Sensors measure physical quantities and convert them into signals that are measurable, such as electrical signals. There are many types of sensors classified based on their measuring quantities like acceleration, pressure, viscosity etc. or their operating mechanisms like capacitive, piezoresistive etc. Microsensors are sensors fabricated using microfabrication techniques to miniaturize their size. Example microsensors discussed are capacitive accelerometers, piezoresistive pressure sensors, and conductometric gas sensors. Commercial applications of microsensors include uses in automotive, biomedical, and consumer electronics.

1. Microsensors
G.K. Ananthasuresh
Professor, Mechanical Engineering
Indian Institute of Science
Bangalore, 560012, India

2. What are sensors?
• Sensors measure something, which we call
a measurand.
• There are lots of sensors
– Based on the measurands
– Based on the way they measure

3. Sensors based on the measurand
• Accelerometer
– Measures acceleration
• Gyroscope
– Measures angular rate
• Pressure sensor
– Measures pressure of a fluid
• Viscosity meter
– Measures viscosity of a fluid
• Anemometer
– Measures wind speed
• Bolometer
– Measures radiation
• Blood analyser
– Measures the presence or quantity of a chemical species
• Virus detector
– Detects the presence of a virus
• Etc.

4. Based on the measurement technique
(e.g., accelerometer)
• An apple and a string
• Capacitive
• Piezo‐resistive
• Fluid level
• Tunnelling current
• Laser interferometry
• Open loop or closed loop
a
tan tan
ma a
a g
mg g
 
   
g

ma
mg
m

5. Based on the measurement technique
(e.g., accelerometer)
• A mango and a string
• Capacitive
• Piezo‐resistive
• Fluid level
• Tunnelling current
• Laser interferometry
• Open loop or closed loop

6. Based on the measurement technique
(e.g., accelerometer)
• A mango and a string
• Capacitive
• Piezo‐resistive
• Fluid level
• Tunnelling current
• Laser interferometry
• Open loop or closed loop

7. Based on the measurement technique
(e.g., accelerometer)
• An apple and a string
• Capacitive
• Piezo‐resistive
• Fluid level
• Tunnelling current
• Laser interferometry
• Open loop or closed loop
a

8. Based on the measurement technique
(e.g., accelerometer)
• A mango and a string
• Capacitive
• Piezo‐resistive
• Fluid level
• Tunnelling current
• Laser interferometry
• Open loop or closed loop

9. Based on the measurement technique
(e.g., accelerometer)
• An apple and a string
• Capacitive
• Piezo‐resistive
• Fluid level
• Tunnelling current
• Laser interferometry
• Open loop or closed loop

10. Sensors are transducers
• Transducers covert one form of energy to
another form.
Sensor
Measurand Output
Colour change
Shape change
State change
Property change
Change in response
etc.

11. A Sensor’s output is usually electrical.
• Sensors usually covert a measurand to an
electrical quantity.
Sensor
Measurand Output
Voltage
Current
Resistance
Capacitance
Inductance
etc.

12. Quantitative vs. qualitative
• Presence or absence of the measurance
– Is it there or not?
• Qualitative
– High or low or medium…?
• Quantitative
– How much is there precisely?
– We want a number

13. Characteristics of a sensor
• Sensitivity
• Resolution
• Range
• Full scale output (FSO)
• Linearity
• Hysteresis
• Response time
• Drift
• Bandwidth
Magnitude of the
output signal per unit
measurand.

14. Characteristics of a sensor
• Sensitivity
• Resolution
• Range
• Full scale output (FSO)
• Linearity
• Hysteresis
• Response time
• Drift
• Bandwidth
Smallest magnitude of
the measurand that
can be reliably and
repetitively detected.

15. Characteristics of a sensor
• Sensitivity
• Resolution
• Range
• Full scale output (FSO)
• Linearity
• Hysteresis
• Response time
• Drift
• Bandwidth
The difference
between the
maximum and
minimum values of
the measurand that
can be detected.

16. Characteristics of a sensor
• Sensitivity
• Resolution
• Range
• Full scale output (FSO)
• Linearity
• Hysteresis
• Response time
• Drift
• Bandwidth
The difference
between the
maximum and
minimum values of
the output signal.

17. Characteristics of a sensor
• Sensitivity
• Resolution
• Range
• Full scale output (FSO)
• Linearity
• Hysteresis
• Response time
• Drift
• Bandwidth
The extent over which
the output signal is
linear with respect to
the measurand.

18. Characteristics of a sensor
• Sensitivity
• Resolution
• Range
• Full scale output (FSO)
• Linearity
• Hysteresis
• Response time
• Drift
• Bandwidth
The difference
between the output
signals for the same
magnitude of the
measurand while the
measurand is
increasing and
decreasing.

19. Characteristics of a sensor
• Sensitivity
• Resolution
• Range
• Full scale output (FSO)
• Linearity
• Hysteresis
• Response time
• Drift
• Bandwidth
The time lag between
the instance the
measurand changes
and the instance the
output signal changes
completely.

20. Characteristics of a sensor
• Sensitivity
• Resolution
• Range
• Full scale output (FSO)
• Linearity
• Hysteresis
• Response time
• Drift
• Bandwidth
The extent of change
in the output even
when the measurand
is constant.

21. Characteristics of a sensor
• Sensitivity
• Resolution
• Range
• Full scale output (FSO)
• Linearity
• Hysteresis
• Response time
• Drift
• Bandwidth
The range of
frequencies of the
time‐varying
measurand over
which the sensor
responds reliably.

22. The same physical element may be
able to sense multiple things…
Boustraw et al. 1990;
flow‐rate sensor
Can also
measure…
Temperature,
Vibration
Sound
Radiation
Chemical species
etc.

23. Some microsensors
• Capacitive accelerometer
• Piezo‐resistive pressure sensor
• Conductometric gas sensor
• Virus‐detecting micro cantilever
• Fibre‐optic crack sensor
• Portable blood analyser

24. Micromachined accelerometers:
two examples
M. Lemkin, M. Ortiz, N. Wongkomet, B. Boser, and J. Smith, ʺA
3‐axis surface micromachined sigma‐delta accelerometer,ʺ Proc.
ISSCC ʹ97, pp. 202‐203, 1997.
ADXL202E by Analog Devices Sandia National Laboratories

25. Measurement of displacement
• There are several other ways of
detection.
– Capacitve
– Piezoelectric
– Piezoresistive
– Magnetic
– Optical
• Single‐axis or multiple axes?
• Cross‐axis sensitivity?
• Over‐range protection?
• Direct mode
• Force‐feedback mode
( )
x t
m
k
b

26. A tradeoff in (micromachined)
accelerometers
mx bx kx ma
  
 
x m
kx ma
a k
  
resonance
k
f
m

Sensitivity
Resonance frequency
High sensitivity implies low resonance frequency;
Low resonance frequency implies small operational range.
At steady state…
But, …
( )
x t
m
k
b
Seismi
c mass
Over‐range
protection
Tradeoff is
necessary

27. The effect of damping
m
k
f
2
b km

2
b km

2
b km
 Critical damping
under damping/resonating
over damping
Over‐damping reduces the useful frequency range.
Under‐damping causes peaking that may lead to mechanical failure.
Thus, damping is usually necessary but not too much or too little.

28. Getting linearity…
0 0 0 0
2 2
0 0
2 2
out
A A A A
xd x
d x d x d x d x
V V V V V for x d
A A d x d
d d
   
 
 
   
    

Good
linearity

29. Capacitance extraction circuit
Chopper stabilization with
boosted gain and correlated
double sampling with cross‐
coupled switches.
This helps reduce 1/f noise and
offset.
1 part per million resolution
2.3 mV noise

30. System level simulation
a+an
x
A demodulator
Low‐pass
filter
Vout
PID
Mechanical
Brownian noise
+
Acceleration
signal
Electronic noise
and offsets
Mass and
suspension
C1
C2
ε0A/(d+x)
ε0A/(d‐x)
+Vm
‐Vm
feedback
4
m
s
V a m
V
k d

Vn

31. A real accelerometer made in IISc
The waveforms show the responses of
the standard STM accelerometer and
IISc’s accelerometer under test when a
0.5 g is applied .
Yellow : Standard accelerometer
Pink : SOIMUMPs accelerometer
Sambuddha Khan
Thejas
Annathasuresh
Bhat
(2009‐2010)

32. Accelerometer: a summary
Summary
Category Sensor
Purpose Measures the acceleration of the body on which this sensor is mounted.
Key words Proof-mass
Suspension
Principle of operation Converts the displacement caused by the inertial force on the proof-mass to a
voltage signal via change in capacitance between movable and fixed parts.
Application(s) Automotive, aerospace, machine tools, bio-medical, etc.

33. A commercial high‐resolution accelerometer
QA2000 Qflex accelerometer
From Honeywell.
~3.2"

34. Piezoresitive pressure sensor
Summary
Category Sensor
Purpose Measures the pressure, typically of gases or liquids.
Key words Piezoresistivity, diaphragm
Principle of operation The external pressure loading causes the deflection, strain, and stress on the
membrane. The strain causes change in the resistance of a material, which is
measured using Wheatstone bridge configuration.
Application(s) Automotive industry, aerospace applications, appliance industry, bio-medical
etc.

35. Motorola’s pressure sensor

36. Conductometric gas sensor
Summary
Category Sensor
Purpose It detects and quantifies the sources of a gas, i.e., its concentration
Key words Catalyst, combustible, adsorption, desorption
Principle of operation The principle is that a suitable catalyst, when heated to an appropriate
temperature, either promotes or reduces the oxidation of the combustible
gases. The additional heat released by the oxidation reaction can be detected.
The fundamental sensing mechanism of a gas sensor relies on a change in the
electrical conductivity due to the interaction process between the surface
complexes such as O-,O2-,H+, and OH- reactive chemical species and the gas
molecules to be detected.
Application(s) Environmental monitoring, automotive application and air conditioning in air
planes, spacecrafts and houses and sensor networks, ethanol for breath
analyzers and food control application etc.

37. Conductometric gas sensors
Nanomaterials Research Inc. gas sensors
Typical materials used: Films of metal oxide like SnO2 and Tio2
Sample fabrication process: Gas sensors are fabricated using the single crystalline SNO2
nanobelts. Nanobelts are synthesized by thermal evaporation of oxide powders under controlled
condition without the presence of a catalyst.

38. A conductometric gas sensor:
how doe it work?
Conductivity: It is a property of material that quantifies the material’s ability to conduct
electric current when an electric potential (difference) is applied. It depends on the number
of free electrons available.
Adsorption: Adsoprtion is the process of collection and adherence of ions, atoms, or
molecules on a surface. This is different from absorption, a much more familiar term. In
absorption, the species enter into the bulk, i.e., the volume. On the other hand, in
adsorption, they stay put on the surface.
Desorption: This is the reverse of adsorption; species (ions, atoms, or molecules) or given
out by the surface.
Combustion: It is a technical term for burning. It is a heat-generating chemical reaction
between a fuel (combustible substance) and an oxidizing agent. It can also result in light
(e.g., a flame).
Pre adsorption of oxygen on semiconducting material surface.
Adsorption of a specific gas that is to be detected.
Reaction between oxygen and the adsorbed gas.
Change in the conductivity of the resistor element.
Desorption of reacted gas on the surface for re‐use.

39. Hand‐held blood analyzer
Abbott Point of Care
http://www.istat.com/
The chip is
small but the
system is
big.
With a few drops of
blood, under a minute it
gives blood analysis:
gases, chemistry, cardiac
markers, etc.

40. Main points
• Sensors are transducers.
• Usually the output is electrical.
• Characteristics of sensors
• Miniaturization helps…
– Because the cost, size, and power consumed are
reduced while improving the performance.
• A lot of commercial microsensors are now
available.
• The scope for further research and
development is unlimited.