RU2434233C1 - Compensation-type accelerometer - Google Patents

Abstract

FIELD: instrument making.

SUBSTANCE: essence of invention consists in spatial division of temperature field of accelerometer into two areas in each of which the temperature is measured with a separate sensor, and compensation of temperature error of scaling factor is performed as per special algorithm. Spatial division of temperature field allows performing separate compensation of temperature errors of scaling factor, which are determined mainly by force sensor and damping liquid, which have various temperatures during acceleration measurement.

EFFECT: accelerometer accuracy improvement.

1 dwg

Description

1. The technical field to which the invention relates.

The invention relates to precision instrumentation and can be used mainly in precision inertial motion control systems, for example, airplanes, missiles, submarines and other objects.

One of the most important tasks solved by control systems is the precision measurement of the apparent acceleration of objects.

Accelerometers are designed to measure acceleration. In order to accurately measure acceleration, it is necessary to significantly reduce their errors due to external and internal disturbing influences.

The main disturbing effect for the accelerometer is temperature. The change in temperature of the accelerometer case occurs due to changes in the ambient temperature, as well as due to the device’s own heating after it is turned on. Due to temperature changes, the accuracy of the accelerometer is reduced due to the temperature error of its scale factor. This decrease is very large, since it is proportional to the measured acceleration. In this regard, the efforts of the developers of accelerometers are focused on reducing the temperature error of the scale factor of the accelerometers.

2. The level of technology

Consider the analogues of the invention.

2.1. Bibliographic data of analogues of the invention

[1] G01P 15/08, 15/13. Accelerometer compensation type. Patent No. 2042955 for application No. 31588843/10 dated 12.31.86. Authors: Kitanin N.G., Abaimov V.V., Grin V.V., Sherstnev V.A.

Published in bull. No.24 08/27/95

[2] G01P 15/08, 15/13. Accelerometer compensation type. Patent No. 2050549 for application No. 3186551/10 dated 12.12.87. Authors: Kitanin N.G., Ivchenko N.N., Grin V.V., Ryazanov A.A.

Published in bull. No. 35 12/20/95

2.2. In the compensation type accelerometer according to the patent of the Russian Federation No. 2042955 [1], common features with the present invention are a sensing element with a winding of a force sensor, a feedback amplifier. In this accelerometer, to achieve the goal of increasing the accuracy of compensating for temperature changes in the scale factor with significant changes in the ambient temperature, an active two-terminal device with a negative input resistance is used, the input of which shunts the winding of the force sensor.

The disadvantage of this accelerometer is the low accuracy of compensation due to the presence of a temperature error of the scale factor proportional to the square of the temperature. This error, as indicated in the description of the invention to the patent [1], is equal to the product of the squares of the coefficient of temperature change of the scale factor and the temperature of the accelerometer.

Closest to the claimed invention, an analogue (prototype) is the invention according to the patent of the Russian Federation No. 2050549 "Compensation-type accelerometer" [2]. In this accelerometer, common features with the present invention are a sensing element with a magnetoelectric force sensor, a position sensor and damping fluid, amplifiers of the force sensor and position sensor, a scale resistor.

In this invention, as well as in the invention according to the patent [1], the linear component of the temperature error of the scale factor of the accelerometer is compensated, moreover, this compensation is made by connecting a voltage converter - current parallel to the winding of the force sensor, and the direction of the converter output current is selected based on the sign temperature change of the scale factor.

This accelerometer has a low measurement accuracy:

- due to the large temperature error due to the presence of a quadratic term in the temperature dependence of the scale factor;

- due to residual temperature errors of the scale factor, except for errors proportional to the temperature in the first and second degree.

3. Disclosure of invention

3.1. The technical result of the invention is to increase the accuracy of the accelerometer by compensating for temperature errors of the scale factor.

The technical result is achieved by a combination of essential features — the introduction of a thermistor, two analog-to-digital converters, a current source, a microcontroller, a voltage divider, a shunt resistor, and the connections of the elements introduced into the accelerometer.

The invention consists in the spatial separation of the temperature field of the accelerometer into two areas, in each of which:

- temperature is measured by a separate sensor;

- compensation of the temperature error of the scale factor is carried out according to a special (different) algorithm.

Spatial separation of the temperature field allows for separate compensation of the temperature errors of the scale factor due to the force sensor and damping fluid, which have different temperatures in the process of measuring acceleration, which improves the accuracy of the accelerometer.

3.2. The invention is aimed at solving the following three problems:

- the formation of a special scheme for obtaining temperature information;

- compensation of errors of the scale factor proportional to the square of the temperature;

- compensation of residual temperature errors of the scale factor, except for errors proportional to the temperature in the first and second degree.

The essential features characterizing the invention and are common with the prototype [2]: a sensing element with a magnetoelectric force sensor, a position sensor and damping fluid, amplifiers of the force sensor and position sensor, a large-scale resistor.

The essential features characterizing the invention and differing from the prototype [2]: the introduced thermistor, two analog-to-digital converters, a current source, a microcontroller, a voltage divider, a shunt resistor and the connection of the introduced elements with each other and with the elements of the prototype.

To solve the first problem - the formation of a special scheme for obtaining temperature information, as a result of introducing new elements into the accelerometer, two new channels for measuring temperature were created.

The source of temperature information for the first channel is the winding of the force sensor (DC). The force sensor in the invention has two functional purposes. First, as in the prototype, the current measured through the winding of the DC is judged on the magnitude of the measured acceleration. Secondly, unlike the prototype, by measuring the resistance of the force sensor winding, the temperature of the DC is determined, by which the second task is solved: compensation of errors of the scale factor proportional to the second degree of temperature. At the same time, they also compensate for errors in the scale factor proportional to the first degree of temperature.

The source of temperature information for the second channel is the introduced thermistor, which mainly measures the temperature of the damping fluid. The second channel of temperature measurement is used to solve the third problem - compensation of errors remaining after compensation of errors proportional to the first and second degrees of temperature.

To solve the third problem, the entire temperature range of the accelerometer is divided into r subbands (r is greater than 5), and each subband uses its own error compensation coefficient. As a result, not only errors of the scale factor proportional to temperature to a degree higher than the second are compensated, but also errors of the scale factor that vary according to the harmonic (for example, sinusoidal) law of temperature.

The solution to the three above problems provides increased accuracy of the accelerometer.

4. Brief Description of the Drawings

The invention is illustrated in the drawing, which shows a block diagram of the proposed device.

In the drawing, the numbers indicate:

1 - winding of the force sensor;

2 - power sensor amplifier;

3 - position sensor;

4 - position sensor amplifier;

5 - scale resistor;

6 - thermistor;

7 - the first analog-to-digital Converter;

8 - the second analog-to-digital Converter;

9 - current source;

10 - microcontroller;

11 - the first resistor of the voltage divider;

12 - the second resistor of the voltage divider;

13 - shunt resistor.

5. The implementation of the invention

5.1. As in the prototype, the proposed compensation type accelerometer contains a sensing element with a force sensor 1 and a position sensor 3, a scale resistor 5, amplifiers of the force sensor 2 and position 4, the first output of the winding of the force sensor 1 is connected to the first output of the scale resistor 5, and the second output of the winding force sensor 1 is connected to the output of the amplifier of the force sensor 2, the output of the position sensor 3 is connected to the input of the amplifier of the position sensor 4.

Unlike the prototype, the proposed accelerometer contains blocks from the sixth to the thirteenth and communication blocks: the output of the position sensor amplifier 4 is connected to the first input of the microcontroller 10, the first output of the scale resistor 5 is connected through the first analog-to-digital converter (ADC) 7 to the second input of the microcontroller 10, the output of the current source 9 is connected to the terminals of the thermistor 6, the shunt resistor 13 and the third input of the microcontroller 10, the first resistor of the voltage divider 11 through the second resistor of the voltage divider 12 is connected to the second output of the winding of the force sensor 1 and through the second analog-to-digital converter (ADC) 8 to the fourth input of the microcontroller 10, and the output of the microcontroller is connected to the input of the amplifier of the force sensor 2.

5.2. The measurement of acceleration using the proposed accelerometer is as follows.

When the accelerometer g is affected by the accelerometer _, a deviation of the sensor element, not shown in the drawing, occurs. This deviation is detected by the position sensor 3, the output signal of which after amplification in the amplifier 4 is supplied to the first input of the microcontroller 10. The microcontroller generates a PWM control signal from the microcontroller output to the input of the amplifier 2. According to this PWM signal, the amplifier 2 generates a current from of its output into the winding of the force sensor 1 and a scale resistor 5 connected in series with it. As a result, the force sensor applies force to the sensor until the deviation of the sensor is zero (until the output -stand position sensor signal).

The current i flowing through the force sensor and a scale resistor is proportional to the measured acceleration g _and :

where K _m - scale factor.

The current i is measured using the first ADC 7.

The voltage U _B1 at the input of the first ADC is proportional to the current i:

where R _m is the resistance of the scale resistor.

The scale resistor has a low temperature coefficient of resistance (TCR).

Taking into account (1), (2) the output (digital) ADC signal 7

where K _c is the coefficient of proportionality.

A digital signal (3) is supplied from the output of the first ADC 7 to the second input of the microcontroller.

The microcontroller corrects the value of U ₁ taking into account compensation for changes in the scale factor of the accelerometer as a function of temperature. The corrected digital signal W from the output of the microcontroller judges the acceleration g _and .

5.3. The correction of the temperature component of the scale factor of the accelerometer is as follows.

Simultaneously with determining the current i flowing through the winding of the force sensor, measure the voltage U _B2 at the input of the second ADC 8. This voltage can be expressed in terms of current i and the parameters of the voltage divider (DN) as follows:

where R _ds - resistance of the winding of the force sensor;

R ₁ , R ₂ - the resistance of the first and second resistors DN, respectively.

The first and second DN resistors have a small TCS, they lead to a voltage drop across the load (R _ds + R _m ) to the input of the second ADC, providing a maximum voltage of no more than permissible.

The output (digital) signal of the ADC 8

The microcontroller calculates the ratio U ₂ / U _{1 of the} signals received at its fourth and second inputs, respectively:

The resistance of the DC winding depends on the temperature:

where R _ds0 - resistance of the winding of the force sensor at nominal temperature;

α is the temperature coefficient of resistance of the winding of the force sensor;

Θ - deviation of the temperature of the winding of the force sensor from the nominal.

Therefore, expression (6) can be represented as:

U ₂ / U ₁ = (R _ds0 + R _m ) R ₁ / R _m (R ₁ + R ₂ ) + R _ds0 R ₁ α · Θ / R _m (R ₁ + R ₂ )

After calibrating the accelerometer (during its manufacture), the component (R _ds0 + R _m ) R ₁ / R _m (R ₁ + R ₂ ) is taken into account and the subsequent temperature correction is carried out using the ratio:

where K _t = R _ds0 R ₁ · α / R _m (R ₁ + R ₂ )

Thus, the signal ratio U ₂ / U _{1 is} proportional to the temperature Θ of the winding of the force sensor, measured relative to the nominal temperature.

The output (digital) signal of the microcontroller, taking into account the compensation of the error of the scale factor proportional to the first and second degrees of temperature Θ, has the form:

where K = g / M ₀ fC _e is the coefficient of accounting for the systematic component of the relative error of the scale factor;

g is the acceleration of gravity at the test site of the accelerometer;

f is the polling frequency of the first and second ADCs;

C _e is the unit price of the least significant bit of the output code;

M ₀ is the calculated value of the output signal of the accelerometer under the action of acceleration lg without taking into account the systematic component of the relative error of the scale factor, and the second index in the designation of the ADC output signals means the position of the device - 1, 2. The position of the device 1 - the sensitivity axis of the device is directed vertically upward. Position of the device 2 - the axis of sensitivity of the device is directed vertically downward;

K ₁ , K ₂ - coefficients of the approximating polynomial.

5.4. The coefficients of the polynomial K ₁ , K _{2 are} determined in the manufacture of the accelerometer from the condition of equality of scale factors of the accelerometer at the edges and in the center of the operating temperature range. The system of equations for determining the coefficients has the form:

where M _In , M _C , M _N - scale factor of the accelerometer at maximum, medium and low temperatures, respectively.

Given the output signal (9), the system of equations (10) can be written as:

The solution of the system of equations for K ₁ , K ₂ :

,

,

where a1 = (U _B1 -U _B12 ) - (U _H11 -U _H12 );

a2 = (U _B1 -U _B12 ) - (U _C11 -U _C12 );

b1 = (U _B21 -U _B22 ) - (U _H21 -U _H22 );

B2 = (U _B21 -U _B22 ) - (U _C21 -U _C22 );

;

;

U _B11 , U _C11 , U _H11 - the output signal of the first ADC at high, medium, low temperature, respectively, in position 1 of the device - the measuring axis is directed vertically upwards;

U _B12 , U _C12 , U _H12 - the output signal of the first ADC at high, medium, low temperature, respectively, in position 2 of the device - the measuring axis is directed vertically downward;

U _B21 , U _C21 , U _H21 - the output signal of the second ADC at high, medium, low temperature, respectively, in position 1 of the device - the measuring axis is directed vertically upwards;

U _B22 , U _C22 , U _H22 - the output signal of the second ADC at high, medium, low temperature, respectively, in position 2 of the device - the measuring axis is directed vertically downward.

The values of the coefficients K ₁ , K ₂ (in the manufacture of the accelerometer) are recorded in the RAM of the microcontroller.

5.5. The residual temperature error of the scale factor is taken into account as follows.

From the output of the current source 9, direct current is supplied to the thermistor, the resistance of which changes in proportion to the current temperature T _t of the thermistor housing. As a result, a voltage proportional to the temperature T _t is formed on the shunt resistance. The signal corresponding to the temperature T _t enters the 3 input of the microcontroller.

The entire temperature range of the accelerometer (during its manufacture) is divided into r sub-bands (r is greater than 5) and for each temperature sub-range from T _i to T _{i + 1} (i from 1 to r + 1), the initial correction coefficient M _{i is} determined (for temperature T _i ) and the final correction coefficient M _{i + 1} (for temperature T _{j + 1} ).

Given the linear dependence of the residual temperature error in the selected sub-range from T _i to T _{i + 1, the} coefficient for taking into account the residual temperature error of the scale factor has the form:

where T _t is the current temperature value measured by a thermistor;

- стартовое (исходное) значение поправочного коэффициента (в момент включения прибора);

- starting (initial) value of the correction factor (at the moment of switching on the device);

- стартовый поправочный коэффициент для i-й температуры;

- starting correction factor for the i-th temperature;

- стартовое значение i-го интервала температуры;

- starting value of the i-th temperature interval;

- начальная температура в момент включения прибора.

- initial temperature at the moment of switching on the device.

The correction factor M _i is determined by the formulas:

,

,

, δµ₁=0, М₁=0,

, δµ ₁ = 0, M ₁ = 0,

- относительная погрешность масштабного коэффициента на i-й температуре T_i (i от 1 до r+1) до и после калибровки акселерометра соответственно. Коэффициенты M_i записывают в оперативную память микроконтроллера.where μ _i ,

- the relative error of the scale factor at the ith temperature T _i (i from 1 to r + 1) before and after calibration of the accelerometer, respectively. The coefficients M _{i are} recorded in the RAM of the microcontroller.

5.6. The resulting output (digital) signal of the accelerometer, taking into account the compensation of temperature errors of the scale factor proportional to the first and second degrees of temperature Θ, as well as the compensation of residual temperature errors of the scale factor using temperature T _t, has the form:

5.7. To implement the proposed accelerometer, a TP-1 semiconductor thermistor and an ATMEGA 8535 microcontroller can be used. The AD7711 chip can be used as an analog-to-digital converter. For the implementation of the resistors of the voltage divider and the shunt resistor, resistors C2-29 V can be used.

5.8. The calculation of the effectiveness of the invention

In the compensation type accelerometer according to the patent of the Russian Federation No. 2050549 [2], an increase in accuracy is achieved by reducing the temperature error, and specifically by reducing the linear component of the temperature dependence of the scale factor, using a thermal compensation amplifier.

However, in real accelerometers, the dependence of the scale factor on temperature differs significantly from linear.

We calculate the temperature error of the scale factor of the accelerometer prototype for the dependence of the scale factor on the temperature, described, for example, by a polynomial of the third degree:

where i is the current in the feedback circuit of the accelerometer [2] without a thermal compensation amplifier;

i ₀ - current in the feedback circuit of the accelerometer [2] without a thermal compensation amplifier at nominal temperature;

D ₁ , D ₂ , D ₃ - temperature coefficients of change of the scale factor of the accelerometer at a temperature in the first, second and third degree, respectively.

In accordance with [2]

and the total current in the feedback circuit of the accelerometer, taking into account the current of the thermal compensation amplifier:

Given (19), the expression for the temperature error of the scale factor of the accelerometer prototype can be written in the form:

Then the temperature error with the following real initial data: D ₁ = 4 · 10 ^-4 1 / ° C, D ₂ = 6 · 10 ^-7 1 / (° С) ² , D ₃ = 5 · 10 ^-9 1 / ( ° C) ³ , Θ = 35 ° C - may be:

δi _k = -16 · 10 ^-8 · 1225 + 6 · 10 ^-7 · 1225-4 · 10 ^-4 · 6 · 10 ^-7 · 42875 + 5 · 10 ^-9 · 42875-

-4 · 10 ^-4 · 5 · 10 ^-9 · 1500625 = 7.4 · 10 ^-4 .

Thus, in the presence of a non-linear dependence of the temperature error of the scale factor, the error of the accelerometer prototype reaches significant values (about 7.4 · 10 ^-4 ).

In the proposed accelerometer, improved accuracy, compared with the prototype, is achieved by reducing the quadratic and higher order components of the temperature dependence of the scale factor.

We calculate how many times (compared with the prototype) increases the accuracy of the proposed accelerometer by reducing (compensating) the temperature component of the scale factor.

Imagine the dependence of the current in the feedback circuit on temperature without compensation in the proposed accelerometer in the form (18).

Compensated feedback current output:

а отношение

пропорционально сопротивлению обмотки датчика силы R_дс, выходную информацию можно представить в виде:Given that

and attitude

in proportion to the resistance of the coil of the force sensor R _ds , the output information can be represented as:

Where

,

,

Given the dependence (7) of the resistance of the winding of the force sensor on temperature, one can record the output information of the accelerometer with compensation for the temperature error in the form of a temperature dependence:

Where

,

,

,

,

.

.

From (22) it follows that, choosing the coefficients C ₁ and C ₂ (during the setup of the accelerometer), it is possible to compensate for the temperature error of the scale factor proportional to the first and second degrees of temperature.

The accelerometer is adjusted (as shown in Section 5.4) according to the condition that the scale factors are equal at the edges and at the midpoint of the operating temperature range.

Let the operating temperature range of the accelerometer be 70 ° С - from minus 5 ° С to + 65 ° С. At the midpoint of the operating temperature range (30 ° С) Θ = 0, at the upper boundary of the operating temperature range the temperature deviation from the average value will be 35 ° С, at the lower - minus 35 ° С. Then we can write a system of two equations:

The solution of the system of equations for C ₁ , C ₂ :

The temperature component of the relative error of the scale factor in the compensation of linear and quadratic terms is described by the expression:

where Θ is the deviation of the current value of the temperature of the device from the temperature at the midpoint of the operating temperature range.

The values of the residual temperature error of the scale factor δi _k at D ₁ = 4 · 10 ^-4 1 / ° C, D ₂ = 6 · 10 ^-7 1 / (° С) ² , D ₃ = 5 · 10 ^-9 1 / (° C) ³ , Θ = 35 ° C are presented in table 1. Moreover, C ₁ = -4.06 · 10 ^-4 1 / ° C, C ₂ = -4.35 · 10 ^-7 1 / (° С) ² .

Table 1 Θ _i , ° С δi _to -35 0 -thirty 4.53 · 10 ^-5 -25 6.95 · 10 ^-5 -twenty 7.63 · 10 ^-5 -fifteen 6.92 · 10 ^-5 -10 5.18 · 10 ^-5 -5 2.7510 ^-5 0 0 5 -2.74 · 10 ^-5 10 -5.1310 ^-5 fifteen -6.8210 ^-5 twenty -7.4810 ^-5 25 -6.7810 ^-5 thirty -4.4010 ^-5 35 0

From table 1 it follows that the residual temperature error does not exceed 7.63 · 10 ^-5 .

Thus, the compensation of the quadratic term of the temperature dependence of the scale factor increases the accuracy of the accelerometer by 7.4 · 10 ^-4 / 7.63 · 10 ^-5 = 9.7 times.

We proceed to consider the effectiveness of the invention in terms of compensating for the residual temperature error of the scale factor δi _k (table 1) using the temperature values T _i measured by the thermistor. The liquid temperature T _i measured by the thermistor may differ by 1 to 5 ° C from the temperature of the force sensor Θ _i .

To compensate for this residual temperature error of the scale factor, the operating temperature range of the accelerometer is divided, for example, into r = 7 sub-ranges (from 8 to 12 ° C each), as shown in table 2.

table 2 Subband Number j T _i , ° C (δi _k ) _i Δ _j one -38 0 -33 4.53 · 10 ^-5 -1.0510 ^-5 -28 6.95 · 10 ^-5 2 -24 7.63 · 10 ^-5 -6.9510 ^-6 -16 6.92 · 10 ^-5 3 -12 5.18 · 10 ^-5 -3.4510 ^-6 -8 2.7510 ^-5 four -3 0 5 · 10 ^-8 2 -2.74 · 10 ^-5 5 10 -5.1310 ^-5 3.5 · 10 ^-6 fourteen -6.8210 ^-5 6 19 -7.4810 ^-5 6.80 · 10 ^-6 24 -6.7810 ^-5 7 28 -4.4010 ^-5 1.01 · 10 ^-5 32 0

In each of these subranges, the temperature change in the scale factor is compensated with its linear coefficient. In this case, the temperature error after compensation of the residual temperature error can be determined (table 2) by the formula:

This temperature error in accordance with table 2 does not exceed the absolute value of 1.05 · 10 ^-5 .

Thus, in comparison with the prototype, the accuracy of the proposed accelerometer in terms of the scale factor increases by an additional 7.63 · 10 ^-5 / 1.05 · 10 ^-5 = 7.26 times. As a result, an increase in accuracy of 9.7 · 7.26 = 70.4 times compared with the prototype allows you to create an accelerometer with high accuracy in a wide temperature range.

Claims (1)

A compensation type accelerometer comprising a sensor with a force sensor and a position sensor, a scale resistor, power and position sensor amplifiers, a first output of the force sensor winding connected to a first output of a scale resistor, a second force sensor winding connected to the output of the force sensor amplifier, position sensor output connected to the input of the position sensor amplifier, characterized in that a thermistor, first and second analog-to-digital converters, a current source, microcontrol are additionally introduced into it ller, first and second resistors of the voltage divider, a shunt resistor, the output of the position sensor amplifier connected to the first input of the microcontroller, the first output of the scale resistor through the first analog-to-digital converter connected to the second input of the microcontroller, the output of the current source connected to the terminals of the thermistor, the shunt resistor, and the third input of the microcontroller, the first resistor of the voltage divider through the second resistor of the voltage divider is connected to the second terminal of the winding of the force sensor and through w There is an analog-to-digital converter to the fourth input of the microcontroller, and the output of the microcontroller is connected to the input of the amplifier of the force sensor.