JP2021027529A - Field apparatus and filter circuit - Google Patents

Abstract

To provide a field apparatus capable of improving the quality of communication and improving noise resistance, and a filter circuit.SOLUTION: A field apparatus 100 is configured connectable to a communication circuit 20 via a pair of transmission lines L1 and L2. The field apparatus 100 comprises: a signal input/output circuit 130 which executes at least one of processing for transmitting a communication signal to the communication circuit and processing for receiving a communication signal from the communication circuit via the pair of transmission lines; and a filter circuit 140 which is connected between the signal input/output circuit and the communication circuit in parallel with the signal input/output circuit. The filter circuit includes a filter unit and a switching unit which are connected in series between a ground point and each of the pair of transmission lines. The switching unit is brought into an OFF state when applying a signal having an amplitude smaller than a predetermined threshold between the ground point and each of the pair of transmission lines, and brought into an ON state in the case where a signal having an amplitude equal to or greater than the predetermined threshold is applied between the ground point and each of the transmission lines.SELECTED DRAWING: Figure 5

Description

The present disclosure relates to field equipment and filter circuits.

Conventionally, a field device capable of communicating with an external device via a filter circuit is known (see, for example, Patent Document 1).

JP-A-2017-67642

When the filter circuit attenuates the common mode noise applied to the transmission line that transmits the communication signal, the impedance between the transmission lines is lowered, which may affect the communication. It is required to improve the noise immunity of equipment while improving the quality of communication.

The present disclosure has been made in view of the above points, and an object of the present disclosure is to provide a field device and a filter circuit capable of improving communication quality and noise immunity.

The field equipment according to some embodiments is configured to be connectable to a communication circuit via a pair of transmission lines, and a process of transmitting a communication signal to the communication circuit via the pair of transmission lines, and the above-mentioned A signal input / output circuit that executes at least one of the processes of receiving a communication signal from the communication circuit is connected in parallel to the signal input / output circuit between the signal input / output circuit and the communication circuit. The filter circuit includes a filter unit and a switching unit, which are connected in series between a grounding point and each line of the pair of transmission lines. When a signal having an amplitude less than a predetermined threshold is applied between the grounding point and each of the pair of transmission lines, the off state is set, and the grounding point and each of the pair of transmission lines are turned off. It is turned on when a signal having an amplitude equal to or higher than the predetermined threshold value is applied between the line and the line. In this way, the filter circuit is turned off when a signal having an amplitude less than a predetermined threshold value is applied to the switching unit, and is turned on for a signal having an amplitude equal to or more than a predetermined threshold value. It is less likely to affect the communication signal while attenuating excessive noise. As a result, communication quality is improved and noise immunity is improved.

In the field equipment according to one embodiment, the filter unit may include a capacitor. By doing so, high frequency noise is attenuated. As a result, the quality of communication is improved. In addition, excessive noise is attenuated to improve noise immunity.

In the field equipment according to one embodiment, the filter unit may include a resistor connected in series with the capacitor. By doing so, the filter unit is protected from excessive noise. As a result, the reliability of field equipment is increased.

In the field apparatus according to one embodiment, the switching unit may include a first diode and a second diode connected in parallel, and the forward direction of the first diode and the forward direction of the second diode. Are opposite to each other, and the predetermined threshold value is determined based on the magnitude of the forward voltage drop of the first diode and the second diode. By doing so, the switching unit is composed of a simple circuit, and the degree of freedom in setting the threshold value is increased. As a result, the convenience of field equipment is improved.

The field device according to the embodiment may be configured to be further connectable to an external power source and may receive DC power from the external power source via the transmission line. By doing so, electric power can be supplied by a common transmission line, and communication signals can be transmitted and received. As a result, the convenience of field equipment is improved.

The filter circuit according to some embodiments includes a filter unit and a switching unit, and the filter unit and the switching unit are located between a grounding point and each line of a pair of transmission lines propagating a communication signal. Connected in series, the switching unit is turned off when a signal having an amplitude less than a predetermined threshold is applied between the grounding point and each line of the pair of transmission lines, and the grounding point is turned off. It is turned on when a signal having an amplitude equal to or higher than the predetermined threshold value is applied between the pair of transmission lines and each line of the pair of transmission lines. In this way, the filter circuit is turned off when a signal having an amplitude less than a predetermined threshold value is applied to the switching unit, and is turned on for a signal having an amplitude equal to or more than a predetermined threshold value. It is less likely to affect the communication signal while attenuating excessive noise. As a result, communication quality is improved and noise immunity is improved.

According to the present disclosure, there are provided field devices and filter circuits that can improve the quality of communication and the noise immunity.

It is a circuit diagram which shows the field equipment which concerns on a comparative example. In the comparative example, an example of the time change of the voltage applied to the transmission line and the time change of the current flowing through the filter circuit is shown. It is a block diagram which shows the structural example of the field apparatus which concerns on one Embodiment of this disclosure. It is a block diagram which shows the structural example of a signal processing circuit. It is a block diagram which shows the structural example of a filter circuit. In one embodiment, an example of the time change of the voltage applied to the transmission line and the time change of the current flowing through the filter circuit is shown.

As shown in FIG. 1, the field device 900 according to the comparative example is connected to an external power source 10 including a voltage source 12 via two transmission lines L1 and L2. The field device 900 operates by the electric power supplied by the voltage source 12. The voltage source 12 is connected in series with the transmission lines L1 and L2. The field device 900 includes a terminal 951 connected to the transmission line L1 and a terminal 952 connected to the transmission line L2.

The field device 900 according to the comparative example includes a sensor 50. The sensor 50 may be configured to be connected to the outside of the field device 900. The field device 900 acquires measurement data from the sensor 50. The sensor 50 operates on the electric power from the field device 900. The sensor 50 measures physical quantities such as pressure, flow rate, and temperature, and outputs the measurement data to the field device 900.

The field device 900 according to the comparative example includes a signal processing circuit 910, a regulator 920, a signal input / output circuit 930, and a filter circuit 940.

The field device 900 is connected to the external power supply 10 via the transmission line L1 and the transmission line L2. The external power supply 10 supplies DC power to the transmission lines L1 and L2. That is, the external power supply 10 applies a DC signal to the transmission lines L1 and L2. The field device 100 operates by receiving electric power from the external power source 10 via the transmission lines L1 and L2. It is assumed that the voltage of the DC signal is represented by _VDC. The external power supply 10 applies a voltage having a reverse sign between the transmission line L1 and the grounding point COM and between the transmission line L2 and the grounding point COM. The potentials of the transmission lines L1 and L2 with respect to the grounding point COM are represented by V1 and V2, respectively. The external power supply 10 _{applies a DC voltage represented by + V DC} / 2 as V1 and a DC voltage represented by −V _DC / 2 as V2. As a result, a DC voltage represented by _VDC is applied between the transmission line L1 and the transmission line L2.

The signal input / output circuit 930, the filter circuit 940, and the regulator 920 are connected in parallel between the transmission line L1 and the transmission line L2. The signal input / output circuit 930 and the filter circuit 940 operate on the electric power supplied from the external power source 10 via the transmission lines L1 and L2. The regulator 920 controls the electric power supplied from the external power source 10 via the transmission lines L1 and L2 so that the voltage or current becomes a predetermined value, and supplies the electric power to the signal processing circuit 910. The signal processing circuit 910 operates on the power supplied from the regulator 920.

The field device 900 is connected to the communication circuit 20 via transmission lines L1 and L2. The communication circuit 20 is connected in parallel to the signal input / output circuit 930. The signal input / output circuit 930 and the communication circuit 20 transmit and receive communication signals to and from each other via transmission lines L1 and L2. One of the signal input / output circuit 930 and the communication circuit 20 superimposes a communication signal on the transmission lines L1 and L2 as an AC signal. The other of the signal input / output circuit 930 and the communication circuit 20 acquires the AC signal superimposed on the transmission lines L1 and L2 as a communication signal. A DC signal as DC power supplied from the external power source 10 is applied to the transmission lines L1 and L2. That is, the signals transmitted on the transmission lines L1 and L2 include a DC signal as DC power and an AC signal as a communication signal. The signal input / output circuit 930 and the communication circuit 20 separate the AC signal as the communication signal from the signals transmitted on the transmission lines L1 and L2.

The field device 900 is connected to the sensor 50 by a signal processing circuit 910. The sensor 50 measures physical quantities such as pressure, flow rate, and temperature, converts the measured data into an electric signal, and outputs the measured data to the field device 900. The signal processing circuit 910 acquires measurement data from the sensor 50.

The signal processing circuit 910 communicates with the sensor 50 based on a predetermined communication standard, and outputs a signal based on the measurement data of the sensor 50 to the signal input / output circuit 930. The signal processing circuit 910 acquires a signal based on the communication signal received from the communication circuit 20 by the signal input / output circuit 930 from the signal input / output circuit 930.

The signal input / output circuit 930 generates a communication signal based on the signal output by the signal processing circuit 910, and superimposes it on the signals transmitted on the transmission lines L1 and L2 as an AC signal. The signal input / output circuit 930 separates the AC signal superimposed on the signal transmitted on the transmission lines L1 and L2 and acquires the communication signal from the communication signal.

The filter circuit 940 attenuates noise superimposed on the signals transmitted on the transmission lines L1 and L2. The filter circuit 940 includes resistors 941 and 943, as well as capacitors 942 and 944. The resistor 941 and the capacitor 942 are connected in series between the transmission line L1 and the grounding point COM. The resistor 943 and the capacitor 944 are connected in series between the transmission line L2 and the grounding point COM.

The potentials of the transmission lines L1 and L2 with respect to the grounding point COM are represented by V1 and V2, respectively. The difference between V1 and V2 corresponds to the voltage of the signal applied between the transmission line L1 and the transmission line L2. When a signal in which a DC signal and an AC signal are superimposed is applied between the transmission line L1 and the transmission line L2, the magnitude of the voltage of the signal corresponds to the difference between V1 and V2.

It is assumed that the communication signals transmitted and received by the signal input / output circuit 930 and the communication circuit 20 are pulse signals represented by square waves. The pulse signal contains two voltage levels. One voltage level is referred to as the H level. The other voltage level is referred to as the L level. When a communication signal is applied to the transmission lines L1 and L2 as an AC signal, the signal input / output circuit 930 and the communication circuit 20 are between the transmission line L1 and the grounding point COM, and between the transmission line L2 and the grounding point COM. A voltage of opposite phase is applied to each. When the H level voltage is represented by + Va, the voltages applied to the transmission lines L1 and L2 to make the communication signal H level are + Va / 2 and -Va / 2, respectively. When the L level voltage is represented by −Va, the voltages applied to the transmission lines L1 and L2 in order to make the communication signal L level are −Va / 2 and + Va / 2, respectively. That is, the voltages applied to the transmission lines L1 and L2 are values having opposite signs to each other.

When an _{H-level communication signal is superimposed on a DC signal having a voltage represented by VDC} , the difference between V1 and V2 corresponding to the potential difference between the transmission line L1 and the transmission line L2 is _VDC + Va. .. When the L level communication signal is superimposed, the difference between V1 and V2 is _{VDC −} Va.

Noise may be superimposed on the transmission lines L1 and L2. The noise includes common mode noise and normal mode noise. The common mode noise is represented as a noise voltage applied in the same phase between each of the transmission lines L1 and L2 and the grounding point COM. The normal mode noise is represented as a noise voltage applied between the transmission line L1 and the transmission line L2.

The current flowing through the filter circuit 940 is determined based on the value of V1 and the value of V2. Capacitors 942 and 944, which are connected in series between the transmission lines L1 and L2 and the grounding point COM in the filter circuit 940, have infinite impedance with respect to the DC signal. On the other hand, the impedance of the capacitors 942 and 944 with respect to the AC signal is lower than the impedance with respect to the DC signal. Further, the higher the frequency of the AC signal, the lower the impedance of the capacitors 942 and 944 with respect to the AC signal. As a result, the filter circuit 940 functions as a low-pass filter and attenuates common mode noise or normal mode noise.

The graph shown in FIG. 2 shows an example of the time change of the voltage applied to the transmission lines L1 and L2 and the time change of the current flowing through the filter circuit 940. The horizontal axis represents the time. The graph (C1) located in the first row from the bottom shows the time change of the signal in which the DC signal and the communication signal are superimposed. The vertical axis of the graph (C1) represents the voltage between transmission lines. The transmission line voltage corresponds to the difference between V1 and V2, and represents the voltage of the signal in which the DC signal and the communication signal are superimposed. The transmission line voltage fluctuates in a cycle represented by Ts. Ts corresponds to the cycle of the communication signal. _{The transmission line voltage is represented as V H} when the communication signal is H level, and is _{represented as V L} when the communication signal is L level. In this case, each V _H and V _L, V _DC + Va, and, corresponding to the V _DC -Va.

The graph (C2) located in the second row from the bottom shows the time change of the noise superimposed on each of the transmission lines L1 and L2 as the common mode noise. The vertical axis of the graph (C2) represents the noise voltage. The graph (C2) includes, as a graph of noise voltage, a graph of time change of noise source voltage and a graph of time change of terminal noise voltage. The noise source voltage is a voltage of common mode noise applied to both transmission lines L1 and L2 by a noise source connected to each of transmission lines L1 and L2 via a CDN (Coupling Decoupling Network). The period of the noise source voltage is represented by Tn. The terminal noise voltage is a voltage propagated to the terminal 951 or 952 as a result of the noise source voltage being attenuated by the filter circuit 940. The terminal noise voltage corresponds to the voltage of the terminal 951 or 952 with respect to the grounding point COM, and corresponds to the value calculated by multiplying the sum of V1 and V2 by 1/2.

The graph (C3) located in the third row from the bottom shows the time change of the current I1 flowing through the capacitor 942 connected between the transmission line L1 and the grounding point COM. It is assumed that the value of the current I1 is positive when the current I1 flows from the transmission line L1 toward the grounding point COM. The graph (C4) located in the fourth row from the bottom shows the time change of the current I2 flowing through the capacitor 944 connected between the transmission line L2 and the grounding point COM. It is assumed that the value of the current I2 is positive when the current I2 flows from the grounding point COM toward the transmission line L2. The vertical axes of the graphs (C3) and (C4) represent the magnitudes of the current I1 and the current I2, respectively. The period in which the magnitudes of the currents I1 and I2 change is the same as the period of the noise source voltage, and is represented by Tn. The noise is attenuated in the filter circuit 940 by the currents I1 and I2 based on the noise source voltage flowing through the capacitors 942 and 944. Due to the noise attenuation, the terminal noise voltage is lower than the noise source voltage in the graph (C2).

On the other hand, a current based on the transmission line voltage applied between the transmission line L1 and the transmission line L2 flows through the filter circuit 940. A current based on the voltage change flows through the filter circuit 940 at the rising or falling edge of the rectangular wave representing the time change of the transmission line voltage. The waveform at the rising edge of the voltage in the graph (C1) is affected by the transient response characteristic of the filter circuit 940, which delays the _{voltage before reaching V H.} Further, the waveform at the time of voltage drop is affected by the transient response characteristic of the filter circuit 940, which delays the _{voltage until it reaches VL.} As a result, the communication signal superimposed on the DC signal deteriorates. Deterioration of communication signals affects the quality of communication.

It is conceivable to reduce the capacitance of the capacitors 942 and 944 in order to reduce the influence of the transient response characteristics. However, by reducing the capacitances of the capacitors 942 and 944, the impedance of each capacitor increases, making it difficult for the filter circuit 940 to attenuate noise. That is, the ability of the filter circuit 940 as a filter is limited. When the capacity of the filter is limited and reduced, excessive common mode noise applied to the transmission lines L1 and L2 may cause the circuit to malfunction or cause the CPU (Central Processing Unit) included in the circuit to run away. is there.

As described above, when the filter circuit 940 attenuates the common mode noise applied between the transmission line L1 and the transmission line L2 for transmitting the communication signal, the filter circuit 940 attenuates the transmission line L1 and the transmission line through the ground point. It is in a state of being connected to L2. By connecting the transmission line L1 and the transmission line L2, the filter circuit 940 can reduce the impedance between the transmission line L1 and the transmission line L2 according to the impedance of the filter circuit 940. A decrease in impedance between the transmission line L1 and the transmission line L2 may cause deterioration of the communication signal. Therefore, when trying to suppress the deterioration of the communication signal by the filter circuit 940, the filtering ability may be limited and deteriorated. Then, common mode noise of a predetermined magnitude or more applied between the transmission line L1 and the transmission line L2 may cause malfunction or runaway of the internal circuit including the CPU, and may affect the equipment. Therefore, it is required to improve the noise immunity of the device while suppressing the influence on the communication such as the deterioration of the communication signal and improving the communication quality.

Therefore, the present disclosure describes a field device and a filter circuit that can improve the quality of communication and the noise immunity.

(Embodiment of the present disclosure)
As shown in FIG. 3, the field device 100 according to the embodiment includes a signal processing circuit 110, a signal input / output circuit 130, and a filter circuit 140. The field device 100 may further include a regulator 120.

The field device 100 is configured to be connectable to the external power supply 10 via the pair of transmission lines L1 and the transmission line L2. The field device 100 may include a terminal 151 connected to the transmission line L1 and a terminal 152 connected to the transmission line L2. The external power supply 10 has a voltage source 12 and supplies DC power to transmission lines L1 and L2. That is, a DC signal is applied to the transmission lines L1 and L2 from the external power supply 10. The external power supply 10 applies a DC voltage represented by _VDC between the transmission line L1 and the transmission line L2. It is assumed that a DC voltage represented by _{+ V DC} / 2 is applied to the transmission line L1. It is assumed that a DC voltage represented by _{−V DC} / 2 is applied to the transmission line L2. That is, the external power supply 10 may be grounded at the midpoint. The field device 100 may operate by receiving electric power from the external power source 10 via the transmission lines L1 and L2. At least a part of the field device 100 may operate with electric power supplied from another power source.

The signal input / output circuit 130, the filter circuit 140, and the regulator 120 are connected in parallel between the transmission line L1 and the transmission line L2. The signal input / output circuit 130 and the filter circuit 140 receive electric power from the external power supply 10 via the transmission lines L1 and L2. The regulator 120 may control the electric power supplied from the external power source 10 via the transmission lines L1 and L2 so that the voltage or current becomes a predetermined value and supply the electric power to the signal processing circuit 110. The signal processing circuit 110 may operate on the power supplied from the regulator 120. The signal processing circuit 110 may operate with electric power supplied from another power source.

The regulator 120 may be configured as a switching regulator. The regulator 120 may be configured as a step-down switching regulator or a step-up switching regulator.

The field device 100 is configured to be connectable to the communication circuit 20 via transmission lines L1 and L2. The communication circuit 20 may be connected in parallel with the signal input / output circuit 130. The signal input / output circuit 130 may transmit a communication signal to the communication circuit 20 via the transmission lines L1 and L2. That is, the signal input / output circuit 130 may output a communication signal to the communication circuit 20. The signal input / output circuit 130 may receive a communication signal from the communication circuit 20 via the transmission lines L1 and L2. That is, the signal input / output circuit 130 may accept the input of the communication signal from the communication circuit 20. The signal input / output circuit 130 may acquire a communication signal from the communication circuit 20. In other words, the signal input / output circuit 130 is at least one of a process of transmitting a communication signal to the communication circuit 20 via the pair of transmission lines L1 and L2 and a process of receiving the communication signal from the communication circuit 20. Executes the processing of. The communication signal may be represented as a voltage signal or a current signal. In the present embodiment, the communication signal is represented as a voltage signal.

The field device 100 may further include a sensor 50. The field device 100 is connected to the sensor 50 by a signal processing circuit 110. The sensor 50 may be configured to be connected to the outside of the field device 100. The signal processing circuit 110 acquires measurement data from the sensor 50 when it is connected to the sensor 50. The sensor 50 measures physical quantities such as pressure, flow rate, and temperature, converts the measured data into an electric signal, and outputs the measured data to the field device 100. The electric signal may include a current signal or a voltage signal. The sensor 50 may operate on the power supplied by the field device 100. The signal processing circuit 110 may supply power to the sensor 50 when it is connected to the sensor 50. The sensor 50 may be operated by electric power supplied from another power source instead of the electric power supplied from the field device 100.

As shown in FIG. 4, the signal processing circuit 110 may include an amplifier 112, an A / D converter 114, and a processor 116.

The signal processing circuit 110 communicates with the sensor 50 based on a predetermined communication standard, and receives an input of an electric signal transmitted from the sensor 50 by the amplifier 112. That is, the amplifier 112 acquires an electric signal transmitted from the sensor 50. The amplifier 112 may acquire an electrical signal as an analog signal. The amplifier 112 may be composed of, for example, an operational amplifier. The amplifier 112 may form a buffer circuit. The buffer circuit converts impedance by having a high input impedance and a low output impedance. When the amplifier 112 constitutes a buffer circuit, the electric signal input from the sensor 50 to the non-inverting terminal of the amplifier 112 is output as it is from the output terminal of the amplifier 112. That is, the amplifier 112 outputs the electric signal transmitted from the sensor 50 as it is.

The A / D converter 114 converts the analog signal output from the amplifier 112 into a digital signal. That is, the A / D converter 114 converts the electric signal input from the sensor 50 into a digital signal.

The processor 116 acquires the measurement data of the sensor 50 based on the digital signal input from the A / D converter 114. The processor 116 outputs a signal based on the measurement data of the sensor 50 to the signal input / output circuit 130. The processor 116 may output a signal based on the measurement data of the sensor 50 to the signal input / output circuit 130 as a digital signal. The processor 116 may output a signal obtained by converting a digital signal into an analog signal to the signal input / output circuit 130.

The processor 116 may acquire the signal from the signal input / output circuit 130. The signal from the signal input / output circuit 130 may include a digital signal converted by the signal input / output circuit 130 receiving the communication signal from the communication circuit 20. The processor 116 may acquire an analog signal from the signal input / output circuit 130. In this case, the processor 116 may include an input circuit that converts an analog signal into a digital signal.

The processor 116 may be configured by a general-purpose integrated circuit such as a CPU. The processor 116 may realize various functions by executing a predetermined program. The processor 116 may have a storage unit. The storage unit may store various information used for the operation of the processor 116, a program for the processor 116 to realize the function of the signal processing circuit 110, and the like. The storage unit may function as a work memory of the processor 116. The storage unit may be composed of, for example, a semiconductor memory or the like.

The signal input / output circuit 130 generates a communication signal based on the digital signal or analog signal output by the signal processing circuit 110, and superimposes it on the signals transmitted on the transmission lines L1 and L2 as an AC signal. The signals transmitted on the transmission lines L1 and L2 include a DC signal corresponding to the electric power supplied from the external power source 10 and a communication signal as an AC signal output by the signal input / output circuit 130. The communication circuit 20 can acquire a communication signal by separating an AC signal from the signals transmitted on the transmission lines L1 and L2.

The signals transmitted on the transmission lines L1 and L2 may include a communication signal as an AC signal output by the signal input / output circuit 130. When the AC signal is superimposed on the signal transmitted by the transmission lines L1 and L2, the signal input / output circuit 130 can acquire the communication signal by separating the AC signal. That is, the signal input / output circuit 130 can acquire the communication signal output by the communication circuit 20 when the communication circuit 20 superimposes the communication signal as an AC signal on the signals transmitted on the transmission lines L1 and L2. The signal input / output circuit 130 may output the communication signal output by the communication circuit 20 to the processor 116 of the signal processing circuit 110. By doing so, the processor 116 can acquire the communication signal from the communication circuit 20.

In other words, in addition to the DC signal applied from the external power supply 10, the communication signal from the signal input / output circuit 130 or the communication circuit 20 is further superimposed as an AC signal on the transmission lines L1 and L2. .. The signal input / output circuit 130 can transmit a communication signal to the communication circuit 20 by superimposing an AC signal on the transmission lines L1 and L2. The signal input / output circuit 130 can receive a communication signal from the communication circuit 20 by separating the AC signal superimposed on the transmission lines L1 and L2 by the communication circuit 20.

It is assumed that the communication signals transmitted and received by the signal input / output circuit 130 and the communication circuit 20 are pulse signals represented by square waves. The pulse signal contains two voltage levels. One voltage level is referred to as the H level. The other voltage level is referred to as the L level. When a communication signal is applied to the transmission lines L1 and L2 as an AC signal, the signal input / output circuit 930 and the communication circuit 20 are between the transmission line L1 and the grounding point COM, and between the transmission line L2 and the grounding point COM. A voltage of opposite phase is applied to each. When the H level voltage is represented by + Va, the voltages applied to the transmission lines L1 and L2 to make the communication signal H level are + Va / 2 and -Va / 2, respectively. When the L level voltage is represented by −Va, the voltages applied to the transmission lines L1 and L2 in order to make the communication signal L level are −Va / 2 and + Va / 2, respectively. That is, the voltages applied to the transmission lines L1 and L2 are values having opposite signs to each other.

Noise may be superimposed on the signals transmitted on the transmission lines L1 and L2. The filter circuit 140 attenuates noise superimposed on the signals transmitted on the transmission lines L1 and L2. The filter circuit 140 may attenuate a component having a predetermined frequency among the signals transmitted on the transmission lines L1 and L2 more than a component having a frequency other than the predetermined frequency. That is, the filter circuit 140 may attenuate a component having a predetermined frequency and allow a component having a frequency other than the predetermined frequency to pass through. The filter circuit 140 may function as a low-pass filter that attenuates components having a frequency equal to or higher than a predetermined frequency.

As shown in FIG. 5, the filter circuit 140 includes resistors 141 and 143, capacitors 142 and 144, and switching units 145 and 146. The capacitors 142 and 144 are also referred to as filter units, respectively. The resistor 141, the capacitor 142, and the switching unit 145 are connected in series between the transmission line L1 and the grounding point COM. The resistor 143, the capacitor 144, and the switching unit 146 are connected in series between the transmission line L2 and the grounding point COM. That is, the filter unit and the switching unit are connected in series between the grounding point COM and each of the pair of transmission lines L1 and L2. The order in which the resistors 141 and 143, the capacitors 142 and 144, and the switching units 145 and 146 are connected may be changed.

The potentials of the transmission lines L1 and L2 with respect to the grounding point COM are represented by V1 and V2, respectively. The difference between V1 and V2 corresponds to the voltage applied between the transmission line L1 and the transmission line L2. The voltage applied between the transmission line L1 and the transmission line L2 is also referred to as a transmission line voltage. When a signal in which a DC signal and an AC signal are superimposed is applied between the transmission line L1 and the transmission line L2, the magnitude of the voltage of the signal corresponds to the difference between V1 and V2.

When _{an H-level communication signal is superimposed on a DC signal having a voltage represented by VDC} , the difference between V1 and V2 corresponding to the potential difference between the transmission line L1 and the transmission line L2 is _VDC + Va. Become. That is, _{a signal in which a DC signal having a voltage represented by VDC} and a communication signal having an H level voltage represented by + Va are superimposed is input to the transmission lines L1 and L2. When the L level communication signal is superimposed, the difference between V1 and V2 is _{VDC −} Va. That is, _{a signal in which a DC signal having a voltage represented by VDC} and a communication signal having an L level voltage represented by −Va are superimposed is input to the transmission lines L1 and L2. It is assumed that the amplitude of the pulse signal in which the voltages of the H level and the L level are + Va and −V, respectively, is expressed as Va.

The noise that can be superimposed on the transmission lines L1 and L2 includes common mode noise and normal mode noise. The common mode noise is represented as a noise voltage applied in the same phase between each of the transmission lines L1 and L2 and the grounding point COM. The normal mode noise is represented as a noise voltage applied between the transmission line L1 and the transmission line L2. For example, the grounding point COM is the ground connected to the field device 100, and corresponds to the destination where the current of the common mode noise superimposed on the transmission line L1 or the transmission line L2 flows through the filter circuit 140.

The current flowing through the filter circuit 140 is determined based on the value of V1 and the value of V2. The capacitors 142 and 144 connected in series between the transmission lines L1 and L2 and the grounding point COM in the filter circuit 140 each have an infinite impedance with respect to the DC signal. On the other hand, the impedance of the capacitors 142 and 144 with respect to the AC signal is lower than the impedance with respect to the DC signal. Further, the higher the frequency of the AC signal, the lower the impedance of the capacitors 142 and 144 with respect to the AC signal. As a result, the filter circuit 140 functions as a low-pass filter that attenuates high-frequency noise, and attenuates common mode noise or normal mode noise.

The switching units 145 and 146 function to turn off when a voltage below a predetermined threshold is applied and turn on when a voltage above a predetermined threshold is applied. The off state may be a state in which a current cannot flow, or a state in which a current less than a predetermined value can flow. The on state may be a state in which a current can flow, or a state in which a current of a predetermined value or more can flow. The predetermined threshold is also referred to as Vth.

Switching units 145 and 146 may include diodes. The diode has a rectifying characteristic. The resistance value of the diode differs depending on whether a voltage equal to or greater than the forward voltage drop is applied in the forward direction, a voltage applied in the reverse direction, or a voltage less than the forward voltage drop is applied in the forward direction. Has. The resistance value when a voltage greater than the forward voltage drop is applied in the forward direction is lower than the resistance value when the voltage is applied in the reverse direction and when a voltage less than the forward voltage drop is applied in the forward direction. .. A diode is considered to be in an on state capable of carrying a large current when a voltage greater than or equal to the forward voltage drop is applied in the forward direction. A diode is considered to be in an off state where almost no current can flow when a voltage is applied in the opposite direction and when a voltage less than the forward voltage drop is applied in the forward direction.

It is assumed that the switching unit 145 includes diodes 145a and 145b. The diodes 145a and 145b are connected in parallel with each other. The diode 145a allows current to flow in the forward direction from the transmission line L1 toward the grounding point COM. The diode 145b allows current to flow in the forward direction from the grounding point COM toward the transmission line L1. That is, the forward direction of the diode 145a and the forward direction of the diode 145b are opposite to each other. Since the forward directions of the diodes 145a and 145b are opposite to each other, the switching unit 145 is configured to be able to transmit signals in both directions. For example, the diodes 145a and 145b can allow current to flow bidirectionally between the transmission line L1 and the grounding point COM.

It is assumed that the switching unit 146 includes diodes 146a and 146b. The diodes 146a and 146b are connected in parallel with each other. The diode 146a passes a current in the forward direction from the transmission line L2 toward the grounding point COM. The diode 146b allows current to flow in the forward direction from the grounding point COM toward the transmission line L2. That is, the forward direction of the diode 146a and the forward direction of the diode 146b are opposite to each other. Since the forward directions of the diodes 146a and 146b are opposite to each other, the switching unit 146 is configured to be able to transmit signals in both directions. For example, the diodes 146a and 146b can allow current to flow bidirectionally between the transmission line L2 and the grounding point COM.

The number of diodes 145a, 145b, 146a, and 146b is two in FIG. 5, respectively. A circuit in which two diodes are connected in series is turned on when a voltage equal to or greater than twice the magnitude of the forward voltage drop is applied in the forward direction. When the forward voltage drop is expressed as Vf, the circuit in which two diodes are connected in series is turned on when a voltage of 2 Vf or more is applied in the forward direction. The switching units 145 and 146 having two diodes connected in series are turned off when a voltage lower than the predetermined threshold is applied, with a predetermined threshold set to 2Vf, and a voltage equal to or higher than the predetermined threshold is applied. It works to turn on when it is done.

The number of diodes 145a, 145b, 146a and 146b may be one or three or more. The numbers of the diodes 145a, 145b, 146a and 146b, respectively, may be the same or different. When the number of diodes connected in series is n, the sum of the magnitudes of their forward voltage drops is represented by Vf × n. That is, the predetermined threshold value in the switching units 145 and 146 is determined based on the product of the magnitude of the forward voltage per diode and the number of diodes connected in series.

One of the diodes 145a and 145b, or one of the diodes 146a and 146b, is also referred to as the first diode. The other diode is also referred to as the second diode. The first diode and the second diode may be connected in parallel so that the forward direction of the first diode and the forward direction of the second diode are opposite to each other. The forward voltage drop of each of the first diode and the second diode is determined by the product of the number of diodes constituting the first diode and the second diode and the forward voltage drop per diode.

The switching units 145 and 146 may include a rectifying diode, a switching diode, a Zener diode, or the like as the diode. The switching units 145 and 146 may be configured to include a diode having a function of protecting the filter circuit 140, such as a TVS (Transient Voltage Suppressor) diode. Since the switching units 145 and 146 are configured to include a diode, a predetermined threshold value of the filter circuit 140 is set by a simple circuit configuration.

In the example of FIG. 5, the number of diodes included in the switching units 145 and 146 is two, but it may be determined according to a set value of a predetermined threshold value. For example, the magnitude of the predetermined threshold value may be set to 3Vf by setting the number of diodes to three. Further, the diode may be replaced with a diode having a different Vf. Since the number of diodes included in the switching units 145 and 146 or the Vf of the diodes can be changed, the degree of freedom in setting a predetermined threshold value of the filter circuit 140 is increased. As a result, the convenience of the field device 100 is improved.

The switching units 145 and 146 may function to have an off state and an off state by being configured to include an active element such as a FET (Field Effect Transistor). By using the active element, the degree of freedom in setting a predetermined threshold value of the filter circuit 140 can be increased. Further, since the switching units 145 and 146 are configured to include an active element, when the voltage applied to the switching units 145 and 146 is a value close to a predetermined threshold value, the on state and the off state are steeply set. Can be switched to. By doing so, the switching characteristics of the switching units 145 and 146 are improved.

The resistor 141 limits the magnitude of the current flowing between the transmission line L1 and the grounding point COM. The resistor 143 limits the magnitude of the current flowing between the transmission line L2 and the grounding point COM. For example, when a suddenly large current such as a lightning surge current flows through the transmission lines L1 and L2, the resistors 141 and 143 can limit the magnitude of the current flowing into the switching units 145 and 146. That is, the resistors 141 and 143 function as protective elements for the switching units 145 and 146. The resistors 141 and 143 are inserted so that the field device 100 meets the explosion-proof standard. The resistance values of the resistors 141 and 143 may be appropriately set according to the explosion-proof standard. The filter circuit 140 is provided with resistors 141 and 143 to protect the filter circuit 140 from excessive noise. As a result, the reliability of the field device 100 is increased. The capacitors 142 and 144 may form a filter unit together with the resistors 141 and 143, respectively.

The graph shown in FIG. 6 shows an example of the time change of the voltage applied between the transmission line L1 and the transmission line L2 and the time change of the current flowing through the filter circuit 140. The horizontal axis represents the time. The graph (1) located in the first row from the bottom shows the time change of the signal in which the DC signal and the communication signal are superimposed. The vertical axis of the graph (1) represents the voltage between transmission lines. The transmission line voltage corresponds to the difference between V1 and V2, and represents the voltage of the signal in which the DC signal and the communication signal are superimposed. The transmission line voltage fluctuates in a cycle represented by Ts. Ts corresponds to the cycle of the communication signal. _VDC corresponds to the voltage of the DC signal. Va corresponds to the amplitude of the communication signal. _{The transmission line voltage is represented as V H} when the communication signal is H level, and is _{represented as V L} when the communication signal is L level. Each V _H and V _L, V _DC + Va, and, corresponding to the V _DC -Va.

The graph (2) located in the second row from the bottom shows the time change of the noise superimposed on each of the transmission lines L1 and L2 as the common mode noise. The vertical axis of the graph (2) represents the noise voltage. The graph (2) includes a graph of the time change of the noise source voltage and a graph of the time change of the terminal noise voltage as a graph of the noise voltage. The noise source voltage is a voltage of common mode noise applied to both transmission lines L1 and L2 by a noise source connected to each of transmission lines L1 and L2 via a CDN (Coupling Decoupling Network). The period of the noise source voltage is represented by Tn. The terminal noise voltage is a voltage propagated to the terminals 151 or 152 as a result of the noise source voltage being attenuated by the filter circuit 140. The terminal noise voltage corresponds to the voltage of the terminal 151 or 152 with respect to the grounding point COM, and corresponds to the value calculated by multiplying the sum of V1 and V2 by 1/2.

In the graph (2), the amplitude of the noise source voltage increases with the passage of time. When the amplitude of the noise source voltage becomes equal to or higher than a predetermined threshold value (Vth), a current based on the noise source voltage starts to flow in the filter circuit 140. In other words, when the noise source voltage becomes Vth or more or −Vth or less, a current based on the noise source voltage starts to flow in the filter circuit 140. It can be said that the filter circuit 140 does not flow a current based on a component having a noise amplitude of less than Vth, but flows a current based on a component having a noise amplitude of Vth or more. As a result, when the amplitude of the noise source voltage becomes Vth or more, the filter circuit 140 causes a current based on a part of the noise to flow between the transmission lines L1 and L2 and the grounding point COM to attenuate the noise. Let me.

The graph (3) located in the third row from the bottom shows the time change of the current I1 (see FIG. 5) flowing through the capacitor 142 connected between the transmission line L1 and the grounding point COM. It is assumed that the value of the current I1 is positive when the current I1 flows from the transmission line L1 toward the grounding point COM. The graph (4) located in the fourth row from the bottom shows the time change of the current I2 (see FIG. 5) flowing through the capacitor 144 connected between the transmission line L2 and the grounding point COM. It is assumed that the value of the current I2 is positive when the current I2 flows from the grounding point COM toward the transmission line L2. The vertical axes of the graphs (3) and (4) represent the magnitudes of the current I1 and the current I2, respectively. The current I1 flows when the amplitude of the noise source voltage applied to the transmission line L1 becomes Vth or more. The current I2 flows when the amplitude of the noise source voltage applied to the transmission line L2 becomes Vth or more. The period in which the magnitudes of the currents I1 and I2 change is the same as the period of the noise source voltage, and is represented by Tn. Noise is attenuated by the noise-based currents I1 and I2 flowing through the capacitors 142 and 144. Due to the noise attenuation, the terminal noise voltage in the graph (2) is lower than the noise source voltage.

By configuring the switching units 145 and 146 so that current can flow in both directions, the filter circuit 140 can attenuate the common mode noise regardless of the polarity of the input common mode noise.

On the other hand, if the voltage amplitude (Va / 2) of the communication signal applied between each of the transmission lines L1 and L2 and the grounding point COM is less than Vth, the filter circuit 140 is turned off and becomes a communication signal. Do not pass the current based on it. As a result, the communication signal is less likely to deteriorate. In other words, if the amplitude (Va) of the voltage of the communication signal applied between the transmission line L1 and the transmission line L2 is less than 2Vth, the communication signal is less likely to deteriorate. Unlike the filter circuit 940 according to the comparative example, the current flowing through the filter circuit 140 is also limited at the timing at which the voltage waveform of the communication signal superimposed on the DC signal in the graph (1) rises and falls. .. Since the filter circuit 140 can limit the current based on the signal whose amplitude is less than Vth, the influence of the transient response characteristic on the communication signal is less likely to be exerted, and the waveform of the communication signal is less likely to be deteriorated. As a result, the filter circuit 140 can improve the quality of the communication signal while attenuating the common mode noise.

When common mode noise is applied between each of the transmission lines L1 and L2 and the grounding point COM, the filter circuit 140 functions as follows.

When the voltage amplitude of the common mode noise is less than Vth, the switching units 145 and 146 cut off the current flowing from the transmission lines L1 and L2, respectively, through the filter unit to the ground point COM. As a result, the filter unit is separated from the transmission lines L1 and L2. As a result, the filter circuit 140 does not attenuate common mode noise. However, in this case, since the noise amplitude is originally small, the influence of the noise on the signal input / output circuit 130 or the communication circuit 20 is limited.

When the amplitude of the common mode noise is Vth or more, the switching units 145 and 146 pass a current from the transmission lines L1 and L2, respectively, through the filter unit to the path toward the grounding point COM. As a result, the filter unit is connected to the transmission lines L1 and L2. As a result, the filter circuit 140 attenuates the component of the common mode noise whose amplitude is Vth or more. By attenuating the common mode noise, the signal input / output circuit 130 or the communication circuit 20 can be protected from excessive noise.

When a communication signal is applied to the transmission lines L1 and L2, the filter circuit 140 functions as follows.

When the voltage amplitude (Va / 2) of the communication signal applied between each of the transmission lines L1 and L2 and the grounding point COM is less than Vth, the current based on the communication signal is from the transmission lines L1 and L2 to the grounding point. It does not flow to the filter circuit 140 including the path to COM. In other words, when the amplitude (Va) of the voltage of the communication signal applied between the transmission line L1 and the transmission line L2 is less than 2Vth, the current based on the communication signal does not flow through the filter circuit 140. By setting Vth based on the magnitude of the voltage amplitude of the communication signal, the influence of the filter circuit 140 on the communication signal can be reduced and the influence can be controlled. As a result, the quality of communication is improved.

The field device 100 according to the present embodiment may be configured such that the impedance of the filter circuit 140 with respect to the communication signal is higher than the impedance with respect to the communication signal with respect to noise. As a result, the field device 100 can realize a configuration that does not easily affect the communication while attenuating the noise contained in the communication signal.

The field device 100 according to the present embodiment can improve the quality of communication with the communication circuit 20 and is noise resistant regardless of whether or not it is connected to the external power supply 10 via the transmission lines L1 and L2. Can be improved. When the field device 100 according to the present embodiment is connected to the external power source 10 via the transmission lines L1 and L2, the power supply and communication can be realized by the common transmission lines L1 and L2. As a result, the convenience of the field device 100 is improved.

The filter circuit 140 according to the present embodiment functions so as not to affect the communication signal while attenuating the superimposed noise in the line for transmitting the communication signal even if it is not included in the field device 100. Can be done. As a result, the quality of communication is improved and the noise immunity is improved.

In the present embodiment, the communication signal is represented as a voltage signal, but the field device 100 and the filter circuit 140 according to the present embodiment may be applied when the communication signal is represented as a current signal. When the communication signal is represented as a current signal, the switching units 145 and 146 of the filter circuit 140 may be configured to be turned on when the magnitude of the amplitude of the current of the signal is equal to or larger than a predetermined value.

Although the embodiments according to the present disclosure have been described based on the drawings and examples, it should be noted that those skilled in the art can easily make various modifications or modifications based on the present disclosure. It should be noted, therefore, that these modifications or modifications are within the scope of this disclosure. For example, the functions included in each component or each step can be rearranged so as not to be logically inconsistent, and a plurality of components or steps can be combined or divided into one. ..

10 External power supply (12: voltage source)
20 Communication circuit 50 Sensor 100 Field equipment 110 Signal processing circuit 112 Amplifier 114 A / D converter 116 Processor 120 Regulator 130 Signal input / output circuit 140 Filter circuit 141, 143 Resistance 142, 144 Capacitor 145, 146 Switching unit 145a, 145b, 146a, 146b diode 151, 152 terminals

Claims (6)

A field device that can be connected to a communication circuit via a pair of transmission lines.
A signal input / output circuit that executes at least one of a process of transmitting a communication signal to the communication circuit and a process of receiving a communication signal from the communication circuit via the pair of transmission lines.
A filter circuit connected in parallel to the signal input / output circuit is provided between the signal input / output circuit and the communication circuit.
The filter circuit has a filter unit and a switching unit connected in series between the grounding point and each line of the pair of transmission lines.
The switching unit
It is turned off when a signal having an amplitude less than a predetermined threshold value is applied between the grounding point and each line of the pair of transmission lines.
A field device that is turned on when a signal having an amplitude equal to or greater than the predetermined threshold value is applied between the grounding point and each line of the pair of transmission lines. The field device according to claim 1, wherein the filter unit includes a capacitor. The field device according to claim 2, wherein the filter unit includes a resistor connected in series with the capacitor. The switching unit includes a first diode and a second diode connected in parallel, and includes a second diode.
The forward direction of the first diode and the forward direction of the second diode are opposite to each other.
The field device according to any one of claims 1 to 3, wherein the predetermined threshold value is determined based on the magnitude of the forward voltage drop of each of the first diode and the second diode. It is configured to be able to connect to an external power supply.
The field device according to any one of claims 1 to 4, wherein DC power is supplied from the external power source via the transmission line. Equipped with a filter section and a switching section
The filter unit and the switching unit are connected in series between the grounding point and each line of a pair of transmission lines propagating a communication signal.
The switching unit
It is turned off when a signal having an amplitude less than a predetermined threshold value is applied between the grounding point and each line of the pair of transmission lines.
A filter circuit that is turned on when a signal having an amplitude equal to or greater than the predetermined threshold value is applied between the grounding point and each line of the pair of transmission lines.

Images