PowerPointPrsentation

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Sensors and Measurement Systems Walter
Lang IMSAS Universität Bremen WS
2007/2008 Stand 4.10.2007
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• Object
• Sensors and measurement systems is an independent
  one semester course which will give you a basic
  understanding in sensors, measurement and
  microsystems technology
• After this course, you should be able to
• Name and explain important sensors
• Apply characterization parameters for sensors
• Choose sensors for a given application and apply
  them
• Analyze sensor systems
• Understand micromachining technologies for
  sensors

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• Content
• Sensors
• Thermal sensors
• Sensor Technology
• Strain, force and pressure
• Inertial sensors accelerometers and gyroscopes
• Flow sensors
• Magnetic sensors, displacement, rotation,
  proximity

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Organisation The course is 2 SWS lecture and 1
SWS tutorial. There are 10 weeks lecture (3
hours) and 4 weeks tutorial (3 hours). There
will be an oral exam You have to attend the
tutorial in order to register for the exam.
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Useful books 1   Ian Sinclair Sensors and
Transducers, ISBN 0 7506 4932 1 Good general
overview in sensors and sensor electronics. Does
not cover technology.   Gregory Kovacs
Micromachined Transducers Sourcebook ISBN 0 07
290722 3 Micromachining Technology and Sensor
Applications   John Bentley Principles of
measurement systems ISBN 0130430285 Pearson
Prentice-Hall Verlag Stephen Senturia
Microsystem Design, ISBN 0 7923 7246 8 Throry,
modelling and layout, but also intersting case
studies. Case study on the ADXL accelerometer  
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Useful books 2   S. Wolf and R. N. Tauber
Silicon processing for the VLSI aera ISBN 0
961672 3 7 Textbook on proces technology for
microelectronics. Thin film technology,
lithography   S. M. Sze VLSI Technology, ISBN 0
07 062686 3 The most famous classical textbook on
semiconductor technology for electronics. Thin
film technology, lithography, etching, but not
micromachining   Ljubisa Ristic sensor
technology and devices, ISBN 0 089006 532
2 Micromachining technology and sensor devices
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1. Thermal sensors Overview How to measure
temperature Thermoelectric effect Resistive
thermometers Thermal Sensors Infrared radiation
thermometry Membrane sensors System and
electronics Sensor characteristics Noise in
sensors
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How to measure T Bi-metal Classical measurement
due to thermal expansion Thermal expansivity of
metals
Still in use for simple thermometers and
thermostats Large hysteresis In microsystems used
as actors There are many unwanted bimetals which
cause thermal stress
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Expansion of a fluid or gas Conventional
thermometer with mercury or alcohol Gas
thermometer is standard method Application T
switch in old fridges Figure from Sinclair
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Thermoelectric effect, Seebeck effect
Thermoelectric Voltage E Seebeck coefficient b
-70...50 µV/K for metals b -500...500 for Si
Highly nonlinear
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Origin of the thermoelectric effect
The energy of the electrons is determied by the
Fermi level, which is different for different
metals. This generates a contact voltage.
Isothermal circuit the contact voltages
cancel Nonisothermal circuit thermoelectric
voltage
Figure by Gerthsen Electrons Too many not
enough T low T high
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Practical use of thermocouples
K-type NiCR/NiAL 4 mV/100C 0C...1100C J-type Fe
/CuNi 5,3 mV/100K 0C...800C R-type PtRh/Pt 0,6
5 mV/100C 0C...1400C Poly-Si metal ? 20
mV/100 K (Depends on doping) Sold as thermowire
or as readymade thermocouples Go to very high T.
Accuracy /1,5C to /-2,5C Small and fast
Wire resistivity unimportant Measure T-difference
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Resistive thermometers
Metal resistor thermometers Pt 100 100O at
0C Also Ni 100, Ni 1000
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Resistive thermometers
Advantages Precise Problems Limited
T-range Self Heating Large, slow Wire resitivity
has an influence (3-lead or 4-lead
technique) Measure T absolute
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Resistive thermometers thermistors
Metals have a rather small T-coefficient
(TCR) Some semiconductors and oxides have very
high TCR NTC negative TCR, oxides and exotic
metals, T lifts electrons into the conductive
band. High R, High sensitivity, very high
nonlinearity PTC positive TCR up to 100/C,
T-switches, ceramic materials, securuty circuits
against overcurrent
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Silicon thermometers
Resistive thermometers highly doped Si, TCR
positive, spreading resistance sensors as
T-sensors p-n junction diode I depends on T,
not very reproducible. Used for T-measurement in
electronic circuit, E.g. an ASIC with a sensor
electronics does T-compensation with an on chip
thermometer
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Pyroelectric effect
Some dielectric materials have a natural dipole
moment (like a permanent magnet has a natural
magentisation). This dipole moment is changed if
the material is deformed (piezoresistive effect)
or if the temperature is chaged (pyroelectric
effect). Only materials with low crystal
symmetry show these effects. Single crystalline
Quartz Polycristalline ceramics like BaTiO3
(Bariumtitanat)
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Sensitivity and time constant
First order time constant. Electrical analogen
RC circuit
P Power T Temperature increase C Thermal
capacity of device(J/K) K Thermal conductivity
of device (W/K) t Thermal time constant
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Sensitivity and time constant
First order time constant. Electrical analogen
RC circuit

• Response of a first-order element to a unit step
• (Figure from J. Bentley Meas. Syst.)
• Consequences
• Tradeoff Sensitivity vs bandwidth
• 2. Sensor too slow
• - Phase shift
• - Signal distortion
• - Reduced sensitivity (Amplitude too small)

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Radiation thermometry
A pyrometer measures the temperature of an object
from the distant. The sensor has a measurement
spot, which is heated by the IR radiation emitted
by the object. The temperature increase of this
spot is measured.

• Thermal membrane sensor
• Substrate (Si Wafer)
• Membrane (Silicon Nitride)
• Functional layers (Temperature measurement, IR
  absorption)
• Electrical contacts
• (Fig. From J. Hoffmann)

IR
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Infrared radiation
At ?10µm most materials show e 0,95...1,0
(except metals, they reflect) The membrane also
emits IR radiation
Fig. by E. Schrüfer (2.139)
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Thermal IR sensors Pyroelectric Low
performance, person detection for light control
and burglar alarm Thermopiles High performace
for T measurement. Ear thermometer, Pyrometry,
microwave ovens, air conditioning Bolometers (Re
sistive T-Measurement) Very low noise, but
need chopping for baseline. Research,
spectrsocopy
Fig. by www.thermografie.de
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Consequence for thermal sensors
K small sensitive, slow C small fast
Tradeoff sensitivity vs response time Thin
insulating membrane (Siliconnitride d300nm,
k2,5 W/mK) gives small C and small K
Thermopile From www.Perkinelmer.com
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Model for a thermopile I Power input IR
radiation Prad Power output Radiation (very
small, neglected) Convection (heat transfer to
the gas) Pconv (Kconv) Conduction in the
membrane and metal Pcond (Kcond) Convection
Newtons law of cooling Heat loss is
proportional to the Temperature. Wall heat
transfer coefficient a.
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Model for a thermopile II Conduction in the metal
(membrane neglected)
a Wall heat transfer coefficient 100W/m2K A
Area 600 µm x 600µm N number of thermocouples
4 x 13 52 l Length of interconnects 350
µm kSi thermal cond. of Si 150 W/mK kAl
thermal cond. of Al 230 W/mK
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Model for a thermopile III
Typical sensitivity of commercial sensors is 20
to 50 V/W
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System and electronics analog Temperature of
sensor head must be corrected. Thermistor in
sensor head. Nonlinearities reduce accuracy
to/-4.
Fig. by www.perkinelmer.com
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System and electronics digital Nonlinearities
corrected by calibration curve /-0,1 accuracy,
used for the ear thermometer
Fig. by www.perkinelmer.com
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• Other thermal sensors
• Many measurement problems are measured by
  measureing the heat (energy) involved in a
  process.
• Flow thermal anemometer
• Electrical power true RMS powermeter
• Chemical composition of combustible gases
  Catalytic gas sensor or pellistor
• Humidity dew point detector
• Pressure thermal vacuum sensor
• Inclination thermal inclinometer
• Analysis of chemical processes microcalorimeter

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Sensor characteristic The characteristic of a
sensor describes the output (Volt, sometimes Ohm)
as a function of the input (physical quantity,
E.g. force) The Responsivity or Sensitivity S
Output signal / input signal Dimension is the
voltage per measured quantity E.g. thermopile
V/W
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Characteristic of a pressure sensor
Characteristics of a 400 mBar pressure sensor
Voltasge V
Pressure mBar
Range 400 mbar Overrange 1000 mbar Sensitivity
175 mV/bar
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The Range or full scale is the maximum value a
system does measure The overrange is the value
which it will shurely survive The Resolution is
the smallest signal which can be distinguished
from a neighboring value Dimension Measured
quantity Limits of resolution may be - A/D
conversion - Display - Noise
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Thermal noise in sensors
The thermal fluctuation (thermal noise) in a
system is correlated with the energy dissipation
(damping, resistivity). (Theorem of dissipation
and fluctuation) For the electrical resistor,
thermal noise is described by Nyquists
formula. Externally measured quantity Noise
voltage UN Dissipation Resistivity
kB Boltzmanns constant
White noise has the same amplitude for all
frequencies (not feasible for large frequencies)
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How to reduce thermal noise

• Reduce R Thermopiles (R50kO) have a higher
  noise level than bolometers (R200O).
• Reduce T In laboratory application, for
  sensitive measurement the detectors are cooled
  with liquid Nitrogen or liquid Helium or by using
  Peltier modules.
• Reduce bandwidth With a small band filter df is
  reduced. Often this is a low pass only which cuts
  of high frequency noise. Very low bandwidth is
  achieved with a sychronous rectifier.
• A noise specification without information about
  the bandwidth is useless.
• The measured quantity which generates an output
  as high as the noise level is called noise
  equivalent signal.

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Noise of a thermopile
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Accuracy The maximum difference between the
sensor output and the physically true value which
is guaranteed under all specified circumstances
(Temperature!) is called accuracy. The accuracy
is specified. Whether a specific sensor is within
this specs, can be measured. A Force sensor
might have A range of 1 N An overrange of 2 N A
noise equivalent force of 0,1 mN After a A/D
conversion of 8 bit a resolution of 4 mN An
accuracy of 10 mN from -20 to 80
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How to define an accuracy Class 1 normally
means maximum deviation is 1 of the full
scale. An accuracy of 1 of full scale is very
inaccurate for small values. An accuracy of 1 of
the measured value is impossible for very small
values. The precise specification is often
defined in sections or given as list or as a
plot. Example Pt100 resistive thermometers are
defined by a list
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Specifications of Pt100 resistive thermometers
DIN EN 60751 www. ephy-mess.de/
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Nonideal characteristic Temperature drift The
characteristic changes with temperature. Long
term drift The characteristic changes with
time. Offset The characterstic does not pass
through the origin. This must not be a problem,
but the offset is often temperature dependent
instable baseline. Nonlinearity Measured by End
point straight line method Make a straight line
from the low end of the range to the high
end. Determine maximum difference from
characteristic to straight line on the output
scale. Divide by difference from the low end to
the high end on the output scale Example
pressure sensor nonlinearity is 0.5
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Nonlinearity End point straight line
Maximum difference at 5 mbar 0.4 V Output
range 9V Nonlinearity 4,4