CN116642602A - Cold junction analog compensation circuit - Google Patents
Cold junction analog compensation circuit Download PDFInfo
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- CN116642602A CN116642602A CN202210691906.7A CN202210691906A CN116642602A CN 116642602 A CN116642602 A CN 116642602A CN 202210691906 A CN202210691906 A CN 202210691906A CN 116642602 A CN116642602 A CN 116642602A
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01K—MEASURING TEMPERATURE; MEASURING QUANTITY OF HEAT; THERMALLY-SENSITIVE ELEMENTS NOT OTHERWISE PROVIDED FOR
- G01K7/00—Measuring temperature based on the use of electric or magnetic elements directly sensitive to heat ; Power supply therefor, e.g. using thermoelectric elements
- G01K7/02—Measuring temperature based on the use of electric or magnetic elements directly sensitive to heat ; Power supply therefor, e.g. using thermoelectric elements using thermoelectric elements, e.g. thermocouples
- G01K7/10—Arrangements for compensating for auxiliary variables, e.g. length of lead
- G01K7/12—Arrangements with respect to the cold junction, e.g. preventing influence of temperature of surrounding air
- G01K7/13—Circuits for cold-junction compensation
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E30/00—Energy generation of nuclear origin
- Y02E30/30—Nuclear fission reactors
Abstract
The application provides a cold end analog compensation circuit which is used for measuring the cold end temperature of a thermocouple. The cold end analog compensation circuit comprises: the first operational amplifier computing circuit receives the cold end temperature signal of the thermocouple and the first reference voltage to perform subtraction operation and output a first output signal, and the first operational amplifier computing circuit is used for eliminating the direct current bias voltage of the cold end temperature signal; the second operational amplifier computing circuit is used for receiving the temperature difference signal of the thermocouple and a second reference voltage to perform addition operation and output a second output signal, and the second output signal is in a positive voltage output range; and a third operational amplifier computing circuit receiving the first output signal and the second output signal for weighted subtraction for outputting a measured temperature signal whose voltage varies in proportion to the measured temperature.
Description
Technical Field
The application relates to the field of cold end compensation of thermocouples, in particular to a cold end analog compensation circuit.
Background
In temperature measurement applications, thermocouples are commonly used because of their robustness, reliability and faster response speed. The thermocouple has a simple structure and can be designed for specific applications. For example, it can be easily inserted into a product in the food industry; it can be welded to the product during the metal heat treatment.
Theoretically, thermocouples were measured at 0 ℃ at the cold end. However, the meter is typically below room temperature at the time of measurement, which results in the cold end of the thermocouple not reaching exactly 0 ℃. The temperature measurement principle of the thermocouple is to sum the cold end temperature of the thermocouple and the temperature corresponding to the measured potential difference of the cold end and the hot end of the thermocouple to obtain the hot end temperature of the thermocouple, namely the absolute temperature of the measured object, so that the cold end of the thermocouple cannot reach 0 ℃ and the absolute temperature of the measured object is not equal to the actual temperature of the measured object. This is the effect of the thermocouple measurement being subject to the float of the cold end temperature. The compensation measure to reduce this error is the cold side temperature compensation.
In the prior art, the cold end compensation technology mainly comprises an ice bath method, a cold end temperature measurement method and a compensation bridge method. For the cold end temperature measurement method, the existing cold end temperature measurement chip mostly comprises a single chip microcomputer, and digital signals, such as a MAX6675 type temperature measurement chip and the like, are output through operation of the single chip microcomputer. However, in order to reduce the cost and package, in some products that do not include a single-chip microcomputer, it is necessary that the cold-end temperature measurement chip be capable of directly outputting an analog signal. In the prior art, the temperature measuring chip of the LTC2997 model can realize similar functions, but the chip has higher price and larger package.
In summary, in order to solve the above-mentioned problems in the prior art, a circuit scheme of cold end compensation is needed in the art, which can overcome the defect that the thermocouple cannot measure the absolute temperature of the object under the condition that the singlechip is not needed to participate in processing the digital signal, so that after cold end compensation, the circuit can output the absolute temperature signal of the measured object in an analog signal manner.
Disclosure of Invention
The following presents a simplified summary of one or more aspects in order to provide a basic understanding of such aspects. This summary is not an extensive overview of all contemplated aspects, and is intended to neither identify key or critical elements of all aspects nor delineate the scope of any or all aspects. Its sole purpose is to present some concepts of one or more aspects in a simplified form as a prelude to the more detailed description that is presented later.
In order to solve the above problems in the prior art, an aspect of the present application provides a cold end analog compensation circuit for measuring a cold end temperature of a thermocouple. The cold end analog compensation circuit comprises: the first operational amplifier computing circuit receives the cold end temperature signal of the thermocouple and the first reference voltage to perform subtraction operation and output a first output signal, and the first operational amplifier computing circuit is used for eliminating the direct current bias voltage of the cold end temperature signal; the second operational amplifier computing circuit is used for receiving the temperature difference signal of the thermocouple and a second reference voltage to perform addition operation and output a second output signal, and the second output signal is in a positive voltage output range; and a third operational amplifier computing circuit receiving the first output signal and the second output signal for weighted subtraction for outputting a measured temperature signal whose voltage varies in proportion to the measured temperature. By adopting the cold end analog compensation circuit, the defect that the thermocouple cannot measure the absolute temperature of an object can be overcome under the condition that a singlechip is not needed to participate in processing digital signals, so that the circuit can output the absolute temperature signal of the measured object in an analog signal mode after cold end compensation.
In an embodiment, the first op-amp calculation circuit in the cold-end analog compensation circuit performs scaling on the cold-end temperature signal, and outputs a first output signal with a voltage linearly varying with respect to the cold-end temperature; the second operational amplifier computing circuit performs scaling on the temperature difference signal, outputs a second output signal with voltage linearly changing relative to the temperature difference signal, wherein the slope of the second output signal is consistent with that of the first output signal, and the third operational amplifier computing circuit performs subtraction operation with a weighting coefficient ratio of 1:1 on the second output signal and the first output signal to output a measured temperature signal with voltage changing in proportion to measured temperature.
In an embodiment, the slope of the second output signal and the slope of the first output signal in the cold-end analog compensation circuit are inconsistent, and the third op-amp calculation circuit performs weighted subtraction on the second output signal and the first output signal, wherein the weighted coefficient ratio of the second output signal and the first output signal is inversely proportional to the slope ratio of the second output signal and the first output signal, so that the coefficient of the cold-end temperature signal and the coefficient of the temperature difference signal are consistent.
In an embodiment, the first operational amplifier calculating circuit in the cold-end analog compensation circuit includes a first differential proportional amplifying circuit, an inverting input terminal of the first differential proportional amplifying circuit receives the cold-end temperature signal, and a non-inverting input terminal of the first differential proportional amplifying circuit receives the first reference voltage, wherein the first reference voltage is equal to a dc bias voltage of the cold-end temperature signal, and the cold-end temperature signal is greater than the first reference voltage so that the first output signal is in a negative voltage output range.
In an embodiment, the slope of the cold side temperature signal in the cold side analog compensation circuit is proportional to the cold side temperature change of the thermocouple, and the dc bias voltage is dependent on the electrical characteristics of the temperature measurement chip used to measure the cold side temperature signal.
In an embodiment, in the cold-end analog compensation circuit, the second operational amplifier computing circuit includes a second differential proportional amplifying circuit, a non-inverting input end of the second differential proportional amplifying circuit receives the temperature difference signal and the second reference voltage, a inverting input end of the second differential proportional amplifying circuit is connected with a resistor, and the other end of the resistor is grounded, and the second differential proportional amplifying circuit performs homodromous addition on the temperature difference signal and the second reference voltage, where the second reference voltage is set so that the second output signal is in a positive voltage output range.
In an embodiment, the third operational amplifier computing circuit in the cold-end analog compensation circuit includes a third differential proportional amplifying circuit, an inverting input terminal of the third differential proportional amplifying circuit receives the first output signal, a non-inverting input terminal of the third differential proportional amplifying circuit receives the second output signal, and a weighted subtraction is performed on the first output signal and the second output signal to obtain a measured temperature signal with a voltage varying in proportion to the measured temperature.
Drawings
The above features and advantages of the present application will be better understood after reading the detailed description of embodiments of the present disclosure in conjunction with the following drawings. In the drawings, the components are not necessarily to scale and components having similar related features or characteristics may have the same or similar reference numerals.
Fig. 1 illustrates a circuit diagram of a cold end analog compensation circuit provided in accordance with an aspect of the present application.
Reference numerals:
100: a cold end compensation circuit;
110: a first differential proportional amplifying circuit;
120: a second differential proportional amplifying circuit;
130: a third differential proportional amplifying circuit;
111: a first operational amplifier;
121: a second operational amplifier;
131: a third operational amplifier;
r1: a first resistor;
r2: a second resistor;
r3: a third resistor;
r4: a first feedback resistor;
r5: a fifth resistor;
r6: a sixth resistor;
r7: a seventh resistor;
r8: a third feedback resistor;
r9: a second feedback resistor;
r10: a tenth resistor;
r11: an eleventh resistor;
r12: a twelfth resistor;
VS1: a negative supply of the first operational amplifier;
VS2: a positive power supply of the first operational amplifier;
VS3: a negative power supply of the second operational amplifier;
VS4: a positive power supply of the second operational amplifier;
VS5: a negative power supply of the third operational amplifier;
VS6: a positive power supply of the third operational amplifier;
VS7: a first reference voltage; and
VS8: a second reference voltage.
Detailed Description
Further advantages and effects of the present application will become apparent to those skilled in the art from the disclosure of the present specification, by describing the embodiments of the present application with specific examples. While the description of the application will be presented in connection with a preferred embodiment, it is not intended to limit the inventive features to that embodiment. Rather, the purpose of the application described in connection with the embodiments is to cover other alternatives or modifications, which may be extended by the claims based on the application. The following description contains many specific details for the purpose of providing a thorough understanding of the present application. The application may be practiced without these specific details. Furthermore, some specific details are omitted from the description in order to avoid obscuring the application.
In the description of the present application, it should be noted that, unless explicitly specified and limited otherwise, the terms "mounted," "connected," and "connected" are to be construed broadly, and may be either fixedly connected, detachably connected, or integrally connected, for example; can be mechanically or electrically connected; can be directly connected or indirectly connected through an intermediate medium, and can be communication between two elements. The specific meaning of the above terms in the present application will be understood in specific cases by those of ordinary skill in the art.
In addition, the terms "upper", "lower", "left", "right", "top", "bottom", "horizontal", "vertical" as used in the following description should be understood as referring to the orientation depicted in this paragraph and the associated drawings. This relative terminology is for convenience only and is not intended to be limiting of the application as it is described in terms of the apparatus being manufactured or operated in a particular orientation.
It will be understood that, although the terms "first," "second," "third," etc. may be used herein to describe various elements, regions, layers and/or sections, these elements, regions, layers and/or sections should not be limited by these terms and these terms are merely used to distinguish between different elements, regions, layers and/or sections. Accordingly, a first component, region, layer, and/or section discussed below could be termed a second component, region, layer, and/or section without departing from some embodiments of the present application.
As mentioned above, the temperature measurement of the thermocouple was performed at the cold end, with 0 c as a standard. However, the instrument is usually at room temperature, which results in that the cold end of the thermocouple cannot reach exactly 0 ℃ so that the absolute temperature of the measured object measured by the thermocouple is not equal to the actual temperature of the measured object, and cold end compensation is needed. In the prior art, the chip for cold end temperature measurement in the cold end compensation method mostly comprises a single chip microcomputer, and digital signals are output through the operation of the single chip microcomputer. However, in order to reduce the cost and package, in some products that do not include a single-chip microcomputer, it is necessary that the cold-end temperature measurement chip be capable of directly outputting an analog signal.
In order to solve the problems in the prior art, the application provides the cold end analog compensation circuit which can overcome the defect that the thermocouple cannot measure the absolute temperature of an object under the condition that a singlechip is not needed to participate in processing digital signals, so that the circuit can output the absolute temperature signal of the measured object in an analog signal mode after cold end compensation.
Referring specifically to fig. 1, fig. 1 illustrates a circuit diagram of a cold-side analog compensation circuit provided in accordance with an aspect of the present application.
In some embodiments of the present application, the cold-side analog compensation circuit 100, as shown in fig. 1, includes a first op-amp calculation circuit, a second op-amp calculation circuit, and a third op-amp calculation circuit. The cold side analog compensation circuit 100 may be used to measure the hot side temperature of a variety of different types of thermocouples, such as a type K thermocouple, a type S thermocouple, and the like.
In order to more clearly describe the cold-side analog compensation circuit 100 protected by the present application, embodiments of the compensation circuit 100 for cold-side compensation for K-type thermocouple thermometry will be described in detail herein below.
In the illustrated embodiment of FIG. 1 of the present application, the cold side temperature signal of the K-type thermocouple may be measured and provided by an on-board temperature measurement chip TC 1047A. TC1047A is a temperature sensor with a linear voltage output whose output voltage is proportional to the measured temperature. TC1047A can accurately measure temperatures in the range of-40℃to +125℃. Typical values of the output voltage of such temperature sensors are: 100mV is output at-40 ℃,500mV is output at 0 ℃, 750mV is output at +25 ℃, and 1.75V is output at +125 ℃. The 10 mV/. Degree.C.output voltage slope of TC1047A allows accurate measurement of temperatures over a wide temperature range. TC1047A provides a 3-pin SOT-23B package suitable for space-critical temperature measurement applications.
It should be understood by those skilled in the art that the type of the temperature measuring chip used in the present application is not limited to the type mentioned in the above embodiments, and a skilled person may select different types of temperature measuring chips according to the type of the thermocouple measured in practical applications, for example, LM20 series of texas instruments, MCP9700 of MICROCHIP may be used to measure cold end temperatures of different types of thermocouples, and output cold end temperature signals of the thermocouples.
In this embodiment, the temperature measuring chip TC1047A may be disposed at the cold end of the K-type thermocouple, and is disposed within a range of about 20mm from the cold end of the K-type thermocouple, where the temperature of the cold end of the K-type thermocouple may be measured by the TC1047A more accurately. The signal output by TC1047A is y=500mV+10mV.T1, wherein T1 is the current temperature of TC1047A, namely the current temperature of the cold end of the K-type thermocouple measured by the temperature measuring chip, 500mV is the direct current bias of the temperature measuring chip TC1047A, and 10 mV/DEG C is the output voltage slope. The cold end temperature signal of the K-type thermocouple is the signal y=500mv+10mv×t1 output by TC 1047A.
Generally, a temperature measurement chip needs a larger signal output voltage slope in order to improve the accuracy. Therefore, the cold end temperature signal of the K-type thermocouple output by the temperature measuring chip TC1047A can be reduced by a certain multiple through the first operational amplifier computing circuit, namely, the slope of the cold end temperature signal is adjusted, and subtraction operation is carried out with the first reference voltage to output a first output signal for eliminating the direct current bias voltage of the cold end temperature signal.
As shown in fig. 1, in some embodiments of the present application, the first op-amp calculation circuit may select a differential proportional amplifying circuit. The cold end temperature signal outputted from TC1047A is inputted as a first input signal Vin1 to an inverting input terminal of a first operational amplifier 111 in the first differential proportional amplifying circuit 110 through a third resistor R3. The first reference voltage VS7 is applied as the second input signal Vin2 to the non-inverting input terminal of the first operational amplifier 111 through the first resistor R1. The output of the first operational amplifier 111 is coupled back to its inverting input via a first feedback resistor R4.
Since in practical application, the problem of the self structure of the operational amplifier can cause the offset voltage to exist at the input end, and the existence of the offset voltage can cause the offset voltage to easily influence or drown the tiny signal in the process of transmitting the tiny signal. The first reference voltage Vin1 may be generally set equal to the dc bias voltage value in the temperature measurement chip. The cold side temperature signal may be set to be greater than the first reference voltage Vin1 so that the first output signal output by the first differential proportional amplifying circuit 110 is in a negative voltage output range for subsequent operation. According to the working principle of the first differential proportional amplifying circuit 110, the second input signal Vin2 (the first reference voltage VS 7) is subtracted from the first input signal Vin1 (the cold end temperature signal), so that the direct current bias in the temperature measuring chip is subtracted by the connected first reference voltage VS7.
In order to ensure that the resistances of the two input terminals of the first operational amplifier 111 of the first differential proportional amplifying circuit 110 to ground are balanced, and in order to avoid reducing the common mode rejection ratio, the first resistor R1 may be equal to the third resistor R3, and the second resistor R2 may be equal to the first feedback resistor R4, that is, the ratio of the second resistor R2 to the first resistor R1 is the same as the ratio of the first feedback resistor R4 to the third resistor R3.
In some embodiments, the resistance of the first resistor R1 may be set to 25k, the resistance of the second resistor R2 to 3.9k, the resistance of the third resistor R3 to 25k, and the resistance of the first feedback resistor R4 to 3.9k. The reverse input end of the first differential proportional amplifying circuit 110 is connected to the cold end temperature signal Vin 1=500 mv+10mv×t1 of the K-type thermocouple, and the non-inverting input end is connected to the first reference voltage VS7 of 500 mV. According to the working principle of the first differential amplifying circuit 110, the output end outputs a first output signalThe resistance values of R1, R2, R3, R4 with the above settings can be obtained, and the first output signal outputted by the first differential ratio amplifying circuit 110 can be +.>
In this embodiment, after passing through the first differential proportional amplifying circuit 110, the slope of the cold end temperature signal of the K-type thermocouple measured by the temperature measuring chip TC1047A is reduced from 10mV/°c at the input to-1.56 mV/°c at the output, and as can be seen from the output result Vout1, the dc bias 500mV in the temperature measuring chip signal is removed by the first differential proportional amplifying circuit 110.
With continued reference to fig. 1, since the slope of the output voltage of the K-type thermocouple is smaller, the temperature difference signal of the cold and hot ends of the K-type thermocouple can be amplified by a certain multiple through the second operational amplifier computing circuit, that is, the slope of the temperature difference signal is adjusted, and the second output signal is outputted by adding to the second reference voltage, so that the second output signal is in the positive voltage output range.
The slope of the temperature difference signal output by the thermocouple is related to the type of thermocouple, and since the temperature difference between the cold end and the hot end of the K-type thermocouple is measured in this embodiment, the temperature difference signal output by the K-type thermocouple is z=0.04 mv×t2, where 0.04mV is the output voltage slope of the K-type thermocouple itself, and T2 is the temperature difference between the hot end and the cold end of the K-type thermocouple. The second operational amplifier calculating circuit may add a plurality of in-phase input signals by providing the in-phase input signals at the in-phase input terminal of the second differential proportional amplifying circuit 120.
Specifically, in the second differential proportional amplifying circuit 120 of fig. 1, the temperature difference signal output by the K-type thermocouple is used as the third input signal Vin3, and is connected to the non-inverting input terminal of the second operational amplifier 121 in the second differential proportional amplifying circuit 120 through the eleventh resistor R11. Meanwhile, the second reference voltage VS8 is used as a fourth input signal Vin4, and is commonly connected to the non-inverting input terminal through a twelfth resistor R12 and the third input signal Vin3 (temperature difference signal). The inverting input terminal of the second operational amplifier 121 is grounded through a tenth resistor R10, and the output terminal thereof is connected back to the inverting input terminal thereof through a second feedback resistor R9. Also, in order to ensure the balance of the resistances of the two input terminals of the second operational amplifier 121 of the second differential proportional amplifying circuit 120 to the ground, and to avoid lowering the common mode rejection ratio, it may be provided that the tenth resistance R10 is equal to the eleventh resistance R11, and the twelfth resistance R12 is equal to the second feedback resistance R9, that is, the ratio of the twelfth resistance R12 and the eleventh resistance R11 is the same as the ratio of the second feedback resistance R9 and the tenth resistance R10.
In some embodiments, the tenth resistor R10 may have a resistance of 1k, the thirteenth resistor R13 has a resistance of 1k, the twelfth resistor R3 has a resistance of 39k, and the second feedback resistor R9 has a resistance of 39k. The third input signal Vin3 input to the non-inverting input terminal of the second differential proportional amplifying circuit 120 is a temperature difference signal Vin 3=0.04 mv×t2 of the cold and hot ends of the K-type thermocouple, and the fourth input signal Vin4 input thereto may be set to 1250mV of the second reference voltage VS8. Because the temperature difference signal of the thermocouple may output a negative voltage, and the comparison circuit connected at the rear stage of the cold end analog compensation circuit 100 does not support the negative voltage input, in order to ensure that the signal finally output by the cold end analog compensation circuit 100 is not a negative voltage, the second output voltage of the second operational amplifier computing circuit may be raised, so as to ensure that the output value of the subsequent third operational amplifier computing circuit is not a negative voltage. In this embodiment, the signal output by the K-type thermocouple can be adjusted to a positive voltage output range by setting a dc bias of 1250mv as the second reference voltage VS8.
It will be appreciated by those skilled in the art that the second reference voltage VS8 includes, but is not limited to, 1250mV as mentioned in the above embodiments, and that the value of the second reference voltage VS8 may be self-adjusting depending on the type of thermocouple. The second reference voltage VS8 may be set to 1250mV for the K-type thermocouple in this embodiment. The second reference voltage VS8 is not necessarily a fixed value, but may be a range interval. For example, it is reasonable that the second reference voltage VS8 of the K-type thermocouple in the above embodiment is set in the range of 1200mV to 1300 mV.
The second output signal outputted by the second differential proportional amplifying circuit 120 according to the working principle of the in-phase addition implemented by the second differential proportional amplifying circuit 120With the resistance values of R9, R10, R11, R12 and the input values of Vin3 and Vin4 set as described above, the second output signal outputted from the second differential proportional amplifying circuit 120 may be ∈>After the second differential proportional amplifying circuit 120 is processed, the slope of the temperature difference signal of the K-type thermocouple is enlarged from 0.04 mV/DEG C at the input to 1.56 mV/DEG C at the output, and the temperature difference signal of the K-type thermocouple output by the second differential proportional amplifying circuit 120 is in a positive voltage output range as can be seen from the output result Vout 2.
In the above embodiment of the present application, since the cold side temperature signal of the thermocouple is obtained by the temperature measuring chip, the output voltage slope of the temperature measuring chip can be regarded as the slope of the cold side temperature signal of the thermocouple. The temperature difference signal of the cold and hot ends of the thermocouple is also a signal that the output voltage is in direct proportion to the measured temperature, so the slope of the temperature difference signal is the slope of the output voltage of the thermocouple.
In general, since the slope of the output voltage of the temperature measurement chip is greater than the slope of the output voltage of the thermocouple, the first differential proportional amplifying circuit 110 reduces the slope of the output voltage of the temperature measurement chip by a certain multiple to obtain a first output signal whose output voltage linearly changes with respect to the temperature of the cold end of the thermocouple, and the second differential proportional amplifying circuit 120 enlarges the slope of the output voltage of the temperature difference signal of the cold end of the thermocouple by a certain multiple to obtain a second output signal whose output voltage linearly changes with respect to the temperature difference of the cold end and the hot end of the thermocouple.
When the first slope value in the first output signal and the second slope value in the second output signal coincide, i.e. the absolute values of the first slope and the second slope coincide. In some embodiments, a subtraction operation with a weighting coefficient ratio of 1:1 may be performed on the second output signal and the first output signal by a third operational amplifier computing circuit to output a measured temperature signal with a voltage that varies in proportion to a measured temperature.
With specific reference to fig. 1, in some embodiments of the present application, the third operational amplifier computing circuit may include a third differential proportional amplifying circuit 130. The first output signal Vout1 outputted from the first differential proportional amplifying circuit 110 is used as an input signal to the inverting input terminal of the third operational amplifier 131 in the third differential proportional amplifying circuit 130. The second output signal Vout2 output from the second differential proportional amplifying circuit 120 is used as an input signal to the non-inverting input terminal of the third operational amplifier 131 in the third differential proportional amplifying circuit 130. The second output signal Vout2 is coupled to the non-inverting input terminal of the third operational amplifier 131 through the fifth resistor R5, and the non-inverting input terminal is grounded through the sixth resistor R6. The first output signal Vout1 is coupled to the inverting input terminal of the third operational amplifier 131 through the seventh resistor R7. The output of the third operational amplifier 131 is connected back to the inverting input thereof through a third feedback resistor R8.
Since the first output signal Vout1 is in the negative voltage range, the second output signal Vout2 is in the positive voltage range, and the absolute values of the first slope and the second slope are identical, the third differential proportional amplifying circuit 130 performs the same-proportion weighting coefficient difference between the first output signal Vout1 and the second output signal Vout2, so as to obtain the actual temperature signal of the measured object whose voltage varies in proportion to the measured temperature, that is, the temperature signal of the hot end of the K-type thermocouple.
For example, in the above embodiment, the first slope in the first output signal is-1.56, and the second slope in the second output signal is 1.56, and the slope values of the two are identical in magnitude, but opposite in sign. At this time, the differential scaling circuit 130 of the third operational amplifier computing circuit may adjust the weight coefficient ratio of the first slope to the second slope to be 1:1. For example, the resistance of the fifth resistor R5 in the differential proportional amplifying circuit 130 may be set to 10k, the resistance of the sixth resistor R6 may be set to 5k, the resistance of the seventh resistor R7 may be set to 10k, and the resistance of the third feedback resistor R8 may be set to 5k. The output signal of the third differential proportional amplifying circuit 130With the above-mentioned resistance values of R5, R6, R7 and R8, the hot-side temperature signal Vout of the thermocouple output from the third differential proportional amplifying circuit 130 is 0.5× [1250mV+1.56mV× (T2+T1)]. The weighted ratio of the first slope to the second slope is 0.5:0.5, i.e. 1:1. The t1+t2 in the output signal Vout of the third differential-scale amplifying circuit 130 is the actual temperature of the hot end of the thermocouple. One skilled in the art can directly find the hot end temperature value t1+t2 corresponding to the hot end voltage signal of the K-type thermocouple output by the third differential proportional amplifying circuit 130 through the national standard K-type thermocouple graduation table.
Preferably, in other embodiments, the third op-amp calculation circuit in the cold-end analog compensation circuit 100 protected by the present application may also select a unit differential proportional amplifying circuit for calculation convenience. Also in the case where the absolute values of the first slope of the first output signal and the second slope of the second output signal are equal, the resistances of the fifth resistor R5, the sixth resistor R6, the seventh resistor R7, and the third feedback resistor R8 may all be set to be the same, for example, four resistors are all set to 10k. The thermocouple's hot side temperature signal vout=vout 2-Vout 1=1250mv+1.56 mv× (t2+t1) output by the unit differential proportional amplifying circuit. The weighted ratio of the second slope to the first slope is directly 1:1. The T1+T2 in the output signal Vout of the unit differential proportional amplifying circuit is the actual temperature of the hot end of the thermocouple, and can be directly found by a person skilled in the art through a national standard K-type thermocouple graduation table.
It will be understood by those skilled in the art that in practical application, the consistency of the absolute values of the first slope and the second slope in the above embodiment may allow a certain deviation between the two slope values, so long as the deviation of the obtained two slope values is within the set range, both belong to the technical solution protected by the present application, and may also be used for executing the technical solution protected by the present application continuously.
Alternatively, in other embodiments of the present application, when the first slope value of the first output signal and the second slope value of the second output signal do not coincide, i.e. the absolute values of the first slope and the second slope do not coincide. In these embodiments, a weighted subtraction may be performed on the second output signal and the first output signal by a third op-amp calculation circuit. The third operational amplifier computing circuit adjusts the weighting coefficient ratio of the first output signal and the second output signal, so that the weighting coefficient ratio is inversely proportional to the slope ratio of the first output signal and the second output signal, and the coefficient of the cold end temperature signal in the first output signal is consistent with the coefficient of the temperature difference signal in the second output signal.
Specifically, as shown in fig. 1, the third operational amplifier computing circuit may select the third differential amplifying circuit 130. In an embodiment, it is assumed that the first output signal Vout1 is-2T 1 and the second output signal Vout2 is 1.25+t2, where the first slope is a coefficient-2 of the cold end temperature signal T1 included in the first output signal, the second slope is a coefficient 1 of the temperature difference signal T2 included in the second output signal, and a ratio of the first slope to the second slope is 2:1. The relationship between the resistance of the eighth resistor R8 and the seventh resistor R7 in the third differential amplifier circuit 130 can be set asThe resistance relationship between the sixth resistor R6 and the fifth resistor R5 is r6=2r5. According to the operation principle of differential amplification implemented by the third differential proportional amplifying circuit 130, the resistors R5, R6, R7 and R8 are connectedSubstituting the relation between the output signals of the third differential proportional amplifying circuit 130 In the formula of the output signal of the third differential proportional amplifying circuit 130, since the corresponding weight coefficient ratio of the first output signal Vout1 to the second output signal Vout2 is 0.5:1=1:2, and the ratio 2:1 of the first slope to the second slope is exactly in inverse relation, the coefficient of the cold end temperature signal T1 in the first output signal and the coefficient of the temperature difference signal T2 in the second output signal are obtained to be consistent, and the common factor can be extracted to obtain the actual temperature t1+t2 of the hot end of the thermocouple.
The operational amplifiers in the cold-side analog compensation circuit 100 protected by the present application, i.e. the first operational amplifier 111, the second operational amplifier 121, and the third operational amplifier 131, may be selected to be dual-power supply precision operational amplifiers with offset voltage less than 25uV, such as OP07C. The first operational amplifier 111 in the first differential proportional amplifying circuit 110 includes a positive power supply VS2 and a negative power supply VS1. The second operational amplifier 121 in the second differential proportional amplifying circuit 120 includes a positive power supply VS4 and a negative power supply VS3. The third operational amplifier 131 in the third differential proportional amplifying circuit 130 includes a positive power supply VS6 and a negative power supply VS5. Because the temperature measuring chip for measuring the cold end temperature of the K-type thermocouple is TC1047A, and the input of TC1047A is negative voltage after passing through the differential proportion circuit, the operational amplifier powered by double power supplies is needed, and the single power supply can not provide negative voltage output.
It should be further understood by those skilled in the art that the resistances of all the resistors in the first differential scaling circuit 110, the second differential scaling circuit 120 and the third differential scaling circuit 130 in the above-mentioned embodiments are not uniquely fixed, and are not limited to the resistances mentioned in the embodiments. The person skilled in the art can select a resistor with a proper resistance value for each amplifying circuit according to the type of the thermocouple for cold end compensation, the type of the temperature measuring chip for cold end temperature measurement and the slope value to be adjusted.
In summary, the application provides a cold end analog compensation circuit, which can provide a thermocouple cold end temperature signal through a temperature measuring chip under the condition that a singlechip is not needed to participate in processing digital signals, and convert the cold end temperature signal into a signal slope the same as that of the thermocouple through an operational amplifier circuit, so as to compensate the cold end temperature signal of the thermocouple, overcome the defect that the thermocouple cannot measure the absolute temperature of an object, and enable the circuit to output the absolute temperature signal of the measured object in an analog signal mode after cold end compensation.
The previous description of the disclosure is provided to enable any person skilled in the art to make or use the disclosure. Various modifications to the disclosure will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other variations without departing from the spirit or scope of the disclosure. Thus, the disclosure is not intended to be limited to the examples and designs described herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
Claims (7)
1. A cold end analog compensation circuit for measuring cold end temperature of a thermocouple, comprising:
the first operational amplifier computing circuit receives the cold end temperature signal of the thermocouple and the first reference voltage to perform subtraction operation and output a first output signal, and the first operational amplifier computing circuit is used for eliminating the direct current bias voltage of the cold end temperature signal;
the second operational amplifier computing circuit is used for receiving the temperature difference signal of the thermocouple and a second reference voltage to perform addition operation and output a second output signal, and the second output signal is in a positive voltage output range; and
and a third operational amplifier computing circuit for receiving the first output signal and the second output signal for weighted subtraction and outputting a measured temperature signal with voltage changing in proportion to the measured temperature.
2. The cold-end analog compensation circuit of claim 1, wherein said first op-amp calculation circuit performs scaling on said cold-end temperature signal, outputting said first output signal having a voltage that varies linearly with respect to said cold-end temperature signal;
the second operational amplifier computing circuit performs scaling on the temperature difference signal, outputs the second output signal whose voltage varies linearly with respect to the temperature difference signal, wherein a slope value of the second output signal is identical to a slope value of the first output signal,
the third operational amplifier computing circuit performs a subtraction operation with a weighting coefficient ratio of 1:1 on the second output signal and the first output signal to output a measured temperature signal in which the voltage varies in proportion to a measured temperature.
3. A cold-end analog compensation circuit according to claim 2, wherein the slope value of said second output signal and the slope value of said first output signal are not identical,
the third op amp calculation circuit performs a weighted subtraction of the second output signal and the first output signal, wherein a weight coefficient ratio of the first output signal and the second output signal is inversely proportional to a slope ratio of the first output signal and the second output signal such that a coefficient of the cold end temperature signal and a coefficient of the temperature difference signal agree.
4. The cold-end analog compensation circuit of claim 1, wherein the first op-amp calculation circuit comprises a first differential proportional-amp circuit having an inverting input receiving the cold-end temperature signal and a non-inverting input receiving the first reference voltage, wherein the first reference voltage is equal to the dc bias voltage of the cold-end temperature signal, the cold-end temperature signal being greater than the first reference voltage such that the first output signal is in a negative voltage output range.
5. The cold-end analog compensation circuit of claim 4, wherein a slope of said cold-end temperature signal varies proportionally with a change in cold-end temperature of said thermocouple, and said dc bias voltage is dependent on an electrical characteristic of a temperature measurement chip used to measure said cold-end temperature signal.
6. The cold-end analog compensation circuit of claim 1, wherein the second op-amp calculation circuit comprises a second differential proportional amplification circuit having a non-inverting input receiving the temperature difference signal and the second reference voltage, a inverting input connected to a resistor and having another end connected to ground, the second differential proportional amplification circuit adding the temperature difference signal and the second reference voltage in the same direction, wherein the second reference voltage is set such that the second output signal is in a positive voltage output range.
7. A cold-end analog compensation circuit according to claim 2 or 3 wherein said third op-amp calculation circuit comprises a third differential proportional amplifying circuit having an inverting input receiving said first output signal and a non-inverting input receiving said second output signal, and performing a weighted subtraction of said first and second output signals to obtain a measured temperature signal having a voltage that varies in proportion to a measured temperature.
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| CN202210691906.7A Pending CN116642602A (en) | 2022-02-15 | 2022-06-17 | Cold junction analog compensation circuit |
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