JP4967433B2 - Switch failure detection circuit - Google Patents

Description

  The present invention relates to a circuit for detecting a switch failure, and more particularly to a switch failure detection circuit suitable for use in a double current input module.

  FIG. 4 shows a configuration of a current input module used in the duplex system. In this figure, only the main part of the invention is shown, and parts not directly related to the invention are omitted. In FIG. 4, 10 is a current input module, and 20 is a current source such as a pressure sensor placed in the field. The current input module 10 includes resistors 11 and 12, a switch element 13, and a control unit 14. The control unit 14 includes a comparison circuit 14a and a control circuit 14b.

  The resistor 11, the switch element 13, and the resistor 12 are connected in series in this order, and this series circuit is connected in parallel to the current source 20. On / off of the switch element 13 is controlled by the control circuit 14b. Sub, which is a connection point between the current source 20 and the resistor 11, and Main voltage, which is a connection point between the switch element 13 and the resistor 12, are input to the comparison circuit 14a.

  The current input module 10 uses two systems, a control side and a standby side. The control circuit 14b switches between the control side and the standby side by turning the switch element 13 on and off. Further, the comparison circuit 14a compares the voltage between the point Sub and the common potential point G, and between the point Main and the common potential point G, thereby causing a disconnection or the like in the middle of the path through which the output current of the current source 20 flows. Sometimes an abnormal location can be detected.

  As an example of a duplex process input / output device that reliably detects an abnormality in a duplexed input / output module, there is one described in Patent Document 1, for example.

JP-A-11-65603

  However, in such a current input module, when the switch element 13 is turned off, the current loop is interrupted, and the operation as the current input module is hindered. Since the switch element 13 must be controlled to be turned off in order to detect the on-fixation failure of the switch element 13, there is a problem that the on-fixation fault detection of the switch element 13 cannot be performed during the control side operation. It was.

  Accordingly, an object of the present invention is to provide a switch failure detection circuit that can detect an on-fixation failure of a switch element without interrupting a current loop.

In order to achieve such a problem, the invention according to claim 1 of the present invention is:
First and second resistors having one end connected in common;
A first switch having one end connected to the other end of the first resistor;
A second switch having one end connected to the other end of the second resistor and the other end connected in common with the other end of the first switch;
A third resistor connected between a common connection point of the first and second switches and a common potential point;
A control circuit for controlling on and off of the first and second switches;
The voltage at the common connection point of the first and second resistors and the voltage at the common connection point of the first and second switches are input, and these voltages are compared to compare the voltages of the first and second switches. A comparison circuit for detecting a failure;
Is provided. Switch failure can be detected without interrupting the current loop.

The invention according to claim 2 is the invention according to claim 1,
The control circuit is configured to turn on both the first and second switches during use other than during failure diagnosis and sequentially turn off one of the first or second switches during failure diagnosis. is there. Internal loss can be reduced.

The invention according to claim 3 is the invention according to claim 1,
The control circuit is configured to turn on one of the first and second switches during use other than during failure diagnosis, and to turn on both the first and second switches during failure diagnosis. It is. Impedance seen from the outside can be made constant.

As is apparent from the above description, the present invention has the following effects.
According to the first, second, and third aspects of the present invention, a circuit in which a resistor and a switch are connected in series is connected in parallel, and the switch is operated so that at least one switch is kept on when a failure is detected.

  Even if one switch is turned off for failure detection, the current loop is not interrupted because the other switch is on. Therefore, even when the control operation is being performed, it is possible to detect the on-fixing failure of the switch by alternately turning off the switch. In addition, when both of the switches are turned off except for failure diagnosis, there is an effect that the impedance of the internal circuit is lowered and the internal loss can be reduced.

  Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of a switch failure detection circuit according to the present invention. In FIG. 1, 30 is a current input module, and 20 is a current source such as a pressure sensor. 31 and 32 are resistors, one end of which is connected in common and connected to the output end of the current source 20. Reference numeral 34 denotes a switch element, one end of which is connected to the other end of the resistor 32. Reference numeral 35 denotes a switch element, one end of which is connected to the other end of the resistor 31. Reference numeral 33 denotes a resistor, one end of which is connected to the other ends of the switch elements 34 and 35 and the other end is connected to a common potential point.

  Reference numeral 36 denotes a control unit, which includes a comparison circuit 36a and a control circuit 36b. The output of the control circuit 36b is input to the gates of the switch elements 34 and 35 to control on / off thereof. The voltage at the common connection point Sub between the current source 20 and the resistors 31 and 32 and the voltage at the common connection point Main between the switch elements 34 and 35 and the resistor 33 are input to the comparison circuit 36a.

  This current input module is also used in the duplex system as in FIG. 4, and the switch elements 34 and 35 are turned on when operating as the control side module, and the switch when used as the standby side module. Both 34 and 35 are controlled off. In addition, description of parts not directly related to the present invention is omitted.

  Next, the operation of this embodiment will be described with reference to FIGS. The current source 20 outputs a current of 20 mA, the resistance values of the resistors 31 and 32 are 500/3 (167.7) Ω, and the resistance value of the resistor 33 is 250/3 (83.3) Ω. To do.

  The voltage values of Main and Sub at this time are shown in FIG. The pattern (1) is a voltage when both the switch elements 34 and 35 are on. Since the resistors 31 and 32 are connected in parallel, the combined resistance is 500/6 = 250 / 3Ω, which is the same as the resistance value of the resistor 33. The voltage of Main is 0.02 × 250/3 = 1.67V, and the voltage of Sub is 3.33V, which is twice that voltage.

(2) and (3) are voltages when one of the switch elements 34 and 35 is on and the other is off. Since one of the resistors 31 and 32 is disconnected, the combined resistance is 500 / 3Ω. Since the resistance value of the resistor 33 does not change, the voltage of Main is 1.67V as in (1).
It is. The voltage of Sub is (500/3 + 250/3) × 0.02 = 5V.

  (4) is a case where both the switch elements 34 and 35 are off, and both the Main and Sub voltages are 0V.

  FIG. 3 is a diagram schematically showing FIG. 2, and symbols (1) to (4) correspond to the symbols of the patterns in the table of FIG. (A) is the Main voltage, and (B) is the Sub voltage. The voltage value is the same as in FIG.

  The comparison circuit 36a monitors the voltages of Main and Sub. When used as a control-side module, both switch elements 34 and 35 are turned on to be in an operating state. The main and sub voltages at this time are 1.67 V and 3.33 V, respectively, as shown in FIG.

  When detecting a failure of the switch element 34, the control circuit 36b controls the switch element 34 to be turned off. Since the switch element 35 remains in the on state, the current input module 30 maintains the operating state.

  When the switch element 34 is turned off as controlled, the state shifts to the state of FIG. 2 (3), and the voltage of the Sub becomes 5V. The comparison circuit 36a detects this voltage and confirms that the switch element 34 is normal. When an on-fixation failure occurs in the switch element 34 and the switch element 34 is not turned off, the Sub voltage is maintained at 3.33V. The comparison circuit 36a determines that an on-fixation failure has occurred in the switch element 34, and outputs an abnormal signal to the outside.

  In addition, since it can carry out by the same operation also in the case of failure detection of the switch element 35, detailed description is abbreviate | omitted. In this embodiment, since both switches are turned on during use other than when a failure is detected, the internal impedance of the current input module 30 can be reduced and the internal loss can be reduced.

In HART communication, which is a type of fieldbus, it is necessary to set the reception resistance of the connected device to a specified value. In this case, when the switch element 34 or 35 is used other than during failure diagnosis , either the switch element 34 or 35 is turned on and the other is turned off, and only the switch elements 34 and 35 are controlled to be turned on only when the switch element failure is detected. What should I do? At this time, if the voltage value of Sub shifts from (2) or (3) to (1), it can be determined that the switch element is normal.

  Further, when the resistors 31, 32, 33 fail, the voltages of Main and Sub monitored by the comparison circuit 36a change according to the state of each failure (open failure, short failure, etc.). 33 faults can also be detected.

  In the embodiment of FIG. 1, the switch elements 34 and 35 are semiconductor elements. However, a switch having a mechanical contact such as a relay may be used.

It is a block diagram which shows one Example of this invention. It is the table | surface which showed the relationship between the state of a switch element, and a voltage value. It is the figure which showed the relationship between the state of a switch element, and a voltage value. It is a block diagram of the conventional current input module.

Explanation of symbols

20 Current source 30 Current input module 31, 32, 33 Resistance 34, 35 Switch element 36 Control unit 36a Comparison circuit 36b Control circuit Main, Sub Voltage measurement point

Claims (3)

First and second resistors having one end connected in common;
A first switch having one end connected to the other end of the first resistor;
A second switch having one end connected to the other end of the second resistor and the other end connected in common with the other end of the first switch;
A third resistor connected between a common connection point of the first and second switches and a common potential point;
A control circuit for controlling on and off of the first and second switches;
The voltage at the common connection point of the first and second resistors and the voltage at the common connection point of the first and second switches are input, and these voltages are compared to compare the voltages of the first and second switches. A comparison circuit for detecting a failure;
A switch failure detection circuit comprising: The control circuit is configured to turn on both the first and second switches during use other than during failure diagnosis, and sequentially turn off one of the first or second switches during failure diagnosis. The switch failure detection circuit according to claim 1, wherein: The control circuit is configured to turn on one of the first and second switches during use other than during failure diagnosis , and turn on both the first and second switches during failure diagnosis. The switch failure detection circuit according to claim 1, wherein:

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