Design of a Hall Probe Pressure Transmitter Using Bellows

excogitation of a Hall poke into extort vector using Bel slumps as Sensor R. Sarkar, Animesh Ghosh, Lipika Ghosh and N. Mandal Asansol Engineering College Vivekananda Sarani, Kanyapur, Asansol-713305 E-mail emailprotected com, ghoshanimesh. emailprotected com, emailprotected com precis Bellows, an elastic type instancy sensor is generally used as a local indicator. To transmit the signal of hollers to a remote standoffishness some technique is needed.In the present paper a Hall see sensor has been used to convert the bellows movement into pentadage signal which potentiometer be converted into 4 20 mA sure signal and convey to a remote indicator. It has been observed that the transducer and vector outputs against wedge have a very good linearity and repeatability. The necessary theoretical equations along with experimental results are inform in the paper. Keywords hale measurement, bellows, Pressure transmitter, Magnet, Hall Probe. I. INTRODUCTION Pressure is an im portant measuring and controlling technical parameter during industrial production process.In order to operate industrial production well, draw should be accurately metric and controlled. Pressure can be measured in price of absolute or gauge. The absolute pressure can be measured in terms of height of a liquid column in a manometer whereas the gauge pressure is measured by different types of sensors 1-4. As for example bourdon tube, diaphragm, capsule, bellow element etc. operate as primary sensing elements for measuring positive or negative gauge pressure. The sensors like strain gauge, piezoresistance, LVDT, capacitive element, inducive element etc. ct as secondary sensors to measure positive or negative gauge pressure. The negative gauge pressure or make clean pressure can also be measured by many other sensors like pirani gauge, ionization gauge, McLeod gauge etc. In industrial drill it is required to transmit the measured pressure to a remote hold. Hence in a pressure t ransmitter, the falsify of sensor parameter due to the change of fluid pressure is converted into an electric or pneumatic signal by using a suitable transducer and that signal after amplification is transmitted to a remote receiver.Thus the pressure transducer is a vital part of any pressure transmitter and its performance determines the reliability of operation of the transmitter. Many works on development of reliable pressure transducer are still being reported by different groups of workers. B. Raveendran et al. 5 have designed and developed a MEMS based receiving primed(p) modular pressure transmitter. A Bourdon tube based pressure transmitter unit using an improved inductance bridge network has been canvass by S. C. Bera et al. 6. Y. Ruan et al. 7 have developed a multipoint radio receiver pressure transmitting system composed of pressure sensor PTB203, A/D converter ADC0804, MCU STC89C52, wireless communication module CC1101, receiver module STC89C52, CC1101 and display m odule LCD1602. Zeng Mingru et al. 8 have developed a HART Protocol based intelligent pressure transmitter which is compatible with both analog and digital signals. K. Subramanian et al. 9 have developed MEMS type capacitive pressure sensor with sensitivity of the order of few fF/ kPa. Universal relative frequency to digital converter (UDFC) technique has been used by S.Y. Yurish 10 to develop an intelligent digital pressure transducer. A multiplexed frequency transmitter technique has been used by R. Vrba et al. 11 to design a reliable pressure transducer using ceramic diaphragm. In the present paper, a manse probe based pressure measurement technique has been developed. In this technique a unceasing drawing card is placed on the tip of the bellows with the Hall probe sensor on the top of the right(prenominal) fitting of bellows chamber as shown in Fig. 1. The movement of the bellows tip is measured by a hall probe sensor.With the change of pressure the distance between magnet and the hall sensor decreases and so the magnetic intensity at the sensor increases. The Hall sensor senses this increase of magnetic sector intensity and accordingly its output potential difference increases with the increase of pressure. This signal is nonlinearly related with the movement of float. But for very small movement of the bellows this voltage depart be intimately linear. The experimental results are reported in the paper. The block plot of the proposed transducer is shown in Fig. 1. Necessary athematical equations have been derived to explain the theory of operation of the transducer as well as transmitter. A prototype unit along with the signal conditioner has been designed and fabricated. The experiments have been performed to find out the static diagnostics of the sensor, transducer and transmitter. The experimental results are reported in the paper. A very good linearity and repeatability of results with adjustable sensitivity of the transducer has been obse rved. pic Fig. 1 Diagram of the proposed transducer along with float and hall probe sensorII. METHOD OF admittance In the present paper the pressure is sensed by a bellows. A magnet is placed on the top of the bellows. And the hall probe on the bellows chamber. The float movement of the bellows is converted into voltage by a hall probe sensor. Output voltage is amplified by an instrumentation amplifier INA101 and whence converted into 4-20 mA current signal using signal conditioning circuit. This signal is then transmitted to remote station with negligible loss. Let the pressure is pic and the corresponding height of the bellows tip from reference is pic.In bellows the height of the tip is proportional to pressure and is written as pic(1) where pic is the constant Now the distance of the hall probe from the magnet is pic(2) where pic is the total length of the hall probe from reference. In the present work the magnet is selected to be a circular permanent magnet. Let the radius an d width of the magnet be pic andpic respectively. Hence magnetic field at the hall probe due to magnet is pic(3) where pic is the constant depending on the pole strength of the magnet, its radius and permeability of air which are all constants.Since pic equation (3) is reduced to pic (4) pic(5) The above equation is equally true for very low pressure also. Since at low pressure pic, so equation (5) is reduced to pic (6) Now the output hall voltage pic of the hall sensor is proportional to pic if the current passing through the sensor be kept constant and hence it is given by pic(7) where pic is the constant of proportionality. Hence from equations (5) & (7) pic (8) or, pic(9) where pic is another constant. Therefore from equations (1), (8) and (9), the output from hall probe is given by pic(10) pic (11) pic(12) Therefore output is linearly related with pressure. III. DESIGN In the present design a cylindrical permanent magnet is selected of inner radius pic, depth pic, width pic. In our present design, pic. The output of hall sensor is amplified by INA101 based instrumentation amplifier. The gain of the instrumentation amplifier is set by external resistor R1. This output signal is first converted into amplified voltage signal picin the set out 1-5 volt D. C. and then into current signalpic in the hold 4-20mA D. C. y a signal conditioner circuit as shown in Fig. 2. After calibration the output of the transmitter becomes 4mA when picis 1 volt and pressurepicis zero psig and 20mA when picis 5 volt and pressurepicis at maximum range picof the bellows. Hence the transmitter voltage outputpic in volt and current outputpicin mA may be written as, pic (13) and pic (14) From (13) and (14), pic (15) where picand pic(16) pic Fig. 2. Block diagram of the proposed pressure transmitter using bellows element as sensing device pic Fig. 3 Circuit diagram of hall probe based pressure indicator IV. EXPERIMENT The experiment is performed in two steps. In the first step, the pr oposed transducer was designed, fabricated and mounted on the outside cover of bellows chamber as shown in Fig. 1. The bellows with the above sensor was first fitted with a dead weight tester and the dead weight of the dead weight tester was change magnitude in steps and in each step the Hall voltage output is measured and the characteristics of the hall sensor based transducer unit is determined.The characteristic graph obtained by plotting Hall voltage against Pressure is shown in Fig. 4. Experiment was repeated both in increasing and decreasing modes for several times and the standard deviation curve for six observations is shown in Fig. 6. In the second step the output of the pressure transmitter is taken in terms of current signal and he characteristic is shown in Fig. 7. pic Fig. 4 Characteristic graph obtained by plotting Hall voltage against Pressure pic Fig. 5 Percentage deviation Curve of the Hall Probe based Pressure Transducer picFig. 6 Standard Deviation Curve of the Hall Probe based Pressure Transducer pic Fig. 7 Characteristic graph of hall probe based pressure transmitter V. DISCUSSION The characteristic of hall probe sensor is nonlinear in nature. But change of hall probe voltage is quite linear as shown in Fig. 4. The linear nature of the curve is due to the fact that the movement of the tip of the bellows for the entire pressure range is generally very small and hall probe voltage due to small change of distance between hall probe and magnet lies almost in the linear zone.The percentage deviation curves from linearity as shown in Fig 5 also indicate that the percentage deviation from linearity also lies within the tolerable limit. A very good repeatability of the experimental data was also observed as shown by the standard deviation curves in Figs. 6. The characteristic of the whole transmitter is almost linear as shown in Fig 7. The design of the system is very simple and the hall probe & the permanent magnet are now available at a very low cost. Hence the cost of the pressure transmitter will be low. References 1 J. P. Bentley, Principles of Measurement Systems, third ed. Longman Singapore Publishers (pvt) Ltd. , Singapore, 1995. 2 E. O. Doeblin, Measurement System Application and Design, 4th ed. , McGraw-Hill, New York, 1990. 3 B. G. Liptak, Process Measurement and Analysis, 3rd ed. , U. K. Butterworth Heinman, Oxford, 1999. 4 D. M. Considine, Process Instruments and Control Hand Book, 2nd ed. , McGraw-Hill, New York, 1974. 5 Raveendran, B. Subhash, K. M. Design of modular pressure transmitter with wireless capability IEEE Conference on Electrical, Electronics and Computer Science (SCEECS), 2012, pp 1 3 6 Bera, S. C. Mandal, N. Sarkar, R. 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