JP7564012B2 - Field Devices - Google Patents

Description

The present invention relates to a field device that transmits data to a higher-level device via a two-wire transmission line.

Field devices that use a two-wire transmission line, such as pressure transmitters, electromagnetic flowmeters, and positioners, output a burnout signal that is different from the normal output signal (4 to 20 mA) when they detect an abnormality in the power supply, clock signal, sensor signal, or output current (see Patent Documents 1 and 2). Depending on the burnout setting by the onboard CPU, the field device outputs a current that is larger than the normal signal (burnout HIGH) as the burnout signal, or a current that is smaller than the normal signal (burnout LOW) as the burnout signal.

Figure 3 shows the configuration of the field device disclosed in Patent Document 1, and Figure 4 shows the configuration of the field device disclosed in Patent Document 2. In the field device 1000 in Figure 3, the CPU 103 monitors the outputs of the sensor unit 101, the A/D converter 102, the CPU 103, the D/A converter 104, the communication unit 105, and the power supply unit 106, and when it detects an abnormality in each of these units as an abnormality occurring within the device itself, it outputs a burnout signal from the communication unit 105 to the transmission line L.

In the field device 1001 in FIG. 4, if an abnormality occurs in the D/A converter 114 and a deviation of a threshold value or more occurs between the value indicated by the digital signal input from the A/D converter 118 and the actual measured value, the CPU 113 determines that the value indicated by the digital signal from the A/D converter 118 is abnormal and switches the signal selector 116. As a result, the signal selector 116 selects the burnout signal (voltage signal) from the burnout signal source 119, and the selected burnout signal is sent to the V/I converter 115, which converts it into a current signal and outputs it to the transmission line L.

However, in the configuration of FIG. 3 disclosed in Patent Document 1, there was a possibility that the abnormality detection unit 103-3 would not be able to detect an abnormality in the communication unit 105. This is because the output of the communication unit 105 becomes a negative potential, and therefore an abnormality in the communication unit 105 cannot be detected by a general electric circuit. In addition, even if an abnormality in the communication unit 105 could be detected, there was a problem in that a burnout signal could not be output if the communication unit 105 was broken.

The configuration of FIG. 4 disclosed in Patent Document 2 has the problem that it is not possible to detect an abnormality in the V/I converter 115, and further has the problem that it is not possible to output a burnout signal if the V/I converter 115 is broken.

In both of the configurations disclosed in Patent Document 1 and Patent Document 2, a prerequisite is that a predetermined burnout signal can be transmitted from the communication unit or the V/I converter to a higher-level device via a transmission line. However, if a malfunction occurs in the communication unit or the V/I converter to the extent that it is not possible to output a burnout signal, there is a possibility that the burnout signal cannot be output to the higher-level device.

JP 2009-266070 A JP 2010-123025 A

The present invention has been made to solve the above problems, and aims to provide a field device that can reliably output a burnout signal even if there is an abnormality in the V/I conversion section.

The present invention is characterized in that, in a field device that transmits data to a host device via a two-wire transmission line, the field device comprises a signal generating unit configured to output a signal corresponding to an output current according to the data to be transmitted to the host device, a V/I conversion unit configured to adjust the output current flowing through the two-wire transmission line to a current within a predetermined current range according to the signal from the signal generating unit, an abnormality detection unit configured to detect a voltage proportional to the output current of the V/I conversion unit and output a burnout control signal when the detected voltage value differs from a normal value, and a burnout signal output unit configured to output a current outside the predetermined current range as a burnout signal instead of the output current of the V/I conversion unit when the burnout control signal is output.

In one configuration example of the field device of the present invention, at least the signal generating unit, a part of the V/I converting unit, and the abnormality detecting unit are mounted on an integrated circuit, the V/I converting unit includes a voltage-to-current converting amplifier configured to convert a signal output from the signal generating unit into a current signal, a transistor configured to convert the current signal output from the voltage-to-current converting amplifier into an output current within the predetermined current range, and a resistor inserted in series into a loop of an output current flowing through the two-wire transmission line, the voltage-to-current converting amplifier of the V/I converting unit is mounted on the integrated circuit, the transistor and the resistor are provided outside the integrated circuit, the abnormality detecting unit detects a voltage generated across the resistor which is proportional to the output current, and the burnout signal output unit is comprised of a first burnout signal output unit mounted on the integrated circuit and a second burnout signal output unit provided outside the integrated circuit as a redundant circuit having the same operation as the first burnout signal output unit.
In addition, in one configuration example of the field device of the present invention, the V/I conversion unit includes a voltage-to-current conversion amplifier configured to convert the signal output from the signal generating unit into a current signal, and a transistor configured to convert the current signal output from the voltage-to-current conversion amplifier into an output current within the predetermined current range.
In one configuration example of the field device of the present invention, the V/I conversion unit includes a resistor inserted in series in a loop of the output current flowing through the two-wire transmission line, and the abnormality detection unit detects a voltage generated across the resistor that is proportional to the output current.

In one configuration example of the field device of the present invention, the abnormality detection unit is characterized by being composed of a first A/D conversion unit configured to convert the voltage proportional to the output current into a digital value, and a judgment unit configured to compare a digital value of a normal value that the voltage proportional to the output current should have with an output value of the first A/D conversion unit, and output the burnout control signal when the values differ.
In one configuration example of the field device of the present invention, the abnormality detection unit further includes a second A/D conversion unit configured to convert the analog output signal of the signal generating unit into a digital value, and the judgment unit compares a digital value of a normal value that the analog output signal of the signal generating unit should have with the output value of the second A/D conversion unit, and outputs the burnout control signal when the values differ.
Moreover, one configuration example of the field device of the present invention is characterized in that it further comprises a CPU configured to output a digital value of the normal value to the abnormality detection section based on a set value of an output current corresponding to data to be transmitted to a higher-level device.

According to the present invention, it is possible to detect an abnormality in the V/I conversion unit, and even if there is an abnormality in the V/I conversion unit, it is possible to reliably output a burnout signal.

FIG. 1 is a block diagram showing the configuration of a field device according to a first embodiment of the present invention. FIG. 2 is a block diagram showing the configuration of a field device according to a second embodiment of the present invention. FIG. 3 is a block diagram showing the configuration of a conventional field device. FIG. 4 is a block diagram showing another configuration of a conventional field device.

[First embodiment]
Hereinafter, embodiments of the present invention will be described with reference to the drawings. Fig. 1 is a block diagram showing the configuration of a field device according to a first embodiment of the present invention. The field device 100 comprises a sensor 1 that outputs an analog signal corresponding to a physical quantity (e.g., pressure) to be measured, an A/D converter 2 that converts the analog signal output from the sensor 1 into a digital signal, and a CPU (Central Processing Unit) that samples the digital signal output from the A/D converter 2 to obtain measurement data of the physical quantity. The field device 100 includes a CPU (CPU 3), a modulation signal generating section 4 for outputting a digital modulation signal to obtain an output current I corresponding to the measurement data obtained by the CPU 3, a D/A conversion section 5 for converting the digital modulation signal output from the modulation signal generating section 4 into an analog signal, a V/I conversion section 6 for adjusting the output current I flowing through the two-wire transmission line L to be within a predetermined current range (4 to 20 mA) according to the signal from the D/A conversion section 5, an abnormality detection section 7 for detecting a voltage proportional to the output current I of the V/I conversion section 6 and outputting a burnout control signal VC when the detected voltage value is different from a normal value, a burnout signal output section 8 for outputting a current outside the predetermined current range as a burnout signal instead of the output current of the V/I conversion section 6 when the burnout control signal VC is output, and a reference voltage generating section 9 for generating a power supply voltage for the field device 100. The modulation signal generating section 4 and the D/A conversion section 5 constitute a signal generating section 11.

The V/I conversion unit 6 is composed of a reference voltage generation unit 60, a voltage-current conversion amplifier 61 whose inverting input terminal is connected to the output terminal of the reference voltage generation unit 60, a transistor Q1 whose base terminal is connected to the output terminal of the voltage-current conversion amplifier 61 via a switch SW1 described later and whose collector terminal is connected to the positive output terminal T1 of the field device 100, a resistor R1 whose one end is connected to the output terminal of the D/A conversion unit 5 and whose other end is connected to the non-inverting input terminal of the voltage-current conversion amplifier 61, a resistor R2 whose one end is connected to the other end of the resistor R1 and whose other end is connected to the negative output terminal T2 of the field device 100, a resistor R3 whose one end is connected to the negative output terminal T2 and whose other end is connected to ground, and a resistor R4 whose one end is connected to the emitter terminal of the transistor Q1 and whose other end is connected to ground.

The abnormality detection unit 7 is composed of an A/D conversion unit 70 that converts the analog signal VA output from the D/A conversion unit 5 into a digital value, an A/D conversion unit 71 that converts the voltage VB proportional to the output current I generated across resistor R4 into a digital value, and a judgment unit 72 that outputs a burnout control signal VC when the digital value of the normal output signal VA that the D/A conversion unit 5 should output differs from the output value of the A/D conversion unit 70, or when the digital value of the normal value that the voltage VB proportional to the output current I should take differs from the output value of the A/D conversion unit 71.

The burnout signal output unit 8 is composed of a switch SW1 inserted between the output terminal of the voltage-current conversion amplifier 61 and the base terminal of the transistor Q1, a switch SW2 inserted between the base terminal of the transistor Q1 and ground, and a switch SW3 inserted between the connection point of the resistors R1 and R2 and ground.

First, the normal operation of the field device 100 will be described. The CPU 3 samples the digital signal from the A/D conversion unit 2 to obtain measurement data, for example, pressure. The CPU 3 then instructs the modulation signal generation unit 4 to output a digital modulation signal corresponding to the current setting value of the output current I according to the measurement data. The CPU 3 executes the processing described in this embodiment according to a program stored in a memory (not shown).

The modulation signal generating unit 4 outputs a digital modulation signal instructed by the CPU 3. The D/A conversion unit 5 converts the digital modulation signal output from the modulation signal generating unit 4 into an analog signal.

The host device 200 is provided with a DC voltage source VSUP, which supplies a drive voltage to the field device 100 via a two-wire transmission line L. The host device 200 is also provided with a load resistor RL, which is inserted in series into the loop formed by the field device 100 and the DC voltage source VSUP.

The voltage-to-current conversion amplifier 61 of the V/I conversion unit 6 converts the analog signal output from the D/A conversion unit 5 into a current signal.
Under normal circumstances, since no burnout control signal VC is output from an abnormality detection unit 7, which will be described later, the switch SW1 of the burnout signal output unit 8 is on and the switches SW2 and SW3 are off. The transistor Q1 converts the current from the voltage-to-current conversion amplifier 61 into an output current I in the range of 4 to 20 mA. The above normal operation is the same as in the conventional system.

Next, the operation of the field device 100 when an abnormality occurs will be described. The A/D conversion unit 70 of the abnormality detection unit 7 converts the output signal VA of the D/A conversion unit 5 into a digital value. The CPU 3 outputs to the judgment unit 72 the digital value of the normal output signal VA that should be output by the D/A conversion unit 5 based on the current setting value of the output current I corresponding to the measurement data.

When the modulation signal generating unit 4 and the D/A conversion unit 5 are normal, the digital value of the normal output signal VA output from the CPU 3 matches the output value of the A/D conversion unit 70. On the other hand, when the values do not match, it can be determined that at least one of the modulation signal generating unit 4 or the D/A conversion unit 5 is abnormal. When the digital value of the normal output signal VA differs from the output value of the A/D conversion unit 70, the judgment unit 72 outputs a burnout control signal VC.

A voltage VB proportional to the output current I is generated across resistor R4, which is provided between the emitter terminal of transistor Q1 and ground. A/D conversion unit 71 converts voltage VB into a digital value. CPU 3 outputs a digital value of the normal value that voltage VB should take to determination unit 72, based on the current setting value of output current I corresponding to the measurement data.

When the V/I conversion unit 6 is normal, the normal value of the voltage VB output from the CPU 3 matches the output value of the A/D conversion unit 71. On the other hand, when the values do not match, it can be determined that the V/I conversion unit 6 is abnormal. When the normal value of the voltage VB and the output value of the A/D conversion unit 71 differ, the determination unit 72 outputs a burnout control signal VC.

When the burnout control signal VC is output, the switch SW1 of the burnout signal output unit 8 is turned off and the switches SW2 and SW3 are turned on. This turns off the function of the V/I conversion unit 6, and a burnout signal with a current value below the lower limit of the normal output current I (4 mA) is output. Specifically, when the switch SW1 is turned off and the switches SW2 and SW3 are turned on, the transistor Q1 is turned off, and a current equivalent to the internal current consumption flows through the reference voltage generating unit 9, and this current consumption flows from the ground of the resistor R3 to the negative output terminal T2 of the field device 100. By keeping the current consumption of the field device 100 below 4 mA, when the transistor Q1 is off, the current consumption becomes a burnout signal (below 4 mA).

In this way, in this embodiment, it is possible to detect abnormalities in the modulation signal generating unit 4, the D/A conversion unit 5, and the V/I conversion unit 6. In addition, in this embodiment, by providing a burnout signal output unit 8 separately from the V/I conversion unit 6, it is possible to reliably output a burnout signal even if there is an abnormality in the V/I conversion unit 6.

[Second embodiment]
Next, a second embodiment of the present invention will be described. Fig. 2 is a block diagram showing the configuration of a field device according to the second embodiment of the present invention. The field device 100a of this embodiment includes a sensor 1, an A/D conversion section 2, a CPU 3, a modulated signal generation section 4, a D/A conversion section 5, a V/I conversion section 6, an abnormality detection section 7, burnout signal output sections 8, 8a, and a reference voltage generation section 9.

The burnout signal output unit 8a is composed of a switch SW4 inserted between the output terminal of the voltage-current conversion amplifier 61 and the base terminal of the transistor Q1, a switch SW5 inserted between the base terminal of the transistor Q1 and ground, and a switch SW6 inserted between the connection point of the resistors R1 and R2 and ground.

In this embodiment, an example is shown in which the A/D conversion unit 2, CPU 3, modulation signal generation unit 4, D/A conversion unit 5, part of the V/I conversion unit 6, and abnormality detection unit 7 are realized in an integrated circuit 10. In this case, it is considered that the burnout signal output unit 8 is also mounted on the integrated circuit 10. When the burnout signal output unit 8 is mounted on the integrated circuit 10, an abnormality may occur not only in the D/A conversion unit 5 and the V/I conversion unit 6, but also in the burnout signal output unit 8.

Therefore, in this embodiment, a burnout signal output unit 8a is provided outside the integrated circuit 10 as a redundant circuit for the burnout signal output unit 8. The operation of switch SW4 of the burnout signal output unit 8a is the same as the operation of switch SW1 of the burnout signal output unit 8, and the operation of switches SW5 and SW6 is the same as the operation of switches SW2 and SW3.

Thus, in this embodiment, even if there is an abnormality in the V/I conversion unit 6 and the burnout signal output unit 8 in the integrated circuit 10, the burnout signal can be reliably output.

This invention can be applied to field devices.

1...sensor, 2, 70, 71...A/D conversion section, 3...CPU, 4...modulation signal generation section, 5...D/A conversion section, 6...V/I conversion section, 7...abnormality detection section, 8, 8a...burnout signal output section, 9, 60...reference voltage generation section, 10...integrated circuit, 11...signal generation section, 61...voltage-current conversion amplifier, 72...determination section, 100, 100a...field device, 200...host device, Q1...transistor, R1-R4...resistors, SW1-SW6...switches.

Claims (7)

In a field device that transmits data to a host device via a two-wire transmission line,
a signal generating unit configured to output a signal corresponding to an output current according to data to be transmitted to a higher-level device;
a V/I conversion unit configured to adjust an output current flowing through the two-wire transmission line so that the output current falls within a predetermined current range in accordance with a signal from the signal generation unit;
an abnormality detection unit configured to detect a voltage proportional to an output current of the V/I conversion unit and output a burnout control signal when the detected voltage value is different from a normal value;
a burnout signal output unit configured to output a current outside the specified current range as a burnout signal instead of the output current of the V/I conversion unit when the burnout control signal is output. 2. The field device of claim 1,
At least the signal generating unit, a part of the V/I converting unit, and the abnormality detecting unit are mounted on an integrated circuit,
The V/I conversion unit is
a voltage-current conversion amplifier configured to convert the signal output from the signal generating unit into a current signal;
a transistor configured to convert a current signal output from the voltage-to-current conversion amplifier into an output current within the predetermined current range;
a resistor inserted in series in a loop of an output current flowing through the two-wire transmission line;
the voltage-to-current conversion amplifier of the V/I conversion unit is mounted on the integrated circuit, and the transistor and the resistor are provided outside the integrated circuit;
the abnormality detection unit detects a voltage generated across the resistor, the voltage being proportional to the output current;
The burnout signal output unit is
a first burnout signal output unit mounted on the integrated circuit;
a second burnout signal output section provided outside the integrated circuit as a redundant circuit having the same operation as the first burnout signal output section. 2. The field device of claim 1 ,
The V/I conversion unit is
a voltage-current conversion amplifier configured to convert the signal output from the signal generating unit into a current signal;
a transistor configured to convert the current signal output from the voltage-to-current conversion amplifier into an output current within the predetermined current range. 4. The field device according to claim 1,
the V/I conversion unit includes a resistor inserted in series in a loop of an output current flowing through the two-wire transmission line,
The field device is characterized in that the abnormality detection unit detects a voltage generated across the resistor that is proportional to the output current. 5. The field device according to claim 2 ,
The abnormality detection unit
A first A/D conversion unit configured to convert a voltage proportional to the output current into a digital value;
a judgment unit configured to compare a digital value of a normal value that a voltage proportional to the output current should take with an output value of the first A/D conversion unit, and output the burnout control signal when the values differ. 6. The field device according to claim 5,
the abnormality detection unit further includes a second A/D conversion unit configured to convert the analog output signal of the signal generation unit into a digital value;
the determination unit compares a digital value of a normal value that the analog output signal of the signal generating unit should have with the output value of the second A/D conversion unit, and outputs the burnout control signal when the values differ. 7. The field device according to claim 5,
A field device further comprising a CPU configured to output a digital value of the normal value to the abnormality detection unit based on a set value of an output current corresponding to data to be transmitted to a higher-level device.

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