Transcript In The Name of Allah The Most Beneficent The

In The Name of Allah
The Most Beneficent The Most Merciful
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ECE 2161:
Instrumentation &
Measurements
Lecture:
Spatial Variables
Measurements
Engr. Ijlal Haider
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Outline
 Spatial Variables
 Position Measurement
 Acceleration Measurement
 Velocity Measurement
 Level Measurement
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Introduction
 Spatial Variables
 relating to the position, area and size of things (Cambridge
Dictionary)
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Level Measurements
 Single point sensing
 actual level of the material is detected when it reaches a
predetermined level, so that the appropriate action can be taken
to prevent overflowing or to refill the container.
 Continuous level sensing
 level of the liquid is measured on an uninterrupted basis. In this
case the level of the material will be constantly monitored and
hence, the volume can be calculated if the cross-sectional area
of the container is known.
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 Direct Measurement of Level
 Indirect Measurement of Level
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Level Sensing
 Sight Glass
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 Floats
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 Probes
 Conductive
 Capacitive
 Ultrasonic
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 Conductive Probes for set level detection
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 Capacitive probes for continuous level
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 Ultrasonic Probes for set level and continuous level detection
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 Radiation Attenuation
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 Bubbler Device
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 Paddle Wheel
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 Load Cells
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Important Relationships for Indirect
Sensing of Level
 Pressure increases as the depth increases in a fluid.
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 The weight of a container can be used to calculate
the level of the material in the container. The volume
V of the material in the container is given by
where r is the radius of the container and d is the depth of the material.
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Position Measurement
 Many Industrial process require linear and rotational position
and distance measurements
 Common applications
 Robots
 Manufacturing Conveyer Belts
 location and position of objects on a conveyor system
 Rolling Mills
 orientation of steel plates in a rolling mill
 Location and position of work piece in automatic milling
operations
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 Linear position is the distance measured with respect to a
reference point.
 Angular position is a measurement of the change in position
of a point about a fixed axis measured in degrees or radians,
where one complete rotation is 360° or 2 pi radians.
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 Absolute position
 Incremental position
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 Potentiometers
 Linear Variable Differential Transformer
 Hall Effect Sensors
 Magneto resistive Sensors
 Magnetostrictive Sensors
 Laser Interference Sensor
 Ultrasonic, IR and Microwave
 Optical Encoders
 Resolvers
 Synchros
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Resistive Displacement Senors
 “Potentiometer” or “Pots”
 Convenient way to convert distance into electrical variable
 The wiper or slider arm of a linear potentiometer can be
mechanically connected to the moving section of a sensor
 Suffer from mechanical wear and tear
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LVDT
 Consists of a single primary
winding positioned between
two identical secondary windings
 The primary winding is energized by a high-frequency 50 Hz
to 20 kHz ac voltage.
 The two secondary windings are made identical by having
an equal number of turns and similar geometry.
 They are connected in series opposition so that the induced
output voltages oppose each other
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Magnetostrictive
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Velocity and Acceleration
 Controlling the speed at which metal stock is fed into a
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machine tool.
Measuring the approach speed of a robotic tool onto its
target.
Monitoring the speed of a generator in an electric power
station.
An airport radar system measuring the speed of an
approaching aircraft using the Doppler effect.
Measuring an automobile’s wheel speed in order to provide
feedback to an antilock brake system.
Acceleration Measurements
 Acceleration is measured by accelerometers as an important
parameter for general-purpose absolute motion
measurements, and vibration and shock sensing.
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Piezoelectric
 A voltage is generated across the crystal when stressed.
 A piezoelectric crystal is spring-loaded with a test mass in
contact with the crystal.
 When exposed to an acceleration, the test mass stresses the
crystal by a force (F = ma), resulting in a voltage generated
across the crystal.
 A measure of this voltage is then a measure of the
acceleration.
 Output levels are typically in the millivolt range.
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Differential Capacitance
 Differential-capacitance accelerometers are based on the principle of change of
capacitance in proportion to applied acceleration. They come in different shapes and
sizes.
 In one type, the seismic mass of the accelerometer is made as the movable element of an
electrical oscillator.
 The seismic mass is supported by a resilient parallel-motion beam arrangement from
the base.
 The system is characterized to have a certain defined nominal frequency when
undisturbed.
 If the instrument is accelerated, the frequency varies above and below the nominal value,
depending on the direction of acceleration.
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Seismic Accelerometer
 Potentiometric
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Seismic Accelerometers
 Strain Gauge
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Velocity Measurement
 The linear velocity of an object, or more correctly a particle, is
defined as the time rate of change of position of the object.
 Vector quantity, meaning it has a direction as well as a
magnitude, and the direction is associated with the direction
of the change in position.
 The magnitude of velocity is called the speed (or pace), and it
quantifies how fast an object is moving.
 The rotational velocity (or angular velocity) of an object is
defined as the time rate of change of angular position, and it
is a measure of how fast an object is turning.
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 There are not many instruments available for velocity
measurements.
 Velocity is often obtained by differentiation of displacement
or integration of acceleration.
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Linear Velocity Transducers
 Commonly called LVT’s
 based on a linear generator. When a coil cuts the magnetic
field lines around a magnet, a voltage is induced in the coil,
and this voltage is dependent on the following relation:
 e BLV
where
e= induced voltage
B= magnetic field strength
L= length of wire in the coil
V= speed of the coil relative to the magnet.
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Light Interference Method
 A beam of monochromatic light is split into two beams.
 One beam is directed onto a stationary mirror.
 The other beam is directed onto a moving target.
 The observer sees the superposition of the two beams.
 As the mirror moves in one direction, summation of the
waves of the two beams will alternately reinforce and cancel
each other.
 The amount of motion for one cycle of light intensity
variation is the wavelength of the light being used. The
frequency of these light-to-dark transitions is proportional to
the velocity of the moving target.
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 “All the effects of Nature are only the
mathematical consequences of a small
number of immutable laws.”
― Pierre-Simon Laplace
(1749-1827)
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Thank You!
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