JPH05196509A - Temperature measuring instrument - Google Patents

Abstract

PURPOSE:To prevent the title instrument from exceeding the forward saturation voltage of a barrier no matter what sort of temperature measuring resistors having different resistances are connected to the instrument by setting a low- current source to a value at which the output voltage of the source does not exceed the clip voltage of the barrier. CONSTITUTION:A temperature measuring resistor 9 is connected to measurement terminals 2, 4, and 6 through a barrier 17. The electric current flowing to the resistor 9 is controlled so that the value IA of the current can be automatically adjusted and the voltage across the resistor 9 does not exceed the voltage across the barrier 17 even when the resistance Rx of the resistor 9 is high. A constant- current source 20 is constituted of an output section 21, setting section 23, and comparing section 25. The section 23 sets the voltage across the resistor 9 to a value which does not exceed the clip voltage Vf across the barrier 17 and the section 25 compares its own output voltage Vo with the voltage Vmax set by the section 23. When the resistance Rx of the resistor 9 is high and the output voltage Vo reaches the set voltage Vmax, the comparing section automatically reduces the value of the constant current IA so that the output voltage Vo cannot exceed the set voltage Vmax.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a temperature measuring device using a resistance temperature detector as a sensor and used in the field of process automation and the like.

[0002]

2. Description of the Related Art A resistance temperature detector is a temperature sensor utilizing the property that the electric resistance value of metal changes with temperature. Particularly, the platinum resistance thermometer is capable of highly accurate temperature measurement, and its performance and characteristics are specified by JIS. The temperature measuring device described in this specification measures the resistance value of such a resistance temperature detector to read the temperature value corresponding to the resistance value. The resistance value Rx is obtained by applying a constant current IA of known value to the resistance temperature detector and measuring the generated voltage VA, from Rx = VA / IA. In the case of platinum resistance thermometer, the current flowing to the resistance thermometer to measure the resistance is JIS
Its standard size is defined as the specified current.

FIG. 4 is a diagram showing an example of a conventional structure of a three-wire type temperature measuring device using a resistance temperature detector as a sensor. That is, when the current IA is made to flow from the constant current source 1 through the path shown in the figure, the amplifier 3
Receives the voltage of V1 = (Rx + rL) · IA. rL is the resistance of the lead 13 (lead 11). The amplifier 5 receives a voltage of V2 = rL · IA (the input impedance of the amplifiers 3 and 5 is high, so no current flows into the measuring terminal 4). Therefore, the amplifier 7 calculates V1−V2 = Rx · IA and takes out the voltage (Rx · IA) generated at the resistance temperature detector.

In fields such as process automation,
Between the resistance temperature detector 9 and the temperature measuring device 15, there are often several hundred meters and a lightning strike may occur. Therefore, in order to prevent damage to the temperature measuring device, measures against lightning are taken. Figure 5
FIG. 4 is a configuration diagram showing measures against lightning strikes, and an energy limiting device called a barrier is provided at the input part of the temperature measuring device to limit the surge voltage applied to the temperature measuring device 15. The barrier 17 in FIG. 5 includes diodes D1 to D4 and three resistors.
It is composed of Rp. When the forward saturation voltage of each diode D1 to D4 is Vf, even if a voltage exceeding Vf is applied, it is clamped to this voltage Vf. As the diodes D1 to D4, Zener diodes may be used instead of the diodes of the rectifier. The barrier 17 should operate (the diode is turned on) only when there is an excessive input, and if it operates during normal measurement, a measurement error will occur.
Therefore, it is necessary to suppress the normal measurement voltage Vd (see FIG. 5) to a low value and to make it equal to or lower than the forward saturation voltage (clamp voltage) Vf of the diode.

[0005]

Generally, the temperature measuring device is designed so that the temperature can be measured even if the resistance temperature detectors having various resistance values are connected. Therefore, there may be a combination of the resistance temperature detector 9 having a resistance value Rx higher than usual and the barrier 17 having a diode having a low forward saturation voltage Vf, and the measurement voltage Vd is Vf <Vd with respect to the clamp voltage Vf. May be. In this case, since the temperature cannot be measured, first measure the resistance value Rx of the resistance thermometer,
-The current value IA such that IA <Vf is calculated, and the constant of the constant current source 1 of the measuring device is changed. However, determining the current value and changing the constant require a lot of man-hours, which is not preferable. On the other hand, it is conceivable to set a low current value IA for all measuring instruments, but the measured signal component decreases and the S / N decreases, which hinders high-accuracy measurement. Since a current is applied, this also poses a problem such as hindering high-precision measurement.

An object of the present invention is to provide a temperature measuring device which does not exceed the forward saturation voltage (clamp voltage) of the barrier regardless of what resistance value the resistance temperature resistor is connected to. is there.

[0007]

The present invention comprises a constant current source,
A resistance temperature detector which is arranged on the object to be measured and to which a current is applied from the constant current source, and a barrier which is provided between the resistance temperature detector and the measurement terminal and acts so that a voltage exceeding the clip voltage is not applied between the measurement terminals. In the device for measuring the temperature by measuring the resistance value of the resistance temperature detector from the voltage applied between the measurement terminals, the current flowing through the resistance temperature detector is introduced, and the resistance value is a known resistance (Rr ), By measuring the voltage of this resistance (Rr), and means for measuring the current flowing through the resistance temperature detector, and, and the constant current source outputs a constant current (IA), The set voltage Vma set so that the output voltage (Vo) of itself does not exceed the clip voltage (Vf) of the barrier.
When x is reached, a constant current source having a function of automatically reducing the value of constant current (IA) is used so that the output voltage (Vo) does not exceed the set voltage Vmax.

[0008]

Since the constant current source outputs the constant current IA, the output voltage Vo of the constant current source increases if the resistance value of the resistance temperature detector connected as a load is large. However, the constant current source used in the present invention has the set voltage Vmax and does not output a voltage higher than the clip voltage at the barrier.
If a resistance temperature detector having a large resistance value is connected and the output voltage reaches the set voltage Vmax set, the value of the constant current IA is set so that the output voltage Vo does not exceed the set voltage Vmax. It has a function to reduce automatically.
Therefore, in a normal measurement state, the resistance temperature detector always operates in a voltage range that does not exceed the barrier clip voltage. Moreover, since the value of the constant current IA can be automatically adjusted,
Does not require manual adjustment. Since the constant current source of the present application automatically changes the value of the output current in this way,
The current value is grasped by the resistance (Rr) whose resistance value is known and the means for measuring its voltage.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram showing a configuration example of a temperature measuring device according to the present invention, FIG. 2 is a diagram showing output characteristics of a constant current source, and FIG. 3 is a configuration example of a resistance value measuring device using the present invention. FIG.

In FIG. 1, reference numeral 30 denotes a temperature measuring device according to the present invention, in which a resistance temperature detector 9 is connected to measuring terminals 2, 4, 6 via a barrier 17. The structure of the barrier 17 is similar to that already described in FIG. The resistance temperature detector 9 is also the same as that described in FIG. In the present invention, even if the resistance value Rx of the resistance temperature detector 9 is large, the current value IA is automatically adjusted so that the clip voltage in the barrier 17 is not exceeded.

The constant current source 20 of the present application comprises an output unit 21 and a setting unit.
23 and a comparison unit 25. The setting unit 23 includes a barrier 17
Is a voltage preset to a value that does not exceed the clip voltage Vf (known) in. The comparison unit 25 compares the output voltage Vo of the constant current source 20 itself with the set voltage Vmax of the setting unit 23, and if Vo <Vmax, a preset value IA = Imax (for example, JI
A signal indicating that a constant current value of 1 mA, 2 mA, or 5 mA specified by S is to be output is added to the output unit 21. When an output voltage Vo that satisfies Vo> Vmax is generated, the comparison unit 25
A control signal for reducing the value of the output current IA is added to the output unit 21 to control so that the output current IA having a value of Vo = Vmax flows. FIG. 2 shows the voltage-current characteristics of the constant current source 20.

Reference numeral 27 is a differential input type AD converter.
The difference between the two signals is calculated, and this difference is calculated as a digital value.
Is to be converted to. AD converter 27 is a switch SW_A
And SW_BFrom the above two signals. Switch SW_AAnd SW
_BSwitching of each contact of is controlled by the microprocessor 29.
Controlled. 29 is a microprocessor (hereinafter simply referred to as μP
2 switch SW _AAnd SW_BControl the contacts of
Then, measure Vxp, Vp, and Vr, which will be described later, and add calculations to them.
In addition, the resistance value (temperature) of the resistance thermometer is calculated.
It Rr is a resistance value in which the current IA flowing through the resistance temperature detector 9 is introduced.
Is a known resistance. This resistance Rr flows to the resistance temperature detector 9.
It is for measuring the current IA.

The operation of FIG. 1 configured as above will be described. Since the constant current source 20 outputs the constant current IA, the output voltage Vo of the constant current source 20 increases as the resistance value Rx of the resistance temperature detector 9 connected as a load increases. The output voltage Vo of the constant current source 20 is expressed by the following equation. Vo = (Rx + Rr + 2 · Rp) · IA (1) Note that Rp is the resistance of the barrier 17 (see FIG. 5). However, the constant current source 20 used in the present invention has the setting voltage Vmax in the setting unit 23 and outputs a voltage larger than the clipping voltage Vf (which is the forward saturation voltage of the diode shown in FIG. 5) in the barrier 17. There is no (this behavior has already been described).

Then, if the resistance temperature detector 9 having a large resistance value Rx is connected, and the output voltage Vo reaches the set voltage Vmax set, the output voltage V
The value of the constant current IA is automatically reduced so that o does not exceed the set voltage Vmax. Therefore, in a normal measurement state, be sure to set the voltage range that does not exceed the clip voltage Vf of the barrier 17,
The resistance temperature detector 9 is operating. Moreover, the constant current is automatically
Since the value of IA can be adjusted, no manual adjustment is required. The constant current source 20 of the present application is
Since the value of is automatically changed, the value of the current flowing through the resistance temperature detector 9 is grasped by measuring the voltage of the resistor Rr whose resistance value is known.

The μP 29 operates as follows to calculate the resistance value of the resistance temperature detector 9. The voltage between measurement terminals 2-4 is Vxp,
Assuming that the voltage across the measurement terminals 4-6 is Vp and the voltage across the resistor Rr is Vr, the voltage Vxp is measured when both switches SW _A and SW _B are at contact point a, and the voltage Vp is measured between switches SW _A and SW _B. Both are measured at the contact b, and the voltage Vr is measured when the switches SW _A and SW _B are both at the contact c. μP29 is the measured V
Using xp, Vp and Vr, the resistance value Rx of the resistance bulb 9
To calculate. Rx ＝ Rr ・ (Vxp−Vp) / Vr (2)

When the value of the resistance Rp (see FIG. 5) in the barrier 17 is very large, the value of the output voltage Vo in the equation (1) becomes Vo> Vmax because of only the resistance value Rp. There is. In this case, the comparison unit 25 adds a control signal for decreasing the value of the output current IA to the output unit 21, and Vo = Vma
Since the output current IA of the value of x is controlled to flow,
A small constant current value flows. As a result, the voltage generated in the resistance temperature detector 9 itself decreases, and the S / N decreases, but the barrier 17 is provided, which is unavoidable.

FIG. 3 shows an example in which the present invention is applied to a potentiometer measuring instrument. The device of FIG. 3 is a potentiometer.
31 shafts are connected to a drive source (not shown),
The sliding piece is slid by this drive source. Therefore, by measuring the resistances R _Y and R _Z of the potentiometer 31, the rotational position of the drive source can be measured. The resistances R _Y and R _Z can be measured by μP29 by the operation already described in FIG.

It should be noted that FIGS. 1 and 3 have been described with reference to an example in which the influence of the wiring resistance rL between the resistance temperature detector 9 and the temperature measuring device 30 and the resistance Rp of the barrier 17 is canceled by the three-wire measuring method. However, this description does not limit the present invention to the three-wire measurement method. For example, the present invention holds true even with a four-wire measurement method, the same operation as described above is performed, and the same effect is obtained.

[0019]

As described above, according to the present invention, if the resistance temperature detector is a normal one, the resistance temperature detector can be measured with a specified current value defined by JIS.
That is, the temperature can be measured with high accuracy. In addition, a high resistance value that exceeds the clip voltage Vf of the barrier 17
The Rx resistance temperature detector 9 is connected so that the constant current IA is automatically reduced and the measurement error is controlled so as not to increase. Further, at this time, unlike the conventional example, in the present invention, it is not necessary to change the measurement method. Therefore, the temperature measuring device of the present invention does not require any setting according to the installation conditions, and the number of steps for that can be reduced.

[Brief description of drawings]

1 is a diagram showing a configuration example of a temperature measuring device according to the present invention, FIG. 2 is a diagram showing an output characteristic of a constant current source, and FIG. 3 is a diagram showing a configuration example of another temperature measuring device according to the present invention. ..

FIG. 1 is a diagram showing a configuration example of a temperature measuring device according to the present invention.

FIG. 2 is a diagram showing output characteristics of a constant current source.

FIG. 3 is a diagram showing a configuration example of a resistance value measuring device using the present invention.

FIG. 4 is a diagram showing a conventional example.

FIG. 5 is a diagram showing a configuration of a barrier.

[Explanation of symbols]

 9 Resistance temperature detector 20 Constant current source 21 Output section 23 Setting section 25 Comparison section 27 AD converter 29 Microprocessor Rr Resistance

Claims (1)

[Claims] 1. A constant current source, a resistance temperature detector which is arranged on a measurement target and to which a current is applied from the constant current source, and a voltage which is provided between the resistance temperature detector and the measurement terminal and which exceeds a clip voltage. In a device that measures the temperature by measuring the resistance value of the resistance temperature detector from the voltage applied between the barrier and the barrier that acts so as not to be applied in between, a current flowing through the resistance temperature detector is conducted. The resistance (Rr) having a known resistance value, and a means (27, 29) for measuring the current flowing through the resistance temperature detector by measuring the voltage (Vr) of this resistance (Rr), When the constant current source outputs a constant current (IA) and the output voltage (Vo) of the constant current source reaches a set voltage Vmax set to a value that does not exceed the clip voltage (Vf) of the barrier, the output is output. The value of constant current (IA) is automatically reduced so that the voltage (Vo) does not exceed this set voltage Vmax. Temperature measuring apparatus which is a constant current source having a capability.