Download Tutorial 5 - Electrical and Computer Engineering

Tutorial 5
Derek Wright
Wednesday, February 16th, 2005
Sensors and Image Systems
•
•
•
•
•
Physical Principles of Sensors
Optical Imaging Systems
IR Imaging Arrays
Electronic Nose
Tactile Sensors and Arrays
Sensor Basics
• Sensors are transducers
• Transducers convert one form of energy to
another
– Alternator in your car turns mechanical into
electrical
– Engine converts chemical to thermal to
mechanical
– Eyes convert light into electrical
Sensor Basics
• Sensors either
– Directly convert one form to another
– Use one form to change (modulate) another
• Direct Conversion:
– Solar panel: Light  Electricity
– Thermocouples: Heat  Electricity
• Modulating:
– Thermoresistive, Optoresistive: Changing resistance
must be have current driven through it to measure
Biological Sensor Arrays - Eyes
• The eye is a
biological form of
a sensor array
• It consists of an
array of
transducers (rods
and cones)
• The signals are
transmitted by
neurons along
axons
Optical Imaging Systems
• Array structures allow multidimensional
measurement to occur
• Optical Imaging Systems:
– Charge Coupled Devices (CCDs)
– CMOS Cameras
– X-ray Imagers
Charge Coupled Devices
• Incident photons cause creation of electron-hole pairs
• Electrons move to insulator boundary under bias for
storage
• Charge is shifted out of a row or column by a shifting
potential
• Cannot be integrated on the same substrate as
accompanying electronic circuits
CCD Operation
CCD Operation
CCD Operation
• http://micro.magnet.fsu.edu/primer/java/photomi
crography/ccd/shiftregister/index.html
• http://www.extremetech.com/article2/0%2C3973
%2C15465%2C00.asp
CMOS Cameras
• Can be created with standard CMOS processes
• Can be integrated with accompanying electronic
circuits
• An incident photon creates an electron-hole pair
in a reverse biased diode
• Configured to cause charge to drain off of a
capacitor
– Photon absorption  capacitor voltage decrease
CCD vs. CMOS Cameras
• CCD has a better Fill Factor (FF)
– Better image quality and photon capture
– Lower noise
• CCD only outputs the analog charge
– Must be converted to digital by another chip
• CMOS has on-chip integration
– Results in high-speed and low-power
– Reduces flexibility, but decreases cost
X-ray Imagers
• Amorphous thin film techniques can
produce large-area x-ray detectors
• Two types:
– Indirect
– Direct
• On-pixel amplification means fewer x-rays
needed to make an image  Safer!
p-i-n Structure
Ec
h
Ev
X-ray Imagers
• Indirect Method:
– A top layer of phosphor turns the x-ray into a
visible discharge
– Visible photons are then detected by
amorphous silicon (a-Si) p-i-n photodiodes
X-ray Imagers
• Direct Method:
– Amorphous selenium (a-Se) absorbs x-rays
– A layer of a-Se with a huge E-field is used
– It converts x-rays into electron-hole pairs
– E-field separates them into current
IR Imagers
• Two detection methods:
– Quantum (photon  e-h pairs)
– Thermal (photon  temp)
• Useful in night vision systems
• Police use them in Ontario to find pot grow houses
IR Imagers
• Quantum Detection:
– Photons have an energy hf = hc/
– If this energy is bigger than the bandgap of a
detector material, e-h pairs will be created
– IR has lower energies than visible, so the
bandgap has to be reduced
– Detector bandgaps can be tuned from 0eV up
– These detectors must operate at very low
temperatures
• Restricted to special uses
IR Imagers
• Thermal Detection:
– IR photons will turn into heat when they hit
certain materials
– The heat can be detected and imaged
– A pyroelectric material will generate a voltage
or current proportional to the IR power shining
on it
Microcalorimetric Sensors
• A heated chamber is kept at a constant
temperature
• An incoming gas flow is burned
• When the gas burns it releases heat energy
• The released heat results in less heat from the
chamber to keep a constant temperature
• Released heat energy can be measure by how
much less the chamber needs to heat the gas
flow
Microcalorimetric Sensors
Electrochemical Cells
• Use a catalyst to convert molecules to be
measured into ions
• Two modes of operation:
– Amperometric: The ions are moved through a
catalyst and electrolyte to create a current
– Potentiometric: The ions charge a capacitor
and appear as a voltage
Electrochemical Cells
Amperometric
Potentiometric
Acoustic Wave Devices
• Tiny free-standing beams are created
through micromachining
• They have a mechanical resonance
frequency ()
• They are coated in a polymer that adsorbs
the specific molecules to be observed
• More molecules stick  mass  
Acoustic Wave Devices
Gas-Sensitive FETs
• A small channel lets gas pass between the
gate and the substrate (channel)
• The underside of the gate can be coated
with a material to adsorb certain gasses
• When the gasses adsorb into the coating,
it changes the threshold voltage
Resistive Semiconductor Gas
Sensors
• O2 can act as a p-type dopant in silicon
• It attacks point defects
• The number of point defects increases
with temperature
– The Si must be heated
• The more O2 in the silicon, the higher the
conductivity
Resistive Touchscreens
• Two flexible resistive layers are separated
by a grid of spacers
• When the two layers are pressed together
the resistance can be measured between
several points
• This determines where the two resistive
layers contacted
Resistive Touchscreens
Capacitive Touchscreens
• A conductive layer is covered with a
dielectric layer
• The finger represents the other plate of the
capacitor
• A kHz signal is transmitted through the
conductive plate, the dielectric, and the
finger to ground
• The current from each corner is measured
to determine the touch location
Capacitive Touchscreens
Ultrasound Touchscreens
• Ultrasonic sound waves (>40 kHz) are
transmitted in both the horizontal and
vertical directions
• When a finger touches the screen, the
waves are damped
• Receivers on the other side detect where
the sound was damped
• Multiple touch locations are possible
Ultrasound Touchscreens
Fingerprint Sensors
• An array of tiny capacitive sensors
• Works similarly to the capacitive
touchscreen
– Finger works as one plate of a capacitor
– Chip works as the other
• Sensors are small enough to determine if
a fingerprint ridge is touching it
• An image is produced
Fingerprint Sensors
Thank You!
• This presentation will be available on the
web.