5. Acoustic Wave Sensors
• To measure chemical compositions in a gas.
• Generate acoustic waves by converting mechanical energy to
electrical.
• Acoustic wave devices are also used to actuate fluid flow in
microfluidic systems
• Actuation energy for this type of sensor is provided by two principal
mechanism: piezoelectric and magnetostrictive.
• Piezoelectric mechanism is more popular method
• Piezoelectricity – transducing mechanical to
electrical energy and viceversa
6. • Piezoelectric materials, under proper electrical bias, can launch elastic
waves in bulk or thin films.
• Two most commonly encountered elastic waves are the
• Surface Acoustic Wave (SAW)
• flexural plate wave (or Lamb wave)
• The SAW occurs on samples of appreciable depth, whereas Lamb
waves occur in thin plates of materials.
Acoustic Wave Sensors
7.
8. BioMEMS
• The term BioMEMS encompasses
i. Biosensors
ii. Bioinstruments and surgery tools
iii. Systems for biotesting and analysis for quick, accurate, and low-
cost testing of biological substances.
• BioMEMS present a great challenge to engineers!
• Two types
• Biomedical sensors and Biosensors
9. Biomedical sensors
• Used to detect biological substances
• Classified as biomedical instruments that are used to measure
biological substances as well as medical diagnosis purpose.
• These sensors can analyze biological samples in quick and accurate
ways.
• Advantages over traditional instruments
Require minute amount of samples
Can perform analysis much faster with virtually no dead volume.
10. Electrochemical sensors
• Certain biological substances such
as glucose in human blood, can
release certain elements by
chemical reaction.
• These elements can alter the
electricity flow pattern in the
sensor, which can be readily
detected.
11. Fig: A biomedical sensor for measuring glucose concentration.
Glucose + O2 gluconolactone + H2O2
12. Biosensors
• Biosensors is a measuring device that contains biological element.
• Work on the principle of the interaction of the analytes that need to
be detected with biologically derived biomolecules, such as enzymes
of certain forms, antibodies, and other forms of protein.
• These biomolecules when attached to the sensing elements, can alter
the output signals of the sensors when they interact with the analyte.
14. Chemical sensors
• Used to sense particular chemical compounds, such as various gas
species.
• Materials are sensitive to chemical attacks.
• Oxygen gas can be sensed by measuring the change of electrical
resistance in a metallic material as a result of the chemical reaction of
oxidation.
• Four types
• Chemiresistor sensors, chemicapacitor sensors, chemimechanical
sensors, Metal oxide gas sensors
15. Chemiresistor sensors
• Organic polymers are used with embedded metal inserts.
• These polymers can cause changes in the electric conductivity of
metal when it is exposed to certain gases.
• Eg. : phthlocycanine is included with copper to sense ammonia (NH3)
and nitrogen dioxide (NO2) gas.
16. Chemicapacitor sensors
• Some polymers can be used as the dielectric material in a capacitor.
• The exposure of these polymers to certain gases can alter the
dielectric constant of the material which in turn changes the
capacitance between the metal electrodes.
• Eg.: polyphylacetylene (PPA) to sense gas species such as CO, CO2, N2
and CH4.
18. Chemimechanical sensors
• There are certain materials, e.g., polymers, that change shape when
they are exposed to chemical (including moisture).
• One may detect such chemicals by measuring the change of the
dimensions of the material.
• E.g.: moisture sensor using pyraline PI-2722
19. • This type of sensor works on a principle similar to that of
chemiresistor sensors.
• Several semiconducting metals, such as SnO2 change their electric
resistance after absorbing certain gases.
• The process is faster when heat is applied to enhance the reactivity
of the measurand gases and the transduction semiconducting
metals.
Metal oxide gas sensors (MOS)
21. Semiconductor Reducing gas Oxidizing gas
Ethanol, CO, NH3, H2 NO2, CO2
N-type Resistance decreases Resistance increases
P-type Resistance increases Resistance decreases
• Electrical conductivity of MOS changes when exposed to certain
oxidizing and reducing gases
Metal oxide gas sensors (MOS)
22. • Better results are obtained if metallic catalysts are deposited on
the surface of the sensor.
• Such deposition can speed up the reactions and hence increase
the sensitivity of the sensor
Table. Available metal oxide gas sensors
23. Optical sensors
• Micro optical sensors have been developed to sense the intensity of
light.
• Solid-state materials that provide strong photon-electron interactions
are used as the sensing material.
• Extremely short response time in generating electrical signals.
24. •Photovoltaic junction
• The photovoltaic junction can produce an electric potential when the more
transparent substrate of semiconductor A is subjected to incident photon
energy.
• The produced voltage can be measured from the change of electrical
resistance in the circuit by an electrical bridge circuit.
25. Photoconductive devices
• A special material that changes its
electrical resistance when exposed to
light.
Photodiodes
• Made up of p- and n- doped semiconductor layers
26. Phototransistors
• Made up of p-, n- and p-doped semiconductor layers
• Incident photon energy can be converted into electric current
output form theses devices.
27. Selection of materials
• Based on quantum efficiency, which is a material’s ability to generate
electron-hole pairs from input photons.
• Eg.: Silicon (Si), Gallium Arsenide (GaAs)
• GaAs has superior quantum efficiency and thus higher gains in the
output, but is more costly to produce.
• Alkali metals such as lithium (Li), sodium (Na), potassium (K), and
rubidium (Rb) are also used
• Most commonly used alkali metal is Cesium (Cs)
28. Pressure sensors
•Micro pressure sensors are used to monitor and measure minute
gas pressure in environments or engineering systems, e.g.
automobile intake pressure to the engine.
•They are among the first MEMS devices ever developed and
produced for “real world” applications.
•Micro pressure sensors work on the principle of mechanical
bending of thin silicon diaphragm by the contact air or gas
pressure.
31. • The strains associated with the deformation of the diaphragm
are measured by tiny “piezoresistors” placed in “strategic
locations” on the diaphragm.
• These tiny piezoresistors are made from doped silicon.
• They work on the similar principle as “foil strain gages” but much
smaller size (in um), but have much higher sensitivities and
resolutions.
Pressure sensors
3
1
1 4 2 3
o in
R
R
V V
R R R R
Wheat stone bridge for signal transduction
Vo= Measured voltage
Vin= Supplied voltage
R1, R2, R3 and R4 – resistance of resistors in wheat stone bridge
32. • Advantages
• These have high gains
• Good linear relationship between the in-plane stress and resistance
change out
• Disadvantages
• Temperature sensitive
Pressure sensors : piezoresitive type
33. Pressure sensors: Capacitive type
• Micro pressure sensing unit utilizing capacitance change for pressure
measurement.
• Two electrodes made of thin metal films are attached to the bottom
of the top cover and the top of the diaphragm
• Any deformation of the diaphragm due to the applied pressure will
narrow the gap between the two electrodes, leading to a change of
capacitance across the electrodes.
• The capacitance of parallel plate capacitor can be given as 0 r A
C
d
35. 2(2 )
o in
C
V V
C C
• The variable capacitance can be measured by measuring the output
voltage and determined from the equation
Where
Vo= Measured voltage
Vin=supplied voltage
C=capacitance change in the capacitor in the micropressure
sensor
C= capacitance of the other capacitors in the bridge
Pressure sensors: Capacitive type
36. • Advantages
• Insensitive to temperature
• Provides excellent linear output signals
• Disadvantage
• Cost to fabrication
Pressure sensors: Capacitive type
37. • Micro pressure sensor using a vibrating beam for signal transfuction.
• A thin n-type silicon beam is installed across a shallow cavity at the
top surface of the silicon die.
• A p-type electrode is diffused at the surface of that cavity under the
beam.
• The p-and n- type silicon layers are doped with are boron and
phosphorus respectively and are conductive.
• Beam is made to vibrate at its resonant frequency by applying an ac
signal to the diffused electrode in the beam before the application of
pressure to the diaphragm
Pressure sensors: resonant type
38. • The stress induced in the diaphragm will be transmitted to the
vibrating beam.
• The induced stress along the beam causes a shift of the resonant
frequency of the beam.
• The shift of the resonant frequency of the beam can be correlated to
induced stress and thus to the pressure applied to the silicon
diaphragm
Pressure sensors: resonant type
40. Major problems in pressure sensors
• system packaging and protection of the diaphragm from the
contacting pressurized media, which are often corrosive,
erosive,and at high temperatures.
41. Thermal sensors
• Thermal sensors are used to monitor, or measure temperature in an
environment or of an engineering systems.
• Common thermal sensors involve thermocouples and thermopiles.
• Thermal sensors work on the principle of the electromotive forces
(emf) generated by heating the junction made by dissimilar materials
(beads).
• The temperature rise at the junction due to heating can be correlated
to the magnitude of the produced emf, or voltage.
43. • The generated voltage (V) by a temperature rise at the bead (∆T) is:
V T
Where
= Seebeck coefficient in V/K
T= Temperature difference between the hot and cold junctions in K
• The Seebeck coefficient depends on the thermocouple wire
materials and the range of temperature measurements
Thermal sensors
45. • Drawback of thermocouples for micro thermal transducers
• Output of thermocouples decreases as the size of the wires and the
beads is reduced.
Thermal sensors
46. Thermopile
• A micro-thermoplile is a more realistic solution for miniaturized heat
sensing.
• Thermopiles operate with both hot and cold junctions, but are
arranged with thermocouples in parallel and voltage output in series.
• Materials for thermopile wires are the same as those used in
thermocouples.
48. • The voltage output form a thermopile can be obtained by the
following expression:
V N T
Where
N = Number of thermocouple pairs in the thermopile
= thermoelectric power (or Seebeck coefficient) of the two
thermocouple materials, V/K
T = temperature difference across the thermocouples, K
Thermopile
49. • Choi and Wise (1986) produced the
micro-thermopile.
• Total of 32 polysilicon-gold
thermocouples used in the
thermopile.
• Overall size 3.6 mm x 3.6 mm x 20 um
• Output signal of 100 mV from 500 K
• Response time 50 ms
Thermopile
50. Microactuation
• An actuator is “a mechanical device for moving or controlling
something”.
• In MEMS devices based on principle of actuation
• Actuation using thermal forces
• Actuation using Shape memory alloys
• Actuation using Piezoelectric crystals
• Actuation using Electrostatic forces
• Electromagnetic actuation?
51. Actuation using Thermal Forces
• Bimetallic strips made by bonding two materials with distinct thermal
expansion coefficients used.
• The strip will bend when is heated or cooled from the initial reference
temperature because of incompatible thermal expansions of the
materials that are bonded together
Eg: microclamps of valves
52. Shape-Memory Alloys (SMA)
• Shape memory alloys are a unique class of alloys that have ability to
‘remember’ their shape and are able to return to that shape even
after being bent.
• SMA are made up of copper-aluminium-nickel and nickel-titanium
(NiTi) (Nitinol),
53. 1. The wire has a memory - for example, if it is folded to form a shape
and then heated above 90 degrees (centigrade) it returns to its
original shape.
Shape-Memory Alloys (SMA)
54. 2. The material can also be ‘programmed’ to remember a shape.
• This can be achieved by folding the wire to a particular shape and
clamping it in position.
• The wire is then heated for a approximately five minutes at precisely
150 degrees or pass an electric current through the SMA wire.
• If the wire is now folded into another shape and then placed in hot
water it returns to the original ‘programmed’ shape.
Shape-Memory Alloys (SMA)
55. • The diagram shows a
steel jig.
• This is used to fold the
SMA wire to shape.
• A battery is then
connected and current
is passed through it.
• The wire has now been
‘programmed’ to its
new shape.
Shape-Memory Alloys (SMA)
57. Actuation using Shape-Memory Alloys (SMA)
• An SMA strip originally in a bent shape at a designed preset
temperature T is attached to a silicon cantilever beam.
• The beam is set straight at room temperature.
• However, heating the beam with the attached SMA strip to the
temperature T would prompt the stirp’s “memory” to return to its
original bent shape.
• The deformation of the SMA strip causes the attached silicon beam
to deform with the strip, and microactuation of the beam is thus
achieved.
59. Actuation using Piezoelectric Crystals
• Certain crystals, such a as quartz, that exist in nature deform with the
application of an electric voltage.
• An electric voltage can be generated across the crystal when an
applied force deforms the crystal.
61. Actuation using Electrostatic Forces
• Coulomb’s law
2
1 '
4
qq
F
r
According to Coulomb’s law, whenever two charged particles A and
B are in an electric field separated by a distance r, the induced
electrostatic force,
- permittivity of the material separating the two particles
62. Electrostatic forces in Parallel plates
• Two charged plates separated by a dielectric material with a gap d,
the induced capacitance is given by
0 r
A
C
d
0 r
WL
d
A – Area of plates,
0 – Absolute permittivity of free space = 8.85 x 10-12 F/m
r – relative permittivity of dielectric material
63. Parallel plate capacitor
• The value of capacitance C between the two plates is defined as
Q
C
V
Where
Q is the amount of stored charge and
V is electrostatic potential
• The electric energy stored by a given capacitor U is expressed as
2
1
2
U CV
2
1
2
Q
C
64. • The capacitor can be used as an actuator to
generate force or displacement (if at least one of
the capacitor plate is suspended or deformable).
• A pair of electrode plates, with one plate firmly
anchored and another one suspended by a
mechanical spring.
• As a differential voltage is applied between the
two parallel plates, an electrostatic attraction force
will develop.
Parallel plate capacitor
65. • The magnitude of the forces equals the gradient of the stored electric
energy U, with respect to dimensional variable of interest.
• The magnitude of the force is given by
U
F
x
2
1
2
C
V
x
x=is the dimensional variable of interest
Parallel plate capacitor
66. 2
0
2
1
2
rWLV
d
• The associated electrostatic force in width (W) and length (L)
directions
2
0
1
2
r
W
LV
F
d
2
0
1
2
r
L
WV
F
d
• If the plates move along their normal axis, the gap between electrodes
changes, then the magnitude of the force can be rewritten as
d
U
F
d
2
1
2
C
V
d
2
1
2
CV
d
67. Q. A parallel capacitor is made of two square plates with the
dimensions L = W= 1000 m. Determine the normal
electrostatic force if the gap between these two plates is d = 2
m. Take dielectric medium between the plates as air.
68. Q. An air-gap capacitor made with two fixed parallel planar plates. At
rest (zero bias) the distance between two parallel plates is 100 m and
the areas of the plates are 400 x 400 m2. The biasing voltage between
these two plates is 5 volts. Calculate the value of capacitance and
magnitude of attractive force. What is the capacitance value if half of
the area is filled with sea water (Take relative permittivity =80).
70. Microgrippers
• The electrostatic forces generated in parallel plates can be used as the
driving forces for gripping objects.
• Designing an electrostatic micro actuator, tradeoff between the range
of movement and available force.
• 2 types
• i) Gripping force provided by normal forces
• ii) Gripping force provided by in-plane forces
72. ii) Gripping force provided by in-plane forces
• uses a pair of misaligned plates
Microgrippers
73. Comb-drive microgripper
• Uses multiple pairs of misaligned plates.
Microgrippers
Advantage
• Reduction in actuation voltage with increases number of plates
74. • Gripping action at the tip of the gripper is initiated by applying a
voltage across the plates attached to the drive arms and the closure
arm.
• The electrostatic force generated by these pairs of misaligned plates
tends to align them, causing the drive arms to bend, which in turn
closes the extension arms for gripping
• Eg: micromanipulators, robots in micro manufacturing processes,
microsurgery
Microgrippers
75. Q. For the comb-driven actuator shown in figure below, determine the
voltage supply required to pull the moving electrode 10 m from the
unstretched position of the spring. The spring constant k is 0.05 N/m. The
comb drive is operated in air. The gap d between the electrodes and the
width W of the electrodes are 2 m and 5 m respectively.
76. Micromotors
• The actuation forces for micromotors are primarily electrostatic forces
• 2 types of micromotors
• Linear motors
• Rotary motors
78. • Figure shows the working principle of the linear motion in a linear motion
between two sets of parallel base plates. Each of the two sets of base
plates contains a number of electrodes made of electric conducting plates.
• All these electrodes have a length W. The bottom base plate has an
electrode pitch of W, whereas the top base plates has a slightly different
pitch, say W+W/3.
• The two sets of base plates are initially misaligned by W/3, as shown in the
figure. The bottom plates are stationary so the top plates can slide over the
bottom plates in the horizontal plane,
• Thus, on energizing the pair of electrodes A and A’ can cause the motion of
the top plates moving to the left until A and ‘A’ are fully aligned. At that
moment, the electrodes B and B’ are misaligned by the same amount, W/3.
Micromotors
79. • One can energize the misaligned pair B-B’ and prompt the top plates
to move by another W/3 distance towards the left.
• Then the C-C’ pair is misaligned by W/3 and the subsequent
energizing of that pair would produce a similar motion of the top
plates to the left by another distance of W/3. The motion will be
completed by yet another sequence of energizing the last pair, D-D’
• It is thus concluded that with carefully arranged electrodes in the top
and bottom base plates and proper pitches, one can create the
necessary electrostatic forces that are required to provide the relative
motion between the two sets of base plates.
Micromotors
81. Micromotors
• Electrodes are installed in the outer surface of the rotor poles and the
inner surface of the stator poles.
• As in the case of linear pitches of electrodes in rotor poles and stator
poles are mismatched in such a way that they will generate an
electrostatic driving force due to misalignment of the energized pairs
of electrodes.
• The reader will notice that the ratio of poles in the stator to those in
the rotor is 3:2.
• The air gap between rotor poles and stator poles can be as small as 2
m
82. • Problem by the faced by engineers in the design and manufacture of
micro rotary motors is wear and lubrication of bearings.
• Speed more than 10,000 rpm.
• With such high rotational speed, the bearing quickly wears off, which
results in wobbling of the rotors.
Micromotors
83. Microvalves
• Precision control of gas flow for manufacturing processes
• Controlling the blood flow in an artery.
• Precision analysis and separation of constituents
85. • The heating of the two electrical resistor rings attached to the top
diaphragm can cause a downward movement to close the passage of glow.
• Removal of heat from the diaphragm opens the valve again to allow the
fluid to flow.
• Removal of heat form the diaphragm opens the valve again to allow the
fluid to flow.
• Circular shape with diameter of 2.5 mm and 10 um thick
• The heating rings are made of aluminum 5 um thick.
• The valve has a capacity of 300 cm3/min at a fluid pressure upto 100 psi
and 1.5 W of power is required.
Microvalves
86. • Microvalve with thermal actuation principle.
• This design is used to control the flow rate from a normally open
valve to a fully closed state.
• The downward bending of the silicon diaphragm regulates the
amount of valve opening.
• Bending of the diaphragm is activated by heat supplied to a special
liquid in the sealed compartment above the diaphragm.
• The heat source in this case is the electric resistance foils attached at
the top of the device.
Microvalves
88. Micropumps
• Working on electrostatic actuation principle
• The deformable silicon diaphragm forms one electrode of a capacitor.
• It can be actuated and deformed toward the top electrode by applying a
voltage across the electrodes.
• The upward motion of the diaphragm increases the volume of the pumping
chamber and hence reduces the pressure in the chamber.
• This reduction of pressure causes the inlet check valve to open to allow
inflow of fluid.
• The subsequent cutoff of the applied voltage to the electrode prompts the
diaphragm to return to its initial position which causes a reduction of the
volume in the pumping chamber.
90. • This reduction of volume increases the pressure of the entrapped
fluid in the chamber.
• The outlet check valve opens when entrapped fluid pressure reaches
a designed value and fluid is released.
• A pumping action can thus be accomplished.
• Diaphragm is of Square shape with size 4 mm x 4 mm x 2 m thick.
• The gap between the diaphragm and the electrode is 4 m.
• The actuation frequency is 1 to 100 Hz.
• At 25 Hz, a pumping rate of 70 L/min is achieved.
Micropumps
91. • Accelerometer is an instrument that measures the acceleration
(or deceleration) of a moving solid.
• Accelerometers are used to measure dynamic forces associated
with moving objects.
• These forces are related to the velocity and acceleration of the
moving objects.
• Traditionally an accelerometer is used to measure such forces.
• Eg: Car’s suspension system, ABS, airbags
Accelerometer
92. Accelerometer
• A typical accelerometer consists of a “proof mass” supported by a spring and a
“dashpot” for damping of the vibrating proof mass.
• The spring and the dashpot are in turn attached to a casing
93. • A minute silicon beam with an attached mass (called seismic mass)
constitutes a spring mass system, and the air in the surrounding space
is use to produce the damping effect.
Accelerometer
94. • The structure that supports the mass acts as the spring.
• The mass is attached to a cantilever beam or plate, which is used as a
spring
• A piezoresistor is implanted on the beam or plate to measure the
deformation of the attached mass, form which the amplitudes and
thus acceleration of the vibrating mass can be correlated.
• Since acceleration (or deceleration is related to the driving dynamic
force that causes the vibration of the solid body to which the casing is
attached, accurate measurement of acceleration can thus enable
engineers to measure the applied dynamic force.
• Signal transducers used in microaccelerometers include piezoelectric,
piezoresisitive, capacitive and resonant members
Accelerometer
98. Microfluidics
• Microfluidics is widely used in biomedical precision manufacturing processes
and pharmaceutical industries.
• Principal applications of microfluidics systems are for
• Chemical analysis
• biological and chemical sensing
• drug delivery
• molecular separation such as DNA analysis
• amplification, sequencing or synthesis of nucleic acids
• environmental monitoring.
• Microfluidics is also an essential part of precision control systems for
automotive, aerospace, and machine tool industries.
99. Advantages
1. Small sample inputs
2. Better performance
3. Can be combined with electronic systems to a single piece as Lab on Chip
4. Disposable after use as batch production is possible
Major components of micro fluidic system
1. Microsensors: to measure fluid properties
2. Actuators: to alter state of fluids- Micro valves ,Micro pumps, compressors are
used.
3. Distribution Channels: regulating flows in various branches in the system-Capillary
network- electrohydrodynamic forces provided by electro osmosis drive the minute
fluid samples through the micro channels.
4. Systems integration: integrating with microsensors, valves and pumps through
microchannel links.
