Sensors for Civil Engineering

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What is Sensor ?
A sensor is a converter that measures a physical quantity
and converts it into a signal which can be read by an
observer or an instrument.
How is it important?
Smart performance of Smart Civil Structures depends on
the quality of the data collected.
What are the important measurement for Civil Structures?
1. Loading sources such as gravity force, wind, seismic and traffic
loading
2. Structural responses such as strain, displacement, inclination and
acceleration and
3. Environmental effects including temperature, humidity, rain and
corrosion
Wind Sensors
Anemometers
Pressure transducers
Wind profile measurements
Propeller
anemometer
3D ultrasonic
anemometer
Doppler radar Doppler sodar

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Seismic Sensors
Seismometers: measure motions of the ground, including those of seismic
waves generated by earthquakes, nuclear explosions and other sources.
• For measurement it requires fixed reference point.
• Its installation can be in the form of a mass-spring system or a horizontal
pendulum system
• Vertical seismometers is used to measure vertical ground motion (works
on principle of mass-spring system) whereas the horizontal one (works on
principle of pendulum system) is used for horizontal ground motion
measurement.
Based on the range of vibration frequency 1) short period seismometers
(short natural frequency and high resonant frequency) and 2) long period
seismometers (measure motions of the ground, including those of seismic
waves generated by earthquakes, nuclear explosions and other sources)
• In Short period Seismometers - The inertial force produced by a seismic
ground motion deflects the mass from its equilibrium position, and the
displacement/velocity of the mass is converted into an electric signal as
the output is proportional to the seismic ground motion.
• Long-period or broadband seismometers - Built according to the force-
balanced principle. The inertial force is compensated by an electrically
generated force (feedback force) so that the mass moves as little as
possible.
• The feedback force (generated with an electromagnetic force transducer
through a servo loop circuit) is strictly proportional to the seismic ground
acceleration and is converted into an electrical signal as the output.

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Load Cells
Load cells are used for the measurement of forces and other structural
loads. According to the type of output signal generated (e.g. hydraulic,
pneumatic and electric signal) load cells can be
• hydraulic load cells
• pneumatic load cells
• strain gauge load cells and
• piezoelectric load cells.
• Hydraulic load cells operates on a mechanical
force-balance principle
• With the increase of the external force, the
pressure of the hydraulic fluid rises accordingly
• The output is linear and relatively stable.
• The accuracy of such load cells can be within
0.25% of full scale or even better
• Mostly used in weighting applications especially
for use in hazardous areas
• A pneumatic load cell is a force-balance device that
operates by measuring changes in air pressure
• Air pressure is applied to one end of the diaphragm and it
escapes through the nozzle placed at the bottom of the load
cell
• The deflection of the diaphragm affect the airflow through
the nozzle as well as the pressure inside the chamber
• To measure the pressure inside the load cell, a pressure
gauge is required; it is usually attached to the cell
• The major advantages of pneumatic load cells are
o cleanliness for the environment
o safety for utilization
o and insensitivity to temperature variations
• Generally used to measure relatively small weights in
industries where cleanliness and safety are of prime
concern.

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• A strain gauge-based load cells are the most commonly used type
• These load cells convert the load acting on them into electrical signals
• When the material of the load cells deforms appropriately (usually in the
linear elastic range), the strain gauge bonded onto this material deforms
accordingly (e.g. stretches or contracts) resulting in the variations of the
electrical resistance of the gauges.
• These variations provide electrical value changes that are relational to
the load applied to the load cell
• The piezoelectric load cell works on similar principle of deformation as the
strain gauge load cells, but the changes of output voltage are generated
through the variations of electrostatic charge, rather than electrical
resistance.
• The electrostatic charge is generated by the quartz crystals of piezoelectric
material and is proportional to the applied force.
• This output is firstly collected on the electrodes sandwiched between the
crystals, and then either converted to a low impedance voltage signal
within the sensor or routed directly to an external charge amplifer for use.
• Most suitable for application in the dynamic loading conditions, where
strain gauge load cells may fail due to high dynamic loading cycles

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Weigh-in-motion
• An effective means of data collection for pavement research and facility
design, traffic monitoring and weight enforcement
• Can measure the axle weight of passing vehicles, and thus the sum of the
weight of the vehicles, the velocity of the vehicles and the distance between
the axles
• No need to stop the vehicle
• WIM systems are less accurate than static scales
Thermometers
• Temperature, including structural temperature and ambient air
temperature, is frequently measured in monitoring systems.
• Changes in temperature significantly influence the overall deflection and
deformation of bridges and buildings even may damage bridges and
buildings
• Temperature is usually non-uniformly distributed over the entire structure
and is different from the ambient temperature, due to heat transfer.
• Sensors include thermocouples, thermistors and resistance temperature
detectors
• Thermocouple consists of two dissimilar conductors that contact each other
(at one or more spots). Electrical potential is created when the temperature
of one of the spots differs.
• Widely used type of temperature sensor for measurement and control
• Thermistors and resistance temperature (made of platinum)detectors are
based on the principle that resistance of a material increases with the
temperature
• More accurate and stable than thermocouples
• Resistance temperature detectors are employed in bridge monitoring
exercises

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Strain Gauges
• The strain gauge is usually attached to an object for strain measurement
• Foil strain gauges, fibre-optic strain gauges and vibrating wire strain
gauges are commonly used sensors measuring strain in civil structures.
Foil strain gauge
• The most common type of strain gauge
• Thin insulating backing which supports
a fine metallic foil
• As the object is deformed, the foil is
stretched or shortened causing the
change in its electrical resistance in
proportion to the amount of strain,
which is usually measured using a
Wheatstone bridge.
• The length of most foil strain gauges is about a few millimetres to a few
centimetres
• Economical and can measure dynamic strains however, is not
recommended in a harsh environment
Vibrating wire strain gauge
• Thin steel wire held in tension between
two end anchorages
• The wire vibrates due to an excitation
with a short pulse, and the resonant
frequency is measured
• When the distance between the
anchorages changes, the tension of the
wire changes, and so too does the
natural frequency
• The data can be transferred to very
long distances without degradation
• Can measure static strain only
• Relatively large and requiring bulky
cablings

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Displacement Sensors
• Displacement of civil structures serves as an effective indicator of their
structural performance condition.
• Large displacements or deformations may affect the structural integrity.
• Temperature is usually non-uniformly distributed over the entire structure
and is different from the ambient temperature, due to heat transfer.
• Equipment measuring the displacement includes linear variable differential
transformers (LVDT), level-sensing stations, tilt beam sensors, GPS,
Infrared Sensors, cameras and so forth.
Commonly used electromechanical
facility for measuring relative
displacements based on the
principle of mutual inductance
Linear Variable Differential Transformer (LVDT)
• LVDTs stroke range: from micrometres to 0.5 m.
• LVDT consists of a hollow metallic tube containing one primary and two
secondary coils and a separate movable ferromagnetic core. Electrical signal
produced by these coils is proportional to position of moving core.
• When the core is at the centre of stoke (named null position), the values of
these two secondary coils’ voltages are equal.
• When the core is moved away from the centre, the induced voltage is
increased in one of the secondary coils but decreased in the other.
• LVDT is a frictionless device because there is no physical contact between
the coil assembly and the movable core
• Very useful in the fatigue-life testing of materials and structures, can be
used in high-performance servo balance instruments and for high-resolution
dimensional gaging systems
• As the null position of an LVDT is stable and repeatable, even over a wide
operating temperature range.
• LVDTs have long mechanical life, environmental robustness and fast
dynamic response.

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Level sensing station
• Works on the principle based on
pressure difference
• Two or more interconnected fluid
filled cells (the fluid is usually
water), and the quasi-static
gravity-induced movement of
liquids for transduction
• It can detect an elevation
difference of about 0.5 mm.
• Designed to measure differential displacement and angular rotation in civil
structures such as bridges, dams, tunnels and buildings
• Installed horizontally to monitor settlement and heave, and when vertically
installed can monitor displacement and rotation.
• When linked in a seriesprovide a detailed profle of differential movement or
settlement
Tilt Beam/ Inclinometer
Camera
• A charge-coupled-device (CCD) camera use a large number of pixels (dots) to
form an image
• Then utilizes image-processing techniques for pixel identification and
subsequent edge detection
• The optical sensor used in these cameras is very sensitive to light hence no
artificial lighting is required to take an image of a civil structure
• sub-pixel displacement-identification measurement method improves
accuracy

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Accelerometers • Piezoelectric type
• Piezoresistive type
• Capacitive type
• Servo-force balance type
Fiber Optic Sensors
Non Contact type Sensors • laser triangulation principle
• eddy current principle
• capacitive principle
• confocal principle.
Whether Stations