Design of a Hall Probe Pressure Transmitter Using Bellows

Design of a Hall Probe Pressure Transmitter using Bellows as Sensor R. Sarkar, Animesh Ghosh, Lipika Ghosh and N. Mandal Asansol Engineering College Vivekananda Sarani, Kanyapur, Asansol-713305 E-mail: [email  protected] com, ghoshanimesh. [email  protected] com, [email  protected] com Abstract: Bellows, an elastic type pressure sensor is generally used as a local indicator. To transmit the signal of bellows to a remote distance some technique is needed.In the present paper a Hall probe sensor has been used to convert the bellows movement into voltage signal which can be converted into 4 – 20 mA current signal and transmitted to a remote indicator. It has been observed that the transducer and transmitter outputs against pressure have a very good linearity and repeatability. The necessary theoretical equations along with experimental results are reported in the paper. Keywords: pressure measurement, bellows, Pressure transmitter, Magnet, Hall Probe. I. INTRODUCTION Pressu re is an important measuring and controlling technical parameter during industrial production process.In order to operate industrial production well, pressure should be accurately measured and controlled. Pressure can be measured in terms 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, inductive element etc. ct as secondary sensors to measure positive or negative gauge pressure. The negative gauge pressure or vacuum pressure can also be measured by many other sensors like pirani gauge, ionization gauge, McLeod gauge etc. In industrial application it is required to transmit the measured pressure to a remote distance. He nce in a pressure transmitter, the change 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 wireless modular pressure transmitter. A Bourdon tube based pressure transmitter unit using an improved inductance bridge network has been studied by S. C. Bera et al. [6]. Y. Ruan et al. 7] have developed a multipoint wireless pressure transmitting system composed of pressure sensor PTB203, A/D converter ADC0804, MCU STC89C52, wireless communication module CC1101, receiver module STC89C52, CC1101 and display module 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 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 hall probe based pressure measurement technique has been developed. In this technique a permanent magnet is placed on the tip of the bellows with the Hall probe sensor on the top of the outside 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 d ecreases and so the magnetic intensity at the sensor increases. The Hall sensor senses this increase of magnetic field intensity and accordingly its output voltage 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 will be almost linear. The experimental results are reported in the paper. The block diagram 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 characteristics 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 observed. [pic] Fig. 1: Diagram of the propos ed transducer along with float and hall probe sensorII. METHOD OF APPROACH 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 then 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 and width of the magnet be ‘ [pic]’ and‘[pic]’ 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 r adius [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 [pic]in the range 1-5 volt D. C. and then into current signal[pic] in the range 4-20mA D. C. y a signal conditioner circuit as shown in Fig. 2. After calibration the output of the transmitter becomes 4mA when [pic]is 1 volt and pressure[pic]is zero psig and 20mA when [pic]is 5 volt and pressure[pic]is at maximum range [pic]of the bellows. Hence the transmitter voltage output[pic] in volt and current output[pic]in mA may be written as, [pic] (13) and [pic] (14) From (13) and (14), [pic] (15) where [pic]and [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. EXPERI MENT The experiment is performed in two steps. In the first step, the proposed 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 increased 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 [pic]Fig. 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 sim ple 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. 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