JPH10149495A - Virtual field unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a virtual field unit which can operate a field by system while the unit of an existed system which cannot correspond to a field bus system is continued. SOLUTION: The virtual field unit connected to the field bus system is provided with a digital signal connection terminal connected to a transmission line, plural analog signal connection terminals where the input/output signals of analog signal-type field units 1a, 1b and 1c are transmitted/received and a signal conversion means which mutually converts signals containing the analog signals transmitted/received from the plural analog signal connection terminals for communication signals transmitted/received from the digital signal connection terminals. Thus, the system can move to the field bus system while the field unit which does not correspond to the field bus used in the existed control system is used as it is.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a field device for transmitting a signal on a two-wire transmission line, and more particularly to a field bus system in which the system can be easily transferred to a field bus and the system can be easily expanded.

[0002]

2. Description of the Related Art Devices called so-called field devices detect physical quantities such as pressure, temperature, and flow rate of various plants, convert the values into electric signals, and transmit the signals to a host device via a transmission line. On the contrary, it is normal to receive a control signal transmitted from a higher-level device and control a valve or the like of a plant.

The transmission of the electric signal is standardized when the signal is an analog signal, and an analog current signal of 4 to 20 mA is transmitted between the field device and the host device. . In general, one-way communication using analog signals is performed between a field device and a higher-level device.

However, in recent years, with the improvement of semiconductor integrated circuit technology, field devices incorporating a microprocessor have been developed and put into practical use. According to this, in addition to one-way analog signal communication on the transmission path, two-way digital signal communication is performed, and it is possible to remotely instruct field range setting, self-diagnosis, and the like. . This type of apparatus is disclosed, for example, in Japanese Patent Application Laid-Open No. 58-48198.
And Japanese Patent Application Laid-Open No. 59-201535.

A specific example will be described with reference to FIG. FIG. 1 shows an example of a device configuration relating to an analog current output type field device requiring an external power supply. The field devices 1a, 1b, and 1c operate with the power supplied from the external power supply 4, output an analog current signal as a constant current source that causes a current corresponding to the detected physical quantity to flow through the transmission path, and the upper receiving device 3 The analog current signal (hereinafter, referred to as an analog signal) flowing through a resistor inserted in series in a path is detected by detecting a potential difference between both ends of the resistor, and is used as an indicated value of the field devices 1a, 1b, 1c. The upper communication device 2 is connected between the field devices 1a, 1b, 1c and the higher device 3 and the external power supply 4, and performs bidirectional communication with the field device 1 by digital signals.

As a method of transmitting a signal through this transmission line, a method of placing a digital signal on an analog signal and communicating the digital signal so as not to affect the analog signal, and switching between an analog signal and a digital signal. A signal transmission method using only digital signals and a signal transmission method using only digital signals are known.

Recently, a field bus system has been proposed as a system in which a plurality of field devices are connected on the same transmission path and communication is performed using only bidirectional digital signals. A typical configuration example of the field bus system will be described with reference to FIG. FIG. 1 shows an example of a device configuration in which a plurality of field devices and a host device are connected in a tree shape via a transmission path. Field device 1
a ', 1b', and 1c 'operate by power supplied from an external power supply 4' via a transmission line 5 ', and sequentially transmit digital signals to the host device 3' via the transmission line 5 '. And performs processing such as transmission of the detected physical quantity and reception of the control value. The upper communication device 2 ′ includes field devices 1 a ′, 1 b ′ and 1 c ′, a higher device 3 ′, and an external power supply 4 ′.
And the field devices 1a ', 1b', 1
Two-way communication is performed with digital signals such as c '. The terminator 7 includes a resistor and a capacitor connected in series, and is connected to both ends of the transmission path 5 '.
In this system, since only digital signals are used for communication, the devices connected to the transmission line 5 'can communicate with each other, and a network must be constructed via the transmission line 5'. Is possible.

When shifting from the existing system of FIG. 4 to the field bus system of FIG. 5, all devices connected to the transmission line 5 need to be changed to those corresponding to the communication mode of the field bus system. However, since the transmission line 5 can be used as it is, the system can be easily shifted, and the number of field devices connected on the transmission line 5 can be increased, so that the system can be easily expanded. Have been.

[0009] Regarding system migration, Japanese Patent Laid-Open No.
A method as disclosed in JP 54599 has also been proposed.
In this example, each of the field devices is provided with an adapter for converting a communication signal into a digital signal.
The system form remains the analog type system of FIG.
It is intended to perform digital two-way communication.

[0010]

However, in the prior art described above, when the existing system is switched to the fieldbus system, it is necessary to change all the components except the transmission path at once.

For this reason, when attempting to use the fieldbus system, there is a problem that excessive equipment costs are required at the beginning. In particular, it is not possible to temporarily use the field devices used in the existing system as it is, and to gradually shift to the fieldbus system.Therefore, all the field devices must be changed. Since equipment costs are unavoidable, it has been difficult to apply them to existing systems in terms of cost performance.

In the example of Japanese Patent Application Laid-Open No. 2-254599,
Although the communication between the field device and the higher-level device can be performed by bidirectional communication using digital signals, the transmission of the analog signal is also performed. Therefore, the connection between the field device and the higher-level device is always a one-to-one connection. Such a form of the field bus system cannot be easily shifted. Therefore, a network cannot be constructed between field devices,
Also, the number of field devices could not be easily increased, and there was difficulty in expandability of the system.

[0013] An object of the present invention is to keep existing system equipment which cannot cope with a fieldbus system,
It is an object of the present invention to provide a virtual field device capable of operating a fieldbus system.

[0014]

A feature of the present invention to achieve the above object is to provide a method for detecting a physical quantity on the field side and / or outputting a manipulated variable via a transmission line composed of a pair of transmission lines. A virtual field device connected to a fieldbus system that communicates by digital signals between the field device and a higher-level device that monitors and / or operates the field device. A digital signal connection terminal for connecting to a channel, a plurality of analog signal connection terminals for transmitting / receiving analog signal type field device input / output signals, and a signal including an analog signal transmitted / received from the plurality of analog signal connection terminals. Signal conversion means for mutually converting communication signals transmitted and received from the digital signal connection terminal. It is to be.

In the case of an analog current output type field device, an output signal of the analog current signal of 4 to 20 mA is converted into a digital signal in a virtual field device according to the present invention, and is virtually transmitted to a transmission line. . Conversely, in the case of an analog current input type field device, the input signal of the analog current signal of 4 to 20 mA is
It is virtually received in the virtual field device according to the present proposal, converted from a digital signal to an analog current signal, and output to the field device.

[0016] This makes it possible to shift to a fieldbus system while using existing analog signal type field devices, so that at least only the upper device and an external power supply are made compatible with the fieldbus system. The system can be realized.

Furthermore, when a field device is exchanged for a field bus, the connection is only changed directly to the connection from the transmission line. Therefore, the field device can be gradually shifted to the field bus system as needed.

[0018]

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows a field bus using the present invention.
FIG. 2 is a device configuration diagram of the system.

In FIG. 1, field devices 1a, 1
b and 1c indicate that the output signal (or input signal) is 4 to 20
It is an analog current signal of mA and can carry out bidirectional communication by putting a digital signal on the signal. It detects physical quantities such as process pressure, temperature and flow rate in various plants and transmits the values. Alternatively, it receives a control amount of a valve or the like. The virtual field device 6 is operated by the power supplied from the external power supply 4 via the transmission line 5, and the transmission line 5 and the field devices 1a, 1
b, 1c. In this embodiment, the virtual field device 6 is connected to the junction on the field side.
An example of installation in a box (relay box) is shown below.
For example, it can be arranged on a higher-level device side.

The virtual field device 6 supplies electric power to the field devices 1a, 1b, 1c by using the transmission lines 5a, 5c.
b, 5c and the field devices 1a,
1b and 1c are received and converted into digital signals, and the values are virtually used as field devices 1a, 1b,
Communication is performed on the fieldbus system as an output signal of 1c. Further, the field devices 1a, 1b, 1
When c is for carrying out bidirectional communication by carrying the analog current signal and the digital signal, the virtual field device 6 establishes transmission lines 5a, 5b, 5c between the field devices 1a, 1b, 1c. Not only analog signals but also digital signals are communicated via the interface, and plays a role of converting a communication protocol between the field bus and the field devices 1a, 1b, 1c. Field device 1a, 1
When b and 1c are only the analog current signals, the virtual field device 6 operates to virtually substitute for the field devices 1a, 1b and 1c with respect to the field bus. For example, when the field device 1c is changed to be compatible with the fieldbus, or a new fieldbus-compatible field device is added and the system is changed or expanded, the fieldbus-compatible field device 1d is
A device connected to the transmission path 5 for the field bus, directly supplied with operating power from the external power supply 4 through the transmission line 5, and connected to the field bus (upper device 3,
Communication with the host communication device 2, etc.). Here, the power consumed by the virtual field device 6 is
And the power consumption of the field devices 1a, 1b, 1c. Therefore, the field device 1
If the power consumed by the virtual field device 6 when the connection such as c is disconnected is controlled, the value of the operating power supplied from the external power supply 4 through the transmission path 5 can be obtained even if the field device 1d corresponding to the field bus is added. , Does not change much.

The higher-level device 3 communicates digital signals with fieldbus-compatible devices such as the virtual field device 6, the field device 1d, and the higher-level communication device 2 via the transmission path 5, and detects various physical quantities (detected by the field device). Pressure, temperature, flow rate, etc.), and a control signal is transmitted to field devices such as valves as control information of the plant.

The upper communicator 2 can be connected to an arbitrary location on the transmission path 5 and operates a display and a keyboard in the upper communicator 2 so as to operate the virtual field devices 6.
Processing such as monitoring and adjustment of the output value of the field device 1d is performed by communicating via the transmission path 5.

Next, the operation of the virtual field device 6 will be described in detail with reference to FIGS. FIG. 2 shows an example of a case where the existing field devices 8a to 8n to be connected are of the analog signal output type and carry out bidirectional communication by carrying digital signals. FIG.
This is an example of a case where the existing field devices 9a to 9n to be connected are of the analog signal input type and perform bidirectional communication by carrying digital signals.

In the present embodiment, only the case where the existing field device to be connected is of the analog signal type and carries out bidirectional communication by carrying a digital signal will be described. In this case, the virtual field device 6 performs only the substitute process virtually, and the fieldbus side regards them all as the same.

In FIG. 2, a DC-DC converter 60
Reference numeral 9 denotes a voltage V for operating the virtual field device 6 itself from a voltage applied from an external power supply via the transmission line.
_DD and non-fieldbus compatible field devices 8
a to 8n, the analog signal receiver 6
, 612n, 12a, 612b,.
Eb, ..., En are created. Here, the DC-DC converter 609 controls the power supplied via the field bus 5 to be constant. The value of this power changes according to the number of connected analog signal type field devices 8a to 8n.

The analog signal receivers 612a, 612b,
, 612n are a power supply for supplying power to the field devices 8a to 8n that are not compatible with the field bus, and 4 to 20 mA as output signals of the field devices 8a to 8n.
The output signal is input to the multiplexer 610 as an analog signal of 1 to 5 V, for example. In the multiplexer 610, the signal is taken into the A / D converter 604 while being sequentially switched according to a command from the I / O interface 605, converted into a digital signal, and stored in the RAM 602. These series of processes are performed by the microprocessor 601 based on data programmed in the ROM 603.
Is controlled by

Further, the modems 613a, 613b,.
Reference numeral 613n denotes an interface circuit for performing bidirectional communication of digital signals by superimposing on the analog signal of a connected existing field device or switching the analog signal. The digital signal is output from the transmission / reception circuit 607 to the corresponding modulator / demodulator 6.
13a, 613b,..., 613n
The modems 613a and 613 selected by the multiplexer 611 according to a command from the I / O interface 605.
, 613n are output as modulated signals. The signals output from the modulators / demodulators 613a, 613b,..., 613n include, for example, two types of frequency signals corresponding to digital signals “1” and “0” as in frequency modulation, and signal as in amplitude modulation. The size of "1",
One corresponding to “0”, one corresponding to two types of phases such as phase modulation, corresponding to “1” and “0”, and the like are used. Conversely, the connected existing field devices 8a to 8n
Is output from the modem 613
a, 613b,..., 613n.
7 is received.

Here, the modulation signal used in the communication between the existing analog signal type field devices 8a to 8n and the virtual field device 6 is a small square or sine wave signal having the same amplitude in the positive and negative directions. , The digital signal is output and superimposed on the analog signal for communication, whereby the analog signal receivers 612a, 612b,.
612n may be left unaffected.

The communication between the existing analog signal type field devices 8a to 8n and the virtual field device 6 is as follows.
Basically, it is executed only when it becomes necessary to communicate with the analog signal type field devices 8a to 8n by communication with the field bus 5, and is not normally used. At this time, the virtual field device 6 is connected to the field bus transmission line 5 and the existing analog signal type field devices 8a to 8a.
n, and plays a role of protocol conversion.

Similarly, in the communication with the transmission path 5 of the field bus, the modulator / demodulator 608 is an interface circuit for performing bidirectional communication of digital signals, and the transmission / reception circuit 60
7 is subjected to encoding processing such as Manchester code, for example, and output to the field bus. Further, a reception signal from the field bus is decoded by the modem 608 and input to the transmission / reception circuit 607.

In such a configuration, the virtual field device 6 does not have a unique address on the transmission path 5 of the field bus, but stores the addresses of the plurality of connected analog signal type field devices 8a to 8n in the RAM 602. If it is stored and matches the address in the communication data from the transmission path 5 of the field bus, the virtual field device 6 virtually processes the analog signal type field devices 8a to 8n. Here, each analog signal type field device 8a
８8n are output to the A / D converter 604 at regular intervals.
/ D converted and stored in the RAM 602, and the setting values of the analog signal type field devices 8a to 8n are also stored in the RAM 602 in advance through communication with the fieldbus transmission line 5, so that the When transmission of an output value is requested from the transmission line 5 of the bus, the virtual field device 6 performs an alternative process without communicating with the analog signal type field devices 8a to 8n. If it is necessary to communicate with each of the analog signal type field devices 8a to 8n in response to a request from the field bus transmission line 5, as described above, the virtual field device 6 Analog signal type field devices 8a to 8
and n performs a protocol conversion.

Next, referring to FIG.
Device that is not compatible with fieldbus 9
An embodiment in which a to 9n are analog signal input types will be described. As in the embodiment of FIG. 2, the DC-DC converter 609 uses the voltage V for operating the virtual field device 6 itself from the voltage applied from the external power supply via the transmission line.
_DD , and a voltage E supplied to the analog signal transmitters 615a, 615b,..., 615n in order to supply power to the field devices 9a to 9n which are not compatible with the field bus.
a ′, Eb ′,..., En ′. The analog signal transmitters 615a, 615b,..., 615n include a power supply for supplying power to the field devices 9a to 9n that are not compatible with the field bus, and a 4 to 20 mA analog current signal which is an input signal of the field devices 9a to 9n. , And the input signal is, for example,
An analog signal of 1 to 5 V is input from the D / A converter 614 via the multiplexer 610. The switching signal of the multiplexer 610 is transmitted to the microprocessor 60
The instruction is input from the I / O interface 605 in response to the instruction 1. The analog signal transmitters 615a, 615b,
, 615n incorporates a sample-hold circuit and a damping circuit, and the input value from the D / A converter 614 is updated when there is an update request from the field bus.

In this embodiment, the communication with the analog signal input type field devices 9a to 9n and the communication with the field bus transmission line 5 are all the same as those described in the embodiment of FIG. .

FIG. 2 shows an example in which the existing field devices to be connected are of the analog signal output type, and FIG. 3 shows an example of the analog signal input type. can do.

According to this embodiment, there is an effect that existing analog signal type field devices and field devices compatible with the field bus can coexist on the same control system. In addition, with only the minimum necessary changes,
There is an effect that the system can be realized.

[0037]

As is apparent from the above description, by installing a virtual field device on the field bus for performing a process of replacing the existing field device, it is not compatible with the field bus used in the existing control system. While using the field device,
The effect is that the system can be migrated.

Further, when replacing a device which is not compatible with the field bus with a device which is compatible with the field bus,
By simply changing the connection, replacement can be easily performed, and replacement with a fieldbus compatible device can be performed without changing the settings of other devices on the fieldbus, so that the control system can be gradually shifted There is also an effect.

[Brief description of the drawings]

FIG. 1 is an apparatus configuration diagram showing one embodiment of a field bus system according to the present invention.

FIG. 2 is a block diagram showing one embodiment of a virtual field device according to the present invention.

FIG. 3 is a block diagram showing another embodiment of the virtual field device according to the present invention.

FIG. 4 is a diagram showing an example of a device configuration of a conventional field device.

FIG. 5 is a diagram showing an example of a field bus system.

[Explanation of symbols]

1a, 1b, 1c ... analog signal type field device, 1
a ', 1b', 1c ', 1d: Fieldbus-compatible field devices, 2: High-order communication device, 3, 3': High-order device,
4, 4 '... external power supply, 5, 5a, 5b, 5c ... transmission line,
6 virtual field device, 7 terminator, 8a-8
n: Analog signal output type field device, 9a to 9n ...
Analog signal input type field device.

Claims (3)

[Claims] 1. A plurality of field devices for detecting a physical quantity on the field side and / or outputting a manipulated value via a transmission path composed of a pair of transmission lines, and a host for monitoring and / or operating the field device. What is claimed is: 1. A virtual field device connected to a fieldbus system for performing communication by digital signals between devices, the virtual field device comprising: a digital signal connection terminal for connecting to the transmission line; and an analog signal type field device. A plurality of analog signal connection terminals through which input / output signals are transmitted and received, and signals including analog signals transmitted and received through the plurality of analog signal connection terminals, respectively, with respect to communication signals transmitted and received through the digital signal connection terminals A virtual field device comprising: signal conversion means for performing mutual conversion. 2. A storage means for storing a communication signal corresponding to each analog signal connection terminal, and controlling communication between the contents stored in the storage means and the digital signal connection terminal. A virtual field device further comprising communication control means. 3. The apparatus according to claim 2, wherein said storage means stores an address indicating a device connected to said analog signal connection terminal, and said control means stores said address in accordance with an address designation from said digital signal connection terminal. A virtual field device for reading or writing a digital signal of a corresponding device stored in a storage means.