PIEZOELECTRIC EFFECT.

The piezoelectric effect was discovered by Pierre and Jacques Curie in 1880. It remained a mere curiosity until the 1940s. The properties of certain crystals to exhibit electrical charges under mechanical loading was of no practical use until very high input impedance amplifiers enabled engineers to amplify their signals. In the 1950's, electrometer tubes of sufficient quality became available and the piezoelectric effect was commercialized.

The charge amplifier principle was patented by W.P. Kistler in 1950 and gained practical significance in the 1960s. The introduction of solid state circuitry and the development of highly insulating materials such as Teflon and Kapton greatly improved performance and propelled the use of piezoelectric sensors into virtually all areas of modern technology and industry.

Piezoelectric measuring systems are active electrical systems. That is, the crystals produce an electrical output only when they experience a change in load. For this reason, they cannot perform true static measurements. However, it is a misconception that piezoelectric instruments are suitable for only dynamic measurements. Quartz transducers, paired with adequate signal conditioners, offer excellent quasistatic measuring capability. There are countless examples of applications where quartz based transducers accurately and reliably measure quasistatic phenomena for minutes and even hours.

APPLICATIONS OF PIEZOELECTRIC INSTRUMENTATION

Piezoelectric measuring devices are widely used today in the laboratory, on the production floor and as original equipment. They are used in almost every conceivable application requiring accurate measurement and recording of dynamic changes in mechanical variable such as pressure, force and acceleration. The list of applications continues to grow and now includes:

• Aerospace: Modal testing, wind tunnel and shock tube instrumentation, landing gear hydraulics, rocketry, structures, ejection systems and cutting force research.
• Ballistics: Combustion, explosion, detonation and sound pressure distribution.
• Biomechanics: Multi-component force measurement for orthopedic gait and posturography, sports, ergonomics, neurology, cardiology and rehabilitation.
• Engine Testing: Combustion, gas exchange and injection, indicator diagrams and dynamic stressing.
• Engineering: Materials evaluation, control systems, reactors, building structures, ship structures, auto chassis structural testing, shock and vibration isolation and dynamic response testing.
• Industrial/Factory: Machining systems, metal cutting, press and crimp force, automation of force based assembly operations and machine health monitoring.
• OEMs: Transportation systems, plastic molding, rockets, machine tools, compressors, engines, flexible structures, oil/gas drilling and shock/vibration testers.
  

PIEZOELECTRIC TRANSDUCERS (Quartz Based)

The vast majority of Kistler transducers utilize quartz as the sensing element. As discussed in other sections of this catalog, Kistler also manufactures transducers which utilize piezo-ceramic elements and micromachined silicon structures. However, the discussion in this section will be limited to quartz applications. Quartz piezoelectric transducers consist essentially of thin slabs or plates cut in a precise orientation to the crystal axes depending on the application. Most Kistler transducers incorporate a quartz element which is sensitive to either compressive or shear loads. The shear cut is used for patented multi-component force and acceleration measuring transducers. Other specialized cuts include the transverse cut for some pressure transducers and the patented polystable cut for high temperature pressure transducers. See Figures 1 and 2 below.

Figure 1 - Quartz Crystal Y Bar

Figure 2 - Piezoelectric Effect

The precisely machined quartz elements are assembled either singly or in stacks and usually preloaded with a spring sleeve. The quartz package generates a charge signal (measured in picoCoulombs) which is directly proportional to the sustained force. Each transducer type which uses a quartz configuration optimized and ultimately calibrated for its particular application (force, pressure, acceleration or strain).

Quartz transducers exhibit remarkable properties which justify their large scale use in research, development, production and testing. They are extremely stable, rugged and compact. Of the large number of piezoelectric materials available today, quartz is employed preferentially in transducer designs because of the following excellent properties:

• high material stress limit, approximately 20,000 psi
• temperature resistance up to 500 degrees C
• very high rigidity, high linearity and negligible hysteresis
• almost constant sensitivity over a wide temperature range
• ultra high insulation resistance (10¹⁴ ohms) allowing low frequency measurements (<1 Hz)

HIGH AND LOW IMPEDANCE

Kistler supplies two types of piezoelectric transducers: high and low impedance. High impedance units have a charge output which requires a charge amplifier or external impedance converter for charge-to-voltage conversion. Low impedance types use the same piezoelectric sensing element as high impedance units and also incorporate a miniaturized built-in charge-to-voltage converter. Low impedance types require an external power supply coupler to energize the electronics and decouple the subsequent DC bias voltage from the output signal.