design and implementation seismic sensor
TRANSCRIPT
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DESIGN AND IMPLEMENTATION OF
SEISMIC SENSOR
by
CHAPTER ONE
INTRODUCTION
1.1 Overview
There are two basic types of seismic sensors: inertial seismometers which measure
ground motion relative to an inertial reference (a suspended mass), and
strainmeters or extensometers 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, strainmeters
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may outperform inertial seismometers. Strainmeters 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
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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. The mutual relations
between the time-domain and frequency-domain. 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.
In digital signal processing, seismic sensors are often represented with other
methods that are efficient and accurate but not mathematically exact, such as
recursive (IIR) filters. Digital signal processing is however beyond the scope of
this section. A wealth of textbooks is available both on analog and digital signal
processing, for example Oppenheim and Willsky (1983) for analog processing,
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Oppenheim and Schafer (1975) for digital processing, and Scherbaum (1996) for
seismological applications.
1.2 Literature Review
As indicated earlier on, the most commonly used description of a seismograph
response in the classical observatory practice has been the magnification curve,
i.e. the frequency-dependent magnification of the ground motion. Mathematically
this is the modulus (absolute value) of the complex frequency response, usually
called the amplitude response. It specifies the steady-state harmonic responsivity
(amplification, magnification, conversion factor) of the seismograph as a function
of frequency. However, for the correct interpretation of seismograms, also the
phase response of the recording system must be known. It can in principle be
calculated from the amplitude response, but is normally specified separately, or
derived together with the amplitude response from the mathematically more
elegant description of the system by its complex transfer function or its complex
frequency response.
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
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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.
Calibrating a seismograph means measuring (and sometimes adjusting) its transfer
properties and expressing them as a complex frequency response or one of its
mathematical equivalents. For most applications the result must be available as
parameters of a mathematical formula, not as raw data; so determining parameters
by fitting a theoretical curve of known shape to the data is usually part of the
procedure. Practically, seismometers are calibrated in two steps.
The first step is an electrical calibration in which the seismic mass is excited with
an electromagnetic force. Most seismometers have a built-in calibration coil that
can be connected to an external signal generator for this purpose. Usually the
response of the system to different sinusoidal signals at frequencies across the
system's passband, to impulses, or to arbitrary broadband signals is observed while
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the absolute magnification or gain remains unknown. For the exact calibration of
sensors with a large dynamic range such as those employed in modern
seismograph systems, the latter method is most appropriate.
1.3 Project Organization
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 most importantly the TL071 JFET op-amp and the
NE555 timer ICs while chapter three looks at piezoelectricity in details. Chapter
four focuses on the design and implementation of the seismic sensor for both
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industrial and domestic application with piezoelectricity with detailed explanation
of the project topic in general as chapter five concludes the project work.
CHAPTER TWO
HARDWARE DESCRIPTION
2.1 Introduction
This chapter will focus on the features of the TL071 Low noise JFET single
operational amplifier such as its description, electrical characteristics and its
operations, and further look also at the NE555 Timer IC such as its overview, pin
outs, pin descriptions, operating overview, electrical/environmental characteristics
and monostable and astable operations.
2.2 TL071Low Noise JFET Single Operational Amplifier
2.2.1Description
The TL071 is a high-speed JFET input single operational amplifier. This JFET
input operational amplifier incorporates well matched, high-voltage JFET and
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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 diagrams below show the pin out configuration and can package of
the IC
(a) (b)
Fig 2.1 (a) can package of TL071 IC, (b) pin connection of Tl071 IC
The following are the description of the individual pin connections of the above IC
as shown in figure 2.1.
1 - Offset null 1
2 - Inverting input
3 - Non-inverting input
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4 - VCC-
5 - Offset null 2
6 - Output
7 - VCC+
8 - N.C.
2.2.2Features of TL071 IC
Tl071 IC is a slightly for powerful JFET single input operational amplifier which
has the following features;
Wide common-mode (up to VCC+) and differential voltage range
Low input bias and offset current
Low noise en = 15nV/ Hz
Output short-circuit protection
High input impedance JFET input stage
Low harmonic distortion: 0.01%
Internal frequency compensation
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Latch-up free operation
High slew rate: 16V /s
All voltage values, except differential voltage, are with respect to the zero
reference level (ground) of the supply voltages where the zero reference level is the
midpoint between VCC+ and VCC. The magnitude of the input voltage must never
exceed the magnitude of the supply voltage or 15 volts, whichever is less.
Differential voltages are the non-inverting input terminal with respect to theinverting input terminal. Short-circuits can cause excessive heating. Destructive
dissipation can result from simultaneous short-circuits on all amplifiers. Rth are
typical values. The output may be shorted to ground or to either supply.
Temperature and/or supply voltages must be limited to ensure that the dissipation
rating is not exceeded. Human body model: 100pF discharged through a 1.5k
resistor between two pins of the device, done for all couples of pin combinations
with other pins floating. Machine model: a 200pF cap is charged to the specified
voltage, then discharged directly between two pins of the device with no external
series resistor (internal resistor < 5 ), done for all couples of pin combinations
with other pins floating. Charged device model: all pins plus package are charged
together to the specified voltage and then discharged directly to the ground. The
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input bias currents are junction leakage currents which approximately double for
every 10C increase in the junction temperature.
2.3 NE555 Timer IC
2.3.1Overview
The 555 Timer IC is an integrated circuit (chip) implementing a variety
oftimerand multivibratorapplications. The IC was designed by Hans R.
Camenzind in1970 and brought to market in 1971 by Signetics (later acquired
by Philips). The original name was the SE555 (metal can)/NE555 (plastic DIP) and
the part was described as "The IC Time Machine". It has been claimed that the 555
gets its name from the three 5 k resistors used in typical early implementations,
[2] but Hans Camenzind has stated that the number was arbitrary. The part is still in
wide use, thanks to its ease of use, low price and good stability. As of 2003, it is
estimated that 1 billion units are manufactured every year.
Depending on the manufacturer, the standard 555 package includes over
20 transistors, 2 diodes and 15 resistors on a silicon chip installed in an 8-pin mini
dual-in-line package (DIP-8). Variants available include the 556 (a 14-pin DIP
combining two 555s on one chip), and the 558 (a 16-pin DIP combining four
slightly modified 555s with DIS & THR connected internally, and TR falling edge
sensitive instead of level sensitive).
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Ultra-low power versions of the 555 are also available, such as the 7555 and
TLC555. The 7555 requires slightly different wiring using fewer external
components and less power.
The 555 has three operating modes:
Monostable mode: in this mode, the 555 functions as a "one-shot".
Applications include timers, missing pulse detection, bounce free switches,
touch switches, frequency divider, capacitance measurement, pulse-width
modulation (PWM) etc
Astable - free running mode: the 555 can operate as an oscillator. Uses
include LED and lamp flashers, pulse generation, logic clocks, tone generation,
security alarms,pulse position modulation, etc.
Bistable mode orSchmitt trigger: the 555 can operate as a flip-flop, if the
DIS pin is not connected and no capacitor is used. Uses include bounce free
latched switches, etc.
2.3.2 Pin Outs & Descriptions
The 555 integrated circuit is a highly accurate timing circuit that is capable of
producing either time delays or oscillation.
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Fig 2.2 Pin out diagram of NE555 Timer IC
V+ is the supply voltage. GND is also Ground (0V) connection for supply
voltage. Threshold is an active high input pin that is used to monitor the charging
of the timing capacitor. Control Voltage is used to adjust the threshold voltage if
required. This should be left disconnected if the function is not required. A 0.01uF
capacitor to Gnd can be used in electrically noisy circuits. The Trigger is also an
active low trigger input that starts the timer. Discharge is the output pin that is used
to discharge the timing capacitor. Out is known as the Timer output pin. Reset is
also an active low reset pin. Normally connected to V+ if the reset function is not
required.
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Fig 2.3 NE555 Timer IC block diagram
2.3.3 Monostable Operation
The circuit diagram illustrates the monostable configuration of the NE555 Timer
IC.
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Fig 2.4 Monostable configuration of Timer IC NE555
In monostable mode the device produces a 'one shot' pulsed output. The pulse is
started by a taking the trigger input from a high (V+) to a low voltage. Once
triggered the circuit remains in this state even if triggered again during the pulse
interval.
The pulse high time is given by: t = 1.1 x R1 x C1
The high to low voltage transition on the trigger input causes the Flip-Flop to
become set. This releases the short circuit (created by holding of the discharge pin
low) across capacitor C1. At this point the output goes high. Capacitor C1 then
begins to charge and the voltage across it begins to increase. When it reaches 2/3
V+ the Flip-Flop is reset. This causes capacitor C1 to discharge very quickly and
the output goes low.
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Minimum output pulse = 5 S
Maximum output pulse = 5 minutes
R1 minimum resistance = 1K ohm
R1 maximum resistance = 1Mohm
2.3.4 Astable Operation
The circuit diagram illustrates the astable configuration of the NE555 Timer IC.
Fig 2.5 Astable configuration of Timer IC NE555
In astable mode the timer continually triggers itself and runs as a multi vibrator.
This results in a continually repeating signal being generated on the output pin.
The external capacitor C1 charges through both R1 and R2 but discharges only
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through R2. Therefore the duty cycle is determined by the ratio of this resistor. If
the value of the two resistors is the same the duty cycle will be 50% and a square
wave will be output.
The 'High' output time is given by: t1 = 0.693 (R1 + R2) x C1
The 'Low' output time is given by: t2 = 0.693 (R2) x C1
Therefore the total period is given by: T = t1 + t2 = 0.693 (R1 + R2) x C1
The frequency of oscillation is given by: f = 1 / T = 1.44 / ((R1 + R2) x C1)
2.3.5 Example Applications
Joystick interface circuit using quad timer 558
The original IBM personal computer used a quad timer 558 in monostable (or
"one-shot") mode to interface up to two joysticks to the host computer. In the
joystick interface circuit of the IBM PC, the capacitor(C) of the RC network (see
Monostable Mode above) was generally a 10nF capacitor. The resistor(R) of the
RC network consisted of thepotentiometerinside the joystick along with an
external resistor of 2.2 kilohms. The joystick potentiometer acted as a variable
resistor. By moving the joystick, the resistance of the joystick increased from a
small value up to about 100 kilohms. The joystick operated at 5 V.
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Software running in the host computer started the process of determining the
joystick position by writing to a special address (ISA bus I/O address 201h). This
would result in a trigger signal to the quad timer, which would cause the capacitor
(C) of the RC network to begin charging and cause the quad timer to output a
pulse. The width of the pulse was determined by how long it took the C to charge
up to 2/3 of 5 V (or about 3.33 V), which was in turn determined by the joystick
position.
Software running in the host computer measured the pulse width to determine the
joystick position. A wide pulse represented the full-right joystick position, for
example, while a narrow pulse represented the full-left joystick position.
Atari Punk Console
One ofForrest M. Mims III's many books was dedicated to the 555 timer. In it, hefirst published the "Stepped Tone Generator" circuit which has been adopted as a
popular circuit, known as the Atari Punk Console, by circuit benders for its
distinctive low-fi sound similar to classic Atari games. The 555 can be used to
generate a variable PWM signal using a few external components. The chip alone
can drive small external loads or an amplifying transistor for larger loads.
CHAPTER THREE
PIEZOELECTRICITY
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3.1 Introduction
In this chapter 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.
3.2 History of Piezoelectricity
3.2.1definition 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 topyroelectricity, 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
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which are piezoelectric, some of which can even be found in human bone. Certain
ceramics andpolymers 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 clockuses 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.
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3.2.2 History
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 jewelers saw. In the scientific circles of the day, this
effect was considered quite a "discovery," and was quickly dubbed as
"piezoelectricity" in order to distinguish it from other areas of scientific
phenomenological experience such as "contact electricity" (friction generated static
electricity) and "pyroelectricity" (electricity generated from crystals by heating).
The Curie brothers asserted, however, that there was a one-to-one correspondence
between the electrical effects of temperature change and mechanical stress in a
given crystal, and that they had used this correspondence not only to pick the
crystals for the experiment, but also to determine the cuts of those crystals. To
them, their demonstration was a confirmation of predictions which followed
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naturally from their understanding of the microscopic crystallographic origins of
pyroelectricity.
The Curie brothers did not, however, predict that crystals exhibiting the direct
piezoelectric effect (electricity from applied stress) would also exhibit the converse
piezoelectric effect (stress in response to applied electric field). This property was
mathematically deduced from fundamental thermodynamic principles by
Lippmann in 1881. The Curies immediately confirmed the existence of the
"converse effect," and continued on to obtain quantitative proof of the complete
reversibility of electro-elasto-mechanical deformations in piezoelectric crystals.
1882 1917 At this point in time, after only two years of interactive work within
the European scientific community, the core of piezoelectric applications science
was established: the identification of piezoelectric crystals on the basis of
asymmetric crystal structure, the reversible exchange of electrical and mechanical
energy, and the usefulness of thermodynamics in quantifying complex
relationships among mechanical, thermal and electrical variables.
In the following 25 years (leading up to 1910), much more work was done to make
this core grow into a versatile and complete framework which defined completely
the 20 natural crystal classes in which piezoelectric effects occur, and defined all
18 possible macroscopic piezoelectric coefficients accompanying a rigorous
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thermodynamic treatment of crystal solids using appropriate tensorial analysis. In
1910 Voigt's "Lerbuch der Kristallphysik" was published, and it became the
standard reference work embodying the understanding which had been reached.
During the 25 years that it took to reach Voigt's benchmark, however, the world
was not holding its breath for piezoelectricity. A science of such subtlety as to
require tensorial analysis just to define relevant measurable quantities paled by
comparison to electro-magnetism, which at the time was maturing from a science
to a technology, producing highly visible and amazing machines. Piezoelectricity
was obscure even among crystallographers; the mathematics required to
understand it was complicated; and no publicly visible applications had been found
for any of the piezoelectric crystals. The first serious applications work on
piezoelectric devices took place during World War I. In 1917, P. Langevin and
French co-workers began to perfect an ultrasonic submarine detector. Their
transducer was a mosaic of thin quartz crystals glued between two steel plates (the
composite having a resonant frequency of about 50 KHz), mounted in a housing
suitable for submersion. Working on past the end of the war, they did achieve their
goal of emitting a high frequency "chirp" underwater and measuring depth by
timing the return echo. The strategic importance of their achievement was not
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overlooked by any industrial nation, however, and since that time the development
of sonar transducers, circuits, systems, and materials has never ceased.
3.3 Applications
Piezoelectric sensors have proven to be versatile tools for the measurement of
various processes. They are used forquality assurance,process control and for
research and development in many different industries. Although the piezoelectric
effect was discovered by Curie in 1880, it was only in the 1950s that the
piezoelectric effect started to be used for industrial sensing applications. Since
then, this measuring principle has been increasingly used and can be regarded as a
mature technology with an outstanding inherent reliability. It has been successfully
used in various applications, such as in medical, aerospace,
nuclearinstrumentation, 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.
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/m. Even though
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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 ortourmaline) have an extreme stability even at high temperature,
enabling sensors to have a working range of up to 1000C. Tourmaline
showspyroelectricity 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 topiezoceramic 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 sensorresistance will
result in a constant loss ofelectrons, 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 300C, special types
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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 500C.
Piezoelectric sensors are also seen in nature. Drybone is piezoelectric, and is
thought by some to act as a biological force sensor.
3.4 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, (eqn. 3.1)
where a is the dimension in line with the neutral axis, b is in line with the charge
generating axis and dis the corresponding piezoelectric coefficient.
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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 elementsthat are mechanically in series and electrically inparallel is the only way to
increase the charge output. The resulting charge is
Cx = dxxFxn, (eqn. 3.2)
where dxx is the piezoelectric coefficient for a charge in x-direction released by
forces applied along x-direction (inpC/N).Fx is the applied Force in x-direction
[N] 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 elements size and shape. Forn elements mechanically in series
and electrically in parallel the charge is
Cx = 2dxxFxn. (eqn. 3.3)
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.
3.5 Electrical Properties
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A piezoelectric transducer has very high DC output impedance and can be modeled
as a proportional voltage source and filter network. The voltage Vat 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.
Fig 3.1 Frequency response of a piezoelectric sensor; output voltage vs applied
force
A detailed model includes the effects of the sensor's mechanical construction and
other non-idealities.[3] The inductanceLm is due to the seismic mass and inertia of
the sensor itself. Ce is inversely proportional to the mechanical elasticity of the
sensor. C0 represents the static capacitance of the transducer, resulting from an
inertial mass of infinite size. Ri is the insulation leakage resistance of the
transducer element. If the sensor is connected to a load resistance, this also acts in
parallel with the insulation resistance, both increasing the high-pass cutoff
frequency.
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Fig 3.2 Equivalent circuit of sensor
For use as a sensor, the flat region of the frequency response plot is typically used,
between the high-pass cutoff and the resonant peak. The load and leakage
resistance need to be large enough that low frequencies of interest are not lost.
Fig 3.3 Schematic symbol and electronic model of a piezoelectric sensor
A simplified equivalent circuit model can be used in this region, in
which Cs represents the capacitance of the sensor surface itself, determined by the
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standard formula for capacitance of parallel plates. It can also be modeled as a
charge source in parallel with the source capacitance, with the charge directly
proportional to the applied force, as above.
3.6 Proposed Circuit Diagram
The circuit diagram below illustrates or shows the proposed circuit diagram for
implementation of the seismic sensor project.
Fig 3.4 Proposed project circuit diagram
CHAPTER FOUR
DESIGN & IMPLEMENTATION OF SEISMIC SENSOR
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4.1 Introduction
This chapter will concentrate on the general architecture and design, circuit
description and operation, come design calculations, the operational flow chart and
the data sheet for the design and implementation of the seismic sensor.
4.2 General Architecture of the Seismic Sensor
The diagram below shows the general architecture of the proposed circuit for the
project.
Fig 4.1 General architecture of seismic sensor using a piezo element
4.3 Circuit Diagram & Description
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XLV1
Input
PIEZO
BUZZER/SPEAKER
AMPLIFIER
CIRCUIT/UNIT
PIEZO
ELEMEN
T
TIMER
CIRCUIT/UNIT
PIEZO
ELEMEN
T
AMPLIFIER
CIRCUIT/UNIT
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The below diagram shows the circuitry for the seismic sensor with its description
below.
Fig 4.2 Circuit diagram for the seismic sensor
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. In the circuit, IC
TLO71 (IC1) is wired as a differential amplifier with both its inverting and non-
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inverting inputs tied to the negative rail through a resistive network comprising R1,
R2 and R3. Under idle conditions (as adjusted by VR1), both the inputs receive
almost equal voltages, which keeps the output low.
TLO71 is a low-noise JFET input op-amp with low input bias and offset current.
The BIFET technology provides fast slew rates. Capacitor C1 is provided in the
circuit to keep the differential input of IC1 for better performance.
4.4 Circuit Operation
When the piezo element is disturbed (by even a slight movement), it discharges the
stored charge. This alters the voltage level at the inputs of IC1 and the output
momentarily swings high as indicated by green LED1. This high output is used to
trigger switching transistor T1, which triggers monostable IC2. The timing period
of IC2 is determined by R7 and C5. With the shown values, it will be around two
minutes. The high output from IC2 activates T2 and the buzzer starts beeping
along with red light indication from LED2.
4.4.1 Design Calculation
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The below calculation is the basic design calculations for the transistors T1 and T2
as well as the timing period for the circuit to produce its beeping sound along with
the red LED.
Biasing Voltage = 1.7v (Theoretical Value)
Vcc = VBE + IRRL
T1; IB = 1.7/R4
= 1.7/330
= 0.0052 A,
T2; IB = 1.7/R8
= 1.7/1 *103
= 0.0017 A,
The timing period of IC2 is determined by R7 and C5.
T = 1.11 *R5 * C7
= 1.11 * 1 x 106
* 100 x10-6
= 111 seconds.
4.5 Flow Chart
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The chart below shows the flow control or processes of operation of the seismic
sensor.
Fig 4.3 Flow chart for seismic sensor using piezo element
4.6 Data Sheet & Cost Analysis
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DISTURBANC
E OF PIEZO
ELEMENT
IC1
VOLTAGE
LEVEL IS
ALTERED
SWITCHING
TRANSISTOR
T2 IS
TRIGGERED
MONOSTABLE
TO IC2 trigger
HIGH OUTPUT
OF IC2
ACTIVATES T2
BUZZER
STARTS TO
BEEP
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Table 4.0 Component list and cost
No.
Name Of Component Specification Quantity
Cost
Gh
1 Resistors R1R2R3R4R5R6R7R8R9
R10VR1
100k10k
100k330k1k
470k1M1k
470k
10k1M
111111111
112 Capacitors C1
C2C3C4C5
C6
10F,25V0.1F
100F,25V0.01F
100F,25V10F,25V
111111
3 Transistor T1T2
npn BC548npn BC548
11
4 Light Emitting Diode LED1LED2
GreenRed
11
5 IC1 TL071 low noiseJFET op-amp
1
6 IC2 NE555 Timer 17 PZ1 Piezo Buzzer 18 PIEZO ELEMENT 19 SWITCH ON/OF 1
CHAPTER FIVE
CONCLUSSION & FUTURE WORKS
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In conclusion, the disturbance made by any moving object using the piezo element
seismic sensor implemented. The disturbance discharges the stored charge. This
caused the IC1to produce a high output. This high output is used to trigger
switching transistor IC2 and the vibration or sound or movement made is caused
the buzzer to beep.
The sound and vibration caused movements can also be detected by new and
growing technology. This project has given any researcher or student to do any
future work on the above project.
REFERENCES
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[2] Scherz, Paul (2000) "Practical Electronics for Inventors," p. 589. McGraw-Hill/TAB Electronics. ISBN: 978-0070580787. Retrieved 2010-04-05.
[3] Ward, Jack (2004). The 555 Timer IC - An Interview with Hans
Camenzind. The Semiconductor Museum. Retrieved 2010-04-05.
[4] van Roon, Fig 3 & related text.
[5] Jung, Walter G. (1983) "IC Timer Cookbook, Second Edition," pp. 4041.Sams Technical Publishing; 2nd ed. ISBN: 978-0672219320. Retrieved2010-04-05.
[6] van Roon, Chapter "Monostable Mode."
[7] van Roon Chapter: "Astable operation."
[8] Engdahl, pg 1.
[9] Engdahl, "Circuit diagram of PC joystick interface"
[10] Engdahl, "Joystick construction".
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[12] Eggebrecht, p. 197.
[13] Eggebrecht, pp. 197-9
[14] Piezocryst website. Retrieved 2006-06-02.
[15] "Interfacing Piezo Film to Electronics" (PDF).Measurement Specialties.March 2006. Retrieved 2007-12-02.
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[18] Ludlow, Chris (May 2008). "Energy Harvesting with PiezoelectricSensors" (PDF). Mide Technology. Retrieved 2008-05-21.
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[20] J. B Gupta, Electrical Technology, 12th Edition, S. K Kataria and Sons,
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[21] D. G. Fink and H. W. Beaty, Standard Handbook for Electrical Engineers,13thEdition, McGraw Hill, Singapore, 1993.
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