Transcript Department of Optical Engineering Zhejiang University

Advanced Sensor Technology
Lecture 7
Jun. QIAN
Department of Optical Engineering
Zhejiang University
A Review of Lecture 6
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Diaphragm deformation equations
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Linear limit:
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center deflection < thickness
Corrugated diaphragm ~10X thickness
Capacitive sensor or strain gauge
Some silicon based example calculations carried
out,
An alternative technology: Kavlico sensor
and learn as much as we can from the way it
is designed, built, packaged, and priced.
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Basic Intent
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Overview basic techniques for sensing
temperature
Some techniques for the measurement of
flow will be briefly highlighted
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Thermometers - traditional techniques
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There are a number of wellknown historical technologies for
the measurement of temperature.
 mercury thermometer, in
which a reservoir of mercury
is sealed in a glass container
under vacuum.
 When the reservoir is heated,
the mercury expands, rising
through a long thin column,
upon which a graded ruler has
been etched.
 What sort of sensitivity can be
expected for such a system?
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Sensitivity and Resolution Concerns
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thermal expansion coefficient (CTE) of mercury ~ 30 PPM/K
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If we assume that dimensions of the container do not change
appreciably
The thermal expansion coeffiecient of fused quartz is about 150x
smaller than that of mercury, so this approximation is roughly valid.
the mercury in the column expands linearly with temperature.
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If we want 1 mm/K at room temperature, and we have a reservoir
volume of 0.1 cm3, we need a 660m column:
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the sensitivity depends very strongly on the diameter of the
column.
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Other traditional techniques
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Bimetal
based on thermal expansion
 very popular even today.
 switches are fairly inexpensive,
 can operate reliably for many cycles,
 may still be the correct choice for temperature
sensing applications.
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Shortcomings
not accurate enough,
 not allow operation over a broad temperature
range.
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Other traditional techniques
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Thermocouple
A metal wire ~ a vessel contains electrons
 Heating one end of the wire:
the effect of heat is to increase the average
velocity of the electrons on the heated end of
the wire
non-uniform distribution of electrons
a voltage across two ends
 Difficulty in measuring the voltage using one
wire
 Heating the joints of two wires of different
materials
thermal couple
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Thermocouple – good for high temp measurement
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The voltages generated by such effects are fairly
small.
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K Type thermocouple:
- 1 0 0℃~ 1 3 0 0℃
J Type thermocouple:
- 1 0 0℃~ 7 6 0℃
A good thermocouple exhibits a voltage signal of
only 10 V/Kelvin.
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accurate measurements of small temp changes
require very well-designed electronics.
For measurements which require accuracy of +/- 10 K,
and need to be carried out at temperatures near
1000K, thermocouples are definitely the way to go.
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Resistance Thermometry
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Platinum wires are commonly used for
resistance thermometry.
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though expensive, it is favored for these
applications for several very good reasons:
reasonably large temperature coefficient,
 not affected by most chemicals, mechanically
stable
 withstand very high temperatures,
 few other metals offer such a favorable collection
of long-term stability performance advantages.
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For narrower temperature ranges, what can
we use?
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Thermistors and
the Steinhart-Hart Equation
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NTC thermister
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depositing a small
quantity of semiconductor paste on to
closely spaced parallel
platinum alloy wires
sintered at a high
temperature at which
time the material forms
a tight bond between
the two wires.
Steinhart-Hart equation
for thermistors
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Glossary of terms - Thermister
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Zero-Power Resistance (Ro):
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Zero Power Temperature Coefficient of Resistance (Alpha):
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The ratio at a specified temperature(T), of the rate of change
of zero power resistance with temperature to the zero power
resistance
Resistance Temperature Characteristic:
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The dc resistance value of a thermistor at a specified
temperature with negligible electrical power to avoid self
heating.
The relationship between the zero power resistance of a
thermistor and its body temperature.
Temperature-Wattage Characteristic:
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The relationship at a specified ambient temperature between
the thermistor temperature and the applied steady state
wattage.
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Products by some
manufacturers
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Resolution Limit
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Resolution limitation imposed by
noise
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R1: thermister, R2: load resistor
if RL>>R1
If the temp of R1 changes by 1 K,
the resistance changes by  R1, the
voltage changes by  (Vin R1/RL).
Noise:
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Improve the Resolution
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by reducing
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the temperature,
the bandwidth, and
the load resistor,
by increasing
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the temperature coefficient
the bias voltage, Vin.
Of all of these parameters, it may be easiest to
increase the bias voltage.
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Self-heating Problem
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The bias causes power to be dissipated in the sense
resistor.
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assume that the sense resistor is attached to the object of
interest with a finite thermal conductance G.
There will be a temperature difference between the sense
resistor and the object of interest given by :
Thermal conductance between the core of the
thermistors and the surface are generally of order 10-2
to 1 W/K
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power dissipation 1mW
T=10-3- 0.1degree C
Self-heating will can cause substantial errors.
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Measurement of changing temperature
with contact temp sensors
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A thermometer is attached to
an object with a thermal
conductance of G (W/K).
Assume that
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The thermometer has a heat
capacity C (J/K).
some power, P, is being
applied to the thermometer
(bias currents)
From energy balance, the
energy into the thermometer
equals the change in energy of
the thermometer:
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Measurement of changing temperature
with contact temp sensors
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A finite temperature offset due to
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bias power (P/G),
oscillation amplitude which varies with frequency.
At low frequency, Ts2  To2,
At higher frequencies, the thermal time constant
associated with the heat capacity of the thermometer
can cause a reduce oscillation and a phase lag.
These issues are important to keep in mind for
measurements of time-varying temperatures.
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Flow Sensors
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There are three basic approaches to
the measurement of flow
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thermal effects to measure fluid motion.
In general, this approach uses a heat source to
deposit heat into the fluid, and a thermometer to
measure the temperature of the fluid. If the heat
source is upstream of the sensor, flow
increases heat transport and causes the sensor
temperature to increase.
 Another possible arrangement is to heat a
thermistor with a fixed power, and measure its
temperature. In this case, fluid flow acts to cool
the thermometer.
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A Commercial Product
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Flow Sensors
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A slightly more complicated approach
relies on Bernoulli's Equation, which
is :
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This principle is applied by measuring
pressure at a pair of points in a fluid.
 When water flows through a pipe
with a varying diameter, the total
flow rate in each region is a
constant , therefore, changes in
tube diameter are compensated for
by changes in fluid velocity.
 By measuring the pressure in
regions with different diameter, it is
possible to measure fluid velocity.
Only work if the flow is not turbulent
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Optical+ Microfluidics
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An Example: Pressure Difference
Based Flowmeter
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Measure the pressure
difference between
the wide and narrow
regions
Pressure drop
techniques only work
if the flow is not
turbulent (dissipative).
Modern MFC (mass
flow controller)
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Doppler effect based flow sensor
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The last technique for flow measurement is
based on measurement of Doppler effects in
sound transport.
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Since sound is carried by pressure waves in a
medium (the fluid), its transport laterally across a
channel is affected by the motion of the fluid.
It is possible to measure the change in sound
frequency due to fluid motion (direct Doppler effect, or
listen for changes in the travel time from transmitter to
receiver.
High sensitivity techniques generally measure
frequency shifts, since excellent accuracy may be
obtained by use of analog or digital signal processing
techniques to measure small frequency shifts.
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Doppler effect based flow sensor
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How to use Doppler effect?
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Wave source:
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Ultrasound
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Microwave
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Freq>= hundreds of KHz
24.125 GHz, less than
1mW/cm2
Required reflection
media
Fine particles
Flow
Rate Range: 0.08 to 12.2 m/sec
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Bubbles
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Pipe Size: Any pipe ID from 12.7 mm to 4.5 m
Accuracy: ±2% of full scale.
Requires solids or bubbles minimum size of 100
microns, minimum concentration 75 ppm.
Repeatability: ±0.1%, Linearity ±0.5% of full scale
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Flow Sensor: Review
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Flow may be measured by
thermal,
 Bernoulli,
 Doppler techniques.
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Thermal techniques are generally least
accurate and least expensive,
Bernoulli techniques can work well, but are
accurate only for non-turbulent flow,
Doppler techniques are potentially most
accurate, but are also generally most
expensive.
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Temperature Sensors: Review
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Temperature Sensors
Bimetal sensor/switch
 Thermocouple
 Thermistor
 Solid-state
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Issues associated with applications
Self-heating
 Thermal response
 Noise limited resolution
 Linearity
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