JPH0334028B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a capacitance measuring device that generates a signal output in response to a change in capacitance to be measured, and in particular can output a measurement signal and supply power to the device using the same signal line. The present invention relates to a two-wire capacitance measuring device.

2. Description of the Related Art Conventionally, as a capacitance measuring device used for detecting a liquid level, etc., there is, for example, one shown in FIG.

In FIG. 1, numeral 1 is an electrode that senses the capacitance to be measured. For example 4~20mA
The measured output is output from the signal output terminal 8a. Note that a resistor 5 provided in the output section 3 detects an output current value for feedback control to stabilize the output current.

On the other hand, power is supplied to the converting section 2 and the output section 3 by stabilizing the DC voltage applied via the power input terminal 8b with a constant voltage circuit 4 and via a filter circuit consisting of a resistor 6 and a capacitor 7. There is. Note that 8c is a common terminal for the power input terminal 8b and the signal output terminal 8a.

By the way, in the conventional device shown in FIG. 1, power supply and signal output are performed using separate signal lines for the following reason.

First, the conversion of the capacitance value into an electrical signal in the converter 2 is performed by pulse-like signal processing, for example, by periodically generating a pulse with a time width corresponding to the value of the capacitance to be measured. , the clock pulses are counted over the time width of these pulses, and the counted digital values are converted into analog and output.

When pulse-like signal processing is performed in the converter 2 in this way, even if an element with low current consumption such as a CMOS-IC is used, a considerably large pulse-like current consumption occurs during operation, resulting in instantaneous It consumes a large amount of current, which causes noise. To prevent this noise, the circuit is generally stabilized by installing a large capacity capacitor to absorb voltage fluctuations, but of course the power consumed by the capacitor is absorbed through the power supply line. The capacitor will be charged, and noise will also be generated by the current flowing into the capacitor from the power supply. Therefore, in a conventional device having a converter 3 that can consume current in a pulsed manner, in order to reliably prevent errors caused by noise entering the signal output line, a dedicated power supply is required separately from the signal output line. There is no choice but to use the so-called three-wire system, which has two lines.

Of course, by using a large-capacity filter capacitor 7 provided on the output side of the constant voltage circuit 4, noise caused by pulsed current consumption can be prevented to some extent even in the case of a two-wire system. This type of equipment used for industrial purposes is required to have an explosion-proof structure from the point of view of safety, and the use of large-capacity capacitors is generally impractical because it is difficult from the point of view of the explosion-proof structure. In addition, noise generated by pulsed current consumption can be suppressed by feedback control of the output current, but this feedback control is
In the case of the A output current, it is effective only when the instantaneous current value is less than the output current value according to the signal, and when it exceeds it, it appears as instantaneous noise. Therefore, it was difficult to create two lines.

The present invention has been made by focusing on such conventional problems, and provides a circuit and a circuit that minimizes pulse-like changes in current consumption in signal processing for converting the value of a capacitance to be measured into an electrical signal. By doing so, we have realized a so-called two-wire system in which power supply and signal output can be performed using the same signal line without using a large-capacity power supply capacitor, and the influence of noise can be virtually ignored. It is an object of the present invention to provide a two-wire capacitance measuring device having an essentially explosion-proof structure, which reduces the amount of output current and allows easy zero and span adjustment of the output current.

Hereinafter, the present invention will be explained based on the drawings.

FIG. 2 is a circuit block diagram showing one embodiment of the present invention.

First, to explain the configuration, numeral 10 is a conversion unit that generates a pulse signal having a time width according to the value of the capacitance to be measured, and the capacitance to be measured 11 is charged in series with the capacitance to be measured 11 represented by a variable capacitor. A variable resistor 12 is provided for this purpose, and the connection point between the capacitance to be measured 11 and the variable resistor 12 is input to one side of the voltage comparator 13. The other end of the voltage comparator 13 is connected to a reference voltage source 14 that generates a predetermined reference voltage Vr. here,
If the terminal voltage of the capacitor to be measured 11 is V _c1 , then Vc1V _r
At this time, the voltage comparator 13 produces an L level output, and when the terminal voltage V _c1 becomes equal to or lower than the reference voltage V _r , the voltage comparator 13 switches to an H level output.

20 is a voltage setting circuit for generating a DC voltage according to the fixed capacitance in order to cancel out the fixed capacitance included in the measured capacitance 11 in the converter 10 and extract only the changed amount, and 20 has a capacitor. A variable resistor 22 for charging is connected in series with the built-in capacitor 21 shown in FIG.
The connection point with the converter 1 is input to one side of the voltage comparator 23, and the other side of the voltage comparator
The reference voltage V _r from the reference voltage source 14 at 0 is input. The operation of the voltage comparator 23 in this voltage setting circuit 20 is similar to that of the voltage comparator 13 of the converter 10, and when the terminal voltage V _c2 of the built-in capacitor 21 exceeds the reference voltage V _r , the output of the comparator 23 is at L level. Therefore, the terminal voltage
When V _c2 becomes lower than the reference voltage V _r , it switches to H level output.

30 is a switch section, which includes a pulse generator 31 that generates a pulse signal of a predetermined frequency, and a switch 3 that is closed by the pulse output of this pulse generator 31.
2 and 33 as the capacitance to be measured 11 and the built-in capacitor 2 in the converter 10, voltage setting circuit 20, respectively.
1, and switches 32 and 33 are connected in parallel with each of switches 32 and 33 by the pulse signal from the pulse generator 31.
By closing, each of the capacitor to be measured 11 and the built-in capacitor 21 is periodically discharged. 40 is a filter section, and each voltage comparator 13, 23 of the conversion section 10 and the voltage setting circuit 20
Low-pass filters 41 and 51 are provided at the outputs of converters 10 and 20 to convert pulse signals corresponding to the respective values of capacitance 11 to be measured and built-in capacitance 21 into DC voltages and output the DC voltages. Reference numeral 60 denotes a differential amplifier section, which includes a differential amplifier 61 into which the DC voltage output from the low-pass filters 41 and 51 is input via resistors 62 and 64, respectively, and feedback resistors 63,
65 and input resistors 62 and 64 to amplify and output a signal according to the difference between both input voltages, and the output of the differential amplifier 61 has a predetermined amplification factor determined by the input resistors 62 and 64. A variable resistor 66 is biased by a reference voltage source 67 and connected.

Reference numeral 70 denotes an output section, in which a DC voltage is applied to the span-adjusted output signal from the differential amplifier section 60 through a variable resistor 76 for creating a divided voltage of a reference voltage source 67 and adjusting the zero point of the output current. The output of the amplifier 71 drives a transistor 72 to control the output current flowing between the power signal terminal 90 and the common terminal 91. Further, a resistor 73 for current detection is connected to the emitter side of the transistor 72, and a resistor 78
A detection voltage proportional to the output current detected by the amplifier 71 is feedback-connected to one of the inputs of the amplifier 71 via a resistor 75, so as to perform feedback control of the output current. Furthermore, 80 is a power supply section, which stabilizes the DC voltage supplied from the power supply/signal terminal 90 to a predetermined power supply voltage using a constant voltage circuit 81 and outputs the stabilized voltage.
A filter circuit consisting of a resistor 82 and a capacitor 83 is provided on the output side of the constant voltage circuit 81, and the converters 10, 20, the switch section 30, the filter section 40, the differential amplifier section 60 and the output are connected through this filter circuit. A power supply voltage is supplied to each of the sections 70.

Next, the operation of the embodiment of the present invention shown in FIG. 2 will be explained. First, the capacitance to be measured 11 in the converter 10
The output effect of a pulse signal having a time width corresponding to the value of will be explained with reference to the time chart of FIG.

The switch 32 is turned on at regular intervals by a pulse signal from the pulse generator 31 in the switch unit 30, and when the switch 32 is turned off, a constant voltage is applied to the capacitor 11 through the variable resistor 12. Power is supplied from the circuit 81.
Now, the power supply voltage supplied from the constant voltage circuit 81 is E
If the reference voltage V _r = aE (a < 1), then from the time the switch 32 is opened, the voltage is applied via the variable resistor 12
The time t _A until charging to V _c1 = aE is (However, since R ₁₂ is the value of the resistor 12 and C ₁₁ is the value of the capacitance to be measured), t _A = R ₁₂・C ₁₁・_lo (1/1-a) ...(1) (However, , _lo indicates the natural logarithm). Here, the capacitance of the measured capacitor 11
C ₁₁ is the sum of the fixed capacitance C _F and the changing capacitance C _V that is to be measured, and is as follows: C ₁₁ = C _F + C _V (2). This fixed capacitance C _F has a value ranging from a fraction to several times _the measured capacitance C _V depending on the object to be measured _. A circuit is provided in which the built-in capacitor 21 and variable resistor 22 in the voltage setting circuit 20 are connected in series, and the built-in capacitor 21 is charged in synchronization with the switch 32 by turning off the switch 33 in the same way as the capacitance to be measured 11. The terminal voltage of the built-in capacitor 21 in this voltage setting circuit 20
The charging time t _B until V _c2 = aE is given as t _B = R ₂₂ ·C ₂₁ · _lo (1/1-a) (3), similar to the above equation (1).

Taking the converter 10 as an example, the measurement of the charging times t _A and t _B of the capacitor 11 to be measured and the built-in capacitor 21 in the converter 10 and the voltage setting circuit 20 is as shown in the time chart in FIG. As shown, when the switch 32 is turned from on to off, charging of the capacitor 11 to be measured starts via the variable resistor 12, and since the value of the variable resistor 12 is constant, the charging time constant It is determined depending on the capacitance C ₁₁ , and the terminal voltage V _c1 of the capacitance to be measured 11 increases in accordance with this time constant. When the terminal voltage V _c1 is lower than the reference voltage V _r from the reference power supply 14, the output of the voltage comparator 13 is at H level, and when the terminal voltage V _c1 exceeds the reference voltage V _r , the output of the voltage comparator 13 goes to L level. When the switch 32 is turned on after a predetermined time, the capacitor 11 to be measured that has been charged up to that point is reset to discharge, and the output of the comparator 13 returns to the H level again. As a result, it is clear from the time chart in FIG. 3 that as the capacitance C 11 of the capacitance to be measured ₁₁ becomes smaller, as indicated by A, the time period during which the comparator 13 falls to the L level becomes longer, and As shown in the figure, when the capacitance C ₁₁ of the capacitor 11 to be measured becomes large, the time for the output of the comparator 13 to fall to the L level becomes shorter, and as a result, the time for the output to reach the H level corresponds to the capacitance C ₁₁ of the capacitor 11 to be measured. A pulse signal whose width changes is output from the comparator 13.

The same effect is applied to the voltage setting circuit 20 to generate a pulse signal having a time width corresponding to the capacitance.

In the conversion section 10 and the voltage setting circuit 20, a pulse signal having a time width corresponding to the respective values of the capacitance to be measured 11 and the built-in capacitance 21 is passed through the filter section 4.
The voltage is converted into a DC voltage by low-pass filters 41 and 51 provided at 0, and is applied to the differential amplification section 60. The differential amplification section 60 outputs a signal based on the difference between the DC voltages from the low-pass filters 41 and 51. In other words, the DC voltage component E _A output from the low-pass filter 41 is calculated as E _A =k・t _A =k・R ₁₂・(C _F +C _V
)・l _o (1/1-a)...(4) (where k is a proportionality constant), and the DC component voltage E _B output from the low-pass filter 51 is calculated using the above equation (3). E _B =k・t _B =k・R ₂₂・C ₂₁・_lo (1/1−a) ...(5). Here, the respective values of resistors 62 to 65 are
When R ₆₁ to R ₆₅ and R ₆₂ /R ₆₄ = R ₆₂ /R ₆₃ , the output of the differential amplifier 61 is E _B −E _A = (CR ₂₂・C ₂₁ −R ₁₂・C _F −R ₁₂・_CV ）
・k・l _o (1/1−a) (6), and a voltage of (E _B −E _A ) appears across the variable resistor 66.

Here, in order to cancel the unnecessary fixed capacitance C _F in measurement, for example, when detecting the liquid level as the capacitance to be measured 11, when the liquid level is at zero level, the voltage setting circuit 20 can be adjusted. The value of the resistor 22 may be adjusted so that R ₂₂・C ₂₁ =R ₁₂・C _F. In this way, the converter 10 and the voltage setting circuit 2
If the relationship between the time constants at 0 is set equal, the above equation (6) becomes E _B −E _A = −R ₁₂・C _V・k・_lo (1/1−a) ……(7) and changes. A DC signal corresponding only to the capacitance C _V can now be obtained.

In this way, the differential amplifier 60 outputs a DC voltage corresponding only to the changing capacitance C _V excluding the fixed capacitance C _F in the capacitance to be measured 11,
This output voltage is input to the amplifier 71 in the output section 70 to drive the transistor 72, and is proportional to the DC signal from the differential amplifier section 60 under feedback control by the current detection resistor 73, feedback resistor 75, and input resistor 74. For example, an output current of 4 to 20 mmA is caused to flow to a predetermined load (such as an indicator) connected between the power signal terminal 90 and the common terminal 91. Further, span adjustment for the output current is performed by a variable resistor 66 in the differential amplifier section 60, and zero point adjustment is performed by a variable resistor 76 provided in the output section 70.

Note that the switches 32, 3 in the embodiment of FIG.
The pulse interval of the pulse signal from the pulse generator 30 that turns off the capacitor 11 is set so that the pulse width is longer than the time t _A until the capacitance to be measured 11 is charged to the terminal voltage V _c1 = aE by the maximum capacitance. It is necessary to define Next, to explain the noise caused by pulse-like current consumption in the embodiment shown in FIG. 2, the circuit sections that operate in a pulse-like manner in the embodiment shown in FIG. is the switch 32,3 in the switch section 30
If a semiconductor switch is used as the switch section 3, the output of the pulse generator 31 requires only a very small amount of power, and the current instantaneously consumed by the switch section 30 can be suppressed to a very small value. Also, the conversion unit 10,
The maximum value of the current flowing in the voltage setting circuit 20 to charge the capacitor 11 to be measured and the built-in capacitor 21 is as follows.
It is equal to the power supply voltage E divided by the minimum value of the variable resistors 12 and 22, and if the minimum value of the variable resistors 12 and 22 is set as a large resistance value, the capacitance to be measured 1
1. The value of the charging current that instantaneously flows to charge the built-in capacitor 21 can be limited to a very small value. Further, since all circuit sections other than the conversion section 10, voltage setting circuit 20, and switch section 30 are circuit sections that consume current in a direct current manner, no pulse-like changes in the current consumption by these circuit sections occur. Therefore, it is only necessary to absorb fluctuations due to extremely small pulse-like current consumption in the converting section 10, voltage setting circuit 20, and switch section 30, so even if the capacitance of the capacitor 83 provided in the power supply section 80 is set to a small value, However, there are almost no fluctuations in the power supply voltage, and when the voltage of the capacitor 83 changes due to pulsed current consumption, the change in the current flowing from the constant voltage circuit 81 to the capacitor 83 via the resistor 82 is small.
The influence of noise on the signal line that also serves as a power supply line can be suppressed to an almost negligible level. Furthermore, since the circuit sections that operate in a pulsed manner are kept to a very small number of circuit sections, such as the conversion section 10, the voltage setting circuit 20, and the switch section 30, the DC current consumed by the circuit sections that operate in a pulsed manner is also reduced. This is very small, and therefore the value of the resistor 82 in the power supply section 80 can be made relatively large, and the change in the current flowing through the resistor 82 can be further reduced. As a result, the power supply signal terminal 90
The change that occurs in the power supply current flowing into the constant voltage circuit 81 via the constant voltage circuit 81 is extremely small.

Furthermore, the span adjustment and zero adjustment of the output current in the embodiment shown in FIG. It is adjusted by dividing the voltage of the DC signal, and on the other hand, for zero point adjustment, the reference voltage of the reference voltage source 67 connected in series with the variable resistor 66 is set by dividing the voltage by the variable resistor 76, and is set independently in this way. Since the output current is controlled by adding the respective DC voltages for the span adjustment and zero adjustment in the amplifier 71, the span adjustment and zero point adjustment do not interfere with each other. It is extremely easy to do.

Furthermore, in order to cancel the influence of the fixed capacitance C _F included in the capacitance to be measured 11 in the converter 10, a voltage setting circuit 2 having the same circuit configuration as the converter 10 is provided.
0 is provided to generate a DC voltage corresponding to the fixed capacitance C _F via the low-pass filter 51, the pulse width of the output waveform of the voltage comparator 13 changes due to the resistance in the converter 10 and the temperature characteristics of the amplifier. Even if this occurs, a similar change will occur in the output waveform of the voltage comparator 23 in the voltage setting circuit 20 having the same configuration. Therefore, such fluctuations in the output waveform depending on temperature characteristics will be caused by the differential amplifier 60 in the next stage. canceled in
It has the advantage of providing stable output against temperature changes. In addition, if temperature stability is not so important, the voltage setting circuit 20 and the filter section 5
0 may be configured with a constant voltage source and a potentiometer to set and output a DC voltage corresponding to the fixed capacitance C _F in the capacitor 11 to be measured.

As explained above, according to the present invention, the configuration includes a charging circuit for charging a capacitance to be measured, a switch means for periodically discharging the capacitance to be measured charged by the charging circuit, and a terminal voltage of the capacitance to be measured. and a reference voltage, and outputs a pulse signal with a time width corresponding to the charging time constant of the capacitor under test, a low-pass filter that converts the output pulse of this comparator into a DC voltage, and a low-pass filter that converts the output pulse of the comparator into DC voltage. A voltage setting circuit sets and outputs a DC voltage according to the fixed capacitance, a differential amplifier circuit extracts the difference between the output voltages of the low-pass filter and the voltage setting circuit, and a measurement output is connected to the output of this differential amplifier circuit. An output circuit that outputs a measurement signal obtained by adding DC voltages that set each of the zero point and span through two output signal lines that also serve as power supply lines is installed, so the circuit part that operates in a pulsed manner This minimizes noise generation due to pulsed power consumption to a practically negligible level, making it possible to realize a two-wire system that uses a signal line that also serves as a power supply line. Since the change in current consumption is small, the capacitance of the capacitor used in the power supply can be reduced, making it essentially explosion-proof, and the zero point and span adjustment of the output current can be made independent of each other. Since this is done, it is possible to easily adjust the zero point and span without causing interference. In addition, as a circuit that generates a DC voltage to cancel the fixed capacitance included in the capacitance to be measured, a circuit having the same circuit configuration as a circuit that generates a DC voltage according to the value of the capacitance to be measured is used to cancel the fixed capacitance. Since a DC voltage is generated, fluctuations in the detected voltage of the capacitance to be measured due to temperature changes can be mutually canceled out, and stability with respect to temperature can be improved.

The capacitance measuring device of the present invention is not limited to the liquid level meter exemplified in the above embodiment, and can be applied as is to any device that can convert a predetermined physical quantity such as distance into capacitance.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an example of a conventional device, FIG. 2 is a circuit block diagram showing an embodiment of the present invention, and FIG. 3 corresponds to the capacitance value in the converter 10 in FIG. FIG. 3 is a time chart diagram showing the generation of a pulse signal having a time width. 10... Conversion unit, 11... Capacity to be measured, 12,
22, 66, 76... variable resistor, 13, 23...
Voltage comparator, 14, 67...Reference voltage source,
20... Voltage setting circuit, 30... Switch section, 3
1... Pulse generator, 32, 33... Switch,
40, 50... Filter section, 41, 51... Low pass filter, 60... Differential amplifier section, 61... Differential amplifier, 70... Output section, 71... Amplifier, 7
2...transistor, 80...power supply section, 81...
Constant voltage circuit, 83... Capacitor, 90... Power supply/signal terminal, 91... Common terminal, 62, 63,
65, 74, 73, 75, 82...resistance.

Claims (1)

[Scope of Claims] 1. A charging circuit for charging a capacitor to be measured; a switch means for periodically discharging the capacitor to be measured charged by the charging circuit; , a comparator that outputs a pulse signal with a time width corresponding to the charging time constant of the capacitance to be measured, a low-pass filter that converts the output pulse of the comparator to a DC voltage, and a comparator that outputs a pulse signal with a time width corresponding to the charging time constant of the capacitance to be measured; A voltage setting circuit that outputs a set DC voltage, a differential amplifier circuit that extracts the difference between the output voltages of the low-pass filter and the voltage setting circuit, and a variable resistor for span adjustment and a zero point to adjust the output voltage of the differential amplifier circuit. A capacitance measurement characterized by comprising: an output circuit that adjusts the span and zero point using a variable resistor for adjustment and then outputs the output as it is or after converting the current through two output lines that also serve as power supply lines. Device. 2. The voltage setting circuit includes a charging circuit that charges a capacitor having a capacity corresponding to the fixed capacitance, another switch means that discharges the capacitor in synchronization with the switch means, and a terminal voltage of the capacitor and the reference voltage. The static electricity supply system according to claim 1, comprising: a comparator that compares the charging time constant of the capacitor and outputs a pulse signal with a time width corresponding to the charging time constant of the capacitor; and a low-pass filter that converts the output pulse of the comparator into a DC voltage. Capacitance measuring device.