JPH0354398B2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention converts the output signal of a detector that detects physical quantities such as pressure and displacement into a corresponding current, and transmits the signal remotely via a two-wire transmission line. The present invention relates to a two-wire transmission circuit for transmitting data to a receiving unit.

<Prior art> As is well known, a two-wire transmission circuit converts the output signal of a detector into a corresponding current and then supplies a two-wire transmission path, and because only two transmission lines are required. Widely used in the field of industrial measurement.

The first embodiment of this type of conventional two-wire transmission circuit is described below.
As shown in the figure. Terminals L ₁ and L ₂ are connected to a power source E _B via a receiver A that receives current. The power source E _B is, for example, a 24 V DC power source. An output transistor Q ₁ and a feedback resistor R ₁ are connected in series to terminals L ₁ and L ₂ . A constant current circuit CC1 and a Zener diode _ZD are connected to both ends of the output transistor _Q1 .
A constant voltage E _Z is obtained across the Zener diode Z _D. SC1 is a sensor circuit that detects physical quantities and converts them into electrical signals, and connects terminals T ₁ and T ₂ by constant voltage E _Z
is energized through. The output of the sensor circuit SC1 is connected from the terminal T ₃ to the operational amplifier Q ₂ via the resistor R ₂
is connected to the non-inverting input terminal of The non-inverting input terminal is also connected to the voltage dividing point of resistor R ₁ via resistor R ₃ , and the voltage obtained by dividing the constant voltage E _Z by resistor R ₄ is applied to the inverting input terminal of operational amplifier Q ₂ . There is. resistance
R ₁ , R ₂ , R ₃ and R ₄ constitute one resistor network. The output transistor _Q1 is connected to be controlled by the output of the operational amplifier _Q2 .

The difference between the voltage signal from sensor circuit SC1 and the zero point setting voltage signal from resistor _R4 controls the current in output transistor _Q1 via operational amplifier _Q2 .
The current flowing through the output transistor Q ₁ and the constant current circuit CC1 flows through the resistor R ₁ , and a voltage signal proportional to this current is fed back to the operational amplifier Q ₂ via the resistor R ₃ , and is applied between both input terminals of the operational amplifier Q ₂ . It operates so that the potential difference between the two becomes zero. As a result, the sensor circuit
The output of SC1 is the transmission current I _L flowing between terminals L ₁ and L ₂
This is displayed on the receiving section A, and functions as a so-called two-wire transmission circuit.

However, in such a conventional two-wire transmission circuit, after the sensor circuit SC1 detects a physical quantity and converts it into an electrical signal, the electrical signal is supplied from the terminal _T3 to the operational amplifier _Q2 , and outputs, for example, 4mA to 20mA. It is converted into a relatively large current signal such as,
Since the output voltage obtained at the terminal _T3 of the sensor circuit SC1 is generally weak, there is a drawback that if this portion receives external induction noise, an error will occur in the corresponding transmission current I _L.

<Objective of the Invention> In view of the above-mentioned conventional technology, the present invention is directed to a sensor circuit that converts a physical quantity into an electrical signal, and even if the output end of the sensor circuit receives induced noise, the effect hardly appears on the transmission current2.
The purpose is to provide a wire transmission circuit.

<Configuration of the Invention> The configuration of the present invention that achieves this object relates to a two-wire transmission circuit that supplies a transmission current to a receiving section via a two-wire transmission path, and adjusts the duty ratio according to the physical quantity to be measured. A sensor circuit section that outputs a pulse signal that changes, a smoothing circuit that averages the pulse signal from this sensor circuit section, a transmission current control section that controls the transmission current using the output of this smoothing circuit, and a transmission current control section that corresponds to the transmission current. The present invention is characterized in that it has a feedback circuit that outputs a feedback voltage, and supplies this feedback voltage to the power source of the sensor circuit section.

<Example> Hereinafter, an example of the present invention will be described in detail based on the drawings. FIG. 2 is a block diagram showing one embodiment of the sensor circuit that constitutes the main part of the present invention, FIG. 3 is a waveform diagram showing waveforms of each part, FIG. 4 is a waveform diagram of output pulses of the sensor circuit, and FIG. The figure is a block diagram showing the overall configuration of an embodiment of the present invention.

First, the sensor circuit shown in FIG. 2 will be explained.
The sensor circuit SC2 is composed of a detection section PCC that converts differential pressure or the like into capacitance, and a capacitance/duty ratio conversion section CDC that converts a change in capacitance in the detection section PCC into a duty ratio.

The detection part PCC has a movable electrode between fixed electrodes SP ₁ and SP ₂ .
An MP is provided such that a mechanical displacement ΔP occurs in response to the physical displacement to be detected.
Fixed electrode SP ₁ is connected to terminal T ₄ , fixed electrode SP ₂ is connected to terminal T ₅
, the movable electrodes MP are respectively connected to the terminals _T6 .

A capacitance C ₁ is formed between the fixed electrode SP ₁ and the movable electrode MP, and a capacitance C ₂ is formed between the fixed electrode SP ₂ and the movable electrode MP. It is designed to vary differentially depending on the mechanical displacement ΔP of MP. If the capacitances C ₁ and C ₂ when ΔP=0 are C ₀ , they are each expressed by the following equations.

C ₁ =C ₀ 1/1−k ₁ ·ΔP (1) C ₂ =C ₀ 1/1+k ₁ ·ΔP (2) However, k ₁ is a constant.

The capacitance/duty ratio converter CDC has the positive power supply terminal T ₇ ,
It has a negative power supply terminal T ₈ and an output terminal T ₉ .
Terminal _T6 is connected to the input end of inverter _G1 and one end of capacitor C _D for voltage division, and its output end is connected to the input end CL of n-bit counter CT via inverter _G2 . There is. The n-bit output terminal Q _o of the counter CT is connected via the inverter G ₃ to the output terminal T ₉ of the capacitance/duty converter CDC, and at the same time is connected to one of the input terminals of the gate G ₄ . Controls switching of C ₁ and C ₂ . Inverter G ₁
The output terminal of is connected to the other terminal of the input terminal of gate G ₄ and at the same time to the input terminal of gate G ₅ .
The output end of the gate _G4 is connected to the terminal _T5 , the output end of the gate _G5 is connected to the terminal _T4 , and the input end of the gate _G6 is connected between them. Gate G ₆
A current limiting circuit is installed between the output terminal of and terminal T ₆ to regulate the charging/discharging current of capacitors C ₁ and C ₂ to a constant current value.
CC2 is connected. In addition, inverter G ₁ ~
The positive power supplies of G ₃ , gates G ₄ to G ₆ , and counter CT are connected to a positive power terminal T ₇ , and the negative power terminals of G 3 , gates G 4 to G 6 , and counter CT are connected to a negative power terminal T ₈ .

The operation of the sensor circuit SC2 configured as described above will be explained with reference to the waveform diagram of FIG. 3.

Now, consider the case where the output terminal _Qo of the counter CT is in the "H" state. At this time, one input terminal of the gate _G4 is in the "H" state. In this state, when the output terminal of inverter _G2 becomes "L" state, each output terminal of gates _G4 and _G5 both becomes "H" state.
As shown in FIG. 3A, the voltage becomes approximately equal to the positive power supply voltage V _DD applied to the terminal T ₇ . Therefore, fixed electrodes SP1 and SP2 are both connected to the positive power supply voltage V _DD
Therefore, the voltage at the input terminal of inverter _G1 ((Figure 3B) is the sum of the positive power supply voltage _VDD and negative power supply voltage _VSS , and the parallel capacitance of capacitances _C1 and _C2 ( _C1 + _C2 )
The voltage suddenly rises almost perpendicular to the voltage divided by the capacitance C _D. On the other hand, the input terminals of gate _G6 are both “H”
Since the output terminal is in the "L" state, it is held at approximately the negative power supply voltage V _SS . Therefore, the charges in the capacitances C ₁ and C ₂ and the capacitance _CD immediately start discharging via the current limiting circuit CC2 and the output impedance of the gate G ₆ .
Since this discharge current is regulated to a constant current value by the current limiting circuit CC2, the voltage at the input terminal of the inverter _G1 decreases linearly as shown in FIG. 3B. When the voltage at the input terminal of inverter _G1 drops to the threshold level _VTH at which its output is inverted, the output of inverter _G1 (Fig. 3C) changes to "H".
As a result, both gates G ₄ and G ₅ go into the “L” state, and the charges in capacitances C ₁ , C ₂ and capacitance C _D are rapidly discharged. The voltage at the input end of G ₁ drops almost vertically. At this time, the voltage at both input terminals of gate _G6 is in the "L" state, so its output terminal is in the "H" state.
state, and the current limiting circuit is activated as shown in Figure 3B.
Capacitance C ₁ , C ₂ and capacitance C _D via CC2
is charged, and the voltage at the input end of inverter _G1 increases linearly. When the voltage at the input end of inverter _G1 reaches the threshold level _VTH , the output end of inverter _G1 turns to "L", and the other end of gate _G4 and the input end of gate _G5 both go to "L". Therefore, both output terminals become “H” and the inverter is turned on again.
The voltage at the input end of _G1 rises rapidly, and the above operation is repeated thereafter.

On the other hand, the output of inverter _G1 is counted by counter CT via inverter _G2 ,
When a certain number of counts are done, the counter CT
The state of the output terminal _Qo changes from the "H" state to the "L" state, and this state is maintained until a certain number of counts are performed again. When the output of counter CT goes to "L" state, one end of gate _G4 becomes "L", so its output end becomes "H" regardless of the state of the other end.
state. Therefore, fixed electrode SP2 is approximately V _DD
The voltage is fixed at . On the other hand, the output terminal of gate G ₅ is inverted depending on the state of the output terminal of inverter G ₁ ,
Correspondingly, the output of gate _G6 is also inverted.
The states of the output terminals of gate _G5 and gate _G6 are opposite to each other. Therefore, this time, the same charging and discharging as described above is performed repeatedly in the capacitance _C1 between terminals _T4 and _T6 , and when the output of the counter CT turns to "H", the above-mentioned charging and discharging is performed again. will be carried out.

Here, the voltage change e ₁ at the input terminal of the inverter G ₁ with reference to the threshold level V _TH is expressed by the following equation.

e ₁ = C ₁ + C ₂ / (C ₁ + C ₂ ) + C _D (V _DD + V _SS ) (3) Also, the voltage change e ₁ is the threshold level
The time t ₁ required to decrease to V _TH is determined by the capacitance C ₁ , C when viewed from the current limit circuit CC2, assuming that i is a constant discharge current regulated by the current limit circuit CC2. ₂ parallel capacitance C ₁ + C ₂ and capacitance
Since C _D is connected in parallel, the following equation is obtained.

i・t ₁ =e ₁ {(C ₁ +C ₂ )+C _D (4) When t ₁ is determined from equations (3) and (4), the following equation is obtained.

t ₁ = ((c ₁ + c ₂ ) V _DD + V _SS /i (5) Note that during repeated charging and discharging, charge corresponding to the threshold level V _TH is accumulated in the capacitance C _D. , since charging and discharging are performed using this as a reference potential, the voltage change e ₁ on the charging side and the voltage change e ₂ on the discharging side are equal, and the charging of this voltage change is controlled by the constant value current i by the current limiting circuit CC2. Therefore, the charging time t ₂ is also equal to the discharging time t _1. These relationships are the same even when the output of the counter CT is "L", but in this case charging and discharging is performed by the capacitance C ₁ Since this is done with and and C _D , equation (5) becomes as follows.

t ₁ = C ₁ V _DD + V _SS /i (6) Putting the above together, the peak value V DD of the output terminal Q _o of the counter CT is in the period T ₁ when it is in the “H” state, and the peak value V _DD is in the period T ₂ when it is in the “L” state. A pulse voltage with peak value V _SS can be obtained. Since the output of the counter CT is inverted by the inverter _G3 , the pulse _Ps obtained at the terminal _T9 of the capacitance/duty ratio converter CDC is as shown in FIG. Period T _H
corresponds to the “H” state of the output terminal Q _o of the counter CT, so it is proportional to the capacitance ((c ₁ + C ₂ ), and the period
Since T _L corresponds to the "L" state of the output terminal Q _o of the counter CT, it is proportional to the capacitance C ₁ . Therefore, if the constant of proportionality is k ₂ , then T _H =k ₂ (c ₁ +c ₂ ) (6) T _L =k ₂ C ₁ (7).

Next, a description will be given of the embodiment shown in FIG. 5 in which a two-wire transmission circuit is constructed using the sensor circuit SC2 described above.

An output transistor Q ₃ , a diode D ₁ and a feedback resistor R ₅ are connected in series to the terminals L ₁ and L ₂ . A constant current circuit CC1 and a Zener diode Z _D are connected in series to both ends of the series circuit of the output transistor Q ₃ and the diode D ₁ , and a constant voltage E _Z is obtained across the Zener diode. Operational amplifier _Q4 is energized by this constant voltage _EZ , and its output voltage controls output transistor _Q3 . Operational amplifier _Q4 and output transistor _Q3 constitute a transmission current control section.
On the other hand, the inverting input terminal of the operational amplifier _Q4 is supplied with a voltage _V1 obtained by dividing the constant voltage _EZ by a resistor _R4 . The pulse output of the sensor circuit SC2 is applied to the non-inverting input terminal of the operational amplifier _Q4 as a smoothed voltage _V2 via a smoothing circuit composed of a resistor _R6 and a capacitor _C3 . A voltage V DD obtained by dividing the constant voltage E _Z by a resistor R ₇ at a voltage division ratio _α is applied to the terminal T ₇ of the capacitance/duty ratio converter CDC. A voltage obtained by subtracting the bias voltage V ₃ from the feedback voltage R ₅ I _L generated across the feedback resistor R ₅ is applied to the terminal T ₈ of the capacitance/duty ratio converter CDC at a voltage dividing ratio β using the resistor R ₈ . Voltage V _SS is applied.

Capacity/duty ratio converter CDC output terminal _T9
Since the pulse P _S shown in Figure 4 is obtained, the resistance
The voltage V ₂ smoothed by R ₆ and capacitor C ₃ is as follows.

V ₂ = T _H / T _H + T _L・(V _DD + V _SS ) (8) Here, the relationship of V _DD = αV _Z , V _SS = β (R ₅ I _L −V ₃ ) is applied to equation (8). Substituting, V ₂ = T _H /T _H + T _L {αV _Z + β(R ₅ I _L −V ₃ ) (8)′. The voltage V ₁ at the voltage dividing point of this voltage V ₂ and resistor R ₄
Since operational amplifier Q ₄ controls output transistor Q ₃ so that V ₂ =V ₁ , the following equation is obtained.

V ₁ =T _H /T _H +T _L {αV _Z +β(R ₅ I _L −V ₃ )} ∴αV _Z +β(R ₅ I _L −V ₃ )=(1+T _L /T _H )V ₂ (9) Here, if we use equations (1), (2), (6), and (7), we get T _L /T _H = 1 + R ₁ ΔP/2 (10), so we substitute this into equation (10) and rearrange it. Then, I _L =V ₃ /R ₅ +1/β・1/R ₅ (3/2V ₁ −αV _Z ) +1/β・R ₁ ΔPV ₁ /2R ₅ (11). The first term of equation (11) is set to V ₃ /R ₅ = 4 mA in the range of I _L = 4 to 20 mA, and the second term is set to α =
It can be made zero by setting it to 3V ₁ /2V _Z. The third term is a term that provides a transmission current I _L proportional to the mechanical displacement ΔP of the movable electrode MP. The span can be adjusted by changing the voltage division ratio β of resistor _R8 .

<Effects of the Invention> As specifically explained above with the examples,
According to the present invention, the output of the sensor circuit is put into the feedback loop as a configuration that feeds back a voltage corresponding to the transmission current to the power supply of the sensor circuit, so that even if the output end of the sensor circuit receives noise, its influence is eliminated. A two-wire transmission circuit that is less likely to appear in the transmission current and is resistant to noise as a whole can be realized.

[Brief explanation of drawings]

Figure 1 is a block diagram showing a conventional embodiment, Figure 2 is a block diagram showing a conventional embodiment.
The figure is a block diagram showing one embodiment of the sensor circuit that constitutes the main part of the present invention, FIG. 3 is a waveform diagram showing waveforms of various parts of the embodiment of FIG. 2, and FIG. FIG. 5 is a waveform diagram showing the waveform of the output pulse. FIG. 5 is a block diagram showing the overall configuration of an embodiment of the present invention. Q ₃ ... Output transistor, Q ₄ ... Operational amplifier,
SC1, SC2...sensor circuit section, PCC...detection section, CDC...capacity/duty ratio conversion section, _G1 , _G2 ,
G ₃ ... Inverter, G ₄ , G ₅ , G ₆ ... Gate, CC
1... Constant current circuit, CC2... Current limiting circuit, CT
……counter.

Claims (1)

[Claims] 1. A two-wire transmission circuit that supplies transmission current to a receiving section via a two-wire transmission path, including a sensor circuit section that outputs a pulse signal whose duty ratio changes depending on a physical quantity to be measured, and the sensor circuit. a smoothing circuit that averages pulse signals from the smoothing circuit, a transmission current control section that controls the transmission current based on the output of the smoothing circuit, and a feedback circuit that outputs a feedback voltage corresponding to the transmission current; A two-wire transmission circuit characterized in that a feedback voltage is supplied to a power source of the sensor circuit section.