JPS6298271A - Digital ohmmeter - Google Patents

Abstract

PURPOSE:To find resistance values of various conductors under different temperatures without any calculation by switching applying a reference voltage which has a constant level regardless of temperature and a reference voltage which varies in level with ambient temperature and supplying the result to an A/D converter. CONSTITUTION:The DC power source VS of a reference voltage generation part 2 generates the constant-level reference voltage for measuring the resistance of a conductor 3 at the current temperature. A temperature/voltage converting circuit 11 has a temperature sensor RT which varies in resistance with the ambient temperature and variation in the ambient temperature is converted by a bridge circuit into a voltage, which is outputted. A temperature coefficient correcting circuit 12 matches output characteristics of the circuit 11 with characteristics equivalent to resistance-temperature variation characteristics of the conductor 3 by an amplifier A3. A reference voltage shift circuit 13 applies an offset voltage to the circuit 12 and shifts its output level to generate the variable-level reference voltage for substituting the resistance of the conductor 3 at the current temperature with resistance at desired temperature. Consequently, the resistance of the conductor 3 at the current temperature is measured and resistance at desired temperature is easily found by being substituted for the resistance at the current temperature.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a digital resistance meter, and more specifically, it is used to measure the resistance of conductive wires such as windings used in electronic parts or electrical equipment. This invention relates to a digital resistance meter that can measure resistance by converting it into resistance at a desired temperature different from the temperature of .

[Technical Background of the Invention] Rotating devices such as electromagnetic relays, transformers, and motors are provided with various windings having different shapes and sizes. The electrical resistance of the conductive wire used in these windings generally has a positive temperature coefficient that increases as the ambient temperature rises, and its value is known to vary depending on the conductive material, and for each material. The temperature coefficient is given for .

When such parts, equipment, or devices in which they are incorporated are operated in a relatively high-temperature machine room or outdoors exposed to direct sunlight, the windings have a large effect on the temperature rise. It is extremely important to investigate the resistance value in advance through indoor tests at room temperature, etc. from the standpoint of accident prevention and reliability design. In addition, if the resistance value at the temperature specified in the specifications can be predicted by measuring the winding resistance of equipment placed in a high or low temperature environment, the resistance value at the temperature specified in the specifications can be estimated using a constant temperature room. Value confirmation tests can be omitted.

In such a case, for example, if you measure the resistance of a conductive wire with a normal digital resistance meter, the value obtained is of course the resistance value at that temperature, and the resistance value at other temperatures is generally Calculations had to be made using mathematical formulas, which was complicated and undesirable for the measurer.

[Purpose of the Invention] This invention was made in view of the above points, and its purpose is to provide temperature correction in which the reference voltage of an A/D converter that digitally converts the resistance value of a conductive wire is switched according to the material of the conductive wire. It is an object of the present invention to provide a digital resistance meter which is equipped with a function and is capable of determining the resistance values of various conductive wires at temperatures different from the current temperature without special calculations.

[Measurement Principle] First, the measurement principle of this digital resistance meter will be explained with reference to FIG. 1 of the accompanying drawings.

An appropriately determined reference temperature is t. This reference temperature t
. If the resistance of the conductive wire to be measured at is Rto, and its temperature coefficient is α, the resistance R at any temperature t is, as is well known, Rt=Rt. (1+α(t-t,)
)−=−(1). In this case, the reference temperature t.

It is practically convenient to set the temperature to, for example, 0°C or 20°C.

Set the reference temperature to this conductive wire. and constant current at any temperature. The voltage generated when flowing is vto
and Vt, V to=I,・Rt,・・・・・・・・・・・・・・・
・・・・・・・・・・・・・・・(2) Vt=I.・Rt・
・・・・・・・・・・・・・・・・・・・・・・・・
- (3).

Substituting equation (1) into equation (3), Vt=1. −Rt, (1+α(t−t,)). Furthermore, by substituting equation (2) into the above equation, Vt=Vt, (1+
α (t to)) (4) is obtained. When formula (4) is changed to -film, V ti = V to
(1+α(ti to)) ・・・・・・(4)'
becomes. However, i=0.1, 2.・・・・・・n.

As shown in FIG. 1, this equation (4)' represents that the resistance of the conductive wire is converted into a voltage Vti with temperature ti as a variable.

Therefore, an A/D that uses a constant level voltage Vs as a reference voltage
When the voltage V t i is digitally converted by a converter, the reference temperature t is obtained. It goes without saying that the resistance RtO of the conductive wire corresponding to the voltage ■to can be found in the above case, and the resistance Rt□ corresponding to the voltage vt1 can be found in the same way for other temperatures and odors.

In FIG. 1, dots indicate reference temperatures on the temperature axis.

and the point vt which indicates the voltage generated in the conductive wire at that temperature
Let P be the point where the straight line connecting O intersects with the straight line representing a constant level reference voltage Vs parallel to the temperature axis, then mark the point □ representing an arbitrary temperature and the voltage generated in the conductive wire at that temperature. Let Q□ be the point where the straight line connecting the represented points Vt and 2 intersects with the straight line passing through the above point P.

Here, the voltage represented by point Q□ is Vs.

Then, this voltage Vs□ and the magnitude of each of the above voltages are, for example, vLo/vs=vt□/■s□・・・・・・・・・・・・
Assuming that the above straight line pQt is drawn so as to have the proportional relationship shown in (5), the voltage Vs at temperature t1
The value obtained by digitally converting □ by using voltage V as the reference voltage of the A/D converter is the reference temperature t. At voltage Vs
It is clear that it is equal to the value obtained by digitally converting the voltage vto using vto as the reference voltage.

In other words, if the current temperature is, for example, the voltage (2) occurring in the conductive wire to be measured is first determined using the reference temperature Vs. , and then switch the base voltage to Vs and measure.The value of the previously measured voltage vt1 is the voltage vt at the reference temperature t.

The result will be the value replaced by . In this case, the reference voltage Vs1 for displacement measurement is calculated using formula (5)% formula% (
5) Also, let the current temperature be t2, and the temperature is as shown in the figure.
When t o < t z < tL, the point where the straight line connecting the point t2 representing the temperature and the point vt2 representing the voltage intersects with the straight line PQ is set as Q2, and the voltage vs2 represented by this Q2 is measured by substitution. For the same reason, the voltage vt2 at the current temperature t2
is converted into a voltage V1 at the reference temperature t0.

In this case, one point Q2 is on the straight line PQ1, so the voltage Vs
, and the magnitude of each voltage above.

V t z / V s z = V t t / V
s t・・・・・・・・・The proportional relationship in (6) is established.

Vs2=Vs, ・V t2/V tl・・・・・・・・・
...(6)'.

In this digital resistance meter, for example, from 0°C to 1
By converting the resistance change of the temperature sensor into a voltage change in a normal practical temperature range such as 00°C, it is now possible to obtain reference voltages for substitution measurements such as Vsl and Vs2 represented by the above straight line PQt. ing.

That is, by substituting equation (5)' into the above (6)', vs2=vs-vt2/vt.

becomes. Generalizing this formula, V s i = V s - V t i / V t.

Substituting equation (4)' into the above equation, Vsi=Vs (1+α(ti-to))...
(7) is obtained.

According to equation (7), the reference voltage for replacement measurement given to the A/D converter is at the reference temperature t. A voltage Vs having the same magnitude as the reference voltage at a constant level, and a voltage Vs i whose change in voltage with respect to a change in temperature has the same value α as the resistance: temperature coefficient of the conductive wire to be measured. If so, I know it's fine.

Note that if the current temperature is t, then the reference voltage is set to V.
If necessary, the voltage Vt measured as s may be expressed as, for example, the current temperature t and the reference temperature t. If the temperature is replaced by a temperature t2 between , the base wall voltage may be set to Vs2 instead of Vs and remeasured.

That is, by transforming Equation 1 (6).

vt, = vti・V s z / V s 1 Substituting the general formulas of formula (4)' and formula (7) into the right side of the above formula, we get V t, x=V to (1+α(t2 t+1
)) ・=...(6)'. According to equation (6)', the voltage Vt2 obtained above is equal to the reference voltage Vt2.
It can be seen that this value is equal to the value obtained by measuring the magnitude of voltage Vt at temperature t2 using s.

[Examples] Hereinafter, the present invention will be described in detail with reference to embodiments shown in the accompanying drawings.

As shown in FIG. 2, this digital resistance meter includes, for example, a measuring section 1 and a reference voltage generating section that supplies a constant level reference voltage and a variable level reference voltage for digital conversion to this measuring section 1. It consists of 2.

First, a conductive wire 3 to be measured is connected between the measurement terminals T1 and 12 of the measuring section 1 through a test lead (not shown), and when a constant DC current is passed through the conductive wire 3 from, for example, a constant current power source 4, the conductive wire 3 is A voltage Vti appears across the line 3 which is proportional to the magnitude of its resistance Rx. This voltage Vti is applied, for example, to A via one terminal A of the analog switch SW1 of the A/D converter 5 connected to the measurement terminal T, and a common terminal C connected to the measurement terminal T2 with a jumper wire J. The signal is added to the integrator A□ of the /D converter, and is further input to the counting section 6 from the integrator A1.

For example, a common contact of a switch SW2 is connected to the other terminal B of the analog switch SW1, and the switching contacts A and B are supplied with a constant level reference voltage Vs and a variable level base 4ffi voltage from the reference voltage generator 2, respectively. Vsi can now be added.

Either one of the reference voltage Vs at a constant level or the reference voltage Vsi at a variable level and the voltage V t i between both ends of the conductive line 3 are alternately switched at a predetermined timing by the switch SW, and the voltage Vti A digital conversion is performed. This conversion data is sent to a measuring circuit 8 including, for example, a display 7, and its value is displayed.

The reference voltage generation section 2 includes, for example, a reference voltage generation circuit 10.
, a temperature/voltage conversion circuit 11, a temperature coefficient correction circuit 12,
and a reference voltage shift circuit 13.

The reference voltage generating circuit 1o uses, for example, the voltage Vs of the DC power source vs as a constant level reference voltage, and is applied to the terminal B of the switch SW1 via the switching contact A of the switch SW2.

The temperature/voltage conversion circuit 11 is a circuit that serves as a voltage source for obtaining a variable level reference voltage V s i that changes in response to changes in ambient temperature, and in this embodiment,
For example, as a medium that replaces temperature changes with voltage changes, does it have a positive temperature coefficient and stable characteristics? A temperature sensor RT consisting of a 1IIJ temperature resistor, a differential amplifier A2 and a resistor R1
It is composed of a bridge circuit consisting of R1, R1, etc.

In this case, a temperature sensor RT and a resistor R1 are connected between, for example, the (-) input terminal of the differential amplifier A2 and its output, and between the DC power supply 78, and the (+)
Between the input terminal and the above DC power supply 78, and the above common terminal C
Resistors R2 and R3 are connected between them, respectively. That is, the voltage Vs of the DC power supply vs is connected to the (-) input terminal of the differential amplifier A2 by the resistor R1 and the temperature sensor RT.
The voltage Vs is divided by resistors R2 and R1 and applied to its (+) input terminal. The output of this differential amplifier A2 is sent to the temperature coefficient correction circuit 12 and applied to the amplifier A via a resistor R4. In addition, the temperature sensor RT
For example, a platinum wire is preferable as the temperature measuring resistor, but other metal wires may also be used.

The temperature coefficient correction circuit 12 receives an output that increases or decreases in proportion to changes in ambient temperature from the differential amplifier A2, and provides a variable level reference having characteristics equal to the output temperature characteristics of the voltage Vti generated in the conductive line 3. It is for forming the voltage Vsi, and includes, for example, an amplifier A, a switch SW1, resistors R4 to R7, and the like. In this embodiment, the (-) input terminal of amplifier A, for example, is connected to resistor R,4.
It is connected to the output terminal of the differential amplifier A2 described above through the (-) input terminal, and one terminal of each of resistors R8 to R7 is connected to this (-) input terminal. The other ends of these resistors R5 to R7 are respectively connected to switching contacts A to C of a switch SW, for example, and the common contact of this switch SW is
It is connected to the output terminal of the second amplifier A. Further, the (+) input terminal of this amplifier A3 is connected to the common terminal C, for example. When the switch SW3 is operated to switch the resistors R6 to R7, the voltage amplification degree of the amplifier A3 is changed, and the voltage applied from the differential amplifier A2 is applied to the conductive wire 3.
It is possible to match the output temperature characteristics of the voltage Vti generated in the In the illustrated example, three resistors R6 to R7 are provided, but if the material of the constant conductivity fi3 of H1'J is one type of copper wire, for example, only one resistor is required.

The reference voltage shift circuit 13 includes the temperature coefficient correction circuit 12.
A group 7 (1B) formed to have the same output temperature characteristics as the voltage Vti generated in the conductive wire 3 to be measured (1B) is for level-shifting the voltage Vsi in accordance with a desired displacement measurement temperature, In this embodiment, the constant level reference voltage Vs is applied as an offset voltage to the negative input terminal of the amplifier A3 via the variable resistor RV. Apply voltages +■ and -■ respectively from the terminals, and the voltage taken out from the movable terminals is sent to amplifier A.

It may be applied as an offset voltage to the negative input terminal of the In this case, the variable resistor RV is preferably a potentiometer or the like having a small temperature coefficient and high resolution.

Next, referring to FIGS. 3 to 5, the reference voltage V for the displacement measurement is set using the resistance change of the temperature sensor RT.
The means for forming si will be explained. Note that FIG. 3 is an excerpt of the temperature/voltage conversion circuit 11 in FIG. 2, and the same reference numerals as in FIG. 1 are given. In the following explanation.

In order to avoid complexity, the reference numeral attached to each resistor will be used as the resistance value of that resistor for convenience.

In Figure 3, the voltage applied from the DC power supply vS is V
s and the current flowing due to this voltage Vs as I
, ,I, and the voltage at each input terminal of the differential amplifier A2 is Vl.
, V2. If the output voltage appearing between the common terminal C and the output terminal OUT is vI, then the electric current is expressed as follows. Here, Rti' represents the resistance of the temperature sensor RT at the temperature ti (i=0, 1, 2...n).

Regarding current I2.

It is expressed as

In this case, the voltage v2 is the direct current voltage Vs between the resistors R2 and R
This is because the pressure is divided by 1.

V2=V s-R, / (R, + R,) −・−(1
0).

Also, from the condition that the potential difference between the two input terminals of differential amplifier A2 is zero.

V, = V, ・・・・・・・・・・・・・・・・・・
・・・・・・・・・・・・・・・・・・・・・・・・(
11).

Here, when formula (lO) is transformed, Vs=V, (R2+R1)/R, -・---...(12
) Substituting equations (11) and (12) into equation (8) and rearranging.

V, [((R,+R3)/R,)-1] Rt i′=
(V knee V,) /R□・・・・・・・・・・・・・
...(13).

Similarly, when formulas (11) and (12) are substituted into formula (9) and rearranged, V, [((R2+R,)/R3)-11R.

”Vt/R-・・・・・・・・・・・・・・・・・・
・・・・・・・・・・・・・・・(14) 1 more
Dividing equation (13) by equation (14) above, regarding the output voltage v0 of differential amplifier A2, VO”Vl (l Rt ” Rtl' / Rz
” R3) is obtained.

Here, for example, if R = R2 is set, the above equation becomes Vo
=V, (1-Rti'/R3) ・--=(15
).

Next, the reference temperature t of the temperature sensor RT. If the resistance value at is Rt0' and its temperature coefficient is β, then at any temperature ti
The resistance value Rti' is calculated as follows from the formula (1) expressing the resistance of the conductive wire 3, Rti' = Rto' (1+
It can be written as β(ti to)).

In this case, if the resistance value R1 is set equal to the resistance value Rt,' of the temperature sensor RT at the reference temperature t0, the above equation becomes Rt i' = R, (1+β(t i to) ), Transforming this gives the following equation.

Rti'/R, = 1 + β (ti - to) - (1
6) 2m・・・・・・・・・・・・・・・・・・
...(16)'However, m=Rti'/R.

Substituting this value of m into equation (15), we get V,=V1(1
-m)・・・・・・・・・・・・・・・・・・・・・
...(15) '.

In equation (15)', if m x 1. That is, Rt
If i'< Rx, the output voltage V of the differential amplifier rPA2.

becomes a positive voltage, m=1, that is, Rti'=
R, then output voltage V. is zero and m〉1゜, that is,
Rti')R, then the output voltage v0 becomes a voltage of negative polarity. This state is shown by the solid line in FIG. When the output voltage vo of the differential amplifier A2 is inverted and amplified by, for example, the amplifier A3 in the temperature coefficient correction circuit 12, the amplified output (■) is obtained. As shown by the dotted line in the figure, it is clear that the positive and negative polarities of '' are reversed with respect to the voltage v0, and the output ■. It goes without saying that the magnitude of ' is determined by the voltage amplification degree of amplifier A.

Therefore, by substituting equation (16) into equation (15) and rewriting it, the output voltage V of the differential amplifier A2 is obtained. ■. =-V□
・β(ti to). If the voltage amplification degree of amplifier A3 is k, then its inverted output V. ′ is V,′=−V,−β(t i −tol・−417
).

Here, for comparison, formula (7) showing the reference voltage Vsi for displacement measurement is reproduced as follows: Vsi=Vs(1+α(t l to))...
・(7) In these equations (7) and (17), the ratio of voltage change to temperature change is expressed as voltage V s i
and V. Expressing the inclination angle 0 and θ' with respect to the temperature axis of ? ...
......(18), and for equation 1 (17), θ' = tan-1(dVo' /d t 1) =
tan-”k・β・vl・・・・・・・・・・・・・・・
・(19).

Voltage ■. In order to match the inclination angle θ' of voltage Vsi with the inclination angle θ of voltage Vsi, by setting θ'=θ, the above equation (18) and the % expression %) can be obtained. Since the left side of equation (20) is a known constant, the value of the voltage amplification degree of the amplifier A can be set. If the conductive wire 3 to be measured is made of several different materials, the temperature coefficient of each wire may be set to α1. α
2. . . . , and therefore the voltage amplification degree also has a different value, Hk2t . . .
In this case, the switch SW may be switched as described above.

Next, if we rewrite equation (17) using equation (20).

vo′=vs・α(t x to) becomes.

Here, if the variable resistor RV of the reference voltage shift circuit 13 is adjusted and an offset voltage of the same magnitude as the reference voltage Vs is added to the voltage V, /, the total voltage becomes ■. 'teeth.

V,'=Vo' +Vs =Vs (1+a, (t i-t,) ),
This means that the same voltage as the reference voltage Vsi shown in the above equation (7) is generated. The interrelationship of these voltages is shown in FIG. 5. Next, the operation of this digital resistance meter will be briefly explained. The conductive wire 3 to be measured is the measurement terminal T
, , T2, and when a constant current is applied from a constant current source 4, a voltage Vti is generated across the conductive wire 3. This voltage vti is alternately switched with a constant level reference voltage Vs via the switch SW1 of the A/D converter 5,
It is converted into a digital value by an integrator A and a counting section 6. In this case, if the ambient temperature is tl, a digital conversion value corresponding to the voltage vt1 is obtained and sent to the measurement circuit 8. In the measuring circuit 8, this digitally converted value is applied to the display 7 via a decoder (not shown) and displayed as a resistance value.

At this point, a variable level reference voltage vs1 corresponding to the temperature t□ has already been formed in the reference voltage generating section 2. Therefore, when the switch SW2 is pressed and its movable piece is connected to the contact B side, the reference voltage of the A/D converter 5 is switched from Vs to Vs, and the temperature of the base 7 is changed to t.
. A digital conversion value corresponding to the voltage 1o when the paper is closed is obtained. This digital conversion value is displayed on the display 7 of the measuring circuit 8 as a resistance value in the same manner as above.

In addition, when replacing the voltage vt at the current temperature t□ with the desired temperature 2, Vt is used as a variable level reference voltage.
The use of s2 is as described in the explanation of the measurement principle. In reality, the variable resistor RV of the reference voltage shift circuit 13 is provided with 11 scales for each 1°C, and the variable resistor RV is adjusted by the amount corresponding to the temperature difference between the current temperature and the desired temperature t,2. By turning the offset voltage, the offset voltage is proportionally shifted and a variable level reference voltage vs2 is set, allowing replacement measurement.

The explanation so far has been based on the reference temperature t. Replace ii+! This was done for the case where the constant temperature range is set on the relatively low temperature side, but as shown in FIG. 6, the temperature t is set on the relatively high temperature side. You can also set In this case, the variable level reference voltage Vsi is shown by the dotted line in the figure, and the reference voltage Vsi is at a constant level at the current temperature t1.
The temperature t at high temperature is determined by switching the voltage vt1 measured at s to a variable level reference voltage shown at point Q1. can be replaced by the voltage vt0 at

The example shown in FIG. 6 is similar to FIG. 1 above.・p-v
This is equivalent to extending, for example, to the left from t, and the measurement principle and operation of each part are the same as in the case of FIG. 1, so a description thereof will be omitted.

Note that when a resistor other than a conductive wire is to be measured, the movable piece of the switch SW2 may be placed on the contact A side as shown in FIG.

[Effects] As explained above, this digital resistance meter uses a constant level reference voltage independent of temperature as the reference voltage of the A/D converter that digitally converts the resistance of the object to be measured, and a reference voltage that It has a reference voltage generator that generates a variable level reference voltage that changes with
The signals are sequentially switched and applied to the A/D converter by W2.

As a result, in addition to the resistance subchamber of the conductive wire at the current temperature, it is now possible to easily find the resistance at a desired temperature by replacing the resistance at the current temperature, and when the device is operated at a low or high temperature different from the current temperature. It is possible to predict the state of heat generation in the event of an accident, prevent accidents from occurring, and improve reliability.

In addition, in some cases, it is possible to eliminate the need for temperature tests, which require a large amount of man-hours, by bringing equipment and parts into a constant temperature room or the like in order to measure the resistance value at a desired temperature, which is also effective in streamlining the process.

[Brief explanation of drawings]

The attached drawings all relate to embodiments of the digital resistance meter according to the present invention, with Fig. 1 being an explanatory diagram of its measurement principle, Fig. 2 being a circuit diagram, and Figs. 3 and 4 showing the operation of the temperature/voltage conversion circuit. FIG. 5 is an explanatory diagram of a temperature coefficient correction circuit and a reference voltage shift circuit, and FIG. 6 is an explanatory diagram of a modified embodiment. In the figure, 1 is a measurement section, 2 is a reference voltage generation section, 3 is a conductive wire to be measured, 4 is a constant current source, 5 is an A/D converter, 8 is a setting circuit, 10 is a reference voltage generation circuit, and 11 is Temperature/voltage conversion circuit, 12 is a temperature coefficient correction circuit, 13 is a reference voltage shift circuit, A2 is a differential amplifier, A is an amplifier, RT is a temperature sensor, RV is a variable resistor, R,, R,, R7 are resistors , sw
,,sw,. SW is a switch, VS is a DC power supply, and Vs and Vsi are reference voltages.

Claims (1)

[Scope of Claims] A digital resistance meter that converts the resistance of a conductive wire to be measured into a voltage, and measures the resistance by converting the voltage to a digital value via an integral type A/D converter, comprising: The A/D converter has a DC power supply that generates a reference voltage at a constant level for measuring the resistance at the current temperature, and a reference voltage source for the A/D converter. A temperature/voltage conversion circuit including a temperature sensor that changes ambient temperature and a bridge that converts changes in ambient temperature into voltage and outputs it, and an amplifier having a predetermined voltage amplification degree, and the amplifier performs the temperature/voltage conversion. A temperature coefficient correction circuit that adjusts the output characteristics of the circuit to characteristics equivalent to the resistance/temperature change characteristics of the conductive wire; and an offset voltage is applied to the temperature coefficient correction circuit to shift its output level, and the current temperature of the conductive wire is adjusted to the current temperature of the conductive wire. and a reference voltage shift circuit for forming a reference voltage of a variable level to replace the resistance of 1 with a resistance at a desired temperature.