Field instrument system

Abstract: A communicator (2) for field instruments (1) connected to a transmission line (5) for transmitting electrical signals from the field instruments (1) to a host instrument (3) operates on electric power fed from an external supply (4) over the transmission line (5). The external power supply (4) is arranged in the transmission line (5). Electric current consumed by the transmission line (5) is set to a constant value.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to a field instrument system having a communicator which is connected to a transmission line which connects field instruments to a host instrument. DESCRIPTION OF THE RELATED ART Instruments known as field instruments have a great variety of sensors incorporated in them, and measure physical quantities, such as pressure, temperature, and flow rate in various plants. They transmit such physical measurements to a host instrument over a transmission line, after having converted the physical quantities into electric signals. The transmission of these electric signals has been standardized. The field instruments output analog current signals of 4-20 mA to the transmission line, and the host instrument receives the analog current signals. The analog signals are transmitted from the field instruments to the host instrument in a one-way communication. Because of improvements in the technique of manufacturing semiconductor ICs, field instruments incorporating microprocessors have been developed and put into practical use in recent years. The field instrument performs two-way communication in digital signals, rather than one-way communication in analog signals as over the above-mentioned transmission line, and is capable of performing processes such as range setting and self-diagnosis of the field instrument even from a remote place. The field instrument also communicates with a communicator exclusively in digital signals, this communicator being connected to any place along the transmission line. Devices of this type are disclosed, for example, in Japanese Patent Laid-Open No. 59-201535, in EP-A-219120, EP-A-212 897 and EP-A-244 808. In the conventional examples mentioned above, as a method of transmitting signals over the transmission line, digital signals are carried on analog signals for simultaneous communication. In addition to this method, there is a method in which analog signals are switched over to digital signals for communication. There is also a method in which communication is performed solely in digital signals. In these conventional examples, the communicator has a built-in power supply such as a battery, and is constructed so as to operate all the built-in circuits on the electric power fed from the built-in power supply. For this reason, it is required to carry out maintenance, such as replacing or charging the built-in battery, after the built-in battery has been used for a predetermined period. The above-described conventional art, however, does not take the case into consideration where the communicator is continuously used for a long period of time for troubleshooting the field instrument or the like. In other words, there is a problem in that since the service time of the power supply incorporated in the communicator is limited, it is impossible to continuously monitor values, such as output values and internal status of the field instrument for prolonged periods. Furthermore, the communicator is not always utilized in an instrument room, but may also be connected to any place along the transmission line for outdoor use. In such a case, when the capacity of the built-in power supply runs out during its service, the built-in power supply must be replaced or charged. This leads to a problem in that maintenance, such as replacing or charging the built-in power supply, becomes troublesome. EP-A-244 808 mentions the possibility of the communicator being operated on electric power drawn from the transmission line. But no specific embodiment is disclosed. Such embodiment would entail that the communicator is no longer freely connectable to or disconnectable from the transmission line without disturbing communication on the line because the physical quantity measured by the field instrument is transmitted to the host instrument in the form of a current value, i.e. as an analog signal. The analog signal may be falsified. SUMMARY OF THE INVENTION It is an object of the invention to provide a field instrument system including a communicator which is convenient to use over prolonged time periods. This object is solved by the field instrument system of each of claims 1 and 2. In one embodiment of the invention, the communicator is connected in parallel to a two-wired transmission line for transmitting electric signals from the field instruments to a host instrument, and operates on electric power fed from an external power supply over the transmission line. In another embodiment, the communicator is connected to the ends of a voltage drop element arranged in series in a transmission line which connects the field instruments to a host instrument, and the communicator operates on electric power fed from an external power supply over the transmission line. In another embodiment, the communicator is connected in series at any place along one of two wires of the transmission line which connects the field instruments to a host instrument, and the communicator operates on electric power fed from an external power supply over the transmission line. The present invention is applicable to a plant monitoring system comprising: a field instrument for measuring physical quantities of a plant; a host instrument for receiving detected signals from the field instrument over a transmission line; a communicator for performing communication with the field instrument and the host instrument; a host controller for controlling the plant based on signals from the host instrument; and a power supply arranged in the transmission line so as to operate the communicator. The present invention is further applicable to a plant monitoring system connected in parallel to a commonly used transmission line, comprising: a plurality of field instruments for measuring physical quantities of a plant; a host instrument for receiving detected signals from the field instruments over the transmission line; a communicator for performing communication with the field instruments and the host instrument; a host controller for controlling the plant based on signals from the host instrument; and a power supply arranged in the transmission line so as to operate the communicator. When communication is performed among the field instruments, the host instrument, and the communicator, electric power to operate the communicator is fed from the transmission line. The field instruments connected to the transmission line are fed with electric power from the external power supply, and are operated on the electric power. For this reason, a constant amount of electric current always passes over the transmission line. When the field instruments communicate with the host instrument, they alter the electric current passing over the transmission line in order to transmit digital signals. This alteration is performed by altering the electric current consumed by the field instruments. The host instrument detects not only alterations in the voltage between the ends of a load resistor connected in series to the transmission line, but also alterations in the voltage between the ends of the transmission line in order to receive the digital signals. When the communicator constructed above is connected to the transmission line, the absolute value of the electric current passing over the transmission line remains altered. If, however, the electric current which the communicator consumes is constant, an alteration in the electric current passing over the transmission line occurs only once. The field instruments connected to the transmission line will not thus erroneously receive digital signals due to that alteration. When the communicator is also engaged in communication, it operates in the same manner as when the field instruments are engaged in communication, so that there is no problem in communication. Moreover, when the voltage drop element is connected in series to the transmission line, and the communicator is then connected to the ends of the voltage drop element, a part of the electric current passing over the transmission line flows to the communicator, thereby allowing the communicator to operate. In addition, even when the communicator is connected in series at any place along one wire of the transmission line, the electric current passing over the transmission line flows to the communicator, thereby also allowing the communicator to operate. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an inner block diagram showing an embodiment of a communicator according to the present invention; Fig. 2 is a view showing the configuration of a communication system unit to which the communicator illustrated in Fig. 1 is connected; Fig. 3 is a view showing the configuration of another communication system unit to which the communicator is connected; Fig. 4 is an inner block diagram showing another embodiment of the communicator according to the present invention; and Fig. 5 is a view illustrating the configuration of a communication system unit to which the communicator shown in Fig. 4 is connected. DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will now be described with reference to the drawings. Fig. 1 is an inner block diagram of a communicator according to the present invention, and Fig. 2 is a view showing the configuration of a communication system unit to which the communicator shown in Fig. 1 is connected. In Fig. 2, field instruments 1 measure, by means of a built-in sensor, physical quantities such as pressure, flow rate, and temperature in various plants. The field instruments 1 operate on the electric power fed from an external power supply 4 arranged in a transmission line 5, and output signals corresponding to the physical quantities. This output is performed by a communication means over the transmission line 5. The communicator 2 has a communication function incorporated in it, and is connected between the field instruments 1 along the transmission line 5 and a host receiving instrument 3 as well as the external power supply 4 in order to communicate with the field instruments 1 in the form of digital signals. The communicator 2 performs processes, such as monitoring and calibrating I/O signals to and from the field instruments 1. The host receiving instrument 3 has a communication function incorporated in it, and receives the physical quantity data which the field instruments 1 measure so as to send the physical quantity data to an unillustrated host controller. This reception is carried out by a communication means over the transmission line 5. The host receiving instrument 3 also communicates with the field instruments 1 to perform processes, such as self-diagnosis and modification to a measurement range. The communicator 2 is detachably attached to any positions along the transmission line 5, and operates, in the same manner as with the field instruments 1, on the electric power fed from the external power supply 4 over the transmission line 5. When the communicator 2 is connected, the electric current "i" passing over the transmission line 5 is the sum of the electric current (i 1 + i 2 + i 3 + .... i n ) which the field instruments 1 consume and the electric current (i c ) which the communicator 2 consumes. When there is no communication performed, this electric current "i" assumes a constant value. For the above reason, when there is no communication performed, the voltage between the ends of the transmission line 5 is the voltage at which the amount proportional to voltage drop (i X R L ) in the host receiving instrument 3 is subtracted from the voltage of the external power supply 4. The voltage between the ends of the transmission line 5 thus becomes a constant value. To perform communication, the field instruments 1 and the communicator 2 alter, in correspondence to communication data, the respective electric current consumption mentioned above, thereby altering the electric current "i" passing over the transmission line 5. Since the voltage between the ends of the transmission line 5 is accordingly altered, the respective devices receive the communication data by detecting alterations in the voltage between the ends of the transmission line 5. The host receiving instrument 3 transmits signals by altering the impedance in a load resistor R L , and detects alterations in the electric current "i" passing through the load resistor R L in order to receive signals. When the communicator 2 is removed from the transmission line 5, the electric current passing over the transmission line 5 is altered. This alteration is, however, not recognized as communication data, so that it does not affect the communication system, so long as the communicator 2 is not removed during communication. Should the communicator 2 be removed from the transmission line 5 even during communication, communication data may be erroneously received. Effect on electric current values, however, can be prevented by carrying out a process such as a retry process, because the communication system is affected only the moment at which the communicator 2 is removed. The detailed operation of the communicator 2 will be described hereinafter with reference to Fig. 1. Inside the communicator 2, a microprocessor (MPU) 202 controls the entire operation of the communicator 2 by means of programs stored in a ROM 204. An input device 208 is composed of a keyboard or the like. When the user inputs information using the keys defined, the input information is transmitted to the microprocessor (MPU) 202 via an I/O interface 206. The microprocessor (MPU) 202 outputs as required a command for communication to a transmitting and receiving circuit (UART) 205, and this command is transmitted to a V/I converter through a modulation circuit 210. The V/I converter sends an electric current corresponding to an input signal to the transmission line 5, and this input signal becomes a transmission signal. If the output signal from the modulation circuit 210 is the same amplitude wave, sine wave or the like in the positive and negative directions, even during communication the electric current which the communicator 2 consumes assumes an approximately constant value with a momentary alteration in the electric current. A response signal from the field instruments 1, which have received the transmission signal, is demodulated in the form of digital signals due to the fact that demodulation circuit 209 detects alterations in the voltage between the ends of the transmission line 5. The response signal is then sent to the microprocessor 202 through the transmitting and receiving circuit (UART) 205. The microprocessor 202 displays the response signal, together with the data stored in a RAM 203, on a display device 207 via the I/O interface 206. Those inner circuits in the communicator 2 operate on the electric power fed from a DC-DC converter 201 over the transmission line 5. At the voltage between the ends of the transmission 5, the DC-DC converter 201 generates voltage (E) capable of operating the respective circuits mentioned above, and feeds the voltage (E) to all the circuits. A constant-current circuit 212 operates so that the electric current, consumed by the inner circuits except the electric current which the V/I converter in the communicator 2 outputs, may always assume a constant value (ic). For this reason, no alteration in the electric current values in any except the electric current which is output as a transmitting signal during communication, occurs in the entire communicator 2. When the communicator 2 is not engaged in communication, the communication of the other devices in the transmission line 5 is therefore not affected. The communicator 2 shown in Fig. 1 may also be used in the system configuration shown in Fig. 3, other than in the system configuration illustrated in Fig. 2. In Fig. 3, the communicator 2 is connected to the ends of a voltage drop element 6. The inner circuits of the communicator 2 operate on part of the electric current "i" passing over the transmission line 5. With such a configuration, it is possible to minimize communication errors when the communicator 2 is connected to the ends of the voltage drop element 6. Fig. 4 illustrates another embodiment of the present invention, and Fig. 5 illustrates an example of the system configuration of the embodiment in Fig. 4. In Fig. 5, the communicator 2 is connected in series to the loop of the transmission line 5, and the inner circuits of the communicator 2 operate on part of the electric current "i" passing over the transmission line 5. When the communicator 2 is connected to the transmission line 5, because it is arranged as a part of the loop of the transmission line 5, a voltage drop occurs in the voltage between the ends of the transmission line 5. However, when the circuits of the communicator 2 are arranged so as to operate by a constant-voltage input so that the voltage drop value may be kept constant, communication is not affected. For the above reason, in the communicator 2 shown in Fig. 4, the voltage on the input side of the DC-DC converter 201 must remain constant. The operation inside the communicator 2 of Fig. 4 is the same as that described in Fig. 1. Since the communicator 2 is connected in series to the transmission line 5, keeping the above-mentioned voltage drop at a constant value renders a constant-current circuit unnecessary. In this embodiment, when the communicator 2 is attached to or removed from the transmission line 5, it is possible to prevent communication on the part of other devices using the same transmission line 5 from being affected. Though the two-wired transmission line has been described in those embodiments, the present invention may also be applied to a four-wired transmission line. As has been explained, according to the present invention, since the communicator does not have a built-in power supply and may be connected to the transmission line, maintenance, such as replacement or charging of the built-in battery, can be omitted. It is also possible to continuously utilize the communicator for prolonged periods, because temporary built-in power supplies such as a battery are no longer necessary. Furthermore, in a plant monitoring system to which the field communicator of the present invention is installed, even when the host controller is removed, it is possible to confirm the operation of the communicator by using an external power supply in the transmission line.

Claims: A field instrument system, comprising: a parallel circuit of field instruments (1) each for measuring a physical quantity, a series circuit of a power source (4) and a host instrument (3) having a resistance (R L ), a transmission line (5) which has two wires and connects the parallel circuit of field instruments with the series circuit of the host instrument and the power source, wherein each field instrument (1) communicates a first digital signal to the host instrument (3) by altering an electrical current through the transmission line (5), and a communicator (2) connectable to the transmission line for monitoring the field instruments and including: a pair of terminals for parallel attachment to the two wires of the transmission line (5) or for parallel attachment to a voltage drop element (6) connected in series with one wire of the transmission line, signal input means (209) connected between said terminals for transforming a variation of the voltage between the terminals into a second digital signal, signal output means (211) connected between said terminals for changing the current flowing between the terminals in accordance with a third digital signal so that the communicator communicates with the field instruments through said signal input means and said signal output means, to monitor the field instruments, a DC-DC converter (201) connected to said pair of terminals to derive the electric power for operating the communicator from the transmission line and a constant current circuit (212) for adjusting the current drawn by the DC-DC converter from the transmission line to a constant value. A field instrument system, comprising: a parallel circuit of field instruments (1) each for measuring a physical quantity, a series circuit of a power source and a host instrument (3) having a resistance (R L ), a transmission line (5) which has two wires and connects the parallel circuit of field instruments with the series circuit of the host instrument and the power source, wherein each field instrument (1) communicates a first digital signal to the host instrument (3) by altering an electrical current through the transmission line (5), and a communicator (2) connectable to the transmission line for monitoring the field instruments and including: a pair of terminals for attachment in series with one wire of the transmission line (5), signal input means (209) connected between said terminals for transforming a variation of the current flowing between the terminals into a second digital signal, signal output means (211) connected between said terminals for changing the current flowing between the terminals in accordance with a third digital signal so that the communicator communicates with the field instruments through said signal input means and said signal output means, to monitor the field instruments, and a DC-DC converter (201) connected to the pair of terminals to derive the electric power for operating the communicator from the transmission line, the DC-DC converter (201) having means for maintaining the voltage across the pair of terminals constant while said electric power is drawn from the transmission line.

Inventors: KOGURE MAKOTO, KOGURE, MAKOTO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1990
Main classification: G08C19
IPC codes: G08C19, H04B3