JPS6310771B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a liquid amount measuring circuit that measures a liquid amount by detecting a change in capacitance of a capacitance sensor as a change in oscillation frequency.

In principle, a device that measures the amount of liquid in a container based on capacitance, such as an automobile fuel level meter, has a pair of electrodes 2A, 2 facing each other in a container 1, as shown in FIG.
B is placed in the depth direction, and the liquid amount is detected by measuring the capacitance between the electrodes 2A and 2B. That is,
The dielectric constant of the liquid 3 in the container 1 is ε _G , the liquid immersion depth of the electrode is l, the height of the electrode is L, and the electrodes 2A, 2
If B is a parallel plate, the liquid volume change υ is proportional to the immersion depth l, and the interelectrode capacitance C is as follows: C=K{(L-l)+ε _G l}=K{(ε _G −1
)l+L}...(1). Here, K is a constant determined by the shape, spacing, etc. of the electrodes 2A and 2B. From this equation (1), the liquid volume change υ can be determined from the following equation (2), which is related to the capacity C with the depth l (liquid level).

1=(C/K-L)/( _εG -1)=A(C-B)...(2) However, A=1/{K( _εG -1)} and B=LK.

A conventional liquid amount measuring circuit based on this principle has a configuration shown in FIG. The capacitance sensor 10, in which a capacitor is formed with the liquid to be measured interposed therebetween, serves as an oscillation frequency control element of the oscillation circuit 11, and changes in its capacitance are detected as changes in the pulse oscillation frequency of the oscillation circuit 11. The pulse output of the oscillation circuit 11 is detected as a pulse period by the pulse width measurement circuit 12, and this periodic signal T is subtracted by a fixed constant B' in the subtraction circuit 13, and this subtraction signal (T-
B') is multiplied by a fixed constant A' in the multiplication circuit 14 to obtain the liquid amount detection output l=A'×(T-B'). Here, constants A' and B' are constants determined in relation to A and B in equation (2) above for the relationship between period T and capacitance C, and this equation is equivalent to equation (2). .

In such a conventional circuit, the oscillation circuit 11
To calculate the liquid volume l from the output pulse width measurement signal T, a binary subtraction circuit 13 and a binary multiplication circuit 14 are required. I had a problem with my computer and needed it.

The present invention uses a counter circuit, a monostable multivibrator, and a reference pulse generation circuit to detect the liquid volume from a pulse with a frequency proportional to the output of a capacitance sensor, and has a simplified configuration that reduces costs and facilitates the use of ICs. The purpose of the present invention is to provide a liquid volume measuring circuit that is easy to use.

FIG. 3 is a block diagram showing one embodiment of the present invention. The oscillation circuit 11 generates a pulse with a period T ₁ (=C×D) proportional to the capacitance C of the capacitance sensor 10, and the pulse output is divided into periods by a frequency divider 15 with a division ratio N ₁ . The frequency is divided into T ₂ (=T ₁ ×N ₁ ) pulses. On the other hand, the reference pulse generation oscillation circuit 16
generates a pulse with a period T ₀ as a reference time signal, and this pulse is divided by the frequency divider 17 into pulses with a period T ₀₁ (T ₀ ×N ₂ ) with a frequency division ratio N ₂ . These pulses are connected to the counter circuit 18 and the comparator 1.
9, the output pulses of the frequency divider 17 are counted by the counter circuit 18 using the output pulses of the frequency divider 15 as reset and counting start command signals. The count value of the counter circuit 18 is used as a comparison input of a comparator 19, and when the count value matches the numerical value _N3 as a comparison standard, the counter circuit 18 is controlled to stop counting by the match detecting power of the comparator 19. Therefore, the monostable multivibrator 20 has a fixed time T ₃ (=N ₃ ×N ₂ ×T ₀ ) until it counts a fixed number N ₃ of pulses of synchronization T ₀₁ , which is _the frequency-divided pulse of reference synchronization T 0. Generates a clock signal.

The counter circuit 21 starts counting the output pulses of the frequency divider 17 at the inverted output of the comparator 19 when the monostable multivibrator 20 finishes timing, and
The counter circuit 18 is counted until the counter circuit 18 is reset by the next pulse output, and the count is proportional to T ₂ -T ₃ . The contents of the counter circuit 21 are stored in a latch circuit 22 before being reset by the output of the comparator 19, and are converted to an analog signal through a digital-to-analog converter 23 and used as an indication input to an analog meter 24. is indicated as an analog quantity that has a linear functional relationship with .

FIG. 4 shows waveforms of various parts in FIG. 3. The output of the oscillation circuit 11 (Fig. 4 a) becomes a pulse with a period T ₁ proportional to the capacitance C of the sensor ₁₀ , and the output of the frequency divider 15 (Fig. 4 b) becomes a pulse with period T ₂ . The output of the oscillation circuit 16 for generating the reference time (Fig. 4c) is the period
The output of the frequency divider 17 (Fig. _4d ) which divides this pulse by the division ratio N ₂ is a pulse with period T _01.
becomes the pulse of The counter circuit 18 starts counting the pulses of the period T ₀₁ at the pulse time t ₁ of the period T ₂ (Fig. 4 e), and at the time t ₂ when the count value reaches N ₃ , the output of the comparator 19 (Fig. 4 Counting is stopped by f). On the other hand, the counter circuit 21 is the counter circuit 18
Counting of pulses with a period T ₀₁ is started from the counting stop time t ₂ (Fig. 4g), and counting is stopped at a time t ₃ when the output of the comparator 19 is inverted, that is, the counter circuit 18 is reset by the pulse with a period T ₂ . . At this time _t3 , the counter circuit 18 starts counting again. Therefore, the counting time T ₁₈ of the counter circuit 18 is N ₃ ×N ₂
×T ₀ becomes a constant value, and the counting time T ₂₁ of the counter circuit 21 becomes as follows.

T ₂₁ =T ₂ −T ₁₈ =N ₁ ×T ₁ −N ₃ ×N ₂ ×T ₀
...(3) Then, the count value N of the counter circuit 21 is as follows.

N=T ₂₁ ×1/T ₀₁ =N ₁ ×T ₁ −N ₃ ×N ₂ ×T ₀ /T ₀ ×N ₂ ...(4) Here, T ₁ is the proportionality constant D to the capacitance C of the sensor 10 From the relationship, the above equation (4) becomes as follows.

N=N ₁ ×D/N ₂ ×T ₀ (C−N ₃ ×N ₂ ×T ₀ /N ₁ ×D) ……(5) It is clear from the comparison of this equation (5) and the above equation (2). As such, N ₁ ×D/N ₂ ×T ₀ = 1/K (ε _G −1)……(6) N ₃ ×N ₂ ×T ₀ /N ₁ ×D=L・K ……(7) By setting N ₁ , N ₂ , N ₃ , D, and T ₀ such that the count value N of the counter circuit 21 matches the liquid amount l.

The count value N of the counter circuit 21 is stored in the latch circuit 22 as a pulse with a period T ₂ (FIG. 4h), converted into an analog signal by the digital-to-analog converter 23 (FIG. 4i), and the meter 24 will be displayed.

According to this embodiment, it is synchronized with a pulse with a period T ₂ proportional to the detection capacitance of the capacitance sensor 10, and is calculated for a certain period T ₁₈ by counting the reference pulse (period T ₀₁ ).
The period is determined by a monostable multivibrator that clocks the
A value is obtained by subtracting a fixed time period _T18 from _T2 , and a count value for constant multiplication is obtained by a counter circuit 21 that counts reference pulses during this subtracted value period. As a result, the measurement accuracy is the same as that of the conventional device, the circuit configuration is reduced to a few circuit elements consisting of a counter circuit, a monostable multivibrator, and a reference pulse generator, and the liquid amount measuring circuit can be made inexpensive and small.

Note that the display means for measuring the liquid amount is not limited to the analog meter shown in the embodiment, but may also be a digital display configured with a numeric display that directly displays the counted value of the counter circuit 21. Further, when the liquid amount measurement result is used for other calculations, the contents of the counter circuit 21 or the latch circuit 22 can be used. For example, in a car's remaining odometer, the distance that can be displayed with the current amount of gasoline is calculated from the amount of gasoline in the fuel tank and the amount of fuel consumed, so the contents of the counter circuit 21 etc. can be used as the measured value of the amount of gasoline. can.

Furthermore, the frequency dividers 15 and 17 in the embodiment are for adjusting the sampling time, and may be omitted depending on the object to be measured by the liquid meter or the required measurement accuracy. However, when used in an automobile fuel gauge, the oscillation circuit 11 is made to oscillate at a high frequency of several 100 KHz in order to avoid the influence of the gasoline dielectric constant on the frequency characteristics. The display changes depending on the order, resulting in an inappropriate display for a fuel gauge. In other words, since the liquid level fluctuates while the car is running, it is desirable to have an averaged display over several minutes that absorbs these fluctuations, and for this stationary display, a frequency divider 15 with a frequency division ratio of about ¹⁰⁸ is used. This can be achieved by providing On the other hand, the frequency divider 17 can be omitted by reducing the frequency of the oscillation circuit 16, but since a capacitor is generally used as a time constant element of the oscillation circuit 16, a large capacitor is required for low frequency oscillation. This results in an expensive and large oscillation circuit. Therefore, it is advantageous in terms of cost and space to divide the high frequency oscillation output by providing the frequency divider 17.

In addition, the liquid level measurement circuit used as a fuel gauge is
The frequency division ratio of the frequency divider 15 is determined so as to have a long sampling time while driving, but the frequency division ratio of the frequency divider 15 is determined to have a long sampling time.
5 and 17 are realized by switching the frequency division ratio using a switching signal from the outside. For example, a vehicle speed sensor signal can be used as the switching signal in this case.

In addition, although the monostable multivibrator 20 is shown to be composed of a counter circuit 18 and a comparator 19, if it is permissible for accuracy, CR
A common monostable multivibrator with a time constant circuit can be used. However, when the liquid amount measuring circuit is configured as one IC circuit, it is preferable to configure the counter circuit 18 and the comparator 19, which eliminates the need for an external capacitor.

As described above, the liquid amount measuring circuit according to the present invention has the following features:
The circuit configuration can be simplified to realize an inexpensive and compact circuit, and in particular, when integrated into an IC, the number of elements is small, which has the effect of reducing the cost.

[Brief explanation of the drawing]

Figure 1 is a diagram for explaining the principle of liquid volume measurement, Figure 2 is a conventional liquid volume measurement circuit diagram, Figure 3 is a circuit diagram showing an embodiment of the present invention, and Figure 4 is a It is a waveform chart of each part of a figure. 10... Capacitance sensor, 11... Oscillation circuit,
15... Frequency divider, 16... Reference pulse generation oscillation circuit, 17... Frequency divider, 18... Counter circuit,
19... Comparator, 20... Monostable multivibrator, 21... Counter circuit, 22... Latch circuit, 23... Digital-analog converter, 24
...Meter.

Claims (1)

[Scope of Claims] 1. A liquid volume measuring device that measures a liquid volume from a capacitance between a pair of electrodes placed in a liquid to be measured, which generates a pulse with a period proportional to the capacitance between the pair of electrodes. a first oscillation circuit, a second oscillation circuit that generates a pulse with a reference period, a monostable multivibrator that generates a pulse for a predetermined period in synchronization with the output pulse of the first oscillation circuit; and a counter circuit that counts the output pulses of the second oscillation circuit from the end of the output until the next output pulse of the first oscillation circuit, and the counter circuit obtains a count value proportional to the liquid amount. Liquid volume measurement circuit. 2. In claim 1, the monostable multivibrator comprises a counter that starts counting the output pulses of the second oscillation circuit in synchronization with the output pulses of the first oscillation circuit, and a count value of this counter. 1. A liquid amount measuring circuit comprising: a comparator that stops counting by the counter when the amount reaches a predetermined value. 3. The liquid amount measuring circuit according to claim 1, wherein the first oscillation circuit includes a frequency divider that can change the ratio between the output pulse period and the capacitance. 4. The liquid amount measuring circuit according to claim 1, wherein the second oscillation circuit includes a frequency divider that can change the output pulse period.