RU2333466C1 - Method of express thermometry - Google Patents

Abstract

FIELD: physics, measurement.

SUBSTANCE: invention concerns medical equipment, in particular methods of thermometry, and is aimed at improving thermometry accuracy and rate. Method of express thermometry is based on active heating of temperature sensor and implies that after temperature sensor contacts with measuring object, temperature of temperature sensor is regulated at level equal to lower limit of thermometry range. Within each N sequential time of duration Δt, integral values I_i of temperature are evaluated and temperature of measuring object is calculated from two formulas for even and odd values of N. Rate is increased due to reduced effect of coetaneous cold irritations in place of its contact with thermometer temperature sensor and extrapolation thermometry algorithm.

EFFECT: improved accuracy of thermometry.

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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, above all, the temperature of the human body.

Most medical thermometers, including modern digital ones, based on the conductive method of heat exchange with the measurement object, have a common drawback - a long measurement time. The complexity of solving this problem is explained by the specifics of a living organism as an object of temperature measurement [1]. How is this problem solved to date? Firstly, the use of non-contact temperature sensors. The most or even exclusively used for these purposes is an infrared 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 E1112-98 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 in a miniature capsule 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 method of thermoresistive measurement of the temperature of warm-blooded organisms [2], based on the active heating of the thermistor, characterized in that before measuring the temperature of the thermistor is stabilized at a level equal to the upper limit of the temperature measurement range, after the thermistor contacts the measurement object the thermistor, at regular intervals, delivers three short pulses of stabilized current, the duration of which is sufficient to determine respectively three values of T _1, T ₂ and T ₃ thermistor temperature, and calculating the object of measurement temperature by the formula

Activity Measure three values T _1, T ₂ and T ₃ thermistor temperature and target temperature calculation is repeated several times with a time shift to the step, a smaller interval between measurements, and the temperature of the object is calculated as the average value of the calculated sample temperature of the object. The time shift of the series of three measurements is made equal to the half-period of the network noise, and each of the temperature values of the thermistor is calculated as the average of n measured values taken with weights proportional to the coefficients of Newton’s binomial of degree n.

The main disadvantage of the prototype method is that its implementation requires a high-precision analog-to-digital converter (ADC) of instantaneous values of the converted voltage, which increases the cost of a thermometer that implements this method. When using a less accurate ADC, accordingly, the accuracy of the thermometer decreases.

The technical result of the invention is to increase accuracy. This is achieved as follows. After the temperature-sensitive element contacts the measurement object, the temperature of the temperature-sensitive element is stabilized at a level equal to the lower limit of the temperature measurement range, during each of N consecutive time intervals of duration Δt, the integral values I _{i of} temperature T _x are determined and the temperature of the measurement object is calculated by the formula

in the case of even N and by the formula

in case of odd N.

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 bridge measuring circuit 1 (IC), consisting of a source of 2 reference voltage (ION), resistors 3 (R ₁ ), 4 (R ₂ ), a source of heating voltage 5 (TIN), switch 6 ( C) of resistor 7 (R ₃ ) and thermistor 8 (R _t ), differential amplifier 9 (DU), ΣΔ-ADC 10, microcontroller 11 (MK) and reading device 12, the microcontroller code input is indicated by numbers 13, 14, 15, microcontroller comparator input and microcontroller control bus. Moreover, the feeding diagonal of the measuring circuit 1 is connected through a switch 6 to the outputs of the sources of reference 2 and heating 5 voltages, the measuring diagonal of the bridge measuring circuit 1 is connected to the input of the differential amplifier 9, the output of which is connected to the input of ΣΔ-ADC 10, the output of which is connected to the code input 13 microcontroller 11, the input 14 of the comparator of the microcontroller is connected to the output of the differential amplifier 9, and the control bus 15 to the control input of the key 6.

Measurement is implemented in software. The microcontroller 11 serves as both a control device and a device for calculating the value of the measured temperature according to formulas (1) and (2). The measurement process is illustrated by a time chart in figure 2, where curve 16 shows the temperature dependence of the thermistor. As an example, the case of an even number N = 8 of the integral temperature values of the thermistor is shown. For definiteness, we believe that before starting the measurement, the temperature T _{os of the} thermistor is equal to the ambient temperature (although this is not a condition restricting the implementation of the method). The measurement process starts at the moment when, at the command of the microcontroller 11, the switch 6 is set to a position where the voltage from the heating current source 5 is supplied to the supply diagonal of the bridge circuit. This voltage is selected so that under the influence of the current flowing through the thermistor, the thermistor quickly heats up . The resistances of the bridge circuit are selected so that when the temperature of the thermistor is equal to the value of T ₀ , the bridge comes into equilibrium, i.e. to satisfy the condition:

where R _{t = Tn} denotes the resistance of the thermistor at a temperature of T ₀ . The value of T ₀ it is advisable to set equal to the lower limit of the temperature measurement range, in the case of measuring the temperature of the human body, for example, T ₀ = 34.5 ° C. Such a value of the initial temperature of the thermistor will reduce the effect of the effect of cold irritation of the skin at the point of contact with the thermosensitive element of the thermometer (namely, the effect of cold irritation is one of the main reasons for the low speed of all existing thermometers based on the conductive method of heat exchange with the measurement object). As soon as the bridge comes to a state of equilibrium, the microcontroller comparator is activated, and switch 5 is set by the microcontroller to the position where the reference voltage source 2 is connected to the supply diagonal of the bridge. Its voltage is selected based on the necessary minimization of the error from the self-heating of the thermistor by the current flowing through it. After that, the microcontroller 11 calculates the temperature of the measurement object by implementing formulas (1) or (2). In the time diagram, the initial integral values for the calculation are denoted as I ₁ ... I ₈ (in this case, N = 8). The values of I ₁ ... I ₈ are sequential readings of the ΣΔ-ADC output code multiplied by a scale factor in order to obtain the measurement result in temperature units.

The value of the interval Δt, in principle, can be any. However, practically this interval should be chosen not less than the period of network interference and a multiple of the half-period of network interference, which allows the use of weighting functions that are effective in terms of suppressing network interference, which are usually implemented in the ΣΔ-ADC at the stage of filtering and decimating a single-bit code sequence from the output of the Δ-modulator.

Formulas (1) and (2) are obtained as follows. Without loss of generality, we find the expression for T _x , assuming that N = 8, so that the derivation of the formulas can be explained using the time diagram in figure 2. If the thermistor has an initial temperature T ₀ and is in thermal contact with the measurement object having a temperature T _x , then the current value of its temperature is expressed by the ratio:

where T _x is the temperature value of the measurement object; τ is the heating time constant of the thermistor.

The integral of expression (3) has the form:

Find the value of the temperature of the thermistor at point t ₁ corresponding to the boundary between the first two intervals of duration Δt (see the timing diagram in figure 2). We define integral temperature 2Δt during the interval from t ₀ to t _2:

As the instantaneous value of the thermistor temperature at the point t ₁ will take the average value of the temperature in this interval:

Since the strong inequality 2Δt << τ is practically satisfied, the average value in the considered interval does not differ from the instantaneous value in the middle of the interval.

интегральное значение температуры за время Δt_u=3Δt на интервале от t₁ до t₄, получим:Denote by

the integral temperature value for the time Δt _u = 3Δt in the interval from t ₁ to t ₄ , we obtain:

We represent the exponent in the form of a truncated series:

We substitute expression (8) into expression (7):

where do we get:

Find the value of the temperature of the thermistor at point t ₅ (see the timing diagram in figure 2). We define integral temperature 2Δt during the interval from t ₄ to t _6:

As the instantaneous temperature value of the thermistor at point t ₅ we take the average temperature in this interval:

Denote by I _{t5 ... t8 the} integral temperature value for the time Δt _u = 3Δt in the interval from t ₅ to t ₈ , we obtain:

We transform expression (10) taking into account expression (8) to the form:

Simplify this expression:

We substitute expression (9) into expression (11):

Got the equation for T _x . We solve it.

It is easy to verify that for a specific value of N = 8, this formula fully coincides with formula (1), given that:

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 in the upper part of the program window allow you to set the bit depth of the ΣΔ-ADC (in this case 16), the value of the ratio Δt _u / τ (in this case it is 0.1). The upper graph shows a bar graph of the absolute error of temperature measurement on the value of the measured temperature in the range from 36 to 41 ° C, excluding the quantization error of the ΣΔ-ADC with a step of 0.05 ° C. As can be seen, in this case, the error is of the order of 0.003 × 10 ^-10 , i.e. is negligible, which indicates the methodological correctness of the proposed measurement method. The lower graph shows the diagram of the dependence of the absolute error on the measured temperature, taking into account the quantization error of the ΣΔ-ADC. The maximum value of the error does not exceed 0.011 ° C and occurs when using a 16-bit ΣΔ-ADC (it is known that such accuracy is far from the extreme accuracy provided by this type of analog-to-digital converters). Moreover, the ratio Δt _u / τ is 0.1, i.e. the measurement time is of the order of 0.2τ. If we take into account that with the traditional measurement method, the waiting time for the end of the transient process of establishing the temperature of the thermistor is at least 4τ (this approximately corresponds to the difference between the temperature values of the thermistor and the measurement object equal to 0.1 ° C), then the measurement time when using the proposed method decreases by 20 time.

, чем и объясняется малое значение методической погрешности, источником которой является погрешность квантования АЦП.In the text box (figure 3) with the label "Maximum temperature increment" it is shown to what maximum value the thermistor has time to warm up during the measurement. The maximum temperature increment does not exceed 0.7 ° C and takes place when the measured temperature is equal to the upper limit of measurement. The corresponding voltage increment at the output of the measuring diagonal of the bridge circuit determines the required dynamic range of the ADC. Thus, an additional advantage of the method is to increase the resolution of measuring the temperature value of the thermistor, in this case equal to

which explains the small value of the methodological error, the source of which is the quantization error of the ADC.

Literature

1. Shakhov E.K., Akinin V.V., Dolgova I.A. Features of measuring the parameters of the human body. // Bulletin of the Samara State Technical University, a series of "Technical Sciences", issue 33, Samara, 2005, p.279-283.

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

Claims (1)

The method of express temperature measurement based on active heating of the thermosensitive element, characterized in that after the contact of the thermally sensitive element with the measurement object, the temperature of the thermally sensitive element is stabilized at a level equal to the lower limit of the temperature measurement range, the integral values are determined for each of N consecutive time intervals of duration Δt I _i temperature and calculate the temperature T _x of the measurement object according to the formula

in the case of even N and by the formula

in case of odd N.

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