Method of compensation of temperature errors of magnetostrictive level instrument



This work considers factors that cause temperature measurement errors of the magnetostrictive level instrument. It considers the ways to reduce temperature errors caused by simultaneous change of electrical and mechanical parameters of the waveguide. There is also a block diagram of the method of compensation of temperature errors and the description how it works.

Keywords: magnetostriction, waveguide, domains, ultrasound, measurement, compensation, error.

One of the important disadvantages of the known magnetostrictive level instruments (MLI) is the inadequacy of compensation of measuring temperature control since it is a non-linear error [1-4]. Besides, the effect of gas environment (GE) and liquids temperature is not considered, since the influence can be different, for example, in summer and winter seasons, or during the night and day and daily changes affect the electrical resistance of the waveguide, which is located in two environments. As the theoretical and experimental studies [5] proved, the change in temperature greatly affects the length ${{l}_{P}}$ of the waveguide, which is changed according to the formula

\[{{l}_{P}}={{t}_{P}}{{v}_{0}}\sqrt{1+2\frac{{{\alpha}_{l}}\left( {{}_{}}-{{}_{0}} \right)}{1+{{\alpha }_{l}}\left( {{}_{}}-{{}_{0}}\right)}},\quad\quad\quad(1)\]

where ${{t}_{P}}$ - is the time of the movement of electric current impulse (ECI) from the generator to the electromagnetic transformer (EMT), which floats on the surface of the liquid;

${{v}_{0}}$ - the velocity of ECI movement along the magnetostrictive waveguide;

${{\alpha }_{l}}$ - the temperature coefficient of the waveguide elongation;

${{}_{}}$ - the temperature of the environment where the waveguide is located;

${{}_{0}}$ - nominal environment temperature.

Reducingthe temperature error is made due to the time of ECI passing of the distance from the generator ECI to EMT, which is on the surface of the liquid, and ultrasonic torsion pulse (UTP) from EMT to ultrasound receiver (USR) with compensation of the nonlinear temperature influence and compensation of the temperature error caused by uneven GE and fluid temperature changes [6,7]. Unlike gauges, whose temperature sensors are located along the length of the waveguide, and amendment to the temperature error of normalized values is formed according to the average readings of the sensors [6,7], the proposed MLI temperature sensors are located along the waveguide so that near the float with EMT at least two temperature sensors are located, one of which measures the temperature of the GE, and another – of the fluid. According to output signals of the measurement information processing unit (MIPU) the average of the temperature is calculated, and subsequently - the adjusting signal to the measurement result. This allows to reduce the temperature error of MLI by 2 times due to the influence of the temperature of the GE on the work of EMT, reduce the temperature error of MLI due to the influence of GE and fluid temperature by 2.5 times on the change of the active resistance of the waveguide and reduce static nonlinearity characteristics when the temperature of the liquid is from minus 400С up to 1200С. Fig. 1 shows the MLI circuit, namely: 1 - the technological apparatus; 2 - waveguide; 3 - the surface of the liquid; 4 - UTP;5 - ECI; 6 - UTP receiver; 7 - ECI amp; 8 - generator; 9 - microprocessor (MP) 10 - meter clock; 11 - UTP receiver amplifier; 12 - adder; 13 - comparator; 14- signal processing unit of the thermocouples (SOUT) 15 - reference voltage source (RVS) 16 - permanent magnet; 17 - float; 18 - protective tube; 19 - shoe.

Picture1 - MLI with compensation of temperature error

Unlike others, the proposed MLI uses chromel-copel thermocouple that are evenly placed along the length of the waveguide so that the distance between both of them in GE and in the fluid is the same and equal to the amount of 5 to 20 percent depending on the length of the waveguide. All thermocouples are arranged in certain chain and periodically they are subject to the survey, and their signals in the form of thermal electromotive force $E(T)\ $ in the proper order are written down in the block 14. Thermocouples 11-13 are located in theGE and 21-23 thermocouples are located in liquid environment. Thus separately thermocouple signals are formed in GE and in liquid. Thermocouple signals of 11 and 21 are sent to SOUT to form amendments due to the changes in shift module, material density and its linear waveguide extension. Signals of the thermocouples 11-13 and 21-23 form the amendment due to the temperature change of the active resistance of the waveguide.

The number of thermocouples that are located in GE and liquid depend on the level. The countdown of the thermocouples begins on the position of the float of the EMT, i.e. the level towards GE or liquid. Thermal electromotive force ${{E}_{1\Gamma}}\left( \right)$ of the thermocouple that is in GE and ${{E}_{1}}\left( \right)$ of the thermocouple that is in fluid are coming to MIPU, where their difference is determined $\Delta {{E}_{1}}\left( \right)={{E}_{1\Gamma }}\left( \right)-{{E}_{1P}}\left( \right)$. Based on the results of measuring the temperature with thermocouples thermoelectric averages are determined. According to the calculated average values ${{\bar{E}}_{\Gamma }}\left( \right)$ and ${{\bar{E}}_{}}\left( \right)$the average temperature of the waveguide is determined. Since during the calibration of MLI the GE and fluid temperature, and accordingly the waveguide were equal to some normalized temperature ${{}_{0}}$, the temperature deviation of the waveguide, which causes the temperature error:

\[\Delta{{\bar{T}}_{}}={{T}_{x0}}\left[ \left( 1-{{{\bar{\delta }}}_{\Gamma }}\right)-{{{\bar{\delta }}}_{C}}{{l}_{P}}/{{l}_{x}} \right],\quad\quad\quad(2)\]

where ${{\bar{\delta }}_{\Gamma }}=\Delta{{\bar{T}}_{\Gamma }}/{{T}_{x0}}$ - relative average change in GE temperature;

${{\bar{\delta }}_{C}}=\Delta{{\bar{T}}_{C}}/{{T}_{x0}}$ - relative change in the average temperature of the liquid.

According to the calculated value of temperature error $\Delta {{\bar{T}}_{}}$ the correction is formed as an electrical signal that goes to MIPU. MLI works as follows. After MLI starts working MP 9 issues control signals to the generator 8 which generates ECI. After amplification in the amplifier 7 ECI is sent to the input waveguide 2. Simultaneously the electric measuring circuit (EMC) is lowered to the zero state and meter clock (MC) 10 is switched on. Thus, ECI, moving along the waveguide 2 at the speed ${{v}_{0}}$ reaches the area of magnetic field of the permanent magnet 16, which is floating on the surface 3 of the float 17. At the same time the electrodynamic force (EDF) appears in EMT, which changes the position of the waveguide material domains. Once the ECI passes the area of the magnetic field of the EMT, the domains start the damped oscillatory process with ultrasonic frequency (USF), thus forming UTP 4. Since the vibrations of the domains take place in the magnetic field of a permanent magnet, there is also an induction current with USF. Since UTP is repelled by the magneticfield, its motion is opposite to the movement of the ECI. This means that the UTP momentum returns back to the beginning of the waveguide 2. At its entrancea USR receiver 6 is located, which converts UTP in electrical voltage which goes to the input of the amplifier 11 and then into the adder 12. The latter is designed to sum up USR signal and temperature probes signal corrections which is formed with the block 14. From the unit adder the value of the measuring signal is sent into the comparator 13, where it is compared with a reference voltage - RVS 15. In case of equality of these voltages MC 10 stops its operation. According to the number of measures impulses the time is determined by which the current value of the liquid is calculated. After the received measurement result is saved in MIPU, a new measurement cycle starts.

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  • Рецензент: д.т.н., проф., зав. каф. Комп'ютерно-інтегрованих систем управління Східноукраїнського національного університету ім. В. Даля Й.І. Стенцель
May 26, 2016