JPH0521198Y2 - - Google Patents

Description

[Detailed description of the invention] [Industrial application field] The present invention detects a physical quantity to be measured by electrical means, for example in a control system of a chemical plant, and sends the measurement signal to a signal processing unit located at a remote location. The present invention relates to field devices used in such cases.

[Conventional technology]

Generally, in a system that measures field data, a detection end is placed at each site, and a signal processing section is often placed centrally in a panel room or the like far from the field. Therefore, on the detection end side, the detection output obtained is amplified to some extent in anticipation of the line resistance to the panel room, and then sent to the signal processing section, and the detection output is primarily a voltage output. However, since current signals are generally more resistant to noise than voltage signals, they are sent out as current signals by providing a voltage/current converter or the like.

[Problem that the invention attempts to solve]

By the way, in such measurement systems, it is generally inevitable that the conversion characteristics of the signal detection element (conversion element) will fluctuate over time due to the nature of the signal detection element (conversion element), requiring occasional or periodic zero adjustment. Zero adjustment required transporting heavy adjustment equipment to a remote field.

In view of the above points, the present invention was devised to solve these problems.The purpose of this invention is to eliminate noise in the field when transmitting the field measurement signal to a signal processing unit located in a remote location. It is an object of the present invention to provide a field device which can reduce the amount of noise and can easily perform zero point adjustment.

[Means for solving the problem] In order to achieve the above object, the present invention uses a comparator to compare the output value of a detection circuit that outputs an electrical signal corresponding to the physical quantity to be measured with a zero value. The count value of the up-down counter is increased or decreased according to the output, and the count value of the up-down counter is converted into an analog signal by a digital-to-analog converter and sent to the detection circuit as a signal for zero adjustment of the conversion element. In the field device equipped with a zero point adjustment mechanism, the detection circuit has two conversion elements, one on the comparison side and the other on the measurement side, and the difference between the first and second detection input voltages obtained from the two conversion elements. first and second operational amplifiers that amplify voltage; an output current source amplifier that receives the output voltages of the first and second operational amplifiers as input and outputs an output current according to the difference voltage; The device is equipped with a zero point adjustment display element that displays the polarity of the output current of the current source amplifier.

[Effect]

When zero adjustment is required, if a clock signal is given to the counter either directly from the signal processing section or by sending a start signal to the clock oscillator built into the field device, the count value will change depending on the polarity of the output current for each input. As a result, the output current value changes toward zero value, and finally the digital
Zero adjustment is performed with an accuracy of less than one bit of the output of the analog converter. At this time, when zeroing by field, each display element that lights up depending on the polarity of the output current can be turned off.
Zero point adjustment can be easily performed without using special measuring equipment.

In addition, by using a differential output current source amplifier to send signals to a remote signal processing section, there is no influence of common mode voltage, and measurements can be made up to the signal processing section without being affected by noise in the field. Can transmit signals.

〔Example〕

FIG. 1 is a block diagram showing one embodiment of the present invention. In the figure, 1 is a detection circuit that has a conversion element that converts a predetermined physical quantity into an electrical quantity and sends a current signal I to a load 10 in the signal processing section of the panel room as an electrical signal corresponding to the physical quantity to be measured, and 2 is a detection circuit for this. A comparator that compares the output value with a zero value, 3 is an up-down counter that increases or decreases the count value depending on the output of this comparator 2, in other words, the polarity of the current signal I, and 4 converts the count value of this counter 3 into an analog signal. This is a digital-to-analog converter that converts the signal into a signal and sends it to the detection circuit 1 as a signal for zero adjustment of the conversion element.

Here, the detection circuit 1 has a conversion element on a comparison side (reference value) and a measurement side (sample side), and configures, for example, a bridge circuit to detect first and second detections obtained from both the conversion elements. The differential voltage between the input voltages is amplified and output. In this embodiment, a circuit configuration as shown in FIG. 2 is used to amplify this differential voltage and convert it into a current signal I for transmission.

FIG. 2 shows only the above-mentioned amplification section and current conversion section of the detection circuit 1.
14 is the first to fourth operational amplifiers, 15 to 22
are resistors, of which 15 to 17 are feedback resistors of the first to third operational amplifiers, respectively. Each resistor has a resistance value shown in the figure. Further, 23 and 24 are light emitting diodes, 25 and 26 are N-P-N and P-N-P transistors, respectively, 27 and 28 are each light emitting diode and a +V power supply,
This is a resistor inserted between the -V power supply.

In the above configuration, the first and second input voltages e 1 and 2 are connected between the non-inverting input terminals and the common terminal of the first and second operational amplifiers 11 and 12 as detection outputs from the detection elements on the reference side and the sample side, for _example . When e ₂ is applied, equation (1) holds between the outputs E ₁ and E ₂ , as is well known.

E ₂ −E ₁ = (1+2R ₁ /R ₂ ) (e ₂ −e ₁ ) ...(1) Now, let the input terminal voltage of the third operational amplifier 13 be es
Assuming that the resistance value of the load 10 is R _L , the inverting input terminal side and the non-inverting input terminal side are respectively E ₂ -es/R ₃ =es-(R _L +r) I/R ₄ ... …(2) [+] side E ₁ −es/R ₃ = es−R _L I/R ₄ … Both equations of (3) hold, and from this E ₂ −E ₁ /R ₃ = −rI/ As R ₄ , I = 1/r・R ₄ /R ₃ (E ₁ −E ₂ ) = 1/r・R ₄ /R ₃ (1+2R ₁ /R ₂ ) (e ₁ −e ₂ )...
(4) holds true.

From equation (4), the output current I is proportional to the difference (e ₁ −e ₂ ) between the input voltages e ₁ and e ₂ and is not affected by the common mode voltage at all. In other words, a differential current output type amplifier has been constructed.

In other words, while conventionally the preamplifier that performs voltage amplification and the voltage/current converter were configured separately as described above, in this embodiment, only the circuit shown in Fig. 2 has the functions of both. This greatly simplifies the configuration compared to the conventional method.

Note that the transistors 25 and 26 are the operational amplifier 1.
However, if a sufficient output current can be supplied from the operational amplifier 13, these transistors are not necessary, and a configuration in which a-a' in the figure is short-circuited may be used. Further, if the direction of current is limited to only one direction, either positive or negative, one of the transistors 25 and 26 can be omitted.

By the way, when zero adjustment is required, in this circuit, if the output current I is positive, the light-emitting diode 2
If the input voltage Vcc and the output voltage Vdc are both negative, the light-emitting diode 24 lights up. Therefore, if the zero adjustment is performed so that both light-emitting diodes are turned off, the zero adjustment can be easily performed without using any special measuring equipment. For example, _e1 and _e2 are output signals from a bridge that includes detection elements on the detection side and reference side, as mentioned above, although they are omitted in the figure.
As shown in FIG. 3, the direction of rotation of the volume knob 31 for bridge balance is indicated on the panel by light emitting diodes 23, 24, so that zero adjustment can be performed very simply, and there is no need to carry a large measuring device to the field as in the past.

The insertion position of these light emitting diodes is the second one.
For example, the fifth
As shown in the figure, it may be connected between the output terminal of the fourth operational amplifier 14 and the common terminal. That is, the output voltage V _A of the operational amplifier 14 is given by the output current I and the load resistance R _L as V _A =IR _L , and depending on the direction of I, one of the light emitting diodes 23 and 24 lights up. In short, the connection method is not particularly limited as long as a predetermined display is made depending on the direction of I. For example, as shown in FIG. It may also be inserted into the line itself. Moreover, it is not limited to light emitting diodes.

In addition, in this embodiment, it is possible to perform zero point adjustment from the panel chamber side using the signal at the terminal 29 which is the output of the fourth operational amplifier 14 in FIG. That is, the signal voltage of this terminal 29
V _A is at the same level as the non-inverting input of the operational amplifier 14, and is expressed as V _A =IR _L (5).

Therefore, if we make V _A = 0,
I=0, that is, zero adjustment can be performed.

Therefore, in this embodiment, as shown in FIG. 6, this output is used as the negative input of the comparator 2 consisting of the fifth operational amplifier, and the other input is connected to the common terminal (zero potential), and the output is used for up-down. Controls up and down of counter 3. That is, depending on the polarity of the output current I, for example, if it is positive, the clock pulse
Each time a CLOCK arrives, the counter 3 increments its count value by "1", and if it is negative, it also decrements by "1". As described above, the digital-to-analog converter 4 sends an analog signal corresponding to this count value to the detection circuit 1 as a zero adjustment signal. Therefore, when performing zero adjustment, make sure that this field device is not in the process of measuring, and apply the clock pulse from the panel chamber side.
When CLOCK is sent out, the output current I changes toward zero value, and finally falls within the accuracy of one bit of the output D/A _OUT of the digital-to-analog converter 4. In other words, as long as there is a clock input, even after the output current I becomes zero, D/
Although the A _OUT output changes, its direction is determined by the output of comparator 2, that is, the polarity of the output current I, so even if the clock input continues, the D/A _OUT
Only one bit (LSB) changes, and I never deviates from the zero point. Therefore, during zero adjustment, the digital-to-analog converter 4
If the number of clock pulses equal to the number of bits is applied, the output current I can be zero-adjusted without fail.

If no clock signal is input, the counter 3 holds its count value and the digital-to-analog converter 4 also keeps its output constant.

Note that the clock signal may not be applied directly from the panel room, but may be generated from a clock oscillation circuit built into the field device, but when a weak measurement signal is amplified, changes such as the clock signal may be generated during measurement. In the case of repetition, it is preferable that the source is not nearby, so even if the latter method is adopted, the clock oscillation circuit is operated by a command from the panel room only during zero adjustment.

Further, the counter 3 has a preset input terminal, and the preset value may be determined to be such that, for example, half of the output of the digital-to-analog converter 4 is output when the power is turned on. This preset value is set by the preset value setting circuit 5.
Loading to the counter 3 is performed by a control signal from the panel room side, and may be configured to be set when the power is turned on.

FIG. 7 shows a specific configuration example when this embodiment is applied to a gas chromatograph. In the figure, 101 is a sample-side conversion element (sensor), 102 is a reference-side conversion element (sensor), 103 is a bridge balance volume, and V _s −V _r is e ₂ − in FIG.
Equivalent to e ₁ . Further, during zero adjustment, a rectangular alternating waveform as shown is sent from the panel chamber side, and clock pulses of "0" and "1" are supplied to the counter 3 via the photocoupler 104. The D/A _OUT output of the digital-to-analog converter 4 changes the current value flowing to the reference side sensor, corrects errors due to changes in the sensor over time, and performs zero adjustment.

[Effect of idea]

As explained above, according to the present invention, the output value is compared with the zero value, the count value of the up-down counter is increased or decreased using the output, and this count value is converted into an analog signal and detected as the zero adjustment signal. By sending the signal to the circuit, zero adjustment can be performed from a remote signal processing section. Furthermore, when zero adjustment is performed in the field, by turning off the respective display elements that are turned on depending on the polarity of the output current, the zero point adjustment can be easily performed without using a special measuring device.

Furthermore, by using a differential output current source amplifier to send the signal to a remote signal processing section, there is no influence of common mode voltage, and measurements can be made up to the signal processing section without being affected by noise in the field. Can transmit signals.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the basic configuration of an embodiment of the present invention, Fig. 2 is a circuit diagram showing an example of the configuration of the detection circuit, Fig. 3 is a diagram for explaining the zero adjustment method using the volume knob, and Fig. 4 5 and 5 respectively show modified examples, FIG. 6 is a diagram for explaining the method of zero adjustment from the panel chamber side, and FIG. 7 shows a specific configuration example when applied to a gas chromatograph. It is a circuit diagram. DESCRIPTION OF SYMBOLS 1...Detection circuit, 2...Comparator, 3...Up-down counter, 4...Digital-to-analog converter, 10...Load on the panel room side, 100...Field device, 101...Sample side conversion element ,
102...Reference side conversion element.

Claims (1)

[Claim for Utility Model Registration] The output value of a detection circuit that outputs an electrical signal corresponding to the physical quantity to be measured is compared with a zero value using a comparator, and the count value of an up-down counter is increased or decreased according to the output. In a field device equipped with a zero point adjustment mechanism that converts the counted value of the up-down counter into an analog signal by a digital-to-analog converter and sends it to the detection circuit as a signal for zero adjustment of the conversion element, the detection circuit comprises: first and second operational amplifiers that have two conversion elements as conversion elements, one on the comparison side and the other on the measurement side, and that amplify the difference voltage between the first and second detection input voltages obtained from the two conversion elements; An output current source amplifier which takes each output voltage of the first and second operational amplifiers as input and sends out an output current according to the difference voltage therebetween, and displays its polarity according to the output current of this output current source amplifier. A field device characterized by comprising a display element for zero point adjustment.