JPS6122764B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an A/D conversion method for converting an analog output of an element having non-linear characteristics, such as a thermistor, into a digital signal and outputting the digital signal. For example, a thermistor has been commonly used as a temperature detection element in a digital thermometer, a digital thermometer, and the like. As is well known, the temperature characteristics of this thermistor are such that the output changes exponentially. In addition to the above-mentioned thermistor, temperature sensing elements include platinum resistors, thermocouples, etc., but they all have temperature-resistance or temperature-resistance
The voltage relationship is not a linear relationship. For this reason, conventionally, as shown in FIG. 1, the output of a probe 1 using a thermistor or the like is added to a linearizer 2 to correct it so that it has linear characteristics. Then, the output of the linearizer 2 is converted into a digital signal via an A/D (analog/digital) conversion circuit 3, and the digital output is counted by a counting circuit 4 and displayed digitally on a display section 5. . Conventionally, the linearizer 2 is used to correct the non-linear characteristics of the temperature sensing element to give it linear characteristics, but even when the linearizer 2 is used, it can be used over a wide temperature range, for example -30°C.
It is extremely difficult to provide linear characteristics over a temperature range of ~200°C due to the characteristics of the circuit elements and the difficulty in adjusting the output of the linearizer. For this reason, when a thermometer is configured to measure a wide temperature range, an error from the true value inevitably appears, which is a problem especially in digital thermometers that require high accuracy. The present invention has been made in view of the above points, and even when the measurement range is wide, when converting the analog output of an element with non-linear characteristics into digital, digital logic operations are performed to convert the true value. It is an object of the present invention to provide an A/D conversion method that can perform digital conversion with high accuracy. The temperature T at which a temperature sensing element such as a thermistor is located
The resistance value R with respect to (°K) can be determined by the following equation. R= _R0exp (1/T-1/ _T0 ) _B0 ...(1) However, _T0 ...Reference temperature (°K). R ₀ ...Resistance value of thermistor at reference temperature T ₀ . B ₀ ...Constant (°K). (When the activation energy is ΔE and the Boltzmann constant is k, it can be determined by the formula B = ΔE/2k.) By modifying the above equation (1), from the resistance value of the thermistor at that time, The temperature T (°K) can be calculated backwards. First, by taking the logarithm of both sides of the above equation (1), the above equation (1) becomes nR=nR ₀ +B ₀ /T−B ₀ /T ₀ (2). If we shift "nR ₀ - B/T ₀ " in the above equation (2) to nR - nR ₀ + B ₀ /T ₀ = B ₀ /T, and take its reciprocal, we get The formula is obtained. Then, when both sides of the above equation (3) are multiplied by "B ₀ " and the denominator and numerator on the right side are multiplied by "T ₀ ", T=B ₀ T ₀ /T ₀ (nR-nR ₀ )+B ₀ …
…(4) becomes. Equation (4) above is for determining the absolute temperature T°K, and when determining the temperature T°C in degrees Celsius (°C), T (°C) B ₀ T ₀ /T ₀ (nR-nR ₀ ) + B
₀ −273.15…… (5) By performing the above equation (5) using an arithmetic circuit, the temperature at that time can be determined from the resistance value of the thermistor. In this case, the above B ₀ constant is temperature dependent, but generally the nominal B
It is expressed as a value between 0°C and 100°C as a constant. Therefore, if the temperature is determined by equation (5) using this nominal B ₀ constant, the error will be large. Therefore, in the present invention, the B ₀ constant is calculated and stored in advance at intervals of, for example, 10°C, and the calculation is performed to obtain the temperature T using the B ₀ constant when the temperature is close to the current temperature T, thereby reducing the error. are doing. Above B ₀
The constant can be obtained by transforming the above equation (2) as shown in the following equation. B ₀ = (nR-nR ₀ )TT ₀ /T ₀ -T...(6
) From the above equation (6), for example, the reference temperature T ₀ = 273.15°K
Using a thermistor with a resistance value R ₀ of 30kΩ,
Calculate B ₀ for 10°C (°K) intervals. in this case,
Measure the temperature T°K and the resistance value RkΩ at that time, and substitute them into equation (6) together with the above values of T ₀ and R ₀ to obtain B ₀ . B ₀ obtained in the above way is
The temperature T ₀ °K and the resistance value R ₀ kΩ corresponding to B ₀ are stored in advance in the memory. This value is shown in the table below.

【table】

[Table] Next, when calculating the temperature with respect to the resistance value R of the thermistor measured using the above three constants T ₀ , R ₀ , B ₀ , use the formula 5 to calculate the above T ₀ , R ₀ , B ₀ , R, but at this time, the above three constants T ₀ , R ₀ , and B ₀ can be calculated using constants in a temperature range close to the measured temperature to calculate a more accurate temperature. It will be possible. The circuit configuration is shown below in the second section.
An embodiment will be described with reference to the drawings. In FIG. 2, reference numeral 11 denotes a temperature detection section using, for example, a thermistor, and the output of this temperature detection section 11 is sent to a VT (voltage-time) conversion section 12. This VT conversion section 12 is configured, for example, by a double integration circuit, and outputs a rectangular wave signal with a time width T that is proportional to the input voltage V from the temperature detection section 11, and the output signal is sent to the signal line 13a. The signal is applied to one input terminal of the AND circuit 14 via. Further, when the VT converter 12 outputs the time signal T, it outputs a clear signal from the signal line 13b prior to outputting the time signal T, and after outputting the time signal T, outputs a transfer command to the signal line 13c. The clear signal is sent to the counting circuit 15, and the transfer command is sent to the C register 16. On the other hand, the other input terminal of the AND circuit 14 is always supplied with a clock pulse of a constant period from the timing signal generating circuit 19. And this AND circuit 1
The output of 4 is sent to the counting circuit 15 and counted, and this counted value is temporarily stored in the C register 16 by a transfer command output from the VT converter 12. The contents stored in this C register 16 are sent to the arithmetic circuit 18 via the input selection circuit 17. The output of this arithmetic circuit 18 is sent to the X register 22, Y
It is input to register 23 and Z register 24. The stored contents of each of these registers 22 to 24 are self-circulated and held within each register, and can be mutually transferred to other registers via a gate circuit 25. The outputs of the registers 22 to 24 are then applied to the input selection circuit 17. The Z register 24 serves as a display register as well as an arithmetic register, and its output is sent to a display section 28 via a gate circuit 26 and a decoder 27 for digital display. Further, the calculation output of the calculation circuit 18 is sent together with the carry data to a judgment circuit 29, where the calculation result is judged. The output of the determination circuit 29 is sent to the address section 30, and the address section 30 specifies the address of the control section 31. This control section 31 gives the next address to the address section 30, and also inputs the input selection circuit 17, the arithmetic circuit 18, the gate circuit 21
Control signals are sent to a to 21c, 25, 26, judgment circuit 29, code generation section 32, etc. to control their operations. The code generating section 32 generates a predetermined code signal in accordance with a command from the control section 31, and the code signal is applied to the input selection circuit 17. Further, the output of the arithmetic circuit 18 is sent to a determination circuit 33. This determination circuit 33 determines which temperature range the temperature calculated by the calculation circuit 18 falls within, and inputs the determination result to the constant address section 34 . The constant address section 34 specifies the address of the constant storage section 35 according to the determination result of the determination circuit 33. As shown in FIG. 3, the constant storage unit 35 stores the three constants T ₀ (°K), R ₀ (kΩ), and B ₀ determined in advance at predetermined temperature intervals, for example, at intervals of 10°C. There is. and,
The data read from the constant storage section 35 is converted into serial data by the parallel-to-serial conversion section 36 and sent to the input selection circuit 17. Next, the operation of the present invention configured as described above will be explained with reference to the flowchart shown in FIG. First, when the power is turned on as shown in step A of FIG. 4, all registers such as the X, Y, and Z registers 22 to 24 are cleared by a command from the control section 31. Further, by turning on the power, a temperature detection signal is output from the temperature detection section 11 and sent to the VT conversion section 12. Before the temperature detection signal is input, the VT converter 12 first outputs a clear signal from the signal line 13b to clear the contents of the counting circuit 15. Next, the VT converter 12 outputs a rectangular wave signal having a predetermined time width T proportional to the voltage V of the temperature detection signal from the signal line 13a, and supplies it to the AND circuit 14. This AND circuit 14 is V-
While the time signal T is being applied from the T converter 12, the gate is opened and the clock pulse input from the timing signal generating circuit 19 is output. The clock pulses output from this AND circuit 14 are sent to a counting circuit 15 and counted. Then, after a predetermined time, when the time signal T of the VT converter 12 becomes "0", the gate of the AND circuit 14 is closed and input to the counting circuit 15 is prohibited. V-T
When the time signal T becomes "0", the conversion unit 12 outputs a transfer command to the signal line 13c, and stores the contents of the counting circuit 15 in the C register 16. In this way, the C register 16 stores a value corresponding to the resistance value R of the thermistor in the temperature detection section 11. The measured value stored in this C register 16 is read out via the input selection circuit 17 in response to a command from the control section 31, as shown in step B in FIG.
The signal is inputted to the X register 22 via. Next, proceeding to step C, the contents of the Y register 23 are read out by the input selection circuit 17 and sent to the determination circuit 33 via the arithmetic circuit 18 for determination. The Y register 23 is designed to store the value of the temperature T determined by calculation, but it is cleared when the power is turned on, so at this point, the Y register 23
The memory content of is "0". Therefore, the determination circuit 33 determines that the stored content of the Y register 23 is "0" and sends the determination result to the constant address section 34. The constant address section 34 specifies the address ki of the constant storage section 35 shown in FIG. 3 according to the determination result, and as shown in step D,
The constants T ₀ , R ₀ , _and B ₀ for
273.15°K, R ₀ = 30000kΩ, B ₀ = 3325.72'' is read out and converted into parallel data by the parallel-to-serial converter 36, and then stored in the X register 22 via the input selection circuit 17, arithmetic circuit 18, and gate circuit 21a. let
Next, proceeding to step E, the measured value R and the constants T ₀ , R ₀ , B ₀ stored in the X register 22 are sequentially read out, and the arithmetic circuit 18 and the X register 22 are read out.
The Y register 23 and the Z register 24 are caused to perform the calculation shown in equation (5) above, and the result of the calculation is stored in the Z register 24. Now, for example, if the ambient temperature is ``-20
℃", and the measured value R, that is, the resistance value R of the thermistor in the temperature detection section 11 is "76.738 kΩ", the calculated value of the temperature T using equation (5) is: T=B ₀ T ₀ /T ₀ (nR-nR ₀ )+B ₀ -273.
15 = 3325.72 x 273.15/273.15 (
n76.738-n30.000)+3325.72
−273.15=−19.56 [℃]. Next, proceed to step F and register Y register 2.
The contents of the Z register 24 are calculated from the contents of 3 by the calculation circuit 1.
8, and the subtraction result is determined by the judgment circuit 29.
The contents of Y register 23 and Z register 24 are sent to
Let the students judge whether the contents of the two are equal or not. If the result of this judgment is "=", the process proceeds to step H and the contents of the Z register 24 are transferred to the gate circuit 26 and decoder 27.
If the judgment result is "≠", the process proceeds to step G. At this point, the contents of the Y register 23 are "0",
The judgment result at step F is "≠" and the process proceeds to step G, where the contents of the Z register 24 are transferred to the Y register 2.
Transfer to 3. Next, the process returns to step C and the operations of steps C to F are performed again. However, at this point, the temperature of "-19.56°C" has been calculated and stored in the Y register 23, so the constants T ₀ , R ₀ , and B ₀ for this temperature are now stored in the constant storage section 35. Read from That is, in step C, the value of temperature T is "-19.56
The determination circuit 33 determines that "℃" is in the range of -20℃≦T<-10℃, the constant address section 34 specifies the address K _i-2 , and the process proceeds to step D.
, the constant setting unit 35 says “T ₀ = 253.15, R ₀
=76.738, B ₀ =3224.23” is read into the X register 22. Next, using the new constant read into the X register 22 in step E, the temperature T is calculated by equation (5) in the same way as in the previous case. In this calculation, the temperature range is the same as the ambient temperature -
Since T ₀ , R ₀ , and B ₀ at 20°C≦T<−10°C are used, the value “T=−20.00°C” is obtained. However, at this point, the contents of the Y register 23 and the Z register 24 are not equal, so step F
From there, the process returns to step C via step G, and the operations up to step F are repeated. At this time, in step G, "-20.00°C" of the Z register 24 is written to the Y register 23, and the following steps C to E are written.
It will be calculated using the same constants as before, so
The calculation result is "-20.00°C", and the judgment result at step F is "=", and the process proceeds to step H, where the content "-20.00°C" of the Z register 24 is sent to the display unit 28 as the true temperature and displayed. Ru. Thereafter, the process returns to step B and the above-described operations are repeated. Next, if we measure the temperature at 172.0°C immediately after measuring the temperature at -20.00°C, the temperature will rise to 172°C (R = 0.21083kΩ) because the thermistor of the temperature detection unit 11 has a small heat capacity. Become. At this time, the value of "R = 0.21083kΩ" is C register 16
_{After being} stored _in the Calculation is performed using equation (5). This calculation yields the value "T=198.27°C," which is determined to be "≠" at step F, and at step C, the determination circuit 33 determines that "190≦T<200," and within this temperature range. "T ₀ = 463.15, R ₀ =
0.1528, B ₀ = 3725.37” is stored in the constant storage unit 35.
are read out to the X register 22, and the temperature T is calculated using these constants. This calculation yields the value "T=172.18℃", which is judged as "≠" in step F, and then "T ₀ = 443.15, R ₀ = 0.2187, B ₀ = 3614.64" for "170≦T<180". '' is read from the constant storage section 35 and calculation is performed. Through this calculation, the value "T=172.00℃" is obtained, and it is judged as "≠" in step F, and the next calculation is "T ₀ , B ₀ , R ₀ ", which is the same as the previous "170≦T<180".
is used, and the value of "T=172.00°C" is obtained. At this time, it is judged as "=" in step F,
Proceed from step F to step H and register Z register 2.
The calculation result “172.00℃” stored in 4 is displayed on the display 28.
is sent to and displayed. In the above embodiment, a case was explained in which a thermistor was used as the temperature sensing element, but it goes without saying that the same implementation is possible using other elements with non-linear characteristics. Further, in the above embodiment, the case where temperature detection is performed is shown, but it can also be implemented in the measurement of other physical quantities, such as when measuring the amount of light using a photoconductive element. Further, the constant storage section in the above embodiment may be a ROM or a RAM, but in the case of a ROM, the resistance value of the thermistor may be adjusted with a variable resistor so that the same ROM can be used even if the characteristics of the thermistor differ. Furthermore, 10
Although constants are stored at intervals of .degree. C., it is needless to say that the constants are not limited to this and can be set as appropriate, and other circuit configurations can be modified in various ways without departing from the gist of the present invention. As described above, according to the present invention, the constant corresponding to the nonlinear element necessary for calculating the measured value is stored in the constant storage section in advance, and when making a measurement, first the constant used last time and the constant corresponding to the measured value are stored in advance. calculate using the value, then read a constant corresponding to the calculated value from the constant storage unit and use it for the next calculation, and
Since the above calculation is performed until the same result as the previously calculated value is obtained, the calculation can be performed using constants most suitable for the measured value, and extremely accurate results can be obtained. Therefore, even if the measurement range is wide, it is possible to digitally convert the analog output of a nonlinear element into a linear characteristic, and to obtain extremely high accuracy.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram showing a conventional digital thermometer, FIG. 2 is a circuit configuration diagram showing an embodiment of the present invention, FIG. FIG. 4 is a flowchart for explaining the operation of this embodiment. 11...Temperature detection section, 12...VT conversion section,
15...Counting circuit, 16, 22-23...Register, 34...Constant address section, 35...Constant storage section.

Claims (1)

[Claims] 1. A detection means for detecting temperature using a non-linear temperature sensing element, a means for converting an analog value outputted by the detection means into a digital value, and a constant necessary for calculation to obtain a measured value divided at predetermined intervals. a constant storage means that stores a plurality of constants in advance in correspondence with the temperature range; a calculation means that performs a predetermined calculation based on the digital value output from the digital conversion means and the constant used in the previous calculation to obtain a measured value; means for storing the calculation result value; means for comparing the calculation result with the previous calculation result; and means for reading a calculation constant from the constant storage means, the comparison means repeatedly performs calculations until the same result is obtained, and the calculated value when the same result is detected is taken as the true measurement value. A/D conversion method.