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Yusupbekov Nodirbek Rustambekovich, Professor, Department of Automation and Control Tashkent State Technical University, Uzbekistan E-mail: [email protected] Jumaev Odil Abdujalilovich, Associate Professor, Department of Automation and Control Navoi State Mining Institute, Uzbekistan E-mail: [email protected] Ismoilov Muhriddin To'lqin og'li, Assistant, Department of Automation and Control Navoi State Mining Institute, Uzbekistan E-mail: [email protected]
IMPROVEMENT MEASUREMENTS OF ELECTROMAGNETIC FLOW METERS
Abstract. An article devoted to the concept of building electromagnetic microprocessor flow meters; takes into account the further development of electromagnetic parameters of flow measurement and increase the accuracy of such devices; the transition to a low-frequency pulsed power magnetic systems; studies of magnetic induction flow meters and the results of their operation; improvement of the automated calculation of the electromagnetic flow signal in the development of a unified device for calibrating instruments.
Keywords: electromagnetic flow meter, increase accuracy, sensor sensitivity, measuring unit, measuring electrodes, measuring signals, operational amplifiers, flow rates, A/D conversion, measured data filtering.
I. Introduction ror of applied electromagnetic flow meters ranges from 0.1
Growing requirements for modern technologies and for to 2.0% [1]. The design of electromagnetic sensors has no resource saving, for environmentally friendly industries, and moving parts. [2].
rising energy prices lead to the need for ever wider use of instruments with the best metrological parameters for measuring the flow of liquids and gaseous streams. These measuring devices are increasingly used to monitor technological parameters, the use of natural resources, internal production processes, and commercial purposes.
II. The problem
The complexity of the task of measuring the flow rate of gas-liquid flows is in the ambiguity of the aggregate state of the measured substance. If to take into account single-phase expenses it is enough to know only one quantity as the quantity of a substance flowing through a certain section per unit time [1], then in gas-liquid mixtures, such information does not give a complete quantitative picture of the flow. It is necessary to know the amount of consumption of each of the components, the ratio between them, etc. The ultimate goal of measuring any technological parameter is determined primarily by the tasks of a specific production, which, in turn, determines the requirements for measuring instruments.
Electromagnetic flow meters are becoming increasingly deserved distribution. A distinctive feature of electromagnetic flow meters is that they can be installed on pipelines of any diameter, ranging from 10 mm to 3000 mm. The given er-
The flow meter does not contain parts that create resistance to flow. They are chemically resistant to almost any kind of liquid and are independent of the readings on viscosity, pressure, temperature, density or conductivity. A linear graded characteristic with an increase in the size of the device, its value does not grow as quickly as with other types of flow meters.
III. Solution
The proposed electromagnetic flow meter task of improving the accuracy is achieved by measuring the flow rate of the medium of a magnetic induction flow meter, containing a measuring tube installed in the line through which the flow flows, equipped with a grounding electrode for potential equalization. In the flow meter, the power is supplied by bipolar pulses with a pause frequency of 5 Hz. Powering the sensor with a bipolar flow, under other equal conditions, doubles the sensitivity of the sensor. Moreover, when using pulsed power, it becomes possible to significantly simplify both the electromagnetic sensor and the measuring circuit. This is due to the following reasons: [3]
- at steady state current in the inductor there is no transformer EMF.;
- there is no phase shift caused by losses in steel, therefore, the inductor supply current is strictly proportional to the flow or induction in the air gap;
- significantly decreases the power consumed by the electromagnetic sensor, since the resistance of the windings with a pulsed power supply is significantly lower than with a sinusoidal voltage supply.
In addition, this sensor power allows for recovery mode, i.e. use the current in the inductor to form the opposite power pulse, which gives a significant reduction in electrical energy consumption.
The next feature of the inductor supply circuit is that it has a separate zero bus relative to the measuring electrodes. This feature allows you to get rid of two types of parasitic capacitances at once: parasitic capacitances between the electrodes and a grounded sensor housing and parasitic capacitances between the inductor and the corresponding electrodes, thus ensuring the possibility of obtaining the most accurate measurement signals without the inherent noise of the converter [4].
Figure 1 Block diagram of the electromagnetic flow meter
The proposed electromagnetic flow meter consists of a primary flow transducer (l) installed in the main pipe (2), with magnetization coils (3), measuring electrodes (4, 5, 6) [measuring unit, I], pre-amplifiers (7, 9), integrated amplifier (8, 10, 11), microcontroller (18), analog-digital converter modules (13, 14, 15), current driver (12), electronic display (16), computer (17) [electronic unit, II].
In the electronic unit II (Fig. 1), preamplifiers 7.9 convert the level of the electrode signal to a predetermined voltage level and provide a ready signal to the microcontroller 18 via amplifiers 8.10 to the medium flow rate measurement cycle. The microcontroller 18 starts the measurement cycle, consisting of the inclusion of a controlled current source 12 for a specified period of time [7]. Through the magnetizing coils, 3 of the block I, a rectangular current pulse is generated, creating a magnetic field in the measuring tube. The magnetic field interacts with the electrically conductive fluid flowing through the measuring tube and, in accordance with the Faraday law, induces an electric field in the liquid, which creates an emf. on the measuring electrodes 4, 5. Induced EMF between the electrodes 4, 5 together with the electrochemical and polarization potentials of these electrodes, relative to the electrode ground 6 potential equalization, is fed to the measuring inputs of operational amplifiers 7.9 of the electronic unit II. From the output of the operational amplifiers 7.9, the total signal of the amplified EMF and the differences of the electrochemical and polarization potentials of the electrodes is fed to the input of the integrating amplifiers 8.10 with a large integra-
tion time constant [6]. Operational amplifiers 7 and 8, form a high-pass filter (HPF), which compensates for the increased difference of the electrochemical and polarization potentials of the electrodes 4, 5 with respect to the ground electrode of the potential equalization 6 at infra-low frequencies. Integrating amplifier 8 amplifies the emf induced by electrodes 4 and 5, as well as the difference of electrochemical and polarized potentials between electrodes k1 (e1-e2) times (Fig 3d). The process of change in time of the electrochemical and polarization potentials of electrodes 4 and 5 is in the region of infra-low frequencies (tenths, hundredths of a Hertz) contains a high-frequency noise component and is shown in (Fig. 3). At the beginning of the measurement, the microcontroller 18 generates a sequence of control signals for the measurement unit, I (Fig. 1). The excitation coils 3 lie on the first diameter of the measuring tube 2. The excitation system serves, during operation, to create a magnetic field B penetrating the pipe wall and flowing medium. It occurs when the current I generated by the electronic unit 12 passes through the excitation coils 3 connected in this case in series excitement. From the controlled current source 12, rectangular current pulses of alternating polarity equal to the amplitude Imax and duration t1 with intermediate off states are fed to the magnetization coils 3 (Fig. 3a). The time duration t1 of the current pulses can be from 40 to 120 milliseconds. The off time must be more than 1 minute and is necessary for the relaxation of partial polarization arising on electrodes 4 and 5 in the measuring tube with a moving medium with a unidirectional low-frequency
pulsed magnetic field (only for liquids with ionic conductivity). Due to the presence of free charge carriers in the liquid, the polarization of the electrodes has a finite relaxation time. With the passage of a current pulse through the magnetization
coils 3, the amplitude of the induced electromotive force (el, e2) on the electrodes 4, 5 increases or decreases in proportion to the average flow velocity in the measuring channel of the flow meter (Fig. 3c).
Figure 2. Functional diagram of the electronic unit of the flow meter
The front and rear edges of the magnetizing rectangular current pulse cause inductive-capacitive and noise high-frequency components of the EMF signal induced on the electrodes, and the process of establishing the magnetic field in the measuring tube (Fig. 3, time interval t2) is accompanied by significant noise in a wide range of high frequencies. Depending on the average flow rate, the magnitude of the induced emf can be three orders of magnitude less than the electrochemical and polarization potentials of the electrodes [6].
From the output of the measuring unit I from the electrode 4, the signal enters the low-pass filter 10 (LPF) through the operational amplifier 9, which amplifies the measuring emf that is k2 times the difference of the electrochemical and polarization potentials of the electrodes 4 and 5 at low frequencies and eliminates high-frequency noise. The signals filtered by high frequencies (Fig. 3e) are fed to the inputs of analog-digital converters 13, 14, 15 from the outputs of which the proportional signals are read by digital codes by the
microcontroller 18. The digital code of each ADC measurement (13, 14, 15) of the corresponding channels is recorded in the memory controller 18. The controller 18 summarizes the data by N measurements for step t1, and then step t2, determining the average data values obtained in each of the stages, respectively, thereby filtering the measured data from low-frequency and high-frequency noise [4].
Analog-to-digital conversion is performed by the method of double integration, i.e. by charging the integrator with voltages Uin and Uop (charging time is a multiple of the period of the mains voltage) and discharging it with the reference
voltage Uref of the corresponding sign. The end of the discharge is fixed by the comparator, the control system generates a pulse, the duration ofwhich is equal to the discharge time of the integrator and is proportional to the measured voltage. In the microcontroller, this duration is converted into a digital form, a fourfold calculation of the time interval corresponding to the input voltage, a single calculation of the time interval corresponding to the reference voltage and the division of the average value are performed. Such an algorithm allows reducing the influence of noise and external influence, as well as the instability of the inductor current [5].
Figure 3. Timing diagrams of measuring signal changes
During direct integration, the integrator output voltage reaches
,T° int .T' int
U0ut=—¡u;„dt or U0ut=—ju;dt (1)
T 0 T 0 The integrator discharge occurs when a reference voltage of opposite polarity is applied to its input. The time interval of the discharge lasts until the processing of the comparator. Wherein
The value of the time interval of the discharge is proportional to the measured signal (U.n or Uop) and for the input signal can be written in the following form [7]
T + = T (4)
xms int U-
T = T
ur
U+
(5)
1 T ms 1 1
U0ut = -ju;„dt or uout = -jUmsdt T 0 T 0
T op
(2)
U.n - the measured voltage at the input of the integrator (corresponding to positive and negative current supply pulses)
\U +| = \U ,r| = |ein|Kpa • Ena
ein - voltage on the electrodes of the electromagnetic sensor;^]
Kpa,Ena - respectively, the transfer coefficients of the preliminary and normalizing amplifiers;
T.nt is the integration time of the input voltage. The total measurement time for one period of the input signal is
The result of the division is proportional to the ratio of the input signal to the reference signal, obtained in ED(led), has the form:
T'
N =-
Kpa ' Kn
K
(9)
f
t= T- + ti = T
U+
_in_
U
f
Un
(6)
f ;
sup int sup
Thus, from the last expression, it follows that the accuracy conversion is determined by the accuracy and stability of the gain in the paths of the separate passage of the measured and reference voltages [3].
The measurement time of the voltage support signal has a similar appearance:
2e. • Kp • E
T _T" Pa
U
(7)
where
|u+sup = \Usup\ = |esup| Ksup
esup is the support voltage at the input of the SA;
K is the gain of the support voltage amplifier; T.nt is the time of direct integration of the reference voltage.
Taking into account the fact that \u+ref = \Uref\ = Uref,
we can write:
2e. • Ka • E
T _T f m pa na
T = T "
sup int
Urf
2e in • Ksup Uref
IV. Conclusion
In conclusion, we can draw the following conclusions that the advantage of this flow meter over other known analogs is as follows:
- the ability to filter from various kinds of interference;
- the ability to view the spectrum as the source data for processing, and not just the result;
- the ability to diagnose and obtain information through a computer;
- control over the position of the signal on the frequency scale to determine the output beyond the metrological range;
- devices with digital electronics and its connection to a computer make it possible to carry out remote diagnostics of the measurement process during operation without stopping the flow.
An additional advantage is the availability of feedback. It allows the device to monitor the integrity of its output circuits.
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