JPH0115919B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a process variable signal transmission method used when converting various process variables into current values flowing through a two-wire line and transmitting the converted signals to a remote receiving section or the like.

As such a transmission method, there are methods using direct current that have been disclosed in the "Two-wire current transmission device" (Japanese Patent Publication No. 49-42936) and the "Displacement/Electrical Signal Converter" (Japanese Patent Publication No. 53-16696). Alternatively, a method using a pulsed binary signal disclosed in the "Two-wire Pulse Transmitter" (Special Publication No. 50-7934) has been adopted, and in both cases, power is supplied from the receiver via a two-wire line. At the same time, signals corresponding to the process amount are transmitted using the same two-wire line.

However, all of these methods require a dedicated receiving section for each method, and separate receiving sections must be provided for the DC current method and the pulse signal method, and the receiving section cannot be used for each method. It had a drawback that it could not be shared.

The present invention has the purpose of fundamentally eliminating such drawbacks of the conventional methods, and provides an extremely convenient process amount signal transmission method that enables reception by either the pulse signal method or the DC signal method in the receiving section. It is something to do.

Hereinafter, details of the present invention will be explained with reference to figures showing examples.

Figure 1 shows a turbine-type positive displacement flowmeter as a sensor.
This is a circuit diagram when used as a TS, and from the sensor TS, which is composed of a turbine 2, a magnet 3, and a pickup coil PC provided inside a flow path 1, the waveforms of each part in FIG. 1 are shown in FIG. a
As shown, a pulse-like signal corresponding to the flow rate is generated, and this signal is applied to each input of the operational amplifier _A1 via the resistors _R1 and _R2 .

The non-inverting input of operational amplifier A ₁ is connected to the reference potential by resistor R ₃ and capacitor C ₁ , and the inverting input is connected to a predetermined bias by resistor R ₄ and connected to resistor R ₅ and capacitor C 1.
Negative feedback is applied by C ₂ , and signal a undergoes waveform shaping to become a pulse signal representing the process amount as shown in Figure 2b, which is used to determine the time constant by resistor R ₆ and capacitor C ₃ . A pulse generator PG such as a monostable multivibrator is driven to generate a pulse signal having a constant pulse width as shown in FIG. 2c.

This pulse signal c controls a field effect transistor (hereinafter referred to as FET) _Q1 to turn it on and off, and is also applied to the non-inverting input of operational amplifier _A2 via resistor _R7 . , is set by potentiometer RV ₁ , is amplified as a difference from the reference voltage applied via resistor R ₈ , controls FET Q ₂ in series with FET Q ₁ , and changes its impedance. It is variable to control the peak value of the current flowing through FET Q ₁ and Q ₂ .

On the other hand, the output terminals TB ₁ and TB ₂ are connected to the receiving section's power supply B and load resistor R _L via a two-wire line, and the voltage from this power supply B is applied to the FET Q ₃
It is stabilized by a constant current circuit and a constant voltage diode ZD, and is used for the power supply voltage of the operational amplifiers A ₁ and A _2. The flow rate as a process amount is determined by the current value passed between the output terminals TB ₁ and TB ₂ . It has become something to express.

Therefore, the current flowing through FETs Q ₁ and Q ₂ and potentiometer RV ₂ is interrupted as FETs Q ₁ turn on and off, and is also affected by the distributed constant of the two-wire line. The pulse current shown _{in FIG} .
It is applied to the non-inverting input of operational amplifier A ₂ , and since capacitors C ₄ and C ₅ are added to the amplifier A ₂ and it also operates as an integrator, the pulse current (c) is A current control loop is configured with negative feedback based on the average value, and the peak value of the pulse current (c) is controlled to be constant.

Therefore, the average value of the pulse current (c) is
The value corresponds to the detected value of the sensor TS, and at the same time, the number of pulses of the pulse current (c) also represents the detected value of the sensor TS. Therefore, the pulse current (c) is averaged by an integrating circuit, or a movable wire type meter, etc. , the detected value of the sensor TS can be obtained by simply counting the number of pulses of the pulse current (c), or further adding arbitrary calculations to the counted value. As a result, the detected value of the sensor TS can be obtained, and can be received by either a DC signal system or a pulse signal system receiving section.

Figure 3 is a circuit diagram when using a differential capacitance type sensor, in which fixed electrodes SP ₁ , SP ₂ and movable electrodes are used.
MP constitutes a differential capacitance sensor DS, and depending on the process amount, the movable electrode MP is replaced by the fixed electrode SP ₁ ,
The capacitance Cs _{1 and Cs 2 change differentially as the capacitance moves between SP 2 and Cs 2 .}

Furthermore, the capacitances Cs ₁ and Cs ₂ are connected to the input of the inverter IN ₁ via FETs Q ₄ and Q ₅ , and the output of the inverter IN ₁ is fed back by the resistor R ₁₀ . When FET Q ₄ and Q ₅ are turned on alternately, a charge/discharge type relaxation oscillator is configured by capacitance Cs ₁ or Cs ₂ , inverter IN ₁ , and resistor R ₁₀ , and capacitance Cs ₁ Alternatively, an oscillation output with a frequency corresponding to Cs ₂ can be obtained from the output of inverter IN ₁ .

Note that the inverter IN ₁ has a hysteresis characteristic in which the input threshold level at which the output is inverted is high at the rising edge and low at the falling edge, and as a result, the capacitance Cs ₁
Oscillation is performed at a frequency f ₁ corresponding to the capacitance Cs 2 and a frequency f ₂ corresponding to the capacitance Cs ₂ .

This frequency f ₁ or f ₂ is determined by counting a certain number of times by a counter CT, and depending on the count output n, the frequency f 1 or f 2 is determined with or without inverter IN ₂ .
Controls FET・Q ₄ and Q ₅ , and counts output n
When FET is “L” (low level), FET Q ₄ turns on and oscillates at frequency f ₁ , which is detected by the counter.
When the CT counts and the count output n becomes “H” (high level) due to the count up, this time
FET Q ₅ turns on and oscillates at frequency f ₂ , and this count up causes the count output n
returns to "L" and repeats this operation.

Therefore, as shown in Figure 4a, which shows the waveforms of each part in Figure 3, the frequency f ₁ is calculated from the count output n.
A pulse signal a is obtained which repeats a count period t ₁ of frequency f ₂ and a count period t ₂ of frequency f 2, and its duty ratio represents the process amount.

Therefore, the pulse current flowing between the output terminals TB ₁ and TB ₂ is affected by the distribution constant of the two-wire line, and becomes as shown in Figure 4b, but the duty ratio varies depending on the process amount. Become something.

In addition, if the amount of negative feedback from the potentiometer RV ₂ is determined appropriately, the pulse peak value of the pulse signal b can be adjusted according to the voltage averaged by the integrating circuit of the resistor R ₇ and the capacitor C ₄ . is controlled, and the average value of the pulse current (b) represents the process amount, which can be received by a direct current receiving section.

In addition, in a pulse signal type receiving section, the process amount can be obtained by shaping the pulse current (b) into a pulse train with a constant peak value and then counting the "H" or "L" period using a clock pulse, etc. Can be done.

In addition, in Fig. 3, if a device with nonlinear characteristics such as a diode circuit is inserted between the counter CT and the operational amplifier _A2 , the pulse signal b
The relationship between the average value AV of be able to.

However, since the pulse period of the pulse current (b) is not affected, it becomes possible to transmit the detected value of the process amount as is, depending on the duty ratio of the pulse current (b). ), the compensated data and the uncompensated data can be transmitted simultaneously, which is extremely convenient.

Note that it is also possible to use other switching elements as FET/Q ₁ and other variable impedance elements as FET/Q ₂ , and the configuration of the sensor and each part can be arbitrarily selected depending on the conditions.
The present invention is open to various modifications.

As is clear from the above explanation, according to the present invention, process amount transmission is realized in a form that is compatible with both DC current type and pulse signal type receiving units.
Since the transmitter can be shared, a significant effect can be obtained in transmitting the detection results of various process quantities.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing waveforms of each part in FIG. 1, FIG. 3 is a circuit diagram showing another embodiment, and FIG. FIG. 5 is a diagram showing the waveforms of each part, and FIG. 5 is a diagram showing the compensation characteristics of nonlinearity. Q ₁ , Q ₂ ...FET (field effect transistor),
A ₂ ... operational amplifier, R ₁ to R ₉ ... resistor, RV ₁ ,
RV ₂ ... Potentiometer, C ₃ , C ₄ ... Capacitor, TB ₁ , TB ₂ ... Output terminal.

Claims (1)

[Claims] 1. A means for generating a pulse signal corresponding to a process amount, and a switching element that uses the pulse signal to intermittent a current flowing through a two-wire line to generate a pulse current;
a control means connected in series to the switching element to which a feedback voltage based on the pulse current and the pulse signal are input, controls a peak value of the pulse current and sends it to the two-wire line; A two-wire process quantity signal transmission system, characterized in that the average value of the pulse current corresponds to the process quantity.