PowerPointPrsentation

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3. Force and pressure
Microphones Measuring sound Types of
microphones 4.10.2007
Force measuremeint The piezoresistive
effect Strain gauges Pressure sensors Measuring
pressure Types of sensors Metal membranes Silicon
membranes
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Strain and stress Definition Strain Change of
length / length Dimension 1 Elastic
deformation - plastic deformation - breaking
Stress Force / Area Dimension N/m2 (same
as pressure)
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Piezoresistive effect Mechanical deformation ?
electrical signal R?l/A R Resistivity ?
Specific resistivity l length A Area When
we pull a wire, it becomes longer and thinner
Piezoresitivity ?R/Rk ?l/l k piezoresistive
Coefficient Assume constant Volume k2 For
metals this is a good approximation
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Thin film strain gauges Metal meander on a
flexible substrate. This is glued on the deformed
body. Can be bought ready made with many
geometries - Analysis of deformation Aircrafts,
cars, machines - Weighing machines - Force
sensors Electrical readout Cross-coupling with
temperature Wheatstone bridge circuit Resistivity
of interconnects 4-wire circuitry
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Geometries
Fig. J. Bentley
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Electrical readout Cross-coupling with
temperature Apply several gauges and use
Wheatstone bridge Signal enhanced, Temperature
effect strongly reduced
tension
Two more gauges on the other side
compression
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Advantages and problems Good resolution of
110-6 1 mg is measured within 1 kg Cheap,
easy to apply, versatile k2 leads to very small
signals Meander resistors do not allow lokal
measurement
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• Technology of strain gauges
• Glue on devices
• Metal thin film on a flexible foil
• Structure by lithography and etching
• Passivateion
• Glue on surface of body to be deformed
• Integrated devices
• Deposit insulating layer
• Deposit metal thin film
• Structure
• Passivate

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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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Mechanical pressure measurement Manometer
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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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Vacuum gauges of Pirani type
Two wires are used. One is heated, the other is
a sensor. At low pressure the heat does not reach
the sensor, since there is no medium for heat
transport any more and thus the thermal
conductivity of the air is reduced. Range 0,1
... 100 Pa Stands athmospheric pressure
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Ionisation vacuum gauges
A hot tungsten filament emits electrons. These
ionise the gas. The number of ions is
proportional to pressure. It is measured by the
current to the anode. Range High vacuum ltlt0,01
Pa. Does not stand high pressure since the hot
filament would oxidise. Increase of sensitivity
lt10-4 Pa Magnetic field ? Electrons go on a
spiral path ? more ionization ? more current.
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Sound Elastic wave of vibrating air
pressure Speed 330 m/s in air, wavelength at 1000
Hz 33 cm Dimension Pa Normally sound is
measured in dB Dezibel (or phone) not a
dimension, but a dimensionless number
p0 0 dB is the faintest sound the human ear can
hear at 1000 Hz. 120 dB is the noise level which
causes pain 20dB is times 10 in sound pressure
physical level 20dB is twice as loud
physiological level
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Sound
lt 30 Hz infrasound. Not audible but causes
injuries gt 20.000 Ultrasound, used for
measurement of distance and flow velocity
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Theory of microphones The diaphragma transduces
the sound pressure p into displacement x
Stiffness must be small ? membrane must be weak
to follow sound wave ?0 10 kHz for Handy, 20
kHz for HiFi, 60 kHz for measurement Tradeoff
resonance frequency vs sensitivity mass must be
small A good microphone needs a light membrane
with low stress.
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Types of microphones Moving coil type Moving
coil in magnetic field. The coil catches 50 Hz
hum add stationary compensating coil
Source Sinclair, Sensors and transducers. Very
good chapter on microphones
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Types of microphones piezoelectric
type Piezoelectric materials have a permanent
dipole moment slike pyroelectric materials. If
they are deformed, they show an electric charge
at the surface. Quartz or piezoceramics Can work
in water, usable also for ultrasound High voltage
output, cheap mass production Not for high
quality, since high impedance of M? hum at 50 Hz
cannot be avoided
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Piezoelectric effect
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Piezoelectric effect applications

• Small headsets, beepers
• Small microphones
• Actuators
• Emitters and sensors for ultrasound

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Types of microphones Capacitor microphone
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Types of microphones Capacitor
microphone Capacitor with constant charge Q Plate
capacity area A and distance d
?C measured without a high frequency
carrier Voltage is in first order linear with
respect to the pressure
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• Electret microphone
• Capacitor microphones need rather high voltages
  to sustain the charge needed to achieve
  sensitivity
• Electrets are materials which are permanently
  charged (like permanent magnets)
• How to make an electret
• Hot plastic, cool down under electric field
• Insulator (E.g. teflon) charged with electron
  beam
• Electret on backplate small voltage, large
  sensitivity
• Very good microphones

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Types of microphones Silicon micromachining Focus
medium quality, cheap mass procudtion.
Application handies. Resonance 10 kHz CalTech,
USA Teflon Electret, loaded by E-beam.
Electrodes Silicon Nitride
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Silicon microphone one chip Source A. Dehe
Sensors Actuators A113 (2007)
(InfineonTechnologies)