Sensor Technologies
Phase Linearity
• Describe how well a system preserves the
phase relationship between frequency
components of the input
• Phase linearity: f=kf
• Distortion of signal
– Amplitude linearity
– Phase linearity
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.
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.
Paradigm of Sensing System Design
Zhang & Aktan, 2005
Instrumentation Considerations
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Sensor technology;
Sensor data collection topologies;
Data communication;
Power supply;
Data synchronization;
Environmental parameters and influence;
Remote data analysis.
Measurement
Physical
phenomenon
Measurement
Output
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.
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
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
Accuracy vs. Resolution
True value
measurement
Accuracy vs. Precision
Precision without
accuracy
Accuracy without
precision
Precision and
accuracy
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
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 decisionmaking.
• 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
Strain Gauges
• Foil strain gauge
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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
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Immune to EM and electrostatic noise
Compact size
High cost
Fragile
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
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
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
Photo courtesy of PCB Piezotronics
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
Photo courtesy of PCB Piezotronics
Accelerometer
Force Sensing
• Metal foil strain-gage based (load cell)
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Good in low frequency response
High load rating
Resolution lower than piezoelectricity-based
Rugged, typically big size, heavy weight
Courtesy of Davidson Measurement
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
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 mm’s to 1 m
available
Photo courtesy of MSI
Displacement Sensing
• Linear Potentiometer
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Resolution (infinite), depends on?
High frequency bandwidth (> 10 kHz)
Fast response speed
Photo courtesy of Duncan Electronics
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
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
Displacement Sensing
• Differential Variable Reluctance Transducers
– Relatively short stroke
– High resolution
– Non-contact between the measured object and sensor
Type of Construction
Standard
tubular
Fixing Mode
by 8mm
diameter
Total Measuring Range 2(+/-1)mm
Pneumatic Retraction
No
Repeatability
0.1um
Operating
-10 to +65
Temperature Limits
degrees C
Courtesy of Microstrain, Inc.
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)
Photo courtesy of Bruel & Kjaer
Photo courtesy of Polytec
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
Shock (high-G) Sensing
• Shock Pressure Sensor
– Measurement range up to 69 MPa (10 ksi)
– High response speed (rise time < 2  sec.)
– High frequency bandwidth (resonant
frequency up to > 500 kHz)
– Operating temperature: -70 to 130 C
– Light (typically weighs ~ 10 g)
Photo courtesy of PCB Piezotronics
• 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)
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.com)
– 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
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
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
MEMS Fabrication Technique
Courtesy of A.P. Pisano, DARPA
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
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.
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.
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
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