Method of and Apparatus for Determining the Level of Liquid in a Vessel

The present invention relates to a method of and an apparatus for determining liquid volume in a vessel and, in particular, the degree of filling of a closed vessel with a liquid.

In many cases it is desirable to determine the degree of filling of a vessl with a liquid and especially the volume of liquid contained in a receptacle. In many cases, the volume of liquid or the degree of filling in a vessel is determined by detecting the position of the level of liquid, i.e. the liquid-gas interface in the vessel.

In systems for determining the residual volume of a fuel in the fuel tank of a vehicle, for example, a float or like level sensor can be provided to signal the degree of filling of the reservoir to the vehicle operator, the degree of filling being registered on a fuel gauge on the dashboard of the vehicle, for example.

In this system, the level sensor is an analog signal generator producing a signal whose amplitude directly represents the degree of filling of the fuel tank and which is connected electrically to the gauge which provides an analog output, i.e. a needle swing of a galvanometer or the like whose angular displacement is proportional to the signal.

In practice this method of displaying the degree of filling of a fuel tank is unsatisfactory because the liquid level and the gauge reading depend upon a variety of factors which represent perturbations of the actual measurement of the residual fuel. For example, the system is sensitive to the inclination of the tank, the sensor and the housing of the automobile, to vibration, to vehicle acceleration and deceleration. The sensor is usually a potentiometer whose resistence value is varied by the float.

Because of these disturbances which can readily affect the measurement, efforts have been made to find other methods and devices which can be effective for the purpose described. Investigations have resulted in piezoelectric, thermal and capacitive devices, some of which are also sensitive to vibrations in the level brought about by different inclinations of the automobile.

For example, in the capacitive approach, two plates are spacedly juxtaposed to form an electrical capacitor and can be disposed vertically in the fuel tank. The fuel in the tank is a dielectric liquid and hence a fluid whose dielectric constant differs from that of the gas (air or vapor) above the liquid-gas interface.

Thus, as the liquid level rises or falls in the tank and the gap between the plates becomes more or less occupied with the dielectric liquid, the electrical capacitance of the device changes and this capacitance can be transformed into an output signal by appropriate circuit means to signal the liquid level. Such devices are sensitive not only to the inclination of the automobile but also to temperature and the quality of the fuel since both temperature and composition of the fuel affect the dielectric constant.

To eliminate these effects, a reference capacitance whose gap is completely flooded by the fuel must be used and the capacitors may require a bridge circuit as part of the sensor. This makes the sensor and the entire device relatively costly without significantly eliminating the problem with respect to variation with inclination. While cost generally does not enter as a significant factor for fuel gauges for aircraft purposes, it is a significant factor for automobiles.

These points have been made slowly to demonstrate that there are numerous level-sensing devices available which have various drawbacks and which can be more or less costly depending upon the purposes to which such devices are put.

In Marine Week, Vol. 6 No. 11 (1979), pages 8 and 9, there is described a device for measuring the level of liquid in a chemical reservoir or tank utilizing a device sensitive to the pressure generated by the column of liquid, namely with a transducer having a vibrating wire whose internal frequency of vibration changes as the pressure changes and which is a function of the tension of the wire which depends upon the pressure of the liquid and hence upon the height of the liquid column, the density of the liquid and the temperature.

However, this approach has not found widespread application in fuel gauges for vehicles because it too is sensitive to the inclination of the tank if the height of the column is measured only at a single point. Measurements at more than one point make the system unduly complicated. Consequently, this system does not solve the problems outlined above.

It is also possible to measure the volume of the gas space remaining in a vessel of known capacity which has been partly filled with a liquid, thereby allowing this measured volume to be subtracted from the known volume of the vessel to give the volume of the liquid. In this method, the gas space is in contact with a diaphragm which partly delimits this space, which is displaced and which is permitted to return to its normal position after such displacement. The time required to reestablish the initial pressure in the chamber after the displacement of the diaphragm, utilizing the admission of air through a calibrated orifice is a measure of the volume of the gas space.

This sytem, while not sensitive to inclination of the liquid surface in the vessel, is sensitive to the vapor pressure in the reservoirs, the imprecision and instability of orifice flow, variations in pressure in the tubes or pipes utilized in the system and the like. In addition, the system requires a variety of tubes, pipes, valves, and above all, an expensive calibrated orifice which contributes to the cost of the system and introduces elements which may adversely affect the reliability thereof.

In Japanese patent No. 146 676/79, the volume of gas in a vessel is detected by acoustic means, the vessel being open to a sonic pressure source or field of predetermined frequency with the pressure and frequency being determined proximal to the opening of the vessel and being a function of the volume of gas therein. This system is, of course, limited in utility to open vessels in which the free gas columns forms a resonant system.

It is the principal object of the present invention to provide a method of and an apparatus for determining the volume of liquid in a closed vessel whereby disadvantages and drawbacks of the earlier system described can be obviated.

Another object of this invention is to provide a method of and an apparatus for detecting the degree of filling of a closed vessel, especially for use as a vehicle fuel gauge, whereby, utilizing low cost means, reliable and accurate measurements can be obtained without perturbations or disturbances introduced by inclination of the vehicle or the like.

These objects and others which will become apparent hereinafter are attained in accordance with the present invention, in a method whereby a temporary variation of the volume of gas in the reservoir is introduced and after the introduction of this change in volume, the natural oscillation of the pressure in the gas space is measured, the period of this oscillation is determined and, since this period is a function of the volume of gas, the volume is determined from the period.

As in the past, this gas volume can then be deducted from the known volume of the vessel to give the volume of liquid therein.

Preferably automatic means is provided for intermittently applying a pulse to a diaphragm or membrane forming a wall of the gas space to initially induce the volume change, this force pulse being thereafter removed to allow the restoring force of the gas pressure to initiate natural oscillation in the manner described.

The period of the oscillation can also be detected automatically and can be transformed, based upon prestored information regarding the relationship of the period to the gas volume, to automatically generate a signal representing this gas volume. The gas volume can, utilizing standard calculator facilities, e.g. under microprocessor control, thus be deducted from the prestored information relating to the predetermined volume of the vessel, to yield the volume of liquid contained therein and this liquid volume can be displayed, e.g. on a digital output module on the dashboard of the vehicle.

The invention, of course, also comprehends an apparatus for this purpose, including force-applying means for applying the aforementioned force impulsively to momentarily change the volume of gas in a vessel, means responsive to the natural oscillation of the gas pressure therein, means for determining the period of this natural oscillation, means for generating a signal representing the gas volume based upon the known relationship of the period to the gas volume (as empirically determined, for example) and means for detecting this signal from a signal representing the total volume of the container and previously stored so as to provide an output representing the liquid volume.

The pulsed force-applying means can include a diaphragm, an electromotive means operatively connected to this diaphragm utilizing an electromagnetic or electrostatic force generator, and circuit means, e.g. a timer, oscillator or other commutator which is capable of periodically applying the force pulse.

According to another feature of the invention, means responsive to the temperature in said vessel is provided and is coupled to the microprocessor-based calculator or central processor unit (CPU) for modifying the output in accordance with any temperature coefficient of the low determining the volume of the gas space from the period of the natural oscillation thereof.

The above and other objects, feature and advantages of the present invention will become more readily apparent from the following description, reference being made to the accompanying drawing in which:

FIG. 1 is a block diagram of a system in accordance with the invention adapted to be used as a fuel gauge for an automotive vehicle, aircraft or other vehicle; and

FIGS. 2-4 are graphs illustrating the description of the invention.

In FIG. 1, we have shown a fuel tank 1 for an automotive vehicle, provided with the filling neck 1a and the usual fuel tank cover 1b. The liquid within the fuel tank is represented at 1c and has a surface 1d above which a gas space 1e is formed.

The upper wall 1f of this tank, defining the gas space 1e, is formed with a perfectly yieldable (nonresilient) diaphragm 2 that is, plastically deformable but reversibly displaceable, which can be of a loudspeaker diaphragm design, i.e. having a thin central region 2a connected to an armature 2b and supported by a corrugated region 2c so that there is little intrinsic resilience in this diaphragm and any change of mass of the latter with time will be insignificant.

The diaphragm 2 is equipped with a motion-inducing element here shown to be an induction coil 3 which is mounted on the armature 2b and which extends into the field of a permanent magnet body 4 which can also be similar to the permanent magnets utilized in loudspeakers, i.e. having a core 4a of one magnetic polarity and a sleeve 4b of the offset magnetic polarity. Thus the magnetic field induced in the coil when an electric pulse is applied to the latter can react with the magnetic field of the permanent magnet to deform the diaphragm 2 and briefly alter the volume of the space 1e, the restoring force generated by the gas spring in this space producing an oscillation which is damped but which has a periodicity determined by the gas volume.

The coil 3 can be connected to a bistable switching device 6 which has been shown schematically and can be any bistable multivibrator which is switched by an input from the commutator or timer 10. The bistable switch 6 can be a flip-flop as described at pages 341-403 of Pulse, Digital and Switching Waveforms, McGraw Hill Book Co., New York, 1965.

The diaphragm 2 is preferably composed of a nonporous material so that its mass does not change with absorption of vapor from the liquid.

The vessel 1 represents the total volume of the actual fuel tank and of the portions in fluid communication therewith, i.e. the total closed volume, including the filling neck 1a and any pipes connected to the fuel tank for filling, emptying or otherwise connecting therewith.

The bistable switch 6 selectively connects the coil 3 of an electromotive means including the coil and the armature to an excitation pulse generator 7 which can be any convenient source of a brief energizing signal, such as a capacitive discharge circuit (see U.S. Pat. Nos. 3,241,756, 3,250,892, and 3,656,946.)

The pulse can be applied via an amplifier 8.

The other position of the bistable switch 6 connects the coil, then serving as a detector of the pressure valves in the space 1e to a zero crossing detector 9 for the voltage from the coil. The zero passage detector may be of the type described at pages 265 ff. of Microcomputer Based Design, McGraw Hill Book Co., New York, 1977.

A temperature sensor 11 within the reservoir 1 is connected by an amplifier 5 to an interface 12 providing the analog-digital interfacing of the analog temperature measurement with the microprocessor based central processor unit (see pages 421 ff. of Digital Computer Circuits and Concepts, Reston Publishing Co., Reston, Va., 1980.)

The CPU 13 receives the pulses from the zero crossing detector 9, with a spacing representing the periodicity of the natural oscillations of the chamber 1e, is also connected to the memory 14 (see pages 170 ff. of Microcomputer Based Design and pages 335 ff. of Digital Computer Circuits and Concepts), and provides outputs to the pulse generator 7, to the commutation unit 10 and an alpha-numeric display 15. The oscillation of the pressure in the space is transformed into a signal of an amplitude varying with the frequency of the oscillation by the interaction of the coil and the armature. The temperature detector 11, its amplifier 5 and the interface 12 allows the CPU 13 to automatically correct the value of the volume of the gas space determined as a function of the period of the latter signal in accordance with the measured value of the temperature. The forceable variation of the volume of the gas space is achieved by applying a force pulse to the diaphragm 2a forming a wall of the gas space 1e.

The diaphragm can be protected by a grill 16 whose aperture size is finer than the size of the bubbles which tend to develop on the surface of the liquid and which, of course, depend upon the surface tension of the liquid. In operation, periodically, with a time spacing greater than the tension interval, the coil 3 is energized to drive the diaphragm from its raised positions shown inwardly and thereby briefly decrease the gas volume in the spaces 1e and increase the pressure therein. The coil is then disconnected from the pulse source and is connected to the detector 9. The oscillations in the gas volume result in a displacement of the diaphragm 2 with a frequency which, although damped, is a function of the volume of gas in the reservoir. For several cycles, the period can be determined and provided as an input to the CPU 13.

The diagram of FIG. 2 shows the signal which may be recorded at the output of the coil following an oscillation in the volume induced in the reservoir. In this graph, the voltage amplitude is plotted along the ordinate against time along the abscissa. Without amplification, the amplitude of the signal at least initially can be about 1.5 volts utilizing a loudspeaker diaphragm with a diameter of 11 cm at the top of a tank which is 31 cm in diameter and 36 cm in height. The excitation current applied to the coil was 0.5 ampere. Since the resistance of the coil was less than 2.9 ohms, the excitation power was less than 0.75 watt and the mean power approximately 0.1 watt. The upper curve represented a liquid volume in the vessel of 18 liters, the lower curve representing a liquid volume of 4 liters. Delta T (.DELTA.T) represents the change in period between the two signals corresponding to a decrease in the liquid volume in the container. In FIG. 3, we have plotted the period in multiseconds along the ordinate against the volume of liquid in the vessel and hence the degree of filling, along the abscissa. From the diagram it will be apparent that the relationship is nonlinear, i.e. that the signal representing the period is a nonlinear function of the volume of the gas. The period drops slowly until about 2/3 of the container is full of liquid and thereafter drops rapidly as the distance between the surface of the liquid and the diaphragm decreases.

Since the gas phase above the liquid is usually an atmosphere saturated with molecules of the liquid in spite of the fact that the period is a function of the diameter and the mean molecular weight of the gas phase, in practice, for any atmosphere and a given liquid, only the temperature dependency need be considered. Accordingly measurement of the temperature provides a complete correction for the period/gas volume law and, with values for the particular tank determined empirically and introduced into the memory 14 for various temperatures, the calculator 13 can readily determine from a given period and temperature, the gas volume and deduct that from the known volume of the receptacle, also stored in the memory, to provide a direct output of the liquid within the tank which is displayed at 15.

It should be noted that the diaphragm is light, has a large diameter and is displaceable with little force so that the sensitivity of the system is high and can provide a measure of the degree of filling practically independently of any perturbations of the liquid surface.

FIG. 4 shows the effect covered over four periods of the response signal, (FIG. 2) of the inclination of the receptacle. The effect is then plus/minus 1% for an inclination of 0.degree. to 20.degree. as plotted along the abscissa. Naturally, the membrane 2 can be activated by other means than those which have been described and indeed by any means which may have been used in connection with loudspeaker diaphragms. Such means can include piezoelectro systems or electrostatic forces generated by forming the membrane 2 as a mobile plate of a capacitor. In this case capacity detection of the movement of the membrane 2 can be utilized to generate the output representing the natural frequency of oscillation of the gas chamber.

One of the advantages of the device of the invention is that the pressure generator and the detector utilize the same elements so that the total number of elements of the device is minimal.

There are few elements susceptible to breakdown which is a particular advantage in the automobile industry. The electrical circuitry can be readily integrated on a simple board or even in a small chip. The invention is also applicable to measuring volumes of liquid in receptacles of other types, for vehicles or any other applications and is practically insensitive to movement of liquid within the vessel, the shape of the vessel or the nature of the liquid.