-production, less materials)
3wider exploitation of IC technology (integration)
3wider applicability to sensor arrays
3 lower weight (greater portability)
-
8/7/2019 Micro Sensors
8/35
-
8/7/2019 Micro Sensors
9/35
Applications ...
s Environmental applications3 determination of concentration of substances (carbon monoxide, heavy
metals, etc.)
s Food industry3 contaminants and impurities
s Process industry
s Robotics3 distance, acceleration, force, pressure, temperature
-
8/7/2019 Micro Sensors
10/35
Pressure sensors
s First microsensors developed and used by industry3 piezoresistive pressure sensor to reduce fuel consumption by a tight
control of the ratio between air and fuel
3disposable blood-pressure sensor to monitor the status of the patientduring operation
s Low production costs, high sensitivity and low
hysteresis
s Commercial products are usually either piezoresistive
or capacitive
-
8/7/2019 Micro Sensors
11/35
Pressure sensors: examples of
operation principles
s Membrane sensors3 deflection of the membrane
3 change in the resonance frequency
s Planar comb structures
s Optical methods (Mach-Zehnder interferometer)
-
8/7/2019 Micro Sensors
12/35
Piezoresistive pressure sensor
s Piezoresistors integrated in the membranes Pressure deflects the membrane
s Resistance changes proportional to deflection and thus to pressure
s Resistance change measured with Wheatstone bridge
-
8/7/2019 Micro Sensors
13/35
Capacitive membrane
pressure sensor
s Membrane deflects when pressure is applied
Distance between the electrodes changes
Capacitance changes
s Capacitive sensors have3 no hysteresis
3 better long-term stability and
3 higher sensitivity but
3 higher production costs
Chip dimensions: 8.4 mm x 6.2 mm
Fabrication: anisotropic etching
-
8/7/2019 Micro Sensors
14/35
Capacitive pressure sensor, based on
comb structure
s Utilizes parallel combstructure
s Force is applied parallel to the
sensor surfaces Force is transformed into
displacement => change in
capacitance
s On one side capacitance
increases and on the other
side decreases => higher
linearity and sensitivity21
21
CC
CCVVIO
+
=
Fabrication: anisotropic etching
-
8/7/2019 Micro Sensors
15/35
Mach-Zehnder interferometer
s Laser light brought into the sensorby optical fiber
s Light is split to two beams
s One light beam crosses a
micromembrane which isdeformed by pressure
s The deformation changes light
properties
s The beams are combined andbrought a photodiode
s Different propagation speeds
result in phase shiftChip size: 0.3 mm x 5 mmOutput: 14 V/mbar
-
8/7/2019 Micro Sensors
16/35
Position and speed microsensors
sApplications3Automobiles
3Robots
3Medical instruments
s Contact-free optical and magnetic methods are
the most significant for MST
-
8/7/2019 Micro Sensors
17/35
Magnetic sensor to measure angular
displacement
s Measurement of joint angle inrobotics
s Hall sensor based measurement of
angular displacement
3 Rotor with a row of teeth3 Stator contains Hall sensors
3 Permanent magnet located under the
sensors
3 Teeth passing by a Hall sensor change
magnetic field
Length: 4 mm
Resolution: 0.028 degrees
-
8/7/2019 Micro Sensors
18/35
Capacitive angular speed sensor
s The fork arrangement is used as aresonator
s The resonator starts to oscillate
when magnetic field and
alternating current are applied(Lorentz force)
s The amplitude of the swing angle
is detected by the capacitance
change between movable andfixed electrodes
Size: 20 mm x 20 mm
Sensitivity: 0.5 mV s/deg
-
8/7/2019 Micro Sensors
19/35
Acceleration microsensors
s Have mostly been used in automotive industry
s Usually detected with capacitive and piezoresistive methods
s An elastic cantilever where a mass is attached is mostly used
s Under acceleration mass displaces the cantilever
s Deflection of the cantilever is detected
s By increasing the mass sensitivity can be increased
-
8/7/2019 Micro Sensors
20/35
Examples of piezoresistive and
capacitive principles
-
8/7/2019 Micro Sensors
21/35
A capacitive accelerometer
by Analog Devices
s A mass-produced capacitive accelerometer was presented in1991
s Microelectronic circuits for signal preamplification,
temperature compensation and system self-test were integratedinto the sensor
s One of the first successful commercial accelerometers
s Currently used in airbag systems
s Range 5 g, resolution 0.005 g
-
8/7/2019 Micro Sensors
22/35
Capacitive cantilever microsensor
s Sensor consists of cantileversacting as one electrode, an
electrode strip and a contact strip
s Sawtooth voltage applied to
gradually increase theelectrostatic force
s Finally cantilever touches the
contact strip
s Acceleration affects the magnitudeof the voltage that is required for
contactCantilever length: 120 - 500 mSensitivity: 0.6 - 100 mV/g
Fabrication: dry etching
-
8/7/2019 Micro Sensors
23/35
Piezoresistive microsensor
with oil damping
s Sensor consists of cantilever beams, a seismic mass and oil.s Oil dampens the resonance of the suspended mass
Cantilever length: 480 m
Seismic mass: 2 mg
Fabrication: wet etching
-
8/7/2019 Micro Sensors
24/35
-
8/7/2019 Micro Sensors
25/35
Chemical sensors ...
s Conventional measurement methods are often verycomplicated and expensive, require laboratory conditions, etc.
s Objectives of microsensors:3
small and inexpensive3 mass-produced
3 accurate and robust
3 use only small amount of reagents
3short response times
-
8/7/2019 Micro Sensors
26/35
Chemical sensors ...
s Research trends (in addition to the development of sensorunits):3 integration of sensors into measurement systems (signal processing)
3 integration of several types of sensors (to test n concentrations)
3 microsystems with several identical sensors (local analysis of a substance,
distribution of a parameter over a certain domain)
s Sensor principles3 potentiometer principle in connection with FET
3 acoustic sensors
3 optical sensors
-
8/7/2019 Micro Sensors
27/35
Structure of a chemical
sensor system
s A sensitive layer is in contact with the substance
s Chemical reaction occurson the sensitive layer
s Due to the reaction physical, optical, acoustic or dielectric properties are
changed
s Transducer transforms the signal into electrical form
-
8/7/2019 Micro Sensors
28/35
Operation principle of interdigital
transducer sensors
s Interdigital transducers usingcapacitive measurement are often
used in chemical sensors
s The capacitance can be adjusted
by changing the dielectricproperties of the sensitive layer
s E.g. resistance of SnO2 sensitive
layer changes when it interacts
with certain substances
2
2
1
=d
UeAF
N
-
8/7/2019 Micro Sensors
29/35
Optical sensor principle
s Optical sensors are inexpensive,easy to sterilize, can handle small
samples and are highly sensitive
s Coupling grid detector
3 substance to be analyzed is in directcontact with the waveguide
3 depending on the concentration of the
substance its index of refraction varies
=> amount of light striking the sensor
depends on the concentration
-
8/7/2019 Micro Sensors
30/35
Ion sensitive FET sensor
s For continuous measurement ofpH value and gases in blood (O2,
CO2)
s A device for external use to make
on-line diagnosis of a patients Consists of a sensor, a blood
sampling and processing part
s Uses ion sensitive FET: gate
potential is proportional to gasconcentration
Chip size: 10 mm x 10 mm
-
8/7/2019 Micro Sensors
31/35
Biosensors
s Measurement principle is similar as with chemicalsensors
s Sensitive layer is biologically sensitive, containing
e.g. enzymes or antibodies
s Interaction between the molecules of the bioelement
and the molecules of the substance changes a physical
or chemical parameter
s Parameter change is converted into electrical signal
s
Signal represents concentration to be measured
-
8/7/2019 Micro Sensors
32/35
Applications of biosensors
s Biological and nutritional research3 to detect e.g. heavy metals or allergens
s Medical applications3
patient data recording for correct and quick diagnosis during surgery
s Integration of biosensors with microfluidic components
=> very small analyzerss Difficulties
3 immobilization of proteins
3 proteins are not stable for a very long time
-
8/7/2019 Micro Sensors
33/35
Metabolism sensors
s Uses biosensitive enzymes tocatalyze a chemical reaction
s Phosphate measurement3
enzyme NP detects phosphate andtriggers chemical reaction
3 one product of the reaction HX is
transformed into XO in another
chemical reaction after consuming
oxygen3 amount of oxygen can be measured
using a chemical sensor
3 phosphate concentration is
proportional to the amount of
consumed oxygen
Nucleoside phosphorylase (NP)
hypoxanthine (XP)
xanthine oxidase (XO)
-
8/7/2019 Micro Sensors
34/35
Immuno-sensors
s Antibody is the biosensitiveelement
s Immobilized antibody molecules
bond with antigen molecules in
the substance (lock and key)s The concentration of antigens can
be measured using for example
interferometric method (light
intensity changes)
-
8/7/2019 Micro Sensors
35/35
Temperature sensors
s Important role in monitoring systems3 process industry
3 medicine
3environmental protection
s Heating and air conditioning systems
s Indirect measurement of other parameters, e.g. in
flow sensorss Error compensation for temperature dependent
sensors and actuators