SU1677650A1 - Temperature compensated parametric transducer - Google Patents

Abstract

The invention relates to measurement technology, automation and can be used, in particular, in tensometric and photometric equipment. The goal was invented - to improve the measurement accuracy by compensating for the temperature errors of the parametric measuring bridge - achieved by the fact that two large-scale algebraic scales were introduced into the converter containing the four-arm measuring bridge, source 1, reference voltage, operational amplifier 2, analog-digital converter 10. an adder 9 and 11 and a current setting resistor 3. To minimize the thermal dependence of the initial bias voltage of the measuring bridge 4, select certain values of the coefficients, va iru values of coefficients, receive the necessary degree of sensitivity of the transducer temperature compensation .1 yl.

Description

N

& four

ABOUT

sl o

The invention relates to the field of measurement technology, automation and telemechanics and can be used, in particular, in devices for measuring pressure of a force of a teneometric type, for example, in differential photometers.

The aim of the invention is to improve the accuracy of measuring the 3 rd by compensating for the temperature errors of the parametric measuring bridge.

The drawing shows a functional electrical circuit of a temperature-compensated parametric converter.

The circuit contains the source 1 of the reference voltage, the operational amplifier 2, the current resistor 3, the parametric measuring bridge 4, consisting of resistors 5-8, which are incremented when exposed to the measured value, and the supply diagonal of the measuring bridge A is connected between the output and the inverting the input of the operational amplifier 2, the output diagonal of the measuring bridge 4 is connected to the first and second inputs of the first large-scale algebraic adder 9, the output of which is connected to the input of the analog-digital converter (ADC)

10, the output of the operational amplifier 2 is connected to the third input of the first 9 and the first input of the second algebraic adders

11, the first output of the source 1 of the reference voltage is connected to the fourth input of the first and second inputs of the second large-scale algebraic adders, a current-generating resistor 3 is connected between the first output of the source 1 of the reference voltage and the inverting input of the operational amplifier 2. the second output of the source 1 of the reference voltage and the non-inverting input of the operational amplifier 2 is connected to the common bus, the output of the second large-scale algebraic adder 11 is connected to the input of the reference voltage of the ADC 10, the output of which is the output of the device va.

The device works as follows.

Output voltage Ug adder 9

U9 2AUK +, (1)

where A U is the useful voltage signal in the output diagonal of the bridge 4;

U2 is the output voltage of op amp 2;

K, Ki - transfer coefficients of the adder 9 by the corresponding inputs;

Ui is the source voltage 1;

K2 is the voltage transfer coefficient of the reference source 1.

The voltage U2 is described by the expression

U2 -Ui

0

five

0

five

0

five

 Ra

R0 (1 + gAt) m U1R3 w

where R3,4 is the equivalent resistance of the supply diagonal of the measuring bridge 4;

RO is the initial resistance of the supply diagonal of the measuring bridge 4 at temperature t0;

a - temperature coefficient of equivalent resistance of the supply diagonal of the measuring bridge 4;

At is the absolute deviation of the temperature of the measuring bridge 4 from the initial value t0;

R3 is the resistance of the current-carrying resistor 3.

At a constant temperature, the equivalent resistance of the measuring bridge 4 with four active arms (moreover, the opposite arms receive the same increments, and the adjacent ones — antiphase increments) does not depend on the value of the external measured parameter, i.e. R3,4 does not depend on Au. This property of the four-shoulder measuring bridge 4 allows using the voltage signal on the supply diagonal (when the bridge is supplied with a constant constant current) as a temperature-dependent value and to perform thermal compensation of the measuring bridge 4: its initial displacement (at zero value of the measured parameter ) and sensitivity.

The expression (1) in accordance with (2) can be written as

Ug 2 AUK- KiUi x R0 (1 + “At)

Rs

+ UiK2.

(3)

When choosing the values of the coefficients Ki and K2 such that the equality Ki

expression (3) takes the form

R3

Ug 2 UK-Kil) “At.

To

(four)

The second term of expression (4) linearly depends on the temperature difference At (between the current and initial values) and does not depend on the signal Au, and consequently, on the measured parameter of bridge 4. By choosing the values of the coefficients Ki and K2, it is possible to compensate for the own temperature coefficient OQ of the initial displacement of the measuring bridge 4.

Using the same principle, it is possible to reduce the temperature error of sensitivity of a parametric converter. For this, the signal from the output of the supply diagonal of the measuring bridge 4 is summed with the reference voltage of the ADC 10

(or with a signal at the input of the operational amplifier 2).

The output code of the ADC 10 is determined by the expression

N N0 -, (5)

where No is the reference number (end of scale);

Un is the reference voltage coming from the output of the adder 11,

Un voltage is defined as

Ul1 U2 K3 + UrK4. (6)

Taking into account (2), (4), expression (5) can be rewritten as

2AU K-Ki Ui - a-At

N

UiK4-Ui K3 + Ui K3aAt

: 2-4U (. Hi V

to

Ui

К - Кз + КзаДг

Ro

) +

+ kz

(one}

Ro

(1-aAt)

Analysis of expression (7) shows the following. The first term of the expression characterizes the useful signal proportional to the voltage A.sub.U. By selecting the values of the coefficients Ks and KU, a minimum temperature coefficient of the sensitivity of the converter is achieved (this achieves a compensation of the temperature coefficient of sensitivity of the measuring bridge A itself).

The second term of the expression does not depend on the useful signal Au, choosing certain values of the coefficients Ki, K2 (as shown) minimizes the thermal dependence of the initial bias voltage of the measuring bridge 4,

In case the initial offset is not required to be compensated (for example, an ADC chip is used with automatic zero correction or manual balancing of the measuring bridge 4), then the coefficients Ki and Ka are chosen equal to 0. Formula (7) will have the form AUК

Ui

N 2

Rs ro

.(eight)

K4-Kz (1-AdO

By varying the values of the coefficients Cs and, the required degree of thermal compensation of the transducer sensitivity is achieved.

It should be noted that the coefficients, Ki, Kz and K /, can have both positive and negative signs.

The coefficient K is chosen based on the need to match the level of useful

signal at the output of the measuring bridge and the sensitivity of the ADC.

The coefficients Ki and K2 are selected based on the compensation of the temperature coefficient 5 of the initial displacement of the measuring bridge (the coefficient o is given in reference books based on 1 V of the supply voltage and 1 ° C). Mathematically, the compensation condition, based on 10 of formula (3), is written as

Ki (ob / aHRa / Ro), (9)

K2 Ki (R0 / R3) o la (10)

In the particular case when R0 Rs, we have Ki K2.

15 Similarly, to compensate for the temperature coefficient d of sensitivity of the bridge, the coefficients Cs and Cs must satisfy the equation obtained from formulas (7) or (8):

201 + d К4 - Кз (1 -a XR3 / Ro), (11)

where do we finally get

K3 (cV2XR3 / Ro), (12)

K4 1 + K3 (R0 / R3) 1 + b / a (13) As it follows from the above formulas 25 (9) - (13), for the exact calculation of the coefficients Ki-K4, the value of the coefficients is necessary (Xo, b of the measurement bridge.) using the empirical tuning method. For this, it is pre-balanced30

sated at normal temperature

the bridge and with the disabled physical (measured) value is placed in a heat chamber. They heat the bridge to the maximum operating temperature and, by adjusting Ki and K2, achieve a minimum unbalance of the bridge.

Then, the same, but with a connected physical quantity, is done with respect to the coefficients Kz and to achieve the same readings at two temperatures.

In order to achieve high accuracy, the adjustment process can be carried out iteratively, in several steps.

In the proposed solution, the signal from the output diagonal is used for the measurement itself, and the signal from the supply diagonal is used for thermal compensation, i.e. essentially the same elements, the same bridge. Neither temporary nor physical

separation of primary channel elements

measurement and channel compensation. This results in higher measurement accuracy than in analogs due to the absence of temperature errors that are unavoidable when these channels are separated, and simplifies the device, since it eliminates the need for using additional thermal compensation channel thermocouples.

Claims (1)

Invention Formula A temperature-compensated parametric converter containing a parametric measuring bridge, a reference voltage source, an operational amplifier, an analog-to-digital converter, characterized in that, in order to improve accuracy, two large-scale algebraic adders and a current-resistor are inserted, output and inverting input of the operational amplifier, the output diagonal of the measuring bridge is connected to the first and second inputs of the first scale th algebraic adder, whose output is connected to the input of analog-to-digital converter, the output of the operational amplifier is connected to a third input of the first and the first input of the second algebraic adders, the first output of the reference source voltage is connected to the fourth input of the first and second input of the second scale algebraic adders, the current supplying resistor includes between the first output of the reference voltage source and the inverting input operational amplifier, the second output of the reference voltage source and non-inverting input of the operational amplifier are connected to the common bus, the output of the second large-scale algebraic adder connected to the input voltage reference voltage of the analog-digital converter, the output of the analog-digital converter is the output of the temperature-compensated parametric converter.