SU1734083A1 - Thermal regulator - Google Patents

Abstract

The thermostat contains 1 resistance thermometer (1), 1 thermostable resistor (2), 1 voltage amplifier (3), 1 power amplifier (4), 1 heater (5), 1 cooling actuator (6), 1 digital programmer ( 7), 1 determining amplifier (8), 2 digital-to-analog converters (9, 10). 1 il.

Description

The invention relates to automatic temperature control, in particular, to precision temperature regulators included in the thermostat systems of measuring cells of devices intended for mass determinations of dielectric characteristics of dielectrics.

A device for temperature control is known that contains a resistive temperature-sensitive bridge, to the output diagonal of which a control unit is connected, which is connected to the second control unit. A platinum resistance thermometer is included in one of the arms of the thermosensitive bridge, and the potentiometric four-digit decade unit is connected to the other.

The disadvantages of the thermostat are, firstly, the complexity of the setpoint switching circuit, secondly, the large error in setting the thermostatting temperature (± 2 ° C).

The closest in technical essence to the present invention is the thermostat BTP-78, which contains a voltage amplifier, to the output of which a power amplifier with executive

elements of heating and cooling, programmer, resistance thermometer included in the first shoulder of the measuring bridge, to the measuring diagonal of which the input of the voltage amplifier is connected, the setpoint switch is turned off to the opposite shoulder of the measuring bridge and is made in the form of two series-connected resistors, between the junction point and the top the supply diagonal of the bridge through the closing contacts of the keys, the control inputs of which are connected with the output of the programmer, include tetrads of binary-weighted resistors, corresponding to tenth units, tenths, and hundreds of degrees celsius.

The disadvantages of the thermostat are complexity, high power consumption and low reliability of the switching part of the temperature setting circuit, which consists of 16 keys and 19 reference resistors, as well as the imperfect matching of the transfer function of the temperature setter with the static characteristic of the resistance thermometer, which limits the accuracy of temperature setting.

(L

WITH

| X | ICO Ј

YU

00 CJ

The balance of the bridge in the thermostat in question is described by the following equation:

K1 + R0 (1 H-At + Bt2) qs + Aqt ,, q4 R2 R3qs + q4 + Aqt l

where Ri is the resistance of the ballast resistor connected in series with the resistance thermometer described by the function Rt Ro (1 + At + Bt2);

c | 5, q4 of conductivity of series-connected resistors forming the opposite shoulder of the bridge;

DC is the conductivity of a resistor included in the tetrad of degrees and corresponding to a change in the temperature setting by one point;

t is the set temperature, ° С;

Ro thermometer resistance at 0 ° C;

R2 and Ra are the resistances of the resistors forming the third and fourth shoulders of the bridge.

Cause qs + q4 Aq t, convert bridge balance equation (1)

Ri + R0 (1 + At + Bt2) qs + Aq -t

Q4-R2-R3 (SC + qs) (1 + Aq)

or

R, + M1 + A, + Bt)

C + - -qЈHqSV

q-t Vq4 + qs

By neglecting the expansion by terms higher than the second order, we obtain a series of equations relating the parameters of the setpoint and the resistance thermometer. To do this, we equate the coefficients at zero, first and second power x t in the right and left parts of equation (2):

Rl + R ° C + i) R2R3;

R0B Aq2 (q4

P2 Hz R0A

q4 + qs .2

(four)

Aq

)

04

R2-R3 q4 + q5y

From equations (W), (4) and (5) we have

A Aq Bq4 + qs

. (6)

q5 Aq

Ro

R2-R3 B2

(7)

d

qs - Aq --g - Q4

(eight)

ten

Substituting (7) and (8) into (3), we find the equation for determining the ballast resistance Ri:

R1

(± + ± 1 Vq4 qs;

-Re

(9)

chen his din

0 ° C;

moat

q)

vyvneka and first

3)

When calculating the parameters of the setpoint adjuster for conductivity regulator's q4 and qs resistor, one has to choose from two contradictory considerations: first, the higher q4 and qs, the better the coupling of the transfer function of the setter to the static characteristic of the thermometer, secondly, an excessive increase in q4 and qs reduces the sensitivity of the thermostat. The choice of thermostat parameters is simplified if q4 qs is assumed in equations (3) - (9).

However, the conductivity q4 cannot be chosen as a large angle, since in this case the quantizing conductivity Aq must also be increased to such limits that it will not be possible

35 binary-weighted tatrads, since

Aq - d-sc.

The aim of the invention is to simplify the thermostat while at the same time

0 increase its reliability and accuracy of temperature.

The goal is achieved by the fact that the thermostat containing the voltage amplifier, the output of which is connected

45 power amplifier with heating and cooling actuators, programmer, resistance thermometer and thermostable resistor, the first terminals of which are connected to the input of the amplifier 50, additional operational amplifier is added, the output of which is connected to the second output of the resistance thermometer and two digital-analogue converters, digital inputs of which connected to the output 55 of the programmer's house, and the output of the first analog-to-analog converter is connected to the first summing input of the operational amplifier and the input the house of the reference signal of the second D / A converter, the output of which is associated with the subtractive

the input of the operational amplifier, the second summing input of which is connected to the source of the reference voltage, the second output of the thermostable resistor and the input of the reference signal of the first digital-to-tax converter,

The introduction of the first D / A converter, whose digital inputs are connected to the output of the programmer, allows a voltage proportional to a given control temperature to be formed.

The introduction of a second DAC, the digital inputs of which are also associated with the output of the programmer, and the input of the reference signal with the output of the first DAC, allows to form an amendment that takes into account the quadratic character of the characteristics of the platinum resistance thermometer, which ultimately improves the accuracy of the control temperature.

The introduction of an inverting operational amplifier allows the resistance thermometer to be supplied with a sum of voltages, namely, the corresponding reference temperature (e.g., 0 ° C) and control proportional to the desired temperature (from the first DAC), and also taking into account the quadratic static characteristic of the resistance thermometer (from the second DAC ). Inverting the voltage supplied to the resistance thermometer with respect to the reference voltage supplied by the thermostable resistors, allows to obtain a zero signal at the input of the voltage amplifier when the object reaches a temperature equal to the specified value.

The drawing shows a schematic diagram of the proposed thermostat.

The thermostat contains a resistance thermometer 1, a thermostable resistor 2, a voltage amplifier 3, a power amplifier 4, a heater 5, an actuator cooling 6, a digital programmer 7, an operational amplifier 8, and digital-analog converters 9 and 10.

The thermostat works as follows,

Suppose that in the initial state at the output of the programmer there is a digital code corresponding to the reference temperature setting of 0 ° C, the temperature in the thermostat is also 0 ° C, and the resistance of thermometer 1 is equal to RO. At the same time, there is no imbalance signal at the input of amplifier 3, since at this moment the resistance of the thermostable resistor 2 and thermometer 1 is equal and opposite voltage voltages are applied to them, which are equal in absolute value.

When the temperature in the thermostat changes (for example, due to a change in the ambient temperature), the resistance of thermometer 1 changes, the balance of the bridge is disturbed, and an unbalance signal appears at the input of amplifier 3, which is amplified by voltage amplifiers 3 and power 4 and depending from the polarity of the signal controls heater 5 or executive

cooling element 6. As a result, the temperature in the thermostat is restored. When the digital signal changes at the output of the programmer 7 (in manual or automatic temperature setting mode)

the digital code corresponding to the preset control temperature is fed to the input of digital-to-analog converters 9 and 10. As a result, the output of the DAC 9 has a voltage proportional to the product of the preset temperature and the reference voltage

Ug kgUt, (1)

where Ug is the voltage at the output of the DAC 9; kg — D / A transfer coefficient 9;

t is the set temperature;

U is the voltage of the reference source that arrives simultaneously at the thermostable resistor 2, the second summing input of the operational amplifier 8 and the input

the reference signal of the DAC 9.

From the output of the DAC 9, the voltage U is fed to the first summing input of the operational amplifier 8 and to the input of the reference signal of the DAC 10, the output of which is set to voltage.

U10 kg kioUt2, (2)

where kio is the transfer ratio of the DAC 10.

The voltage Uio is fed to the subtracting input of the operational amplifier 8, the output voltage of which is

U8 -k8iU-k82Ug + k83Uio, (3) where ketch, K82, K83 - transfer coefficients

operational amplifier 8 for the first and second summing inputs and the third subtractive, respectively. The voltage Us is supplied to the resistance thermometer. The balance of the bridge (zero potential at the input of the voltage amplifier 3) will take place when the currents flowing through the resistance thermometer 1 and the thermostable resistor 2 are equal:

55 U8,

Ж + ТҐ- °

(four)

where Rt is the resistance of the thermometer at a given temperature;

R2 resistance thermostable resistor 2.

The resistance of a platinum thermometer is quite accurately described by a quadratic function in a wide range of temperatures

Rt Ro (1 + At + Bt2), (5) where Rt is the resistance of the thermometer at temperature t;

Ro is the resistance of the thermometer at the reference temperature to, usually equal to 0 ° С;

A - 3,908-10 3; B -5,802-10 7 - coefficients that take into account the curvature of the static characteristic of the resistance thermometer.

Substituting in (4) the expressions (3) and (5) and taking into account (1) and (2), we obtain

+ k82 K9-t -ksa kg-kio t2

Ro (1 + At + Bt2)

(6) 20

15

From (b) it is obvious that the set temperature in the thermostat will be established when the following conditions are met:

Ro Ro. 1 82-K9 d, K81 A

k83 k9 k-io kei

about

(7)

Equations (7) are of practical value, since they allow calculating the parameters of the thermostat elements, which provide an ideal conjugation of the transfer function of the temperature setter with the static temperature characteristic

resistance meter and to ensure higher temperature accuracy.

The introduction of two D / A converters and an operational amplifier instead allows

5 switching elements and reference resistors of binary weighted tetrads, which simplifies the device, improves reliability and reduces power consumption and mass and size characteristics of the thermostat.

Claims (1)

0 claims A thermostat containing a voltage amplifier, to the output of which a power amplifier with heating and cooling actuators is connected, a pro grammator, a series-connected resistance thermometer and a thermostable resistor, the combined terminals of which are connected to the input of a voltage amplifier, characterized in that while increasing the reliability and accuracy of the temperature setting, an operational amplifier is added to it, the output of which is associated with the second output of the thermometer 5 resistors, and two digital-to-analog converters, the digital inputs of which are connected to the output of the programmer, and the output of the first digital-to-analog converter is connected to the first summing 0 the input of the operational amplifier and the input of the reference signal of the second digital-to-analog converter, the output of which is connected to the subtractive input of the operational amplifier, the second summing input of which is connected to the reference voltage source, the second output of the thermostable resistor and the input of the reference signal of the first digital-analog converter. & u & -