Electric Water Heater And Piezoelectric Force Transducer Essay, Research Paper

What is a piezoelectric force transducer (or sensor)?

A transducer is a device that transmits power from one system to another. In the case of the piezoelectric, they power comes in the form of electricity from a piezoelectric material acted on. Piezoelectricity may be described as the property of some materials that become electrically charged when subject to mechanical stress. These special materials also show a similar effect by undergoing mechanical deformation due to the application of an electric field. This effect was first discovered in 1880 in naturally occurring single crystals such as quartz. The reason certain materials have piezoelectric effects is related to the lack of symmetry in the unit cell. With the lack of symmetry, electric dipoles form due to mechanical distortion.

Piezoelectric material characteristics depend on stress, strain, displacement and the applied electric field on the material. Table 1 shows a summery of the basic parameters of a piezoelectric material. Figure 1 Symbols and Terminology, expands the basic parameters. The materials direction of polarization is designated as the z-axis of an orthogonal crystallographic system. The x.y.z axes are represented as 1,2,3 directions and the shear about these axes is represented as 4,5,6. The various piezoelectric material constants are expressed with subscripts. The first subscript gives direction of the electric field associated with theapplied voltage or charge produced. The second subscript gives direction of mechanical stress or strain. The superscripts show a constant mechanical or electrical boundary condition.

Piezoelectric materials constant are important to know. The density of the material effects is piezoelectric ability. Another constant is known as Curie temperature. The structure the material changes at Curie temperature from piezoelectric (non-symmetrical) to a non-piezoelectric (symmetrical) form. Along with the phase change peak in the dielectric constant-temperature curve at that temperature. The Curie temperature noted, Tc, is expressed in degrees Celsius. The dielectric constant, relative permittivity, is defined as the ratio of the permittivity of the material to the permittivity of free space. The dielectric loss factor is another constant representing the ratio of resistance to reactance of parallel equivalent circuit of the material element see Figure 2 Notation for Relative Permittivity. Other constants of piezoelectric material include frequency, coupling coefficient, charge coefficient, voltage coefficient, elastic, and aging rate.

The piezoelectric force transducer is a special material that transmits power. The characteristics of the material and how the material is effected govern the power transmission magnitude.

How does Piezoelectric Force Transducer measure force? Can they measure static forces?

When the piezoelectric transducer material undergoes shock, a force, pressure or acceleration, the piezoelectric effect occurs. A small amount of electricity is discharged from the material a proportional direction to the input stress. Generally piezo material in very stiff. Because of the high stiffness it is possible to make sensors with very high resonant frequencies making them capable of measuring rapidly changing dynamic phenomena shock, pressure wavefronts, high frequency hydraulic and other very sensitive forces.

The challenge for the measurement system is to couple information contained within the small amount of electric charge released by the material to the outside world, without dissipating it or otherwise changing it. There are two methods of system to measure the dynamic metrology. The Charge Mode System, and Low Impedance Voltage Mode (LIVM System). Under the charge mode system, the sensors are combined with a charge amplifier. The charge amplifier acts as a special type of amplifier designed specifically to measure electric charge. Figure 3 A Charge Mode Accelerometer System, shows the basic setup for this type of configuration. The second, LIVM System, different to the charge mode system because an amplifier isn t used. Instead a special Current Source Power Unit is used. Figure 4 The LIVM System, shows the basic setup for this type of configuration.

Because the type of forces the piezoelectric transducer can sense are dynamic, static forces are not measurable instantly. The response of the piezoelectric sensor to a static force can be characterized by a graph over time after sensing a dynamic force. This information can be used to calibrate the sensor.

How are Piezoelectric Force Transducers Calibrated?

As discussed, when the piezoelectric material experiences compression by an outside force, the material discharges electricity. The charges is usually around 1 X 10^-12 Coulombs (picocoulomb, pC). A metering device that has sensitivity in this range is needed. Or a special amplifier can be used to change the energy. There are different meter setups manufactured for use with a piezoelectric force transducer. Figure 5 Unholtz Dickie Vibration Testing System, Model 310, and Figure 6 & 7 Meter control picture, show equipment manufactured for piezoelectric force determination. Whatever equipment used the underlying principal is the same for calibrating the piezoelectric force transducer with the equipment. For an analog meter display a known vibration (generated with a vibration generator) can be applied to the piezo. The sensitivity or needle deflection of the meter can be adjusted to the value of amplitude of the vibration. For a digital meter, the maximum number shown on the display should be adjusted till it is equal with the know force caused by the vibrations. An exact calibration method can usually be obtained from the manufacturer.

References

COLUMBIA RESEARCH LABORATORIES, INC., Piezoelectric Product Division, 1925 Mac Dade Blvd. Woodlyn, PA 19094. Or, www.columbiaresearchlab.com

DYTRAN INSTRUMENTS, INC., Piezoelectric Sensors for Dynamic Measurements-Force, Pressure, Vibration, Shock 1998-1999 General Catalog & Instrumentation Handbook. 21592 Marilla Street, Chatsworth, CA 91311. Or e-mail [email protected].

SENSOR TECHNOLOGY LIMITED, BM Hi-Tech Division, PO Box 97,20 Stewart Road, Collingwood, Ontario Canada L9Y3Z4. Or www.sensortech.ca

AM 317 lab manual, California State University, Northridge. Lab #9 Calibration of a Piezoelectric Force Transducer.