GB1598447A

GB1598447A - Liquid level indicators

Info

Publication number: GB1598447A
Application number: GB1832/77A
Authority: GB
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1977-01-18
Filing date: 1977-01-18
Publication date: 1981-09-23
Also published as: FR2377615B1; ES465640A1; IT1101937B; IT7847646A0; DE2801586A1; FR2377615A1

Description

(54) LIQUID LEVEL INDICATORS (71) We, LUCAS INDUSTRIES LIMITED, a British Company of Great King Street, Birmingham, B19 2XF, England, do hereby declare the invention for which we pray that a Patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to a liquid level indicator for detecting the level of the hydraulic fluid in a brake or clutch fluid reservoir on a road vehicle.

It is an object of the present invention to provide a new or improved liquid level indicator.

A liquid level indicator according to the present invention comprises a probe mounted in a reservoir containing hydraulic clutch or brake fluid, the level of which is to be detected, an electric circuit for detecting the high frequency dielectric damping of the fluid as sensed by the probe when it is immersed in the fluid, and a warning means controlled by the circuit for giving a warning when the probe is not immersed in the fluid.

The circuit may include a high frequency oscillator incorporating a winding coupled to the probe, said oscillator having a predetermined frequency, but an amplitude which is substantially altered depending on whether the probe is immersed in air or the fluid.

The present invention will now be described in more detail with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram illustrating one example of the invention, and Figure 2 shows a preferred arrangement of the various parts in Figure 1.

Referring to Figure 1, a road vehicle includes a battery 11 having its negative terminal connected to an earthed supply line 12 and its positive terminal connected through the vehicle ignition switch 13 to a positive supply line 14. A probe which is immersed in the brake fluid reservoir of the vehicle is indicated at 15, and is electrically connected to one end of a winding 16 at the other end of which is connected to the line 12. A point on the winding 16 is connected through a capacitor 17 and a resistor 18 in parallel to the base of an n-p-n transistor 19 having its emitter connected to a further point on the winding 16, its base connected through a pair of resistors 21, 22 in series to the line 14 and its collector connected through a resistor 23 to the line 14.The junction of the resistors 21 and 22 is connected through a diode 24 to the collector of the transistor 19, which is further connected to the base of an n-p-n transistor 25.

The transistor 25 has its base connected through a capacitor 26 to the line 12, and its collector connected through a resistor 27 to the line 14, and also connected to the base of a p-n-p transistor 28. The emitter of the transistor 25 is connected to the junction of a pair of resistors 29, 31 connected in series between the lines 14, 12, whilst the emitter of the transistor 28 is connected to the line 14. The collector of the transistor 28 is connected through a resistor 32 to the base of the transistor 25, and through a resistor 33 and a warning lamp 34 in series to the line 12. Finally, the point on the winding 16 which is connected to the base of the transistor 19 is further connected through a capacitor 35 to the line 12.

The transistor 19 together with the capacitors 17 and 35 and the winding 16 form a high frequency Hartley oscillator having a high frequency determined primarily by the capacitor 35 and the portion of the winding 16 bridged by the capacitor 35. In order to ensure that the oscillator does not fail to operate initially as a result of saturation of the transistor 19, the base bias of the transistor 19 is provided by a potentiometer network comprising the resistors 18, 21 and 22, and the diode 24 couples the junction of the resistors 21 and 22 to the collector of the transistor 19. If the transistor 19 is not approaching saturation, the diode 24 does not conduct, but if saturation is approached, the diode 24 conducts to prevent saturation of the transistor 19, thereby ensuring that the circuit has the correct bias to commence oscillating.

Although the frequency of the oscillator is substantially constant, its amplitude will depend on whether or not the probe is immersed in the brake fluid as will now be explained. It has been found that the dielectric properties of brake fluid show substantial dielectric damping at high frequencies.

As a result of this high frequency dielectric damping of the brake fluid, and also as a result of transformer action in the winding 16, when the probe 15 is immersed in hydraulic fluid there will be substantial damping of the amplitude of the oscillator. The damping will of course determine the collector current of the transistor 19, and hence the collector potential of the transistor 19.

The AC signal at the collector of the transistor 19 is de-coupled to earth by way of the capacitor 26, but there will be a DC level which is determined by whether or not the probe 15 is immersed in the brake fluid.

The arrangement is such that provided the probe is immersed in the liquid, the collector potential of the transistor 19 is relatively low and the transistor 25, the emitter potential of which is determined by the resistors 29 and 31, is off. As a result, the transistor 28 is also off and the lamp 34 is not illuminated. However, if the probe 15 is not immersed in the liquid, then the collector potential of the transistor 19 rises substantially, turning on the transistor 25, so that base current is provided to the transistor 28 by way of the resistor 31 and the transistor 25, turning the transistor 28 on.

Positive feedback is applied to the base of the transistor 25 through the resistor 32, so that the circuit switches rapidly to a state in which the warning lamp 34 is illuminated to give a warning to the driver. The warning could of course be given in other ways.

The lamp 34 will be extinguished again when the potential of the collector of the transistor 19 falls, but the level to which the potential must fall will be lower than the level at which the lamp 34 turns on by an amount which is determined by the resistors 29,31 and 32.

Figure 2 shows the preferred mechanical arrangement, with a reservoir 41 for the liquid, and a lid 42 closing the reservoir and carrying a printed circuit board 43 from which the probe 15 extends. On the printed circuit board 43 is a spirally wound printed circuit constituting the winding 16, and the remaining components of the circuit, with the exception of the warning lamp 34, are mounted on a further printed circuit board 44 positioned above the board 43, the required connections between the boards 43 and 44 being made in any convenient manner. Input terminals are provided on the lid 42 and are indicated by the numerals 14 and 12, in Figure 1. A third terminal 45 is connected to the warning lamp 34, which of course is positioned so that it is visible to the driver.

By taking advantage in the present invention of the substantial damping present in brake fluid at high frequencies, the level of fluid in a brake reservoir may be easily detected by using a compact probe.

The compact nature of the probe of the present invention makes it particularly suitable for measuring the fluid level in a brake reservoir divided into two chambers by a partition and in which the desired level of fluid is above the top of the partition, as the space available for a probe in such a reservoir is particularly limited.

The present invention may also be used to detect the fluid level in a hydraulic clutch fluid reservoir and may be used to detect the level of any fluid which shows substantial damping at high frequencies.

It has been found that one important group of fluids which exhibit this high frequency damping is the castor oil based group of fluids, and that with this group of fluids, the damping becomes significant at 10 MHz and high at 60 MHz.

WHAT WE CLAIM IS: 1. A liquid level indicator comprising a probe mounted in a reservoir containing hydraulic clutch or brake fluid, the level of which is to be detected, an electric circuit for detecting the high frequency dielectric damping of the fluid as sensed by the probe when it is immersed in the fluid, and a warning means controlled by the circuit for giving a warning when the probe is not immersed in the fluid: 2. A liquid level indicator according to claim 1 wherein the circuit includes a high frequency oscillator incorporating a winding coupled to the probe, said oscillator having a predetermined frequency, but an amplitude which is substantially altered depending on whether the probe is immersed in air or fluid.

3. A liquid level indicator according to claim 2 wherein the circuit further includes a capacitor connected across the output of the oscillator, a first transistor controlled by the voltage across the capacitor, and a second transistor controlled by the first transistor and controlling the warning means.

4. A liquid level indicator according to claim 3 wherein positive feedback is provided from the second transistor to the first transistor.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. depend on whether or not the probe is immersed in the brake fluid as will now be explained. It has been found that the dielectric properties of brake fluid show substantial dielectric damping at high frequencies. As a result of this high frequency dielectric damping of the brake fluid, and also as a result of transformer action in the winding 16, when the probe 15 is immersed in hydraulic fluid there will be substantial damping of the amplitude of the oscillator. The damping will of course determine the collector current of the transistor 19, and hence the collector potential of the transistor 19. The AC signal at the collector of the transistor 19 is de-coupled to earth by way of the capacitor 26, but there will be a DC level which is determined by whether or not the probe 15 is immersed in the brake fluid. The arrangement is such that provided the probe is immersed in the liquid, the collector potential of the transistor 19 is relatively low and the transistor 25, the emitter potential of which is determined by the resistors 29 and 31, is off. As a result, the transistor 28 is also off and the lamp 34 is not illuminated. However, if the probe 15 is not immersed in the liquid, then the collector potential of the transistor 19 rises substantially, turning on the transistor 25, so that base current is provided to the transistor 28 by way of the resistor 31 and the transistor 25, turning the transistor 28 on. Positive feedback is applied to the base of the transistor 25 through the resistor 32, so that the circuit switches rapidly to a state in which the warning lamp 34 is illuminated to give a warning to the driver. The warning could of course be given in other ways. The lamp 34 will be extinguished again when the potential of the collector of the transistor 19 falls, but the level to which the potential must fall will be lower than the level at which the lamp 34 turns on by an amount which is determined by the resistors 29,31 and 32. Figure 2 shows the preferred mechanical arrangement, with a reservoir 41 for the liquid, and a lid 42 closing the reservoir and carrying a printed circuit board 43 from which the probe 15 extends. On the printed circuit board 43 is a spirally wound printed circuit constituting the winding 16, and the remaining components of the circuit, with the exception of the warning lamp 34, are mounted on a further printed circuit board 44 positioned above the board 43, the required connections between the boards 43 and 44 being made in any convenient manner. Input terminals are provided on the lid 42 and are indicated by the numerals 14 and 12, in Figure 1. A third terminal 45 is connected to the warning lamp 34, which of course is positioned so that it is visible to the driver. By taking advantage in the present invention of the substantial damping present in brake fluid at high frequencies, the level of fluid in a brake reservoir may be easily detected by using a compact probe. The compact nature of the probe of the present invention makes it particularly suitable for measuring the fluid level in a brake reservoir divided into two chambers by a partition and in which the desired level of fluid is above the top of the partition, as the space available for a probe in such a reservoir is particularly limited. The present invention may also be used to detect the fluid level in a hydraulic clutch fluid reservoir and may be used to detect the level of any fluid which shows substantial damping at high frequencies. It has been found that one important group of fluids which exhibit this high frequency damping is the castor oil based group of fluids, and that with this group of fluids, the damping becomes significant at 10 MHz and high at 60 MHz. WHAT WE CLAIM IS:

1. A liquid level indicator comprising a probe mounted in a reservoir containing hydraulic clutch or brake fluid, the level of which is to be detected, an electric circuit for detecting the high frequency dielectric damping of the fluid as sensed by the probe when it is immersed in the fluid, and a warning means controlled by the circuit for giving a warning when the probe is not immersed in the fluid:

2. A liquid level indicator according to claim 1 wherein the circuit includes a high frequency oscillator incorporating a winding coupled to the probe, said oscillator having a predetermined frequency, but an amplitude which is substantially altered depending on whether the probe is immersed in air or fluid.

5. A liquid level detector substantially

as hereinbefore described with reference to and as shown in the accompanying drawings.