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INTRODUCTION
There are two basic types of seismic sensors: inertial seismometers which measure ground motion relative to an inertial reference (a suspended mass), and strain meters or which measure the motion of one point of the ground relative to another. Since the motion of the ground relative to an inertial reference is in most cases much larger than the differential motion within a vault of reasonable dimensions, inertial seismometers are generally more sensitive to earthquake signals.
However, at very low frequencies it becomes increasingly difficult to maintain an inertial reference, and for the observation of low-order free oscillations of the Earth, tidal motions, and quasi-static deformations, strain meters may outperform inertial seismometers. Strain meters are conceptually simpler than inertial seismometers although their technical realization and installation may be more difficult.
An inertial seismometer converts ground motion into an electric signal but its properties cannot be described by a single scale factor, such as output volts per millimeter of ground motion. The response of a seismometer to ground motion depends not only on the amplitude of the ground motion (how large it is) but also on its time scale (how sudden it is). This is because the seismic mass has to be kept in place by a mechanical or electromagnetic restoring force. When the ground motion is slow, the mass will move with the rest of the instrument, and the output signal for a given ground motion will therefore be smaller. The system is thus a high-pass filter for the ground displacement. This must be taken into account when the ground motion is reconstructed from the recorded signal, and is the reason why we have to go to some length in discussing the dynamic transfer properties of seismometers.
The dynamic behavior of a seismograph system within its linear range can, like that of any linear time-invariant (LTI) system, be described with the same degree of completeness in four different ways: by a linear differential equation, the Laplace transfer function, the complex frequency response, or the impulse response of the system. The first two are usually obtained by a mathematical analysis of the physical system (the hardware). The latter two are directly related to certain calibration procedures and can therefore be determined from calibration experiments where the system is considered as a black box (this is sometimes called an identification procedure). However, since all four are mathematically equivalent, we can derive each of them either from knowledge of the physical components of the system or from a calibration experiment.
Practically, the mathematical description of a seismometer is limited to a certain bandwidth of frequencies that should at least include the bandwidth of seismic signals. Within this limit then any of the four representations describe the system's response to arbitrary input signals completely and unambiguously. The viewpoint from which they differ is how efficiently and accurately they can be implemented in different signal-processing procedures.
While for a purely electrical filter it is usually clear what the amplitude response is - a dimensionless factor by which the amplitude of a sinusoidal input signal must be multiplied to obtain the associated output signal - the situation is not always as clear for seismometers because different authors may prefer to measure the input signal (the ground motion) in different ways: as a displacement, a velocity, or an acceleration. Both the physical dimension and the mathematical form of the transfer function depend on the definition of the input signal, and one must sometimes guess from the physical dimension to what sort of input signal it applies. The output signal, traditionally a needle deflection, is now normally a voltage, a current, or a number of counts.
PROJECT ORGANISATION
This project work presented the design and implementation of seismic sensors for industrial and domestic purpose using the piezo element and a piezo buzzer with its underlining principle of piezoelectricity. The circuit uses readily available components and the design is straight forward.
A standard Piezo sensor is used to detect vibrations/sounds due to pressure changes. The piezo element acts as a small capacitor having a capacitance of a few nanofarads. Like a capacitor, it can store charge when a potential is applied to its terminals. It discharges through VR1, when it is disturbed.
The project work is organized as follows: chapter two will concentrate on the hardware description which is mostly on TL071 JFET op-amp, NE555 timer ICs, Resistors, Transistors, Capacitors and piezoelectricity in detail, chapter three focuses on the design and implementation of the seismic sensor with piezoelectricity along with detailed explanation of the project. Chapter four, chapter five and chapter six discuss about Applications, Conclusions and Future scope, and References respectively.
HARDWARE DESCRIPTION
2.1 PIEZO ELEMENT
Introduction
This is a focus on piezoelectricity as the backbone behind the operation of this proposed circuit. We are going to look at the history of piezoelectricity, features of Piezo element, buzzer, the proposed circuit diagram and some applications of the piezoelectricity.
A piezoelectric sensor shown in Fig 2.1 is a device that uses the piezoelectric effect to measure pressure, acceleration, strain or force by converting them to an electrical signal.
History of Piezoelectricity
Piezoelectricity is a form of electricity created when certain crystals are bent or otherwise deformed. These same crystals can also be made to bend slightly when a small current is run through them, encouraging their use in instruments for which great degrees of mechanical control are necessary. This is called converse piezoelectricity. For example, scanning tunneling microscopes (STMs) use piezoelectric crystals to “scan” the surface of a material and create images of great detail. Piezoelectricity is related to pyroelectricity, in which a current is created by heating or cooling the crystal.
The property of piezoelectricity is dictated by both the atoms in the crystal and the particular way in which that crystal was formed. Some of the first substances that were used to demonstrate piezoelectricity are topaz, quartz, tourmaline, and cane sugar. Today, we know of many crystals which are piezoelectric, some of which can even be found in human bone. Certain ceramics and polymers have exhibited the effect as well.
A piezoelectric crystal consists of multiple interlocking domains which have positive and negative charges. These domains are symmetrical within the crystal, causing the crystal as a whole to be electrically neutral. When stress is put on the crystal, the symmetry is slightly broken, generating voltage. Even a tiny bit of piezoelectric crystal can generate voltages in the thousands.
Piezoelectricity is used in sensors, actuators, motors, clocks, lighters, and transducers. A quartz clock uses piezoelectricity, as does any cigarette lighter without a flint. Medical ultrasound devices create high-frequency acoustic vibrations using piezoelectric crystals. Piezoelectricity is used in some engines to create the spark which ignites the gas. Loudspeakers use piezoelectricity to convert incoming electricity to sound. Piezoelectric crystals are used in many high-performance devices to apply tiny mechanical displacements on the scale of nanometers. Even though a piezoelectric crystal never deforms by more than a few nanometers when a current is run through it, the force behind this deformation is extremely high, on the order of meganewtons. This deformational power is used in mechanics experiments and for aligning optical elements many times heavier than the piezoelectric crystal itself.
The first experimental demonstration of a connection between macroscopic piezoelectric phenomena and crystallographic structure was published in 1880 by Pierre and Jacques Curie. Their experiment consisted of a conclusive measurement of surface charges appearing on specially prepared crystals (tourmaline, quartz, topaz, cane sugar and Rochelle salt among them) which were subjected to mechanical stress. These results were a credit to the Curies' imagination and perseverance, considering that they were obtained with nothing more than the foil, glue, wire, magnets, and a jeweler’s saw.
Sensor Design
Based on piezoelectric technology various physical quantities can be measured; the most common are pressure and acceleration. For pressure sensors, a thin membrane and a massive base is used, ensuring that an applied pressure specifically loads the elements in one direction. For accelerometers, a seismic mass is attached to the crystal elements. When the accelerometer experiences a motion, the invariant seismic mass loads the elements according to Newton’s second law of motion
F = ma.
The main difference in the working principle between these two cases is the way forces are applied to the sensing elements. In a pressure sensor a thin membrane is used to transfer the force to the elements, while in accelerometers the forces are applied by an attached seismic mass.
Sensors often tend to be sensitive to more than one physical quantity. Pressure sensors show false signal when they are exposed to vibrations. Sophisticated pressure sensors therefore use acceleration compensation elements in addition to the pressure sensing elements. By carefully matching those elements, the acceleration signal (released from the compensation element) is subtracted from the combined signal of pressure and acceleration to derive the true pressure information.
Vibration sensors can also be used to harvest otherwise wasted energy from mechanical vibrations. This is accomplished by using piezoelectric materials to convert mechanical strain into usable electrical energy. The Fig 2.2 shows metal disks with Piezo material.
Sensing Materials
Two main groups of materials are used for piezoelectric sensors: piezoelectric ceramics and single crystal materials. The ceramic materials (such as PZT ceramic) have a piezoelectric constant or sensitivity that are roughly two orders of magnitude higher than those of single crystal materials and can be produced by inexpensive sintering processes. The Piezo effect in Piezo ceramics is trained, so unfortunately their high sensitivity degrades over time. The degradation is highly correlated with temperature. The less sensitive crystal materials (gallium phosphate, quartz, and tourmaline) have a much higher – when carefully handled, almost infinite – long term stability.
Principle of Operation
Depending on how a piezoelectric material is cut, three main modes of operation can be distinguished: transverse, longitudinal, and shear.
• 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.
• 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. Fx is the applied Force in x-direction and ‘n’ corresponds to the number of stacked elements.
• 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.
In contrast to the longitudinal and shear effects, the transverse effect opens the possibility to fine-tune sensitivity on the force applied and the element dimension.
Electrical Properties
A piezoelectric transducer has very high DC output impedance and can be modeled as a proportional voltage source and filter network. The voltage V at the source is directly proportional to the applied force, pressure, or strain. The output signal is then related to this mechanical force as if it had passed through the equivalent circuit. Here Fig 2.3 shows the frequency response of a piezoelectric sensor.
• Applications
Piezoelectric sensors have proven to be versatile tools for the measurement of various processes. They are used for quality assurance, process control and for research and development in many different industries.
It has been successfully used in various applications, such as in medical, aerospace, nuclear instrumentation, and as a pressure sensor in the touch pads of mobile phones. In the automotive industry, piezoelectric elements are used to monitor combustion when developing internal combustion engines. The sensors are either directly mounted into additional holes into the cylinder head or the spark/glow plug is equipped with a built in miniature piezoelectric sensor.
• Advantages
The rise of piezoelectric technology is directly related to a set of inherent advantages. The high modulus of elasticity of many piezoelectric materials is comparable to that of many metals and goes up to 105 N/m². Even though piezoelectric sensors are electromechanical systems that react to compression, the sensing elements show almost zero deflection. This is the reason why piezoelectric sensors are so rugged, have an extremely high natural frequency and an excellent linearity over a wide amplitude range.
Additionally, piezoelectric technology is insensitive to electromagnetic fields and radiation, enabling measurements under harsh conditions. Some materials used (especially gallium phosphate or tourmaline) have an extreme stability even at high temperature, enabling sensors to have a working range of up to 1000°C. Tourmaline shows pyroelectricity in addition to the piezoelectric effect; this is the ability to generate an electrical signal when the temperature of the crystal changes. This effect is also common to piezo ceramic materials.
One disadvantage of piezoelectric sensors is that they cannot be used for truly static measurements. A static force will result in a fixed amount of charges on the piezoelectric material. While working with conventional readout electronics, imperfect insulating materials, and reduction in internal sensor resistance will result in a constant loss of electrons, and yield a decreasing signal. Elevated temperatures cause an additional drop in internal resistance and sensitivity. The main effect on the piezoelectric effect is that with increasing pressure loads and temperature, the sensitivity is reduced due to twin-formation. While quartz sensors need to be cooled during measurements at temperatures above 300°C, special types of crystals like GaPO4 gallium phosphate do not show any twin formation up to the melting point of the material itself.
However, it is not true that piezoelectric sensors can only be used for very fast processes or at ambient conditions. In fact, there are numerous applications that show quasi-static measurements, while there are other applications with temperatures higher than 500°C.
Piezoelectric sensors are also seen in nature. Dry bone is piezoelectric, and is thought by some to act as a biological force sensor.
• Mechanical
A joy buzzer is an example of a purely mechanical buzzer.
• Electromechanical
Early devices were based on an electromechanical system identical to an electric bell without the metal gong. Similarly, a relay may be connected to interrupt its own actuating current, causing the contacts to buzz. Often these units were anchored to a wall or ceiling to use it as a sounding board. The word "buzzer" comes from the rasping noise that electromechanical buzzers made.
• Electronic
A piezoelectric element may be driven by an oscillating electronic circuit or other audio signal source. Sounds commonly used to indicate that a button has been pressed are a click, a ring or a beep. Electronic buzzers find many applications in modern days.
2.3 TL071
This will focus on the features of the TL071 Low noise JFET single operational amplifier such as its description, electrical characteristics and its operations.
Description
The TL071 is a high-speed JFET input single operational amplifier. This JFET input operational amplifier incorporates well matched, high-voltage JFET and bipolar transistors in a monolithic integrated circuit. The device features high slew rates, low input bias and offset currents, and low offset voltage temperature coefficient. The Fig 2.7 shows the pin out configuration of the IC.