JPH03135733A - Liquid level detecting device - Google Patents

Abstract

PURPOSE:To improve the accuracy and reliability of liquid level detection by arranging platinum thin film resistance bodies at the upper and lower end parts of a heat-conductive insulating substrate arranged in a liquid tank, further arranging a heat generating body on the thin film resistance body at the upper end part and connecting the thin film resistance bodies to constant-current sources respectively, and detecting the difference between their terminal voltages. CONSTITUTION:The upper resistance body 2 made of the heat generating body 4 and a platinum thin film is provided at the upper end part of the ceramic substrate 1 and the lower resistance body 3 is arranged at the lower end part at well. Those resistance bodies are connected to the constant-current sources respectively and voltages developed across the resistance bodies are inputted to an operational amplifier 6, which detect their difference voltage. The heat- conductive insulating substrate is dipped in liquid and the temperature difference between the upper and lower end parts of the substrate is detected electrically to measure the liquid level; and neither mechanical constitution nor variable resistance which causes a measurement error is required and the high-accuracy and high-reliability detecting device is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention can be used as a liquid level detection device for detecting the remaining amount of gasoline in an automobile.

Conventional technology Conventionally, the remaining amount of gasoline can be detected by placing a float in the gasoline.One end of a rod is connected to the float, the other end is connected to a fulcrum, and a variable resistance slider is moved in the middle of this rod. The vertical position was converted into an electrical resistance value and detected electrically.

Problems to be Solved by the Invention The above conventional examples require mechanical structures such as floats and rods, and do not have sufficient long-term reliability, accuracy, reproducibility, etc.

Variable resistors also have problems with linearity, temperature characteristics, and long-term reliability, and increasing the precision and reliability of conventional configurations poses problems such as increased costs.

Means for Solving the Problems In order to achieve the above objects, the present invention provides platinum-based thin film resistors on the upper and lower ends of a thermally conductive insulating substrate disposed in a liquid tank, and further provides insulation. A heating element is disposed on the platinum-based thin film resistor at the upper end of the substrate, each of the platinum-based thin film resistors is connected to a constant current source, and an operational amplifier is installed to detect the difference in terminal voltage of each resistor. It is characterized by the fact that it has been provided.

According to the present invention, it is possible to eliminate the mechanical structure and variable resistance of a conventional liquid level detection device and replace them all with an electrical structure, thereby significantly improving the accuracy, reliability, and reproducibility of liquid level detection. improves.

Example An embodiment of the present invention will be described with reference to FIGS. 1 and 2.

The insulating substrate only needs to have homogeneous thermal conductivity, and a poor conductor of heat is not desirable. A ceramic substrate is suitable. A heating element 4 and an upper resistor 2 made of a platinum-based thin film are disposed at the upper end of the ceramic substrate 1.
, and a lower resistor 3 is similarly arranged at the lower end. These resistors are each connected to a constant current source, and the voltage generated in each resistor is inputted to an operational amplifier, thereby detecting the voltage difference between them.

In this example, the thermally conductive insulating substrate is immersed in liquid, and the liquid level is measured by electrically detecting the temperature difference between the upper and lower ends of the substrate. It is possible to provide a highly accurate and highly reliable level detection device without the need for a variable resistor that may cause measurement errors or other configurations.

Figure 2 shows an embodiment in which the ceramic substrate is made thinner toward the bottom in order to maintain linearity in the relationship between the liquid level position (H-h) of the ceramic substrate and the differential voltage. . The same results can be obtained by using other resistance thermometers for temperature measurement.

In addition, as shown in Figure 3a, a platinum-based thin film resistor is formed from the upper end to the lower end of the insulating substrate, and the part immersed in the liquid is cooled by the heat absorption of the liquid and the resistance value decreases. By measuring the resistance of the body, the liquid level can be detected. Furthermore, as shown in Figure 3, a similar detection can be performed by pasting a large number of platinum-based thin film resistor chips from the upper end to the lower end of an insulating substrate and connecting them in series. Can be done. In these cases, the platinum-based thin film resistor also serves as a heating element. In Figure 3a, a platinum-based thin film resistor must be formed on the entire surface of the insulating substrate.
This leads to an increase in the size of the film deposition equipment, and because the uniformity of the resistor film thickness is directly linked to the linearity of level detection, detailed manufacturing considerations are required to achieve a uniform film thickness over the entire surface of the resistor. Since a large amount of expensive platinum-based material is deposited, the material cost is also high. As shown in the figure, the uniformity of the resistance values of a large number of resistor chips is directly linked to the linearity of level detection, so it is a manufacturing issue to reduce variations in resistance values and resistance temperature coefficients. A large number of resistor chips are attached to an insulating substrate,
Connecting these in series also requires a lot of man-hours and is a manufacturing issue.

However, in this example, the platinum-based thin film resistors formed on the thermally conductive insulating substrate are limited to one each on the upper and lower ends of the insulating substrate, and since a small resistance shape is sufficient, the resistor chip is It can also be pasted, which solves the problem of increasing the size of film deposition equipment. Since the deposited film is localized, uniformity of the film thickness over a wide area is not required. Since there are two resistors,
The amount of material used is small, and even if a resistor chip is attached, the number of man-hours is small and it can be manufactured at low cost.

Now, the principle of this liquid level detection device will be explained in detail.

When the heating element at the upper end of the insulating substrate is driven with constant power, the generated heat travels through the insulating substrate and reaches the lower end. Assuming that the height of the upper end resistor 2 is immersed in the liquid up to the height of the H1 insulating substrate, the temperature of this portion is constant at the same temperature as the temperature of the liquid. Therefore, the temperature T of the upper end resistor 2 is T, =r (H-h) 1-1-T2-=.
(1) r: Thermal resistance per unit length of the insulating substrate 1: Determined by the heat flow of the insulating substrate. The resistors at the upper and lower ends are connected to a constant current power supply, and the voltage V generated in each resistor is
, V2 is detected by the operational amplifier 6, v,=R4×I=Ro(1-4-ttT,)X −・
-(2)V2=R2XI=R(1(1+αT 2)I
...-=-(3) R1: Resistance value of the resistor at temperature T1 R2: Resistance value of the resistor at temperature 2 Ro22 Resistance value of the resistor at humidity temperature Any current value α: Platinum-based Temperature coefficient of thin film resistor Subtracting equation (3) from equation (2) gives d v=v 1-V2=RoXIXα(T,-T2)
・------・(4) Substituting equation (1) into equation (4), dv=Ro x engineering x α x rxi (H-h)...
・(6) From equation (6), the differential voltage aV is determined by the position of the liquid level (H-h
) is proportional to As (H-h) increases and T increases, the flow of heat from the heating element is not only caused by heat conduction through the insulating substrate, but also by heat radiation from the top of the insulating substrate, which is ignored compared to heat conduction. become unable. In this case, the apparent thermal resistance becomes smaller, so the width of the insulating board is made thicker at the upper end.
By narrowing toward the lower end, the linear proportional relationship of equation (6) can be maintained while keeping r constant.

Effects of the Invention According to the present invention, the mechanical structure and variable resistance of the conventional example can be abolished and all replaced with an electrical structure. Therefore, it is possible to significantly improve the accuracy, reliability, and reproducibility of liquid level detection. According to the present invention, it is only necessary to arrange small resistors at the upper and lower ends of the ceramic substrate, and it is sufficient that these two resistors have the same R8 and α. Ro
The variation in α can be reduced by adjusting the constant current regulator, and the variation in α can be reduced by adjusting the peripheral resistance of the operational amplifier.
Each can be corrected.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a liquid level detection device according to an embodiment of the present invention, Fig. 2 is a block diagram of a ceramic substrate according to an embodiment of the present invention, and Fig. 3 is a diagram of the invention that led to the creation of the present invention. It is a block diagram of a ceramic board. DESCRIPTION OF SYMBOLS 1... Ceramic substrate, 2... Upper end resistor, 3... Lower end resistor, 4... Heating element, 5...・Constant current source. 6... Operational amplifier.

Claims (4)

[Claims] (1) Platinum-based thin film resistors are placed on the upper and lower ends of a thermally conductive insulating substrate placed in the liquid tank, and a heating element is placed on the platinum-based thin film resistor on the upper end of the insulating substrate. A liquid level detection device characterized in that each of the platinum-based thin film resistors is connected to a constant current source, and an operational amplifier is provided for detecting a difference in terminal voltage of each resistor. (2) The liquid level detection device according to claim 1, wherein a ceramic substrate is used as the thermally conductive insulating substrate. (3) The liquid level detection device according to claim 1 or 2, wherein the ceramic substrate on which the heating element is located is thick at the upper end and gradually becomes thinner toward the lower end. (4) The liquid level detection device according to claim 1, wherein a temperature measuring resistor other than platinum-based is used in place of the platinum-based thin film resistor.