Fiber Optic Sensors

What are fiber optic sensors?

The fundamental characteristic of all fiber optic sensors is that they depend on some optical properties, such as intensity, phase, state of polarization and wavelength, to be modulated by measurands. Measurands could be pressure, temperature, electromagnetic field or displacement.

All fiber optic sensors have an optical element that is sensing these property changes. For most sensors, this element is the optical fiber itself or a non-fiber optical element.

Fiber optic sensors with optical fiber as sensor element are called "intrinsic fiber sensor" and sensors with a non-optical fiber sensing element are called "extrinsic fiber sensor".

1. Intrinsic Fiber Sensors

In the intrinsic fiber sensor, external measurands such as pressure, vibration, temperature interact with optical fiber element and cause fiber bending, fiber distortion and a change in the refractive index of the sensing fiber.

Because of the refractive index change, lights that travel through the fiber are affected accordingly. The changes in light properties, such as light intensity, light wavelength and light phase are then detected. The magnitude of measurands interacting with the fiber can then be determined.

2. Extrinsic Fiber Sensors

Birefringent crystal, intensity mask or thin film absorbers are most often used as sensor elements in extrinsic fiber sensors. Usually they are integrated into the optical path.

When the external force interacts with the sensing element, the light properties are modulated as well. The sensor has light source, optical path and photo detector parts. The magnitude of measurands is detected similar to intrinsic fiber sensors.

The Applications of Fiber Optic Sensors

Wide Area Sensing and Monitoring

Because of optical fiber's immune to electromagnetic field, fiber sensors have a big potential in these areas. They are widely used in temperature sensing in building, leakage monitoring along oil pipelines and so on.

The above mentioned applications are called wide area sensing or monitoring. The name means that the sensing covers a wide area. In this area, fiber sensors are divide into two categories: distributed sensor and quasi-distributed sensor.

1. Distributed Sensor

Distributed sensors sense measurands continuouly over the entire length of the fiber. The most important criteria is that sensor fibers must be very sensitive to measurands.

A typical example of distributed sensors is a temperature sensor utilizing Raman scattering effect in optical fibers. Another example is OTDR (Optical Time Domain Reflectometer) which senses signal reflection in the whole length of an optical path.

2. Quasi-Distributed Sensor

Quasi-distributed sensors use discrete sensor elements that are carefully arranged in the fiber network. This type if sensor needs to be small size, low cost and high reliability.

High Sensitivity Measurements

Another area for fiber sensors is the high sensitivity measurement applications. This type of sensors typically utilize light interference's extremely high sensitivity property.

A number of interferometric fiber sensors have been used for measurement of temperature, pressure, vibration and so on. The fiber optic gyroscope is one typical example of this type of applications.

Harsh Environment Measurement

Some extreme environment has no choice but fiber optic sensors. This kind of applications include high temperature, immersion into chemical reagents, radioactive rays factories and so on. The fiber optic sensor's resistant to this type of harsh environment is extremely important.

Colin Yao is an expert on fiber optic communication technologies and products. Learn about fiber optic ST, ST connectors, ST fiber connector on Fiber Optics For Sale Co. web site.

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Logic Gates

Logic gates take binary values and perform functions on them, similar to the functions found in simple algebra. Binary algebra is the set of mathematical laws that are valid for binary values. A binary value can only be a 1 or a 0. 1 is a high value, representing true and high voltage. 0 is a low value, representing a false value and low voltage.

Logic gates are typically packaged in integrated circuits, although they can be constructed using analogue components. Integrated circuits allow multiple logic gates to be packaged in one chip and are usually quite reliable. Logic gates typically come in two flavours, TTL (transistor-transistor logic) and CMOS (Complementary Metal Oxide Semiconductor). One must be careful mixing the two types, there logic low and logic high are different voltages. A CMOS might take a TTL high as a LOW and a TTL will accept a CMOS low as a high. Because of this they are generally incompatible, but there are a few CMOS that can accept TTL inputs and vice versa.

The buffer and NOT gates are the simplest of the logic gates. The buffer would be used as a digital signal booster, if a logic signal was to travel for some distance voltage drop from wire resistance would lower a logic high voltage so low that when it reaches its destination its read as a logic low, putting this in between would solve that problem. The buffers algebra function is B = A

NOT gates simply change the input from a 1 to a 0 or vice versa. It is also called an inverter and has many uses in logic circuits. For example, you have 2 lights, but you only want 1 on at any one time, you would put a NOT gate between one light so when there is a logic high input 1 light is on and the other connected to the NOT gate is off and when there is a logic low input the second comes on because of the NOT gate. The circle on the end of the triangle indicates that it’s an inverting gate and you can recognise any inverting logic device by this circle.

The equivalent binary algebra function is B = A’, where B is the output and A is an input value.



The AND gate most commonly come in IC packages of 2 and 3 input versions. The output only produces a logical 1 when all of the inputs are 1. An AND gate could be used in an alarm circuit, where input A would be a reed switch input and B would be an armed control, So the alarm would only be activated if the alarm was active AND the reed switch was circuit was opened (opened door ect.).



The OR gate has a minimum of two inputs and produces an output of 1 if at least one of the inputs has a value of 1. An OR gate could be used to expand the number of reed switches in the previous example

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Relays

The RELAY is a device that acts upon the same fundamental principle as the solenoid. The difference between a relay and a solenoid is that a relay does not have a movable core (plunger) while the solenoid does. Where multipole relays are used, several circuits may be controlled at once. Relays are electrically operated control switches, and are classified according to their use as POWER RELAYS or CONTROL RELAYS. Power relays are called CONTACTORS; control relays are usually known simply as relays. The function of a contactor is to use a relatively small amount of electrical power to control the switching of a large amount of power.

The contactor permits you to control power at other locations in the equipment, and the heavy power cables need be run only through the power relay contacts. Only lightweight control wires are connected from the control switches to the relay coil. Safety is also an important reason for using power relays, since high power circuits can be switched remotely without danger to the operator. Control relays, as their name implies, are frequently used in the control of low power circuits or other relays, although they also have many other uses. In automatic relay circuits, a small electric signal may set off a chain reaction of successively acting relays, which then perform various functions. In general, a relay consists of a magnetic core and its associated coil, contacts, springs, armature, and the mounting. Figure 3-19 illustrates the construction of a relay. When the coil is energized, the flow of current through the coil creates a strong magnetic field which pulls the armature downward to contact C1, completing the circuit from the common terminal to C1. At the same time, the circuit to contact C2, is opened.

A relay can have many different types of contacts. The relay shown in figure 3-19 has contacts known as "break-make" contacts because they break one circuit and make another when the relay is energized. Figure 3-20 shows five different combinations of relay contacts and the names given to each.
A single relay can have several different types of contact combinations. Figure 3-21 is the contact arrangement on a single relay that has four different contact combinations. (The letters next to the contacts are the "forms" shown in figure 3-20.)
One type of relay with multiple sets of contacts is the clapper relay shown in figure 3-22. As the circuit is energized, the clapper is pulled to the magnetic coil. This physical movement of the armature of the clapper forces the pushrod and movable contacts upward. Any number of sets of contacts may be built onto the relay; thus, it is possible to control many different circuits at the same time. This type of relay can be a source of trouble because the motion of the clapper armature does not necessarily assure movement of all the movable contacts. Referring to figure 3-22, if the pushrod were broken, the clapper armature might push the lower movable contact upward but not move the upper movable contact.
Some equipment requires a "warm-up" period between the application of power and some other action. For example, vacuum tubes (covered later in this training series) require a delay between the application of filament power and high voltage. A time-delay relay will provide this required delay. A thermal time-delay relay (fig. 3-23) is constructed to produce a delayed action when energized. Its operation depends on the thermal action of a bimetallic element similar to that used in a thermal circuit breaker. A heater is mounted around or near the element. The movable contact is mounted on the element itself. As the heat causes the element to bend (because of the different thermal expansion rates), the contacts close.
Relays can be described by the method of packaging; open, semisealed, and sealed. Figure 3-24 shows several different relays and illustrates these three types of packaging. Figure 3-24 (E), (G) and (H) are open relays. The mechanical motion of the contacts can be observed and the relays are easily available for maintenance. Figure 3-24 (A), (B) and (C) are semisealed relays. The covers provide protection from dust, moisture, and other foreign material but can be removed for maintenance.
The clear plastic or glass covers provide a means of observing the operation of the relay without removal of the cover. Figure 3-24 (D) and (F) are examples of a hermetically sealed relay. These relays are protected from temperature or humidity changes as well as dust and other foreign material. Since the covers cannot be removed, the relays are also considered to be tamper-proof. With metal or other opaque covers, the operation of the relay can be "felt" by placing your finger on the cover and activating the relay.

Source: freeinfosociety.com

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Opamps: Operational Amplifiers

Operational amplifiers, often known as opamps, are nifty little integrated circuits that contain several transistors. These transistors are used to amplify or attenuate a signal, depending on the circuit that the opamp is placed in.

Why use an opamp when you can use a transistor? Because opamp packages are easier to control, generally more powerful, and also more versatile. An equivalent amplifier circuit might take numerous components, but the opamp places them all into a compact package, reducing the amplifier’s circuit board footprint and the number of components that could fail.

The classic opamp, and perhaps the most widely used, is the UA741. This device comes in several packages, including a compact 8 pin DIP as well as a 10, 14, and 20 pin version for other purposes. All of these are quite cheap, with the 8 pin version costing only $0.30 or so at most online retailers. You would pay considerably more to construct an amplifier circuit using pure transistors.

Opamps are used very frequently for signal conditioning circuits. Let’s say you have a sensor, perhaps a heartbeat sensor, and it is picking up a signal of several millivolts. You want the signal to be windowed to 0-5V so you can hook it up to an analog-to-digital converter. Sounds hard right? Not so!

The first step in designing your circuit is to determine the gain you will need. Let’s assume that the signal from your sensor has a peak of 5 millivolts. In that case, you can calculate the gain as:

To find the gain you need, you divide the desired output by the input, in this case 5 and 5E-3 (.005) volts, respectively. Here, we need a gain of one thousand. Can the opamp handle that magnitude? Yes!

Now we need to construct our circuit. The opamp is represented as a triangle:


The leads on the left are inputs. One of these has to be grounded and which one depends on whether your signal is inverted or not. To avoid inverting (flipping) your signal, you need to ground IN- and connect your signal to IN+.

The other goofy thing about the opamp is that it needs two VCC signals, unlike most other integrated circuits. They should be identical in magnitude, but opposite in polarity. For example, you might connect positive 12V to VCC+ and negative 12V to VCC-. Finally, we have Vout, which is the output signal.

Now, to perform the amplification, we need to tell the opamp what gain we want. To do this, we but two resistors in a circuit with it, forming the classic non-inverting amplifier circuit:
Here, we see the two resistors, R1 and R2. The approximate gain of the amplifier in this case would be the ratio of R2 to R1. The two resistors can be any values as long as they maintain that ratio. In our case, we could use a 1000 ohm resistor for R1 and a 1 megaohm resistor for R2.

Now if this circuit was actually constructed, Vin could be connected to the sensor and Vout could be connected to the ADC. By looking at the input and output signals on an oscilloscope, you would be able to see that the signal was clearly amplified. If no amplification is occurring or you only see noise, ensure that your circuit is connected properly, you have +12 and -12 (not ground) volts connected to the opamp’s VCC, and also make sure that everything has a common ground. These are the most common mistakes that I have committed when working with opamps.

Now you should be able to go out and use the opamp effectively. We have looked at the most common circuit configuration, but it is also possible to configure the opamp to do frequency-based filtering and other nifty things.

Artikel From : freeinfosociety.com

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LED

Light from a Light Emitting Diode (LED) is created in much the same way that light is created in a flourescent tube or neon sign. In an LED crystal the electrons of its atoms are pumped up to higher energy states, and when they fall back down again, each atom gives off a particle/wave of light. However, the electrons in an LED are not exactly the same as the gas molecules in a neon sign. They are not in orbitals stuck to individual atoms.

Instead the electrons occupy a contiguous "sea of charge," and they continually wander among all the atoms in the material. But while they do this, they maintain a particular energy level just like
they do when stuck to individual atoms. It's as if each electron in an LED crystal was "orbiting" among all the atoms of the substance as a whole, and the electron always "orbits" at a particular "height" above each of the atoms it passes.

Be aware that *all* substances contain electrons. The electrons I'm discussing here are not supplied by the battery, they instead occur naturally in the wires, crystals, etc. They are in the LED all the time, even when the battery is not connected. Don't make my original mistake by imagining that electrons are injected into the LED by the power supply. In fact, they are already in the material, and the power supply simply forces them to flow.

To create LED light, first we connect two conductive crystals of different characteristics together. Both types of crystal contain movable electrons. In one type of crystal the electrons "orbit" naturally at a high energy level, and in the other, they always "orbit" low. When a voltage is applied across the joined crystals, the electrons inside are forced to flow across the boundary between the pair of crystals. If the flow direction is correct, electrons in the "high" crystal flow into the "low" crystal and must begin orbiting at the lower energy level. As they fall to the lower energy level, they give off light. The frequency of the light (which we see as the color) is determined by the difference in energy levels between the two crystals. By manufacturing different types of crystals having different natural energy levels, various colors of light can be created. Crystals with similar levels create low-energy photons of red light or even infrared light. With a larger difference in energy levels, green light can be created. An even larger energy-step can create blue light.

The "high" and "low" crystals are usually called "n-type" and "p-type." In n-type crystals the movable electrons wander around while staying at the upper energy level of an unfilled outer atomic orbital. During an electric current they travel at this level. In "p-type" crystals the mobile electrons naturally exist at a deeper orbital level. When the two crystals are connected to each other and then connected properly in a circuit with a battery, the battery creates a current in the entire circuit. It sucks electrons out of the end of the p-type crystal and into the wire. At the same time it pushes electrons into the far end of the n-type crystal. The electrons already in the n-type crystal then are forced to flow across the crystal junction, fall down in energy, emit light, and end up back in the p-type crystal.

Where did the electrons get the energy to emit light? How do they get to a higher energy level so they can enter the n-type crystal? Well, in order for the battery to push electrons through the LED, it had to apply electrical attraction and repulsion forces to the electrons in the crystal. To apply force to the electrons in the crystal, it had to apply a force to the electrons in the negative wire. This squeezes all the electrons on the surface of the negative wire together, which raises the voltage of the entire wire. (If electrons were like water, then the wire is like a long trough. The battery pumps water into one end of the trough, and this makes the water level 'voltage' rise everywhere in the trough.) When the negative wire's electrons get to the energy level equal to the n-type crystal, they start flowing into the crystal and falling "down" the junction, emitting light as they go. (This analogy is incomplete: at the same time that the battery was pumping up the "water level" of the negative wire to match the n-type crystal level, it also was REDUCING the "water level" of the positive wire so that the low-energy electrons of the p-type crystal could be sucked into the wire.)

Here's another way to visualize LEDs. In a neon sign, the electrons around each neon atom get pumped up in energy as they're whacked by incoming high-speed electrons. In an LED the battery pumps up the electrons directly. In a neon sign, each atom emits light when an electron falls back to its original energy level. In an LED, the whole crystal junction emits light as electrons drop back to a lower level. Therefor an LED resembles a gigantic single neon atom! An LED/atom is so large that we can connect its electron cloud directly to a battery with wires. It's so large that we can build in different characteristics, and change the color of its flourescence.

Light Emitting Diodes are much like solar cells. Both devices use n-type and p-type crystals, but in a solar cell the process runs backwards: instead of falling down in energy and emitting light, light hitting the solar cell causes electrons in the p-type crystal to jump upwards in energy. If these electrons are near the crystal junction, they can end up in the n-type crystal, and they can flow through wires to the outside world, falling down in energy as they do. In fact, if light shines on an LED, the LED behaves as a tiny, inefficient solar cell. And conversely, if a battery is used to create a current in a solar cell, the solar cell can emit a very tiny amount of (mostly infrared) light. An LED gives light when charge is pumped through it, and when light shines on a solar cell, the solar cell becomes a charge pump.

Light Emitting Diodes are also like thermocouples. N-type and p-type crystals are not the only materials whose electrons "orbit" at different energy levels. Different metals have different levels too. If a copper wire is twisted together with an iron wire, a junction is formed between them which contains an energy-step like that of an LED. The energy-step in a thermocouple is much smaller than in an LED. If electrons are forced to flow across the thermocouple's energy step, they fall down in energy level and emit energy. But what do they emit? Longwave Infrared light and crystal vibrations. Together we call these by the name "heat energy". The energy step in a thermocouple is too small, so it cannot emit photons of visible light. Instead it creates "heat." And conversely, if heated, a thermocouple can create an electric current. When operated one way, a thermocouple is a bit like an LED which emits heat. When operated the other way, it acts a bit like a "solar battery" and becomes a "heat battery."

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