BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the technology of a calculation device and a calculation method, and more particularly to an operation device and an operation method of obtaining thermo-couple input temperature from thermo-couple output electrical potential and surrounding temperature. 
     2. Description of Related Art 
     Conventionally, the expressions of obtaining thermo-couple input temperature are given in an expression 1 (expression (1) is used hereinafter),              Z   =           c   *   x     -       (     y   +   273.15     )     4       4     -   273.15             (   1   )                                
     where 
     c: constant, 
     x: thermo-couple output, 
     y surrounding temperature [° C.], 
     z thermo-couple input temperature [° C.]. 
     Further, for obtaining thermo-couple input temperature, there is a method of obtaining it by immediately calculating using the required expression (1) and a method of obtaining it by calculating using expression 2 (expression (2) is used hereinafter), 
     
       
           Z=c 1+( c 2 +c 3 *x )* x +( c 4 +c 5 *y )* y   (2)  
       
     
     where 
     C1˜c5: constant, 
     x: thermo-couple output [V], 
     Y: surrounding temperature [° C.], 
     z: thermo-couple input temperature [° C.], 
     which is an approximate expression of the expression (1). 
     Here, FIGS. 6 and 7 are simulation results in which calculations are performed using the expression (1) and the expression (2) in the lower frequency of 520 kHz of the high speed clock. FIG. 7 is a graph of the simulation results expressed in FIG.  6 . However, in the conventional calculation method, in order to calculate the expression (1), it is required to perform multiplication/division calculation thrice, adding/subtraction calculation thrice, and square root calculation twice. Here, for each individual calculation, multiplication/division calculation requires 17 msec, adding/subtraction calculation requires 0.4 msec, and square root calculation requires 25 msec. Namely, in order to calculate the expression (1), total calculation requires 102.2 msec. Here, according to the expression (1), in the case where thermo-couple output electrical potential is 0.02 [V] and the surrounding temperature is 18 [° C.], thermo-couple input temperature is 122.83 [° C.]. 
     On the other hand, in order to calculate the expression (2), it is required to perform multiplication/division calculation thrice, adding/subtraction calculation thrice, and square root calculation twice. Namely, in order to calculate the expression (2), total calculation requires 69.2 msec. Here, according to the expression (2), in the case where thermo-couple output electrical potential is 0.02 [V] and the surrounding temperature is 18 [° C.], thermo-couple input temperature is 121.89 [° C.]. 
     In this way, when calculation is performed using the expression (1), calculation requires 102.2 msec, although the value of thermo-couple input temperature that is required is accurate. On the other hand, when calculation is performed using the expression (2), the result of calculation becomes 121.89 [° C.] by causing an error of 0.94 [° C.] from the expression (1) although calculation requires 69.2 msec by a reduction of 33 msec. 
     Namely, the time for calculation becomes long although the result of the calculation is accurate when the expression (1) is calculated, and the result of the calculation causes an error although the time for the calculation becomes short when the expression (2) is calculated. 
     The purpose of the present invention is to provide a calculation device and a calculation method in which the error caused in the results of the calculation is reduced as much as possible and the time for calculation is small. 
     SUMMARY OF THE INVENTION 
     The summary of representatives is explained in the following based on the inventions disclosed in the present application. 
     A calculation device of the present invention, in which thermo-couple input temperature is calculated from thermo-couple output electrical potential and the surrounding temperature, includes a memory for storing a plurality of values of thermo-couple input temperature being calculated using a plurality of values of thermo-couple output electrical potential and a plurality of values of surrounding temperature; and a calculation unit for inputting a predetermined value of thermo-couple output electrical potential and a predetermined value of surrounding temperature, reading out from the memory a plurality of values of stored thermo-couple output electrical potential corresponding to a predetermined value of inputted thermo-couple output electrical potential, reading out from the memory a plurality of values of stored surrounding temperature corresponding to a predetermined value of inputted surrounding temperature, reading out from the memory a plurality of values of stored thermo-couple input temperature selected from a plurality of values of read out thermo-couple output electrical potential and a plurality of values of read out surrounding temperature, and calculating thermo-couple input temperature corresponding to a predetermined value of inputted surrounding temperature and a predetermined value of inputted thermo-couple output electrical potential using an interpolation. 
     In accordance with the above-described system arrangements, it is possible to reduce an error caused in the results of calculation as much as possible and shorten the time for calculation as much as possible. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the invention, the object, features and advantages thereof will be better understood from the following description taken in connection with the accompanying drawings in which: 
     FIG. 1 is a system diagram of a calculation device of the first preferred embodiment of the present invention; 
     FIG. 2 is a flowchart of calculation procedures of the first preferred embodiment of the present invention; 
     FIG. 3 is a conceptual drawing of calculation procedures of the first preferred embodiment of the present invention; 
     FIGS. 4A and 4B are a conceptual drawing and a graph, respectively, of calculation procedures of the second preferred embodiment of the present invention; 
     FIGS. 5A and 5B are a conceptual drawing and a graph, respectively, of calculation procedures of the third preferred embodiment of the present invention; 
     FIG. 6 shows simulation results in which calculation is performed using expressions (1) and (2) with the lower frequency of 520 kHz of the fast clock; and 
     FIGS. 7A and 7B are graphs showing the simulation results described in FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will now be described based on the preferred embodiments. This does not intend to limit the scope of the present invention, but rather exemplify the invention. All of the features and the combinations thereof described in the embodiments are not necessarily essential to the invention. 
     In the following, the embodiment of the present invention is explained in detail with reference to the drawings. Here, in all drawings for explaining the embodiments of the present invention, the same reference number is used to the element having the same function, thus repetitive explanation is omitted. 
     First Preferred Embodiment 
     FIGS. 1 to  3  show a calculation device and a calculation method of the first preferred embodiment of the present invention. FIG. 1 is a system block diagram of the calculation device of the first preferred embodiment of the present invention. FIG. 2 is a flowchart of a calculation procedure of the first preferred embodiment of the present invention. FIG. 3 is a conceptual view showing the calculation procedure of the first preferred embodiment of the present invention. 
     First of all, the calculation device of the first preferred embodiment of the present invention is explained with reference to FIG.  1 . The calculation device of the first preferred embodiment of the present invention includes a memory device  101  (“memory  101 ” is used hereinafter) having EEPROM, etc. and a calculation unit  102  for calculating a thermo-couple input temperature. Here, the calculation unit  102  includes a ROM  103  for storing procedures for calculating the thermo-couple input temperature. 
     The memory  101  calculates using the expression (1) from thermo-couple output electrical potential x and a surrounding temperature y as indicated in FIG. 6A, and then stores a thermo-couple input temperature z thus calculated as a table format. In the memory  101 , thermo-couple output electrical potential x, the surrounding temperature y, and thermo-couple input temperature z are stored. Further, the thermo-couple input temperature z is accessed by two values of thermo-couple output electrical potential x and the surrounding temperature y. 
     In the ROM  103 , expressions 3 to 7 (“expressions (3) to (7)” are used hereinafter) are programmed and stored. 
     
       
           ofs   —   x =( x−x 1)/( x 2 −x 1)  (3)  
       
     
     
       
           ofs   —   y =( y−y 1)/( y 2 −y 1)  (4)  
       
     
     
       
           z   —   r 1 =z 1+( z 2 −z 1)* ofs   —   x   (5)  
       
     
     
       
           z   —   r 2 =z 3+( z 4− z 3)* ofs   —   x   (6)  
       
     
     
       
           z=z   —   r 1+( z   —   r 2 −z   —   r 1)* ofs   —   y   (7)  
       
     
     Stored programs are read out by an indication of the calculation unit  102 . 
     Next, the calculation method of the first preferred embodiment of the present invention is explained with reference to FIGS. 2 and 3. Here, as an example, the case in which thermo-couple output electrical potential x is obtained for thermo-couple output electrical potential of 0.02 [V] and the surrounding temperature of 18 [° C.], is explained. 
     First of all, at a step  201 , the calculation unit  102  inputs the surrounding temperature y (=18.0) and the thermo-couple output electrical potential x (=0.02) corresponding to the thermo-couple input temperature z. 
     Next, at a step  202 , the calculation unit  102  selects values of x1 and x2 which are the closest to inputted thermo-couple output electrical potential x and satisfies “x1≦inputted thermo-couple output electrical potential x≦x2” from the memory  101  in which the calculation results of expression (1) are stored in the table format. That is to say, values of x1 and x2, which are the closest to values of inputted thermo-couple output electrical potential x, are selected from the memory  101 . In this embodiment, the calculation unit  102  selects 0.018 as x1 and 0.022 as x2, but does not select 0.013 as x1 and 0.027 as x2. Because, when 0.013 as x1 and 0.027 as x2 are selected, the inaccuracy of the thermo-couple input temperature z, which should be obtained, becomes large, compared with the case of selecting 0.018 as x1 and 0.022 as x2. 
     In the same way, the calculation unit  102  selects values of y1 and y2, which are the closest to the inputted surrounding temperature y, and satisfies “y1≦inputted surrounding temperature y≦y2.” That is to say, values of y1 and y2, which are the closest to the inputted surrounding temperature y, are selected from the memory  101 . In this embodiment, the calculation unit  102  selects 14.0 as y1 and 20.0 as y2 from the memory  101 , but does not select 9.0 as y1 and 26.0 as y2. Because, when 9.0 as y1 and 26.0 as y2 are selected, the inaccuracy of the thermo-couple input temperature z, which should be obtained, becomes large, compared with the case of selecting 14.0 as y1 and 20.0 as y2. 
     Next, the calculation unit  102  selects the thermo-couple input temperature z1 (=113.97) corresponding to selected x1 (=0.0018) and y1 (14.0) from a table of the memory  101 . In the same way, the calculation unit  102  selects the thermo-couple input temperature z2 (=128.17) corresponding to selected x2 (=0.022) and y1 (=14.0), the thermo-couple input temperature z3 (=116.47) corresponding to selected x1 (=0.018) and y2 (=20.0), and the thermo-couple input temperature z4 (=130.42) corresponding to selected x2 (=0.022) and y2 (=20.0) from the table of the memory  101 . 
     Next, at a step  203 , the calculation unit  102  calculates the expression (3) and obtains a fluctuation rate ofs_x (=0.5) of thermo-couple output electrical potential using inputted thermo-couple output electrical potential x (=0.02) and selected thermo-couple output electrical potential x1 (=0.018) and x2 (=0.022). 
     Next, the calculation unit  102  calculates the expression (4) and obtains a fluctuation rate ofs_y (=0.6667) of the surrounding temperature using the inputted surrounding temperature y (=18.0) and the surrounding temperature y1 (=14.0) and y2 (=20.0). 
     Next, the calculation unit  102  calculates the expression (5) using the selected thermo-couple input temperature z1 (=113.97) and z2 (=128.17) and the calculated fluctuation rate ofs_x (=0.5) of the thermo-couple output electrical potential. Further, the first forecast thermo-couple input temperature z_r1 (=121.07) of the fluctuation rate of thermo-couple output electrical potential to the surrounding temperature y1 (=14.0 is obtained. 
     Next, the calculation unit  102  calculates the expression (6) using the selected thermo-couple input temperature z3 (=116.47) and z4 (=130.42) and the calculated fluctuation rate ofs_x (=0.5) of the thermo-couple output electrical potential. Further, the second forecast thermo-couple input temperature z_r2=123.445) of the fluctuation rate of the thermo-couple output electrical potential to the surrounding temperature y2 (=20.0) is obtained. 
     Next, the calculation unit  102  calculates the expression (7) using the calculated first forecast thermo-couple temperature z_r1 (=121.07), the calculated second forecast thermo-couple input temperature z_r2 (=123.445), and the calculated fluctuation rate ofs_y (=0.6667) of the surrounding temperature. Then, the thermo-couple input temperature z (=122.653) is obtained. 
     In accordance with the calculation device and the calculation method of the first preferred embodiment of the present invention, the following advantageous results are obtained. 
     (1) In the first preferred embodiment of the present invention, when expression (3) to expression (7) are calculated, multiplication/division calculation is performed five times and adding/subtraction calculation is performed ten times, and square root calculation is performed ten times. Here, if multiplication/division calculation requires 17 msec and adding/subtraction calculation requires 0.4 msec in order to perform each individual calculation, total calculation requires 89 msec. Namely, the calculation of the first preferred embodiment of the present invention reduces 13 msec of the calculation time, compared with the calculation of the conventional expression (1) Further, the result of the calculation of the first preferred embodiment of the present invention is 122.653 [° C.], and the error thereof is only 0.177 [° C.], compared with the result (122.83 [° C.]) of the calculation of conventional expression (1). This is a great improvement, compared with the error (0.94 [° C.]) of the result of the calculation between the conventional expression (1) and the conventional expression (2). 
     (2) Further, when both of the inputted thermo-couple output electrical potential x and the inputted surrounding temperature y are in the memory  101 , it is possible to obtain the thermo-couple input temperature z from the memory  101  without performing the above-mentioned step 3. In this case, it is possible to reduce the calculation time for obtaining the thermo-couple input temperature z since it can be reduced to perform the step 3. 
     (3) In the calculation device of the first preferred embodiment of the present invention, the result, which is obtained by calculating a function for obtaining the thermo-couple input temperature z from the thermo-couple output electrical potential x and the surrounding temperature y, is stored in a readable and writable memory device. Thus, in the case where it is required to change the function corresponding to applications, it is enough to alter data stored in the memory device, but it is not required to alter the calculation device. Further, products adopting the present invention can rewrite data by on-board even if manufacturing has finished. 
     Second Preferred Embodiment 
     FIG. 4 is a view for the calculation method and the calculation device of the second preferred embodiment of the present invention. FIG. 4A is a conceptual view showing the calculation procedures of the second preferred embodiment of the present invention. FIG. 4B is a graph of the table FIG.  4 A. 
     In the calculation method and the calculation device of the second preferred embodiment of the present invention, the memory  101 , as shown in FIG. 4A, calculates using the expression (1) using the thermo-couple output electrical potential x and the surrounding temperature y, and stores obtained the thermo-couple input temperature z in the table format. In the table used in the second preferred embodiment of the present invention, only the area where a highly resolved operation is required is calculated in detail. Namely, in the area where the highly resolved operation is required, it is stored in the memory  101  in the table format by subdividing the range of the thermo-couple output electrical potential x and the surrounding temperature y and by obtaining the thermo-couple input temperature z corresponding thereto. 
     In accordance with the calculation method and the calculation device of the second preferred embodiment of the present invention, the following effects are obtained, in addition to the effects (1) to (3) which can be obtained by the calculation method and the calculation device of the second preferred embodiment of the present invention. 
     (4) In the calculation method and the calculation device, in the area where high resolution is required, it is possible to obtain a highly resolved thermo-couple input temperature. Further, for the area, except for the area where the high resolution is required, it is possible to save the capacity of the memory  101  by setting a rough setting. 
     Third Preferred Embodiment 
     FIG. 5 is a view showing the calculation method and the calculation device of the third preferred embodiment of the present invention. FIG. 5A is a conceptual view showing the calculation procedures of the third preferred embodiment of the present invention. FIG. 5B is a graph of the table of FIG.  5 A. 
     In the calculation method and the calculation device of the third preferred embodiment of the present invention, the memory  101 , as shown in FIG. 5A, calculates using the expression (1) using the thermo-couple output electrical potential x and the surrounding temperature y, and stores obtained the thermo-couple input temperature z in the table format. In the table used in the third preferred embodiment of the present invention, in addition to the table shown in the first preferred embodiment and the second preferred embodiment, the thermo-couple input temperature z of the area where the error is caused severely is stored. Namely, the thermo-couple input temperature z of the area where the error becomes large is obtained in advance, and then it is stored in the memory  101  in the table format. 
     In accordance with the third preferred embodiment of the present invention, in addition to the effects (1) to (3) obtained in the calculation method and the calculation device of the first preferred embodiment of the present invention, the following advantageous effects are obtained. 
     (5) By storing the area where the error is caused severely in the memory in advance by formatting in the table, it is avoidable to calculate in the area thereof and then to cause the error. Further, it is possible to shorten the calculation time for obtaining the thermo-couple input temperature z. 
     Thus, the invention invented by the present inventor has been explained in a concrete manner based on the preferred embodiments, the present invention is not limited to the above-described preferred embodiment, but it goes without saying that various modifications are possible within the scope of the spirit of the subject matter. 
     The advantageous effect, which is obtained by the representative embodiment of the invention disclosed in the present application, is explained in the following. 
     In accordance with the present invention, the calculation time for obtaining the thermo-couple input temperature is reduced, and further the calculation precision is enhanced.
A calculation device includes a memory for storing a plurality of values of thermo-couple input temperature calculated using a plurality of values of thermo-couple output electrical potential and a plurality of values of surrounding temperature; and a calculation unit for inputting a value of thermo-couple output electrical potential and a value of surrounding temperature, reading from the memory a plurality of values of stored thermo-couple output electrical potential corresponding to the inputted thermo-couple output electrical potential, reading from the memory a plurality of values of stored surrounding temperature corresponding to the inputted surrounding temperature, reading from the memory a plurality of values of stored thermo-couple input temperature corresponding to the read values of the thermo-couple output electrical potential and the read values of surrounding temperature, and calculating by interpolation the thermo-couple input temperature corresponding to the inputted surrounding temperature and the inputted thermo-couple output electrical potential.