Piezoelectric Accelerometer

1. Piezoelectric Accelerometer ECE 5320 MECHATRONICS“Sensor Tutorial” Prepared by: Jaime Fernandez Department of Electrical and Computer Engineering Utah State University “Robert H. Bishop”, Mechatronics Handbook, figure 19.3

2. Outline • Reference List • History • Piezoelectric Notions • Acceleration Sensing Notions • Major Applications • Basics Principles • Piezoelectric Accelerometer Design • Major specifications

3. Reference List • “Robert H. Bishop”, Mechatronics Handbook, 2002, CRC Press LLC • “Wikipedia”, http://en.wikipedia.org/wiki/Piezoelectricity • “Alexander D. Khazan”, Transducers and Their Elements, Prentice Hall PTR • “Robert H. Bishop”, Mechatronics Handbook, section 19.2 • “Curtis D. Johnson”, Process Control Instrumentation Technology, Prentice Hall PTR, National Instruments.

4. History • A related property known as pyroelectricity, the ability of certain mineral crystals to generate electrical charge when heated, was known of as early as the 19th century, and was named by David Brewster in 1824. In 1880, the brothers Pierre Curie and Jacques Curie predicted and demonstrated piezoelectricity using tinfoil, glue, wire, magnets, and a jeweler's saw. • They showed that crystals of tourmaline, quartz, topaz, cane sugar, and Rochelle salt (sodium potassium tartrate tetrahydrate) generate electrical polarization from mechanical stress. “Wikipedia”, http://en.wikipedia.org/wiki/Piezoelectricity

5. Quartz and Rochelle salt exhibited the most piezoelectricity. Converse piezoelectricity was mathematically deduced from fundamental thermodynamic principles by Lippmann in 1881. • The Curies immediately confirmed the existence of the "converse effect," and went on to obtain quantitative proof of the complete reversibility of electro-elasto-mechanical deformations in piezoelectric crystals. (1) “Wikipedia”, http://en.wikipedia.org/wiki/Piezoelectricity

6. Piezoelectric Notions • “Piezoelectricity is the ability of certain crystals to generate a voltage in response to applied mechanical stress. The word is derived from the Greek piezein, which means to squeeze or press. The piezoelectric effect is reversible in that piezoelectric crystals, when subjected to an externally applied voltage, can change shape by a small amount. The deformation, about 0.1% of the original dimension in PZT, is of the order of nanometers, but nevertheless finds useful applications such as the production and detection of sound, generation of high voltages, electronic frequency generation, and ultra fine focusing of optical assemblies.” (1) • “In a piezoelectric transducer, the acceleration acts on the seismic mass that develops a force on piezoelectric quartz, or ceramic crystal, or on several crystals. The force causes charges on the crystals proportional to the acceleration.” (2) “Wikipedia”, http://en.wikipedia.org/wiki/Piezoelectricity “Alexander D. Khazan”, Transducers and Their Elements, Prentice Hall PTR “Robert H. Bishop”, Mechatronics Handbook, section 19.2

7. Acceleration Sensing Notions • Direct measurement: this is done by sensing the acceleration directly. • Indirect measurement: this is done differentiating the velocity. In the real world is more common integrating a measurement than differentiating it. • “The applicability of these techniques depends on the type of motion (rectilinear, angular, or curvilinear motion) or equilibrium centered vibration. For rectilinear and curvilinear motions, the direct measurement accelerometers are preferred. However, the angular acceleration is usually measured by indirect methods”. (3) “Robert H. Bishop”, Mechatronics Handbook, section 19.2 “Curtis D. Johnson”, Process Control Instrumentation Technology, Prentice Hall PTR, National Instruments

8. Major Applications • Piezoelectric accelerometer are good sensing device for the application were is needed: • high bandwidth • low power consumption • ruggedness “As acceleration acts on the mounting surface of a flat plate shear sensing configuration, the piezoelectric ceramic element is put into shear relative to the attached mass.” “Robert Shear, Endevco” Kavlico Corporation Piezoelectric Sensors for OEM Applications, figure 3

9. Applications: • Machine Monitoring. Bearing and gear mesh wear typically have characteristic frequencies in the 2­10 kHz range. • Shock Detection. For applications such as shipment monitoring, impact detection, or drop testing. • Vehicle Dynamics. handling parameters such as ride smoothness, vehicle performance, cargo transport, and asset tracking in vehicle. • Structural Dynamics: detect fatigue, resonance, and response to load.

10. Basics Principles • These devices utilize a mass in direct contact with the piezoelectric component or crystal as shown in When a varying motion is applied to the accelerometer, the crystal experiences a varying force excitation (F = ma), causing a proportional electric charge q to be developed across it. • So, where q is the charge developed and dij is the piezoelectric coefficient of the material. As this equation shows, the output from the piezoelectric material is dependent on its mechanical properties, dij.

11. These accelerometers are useful for high-frequency applications. The roll-off typically starts near 100 Hz. These active devices have no DC response. Since piezoelectric accelerometers have comparatively low mechanical impedances, their effect on the motion of most structures is negligible.

12. Mathematically, their transfer function approximates a third-order system that can be expressed as where Kq is the piezoelectric constant related to charge (C cm), τ is the time constant of the crystal, and s is the Laplace variable. It is worth noting that the crystal itself does not have a time constant τ, but the time constant is observed when the accelerometer is connected to an electric circuit, for example, an RC circuit. “Robert H. Bishop”, Mechatronics Handbook, “Robert H. Bishop”, Mechatronics Handbook, fig 19.22

13. The low-frequency response is limited by the piezoelectric characteristic τs / (τs + 1), while the high frequency response is related to mechanical response. The damping factor  is very small, usually less than 0.01 or near zero. Accurate low-frequency response requires large τ, which is usually achieved by use of high-impedance voltage amplifiers. At very low frequencies thermal effects can have severe influences on the operation characteristics. “Robert H. Bishop”, Mechatronics Handbook,

14. Piezoelectric Accelerometer Design “Manfred Weber”, http://www.mmf.de/

15. “Manfred Weber”, http://www.mmf.de/

16. Major specifications “Endevco” “Piezoelectric accelerometer”, model 22, <http://www.bksv.com/pdf/22.pdf>

17. “Endevco” “Piezoelectric accelerometer”, model 22, <http://www.bksv.com/pdf/22.pdf>

18. “Endevco” “Piezoelectric accelerometer”, model 22, <http://www.bksv.com/pdf/22.pdf>

19. Advantages of piezoelectric accelerometer

20. Suppliers Information • http://www.columbiaresearchlab.com/index.asp/ • http://www.endevco.com/ • http://www.meas-spec.com/myMeas/default/index.asp/ • http://www.summitinstruments.com/ • http://www.ferroperm-piezo.com • http://www.matsysinc.com/ • http://www.piezosolutions.net/ • http://www.sensortech.ca/ • http://www.vfultrasound.com/ • http://www.xinetics.com/home.html/