JPH0810167B2 - Method for detecting wire breakage in RTD input processor - Google Patents

Description

Detailed Description of the Invention [Industrial applications]

The present invention relates to an input processing device for input-processing a signal detected by a resistance temperature detector which is made of copper, platinum or the like and is used as a temperature sensor, wherein the resistance temperature detector detects a disconnection of a conductor wire connecting the resistance temperature detector. The present invention relates to a disconnection detection method for an input processing device.

[Prior art]

FIG. 3 shows a conventional 3-wire resistance temperature detector input processing device (hereinafter referred to as a device) 1 having a disconnection detection function. In the figure, the resistance temperature
vise, hereinafter referred to as RTD) 2 is a terminal 3 provided on the device 1,
It is connected to 4,5 through three conducting wires 6,7,8. Terminal 3
A constant current source 9 is connected to and one end of a resistor R ₁ is connected. A negative voltage -V is applied to the terminal 4 via a high resistance R _{H of the} order of MΩ, and one end of the resistance R ₂ is connected. The terminal 5 is connected to a reference potential such as a ground potential and one end of a resistor R ₃ is connected. A capacitor C ₁ is connected between the other end of the resistor R _{1 and} the other end of the resistor R ₃ , and a capacitor C ₂ is connected between the other end of the resistor R _{2 and} the other end of the resistor R ₃ . . The voltage V ₁ across the capacitor C ₁ is applied to the switches 10 ₁ and 10 ₂ forming the selector 10, and the voltage V ₂ across the capacitor C ₂ is applied to the switches 10 ₃ and 10 ₄ forming the selector 10. The selector 10 alternately closes the switches 10 ₁ and 10 ₂ and the switches 10 ₃ and 10 ₄ at a predetermined cycle in response to a select signal given from the CPU 11 to thereby generate the above voltages V ₁ and V ₂
And are taken out in time division and added to the differential amplifier 12. The voltages V ₁ and V ₂ amplified by the differential amplifier 12 are converted into digital data by the A / D converter 13, and this digital data is sent to the CPU 11. CPU11 is program ROM14 and work RAM15
Is used to perform a predetermined calculation process based on the digital data, thereby obtaining a voltage Ri across the RTD 2 described later, and converting this Ri into temperature. Next, the principle of temperature measurement when the RTD2 is used as a temperature sensor will be described. When the resistance of RTD2 is R, this resistance R increases as the temperature rises. In FIG. 3, in the steady state, the current i from the constant current source 9 flows through the path from the terminal 3 to the lead wire 6, RTD2, lead wire 8, terminal 5 and ground as shown. Moreover, since a high resistance R _H is connected to the conductor 7, no current flows. The capacitors C ₁ and C ₂ are charged through the resistors R ₁ and R ₂ , and the voltages V ₁ and V ₂ appear across them. In addition,
The capacitors C ₁ and C ₂ and the resistors R ₁ , R ₂ and R ₃ constitute a low pass filter that absorbs the voltage fluctuation. In the state where the current i is flowing, However, R: resistance of RTD2 r: resistance of conductors 6, 7, 8 Clear the r from equation (1), when obtaining the voltage across Ri of RTD2, the _{Ri = V 1 -2V 2 ...... (} 2). That is, by performing the calculation of the above formula (2),
The influence on the measured value of the conductor resistance r can be eliminated. Therefore, the CPU 11 obtains Ri by the calculation of the equation (2) and converts this Ri into temperature. Next, a method of detecting disconnection of the conducting wires 6, 7, 8 will be described. (1) When the conducting wire 6 or 8 is broken The current i flows through the route of the resistor R ₁ → the capacitor C ₁ → the resistor R ₃ , so that the voltage V ₁ across the capacitor C ₁ rises sharply. Therefore, it is possible to know the disconnection of the conducting wire 6 or 8 by detecting that the voltage V ₁ has become equal to or higher than a predetermined value. (2) When the conducting wire 7 is disconnected Since the upper end of the capacitor C _{2 in} the figure is pulled to the negative voltage −V side via the resistance R ₂ and the high resistance R _H , the voltage V ₂ becomes negative.
Therefore, the disconnection of the conducting wire 7 can be known by detecting that V ₂ <0. In addition, V ₂ ≧ 0 in a normal state.

[Problems to be Solved by the Invention]

Among the above-described disconnection detection methods, the disconnection detection method for the conducting wire 6 or 8 has the following problems. A curve a shown by a solid line in FIG. 2 shows an example of the value of the voltage V ₁ sampled at each sampling point n−1, n, n + 1, n + 2 ... When the CPU 11 samples data at the cycle Ts. In this figure, the reference voltage for disconnection determination is V _T , and when V ₁ > V _T , it is determined that the conductor 6 or 8 is disconnected. now,
If a disconnection occurs at the time point t ₁ between the sample points n and n + 1, the voltage V ₁ across the capacitor C ₁ rapidly increases from V _{n with} the time constants of R ₁ , R ₃ and C ₁ at the same time when the disconnection occurs. And V ₁
The value of becomes V _{n + 1} (<T _T ) at sample point n + 1,
At the next sample point n + 2, V _{n + 2} > T _T, and it is determined that the wire is broken. However, actually n + 1
At the point of time, the disconnection has already occurred, and although the value of V _{n + 1} taken in at that time is abnormal data, since it is smaller than V _T , it is input and processed as normal data. As a result, an abnormally high temperature is treated as a correct temperature. As a countermeasure against this, it is conceivable to lower the judgment reference voltage V _T or shorten the sampling period T _S. However, if V _T is lowered, the temperature measurement range is narrowed, and in the case of sequentially sampling each measurement value using a large number of temperature sensors, the sampling period T _S needs to be longer than a certain length. The present invention has been made to solve the above problems, and in a 3-wire resistance temperature detector input processing device,
Disconnection of the resistance thermometer input processing device is designed to prevent abnormal data generated by the disconnection of the lead wire connecting the resistance temperature detector before it reaches the reference value for disconnection judgment and to prevent it from being treated as normal data. A detection method is provided.

According to the present invention, the time constant of a low-pass filter connected between a first terminal and a third terminal is set such that a voltage across a capacitor that constitutes the low-pass filter when a disconnection occurs is a predetermined value. In the sampling period, select a value that exceeds the predetermined disconnection judgment reference value, check whether the sampled voltage is higher than the previously sampled value by more than a predetermined amount, and exceed the predetermined value. If the voltage has not risen, the sampled voltage is set to a correct value, and if it exceeds the predetermined amount, the value of the previously sampled voltage is held.

[Action]

If the sampled voltage rises above the previously sampled value by more than the predetermined amount and the cause is an actual temperature rise, the voltage will be less than the predetermined value at the next sample. At this time, the held voltage value is treated as correct data. If the cause is disconnection, the voltage becomes higher than a predetermined value in the next sample, and it is determined to be disconnection.

【Example】

An embodiment of the present invention will be described below with reference to the drawings. First, the above voltage that suddenly rises when a break occurs in the conductor 6 or 8.
The magnitude of the time constant due to the resistors R ₁ , R ₃ , and C ₁ that determine the rising speed of V ₁ is set as follows. That is, V _{1 in} one sample period T _S
The time constant is set so that the rising amount of the voltage exceeds the reference voltage V _T for disconnection determination. Hereinafter, this is referred to as condition A. Under the above condition A, disconnection is determined by the procedure shown in the flowchart of FIG. In FIG. 1, when the disconnection determination is started, the step
First the V ₁ obtained at the sample point and the V in ST1
Compare with _T. If V ₁ <V _T , the process proceeds to step ST2, where it is checked whether or not the V ₁ sampled this time exceeds the V ₂ sampled last time by a predetermined amount ΔV ₁ or more. If not elevated captures V ₁ which is currently the sample in step ST4 as the current normal data. Next, returning to step ST1, V ₁ at the next sampling point is sampled. If the V ₁ sampled this time has exceeded ΔV ₁ from the previous time in step ST 3 above, then step ST 3
Proceed to 5, wherein after holding the V ₁ of the last as the current V _1, the flow returns to step ST1. Also, in step ST2 above, V ₁ <
If it is not V _T , it is determined as a disconnection in step ST6. This flowchart has three paths, and each path (1), (2), and (3) will be described below. Path (1): Step ST2 → ST3 → ST4 Path This is the path when the resistance value R of the RTD2 does not change suddenly in the non-disconnected state. In most cases, this path will be considered. Path (2): Path of steps ST2 → ST3 → ST5 There are two possible causes for this path. The first is the case where the temperature rises rapidly and the resistance value R of the RTD 2 increases, so that the increase in V ₁ increases as shown by the curve b in FIG. The second is a disconnection condition, V ₁
<V _T , but when V ₁ is rising rapidly, the increase in V ₁ is large. However, since it is not possible to determine here whether the cause is the former or the latter, V ₁ retains the previous value for the time being, with the suspicion that it is the latter. Here, if it is due to the disconnection, V ₁ > V _T should be satisfied at the next sample point according to the condition A described above, and therefore, the step
Go to ST6 and it is judged as a wire break. If the cause is a sharp rise in temperature as shown by the curve b, the path 1 is always passed at the next sample point or subsequent sample points, and the value of V ₁ is updated. Pass (3): Step ST2 → ST6 Pass It is judged as a disconnection. Next, the case where the above-mentioned flowchart is applied to the curve a in FIG. 2 will be described. First, in step ST2 for _{n + 1} , since V _{n + 1} <V _T , the disconnection state is not determined. But step ST
In (3), (V _{n + 1} −V _n )> ΔT _1, so the process proceeds to step ST5 and the previous V ₁ is held. Next, at n + 2, V _{n + 2} > V _T is always satisfied due to the condition A, so it is determined that the wire is disconnected at step ST6. The effects in this case are as follows. Before the application of the present invention, V ₁ of each of n, n + 1 and n + 2 in FIG. 2 is V _n (normal), V _{n + 1} (abnormal) and V _{n + 2} (disconnection). After application, V _n (normal), V _n (normal) (previously held V _n ), V _{n + 2} (broken line), and abnormal (in the sense of a value far apart from actual) data can be removed. it can.

【The invention's effect】

According to the present invention, the time constant of the low-pass filter connected between the first and third terminals is determined so that the voltage across the capacitor that constitutes the low-pass filter when a disconnection occurs is a predetermined disconnection at a predetermined sampling period. A value that exceeds the reference value is selected, and it is checked whether the sampled voltage is higher than the previously sampled value by more than a predetermined amount. If the voltage is not higher than the predetermined amount, the above value is determined. The sampled voltage is set to a correct value, and when it exceeds the predetermined amount, the value of the previously sampled voltage is held. Judging whether it is due to temperature rise or wire breakage, if the temperature rises, the sampled voltage is regarded as normal data, and if it is due to wire breakage, the sampled voltage is sampled. The voltage is removed as abnormal data, the voltage sampled the previous time is regarded as normal data, and it is possible to determine that there is a disconnection at the next sample, which prevents the abnormal data from being taken in as normal data. The effect is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a flow chart showing a procedure of a wire breakage detection method of a resistance temperature detector input processing apparatus according to an embodiment of the present invention;
FIG. 3 is a characteristic diagram showing the voltage value at each sample point, and FIG. 3 is a configuration diagram showing a conventional 3-wire resistance temperature detector input processing device to which the present invention can be applied. 2 is a resistance temperature detector, 3, 4 and 5 are terminals, 6 and 7 and 8 are lead wires, 9 is a constant current source, C ₁ and C ₂ are capacitors, R ₂ , R ₂ and R ₃ are resistors, and V ₁ Is the voltage across capacitor C ₁ and V ₂ is the voltage across capacitor C ₂ .

Claims (1)

[Claims] 1. A resistance temperature detector is connected to first, second and third terminals via first, second and third conducting wires, and the first
A power source is connected to the terminal, a first low-pass filter including a first capacitor is connected between the first and third terminals, and a second capacitor including a second capacitor is connected between the second and third terminals. 2 low-pass filters are connected, the first voltage obtained across the first capacitor and the second voltage obtained across the second capacitor are sampled at a predetermined sampling period, and sampled. The temperature is obtained by performing a predetermined calculation based on the first and second voltages, and a disconnection occurs in the first or third conductor when the sampled first voltage exceeds a predetermined value. In the resistance thermometer input processing device configured to determine that the time constant of the first low-pass filter, the first voltage when the disconnection occurs is the predetermined value at the sampling period. Cross over While selecting such a value, it is checked whether or not the sampled first voltage is higher than the previously sampled value by more than a predetermined amount, and if it is not higher than the predetermined amount, it is sampled by the above. A method for detecting disconnection of a resistance temperature detector input processing device, wherein the first voltage is set to a correct value, and when the predetermined value is exceeded, the previously sampled first voltage value is held.