RU2315269C1 - Mode of compensation of additive temperature error of the sensor with a vibrating element - Google Patents

Abstract

FIELD: the invention refers to the field of instrument making and may be used in measuring technique at measuring power, pressure, acceleration etc.

SUBSTANCE: the initial frequencies of the generator are measured. The value of a thermal dependent resistance R_β with a temperature coefficient of resistance α_β located nearby to the vibrating element depends from the temperature of the measuring converter. The scheme of upkeep of vibrations is preliminary tuned at maximum working temperature. The initial level of the frequency of the generator of harmonic vibrations at minimal and maximal working temperatures is fixed. Besides the value of the resistance R₁₊ in the circuit of the inverting adder at maximal working temperature corresponding to the initial frequency of the generator is fixed.

EFFECT: increases accuracy of compensation of additive temperature error of the sensor with a vibrating element in the conditions of stationary temperature regimes.

4 dwg

Description

The invention relates to the improvement of transducers with vibrating elements and can be used in measuring technique for measuring force, pressure, acceleration, etc.

When the ambient temperature changes, the temperature of the sensor changes, which affects the change in the output signal, therefore, an additional measurement error appears. The change in the natural frequency of vibrations of a vibrating element is influenced both by a change in the geometric dimensions of the primary transducer due to the presence of a linear expansion coefficient for any material and to an elastic modulus due to the temperature coefficient of elasticity inherent in any material. A change in the first leads to the appearance of an additive temperature error, and a change in the elastic modulus leads to a multiplicative temperature error.

The simulation using the finite element method of the initial frequency of natural vibrations and the deviation of the frequency of natural vibrations of the vibrating element from the influence of the measured parameter depending on the temperature effect for the design of the vibrating element, presented in US patent No. 4813271 from 03/21/1989, showed the following results:

- at a temperature of minus 60 ° C, the initial frequency of natural vibrations f _-60 was 53019 Hz;

- at a temperature of + 60 ° C, the initial frequency of natural vibrations f ₊₆₀ amounted to 52095 Hz;

- the deviation of the initial natural frequency Δf temperature range ΔT = 120 ° C was 924 Hz.

, или переводя в аддитивную температурную чувствительность

Полученные количественные оценки дополнительной аддитивной температурной погрешности не позволяют использовать подобные конструкции без применения специальных методов компенсации температурной погрешности, в особенности в высокоточных датчиках.If we take the deviation of the frequency of natural vibrations of the vibrating element from the measured parameter (for example, a pressure of 3 atm) f _H = 2500 Hz, then the additional temperature additive error will be

, or translating into additive temperature sensitivity

The obtained quantitative estimates of the additional additive temperature error do not allow the use of such structures without the use of special methods for compensating the temperature error, especially in high-precision sensors.

Currently, the most widely used method of compensating for temperature errors is the introduction of a temperature-dependent element (for example, a thermistor) into the sensor design, from which temperature information is taken, with its subsequent processing and correction of the information signal (for example, US Patent No. 4,724,707 dated 08.20. 1986). However, the use of this compensation method has several disadvantages:

1. An additional temperature measurement channel is introduced.

2. Requires mathematical processing of the signal from the additional channel and the adjustment of the information signal taking into account the additional signal.

3. There is no separate compensation for the additive and multiplicative temperature errors, and as a result, the specified accuracy of temperature measurement is not provided to compensate for the temperature errors.

The biggest disadvantage of this method is the provision of a given accuracy of temperature error compensation. So, for high-precision sensors of class no more than δ≤0.05%, the additive temperature sensitivity should be S _ot ≤0.5 · 10 ^-5 1 / ° С, which is an order of magnitude less than the obtained quantitative estimate of the total temperature error of sensors with a single-crystal vibrating element. Then, to ensure the given accuracy, the channel for measuring temperature should have an error of not more than 0.06 ° С (for the previously considered case of temperature changes in the range of 120 ° С), which is problematic with existing methods of measuring temperature.

According to the operating mode, converters with a frequency output are classified as follows: operating in the mode of free oscillations, self-oscillations and forced oscillations. The proposed method considers converters with a frequency output operating in self-oscillation mode, which include a vibrating element (resonator) and a circuit for maintaining the vibrations of the vibrating element, which consists of a signal receiver for registering vibrations of the vibrating element, the tuning circuit of the generator, harmonic oscillator and system excitation (for example, see Bodner, V.A. Instruments of primary information: Textbook for aviation universities. - M.: Mechanical Engineering, 1981). The operation scheme of such converters is as follows: the signal from the harmonic oscillation generator is supplied to the oscillation excitation system. Since the frequency of the excitation signal is close to the frequency of the natural oscillations of the resonator, the resonator begins to oscillate at one of the harmonics, and the amplitude of the oscillations corresponds to the resonance curve of this resonator. The maximum oscillation amplitude of the resonator is achieved when the frequency of the excitation signal is equal to the frequency of the natural oscillations of the resonator. Oscillations of the resonator are perceived by the receiver of signals, then the captured signal is converted by a normalizing amplifier. The signal from the output of the normalizing amplifier is a harmonic signal of the required amplitude, the frequency of which corresponds to the resonator natural frequency. In addition, the normalizing amplifier can be considered as part of the tuning circuit of the harmonic oscillator. From the output of the normalizing amplifier, the signal previously converted into a constant voltage signal is supplied to the generator, and the frequency of the generator is determined by the value of the supplied signal. Under the influence of external factors (pressure, force, temperature, etc.), the frequency of the natural oscillations of the resonator changes, the value of the feedback signal (control signal) supplied to the harmonic oscillator changes accordingly, the frequency of the generator changes in proportion to the change in the value of the control signal. Since the frequency of the generator becomes equal to the changed frequency of the natural oscillations of the resonator, the oscillations of the resonator again occur with a maximum amplitude in resonance. Thus, the generation mode is due to the supply of a positive feedback signal from the resonator after amplification again to the resonant circuit, while the feedback includes a generator tuning circuit and a voltage controlled harmonic oscillator.

The invention consists in the following.

The problem to which the claimed invention is directed is to develop a method for compensating the additive temperature error of a sensor with a vibrating element with an output signal in the form of a frequency deviation, which would improve the accuracy of minimizing the additive temperature error under stationary temperature conditions.

The technical result consists in increasing the accuracy of minimizing the additive temperature error of a sensor with a vibrating element in stationary temperature conditions.

The specified technical result is achieved as follows:

- the inverting adder is included in the feedback, which adds two signals: a constant voltage to provide the initial frequency of the resonator's own oscillations and an output signal from a detector that converts the sinusoidal voltage coming from the normalizing amplifier into a constant voltage, the value of which is proportional to the amplitude of the sinusoidal voltage;

- the additive component of the temperature error is compensated by introducing a thermally dependent resistance R _β into the inverting adder circuit, which changes the initial level of the generator frequency with temperature so that the frequency of the signal from the generator does not depend on the temperature of the transmitter where the thermally dependent resistance is located, in the absence of the measured parameter .

Figure 1 shows the structural diagram of the Converter: 1 - a harmonic oscillator controlled by voltage, 2 - a vibrating element (resonator), 3 - a normalizing amplifier, 4 - a detector that converts a sinusoidal signal to a constant voltage, 5 - thermally dependent resistance R _β for compensation additive temperature error, 6 - inverting adder, T - external factor - temperature, P - measured parameter, for example pressure. The converter output is a sinusoidal signal whose frequency deviation corresponds to the deviation of the oscillation frequency of the vibrating element 2 from the measured parameter R.

Figure 2 presents the resonance curve, which describes the dependence of the oscillation amplitude X of the vibrating element on the excitation frequency f (in this case, the oscillator frequency). The shape of the resonance curves depends on the characteristics of the material used, in particular, the coefficient of internal friction (damping).

Figure 3 presents a variant of a voltage-controlled rectangular pulse generator, which is implemented on two operational amplifiers of the LM301 type: 7 - a source of control voltage (hereinafter "E ₁ "), 8 - resistance (hereinafter "R _1G "), 9 - thyristor (for example, 2N6087), 10 - capacitance (hereinafter referred to as “C _1G ”), 11, 14 - operational amplifier LM301, 72 - capacitance, 13 - reference voltage source (hereinafter “E ₂ ”), 15 - resistance of 100 kOhm, 16 - resistance of 35 kOhm.

Figure 4 shows an example of an inverting adder: 17 - voltage E ₀ corresponding to the initial level of the generator frequency, 18 - resistance R ₁ , 19 - voltage E _Δ proportional to the oscillation amplitude of the vibrating element, 20 - resistance R ₂ , 21 - feedback resistance R _a, 22 - an operational amplifier 23 - the load resistance R _H.

Consider the method of compensating the additive temperature error on the example of the resonance curve shown in figure 2. At the rated temperature and the absence of the measured parameter, the oscillation amplitude X _{0 of the} vibrating element is amplified by an amplifier and converted to direct voltage by a detector of such a magnitude that the generator frequency f ₀ is equal to the natural frequency of the vibrating element, i.e. vibrations of the vibrating element occur in resonance. With an increase in the measured parameter (for example, pressure) due to the mismatch of the generator frequency f _{0 with the} natural frequency of the vibrating element, which increases with increasing the measured parameter and becomes equal to a certain frequency f ₁ (for which the vibration amplitude of the vibrating element decreases to X ₁ without taking into account the influence of feedback ) The voltage supplied to the input of the voltage-controlled generator decreases. The frequency of the generator in accordance with the change in the control voltage will also change by a value determined by the functional dependence of the frequency of the generator on the control voltage, and becomes equal to the natural frequency f _{1 of the} vibrating element. The operation of the converter at the nominal temperature always corresponds to the vibrations of the vibrating element in resonance (since the natural frequency of the vibrating element coincides with the excitation frequency of the generator controlled by voltage), the amplitude of the vibrations of the vibrating element is constant. The range of operation of the converter is from the initial frequency of natural oscillations f _{0 of the} vibrating element (in the absence of the measured parameter) to the maximum value of the natural frequency of oscillations f _{max of the} vibrating element (under the action of the maximum permissible value of the measured parameter).

On the other hand, a change in the temperature of the measuring transducer in the absence of a measured parameter leads to a change in the frequency of the natural vibrations of the vibrating element, there is a mismatch between the frequency of the generator and the natural frequency of the vibrating element, i.e. the vibrations of the vibrating element do not occur in resonance, which means that the amplitude of the vibrations of the vibrating element decreases in accordance with the resonance curve, see figure 2, and the feedback will work out the change in the frequency of the generator signal. This leads to the appearance of an additional additive temperature error. To reduce this error, a decrease in the amplitude of vibrations of the vibrating element with a change in temperature should not affect the signal frequency at the output of the voltage-controlled generator, i.e. the generator frequency should remain the same. This is achieved by introducing a thermally dependent element (for example, resistance or capacitance) into the control circuit of the generator and selecting a functional connection between the change in the value of the thermally dependent element and the temperature of the converter so that the voltage supplied to the input of the voltage-controlled generator remains constant when the temperature changes, this ultimately allows you to compensate for the additive temperature error.

Therefore, the vibrations of the vibrating element occur in resonance at any value of the measured parameter, but only at the nominal temperature. When the temperature changes, the initial frequency of the generator will remain constant and will no longer correspond to the frequency of natural vibrations of the vibrating element. This leads to a decrease in the amplitude of the vibrations of the vibrating element without affecting the measured parameter (see point A, which corresponds to resonance, and A ', which correspond to the case of a mismatch between the generator frequency and the frequency of natural vibrations of the vibrating element when the temperature changes). And since the output signal is the frequency, the change in amplitude does not affect the measurement accuracy.

The disadvantage of this method is the occurrence of uncertainty when the temperature changes in the absence of a measured parameter, namely that the amplitude of the vibrating element changes when the temperature changes in accordance with the resonance curve, see points A ', i.e. decreases both with decreasing and with increasing temperature. Then the direction of the change in the frequency of natural vibrations of the vibrating element cannot be determined by the sign of the change in the amplitude of the oscillations, and therefore, by the sign of the change in the control voltage. In this case, it is necessary to use only one branch of the resonance curve: the left - from the frequency f _min to the frequency f ₀ or the right - from the frequency f ₀ to the frequency f _max . In the case of using the left branch of the resonance curve, the frequency f _min corresponds to the minimum initial operating frequency of the resonator, i.e. at the maximum temperature of the converter and in the absence of a measured parameter, and the frequency f ₀ corresponds to the maximum operating frequency of the converter, i.e. at the minimum temperature of the converter and at the nominal value of the measured parameter. In the case of using the right branch of the resonance curve, the frequency f ₀ corresponds to the minimum initial frequency of the generator, i.e. at the maximum temperature of the converter and in the absence of a measured parameter, and the frequency f _max corresponds to the maximum operating frequency of the converter. The choice of the branch of the resonance curve, along which the transducer subsequently works, depends on the conversion function of the harmonic oscillator.

An example of a voltage-controlled square-wave generator is shown in FIG. 3, which is implemented on two operational amplifiers of the LM301 type. The conversion function of this generator, which gives the connection of the generation frequency f with the control voltage E ₁ , has the form:

The inverting adder, see figure 4, has the following conversion function:

where U is the output voltage;

R _OC is the resistance in the feedback of the operational amplifier;

E ₀ - input voltage corresponding to the initial level of the oscillator generation frequency;

R ₁ - resistance in the circuit E ₀ ,

E _Δ is the input voltage proportional to the amplitude of the vibrations of the vibrating element;

R ₂ is the resistance in the circuit E _Δ .

As can be seen from expression (2) by changing the nominal values of the resistances R ₁ , R ₂ and R _OC, you can adjust the output voltage of the adder, and hence the frequency of the generator. In this case, the output voltage of the inverting adder U is the control voltage E _{1 of} the rectangular pulse generator.

Consider the case when the pressure-dependent element is the resistance R _{1 of the} inverting adder to compensate for the additive temperature error.

In accordance with the generator conversion function (1), the frequency f increases with increasing control voltage E ₁ ; therefore, an inverting adder must be introduced into the generator control circuit, which allows increasing the control voltage E ₁ with decreasing vibration amplitude of the vibrating element, which corresponds to an increase in the natural vibration frequency. In this case, the range of operation of the converter will be the right branch of the resonance curve. In this case, the generator is tuned in resonance with the vibrating element at the maximum operating temperature and the absence of the measured parameter, since at the maximum operating temperature the natural frequency of the vibrating element is minimal. Then, with an increase in the measured parameter, the frequency of natural vibrations of the vibrating element increases, which leads to a decrease in the amplitude of the oscillations in accordance with the resonance curve. This change in the amplitude of the vibrations of the vibrating element is worked out by positive feedback and the frequency of the generator again becomes equal to the frequency of the natural vibrations of the vibrating element. Thus, the vibrations of the vibrating element occur in resonance, but only for the temperature at which the oscillation maintenance circuit was set up, i.e. for maximum operating temperature. When the temperature changes, the vibrations of the vibrating element do not occur in resonance, but since the output signal of the converter is a dependence of the frequency deviation on the measured parameter, the change in the amplitude of the vibrations of the vibrating element does not affect the measurement accuracy.

We introduce the following notation:

- the initial frequency of the generator f ₀₊ at the maximum operating temperature:

where E ₁₊ is the control voltage supplied to the generator at the maximum operating temperature and the absence of the measured parameter;

- the initial frequency of the generator f _0- at the minimum operating temperature:

where E _1- is the control voltage supplied to the generator at the minimum operating temperature and the absence of the measured parameter;

- the control voltage E ₁₊ supplied to the generator at the maximum operating temperature and the absence of the measured parameter is determined by the following formula in accordance with (2):

where R ₁₊ - resistance R ₁ at maximum operating temperature;

E ₀ is the voltage value corresponding to the initial level of the generator frequency at the maximum operating temperature, i.e. which is obtained when tuning the circuit to resonance, this voltage is constant for all operating modes;

E _{Δ +} - voltage value proportional to the amplitude of vibrations of the vibrating element at the maximum operating temperature and the absence of the measured parameter;

- the control voltage E _1- supplied to the generator at the minimum operating temperature and the absence of the measured parameter is determined by the following formula:

where R _1- is the resistance R ₁ at the minimum operating temperature;

E _Δ is the voltage value proportional to the amplitude of the vibrations of the vibrating element at the minimum operating temperature and the absence of the measured parameter.

The condition for compensating the additive temperature error is that the frequency of the generator remains constant when the temperature changes, i.e. f ₀₊ = f _0- , which is equivalent to maintaining the magnitude of the control voltage supplied to the generator with a change in temperature E ₁₊ = E _1- :

To obtain the required nominal value of the compensation element and the required magnitude of the change in the nominal value of the compensation element when the temperature changes, in most cases a serial (or parallel) connection of a thermally independent element and a thermally dependent element is used. Therefore, the resistance R ₁ consists of thermally independent resistance R _1H and thermally dependent resistance R _β connected in series, and the total resistance is defined as R ₁ = R _1H + R _β . In this case, the value of the thermally dependent resistance R _β to compensate for the additive temperature error varies with temperature according to the dependence R _βt = R _β · (1 + α _β · ΔT), where R _β is the initial value of the thermally dependent resistance, i.e. the resistance value at normal temperature, α _β is the temperature coefficient of resistance (TCR) of the thermally dependent resistance R _β , ΔT is the temperature range.

We introduce the following notation: the value of the thermally dependent resistance R _β at the maximum working temperature R _{β +} = R _β (1 + α _β · ΔТ ₁ ), where ΔT ₁ is the range of temperature changes from normal to maximum working temperature; the value of the thermally dependent resistance R _β at the minimum operating temperature R _β- = R _β (1 + α _β · ΔТ ₂ ), where ΔТ ₂ is the range of temperature changes from normal to minimum operating temperature.

Since the configuration of the circuit under consideration is performed at the maximum operating temperature, the resistance value R ₁ obtained from the preliminary setup is equal to R ₁₊ = R _1H + R _{β +} .

In the expression (7) we substitute the resistance R ₁₊ , R _1- and, simplifying the expression obtained:

from which we obtain the following quadratic equation:

Where

a = (E _Δ- -E _{Δ +} ) · (1 + α _β · ΔТ ₁ ) · (1 + α _β · ΔТ ₂ ),

b = E ₀ · R ₂ · α _β · (ΔT ₁ -ΔT ₂ ) + E _Δ- · R _1H · (2 + α _β · (ΔT ₂ + ΔT ₁ )) - E _{Δ +} · R _1H · (2 + α _β · (ΔT ₂ + ΔT ₁ )),

Solving a system of equations

find the required value of the thermally dependent resistance R _β based on the structurally obtained value of the TCS α _β . For semiconductor materials, a negative TCS can be obtained, the absolute value of which lies in the range 0.003 ÷ 0.2 1 / ° С, and for the nichrome Х20Н80 used in the formation of sprayed resistances, the TCS does not exceed 1 · 10 ^-3 1 / ° С, for the alloy Х20Н75Ю TCS does not exceed 5 · 10 ^-4 1 / ° С, for an alloy based on silicides P65XC TCS does not exceed 1 · 10 ^-4 1 / ° С. Since the required nominal value of the thermally dependent resistance R _β always differs from the nominal value of the actually sprayed resistance, it is made of a known lower value with its subsequent adjustment of the nominal value to the calculated value, for example, using electroerosive or laser fitting methods.

Example.

Пусть величины сопротивлений R_OC=R₁₊=R₂=1 кОм. Предположим, что выходное напряжение с детектора составляет Е_Δ+=0.5 В (величина выходного напряжения зависит от схемной реализации детектора, выходное напряжение которого пропорционально амплитуде входного напряжения). Тогда требуемое значение напряжения Е₀ в соответствии с последним выражением равно минус 2.5 В для получения управляющего напряжения E₁=2 В. Для получения частоты генератора 53019 Гц при минимальной рабочей температуре величина выходного напряжения с детектора должна быть равна

To obtain a generator frequency of 52095 Hz, which corresponds to the initial frequency level at the maximum operating temperature of the converter in question, we select the following values of the generator elements in accordance with the conversion function (1): resistance R _1G = 1 kOhm, capacitance C _1G = 7.678 nF, reference voltage E ₂ = 6 V, control voltage E ₁ = 2 V. Then, in accordance with the expression (2)

Let the resistance values R _OC = R ₁₊ = R ₂ = 1 kOhm. Suppose that the output voltage from the detector is E _{Δ +} = 0.5 V (the magnitude of the output voltage depends on the circuitry of the detector, the output voltage of which is proportional to the amplitude of the input voltage). Then, the required voltage value E ₀ in accordance with the last expression is minus 2.5 V to obtain a control voltage E ₁ = 2 V. To obtain a generator frequency of 53019 Hz at a minimum operating temperature, the value of the output voltage from the detector must be equal

Then, solving the system of equations (9), the required value of the thermally dependent resistance R _β and the thermally independent resistance R _{1H is found} , based on the parameters of the circuit during presetting and the value of TCS α _β obtained in the manufacture. For the case under consideration, the system of equations (9) has a solution for a negative TCS α _β . Suppose that in the manufacture of TCS α _β turned out to be minus 0.003 1 / K. Then the required resistance value in accordance with the system of equations (9) R _β = 39.981 Ohms (at normal temperature), R _1H = 964.817 Ohms.

Substituting the found resistance values into expression (7):

видим, что управляющее напряжение Е₁ при максимальной и минимальной рабочих температурах равно 2 В, что соответствует частоте генератора 52095 Гц, т.е. начальный уровень частоты генератора не зависит от температуры, что означает компенсацию аддитивной температурной погрешности при максимальной и минимальной рабочих температурах.

we see that the control voltage E ₁ at maximum and minimum operating temperatures is 2 V, which corresponds to a generator frequency of 52095 Hz, i.e. the initial level of the generator frequency is independent of temperature, which means compensation of the additive temperature error at the maximum and minimum operating temperatures.

In this way, the additive temperature error of the transducer under consideration with the vibrating element is compensated.

Claims (1)

A method for compensating the additive temperature error of a sensor with a vibrating element, which includes a voltage-controlled harmonic oscillator, a normalizing amplifier, a detector, an inverting adder in the positive feedback circuit and a temperature-dependent resistance with a known temperature coefficient of resistance (TCS) α _β installed directly in the zone vibrating element, and a frequency output signal, characterized in that the TCS of the installed thermally dependent resistance should be from itsatelnym and its denomination should be clearly less than the nominal temperature dependent resistance R _β, necessary to compensate for the additive temperature error produce presetting circuit maintaining the vibrating member oscillations at the maximum operating temperature, fixed initial frequency generator harmonic oscillations at the minimum and maximum operating temperatures, and R ₁₊ resistance value in the circuit of the inverting adder with the maximum operating temperature are required The values are temperature dependent resistance R _β and termonezavisimogo resistance R _1H, solving the system of equations

where a = (E _Δ- -E _{Δ +} ) · (1 + α _β · ΔТ ₁ ) · (1 + α _β · ΔТ ₂ ); b = E ₀ · R ₂ · α _β · (ΔT ₁ -ΔT ₂ ) + E _Δ- · R _1H · (2 + α _β · (ΔT ₂ + ΔT ₁ )) - E _{Δ +} · R _1H · (2 + α _β · (ΔT ₂ + ΔT ₁ ));

- the magnitude of the voltage from the detector, proportional to the amplitude of the vibrations of the vibrating element at the minimum operating temperature and the absence of the measured parameter;

- the magnitude of the voltage from the detector, proportional to the amplitude of the vibrations of the vibrating element at the maximum operating temperature and the absence of the measured parameter; E ₀ is the input voltage of the inverting adder corresponding to the initial level of the oscillator generation frequency; R _1H is a thermally independent resistance, connected in series with a thermally dependent resistance R _β , in the inverting adder circuit, corresponding to the initial level of the generator frequency; R ₁₊ is the resistance value in the inverting adder circuit, selected when pre-setting the circuit to maintain oscillations at maximum temperature, corresponding to the initial level of the generator frequency; ΔТ ₁ - the range of temperature changes from normal to maximum operating temperature; R _1G - resistance in the circuit of the harmonic oscillator; C _1G - capacity in the circuit of the harmonic oscillator; R _oc is the resistance in the feedback of the operational amplifier of the inverting adder; R ₂ is the resistance in the inverting adder circuit; E ₂ is the reference voltage of the harmonic oscillator; f ₀₊ is the initial level of the frequency of the harmonic oscillator at the maximum operating temperature; ΔТ ₂ - the range of temperature changes from normal to maximum operating temperature; f _0- - the initial level of the frequency of the harmonic oscillator at a minimum operating temperature, make the nominal value of the thermally dependent resistance R _β to the calculated value, connect the thermally dependent resistance R _{β in} series with the thermally independent resistance R _1H in the inverting adder circuit.

Images