CN103542954A - High-precision thermocouple input module and measurement method - Google Patents
High-precision thermocouple input module and measurement method Download PDFInfo
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Abstract
The invention relates to a high-precision thermocouple input module comprising an isolator, an amplifier, an analog-digital conversion unit, a controller, a digital-analog converting unit and an electronic switch. The isolator is connected with a thermocouple, the output end of the amplifier is connected with the analog-digital conversion unit, and the output end of the analog-digital conversion unit is connected to the controller. The module is characterized in that the grounding end of the isolator is grounded and outputs a grounding joint outwards; the voltage output end of the isolator outputs two joints outwards; the controller outputs feedback signals to the digital-analog converting unit according to signals acquired from the analog-digital conversion unit; and the output end of the digital-analog converting unit outputs an input joint outwards. Another technical scheme of the invention provides a high-precision thermocouple input measuring method adopting the thermocouple input module. The module has the advantages that the entire absolute accuracy of thermocouple signals is guaranteed to range from 0.1 to 0.2 DEG C, the absolute precision can achieve 0.005 to 0.01%, and actual requirements of the industrial control process can be met.
Description
Technical field
The present invention relates to a kind of high-precision thermopair load module and measuring method, for improving the occasionally measuring accuracy of the small-signal such as millivolt of thermoelectricity, belong to instrument for automatic control technical field.
Background technology
Thermopair is that the metal wire by two kinds of different components connects to form closed-loop path in its end, directly the one end as measuring media temperature is called working end (also referred to as measuring junction), the other end is called cold junction (also referred to as compensation end), when there is temperature difference between working end and cold junction, in loop, just having electric current passes through, now between two ends, just there is thermopower (Seebeck effect), according to circumstances select different metal materials just can draw approximately Voltage-output proportional to temperature difference.But this voltage is all very very little, for example more modal nickel chromium triangle-nisiloy K type thermocouple-270 ℃～1372 ℃ of corresponding thermoelectrical potentials are-6.458mV～54.886mV, and nickel chromium triangle-cupronickel E type thermocouple-270 ℃～1000 ℃ of corresponding thermoelectrical potentials are-9.835mV～76.373mV.
The signal processing method of the thermopair load module of scattered control system (DCS) or programmable control system (PLC) dispatch control system is to obtain the thermopower (millivolt voltage signal) being produced by thermopair, using it as analog electrical signal, through amplifier, be amplified to volt voltage signal, via analog to digital converter (ADC), be input in the numerical data of control chip again, by control chip or transfer to host computer table look-up according to calibration number or adopt temperature estimation model formation to process to convert to actual temperature.
Because semiconductor technology is relatively backward, in order to overcome the problem that control chip processing speed is slow and memory span is little, adopt various temperature estimation model formations to process in early days.Along with the technical indicator of control chip processing speed and memory capacity significantly promotes, adopt the most original tabling look-up to overcome the small error of temperature estimation model formation and actual temperature.
From surface, thermocouple signal measuring accuracy can focus on analog to digital converter (ADC), general analog to digital converter adopts 12, although analog to digital converter theoretical resolution is 1/4096 (2/10000ths), but due to the temperature of the components and parts such as the reference voltage of the components and parts such as amplifier and analog to digital converter and the resistance reason of wafing etc., the measuring accuracy of in fact whole thermocouple signal is generally between 0.1～0.2%, even if some producer brings up to 13 or 14, even 15 by analog to digital converter, but overall measurement accuracy can not improve how many.
In industrial process is controlled, this relative accuracy is for the valve seat opening of topworks, pressure, flow, liquid level, electric current and voltage etc. process variable problem little, but for temperature signal, because it does not have concrete range ability, generally thermopair load module is the thermocouple signal of compatible various calibration number, the thermoelectrical potential signal adopting has enough upper limit nargin, be converted to the temperature range upper limit and can reach 2000 ℃, at this moment relative accuracy is converted to absolute value and can arrives 2 ℃～4 ℃.And in fact, low temperature generally can occur in device start process, when industrial process normally moves, main temperature course parameter can be carried out variation by a small margin at a higher temperature.For example 600MW ultra supercritical thermal power generation unit boiler superheater outlet temperature is in the temperature operation between 560 ± 10 ℃ substantially when above of 50% declared working condition; range of temperature is at 20 ℃; if and used as boiler safety guard signal; jump stove value and be also only 597 ℃, the absolute error that temperature variation is+37 ℃ at this moment 2 ℃～4 ℃ just seems very serious.
Summary of the invention
The technical problem to be solved in the present invention is the precision that guarantees thermocouple signal.
In order to solve the problems of the technologies described above, a technical scheme of the present invention has been to provide a kind of high-precision thermopair load module, comprise isolator, amplifier, AD conversion unit, controller, D/A conversion unit and electronic switch, isolator is connected with thermopair, the output terminal AD conversion unit of amplifier, the output terminal of AD conversion unit connects controller, it is characterized in that: the earth terminal of isolator is outwards drawn ground interface in ground connection, the voltage output end of isolator is outwards drawn two interfaces simultaneously, be respectively the first Voltage-output interface and second voltage output interface, the signal that controller basis is obtained by AD conversion unit is to D/A conversion unit output feedback signal, the output terminal of D/A conversion unit is outwards drawn output interface, action by electronic switch makes the in-phase input end of amplifier be connected with the first Voltage-output interface, the inverting input of amplifier is connected with ground interface simultaneously, or the in-phase input end of amplifier is connected with output interface, the inverting input of amplifier is connected with ground interface simultaneously, or the in-phase input end of amplifier is connected with output interface, the inverting input of amplifier is connected with second voltage output interface simultaneously.
Preferably, described amplifier is programmable operational amplifier.
Preferably, described electronic switch has two, is single-pole double-throw (SPDT) electronic switch.
Preferably, described AD conversion unit and described D/A conversion unit are selected the converter of 12 precision.
Another technical scheme of the present invention has been to provide a kind of high-precision thermopair input measurement method, adopts above-mentioned thermopair load module, and its basic ideas are:
The enlargement factor of step-down amplifier, is divided into small-range scope by the bound range reference point of temperature from the section of super large range ability after prediction actual measurement temperature value.Controller utilizes digital to analog converter to form a closed loop as feedback end and input end, adjusts the reference data value of amplifier to overcome the composition error of input end and feedback end.Increase substantially the enlargement factor of amplifier, in conjunction with the reference data value of corrected amplifier, through the temperature value that high-precision thermocouple signal is corresponding that calculates of controller, be characterised in that, concrete steps are:
The first step, operation electronic switch, the in-phase input end of amplifier is connected with the first Voltage-output interface, the inverting input of amplifier is connected with ground interface simultaneously, the enlargement factor of amplifier is set to A, controller obtains the initial value by the output voltage V p of AD conversion unit, according to the initial value of output voltage V p and corresponding temperature Tp thereof, the upper limit range value of thermopair load module is defined as to Tp+A ℃, lower limit range value is defined as Tp-A ℃, according to calibration number, tables look-up and finds the thermopower V0 corresponding with Tp+A ℃;
Second step, operation electronic switch, the in-phase input end of amplifier is connected with output interface, the inverting input of amplifier is connected with ground interface simultaneously, adjust the feedback voltage V f of controller, make the reference voltage Vref=V0+ Δ 1 on output interface, Δ 1 is the input composition error of amplifier and AD conversion unit;
The 3rd step, operation electronic switch, the in-phase input end of amplifier is connected with output interface, the inverting input of amplifier is connected with second voltage output interface simultaneously, the enlargement factor of amplifier is set to B, B is much larger than A, now, reference voltage Vref=V0+ Δ 1 on the in-phase input end of amplifier, voltage on the inverting input of amplifier is the output voltage V i of isolator, end difference is V0+ Δ 1-Vi, deduct after the input composition error Δ 1 of amplifier and AD conversion unit, the output voltage V p=V0+ Δ 1-Vi-Δ 1=V0-Vi of final AD conversion unit, the temperature T i that this thermopower is corresponding, final temperature value is Tp+A-Ti.
Advantage of the present invention is: guarantee that the whole absolute precision of thermocouple signal is at 0.1～0.2 ℃, relative accuracy can reach 0.005～0.01%, can meet the real needs of industrial control process.
Accompanying drawing explanation
Fig. 1 is a kind of high-precision thermopair load module schematic diagram provided by the invention.
Embodiment
For the present invention is become apparent, hereby with preferred embodiment, and coordinate accompanying drawing to be described in detail below.
As shown in Figure 1, a kind of high-precision thermopair load module provided by the invention, comprise isolator, amplifier, AD conversion unit ADC, controller, D/A conversion unit DAC and electronic switch, isolator is connected with thermopair, the output terminal AD conversion unit ADC of amplifier, the output terminal of AD conversion unit ADC connects controller, it is characterized in that: the earth terminal of isolator is outwards drawn ground interface 4 in ground connection, the voltage output end of isolator is outwards drawn two interfaces simultaneously, be respectively the first Voltage-output interface 1 and second voltage output interface 3, the signal that controller basis is obtained by AD conversion unit ADC is to D/A conversion unit DAC output feedback signal, the output terminal of D/A conversion unit DAC is outwards drawn output interface 2, action by electronic switch makes the in-phase input end SA of amplifier be connected with the first Voltage-output interface 1, the inverting input SB of amplifier is connected with ground interface 4 simultaneously, or the in-phase input end SA of amplifier is connected with output interface 2, the inverting input SB of amplifier is connected with ground interface 4 simultaneously, or the in-phase input end SA of amplifier is connected with output interface 2, the inverting input SB of amplifier is connected with second voltage output interface 3 simultaneously.
Amplifier is programmable operational amplifier.
Electronic switch has two, is single-pole double-throw (SPDT) electronic switch.
AD conversion unit ADC and described D/A conversion unit DAC select the converter of 12 precision, and those skilled in the art also can select the converter of other precision as required.
The thermopair input measurement method of utilizing above-mentioned module, the steps include:
The first step, operation electronic switch, the in-phase input end SA of amplifier is connected with the first Voltage-output interface 1, the inverting input SB of amplifier is connected with ground interface 4 simultaneously, the enlargement factor of amplifier is set to A, controller obtains the initial value by the output voltage V p of AD conversion unit ADC, according to the initial value of output voltage V p and corresponding temperature Tp thereof, the upper limit range value of thermopair load module is defined as to Tp+A ℃, lower limit range value is defined as Tp-A ℃, according to calibration number, tables look-up and finds the thermopower V0 corresponding with Tp+A ℃;
Second step, operation electronic switch, the in-phase input end SA of amplifier is connected with output interface 2, the inverting input SB of amplifier is connected with ground interface 4 simultaneously, adjust the feedback voltage V f of controller, make the reference voltage Vref=V0+ Δ 1 on output interface 2, Δ 1 is the input composition error of amplifier and AD conversion unit ADC;
The derivation of above-mentioned second step is:
If adjusting the feedback voltage V f of controller is thermopower V0, Vref reference voltage=V0-Δ 2, Δ 2 is feedback integration error, output voltage V p=Vref-Δ 1=V0-Δ 2-Δ 1.If make output voltage V p=V0,, by backwards calculation, the output feedback voltage V f of controller need to be adjusted into V0+ Δ 2+ Δ 1, now, Vref=Vf-Δ 2=V0+ Δ 2+ Δ 1-Δ 2=V0+ Δ 1.
The 3rd step, operation electronic switch, the in-phase input end SA of amplifier is connected with output interface 2, the inverting input SB of amplifier is connected with second voltage output interface 3 simultaneously, the enlargement factor of amplifier is set to B, B is much larger than A, now, reference voltage Vref=V0+ Δ 1 on the in-phase input end SA of amplifier, the output voltage V i that voltage on the inverting input SB of amplifier is isolator, end difference is V0+ Δ 1-Vi, deduct after the input composition error Δ 1 of amplifier and AD conversion unit ADC, the output voltage V p=V0+ Δ 1-Vi-Δ 1=V0-Vi of final AD conversion unit ADC, the temperature T i that this thermopower is corresponding, final temperature value is Tp+A-Ti.
Below in conjunction with data, further illustrate the present invention.
Utilize the first passage output 27.025mV (this thermopower K calibration correspondence 650 ℃) of high-precision signal generator to thermopair load module, the course of work following (for ease of understanding, during computation and measurement value, enlargement factor is left in the basket) of thermopair load module:
The first step, operation electronic switch, the in-phase input end SA of amplifier is connected with the first Voltage-output interface 1, the inverting input SB of amplifier is connected with ground interface 4 simultaneously, the enlargement factor that amplifier is set is 50, obtain the initial value 26.856mV of Vp, according to the initial value 26.878mV of Vp and corresponding temperature Tp (646.53 ℃), its upper limit range value is defined as to 28.984mV (696.53 ℃), lower limit range value is defined as 24.758mV (596.53 ℃).
Second step, operation electronic switch, the in-phase input end SA of amplifier is connected with output interface 2, the inverting input SB of amplifier is connected with ground interface 4 simultaneously, the feedback voltage V f of controller is 28.984mV, the Vp obtaining by input end is 28.704mV, by backwards calculation, if will make Vp, be 28.984mV, the feedback voltage V f of controller is set as to 29.264mV.
The 3rd step, operation electronic switch, the in-phase input end SA of amplifier is connected with output interface 2, and the inverting input SB of amplifier is connected with second voltage output interface 3 simultaneously, and the enlargement factor that amplifier is set is 500, now, Vref is 29.131mV, the actual input 27.025mV that Vi is signal generator, and end difference is 2.106, finally obtaining Vp is 1.959mV, the thermopower of converting final measured signal is 27.025, and corresponding temperature is 650 ℃, and measuring error is 0%.
Claims (5)
1. a high-precision thermopair load module, comprise isolator, amplifier, AD conversion unit (ADC), controller, D/A conversion unit (DAC) and electronic switch, isolator is connected with thermopair, the output terminal AD conversion unit (ADC) of amplifier, the output terminal of AD conversion unit (ADC) connects controller, it is characterized in that: the earth terminal of isolator is outwards drawn ground interface (4) in ground connection, the voltage output end of isolator is outwards drawn two interfaces simultaneously, be respectively the first Voltage-output interface (1) and second voltage output interface (3), the signal that controller basis is obtained by AD conversion unit (ADC) is to D/A conversion unit (DAC) output feedback signal, the output terminal of D/A conversion unit (DAC) is outwards drawn output interface (2), by the action of electronic switch, make the in-phase input end (SA) of amplifier be connected with the first Voltage-output interface (1), the inverting input (SB) of amplifier is connected with ground interface (4) simultaneously, or the in-phase input end (SA) of amplifier is connected with output interface (2), the inverting input (SB) of amplifier is connected with ground interface (4) simultaneously, or the in-phase input end (SA) of amplifier is connected with output interface (2), the inverting input (SB) of amplifier is connected with second voltage output interface (3) simultaneously.
2. a kind of high-precision thermopair load module as claimed in claim 1, is characterized in that: described amplifier is programmable operational amplifier.
3. a kind of high-precision thermopair load module as claimed in claim 1, is characterized in that: described electronic switch has two, is single-pole double-throw (SPDT) electronic switch.
4. a kind of high-precision thermopair load module as claimed in claim 1, is characterized in that: described AD conversion unit (ADC) and described D/A conversion unit (DAC) are selected the converter of 12 precision.
5. a high-precision thermopair input measurement method, adopts thermopair load module as claimed in claim 1, it is characterized in that, step is:
The first step, operation electronic switch, the in-phase input end (SA) of amplifier is connected with the first Voltage-output interface (1), the inverting input (SB) of amplifier is connected with ground interface (4) simultaneously, the enlargement factor of amplifier is set to A, controller obtains the initial value by the output voltage V p of AD conversion unit (ADC), according to the initial value of output voltage V p and corresponding temperature Tp thereof, the upper limit range value of thermopair load module is defined as to Tp+A ℃, lower limit range value is defined as Tp-A ℃, according to calibration number, table look-up and find the thermopower V0 corresponding with Tp+A ℃,
Second step, operation electronic switch, the in-phase input end (SA) of amplifier is connected with output interface (2), the inverting input (SB) of amplifier is connected with ground interface (4) simultaneously, adjust the feedback voltage V f of controller, make the reference voltage Vref=V0+ Δ 1 on output interface (2), Δ 1 is the input composition error of amplifier and AD conversion unit (ADC);
The 3rd step, operation electronic switch, the in-phase input end (SA) of amplifier is connected with output interface (2), the inverting input (SB) of amplifier is connected with second voltage output interface (3) simultaneously, the enlargement factor of amplifier is set to B, B is much larger than A, now, reference voltage Vref=V0+ Δ 1 on the in-phase input end of amplifier (SA), the output voltage V i that voltage on the inverting input of amplifier (SB) is isolator, end difference is V0+ Δ 1-Vi, deduct after the input composition error Δ 1 of amplifier and AD conversion unit (ADC), the output voltage V p=V0+ Δ 1-Vi-Δ 1=V0-Vi of final AD conversion unit (ADC), the temperature T i that this thermopower is corresponding, final temperature value is Tp+A-Ti.
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Citations (5)
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| US5121051A (en) * | 1988-09-22 | 1992-06-09 | U.S. Philips Corporation | Method and apparatus for measuring small electrical signals |
| CN202018345U (en) * | 2011-03-07 | 2011-10-26 | 济南大学 | Novel thermocouple temperature measurement module |
| CN102426066A (en) * | 2011-09-26 | 2012-04-25 | 天津成科自动化工程技术有限公司 | Switching detection circuit using multi-channel PT100 thermistors as temperature sensor |
| CN202814584U (en) * | 2012-09-26 | 2013-03-20 | 深圳市华煜芯科技有限公司 | Temperature measuring system of temperature control device |
| CN203672507U (en) * | 2013-10-10 | 2014-06-25 | 上海发电设备成套设计研究院 | High-precision thermocouple input module |
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Patent Citations (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5121051A (en) * | 1988-09-22 | 1992-06-09 | U.S. Philips Corporation | Method and apparatus for measuring small electrical signals |
| CN202018345U (en) * | 2011-03-07 | 2011-10-26 | 济南大学 | Novel thermocouple temperature measurement module |
| CN102426066A (en) * | 2011-09-26 | 2012-04-25 | 天津成科自动化工程技术有限公司 | Switching detection circuit using multi-channel PT100 thermistors as temperature sensor |
| CN202814584U (en) * | 2012-09-26 | 2013-03-20 | 深圳市华煜芯科技有限公司 | Temperature measuring system of temperature control device |
| CN203672507U (en) * | 2013-10-10 | 2014-06-25 | 上海发电设备成套设计研究院 | High-precision thermocouple input module |
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