Transcript Force Sensing Resistors

Piezoresistors
Piezoresistors can be fabricated using wide variety of
piezoresistive materials. The simplest form of
piezoresistive silicon sensors are diffused resistors.
Piezoresistors consist of a simple two contact diffused nor p-wells within a p- or n-substrate..
As the typical square
resistances of these
devices are in the range
of several hundred
ohms, additional p+ or
n+ plus diffusions are
necessary to facilitate
ohmic contacts to the
device
Physics of operation
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Signal analyzer
Cantilever Calibration
Laser vibrometer
Vdisplacemen
t
15V
Vstrain
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Piezoresistor Bridge Voltage vs. Displacement
 Measure at resonant frequency of cantilever
 Typical sensitivity ~ 1mV/µm
Noise spectrum of piezoresistor
 < 0.1µV/Hz or ~80pN/  Hz at 1Hz
Force Sensing Resistors
As their name implies, force sensing resistors use
the electrical property of resistance to measure
the force (or pressure) applied to a sensor. A force
sensing resistor is made up of two parts. The first
is a resistive material applied to a film. The
second is a set of digitating contacts applied to
another film. Figure shows this configuration. The
resistive material serves to make an electrical
path between the two sets of conductors on the
other film. When a force is applied to this sensor,
a better connection is made between the
contacts, hence the conductivity is increased.
switches using FSRs. Above this region, the force
is approximately proportional to until a saturation
region is reached. When forces reach this
magnitude, additional forces do not decrease the
resistance substantially
Piezoresistive for pressure
Piezoelectric pressure sensors:
As active designs, piezoelectric
sensors can only be used for
quasistatic rather than truly static
measurement. However they are
ideal for dynamic applications.
Piezoelectric pressure sensors can
be employed wherever rapidly
changing pressures at temperatures
of up to 400°C have to be measured
as accurately as possible
Piezoresistive for pressure
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The piezoresistive principle is
based on the semiconductor effect
first described in 1954, which
states that under mechanical
stress semiconductors change
their electrical resistance.
Compared with the conventional
strain gage measurement of the
time, this opened up completely
new applications. Since then
similar breakthroughs have
included the thin film technique on
metal and its thick layer
counterpart on ceramic.
Piezoresistive sensors measure
static pressures in gases and
liquids. The results achieved under
even the most adverse conditions
are precise and repeatable.
Photo-electric sensors
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For non-contact detection of targets at a
distance regardless of material.
Photoelectric sensors emit invisible infrared or visible red light to detect the
presence of an object. The target either
breaks a beam of light or reflects it back
to the detector to activate the sensor
output
Photo-electric sensors
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Advantages of photoelectric sensors
include longer standoff distances than
inductive proximity sensors, ability to
detect virtually any target material, ability
to differentiate between targets of
different color or surface characteristics,
and the ability to operate in different
sensing modes such as thru-beam, retroreflective, or diffuse
Photo-electric sensors
Counting Product
Counting a Product
on the Conveyor
Belt, while numbers
of interruptions of
light are counted
electronically with
the help of electronic
circuits coupled with
sensors.
Thermistor
A thermistor is a type of resistor whose resistance varies
with temperature.
Thermistors are widely used as inrush current
limiters, temperature sensors, self-resetting
overcurrent protectors, and self-regulating heating
elements
Thermistors differ from resistance temperature
detectors (RTD) in that the material used in a
thermistor is generally a ceramic or polymer, while
RTDs use pure metals. The temperature response
is also different; RTDs are useful over larger
temperature ranges, while thermistors typically
achieve a higher precision within a limited
temperature range [usually -90C to 130C]
Basic operation of Thermistor
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Assuming, as a first-order approximate
that the relationship between resistance
and temperature is linear, then:
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ΔR = k.ΔT
where
ΔR = change in resistance
 ΔT = change in temperature
 k = first-order temperature coefficient of
resistance
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Types of Thermistor
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Thermistors can be classified into two types depending
on the sign of k. If k is positive, the resistance increases
with increasing temperature, and the device is called
a positive temperature coefficient (PTC) thermistor,
or posistor. If k is negative, the resistance decreases
with increasing temperature, and the device is called
a negative temperature coefficient (NTC) thermistor.
Resistors that are not thermistors are designed to have
a k as close to zero as possible, so that their resistance
remains nearly constant over a wide temperature range
Applications of NTC
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NTC thermistors are used as resistance thermometers in lowtemperature measurements of the order of 10 K.
NTC thermistors can be used as inrush-current limiting devices in
power supply circuits. They present a higher resistance initially
which prevents large currents from flowing at turn-on, and then heat
up and become much lower resistance to allow higher current flow
during normal operation. These thermistors are usually much larger
than measuring type thermistors, and are purposely designed for
this application.
NTC thermistors are regularly used in automotive applications. For
example, they monitor things like coolant temperature and/or oil
temperature inside the engine and provide data to the ECU and,
indirectly, to the dashboard.
Thermistors are also commonly used in modern digital
thermostats and to monitor the temperature of battery packs while
charging.
Applications of PTC
PTC thermistors are temperature dependent resistors
manufactured from barium titanate and should be chosen
when a drastic change in resistance is required at a
specific temperature or current level. PTCs can operate
in the following modes:
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Temperature sensing, switching at temperatures ranging from
60°C to 180°C, e.g. protection of windings in electric motors and
transformers.
Solid state fuse to protect against excess current levels, ranging
from several mA to several A (25°C ambient) and continuous
voltages up to 600V and higher, e.g. power supplies for a wide
range of electrical equipment.
Liquid level sensor.
Look-Like
Thermocouple
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A thermocouple or thermocouple
thermometer is a junction between two
different metals that produces a voltage related
to a temperature difference. Thermocouples are
a widely used type of temperature sensor for
measurement and control[1] and can also be
used to convert heat into electric power. They
are inexpensive[2] and interchangeable, are
supplied fitted with standard connectors, and
can measure a wide range of temperatures. The
main limitation is accuracy: system errors of
less than one kelvin (K) can be difficult to
achieve
Applications
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Thermocouples are suitable for measuring over
a large temperature range, up to 2300 °C. They
are less suitable for applications where smaller
temperature differences need to be measured
with high accuracy, for example the range 0–100
°C with 0.1 °C accuracy. For such
applications thermistors and resistance
temperature detectors are more suitable.
Applications include temperature measurement
for
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kilns,
gas turbine exhaust,
diesel engines, and
other industrial processes
Types of Thermo-couples
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Type K (Chromel / Alumel)
Type E (Chromel / Constantan)
Type J (Iron / Constantan)
Type N (Nicrosil / Nisil)
Type B (Platinum / Rhodium)
Type R (Platinum / Rhodium)
Type S (Platinum / Rhodium)