JPH04329319A - Liquid level measuring by heat radiating type level sensor - Google Patents

Abstract

PURPOSE:To compensate an error caused by a difference between a sensor temperature and an ambient temperature. CONSTITUTION:In a liquid level measuring method by a heat radiating type level sensor, constant current is passed intermittently through a sensor FL which is a resistor submerged in the liquid so as to store the initial output voltage which is outputted from the sensor FL. The voltage outputted in lapse of a prescribed time from the initial condition is divided by the initial output voltage, and a steady output voltage stabilized from a rising inclination in proportion to the output time determined by the above division is computed for estimation. At the same time, the corrected steady output voltage V'' to for which a difference between a sensor temperature and the ambient temperature is compensated by the following corrective expression is calculated to be known so that it may be taken as the level measuring data of liquid. The correction expression is V''tc=Vtc+(V1-V'1)XG, in which V1 is the initial voltage, V'1 is the initial voltage in the previous measuring, Vtc is the abovementioned steady output voltage and G is a preset constant.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid level measuring method using a heat dissipation type level sensor suitable for detecting the level of fuel in a vehicle fuel tank, for example.

0002

2. Description of the Related Art A heat dissipation type level sensor utilizes the fact that the resistance of the sensor, which is a resistor, changes depending on the depth of immersion in the liquid level. To briefly describe the method using pulse constant current, a constant current is applied for several seconds in the pulse method. The sensor voltage increases as a result of current application, and the amount of increase is proportional to the liquid level. However, the voltage difference between FULL and EMPTY at the end of current supply is small, and practical resolution cannot be obtained. Therefore, it was decided to obtain an output proportional to the liquid level from the average slope of the voltage rise instead of the above-mentioned rise amount. Specifically, the sensor voltage was digitally input every few milliseconds, and a first-order approximation process was performed using a microcomputer to obtain the slope, thereby increasing the output resolution. This will be explained with reference to FIGS. 12(a) and 12(b). FIG. 12(b) is a diagram in which the horizontal axis represents time and the vertical axis represents current.
It shows a pulse current that intermittently repeats a constant current that becomes stable at tf and turns off at time tf. FIG. 12(a) shows a sampled state of the sensor output during this energization, and shows a state in which the sensor output is sampled at time t.
Starting from the initial voltage V1 at 1, V2, V3
,...Vf is obtained. Therefore, each output voltage is set to the initial voltage V
Divide by 1 to compensate for the ambient temperature. Then, the rising slope of this output with respect to time is subjected to first-order approximation processing to obtain the slope as shown in FIG. 13, and from this, the steady output voltage Vtc is obtained and used as liquid level measurement data.

[0003] Also, to explain the above output, Fig. 4 (
a) and (b) show the slope a of the change in sensor resistance (which has the same meaning as the output voltage) due to energization when the ambient temperature is high, and the slope b of the change in sensor resistance when the ambient temperature is low.
In the initial stage of energization, the temperature of the sensor is considered to be the same as the ambient temperature, so the higher the ambient temperature, the greater the output.

As mentioned above, FIGS. 5(a) and 5(b) show the slope c when each output is divided by the initial voltage to compensate for the difference in ambient temperature shown in FIG. The slopes are almost the same regardless of whether it is high or low.

FIG. 6 is a diagram comparing the slope d of resistance change when the ambient temperature is the same as the sensor temperature and the slope e of resistance change when the ambient temperature is lower than the sensor temperature. Although the above-mentioned compensation is performed for both resistance changes and the initial resistance is the same, it can be seen that the output (resistance) is lower with the slope e.

In other words, the above-mentioned temperature compensation is a compensation when the initial temperature of the sensor is always the same as the ambient temperature. For example, when the ambient temperature suddenly changes, the temperature of the sensor is Due to its heat capacity, the temperature does not follow the change, and the output changes as shown in FIG. 6, for example, and there is a possibility that a measurement error may occur.

[0007]

SUMMARY OF THE INVENTION As described above, the liquid level measuring method using a pulse-type heat dissipation type level sensor has the risk of generating measurement errors due to sudden changes in ambient temperature.

The present invention has been made in order to solve the above-mentioned problems, and it is an object of the present invention to provide a liquid level measuring method using a heat dissipation type level sensor that does not cause measurement errors even when the ambient temperature suddenly changes. With the goal.

[0009]

[Means for Solving the Problems] In order to achieve the above object, the present invention intermittently supplies a constant current to a sensor, which is a resistor immersed in a liquid, and stores the initial output voltage output from the sensor. Then, divide the output voltage from the initial state until a predetermined time elapses by the initial output voltage, calculate and predict the steady output voltage that will be in the steady state from the rising slope of the output obtained by this division with respect to time, and use the following modified formula. A liquid level measuring method using a heat dissipation type level sensor, characterized in that a corrected steady output voltage V″tc which compensates for the difference between the sensor temperature and the ambient temperature is calculated and used as liquid level measurement data.

[0010]V″tc=Vtc+(V1−V′1)
×G……Modified formula However, the above initial voltage is V1
, the initial voltage in the previous measurement is V'1, the steady output voltage is Vtc, and a preset constant is G.

[0011]

[Operation] To explain the above correction formula in detail, for errors caused by the difference between the sensor temperature and the ambient temperature, it is necessary to determine the amount of compensation for the output by determining the change in ambient temperature and its speed. Consider inserting an object at a different temperature into air at a constant temperature. At this time, the heat transfer, that is, the heat flow rate (the amount of heat transferred per unit volume and unit length) is proportional to the temperature difference between the object and the surrounding medium (air). By measuring the temperature of the object at regular time intervals, it is possible to estimate the temperature difference between the surrounding medium and the object at that point in time (more precisely, between each measurement). For example, the larger the difference in temperature of an object at two measured points in time, the different the temperature of the object from the surroundings. When considering this, the temperature of the object (sensor) is determined from the initial resistance, and the temperature difference between the object (sensor) and the surroundings, that is, the change in ambient temperature and its speed, are calculated using the initial resistance value measured last time and the initial resistance measured this time. Judgment was made from the difference in values. Then, the difference in initial resistance was multiplied by a constant G obtained from experiments, and added to the sensor output (steady output voltage) for compensation. This constant must be determined through experiments, as it varies depending on the sensor shape, heat capacity, coating material, etc.

(1) Detection of the difference between the sensor temperature and the ambient temperature will be described.

FIG. 7 shows that the sensor temperature and the ambient temperature are equal;
Moreover, when the temperature is constant, the sensor temperature, that is, the initial resistance value does not change. Figure 8 shows the followability of the sensor temperature when the ambient temperature changes.
1, T2, ... indicate the ambient temperature, Ts1, Ts
2, . . . indicate the temperature of the sensor (initial resistance value R). When the ambient temperature changes, a temperature difference occurs in the sensor.

In FIG. 9(a), the vertical axis shows the difference between the sensor temperature and the ambient temperature (sensor temperature - ambient temperature), and in FIG. 9(b), the vertical axis shows the difference in initial resistance value, that is, the difference in the current measurement. The initial resistance value is R2, the initial resistance value of the previous measurement is R1, and the difference R2 - R1 is shown corresponding to the temperature difference position in FIG. 9(a).

According to these correspondences, the difference between the sensor temperature and the ambient temperature is approximately proportional to the difference in initial resistance values.

(2) The difference between the sensor temperature and the ambient temperature and the output change will be described.

When there is a temperature difference, the output changes compared to when there is no temperature difference. That is, as can be seen from FIGS. 5 and 6, the output changes in proportion to the difference in temperature.

From the above, as shown in FIG. 10, the difference between the sensor temperature and the ambient temperature is proportional to the difference in initial resistance value,
As shown in Figure 11, the difference between the sensor temperature and the ambient temperature is
Proportional to output change. Therefore, the difference between the initial voltage at the previous measurement and the current initial voltage is determined by the constant G
By multiplying by and adding this to the steady-state output voltage calculated in this measurement, it is possible to compensate for the effect that the difference between the sensor temperature and the ambient temperature has on the output. (In this case, G is negative.)

0
019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be explained below with reference to the drawings and an apparatus in which it was carried out.

First, an apparatus implementing the method of the present invention will be described with reference to FIGS. 1 to 3, and its operation and embodiments will be described as examples of the present invention.

1 is a fuel tank, and FL is a sensor serving as a resistor immersed in the tank 1. A constant current I is applied to both ends of the sensor FL through a constant current pulse circuit 2. The output voltage Vp generated across the level sensor FL by flowing this current is taken into the CPU 4 through the A/D converter 3.

The current I generated from the pulse circuit 2 is shown in FIG.
As shown in b), a large period having a period from t0 to tf is repeated after a cooling time,
The entire period is set to about 3 seconds. Therefore, as shown in FIG. 2(a), the voltage Vp rises from the initial state at a gradient corresponding to the level of the liquid level (the lower the level, the higher the slope is, and the higher the level, the lower the slope). This is repeated, sequentially taken into the CPU 4, and sequentially stored in the storage section of the CPU 4 together with the time data (enlarged portion in FIG. 1).

Here, the initial voltage, ie, the output voltage V1 at t1, can be regarded as the output voltage in a state where the sensor FL is not yet heated by the current I.

In other words, it can be regarded as an output voltage based on a resistance value similar to that of a conventional temperature compensation resistor, and the CPU 4
stores this initial output voltage V1, and divides the values of successively input voltages V2 to Vn by this initial voltage V1 using an arithmetic means consisting of a CPU, and a steady state is reached from the rising slope with respect to these times. The steady-state output voltage Vtc is calculated and calculated. This is liquid level measurement data that has been compensated for the ambient temperature, but it has also been added to the value obtained by subtracting the initial voltage value (initial resistance value) from the previous measurement from the initial voltage value (initial resistance value) for this measurement. The value multiplied by the set constant G is the steady output voltage Vt.
In addition to c, a corrected steady output voltage V″tc is calculated. Then, this is used as liquid level measurement data that compensates for the output difference due to the temperature difference between the sensor FL and the surroundings, and this is displayed on the display unit 5. Since the heat capacity of sensor FL is large, sampling voltages from V0 to Vf do not reach a steady voltage level.

On the other hand, if the period from t1 to tf is, for example, 3 seconds and the sampling interval is 10 msec, 300 sampling voltages Vp can be obtained.

Therefore, as shown in FIG. 3, the CPU 4 includes a program for calculating the approximate steady-state output voltage Vtc at the time tc when the steady state is reached from the rising slope of the first-order approximate straight line, and a program for calculating the approximate steady-state output voltage Vtc at the time tc when the steady state is reached, and the modified steady-state output using the correction formula. It has a built-in calculation program to calculate voltage.

[0027]

[Effects of the Invention] As detailed above, the present invention compensates for output changes due to differences in sensor temperature and ambient temperature.
Even in cases such as sudden changes in ambient temperature, highly reliable and accurate liquid level measurement data can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of an apparatus that implements the measurement method of the present invention.

FIG. 2 is a diagram showing the relationship between constant current and sensor output in the same embodiment.

FIG. 3 is a diagram illustrating the relationship between steady output voltage and slope in the same embodiment.

FIG. 4 is a diagram illustrating the relationship between sensor resistance change and ambient temperature.

FIG. 5 is a diagram showing the resistance change of a temperature compensated sensor.

FIG. 6 is a diagram showing resistance changes when sensor temperature and ambient temperature are different.

FIG. 7 is an explanatory diagram of the relationship between ambient temperature and sensor temperature when the ambient temperature is constant.

FIG. 8 is a diagram illustrating the followability of sensor temperature to changes in ambient temperature.

FIG. 9 is a diagram showing the relationship between the temperature difference between the sensor and the surroundings and the difference in initial resistance values before and after.

FIG. 10 is a diagram showing that the temperature difference between the sensor and the surroundings is proportional to the difference in initial resistance values.

FIG. 11 is a diagram showing that the temperature difference between the sensor and the surroundings is proportional to the output change.

FIG. 12 is an explanatory diagram of the relationship between sensor current and sensor output voltage.

FIG. 13 is an explanatory diagram of steady output voltage.

[Explanation of symbols]

FL Sensor V1, V'1 Initial output voltage V1, V2,...Vn Output voltage Vtc Steady output voltage V''tc Modified steady output voltage

Claims (1)

[Claims] 1. A constant current is intermittently applied to a sensor which is a resistor immersed in a liquid, an initial output voltage outputted from the sensor is memorized, and the output voltage from the initial state until a predetermined period of time has elapsed is stored as the initial output voltage. Corrected steady-state output by dividing by the output voltage, calculating and predicting the steady-state output voltage from the rising slope of the output over time obtained by this division, and compensating for the difference between the sensor temperature and the ambient temperature using the correction formula below. A liquid level measurement method using a heat dissipation type level sensor, characterized in that a voltage V″tc is calculated and obtained and this is used as liquid level measurement data.V″tc = Vtc + (V1 − V′1)
×G……Modified formula However, the initial voltage is V1,
Let V'1 be the initial voltage in the previous measurement, Vtc be the steady output voltage, and G be a preset constant.