JPS63111432A - Temperature measuring instrument - Google Patents

Abstract

PURPOSE:To remove the influence of thermoelectromotive force and to eliminate the influence of reference power source variation by varying a current which flows to a thermoresistance element corresponding to variation in source voltage and employing a polarity switching system. CONSTITUTION:Constant voltage generating circuits 1 and 2 generate equivalent plus and minus constant voltages VREF, one of which is outputted through a changeover switch 3. This output voltage is converted by a voltage-current converting circuit 4 so as to output a constant current I, which flows through the thermoresistance element with resistance Rt. The element 5 indicates a resistance value proportional to ambient temperature and its terminal voltages V1 and V2 are inputted to a differential amplifier circuit 6. The output VX of the circuit 6 becomes VX=-A(V1-V2)+BVREF=(-ARt/R0+B)VREF by using coefficients A and B and is converted by a signal processing part 7 into a voltage VX/VREF, i.e. a signal regarding only the resistance Rt, thereby finding temperature from a resistance Rt-temperature characteristic.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a temperature measuring device using a temperature measuring resistance element whose resistance value changes depending on temperature.

(Prior art) As a method of temperature measurement using a resistance thermometer element...
) A method in which a standard resistor and a temperature-measuring resistance element are connected in series, a DC voltage is applied, and the voltage between the standard voltage and the temperature-measuring resistance element is alternately measured to determine the resistance value of the resistance element, that is, its temperature ( Japanese Industrial Standards 28704) (2) A transformer-type ratio bridge system using alternating current that is not affected by fluctuations in reference voltage is known.

(Problems to be Solved by the Invention) However, in the method (1) above, ■ an error occurs due to thermal electromotive force; ■ automatic measurement is not possible; ■ when water leaks inside the sensor, It has disadvantages such as causing electrolytic corrosion, and in the method (2) above, the impedance changes depending on the inductance and capacitance when the cable is extended, which causes measurement errors.

The present invention has been made in view of the above, and uses a polarity switching method to take the average value, eliminate the influence of thermoelectromotive force, and prevent fluctuations due to cable length and electrolytic corrosion. The present invention provides a temperature measuring device that does not require

(Means for Solving the Problems) A temperature measuring device according to the present invention includes: a constant voltage source that generates a bipolar voltage; and a constant current generating means that converts and forms a current proportional to the voltage of the constant voltage source. , a switching means interposed between the constant voltage source and the constant current generating means for switching the polarity of the constant voltage, a temperature measuring resistance element through which the constant current flows, and a voltage generated across the temperature measuring resistance element. a differential amplifier circuit configured to send out an output proportional to the constant voltage from the constant voltage; and a signal corresponding to the differential amplifier circuit output/constant voltage from the differential amplifier circuit output and the constant voltage. The relationship between the resistance value of the resistance temperature measuring element and the temperature is written in advance, and
and converting means for converting the output of the signal processing section into temperature data.

(Function) FIG. 1 is a block diagram showing the basic configuration of the present invention, in which a constant voltage generation circuit 1.2 generates constant voltages VREF of equal value + and -, and a changeover switch 3 selects one of the voltages. is output. This output voltage is converted to a constant current I by the voltage-current conversion circuit 4.
The constant current I flows through the temperature measuring resistance element 5.

The temperature-measuring resistance element 5 exhibits a resistance value proportional to the ambient temperature, and potentials Vl and V2 at both ends thereof are guided to a differential amplifier circuit 6. This differential amplifier circuit 6 is preferably, for example, an instrumentation amplifier (measurement amplifier), and generally has the configuration shown by the dotted chain line in FIG. 2.

In the same configuration, output ■8 is vx=-A (V,-V2)+BV, .

=RtI (Rt- is the resistance value of the element 5). Therefore, VX = -A'Rt I +BV*tr However, Ro is U
Resistor connected to the inverting terminal of 4.

The output (2) 8 is converted into Vx/VREr, that is, a signal related only to Rt, in the signal processing section 7, and the temperature is further calculated from the Rt-temperature characteristic.

The control unit 8 sends out timing signals for each circuit, and also performs conversion based on the relationship between R1 and temperature.

(Example) FIG. 2 is a circuit diagram showing one embodiment of the present invention.

In the figure, Zh is a Zener diode that generates a predetermined constant voltage, Ul is an operational amplifier that does not change in level with respect to changes in load impedance, making it close to the so-called ideal constant voltage source, and U2 forms a constant voltage source with the opposite polarity. It is an operational amplifier.

Reference numeral 3 denotes a switch which is alternately switched by a control pulse (for example, signal a, see FIG. 3) from the control section 8.

Reference numeral 4 denotes a voltage-current conversion circuit, which includes operational amplifiers U3 and U4, transistors Ql and Q2, a resistor Ro, and the like. The operational amplifier U3 functions as a voltage follower for impedance conversion, which minimizes the output impedance caused by the analog switch 3, and the output is always VREF (
or -VREI'').The output of this operational amplifier U3 is sent to the non-inverting terminal (+ side) of operational amplifier U4.The operational amplifier ideally has an inverting terminal (-) and a non-inverting terminal (+). Therefore, when switch 3 outputs 10 VREF', both terminals are at +V.
It becomes REF. That is, a current 1=VRεr/Ra flows through the resistor R0 connected to the inverting terminal of the operational amplifier U4. The operational amplifier U4 changes its output so that the same current value is maintained even if the resistance value Rt of the temperature measuring resistance element 5 changes. Transistor Q1. Q2 is used for current amplification.

In this way, a constant current ■ always flows through the temperature sensing resistance element 5, and the voltages (vl, V2 mentioned above) between its ends are applied to the non-inverting terminals of the operational amplifiers U;, , U6 of the differential amplifier circuit 6, respectively. The operational amplifier U7 outputs the voltage signal of formula (1) as the output vX. Obeazop Us, Us
If one with a high input impedance is selected as the resistance temperature sensing element 5, the impedance of the connection cable of the resistance temperature measuring element 5 can be ignored.

The signal processing section 7 is for obtaining the above-mentioned V x /V REF, and it is possible to perform A-D conversion and simply divide, or as in this embodiment, it is possible to use an integration circuit 71 and a counter 7.
A configuration consisting of 2 is also conceivable. This integrating circuit 7
1 has operational amplifiers Ul, VX and V from U7, respectively.
REr is input, and a lamp output proportional to the input terminal is generated. Therefore, when Vx is input, the slope differs depending on its value, but ■□8. The circuit is designed to always have a constant slope, and this type of circuit is not new and has been in common use for several years.

The first integration is performed by applying the voltage ■× by the pulse a (see Figure 3) from the control circuit 8, and the second integration is performed by applying the voltage VREF by the pulse b (see Figure 3) from the control circuit 8. (Figure 3, C).

The switch 3 is switched by a signal from the control circuit 8 at the same time as the pulse a or just before the pulse a, so that VX (or -VX) is stably applied from the start of integration.

The first integration period (t2-t+) is a preset time.

Now, if we set t1=0, the first integral waveform e is e=kVx t ・・・・・・・・・・・・・・・・・・
... (2) k is expressed as a proportionality constant, and at time t2, e=kVxt2 ......
(2)′.

The integration in FIG. 2 is performed from time t2 to time t3 when the output becomes OV with VIIEF applied with opposite polarity.

The integral waveform e during this period is e = k Vpcr (t - t2) + ex
It can be expressed as “”” (3).

Then, at time t3, 0''-k VnEr(t3-t,)+ax-',
ex = k VRtr (ts - t2) ・”=
(3)'Here, substituting (2)' into (3)', we get
It is expressed as a function of t3 (t2 is constant) as follows.

That is, from this formula (4), t, ~t.

By measuring the time, V X / V REF can be calculated backwards.

Note that in the next cycle, the switch 3 is switched to generate a lamp output of opposite polarity to eliminate the influence of thermoelectromotive force. The period T can be measured from about several hundred milliseconds.

The integrating circuit 71 also has a zero-cross detection circuit (not shown), and detects time 1. ．． 13 timing is detected. This detection signal is led to the next stage counter 72, during which time pulses d (FIG. 3) are counted to form a signal corresponding to time.

The control circuit 8 is equipped with a calculation formula or conversion table of the above-mentioned formula (1) and a conversion formula or conversion table between the resistance Rt and the periodic temperature, and calculates the ambient temperature from the above-mentioned time signal. The two types of temperature data in states with different polarities may be further averaged and output and displayed.

In this embodiment, the V R1F' input to the integrating circuit 71 is from the operational amplifier U1;
In conjunction with, ie, operational amplifier U.

If it is set so that and U2 are switched, the +- voltage VR1
do not necessarily have to be equivalent.

(Effects of the Invention) As explained above, according to the present invention, the operational amplifier U,
, U, output V Ill:F is Zener diode Z
Even if h fluctuates somewhat due to aging or other factors, the current I flowing through the resistance temperature sensing element 5 changes in conjunction with the fluctuation, and the input terminal to the operational amplifier L+7% integration circuit 71 also changes to V RE
Since it is F itself, it has no effect on the above equations (1) and (4), and accurate temperature measurement is always possible.

Further, since the impedance is changed, the impedance of the connection of the resistance temperature measuring element 5 can be effectively ignored, so that highly accurate temperature measurement can be expected.

Further, since the switch 3 is switched to automatic mode, rapid temperature measurement is possible, and since + and - currents are used, errors caused by thermoelectromotive force and problems of electrolytic corrosion are eliminated.

Also, since it uses an integrating circuit, it is less susceptible to noise and the like.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the basic configuration of the present invention. FIG. 2 is a circuit diagram showing one embodiment of the present invention. FIG. 3 is a waveform diagram.

Claims (1)

[Claims] A constant voltage source that generates voltages + and -V_R_E_F;
a constant current generating means that is proportional to the voltage of the resistance temperature measuring element and the constant voltage source and supplies a constant current to the resistance temperature measuring element; and a constant voltage source interposed between the constant voltage source and the constant current generating means. an amplifier circuit that outputs a voltage V_X proportional to V_R_E_F from the voltage across the temperature-measuring resistance element and a constant voltage V_R_E_F; and an amplifier circuit that outputs a signal corresponding to V_X/V_R_E_F. The relationship between the resistance value and temperature of the signal processing means and the resistance temperature measuring element is written in advance,
and converting means for converting the output of the signal processing means into temperature data.