RU2330243C2 - Method for temperature compensation of differential transmitters having linear characteristics - Google Patents

Abstract

FIELD: measuring instruments.

SUBSTANCE: constant values k and C are determined from experimental data obtained in a static mode at a variable ambient temperature by the following linear regression function y=k-F(X₀,T)+k_1X-X+C, where y is the difference between signals of a reference primary element (PE) and a measurement one of the transmitter. The transmitter signals are converted from analog into digital ones during the measurement. A computing device calculates the value of a measured quantity X from the formula:

where F(X₀,T) is a numerical value of voltage from the reference PE, F(X₁,T) is a numerical value of voltage from the measuring PE and 1/k_1X is a proportionality coefficient, determined during calibration of the transmitter.

EFFECT: increasing the accuracy of measurements.

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Description

The invention relates to measuring equipment and can be used when measuring physical quantities using differential sensors based on primary measuring transducers with separate electrical outputs and linear characteristics.

An analogue of this invention is a method of temperature compensation of strain gauge relative pressure sensors with a sealed internal cavity and a bridge measuring circuit (RF patent No. 2267756 RU), characterized in that they determine the temperature coefficients of resistance (TCS) of all shoulders of the bridge circuit, the initial imbalance of the sensor and the sensor output signal at nominal pressure, then the equivalent value of the TCS of the sensor bridge circuit from the temperature expansion of the fixed value of the gas pressure is calculated, etizirovannogo the interior cavity of the sensor, and determine the value termonezavisimogo compensation resistor, which include one of the arms of the bridge circuit.

The disadvantage of this method is that to determine the shoulder, to which it is necessary to bring the change from the pressure sealed in the sensor when the temperature and the spread of the TCS strain gages change, it is necessary to evaluate the equivalent value of the TCS from the change in internal pressure and the TCS of the bridge measuring circuit due to the technological spread of the TCS strain gages .

The prototype of this invention is a method of temperature compensation of strain gauge sensors (RF patent No. 2027142 RU), characterized in that they measure the initial output signals of the sensor at two values of the operating temperature range, include a temperature-sensitive resistor in one of the arms of the bridge circuit and seal the sensor in normal climatic conditions.

The disadvantage of this method is that the sensor must be sealed in normal climatic conditions, since failure to fulfill this condition leads to an increase in measurement error, and the use of a thermosensitive resistor made of material other than strain gauge sensors significantly limits the dynamic range of the measured values.

An object of the invention is to increase the measurement accuracy and simplify the calibration process.

Figure 1 shows a structural diagram of an automatic temperature compensation of a differential sensor, where

1, 10 - adders;

2 - the source of the reference value (JOB);

3 - reference primary measuring transducer (D ₀ );

4 - primary measuring transducer (D ₁ );

5 - display device (UI);

X ₀ - input reference value;

X is the measured value;

F (X ₀ , T) is the output signal of the reference PIP;

F (X ₁ , T) is the output signal of the measuring PIP;

y is the output signal of the differential sensor;

T is the ambient temperature;

In Fig. 1, the signal X _{0 of the} source of the reference value 2 and X of the measured value are fed to the adder 1, from the output of which the signal X _{1 is} supplied to the primary measuring transducer (PIP) 4. The signal X _{0 is} supplied to the reference PIP 3. Received output signals F ( X ₀ , T) and F (X ₁ , T) fall into the adder 10, from the output of which the signal y goes to the indicating device (UI) 5.

In the case of automatic temperature compensation of the differential sensor, we have:

X ₀ = const

F (X ₀ , T) = k _0X · X ₀ + k _0T · T + C, where k _0X = const, k _0T = const, С = const.

Hence,

,

,

where C ₀ = const

,

,

where k _1X = const, k _1T = const, C ₁ = const

A prerequisite for the physical implementation of automatic temperature compensation of a differential sensor is the identity of the characteristics of the reference and measuring sensors, that is, in this case:

Thus, when substituting (2) and (3) in (4), taking into account (5), we obtain:

y = -k _1XX + C, ^'

that is, the difference between the signals of the reference and measuring sensors is directly proportional to the measured value of X and does not depend on the ambient temperature. Such a technical solution is very difficult technologically, and in some cases impossible, because sensors with identical properties cannot be manufactured.

The use of numerical methods of temperature compensation is associated with the calibration of the sensor along the temperature channel, with the measurement of the temperature of the sensor and, therefore, leads to an additional measurement error.

Thus, in the general case

k _0T ≠ k _1T

and as a result of the transformations we get

k = const, k _1X -const, C = const

Substituting (4) into (6), we obtain

The values of k and C are determined by processing the data obtained in the static mode (i.e., at X = 0) when the ambient temperature T varies within the operating range, performing linear regression of function (6), and the measurement of T in this case is not necessary. The proportionality coefficient 1 / k _1X is determined when calibrating the sensor.

For the physical implementation of this method, we offer structural diagrams of numerical methods of temperature compensation shown in FIG. 2 and FIG. 3, where

1 - adder;

2 - the source of the reference value (JOB);

3 - reference primary measuring transducer (D ₀ );

4 - primary measuring transducer (D ₁ );

5 - display device (UI);

6 - analog-to-digital Converter (ADC);

7 - computing device (WU);

8 - analog switch (AK);

9 - digital switch (CC);

X ₀ - input reference value;

X is the measured value;

F (X ₀ , T) is the output signal of the reference PIP;

F (X ₁ , T) is the output signal of the measuring PIP;

T is the ambient temperature.

In Fig.2, the signal X _{0 of the} source of the reference value 2 and X of the measured value are fed to the adder 1, from the output of which the signal X _{1 is} supplied to the primary measuring transducer (PIP) 4. The signal Xo is supplied to the reference PIP 3. Received output signals F (X ₀ , T) and F (X ₁ , T) fall into the adder 1, from the output of which the signal y enters the analog switch (AK) 8, then each of them is converted into a digital signal in an analog-to-digital converter (ADC) 6, after why both fall into the computing device (WU) 7, where the calculation of measured in masks X by the formula (7), and then the result is displayed on the display device (M) 5.

In Fig. 3, the signal X _{0 of the} source of the reference value 2 and X of the measured value are fed to the adder 1, from the output of which the signal X _{1 is} supplied to the primary measuring transducer (PIP) 4. The signal X _{0 is} supplied to the reference PIP 3. Received output signals F ( X ₀ , T) and F (X ₁ , T) are fed to separate analog-to-digital converters (ADCs) 6, then to a digital switch (CC) 9, after which both end up in a computing device (WU) 7, where the measured X value by the formula (7) and then the result is displayed on the display device (UI) 5.

Figure 4 shows a General view of a differential sensor for implementing this method of temperature compensation, which is an optocoupler with an open communication channel. The device operates as follows. Shutter 1 (in this case, the adder) at one end is rigidly fixed to the studied object. Under the action of radiation from the source of the reference value 2 (in this case, the emitter) in the reference PIP 4 and measuring PIP 3 (photodiodes with linear characteristics), a photocurrent occurs. Under the influence of the measured value X, the shutter 1 moves, which causes a change in the photocurrent in the measuring PIP 3. A change in the photocurrent in the reference PIP 4 occurs when the ambient temperature changes. Further, the processing of the received signals (F (X ₀ , T) and F (X ₁ , T)) is carried out in accordance with the description of the structural diagrams in Figure 2 or Figure 3.

In order to increase the convenience, efficiency and accuracy of the measurement, as well as the use of this sensor in digital automated systems and devices, the calculation of the measurement results is performed by a computing module connected to the sensor in a single electronic circuit by an interface device. One of the options for the device interface is shown in Fig.5. In this case, a PC of the type IBM PC is used as a computing module, which is connected to the device via the RS232 serial port. The device operates as follows. When the power is connected, the radiation from the VD1 emitter (Vishay Telefunken TSAL 6200 infrared LED) is incident on the VD2 and VD3 photodetectors (Vishay Telefunken BPW34 single-crystal silicon PIN photodiodes) with linear characteristics. The characteristics and operation mode of VD1 allow us to assume that X ₀ = const. From the photodetectors, the signals are fed to the normalizing amplifiers DA3 and DA4 (LM135 from National Semiconductor). The output of each normalizing amplifier is connected to the inputs of an analog switch (pins 2 and 3 DD2 - chip K561KT3). The state of the trigger DD1.1 (chip K561TM2) is checked by software and, if necessary, it is set to the logical state “1” at pin 1, which corresponds to the measurement mode of the reference sensor signal.

Further, the normalized voltage from terminal 7 of DA4.2 is supplied through the corresponding channel DD2 and the voltage divider on resistors R9 and R10 to the input of the DD3 ADC (in this case, a 12-bit ADC with the Burr-Brown ADS1286 serial interface is used). The DA5 chip (LM136-2.5V from National Semiconductor) is the source of the ADC voltage reference. Then, in accordance with the ADC communication protocol, the measurement, conversion and transmission of data to the PC is performed. To convert the voltage values of the TTL standard, in which the ADC operates, to the values of the RS232 protocol (and vice versa), the IC DD4 and DD5 are used in this circuit. DD4 (chip КР1561ТЛ1) is a set of Schmitt triggers with inversion, and DD5 is a universal asynchronous transceiver for interfacing with asynchronous serial data channels with an integrated doubler and voltage inverter (ST232 chip from STMicroelectronics). Then, according to the recession of the positive pulse at pin 3 DD1.1 at pin 1 DD1.1, the logical value “0” is set, and at pin 2 DD1.1 logical “1”, which corresponds to the voltage measurement mode at the output of the normalizing amplifier (pin 7 DA3 .2), to the input of which (pin 2 DA3.1) a photodetector VD2 is connected. In the future, the process of measuring the voltage at the output of the normalizing amplifier of the measuring sensor is similar to measuring the voltage at the reference sensor. Then, in the computing module, the weight values are calculated with the substitution of the corresponding values in formula (7). The stabilization of +5 volt and +9 volt supply voltages is carried out in the interface device, respectively, with DA1 chips (National Semiconductor chip LM78L05) and DA2 (National Semiconductor chip LM78L09 chip). The device is powered from a constant voltage source of 11.2 ÷ 30 volts and a load current of at least 25 mA.

Claims (1)

A method of temperature compensation of a differential sensor with linear characteristics, containing two primary converters (PP) with non-identical linear characteristics having a separate electrical output, moreover, one of the converters is a reference one and is designed to convert a signal from a reference value source to an output signal, and the other measuring is intended to convert the total signal of the source of the reference quantity and the measured quantity into an output signal, at which the formation of the output signals of the PP from analog to digital, then digital signals are sent to a computing device, where the calculation of the measured value of X is carried out according to the formula

where F (X ₀ , T) is the numerical value of the voltage from the reference PP, F (X ₁ , T) is the numerical value of the voltage from the measuring PP, 1 / k _1X is the proportionality coefficient determined by the calibration of the sensor, k and C are constant values for this sensor, determined by the experimental data obtained in the static mode (at X = 0) at a changing ambient temperature by linear regression of the function y = k · F (X ₀ , T) + k _1X · X + C, where y is the difference signals of reference and measuring PP.

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