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Transcript of 7 Sensors Technology
7th Meeting
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.
Sensor technology; Sensor data collection topologies; Data communication; Power supply; Data synchronization; Environmental parameters and
influence; Remote data analysis.
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.
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.
Zhang & Aktan, 2005
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
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
True value
measurement
Precision without accuracy
Accuracy without precision
Precision and accuracy
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
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
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
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
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
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
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
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
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
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
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
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
Differential Variable Reluctance Transducers • Relatively short stroke• High resolution• Non-contact between the measured object and sensor
Type of Construction Standard
tubular
Fixing Modeby 8mm
diameter
Total Measuring Range
2(+/-1)mm
Pneumatic Retraction No
Repeatability 0.1um
Operating Temperature Limits
-10 to +65 degrees C
Courtesy of Microstrain, Inc.
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
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 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)
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
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
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
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
Courtesy of A.P. Pisano, DARPA
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
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.
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 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
ReferenceZhang, 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