Piezoelectric Sensors

1. Piezoelectric Sensors Seth R. Hills ECE5320 Mechatronics Assignment #1

2. Outline • Reference list • Links for more information • Major applications • Basic working principle illustrated • A typical sample configuration in application • Major specifications • Limitations • Selection Criteria • Cost information • Where to buy

3. References: • http://www.sensorsmag.com/articles/0204/27/main.shtml • http://www.media.mit.edu/resenv/classes/MAS836/Readings/MSI-techman.pdf • http://www.princeton.edu/~cml/html/publicity/TRN20001122/Piezoelectric%20sliver%20.htm • http://www.sensorsweb.com/taxonomy/term/46 • http://www.davidson.com.au/products/pressure/pcb/theory/piezo-theory.asp

4. To Explore Further Visit these websites or check out these articles: • http://www.iupac.org/publications/pac/2004/pdf/7606x1139.pdf • http://www.ndt.net/article/yosi/yosi.htm

5. Major applications • Sonar • Hearing Aids. Low frequency • Ultrasound. Ultrasonic actuator • Night vision. Pyroelectric sensor effect • Traffic Sensors • Music Pickups • Machine Monitoring • Bearing Wear Sensors • Thread Break Sensor • Accelerometers • Aerospace. Modal testing, wind tunnel, and shock tube instrumentation; • Ballistics. Combustion, explosion, and detonation • Engine Testing. Combustion and dynamic stressing • Shock/Vibration • Implantables:Pacemaker Activity Monitor, Implantable Switch, Vascular Graft Monitor, Micropower Source • And many, many more! http://www.sensorsmag.com/articles/0204/27/main.shtml#sidebar1

6. Specific Applications • To detect sound, e.g. piezoelectric microphones (sound waves bend the piezoelectric material, creating a changing voltage) and piezoelectric pickups for electrically amplified guitars. • Piezoelectric elements are also used in the generation of sonar waves. Piezoelectric microbalances are used as very sensitive chemical and biological sensors. • Piezoelectric elements are used in electronic drum pads to detect the impact of the drummer's sticks. http://www.sensorsweb.com/piezoelectric_sensors

7. Basic working principle • In 1880, brothers Pierre and Jacques Curie demonstrated the relationship between a mechanical load on a crystal and the electric charge resulting from it. • Piezoelectricity is a linear electromechanical interaction between the mechanical and electrical states of a material. • The piezoelectric effect results from a deformation of the crystal lattice by some external force that pushes the positive and negative lattice points against one another and thus produces a dipole moment and an electric charge. http://www.sensorsmag.com/articles/0204/27/main.shtml#sidebar1

8. Electrets are solids which have a permanent electrical polarization. The electrical analog of magnets (Figure 3). 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. Electrets Figure 3: Internal Structure of an electret http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node7.html

9. Piezoelectric Crystals • Permanent polarization as in the case of the electrets is also observed in crystals. • Each cell of the crystal has an electric dipole, and the cells are oriented such that the electric dipoles are aligned. Again, this results in excess surface charge which attracts free charges from the surrounding atmosphere making the crystal electrically neutral. • If a sufficient force is applied to the piezoelectric crystal, a deformation will take place. This deformation disrupts the orientation of the electrical dipoles and creates a situation in which the charge is not completely canceled. This results in a temporary excess of surface charge, which subsequently is manifested as a voltage which is developed across the crystal. http://ccrma.stanford.edu/CCRMA/Courses/252/sensors/node7.html

10. Description Continued… • In a piezoelectric crystal, the positive and negative electrical charges are separated, but symmetrically distributed, so that the crystal overall is electrically neutral. When a stress is applied, this symmetry is disturbed, and the charge asymmetry generates a voltage. A 1 cm cube of quartz with 500 lb (2 kN) of correctly applied pressure upon it, can produce 12,500 V of electricity. http://www.sensorsweb.com/taxonomy/term/46

11. A typical sample configuration in application Piezo Film Textile Sensors A piezoelectric film vibration sensor, mounted to a thin steel beam, monitors the acoustic signal caused by the abrasion of the thread running across the beam, analogous to a violin string. The absence of the vibration instantly triggers the machinery to stop. Thread tension and defects in man-made fibers are also monitored with piezo film textile sensors. http://www.media.mit.edu/resenv/classes/MAS836/Readings/MSI-techman.pdf

12. The sensing element isa crystal which has the property of emitting a chargewhen subjected to a compressive force. In the accelerometer, this crystal is bonded to a masssuch that when the accelerometer is subjected to a 'g'force, the mass compresses the crystal which emits asignal. This signal value can be related to the imposed 'g' force THE PIEZOELECTRIC ACCELEROMETER (Compression Type) http://www.sensorland.com/HowPage003.html

13. piezoelectric cantilevers to measure viscosity Description:  This sliver of piezo- electric material forms a cantilever that can be used to measure the density of a liquid. Forces acting on the microcantilever cause corresponding changes in its’ electrical resistance. http://www.princeton.edu/~

14. Major specifications • 1. Electro-Mechanical Conversion (1 direction) 23 x 10-12m/V, 700 x 10-6N/V (3 direction) -33 x 10-12m/V • 2. Mechano-Electrical Conversion (1 direction) 12 x 10-3V per microstrain, 400 x 10-3V/ (3 direction) 13 x 10-3V/N • 3. Pyro-Electrical Conversion 8V/ o K (@ 25 o C) • 4. Capacitance 1.36 x 10-9F; Dissipation Factor of 0.018 @ 10 KHz; • 5. Maximum Operating Voltage DC: 280 V (yields 7 µm displacement in 1 direction) AC: 840 V (yields 21 µm displacement in 1 direction) • 6. 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

15. Pros and Cons • Piezoelectric sensors offers unique capabilities which are typically not found in other sensing technologies. • There are certain advantages: • wide frequency and amplitude range • and disadvantages: • no static measuring capability • This all depends on the particular application. Therefore, when choosing a specific sensor or sensor technology, it is important to pay close attention to the performance specifications. http://www.davidson.com.au/products/pressure/pcb/theory/piezo-theory.asp

16. Limitations • Low frequency performance (below 500Hz) tends to be limited • Limited displacements • Quasistatic force sensing • Requires high impedance amplification of signal

17. Selection Criteria • Dependent on Application • Range • Resolution • Sensitivity • Error • Repeatability • Linearity and Accuracy • Impedance • Nonlinearities • Static and Coulomb Friction

18. Selection Criteria • Eccentricity • Backlash • Saturation • Deadband • System Response • First-Order System Response • Underdamped Second-Order System Response • Frequency Response

19. Cost Information There are many manufacturers and distributors of piezoelectric devices here are a few: • Piezoelectric microphone: $94.84 – digikey.com • Piezoelectric axle sensor: Request a quote - www.triggindustries.com • 28 Micron Piezo Film Sheet: $55.00+ at www.msiusa.com

20. Where to Buy: • http://www.triggindustries.com/piezoelectric_sensors.htm • http://www.piezo-products.com/?mp-pvid=1-1gLgd-0iQ-1D8vA9 • http://www.sparklerceramics.com/ • www.msiusa.com • www.digikey.com