Endüstriyel ölçme ve kontrol
Industrial measurement and control
Yazar: YAKUP KADRİ GÜLBAZ
Danışman: DOÇ. DR. KENAN KUTLU
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü
Derece: Yüksek Lisans, 1994
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: Denetim = Control ; Endüstriyel ölçüm cihazları = Industrial measurement equipments ; Ölçme-değerlendirme = Measurement and evaluation

Özet:
Günümüzde teknoloji ve otomasyonun gelişmesi ile birlikte, tasarımlarda kullanılan ürünlerin nitelik ve niceliği de değişime uğramaktadır. Hızlı teknolojik gelişme ve mikroelektronik sistemler kullanılması ile birlikte ürünlerin boyutları küçülürken işlevleri artmaktadır. Dünyadaki değişime paralel olarak Türkiye'de bu tür cihazların kullanılması yaygınlık kazanmaktadır. Daha az maliyetle fazla üretim amacından yola çıkarak otomasyon hayatımızın her alanına girmektedir. Tek merkezden tüm bir üretimi veya prosesi, kontrol etmek, insanlardan kaynaklanabilecek pekçok soruna bu şekilde çözüm bulmak ve hassas ölçümler sonucu çok hızlı müdahalelerde bulunmak artık günümüzde bilinen ve uygulanan bir yöntemdir. Danimarka'da kurulu Danfoss firmasının Nordborg ve Greasten'deki endüstriyel ölçüm cihazları, frekans konvertörleri üreten fabrikalarında, kalibrasyon ünitelerinde yaptığım incelemeler ve bu ürünlere ait teknik bilgiler çalışmanın temelini oluşturmaktadır. Bu çalışmada sözü geçen cihazların hepsi elektronik esaslı olup, ölçüm sonuçlarını standart sinyaller olarak vermektedirler. Bu nedenle istendiğinde bilgisayar veya PLC ile haberleşme sağlanabilir. Ayrıca klasik debimetreler ile karşılaştırıldığında bu cihazların ölçüm değerlerinin hassasiyeti çok yüksektir. Cihazların çalışma prensiplerine ait teorik bilgiler pekçok kaynakta yer almaktadır. Ancak bunların üretim yön temleri ve kalibrasyonlarına ait bilgiler sınırlıdır. Bu nedenle çalışmada kalibrasyon işlemi geniş olarak anlatıl makta ve debimetre cihazlarının kalibrasyonu sonucu bilgi sayarda elde edilen değerler ve bunlara aitgrafikler etler bölümünde yer almaktadır. Cihazların kullanımına ait gerçek uygulamalar ve bunlara ait sonuçlar teknolojik gelişmenin boyutları konusunda fikir vermektedir. ıx

Abstract:
The demand for improved purification quality and accurate measurement has led to an increased need to control, regulate and monitor the different processes in Water works and purification installations. Because measurements within this sector are very different in character, different principles of measurement are reguired. The range of equipment for the water treatment sector which is included in this thesis is therefore based on three proven principles: - Electromagnetic flow measurement - Ultrasonic flow measurement of filled pipes. - Ultrasonic flow measurement in open channels, These principles of measurement are obstruction-free (no pressure/ energy loss) and involve no mechanical moving parts, i.e. they give good operational economy. Flowmeters are used within water supply for: - Mesuring raw water quantity - Dosing (chemicals, polymers) - Monitoring filters - Measuring flush water - Measuring supplied quantities - Monitoring leakage Flowmeters are used within waste water purification for: - Measuring the supply yto the installation - Dosing (chemicals, polymer) - Measuring return sludge - Measuring sludge for digestion tank - Measuring discharge from the installation - Measuring over flowElectromagnetic flowmeters Electromagnetic flowmeters measure flow in full pipes, The principle of measurement is an extension of Faraday's law: the voltage induced in an electric conductor moving through a magnetic field is proportional to the speed at which the conductor moves. An electrical charge (Q) moved at a velocity (V) per pendicular to a magnetic field (B) is effected by a force (F) 1) F= QxVxB When both positive and negative charges are present, the force causes separation of the charges. This force is counteracted by the electrostatic force (E) 2) F=QxE Faraday's law: Equations 1 and 2 represent Fraday's law of induction. 3) E=VxB The law can be formulated thus: When an electrical conductor of length L is moved at velocity V, perpendicular to the lines of flux through a magnetic field B, voltage U. is induced across the end points of the conductor. U.= BxLxV i The sensor of the electromagnetic flowmeter consists of ceramic measuring pipe in which two electrodes are mounted opposite to each other. The magnetic coils are fitted around the meter and are supplied by a pulsating direct current. Thus a pulsating magnetic field is produced across the liquid. When an electrically conduc tive liquid moves in magnetic field, a voltage is induced across the liquid at right angles to that field. The electrode voltage is led to the signal converter, where it is converted to a standard analogue output signal of 0/4-20 mA. xiUltrasonic Flowmeters Ultrasonic flowmeter also measure flow in full pipes. The principle of measurement is based on the fact that the speed of sound increases when it is transmitted in the direction of flow and decreases when transmitted against the direction of flow. The difference in upstream and signal transmission time is proportional to the flow velocity. Ultrasonic flow measurement can be used for both conductive and non-conductive liquids. In the measuring pipe the ultrasonic wave is transmit ted across the axis of the pipe to the opposite transducer in what we call the sound track. The ultrasonic signall will thus pass through water layers at varying velocities. The measuring system compensates so the registrated transit times indicate mean times for the travel of the sound in the sound track. If two sound tracks had been used, i.e. four trans ducers, better transit time averaging and higher accuracy would have achieved. Flow Metering In Open Channels Open channel flow measurement is used to measure flow in pipes that are not filled an in channels in severe environments. The complete system for flow measurement in channels consists of a signal converter, a transducer and a prefabricated fibreglass flume. Flow measurement in open channels involves creating a controlled relationship between liquid height and liquid flow. The depth is measured by ultrasonic sound transmitted from a transducer mounted above the measuring flume. The transmission time, the time the signal takes to reach the surface of the water and return, is inversely proportional to the depth and therefore the flow. A reference bar is used to compensate for the effect of air temperature on sound speed. The signal transmitter contains a linearisation system (level/flow conversion) which together with the reference bar creates optimum system accuracy. Typical application areas are the inlet and outlet from municipal purification installations, industrial water purification and outfall monitoring. xnIn addition to the mentioned flowmeter types, the mass flowmeters will be explained here below. Mass Flowmeter The mass flowmeter measures the mass flow direct in kg/s unlike the ultrasonic flowmeter, the magnetic flow meter and other volumetric flowmeters which measure velocity and compute the volume flow in for example, litres/second. An important reason for measuring the mass instead of volume is that the mass does not change due to external influences whereas the volume changes with the temperature. Formerly the mass flow was traditionally measured indirectly by multiplying a volume flow measurement by the density of the medium. This method demands therefore as a supplement to the volume flow measurement, density measurement or density determination from a temperature measurement; and more measurement will always increase the uncertainty. Therefore direct mass flow measurement is preferable. Mass flowmeters measuring the mass flow directly often use the force which occurs when a mass in movement changes its velocity and direction, called the coriolis force. Here below, there is an explanation of how the coriolis force can be exploited for direct mass flow measurement. The medium is equally divided through two parallel pipes. The two pipes are made to oscillate by an electro magnet. Due to the ascillations each mass particle of the medium will change direction and alter its velocity the coriolis force will affect the deflections of the pipes. The variations in the deflections, which are a measureof the mass flow, can be tracked by means of (magnetic) sensors whose signals are converted into an electrical output signal proportional to the mass flow. Frequency Converters T? 3. frequency converter has developed much since the first uait was launced at the end of the 60 *s. Today's advanced microprocessors and semi-conductors have improved the frequency converter substantially. The frequency converter can be divided up into four main components: Xlll1- The rectifier converts the three-phase a.c. voltage from the supply mains to a pulsating d.c. voltage. 2- The intermediate circuit. There are there different types. One type converts the voltage of the rectifier into a d.c. current. The other type stabilizes the pulsating d.c. voltage and sends this on to the inverter. The third type of intermediate circuits converts a constant d.c. voltage from the, rectifier into a variable value. 3- The inverter controls the frequency of the motor voltage. One type of inverter also converts the constant d.c. voltage into a variable a.c. voltage. 4- The electronics of the control circuit can trans mit signal to both the rectifier, the intermediate circuit and the inverter The frequency converter monitors the process cont rolled and takes action in case of operational disturban ces. There are three types of monitoring: Plant Monitoring: The frequency converter monitors the plant on the basis of the output frequency, output currents and motor torque. On the basis of these values it is possible to set a number of limit values for the control, such as min. allowable speed or max. allowable motor current. Motor Monitoring: The frequency converters monitors the motor on the basis of a calculation of the thermal conditions. Like a thermal relay the frequency converter takes care that the motor is not overloaded. It also takes the output frequency into account, that ensures that the motor is not overloaded at low speeds, where the self ventilation of the motor is reduced. Unit Monitoring: The frequency converter cuts out in case of o ver cur rent. Some frequency converters can yield a momentary overcurrent. The fast micro processors used in the frequency can sum up the motor current and the time, that ensures opti mum utiliaziton without overloading the frequency converter, xiv