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Dept. of Electrical and Computer Engineering

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1 Dept. of Electrical and Computer Engineering
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 ; Tel: (435) ; 03/11/2010

2 Outline References Basic Information
Basic Working Principle illustrated Major applications Major specifications Pros and Cons Selection Criteria

3 References 1. "Piezoelectric sensors". Piezocryst website. Retrieved 03/10/10.  Alfredo Vázquez Carazo (January 2000). Novel Piezoelectric Transducers for High Voltage Measurements. Universitat Politècnica de Catalunya. pp. 242.  Karki, James (September 2000). "Signal Conditioning Piezoelectric Sensors" (PDF). Texas Instruments. Retrieved 3/10/10. 

4 Other Places for Information

5 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]

6 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.

7 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

8 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

9 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.”

10 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.”

11 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.”

12 Sensor Circuit Models Figure 3: Piezoelectric Sensor Circuit Models

13 Major Applications Medical Hearing aids Ultrasound Aerospace
Modal testing Wind tunnel instrumentation Shock tube instrumentation Power Generation Nuclear instrumentation Shock/Vibration monitoring

14 Major Applications Automotive Internal Combustion Engines
Traffic Sensing Bearing wear sensors Quality Assurance Process Control Machine monitoring

15 Major Applications Mobile electronics Touch-pads Accelerometers
Military Sonar Night vision Ballistics AND MANY MORE!!!!!

16 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 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)

17 Principle Comparisons
Strain Sensitivity [V/µ*] Threshold [µ*] Span to threshold ratio Piezoelectric 5 100,000,000 Piezoresistive 0.0001 2,500,000 Inductive 0.001 0.0005 2,000,000 Capacitive 0.005 750,000

18 Pros Cons High sensitivity Wide input frequency range
High input amplitude range Low power consumption Unique applications Low bandwidth Limited displacement High cost Requires high impedance amplification No Static measurements.

19 Selection Criteria Nonlinearities Frequency Response Sensitivity
Impedance Repeatability System Response Error Linearity and Accuracy

20 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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