RU2307997C1 - Method of adjusting strain-gages - Google Patents

Abstract

FIELD: measuring technique.

SUBSTANCE: method comprises connecting the compensating resistor with maximum permissible temperature resistance coefficient and strain-gage sensitivity close to unity to the circuit for power supply to the bridge measuring circuit, setting the pickup in the connecting pipe, shutting the temperature-dependent compensating resistor with a cross-piece, determining deviation of the output signal from the nominal measured parameter, measuring input resistance of bridge circuit, removing cross-piece, and determining deviation of the output signal of the gage.

EFFECT: enhanced precision.

Description

The invention relates to measuring equipment and can be used to configure strain gauge sensors with a bridge measuring circuit for a multiplicative temperature error.

A known method of compensating for the multiplicative temperature error of a bridge circuit (see "Designing sensors for measuring mechanical quantities" edited by E.P. Osadchogo, 1979), which consists in installing a temperature-dependent compensation resistor R _α in the supply circuit of the bridge measuring circuit, the nominal value of which provides multiplicative compensation maximum temperature error of the sensor manufacturing technology adopted for parallel termonezavisimy which includes an adjustable resistor R _d, and calc those par last through the physical characteristics of the elements that make up the sensor.

However, the use of this method when configuring sensors has several disadvantages. As can be seen from the analytical expression, to determine the value of the thermally dependent compensation resistor, it is necessary to know a number of physical parameters of the elements that make up the sensor. For example, to determine the temperature coefficient of resistance (TCR) α _{to a} temperature-dependent compensation resistor R _α mounted on an elastic element (UE), it is necessary to know such physical parameters as: TCR of the material of the resistor α _r , temperature coefficients of linear expansion (TEC) of the resistor α _n and RE - α _e . To determine the temperature coefficient of sensitivity (TCR) of the sensor α _d, it is necessary to know such physical parameters as: coefficient of strain sensitivity K _about ; TKH strain gauges α _t installed on the UE; the temperature coefficient of the elastic modulus η _{e of the} UE material and others. As a rule, this information can be obtained from certificates for the materials used, however, the existing technological scatter in the manufacture of these materials does not allow the use of this information due to the significant scatter obtained in determining the values of the compensation elements. In addition, if the sensor is made using microelectronic technology (metal-film or semiconductor diffusion sensors), this information is not available at all. Therefore, in order to use analytical expressions, it is necessary to experimentally determine the required parameters of the elements that make up the sensor.

However, direct methods for determining the physical parameters of the compensation elements and the elements of the measuring circuit of the sensor represent a certain complexity.

Firstly, these are purely constructive and technological difficulties associated with the fact that:

- determination of all physical parameters - both elements of the measuring circuit and compensation elements must be carried out in the assembled sensor, since the existing temperature and temperature deformations in the sensor body can cause unacceptably large errors in the determination of compensation elements;

- determination of all physical parameters of individual elements requires, as a rule, violation of electrical connections in the assembled sensor, which can lead to a significant deterioration in metrological characteristics (in particular, sensors made using microelectronic technology, since these connections were established at the very early stages of formation measuring circuit).

Secondly, direct methods of measuring these parameters, in addition to the high complexity, do not provide the required accuracy. So, when measuring TCS strain gages of the order of 1 · 10 ^-5 1 / ° C with an accuracy of 5%, resistance measurement with a nominal value of 1000 Ohms is required at a temperature difference of 50 ° C with an accuracy of 0.025 Ohms, which is a measurement accuracy of 0.0025% and requires application of special measurement methods.

- выходной сигнал при воздействии номинального измеряемого параметра соответствует суммарному изменению сопротивлений плеч мостовой цепи в 10 Ом. Тогда для обеспечения замера суммарного изменения сопротивлений плеч мостовой схемы в 10 Ом через выходные сигналы датчика с точностью 0,025 Ом потребуется использование вольтметра класса не выше 0,25%, что доступно в настоящее время на любом производстве. Увеличение же класса точности используемых приборов позволит повысить точность определения ТКС и, соответственно, точность расчета компенсационных резисторов.Therefore, there was a need to develop a methodology that allows the replacement of direct methods for measuring the physical parameters of structural elements of a sensor by measuring its output signals at various temperatures. This not only simplifies the configuration of the sensors, but also the transition to the implementation of the sensor output signals by at least two orders of magnitude increases the accuracy of measurement, and, accordingly, the calculation and compensation of temperature errors. Indeed, for the example under consideration, when evaluating the TCS of a bridge circuit with a relative output signal

- the output signal when exposed to a nominal measured parameter corresponds to a total change in the resistance of the arms of the bridge circuit of 10 ohms. Then, to ensure the measurement of the total change in the arm resistances of the bridge circuit of 10 Ohms through the sensor output signals with an accuracy of 0.025 Ohms, a voltmeter of class not higher than 0.25% will be required, which is currently available at any plant. An increase in the accuracy class of the instruments used will increase the accuracy of determining the TCS and, accordingly, the accuracy of the calculation of compensation resistors.

In addition, when moving on to assessing the physical parameters of the elements of the measuring circuit through the output signals, another positive property appears. This property is expressed in the possibility of simultaneous compensation of not only the multiplicative temperature error of the sensor from the discrepancy between the TCD strain gages and the temperature coefficient of elastic modulus of REs, but also additional multiplicative temperature errors from the sensitivity of the sensor to the tightening moment. Indeed, when carrying out the temperature adjustment of the sensor with its installation in a technological fitting with a nominal tightening torque, a change in the axial force in the threaded part of the sensor when the temperature changes will directly affect the change in the sensor output signal. That is, the change in the output signal of the sensor will depend not only on the discrepancy between the TCD strain gages and the temperature coefficient of the elastic modulus of the RE, but also on its sensitivity to the tightening torque. If at the same time the TFC of the sensor is estimated through a change in the output signal with a change in temperature, then the calculated value of the TFC of the sensor will take into account the influence of the sensor sensitivity to the tightening torque.

Another significant drawback of this method is the low accuracy of the rating of the compensation resistor when using available analytical expressions for its calculation. To confirm this, consider an example of compensation for the multiplicative temperature error according to the prototype.

Example

.To minimize the multiplicative temperature error of the strain gauge sensor with a bridge measuring circuit, the input resistance of which R _in = 1000 Ohms, the TCS of the input resistance α _m = 0.5 · 10 ^-3 1 / ° С, the temperature coefficient of sensitivity α _d = 1 · 10 ^-3 1 / ° С, thermally dependent compensation resistor R _α = 500 Ohm, and its TCS α _к = 4 · 10 ^-3 1 / ° С, supply voltage U _n = 10 V, temperature range ΔТ = 100 ° С, total relative change in resistance bridge circuit at the nominal value of the measured parameter

.

Decision

The value of the compensation thermally independent resistor R _d to compensate for the multiplicative temperature error can be determined in accordance with the prototype

.

.

When installing a compensation resistor in the power circuit, the output signal from the sensor can be determined by the formula

.

.

When exposed to temperature, the output signal from the sensor can be determined by the formula

Then the multiplicative temperature sensitivity is defined as:

.

.

It can be seen that the existing methods for compensating the multiplicative temperature error do not provide the required accuracy and, as a result, lead to a phased minimization of this error, which greatly complicates the technology of temperature adjustment of the sensor.

The invention consists in the following.

The task to which the claimed invention is directed is to develop a method for tuning strain gauge sensors with a bridge measuring circuit by a multiplicative temperature error, which would improve the manufacturability and accuracy of compensating for a multiplicative temperature error during the setup process.

The technical result is an increase in manufacturability and accuracy in the process of tuning strain gauge sensors with a bridge measuring circuit for a multiplicative temperature error.

The specified technical result is achieved by the fact that:

- the temperature setting of the sensor is carried out after its preliminary assembly and installation in the process fitting with a nominal value of the tightening torque, which allows the compensation process to compensate for additional multiplicative temperature errors of the sensor from sensitivity to the tightening torque when the temperature changes;

- determination of the physical parameters of the elements that make up the sensor is carried out by determining the deviation of the sensor output signals at different temperatures without connecting and when connecting a temperature-dependent compensation resistor;

- the manufacture and installation of a temperature-dependent compensation resistor is carried out according to the manufacturing technology of strain gauges;

- the material from which the technological thermally dependent compensation resistor is made must have a maximum TKS and TKH close to unity, and its nominal value must be such as to compensate for the maximum possible multiplicative temperature error of the sensor made by the adopted technology;

- Calculation of the adjustable thermally independent resistor is performed according to experimentally determined deviations of the sensor output signal from the nominal value of the measured parameter at different temperatures with and without connection of a thermally dependent compensation resistor;

- installation in the sensor of the calculated value of the adjustable thermally independent resistor is carried out in places with the lowest heat flux during operation of the sensor (for example, in the secondary converter).

The method for determining the adjustable thermally independent resistor R _d through the deviation of the sensor output signals consists in determining the TFC of the sensor α _d , the TCS α _{to the} resistor R _α , expressed through the deviations of the sensor output signals, and the TCS α _{m of the} input resistance of the bridge circuit R _I , determined through measurements of the input resistance bridge at normal temperature and operating temperature and the subsequent calculation of the value of the compensation resistor R _d .

Due to the fact that the analytical expression for calculating a fitting thermally independent resistor has insufficient accuracy (see an example), it is first necessary to derive an analytical expression for its calculation depending on the physical parameters of the sensor for given values of R _α , α _k , α _d , α _m .

The output signal from the sensor at a temperature T _o is defined as:

.

.

The output signal from the sensor at temperature T is defined as:

Since the condition for compensation of the multiplicative temperature error is the equality of the sensor output signal at different temperatures, then, equating the last two expressions and writing down the resulting equation with respect to the adjustable resistor R _d , we obtain

The resulting expression allows you to determine the fitting resistor R _d through the physical parameters of the elements and is the main when determining it through the output signals of the sensor.

To fulfill the requirements for evaluating the physical parameters through deviation of the output signals on the UE, directly in the installation zone of the strain gages, install a temperature-dependent compensation resistor R _α , a value known to be larger than necessary to compensate for the multiplicative temperature error of the sensor, for example, for metal film sensors, its value should be within 300-500 ohms. At the same time, the technology for installing a temperature-dependent compensation resistor on the UE should be the same as the technology for installing working strain gages. The place and technology of installation of the resistor R _{α are} determined by the requirement to obtain minimal deviations of its temperature conditions from the working strain gages during operation. The material from which the thermally dependent compensation resistor is made must have a maximum TCS value in order to ensure maximum compensation efficiency for the selected technology of its manufacture and installation on the UE and a strain sensitivity coefficient close to unity, to exclude the effect of deformation at its installation location on the UE. The resistor R _α made in this way is included in the power supply circuit of the bridge measurement circuit.

To determine TCF α _d probe shorted resistor R _α is removed and a jumper deviation values U and U _{vyho vyht} sensor output signal at the nominal value of the measured parameter X _n and temperature T _o and T, respectively.

But, on the other hand, the deviation of the output signals of the sensor under the influence of the measured parameter X _n can be represented as:

- at a temperature T _o

- at temperature T

where S _d - the sensitivity of the sensor to the measured parameter.

Dividing the second equation by the first one, it is possible to determine the TFC of the sensor, expressed through the deviation of the output signals at different temperatures:

TCR of the input resistance bridge circuit can be determined by experimentally removed at temperatures T _a and T values of the input resistance _Rin R and R _vht respectively.

и

при воздействии номинального значения измеряемого параметра Х_н и температурах Т_о и Т соответственно.To determine α _to remove the jumper from the thermally dependent resistor R _α and determine the output signals of the sensor

and

when exposed to the nominal value of the measured parameter X _n and temperatures T _about and T, respectively.

Then the expressions (2) and (3) for the deviation of the output signals of the sensor with the connected thermally dependent compensation resistor R _α will have the form:

- at a temperature T _o

- at temperature T

Dividing the second equation by the first one, substituting expressions (4) and (5) and solving it with respect to α _к , we can obtain the analytical expression of the TCS of the thermally dependent compensation resistor R _α , expressed in terms of the value of the thermally dependent compensation resistor R _α and the deviation of the sensor output from the measured parameter, taken at different temperatures:

Substituting formulas (4), (5) and (8) into equation (1) and solving with respect to R _d , we can calculate the nominal value of the thermally independent resistor to compensate for the multiplicative error, determined through the value of the thermally dependent resistor R _α and the deviation of the sensor output signals taken at different temperatures.

An adjustable thermally independent resistor of the calculated value is installed in parallel with a thermally dependent compensation resistor in such a place in the sensor construction where minimal heat fluxes are present during operation (for example, in a secondary converter). This reduces the effect of temperature on the adjustable thermally independent resistor.

To assess the accuracy of minimizing the multiplicative temperature error of the proposed method, we will calculate the multiplicative temperature sensitivity of the sensor using an example.

Example

To minimize the multiplicative temperature error of the strain gauge sensor with a bridge measuring circuit, the input resistance of which R _in = 1000 Ohms, the thermally dependent compensation resistor R _α = 500 Ohms, the supply voltage U _n = 10 V, the temperature range ΔT = 100 ° C, the total relative change in resistance bridge circuit when exposed to the nominal value of the measured parameter

Decision

To calculate the temperature-dependent compensation resistor R _α, it is necessary to experimentally determine the deviation of the sensor output signals at temperatures T _o and T for two cases:

- without including R _α (U and U _{vyho vyht)} compensating process are temperature dependent resistor;

- when the technological thermally dependent compensation resistor is turned on

Additionally, we assume that the TCS of the input resistance α _m = 0.5 · 10 ^-3 1 / ° С is experimentally determined through measurements of the input resistance of the bridge circuit at temperatures T _about and T.

However, if you simply set these parameters, you can get unrealistic values of the thermally dependent compensation resistor R _α . Therefore, we will calculate the deviation of the output signals based on the real values of the physical parameters of the sensor elements. We set the following parameters: TFC of the sensor α _d = 1 · 10 ^-3 1 / ° С, TKS of a thermally dependent compensation resistor α _к = 4 · 10 ^-3 1 / ° С. We use the accepted values of the physical parameters only to calculate the deviation of the output signals, which must be determined experimentally, and in the future, when calculating the value of the adjustable thermally independent compensation resistor R _d and the multiplicative temperature sensitivity, we will not take them into account, and we will evaluate them in accordance with the proposed methodology .

The deviation of the sensor output signals for these cases according to the accepted physical parameters can be calculated as follows:

;

;

To calculate the value of the adjustable thermally independent resistor, it is necessary to determine the TFC of the sensor and the TCS of the compensation resistor through the deviation of the output signals according to formulas (4) and (8), respectively:

;

;

Using the obtained values of the physical parameters, we calculate by formula (1) the nominal value of the adjustable thermally independent resistor

.

.

From where R _d = 26739.3 Ohms.

To assess the accuracy of compensation in accordance with the accepted method, we calculate the output signals of the sensor at different temperatures after installing the compensation elements in the bridge power circuit in the form of a parallel connection of resistors R _α and R _d = 26739.3 Ohms.

According to the obtained values of the deviation of the output signals after connecting the calculated value of the resistor R _d we determine the multiplicative temperature sensitivity of the sensor

The resulting value of the multiplicative temperature sensitivity of the sensor indicates that the accuracy of the settings for the multiplicative temperature error of the proposed method of compensation increased by more than an order of magnitude compared with the prototype. In addition, the proposed method of compensating for the multiplicative temperature error allows you to simultaneously minimize the additional temperature error of the sensor from the influence of its sensitivity to the moment of tightening.

Claims (1)

An indirect way of adjusting strain gauge sensors with a bridge measuring circuit by a multiplicative temperature error, which consists in installing a temperature-dependent compensation resistor R _α in the power supply circuit of the bridge measuring circuit, the nominal value of which compensates for the maximum multiplicative temperature error of the sensor for the adopted manufacturing technology, in parallel with which a thermally independent resistive fitting R _d, characterized in that produce preliminary assembly dates ika is set in the supply circuit of the bridge measuring circuit temperature-dependent compensation resistor R _α with the maximum possible temperature coefficient of resistance (TCR) and the gauge factor (TCF) is close to unity, which can be accessed by the selected material and manufacturing technology, temperature-dependent compensation resistor manufactured by technology working strain gauges and install on the elastic element in the installation area of the working strain gages, install the sensor in the technological fitting with the nominal value of the torque shorted temperature-dependent compensation resistor R _α crosspiece and determining the deviation of the sensor output signal from the nominal parameter being measured at ambient temperature T _a and the working temperature T - U _vyho and U _vyht respectively and simultaneously measure the input impedance bridge circuit at specified temperatures - R _I and R _I respectively, remove the jumper and again determine the deviation of the sensor output from the nominal measured parameter at normal temperature T _o and operating temperature T -

accordingly, the TFC of the sensor α _d , the TCS of the input resistance of the bridge circuit α _m , the TCS of the thermally dependent compensation resistor α _{k are} calculated and the nominal value of the adjustable thermally independent resistor is determined by solving the quadratic equation

Where

ΔT = TT _o - temperature deviation during the test, install a adjustable thermally independent resistor, of the calculated value, in parallel with the thermally dependent compensation resistor in such a place in the sensor construction where minimal heat fluxes are present during operation, for example, in a secondary converter.