JPS63134967A - Disconnection detecting circuit - Google Patents

Abstract

PURPOSE:To cancel an error caused by a disconnection detecting part, by providing a resistance network between a disconnection detecting power source and an analog signal line. CONSTITUTION:A thermocouple 1 is connected to a cold junction part 3 through a thermocouple compensating conductor 2, and a voltage corresponding to a temperature difference of the thermocouple 1 and the cold junction part 3 is inputted to an analog input device 4, through an analog signal line to which resistors RP, RN are connected in series. To the cold junction part 3 side of the analog signal line, a disconnection detecting part 5 consisting of a DC power source ES and resistors R1, R2 is connected, and in case the line of the thermocouple 1 side is disconnected, an input voltage of the input device 4 is scaled out and the disconnection can be detected. Also, to the input device 4 side of the analog signal line, a resistance network 6 connected to the DC power source ES is connected, and an error influenced over a measuring voltage by the DC power source ES at the time of non-discronnection is cancelled.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention relates to disconnection detection for detecting disconnection of an analog signal cable, such as a thermocouple input, that is input to an industrial computer system or a measuring instrument. It is related to circuits.

(Conventional technology) Conventionally, there has been a device of this type as shown in FIG.

In the figure, (1) is a thermocouple, (2) is a thermocouple compensation conductor, (3) is a cold junction, (4) is an analog input device such as an industrial computer system, R, R2 is a resistor, and ES is a DC power supply for scaling out the input voltage to the analog input device in the event of a disconnection, and R,,112,Es
This constitutes a thermocouple disconnection detection section (5).

Next, the operation will be explained. In Figure 4, the thermocouple (
1) connects the cold junction ('l) via the thermocouple compensation conductor (2).
When connected, a voltage corresponding to the difference between the temperature of the thermocouple (1) and the temperature of the cold junction (3) is input to the analog input device (4). If the thermocouple (1) and thermocouple compensation conductor (2) are not disconnected, their DC resistance values are relatively small (for example, +00Ω), and the resistance values of R1 and R2 are increased (for example, 5MΩ), and the DC resistance value is relatively small (for example, +00Ω). By appropriately selecting the voltage of the power source E3 (for example, IOV), the influence of the disconnection detection section (5) can be reduced to some extent.

For example, 1τ (difference is xlO(v) = -100(μv) 5x, l116X i which occurs in the case of a constant within ``bis()), and the rated input terminal to the analog input device (4) is 2
If it is 0mV, an error of will occur.

However, as shown in Figure 5, if a disconnection occurs at point (a), the DC resistance value on the thermocouple (1) side increases (for example, IMΩ), and scale-out occurs in the analog input device (4). A voltage is applied.

By determining this scale-out, disconnection can be detected.

[Problem that the invention seeks to solve]

Since the conventional disconnection detection circuit is configured as described above, an error occurs when the total resistance value of the 8-couple compensation conducting wire (2) and the thermocouple (1) is not a sufficiently high value. Then, in order to prevent errors from occurring even if the resistance value is large to a certain extent, the resistance values of the resistors R, R2 are set to a high value,
There is a problem in that if the voltage of the DC power supply Es is reduced, it becomes impossible to completely detect a disconnection.

This invention was made in order to solve the problems of the conventional ones as described above, and it is possible to detect disconnection without causing an error even when the total resistance value of the thermocouple compensation conductor and the thermocouple is large to some extent. The object of the present invention is to obtain a disconnection detection circuit that can reliably detect wire breakage.

[Means for solving problems]

The disconnection detection circuit according to the present invention cancels the error voltage generated in the disconnection detection section using an error canceling section consisting of a DC power supply used in the disconnection detection section and a resistor network, thereby ensuring a burnout voltage at the time of disconnection. Therefore, a resistor is inserted in the analog signal line between the disconnection detection section and its error cancellation section.

[Effect]

The error canceling section, which is a resistor network in the present invention, correctly divides the DC voltage for wire breakage detection, and works with the wire breakage detection section and the resistor of the analog signal line so that when the wire breaks, the wire breakage detection section acts largely. When the wire breaks, the impedance on the thermocouple side is small, so the effect is suppressed and the error component is canceled.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. 1st
In the figure, RP and RN are resistors inserted in series with the analog signal line from the disconnection detection section (5),
Together with an error canceling section (6) consisting of a resistance network including resistors R5, R6, 11+, R4, and VRI, the error voltage at the time of non-disconnection is canceled. In addition, thermocouple (1), heat'i mini compensation conductor (2)
, the resistors R, , It2 in the disconnection detection section (5), the power source Es for scaling out in the event of a disconnection, and the analog input device (4) are the same as those in the conventional example.

Next, the operation of the upper assembly configuration will be explained.

The thermocouple (1) is connected to the cold junction (
3), and a voltage corresponding to the temperature difference between the thermocouple (1) and the cold junction (3) is input to the analog input device (4), as in the conventional example.

Therefore, the resistors RP, R,
is inserted in order to ensure error canceling operations by the disconnection detection section (5) and the error cancellation section (6) consisting of a resistor network. That is, when there is no disconnection, the error canceling section (6) is connected to the DC power supply E8 and the analog inputs of the resistors Rp and RN so that the offset error generated in the disconnection detecting section (5) is in the opposite polarity to the polarity of the offset error. It is connected to the device (4) side, and the error canceling section (
The voltage across the variable resistor VR in 6) is balanced at a value smaller than the disconnection detection power supply voltage E. On the other hand, when the line is disconnected, the analog signal line becomes open and the RP,
Since RP and RN are set to the constant of RN (R), there is almost no relationship between RP and RN. Therefore, if the voltage across the variable resistor VR is set to α・Es (α<1), the analog input device ( The voltage to the analog input device (4) at the time of disconnection can be made to be higher than scale-out by appropriately selecting various constants.

The above can be expressed numerically as follows, with reference to the equivalent circuits shown in FIGS. 2(a) and 2(b) when the wire is not broken and when the wire is broken. Note that the electromotive force of the thermocouple is assumed to be sufficiently smaller than Es, and is omitted from the equivalent circuit. Also, I(1=R
2=L=11. Let +=1(/2.R) r.

・If V,, -O when there is no disconnection, α・Es=R−1I...(1) α・ES++1=R−1+(Rp”RN)!++R(1
+”12)...(2)lEs=ri2”R(i+”1
2) "" (3) Therefore, If the size of RP+RN is set in consideration of the value of r, it is easy to set α to about 0.9, for example.

・When disconnection occurs, RP, I(N(R), vio=ε3−α−Es=(1−α)ES, for example, when E3=lOv, α=0.9, V, .=−1v. The burnout voltage can be obtained, and the full scale value of the analog input device (4) can be sufficiently scaled out in the range of 10mvNI00mv, which is often used for thermocouple input.

In the above embodiment, the resistors in the resistance network are connected as shown in Fig. 1, but as shown in Fig. 3, the analog signal line is directly connected to the variable resistors VR, VR2 (however, VR,, VR2>R+, R
2) may be used.

Further, in the above embodiment, a thermocouple is used, but an analog signal from another sensor may be used, and the same effects as in the above embodiment can be obtained.

〔Effect of the invention〕

As described above, according to the present invention, a virtual error canceling power supply is formed by a resistor network in which the error voltage generated in the disconnection detection section is connected to the disconnection detection power supply, and a series resistor is inserted into the analog signal line. This has the effect of eliminating errors.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a thermocouple disconnection detection circuit according to an embodiment of the present invention, and FIGS.
FIG. 3 is an equivalent circuit diagram of a disconnection detection circuit showing another embodiment of the present invention, and FIG.
The figure is a circuit diagram showing a conventional thermocouple disconnection detection circuit, and FIG. 5 is a circuit diagram for explaining the case where disconnection occurs in FIG. 4. In the figure, (1) is a thermocouple, (2) is a thermocouple compensation conductor, and (3
) is the cold junction part, (4) is the analog input device, (5)
is the disconnection detection section, (6) is the error cancellation section (resistance network), R+
, Rz, R3, R4, Rs, Re, Rp, and IIN are fixed resistors, Es is a fixed power supply, and VR, , VR2, and VR3 are variable resistors. In each figure, the same reference numerals indicate the same or corresponding parts.

Claims (4)

[Claims] (1) One side of a resistor is connected to each of a pair of analog signal lines that receive sensor input, and a DC power supply for disconnection detection is connected to the other side of the resistor to form a disconnection detection section, and when a disconnection occurs, the In a disconnection detection circuit that detects disconnection by scaling out the input voltage to an analog input device that receives sensor input via an analog signal line, a coupling resistor is installed on each analog signal line between the disconnection detection section and the analog input device. In addition to providing a resistor, an error canceling section is provided between the resistor, the analog signal line of the analog input device, and the DC power source, forming a virtual error canceling power source using a resistor network. Disconnection detection circuit. (2) The error canceling section connects a resistor in series to each of the analog signal lines, and connects two resistors from both ends of the DC power supply to each of the other sides of the series resistors. 2. The disconnection detection circuit according to claim 1, wherein the resistance network is constructed by connecting two series-connected resistors and inserting a variable resistor between the two series-connected resistors. (3) The disconnection detection circuit according to claim 1, wherein the error canceling section is configured by connecting variable resistors from both ends of the DC power supply, and connecting one of the variable resistors to each signal line to form a resistance network. . (4) The disconnection detection circuit according to any one of claims 1 to 3, wherein the sensor is a thermocouple.