Seismic instruments

Seismic instruments used for registration of ground motion caused by earthquakes are essential for studying seismology.

How does a seismometer function

The seismometer together with the unit recording the signal is called a seismograph. The seismometer senses the ground vibration and converts this to a signal that can be recorded. Modern seismographs can measure movements smaller than one nm (one millionth of a millimetre)

How it works: When the ground is moving rapidly, the spring ("Fjær") suspended mass ("Masse") will keep quiet due to inertia and we will get a measurement on the scale ("Måleskala") to the right. This is the principle of the mechanical seismograph. The seismograph in this figure measures the vertical ground motion. In newer seismometers there are electrical coils around the mass, which is magnetic, so that an electrical signal is generated when the mass moves.

Horizontal motion

Earthquakes generate both vertical and horizontal motions. In order to measure a horizontal movement, we need a mass which can swing in the horizontal plane.

A simple horizontal pendulum. When the ground moves to the right, the mass will swing to the left and the ground motion will be recorded on the paper, which moves down. The mass can swing in all directions and must be suspended on a very long string in order to be able to record low frequencies. In order to avoid this, a ‘garden gate’ or inverted pendulum is used (see next figures).

Inverted pendulum. The mass can swing in all horizontal directions. This is the principle of the Wiechert seismograph used in Bergen from 1921 to 1968, see figure below.

The Wiechert seismograph. The mass horizontal motion is captured by two arms that, by using a system of levers, can amplify the motion and record it on two rotating drums (R). The seismograph can record horizontal motions in East-West and North-South directions at the same time.

"Garden gate" pendulum. The mass only swings horizontally in one direction. It hangs at an inclined angle in order to make it swing more slowly (like a door hanging at an angle). This principle is used in the Bosch seismograph.

Moderen seismic sensors

In new sensors, the mass barely moves. The mass is suspended by a spring ("Fjær"). Departure from the mass center position is measured with a distance measurement device ("Avstandsmåler"). As soon as the mass tries to move, the distance measurement device will send a current through the coil ("Spole"), which will oppose the motion so that the mass remains stationary. The larger the force on the mass, the larger the current. The size of the current will therefore be a measure of the ground motion (more correctly the ground acceleration). Such instruments can be built compact and very sensitive and are called accelerometers. They are also widely used for other purposes, such as releasing

Accelerometer on an electronic chip. The distance measurement device is a capacitor, the spring is a torsion bar and the mass is the upper capacitor plate. The chip has a dimension of 2x2 mm.

Frequencies measured

Seismometers measure signals with frequencies between 0.001 Hz and 100 Hz It is relatively simple to construct seismometers that measure the higher frequencies higher

than 0.1 Hz (short period seismometer) Seismometers that measure the low frequencies (less than 0.1 Hz) are more difficult to make (long period seismometers) Modern (and expensive) seismometers measure both low and high frequencies (broad band seismometers). Technically they are based on the principle of the accelerometer.

Recording of seismic signals

The sensor generates a signal. This was recorded mechanically on older seismographs. All modern sensors give out an electrical signal that can be recorded in several different ways. A recording on paper, usually lasting 24 hours, is called a seismogram.

Optical recording. The electrical signal is sent to a galvanometer with a mirror. A light beam is reflected from the mirror and recorded on a rotating drum with light-sensitive paper. This system was used from early 1900 up to a few years ago.

Pen recording. Instead of a ‘lightbeam’ pen, an electrical pen, also recording on paper on a rotating drum, is used. The pen can record with ink, scratch on smoked paper, or develop heat and write on heat-sensitive paper.

Digital recording. Today, nearly all seismographs record digitally. The electrical signal is transformed to a digital signal that can be recorded by a computer.

Optical recording. The electrical signal is sent to a galvanometer with a mirror (H). A light beam (L) is reflected from the mirror (H), and recorded on a drum with optical paper (R). In front of the drum, there is an optical lens focusing the beam.

Pen recording of seismic signals. The signal from the seismometer is amplified ("Forsterker") and sent to a drum recorder ("Trommel til registrering"). The time signal from a GPS (Global Positioning System) receiver is used to generate minute and hour pulses recorded together with the signal. Before time signals were available by radio, time pulses were generated by a mechanical clock.

Analog to digital converter. The electrical input signal is continuous: In other words, we know the size (amplitude) of the signal at any time. The analog to digital converter (AD) measures the amplitude at regular time intervals (?t) and gives out the numerical values for the amplitude as a sequence of numbers. These are then read by the computer. For seismic signals, the amplitudes are usually read 100 times per second. For digital music recorded on CD, we have ca 44 000 values per second.

Digitial seismic station. The signal is sent to the analog to digital converter ("Analog til digital omformer"), which converts the signal to a digital signal. It is then transferred to a computer ("Datamaskin"), where it is recorded and stored ("Datalagring"). Via Internet, the signal is sent from there to a central data center. "GPS tidsmarkering" means making a time stamping by GPS, and "Strømforsyning", power supply.

Accelerometer Principles

Overview

This tutorial is part of the National Instruments Measurement Fundamentals series. Each tutorial in this series will teach you a specific topic of common measurement applications by explaining theoretical concepts and providing practical examples. There are several physical processes that can be used to develop a sensor to measure acceleration. In applications that involve flight, such as aircraft and satellites, accelerometers are based on properties of rotating masses. In the industrial world, however, the most common design is based on a combination of Newton's law of mass acceleration and Hooke's law of spring action.

Table of Contents

1. Spring-Mass System 2. Natural Frequency and Damping 3. Vibration Effects 4. Relevant NI Products 5. Buy the Book

Spring-Mass System

Newton's law simply states that if a mass, m, is undergoing an acceleration, a, then there must be a force F acting on the mass and given by F = ma. Hooke's law states that if a spring of spring constant k is stretched (extended) from its equilibrium position for a distance Dx, then there must be a force acting on the spring given by F = kDx.

FIGURE 5.23 The basic spring-mass system accelerometer.

In Figure 5.23a we have a mass that is free to slide on a base. The mass is connected to the base by a spring that is in its unextended state and exerts no force on the mass. In Figure 5.23b, the whole assembly is accelerated to the left, as shown. Now the spring extends in order to provide the force necessary to accelerate the mass. This condition is described by equating Newton's and Hooke's laws:

ma = kDx (5.25)

where k = spring constant in N/mDx = spring extension in m m = mass in kg a = acceleration in m/s2

Equation (5.25) allows the measurement of acceleration to be reduced to a measurement of spring extension (linear displacement) because

If the acceleration is reversed, the same physical argument would apply, except that the spring is compressed instead of extended. Equation (5.26) still describes the relationship between spring displacement and acceleration.

The spring-mass principle applies to many common accelerometer designs. The mass that converts the acceleration to spring displacement is referred to as the test mass or seismic mass. We see, then, that acceleration measurement reduces to linear displacement measurement; most designs differ in how this displacement measurement is made.

Natural Frequency and Damping

On closer examination of the simple principle just described, we find another characteristic of spring-mass systems that complicates the analysis. In particular, a system consisting of a spring and attached mass always exhibits oscillations at some characteristic natural frequency. Experience tells us that if we pull a mass back and then release it (in the absence of acceleration), it will be pulled back by the spring, overshoot the equilibrium, and oscillate back and forth. Only friction associated with the mass and

base eventually brings the mass to rest. Any displacement measuring system will respond to this oscillation as if an actual acceleration occurs. This natural frequency is given by

where fN = natural frequency in Hz k = spring constant in N/m m = seismic mass in kg

The friction that eventually brings the mass to rest is defined by a damping coefficient , which has the units of s-1. In general, the effect of oscillation is called transient response, described by a periodic damped signal, as shown in Figure 5.24, whose equation is

XT(t) = Xoe-µt sin(2pfNt) (5.28)

where Xr(t) = transient mass position Xo = peak position, initially µ = damping coefficient fN = natural frequency

The parameters, natural frequency, and damping coefficient in Equation (5.28) have a profound effect on the application of accelerometers.

Vibration Effects

The effect of natural frequency and damping on the behavior of spring-mass accelerometers is best described in terms of an applied vibration. If the spring-mass system is exposed to a vibration, then the resultant acceleration of the base is given by Equation (5.23)

a(t) = -w2xo sin wt

If this is used in Equation (5.25), we can show that the mass motion is given by

where all terms were previously denned and w = 2pf, with/the applied frequency.

FIGURE 5.24 A spring-mass system exhibits a natural oscillation with damping as response to an impulse input.

FIGURE 5.25 A spring-mass accelerometer has been attached to a table which is exhibiting vibration. The table peak motion is xo and the mass motion is Dx.

To make the predictions of Equation (5.29) clear, consider the situation presented in Figure 5.25. Our model spring-mass accelerometer has been fixed to a table that is vibrating. The xo in Equation (5.29) is the peak amplitude of the table vibration, and Dx is the vibration of the seismic mass within the accelerometer. Thus, Equation (5.29) predicts that the seismic-mass vibration peak amplitude varies as the vibration frequency squared, but linearly with the table-vibration amplitude. However, this result was obtained without consideration of the spring-mass system natural vibration. When this is taken into account, something quite different occurs.Figure 5.26a shows the actual seismic-mass vibration peak amplitude versus table-vibration frequency compared with the simple frequency squared prediction.You can see that there is a resonance effect when the table frequency equals the natural frequency of the accelerometer, that is, the value of Dx goes through a peak. The amplitude of the resonant peak is determined by the amount of damping. The seismic-mass vibration is described by Equation (5.29) only up to about fN/2.5.Figure 5.26b shows two effects. The first is that the actual seismic-mass motion is limited by the physical size of the accelerometer. It will hit "stops" built into the assembly that limit its motion during resonance. The figure also shows that for frequencies well above the natural frequency, the motion of the mass is proportional to the table peak motion, xo, but not to the frequency. Thus, it has become a displacement sensor. To summarize:

1. f < fN - For an applied frequency less than the natural frequency, the natural frequency has little effect on the basic spring-mass response given by Equations (5.25) and (5.29). A rule of thumb states that a safe maximum applied frequency is f < 1/2.5fN.

2. f > fN - For an applied frequency much larger than the natural frequency, the accelerometer output is independent of the applied frequency. As shown in Figure 5.26b, the accelerometer becomes a measure of vibration displacement xo of Equation (5.20) under these circumstances. It is interesting to note that the seismic mass is stationary in space in this case, and the housing, which is driven by the vibration, moves about the mass. A general rule sets f > 2.5 fN for this case.

Generally, accelerometers are not used near the resonance at their natural frequency because of high nonlinearities in output.

FIGURE 5.26 In (a) the actual response of a spring-mass system to vibration is compared to the simple w2 prediction In (b) the effect of various table peak motion is

shown

EXAMPLE 5.14An accelerometer has a seismic mass of 0.05 kg and a spring constant of 3.0 X 103 N/m Maximum mass displacement is ±0 02 m (before the mass hits the stops). Calculate (a) the maximum measurable acceleration in g, and (b) the natural frequency.

SolutionWe find the maximum acceleration when the maximum displacement occurs, from Equation (5.26).a.

or because

b. The natural frequency is given by Equation (5.27).

Seismometers (in Greek seismos = earthquake and metero = measure) are used by seismologists to measure and record the size and force of seismic waves. By studying seismic waves, geologists can map the interior of the Earth, and measure and locate earthquakes and other ground motions. Seismograph is often interchangeably with seismometer

John Milne invented the horizontal pendulum seismograph at the Imperial College of Engineering in Japan in 1880. This marked the beginning of modern seismology

Basic principles

Seismometers have:

The foundation is critical, and often the most expensive part of a seismic station An inertial mass , using springs or gravity to establish a steady-state reference

position A damper system to prevent long term oscillations in response to an event A means of recording the motion or force of the mass relative to the frame

Passing seismic waves move the frame, while the mass tends to stay in a fixed position due to its inertia. The seismometer measures the relative motion between the frame and the suspended mass

Early seismometers used optics, or motion-amplifying mechanical linkages. The motion was recorded as scratches on smoked glass, or exposures of light beams on photographic paper

Modern instruments use electronics. Usually, the proof mass is held motionless by an electronic negative feedback loop that drives a coil. The distance moved, speed and acceleration of the mass are directly measured. The measurements are often digitized and stored using a computer, and then are sometimes automatically interpreted by computer programs to locate earthquakes

A cruder system is used for geologic surveys: The geophones in surveys just have a heavy magnet suspended in a coil. When the ground shakes, the frame and coil move, while the heavy magnet stays. The magnet's field therefore cuts the coil and induces a measurable electric current in the coil

Professional seismic observatories usually have instruments measuring three axes, north-south, east-west, and up-down. Seismologists generally prefer a vertical seismograph if only one instrument is available

A professional station is sometimes mounted on bedrock with an uncracked connection to a continental plate. The best mountings may be in deep boreholes, which avoid thermal effects, ground noise and tilting from weather and tides. Amateur, or less exotic instruments are often mounted in insulated enclosures on small buried piers of unreinforced concrete. Reinforcing rods and aggregates would distort the pier as the temperature changes. A site should always be surveyed for ground noise with a temporary installation before pouring the pier and laying conduit

Zhang Heng's Seismometer

In 132, Zhang Heng of China's Han dynasty invented the first seismometer, called Houfeng Didong Yi (lit. instrument for measuring the seasonal winds and the movements of the Earth). By use of a mechanical chain reaction caused by the earth's heavy vibration during an earthquake, a pendulum mechanism within the copper-framed, urn-shaped seismometer would sway and activate a series of levers. This in turn would ultimately drop a spherical brass ball from an artificial dragon-mouth of the urn's top into an artificial toad-mouth below, signifying the cardinal direction of the earthquake. Use of this device was recorded in the historical text of the Book of Later Han

An early example

The principle can be shown by an early special purpose seismometer. This consisted of a large stationary pendulum, with a stylus on the bottom. As the earth starts to move, the heavy mass of the pendulum has the inertia to stay still in the non-earth frame of reference. The result is that the stylus scratches a pattern corresponding with the earth's movement. This type of strong motion seismometer recorded upon a smoked glass (glass with carbon soot). While not sensitive enough to detect distant earthquakes, this instrument could indicate the direction of the initial pressure waves and thus help find the epicenter of a local earthquake — such instruments were useful in the analysis of the 1906 San Francisco earthquake. Further re-analysis was performed in the 1980s using these early recordings, enabling a more precise determination of the initial fault break location in Marin county and its subsequent progression, mostly to the south

Early designs

After 1880, most seismometers were descended from those developed by the team of John Milne, James Alfred Ewing and Thomas Gray, who worked together in Japan from 1880-1895. These seismometers used damped horizontal pendulums. Later, after World War II, these were adapted into the widely used Press-Ewing seismometer

Later, professional suites of instruments for the world-wide standard seismographic network had one set of instruments tuned to oscillate at fifteen seconds, and the other at ninety seconds, each set measuring in three directions. Amateurs or observatories with limited means tuned their smaller, less sensitive instruments to ten seconds

The basic damped horizontal pendulum seismometer swings like the gate of a fence. A heavy weight is mounted on the point of a long (from 10 cm to several meters) triangle, hinged at its vertical edge. As the ground moves, the weight stays unmoving, swinging the "gate" on the hinge

The advantage of a horizontal pendulum is that it achieves very low frequencies of oscillation in a compact instrument. The "gate" is slightly tilted, so the weight tends to slowly return to a central position. The pendulum is adjusted (before the damping is installed) to oscillate once per three seconds, or once per thirty seconds. The general-purpose instruments of small stations or amateurs usually oscillate once per ten seconds. A pan of oil is placed under the arm, and a small sheet of metal mounted on the underside of the arm drags in the oil to damp oscillations. The level of oil, position on the arm, and angle and size of sheet is adjusted until the damping is "critical," that is, almost having oscillation. The hinge is very low friction, often torsion wires, so the only friction is the internal friction of the wire. Small seismographs with low proof masses are placed in a vacuum to reduce disturbances from air currents

Zollner described torsionally-suspended horizontal pendulums as early as 1869, but developed them for gravimetry rather than seismometry

Early seismometers had an arrangement of levers on jeweled bearings, to scratch smoked glass or paper. Later, mirrors reflected a light beam to a direct-recording plate or roll of photographic paper. Briefly, some designs returned to mechanical movements to save money. In mid-twentieth-century systems, the light was reflected to a pair of differential electronic photosensors. The recording device in most such machines was paper on a slowly-turning drum

Modern instruments

Modern instruments use electronic sensors, amplifiers, and recording instruments. Most are broadband, operating on a wide range of frequencies

defining difference between modern and obsolescent seimometers is that modern ones utilize so-called zero-length springs that violate Hooke's law to get very long resonant periods with relatively small physical instruments

A zero length spring is actually a spring that has a constant force for some part of its stretch. So a pendulum driven by the spring can have (theoretically) an infinitely long period. It is thus able to sense very long-period seismic waves that easily penetrate great distances through the earth

Practical zero-length springs are odd mechanical systems that require special compliant mountings, and have limited physical ranges. Also, the force they generate is never exactly constant, but it can be a good approximation

As a result, some commercially-available research seismometers receive frequencies from 30 Hz (0.03 seconds per cycle) to 1/850 Hz (850 seconds per cycle

Seismometers unavoidably introduce some distortion into the signals they measure, but professionally-designed systems have carefully-characterized frequency transforms

Modern sensitivities come in three broad ranges: geophones, 50 to 750 V/m; local geologic seismographs, about 1,500 V/m; and teleseismographs, used for world survey, about 20,000 V/m. Instruments come in three main varieties: short period, long period and broad-band. The short and long period measure velocity and are very sensitive, however they 'clip' or go off-scale for ground motion that is strong enough to be felt by people. A 24-bit analog-to-digital conversion channel is commonplace. Practical devices are linear to roughly a part per million

Delivered seismogmeters come with two styles of output: analog and digital. Analog seismographs require analog recording equipment, possibly including an analog-to-digital converter. Digital seismographs simply plug in to computers. They present the data in standard digital forms (often "SE2" over ethernet

Teleseismometers

The modern broad-band seismograph can record a very broad range of frequencies. It consists of a small 'proof mass', confined by electrical forces, driven by sophisticated electronics. As the earth moves, the electronics attempt to hold the mass steady through a feedback circuit. The amount of force necessary to achieve this is then recorded

Most designs are proprietary, but in one publicly-available professional design[4], the electronics holds a mass motionless relative to the frame. Basically, the distance between the mass and some part of the frame is measured very precisely, by a linear variable differential transformer. Many instruments use a linear variable differential capacitor

That measurement is then amplified by electronic amplifiers attached to parts of an electronic negative feedback loop. One of the amplified currents from the negative feedback loop drives a coil very like a loudspeaker, except that the coil is attached to the mass, and the magnet is mounted on the frame

The result is that the mass stays nearly motionless

Most instruments directly measure the ground motion using the distance sensor

The voltage generated in a sense coil on the mass by the magnet directly measures the instantaneous velocity of the ground.

The current to the drive coil provides a sensitive, accurate measurement of the force between the mass and frame, thus directly measuring the ground's acceleration (using F=MA of basic physics

One of the continuing problems with sensitive vertical seismographs is the buoyancy of their masses. The uneven changes in pressure caused by wind blowing on an open window can easily change the density of air in a room enough to cause a vertical seismograph to show spurious signals. Therefore, most professional seismographs are sealed in rigid gas-tight enclosures. For example, this is why a common Streckheisen model has a thick glass base that must be glued to its pier without bubbles in the glue

It might seem logical to make the heavy magnet serve as a mass, but that subjects the seismograph to errors when the Earth's magnetic field moves. This is also why seismograph's moving parts are constructed from a material that minimally interacts with magnetic fields

A seismograph is also sensitive to changes in temperature, and many instruments are constructed from low expansion materials such as nonmagnetic invar

The hinges on a seismograph are usually patented, and by the time the patent has expired, the art has improved. The most successful public domain designs use thin foil hinges in a clamp

Another issue is that the transfer function of a seismograph must be accurately characterized, so that its frequency response is known. This is often the crucial difference between professional and amateur instruments. Most instruments are characterized on a variable frequency shaking table

Strong-motion seismometers

Another type of seismometer is a digital strong-motion seismometer, or accelerograph. This data is essential to understand how an earthquake affects human structures

A strong-motion seismometer measures acceleration. This can be mathematically integrated later to give velocity and position. Strong-motion seismometers are not as sensitive to ground motions as teleseismic instruments but they stay on scale during the strongest seismic shaking

Other forms

Accelerographs and geophones are often heavy cylindrical magnets with a spring-mounted coil inside. As case moves, the coil tends to stay stationary, so the magnetic field cuts the wires, inducing current in the output wires. They receive frequencies from several hundred hertz down to 4.5 Hz (cheap) to as low as 1 Hz (pretty expensive). Some have electronic damping, a low-budget way to get some of the performance of the closed-loop wide-band geologic seismographs

Strain-beam accelerometers constructed as integrated circuits are too insensitive for geologic seismographs (2002), but are widely used in geophones

Some other sensitive designs measure the current generated by the flow of a non-corrosive ionic fluid through an electret sponge or a conductive fluid through a magnetic field

Modern recording

Today, the most common recorder is a computer with an analog-to-digital converter, a disk drive and an internet connection. Many observatories now use computers. For amateurs, a PC with a sound card and software is adequate, and saves a lot of paper.

An algorithm often used to eliminate insignificant observations uses a short-term average and a long term average. When the short term average is statistically significant compared to the long term average, the event is worth recording.

Interconnected seismometers

Seismometers spaced in an array can also be used to precisely locate, in three dimensions, the source of an earthquake, using the time it takes for seismic waves to propagate away from the hypocenter, the initiating point of fault rupture (See also Earthquake location). Interconnected seismometers are also used to detect underground nuclear test explosions

In reflection seismology, an array of seismometers images sub-surface features. The data are reduced to images using algorithms similar to tomography. The data reduction methods resemble those of computer-aided tomographic medical imaging X-ray machines (CAT-scans), or imaging sonars

A world-wide array of seismometers can actually image the interior of the Earth in wave-speed and transmissivity. This type of system uses events such as earthquakes, impact events or nuclear explosions as wave sources. The first efforts at this method used manual data reduction from paper seismograph charts. Modern digital seismograph records are better adapted to direct computer use. With inexpensive seismometer designs and internet access, amateurs and small institutions have even formed a "public seimograph network

Seismographic systems used for petroleum or other mineral exploration historically used an explosive and a wireline of geophones unrolled behind a truck. Now most short-range

systems use "thumpers" that hit the ground, and some small commercial systems have such good digital signal processing that a few sledgehammer strikes provide enough signal for short-distance refractive surveys. Exotic cross or two-dimensional arrays of geophones are sometimes used to perform three-dimensional reflective imaging of subsurface features. Basic linear refractive geomapping software (once a black art) is available off-the-shelf, running on laptop computers, using strings as small as three geophones. Some systems now come in an 18" (0.5 m) plastic field case with a computer, display and printer in the cover

Small, inexpensive seismic imaging is now sufficiently inexpensive that it is used by civil engineers to survey foundation sites, locate bedrock, and find subsurface water.

Accelerograph: A seismograph whose output is proportional to ground acceleration (in

comparison to the usual seismograph whose output is proportional to ground velocity). Accelerographs are typically used as instruments designed to record very strong ground motion useful in engineering design; seismographs commonly record off scale in these circumstances. Normally, strong motion instruments do not record unless triggered by strong ground motion.

Seismogram: A graph showing the motion of the ground versus time. Seismograms are the records (paper copy) produced by seismographs used

to calculate the location and magnitude of an EQ. They show how the ground moves with the passage of time. On a seismogram, the HORIZONTAL axis = time (measured in seconds) and the VERTICAL axis= ground displacement (usually measured in millimeters). When there is NO EQ reading there is just a straight line except for small wiggles caused by local disturbance or "noise" and the time markers. --

Seismograph: A sensitive instrument that can detect, amplify, and record ground

vibrations too small to be perceived by human beings. Seismographs are instruments used to record the motion of the ground

during an EQ--installed in the ground throughout the world and operate as seismographic network. The first one was developed in 1890. The earliest "seismoscope" was invented by the Chinese philosopher Chang Heng in A.D. 132. This did not record earthquakes, however. It only indicated that there was one occurring. A seismograph is securely mounted onto the surface of the earth so that when the earth shakes, the entire unit shakes with it, EXCEPT for the mass on the spring which has inertia, and remains in the same place. As the seismograph shakes under (in the example below) the mass, the recording device on the mass records the realtive

motion between itself and the rest of the instrument, thus recording the ground motion. In reality, these mechanisms are no longer manual, but instead work by measuring electronic changes produced by the motion of the ground with respect to the mass.

Seismometer: A seismometer is the internal part of the seismograph, which may be a

pendulum or a mass mounted on a spring; however, it is often used synonymously with "seismograph". --

The cathode ray tube (CRT), invented by German physicist Karl Ferdinand Braun in 1879, is an evacuated glass envelope containing an electron gun (a source of electrons) and a fluorescent screen, usually with internal or external means to accelerate and deflect the electrons. When electrons strike the fluorescent screen, light is emitted

The electron beam is deflected and modulated in a way which causes it to display an image on the screen. The image may represent electrical waveforms (oscilloscope), pictures (television, computer monitor), echoes of aircraft detected by radar, etc

The single electron beam can be processed in such a way as to display moving pictures in natural colors

The generation of an image on a CRT by deflecting an electron beam requires the use of an evacuated glass envelope which is large, deep, heavy, and relatively fragile (this has earned it the nickname "Fishbowl"). The development of imaging technologies without these disadvantages has caused CRTs to be largely displaced by flat plasma screens, liquid crystal displays, DLP, OLED displays, and other technologies

An exception to the typical bowl-shaped CRT would be the flat CRTs[1][2] used by Sony in their Watchman series (the FD-210 was introduced in 1982). One of the last flat-CRT models was the FD-10A (last produced in 1989 as Sony moved to LCD displays in 1990). The CRT in these units was flat with the electron gun located roughly at right angles below the display surface thus requiring sophisticated electronics to create an undistorted picture free from keystoning and the like

A transducer is a device, usually electrical, electronic, electro-mechanical, electromagnetic, photonic, or photovoltaic that converts one type of energy to another for various purposes including measurement or information transfer (for example, pressure sensors). In a broader sense (for example in the Viable System Model) a transducer is sometimes defined as any device that converts a signal from one form to another. A very common device is an audio speaker, which converts electrical voltage variations representing music or speech, to mechanical cone vibration. The speaker cone in turn vibrates air molecules creating acoustical energy

Types of transducers

This list is confined to the narrower definition of the term.

Electromagnetic:

Antenna - converts electromagnetic waves into electric current and vice versa.

Cathode ray tube (CRT) - converts electrical signals into visual form

Fluorescent lamp, light bulb - converts electrical power into visible light

Magnetic cartridge - converts motion into electrical form

Photocell or light-dependent resistor (LDR) - converts changes in light levels into resistance changes

Tape head - converts changing magnetic fields into electrical form

Hall effect sensor - converts a magnetic field level into electrical form

Electrochemical:

pH probe ,Electro-galvanic fuel cell ,Electromechanical (electromechanical output devices are generically called actuators): ,Electroactive polymers ,Galvanometer, MEMS ,Rotary motor, linear motor ,Vibration powered generator ,Potentiometer when used for measuring position ,Load cell converts force to mV/V electrical signal using strain gauges ,Accelerometer ,Strain gauge ,Switch ,String Potentiometer ,Air flow sensor ,

Electroacoustic: ,

Geophone - converts ground movement (displacement) into voltage ,Gramophone pick-up

Hydrophone - converts changes in water pressure into an electrical form

Loudspeaker, earphone - converts changes in electrical signals into acoustic form

Microphone - converts changes in air pressure into an electrical signal

Piezoelectric crystal - converts pressure changes into electrical form

Tactile transducer

Photoelectric:

Laser diode, light-emitting diode - convert electrical power into forms of light

Photodiode, photoresistor, phototransistor, photomultiplier tube - converts changing light levels into electrical form

Electrostatic:

Electrometer ,Liquid crystal display (LCD)

Thermoelectric:

RTD Resistance Temperature Detector ,Thermocouple ,Peltier cooler ,Thermistor (includes PTC resistor and NTC resistor)

Radioacoustic:

Geiger-Müller tube used for measuring radioactivity. ,Receiver (radio)

cathode ray tube (CRT):

The cathode ray tube (CRT), invented by German physicist Karl Ferdinand Braun in 1879, is an evacuated glass envelope containing an electron gun (a source of electrons) and a fluorescent screen, usually with internal or external means to accelerate and deflect the electrons. When electrons strike the fluorescent screen, light is emitted.

The electron beam is deflected and modulated in a way which causes it to display an image on the screen. The image may represent electrical waveforms (oscilloscope), pictures (television, computer monitor), echoes of aircraft detected by radar, etc

The single electron beam can be processed in such a way as to display moving pictures in natural colors

The generation of an image on a CRT by deflecting an electron beam requires the use of an evacuated glass envelope which is large, deep, heavy, and relatively fragile (this has earned it the nickname "Fishbowl"). The development of imaging technologies without these disadvantages has caused CRTs to be largely displaced by flat plasma screens, liquid crystal displays, DLP, OLED displays, and other technologies

An exception to the typical bowl-shaped CRT would be the flat CRTs[1][2] used by Sony in their Watchman series (the FD-210 was introduced in 1982). One of the last flat-CRT models was the FD-10A (last produced in 1989 as Sony moved to LCD displays in 1990). The CRT in these units was flat with the electron gun located roughly at right angles below the display surface thus requiring sophisticated electronics to create an undistorted picture free from keystoning and the like

load cell:load cell is typically an electronic device (transducer) that is used to convert a force into an electrical signal. This conversion is indirect and happens in two stages. Through a mechanical arrangement, the force to be sensed is used in deforming a strain gauge. The strain gauge converts the deformation (strain) to electrical signals. Normally, a load cell consists of four strain gauges in a wheatstone bridge configuration, but are also available with one or two strain gauges. The electrical signal output is normally in the order of a few millivolts and requires amplification by an instrumentation amplifier before it can be used. The output of the transducer is plugged into an algorithm to calculate the force applied to the transducer

Although strain gauge load cells are the most common, there are other types of load cells as well. In industrial applications, hydraulic (or hydrostatic) is probably the second most common, and these are utilized to eliminate some problems with strain gauge load cell devices. As an example, a hydraulic load cell is immune to transient voltages (lightning) so might be a more effective device in outdoor environments

Other types include piezo-electric load cells (useful for dynamic measurements of force), and vibrating wire load cells, which are useful in geomechanical applications due to low amounts of drift

Every load cell is subject to "ringing" when subjected to abrupt load changes. This stems from the spring-like behavior of load cells. In order to measure the loads, they have to deform. As such, a load cell of finite stiffness must have spring-like behavior, exhibiting vibrations at its natural frequency. An oscillating data pattern can be the result of ringing. Ringing can be suppressed in a limited fashion by passive means. Alternatively, a control system can use an actuator to actively damp out the ringing of a load cell. This method offers better performance at a cost of significant increase in complexity.

Strain gauge:A strain gauge (alternatively: strain gage) is a device used to measure deformation (strain) of an object. Invented by Edward E. Simmons in 1938, the most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern. The gauge is attached to the object by a suitable adhesive, such as cyanoacrylate[1]. As the object is deformed, the foil is deformed, causing its electrical

resistance to change. This resistance change, usually measured using a Wheatstone bridge, is related to the strain by the quantity known as the gauge factor.

The gauge factor GF is defined as where RG is the resistance of the undeformed gauge, ΔR is the change in resistance caused by strain, and ε is strain. For metallic foil gauges, the gauge factor is usually a little over 2[2]. For a single active gauge

and three dummy resistors, the output v from the bridge is where BV is the bridge excitation voltage

Foil gauges typically have active areas of about 2-10 mm in size. With careful installation, the correct gauge, and the correct adhesive, strains up to at least 10% can be measured

Accelerometer

An accelerometer is a measuring device for specific external force. Specific external force is the sum total of external forces acting on an object divided by the mass. Accelerometers do not measure internal forces such as gravity. An acceleromter sitting still on a table top will read one gee of specific force due to the table pushing up on the accelerometer equal to its weight. If dropped in a vacumm it will read zero. For computation of position, the Inertial Navigation System adds in the estimate of the local gravity acceleration to the signal coming off of the acceleromters. Accelerometers may be part of an Inertial Navigation System, used to detect and measure vibrations, or for measuring acceleration due to gravity (inclination). An accelerometer inherently measures its own motion (locomotion), in contrast to a device based on remote sensing

Accelerometers are perhaps the simplest MEMS device possible, sometimes consisting of little more than a suspended cantilever beam or proof mass (also known as seismic mass) with some type of deflection sensing and circuitry. MEMS Accelerometers are available in a wide variety of ranges up to thousands of gn's. Single axis, dual axis, and three axis models are available

Accelerometers can be used to measure vibration on cars, machines, buildings, process control systems and safety installations. They can also be used to measure seismic activity, inclination, machine vibration, dynamic distance and speed with or without the influence of gravity. Applications for accelerometers that measure gravity, wherein an accelerometer is specifically configured for use in gravimetry, are called gravimeters.

Accelerometers are being incorporated into more and more personal electronic devices such as media players and handheld gaming devices. In particular, more and more smartphones (such as Apple's iPhone) are incorporating accelerometers for step counters, user interface control, and switching between portrait and landscape modes

Accelerometers are used along with gyroscopes in inertial guidance systems, as well as in many other scientific and engineering systems. One of the most common uses for micro electro-mechanical system (MEMS) accelerometers is in airbag deployment systems for modern automobiles. In this case the accelerometers are used to detect the rapid negative acceleration of the vehicle to determine when a collision has occurred and the severity of the collision.

The widespread use of accelerometers in the automotive industry has pushed their cost down dramatically

Transducer TypesUltrasonic transducers are manufactured for a variety of applications and can be custom fabricated when necessary. Careful attention must be paid to selecting the proper transducer for the application. A previous section on Acoustic Wavelength and Defect Detection gave a brief overview of factors that affect defect detectability. From this material, we know that it is important to choose transducers that have the desired frequency, bandwidth, and focusing to optimize inspection capability. Most often the transducer is chosen either to enhance the sensitivity or resolution of the system.

Transducers are classified into groups according to the application.

Contact transducers are used for direct contact inspections, and are generally hand manipulated. They have elements protected in a rugged casing to withstand sliding contact with a variety of materials. These transducers have an ergonomic design so that they are easy to grip and move along a surface. They often have replaceable wear plates to lengthen their useful life. Coupling materials of water, grease, oils, or commercial materials are used to remove the air gap between the transducer and the component being inspected.

Immersion transducers do not contact the component. These transducers are designed to operate in a liquid environment and all connections are watertight. Immersion transducers usually have an impedance matching layer that helps to get more sound energy into the water and, in turn, into the component being inspected. Immersion transducers can be purchased with a planer, cylindrically focused or spherically focused lens. A focused transducer can improve the sensitivity and axial resolution by concentrating the sound energy to a smaller area. Immersion transducers are typically used inside a water tank or as part of a squirter or bubbler system in scanning applications.

More on Contact Transducers.

Contact transducers are available in a variety of configurations to improve their usefulness for a variety of applications. The flat contact transducer shown above is used in normal beam inspections of relatively flat surfaces, and where near surface resolution is not critical. If the surface is curved, a shoe that matches the curvature of the part may

need to be added to the face of the transducer. If near surface resolution is important or if an angle beam inspection is needed, one of the special contact transducers described below might be used.

Dual element transducers contain two independently operated elements in a single housing. One of the elements transmits and the other receives the ultrasonic signal. Active elements can be chosen for their sending and receiving capabilities to provide a transducer with a cleaner signal, and transducers for special applications, such as the inspection of course grained material. Dual element transducers are especially well suited for making measurements in applications where reflectors are very near the transducer since this design eliminates the ring down effect that single-element transducers experience (when single-element transducers are operating in pulse echo mode, the element cannot start receiving reflected signals until the element has stopped ringing from its transmit function). Dual element transducers are very useful when making thickness measurements of thin materials and when inspecting for near surface defects. The two elements are angled towards each other to create a crossed-beam sound path in the test material.

Delay line transducers provide versatility with a variety of replaceable options. Removable delay line, surface conforming membrane, and protective wear cap options can make a single transducer effective for a wide range of applications. As the name implies, the primary function of a delay line transducer is to introduce a time delay between the generation of the sound wave and the arrival of any reflected waves. This allows the transducer to complete its "sending" function before it starts its "listening" function so that near surface resolution is improved. They are designed for use in applications such as high precision thickness gauging of thin materials and delamination checks in composite materials. They are also useful in high-temperature measurement applications since the delay line provides some insulation to the piezoelectric element from the heat.

Angle beam transducers and wedges are typically used to introduce a refracted shear wave into the test material. Transducers can be purchased in a variety of fixed angles or in adjustable versions where the user determines the angles of incidence and refraction. In the fixed angle versions, the angle of refraction that is marked on the transducer is only accurate for a particular material, which is usually steel. The angled sound path allows the sound beam to be reflected from the backwall to improve detectability of flaws in and around welded areas. They are also used to generate surface waves for use in detecting defects on the surface of a component.

Normal incidence shear wave transducers are unique because they allow the introduction of shear waves directly into a test piece without the use of an angle beam wedge. Careful design has enabled manufacturing of transducers with minimal longitudinal wave contamination. The ratio of the longitudinal to shear wave components is generally below -30dB.

Paint brush transducers are used to scan wide areas. These long and narrow transducers are made up of an array of small crystals that are carefully matched to minimize variations in performance and maintain uniform sensitivity over the entire area of the transducer. Paint brush transducers make it possible to scan a larger area more rapidly for discontinuities. Smaller and more sensitive transducers are often then required to further define the details of a discontinuity.

Cathode ray oscilloscope:

The cathode-ray oscilloscope (CRO) is a common laboratory instrument that provides accurate time and aplitude measurements of voltage signals over a wide range of frequencies. Its reliability, stability, and ease of operation make it suitable as a general purpose laboratory instrument. The heart of the CRO is a cathode-ray tube shown schematically in Fig.

The cathode ray is a beam of electrons which are emitted by the heated cathode (negative electrode) and accelerated toward the fluorescent screen. The assembly of the cathode, intensity grid, focus grid, and accelerating anode (positive electrode) is called an electron gun. Its purpose is to generate the electron beam and control its intensity and focus. Between the electron gun and the fluorescent screen are two pair of metal plates - one oriented to provide horizontal deflection of the beam and one pair oriented ot give vertical deflection to the beam. These plates are thus referred to as the horizontal and vertical deflection plates. The combination of these two deflections allows the beam to reach any portion of the fluorescent screen. Wherever the electron beam hits the screen, the phosphor is excited and light is emitted from that point. This coversion of electron energy into light allows us to write with points or lines of light on an otherwise darkened screen.

          In the most common use of the oscilloscope the signal to be studied is first amplified and then applied to the vertical (deflection) plates to deflect the beam vertically and at the same time a voltage that increases linearly with time is applied to the horizontal (deflection) plates thus causing the beam to be deflected horizontally at a uniform (constant> rate. The signal applied to the verical plates is thus displayed on the screen as a function of time. The horizontal axis serves as a uniform time scale.

          The linear deflection or sweep of the beam horizontally is accomplished by use of a sweep generator that is incorporated in the oscilloscope circuitry. The voltage output of such a generator is that of a sawtooth wave as shown in Fig. 2. Application of one cycle of this voltage difference, which increases linearly with time, to the horizontal plates causes the beam to be deflected linearly with time across the tube face. When the voltage suddenly falls to zero, as at points (a) (b) (c), etc...., the end of each sweep - the beam flies back to its initial position. The horizontal deflection of the beam is repeated periodically, the frequency of this periodicity is adjustable by external controls.

          To obtain steady traces on the tube face, an internal number of cycles of the unknown signal that is applied to the vertical plates must be associated with each cycle of the sweep generator. Thus, with such a matching of synchronization of the two deflections, the pattern on the tube face repeats itself and hence appears to remain stationary. The persistance of vision in the human eye and of the glow of the fluorescent screen aids in producing a stationary pattern. In addition, the electron beam is cut off (blanked) during flyback so that the retrace sweep is not observed.

CRO Operation:  A simplified block diagram of a typical oscilloscope is shown in Fig. 3. In general, the instrument is operated in the following manner. The signal to be displayed is amplified by the vertical amplifier and applied to the verical deflection plates of the CRT. A portion of the signal in the vertical amplifier is applied to the sweep trigger as a triggering signal. The sweep trigger then generates a pulse coincident with a selected point in the cycle of the triggering signal. This pulse turns on the sweep generator, initiating the sawtooth wave form. The sawtooth wave is amplified by the horizontal amplifier and applied to the horizontal deflection plates. Usually, additional provisions signal are made for appliying an external triggering signal or utilizing the 60 Hz line for triggering. Also the sweep generator may be bypassed and an external signal applied directly to the horizontal amplifier.

THE LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)

The Linear Variable Differential Transformer (LVDT) is a displacement measuring instrument and is not a strain-based sensor.

The LVDT models closely the ideal Zeroth-order displacement sensor structure at low frequency, where the output is a direct and linear function of the input.

The LVDT is a variable-reluctance device, where a primary center coil establishes a magnetic flux that is coupled through a mobile armature to a symmetrically-wound secondary coil on either side of the primary.

Two components comprise the LVDT: the mobile armature and the outer transformer windings. The secondary coils are series-opposed; wound in series but in opposite directions.

When the moving armature is centered between the two series-opposed secondaries, equal magnetic flux couples into both secondaries and the voltage induced in one half of the secondary winding is balanced and 180 degrees out-of-phase with, the voltage induced in the other half of the secondary winding.

The balanced condition provides total cancellation of secondary voltages and therefore zero voltage output. When the moveable armature is displaced from the balanced condition, more magnetic flux will couple into one half of the secondary than into the other producing an imbalance voltage output at the primary coil excitation frequency. The output voltage of the LVDT is therefore a direct function of the displacement of the mobile magnetic armature. The LVDT is, by definition, a transformer and requires an oscillating primary coil input.

The DC LVDT is provided with onboard oscillator, carrier amplifier, and demodulator circuitry. The AC LVDT requires these components externally. Due to the presence of internal circuitry, the DC LVDT is temperature limited operating from typically -40 C to +120 C.

The AC LVDT is able to tolerate the extreme variations in operating temperature that the internal circuitry of the DC LVDT could not tolerate. Typically, LVDT’s will be excited by a primary carrier voltage oscillating at between 50 hertz and 25 Kilohertz with 2.5 Kilohertz as a nominal value. The carrier frequency is generally selected to be at least 10 times greater than the highest expected frequency of the core motion.

The external housing of the LVDT is fabricated of material having a high-magnetic permeability therefore desensitizing the device from the effects of external magnetic fields.

No sensing spring element exists within an LVDT and therefore, the output of the sensor is hysteresis-free. Some LVDT displacement measuring sensors are, however, provided with internal armature return springs to allow profile measurement. When there exists no direct contact with the moving armature is allowed no mechanical wear results. The provision of linear bearings to prevent armature to coil structure contact and to limit wear can greatly extend LVDT operating life expectancies.

The strong relationship between core position and output voltage yields a sensor design that shows excellent resolution, limited more by the associated circuitry than the sensing method.

The internal core of the LVDT is generally constructed of an annealed nickel iron alloy with the high-temperature limitations of the device limited to the curie point of the core and the winding insulations used.

The thermal response characteristics of the LVDT are excellent for static and quasi-static thermal environments due to the physical and electrical symmetry of these devices. The physical symmetry also contributes to excellent zero repeatability over time and temperature. Most thermal-sensitivity shift errors result from the significant thermal coefficient of resistance (TCR) of the copper transformer windings. With increasing temperature, the primary coil resistance will increase causing a decrease of the primary current in the constant-voltage-excited case and therefore decreasing the magnetic flux generated and voltage output correspondingly.

The use of constant-current excitation will ensure a constant primary flux regardless of the coil resistance. Since the equivalent circuit of the constant-current source is a voltage source with an infinite series resistance, the use of a low-TCR resistance, in series with the primary, will function in much the same manner as the piezoresistive span-compensation resistor by causing the primary voltage to increase as a function of temperature thus offsetting the TCR-induced losses. The use of the series low-TCR resistor in the primary circuit allows the constant-voltage source to appear to the LVDT as a constant-current source.

Other thermally-active methods may also be used to compensate for the primary winding TCR by causing the primary voltage to increase, with rising temperature, in proportion to the increase in the primary coil resistance. The temperature coefficient of magnetic permeability is another contributor to the thermal-sensitivity shift and is compensated out as a net effect by the means described above. Within approximately 2 seconds of power application the LVDT oscillator and demodulator circuitry will stabilize sufficiently for dynamic measurement.

Due to self-heating of the primary coil, warm-up times for high precision static measurement are comparable to strain gaged sensors and are dependent upon the thermal stability of the measuring environment.

Lvdt

An LVDT Displacement Transducer comprises 3 coils; a primary and two secondaries.

The transfer of current between the primary and the secondaries of the LVDT displacement transducer is controlled by the position of a magnetic core called an armature.

On our position measurement LVDTs, the two transducer secondaries are connected in opposition.

At the centre of the position measurement stroke, the two secondary voltages of

the displacement transducer are equal but because they are connected in opposition the resulting output from the sensor is zero.

As the LVDTs armature moves away from centre, the result is an increase in one of the position sensor secondaries and a decrease in the other. This results in an output from the measurement sensor.

With LVDTs, the phase of the output (compared with the excitation phase) enables the electronics to know which half of the coil the armature is in.

The strength of the LVDT sensor's principle is that there is no electrical contact across the transducer position sensing element which for the user of the sensor means clean data, infinite resolution and a very long life.

Our range of signal conditioning electronics for LVDTs handles all of the above so that you get an output of voltage, current or serial data proportional to the measurement position of the displacement transducer

Accelerometer

Another type of transducer is the accelerometer. The accelerometer is not self-generating and requires an input voltage excitation.

A crystal cut to resonate at a specific frequency is supported within a case. Mounted above the crystal is a mass. As the case of the pickup vibrates, the force of gravity acting on the mass attempts to deform the crystal. The crystal is a piezoelectric element, so it generates an electrical charge when a mechanical force is applied to it.

Although an accelerometer does not measure velocity directly, an electronic integration can be performed on the output signal of the accelerometer to obtain velocity. Accelerometers generate high-frequency responses suitable for high-speed machinery and are very useful for monitoring vibrations in high temperature areas.

Transducer installation depends on the mechanical configuration of the assembly. Anti-friction bearings usually require contact pickups due to the low casing damping. Journal bearings generally need non-contact pickups. In this fashion, we are looking at relative motion of the shaft.

Typical Transmitter Connection Diagram

Generally, two velocity pickups, one mounted on the inboard bearing for radial vibration and one mounted on the outboard bearing in the axial direction, are adequate for determining misalignment and axial thrust. The inboard bearing usually consists of two non-contact probes, one for readings in the horizontal direction, and one for readings in the vertical direction.

Accelerometers are absolute transducers. To make a relative velocity measurement two transducers are used and the output of one is subtracted from the other. Making acceleration measurements by means of differentiating the output from a velocity transducer, or double differentiating the output from a displacement transducer, is rare. To make a relative acceleration measurement two accelerometers are used and the output of one is subtracted from the other. Accelerometers are also available with internal integrators; these are sometimes termed velometers.