SU1312405A1 - Thermoresistive temperature meter with digital indication - Google Patents

Abstract

The invention relates to temperature measurement and can be used in automated systems for measuring and controlling ambient temperature. The purpose of the invention is to improve the measurement accuracy. The device contains a bridge circuit 1, an unbalance amplifier 2 of a bridge circuit 1 with a temperature-sensitive element R, a controlled frequency generator 3, a shaper of 4 bipolar pulses, a pulsed signal attenuator 5, an adjustable DC voltage source 6, a reference frequency generator 7 pulse sequences and frequency meter 9. The introduction of new elements and the formation of new connections between the elements of the device allows to compensate for the non-linearity of the characteristics of the temperature-sensitive element R-p and. allows the use of a device for controlling rapidly changing temperatures. 1 il. with b (/) Ll.J

Description

113

The invention relates to temperature measurement and can be used in an automated system with rhenium and temperature control of the surrounding medium.

The purpose of the invention is to improve the measurement accuracy.

The drawing shows a block diagram of a thermal resistance temperature meter with a digital readout.

Thermal resistance temperature meter with digital readout contains a bridge circuit 1 with a thermistor R in one of its arms, connected by its output to the input of the unbalance amplifier of the bridge circuit 2, a generator

3 controlled frequency shaper

4 bipolar pulses, an attenuator 5 of a pulse signal 5, an adjustable source 6 of DC voltage, a generator 7 of the reference frequency, a subtractor of 8 pulse sequences, a meter 9 of frequency.

. Thermal resistance temperature meter with digital readout works as follows.

Blocks 1-5 form a self-balancing bridge circuit providing an isothermal mode of operation of the thermistor R (i.e. the constancy of its temperature), when exposed to any external disturbing factors. The voltage of the unbalance of the bridge after amplification by amplifier 2 increases the frequency of generator 3 until the bridge is balanced by the action of rectangular pulses arriving at the bridge from driver 4.

At the same time, when the amplitude and the duration G of the supplying pulses are constant, the output frequency Fg, of generator 3, is connected to the heating power P of the thermistor R by a linear dependence

8

- JLML Sp

(one)

where R is the resistance of the thermistors at the selected operating point determined by the bridge shoulder resistors;

about t

p, t2 g - sensitivity pavement

 power schemes. When a thermistor is subjected to a varying ambient temperature that is adequate to the additional power of its heating, equal to РР, the first 24052

bridge imbalance occurs initially,. However, the resulting signal unbalance leads to a change in the frequency of the generator 3, and hence to

5, the change in the heating power of the thermistor R by a pulse signal is also an & signal, so that the bridge returns to a balanced state.

fO In this case, the change in the frequency of the generator 3, taking into account expression (1), is determined by the relation

J5

uF.

Sp. ivP.

(2)

Since the thermistor is in the B isothermal mode, i.e. works at one point of its characteristic, the pavement transformation function

power schemes are theoretically absolutely linear with any non-linearity of the thermistor characteristic.

It is known that a change in the ambient temperature by U.T is equivalent to the supply to the thermistor (or selection from it) of the additional power of its heating DR, defined by the ratio

thirty

LR H. & (

(3)

where H BT / ° CJ is constant dissipated

thermistor.

Consequently, a change in the ambient temperature causes a linear change in the frequency of the generator 3, which, taking into account expressions (2 and 3), is determined by the ratio

40

uF ,, u0-H Sp.

(four)

Thus, the current value of the ambient temperature 0 (t) is related to the linear functional dependence of -c with the output frequency of the generator 3, i.e. Fg (, | (t) K9 (t)., However, from relation (4) it follows that the coefficient K may take different values when the thermistor R is replaced by one instance with another, which is due to the spread of the constant value of the H resistor in wide limits even for the thermistor of the same

type

In order to provide a constant, specific value of the transmission coefficient K for all possible instances of dyagny type thermistors, between

pulse generator A and the input diagonal of bridge 1, an attenuator 5 is installed, which provides adjustment of the amplitude of the supply pulses of the bridge and, as follows from relation (1), a change in its sensitivity Sy.

Thus, by varying the amplitude of the bridge supply pulses using an attenuator 5, it is possible to provide the required value of the conversion coefficient K for the bridge circuit and, consequently, for the entire measuring device as a whole, for any values of the constant dissipation of the thermoresistor, i.e. . eliminate multiplicative measurement error.

Adjustable voltage source 6 provides additional power for heating the thermistor. This makes it possible to significantly reduce the heating power of a thermistor by pulsed voltage and thereby significantly increase the sensitivity of the bridge circuit. This is due to the fact that the main thermal energy, which is necessary for balancing the bridge circuit, is supplied to the thermistor from an adjustable source of direct current voltage. And only a part of the energy .u0, determined by the range of measured temperatures u9 and the value of the constant dissipation H of the thermistor, is provided by a pulse signal that carries information about the current value of the ambient temperature. And since the value of LR, as a rule, is many times less than the total energy P necessary for balancing the bridge circuit, the sensitivity of the device also increases by many times.

at PE l times n - times).

In addition, the adjustable voltage source 6 compensates for the variation in the initial heating power of the thermistor at the operating point, i.e. eliminates additive measurement error.

Setup of the meter is as follows.

With the help of an attenuator 5 the required value of the coefficient

that transform to the bridge

scheme, i.e. the required LR value is provided

the frequency of the bridge circuit at a very specific value of the change in ambient temperature A9.

Then, the additional heating power of the thermistor with the help of the regulated voltage source 6 is changed, the output frequency Fg ,, .. is achieved, the bridge circuit in which the difference frequency at the output of the subtractor 8 of the pulse sequences corresponds to the current value of the ambient temperature 9 (t).

After these settings, the meter is ready for operation and its indication, i.e. the results of measuring the output signal of the subtractor 8 pulsed probes using the frequency meter 9 unambiguously determine the current value of the ambient temperature 9 (t).

Since the heating of the thermistor is constant and pulsed to eliminate the correlation between these voltages (i.e., to eliminate the effect on the conversion factor K of the DC voltage source of the regulated voltage source 6 to the bridge circuit) formed by the pulse shaper must be bipolar and do not contain a constant component.

Since an increase in the ambient temperature causes a decrease in the output frequency of the bridge circuit (and vice versa), the output frequency of the reference frequency generator 7 must be greater than the maximum possible frequency in the measured temperature range.

The advantage of the proposed device is also its high speed, due to the fact that the thermistor is in isothermal mode, i.e. its temperature is almost constant due to negative power feedback from the thermistor. In this case, is equivalent to the time constant of the bridge circuit T, c in Kgj. times less than the thermal time constant of thermistor C (K ,, j. is the coefficient of negative feedback of the bridge circuit), i.e. almost a hundred times less than the value of. This allows the proposed device to be used to control rapidly changing temperatures.

5ГЗ

Claims (1)

Invention Formula A thermo resistive temperature meter with a digital readout, containing a bridge circuit with a thermistor in one of its arms, connected by a measuring diagonal to the input of an unbalance amplifier of the bridge circuit, a voltage source connected to the diagonal of the bridge power supply. and so that, in order to improve the accuracy of the measurement. To compensate for the nonlinearity of the characteristics of the temperature-sensitive element, a reference frequency generator, connected in series with a controlled frequency generator, calculated an impulse sequence generator and a frequency meter connected in series to a bipolar pulse generator and a pulse signal attenuator, the output of the unbalance amplifier of the bridge circuit connected to the input of the controlled frequency generator, the output of which is connected to the input of the bipolar pulse former, the output of the reference frequency generator is connected to the second input the pulse sequence subtractor, the pulse signal attenuator output is connected to the diagonal of the bridge power supply, and the voltage source is adjustable.