Integrated Technology Systems - Assignment Example

 Abstract The need for accurate information coupled with concerns over anthropogenic climate change has heightened technological advancement in rainfall measurements. The need for accuracy, timely and convenient access to information and data has resulted into development of advanced sensors. This discussion presents a review of the main temperature sensors for control systems; thermocouples, Resistance Temperature Detectors (RTD) and thermistors outlining the characteristics of each type and providing an explanation and examples of the types of systems they are most suited to. This paper also provides a summary of the typical sensors for measurement of rainfall. These sensors are compared with the latest developments in rainfall measurement sensor technology such as Vaisala RAINCAP, optical sensor, remote sensing and satellite technology including a discussion on the key drivers supporting the development and uptake of the newer sensors. Introduction A sensor can be defined as electronic equipment that measures physical quantity and converts the same into a signal which can read easily by a technician or operator of machine or equipment. In this regard, a sensor is equipment that responds to an input quantity by generating an equivalent optical or electrical output signal. It is therefore important to note that a sensor sensitivity is measured in terms of how much its output signal changes when the measured quantity changes. For example, when a mercury thermometer moves by one centimeter when there is increase or decrease in temperature by 10C, then the sensitivity is 1cm/0C. Sensors that measure very small changes in the physical quantities are therefore very sensitive and vice versa. This is referred to as sensor resolution and it is the smallest change that a sensor can detect in the change in the physical quantity that it measuring (Sinclair, 2001). Sensor resolution has formed the basis for technological advancements and more sensors are being manufactured with respect to improving their sensitivity. According to Sinclair (2001), sensors should be designed to have very little effect on what they are designed to measure since this has effect on their resolution. He also stated that sensor resolution can be improved by making sensor smaller. In this regard, technological advancement has made it easier for design and manufacture of microsensors using Microelectromechanical systems (MEMS) technology. However, a good sensor can be defined by three characteristics: sensitivity to the measured property alone not influenced by the measured property and sensitivity to any other property likely to be encountered in its application. This discussion focusses on a review of the main temperature sensors for control systems: Thermocouples, Resistance Temperature Detectors-RTD and Thermistors outlining the characteristics of each type and providing an explanation and examples of the types of systems they are most suited. Thermocouples Thermocouples are temperature measuring sensors that are made up of two dissimilar metals mainly joined together at one end. The joint of the two metals are sensitive to temperature change in that when they are heated or cooled, they generate voltage that can be converted into signals and correlated back to the temperature. The principle of operation of the thermocouple temperature sensors is that dissimilar metals often exhibit contact potential between them which varies with the variance in temperature. A contact potential cannot be measured for a single connection, however, when two junctions are within the circuit at different temperatures, then a voltage of few millivolts can be generated and detected. The generated voltage is zero when there are no temperature differences but changes with varying temperatures until a peak is reached. In this respect, thermocouple is limited to the range of temperatures that it can detect due to the nonlinear shape of the characteristics. Moreover, at temperatures higher than the turn-over point, a reversal takes place thus limits the use of thermocouple temperature sensors. Flow of electric current through a conductor with ends kept at different temperatures result into the generation of heat. The rate of heat release is proportional to the product of temperature gradient and current flow (Sinclair, 2001). Like other circuits, the design of a thermocouple contain more than two junctions of different metals, however, the design is such that only the intended junctions are at different temperatures. The quantity of voltage generated by the thermocouple is often very little to the tune of millivolts for a 100C change in temperatures (Mukhopadhyay et al, 2009). The voltage is also affected by the metals used in the thermocouple. For this reason therefore, Copper/Constantan is mainly used for lower range while platinum/rhodium type is used for the higher ranges of temperatures. The small voltages output are also amplified using a chopper amplifier or an operational amplifier. Amplifier must be chosen carefully in order to ensure good drift stability and the device must be calibrated at frequent intervals. In situations where thermocouple is subjected to on and off switching, it is normally fitted with controller that enables pre-set switching temperatures. Thermocouple temperature sensors are widely used for industrial purposes due to its suitability to measure high temperatures. Despite the many metal options that can be used for thermocouple sensors, only a few are used due to the need to withstand high temperatures and corrosion and provide a linear scale in measurement. In this regard, they are often divided into two groups, the base metals e.g. iron-constantan and the platinum-rhodium. The latter combination is used for higher temperatures though it has low output levels and requires sheathing of ceramic to avoid damage by oxidation. It is also resistant to all known acids which make it typical for industrial use. Iron-constantan on the other hand is prone to rusting and oxidation in general and must be protected. Thermocouple is ideal for industrial use because the elements themselves are very small and can be inserted in very tiny places to sense rapidly changing temperatures (Mukhopadhyay et al, 2009). The electrical nature of the thermocouple’s circuitry also enables remote reading of the thermocouple’s output. Resistance Temperature Detectors-RTD Resistance temperature detectors operate on the principle that that electrical resistance is dependent on temperature changes. This type of temperature sensor has a temperature dependent electrical resistance element (Sinclair, 2001). In this regard, the resistance of this element increases with increasing temperature in a linear manner but within the temperature range of the RTD in question. Resistance temperature detectors are made from either metals or metal alloys with platinum being the most common metal used. This is because it is stable within a wide range of temperatures and the fair linear resistance characteristics (Park & Mackay, 2003). The specific and most platinum used in RTD is the film of platinum PT100 (DIN 43760 standard) which exhibit a nominal resistance of 100 ohms and can operate within a temperature range of -2700C to 6500C but still maintain a fairly linear characteristics (Park & Mackay, 2003). In very high temperatures applications, tungsten is sometimes used due to its high resistance of about 1000 ohms. The RTD element is not supposed to be stressed mechanically or be contaminated because this also changes the resistance of the conductor (Park & Mackay, 2003). There are two forms of Resistance temperature detectors’ devices; metal film or wire wound. Wire wound devices consist of a length of wire wound on a neutral core then housed in a protective sleeve. Metal film RTDs on the other hand comprises of a ceramic substrate that houses the sensitive element which is normally a millimeter thick zigzag metallic track. The resistance of the metallic device can be controlled by laser trimming of the metal track. Normally, when the size is reduced, the resistance is increased hence the thermal inertia is very much reduced. This is favorable since it results into increased response and good sensitivity as well. One major advantage of Resistance temperature detectors is that the change in resistance as a result of change in temperature is more linear for a wider temperature range compared to both thermocouple and thermistors. The specifications for RTD must therefore include: temperature coefficient, sensor material, and a reference resistance. Resistance temperature detectors were initially used for laboratory analytical work. However, advancement in technology has resulted into these thermometers being used in other areas where they were not acceptable such as industrial. According to Sinclair (2001), some of the industrial processes that required holding temperatures at a very close to 100C are now needed to be held even closer. Emphasis on uniformity and quality control now demands for temperatures to be controlled within closer limits than before hence there was need to advance RTD. Resistance temperature detectors are now used both in the laboratory as well as in industrial processes such as manufacture of food and pharmaceuticals which are very sensitive to temperatures and require closer limits monitoring. . Thermistors Thermistor is a type of temperature sensor that is made of a thermistor. Thermistors are special type of semiconductor resistor formed by combining oxides from various metals like nickel, manganese and cobalt among others. The types of semiconductors used in thermistor depend greatly on the level of temperature coefficient and the resistance value required (Sinclair, 2001). Most common thermistors have negative temperature coefficient and exhibit generally logarithmic law and have no violent changes in resistance. The negative temperature coefficient thermistors also exhibit a high degree of sensitivity to small changes in temperature with resolution of the sensor at 4% per 0C (Mukhopadhyay et al, 20089). Thermistors are more accurate than thermocouples and not RTDs and best suited to low temperature ranges as well. As stated by Mukhopadhyay et al (2009), thermistors are non-linear temperature sensors and are directly applicable to temperature ranges between -800C up to 2500C. However, technological advancement has improved this nonlinearity by incorporating stand-alone data loggers or PCs. Thermistors are divided into two major groups namely Positive Temperature Coefficient (PTC) thermistors and Negative Temperature Coefficient (NTC) thermistors; however, the latter is the mainly used thermistor for precision temperature measurements. NTC thermistor is the main conventional metal-oxide thermistor in which the resistance reduces with the increasing temperatures. The resistance of NTC thermistor is determined by its physical dimension and the nature of materials used due to sensitivity differences exhibited by different materials. However, the relation between temperature and resistance for NTC thermistors is often very nonlinear and are commonly incorporated in temperature sensing circuits. PTC thermistors are the most currently developed and are used mainly in the protection circuits to sensing current or temperatures. Unlike the NTC thermistors, PTC thermistors exhibit current-voltage characteristics that sense change in direction. There are two main types of PTC thermistors that are used though made from the compounds; lead, barium and strontium titanates. The first is the over-temperature protection which as a switchover point at the reference temperature. In these types of devices, at a lower temperatures below the reference temperatures, the resistance of the PTC devices are fairly constant, however, when the temperature rises close to the reference temperature, the PTC takes over and this result into rapid increase in resistance as the temperature rises (Mukhopadhyay et al, 2009). The other type of PTC is used for overcurrent protection and applies the same principle as discussed above. Typical Sensors for Measurement of Rainfall Measurements of rainfall can be done using different methods and observing systems and associated algorithms such as direct site sensors like rain gauge and disdrometers, ground based remote sensors such as infrared ground radars. It is however important to note that each sensor has its own strength and limitations. Disdrometers and Rain gauges are the oldest rainfall sensors and provide a direct in situ measurement of properties of rainfall at a high temporal resolution. Despite the fact that it is old and has been used for a long period of time, rain gauge still remains the basis for calibrating rainfall remote sensing algorithms (Testik & Gebremichael, 2010). However, rain gauges are subject to both random and systematic measurement errors. The first errors that are associated with rain gauges is the local systematic errors. These errors are related to wind-induced errors, losses due to evaporations, wetting, and lack of proper calibration and splashing. These errors cannot be fairly estimated and the correction is often not accurate. Winds also contribute to other effects resulting into under catchment. As stated by Testik & Gebremichael (2010), rainfall gauges require correction using numerical modeling techniques and empirical formulas with regards to field interpretations. There are different formulas depending on the type of rain gauge and the altitude where measurements are done. Calibration of rain gauges and disdrometers is simple thus there is tendency to miss the accuracy even after calibration has been done. These are some of the limitations that has driven the advancement in technology and adoption of more modern technology in rainfall measurements. The need for accurate information coupled with concerns over anthropogenic climate change has increased the need to understand more about temporal and spatial variations in the amount of rainfall on the earth’s surface. The estimation of large scale precipitations can either be done using traditional surface gauges measurements by the use of satellite remote sensing. However, these technologies have their own advantages as well as shortcomings. According to Hulme et al (2002), rainfall gauges have been used to provide measurements for trends as well as variability of monthly rainfalls throughout the entire twentieth century but due to certain shortcomings, new technologies are being advanced. Moreover, this information was only, limited to about thirty percent of the earth’s surface. The need to provide a complete coverage of the earth’s surface led to the invention of satellite remote sensing technologies. As stated by Hulme et al (2002), multi-platform satellite measurements provides a complete coverage at monthly to sub daily resolution which is very important for the modern estimation of rainfall. However the merger of both the gauge and satellite data sets is very important in minimizing the shortcomings of one another while maximizing the advantages of each technology. Vaisala RAINCAP Sensor Technology Vaisala RAINCAP Sensor Technology is one of the latest unique and new approaches to measurement and estimating of rainfall amounts. This technology is more accurate than the previous rain gauge and since it measures the individual rain’s drops. The intensity of the raindrops on the surface of a stainless steel is estimated using a piezoelectric detector. In this way, the sensor provide accurate and timely information on the duration, intensity and accumulated rainfall (Kampfer, 2012).Vaisala RAINCAP Sensor Technology major advantage compared to the other technologies such as disdrometers and rain gauge is that it has the capacity to differentiate between the rainfall and hailstones. Unlike the traditional rain gauges, it has no moving parts and thus do not require cleaning and emptying thus is essentially self maintaining. The components of a RAINCAP are 90mm stainless steel cover in a round shape, a rigid frame and a piezoelectric detector. The round stainless steel is mounted on the rigid frame. Piezoelectric detector is then mounted under the cover and consists of other electronic systems also mounted beneath the detector. The principle of operation is that the when the rain drops hit the RAINCAP at a terminal velocity, the acoustic detection of the rain drops as it impacts the sensor cover is realized (Testik & Gebremichael, 2010). The velocity of each raindrop is a function of the diameter hence more intense signals are created by larger drops than the smaller drops. The acoustic signals are then converted into voltages by piezoelectric detector. The total amount of rainfall is then calculated from the total amounts voltage signals and the known surface area of the RAINCAP sensor. Other rainfall parameters such as the duration, intensity and hails can also be calculated from this information. The RAINCAP sensor has been tested and validated in different sites in both tropical and temperate conditions. The major advantages of RAINCAP over other rain gauges are three; it can distinguish between hail and rain with data support for the two. It gives rainfall data in time and not affected by temperature or wind and last but not least, it does not require daily maintenance hence suitable for remote conditions (Kampfer, 2012). Optical Rain Sensor Optical sensors are small sized equipments that use the infrared beams to detect the drops of rain on the surface of the equipment. The reading is then correlated to the specific amount of water while the frequent readings are correlated to intensity of the rainfall. Optical rain sensors were born from the technology that automatically triggers the windshield wipers of automobiles. Optical rain sensor is a compact device that is not maintained regularly and can be mounted on a structure or a pole to measure rainfall. The rain gauge consists of several modes and individual users can select depending on their interests. It also consists of different levels of sensitivity that can be chosen from The major advantage of the optical rain sensor is that it eliminates the problem of tipping. However as Gao et al (2006), states, it doesn’t have accuracy in heavy rains as in other equipments discussed above. While it solves the problems of tipping baskets, it does not give accurate data. Another advantage is that it is best suited to provide estimates when all the other devices used fails. This is because optical rain sensor is more of a qualitative device than a quantitative device. The diversity with which the optical sensor can be located also makes it suitable for application in different scenarios. According to Kampfer (2012), optical rain sensor can be installed in areas such as mountains where the other equipments are difficult to install since it is not affected by lightening. Recent developments are looking for other opportunities to take advantage of optical fiber sensor innovations to apply this technology in other fields. Remote Sensing Remote sensing refers to the science of getting information concerning an object mainly by analyzing data acquired by another device called a sensor note in contact with the object. According to Kampfer (2012), we can get information about the objects that we want to study by measuring the electromagnetic waves from Gamma rays to microwaves. Satellites are used to accomplish this by being position approximately 36000 kilometers above the earth’s surface thus sees the same view as full earth. As stated by Gao et al (2006), sensors are sensitive to different wavelength bands within the electromagnetic spectrum and hence provide detailed information about the atmospheric conditions of the earth. These wavelengths can be interpreted and correlated to the earth surface and most importantly give information about the amounts of rainfalls. Satellites form the latest technological development in rainfall measurements. Even though there are no satellites that can be used to reliably measure rainfall amounts in all circumstances, satellite contains sensors that can measure different aspects of the atmospheric earth such as the cloud thickness (Gao et al 2006). The major advantages of satellites and their sensors is that they are cheap since most of the images from satellite are available on the internet and can be downloaded. Another advantage is that it gives a real time estimation data since geostationary satellites are generally available in linear real time and lastly, satellites can give estimates about rainfall information over a wider area. However, satellites are not reliable since several environmental factors are indirectly sensed and may not be used in all circumstances and sometimes demands for skilled operator interpretations. References Gao, W., Qu, J. J., Kafatos, M., Murphy, R. E. & Salomonson, V. V. (2006). Earth Science Satellite Remote Sensing: Data Computational Processing, and Tools (Volume Two). Berlin: Springer-Verlag. Hulme, M., New, M., Todd, M. & Jones, P. (2002). “Precipitation measurements and trends in the twentieth century.” International Journal of Climatology, Vol 21, (15) 1889-1922 Kampfer, N. (2012). Monitoring Atmospheric water Vapor: Ground-Based Remote Sensing and In-situ Methods. New York: Springer. Mukhopadhyay, S. C., Gupta, G. S. & Huang, R. Y. (2009). Recent Advances in Sensing Technology. Berlin: Springer-Verlag. Park, J. & Mackay, S. (2003). Practical Data Acquisition for Instrumentation and Control Systems. Burlington, MA: Newnes Publishers Sinclair, I. (2001). Sensors and Transducers: Third Edition. Oxford, OX2: Butterworth-Heinemann Testik, F. Y. & Gebremichael, M. (2010). Rainfall: State of the Science. Washington, DC: American Geophysical Union. Read More