Piezoelectric Sensors - Utah State University

Report
ECE5320 Mechatronics
Assignment#01: Literature Survey on Sensors and
Actuators
Topic: Piezoelectric Sensors
Prepared by:
Luke Peacock
Dept. of Electrical and Computer Engineering
Utah State University
Email:[email protected] ; Tel: (435)820-0462;
03/11/2010
Outline
References
Basic Information
Basic Working Principle illustrated
Major applications
Major specifications
Pros and Cons
Selection Criteria
References
1. "Piezoelectric sensors". Piezocryst website.
http://www.piezocryst.com/piezoelectric_sensors.php. Retrieved 03/10/10.
2. Alfredo Vázquez Carazo (January 2000). Novel Piezoelectric Transducers for
High Voltage Measurements. Universitat Politècnica de Catalunya. pp. 242.
3. Karki, James (September 2000). "Signal Conditioning Piezoelectric Sensors"
(PDF). Texas Instruments.
http://focus.ti.com/lit/an/sloa033a/sloa033a.pdf. Retrieved 3/10/10.
4. http://en.wikipedia.org/wiki/Piezoelectric_sensor
5. http://www.media.mit.edu/resenv/classes/MAS836/Readings/MSItechman.pdf
6. http://www.davidson.com.au/products/pressure/pcb/theory/piezotheory.asp
Other Places for Information
• http://upload.wikimedia.org/wikipedia/commons/c/c4/Sc
hemaPiezo.gif
• http://www.pcb.com/techsupport/tech_gen.php
• http://www.openengineering.com/index.php/eng/Products/OofelieMultiphysics-Specifications/PiezoElectric/Actuators-andsensors
Basic Information
“The word piezo comes from the Greek word piezein,
meaning to press or squeeze. Piezoelectricity refers to the
generation of electricity or of electric polarity in dielectric
crystals when subjected to mechanical stress and
conversely, the generation of stress in such crystals in
response to an applied voltage. In 1880, the Curie brothers
found that quartz changed its dimensions when subjected to
an electrical field and generated electrical charge when
pressure was applied. Since that time, researchers have
found piezoelectric properties in hundreds of ceramic and
plastic materials.“ [2-Carazo]
Basic Information Continued:
• Although it was discovered in late 1800’s by the Curie
brothers, it was nearly the 1950s before the piezoelectric
effect was used for industrial sensing applications.
• It is a versatile tool for the measurement of various
processes
• Useful in the determination of pressure, acceleration,
strain or force in quality assurance, process control and
development across many different industries.
Electrets
Electrets are solids which have a
permanent electrical polarization.
The electrical analog of magnets
(Figure 1). In general, the alignment
of the internal electric dipoles
would result in a charge which
would be observable on the surface
of the solid. In practice, this small
charge is quickly dissipated by free
charges from the surrounding
atmosphere which are attracted by
the surface charges.
Figure 1: Internal structure of an electret
http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node7.html
Principle of Operation
Depending on the way a piezoelectric material is cut, three main types of operations can be
distinguished 1. transversal 2. longitudinal 3. shear.
Figure 2: Piezoelectric modes of operation
http://www.piezocryst.com/piezoelectric_sensors.php
Transverse Effect
“A force is applied along a neutral axis (y) and the
charges are generated along the (x) direction,
perpendicular to the line of force. The amount of
charge depends on the geometrical dimensions of
the respective piezoelectric element. When
dimensions a, b, c apply, Cx = dxyFyb / a, where a is
the dimension in line with the neutral axis, b is in line
with the charge generating axis and d is the
corresponding piezoelectric coefficient.”
http://www.piezocryst.com/piezoelectric_sensors.php
Longitudinal Effect
“The amount of charge produced is strictly proportional to the
applied force and is independent of size and shape of the
piezoelectric element. Using several elements that are
mechanically in series and electrically in parallel is the only way
to increase the charge output. The resulting charge is Cx =
dxxFxn, where dxx is the piezoelectric coefficient for a charge in
x-direction released by forces applied along x-direction (in
pC/N). Fx is the applied Force in x-direction [N] and n
corresponds to the number of stacked elements.”
http://www.piezocryst.com/piezoelectric_sensors.php
Shear Effect
“Again, the charges produced are strictly
proportional to the applied forces and are
independent of the element’s size and shape. For
n elements mechanically in series and
electrically in parallel the charge is Cx = 2dxxFxn.”
http://www.piezocryst.com/piezoelectric_sensors.php
Sensor Circuit Models
Figure 3: Piezoelectric Sensor Circuit Models
Major Applications
Medical
Hearing aids
Ultrasound
Aerospace
Modal testing
Wind tunnel instrumentation
Shock tube instrumentation
Power Generation
Nuclear instrumentation
Shock/Vibration monitoring
Major Applications
Automotive
Internal Combustion Engines
Traffic Sensing
Bearing wear sensors
Quality Assurance
Process Control
Machine monitoring
Major Applications
Mobile electronics
Touch-pads
Accelerometers
Military
Sonar
Night vision
Ballistics
AND MANY MORE!!!!!
Major Specifications
Electro-Mechanical Conversion
(1 direction) 23 x 10-12m/V, 700 x 10-6N/V
(3 direction) -33 x 10-12m/V
Mechano-Electrical Conversion
(1 direction) 12 x 10-3V per microstrain, 400 x 10-3V/
(3 direction) 13 x 10-3V/N
Pyro-Electrical Conversion
8V/ o K (@ 25 o C)
Capacitance
1.36 x 10-9F; Dissipation Factor of 0.018 @ 10 KHz;
Maximum Operating Voltage
DC: 280 V (yields 7 µm displacement in 1 direction)
AC: 840 V (yields 21 µm displacement in 1 direction)
Maximum Applied Force (at break, 1 direction)
6-9 kgF (yields voltage output of 830 to 1275 V)
http://www.media.mit.edu/resenv/classes/MAS836/Readings/MSI-techman.pdf
Principle Comparisons
Principle
Strain
Sensitivity
[V/µ*]
Piezoelectric
5
0.00001
100,000,000
Piezoresistive
0.0001
0.0001
2,500,000
Inductive
0.001
0.0005
2,000,000
Capacitive
0.005
0.0001
750,000
Span to
Threshold [µ*] threshold ratio
http://en.wikipedia.org/wiki/Piezoelectric_sensor
Pros
• High sensitivity
• Wide input frequency
range
• High input amplitude range
• Low power consumption
• Unique applications
Cons
Low bandwidth
Limited displacement
High cost
Requires high impedance
amplification
• No Static measurements.
•
•
•
•
Selection Criteria
Nonlinearities
Frequency Response
Sensitivity
Impedance
Repeatability
System Response
Error
Linearity and Accuracy
Selection Criteria
Eccentricity
First-Order System Response
Under-damped Second-Order System Response
Saturation
Backlash
Resolution
Range
Static and Coulomb Friction

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