RU2326354C1 - Method of iterative thermoresistant thermometry - Google Patents

Abstract

FIELD: medical equipment; thermometry.

SUBSTANCE: method is based on active heat of thermoresistor, which is in thermal contact with measured object. Result is achieved due to the fact that after thermal contact with measured object values T₁ - T₇ of thermoresistor temperature is estimated on borders of the same consecutive time intervals of duration Δt₁ of lower thermal time constant of thermoresistor, the first, the second and the third evaluation of measured temperature is calculated by certain formula.

EFFECT: increased performance.

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Description

The present invention relates to medical equipment and can be used to measure the temperature of living warm-blooded organisms, and especially the temperature of the human body.

One of the urgent problems of medical instrumentation is the problem of rapid measurement of human temperature. The complexity of solving this problem is explained by the specificity of a living organism as an object of temperature measurement. It is known that thermistors (semiconductor thermal resistance) due to their small size and mass have a small intrinsic time constant - of the order of a second or less. However, the real time of establishing the temperature of the thermistor equal to (with an acceptable error) the temperature of the surface layer of the skin is calculated in minutes. This phenomenon is explained by the peculiarities of the mechanism of heat exchange between the body and the environment. Usually, the initial temperature of the thermometer sensing element is equal to the ambient temperature, which, naturally, is lower than body temperature. Therefore, when a thermosensitive element touches the surface of the skin, cold irritation of the corresponding area of the skin occurs. In response to cold irritation, the vessels of the superficial plexus tend to narrow, and the deep, on the contrary, expand. This leads to a decrease in the temperature of the outer layers of the skin, and therefore to a decrease in heat transfer. Those. the body is insulated from a cold object.

How is this problem solved to date? Firstly, the use of non-contact temperature sensors. The most or even exclusively applicable for these purposes is the infrared radiation sensor. Digital infrared thermometers are produced by a number of foreign companies. You can call, for example, the famous Japanese company OMRON, which produces the TEM-004 ear infrared digital thermometer, the measurement time of which is 1 second. Thermometer "ThermoTek" model 820 Israeli company "SAAT" provides for measuring the temperature of the frontal part of the human head. Since it differs significantly from the temperature of the core of the body, the device automatically introduces corrections, so that the measured value corresponds to the oral temperature. The disadvantages of infrared thermometers are relatively high cost and low accuracy. There are also conductive versions of digital thermometers that provide relatively high speed. An example is the ThermoTek thermometer model 0482 of the same Israeli company, SAAT. The thermometer has a fairly high accuracy (the error according to ASTM E 1112-98 is not more than ± 0.1 ° C) and speed (measurement time of about 10 seconds). The appearance of the device shows that the developers took all the necessary measures to reduce the thermometer’s own time constant. For this purpose, a thermistor is used as a sensor, which is placed on the tip of a long holder with a small cross section, which reduces the outflow of heat into the environment through the body of the thermometer.

The TEM-003 digital thermometer of the Japanese company OMRON has approximately the same characteristics.

Of the known closest in technical essence is the compensation method of measuring temperature [1], based on a step-by-step change in the temperature of a thermistor in thermal contact with the measurement object, from a value slightly exceeding the upper limit of the temperature measurement range to a value different from the temperature of the measurement object by an amount not exceeding the permissible value by changing the setpoint of the temperature stabilization system of the thermistor.

The main disadvantage of the prototype method is that the rate of change of the setpoint of the temperature stabilization system of the thermistor should not exceed the rate of natural cooling of the thermistor, depending on the temperature difference of the thermistor and the measurement object. Since the rate of natural cooling depends on the temperature difference between the thermistor and the measurement object, when approaching the state of the temperature balance, the cooling rate slows down more and more. This leads to low performance achieved using the prototype method, especially with the high required measurement accuracy. In addition, the disadvantage of the prototype method is a rather complicated implementation due to the presence of such a node as a temperature stabilization system of a thermistor.

The task of the invention is to improve performance and simplify implementation. In the proposed method of iterative thermoresistive temperature measurement based on the active heating of a thermistor in thermal contact with the object, according to the invention, after thermal contact with the measurement object, the temperature values T ₁ , T ₂ , T _{3 of} the thermistor are determined at the boundaries of two identical consecutive time intervals with a duration Δt less than the thermal time constant of the thermistor, the first estimate of the measured temperature is calculated by the formula

,

,

the thermistor is heated by a current pulse to a temperature equal to T _x1 -ΔT, where ΔТ is the value equal to the maximum error of the first estimate, the values of T ₄ , T _{5 of the} temperature of the thermistor are determined at the boundaries of an interval of duration Δt adjacent to the trailing edge of the heating pulse, the second estimate of the measured temperature according to the formula

,

,

, импульсом тока нагревают терморезистор до температуры, равной Т_х2-ΔТ, определяют значения Т₆, Т₇ температуры терморезистора на границах интервала длительностью Δt, примыкающего к заднему фронту второго импульса нагрева, вычисляют третью оценку измеряемой температуры по формулеWhere

, the thermistor is heated with a current pulse to a temperature equal to T _x2 -ΔT, the values of T ₆ , T _{7 of the} temperature of the thermistor are determined at the boundaries of an interval of duration Δt adjacent to the trailing edge of the second heating pulse, a third estimate of the measured temperature is calculated by the formula

which is taken as the measured value of the temperature of the object.

An example of a functional diagram of a device that implements the proposed method is presented in figure 1. Figure 2 shows the timing diagram of the operation of the device. Figure 3 shows the window of the program that implements a simulation model of the device. Functional diagram (figure 1) includes a measuring circuit 1 (IC), consisting of a source of heating current 4 (INT), a key 5 (C) and a thermistor 6 (R _t ), and a microcontroller 8 (MK), in which figures 10 and 11, respectively, the output of the key management bus 5 and the ADC input are indicated. Moreover, the thermistor 6 through a key 5 is connected to the output of the heating current source 4, the control input of which is connected to the output 10 of the control bus of the microcontroller 8, and 11 of which the ADC is connected to the output of the measuring circuit 1.

We believe that the measurement is implemented in software. The microcontroller 8 serves as both a control device and a device for evaluating the voltage value at the output of IC 1. The measurement process is illustrated by the time diagram in figure 2, where line 12 shows the measured temperature T _x , curve 13 shows the temperature of the thermistor and curve 14 shows the heating current pulses. For definiteness, we believe that before starting the measurement, the temperature of the thermistor is equal to the ambient temperature (although this is not a condition limiting the implementation of the method). The ambient temperature is indicated in figure 2 through T _c . The reference current source 2 and resistor 3 set the reference point of the voltage across the thermistor 6, since the difference in voltage drops across the thermistor 6 and resistor 3, amplified by the differential amplifier 7, is supplied to the ADC input of the microcontroller. The thermal contact of the thermistor with the measurement object is provided. At the command of the microcontroller 8 (Fig. 1), the key 5 is closed (moment t ₁ , Fig. 2) for a short interval equal to the time of establishment of the output voltage of the I / O (sampling and storage device), which is usually turned on at the input of the ADC of the microcontroller. The output code of the microcontroller ADC is converted to the value T _{1 of the} temperature of the thermistor corresponding to the moment t ₁ . After a predetermined interval Δt (time t ₂ , figure 2), the temperature T _{2 is} similarly determined. After another interval Δt (moment t ₂ , figure 2), the key 5 closes. At time t ₃ , the temperature T _{3 of the} thermistor is determined.

From the three values of the temperature of the thermistor T ₁ , T ₂ , T ₃ , the first estimate T _x1 is determined (rough, since the intervals Δt are short and therefore the corresponding temperature increments of the thermistor are also small)

Formula (1) is obtained as follows. If the thermistor has an initial temperature T _n and is in thermal contact with the measuring object having a temperature T _x , then the temperature increment of the thermistor for the time interval Δt (during passive heat transfer) is expressed by the known relation

where τ is the time constant of the thermistor characterizing its thermal inertia. Those. for a given time interval Δt, the temperature increment of the thermistor is directly proportional to the initial temperature difference of the thermistor and the measurement object.

, для приращений ΔT₁ и ΔТ₂ (см. фиг.2) в соответствии с выражением (2) получим:If Δt = t ₃ -t ₂ = t ₅ -t ₄ = Const, then, introducing the notation

, for increments ΔT ₁ and ΔT ₂ (see figure 2) in accordance with the expression (2) we obtain:

Subtract (4) from (3):

ΔT ₁ -ΔT ₂ = (T ₂ -T ₁ ) k,

whence for the coefficient k we get

According to (4)

Substituting (5) into (6) and taking into account that ΔT ₁ = T ₂ -T ₁ , ΔT ₂ = T ₃ -T ₂ , we obtain the formula (1).

The value of k obtained by formula (5) does not take into account the error in determining the values of ΔT ₁ and ΔT ₂ and was used only to derive formula (1). Therefore, the real value of k is determined from the expression

whence follows

Key 5 opens at time t ₄ when the temperature of the thermistor reaches the value T ₄ = T _x1 -ΔT, where ΔT is a value that exceeds the maximum possible error in determining the first estimate T _{x1 of the} measured temperature. The specific value of ΔT does not matter, for example, it can be taken equal to 10ΔT _q , where ΔT _q is the main component of the error in estimating T _x1 values of the measured temperature, due to an error in the quantization of the ADC of the microcontroller. Further, in the interval from t ₄ to t ₆ the same processes occur as in the interval from t ₂ to t ₄ . Those. the thermistor temperature value T ₅ is determined at time t ₅ , the second estimate T _{x2 of} the object temperature is calculated, the thermistor is heated to a temperature T ₆ = T _x2 -ΔT. Evaluation is performed according to a formula different from (1). On the interval from t ₄ to t ₅ we have

Where

this is the value that was determined at the first stage by the formula (8).

From (9) we find the updated value of T _x2

On the interval from t ₆ to t ₇ we have

From (12) we find the updated value of T _x3

Obviously, the third estimate will be more accurate than the second, since the values of T ₇ , T _{6 are} known with an accuracy determined by the resolution of the ADC of the microcontroller, i.e. we determine, with an error in estimating the value of the coefficient k, not the temperature itself, but its increment, expressed by the second term of formula (13).

To study the process of measuring temperature by the proposed method, a simulation model of a thermometer was created by software. The program interface is presented in figure 3. The interface elements on the upper panel “Model Parameters” of the program window allow you to set the Δt / τ ratio (in this case it is 0.1), the width of the ADC conversion range of the microcontroller, reduced to the temperature scale (in this case, it is 41-20 = 21 ° C), and the resolution of the ADC (in this case, it is 11). In the middle panel "Single Measurement Results", using the text box with the designation "Tx", you can enter the value of the measured temperature (in this case 38 ° C). The text box with the designation “First estimate of Tx without quantization” displays the value of the first estimate of the temperature of the measurement object, calculated by formula (1) with the accuracy that the computer provides. As you can see, the error in determining the measured temperature by the formula (1) is practically absent, which indicates the methodological correctness of this formula. In the remaining text windows of the middle panel, the values of three successive estimates of the temperature of the object obtained in accordance with the above algorithm, taking into account the quantization error of the ADC of the microcontroller, are displayed. The lower panel shows a graph of changes in measurement error over the measuring range from 37 to 41 ° C. As can be seen, with the accepted parameters of the model (quite acceptable in the practical implementation of the method), the maximum absolute error does not exceed 0.07 ° C.

The temperature measurement time using the proposed method directly depends on the parameters of the thermistor. If we use thermistors, for example, type B57311V from EPCOS, for which the thermal inertia is characterized by a time constant of the order of 4 seconds, then when choosing the ratio Δt / τ = 0.1, the total measurement time will not exceed 1.5 seconds.

Literature

1. Shakhov E.K. Compensation method for measuring temperature. RF patent №2257553. Published: July 27, 2005, Bull. No. 21.

Claims (1)

A method of iterative thermoresistive temperature measurement, based on the active heating of a thermistor in thermal contact with the measurement object, characterized in that after thermal contact with the measurement object determine the values of T ₁ , T ₂ , T ₃ temperature of the thermistor at the boundaries of two identical consecutive time intervals of duration Δt, less than the thermal time constant of the thermistor, calculate the first estimate of the measured temperature according to the formula

, the thermistor is heated by a current pulse to a temperature equal to T _x1 -ΔT, where ΔТ is the value equal to the maximum error of the first estimate, the values of T ₄ , T _{5 of the} temperature of the thermistor are determined at the boundaries of an interval of duration Δt adjacent to the trailing edge of the heating pulse, the second estimate of the measured temperature according to the formula

Where

using a current pulse, heat the thermistor to a temperature equal to T _x2 -ΔT, determine the values of T ₆ , T _{7 of the} temperature of the thermistor at the boundaries of an interval of duration Δt adjacent to the trailing edge of the second heating pulse, calculate the third estimate of the measured temperature by the formula

which is taken as the measured value of the temperature of the object.

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