SU970134A1 - Digital temperature meter - Google Patents

Description

The invention relates to electrothermometry and can be widely used in the construction of digital temperature and thermo-emf meters, complete with low-sensitivity thermocouples.

Temperature meters are known that have an amplifier, a resistor, a switch, a selector, a result counter and a digital reading device connected in series, the output of the clock generator, the output of the automatic frequency control device, and the common connection point of the output are connected to another selector input. the key and the integrator input is connected through the second resistor and the second key output of the source of the reference voltage l.

However, the known meters have a strictly linear conversion function and are therefore unsuitable for direct temperature measurement, since the calibration characteristics of thermocouples are non-linear.

Closest to the proposed technical entity is a device that contains a series-connected thermocouple.

amplifier, resistor, switch, integrator, comparison unit, selector, result counter, digital reading device, clock generator, the output of which is connected to the second input of the selector, and the input connected to the output of the automatic frequency control device, as well as n blocks piecewise -linear

10 approximations consisting of n sources of the reference voltage, the outputs of which, through the corresponding resistors of each of the pieces of the piecewise linear approximation, are connected to the movable contacts of the cross-connectors of the approximation sections, are not the first. movable contacts which are interconnected and with movable contacts of switches corresponding

20 approximation blocks, the first fixed contacts of which are interconnected and connected to the integrator input, and the second fixed contacts of these switches and switches of the approximation sections in each of the linearization blocks are connected to a common bus, and the outputs of the control unit are digital control unit and control inputs

all switches, and the signal input of the control unit is connected to the output of the clock pulse generator 2.

However, in this device, primary temperature measuring transducers, such as thermocouples, have non-linear calibration characteristics, which, when measuring accuracy, necessitates an increase in the number of approximation sections, and thus an increase in the number of linearization blocks and, accordingly, the number of resistors and switches connected to the input integrator in the second cycle of integration. Consequently, an additional source of error appears - reverse key currents that affect the value of the integrator's health-discharge currents, and hence the measurement result. In addition, some thermocouples have very low sensitivity at certain temperature levels. For example, a thermolar copper-constantan at 20 K has a sensitivity of approximately 5 µV / K, and at 10 K its sensitivity is even lower and is only. 2.5 µV / K. This leads to the fact that in order to achieve a linear conversion of the measured temperature into a code, it is necessary to increase the values of the discharge resistors of the linearization blocks.

Indeed, the values of the discharge resistors are read by the formula

R .. /

1 b, U; RT, fo

where E is the voltage source of the reference voltage of the linearization unit;

R, j is the resistance of the integrator charge resistor in the first integration cycle; K is the gain of the amplifier; T - the duration of the first measure

integration;

sy - the thermocouple EMF values of the thermocouple at the i-cM plot of the approximation;

p. is the number of pulses in the i-th area of the approximation, corresponding to the value of the measured temperature in this area; f- frequency clock generator

pulses. ,

For a serial digital voltmeter Sch 68000 and a copper-constantan thermocouple in the range of 10-20 K, the value of R is 2.5 MΩ. The inclusion of such large resistances at the input of the integrator leads to a sharp increase in the error, since their values become comparable with the leakage resistances. It is possible to reduce the values of the discharge resistors, for example, by increasing the clock pulse frequency f, however, this reduces the measurement noise immunity, since it is difficult to perform the clock phase clock frequency auto-tuning with a given accuracy. In addition, when measuring the temperature of the low-sensitivity,

It is impossible to combine two functions in one device: temperature measurement and measurement of thermo-EMF, which is especially important in thermometry.

5 The purpose of the invention is to increase the accuracy of temperature measurement while expanding the functionality.

The goal is achieved by introducing a scheme into the device.

0 OR, second result counter, frequency divider, reversible counter, second digital reading device and AND circuit, one of the inputs of which is connected to the output of the reversible counter, and the other two are connected respectively to the output of the clock generator and one of the outputs of the control unit, output circuit OR connected to the input of a digital linearization circuit,

Q output of which through the second result counter is connected to the input of the second digital reading device, and one of the inputs of the OR circuit is connected to the output of the selector and the subtractive input of the reversing counter, the other

 the input is with the output of the AND circuit and the input of the frequency divider, the output of which is connected to the summing input of the reversible counter.

The drawing shows a functional diagram of a device for measuring temperature.

The device contains a series-connected primary measuring transducer 1, an amplifier 2,

5 a resistor 3, a switch 4, an integrator 5, a comparison block b, a selector 7, a result counter 8 and a digital reading device 9. The output of the reference voltage source 10 is connected to the common connection point of the switch 4 and the integrator 5 via a second resistor 11 and a second switch reader 12, and to the second input of the selector 7 is connected to the generator output 13 clock pulses, the input of which is connected to the output of the device 14 automatic frequency control. The output of the generator 13 clock pulses and one of the outputs of the control unit 16 are connected to the two inputs cxeNM And 15, respectively, and the output of the reversible counter 17 is connected to its third input, to the summing input of which the output of the frequency divider 18 is connected, and to the subtracting input - the output of the selector 7 The input of the frequency divider 18 is connected with the output of the I-15 circuit and with one of the inputs of the OR 19 circuit, the other input of which is connected to the output of the selector 7. The output of the OR 19 circuit through the digital linearization circuit 20 and the second counter 21 of the result is connected to the input second digit readout device 22, and the outputs of the control unit 16 are connected to the control inputs of switches 4 and 12, the selector 7, and the generator 13 of clock pulses. The device works as follows. The output thermo-emf EX of thermocouple 1 is amplified by amplifier 2. up to the nominal level of CUE (where Ku is the gain factor of amplifier 2). For time Tj tT, multiple to the period T of the network voltage of 50 Hz, the control unit 16 opens its switch 4, as a result of which the integrator 5 begins to integrate the output voltage Kue of the amplifier 2. By ms the voltage of the end of the interval at the output of the integrator 6 results in the voltage ewxiIirK J T7 (1) о where R is the resistance of the resistor 3; C is the capacitor of the integrator capacitor 5. In the second cycle (after the end of the interval T) the control unit 16 with its signal closes the switch 4 thereby disconnecting the output of amplifier 2 from the input of the integrator 5, the key 12 is opened, connected to the input of the integrator 5, the output of the source 10 voltage, and the selector 7 opens for the passage of pulses from the generator 13 clock pulses to the input of the counter 8 of the result. In the case of ems, the discharge of the integrator 5 occurs until its return to the initial state, which is fixed by the Comparison unit b. In the second cycle, at the output of the integrator 5, the voltage V «.- poai-g. (2) where R is the resistance of the resistor 11; BD is the output voltage of the source 10 of the reference voltage; TJ is the time interval from the beginning of the second integration cycle to the moment of integrator 5 returning to the initial state, i.e. until the moment when the equality Ugyxi Uenxi- holds. . Equating the expressions (1) and (2), we define the duration Tj of the second cycle of integration EX-- E). where K -% -. - analog conversion factor. At the time of the end of the interval TU, the comparison unit 6 is activated and closes the selector 7 with its output signal, as a result of which the pulses from the generator 13 clock pulses to the input of the result counter 8 are stopped. Thus, in the counter 8, a number code is fixed, which is proportional to the measured thermocouple EMF Ejc of the thermocouple 1 t, where f is the pulse frequency of the generator 13 clock pulses. At the time of the end of interval T ;, pulses with a frequency from the generator output of 13 clock pulses through the selector 7 arrive at the result counter 8 and simultaneously to the subtracting input of the reversible counter 17 and to one of the inputs of the OR circuit 19.; At the same time, the signal from block 16 control arriving at one of the inputs of the AND 15 circuit, pulses of the frequency fo from the generator output 13 clock pulses through the AND 16 circuit arrive at the input of the frequency divider 18 and to the second input of the OR circuit 19. From the output of the frequency divider 18, the pulses of frequency fj, (; n .-1) (where (p-1) - the coefficient is divided and the frequencies of the 18 frequency divider) are fed to the summing input of the reversible counter 17. These pulses will arrive until it passes through zero. Thus, the summing input of the reversible counter 17 must receive a number of pulses N of frequency d (n-1), since N pulses of frequency fft / arrived at the subtracting input, because at the moment Tj ends at the comparison unit signal, the selector 7 closes f to its subtracting input is terminated. At the moment of switching the reverse counter 17 through zero, its output signal arriving at the third input of the AND circuit, the latter prohibits the passage of .. pulses from the generator output 13 clock pulses to the input of the frequency divider 18, and hence to the second input of the OR circuit. Thus, N pulses go to one input of the OR circuit 19, and (n-l) Ng pulses to the second. Therefore, from the output of the circuit OR 19 to the input of the digital circuit 20 linearization receives nNg pulses. Digital linearization scheme can be built on any known principle. From the output of the digital linearization circuit 20 to the input of the second counter 21 results

N pulses are received, the number of which is directly proportional to the measured temperature 9, Measurement results B and V are displayed, respectively, by digital reading devices 9 and 22.

Then the measurement cycle is repeated.

Thus, due to the introduction of additional nodes and connections in the proposed device, compared with the known, the accuracy of temperature measurement increases and the functionality significantly increases, since it is simultaneously possible to obtain the results of measuring thermo-EMF and temperature.

Claims (2)

1. USSR author's certificate number 347909, cl. G 01 K 7/02, 1970. 2. USSR author's certificate / as per draft 2787095 / 18-10 ,, cl. G 01 K 1 / 02f 1979 (prototype). ff