JP2007113921A - Conversion arithmetic device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a conversion arithmetic device allowing accurate and high-speed conversion processing without requiring a memory resource for storing a conversion table online. <P>SOLUTION: The conversion arithmetic device for converting the resistance of a thermistor for measuring temperature into temperature comprises a coefficient keeping means for keeping the coefficient of each term of a high-order polynomial approximating the theoretical expression showing the relation between the resistance and temperature of the thermistor, and a temperature arithmetic means for reading the coefficient of each term from the coefficient keeping means, setting it in the high-order polynomial, substituting the resistance input from the thermistor into the high-order polynomial having the set coefficient, and calculating the temperature. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a conversion operation device that converts a resistance value of a thermistor that measures temperature into temperature.

  In addition to electronic devices that measure temperature as a measurement / control target, there are many electronic devices that need to know the temperature for indirect purposes, such as compensating the operation of the electronic device by temperature. There are many examples in which a thermistor is used as a sensor when measuring these temperatures.

  FIG. 2 is a functional block diagram showing a configuration example of a conventional conversion arithmetic device. The conversion calculation method of this configuration example searches for two resistance values sandwiching the resistance value R obtained by measurement from a conversion table representing the correspondence between the resistance value and temperature defined for each thermistor. The temperature T is estimated and calculated by mathematically dividing (linear approximation) the temperature values at two points corresponding to the resistance values.

Reference numeral 1 denotes a thermistor that measures temperature, and measures a resistance value by a voltage drop E when a constant current is passed. Reference numeral 2 denotes AD conversion means, which inputs the voltage E and digitally outputs a resistance value R obtained by AD conversion. 3 is a search means, which inputs a resistance value R, accesses a conversion table 4 of the thermistor 1 held in an appropriate memory means, searches for two resistance values R1 and R2 sandwiching the resistance value R, The temperatures T1 and T2 corresponding to R1 and R2 defined in the conversion table 4 are acquired.

  FIG. 3 is a table showing an example of a conversion table provided by the manufacturer. In FIG. 3, when the measured resistance value R is 4.5 kΩ, the resistance values R1 and R2 sandwiching the resistance value are 4.16 kΩ and 4.911 kΩ, and the temperatures T1 and T2 corresponding to these resistance values are 50 ° C and 45 ° C.

  5 is an internal division calculation means for inputting temperatures T1 and T2 and resistance values R, R1 and R2 passed from the search means, and calculating the temperature T by mathematically dividing these values internally (linear approximation). To do. FIG. 4 is a schematic diagram for explaining the internal division calculation. The temperatures T1 and T2 of R1 and R2 across R are approximated by a straight line, and the temperature T is estimated and calculated from R and the approximate straight line. In order to increase the accuracy of the calculation, in addition to the two points sandwiching the resistance value R, there are cases where the four known points at both ends are approximated by a curve.

The thermistor is a semiconductor resistance element having a negative temperature coefficient of resistance. When the resistance values at the two temperatures T1 and T2 are R1 and R2 and the thermistor constant in this range is B12, the following relationship is generally established between the resistance value and the absolute temperature.
B12 = (logR1-logR2) / (1 / T2-1 / T1) (1)
Here, T and R under the condition of T1 ≦ T ≦ T2 (R2 ≦ T ≦ R1) are related by the following equation using the thermistor constant B12 between T1 and T2.
R = R1 · expB12 (1 / T-1 / T1) (2)

  From this, in order to obtain the temperature T from an arbitrary resistance value R obtained by measurement, a known temperature and resistance value in the vicinity of the temperature to be measured are searched, and the thermistor constant in this range based on those values. B12 can be obtained and calculated by adding a resistance value corresponding to a difference from the known resistance value.

  FIG. 5 is a functional block diagram showing another configuration example of a conventional conversion arithmetic device using this technique. Differences from the configuration of FIG. 2 will be described. Reference numeral 6 denotes a thermistor constant calculation means which obtains R1, R2 and T1, T2 from the search means 3, calculates the equation (1), and outputs the thermistor constant B12 in this range.

  Reference numeral 7 denotes an internal division calculation means which obtains the thermistor constant B12 from the thermistor constant calculation means 6 and R, R1 and T1 from the search means 3, calculates the equation (2), and outputs the temperature T. FIG. 6 is a schematic diagram for explaining the internal division calculation. A straight line given by a negative gradient B12 from the intersection of logR1 and 1 / T1 is estimated by calculating internal division using logR, logR1 and 1 / T1, and a temperature T obtained by reciprocal calculation is output.

  The definition content of the conversion table illustrated in FIG. 3 is different for each thermistor. Therefore, in the conversion arithmetic devices of FIGS. 2 and 5, a plurality of conversion tables 4 are held in advance corresponding to the types of thermistors 1 that can be used, and the search means 3 corresponds to the thermistors to be used. Refer to by switching.

  Patent Document 1 describes an electronic thermometer that uses a thermistor as a temperature sensing element and calculates a temperature by performing a correction operation.

Japanese Patent No. 2504753

The conventional apparatus shown in FIGS. 2 and 5 has the following problems.
(1) It is necessary to keep a conversion table online and to access a conversion table of known resistance value and temperature for each conversion operation.

(2) An algorithm for providing a search unit and searching for a known value in the vicinity of the measured resistance value every time conversion processing is required.

(3) Based on the search result, it is necessary to calculate a parameter for approximation every time a conversion calculation is performed.

as a result,
(1) A memory resource for holding the conversion table is required, and a larger memory resource is required when measuring a wide range of temperatures or when a highly accurate conversion is required.

(2) The calculation time becomes longer, and time unevenness occurs depending on the converted resistance value (temperature). Furthermore, when measuring a wide range of temperatures or when high-accuracy conversion is required, the time required for the search increases as the conversion table increases.

(3) A large memory capacity is required for the arithmetic processing, and the processing time becomes long.

  The present invention has been made to solve the above-described problems, and does not require a memory resource for holding a conversion table online, and realizes a conversion operation device that enables high-precision and high-speed conversion processing. The purpose is to do.

In order to achieve such a subject, the present invention has the following configuration.
(1) In a conversion operation device that converts the resistance value of a thermistor that measures temperature into temperature,
Coefficient holding means for holding the coefficient of each term of a high-order polynomial approximating a theoretical expression representing the relationship between the resistance value of the thermistor and temperature;
Read the coefficient of each term from this coefficient holding means and set it to the high-order polynomial, substitute the resistance value input from the thermistor to the high-order polynomial set coefficient, temperature calculation means to calculate the temperature,
A conversion operation device comprising:

(2) Coefficient calculation means for calculating the coefficient of each term of the high-order polynomial based on a conversion table defined for each thermistor;
Coefficient setting means for setting the calculated coefficient in the coefficient holding means;
The conversion operation device according to (1), further comprising: a coefficient generation unit.

(3) The conversion calculation apparatus according to (2), wherein the coefficient generation unit generates a coefficient offline in advance for each thermistor used.

(4) The conversion arithmetic apparatus according to (2) or (3), wherein the coefficient calculation means identifies the coefficient by a least square method based on the conversion table.

(5) The temperature calculation means uses the logarithm (log R) of the resistance value R as a variable, and based on given coefficients a0, a1, a2, a3,.
T = a0 + a1 (logR) + a2 (logR) ² + a3 (logR) ³ +
... an (logR) ⁿ
The conversion operation device according to any one of (1) to (4), wherein the temperature T is calculated by calculating a higher-order polynomial.

(6) The conversion calculation device according to any one of (1) to (5), wherein the temperature calculation means calculates the high-order polynomial by a Horner method.

As is apparent from the above description, the present invention has the following effects.
(1) It is only necessary to store the coefficient values of (degree + 1) number of polynomials that are previously calculated off-line based on the conversion table, and the memory resource can be greatly reduced as compared to the case where the entire conversion table is stored online. I can plan. This is particularly effective when measuring a wide temperature range.

(2) Since a conversion operation can be performed only with a simple logarithmic calculation and a polynomial operation, the time required for the operation can be shortened and equalized compared with the search, approximate constant calculation, and approximate calculation. In addition, the memory resources required for the operation are remarkably small, and the procedure can be easily created and verified.

(3) When the order of the approximate polynomial is made higher to improve the conversion accuracy, the memory resource associated with the change is only an increase of the coefficient for the additional order, and the computation time is also increased by using the Horner method. The addition and multiplication need only be increased by the additional order, and the memory resource does not increase and the search time does not increase.

  Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a functional block diagram showing an embodiment of a conversion arithmetic device to which the present invention is applied. The same elements as those of the conventional apparatus described with reference to FIGS. Hereinafter, the characteristic part of the present invention will be described.

  In FIG. 1, reference numeral 100 denotes a temperature calculation means, which inputs a measured resistance value R, calculates this, and outputs a temperature T. As a calculation method, a high-order polynomial approximating a theoretical expression representing the relationship between the resistance value of the thermistor and the temperature is used.

  Reference numeral 200 denotes coefficient holding means, which holds the coefficient of each term of the high-order polynomial calculated by the temperature calculation means 100 in the coefficient table 201. When there are a plurality of types of thermistors 1 to be used, a plurality of coefficient tables different for each thermistor are provided. The temperature calculation means 100 calculates a high-order polynomial by substituting a coefficient referring to the data of this coefficient table.

The relationship between the resistance value R and the temperature T is expressed by the following theoretical formula with C1 and C2 as constants.
R = C1 · exp (C2 / T) (C2 / T) (3)
In order to approximate a wide range of temperatures T as a single expression using a high-order polynomial with the resistance value R as a variable, it is necessary to take a large number of orders in order to ensure the required accuracy, which is problematic in practice.

Therefore, using the logarithm of R (log R) as a variable, based on the coefficients a0, a1, a2, a3,.
T = a0 + a1 (logR) + a2 (logR) ² + a3 (logR) ³ +
... an (logR) ⁿ (4)
It is approximated by a high-order polynomial given by.

  Reference numeral 300 denotes coefficient generation means. Reference numeral 301 denotes coefficient calculation means, which calculates the coefficients a0, a1, a2, a3,... An of each term of the high-order polynomial based on the conversion table 302 defined for each thermistor. Reference numeral 303 denotes a coefficient setting unit that sets the calculated coefficient in the coefficient holding unit 200.

  In the present invention, the coefficient generation means 300 is characterized in that the coefficient is generated in advance by off-line engineering based on the conversion table for each thermistor used, and it is necessary to hold the conversion table online as in the conventional method. There is no.

  The coefficient calculation based on the conversion table 302 by the coefficient calculation means 301 can identify a coefficient with high accuracy by adopting a method for identifying each coefficient by a known least square method. The degree of the polynomial may be determined according to the required conversion accuracy, but the conversion accuracy is naturally limited by the number of significant digits of the original conversion table itself, and may be determined practically accordingly.

  According to the verification by the applicant, it has been confirmed that the necessary conversion accuracy can be ensured by the calculation of the polynomial up to the fourth order using the equation (4) by the coefficient identification based on the conversion table shown in FIG. A specific calculation example is shown below.

When the coefficients a0, a1, a2, a3, a4 are calculated by the least square method from the conversion table of FIG.
a0 = 98.9524230619253
a1 = -38.7221049572286
a2 = 3.431439511687622
a3 = -0.272348491790581
a4 = 0.0117614997709552
Is obtained.

  If the resistance value R = 4.500 kΩ is obtained by measuring the temperature of the thermistor 1, the variable logR is logR = log4.5 = 1.504077. If this value and the values of the coefficients a0 to a4 are substituted into the equation (4), T = 47.60765 (° C.) ≈47.608 (° C.) is obtained.

  Using the resistance value R = 4,500 kΩ, the temperature T obtained by the conventional apparatus of FIG. 5 is T = 47.6014 (° C.), and the error is about 0.01%. Therefore, it can be understood that the conversion calculation value according to the present invention is converted with a reasonable and sufficient accuracy.

  As described above, the order of the approximate polynomial is practically 4th as described above. However, when the order of the approximate polynomial is made higher in order to improve the calculation accuracy, the memory resource associated with the change is the coefficient holding means. The increase in the memory resource is extremely small as the coefficient for the additional order of 200 only increases. Further, by using the well-known Horner method for the higher-order calculation, the calculation time can be increased by the additional order, and the increase in processing time accompanying the increase in order is small.

It is a functional block diagram which shows one Embodiment of the conversion calculating device to which this invention is applied. It is a functional block diagram which shows the structural example of the conventional conversion calculating apparatus. It is a table which shows an example of a conversion table. It is a schematic diagram explaining the internal division calculation of FIG. It is a functional block diagram which shows the other structural example of the conventional conversion calculating apparatus. It is a schematic diagram explaining the internal division calculation of FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Thermistor 2 AD conversion means 100 Temperature calculation means 200 Coefficient holding means 201 Coefficient table 300 Coefficient generation means 301 Coefficient calculation means 302 Conversion table 303 Coefficient setting means

Claims (6)

In a conversion operation device that converts the resistance value of a thermistor that measures temperature into temperature,
Coefficient holding means for holding the coefficient of each term of a high-order polynomial approximating a theoretical expression representing the relationship between the resistance value of the thermistor and temperature;
Read the coefficient of each term from this coefficient holding means and set it to the high-order polynomial, substitute the resistance value input from the thermistor to the high-order polynomial set coefficient, temperature calculation means to calculate the temperature,
A conversion operation device comprising: Coefficient calculation means for calculating the coefficient of each term of the high-order polynomial based on a conversion table defined for each thermistor;
Coefficient setting means for setting the calculated coefficient in the coefficient holding means;
The conversion operation apparatus according to claim 1, further comprising: a coefficient generation unit comprising:   The conversion arithmetic apparatus according to claim 2, wherein the coefficient generation unit generates a coefficient offline in advance for each thermistor used.   The conversion arithmetic device according to claim 2 or 3, wherein the coefficient calculation means identifies the coefficient by a least square method based on the conversion table. The temperature calculation means uses the logarithm (logR) of the resistance value R as a variable, and based on given coefficients a0, a1, a2, a3,.
T = a0 + a1 (logR) + a2 (logR) ² + a3 (logR) ³ +
... an (logR) ⁿ
5. The conversion calculation device according to claim 1, wherein the temperature T is calculated by calculating a high-order polynomial of 6. The conversion calculation device according to claim 1, wherein the temperature calculation unit calculates the high-order polynomial by a Horner method.

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