DE2710782C2 - Device for measuring temperature differences - Google Patents

Description

- One by a sequence control (20) for sequentially connecting one thermal network in each case (10, 11) circuit arrangement controllable with the injection voltage source (13)

- a controllable, at its control input by the activated thermistor network (10, 11) receiving the flowing current and impressing it in a charging capacitor (15) Constant current injection device (14),

- and a comparator (17) which compares the voltage (U _c ) dropping across the charging capacitor (15) with a reference voltage (Urcf) and emits a control signal if these input voltages are equal,

- That the reference voltage (U _n t) is in a fixed ratio to the impression voltage (Ud _n ) applied to the respective thermistor (10, 11), and that

- The period of time between the start of charging of the charging capacitor (15) and the Timing circuit (18,19) measuring the control signal of the comparator (17) and

- One the result of the first measurement in each case up to the end of the second measurement in each case storing and the difference between the two measurement results forming memory and arithmetic circuit (18) are provided.

2. Apparatus according to claim 1, characterized in that the timing circuit has a Up / down counter counting pulses from an oscillator (19) (18) and that the up / down counter (18) by the sequence control (20) to form the temperature difference value to be determined when recording the> o first measured temperature value on counting-up mode and when the second measured temperature value is recorded can be switched to countdown mode.

The invention relates to a device for measuring temperature differences according to the preamble of claim 1 laid down Art. bo

In such known devices, a constant is applied to the two thermistor networks at the same time Voltage is applied so that through each of them one uniquely determined by the associated temperature Current flows that is in the microampere range. Therefore, each of the two thermistor networks is an operational amplifier downstream, the output signal of which changes proportionally to the input current. To the To obtain the temperature difference sought, the two signals preamplified in this way become one difference-forming member, for example a differential amplifier with the gain 1 supplied, whose The output signal then has to be amplified again before the temperature difference thus obtained for the sought-after temperature difference proportional analog signal from an analog / digital converter can be supplied and converted by this into a digital measured value.

This known measuring device has the disadvantage that it is used against drift phenomena Operational amplifier is extremely sensitive, so that to achieve more useful or a very high circuit complexity to avoid or compensate for the drift effects got to.

In addition to an increase in costs, this leads to the further disadvantage of a relatively high power consumption the entire measuring arrangement, which makes battery operation economically unattractive because the The energy supply of a usable size battery depleted far too quickly and a drop in the Supply voltage quickly leads to a falsification of the measurement results.

In contrast, the invention is based on the object of providing a measuring device of the type described at the outset Art to create the simplest possible, only a few active components and therefore has a low power consumption construction and at the same time against drift and Is largely insensitive to interference.

To achieve this object, the invention provides the features summarized in claim 1.

By these measures according to the invention, a single-channel measuring arrangement is created in which the two thermistor networks required to measure a temperature difference not at the same time, but rather are activated one after the other, which is done with the help of the switch arrangement that of analog switches z. B. MOS field effect transistors can be formed, which can be controlled with extremely small currents. This makes it possible to put both thermistor networks in parallel to one and the same measuring circuit, whereby all associated with a different drift of the two measuring channels of the known device Problems are avoided and that in the known measuring device for eliminating these There are no problems with the circuitry required.

In addition, however, the drift phenomena of the remaining one measuring channel, insofar as they occur at all in their influence on the measurement result eliminated. This is done by the fact that the thermistor network connected to the impressing voltage flowing, extremely small, representing a measure of the temperature of the thermistor network Current directly to control a controllable constant current injection device is used, which emits the same current at its output that is fed to it at the control input, but different from the it controlling thermistor, has an almost infinitely high internal resistance on the output side. Through this it is achieved that in the downstream charging capacitor one of the temperature of the thermistor dependent, constant current is impressed, so that the voltage drop across this capacitor during the charging process, not according to the usual e-function, but linearly with time

increases

The comparator connected downstream of the charging capacitor compares the capacitor voltage, which increases from zero with a reference voltage that has a fixed predetermined ratio to the impression voltage stands what z. B. achieved with the help of a voltage divider can be that of selected resistors with a low tendency to age and very small Temperature resistance coefficients (metal film resistances)

If its two input voltages are the same, the comparator emits a signal that has been one since Start of the charging process, current time measurement ended; the temperature thus obtained, the temperature to be measured representative timing value, which is advantageous according to claim 2, but not necessary in digital form, for. B. is obtained in that the oscillation periods occurring in the period to be measured one high-frequency crystal oscillator are counted, is then stored until a corresponding Measured value for the second thermistor network at the other temperature for the desired one Difference formation is available.

The relationship between the temperature difference to be measured and the measured time difference can be represented mathematically by the following equations:

If one assumes that one of the two thermistor networks is at the temperature Ti, then the following applies the equation for the variable resistance of this thermistor network

Ri = Z) ₁ - M \ T _X (1)

The impression voltage Uan is now applied to this resistor so that the current flows through it

(2)

flows. This current is fed in an unchanged manner by the constant current injection device to the charging capacitor, for whose voltage U ₁ , which increases from zero at any point in time t, applies:

where Q (i) is the for. Point in time / charge on the capacitor.

If the comparator indicates at time Z ₁ that the capacitor voltage U _{1 is} equal to the reference voltage U "f , then the following applies

Solving equation (4) for Z ₁ and substituting the value from equation (2) for Ζ, one obtains

Uni ■

u _n

(5)

or, since £ / "" and U _nf are strictly proportional to each other (proportionality factor ρ )

ρ -

(6)

The same applies to the second thermizer _{at temperature T 2}

I ₂ = p C (Z) ₂ -M ₂ F ₂ ),

(7)

so that for the temperature difference A T of interest there is the relationship

AT = T ₂ -T ₁ =

I.
pC

h_ M ₂

A /, M ₂

₁ M ₁ / (8)

results.

Since paired thermistors are readily available in the Hrndel, the parameters of which M \ and M ₂ or b \ and bi coincide very precisely over the entire measuring range, the temperature difference sought is strictly proportional to the measured time difference t? -1 \ , whereby the proportionality factor \ IpCMx ^ ₂ depends only on component parameters which, with a suitable selection of the components, are practically not subject to any drift or aging phenomena.

In particular, it is important that in the measurement result obtained in this way, the absolute values of the Impression voltage and the reference voltage not t> o come in more, so that even a slowly falling battery voltage with prolonged use does not have any disturbing effects Able to exert influences.

The invention is described below using an exemplary embodiment with reference to the drawing described. In this the single figure shows the block diagram of the device for measuring Temperature differences,

In the case of the FIG. 1, thermistor networks 10, 11 can be applied one after the other to the fixed impression voltage Ud _n supplied by an operational amplifier circuit (not shown) with the aid of an analog switch arrangement 13, a sequence controller 20 actuating the analog switches 13, for example, so that the one exposed to the lower temperature is the first Thermistor network 10 and then the thermistor network 11 exposed to the higher temperature is connected to the impression voltage source.

If the thermistor network 10 is connected to the impression voltage source, then flows through the thermistor network 10 is a current depending on the temperature in question, which according to the above equation (2) falls with decreasing temperature and increases with increasing temperature. This stream becomes one downstream constant current impressing device 14 is supplied via a line 9 and controls this Constant current impressing device 14 so that it is connected for one between its output terminals

Charging capacitor 15 also clearly represents the temperature to be measured, constant charging current supplies.

A switch 16 which short-circuits the charging capacitor 15 for discharging is now opened by the sequence control 20, and a voltage begins to build up on the charging capacitor 15, which increases linearly over time. The capacitor voltage U _c is fed to a comparator 17. If the capacitor voltage reaches the reference voltage U _r cf specified at the second input of the comparator 17 i <>, the comparator emits a switching signal to stop a counter 18, which was started at the time when the switch 16 to enable the constant current charging of the charging capacitor 15 had been opened. \ -, Here, since the first to a smaller charging current, a longer charging time that is leading lower temperature should be measured had been switched by the flow controller 20 of the counter 18 'to the operating state count up ". ? ₍ i

With the correct choice of the counter clock frequency emitted by a quartz-controlled oscillator 19, suitable definition of the impression voltage U _em and appropriate selection of the charging capacitor 15 and the reference voltage LVc /, it is achieved that in the 1-, measurement result each digital counter step has a defined value in the decimal scale Temperature measurement. For example, with commercially available components it is possible to digitally obtain a temperature value with an accuracy of ± 0.1 ° K. _{J (l}

In this way, a temperature measurement becomes a time measurement, because the time is measured which elapses until the charging capacitor voltage has reached the reference voltage U _rc f js supplied to the second input of the comparator 15 after the switch 16 has been opened.

The digital result contains a constant that depends on the size of the charging capacitor 15 and on the impression voltage i / _n and the reference voltage Ure! This constant is proportional to the 4η reference voltage LW and inversely proportional to the impression voltage U _C m and also proportional to the resistance value b \ of the linearized thermistor network 10 at 0 ° C = 273 ° K.

In addition, the digital result also contains the lower temperature value applied to the temperature sensor 10, which is provided with a constant factor, which in turn is proportional to the capacitance C and to the reference voltage U _n / and inversely proportional to the impression voltage U ^ _n . but in addition it is proportional to the slope M \ of the strong star temperature straight line of the inconsistent thermistor network (see above equation (5)).

After receiving the measurement result representing the lower temperature, the same measurement is carried out with the temperature sensor in the second thermistor network 11, which is exposed to the higher temperature, with the impressing voltage Ucm now being applied to the thermistor network 11 by the sequence control 20 via the analog switch arrangement 13 and via the same constant current impressing device 14 the same charging capacitor 15 is charged as above. This time, however, the counter 18 is switched to the "counting down" mode

The digital counting result obtained after completion of this second measuring process now contains only one value which is directly proportional to the difference AT between the different temperatures prevailing at the two temperature sensors. The constant of proportionality here is proportional to the reference voltage Urcf and inversely proportional to the impression voltage Uein, as well as proportional to the capacitance C and to the slope Λίι_2 of the resistance temperature line of the linearized thermistor networks 10,11.

With a suitable selection of the charging capacitor 15, the accuracy of the measured value can only depend on changes in the slope value M _{1 ,?} the resistance temperature line can be influenced. A drifting of the voltage supply has no influence on the accuracy of the measurement result, since the voltages i / _e , „and LW are derived from the same voltage supply, ie are strictly proportional to each other and U _re t in the numerator and U _e in in the denominator of the expression for the proportionality constant stand.

A value for the temperature difference Δ Τ is now given in digital form. It is also possible to measure the temperature of each of the two measuring sensors 10, 11 also in absolute terms. For this purpose, a metal film resistor 12 is provided parallel to the linearized thermistor networks 10, 11, the value of which is equal to the resistance of the two same linearized thermistor networks 10, 11 at ⁰ ° C. This metal film resistor 12 is first applied to the impression voltage Uem by the analog switch arrangement 13 on the basis of a control signal coming from the sequence controller 20 and a digital count value is obtained from the counter 18 switched to the "counting up" operating state according to the method described above. In the second measuring step, the thermistor network 10 or U is connected to the analog switch arrangement 13 and the counter 18 is switched to the "counting down" operating state. The digital counting result obtained is a measure of the absolute temperature actually prevailing at the sensor.

With the described circuit arrangement may include any number of sensors 10, 11 ... to the same measurement channel connected η, the temperature differences or their absolute temperatures can be determined on this one channel.

It is important that the current flowing through the sensor is chosen so that it never exceeds the value that could lead to self-heating of the sensor, i.e. that the impression voltage t / _e / _n is admitted so that for the highest temperature to be measured the sensor current remains below the self-heating value

As can be seen from the figure, from the sequence control 20 to the start input of the counter 18 and leading to the control input of the teaser 16 Line switched on a delay element 21, which ensures that the respectively controlled switch of the Analog switch arrangement 13 has actually closed before the short-circuit switch 16 opened and with the count of the output from the quartz-controlled oscillator 19 during the charging time of the charging capacitor 15 Impulse is started.

The sequence control 20 is under the control of a higher-level control unit 22, which is also responsible for this ensures that after the completion of a complete temperature difference measurement at the data output of the Counter 18 appearing digital measurement result in a Buffer 23 taken over and a further processing, for example the representation by a Display unit 24 is supplied.

For this 1 sheet of drawings

Claims (1)

Patent claims: 1. Device for measuring temperature differences with at least two linearized thermistor networks, to which an impression voltage can be applied and of which one of the higher and the other is exposed to the lower temperature, and with one the strength of the by the Thermistor networks of flowing current as a measure for ι ο the relevant temperature-evaluating measuring circuit, characterized in that the measuring circuit