CN108955933B - Measuring circuit for improving temperature measurement precision of steel rail and parameter selection method - Google Patents
Measuring circuit for improving temperature measurement precision of steel rail and parameter selection method Download PDFInfo
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- CN108955933B CN108955933B CN201810930631.1A CN201810930631A CN108955933B CN 108955933 B CN108955933 B CN 108955933B CN 201810930631 A CN201810930631 A CN 201810930631A CN 108955933 B CN108955933 B CN 108955933B
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- 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
- G01K7/16—Measuring temperature based on the use of electric or magnetic elements directly sensitive to heat ; Power supply therefor, e.g. using thermoelectric elements using resistive elements
- G01K7/22—Measuring temperature based on the use of electric or magnetic elements directly sensitive to heat ; Power supply therefor, e.g. using thermoelectric elements using resistive elements the element being a non-linear resistance, e.g. thermistor
- G01K7/24—Measuring temperature based on the use of electric or magnetic elements directly sensitive to heat ; Power supply therefor, e.g. using thermoelectric elements using resistive elements the element being a non-linear resistance, e.g. thermistor in a specially-adapted circuit, e.g. bridge circuit
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Abstract
The invention discloses a measuring circuit for improving the measurement precision of steel rail temperature and a parameter selection method, wherein the circuit comprises the following steps: three output ends of the three-output thermal resistor R are respectively provided with lead resistors R1, R2 and R3, the three-wire bridge circuit comprises resistors R1, R2, R3 and R4 which are connected in a ring shape, wherein the common connection point of the R1 and the R2 is connected with a reference power supply E, and the common connection point of the R3 and the R4 is grounded; the A end of the three-output thermal resistor R is connected with R1 through R1, the A end of the three-output thermal resistor R is also connected with the positive input end of the operational amplifier circuit through R2, and the B end of the three-output thermal resistor R is connected with R4 through R3; the common connection point of R2 and R3 is also connected with the negative input end of the operational amplifier circuit. The invention does not need to measure the values of the lead wire and the contact resistance of the sensor, can greatly reduce the measurement error caused by the lead wire and the contact resistance, improves the precision, the stability and the anti-interference capability of the acquisition circuit, and also simplifies the field installation process.
Description
Technical Field
The invention relates to the field of railway detection, in particular to a measuring circuit for improving the measurement precision of steel rail temperature and a parameter selection method.
Background
The railway environment condition is complex, the wireless scheme is limited by the railway environment (installation mode-driving safety problem may be caused, wireless transmission-interference on the railway, shielding interference of the train is more, power supply-railway electricity is a special line, water-proof and dust-proof, etc.), therefore, the mode of separating the sensor and the acquisition equipment is usually adopted, the sensor is fixed on the steel rail by a clamp, the acquisition equipment is installed outside the safe distance beside the railway, thus the acquisition system and the railway steel rail can be electrically isolated, and the railway operation cannot be damaged by factors such as sensor falling off and electromagnetic interference.
However, in the above-mentioned structure in which the sensor is usually separated from the collecting device, the length and thickness of the lead of the sensor at the time of installation are determined by field construction, and the resistances due to the length and thickness of the lead are not uniform, which causes a decrease in the temperature measurement accuracy. Longer sensor leads and contact impedances can introduce greater errors into the measurements, while longer leads can also make environmental disturbances more pronounced. Specifically, the resistance of the lead directly causes the overall deviation of the measured temperature, and since the resistance of the lead resistance is uncertain, the overall deviation of the measured temperature is uncertain, and as a result, the measurement accuracy is reduced.
The traditional measuring method in the prior art is to directly measure and then subtract the temperature values corresponding to the lead and the contact resistor, and the lead and the contact resistor are measured in the mode each time of installation, so that the field operation is inconvenient, and the measurement error is huge due to the influence of the precision of the measuring instrument and the awareness of the person in charge of the measuring personnel.
Disclosure of Invention
The invention aims to overcome the defects of the prior art, provides a measuring circuit for improving the temperature measuring precision of a steel rail and a parameter selection method, and solves the problems in the prior art.
The purpose of the invention is realized by the following technical scheme: a measuring circuit for improving the measurement precision of the temperature of a steel rail comprises a three-output thermal resistor R, a three-wire system bridge circuit and an operational amplifier circuit;
three output ends of the three-output thermal resistor R are respectively provided with lead resistors R1, R2 and R3, the three-wire bridge circuit comprises resistors R1, R2, R3 and R4 which are connected in a ring shape, wherein the common connection point of the R1 and the R2 is connected with a reference power supply E, and the common connection point of the R3 and the R4 is grounded; wherein R is2＝R3And when the three-wire bridge circuit is balanced, (R)1+r1+r2)·R3＝(R4+r2+r3+R)·R2；
The A end of the three-output thermal resistor R is connected with R1 through R1, the A end of the three-output thermal resistor R is also connected with the positive input end of the operational amplifier circuit through R2, and the B end of the three-output thermal resistor R is connected with R4 through R3; the common connection point of the R2 and the R3 is also connected with the negative input end of the operational amplification circuit;
the output end of the operational amplification circuit outputs a measurement signal.
Furthermore, the type of the three-output thermal resistor R is PT 100.
Furthermore, the lead resistors r1, r2 and r3 are equal in size.
Furthermore, the measuring circuit also comprises a common-mode signal suppression circuit which is used for generating filtering suppression effect on the input common-mode signal; the common mode signal suppression circuit comprises a capacitor C1 connected between the positive input end and the negative input end of the operational amplification circuit.
Furthermore, the measuring circuit further includes a first low-pass filter circuit respectively disposed at the positive input end of the operational amplifier circuit, and a second low-pass filter circuit disposed at the negative input end of the operational amplifier circuit.
Furthermore, the first low-pass filter circuit comprises R5, C2 and C4, wherein one end of R5 is connected with R2, the other end of R5 is respectively connected with the positive input end of the operational amplifier circuit, one end of C2 and one end of C4, and the other ends of C2 and C4 are grounded; the second low-pass filter circuit comprises R6, C3 and C5, wherein one end of R6 is respectively connected with R2 and R3, the other end of R6 is respectively connected with a negative input end of the operational amplifier circuit, one end of C3 and one end of C5, and the other end of C3 and the other end of C5 are grounded.
The invention also provides a parameter selection method of the measuring circuit for improving the measurement accuracy of the temperature of the steel rail, which comprises the following steps:
s1: determining the range of the temperature of the steel rail which needs to be measured actually, and determining the resistance value range of the three-output thermal resistor R according to the range;
s2: selecting a base point according to the difficulty of finding the resistance value of the actual resistor and considering a certain fault tolerance, and obtaining the resistance value of the three-output thermal resistor R at the base point; wherein the three-wire bridge circuit is in a balanced state when at a base point;
s3: according to R2＝R3And when the three-wire bridge circuit is balanced (R)1+r1+r2)·R3＝(R4+r2+r3+R)·R2The actual sizes of the lead resistors r1, r2 and r3 and the size of the three-wire bridge circuit resistor in consideration of the general precision resistor which can be purchased are selected: firstly, selecting the sizes of R2 and R3, and selecting the sizes of the resistors R1 and R4 according to the resistance of the three output thermal resistor R at a base point;
s4: obtaining the voltage of the input end of the operational amplifier circuit at the maximum value and/or the minimum value of the range of the temperature of the steel rail according to the selected sizes of the resistors R1, R2, R3 and R4;
s5: and determining the amplification factor of the operational amplification circuit according to the sampling range of the subsequent AD conversion circuit.
Further, the method further comprises the following steps:
s6: and selecting parameters of the low-pass filter circuit arranged at the input end of the operational amplification circuit by considering the low-pass filter bandwidth range of the signal input at the input end of the operational amplification circuit.
Further, the method further comprises the following steps:
s7: and selecting the parameter of the common-mode signal elimination capacitor arranged between the two input ends of the operational amplification circuit.
Further, in general, the lead resistances r1, r2 and r3 are equal in size, so that the actual sizes of the lead resistances r1, r2 and r3 do not need to be obtained in step S3.
The invention has the beneficial effects that:
(1) the invention adopts the three-wire system horizontal bridge circuit, does not need to measure the values of the lead wire and the contact resistance of the sensor, can greatly reduce the measurement error brought by the lead wire and the contact resistance under the condition of specific circuit parameters, improves the precision, the stability and the anti-interference capability of the acquisition circuit, and also simplifies the field installation process.
(2) The circuit structure can eliminate or greatly reduce the influence caused by the lead resistance of the sensor, the specific circuit parameter performance in the parameter selection method is better, and peripheral compensation resistors (which need to be adjusted according to the field installation requirement) are not needed for compensation, so that the influence of the lead resistance of the sensor is eliminated.
(3) The parameter selection method of the invention ensures that the invention can be determined by the circuit parameters without a circuit specially used for adjusting zero point and full scale, and the circuit parameters are determined after the application environment and technical indexes of one circuit are determined. In the prior art, according to the actual situation of the field (for example, lead wires with different lengths and thicknesses are adopted), the compensation resistor needs to be adjusted to eliminate lead wire errors.
(4) The invention has the filtering and inhibiting effect on input common-mode signals (the capacitor C1 inhibits the common-mode signals) and the filtering and inhibiting effect on input alternating-current signals (R5, C2 and C4 perform low-pass filtering on signals at the positive input end of the operational amplifier, and R6, C3 and C5 perform low-pass filtering on signals at the negative input end of the operational amplifier).
(5) The required devices are few, the cost is low, the circuit does not need to be adjusted after being welded, and the circuit can be directly applied in batches.
Drawings
FIG. 1 is a schematic circuit diagram of the present invention;
FIG. 2 is a schematic diagram of a horizontal bridge.
Detailed Description
The technical solutions of the present invention will be described clearly and completely with reference to the accompanying drawings, and it should be understood that the described embodiments are some, but not all embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
In the description of the present invention, it should be noted that directions or positional relationships indicated by "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", and the like are directions or positional relationships described based on the drawings, and are only for convenience of description and simplification of description, and do not indicate or imply that the device or element referred to must have a specific orientation, be configured and operated in a specific orientation, and thus, should not be construed as limiting the present invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance.
In the description of the present invention, it should be noted that, unless otherwise explicitly stated or limited, the terms "mounted," "connected," and "connected" are to be construed broadly, and may be, for example, fixedly connected, detachably connected, or integrally connected; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meanings of the above terms in the present invention can be understood in specific cases to those skilled in the art.
In addition, the technical features involved in the different embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
Example 1
A measuring circuit for improving the measurement accuracy of the temperature of a steel rail is used for measuring the temperature data of the steel rail of a railway in a wired mode, three output thermal resistors R of the measuring circuit are clamped on the steel rail, and the rest part of the measuring circuit passes through a measuring line. Specifically, as shown in fig. 1, a measuring circuit for improving the measurement accuracy of the temperature of the steel rail comprises a three-output thermal resistor R, a three-wire bridge circuit and an operational amplifier circuit; three output ends of the three-output thermal resistor R are respectively provided with lead resistors R1, R2 and R3, the three-wire bridge circuit comprises resistors R1, R2, R3 and R4 which are connected in a ring shape, wherein the common connection point of the R1 and the R2 is connected with a reference power supply E, and the common connection point of the R3 and the R4 is grounded; wherein R is2＝R3And when the three-wire bridge circuit is balanced, (R)1+r1+r2)·R3＝(R4+r2+r3+R)·R2(ii) a The A end of the three-output thermal resistor R is connected with R1 through R1, and the A end of the three-output thermal resistor R also passes throughR2 is connected with the positive input end of the operational amplifier circuit, and the B end of the three-output thermal resistor R is connected with R4 through R3; the common connection point of the R2 and the R3 is also connected with the negative input end of the operational amplification circuit; the output end of the operational amplification circuit outputs a measurement signal.
In the prior art, the traditional measurement method in the prior art is to directly measure and then subtract the temperature value corresponding to the lead and the contact resistance, and the lead and the contact resistance are measured in the mode each time of installation, so that the field operation is inconvenient, and the measurement error is huge because the measurement method is easily influenced by the precision of a measuring instrument and the awareness of a measuring person.
In the embodiment, the three-wire horizontal bridge circuit is adopted, the values of the lead wire and the contact resistance of the sensor do not need to be measured, the measurement error caused by the lead wire and the contact resistance can be greatly reduced under the condition of specific circuit parameters, the precision, the stability and the anti-interference capability of the acquisition circuit are improved, and the field installation process is simplified.
Specifically, the following defines the model number of the three-output thermal resistor R as PT100, and the rest of the achievable three-output thermal resistors R can be replaced.
The horizontal bridge is one of unbalanced bridges, as shown in FIG. 2, R1＝RX、R2＝R3And R is1≠R2When the bridge is balanced, the output pressure difference U is0Is 0.
An unbalanced bridge typically has two modes, voltage output and power output. In the acquisition system, the impedance of the access devices at the output end of the bridge is high, so that a voltage output mode is generally adopted.
As can be seen from fig. 2, in the case that the input impedance of the bridge output terminal access device approaches infinity:
let RX＝RX0+ΔR，RXFor the resistance to be measured, RX0As its initial value, Δ R is its resistance change amount,from equation (1) we can obtain:
for a horizontal bridge, equation (2) can be simplified as:
from the formula (3), when RX0When the voltage is large enough, the output voltage value U is0And the resistance change amount deltar is in a linear relation.
And correspondingly, the present embodiment employs the three-wire system horizontal bridge circuit described in fig. 1. When the bridge is balanced, (R)1+r1+r2)·R3＝(R4+r2+r3+RPT100)·R2Wherein r is1、r2、r3Is a PT100 sensor lead and is at r1＝r2＝r3In the case of r (normal case), the characteristics of the horizontal bridge are known as follows: r1＝R4+RPT100＝RX。
The output voltage of the output horizontal bridge is:
introducing a resistance increment Δ R, which can be obtained from equation (4):
wherein R isX0＝R1＝(R4A + PT100 reference resistance), the PT100 reference resistance is selected according to the actual application.
From the formula (5), when RX0When the value is large and the delta R resistance variation is small, the lead resistance R (R refers to the sensor)Single-ended lead impedance, the impedance of the lead at both ends of the sensor is 2r in practical application) will not have too much influence on the output voltage (in practical application, r is usually less than 1 Ω, and in extreme cases will not exceed 3 Ω).
In addition, preferably, as shown in fig. 1, in this embodiment, the measurement circuit further includes a common mode signal rejection circuit, configured to generate a filtering rejection effect on the input common mode signal; the common mode signal suppression circuit comprises a capacitor C1 connected between the positive input end and the negative input end of the operational amplification circuit.
Meanwhile, preferably, as shown in fig. 1, in the present embodiment, the measurement circuit further includes a first low-pass filter circuit disposed at the positive input end of the operational amplifier circuit and a second low-pass filter circuit disposed at the negative input end of the operational amplifier circuit, respectively.
Specifically, as shown in fig. 1, the first low-pass filter circuit includes R5, C2 and C4, wherein one end of R5 is connected to R2, the other end of R5 is respectively connected to the positive input end of the operational amplifier circuit, one end of C2, one end of C4, and the other ends of C2 and C4 are grounded; the second low-pass filter circuit comprises R6, C3 and C5, wherein one end of R6 is respectively connected with R2 and R3, the other end of R6 is respectively connected with a negative input end of the operational amplifier circuit, one end of C3 and one end of C5, and the other end of C3 and the other end of C5 are grounded.
Example 2
Based on the implementation of the foregoing embodiment, this embodiment further provides a parameter selection method for a measurement circuit that improves the measurement accuracy of a rail temperature, where the method is used to select values of various components in the measurement circuit. The method comprises the following steps:
s1: and determining the range of the temperature of the steel rail which needs to be measured actually, and determining the resistance value range of the three-output thermal resistor R according to the range.
Specifically, in this embodiment, the rail temperature ranges from-40 ℃ to +100 ℃.
The lowest temperature point at which the temperature is usually measured is called zero (the lowest voltage corresponding to the AD sampling range connected to the output terminal of the operational amplifier circuit), and the highest temperature point is called full (the highest voltage corresponding to the AD sampling range connected to the output terminal of the operational amplifier circuit). However, in practical applications, a certain margin is usually left to improve the accuracy. For example, if the AD sampling range is 0 to 4.5V, the actual zero point is set to about 0.1V, and the fullness is set to about 4.4V.
When the temperature of the steel rail ranges from minus 40 ℃ to plus 100 ℃, the resistance value of the corresponding PT100 is 84.27-138.51 omega.
S2: according to the difficulty of finding the resistance value of the actual resistor and considering certain fault tolerance, the base point is positioned at the minimum value of the range of the temperature of the steel rail, and the resistance value of the three output thermal resistors R at the base point is obtained; wherein the three-wire bridge circuit is in a balanced state when in a base point.
Therefore, in this embodiment, the resistance value 83.08 Ω corresponding to-43 ℃ is selected as the base point.
As can be seen from the formula (5) given in example 1 and subsequent test data, the error theory is 0 at the base point, and the accumulated error becomes larger the farther away from the base point temperature. Therefore, the base point is optimally set at 84.27 Ω corresponding to-40 ℃, but the resistor with the resistance value is difficult to find in practice, and the base point is set at-43 ℃ in consideration of a certain fault tolerance range.
If the base point is selected to be at the central point of the temperature measurement range, the temperature measurement values at the two ends of the central point have accumulated errors, but compared with the case that the base point is arranged at one end, the accumulated errors are reduced by half, and an offset circuit needs to be added at the rear end of the operational amplifier or an AD converter capable of sampling positive and negative values is directly selected.
S3: according to R2＝R3And when the three-wire bridge circuit is balanced (R)1+r1+r2)·R3＝(R4+r2+r3+R)·R2The actual sizes of the lead resistors r1, r2 and r3 and the size of the three-wire bridge circuit resistor in consideration of the general precision resistor which can be purchased are selected: the sizes of R2 and R3 are selected firstly, and the sizes of the resistors R1 and R4 are selected according to the resistance values of the three output thermal resistors R at the base point.
Specifically, because the bridge is a horizontal bridge, R2 ═ R3, R1+ R1 ═ R4+ R3+ PT100, in order to reduce errors caused by the circuit itself, the value of R2 and the value of R1+ R1 cannot be greatly different, and in consideration of the general precision resistor that can be purchased, R2 ═ R3 ═ 1k Ω is selected.
For the sizes of the resistors R1 and R4, R1 is 825 Ω, and R4 is 908 Ω, so that R4-R1 is 908 Ω -825 Ω, and corresponds to the resistance value of PT100 at the base point of-43 ℃ (the resistance value of PT100 at-43 ℃ is 83.08 Ω), and at the base point, the bridge is in a balanced state, the error is zero, and as the resistance value of PT100 becomes larger (i.e., the temperature rises), the bridge is in an unbalanced state, the accumulated error also gradually increases, and the error is the maximum at 100 ℃ (the theoretical support that the error gradually increases is formula (5)).
When the base point is selected and the circuit parameters are selected, the zero point of the whole circuit is also determined without adjustment.
S4: according to the selected sizes of the resistors R1, R2, R3 and R4, the voltage size of the input end of the operational amplifier circuit at the maximum value of the range of the temperature of the steel rail is obtained.
When the bridge measures the highest temperature, the output voltage is obtained (here 100 ℃), which in this embodiment is 0.0725V.
S5: and determining the amplification factor of the operational amplification circuit according to the sampling range of the subsequent AD conversion circuit.
After the zero point is determined, the full scale is the output voltage (100 ℃) multiplied by the operational amplifier when the bridge measures the highest temperature, the output voltage is 0.0725V multiplied by the operational amplifier 62 is 4.495V when the bridge measures the highest temperature, the voltage sampling range of the AD converter is satisfied, and therefore the amplifier 62 is directly given.
Since the operational amplifier circuit belongs to the prior art, the structure thereof is not described in detail.
More preferably, in this embodiment, the method further includes:
s6: and selecting parameters of the low-pass filter circuit arranged at the input end of the operational amplification circuit by considering the low-pass filter bandwidth range of the signal input at the input end of the operational amplification circuit.
For the first low-pass filter circuit and the second low-pass filter circuit in the preferred embodiment of embodiment 1, the values of R5, R6, C2, C3, C4, and C5 are selected in a low-pass filter bandwidth range mainly considering the input of the operational amplifier input signal, R5 ═ R6 ═ 10k Ω, C2 ═ C3 ═ 0.1uF, and C4 ═ C5 ═ 4.7uF, and mainly because the temperature changes slowly, the corresponding voltage signal is relatively stable, so that the ac signal is suppressed.
More preferably, in this embodiment, the method further includes:
s7: and selecting the parameter of the common-mode signal elimination capacitor arranged between the two input ends of the operational amplification circuit.
And C1 ═ 0.001uF is a common mode signal cancellation capacitor.
More preferably, in the present embodiment, in general, the sizes of the lead resistors r1, r2 and r3 are equal, so that the actual sizes of the lead resistors r1, r2 and r3 do not need to be obtained in step S3.
According to the parameters, in practical application (the single-end resistance r of the sensor is less than 1 omega, the resistance of the copper wire with 0.2 square is about 0.86 omega when the length is 10 meters), the measurement accuracy is less than 0.2 ℃ (corresponding data exists in a cross-bottom book), and the practical application can be met.
In addition, in order to ensure the measurement accuracy and the anti-interference capability, the circuit performance can be further improved by adopting the following modes according to the practical application environment:
(1)R1and R2The difference of the values of the two arms is as small as possible, so that the impedance of the two arms of the bridge is consistent, and the resistance to the power supply and the external interference can be greatly improved.
(2)R1The value of the voltage is selected properly, and too small value not only increases the power consumption of the circuit, but also increases the accumulative error of measurement; too large can reduce the measurement accumulated error of the circuit, but the amplification factor of the rear-end operational amplifier is increased, the single-stage amplification is difficult to meet the requirement, a multi-stage amplification circuit is needed, and the noise of the circuit can be brought in.
(3) The back end OP amplifier can adopt an instrument amplifier according to specific requirements, and the anti-interference capability of the circuit is further improved. The reference power supply E adopts a high-precision reference power supply, so that the stability and the precision of a temperature acquisition channel can be further improved.
(4) Reducing the temperature measurement range can reduce the accumulated error. By this idea, R can be converted1The high-precision digital potentiometer is replaced, the base point is set in different temperature sections, the measurement precision is improved, and the accumulated error is reduced.
(5) The base point is selected at the central point of the temperature measurement range, the temperature measurement values at two ends of the central point have accumulated errors, but compared with the base point arranged at one end, the accumulated errors are reduced by half, and an offset circuit needs to be added at the rear end of the operational amplifier or an AD converter capable of sampling positive and negative values is directly selected.
Example 3
In this embodiment, a corresponding measurement circuit is built according to the parameters selected in embodiment 2, and data is acquired, specifically:
the rail temperature is usually measured in the range of-40 ℃ to +100 ℃, the corresponding PT100 resistance value is 84.27-138.51 Ω, the resistance value corresponding to-43 ℃ is 83.08 Ω as the base point, and the bridge is in balance at the point. In which R is1＝825Ω，R2＝R3＝1kΩ，R4＝908Ω，R5＝R6＝10kΩ，C1＝0.001uF，C2＝C3＝0.1uF，C4＝C5The reference power supply E is 5V, the number of AD converter bits is 16 bits, and the sampling voltage range is 4.5V when 4.7uF is obtained. The output voltage value U of the horizontal bridge can be obtained according to the formulas (3) and (5)0As shown in the following table:
| temperature (. degree.C.) | r＝0Ω | r＝0.25Ω | r＝0.5Ω | r＝1Ω | r＝3Ω | r＝5Ω |
| -40 | 0.00055 | 0.00055 | 0.00055 | 0.00055 | 0.00055 | 0.00055 |
| 20 | 0.01081 | 0.01081 | 0.01081 | 0.01080 | 0.01078 | 0.01076 |
| 0 | 0.02147 | 0.02146 | 0.02146 | 0.02145 | 0.02140 | 0.02135 |
| 20 | 0.03196 | 0.03196 | 0.03195 | 0.03193 | 0.03186 | 0.03179 |
| 40 | 0.04232 | 0.04231 | 0.04230 | 0.04227 | 0.04218 | 0.04209 |
| 60 | 0.05252 | 0.05251 | 0.05249 | 0.05246 | 0.05235 | 0.05224 |
| 80 | 0.06259 | 0.06257 | 0.06255 | 0.06252 | 0.06239 | 0.06225 |
| 100 | 0.07250 | 0.07248 | 0.07247 | 0.07243 | 0.07227 | 0.07212 |
TABLE 1 horizontal bridge output Voltage value U0
Output voltage U of horizontal bridge0Amplified to the effective collection voltage range of the AD converter by an amplifier OP, wherein U is K.U0And K is the operational amplification factor. According to the above parameters, K is 62. The value of the AD converter acquisition voltage U is as follows:
| temperature (. degree.C.) | r＝0Ω | r＝0.25Ω | r＝0.5Ω | r＝1Ω | r＝3Ω | r＝5Ω |
| -40 | 0.034 | 0.034 | 0.034 | 0.034 | 0.034 | 0.034 |
| 20 | 0.671 | 0.670 | 0.670 | 0.670 | 0.668 | 0.667 |
| 0 | 1.331 | 1.331 | 1.330 | 1.330 | 1.327 | 1.324 |
| 20 | 1.982 | 1.981 | 1.981 | 1.980 | 1.975 | 1.971 |
| 40 | 2.624 | 2.623 | 2.622 | 2.621 | 2.615 | 2.610 |
| 60 | 3.256 | 3.255 | 3.255 | 3.253 | 3.246 | 3.239 |
| 80 | 3.880 | 3.879 | 3.878 | 3.876 | 3.868 | 3.860 |
| 100 | 4.495 | 4.494 | 4.493 | 4.490 | 4.481 | 4.471 |
TABLE 2 acquisition of the voltage value U by the AD converter
From tables 1 and 2, it can be seen that: under the condition that the impedance of the lead is not changed, the measurement error is gradually increased along with the increase of the measurement temperature; and under the condition of unchanged temperature, the impedance of the lead gradually increases, and the measurement error gradually increases.
Thus, at 100 ℃ away from the base point (i.e., the bridge balance point), the error of the measurement is greatest; at 0 c close to the base point, the measurement error is small and close to zero.
The value of the voltage U collected by the AD converter is quantized by AD and converted into a corresponding temperature value T, and the result is shown in the following table:
| 100℃ | r＝0Ω | r＝0.25Ω | r＝0.5Ω | r＝1Ω | r＝3Ω | r＝5Ω |
| AD value (16bit) | 65463 | 65449 | 65434 | 65390 | 65259 | 65113 |
| T(℃) | 100.00 | 99.97 | 99.94 | 99.84 | 99.55 | 99.23 |
| ΔT(℃) | 0 | 0.03 | 0.06 | 0.16 | 0.45 | 0.77 |
| T(℃) | 0 | 1.25 | 2.5 | 5 | 15 | 25 |
TABLE 3 conversion of the corresponding temperature values T
In Table 3, Δ T represents the difference in measured temperature between the resistance of the lead-in and the resistance of the lead-out, TrThe temperature value corresponding to the double-end lead and the contact resistance of the sensor during direct measurement is shown (in the temperature range of-40 ℃ to +100 ℃, 0.4 omega approximately corresponds to 1 ℃).
Thus, from Table 3, it can be seen that the circuit configuration and circuit parameters provided by the above-described embodiments are employed, under normal conditions (r)<1 Ω), the maximum error of the measurement does not exceed 0.2 ℃, in the limiting case (r)<3 omega) and the maximum error of the measurement does not exceed 0.5 ℃. Temperature value T introduced by lead and contact resistance in direct measurementrIn contrast, the measurement error is greatly reduced.
It is to be understood that the above-described embodiments are illustrative only and not restrictive of the broad invention, and that various other modifications and changes in light thereof will be suggested to persons skilled in the art based upon the above teachings. And are neither required nor exhaustive of all embodiments. And obvious variations or modifications of the invention may be made without departing from the spirit or scope of the invention.
Claims (4)
1. A parameter selection method of a measuring circuit for improving the measurement accuracy of steel rail temperature is characterized by comprising the following steps: the measuring circuit comprises a three-output thermal resistor R, a three-wire system bridge circuit and an operational amplifying circuit;
three output ends of the three-output thermal resistor R are respectively provided with lead resistors R1, R2 and R3, the three-wire bridge circuit comprises resistors R1, R2, R3 and R4 which are connected in a ring shape, wherein the common connection point of the R1 and the R2 is connected with a reference power supply E, and the common connection point of the R3 and the R4 is grounded; wherein R is2＝R3And when the three-wire bridge circuit is balanced, (R)1+r1+r2)·R3＝(R4+r2+r3+R)·R2；
The A end of the three-output thermal resistor R is connected with R1 through R1, the A end of the three-output thermal resistor R is also connected with the positive input end of the operational amplifier circuit through R2, and the B end of the three-output thermal resistor R is connected with R4 through R3; the common connection point of the R2 and the R3 is also connected with the negative input end of the operational amplification circuit;
the output end of the operational amplification circuit outputs a measurement signal;
the measuring circuit also comprises a first low-pass filter circuit and a second low-pass filter circuit, wherein the first low-pass filter circuit is arranged at the positive input end of the operational amplifier circuit, and the second low-pass filter circuit is arranged at the negative input end of the operational amplifier circuit;
the first low-pass filter circuit comprises R5, C2 and C4, wherein one end of R5 is connected with R2, the other end of R5 is respectively connected with the positive input end of the operational amplifier circuit, one end of C2 and one end of C4, and the other end of C2 and the other end of C4 are grounded; the second low-pass filter circuit comprises R6, C3 and C5, wherein one end of R6 is respectively connected with R2 and R3, the other end of R6 is respectively connected with a negative input end of the operational amplifier circuit, one end of C3 and one end of C5, and the other ends of C3 and C5 are grounded;
the measuring circuit also comprises a common-mode signal suppression circuit which is used for generating a filtering suppression effect on the input common-mode signal; the common-mode signal suppression circuit comprises a capacitor C1 connected between the positive input end and the negative input end of the operational amplification circuit;
the back-end operational amplification circuit adopts an instrument amplifier, so that the anti-interference capability of the circuit is further improved;
the temperature measurement range is reduced, the accumulated error is reduced, so that R1 is replaced by a high-precision digital potentiometer, base points are arranged in temperature sections, the temperature measurement range is reduced, the measurement precision is improved, and the accumulated error is reduced;
the method comprises the following steps:
s1: determining the range of the temperature of the steel rail which needs to be measured actually, and determining the resistance value range of the three-output thermal resistor R according to the range; the temperature of the steel rail ranges from minus 40 ℃ to plus 100 ℃, and the resistance value of the corresponding thermal resistor is 84.27-138.51 omega;
s2: selecting a base point according to the difficulty of finding the resistance value of the actual resistor and considering a certain fault tolerance, and obtaining the resistance value of the three-output thermal resistor R at the base point; wherein the three-wire bridge circuit is in a balanced state when at a base point; selecting a resistance value of 83.08 omega corresponding to-43 ℃ as a base point; the base point is selected at the central point of the temperature measurement range, the temperature measurement values at two ends of the central point have accumulated errors, but compared with the base point arranged at one end, the accumulated errors are reduced by half, and an offset circuit is required to be added at the rear end of the operational amplifier or an AD converter for sampling positive and negative values is directly selected;
s3: according to R2＝R3And when the three-wire bridge circuit is balanced (R)1+r1+r2)·R3＝(R4+r2+r3+R)·R2The actual sizes of the lead resistors r1, r2 and r3 and the size of the three-wire bridge circuit resistor in consideration of the general precision resistor which can be purchased are selected: firstly, selecting the sizes of R2 and R3, and selecting the sizes of the resistors R1 and R4 according to the resistance of the three output thermal resistor R at a base point; the difference between the values of R1 and R2 is as small as possible, so that the impedance of two arms of the bridge is consistent, and the resistance to power supply and external interference can be greatly improved;
selecting R2-R3-1 k omega, selecting R1-825 omega and R4-908 omega for the sizes of the resistors R1 and R4, so that R4-R1-908 omega-825 omega are 83 omega, and the resistance value of the thermal resistor is just 83.08 omega when the base point is minus 43 ℃, and at the base point, the bridge is in a balanced state, the error is zero, and the bridge is in an unbalanced state as the resistance value of the thermal resistor is larger, namely the temperature is increased; when the base point is selected and the circuit parameters are selected, the zero point of the whole circuit is also determined without readjustment;
s4: obtaining the voltage of the input end of the operational amplifier circuit at the maximum value and/or the minimum value of the range of the temperature of the steel rail according to the selected sizes of the resistors R1, R2, R3 and R4;
s5: determining the amplification factor of the operational amplification circuit according to the sampling range of the subsequent AD conversion circuit;
s6: selecting parameters of a low-pass filter circuit arranged at the input end of the operational amplification circuit by considering the low-pass filter bandwidth range of the signal input at the input end of the operational amplification circuit; the values of R5, R6, C2, C3, C4, and C5 are selected mainly in consideration of the low-pass filtering bandwidth range of the input signal of the operational amplifier input terminal, and R5 ═ R6 ═ 10k Ω, C2 ═ C3 ═ 0.1uF, and C4 ═ C5 ═ 4.7uF, and mainly because the temperature is slowly changed and the corresponding voltage signal is relatively stable, the ac signal is suppressed;
s7: selecting a parameter of a common mode signal eliminating capacitor arranged between two input ends of the operational amplification circuit; c1 ═ 0.001uF is a common mode signal cancellation capacitor;
by adopting the circuit structure and the circuit parameters provided by the above, the maximum error of measurement does not exceed 0.2 ℃ under the normal condition that r is less than 1 omega, and the maximum error of measurement does not exceed 0.5 ℃ under the limit condition that r is less than 3 omega.
2. The parameter selection method for the measuring circuit for improving the measurement accuracy of the temperature of the steel rail according to claim 1, is characterized in that: the type of the three-output thermal resistor R is PT 100.
3. The parameter selection method for the measuring circuit for improving the measurement accuracy of the temperature of the steel rail according to claim 1, is characterized in that: the lead resistors r1, r2 and r3 are equal in size.
4. The parameter selection method for the measuring circuit for improving the measurement accuracy of the temperature of the steel rail according to claim 1, is characterized in that: the lead resistors r1, r2 and r3 are equal in size, so that the actual sizes of the lead resistors r1, r2 and r3 do not need to be obtained in step S3; and in order to reduce the error caused by the circuit, the value of R2 and the value of R1+ R1 cannot be greatly different.
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| CN203224306U (en) * | 2013-01-16 | 2013-10-02 | 燕山大学 | Single-chip microcomputer control-based intelligent flat-panel acquisition system for thermal electromotive force |
| CN104344908B (en) * | 2013-08-02 | 2017-12-29 | 上海微电子装备（集团）股份有限公司 | A kind of three-wire system thermal resistance measuring circuit |
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