Modern Instrumentation PHYS 533CHEM 620

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Modern InstrumentationPHYS 533/CHEM 620

• Lecture 11
• Light, Force, Strain, and Pressure Sensors
• Amin Jazaeri
• Fall 2007

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Classification of Sensors

• Proprioceptive (Internal state) v.s.
  Exteroceptive (external state)
• measure values internally to the system (robot),
  e.g. battery level, wheel position, joint angle,
  etc,
• observation of environments, objects
• Active v.s. Passive
• emitting energy into the environment, e.g.,
  radar, sonar
• passively receive energy to make observation,
  e.g., camera
• Contact v.s. non-contact
• Visual v.s. non-visual
• vision-based sensing, image processing, video
  camera

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Light energy

• For a sensor, were interested in the light power
  that falls on a unit area, and how well the
  sensor converts that into a signal.
• A common unit is the lux which measures apparent
  brightness (power multiplied by the human eyes
  sensitivity).
• 1 lux of yellow light is about 0.0015 W/m2.
• 1 lux of green light (50 eff.) is 0.0029 W/m2.
• Sunlight corresponds to about 50,000 lux
• Artificial light typically 500-1000 lux

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Electromagnetic Spectrum
Visible Spectrum
700 nm
400 nm
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Light sensors

• Simplest light sensor is an LDR (Light-Dependent
  Resistor).
• Optical characteristics close to human eye.
• Can be used to feed an A/D directly without
  amplification (one resistor in a voltage
  divider).
• Common material is CdSSensitivity dark 1 M?,10
  lux 40 k?,1000 lux 400 ?.

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(a) General block diagram of an optical
instrument. (b) Highest efficiency is obtained by
using an intense lamp, lenses to gather and focus
the light on the sample in the cuvette, and a
sensitive detector. (c) Solid-state lamps and
detectors may simplify the system.
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Light sources and detectors

• Sources
• Incandescent bulb
• Light emitting diode (LED)
• Gas and solid state lasers
• Arc lamp
• Fluorescent source

• Detectors
• Thermal detector (pyroelectric)
• Photodiode
• Phototransistor
• Charge-coupled device (CCD)
• Photoconductive cell
• Photomultiplier tube

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• Block diagram of a single beam spectrophotometer.
  The prism serves as the dispersing device while
  the monochromator refers to the dispersing device
  (prism), entrance slit, and exit slit. The exit
  slit is moveable in the vertical direction so
  that those portions of the power spectrum
  produced by the power source (light source) that
  are to be used can be selected.

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Photoemissive Sensors
Phototube have photocathode coated with alkali
metals. A radiation photon with energy cause
electron to jump from cathode to anode. Photon
energies below 1 eV are not large enough to
overcome the work functions, so wavelength over
1200nm cannot be detected.
Photomultiplier An incoming photon strikes the
photocathode and liberates an electron. This
electron is accelerated toward the first dynode,
which is 100 V more positive than the cathode.
The impact liberates several electrons by
secondary emission. They are accelerated toward
the second dynode, which is 100 V more positive
than the first dynode, This electron
multiplication continues until it reaches the
anode, where currents of about 1 mA flow through
RL. Time response lt 10 nsec
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Photoconductive Cells
Photoresistors a photosensitive crystalline
materials such as cadmium Sulfide (CdS) or lead
sulfide (PbS) is deposited on a ceramic
substance. The resistance decrease of the
ceramic material with input radiation. This is
true if photons have enough energy to cause
electron to move from the valence band to the
conduction band. light-dependent resistors
(LDRs) are slow, but respond like the human eye
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Photojunction Sensors
Photojunction sensors are formed from p-n
junctions and are usually made of silicon. If a
photon has enough energy to jump the band gap,
hole-electron pairs are produced that modify the
junction characteristics.
Photodiode With reverse biasing, the reverse
photocurrent increases linearly with an increase
in radiation. Phototransistor radiation
generate base current which result in the
generation of a large current flow from collector
to emitter. Response time 10 microsecond
Voltage-current characteristics of irradiated
silicon p-n junction. For 0 irradiance, both
forward and reverse characteristics are normal.
For 1 mW/cm2, open-circuit voltage is 500 mV and
short-circuit current is 8 mA.
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Photovoltaic Sensors
Photovoltaic sensors is a p-n junction where the
voltage increases as the radiation increases.
Spectral characteristics of detectors, (c)
Detectors. The S4 response is a typical phototube
response. The eye has a relatively narrow
response, with colors indicated by VBGYOR. CdS
plus a filter has a response that closely matches
that of the eye. Si p-n junctions are widely
used. PbS is a sensitive infrared detector. InSb
is useful in far infrared. Note These are only
relative responses. Peak responses of different
detectors differ by 107.
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Photovoltaic Sensors

• Photovoltaic
• light falling on a pn-junction can be used to
  generate electricity from light energy (as in a
  solar cell)
• small devices used as sensors are called
  photodiodes
• fast acting, but the voltage produced is not
  linearly related to light intensity

A typical photodiode
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Photodiode vs. Photoresistor

• Photoresistor simple but slow
• Photodiode/phototransistor complex but fast
• Phototransistor vs. Photodiode
• Higher current
• Slower (100KHz)
• Higher dark current

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Light sensors high end

• At the cutting edge of light sensor sensitivity
  are Avalanche photodiodes.
• Large voltages applied to these diodes accelerate
  electrons to collide with the semiconductor
  lattice, creating more charges.
• These devices have quantum efficienciesaround
  90 and extremely low noise.
• They are now made withlarge collection areas
  andknown as LAAPDs (Large-Area Avalanche
  Photo-Diode)

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Light sensors cameras

• Two solid-state camera types CCD and CMOS.
• CCD is the more mature technology, and has the
  widest performance range.
• 8 Mpixel size for cameras
• Low noise/ high efficiency for astronomy etc.
• Good sensitivity (low as 0.0003 lux, starlight)
• CCDs require several chips,but are still cheap
  (50 )
• Most CCDs work in near infraredand can be used
  for night visionif an IR light source is used.

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Light sensors cameras

• CMOS cameras are very compact and inexpensive,
  but havent matched CCDs in most performance
  dimensions.
• Start from 20(!)
• Custom CMOS camerasintegrate image
  processingright on the camera.
• Allow special functions likemotion detection,
  recognition.

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Polarized Light

• Normal light light wave travels at all
  orientation (w.r.t. horizon)
• Polarized light all the light traveling in a
  given orientation.
• Two normal filters gt no passing light
• The amount of light can be controlled

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Applications

• object presence detection
• object distance detection
• surface feature detection (finding/following
  markers/tape)
• wall/boundary tracking
• rotational shaft encoding
• bar code decoding

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Sensor limitations

• Light reflectivity
• Surface color
• Black does not reflect
• White reflects
• Texture
• Ambient light
• Measure with and w/o emitter
• Subtract from each other
• Sensor calibration
• Calibration to be done repeatedly. Why?
• gt Partially observable

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Force Sensing
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Sensing Element in Force Sensors

• There are many types of sensors can be used to
  measure force (or torque)! Resistive type force
  sensors, such as strain gages ands load cells,
  are very commonly used in force measurements.

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Load Cell (Example of Spec. Items)
Performance Load range 5 to 250 lbs
Non-Linearity ?0.05 F.S. Hysteresis
?0.03 F.S. Non-Repeatability ?0.03 F.S.
Output 3 mV/V Resolution Infinite Environmen
tal Temp. operating 0 to 130 F Temp.
compensated 30 to 130 F Mechanical Static
overload 50 over capacity
Full Scale
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Force sensors - Strain Gauges

• Strain gauge - The main tool in sensing force.
• Strain gauges, measure strain
• Strain can be related to stress, force, torque
  and a host of other stimuli including
  displacement, acceleration or position.
• At the heart of all strain gauges is the change
  in resistance of materials due to change in their
  length due to strain.

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Strain Gages
Characteristics 1) able to measure strains of
?1mm/m 2) small in size and light in weight 3)
able to response to high frequency signals 4)
wide range of linear response 5) has stable
calibration constant (gage factor) 6) flexible
in use and wide range applications 7) low in
cost 8) easy compensation to various factors
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Fundamentals of Strain Gages
eT
Transverse strain
Axial strain
eL
F
F
A
l
?l
Poissons ratio
Material resistivity
Elastic Modulus
Element length
The resistance of a strain gage
Cross section area
When a strain gage is strained, the change in
resistance is
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Strain Gages
Relative change in resistance
Because
Poissons ratio
Then
Define a Gage factor G
Gage factor of a strain gage
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Example Specs of Strain Gages
Temperature Range Normal -100 to 350
F Short-Term -320 to 400 F Strain Range 3
for gage lengths under 1/8 in 5 for 1/8 in and
over Fatigue Life 105 cycles at 1500
microstrain 106 cycles at 1500 microstrain
with low modulus solder.
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Strain Gauge

• For any given strain gauge the gauge factor is a
  constant
• Ranges between 2 to 6 for most metallic strain
  gauges
• From 40-200 for semiconductor strain gauges.
• The strain gauge relation gives a simple linear
  relation between the change in resistance of the
  sensor and the strain applied to it.

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Stress and Strain
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Two-axis strain gauge
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120 degree rosette
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45 degree rosette
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45 degree stacked rosette
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membrane rosette
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Semiconductor strain gauges

• Operate like resistive strain gauges
• Construction and properties are different.
• The gauge factor for semiconductors is much
  higher than for metals.
• The change in conductivity due to strain is much
  larger than in metals.
• Are typically smaller than metal types
• Often more sensitive to temperature variations
  (require temperature compensation).

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Semiconductor strain gauges

• All semiconductor materials exhibit changes in
  resistance due to strain
• The most common material is silicon because of
  its inert properties and ease of production.
• The base material is doped, by diffusion of
  doping materials (usually boron or arsenide for p
  or n type) to obtain a base resistance as needed.
  
• The substrate provides the means of straining the
  silicon chip and connections are provided by
  deposition of metal at the ends of the device.

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Amplification for Strain Gages

• Sensitive instrumentation is required to measure
  the small changes in resistance produced by
  strain gauges.
• Wheatstone bridge is typically used to measure
  resistances accurately and dynamically over a
  very large range (1 to 1,000,000 W).

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Application of Strain Gages
Strain gages are used in cantilever type load
cells
R1
R3
l
x
e
R2
w
R4
t
-e
F

Vs
-
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Application of Strain Gages
F
Strain gages are used in pillar type load cells
R3
, R1
eT
, R4
R2
eL
F
Poissons ratio
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Application of Strain Gages
Strain gages are used in torque cells
Strained compressed
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Common Strain Gage Arrangements
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Practical Implementation of Strain Gages
Force Input
Strain gages
Bridge
Amplifier / filter
Computer

• Preparation
• clean with sandpaper
• degreaser solvent
• glue adhesive curing agent
• clams for curing cycle (24 h)

Parallel
Most critical step !
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Practical Implementation of Strain Gages

• Wheatstone bridge and amplification circuit

• Specifications
• Chip 741
• R1 R2
• R3 RF
• Gain RF / R1

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Tactile sensors

• Tactile sensors are force sensors but
• Definition of tactile action is broader, the
  sensors are also more diverse.
• One view is that tactile action as simply sensing
  the presence of force. Then
• A simple switch is a tactile sensor
• This approach is commonly used in keyboards
• Membrane or resistive pads are used
• The force is applied against the membrane or a
  silicon rubber layer.

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Tactile sensors

• In many tactile sensing applications it is often
  important to sense a force distribution over a
  specified area (such as the hand of a robot).
• Either an array of force sensors or
• A distributed sensor may be used.
• These are usually made from piezoelectric films
  which respond with an electrical signal in
  response to deformation (passive sensors).

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A tactile sensor
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Tactile sensors

• Operation
• The polyvinylidene fluoride (PVDF) film is
  sensitive to deformation.
• The lower film is driven with an ac signal
• It contracts and expands mechanically and
  periodically.
• When the upper film is deformed, its signal
  changes from normal and the amplitude and or
  phase of the output signal is now a measure of
  deformation (force).

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Pressure Sensors What is pressure? Pressure
force per area in fluids 1 N/m2 1 Pa
Pascal Engineers use the bar 1 bar 105 Pa 1
athmosphere 10 m of water column Ranges of
pressure measurement Athmosphere 1
bar Hydraulics, pneumatics 6 -10 bar Car
industry 1 - 5 bar (tyre), 20 bar (air
conditioning) Medicine Blood 100 mbar in the
human body 10 to 100 mbar Deep sea level 4.000m
400 bar Thin film processes 1 to 1000 Pa (0,01
to 10 mbar) Rough vacuum 10 Pa (changing pumps)
High vacuum down to 10-8 Pa
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Pressure sensors

• Pressure sensors come in four basic types
• Absolute pressure sensors (PSIA) pressure sensed
  relative to absolute vacuum.
• Differential pressure sensors (PSID) the
  difference between two pressures on two ports of
  the sensor is sensed.
• Gage pressure sensors (PSIG) the pressure
  relative to ambient pressure is sensed. (Most
  common)
• Sealed gage pressure sensor (PSIS) the pressure
  relative to a sealed pressure chamber (usually 1
  atm at sea level or 14.7 psi) is sensed.

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Piezoresistive pressure sensors

• Piezoresistor is a semiconductor strain gauge
• Most modern pressure sensors use it rather than
  the conductor type strain gauge.
• Resistive (metal) strain gauges are used only at
  higher temperature or for specialized
  applications
• May be fabricated of silicon
• simplifies construction
• allows on board temperature compensation,
  amplifiers and conditioning circuitry.

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Piezoresistive pressure sensors

• Basic structure
• two gauges are parallel to one dimension of the
  diaphragm
• The two gauges can be in other directions

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Piezoresistive pressure sensors

• The change in resistance of the two piezoresistos
  is

? is an average sensitivity (gauge) coefficient
and ?x and ?y are the stresses in the transverse
directions
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Piezoresistive pressure sensors

• Piezoresistors and the diaphragm are fabricated
  of silicon.
• A vent is provided, making this a gage sensor.
• If the cavity under the diaphragm is hermetically
  closed and the pressure in it is P0, the sensor
  becomes a sealed gage pressure sensor sensing the
  pressure P-P0.
• A differential sensor is produced by placing the
  diaphragm between two chambers, each vented
  through a port (figure).

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Differential pressure sensor
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Piezoresistive pressure sensors

• A different approach is to use a single strain
  gauge
• A current passing through the strain gauge
• Pressure applied perpendicular to the current.
• The voltage across the element is measured as an
  indication of the stress and therefore pressure.

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Construction

• Many variations
• Body of sensor is particularly important
• Silicon, steel, stainless steel and titanium are
  most commonly used
• Ports are made with various fittings
• The contact material is specified (gas, fluid,
  corrosivity, etc.)

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Various pressure sensors
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Miniature pressure sensors
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Pitran pressure sensors (absolute)
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150 psi differential pressure sensor
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100 psi absolute pressure sensor (TO5 can)
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15 and 30 psi differential pressure sensors
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Capacitive pressure sensors

• The deflection of the diaphragm constitutes a
  capacitor in which the distance between the
  plates is pressure sensitive.
• The basic structure (not shown) consists of two
  metalic plates
• These sensors are very simple and are
  particularly useful for sensing of very low
  pressure.
• At low pressure, the deflection of the diaphragm
  may be insufficient to cause large strain but can
  be relatively large in terms of capacitance.

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Capacitive pressure sensors

• The capacitance may be part of an oscillator,
• The change in its frequency may be quite large
  making for a very sensitive sensor.
• Other advantages
• less temperature dependent
• stops on motion of the plate may be incorporated,
  - not sensitive to overpressure.
• Overpressures of 2-3 orders of magnitude larger
  than rated pressure may be easily tolerated
  without ill effects.
• The sensors are linear for small displacement but
  at larger pressures the diaphragm tends to bow
  causing nonlinear output

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Magnetic( Inductive) pressure sensors

• A number of methods are used
• In large deflection sensors an inductive position
  sensor may be used or an LVDT attached to the
  diaphragm.
• For low pressures, variable reluctance pressure
  sensor is more practical.
• The diaphragm is made of a ferromagnetic material
  and is part of the magnetic circuit shown in
  Figure 6.32.

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Variable reluctance pressure sensor
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Magnetic pressure sensors

• The reluctance is directly proportional to the
  length of the air gap between the diaphragm and
  the E-core.
• Gap changes with pressure and the inductance of
  the two coils changes and sensed directly.
• A very small deflection can cause a very large
  change in inductance of the circuit making this a
  very sensitive device.
• Magnetic sensors are almost devoid of temperature
  sensitivity allowing these sensors to operate at
  elevated temperatures.

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Other pressure sensors

• Optoelectronic pressure sensors - Fabri-Perot
  optical resonator to measure small displacements.
  
• light reflected from a resonant optical cavity is
  measured by a photodiode to produce a measure of
  pressure sensed.
• A very old method of sensing low pressures (often
  called vacuum sensors) is the Pirani gauge.
• based on measuring the heat loss from gases which
  is dependent on pressure. The temperature is
  sensed and correlated to pressure, usually in an
  absolute pressure sensor arrangement.

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Pressure sensors - properties

• Semiconductor based sensors can only operate at
  low temperatures (?50 to 150?C).
• Temperature dependent errors can be high unless
  properly compensated (externally or internally).
• The range of sensors can exceed 50,000 psi and
  can be as small as a fraction of psi.
• Impedance is anywhere between a few hundred Ohms
  to about 100 k?, depending on device.
• Linearity is between 0.1 to 2 typically

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Pressure sensors - properties

• Other speciffications include
• Maximum pressure, burst pressure and proof
  pressure (overpressure)
• electrical output - either direct (no internal
  circuitry) or after conditioning and
  amplification.
• Digital outputs are also available.
• Materials used (silicon, stainless steel, etc.)
  and compatibility with gases and liquids are
  specified
• port sizes and shapes, connectors, venting ports
• cycling of the pressure sensors is also specified
  
• hysteresis (usually below 0.1 of full scale)
• repeatability (typically less than 0.1 of full
  scale).

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• How to construct a pressure sensor
• Pressure is transformed into deflection of a
  membrane
• Tasks to do
• Understand the elastic deformation of a membrane
• Construct a membrane
• Sense the deflection
• Construct a sensor housing
• Build an electronic circuit

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Metal membrane pressure sensor
Figure from Hesse, Schnell Sensoren für die
Prozess- und Fabrikautomation
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Capacitive pressure sensor Capacity of two plates
A Area d distance

• Advantages
• Sensitive
• Stable, small T-drift (ceramic technology)
• Small, E.g. 2mm Si chip size for eye pressure
  sensor
• Disadvantages
• Nonlinear (1/d)
• Electronics complicated
• Capacitive bridge circuit
• C - Frequency conversion

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Inductive pressure sensor Inductive bridge
circuit Very T-stable Large devices, large
membranes for small differential pressure
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• Si micromachined pressure sensor
• Why Si technology?
• Batch process gt1000 chips on a wafer
• Precise control of technology
• k-factor in Si is 100 (gtgt2!)
• Monolithic integration with electronics
• Very advanced and available technology
• Housing processes well known

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Piezoresistive effect in Si Semiconductors
deformation changes band structure ? large
change in conductivity ? k positive or negative ?
k sensitive on temperature, doping and crystal
orientation Longitudinal Current parallel to
strain ?? ?i Transversal Current vertical to
strain ?? ?i Rule of thumb for p-doped
Si Longitudinal k 100 Transversal k -100
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Principle of Si p-sensor
A longitudinal B transversal
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Silicon piezoresistive pressure sensor
Metal
Nitride
Piezoresistor
Oxide
Si
Si
Pyrex glass
Metal socket
Figure from Bonfig, Sensoren
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Low pressure vacuum gauges
Ranges Rough vacuum at 1 Pa to 100 Pa. In this
range, mechanical pumping is switched to turbo
pumping for high vacuum. A sensor is needed to
trigger the valves. High vacuum below 0,1 Pa.
This is measured for process control. Usually,
a vacuum system has one sensor for control of the
pumping down and pump control (pirani type) and
one sensor for the control of the final pressure
(ionisation type).
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Quartz Pressure Sensor

• A typical Quartz crystal sensor with inbuilt
  micro-electric circuitry and a diaphragm.
• These sensors measure dynamic pressures, and are
  not generally used for static pressure sensing.
• Proper and accurate alignment of the sensor is
  very important for higher sensitivity.
• Sensors used in high temperature conditions(e.g.
  combustion chamber of an engine) use either
  recess mounting, baffled diaphragm or thermal
  protection coatings to reduce negative signal
  effects.

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Pros and Cons

• Have a high Stiffness value and produce a high
  output with very little strain.
• Ideal for rugged use.
• Excellent linearity over a wide amplitude.
• Ideal for continuous online condition monitoring
  smart systems.

• Can be used only for dynamic pressure sensing as
  in case of static sensing the signals will decay
  away.
• Operation over long cables may affect frequency
  response and introduce noise and distortion, the
  cables need to be protected.

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Typical Application-Combustion Monitoring

• Pressures developed during the combustion process
  is continuously measured by sensors mounted on
  the cylinder heads.
• Continuous Pressure monitor(CPM) systems are the
  basic data acquisition and data reduction
  software and hardware units.