Transcript CEE125 Lecture 2

Instrumentation Selection
Strategies
Robert Nigbor
nees@UCLA
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OUTLINE
• Some sources of instrumentation selection
information
• Common types of measurements in NEES
research
• Introduction to the Operating Range concept
for sensor + data acquisition selection
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Training & Information on
Instrumentation Selection
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A question for the audience – how many
universities have hands-on instrumentation
courses as part of the CE curriculum?
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Professional Organizations
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Example: Prof. Muratore’s ME Instrumentation & Data
Acquisition course at Rice
ASCE
IEEE
Vendors
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IOTech
National Instruments
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www.mccdaq.com/handbook/handbook.aspx
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www.mccdaq.com/handbook/resource_center.aspx
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www.ni.com/academic/measurements_curriculum.htm
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Review of Basic Instrumentation
Issues
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Basic Instrumentation Blocks
Graphics from www.ni.com DAQ Fundamentals
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Typical Analog-to-Digital Instrumentation System
Signal Conditioner
Excitation
Volts
Sensor
Bridge
Completion
Microvolts to
Volts
depending on
the sensor
Sample
and
Hold
Analog
To
Digital
Converter
Amplification
Multiplexing
and
data
transmission
Low Pass
Filter
Bits
Recording,
Storage
and Display
Data Acquisition Unit
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From John F. Muratore’s course
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What it really looks like!
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NEES Experiments typically use tens or hundreds of sensor
channels, compared to hundreds or thousands in Aerospace,
Mechanical, Physics, and Geophysics applications.
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The Most Common NEES Measurements:
Position, Motion, Strain, Force, Pressure
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Linear Position
Angular Position
Linear Velocity
Angular Velocity/Rate
Acceleration
Strain in steel & concrete
Force via Strain
Pressure via strain
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Position
Velocity
Acceleration –
proportional to force
Linear
Position – Direct measurement
by moving electrical element
(potentiometer, Linear Variable
differential transformer
(LVDT)), measuring time of
flight (laser, sonar),
Velocity- Direct
measurement by
Doppler (sound or
radio),
Integrate
acceleration
(inertial) or
differentiate
position
Acceleration –
accelerometer –
measure a force, divide
by mass
Pressure – force per
unit area
Angular
Angle –Angle shaft encoder,
synchro-resolver, rate
integrating gyro, Rotary
Variable Differential
Transformer (RVDT)
Angle rate – direct
measurement by
rate gyro,
Angular acceleration –
differentiate rate gyro.
Measure a torque,
divide by moment of
inertia
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From John F. Muratore’s course
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String Potentiometer
From www.spaceagecontrol.com
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Linear Variable Differential
Transformer (LVDT)
Typically core is
attached by a shaft to
the object whose
position is being
measured
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Core left –
magnitude is a
function of position
in same phase as
Ein
LVDT core centered – no signal
Core right –
magnitude is a
function of
position in
opposite phase
as Ein
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Accelerometer
Types of Accelerometers:
Electronic : transducers
produce voltage output
Servo controlled: use
suspended mass with
displacement transducer
Piezoelectric: Mass attached
to a piezoelectric material,
which develops electric
charge on surface.
Principle: An acceleration a will cause
the mass to be displaced by ma/k or
alternatively, if we observe a
displacement of x, we know that the
mass has undergone an acceleration of
kx/m.
Generally accelerometers are placed in three orthogonal directions
to measure accelerations in three directions at any time. Sometimes
geophones (velocity transducers) are attached to accelerometers to
measure the seismic wave velocities.
CEE125, Spr10, Lecture 4
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Earthquake Sensors –
Accelerometer Example:
Kinemetrics EpiSensor
CEE125, Spr10, Lecture 4
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Strain Gage
As the material to which the gage is bonded increases in
length (tension), the cross sectional area of the wire in the
strain gage decreases. As area decreases, the resistance
increases because resistance is inversely proportional to
wire cross sectional area
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Typical Implementation via Bridge Circuit:
Change in Resistance  Change in Voltage
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Load Cells – Linear Force
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Strain gage bridge
measures elastic strain in
material due to applied
force
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Basic Pressure Sensor Types
Capacitance Pressure Transducer
Piezoelectric Pressure Transduce
Strain gage pressure transducer
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From John F. Muratore’s course
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Measurement Needs and Constraints
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Instrumentation design must consider
amplitude, time, and frequency needs and
constraints
ALL measurement systems have limitations
in all three domains
A large part of the ART of instrumentation
design and implementation is the optimization
of needs and constraints
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Example:
Static versus
Dynamic
Displacements
LVDT’s here work for
slow (quasi-static)
motions but not for
vibrations/dynamic
motions, due to
resonance of sensor and
frame
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Example:
Piezoelectric
Accelerometers &
Earthquake Motions
Not Sensitive to <1Hz Motions, an
important part of earthquake shaking
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“Operating Range”
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Tool for considering both amplitude &
frequency ranges in an instrumentation
system
Tool for comparing instrumentation operating
range with measurement need
Time must be considered separately, but this
is often a data acquisition issue instead of a
sensor issue
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Amplitude, usually in narrow band like 1/3-octave
Operating Range Diagram
Maximum or Clip Level
Operating Range of
Component or System
Resolution or Noise Level
Lower Corner
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Frequency
Upper Corner
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Amplitude, usually in narrow band like 1/3-octave
Operating Range Diagram
Maximum or Clip Level
Phenomenon 1:
Within Operating
Range, is OK
Operating Range of
Component or System
Resolution or Noise Level
Lower Corner
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Frequency
Upper Corner
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Amplitude, usually in narrow band like 1/3-octave
Operating Range Diagram
Maximum or Clip Level
Phenomenon 2:
Outside Operating
Range, not OK
Operating Range of
Component or System
Resolution or Noise Level
Lower Corner
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Frequency
Upper Corner
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Instrumentation Selection Strategy
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Understand amplitude, time & frequency
limitations/constraints of potential instrumentation
components and systems (sensor + signal
conditioning + digitizer).
Compare with amplitude, time & frequency needs of
your particular measurement challenge.
Make sure your particular phenomenon lies within
the operating range of the instrumentation
The Operating Range Diagram is a useful tool for
this comparison
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