introduction to Sensor

1. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Introduction to
Sensors
Sookram Sobhan

2. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Overview
 What are Sensors?
 Detectable Phenomenon
 Physical Principles – How Do Sensors Work?
 Need for Sensors
 Choosing a Sensor
 Sensor Descriptions
 Temperature Sensor
 Accelerometer
 Light Sensor
 Magnetic Field Sensor
 Ultrasonic Sensor
 Photogate
 CO2 Gas Sensor

3. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
What are Sensors?
 American National Standards Institute (ANSI) Definition
 A device which provides a usable output in response to a
specified measurand
 A sensor acquires a physical parameter and converts it into a signal
suitable for processing (e.g. optical, electrical, mechanical)
 A transducer
 Microphone, Loud Speaker, Biological Senses (e.g. touch, sight,
…ect)

4. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Detectable Phenomenon
Stimulus Quantity
Acoustic Wave (amplitude, phase, polarization), Spectrum, Wave
Velocity
Biological & Chemical Fluid Concentrations (Gas or Liquid)
Electric Charge, Voltage, Current, Electric Field (amplitude,
phase,
polarization), Conductivity, Permittivity
Magnetic Magnetic Field (amplitude, phase, polarization), Flux,
Permeability
Optical Refractive Index, Reflectivity, Absorption
Thermal Temperature, Flux, Specific Heat, Thermal Conductivity
Mechanical Position, Velocity, Acceleration, Force, Strain, Stress,
Pressure, Torque

5. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Physical Principles
 Amperes’s Law
 A current carrying conductor in a magnetic field experiences a force
(e.g. galvanometer)
 Curie-Weiss Law
 There is a transition temperature at which ferromagnetic materials
exhibit paramagnetic behavior
 Faraday’s Law of Induction
 A coil resist a change in magnetic field by generating an opposing
voltage/current (e.g. transformer)
 Photoconductive Effect
 When light strikes certain semiconductor materials, the resistance of the
material decreases (e.g. photoresistor)

6. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Need for Sensors
 Sensors are omnipresent. They embedded in
our bodies, automobiles, airplanes, cellular
telephones, radios, chemical plants, industrial
plants and countless other applications.
 Without the use of sensors, there would be no
automation !!
 Imagine having to manually fill Poland Spring bottles

7. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Choosing a Sensor

8. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Temperature Sensor
 Temperature sensors appear in building,
chemical process plants, engines, appliances,
computers, and many other devices that require
temperature monitoring
 Many physical phenomena depend on
temperature, so we can often measure
temperature indirectly by measuring pressure,
volume, electrical resistance, and strain

9. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Temperature Sensor
 Bimetallic Strip
 Application
 Thermostat (makes or
breaks electrical
connection with
deflection)
)]T-(T1[ 00 β+= LL

10. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Temperature Sensor
 Resistance
temperature device.






−
=
+=
0
11
0
00 )]T-(T1[
TT
eRR
RR
γ
α

11. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Accelerometer
 Accelerometers are used to
measure along one axis and is
insensitive to orthogonal
directions
 Applications
 Vibrations, blasts, impacts,
shock waves
 Air bags, washing machines,
heart monitors, car alarms
 Mathematical Description is
beyond the scope of this
presentation. See me during
lunch if interested

12. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Light Sensor
 Light sensors are used in
cameras, infrared
detectors, and ambient
lighting applications
 Sensor is composed of
photoconductor such as a
photoresistor,
photodiode, or
phototransistor

13. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Magnetic Field Sensor
 Magnetic Field
sensors are used for
power steering,
security, and current
measurements on
transmission lines
 Hall voltage is
proportional to
magnetic field
tqn
BI
VH
⋅⋅
⋅
=

14. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Ultrasonic Sensor
 Ultrasonic sensors are
used for position
measurements
 Sound waves emitted are
in the range of 2-13 MHz
 Sound Navigation And
Ranging (SONAR)
 Radio Dection And
Ranging (RADAR) –
ELECTROMAGNETIC
WAVES !!

15. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Photogate
 Photogates are used in
counting applications
(e.g. finding period of
period motion)
 Infrared transmitter and
receiver at opposite
ends of the sensor
 Time at which light is
broken is recorded

16. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
CO2 Gas Sensor
 CO2 sensor measures
gaseous CO2 levels in an
environment
 Measures CO2 levels in
the range of 0-5000 ppm
 Monitors how much
infrared radiation is
absorbed by CO2
molecules

17. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Sensors
 Resistive, Capacitive
 Strain gauges, piezoresistivity
 Simple XL, pressure sensor
 ADXL50
 Noise

18. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Resistive sensors
 R(x) = R0(1+αx)
E.g. TCR, gauge factor
 Generate thermal noise
 Wheatstone bridge
minimizes sensitivity to
Nominal resistance
value
Power supply variation
Other inputs
 R(x) = R0(1+αx) (1+βy)
R0R(x)
R0R0
Vx
V+ V-

19. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Capacitive sensors
 Typically used to measure displacement
 C ~= ε0 A/d
 Can be used in Wheatstone bridge (with
AC excitation)
 Sensitive to environmental coupling
Typically want amplifier very close
Typically need to shield other varying
conductors
Definitely don’t want charge-trapping
Separation (d)
Area (A)

20. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Strain Sensors
 Shape change
δL/L = ε
δa/a = -νε (Poisson’s ratio)
R(a,b,L) = ρ L/A
R(ε) = R0(1+(1+2ν) ε )
R(ε) = R0(1+G ε )
 Piezoresistive
ρ(ε) = ρ0(1+ GP ε )
R(ε) = R (1+(G +G) ε )
F F

21. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Simple piezoresistive pressure
sensor

22. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Simple piezoresistive
accelerometer

23. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Simple capacitive
accelerometer
 Cap wafer may be micromachined silicon, pyrex, …
 Serves as over-range protection, and damping
 Typically would have a bottom cap as well.
C(x)=C(x(a))
Cap wafer

24. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Simple capacitive pressure
sensor
C(x)=C(x(P))

25. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
ADXL50 Accelerometer +-50g
 Polysilicon
MEMS &
BiCMOS
 3x3mm die

26. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
ADXL50 Sensing Mechanism
 Balanced differential capacitor output
 Under acceleration, capacitor plates move changing
capacitance and hence output voltage
 On-chip feedback circuit drives on-chip force-feedback to
re-center capacitor plates.

27. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Analog Devices Polysilicon
MEMS

28. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
ADXL50 – block diagram

29. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Sense
Circuit
Electrostati
c Drive
Circuit
Proof
Mass
Digital
Output
MEMS Gyroscope Chip
Rotation
induces
Coriolis
acceleratio
n
J. Seeger, X. Jiang, and B. Boser

30. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
MEMS Gyroscope Chip
1µm
Drive
0.01Å Sense
J. Seeger, X. Jiang, and B. Boser

31. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Thermal Noise
 Fundamental limitation to sensor
performance due to thermal noise
 “White” noise, Johnson noise, Brownian
motion all the same
Not the same as flicker, popcorn, 1/f noise
 Equipartition theorem (energy perspective)
every energy storage mode will have ½ kBT of
energy
 Nyquist (power perspective):

32. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Equipartition ½ kBT = 4x10-21
J @ room temperature
(300K)
 ½ C V2
= ½ kBT
C=1pF  Vn = 60uV (RMS value)
 ½ k x2
= ½ kBT
K = 1N/m  xn = 0.06nm
 ½ m v2
= ½ kBT

33. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Nyquist
 PN = 4 kBT B
 In a resistor
PN = VN
2
/R = 4 kBT B
VN = sqrt(4 kBT R B)
 = sqrt (4 kBT R) sqrt(B)
If R = 1kZ then
VN = 4nV/sqrt(Hz) sqrt(B)

34. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Summary
 Resistive and capacitive sensors most
common
 Sensing, amplification, filtering, feedback
on the same chip ~$2
 Minimum detectable signal limited by
thermal noise

35. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Thank You

36. RAISE Teachers Workshop @ Polytechnic University Monday August 29, 2005
Thank You