The development of fiber-optic technology was mainly driven by the requirement of the telecommunications industry. Nonetheless one should not overlook that telecommunications is not the only application of fiber optics. The other major application area is in metrology and data acquisition.
Why Sensors? Why Fiber-Optics?
It used to be that in any major machinery or installation, gauges were located wherever that relevant information was present: a thermometer at the boiler, a tachometer at the shaft, a fuel gauge at the tank, etc. Staff could then go to these locations and take readings. Meanwhile the trend is that data acquisition and display are separated. For example, consider an airplane: Sticking out a mercury thermometer is obviously not a good idea for measuring the outside temperature. Fuel tanks are in the wings; who would climb out there to check a level tube? Instead,a ll data of interest are acquired at their respective location with sensors. The sensor's response is transmitted, usually by cable, to a central monitoring station where all displays are side by side to provide an overview. In the airplane this location is in the cockpit where the pilot can check all instruments without leaving his seat.
Industrial installations, too, have a central control room where all information comes together. It is not only time.saving when staff do not need to walk around the premises to take instrument readings, but it also minimizes risks to human because often date are taken in hard-to-reach or dangerous places, such as inside chimneys, in high-voltage apparatus, or in numerous places inside nuclear power stations.
To go by such a remote sensor concept, there are three ingredients required:
- Sensors for any physical quantity that may be of interest. This includes temperature, pressure, stress and strain, distance, filling level, speed, force, vibration, etc. The sensors must translate such quantities into a format that can easily be transmitted.
- Transmission lines.
- Displays that translate the transmitted data into a format accessible to human senses, i.e., typically make them visible or audible.
Of course, the scheme also facilitates the keeping of records of relevant data; witness the "flight recorder" which is of central importance after a plane crash. It has often been taken for granted that for the transmission one uses an electric quantity: most often a voltage, but there is at least one standard where this is a current. The lines are then usually copper cables. The advantages of this approach is that there are innumerably many suppliers, and sensors can be picked from an unfathomable variety of hardware. Also, there is an abundant supply of well-trained engineers and technicians who are knowledgeable about this technology and can use it very efficiently.
Now enter optical fiber. First, one might have the idea of using sensors that do not translate the original data into an electric format, but rather into some optical format, like a light intensity or wavelength. There is no difficulty in converting this to a display because optical formats are easily assessed at the receiver. All it takes is a photodetector, and one is back to a voltage or current that can be displayed in a routine way. Of course, the question is: if one eventually converts to electrical anyway, why bother with optics?.
The point is that during transmission, the data are in an optical format. While on its way across the distance, plenty of adverse effects can act on the transmitted signal. In the case of electric cables, one severe problem is interference from external electromagnetic fields. To avoid such difficulty, one usually provides shielding, which in the case of strong external fields is quite involved. Optical fiber, by contrast, is immune to that kind of interference.
There are some other properties of optical fibers that are advantageous in this context. As we saw earlier, they are small and lightweight. The accompanying savings in space and weight can be quite important, e.g., in vehicles, in particular in aircraft or spacecraft. Also, optical fibers withstand extreme temperatures better than electrical cables. They are also more robust in the presence of aggressive chemicals. Finally, fibers provide perfectly separated electrical potentials, a fact that is greatly appreciated, e.g., in petrochemical installations. We see, one might have benefits from an optical technology. It is good news that a wide variety of optical sensors is available. There is hardly any physical quantity for which no optical sensor exist. New sensor are added all the time for chemical and other quantities, too.
When we look at these fiber-optic sensors, we nee to broadly distinguish two classes (figure 1): There are sensors that are mounted in front of, next to, or in proximity of the fiber, read the quantity under investigation, and launch a corresponding light signal into the fiber. In this case, the fiber is merely the transmission medium and has nothing to do with the acquisition of the original quantity. Such sensors are called extrinsic. In contrast, intrinsic sensors use the fiber itself or part of it directly to read the original quantity. Then the fiber is both sensor and cable at he same time. We will look at examples of both types.
 |
| Figure 1.- Classification of sensor types. extrinsic and intrinsic sensors. |
In this case an optical fiber leads up to a "black box" that impresses information onto the light beam in response to an environmental effect. The information could be impressed in terms of intensity, phase, frequency, polarization, spectral content, or other methods. An optical fiber then carries the light with the environmentally impressed information back to an optical and/or electronic processor.In some cases the input optical fiber also acts as the output fiber. The intrinsic or all-fiber sensor shown in figure 1 (a) uses an optical fiber to carry the light beam, and the environmental effect impresses information onto the light beam it is in the fiber. Each of these classes of fibers in turn has many subclasses with, in some cases, sub-subclasses that consist of large numbers of fiber sensors. In some respects the simplest type of fiber optic sensor is the hybrid type that is based on intensity modulation.
References
Ingeniero en electronica con aficiones a escribir y compartir todo aquello que le llama la atencion o que su curiosidad atrapa..
#Curioseando #Perdiendoeltiempo #sinnadamejorquehacer.
No comments:
Post a Comment