RU2389984C2 - Thermometric cable and calibration method thereof - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention is meant for measuring temperature using a contact method simultaneously in a group of locations on which the measuring cord of the thermometric cable can be laid. There are temperature sensors in the measuring cord. The cable has an electronic part made in form of an electronic unit having a connector for electrically connecting the cable to an external display device through a digital bus. Transmission over the digital bus is enabled by a microcontroller (or FPLD) in the electronic unit. The transmission format enables connection of a group of cables to the common digital bus. There are different versions of circuits using galvanic isolation, different synchronous and asynchronous data transfer protocols, different types of temperature sensors, as well as versions of the design of the measuring cord. The electronic unit has PROM in which calibration coefficients are recorded, from which temperature dependence of the thermal signal can be reconstructed, e.g. volt-temperature characteristic for each sensor of the cable, which is necessary for calculating measurement results in degrees. The said device implements the corresponding remote temperature measurement method.

EFFECT: increased measurement accuracy, increased compactness and simple manufacture and use.

17 cl, 6 dwg

Description

The invention relates to thermometry and is intended to measure temperature in a contact manner simultaneously in a group of locations along which a measuring cord of a thermometric braid can be laid. The invention can be used in geophysical surveys, in the agricultural and chemical industries for measuring the temperature distribution along the depth of the well, on the surface of the soil, in the volume of gaseous, liquid or granular media, along the length of the pipeline, in the volume of the room, for example, a greenhouse. The measurement is provided by digitizing the signals from a group of temperature sensors located at various points along the length of the measuring cord.

TERMINOLOGICAL DESIGNATIONS

- measuring cord - a group of temperature sensors connected by electric wires to the measuring circuit and mounted so that each sensor is located at a given length from the beginning of the spit, and the electrical connection of the wires to the measuring unit or device is located at the beginning of the spit;

- electronic unit - an electrical circuit structurally designed in the housing and having at least a connector for connecting to an external device, for example, to a display measuring device;

- thermometric braid - one or more measuring cords connected to an electronic unit;

- PC - personal computer;

- ADC - analog-to-digital converter;

- ROM - electronic digital non-volatile (permanent) storage device;

- EPROM - rewritable ROM;

- TKS - temperature coefficient of resistance [% / ° C];

- dynamic range - in this description means the absolute dynamic range, for example, the dynamic range of the sensor signal - this is the increment of the sensor signal, expressed in units of the signal, for example, millivolts (Umax-Umin), corresponding to the change in the measured value, for example temperature, in the entire operating range. The calculation of the relative dynamic range, expressed in decibels, with a linear assumption of the dependence of the sensor signal on the measured value is possible by the formula:

D = 20lg ((Umax-Umin) / U0) [dB],

where U0 is the minimum voltage value recorded by a particular equipment. Depending on the required variant of the relative dynamic range, for example, the voltage value corresponding to the value of the least significant bit of the ADC can be taken as U0, or the noise value can be taken as U0. In the latter case, the result will be the signal-to-noise ratio, expressed in decibels.

- digital bus - one or more electrical wires, not counting the common wires and wires for supplying power (as an option bus with power supply) connecting one or more devices capable of exchanging data through such an electrical connection;

- digital bus controller - an electronic circuit for generating the required levels of electrical signals for receiving / transmitting data on a digital bus;

- thermosignal - a signal received from a temperature sensor, analog, the voltage of which corresponds to the measured temperature, or a digital signal that carries a numerical value that corresponds to the measured temperature;

- discrete function - this refers to the option of representing the discrete function in the form of a set of thermosignal-temperature pairs;

- calibration of the thermometric braid - a procedure that includes determining the sequence of sensors in the measuring cord, as well as calibrating the sensors;

- sensor calibration - the procedure for determining calibration coefficients for the temperature dependence of the thermal signal;

- calibration coefficients - a series of numerical values by which it is possible to restore the temperature dependence of the thermosignal as a continuous function.

BACKGROUND

Known options for electrical connection of a group of temperature sensors in a garland with the aim of measuring temperature at various points corresponding to the location of the sensors [1 ... 4]. Often in practice it is required that this garland of sensors with connecting wires be flexible [1, 3]. Such a garland of sensors in the literature from the field of geophysical surveys has received the name - thermometric braid, or thermocosa. The sensors connected in a garland can be analog [1 ... 5] or having a digital output [8, 9]. As secondary converters for analog sensors, switch [4] and digital [2] voltage measuring instruments are used. Switch devices require a linear characteristic of the thermal signal, calibration of the scale of the device, or the use of linearizing amplifiers [5], which currently limits their use. Digital imaging devices containing a microcontroller allow you to register measurement data and perform mathematical processing of the thermal signal digitized using the ADC in order to calculate the temperature in degrees. The disadvantages of circuits with analog sensors include measurement errors due to the non-linearity of the volt-temperature characteristics of the thermosignal, a serial spread of the parameters of the sensors; errors due to spurious EMF at connection points, for example, connecting wires, as well as connectors, by the mutual influence of sensors connected to a common wire; loss of accuracy with increasing length of connecting wires. The disadvantage may be a large number of wires in the thermometric braid, for example, in the case of using a 4-wire connection scheme for each sensor [4, 5], which leads to an increase in the pseudo-diameter of the group of wires in the braid and complicates the process of its manufacture. The disadvantages of circuits with digital sensors include a limitation on the number of sensors connected to the digital bus [9], the maximum length of the wires of the digital bus [8] and the time it takes to convert the temperature into a digital code [8, 9].

The sensor connection scheme recommended by GOST 25358-82 [1] is the closest to that proposed in the present invention and is selected as a prototype.

SUMMARY OF THE INVENTION

The technical result of the present invention is to improve measurement accuracy, increase compactness, simplify manufacturing and operation, in particular, the ability to connect thermometric braids to a common digital bus for the purpose of remote data exchange.

Improving the measurement accuracy is achieved simultaneously by several elements of the fundamental construction of the thermometric braid:

1. The non-linearity and variation in the characteristics of the group of sensors are compensated by the mathematical processing of the thermosignal using a previously known temperature dependence of the thermosignal. These dependencies, examples of which are presented in Fig.6, for each sensor are determined during the calibration of the thermometric braid.

2. The circuit of the passage of all analog signals is constructed so that the passage of current through the analog switch and connector is excluded. There is no need for a connector through which analog signals would pass, since an ADC is installed directly in the electronic block of the thermometric braid (Fig. 1), and data is exchanged with an external device, such as a device that displays the measurement results on the display, via a digital receive bus / Transmission via microcontroller and digital bus controller. The analog switch, connecting any input to the analog output, transfers only the EMF of the thermal signal, i.e. only voltage potential. In this case, the current passing through the analog switch is present only at the time of switching the channel as a result of the transient process of recharging the total capacitance of the channel of the switch and the input capacitance of the ADC. The direct current passing through the switch is limited only by the leakage current, which is extremely small for making a noticeable error in the measurement of the thermal signal, because ADC input resistance can be of the order of 1000 [MΩ], which is documented in [7]. Thus, the influence of the channel impedance of the analog switch channel, the temperature drift of which is comparable to the thermistor TCS, is minimized. For example, for the ADG508A multiplexer, the drift is 0.6 [% / ° C] with an open channel resistance of 280 [Ohms], which is documented in [6].

Example: A change in the channel impedance of an analog switch channel when heating the housing of the microcircuit by 20 [° C] can be 2 [Ohm]. When the thermistor TCS 0.4 [% / ° C], the thermistor resistance 1000 [Ohm], the error introduced by the switch, the channel of which is connected in series with the thermal resistance, is estimated to be:

2 [Ohm] / (1000 [Ohm] · 0.4 [% / ° C] / 100 [%]) = 0.5 [° C].

At lower values of thermal resistance and TCS, characteristic of metal thermal resistance, the error will be greater.

3. Since all sensors are constantly included in the measuring circuit, when switching the channel of the analog switch, the sensors do not connect or disconnect from the supply voltage. Thus, the sensors are in a steady electric and thermal mode, which eliminates the measurement error, which can be caused, for example, by a change in the temperature of the sensor element of the sensor, for example, a semiconductor crystal of a thermistor or an integrated circuit chip as a result of supplying a voltage or current to the sensor .

4. Since the thermal signals are switched directly to the ADC, the analog signal transmission circuit does not contain active elements for analog processing of the thermal signal. In particular, the circuit does not contain an amplifier of signals from sensors, which could introduce an additional error, for example, related to the zero drift of an operational or instrumental amplifier.

5. The absence of a connector in the path of analog signals eliminates possible deviations of the contact resistances, as well as possible parasitic resistances, for example, when water condensates in the form of dew, which forms a resistive surface on the dielectric material of the connector. In addition to the above, it becomes possible to completely seal both the elements of the analog circuit and all elements of the electronic circuit, with the exception of the connector for connecting the thermometric spit to the digital bus. Sealing of the electronic unit can be done by filling the case with a casting compound, and sealing the measuring cord using a sealed tube as a sheath for the cord, in particular, heat-shrink, and sealing it at the ends, for example, using fusible polymer (hot melt adhesive) or polymerizable sealant. Sealing the measuring cord is also possible by extrusion coating a polymer material, for example a polymerizable sealant, on wires with sensors in order to obtain a monolithic structure. To protect the measuring cord from mechanical damage, it is advisable to additionally frame its shell with a metal or polymer flexible sleeve in the form of a corrugated tube, a wire braid or a wire or tape spiral. Since all wires and sensors are in a single shell, the measuring cord becomes more compact compared to a cord made of separate wires in its own shell, coming from each sensor. A measuring cord of a smaller cross-section can be laid, for example, inside a casing of a thermometer well of a smaller diameter, which reduces the cost of the latter. An embodiment of the thermometric braid is presented in Fig.5. Pin 21 is used to secure a removable load at the end of the spit to immerse the spit in the thermal well under its own weight.

6. The analog signal path does not contain connection points at which spurious emf of unpredictable magnitude could occur, which could not be compensated. As a rule, the emf is parasitic, which occurs at the connection points of the connector contacts and at the soldering points of the wires and other elements of the electrical circuit. The nature of such an EMF is the effect of a thermocouple that occurs at the connection points of conductors made of materials that differ in composition. In this case, such materials may be solder and copper wire. In the ideal case, the EMF of a thermocouple can be compensated for by the same EMF of opposite polarity from another thermocouple at the same connection point. For example, when soldering a copper wire to a copper contact pad, a copper-solder-copper structure is formed on the printed circuit board. In this structure, two EMFs of opposite polarity arise, which are completely compensated if the chemical compositions and crystal structure of the material of the copper wire and the copper contact pad are identical. A similar thermoEMF occurs at the places of soldering the microcircuit leads to the contact pads on the printed circuit board with the formation of the steel-solder-copper structure. In the latter case, the total EMF value is much larger [4, 5]. Also, a significant value of thermoEMF occurs at the soldering points of the wires to the connector pins made, for example, of silver or gilded brass. The reason for the change in the chemical composition and structure of the material of the contacts of the connector can also be their wear, which will lead to an additional change in thermopower.

Example: a change in the parasitic EMF when heated by 20 [° C] soldering points with POS-61 solder of a copper wire to a tinned steel terminal of a microcircuit or a connector pin can be 500 [μV]; with a dynamic range of a thermal signal equal to 250 [mV], corresponding to a range of measured temperatures of -50 ... + 50 [° C], the error introduced by this parasitic EMF will be:

(500 [μV] / 1000) [mV] / 250 [mV] · (50 - (- 50)) [° C] = 0.2 [° C].

Since it is not possible to thermostat the body of the electronic unit under the actual operating conditions of the thermometer, it is necessary to compensate for the effect of thermopower by directly incorporating special thermosensitive elements into the analog measuring circuit [4, 5] or to compensate for the effect of thermopower by mathematical processing of digital data of measurements of the thermal signal, having information on the values in advance spurious thermoEMF.

In the present invention, the problem of the influence of spurious EMF is solved by calibrating the thermometric braid, during which the total effect of all elements of the analog circuit, both the EMF and the resistances, for example, the resistances of the connecting wires, is taken into account. The purpose of calibration is to obtain in digital format the temperature dependences of the thermal signals from each sensor, and also, if necessary, to obtain additional temperature dependences of the thermal signals on the temperature of the electronic unit. To measure the temperature of the electronic unit, an additional analog or digital temperature sensor is installed in it, which is connected to the ADC or to the microcontroller of the thermometric braid. Thus, it becomes possible to compensate for the influence of spurious EMFs by mathematical processing of digital measurement data using a microcontroller in the electronic unit of the thermometric spit or by means of an external device to which the spit is connected.

7. It is possible to detect a temperature gradient in the body volume of the electronic unit. The uneven temperature in the electronic unit introduces an additional measurement error. The presence of a temperature gradient indicates the ongoing process of uneven heating or cooling of the housing of the electronic unit, which will lead to a change in the characteristics of microcircuits operating with analog signals. In this case, it is necessary to withstand the electronic unit until the thermal regime is established and the ADC is automatically calibrated by voltage. To detect the temperature gradient, it is advisable to install several temperature sensors in various locations of the electronic unit.

8. The nominal value and TCS of the thermistor or the sensitivity of the integrated sensor are selected providing the maximum value of the signal-to-noise ratio. As noted above, this ratio depends on the absolute value of the dynamic range of the thermal signal. The optimal value is the dynamic range of the analog thermosignal, comparable with the range of the input voltage of the ADC, which, as a rule, is equal to the supply voltage of the ADC. To eliminate possible signal distortion by the analog switch, it is advisable to further limit the range of the analog thermal signal. For example, with the ADC supply voltage equal to 5 [V], the optimal range of the thermal signal will be the interval from 1 to 4 [V]. In the circuit of the half-bridge resistive divider (Fig. 1) from the thermal resistance 1 and the constant resistor 5, the range of the thermal signal will depend on the value of the reference voltage Uref, the nominal value of the resistors and the TCS of the thermistor. The minimum resistance value of the thermistor is limited by the power consumption current, the maximum current of the reference voltage source, the power generated by the sensor in the form of heat, and the resistance of the connecting wires, which determines the magnitude of the mutual influence of the sensors connected to the common wire in the measuring cord. The maximum resistance value of the thermistor is limited by the insulation resistance, input resistance of the analog switch and ADC, as well as the value of spurious interference on the wires in the measuring cord. For a measuring cord with a length of 10 to 50 m with a temperature range from minus 50 to 50 [° C], the resistance of the thermistor is from 1 to 50 [kOhm] or the sensitivity of the integrated sensor is from 2.5 to 20 [mV / ° C].

9. Since the same reference voltage is used as a reference for the ADC and for supplying half-bridges of resistive dividers from thermistors 1 and constant resistances 5, the circuit is not very sensitive to deviations of the reference voltage.

10. To suppress spurious interference to the analog circuit, an integrating type ADC and / or an integrating stage at the input of the ADC are used, for example, an integrating RC circuit.

11. To reduce spurious interference from the digital bus, galvanic isolation of the electrical connection of the thermometric spit is used. Digital signals are transmitted via inductive or optical communication using a transformer or optocoupler, respectively, and thermometric spit power is supplied through a galvanically isolated voltage converter.

12. The digital format for transmitting measurement data from a thermometric spit to a recording / display instrument eliminates the loss of accuracy with increasing length of the connecting wire.

Simplification of the manufacture of the thermometric braid is associated with the absence of the need to monitor the sequence of wires from each sensor, which greatly simplifies and speeds up the manufacturing process of the measuring cord. The order of the sensors is determined during the calibration of the thermometric spit.

Simplification of operation of the thermometric braid is associated with the use of a digital format for exchanging data of the braid and an external device, which makes it possible to remotely access, connect a group of braids to a common digital bus, record measurement data in an external device, and when choosing the appropriate data transfer protocol and connect a radio modem data transmission over the air. A radio modem can also be part of an electronic unit, for example, when connecting a microcontroller to a transceiver.

Figure 1 shows a variant of the electrical circuit thermometric braids with analog temperature sensors. The number of switch channels for an example is chosen equal to four. Thermal resistance 1 is placed in the measuring cord 10, one terminal of each of which is connected to a common wire. In the electronic unit 20, each of the constant resistors 5 is included with the corresponding thermal resistance 1 in the half-bridge circuit of the resistive divider of the reference voltage Uref, formed by the reference voltage source 17. The voltage of the thermosignal from each resistive divider is supplied to the corresponding input X0 ... X3 of the analog switch 15. The channel number of the analog switch 15, the corresponding input of which is connected to output Y, is selected by a digital signal from microcontroller 14, for example, in the form of a binary code for address inputs A 0, A1, connected to the pins PA0, PA1 of the input / output port of the microcontroller 14. With the appropriate type of analog switch, this digital signal from the microcontroller can be transmitted in serial format, for example, corresponding to the SPI or I2C standard, as well as in the format of the position code. It is advisable to use the position code in the case of using an analog switch assembled on discrete elements, for example, if each channel is switched by one or a pair of complementary field-effect transistors. The output Y of the analog switch 15 through the integrating stage 18, made on an RC circuit, is fed to the ADC 13 at the input Ain1. The output of the analog switch 15 can be connected directly to the ADC 13 without an integrating stage 18 in the absence of the need for additional noise suppression of the thermal signal, for example, in the case of an ADC of the integrating type and / or due to the sufficiently large stray capacitance of the switch channel, which together with resistances 1 and 5 also creates an integrating RC chain. The thermistor 6 located in the electronic unit 20 is included in a similar circuit for the formation of a thermal signal supplied to the ADC 13 at its second input Ain2. The thermal signal from the thermistor 6 can also be connected to the free input of the analog switch 15. The thermistor 6 is used to determine the temperature of the electronic unit in order to compensate for the influence of spurious thermoelectric power by mathematical processing of the digitized thermal signals. Instead of a thermistor, it is possible to use a digital temperature sensor connected directly to the microcontroller 14. Signals from analog sensors can also be connected directly to the microcontroller 14, if the microcontroller has an integrated ADC, as shown in Fig. 2. To determine the temperature gradient by the volume of the electronic unit, it is necessary to install two or more digital or analog sensors in it, similar to sensor 6. Data exchange between the ADC 13 and microcontroller 14 is carried out via a digital interface, for example, through a two-wire serial synchronous protocol like SPI using signals C and D connected to the terminals PB0 and PB1 of the input / output port of the microcontroller 14, as shown in figure 1. EPROM 16 is similar to the ADC 13 is connected to the microcontroller 14 for data exchange via any digital protocol, such as I2C. To ensure the required amount of data, several EPROM chips can be included in the circuit. EEPROM 16 is necessary if the microcontroller 14 does not have a built-in user EEPROM, for example of type Flash, or its volume is insufficient. An EPROM in a separate microcircuit or as part of a microcontroller is necessary for storing calibration coefficients and other information necessary for the operation of the thermometer, for example, the serial number of the braid, text comments, etc. For connecting the thermometer to an external device, connector 7 is provided, which has contacts for digital data buses, for example, standard RS485, as well as contacts for supplying power to the thermometric spit. Figure 1 presents a variant of the circuit of an electronic unit with galvanic isolation for power and digital signals. The galvanic isolation of the power supply is performed on the voltage converter 11, the input terminals of which are supplied with voltage from the connector 7, and the output terminals of the converter 11 are connected to a common wire and the internal power supply bus Vcc. The Converter 11 must also provide voltage stabilization at its output. The galvanic isolation of digital signals is performed on optocouplers 12, instead of which transformers for inductive signal transmission can also be used. The microcontroller 14 through the signals T × D and R × D through the optocouplers 12 and the controller of the digital bus 9 communicates with an external device. The digital bus controller 9 is used to convert signals of the RS232 standard to RS485 and provides data exchange via an external digital bus connected to connector 7. The control of the data transmission direction signal necessary for the controller 9 to work is performed by the microcontroller 14 similarly to the T × D signal and is not shown in the diagram shown. In the case of data exchange via a digital bus according to the RS232 standard, T × D and R × D signals can be connected to connector 7 via optocouplers or directly in the absence of galvanic isolation. The RS232 and RS485 standards are asynchronous and, unlike synchronous ones, such as I2C and SPI, require exact matching of the data transfer speed. However, since the RS485 uses a differential pair as a data bus, this standard provides data transmission over larger distances. The supply voltage for the controller 9 forms a voltage stabilizer 8, which can be performed both on an integrated circuit and on discrete elements, for example, on a chain of a resistor and a zener diode connected to a common wire. The stabilizer 8 is necessary in case of excessive voltage coming from the connector 7, which can be specially provided for compensating for the voltage drop along the length of the wire when connecting a group of thermometric braids to one data and power bus. Figure 2 shows a simplified version of the electrical circuit thermometric braids without galvanic isolation. An ADC with an analog switch, an EPROM, and a voltage reference are part of the microcontroller 14. The functions of the digital bus controller are also performed by the microcontroller 14, from the input / output port of which the signals PB0 and PB1 are connected through a cascade 19 of resistances and zener diodes that limit the voltage to the digital bus via connector 7. The transmission format on the digital bus can correspond to a synchronous two-wire protocol of type I2C [9] or SPI, or a single-wire type of Micro-LAN [8]. In the latter case, one PB0 pin is sufficient to connect to the digital bus. The voltage stabilizer 8 provides power to the microcontroller, and is also used as a reference voltage source for the thermal signal generation circuit. The stabilizer 8 can be performed on an integrated circuit, for example, a general-purpose linear voltage stabilizer or on discrete elements. The stabilizer 8 may be absent if a stabilized supply voltage is supplied to the connector 7. In figure 2, as temperature sensors located in the measuring cord 10, integrated circuits 2 are used, which are included in the measuring circuit similarly to the thermistors 1. In addition, the measuring cord 10 in figure 2 is characterized in that the resistors 5 connected directly to it are located to a wire with a reference voltage passing along the length of the measuring cord. Figure 3 shows a variant of the circuit of the measuring cord 10, in which integrated circuits 3 are used as temperature sensors, which are connected to the power bus with two leads and form a voltage signal on a separate terminal. Instead of integrated circuits, hybrid circuits or discrete element assemblies consisting of thermal resistance and an analog signal amplifier can be used. Depending on the required measurement accuracy, the same voltage Vcc can be used to power the sensors 3 as for supplying the microcontroller 14 or a specially stabilized reference voltage Uref. The analog switch is part of the ADC 13, so the thermal signals from the sensors 3 are connected to the terminals of the ADC 13 controlled by the microcontroller 14.

Figure 4 shows a variant of the electrical circuit of a thermometric braid with digital temperature sensors connected to the microcontroller 14 via a single-wire interface with a data transfer format that allows connecting a group of sensors to a common digital bus. In the case of using digital sensors, the measurement error will be determined by the discreteness of the digital format in which the sensor transmits the thermal signal, as well as the noise of the thermal signal, including the reproducibility of the measurements. The nonlinearity of the thermosignal of digital sensors is significant in the range of negative temperatures and, in contrast to the temperature dependence of the thermosignal of analog sensors, is characterized by the presence of many extrema. The non-linearity of the thermosignal of digital sensors can also be compensated by mathematical processing using the previously known temperature dependence of the thermosignal. At the same time, for storing calibration coefficients, a separate EPROM located in the electronic unit is also necessary, since in digital sensors, the user EPROM is either absent or its volume is insufficient. It is possible to use digital sensors with various formats that provide connection to a common digital bus, for example, I2C [9]. However, in the latter case, as a rule, the format of these sensors does not allow connecting more than 8 sensors to one bus. The problem of limiting the number of sensors can be solved by using digital sensors that provide a chain mode of sensor selection. EPROM 16, located in the electronic unit, can be connected to the microcontroller 14 via the same digital bus as the sensors 4, or to the individual terminals of the microcontroller 14. The sensors 4 can be powered by a separate wire laid in the measuring cord, or parasitically by digital to the bus.

The options for connecting the microcontroller 14 to the remaining elements of the electrical circuit of the electronic unit 20, which are not shown in FIGS. 3, 4, are similar to the above options shown in FIGS. 1, 2. In all the described options, instead of the microcontroller 14, a FPGA programmable logic chip can be applied, performing the same functions.

The process of interaction of the thermometric braid with an external device occurs by executing commands that the external device sends, such as a request to read the EPROM data, write to the EPROM, read the result of ADC digitization, start the ADC internal auto-calibration by voltage, select the required analog switch channel, turn on / off power saving mode. Moreover, the thermometric braid answers only a request in which its serial number is present, which makes it possible to build a network of thermometric braids by connecting them to a common digital bus.

During operation of the thermometric spit, in order to obtain the measurement result in degrees, numerical mathematical processing of the digitized thermosignal is required, during which the temperature is calculated using the previously known functional dependence for each sensor. These functional dependencies should be stored digitally in the ROM of the thermometric spit. Calculations can be performed by the microcontroller of the thermometric braid or the microcontroller or microprocessor of an external device. The functional dependence of temperature on the voltage of the thermosignal is determined during the calibration of the thermometric braid. The dimension and digital format of the thermal signal voltage can be in volts or millivolts, digitized in a floating-point format or other units, for example, in the format of integer data that microcontroller 14 receives from ADC 13, or in the format that digital temperature sensor 4 sends.

Figure 6 presents examples of volt-temperature characteristics of thermal signals obtained from various analog sensors during the calibration of two thermometric braids. The group of curves 1 was obtained during the calibration of the braid, in the measuring cord of which there were placed semiconductor thermistors with negative TCS, included in the circuit in figure 1. A linear relationship 3 is obtained with one of the integrated circuits 3 included in the circuit of the measuring cord 10 in figure 3.

Calibration of the thermometric braid includes determining the sequence of sensors in the measuring cord and calibrating the sensors, during which the volt-temperature characteristics of the thermal signals from the sensors are determined.

To determine the temperature dependences of the thermosignals, all the measuring cords of the thermometric braid are placed in the thermostat chamber, in which the required temperature is set, and the digitized values of the thermosignals are obtained from all sensors. The cycle of receiving thermal signals from the sensors and the corresponding temperature in the thermostat chamber is repeated, each time changing the temperature, and thereby the temperature dependences of the thermal signals of each sensor are obtained in the form of discrete functions in the experimentally studied temperature range. In the case of receiving thermal signals only at two temperatures of the thermostat chamber, the temperature dependences of the thermal signals will be obtained in the approximation of a linear function. The thermometric braid is connected via a digital bus to a recording device, by means of which a digitized value of the thermosignal is obtained from each sensor. The calibration process can be controlled by an external recording and / or display device or a PC connected to it. The temperature value established in the thermostat chamber at the time of the ADC survey is obtained either by manually entering from the keyboard of the recording device or from the PC keyboard, or by obtaining in digital format the temperature value from the previously calibrated reference thermometer, similarly connected via a digital bus to the recording device.

The obtained temperature dependences of the thermosignals are recorded in the EPROM of the thermometric braid in the form of a series of calibration coefficients in a digital format, the structure of which can be different. A number of numerical values by which it is possible to restore the temperature dependence of the thermosignal as a continuous function are called calibration coefficients in the present description.

The structure of the digital format of the calibration coefficients may correspond to the aforementioned discrete function, in which a series of digitized values of thermal signals are compared to a number of temperatures. Such pairs of thermosignal and temperature are actually experimental data. In Fig.6 they are indicated by points through which a continuous line of the voltage-voltage characteristics of the signal from a particular sensor passes, which is required to be fixed in the ROM of the thermometric spit.

The structure of the digital format of the calibration coefficients may correspond to a series of coefficients of a continuous function in which the calibration coefficients are coefficients of the functional dependence of temperature on the thermal signal. For example, a functional relationship may be polynomial:

T = A ₀ + A ₁ · x + A ₂ · x ² + ... + A _n · x ⁿ ,

where T is the temperature, x is the value of the thermal signal, A ₀ ... A _n are the calibration coefficients. In this case, the calibration coefficients are the coefficients of the equation. The coefficients are obtained by regression mathematical processing of the experimentally obtained discrete function of the temperature dependence of the thermal signal.

To compensate for the influence of parasitic EMFs at points of electrical connections, such as places of soldering wires and microchips, as well as the influence of undesirable temperature deviations of the parameters of circuit elements, such as the temperature deviation of the resistances of the constant resistors 5 located in the electronic unit, it is necessary to experimentally obtain the functional dependence of the thermal signals from the temperature of the electronic unit at a constant temperature of the measuring cord. It is advisable to obtain the dependence of the increments of the thermal signals on the temperature of the electronic unit, taking for zero values the increment data at the temperature of the electronic unit at which the sensors were calibrated in the measuring cord. The obtained dependences of the temperature increments will be arithmetic corrections for the corresponding thermal signals and, when added, will form values for a more accurate calculation of the sensor temperatures. Since this temperature increment of the thermosignal is insignificant compared to the dynamic range of the thermosignal, it is possible to take the dependence of the thermosignal increment on the temperature of the electronic unit linear and limit ourselves to two experimental points for the two temperatures of the electronic unit. At one of these temperatures, as mentioned above, the temperature increments are zero, because at a given temperature of the electronic unit, the dependences of the thermal signals on the temperature of the sensors are obtained. Thus, it remains to obtain the mentioned temperature increments at a different temperature of the electronic unit. For example, the calibration of the sensors of the measuring cord was performed at room temperature of the housing of the electronic unit 25 [° C], the temperature of 35 [° C] was set in the thermostat chamber. Without removing the measuring cords from the thermostat, place the electronic unit in the second thermostat, set the temperature in it to minus 30 [° C] and maintain until the temperature gradient in the volume of the body of the electronic unit becomes insignificant, which can be determined either by the exposure time or by temperature differences from several sensors installed at various points of the electronic unit. They re-calibrate the sensors of the measuring cord at the current temperature of 35 [° C] and find the increments of the thermal signals from all sensors as the difference of the thermal signals at different temperatures of the electronic unit. Knowing the temperature difference of the electronic unit 25 - (- 30) = 55, the dependence of the thermal signal increment on the temperature of the electronic unit is determined as a linear function represented by two coefficients. These coefficients for all thermal signals are recorded in the ROM. Without assuming the linear dependence of the thermal signals on the temperature of the electronic unit, to obtain the coefficients of the function of two variables (two temperatures: the sensors of the measuring cord and the electronic unit), multiple calibration of the sensors of the measuring cord at a number of different temperatures of the electronic unit will be required and a larger amount of ROM data will be required to record calibration coefficients, which may be applicable only in exceptional cases.

In the manufacturing process of the measuring cord, the process of tracking the sequence of connecting wires coming from each analog sensor to the electronic unit is time-consuming, or the process of determining and tracking the sequence of serial numbers of digital sensors. To optimize production, it is advisable to connect these connecting wires (for example, solder to the pads of the printed circuit board) to the inputs of the analog switch in random order or mount digital sensors without tracking their serial numbers. Due to the arbitrary order of connecting the sensors to the inputs of the analog switch, it will be necessary to determine the correspondence of the channels of the analog switch to the sensors connected to them or to determine the sequence of serial numbers of digital sensors, which can be done by responding to the temperature signals of the sensors, for example, by heating a portion of the measuring cord. To do this, the sensors are sequentially heated in the order of their location in the measuring cord by locally heating a portion of the measuring cord with a length shorter than the distance between the sensors, and at the same time changes in all thermal signals are recorded. The thermometric braid is connected via a digital bus to a recording device, by which the result of digitizing the thermal signals of all sensors in series is obtained. Moreover, the duration of the shift of the heating section of the measuring cord by an amount equal to the distance between the sensors should be at least twice as long as the cycle for obtaining the digitized values of all thermal signals. Analyze the obtained data in order to detect the response of the thermal signal to the heating of the sensor. In the process of sequential heating of the sensors, a numerical series of channel numbers of the switch or a series of serial numbers of digital sensors are composed in the sequence in which the responses of the thermal signals to heating were recorded, and this numerical series is recorded in the ROM of the thermometer.

LITERATURE

[1] GOST 25358-82, Soils, Field method for temperature determination (Appendix 3).

[2] SU 1234730, 1986.05.30, IPC G01K 7/02, Multichannel digital thermometer.

[3] US 6431750, 2002.08.13, IPC G01K 7/00, Flexible temperature sensing probe.

[4] Preobrazhensky V.P. Thermotechnical measurements and devices. - M .: Energy, 1978, 704 p.

[5] Wigleb G. Sensors - M.: Mir, 1989, 196 p.

[6] Datasheet ADG508A, Analog Devices, www.analog.com

[7] Datasheet AD7714, Analog Devices, www.analog.com

[8] Datasheet DS18S20, Dallas Semiconductor, www.dalsemi.com

[9] Datasheet DS1624, Dallas Semiconductor, www.dalsemi.com

Claims (17)

1. Thermometric braid, including one or more measuring cords, in each of which there is one or more temperature sensors, including also a non-volatile rewritable storage device EPROM, characterized in that the EPROM contains calibration coefficients of the functions of the temperature dependences of the thermal signals of the sensors, as well as the information necessary for operation of the thermometric braid, at least the serial number of the braid and a number of sensor identifiers, while the identifier for the analog sensor is the channel number of the switch to which the corresponding sensor is connected, and for the digital sensor is the serial number of this sensor, which is the address of the sensor on the digital bus, and the ROM is either in separate cases of one or more microcircuits or is part of a microcontroller or microcircuit programmable logic FPGA. 2. The thermometric spit according to claim 1, characterized in that it further comprises a microcontroller or FPGA programmable logic chip that provides mathematical processing of the thermosignal using calibration coefficients recorded in the ROM, and / or providing data exchange with temperature sensors and / or an external device, as well as a digital bus controller that provides data exchange between the thermometric scythe and an external digital device in the form of a display or recording device and allows connecting a group of thermometric braids to a single digital data bus, where the EPROM is either in separate cases of one or more microcircuits or is part of a microcontroller or FPGA, and the digital bus controller is either in a separate case of microcircuit and / or on discrete elements or is part of a microcontroller or FPGA, moreover, to exchange data with an external device, a connector is used for electrical connection of the thermometric spit, which has terminals for power supply and terminals for a digital bus s to the digital bus controller, and / or transceiver is capable of receiving and transmitting data across radio waves. 3. The thermometric spit according to claim 1, characterized in that the electronic unit additionally contains cascades of galvanic isolation, including a galvanically isolated voltage converter, as well as cascades of inductive and / or optronic transmission of digital signals, made using transformers for inductive signal transmission or optocouplers . 4. Thermometric braid according to claims 1 to 3, characterized in that it further comprises one or more sensors for measuring the temperature of the electronic unit. 5. The thermometric spit according to claim 2, characterized in that the digital bus controller is configured to transmit data using an asynchronous data transfer protocol, such as RS-232 or RS-485, or synchronous data transfer protocols, such as SPI or I2C, or single-wire data protocol, such as Micro-LAN. 6. Thermometric braid according to claim 1, characterized in that the measuring cord is additionally framed by a metal or polymer flexible sleeve in the form of a corrugated tube, wire braid or wire or tape spiral. 7. The thermometric braid according to claim 1, characterized in that thermal analogs are used as analog temperature sensors, in particular metal thermistors or semiconductor thermistors, or integrated circuits similarly included in the measuring circuit, each of which is connected to a common wire through one terminal passing through the length of the measuring cord, and the other terminal is connected to a constant resistor separate for each sensor and connected according to the resistive divider circuit of the reference voltage. 8. Thermometric braid according to claim 7, characterized in that the constant resistors of the measuring circuit are located in the electronic unit of the thermometric braid. 9. The thermometric spit according to claim 7, characterized in that the constant resistors of the measuring circuit are located in the measuring cord, in which, in addition to the common wire, a wire with a potential of a reference voltage passes to which constant resistors of the measuring circuit or thermal resistance are connected. 10. The thermometric braid according to claim 7, characterized in that the ohmic value of the thermal resistance when the temperature changes within the measured range is not less than 1 kOhm and not more than 50 kOhm. 11. The thermometric spit according to claim 1, characterized in that integrated microcircuits or assemblies on discrete elements consisting of thermal resistance and an analog signal amplifier that generate temperature-dependent voltages are used as analog temperature sensors, and the microcircuits or assemblies are connected to the power bus passing along the length of the measuring cord and consisting of a common wire and a wire with a potential of the reference voltage. 12. Thermometric braid according to claim 11, characterized in that the sensitivity of the analog sensors is in the range from 2.5 to 20 [mV / ° C]. 13. A method for calibrating a thermometric spit, including determining the sequence of sensors in the measuring cord and determining the temperature dependences of the signals of all sensors, characterized in that the sequence of sensors in the measuring cord is determined by sequentially heating the sensors in the order they are located in the measuring cord by local heating of the measuring section cord with a length less than the distance between the sensors, and simultaneously record changes in signals from all sensors so that The thermometric braid is connected via a digital bus to the recording device, by which signals are received sequentially from all sensors, the received digital data is analyzed to detect the response of the signal to the sensor’s heating, they comprise a numerical series of switch channel numbers or a series of serial numbers of digital sensors of the measuring cord in the sequence in which the responses of the signals to heating were recorded, and this numerical series is recorded in the ROM of the thermometric braid, and the temperature dependences the signals of all sensors are determined by placing the measuring cord of the thermometric braid in the thermostat chamber, setting the required temperature, connecting the thermometric braid via a digital bus to the recording device, by means of which the digitized signal value from each sensor of the measuring cord is obtained, the temperature value in the thermostat chamber is obtained, or by manual input from the keyboard of a recording device or keyboard of a personal computer, or via a digital bus from a pre-calibrated raztsovoy thermometric streamer connected to the recording device, processes the received digital values thermostat and sensor to produce calibration coefficients which are recorded in the PROM thermometric braids, and the digitized value of the signal from each sensor is obtained at least at two different temperatures in the thermostat chamber. 14. The method of calibrating a thermometric braid according to claim 5, characterized in that the calibration coefficients are discrete functions of the temperature dependences of the sensor signals in the form of a series of signal values of each sensor and the corresponding temperatures in the thermostat chamber at the time of the ADC survey. 15. The method for calibrating a thermometric spit according to claim 5, characterized in that the calibration coefficients are coefficients of continuous functions of the temperature dependences of the sensor signals, the coefficients being determined by regression mathematical processing of the discrete functions of the temperature dependences on the sensor signals obtained experimentally as a result of sensor calibration. 16. The method for calibrating a thermometric spit according to claim 7, characterized in that the functional dependence is a polynomial dependence of the temperature on the sensor signal:
T = A ₀ + A ₁ · x + A ₂ · x ² + ... + A _n · x ⁿ ,
where T is the temperature, x is the value of the sensor signal, A ₀ ... A _n are the calibration coefficients. 17. The method of calibrating a thermometric braid according to any one of claims 5-8, characterized in that it further determines the dependence of the analog signals of the sensors of the measuring cords on the temperature of the electronic unit, for which the electronic unit is placed in the chamber of the second thermostat, set the required temperature in it and get digitized the values of the signals from each sensor of the measuring cords at least at two different temperatures of the electronic unit.

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