Sensor Technologies

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Sensor Technologies
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Phase Linearity

• Describe how well a system preserves the phase
  relationship between frequency components of the
  input
• Phase linearity fkf
• Distortion of signal
• Amplitude linearity
• Phase linearity

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Sensor Technology - Terminology

• Transducer is a device which transforms energy
  from one type to another, even if both energy
  types are in the same domain.
• Typical energy domains are mechanical,
  electrical, chemical, magnetic, optical and
  thermal.
• Transducer can be further divided into Sensors,
  which monitors a system and Actuators, which
  impose an action on the system.
• Sensors are devices which monitor a parameter of
  a system, hopefully without disturbing that
  parameter.

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Categorization of Sensor

• Classification based on physical phenomena
• Mechanical strain gage, displacement (LVDT),
  velocity (laser vibrometer), accelerometer, tilt
  meter, viscometer, pressure, etc.
• Thermal thermal couple
• Optical camera, infrared sensor
• Others
• Classification based on measuring mechanism
• Resistance sensing, capacitance sensing,
  inductance sensing, piezoelectricity, etc.
• Materials capable of converting of one form of
  energy to another are at the heart of many
  sensors.
• Invention of new materials, e.g., smart
  materials, would permit the design of new types
  of sensors.

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Paradigm of Sensing System Design
Zhang Aktan, 2005
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Instrumentation Considerations

• Sensor technology
• Sensor data collection topologies
• Data communication
• Power supply
• Data synchronization
• Environmental parameters and influence
• Remote data analysis.

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Measurement

• Measurement output
• interaction between a sensor and the environment
  surrounding the sensor
• compound response of multiple inputs

• Measurement errors
• System errors imperfect design of the
  measurement setup and the approximation, can be
  corrected by calibration
• Random errors variations due to uncontrolled
  variables. Can be reduced by averaging.

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Sensors

• Definition a device for sensing a physical
  variable of a physical system or an environment
• Classification of Sensors
• Mechanical quantities displacement, Strain,
  rotation velocity, acceleration, pressure,
  force/torque, twisting, weight, flow
• Thermal quantities temperature, heat.
• Electromagnetic/optical quantities voltage,
  current, frequency phase visual/images, light
  magnetism.
• Chemical quantities moisture, pH value

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Specifications of Sensor

• Accuracy error between the result of a
  measurement and the true value being measured.
• Resolution the smallest increment of measure
  that a device can make.
• Sensitivity the ratio between the change in the
  output signal to a small change in input physical
  signal. Slope of the input-output fit line.
• Repeatability/Precision the ability of the
  sensor to output the same value for the same
  input over a number of trials

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Accuracy vs. Resolution
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Accuracy vs. Precision
Precision without accuracy
Accuracy without precision
Precision and accuracy
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Specifications of Sensor

• Dynamic Range the ratio of maximum recordable
  input amplitude to minimum input amplitude, i.e.
  D.R. 20 log (Max. Input Ampl./Min. Input Ampl.)
  dB
• Linearity the deviation of the output from a
  best-fit straight line for a given range of the
  sensor
• Transfer Function (Frequency Response) The
  relationship between physical input signal and
  electrical output signal, which may constitute a
  complete description of the sensor
  characteristics.
• Bandwidth the frequency range between the lower
  and upper cutoff frequencies, within which the
  sensor transfer function is constant gain or
  linear.
• Noise random fluctuation in the value of input
  that causes random fluctuation in the output value

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Attributes of Sensors

• Operating Principle Embedded technologies that
  make sensors function, such as electro-optics,
  electromagnetic, piezoelectricity, active and
  passive ultraviolet.
• Dimension of Variables The number of dimensions
  of physical variables.
• Size The physical volume of sensors.
• Data Format The measuring feature of data in
  time continuous or discrete/analog or digital.
• Intelligence Capabilities of on-board data
  processing and decision-making.
• Active versus Passive Sensors Capability of
  generating vs. just receiving signals.
• Physical Contact The way sensors observe the
  disturbance in environment.
• Environmental durability will the sensor robust
  enough for its operation conditions

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Strain Gauges

• Foil strain gauge
• Least expensive
• Widely used
• Not suitable for long distance
• Electromagnetic Interference
• Sensitive to moisture humidity
• Vibration wire strain gauge
• Determine strain from freq. of AC signal
• Bulky
• Fiber optic gauge
• Immune to EM and electrostatic noise
• Compact size
• High cost
• Fragile

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Strain Sensing

• Resistive Foil Strain Gage
• Technology well developed Low cost
• High response speed broad frequency bandwidth
• A wide assortment of foil strain gages
  commercially available
• Subject to electromagnetic (EM) noise,
  interference, offset drift in signal.
• Long-term performance of adhesives used for
  bonding strain gages is questionable
• Vibrating wire strain gages can NOT be used for
  dynamic application because of their low response
  speed.
• Optical fiber strain sensor

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Strain Sensing

• Piezoelectric Strain Sensor
• Piezoelectric ceramic-based or Piezoelectric
  polymer-based (e.g., PVDF)
• Very high resolution (able to measure nanostrain)
• Excellent performance in ultrasonic frequency
  range, very high frequency bandwidth therefore
  very popular in ultrasonic applications, such as
  measuring signals due to surface wave propagation
• When used for measuring plane strain, can not
  distinguish the strain in X, Y direction
• Piezoelectric ceramic is a brittle material (can
  not measure large deformation)

Courtesy of PCB Piezotronics
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Acceleration Sensing

• Piezoelectric accelerometer
• Nonzero lower cutoff frequency (0.1 1 Hz for
  5)
• Light, compact size (miniature accelerometer
  weighing 0.7 g is available)
• Measurement range up to /- 500 g
• Less expensive than capacitive accelerometer
• Sensitivity typically from 5 100 mv/g
• Broad frequency bandwidth (typically 0.2 5 kHz)
• Operating temperature -70 150 C

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Acceleration Sensing

• Capacitive accelerometer
• Good performance over low frequency range, can
  measure gravity!
• Heavier ( 100 g) and bigger size than
  piezoelectric accelerometer
• Measurement range up to /- 200 g
• More expensive than piezoelectric accelerometer
• Sensitivity typically from 10 1000 mV/g
• Frequency bandwidth typically from 0 to 800 Hz
• Operating temperature -65 120 C

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Accelerometer
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Force Sensing

• Metal foil strain-gage based (load cell)
• Good in low frequency response
• High load rating
• Resolution lower than piezoelectricity-based
• Rugged, typically big size, heavy weight

Courtesy of Davidson Measurement
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Force Sensing

• Piezoelectricity based (force sensor)
• lower cutoff frequency at 0.01 Hz
• can NOT be used for static load measurement
• Good in high frequency
• High resolution
• Limited operating temperature (can not be used
  for high temperature applications)
• Compact size, light

Courtesy of PCB Piezotronics
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Displacement Sensing

• LVDT (Linear Variable Differential Transformer)
• Inductance-based ctromechanical sensor
• Infinite resolution
• limited by external electronics
• Limited frequency bandwidth (250 Hz typical for
  DC-LVDT, 500 Hz for AC-LVDT)
• No contact between the moving core and coil
  structure
• no friction, no wear, very long operating
  lifetime
• Accuracy limited mostly by linearity
• 0.1-1 typical
• Models with strokes from mms to 1 m available

Photo courtesy of MSI
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Displacement Sensing

• Linear Potentiometer
• Resolution (infinite), depends on?
• High frequency bandwidth ( 10 kHz)
• Fast response speed
• Velocity (up to 2.5 m/s)
• Low cost
• Finite operating life (2 million cycles) due to
  contact wear
• Accuracy /- 0.01 - 3 FSO
• Operating temperature -55 125 C

Photo courtesy of Duncan Electronics
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Displacement Transducer

• Magnetostrictive Linear Displacement Transducer
• Exceptional performance for long stroke position
  measurement up to 3 m
• Operation is based on accurately measuring the
  distance from a predetermined point to a magnetic
  field produced by a movable permanent magnet.
• Repeatability up to 0.002 of the measurement
  range.
• Resolution up to 0.002 of full scale range (FSR)
• Relatively low frequency bandwidth (-3dB at 100
  Hz)
• Very expensive
• Operating temperature 0 70 C

Photo courtesy of Schaevitz
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Displacement Sensing

• Differential Variable Reluctance Transducers
• Relatively short stroke
• High resolution
• Non-contact between the measured object and
  sensor

Courtesy of Microstrain, Inc.
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Velocity Sensing

• Scanning Laser Vibrometry
• No physical contact with the test object
  facilitate remote, mass-loading-free vibration
  measurements on targets
• measuring velocity (translational or angular)
• automated scanning measurements with fast
  scanning speed
• However, very expensive ( 120K)

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Laser Vibrometry

• References
• Structural health monitoring using scanning laser
  vibrometry, by L. Mallet, Smart Materials
  Structures, vol. 13, 2004, pg. 261
• the technical note entitled Principle of
  Vibrometry from Polytec

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Shock (high-G) Sensing

• Shock Pressure Sensor
• Measurement range up to 69 MPa (10 ksi)
• High response speed (rise time
• High frequency bandwidth (resonant frequency up
  to 500 kHz)
• Operating temperature -70 to 130 C
• Light (typically weighs 10 g)
• Shock Accelerometer
• Measurement range up to /- 70,000 g
• Frequency bandwidth typically from 0.5 30 kHz
  at -3 dB
• Operating temperature -40 to 80 C
• Light (weighs 5 g)

Photo courtesy of PCB Piezotronics
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Angular Motion Sensing (Tilt Meter)

• Inertial Gyroscope (e.g., http//www.xbow.com)
• used to measure angular rates and X, Y, and Z
  acceleration.
• Tilt Sensor/Inclinometer (e.g.,
  http//www.microstrain.com)
• Tilt sensors and inclinometers generate an
  artificial horizon and measure angular tilt with
  respect to this horizon.
• Rotary Position Sensor (e.g., http//www.msiusa.co
  m)
• includes potentiometers and a variety of magnetic
  and capacitive technologies. Sensors are designed
  for angular displacement less than one turn or
  for multi-turn displacement.

Photo courtesy of MSI and Crossbow
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MEMS Technology

• What is MEMS?
• Acronym for Microelectromechanical Systems
• MEMS is the name given to the practice of making
  and combining miniaturized mechanical and
  electrical components. K. Gabriel, SciAm,
  Sept 1995.
• Synonym to
• Micromachines (in Japan)
• Microsystems technology (in Europe)
• Leverage on existing IC-based fabrication
  techniques (but now extend to other non IC
  techniques)
• Potential for low cost through batch fabrication
• Thousands of MEMS devices (scale from 0.2 ?m
  to 1 mm) could be made simultaneously on a single
  silicon wafer

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MEMS Technology

• Co-location of sensing, computing, actuating,
  control, communication power on a small
  chip-size device
• High spatial functionality and fast response
  speed
• Very high precision in manufacture
• miniaturized components improve response speed
  and reduce power consumption

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MEMS Fabrication Technique
Courtesy of A.P. Pisano, DARPA
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Distinctive Features of MEMS Devices

• Miniaturization
• micromachines (sensors and actuators) can handle
  microobjects and move freely in small spaces
• Multiplicity
• cooperative work from many small micromachines
  may be best way to perform a large task
• inexpensive to make many machines in parallel
• Microelectronics
• integrate microelectronic control devices with
  sensors and actuators

Fujita, Proc. IEEE, Vol. 86, No 8
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MEMS Accelerometer

• Capacitive MEMS accelerometer
• High precision dual axis accelerometer with
  signal conditioned voltage outputs, all on a
  single monolithic IC
• Sensitivity from 20 to 1000 mV/g
• High accuracy
• High temperature stability
• Low power (less than 700 uA typical)
• 5 mm x 5 mm x 2 mm LCC package
• Low cost (5 14/pc. in Yr. 2004)

Courtesy of Analog Devices, Inc.
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MEMS Accelerometer

• Piezoresistive MEMS accelerometer
• Operating Principle a proof mass attached to a
  silicon housing through a short flexural element.
  The implantation of a piezoresistive material on
  the upper surface of the flexural element. The
  strain experienced by a piezoresistive material
  causes a position change of its internal atoms,
  resulting in the change of its electrical
  resistance
• low-noise property at high frequencies

Courtesy of JP Lynch, U Mich.
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MEMS Dust

• MEMS dust here has the same scale as a single
  dandelion seed - something so small and light
  that it literally floats in the air.

Source Distributed MEMS New Challenges for
Computation, by A.A. BERLIN and K.J. GABRIEL,
IEEE Comp. Sci. Eng., 1997
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Sensing System

• Reference
• Zhang, R. and Aktan, E., Design consideration
  for sensing systems to ensure data quality,
  Sensing issues in Civil Structural Health
  Monitoring, Eded by Ansari, F., Springer, 2005,
  P281-290