CN101603863B - Realization of temperature compensation of cold junction of thermocouple based on temperature field fitting method - Google Patents

Abstract

The invention provides a method for realizing the temperature compensation of a cold junction of a thermocouple by a temperature field fitting method, which comprises the steps of: establishing a mathematical model of temperature distribution on a plane determined by three points only through a distribution change discipline of the temperature of the three points, wherein the independent variable is the known temperature of the three points, and a function value is a temperature value of any point on the rectangular plane determined by taking the three points as vertexes; processing experimental data by a method of least squares to solve the undetermined coefficient of the model; and realizing the temperature compensation of thermocouples of all channels on the plane by measuring the temperature of the three points.

Description

Realize the method for thermocouple cold junction compensation based on the temperature field match

Technical field

The invention belongs to the temperature instrument design field, propose a kind of method that realizes thermocouple cold junction compensation based on the temperature field approximating method.

Background technology

Common cold junction compensation has bridge method, cryoscopic method and temperature sensors of high precision penalty method.Comparatively speaking, adopt the temperature sensors of high precision compensation, have advantages such as precision height, good stability.Conventional method all is that one tunnel thermocouple measurement passage uses a temperature sensor to compensate, and like this, the cost of product will significantly improve along with the increase of thermocouple measurement passage.

Summary of the invention

At the problems referred to above, the present invention proposes the method that a kind of temperature field approximating method is realized thermocouple cold junction compensation, only pass through the changes in distribution rule of 3 temperature, foundation is by the mathematical model of Temperature Distribution on these 3 definite planes, and the method by least square handles experimental data, tries to achieve the undetermined coefficient of model.Under the prerequisite that guarantees high compensation precision, in product, reduce the quantity of the temperature sensor that is used to compensate, thereby reduce cost of products with more thermocouple measurement passage.

Technical scheme of the present invention is as follows:

Set up two dimensional surface Temperature Distribution mathematical model, independent variable is 3 known temperature, and functional value is with these 3 temperature values by any point on the definite rectangle plane in summit.Cold junction connection terminal for thermopair is in a product on the plane, only needs to measure wherein 3 temperature, can realize the electric thermo-couple temperature compensation of all passages on this plane.

At the concrete distribution situation of thermocouple terminal of most of instrument, setting up with ABCD is the rectangle plane on summit, makes T _A, T _BAnd T _DBe respectively the actual temperature that records of 3 of A, B and D, O be rectangle ABCD inside more arbitrarily, E and F are respectively the projection of O point on limit AD and AB.

Suppose that on this plane on the some E online AD, the closer to the A point, then the influence that ordered by A of E point temperature is big more, the temperature effect that ordered by D is more little.When the E point is in the mid point of AD, be subjected to the temperature effect weight of 2 of AD identical.Definition Lad, Lab, Lae and Laf are respectively the distance of AD, AB, AE and AF, T _E, T _FAnd T _OBe respectively the temperature that E, F and O are ordered.Introduce weighted-average method, can obtain T _E:

$T_E=\frac{L_{\mathrm{ad}}-L_{\mathrm{ae}}}{L_{\mathrm{ad}}}\·T_A+\frac{L_{\mathrm{ae}}}{L_{\mathrm{ad}}}\·T_D......\left(1\right)$

$T_F=\frac{L_{\mathrm{ab}}-L_{\mathrm{af}}}{L_{\mathrm{ab}}}\·T_A+\frac{L_{\mathrm{af}}}{L_{\mathrm{ab}}}\·T_B......\left(2\right)$

T _A-T _E＝T _F-T _O ......(3)

T _O＝T _E+T _F-T _A ......(4)

In formula (1) and (2) substitution (4), then have:

$T_O=(1-\frac{L_{\mathrm{af}}}{L_{\mathrm{ab}}}-\frac{L_{\mathrm{ae}}}{L_{\mathrm{ad}}})\·T_A+\frac{L_{\mathrm{af}}}{L_{\mathrm{ab}}}\·T_B+\frac{L_{\mathrm{ae}}}{L_{\mathrm{ad}}}\·T_D......\left(5\right)$

By formula (6) as can be known, the interior temperature value T of any arbitrarily of plane ABCD _OAll be about O point relative position and T _A, T _BAnd T _DFunction.Promptly as long as the position that O is ordered determines that promptly the coefficient in the equation (6) is determined, so just can try to achieve the temperature that O is ordered according to three summit temperature.

When the O point is on the line CD, then have: L _Ae=L _Ad, being updated in the formula (5) has:

$T_O=T_D+\frac{L_{\mathrm{af}}}{L_{\mathrm{ab}}}\·(T_B-T_A)......\left(6\right)$

When the O point is positioned on the line AB, then have: L _Ae=0, substitution formula (5):

$T_O=T_A+\frac{L_{\mathrm{af}}}{L_{\mathrm{ab}}}\·(T_B-T_A)......\left(7\right)$

Order

Then, the O point has when line CD goes up when being in:

T _O＝T _D+k·(T _B-T _A) ......(8)

When being positioned at, the O point has when line AB goes up:

T _O＝T _A+k·(T _B-T _A) ......(9)

Wherein, coefficient k and O point is to the distance dependent of AD.Like this, on a rectangle plane, only need the temperature value on known three summits, just can pass through this mathematical model, try to achieve the temperature value of any point on the plane.

The present invention is based on the principle of least square, the cold junction temperature distribution situation of match thermopair, reach the effect of using three road PT100 compensation systems to realize the compensation of No. eight thermopairs, compensation precision is ± 1 ℃, and cost is saved cost than conventional PT100 compensation scheme and reached 62.5%.

Description of drawings

Fig. 1 is a temperature profile on the two dimensional surface.

Fig. 2 is the thermopair passage cold junction temperature distribution plan of instrument.

Fig. 3 is instrument mould model ME MAX453-3 KMGY, brand Phoenix, the sub-front elevation of thermocouple terminal.

Fig. 4 is the instrument layout viewing, and temperature field test equipment (DUT) influences the normal miscellaneous equipment (OTHER DEVICE) that distributes of DUT internal temperature and is positioned at the left side on the right side.

Fig. 5 is the instrument layout viewing, and temperature field test equipment (DUT) influences the normal miscellaneous equipment (OTHER DEVICE) that distributes of DUT internal temperature and is positioned at the right side in the left side.

Fig. 6 is the instrument layout viewing, and temperature field test equipment (DUT) is positioned at both sides, influences the normal miscellaneous equipment (OTHER DEVICE) that distributes of DUT internal temperature and is positioned at middle.

Embodiment

According to the method in the technical scheme, choose instrument and verify this method with eight thermocouple signal inputs.Its eight tunnel analog input channel AI0～AI7 as shown in Figures 2 and 3, gets wherein 0A/0B, 4A/4B and the actual cold junction temperature point of 7A/7B, uses 3 temperature of PT100 sensor measurement, temperature compensation sensor is pasted be put on the terminal.In actual use, the factor that has influence on each channel temperature distribution can be divided into three kinds, from the installation site of instrument, is divided into the left side that is positioned at miscellaneous equipment, and right side and centre are as Fig. 4, Fig. 5 and shown in Figure 6; On environment temperature, it is 16 ℃～40 ℃; From the angle of air flow, be divided into external fan is arranged, air flow is good and do not have a fan, the occasion that air flow is relatively poor.Instrument DUT to be measured is placed respectively under three kinds of conditions, connect thermopair, and the hot junction of thermopair is placed room temperature environment, the cold junction temperature value T that record records _MAI0～T _MAI7Make again that the cold junction temperature match value that is obtained by mathematical model is T _AI0～T _AI7, then have:

T _AI1＝T _AI0+k ₁·(T _AI7-T _AI4) ......(10)

T _AI2＝T _AI0+k ₂·(T _AI7-T _AI4) ......(11)

T _AI3＝T _AI0+k ₃·(T _AI7-T _AI4) ......(12)

T _AI5＝T _AI4+k ₅·(T _AI7-T _AI4) ......(13)

T _AI6＝T _AI4+k ₆·(T _AI7-T _AI4) ......(14)

Need to determine the undetermined coefficient in equation (10)～(14) so, this coefficient choose the concrete condition of work of wanting in the integrative instrument use, therefore, the definite of COEFFICIENT K j must take into account three top influence factors.Promptly determine COEFFICIENT K, make in the above under three kinds of external conditions match value and actual value sum of square of deviations ∑ (T _MAIi-T _AIi) ²Minimum.The data that will record under different external conditions gather, and use EXCEL neutral line regression function LINEST, and match obtains the coefficient of passage 1～passage 6, and is as shown in table 1.And according to the additional statistical value assessment fitting effect that returns.

Table 1, COEFFICIENT K fitting result

Passage	COEFFICIENT K	sel	r2	ssreg	ssresid
						1	-0.10420	0.00414	0.95444	142.51363	6.80324
2	-0.32138	0.00601	0.97870	658.96757	14.34296
						3	-0.42094	0.00410	0.99180	804.79776	6.65485
5	-0.11913	0.00316	0.97912	186.28380	3.97322
						6	0.20269	0.00525	0.97604	444.77963	10.91984

Wherein K is the coefficient that obtains, and sel is the standard variance of K, and r2 is a coefficient of determination, and ssresid represents residual sum of squares (RSS), is the quadratic sum of actual value and match value.Sstotal is a total sum of squares, the difference of two squares sum of actual value and mean value.Ssreg is a regression sum of square, ssreg=sstotal-ssresid, and r2=ssreg/sstotal, visible coefficient of determination r2 represents then that more near 1 fitting effect is good more.

Then can get in these coefficient substitution equation (10)～(14):

T _AI1＝T _AI0-0.104196·(T _AI7-T _AI4) (15)

T _AI2＝T _AI0-0.321384·(T _AI7-T _AI4) (16)

T _AI3＝T _AI0-0.42094·(T _AI7-T _AI4) (17)

T _AI5＝T _AI4-0.119127·(T _AI7-T _AI4) (18)

T _AI6＝T _AI4+0.202694·(T _AI7-T _AI4) (19)

At last, this mathematical model by being write as the form of code, is embedded in the program of electric thermo-couple temperature measurement products, can realizes only finishing multi-channel thermocouple cold junction compensation problem with three compensation points.

For the checking compensation precision, write the driver of instrument again according to this compensation method, after debugging finishes, eight road thermocouple signals are inserted acquisition channel, and insert in the calibration cell in the hot junction, instrument is placed under the different environment again, read the temperature value that thermopair records, can verify compensation precision.Actual test, the compensation precision of this method is ± 1 ℃.

Claims (1)

1. realize the method for thermocouple cold junction compensation based on the temperature field match for one kind, it is characterized in that: the connection terminal of measuring thermocouple signal is distributed, and setting up with ABCD is the rectangle plane on summit, makes T _A, T _BAnd T _DBe respectively the actual temperature that records of 3 of A, B and D, O be rectangle ABCD inside more arbitrarily, E and F are respectively the projection of O point on limit AD and AB, on the some E online AD; Definition Lad, Lab, Lae and Laf are respectively the distance of AD, AB, AE and AF, T _E, T _FAnd T _OBe respectively the temperature that E, F and O are ordered;

$T_E=\frac{L_{\mathrm{ad}}-L_{\mathrm{ae}}}{L_{\mathrm{ad}}}\·T_A+\frac{L_{\mathrm{ae}}}{L_{\mathrm{ad}}}\·T_{\text{D}}$

$T_F=\frac{K_{\mathrm{ab}}-L_{\mathrm{af}}}{L_{\mathrm{ab}}}\·T_A+\frac{L_{\mathrm{af}}}{L_{\mathrm{ab}}}\·T_{\text{B}}$

T _A-T _E＝T _F-T _O

$T_O=(1-\frac{L_{\mathrm{af}}}{L_{\mathrm{ab}}}-\frac{L_{\mathrm{ae}}}{L_{\mathrm{ad}}})\·T_A+\frac{L_{\mathrm{af}}}{L_{\mathrm{ab}}}\·T_B+\frac{L_{\mathrm{ae}}}{L_{\mathrm{ad}}}\·T_D$

The temperature that O is ordered is tried to achieve according to three summit temperature after determining in the position that O is ordered.

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