JPH0532800B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention applies pressure, which is a process of various plants,
The present invention relates to a two-wire transmission device that transmits physical quantities such as flow rate and temperature.

In general, in various plants, it is necessary to accurately grasp changes in the state of each process from the viewpoint of operating the plant properly and producing products of desired quality. Therefore, in each plant, various process physical quantities (hereinafter referred to as measurable quantities) are collected from various parts of the plant, and for the purpose of managing each process, the collected measurable quantities are sent to a central control room in a remote location, etc. The data is transmitted to receiving units centrally installed in the area.

The means of transmitting these measured quantities is 4 mA based on international standards, which transmits measured quantities collected from various parts of the plant.
It is converted into an electrical signal of ~20mA and transmitted to a receiving unit such as a central control room through a cable.At this time, from the viewpoint of ease of construction and cost, two transmission lines are used to transmit the signal. Wire transmission equipment is used.

FIG. 1 is a diagram showing the configuration of such a conventional two-wire transmission device. This transmission device has a transmitter 2 that outputs a current signal corresponding to a measured quantity 1 of the process.
In this configuration, a receiving section 4 is connected via a wire type cable (transmission line) 3. This transmitter 2 includes a receiving section 4
It receives an amount of electricity from the side power supply section 5, changes the amount of electricity from the power supply section 5 according to the amount to be measured in the process,
This is transmitted to the receiving section 4 through the cable 3 as an analog measured quantity signal.

On the other hand, on the receiving part 4 side, the measured quantity signal sent from the transmitter 2 is converted into voltage by a current/voltage conversion resistor 6, and then sent to a receiving instrument 7, where it is converted into a digital signal. . Furthermore, this digital signal is converted into a process control signal by the control computer 8, and is used for purposes such as process control.

Therefore, the transmitter 2 having the above configuration
is configured such that the amount of electricity received from the power source section 5 is changed in accordance with the quantity to be measured 1 and is transmitted to the receiving section 4. Therefore, since the amount of electricity supplied to the transmitter 2 as the power source also serves as the signal transmission, the two transmission lines 3 are used to transmit an analog measurand signal corresponding to the measurand of the process. Moreover, the installation work is simple and can be realized at a very low cost.

Therefore, since it has such advantages, the transmitter 2 is widely used for process control in various plants. Furthermore, the transmitted signal format is unified, and consistency with the receiving side is high.

However, in this two-wire transmission device, the information to be transmitted is limited to the measured quantity 1, and the information necessary for managing the transmitter 2, such as the setting span and self-diagnosis information of the transmitter 2 (hereinafter referred to as There is a problem in that it is not possible to transmit additional information (referred to as additional information).

Therefore, when transmitting the analog measured quantity 1 and digital additional information, it is generally considered to send the measured quantity 1 and the additional information separately.

However, such data transmission means require separate installation of transmitting/receiving equipment, transmission lines, etc., making the configuration extremely complex, and inevitably increasing the size of the device. As a result, four transmission lines are used, which is not practical.

Therefore, it is required to transmit the measured quantity 1 and additional information by reducing the number of transmission lines as much as possible.
In this case, if one line is drawn from the transmitting side to the receiving side as a common line and the common line is used to switch between the measured quantity 1 and the additional information, 3
Although it becomes possible to receive the measured quantity 1 and additional information using a wire type transmission line, the advantages of a two-wire type transmitter cannot be utilized in this way.

If possible, use a two-wire transmitter to
A simple configuration for switching and transmitting information and additional information was desired.

However, if a two-wire transmitter is used to switch and transmit the measured quantity 1 and additional information, problems with measurement accuracy remain. It is conceivable to provide an information receiver and connect directly to the transmission line while switching between the measurand quantity transceiver and the additional information transceiver. It was also necessary to provide a switching means for switching between the measured quantity transceiver and the additional information transceiver.

It is also possible to transmit in synchronization, but this would require a separate synchronization signal line.
Furthermore, it is necessary to match the transceiver for the measured quantity and the transceiver for additional information, but this is extremely difficult.

In any case, the configuration is very complicated, and since the receiver 4 side receives the measurand signal during the transmission of the digital signal as additional information, the received measurand signal is There is a problem that errors occur in the measured quantity.

Therefore, the present invention has been made in view of the above-mentioned circumstances, and is a two-wire transmission system that can reliably transmit a measured quantity and any additional information with a simple configuration without sacrificing the advantages of a two-wire transmitter. The purpose is to provide equipment.

Another object of the present invention is to correct the measured quantity signal and the receiving side by correcting the measured quantity without sacrificing the advantages of the two-wire transmitter and while making effective use of digital additional information. The object of the present invention is to provide a two-wire transmission device that can transmit data while ensuring consistency, and can receive a measured quantity signal and additional information with high accuracy on the receiving side.

An embodiment of the present invention will be described below with reference to FIGS. 2 and 3.

This transmission device includes a current control circuit 12 that takes in the measurand signals 10 of each process sampled from various parts of the plant through a subtraction circuit 11, performs necessary signal control, and outputs the signal, and a set span of the transmission device. a time-series signal conversion circuit 14 that converts an additional information signal 13 such as a value into a time-series signal and outputs the signal;
a selection circuit 15 that selects the output of the current control circuit 12 and the output of the time-series signal conversion circuit 14 at an appropriately determined selection period; and a time-series signal or current control circuit 12 selected by the selection circuit 15. and a current output circuit 16 that sends out the measured quantity signal from the oscilloscope to the transmission line 17 as a current signal.

The output side of this current output circuit 16 is connected to the transmission line 17 as well as the current control circuit 12 as described above, and the current control circuit 12 and the current output circuit 16 constitute a feedback circuit. Further, the selection time of the selection circuit 15 is arbitrarily set by a timer circuit 18. When the selection circuit 15 receives a selection signal from the timer circuit 18 and selects the time-series signal conversion circuit 14 side, an operation timing signal is transmitted from the timer circuit 18 to the time-series signal conversion circuit 14 via the operation signal line 19. The time-series signal conversion circuit 14 starts operating for the first time.

This time series signal conversion circuit 14 includes a counting circuit 20.
is connected. This counting circuit 20 counts the digital signals that are the time series signals of the time series signal conversion circuit 14 at the time when the time series signal conversion circuit 14 starts operating. Here, the counted digital signals are held until the time series signal conversion circuit 14 starts operating based on the next selected time, and at the same time are averaged by the DA conversion circuit 21 at the next stage of the counting circuit 20. The signal is input to the negative terminal of the subtraction circuit 11 at the next stage. Therefore, this subtraction circuit 11 subtracts the averaged signal from the DA conversion circuit 21 from the measured quantity signal, and introduces the obtained subtraction output to the current control circuit 12.

On the other hand, on the receiving side 22, in addition to the receiving section 4 which receives the analog measured quantity signal sent through the current control circuit 12, selection circuit 15, current output circuit 16 and transmission line 17, the transmission line 17
A voltage drop element such as a current/voltage conversion resistor 6 and a high pass filter 23, for example, inserted in the
A digital signal receiving means consisting of a comparator 24 and the like is connected to receive the time series signal which is the additional information.

Therefore, this transmission device basically converts the measurand signal 10 obtained under feedback control and the time-series signal obtained by converting the additional information signal in a time-series manner from the timer circuit 18. The selection circuit 15 alternately selects and transmits the selected signals to the transmission line 17 based on the selection signal.

Next, the operation of the apparatus configured as described above will be explained with reference to FIG. 3. Now, selection circuit 1
5, if the time during which the time-series signal conversion circuit 14 is selected is T ₁ and the time during which the current control circuit 12 side is selected is T ₂ , one cycle of the selection circuit 15 is T.
is in the relationship T=T ₁ +T ₂ .

Therefore, first, upon receiving the selection signal from the timer circuit 18, the selection circuit 15 selects the time series signal conversion circuit 14 for the time _T1 . The series signal conversion circuit 14 starts operating, takes in the additional information signal 13, and converts it into a time series signal. This time-series signal is sent to the current output circuit 16 via the selection circuit 15, where it is converted into a current and sent to the transmission line 17.

At this time, the counting circuit 20 counts the digital signals, which are time-series signals, when the time-series signal conversion circuit 14 starts operating. This count value is the following D-
The A conversion circuit 21 converts the digital signal into an analog signal that is the average value of the digital signal during time _T1 . This analog signal is input to the negative terminal of the subtraction circuit 11. In other words, the time of the time series signal
The average value at T ₁ will be input to the negative terminal of the subtraction circuit 11. By the way, the counting circuit 20
The count value is held until the start of the next operation of the time-series signal conversion circuit 14, that is, for one cycle T of the selection circuit 15. Therefore, the average value of the digital signal from the analog signal from the A-D conversion circuit 21 is also input to the negative terminal of the subtraction circuit 11 during one period T.

Next, when the selection circuit 15 selects the current control circuit 12 in response to the selection signal from the timer circuit 18, since the measured quantity signal 10 has already been input to the positive terminal of the subtraction circuit 11, this subtraction circuit 11
Now, subtraction processing is performed from the measured quantity signal 10 using an analog signal that is the average value of the time series signal at time _T1 . Then, this subtracted output is the current control circuit 1
2 is sent to the current output circuit 16 via the selection circuit 15, and from the current output circuit 16 is sent to the transmission line 17 as a measured quantity signal. Thereafter, when the selection circuit 15 selects the time-series signal conversion circuit 14 side again, the counting circuit 20 starts counting anew, and the same process is repeated thereafter.

By the way, if the above-described correction process by subtraction is performed, the influence of the additional information signal on the measurand signal can be eliminated, and the selection circuit 15 on the receiving side 22
The average value of the received signal in one period T becomes proportional to only the measured quantity signal. The reason for this will be explained using the time chart of FIG. now,
The selection time of the additional information signal output from the time-series signal conversion circuit 14 is T ₁ , the selection time of the corrected measurand signal output from the current control circuit 12 is T ₂ ,
One period of the selection circuit 15 is T, the time series signal of the additional information is D ₁ , D ₂ , the measurand signal after correction is A ₁ , A ₂ , and the selection time T ₁ of the time series signal D ₁ is The average value is x ₁ , the average value at the selection time T ₂ of the time series signal D ₂ is x ₂ , the selection time T ₁ of the measured quantity signal 10
Let x ₀ be the average value of , then the receiving unit 4 performs the following receiving process.

That is, when receiving the measurand signal, the receiver 4 responds to the average value of the received signal over one period T, so that the additional information signal of each period T ₁ is different between D ₁ and D ₂ . In the case of content, the average value over one period T changes, which naturally causes problems. Therefore, the additional information signal D ₁ for each selection time T ₁ ,
The selection time for each average value x ₁ and x ₂ of D ₂
It is sufficient to correct the measurand signal at T ₂ . In other words, the average value x ₁ of the additional information signals D ₁ and D ₂ ,
Between x ₂ , the average value of D ₂ x ₂ is higher than the average value of D ₁ x ₁
higher than Therefore, it is only necessary to make the value of the measured quantity signal A ₁ higher than A ₂ by the difference between the average values x ₁ and x ₂ . In this way, each period
The difference in the average value of T ₁ is only the difference in the measured quantity signal.

Further, the current control circuit 12 includes a current output circuit 16
This constitutes a feedback circuit, which controls the measured quantity signal based on the output of the current output circuit 16 and then sends it to the current output circuit 16. In this way, the current output circuit 16 outputs a signal that is proportional to the measured quantity signal 10 within the internationally standardized lower limit of the transmission signal, such as 4 mA, and upper limit, such as 20 mA, and which is received by the receiver 4. A current that matches the measured quantity signal and the additional information signal is outputted and sent to the receiving side 22 through the transmission line 17. The receiving section 4 of the receiving side 22 receives the measured quantity signal and measures the averaged signal as in the conventional case.

Further, the additional information signal is converted into a voltage by the current/voltage conversion resistor 6, extracted by the high pass filter 23, and then sent to the comparator 2 in the subsequent stage.
4 will be introduced. In this comparator 24, the time-series signal conversion circuit 14 converts the signal back into a digital signal converted into a time-series signal. In this way, the measured quantity and additional information are obtained.

In addition, considering the effect that transmitting the additional information according to the present invention has on conventional receiving instruments, transmitters and receiving instruments used for process control are normally used for several seconds to avoid being affected by disturbances to the process. It has a low-pass characteristic with a time series of ~10 seconds. On the other hand, since the transmission speed of digital signals is approximately 1 Kbit/s to 1 Mbit/s, the transmission apparatus of the present invention temporarily transmits 10 bits of additional information at a transmission speed of 10 bits during the selection time _T1 of the time-series signal. When transmitting a 1K bit/second digital signal, the selection time _T1 is 10 milliseconds. Therefore, if the timer circuit 18 is set so that the selection time T ₂ on the current control circuit 12 side is sufficiently longer than T ₁ , the selection time T ₁ will be very short compared to one cycle T of the selection circuit 15. It's time.

Therefore, considering the low-pass characteristics of the receiving instrument, the receiving instrument does not respond to information sent in an extremely short period of time as described above, but responds to the time average value of the signal over the period T. Therefore, when receiving a measured quantity using the two-wire transmission device according to the present invention, the conventional receiving instrument itself does not require any changes.

Note that the present invention is not limited to the embodiments described above. For example, in the above embodiment, the amount of additional information is converted into a digital signal and transmitted, but instead of 0 and 1 of this digital signal, two types of frequencies corresponding to 0 and 1 are used to transmit the amount of additional information as a signal. Alternatively, it is possible to set frequencies corresponding to various values of the amount of additional information and transmit those frequencies as the amount of additional information, and further to correspond to the ratio of the pulse widths of two types of pulses for the various values of the amount of additional information. Let me,
It is also possible to transmit two types of pulses with different pulse width ratios depending on the amount of additional information. Further, the content of the additional information is not limited to the span setting value and self-diagnosis information of the transmission device, but also, for example, the amount to be measured can be converted into a digital signal and transmitted as additional information. By doing this, the receiving instrument can:
Originally, the measured quantity is received as an analog signal, but
If you collect additional information to know the exact value,
By obtaining the measured quantity converted into a digital signal, it is possible to accurately grasp the measured quantity.

Therefore, according to the above configuration, the following various effects can be achieved.

First, according to the invention of claim 1,
On the transmitting side, the analog measured quantity and digital additional information are selectively switched at an appropriately determined selection cycle and transmitted to the two-wire transmission line, without sacrificing the advantages of the two-wire transmitter. , it is possible to reliably transmit the measured quantity and any additional information with a simple configuration.

Incidentally, in the case where an analog signal generator and a pulse signal generator are connected in series without employing a switching means as in the present invention, and a pulse signal is superimposed on the analog DC signal and transmitted. When one signal generator is abnormal or malfunctions, the other signal generator also becomes abnormal or malfunctions at the same time, and the amplitude of the pulse signal is the difference between the pulse signal and the noise level when the analog DC signal level is low. Nozzle resistance decreases.

In contrast, the device of the present invention employs a switching means, so even if one signal generation system is abnormal or malfunctions, the other signal generation system can operate normally, and it does not depend on the measured quantity. Since the additional information is transmitted independently without any interference, noise resistance can be improved. Furthermore, the device of the present invention can reliably separate and receive the measured quantity and additional information on the receiving side, and since the analog signal receiving means and the digital signal receiving means are connected in parallel to the transmission line, the transmitting side Similarly to the above case, an abnormality or failure of one receiving means does not affect the other receiving means.

Next, the invention of claim 2 corrects the measurand quantity without sacrificing the advantages of the two-wire transmitter and makes effective use of digital additional information. It is possible to transmit data while ensuring consistency with the data.

Further, the invention of claim 2 is an improvement on the invention of claim 1 from the aspect of measurement accuracy. In other words, the invention of claim 2 is effective when a digital signal is added for a short time to the analog signal that is the measured quantity, whereas the invention of claim 1 is Even when digital signals are added over a long period of time, measurement accuracy is not impaired.

The reason for this is that since errors caused by the transmission and reception of digital signals are corrected when outputting analog signals, it is possible to transmit and receive information under test without any errors caused by digital signals, and as a whole, it is possible to accurately match the signal under measurement. Additional information can be received.

Therefore, in the invention of claims 1 and 2, the set span value and self-diagnosis information of the transmitter installed at the plant site can be obtained in an extremely short time, along with the measured quantities of processes in various plants. This allows the far-away receiving side to manage the current status of the transmission equipment, and this allows the receiving side to manage the current status of the transmission equipment. , it is possible to realize more reliable process control and ensure safe and proper operation of the plant. Furthermore, even though the amount of information to be transmitted has increased, it can be done using two transmission lines, so the ease of installing signal lines and the low cost of construction are not compromised. Furthermore, since the additional information does not have any influence on the signal of the measured quantity,
It has excellent compatibility in that it can be adapted to internationally standardized two-wire transmission signal formats.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a transmission device using a conventional two-wire transmitter, FIG. 2 is a block diagram showing an embodiment of a two-wire transmission device according to the present invention, and FIG. 3 is a time chart showing the relationship between the measured quantity signal and the additional information signal in the selection cycle of the selection circuit. 4... Receiving section, 6... Voltage drop element (current/voltage conversion resistor), 10... Measured quantity signal, 11... Correction means (subtraction circuit), 12... Current control circuit, 13...
Additional information signal, 14... Time series signal conversion circuit, 15
... selection circuit, 16 ... current output circuit, 17 ... transmission line, 18 ... timer circuit, 20, 21 ... averaging means (20: counting circuit, 21: D/A conversion circuit),
22...Receiving side, 23, 24...Digital signal receiving means.

Claims (1)

[Claims] 1. A two-wire system that changes the amount of electricity supplied from a power supply according to the quantity to be measured in the process, and sends out the information to be measured as an analog signal corresponding to the quantity to be measured to two transmission lines. The transmission device includes a digital signal generating means for outputting additional information regarding the transmission device as a time-series digital signal, and selectively sending the digital signal and the analog signal to the transmission line at an appropriately determined selection period. a voltage drop element inserted into the transmission line; digital signal extraction means for extracting only the digital signal from among the signals input from the transmission line through the voltage drop element; a digital signal receiving means for receiving a digital signal taken out by the means; and an analog signal receiving means for receiving the analog signal inputted through the transmission line, and receiving the digital signal and the analog signal. Characteristic 2-wire transmission device. 2. In a two-wire transmission device that changes the amount of electricity supplied from a power source according to a measured quantity of a process and sends measured information to a two-wire transmission line as an analog signal corresponding to the measured quantity, the transmission digital signal generating means for outputting additional information regarding the device as a time-series digital signal; averaging means for averaging the digital signal generated by the digital signal generating means over its output time; and the analog signal. a selection means for selectively sending out the digital signal and the corrected analog signal to the transmission line at an appropriately determined selection period; a voltage drop element inserted at the other end of the line; digital signal receiving means for receiving the digital signal input from the transmission line through the voltage drop element; and receiving the analog signal input through the transmission line. 1. A two-wire transmission device, comprising: analog signal receiving means, and receiving the digital signal and the analog signal.