RU2298147C1 - Mode of tuning of resistive-strain sensors with a bridge measuring circuit according to an additive temperature error - Google Patents

Abstract

FIELD: the invention refers to measuring technique and may be used at tuning of resistive-strain sensors with a bridge measuring circuit according to an additive temperature error.

SUBSTANCE: the essence is in determining of an arm to which a balancing resistor after tuning of a gauge, and temperature coefficient of resistance of a compensating thermo dependent and balancing resistor would be connected in dependence of their manufacturability and the place of their installation in the gauge. The value of the compensating thermo dependent resistor is calculated taking into consideration the following balancing of the bridge circuit with the balancing resistor with chosen temperature coefficient of resistance. After installation of the compensating resistor into the arm of the bridge scheme balancing of the bridge scheme is made by connecting of the balancing resistor with given temperature coefficient of resistance in-series with a working resistive-strain sensor with its following installation in a previously determined place in the gauge. The temperature coefficient of resistance of the balancing resistor is chosen in dependence of manufacturability and the place of its installation in the gauge.

EFFECT: increases manufacturability, reliability and accuracy in the tuning process.

Description

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

There is a method of compensating the additive temperature error of the bridge circuit (see Patent for invention RU 2265802 C1, G01B 7/16 "Method of tuning strain gauge sensors with a bridge measuring circuit for the additive temperature error", registered December 10, 2005), which consists in pre-balancing the bridge circuits within ± 0.5% of the nominal output signal, determining the temperature coefficient of resistance (TCS) of all shoulders of a balanced bridge circuit and installing in a specific arm, in series with the working strain gauge, compensation thermally dependent resistor R _β calculated value with subsequent balancing of the bridge circuit without changing the TCS of the balanced arm.

However, the use of this method when configuring sensors has a number of disadvantages, since balancing the bridge circuit without changing the TCS of the resistance of the balanced arm can be achieved either by selecting a balancing resistor R _b with a TCS equal to the TCS of the balanced arm, or by laser or EDM adjustment of the strain gauge at microelectronic performance of the sensor.

In the first case, the equality of the TCS of the working strain gauge and the balancing resistor is extremely difficult for the following reasons:

1. To eliminate the temperature gradient between the working strain gage and the balancing resistor, the latter must be installed directly on the elastic element in the installation zone of the strain gages, which greatly complicates the design of the sensor.

2. To select the TCS of the balancing resistor equal to the TCS of the working strain gauge with an accuracy of at least ± 1% is possible only when the balancing resistor is made of material and using the technology of the working strain gauge. However, even in this case, obtaining the equality of the TCS of the balancing resistor and the working strain gauge with a given accuracy is a problematic task, since this is possible either using selective selection, which significantly increases the complexity and, consequently, the cost of the sensor, or using high-quality equipment with high technological process , which also leads to an increase in the cost of the sensor. Indeed, even with a thin-film design of strain gauges in a single vacuum cycle from a single sample, the spread of the TCS on one elastic element reaches ± 10% or more, which leads to the need to compensate for the additive temperature error.

In the second case, the invariance of the TCS of the balanced shoulder of the bridge circuit is achieved due to laser or electroerosive adjustment of the strain gauges of the sensors in microelectronic design, which is quite simple. However, this technology for fitting strain gages has its drawbacks:

- significantly increases the temporary instability of the modified strain gauge due to violation of the surface layer of the film;

- significantly reduced reliability due to the occurrence of microcracks in the cutting zone, which quickly develop when exposed to deformations of the elastic element during operation.

The effect of the mismatch of the TCS of the balancing resistor of the TCS of the working arm on the additive temperature error will be considered as an example.

Example

Determine the change in the additive temperature sensitivity of the strain gauge sensor with an equal-arm bridge measuring circuit when balancing the sensor with a resistance with a TCS different from the TCS of the balanced arm, if it is known:

- shoulder resistance R = 1000 Ohms;

- TCS strain gages respectively α ₁ = 4,5 · 10 ^-4 1 / ° C; α ₂ = α ₃ = α ₄ = 4.0 · 10 ⁻⁴ 1 / ° C;

- TCS compensation thermally dependent resistor α _β = 40 · 10 ^-4 1 / ° C;

;- the total relative change in the resistance of the bridge circuit from the nominal value of the measured parameter

;

- TCS balancing resistor α _b = 3.5 · 10 ^-4 1 / ° C;

- the range of temperature changes Δt = 100 ° C;

- the permissible value of the additive temperature sensitivity of the sensor after adjustment should be within S _ot = ± 1 · 10 ^-4 1 / ° С.

Decision

According to Patent RU 2265802 C1, G01В 7/16, assuming that in the initial state the bridge measuring circuit is balanced (according to the condition of the task, all shoulders are equal), and since (α ₁ + α ₄ ) - (α ₂ + α ₃ ) = 0 5 · 10 ^-4 1 / ° C, then with increasing temperature the initial level of the output signal will change in a positive direction. Therefore, a compensating thermally dependent resistor must be included in the shoulders perceiving compression deformation from the measured parameter, that is, in series with strain gauges R ₂ or R ₃ . Then the balancing must be done by turning on the resistor R _{b in} series with the strain gages R ₁ or R ₄ . Choose the arm of the compensation thermally dependent resistor R ₂ , and the arm of the balancing resistor R ₁ . The value of the compensation thermally dependent resistor can be calculated by the formula

For the case of balancing the bridge circuit without changing the TCS of the arm R _1, its nominal value will be R _1total = R ₁ + R _b = 1000 + 14.0845 = 1014.0845 Ohm, where R _b = R _β is a balancing resistor with α _b = α ₁ = 4.5 · 10 ^-4 1 / ° C. Then you can evaluate the temperature change in the initial level of the output signal and the additive temperature sensitivity of the sensor:

1. Without configuration

- initial sensor output at normal temperature

- initial sensor output signal at Δt = 100 ° С

- change in the initial output signal from temperature changes

ΔU _ot = U _ot -U _o = 12,225 · 10 ^-4 U _n ,

- additive temperature sensitivity of the sensor

where U _n is the voltage of the sensor;

k = R ₁ / R ₂ = R ₃ / R ₄ is the symmetry coefficient of the bridge circuit;

U _nom - the nominal output signal of the sensor at the nominal value of the measured parameter.

2. After setting

- initial sensor output at normal temperature

U _o = 0,

- initial sensor output signal at Δt = 100 ° С

- additive temperature sensitivity of the sensor

S _ot = 0

Thus, the adjustment method used allows one to obtain full compensation of the additive temperature error. However, the technological accuracy of manufacturing compensation thermally dependent and balancing resistors allows you to get the values of these resistances within R _β = R _b = 14.1 ± 0.05 Ohms.

Take R _β = 14.15 Ohms and R _b = 14.1 Ohms.

In this case: U _o = -0.12 · 10 ^-4 U _p ; U _ot = 0.03 · 10 ^-4 U _p ; ΔU _ot = 0.15 · 10 ^-4 U _p ; S _ot = 0.6 · 10 ^-4 1 / ° C. The results obtained indicate the need for high-precision performance of the values of the compensation and balancing resistors.

For the case of balancing the bridge circuit by resistance with a TCS different from the TCS of the balanced arm, in accordance with the initial data without taking into account the technological capabilities of manufacturing resistors.

In this case: U _o = 0; ΔU _ot = U _ot = 0.332 · 10 ^-4 U _p ; S _ot = 1.33 · 10 ^-4 1 / ° C. Taking into account the technological capabilities of manufacturing compensation and balancing resistors will give even greater temperature uncompensation of the additive error, which can be estimated within S _ot = ± 2.0 · 10 ^-4 1 / ° С.

Thus, the spread of the TCS of the balancing resistor and the working strain gauge even within 10% does not allow us to obtain the required degree of compensation of the additive temperature error. This leads to the conclusion that it is necessary to develop a method for compensating the additive temperature error taking into account the different values of the TCS of the balancing resistor and the strain gauge of the balanced arm.

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 for an additive temperature error, which would improve the reliability, manufacturability and accuracy of compensating additive temperature errors during the setup process.

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

The specified technical result is achieved by the fact that the calculation of the compensation resistor is performed for a pre-balanced bridge circuit from the condition of equalizing the sums of TCS strain gages that are pairwise located in opposite shoulders of the bridge circuit taking into account changes in the TCS of the balanced arm when the balancing resistor with TCS is connected in series with the strain gauge different from the TCS of the strain gauge .

This is achieved by the fact that after determining the connection arm of the compensation thermally dependent resistor, the connection arm of the balancing resistor is determined from the condition of the sign of the initial output signal when the compensation thermally dependent resistor is connected. When a compensating thermally dependent resistor is connected to the arms receiving tensile strain (arm R ₁ or R ₄ ), the initial output signal will change in the positive direction, then the balancing resistor must be included in the adjacent arms relative to the arm connecting the compensating resistor, that is, to the arms that receive compression deformation ( shoulder R ₂ or R ₃ ). When a compensating thermally dependent resistor is connected to the shoulders receiving compression deformation (shoulder R ₂ or R ₃ ), the initial output signal will change in the negative direction, then the balancing resistor must be included in the adjacent shoulders relative to the connecting arm of the compensating thermally dependent resistor, that is, to the shoulders receiving the tensile strain (shoulder R ₁ or R ₄ ).

The calculation of the nominal compensation temperature-dependent resistor is based on the fact that the additive temperature error of the balanced bridge circuit depends only on the equality of the sums of TCS strain gages, located in pairs in opposite shoulders of the bridge circuit. In this case, the TCS of the arms are calculated, to which the compensated thermally dependent and balancing resistors are connected, since their connection changes the TCS of the same arms, and taking into account the changes in their TCS, the nominal value of the compensated thermally dependent resistor is calculated for the specified values of the TCS of the compensated thermally dependent and balancing resistors.

After connecting the calculated value of the thermally dependent compensation resistor with the specified TCS to the previously selected arm, the bridge circuit is balanced by connecting the balancing resistor with the specified TCS to the previously selected arm. The value of the balancing resistor is determined experimentally, for example, after connecting a variable resistor to the selected arm.

Depending on the design of the sensor, the balancing resistor can be installed either directly on the elastic element (RE) in the installation zone of the working strain gages, or outside it, in the absence of temperature influence of the measured medium (for example, in the secondary converter). This imposes its specific requirements on both the design and the choice of a TCS balancing resistor. When installing temperature-dependent compensation and balancing resistors (compensation elements) on the UE, the following requirements are imposed on them:

- to exclude the temperature gradient between the working strain gages and the compensation elements, the latter should be located on the elastic element in the installation zone of the working strain gages and be manufactured using the same technology as the working strain gages;

- in order to obtain the maximum compensation efficiency for the additive temperature error, the TCS of the compensating thermally dependent resistor should have the maximum possible value for the selected manufacturing technology.

- to obtain maximum compensation efficiency for the additive temperature error of the TCS of the balancing resistor, α _b must be within the TCS of the working strain gages α _i , that is, within the limits of α _b = 0 ÷ α _i .

If the TCS of the balancing resistor goes beyond the specified limits, then the compensation efficiency will decrease sharply. Indeed, since the connection of a balancing resistor with a lower TCS than α _i leads to a decrease in the TCS of the bridge arm compared to the TCS of the strain gage, it is necessary to use a compensation thermally dependent resistor of a lower nominal value, i.e., the compensation efficiency increases. When the TCS of the balancing resistor is greater than α _I , the total TCS of the arm increases, which leads to an increase in the compensation thermally dependent resistor, that is, the compensation efficiency decreases, and the nominal value of the compensation thermally dependent resistor can reach unacceptably large values. When using a balancing resistor with a negative TCR value, an overcompensation of the additive temperature error can occur and even the moment of changing the arm of the inclusion of a compensation thermally dependent resistor may occur.

For the second case of the design of the balancing resistor, when a temperature gradient is necessarily present between the working strain gages and the balancing resistor, there is completely no requirement for the manufacturing technology of the balancing resistor. However, there is a strict requirement for TCS, which should be as close to zero as possible and should be no more than ± 0.5% α _i .

The method is as follows.

In the proposed method, as in the case of compensation according to the prototype, compensation is achieved by aligning the TCS strain gages included in the opposite shoulders of the bridge measuring circuit while balancing the bridge circuit.

It is based on the fact that a change in the initial level of the output signal of a balanced bridge circuit depends only on the spread of the TCS resistors. This means that to compensate for the additive temperature error, it is necessary to fulfill the balance condition of the bridge circuit under the influence of temperature, which will be determined by the equality of the sums of TCS of the opposite shoulders (α ₁ + α ₄ ) - (α ₂ + α ₃ ) = 0, that is, it is necessary to align the TCS shoulders of the bridge chain.

But since the series connection of compensation elements to the working strain gauges changes the TCS of the arms to which they are connected, then to perform the temperature balance of the bridge circuit while balancing it, it is necessary to calculate the TCS of these arms from the balance condition of the bridge circuit.

Here is the derivation of the expression for calculating the nominal value of the compensation thermally dependent resistor R _β connected to the shoulder R ₁ , while balancing the bridge circuit balancing resistor R _b connected to the shoulder R ₂ .

When the compensation resistor R _{β is} connected in series to the strain gauge R _{1, the} total arm resistance becomes

R _1total = R ₁ + R _β .

Change in total shoulder resistance with temperature

ΔR 1 _total = R ₁ · α ₁ · Δt + R _β · α _β · t,

where α ₁ and α _β are the TCS of the strain gauge R ₁ and the compensation resistor R _β ;

Δt is the temperature range.

TCS total shoulder resistance

When the balancing resistor R _{b is} connected in series to the strain gauge R ₂ while the compensation resistor is connected at the same time, the bridge circuit balancing condition will take the form

(R ₁ + R _β ) · R ₄ = (R ₂ + R _b ) · R ₃ .

Where does the value of the balancing resistor be defined as

Similarly, with the derivation of expression (1), the total TCS of the shoulder R _2total after connecting the balancing resistor will be

where α _b and α ₂ - TCS balancing resistor and shoulder R _{2 of the} bridge circuit, respectively.

Substituting expression (2) into the obtained equation, we obtain

Knowing the TCS of all the arms of the bridge circuit, taking into account the connection of the compensation elements, using expressions (1) and (3), we can draw up the equation of the temperature balance of the bridge circuit

where α ₃ , α ₄ - TCS of the corresponding shoulders of the bridge chain.

Solving equation (4) with respect to the compensation thermally dependent resistor R _β , we obtain

The resulting expression (5) of the resistor allows you to calculate the value of the compensation thermally dependent resistor taking into account the subsequent balancing of the bridge circuit by the resistor R _b with a given TCS (α _b ).

Arguing in the same way, we can derive expressions for calculating the nominal values of compensating thermally dependent resistors for all possible options for including compensation elements.

When you include R _β in the shoulder R ₁ and R _b in the shoulder R ₃

When you include R _β in the shoulder R ₄ and R _b in the shoulder R ₂

When you include R _β in the shoulder R ₄ , and R _b in the shoulder R ₃

The inclusion options considered cover all options when a compensating thermally dependent resistor is connected in series with strain gauges that sense tensile strain, and the inclusion of a balancing resistor in series with strain gauges that sense compression strain. An analysis of the obtained expressions allows us to derive a generalized expression for calculating the nominal value of a compensating thermally dependent resistor for all considered cases. Thus, when a compensating thermally dependent resistor is included in any arm that is sensitive to tensile strain, and the inclusion of a balancing resistor in any arm that is sensitive to compressive strain, the compensating thermally dependent resistor can be calculated by the formula

where R _to and R _p - the nominal resistance of the strain gauge of the shoulder, which is connected to a compensating thermally dependent resistor and the opposite shoulder, respectively;

R _sb and R _s are the resistance value of the strain gage to which the balancing resistor is connected, adjacent to the strain gage to which the compensation thermally dependent resistor and the second adjacent arm are connected, respectively;

α _s and α _sb are temperature coefficients of resistance of strain gages respectively R _s and R _{sb of} adjacent arms of the circuit relative to the arm to which a compensation thermally dependent resistor is connected;

α _to and α _p - temperature coefficient of resistance of the strain gage of the shoulder, to which is connected a compensating thermally dependent resistor and the opposite shoulder, respectively;

α _β and α _b are the temperature coefficients of the resistances of the compensation thermally dependent R _β and balancing R _b resistors, respectively;

When you enable R _β in the shoulder R ₂ and R _b in the shoulder R ₁

When you include R _β in the shoulder R ₂ and R _b in the shoulder R ₄

When you enable R _β in the shoulder R ₃ and R _b in the shoulder R ₁

When you include R _β in the shoulder R ₃ and R _b in the shoulder R ₄

The inclusion options considered cover all options when a compensating thermally dependent resistor is connected in series with strain gauges that sense compression deformation, and the inclusion of a balancing resistor in series with strain gauges that sense tensile strain. An analysis of the obtained expressions allows us to derive a generalized expression for calculating the nominal value of a compensating thermally dependent resistor for all considered cases. Thus, when a compensating thermally dependent resistor is included in any arm that receives compression deformation, and the inclusion of a balancing resistor in any arm that accepts tensile strain, the compensated thermally dependent resistor can be calculated by the formula

Comparing equations (6) and (7), we can derive a general expression for calculating a compensation thermally dependent resistor for all possible cases of connecting compensation elements

where the arithmetic signs (+) and (-) are taken from the upper values when installing a compensating thermally dependent resistor in the shoulders, perceiving tensile strain, and by the lower values when installing a compensating thermally dependent resistor in the shoulders, perceiving compression deformation.

After installing a compensating thermally dependent resistor with a rated value and previously determined TCS values and the installation arm, the bridge circuit is balanced by connecting a balancing resistor with the previously selected TCS arm with a previously selected TCS equal to α _b . The value of the balancing resistor is determined experimentally, for example, after connecting a variable resistor to the selected arm.

The proposed compensation method allows simple methods to choose a compensation scheme option. Improving the accuracy of compensation of the additive temperature error is achieved by the fact that:

- in the process of determining the nominal value of the compensation resistor, the influence on the TCS of the balanced arm of the nominal and the TCS of the balancing resistor is taken into account;

- the criteria for the selection of TCS compensation elements are determined depending on the manufacturing technology and their installation location in the sensor design.

The increase in reliability during operation of the sensor in microelectronic design is due to the exclusion of the influence of laser or electroerosive fitting of strain gauges.

Improving manufacturability in the manufacturing process of the sensor is achieved by eliminating the stringent requirements for obtaining a given TCS in the manufacture of a balancing resistor, it is sufficient to determine the TCS with an accuracy of ± 0.5%.

To verify the accuracy of the proposed method, we consider an example of tuning the sensor for additive temperature error.

Example

Compensate for the additive temperature error of the strain gauge sensor with a parallel-symmetric bridge measuring circuit, if it is known:

- the resistance of the strain gauges of the bridge circuit are respectively equal to R ₁ = R ₃ = 1000 Ohms; R ₂ = R ₄ = 500 Ohms;

- TCS strain gages are respectively equal to α ₁ = 4.4 · 10 ^-4 1 / ° C; α ₂ = α ₃ = α ₄ = 4 · 10 ^-4 1 / ° C;

- TCS compensation thermally dependent resistor α _β = 40 · 10 ^-4 1 / ° C;

;- the total relative change in the resistance of the bridge circuit from the nominal value of the measured parameter

;

- TCS balancing resistor α _b = 4 · 10 ^-6 1 / ° C;

- the range of temperature changes Δt = 100 ° C;

- the permissible value of the additive temperature sensitivity of the sensor after adjustment should be within S _ot = ± 1 · 10 ^-4 1 / ° С.

Decision

Assuming that in the initial state the bridge measuring circuit is balanced (according to the condition of the problem, the adjacent arms are equal), and since (α ₁ + α ₄ ) - (α ₂ + α ₃ ) = 0.5 · 10 ^-4 1 / ° C, then with increasing temperature, the initial level of the output signal will change in a positive direction. Therefore, a compensating thermally dependent resistor must be included in the shoulders perceiving compression deformation from the measured parameter, that is, in series with strain gauges R ₂ or R ₃ . Then the balancing must be done by turning on the resistor R _{b in} series with the strain gages R ₁ or R ₄ . Choose the arm of the compensation thermally dependent resistor R ₂ , and the arm of the balancing resistor R ₁ . The value of the resistor R _β can be calculated by the formula (7)

The value of the balancing resistor can be calculated by the formula

The initial level of the output signal of the bridge circuit after the temperature setting can be determined by the formula

The initial level of the output signal of the bridge circuit when the temperature changes after the temperature setting can be determined by the formula

The change in the initial level of the output signal from a change in temperature after the temperature setting can be defined as

ΔU _ot = U _ot -U _o = U _n · (-4 + 1) · 10 ^-7 = -3 · 10 ^-7 U _n .

Additive temperature sensitivity of the sensor after temperature setting

From the above calculation it is seen that the proposed compensation method provides the required accuracy of minimizing the additive temperature error.

Claims (1)

The method of tuning strain gauge sensors with a bridge measuring circuit for an additive temperature error, which consists in pre-balancing the bridge circuit within ± 0.5% of the nominal output signal, determining the temperature resistance coefficients (TCS) of all arms of the bridge circuit, taking into account the introduction of a balancing resistor during preliminary balancing and determining the shoulder of a balanced sensor, in which a thermally dependent compensation resistor is connected in series with the working strain gauge ru, characterized in that they determine the arm into which the balancing resistor will be connected after the sensor is configured, and the TCSs of the compensated thermo-dependent and balancing resistors from the conditions: when the compensated thermo-dependent resistor is connected to the selected shoulder of the bridge circuit, the balancing resistor will be connected to the adjacent arm, which receives the deformation of the opposite sign relative to the arm of the connection of the compensation thermally dependent resistor; compensating thermally dependent resistor is made according to the manufacturing technology of working strain gages with the highest possible value of TCS and is installed on an elastic element in the installation zone of working strain gages; when installing a balancing resistor on an elastic element in the installation zone of the working strain gages, it should be manufactured using the manufacturing technology of working strain gages, and its TKS should be in the range from zero to TKS of the strain gages; when installing a balancing resistor outside the range of the temperature field of the measured parameter, for example, in a secondary converter, additional requirements for its manufacturing technology are not imposed, and its TKS should be as low as possible in the range of ± 0.5% TKS strain gages, calculate the value of the compensation thermally dependent resistor the formula

where R _β is the resistance of the compensation thermally dependent resistor; R _to and R _p - resistance of the strain gauges, to which are connected a compensation thermally dependent resistor and the opposite shoulder, respectively; R _sb and R _s are the resistance of the strain gages to which the balancing resistor is connected, adjacent to the strain gage to which the compensation thermally dependent resistor is connected, and the second adjacent arm, respectively; α _s and α _sb - TKS of strain gages respectively R _s and R _sb , adjacent arms of the circuit relative to the arm to which a compensation thermally dependent resistor is connected; α _to and α _p - TCS strain gages, which is connected to the compensation thermally dependent resistor and the opposite shoulder, respectively; α _β and α _b - TCS compensating thermally dependent R _β and balancing R _b resistors, respectively; arithmetic signs (+) and (-) are taken according to the upper values when installing a compensating thermally dependent resistor in the shoulders that are perceiving tensile strain, and the lower values when installing a compensating thermally dependent resistor in the shoulders that are receiving compression deformation, after installing the compensated thermally dependent resistor of the calculated value to a certain previously the shoulder of the bridge circuit, it is balanced by connecting a balancing resistor with a given TCS to the previously defined shoulder.