CN103542954B - A kind of high-precision thermocouple input module and measuring method - Google Patents

Abstract

The invention relates to a high-precision thermocouple input module, which includes an isolator, an amplifier, an analog-to-digital conversion unit, a controller, a digital-to-analog conversion unit, and an electronic switch. , the output end of the analog-to-digital conversion unit is connected to the controller, which is characterized in that: the grounding end of the isolator leads out to the grounding interface while being grounded, and the voltage output end of the isolator leads out to two interfaces at the same time. The signal obtained by the digital-to-analog conversion unit outputs a feedback signal to the digital-to-analog conversion unit, and the output terminal of the digital-to-analog conversion unit leads out to an output interface. Another technical solution of the present invention is to provide a high-precision thermocouple input measurement method, using the above-mentioned thermocouple input module. The invention has the advantages of ensuring that the overall absolute accuracy of the thermocouple signal is 0.1-0.2°C, and the relative accuracy can reach 0.005-0.01%, which can meet the real needs of the industrial control process.

Description

A high-precision thermocouple input module and measurement method

technical field

The invention relates to a high-precision thermocouple input module and a measurement method, which are used to improve the measurement accuracy of small signals such as thermocouples or millivolts, and belong to the technical field of automatic control instruments.

Background technique

A thermocouple is connected by two metal wires of different components to form a closed loop. The end directly used to measure the temperature of the medium is called the working end (also called the measuring end), and the other end is called the cold end (also called the compensation end). end), when there is a temperature difference between the working end and the cold end, a current will flow through the circuit, and there will be a thermal electromotive force (Seebeck effect) between the two ends. produces a voltage output approximately proportional to the temperature difference. But this voltage is very small. For example, the most common nickel-chromium-nickel-silicon K graduation thermocouple -270℃～1372℃ corresponds to a thermoelectric potential of -6.458mV～54.886mV, and the nickel-chromium-copper-nickel alloy E graduation The thermoelectric potential corresponding to the thermocouple -270℃～1000℃ is -9.835mV～76.373mV.

The signal processing method of the thermocouple input module of the control system such as the distributed control system (DCS) or the programmable control system (PLC) is to obtain the thermal electromotive force (millivolt voltage signal) generated by the thermocouple, and pass it through the amplifier as an analog electrical signal It is amplified to a volt voltage signal, and then input to the digital data of the control chip through the analog-to-digital converter (ADC), and converted into the actual temperature by the control chip or by the host computer according to the graduation number look-up table or by using the temperature estimation model formula. .

In the early days, due to the relative backwardness of semiconductor technology, in order to overcome the problems of slow processing speed and small memory capacity of the control chip, various temperature estimation model formulas were used to deal with it. As the technical indicators of control chip processing speed and storage capacity have been greatly improved, the most primitive look-up table is used to overcome the slight error between the temperature estimation model formula and the actual temperature.

On the surface, the measurement accuracy of thermocouple signals will be concentrated on the analog-to-digital converter (ADC). Generally, the analog-to-digital converter uses 12 bits, although the theoretical resolution of the analog-to-digital converter is 1/4096 (that is, 2/10,000). , but due to the reference voltage of components such as amplifiers and analog-to-digital converters and the temperature drift of components such as resistors, etc., in fact, the measurement accuracy of the entire thermocouple signal is generally between 0.1 and 0.2%. The digital converter is improved to 13 or 14, or even 15, but the overall measurement accuracy cannot be improved much.

In industrial process control, this relative accuracy is not a big problem for process variables such as valve opening, pressure, flow, liquid level, voltage and current of the actuator, but for temperature signals, because it has no specific range, Under normal circumstances, the thermocouple input module is compatible with thermocouple signals of various graduation numbers, and the thermoelectric potential signal used will have enough upper limit margin, and the upper limit of the converted temperature range will reach 2000°C. At this time, the relative accuracy is converted into an absolute value. It will reach 2°C to 4°C. In fact, low temperature generally occurs during the start-up process of equipment. When the industrial process is running normally, the main temperature process parameters will undergo small changes at a higher temperature. For example, when the outlet temperature of the boiler superheater of a 600MW ultra-supercritical thermal power generation unit is above 50% of the rated working condition, the temperature basically operates between 560±10°C, and the temperature variation range is 20°C. If it is used as a boiler safety protection signal, the furnace will trip The value is only 597°C, and the temperature change is +37°C. At this time, the absolute error of 2°C to 4°C is very serious.

Contents of the invention

The technical problem to be solved by the invention is to ensure the accuracy of the thermocouple signal.

In order to solve the above technical problems, a technical solution of the present invention is to provide a high-precision thermocouple input module, including an isolator, an amplifier, an analog-to-digital conversion unit, a controller, a digital-to-analog conversion unit, and an electronic switch. The isolator and The thermocouple is connected, the output end of the amplifier is connected to the analog-to-digital conversion unit, and the output end of the analog-to-digital conversion unit is connected to the controller. Lead out two interfaces, namely the first voltage output interface and the second voltage output interface, the controller outputs a feedback signal to the digital-analog conversion unit according to the signal obtained by the analog-to-digital conversion unit, and the output terminal of the digital-to-analog conversion unit is outward The output interface is drawn out, and the non-inverting input terminal of the amplifier is connected to the first voltage output interface through the action of the electronic switch, and the inverting input terminal of the amplifier is connected to the grounding interface at the same time, or the non-inverting input terminal of the amplifier is connected to the output interface. The inverting input terminal is connected to the ground interface, or the non-inverting input terminal of the amplifier is connected to the output interface, and the inverting input terminal of the amplifier is connected to the second voltage output interface.

Preferably, the amplifier is a programmable operational amplifier.

Preferably, there are two electronic switches, both of which are single-pole double-throw electronic switches.

Preferably, the analog-to-digital conversion unit and the digital-to-analog conversion unit use 12-bit precision converters.

Another technical solution of the present invention is to provide a high-precision thermocouple input measurement method, using the above-mentioned thermocouple input module, the basic idea is:

Reduce the magnification of the amplifier, predict the actual measured temperature value, and divide the upper and lower limit range reference points of the temperature from the ultra-large range into small range slices. The controller uses the digital-to-analog converter as a feedback terminal to form a closed-loop loop with the input terminal, and adjusts the reference value of the amplifier to overcome the comprehensive error of the input terminal and the feedback terminal. Significantly increase the magnification of the amplifier, combined with the reference value of the amended amplifier, and calculate the temperature value corresponding to the high-precision thermocouple signal through the calculation of the controller. The characteristic is that the specific steps are:

The first step is to operate the electronic switch so that the non-inverting input terminal of the amplifier is connected to the first voltage output interface, and the inverting input terminal of the amplifier is connected to the grounding interface at the same time, the amplification factor of the amplifier is set to A, and the controller obtains The initial value of the output voltage Vp of the unit, according to the initial value of the output voltage Vp and its corresponding temperature Tp, the upper limit range value of the thermocouple input module is defined as Tp+A°C, and the lower limit range value is defined as Tp-A°C, according to Look up the index number to find the thermal electromotive force V0 corresponding to Tp+A°C;

The second step is to operate the electronic switch, so that the non-inverting input terminal of the amplifier is connected to the output interface, and at the same time, the inverting input terminal of the amplifier is connected to the grounding interface, and the feedback voltage Vf of the controller is adjusted so that the reference voltage Vref on the output interface is Vref=V0+ Δ1, Δ1 is the input comprehensive error of the amplifier and the analog-to-digital conversion unit;

The third step is to operate the electronic switch so that the non-inverting input terminal of the amplifier is connected to the output interface, and at the same time the inverting input terminal of the amplifier is connected to the second voltage output interface, and the amplification factor of the amplifier is set to B, which is much larger than A. At this time , the reference voltage Vref on the non-inverting input terminal of the amplifier=V0+Δ1, the voltage on the inverting input terminal of the amplifier is the output voltage Vi of the isolator, and the terminal difference is V0+Δ1-Vi, minus the amplifier and the analog-to-digital conversion After the input comprehensive error Δ1 of the unit, the final output voltage of the analog-to-digital conversion unit Vp=V0+Δ1-Vi-Δ1=V0-Vi, and the temperature Ti corresponding to the thermal electromotive force, the final temperature value is Tp+A-Ti.

The invention has the advantages of ensuring that the overall absolute accuracy of the thermocouple signal is 0.1-0.2°C, and the relative accuracy can reach 0.005-0.01%, which can meet the real needs of the industrial control process.

Description of drawings

Fig. 1 is a schematic diagram of a high-precision thermocouple input module provided by the present invention.

Detailed ways

In order to make the present invention more comprehensible, preferred embodiments are described in detail below with accompanying drawings.

As shown in Figure 1, a high-precision thermocouple input module provided by the present invention includes an isolator, an amplifier, an analog-to-digital conversion unit ADC, a controller, a digital-to-analog conversion unit DAC and an electronic switch, and the isolator is connected to the thermocouple , the output terminal analog-to-digital conversion unit ADC of the amplifier, the output terminal of the analog-to-digital conversion unit ADC is connected to the controller, and it is characterized in that: the ground terminal of the isolator leads out the ground interface 4 while being grounded, and the voltage output terminal of the isolator simultaneously Two interfaces are drawn out, namely the first voltage output interface 1 and the second voltage output interface 3. The controller outputs a feedback signal to the digital-to-analog conversion unit DAC according to the signal obtained by the analog-to-digital conversion unit ADC, and the digital-to-analog conversion unit DAC The output terminal of the amplifier leads out to the output interface 2, and the non-inverting input terminal SA of the amplifier is connected to the first voltage output interface 1 through the action of the electronic switch, and the inverting input terminal SB of the amplifier is connected to the grounding interface 4 at the same time, or the non-inverting input terminal SA of the amplifier is connected to the grounding interface 4, or the non-inverting input terminal SA of the amplifier is connected The input terminal SA is connected to the output interface 2, and the inverting input terminal SB of the amplifier is connected to the grounding interface 4, or the non-inverting input terminal SA of the amplifier is connected to the output interface 2, and the inverting input terminal SB of the amplifier is connected to the second voltage output Interface 3 is connected.

The amplifier is a programmable operational amplifier.

There are two electronic switches, both of which are single-pole double-throw electronic switches.

The analog-to-digital conversion unit ADC and the digital-to-analog conversion unit DAC use 12-bit precision converters, and those skilled in the art can also select other precision converters as required.

Using the thermocouple input measurement method of the above module, the steps are:

The first step is to operate the electronic switch so that the non-inverting input terminal SA of the amplifier is connected to the first voltage output interface 1, and at the same time the inverting input terminal SB of the amplifier is connected to the grounding interface 4, and the amplification factor of the amplifier is set to A, and the controller obtains From the initial value of the output voltage Vp of the analog-to-digital conversion unit ADC, according to the initial value of the output voltage Vp and its corresponding temperature Tp, the upper limit value of the thermocouple input module is defined as Tp+A°C, and the lower limit value is defined as Tp -A°C, look up the table according to the graduation number to find the thermal electromotive force V0 corresponding to Tp+A°C;

The second step is to operate the electronic switch so that the non-inverting input terminal SA of the amplifier is connected to the output interface 2, and at the same time the inverting input terminal SB of the amplifier is connected to the grounding interface 4, and the feedback voltage Vf of the controller is adjusted so that the reference on the output interface 2 Voltage Vref=V0+Δ1, Δ1 is the input comprehensive error of the amplifier and the analog-to-digital conversion unit ADC;

The derivation process of the above second step is:

If the feedback voltage Vf of the adjustment controller is thermal electromotive force V0, then the Vref reference voltage=V0-Δ2, and Δ2 is the feedback comprehensive error, then the output voltage Vp=Vref-Δ1=V0-Δ2-Δ1. To make the output voltage Vp=V0, through reverse calculation, the output feedback voltage Vf of the controller needs to be adjusted to V0+Δ2+Δ1, at this time, Vref=Vf-Δ2=V0+Δ2+Δ1-Δ2=V0 +Δ1.

The third step is to operate the electronic switch so that the non-inverting input terminal SA of the amplifier is connected to the output interface 2, and the inverting input terminal SB of the amplifier is connected to the second voltage output interface 3 at the same time, and the amplification factor of the amplifier is set to B, which is much larger than A. At this time, the reference voltage Vref on the non-inverting input terminal SA of the amplifier=V0+Δ1, the voltage on the inverting input terminal SB of the amplifier is the output voltage Vi of the isolator, and the terminal difference is V0+Δ1-Vi, minus After removing the input comprehensive error Δ1 of the amplifier and the analog-to-digital conversion unit ADC, the output voltage of the final analog-to-digital conversion unit ADC Vp=V0+Δ1-Vi-Δ1=V0-Vi, the temperature Ti corresponding to the thermal electromotive force, and the final temperature The value is Tp+A-Ti.

The present invention is further described below in conjunction with data.

Use a high-precision signal generator to output 27.025mV to the first channel of the thermocouple input module (this thermal electromotive force corresponds to 650°C in the K scale), and the working process of the thermocouple input module is as follows (for easy understanding, when calculating the measured value magnification is ignored):

The first step is to operate the electronic switch so that the non-inverting input terminal SA of the amplifier is connected to the first voltage output interface 1, and the inverting input terminal SB of the amplifier is connected to the grounding interface 4 at the same time, and the amplification factor of the amplifier is set to 50 to obtain the initial value of Vp The value is 26.856mV, according to the initial value of Vp 26.878mV and the corresponding temperature Tp (646.53°C), its upper limit range value is defined as 28.984mV (696.53°C), and its lower limit range value is defined as 24.758mV (596.53°C).

The second step is to operate the electronic switch so that the non-inverting input terminal SA of the amplifier is connected to the output interface 2, and the inverting input terminal SB of the amplifier is connected to the grounding interface 4 at the same time. The feedback voltage Vf of the controller is 28.984mV, which is obtained through the input terminal Vp is 28.704mV. Through reverse calculation, if Vp is to be 28.984mV, the feedback voltage Vf of the controller is set to 29.264mV.

The third step is to operate the electronic switch so that the non-inverting input terminal SA of the amplifier is connected to the output interface 2, and the inverting input terminal SB of the amplifier is connected to the second voltage output interface 3 at the same time, and the amplification factor of the amplifier is set to 500. At this time, Vref is 29.131mV, Vi is the actual input of the signal generator 27.025mV, the terminal difference is 2.106, and the final Vp is 1.959mV, the thermal electromotive force converted to the final measured signal is 27.025, the corresponding temperature is 650°C, and the measurement error is 0 %.

Claims (3)

1. a high-precision thermocouple input 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 of amplifier connects AD conversion unit (ADC), the output terminal connection control device of AD conversion unit (ADC), it is characterized in that: the earth terminal of isolator outwards draws ground interface (4) while ground connection, the voltage output end of isolator outwards draws two interfaces simultaneously, be respectively the first voltage output interface (1) and the second voltage output interface (3), controller according to the signal obtained by AD conversion unit (ADC) to D/A conversion unit (DAC) output feedback signal, the output terminal of D/A conversion unit (DAC) outwards draws output interface (2), by the action of electronic switch, the in-phase input end of amplifier (SA) is 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 of amplifier (SA) 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 of amplifier (SA) is connected with output interface (2), the inverting input (SB) of amplifier is connected with the second voltage output interface (3) simultaneously,

Described amplifier is programmable operational amplifier;

Described electronic switch has two, is single-pole double-throw (SPDT) electronic switch.

2. a kind of high-precision thermocouple input module as claimed in claim 1, is characterized in that: described AD conversion unit (ADC) and described D/A conversion unit (DAC) select the converter of 12 precision.

3. a high-precision thermopair input measurement method, adopt thermocouple input module as claimed in claim 1, it is characterized in that, step is:

The first step, operation electronic switch, the in-phase input end of amplifier (SA) 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 of the output voltage Vp of AD conversion unit (ADC), according to the initial value of output voltage Vp and the temperature Tp of correspondence thereof, the upper limit range value of thermocouple input module is defined as Tp+A DEG C, lower limit range value is defined as Tp-A DEG C, table look-up according to Graduation Number and find the thermopower V0 corresponding with Tp+A DEG C,

Second step, operation electronic switch, the in-phase input end of amplifier (SA) 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 adjustment 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);

3rd step, operation electronic switch, the in-phase input end of amplifier (SA) is connected with output interface (2), the inverting input (SB) of amplifier is connected with the 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, voltage on the inverting input (SB) of amplifier is the output voltage Vi of isolator, end difference is V0+ Δ 1-Vi, after deducting the input composition error Δ 1 of amplifier and AD conversion unit (ADC), the output voltage Vp=V0+ Δ 1-Vi-Δ 1=V0-Vi of final AD conversion unit (ADC), the temperature Ti that this thermopower is corresponding, then final temperature value is Tp+A-Ti.