GB2146770A - Device for determining liquid volume - Google Patents

Abstract

A device for determining the volume of liquid in a vessel comprises a vibrator (5) to vibrate the vessel (1) and contents (2) at its natural frequency, a sensor (3) to detect the resulting vibrations and to deliver a signal dependent upon the frequency of those vibrations, a source of a second signal (10) at a predetermined fixed reference frequency higher than that of the first signal, counting means (9, 11) for comparing the two signals and presenting in digital form the number of cycles of the reference frequency equal to one (or more) cycles of the first signal frequency, and calculator means e.g. a computer to derive therefrom a measure of the volume of liquid in the vessel. The invention also includes a method for measuring the volume of liquid in a vessel, for which method the above device may be used. <IMAGE>

Description

SPECIFICATION Device for determining liquid volume The present invention is a device for determining the volume of liquid contained in a vessel.

The need to determine the volume of liquid contained in a vessel at a given moment arises frequently in practice. For example, when a vessel is being filled or emptied it may be desired to establish when the vessel is almost full (to avoid overflowing) or almost empty (to enable a fresh supply of liquid to be provided). In other situations, an indefinite quantity of liquid may have been delivered to a vessel, or dispensed therefrom, and an accurate determination of the volume of liquid delivered or dispensed may be required, for example for charging or costing purposes.

Often visual determination of the volume of liquid in a vessel is impossible, for example when the vessel is opaque, or it may be difficult or inaccurate. Thus the determination of the volume of liquid fuel in an opaque freestanding container, for example a pressure cylinder, has to be carried out by a non-visual method. In another example, when the volume of milk delivered by a given cow to a glass vessel in a milking machine has to be determined, the apparently simple task of reading a graduated scale becomes much iess simple in the practical conditions obtaining in a milking-shed and, in any case, the high standards of accuracy required by the Milk Marketing Board may be unattainable.

Alternative, non-visual methods of determining liquid volume suffer from various disadvantages. Load cells have, in general, proved unreliable in practice and the results obtained have been not very accurate. Flow meters have some merit for determining the volume of liquid flowing past a given point in a given period of time but, when the flow is a varying one (for example, the pulsed flow of liquid delivered by a cow), flow measurements of greater accuracy than + 5 per cent are difficult, if not impossible, to achieve. Similar levels of accuracy usually result when resistance probes are employed for the determination of the volume of a liquid.

A prime object of our invention is therefore to provide an alternative device for determining the volume of liquid in a vessel, which device can be used with opaque vessels and is capable of yielding results of improved accuracy, even in difficult conditions.

According to our invention, a device for determining the volume of liquid in a vessel comprises a vibrator to vibrate the vessel and contents at its natural frequency, a sensor to detect the resulting vibrations and to deliver a signal dependent upon the frequency thereof, a source of a second signal at a predetermined fixed reference frequency higher than the frequency of the first signal, counting means for comparing the two said signals and presenting in digital form the number of cycles of said reference frequency equal to one cycle (or a predetermined number of cycles) of said first signal frequency, and calculator means to derive therefrom a measure of the volume of liquid in said vessel.

Our invention is based on the fact that a vessel and its liquid contents together have a natural frequency of vibration, which varies with the quantity of liquid introduced but which, for a given liquid, is largely independent of other factors. It is recognised that ambient conditions such as temperature and air density have an influence but in most situations they do not vary greatly and the influence of such variations is small, so it will often be acceptable to ignore these variables.

The natural frequency at which a vessel and its liquid contents together vibrate is therefore an accurate indication of the quantity of liquid contained therein.

The vibrator to vibrate the vessel and its contents may be any conventional vibrator of suitable frequency range and dimensions, depending upon the characteristics of the vessel and contents. We particularly prefer to employ a feed-back arrangement in which the vibration of the vessel and liquid therein initiates a signal which is fed back to the vibrator, preferably via an amplifer, and causes the vibrator to vibrate at the frequency of the signal, thereby maintaining the vibrations at the natural frequency of vessel and contents.

Our invention requires a sensor to deliver a signal dependent upon the frequency of the vibrations, which signal is subsequently used to determine the volume of liquid in the vessel. Advantageously, the same sensor may also be a feature of a feed-back arrangement of the type described above, so that a single sensor performs the two functions of activating the vibrator as described and also providing the signal to be compared with the reference signal. Alternatively, two separate sensors may be employed, each to perform one of said two functions.

The signal delivered by the sensor, which is dependent upon the frequency of the vibrations, will normally be sinusoidal in shape.

Before comparing this with the reference signal, it is desirable that it be converted to a square-wave form to render it transistor-transistor-logic (TTL) compatible. A conventional squaring amplifier can be used for this purpose.

As indicated above, the reference signal should be at a frequency higher than that of the vibrations, as the accuracy of the device according to our invention requires that the number of reference frequency cycles within one (or more) vibration frequency cycle be counted. Clearly it is important that the reference signal should be delivered at a stable frequency if reliable results are to be obtained.

The reference signal, like the signal to be measured against it, should be TTL-compatible.

The counting means, required to make the comparison of the two signals, may usefully take the form of a logic gate and a digital counter, working in combination. Thus the logic gate may identify those moments when both square-wave signals are in the "high" condition and deliver pulses of the reference frequency to the counter for each cycle of the vibration frequency signal. A straight count of the number of reference frequency cycles in each one cycle of the vibration frequency signal (or in a predetermined number of such cycles) is a direct indication of the period of the vibration frequency signal. That is, when the number of reference frequency cycles is higher, the period of the vibration frequency signal is longer and the frequency thereof is lower.

The result of the count is presented in digital form. Thus a binary number is obtained which is proportional to the period of the vibrations, which in turn is an indication of the volume of liquid in the vessel. The volume itself must be calculated by reiating the period to that of the vibrations of the vessel alone.

The relationship is such that, except possibly for very small frequency variations and where a high standard of accuracy is not required, calculator means are required to perform the appropriate calculations. Thus conveniently a computer may be employed for the purpose, whereby a direct and accurate reading of liquid volume may be presented.

The relationship between the measured value of signal period and the liquid volume may be determined theoretically or by direct observation, in either case taking into account only such of the previously-mentioned variables as accuracy and relevance requires.

Once the relationship has been specified for a given situation, the computer may be programmed with the required formula and thereby the results may be directly presented as a volume of liquid, expressed as desired.

The invention will now be further described with reference to the accompanying drawing, which is a flow-sheet representing the treatment of a signal arising using one form of device according to our invention.

In the drawing, a vessel 1 is shown containing liquid 2. A sensor 3, mounted against a wall (e.g. the side wall or the bottom) of the vessel 1, picks up vibrations from the vessel and its contents and emits a voltage signal which varies at the frequency of these vibrations. This signal is fed to an amplifier 4 (the feedback amplifier) and the amplified signal is in turn fed to a vibrator 5, also mounted against the wall of the vessel 1. In this way, the vibrator 5 is caused to vibrate the vessel 1 at the frequency of the input signal and in this way the vessel is vibrated continuously at the natural frequency of the vessel 1 and the liquid 2 together.

The sensor 3 and the vibrator 5 may be supported at opposite ends of a spring yoke (not shown in the drawings) by means of which they are held in contact with the vessel wall. Thus the vessel 1 may readily be removed and replaced if desired.

The output from the amplifier 4, which is in the form of a sinusoidal wave at the frequency of the vibrations, is also fed to a squaring amplifier 6, by means of which it is converted to a square-wave signal at the same frequency, switching between zero and 5 volts.

This signal is now TTL compatible.

The signal from amplifier 6 is fed direct to an 8-bit counter 7, coupled to eight switches shown as a block at 8. The counter 7 produces square-wave signals at eight different frequencies, namely that of the vibrations (''f'') ) and successive sub-multiples thereof, that is f/2, f/4, f/8, f/16, f/32, f/64 and f/128. By means of the switches 8, the frequency range and accuracy of the device overall may be selected.

The signal at the frequency selected by switches 8 is now fed to a logic gate 9, which also, by line 10, receives a fixed, stable square-wave signal of 1 mega Hertz frequency (the reference signal). The logic gate 9 identifies those moments when both input signals are in the "high" condition and thereby delivers pulses at 1 m Hz frequency throughout each separate cycle of the vibration frequency signal. These pulses are fed to a sixteen-bit counter, designated generally in the drawing by the reference numeral 11. The counter 11 consists of two eight-bit counters 1 2 and 1 3, each with an eight-bit latch (14 and 1 5 respectively), and also a latch/reset logic unit 16.

At the end of each cycle, the count is latched by latches 14 and 1 5 and the counters 12 and 1 3 are reset by the unit 1 6 ready to count the pulses in the next cycle. The latch 1 4 registers the least significant byte (LSB) and the latch 1 5 registers the most significant byte (MSB). Thus the latches 14 and 1 5 may be read by a computer to give a sixteen-bit binary number proportional to the period (1 /f) of the natural vibration of the vessel and contents.

The volume of liquid contained within the vessel 1 may be calculated as a function of the difference between the period measured with liquid in the vessel and that obtained when the vessel is empty. That is V is a function of (1 /f1 - 1 /f0) where V is the volume of liquid, f0 is the frequency when empty, and f, is the frequency at volume V.

As indicated above, the specific relationship can be calculated theoretically or it may be determined by experiment.

Using a 1 m Hz reference frequency, it is possible to measure the period of oscillation at least eight times per second, thereby obtain ing a high degree of accuracy. Thus using the above apparatus we have been able to obtain results to within less than 0.1 per cent of the true value. We may observe that, in the case of the measurement of milk volumes in vessels in a milking machine, the Milk Marketing Board specifies that milk volumes be measured to the nearest 0.2 litre. Since the vessel sizes used are typically of about 27 litres capacity, this implies a required accuracy of + 0.4 per cent.

The device according to the present invention thus is able to give an indication of the volume of liquid (not the level) in a vessel with a degree of accuracy which is high compared with earlier methods. It can be applied to an existing vessel and can easily be calibrated. Because the method uses digital techniques, a computer may be used to monitor and record the volumes of liquid in a large number of vessels easily and quickly.

Furthermore, because the device of the present invention enables liquid volumes to be determined quickly, it becomes possible to take very frequent readings, for example eight times per second as indicated above. It then becomes easily possible to monitor the rate of change of volume of liquid, which is a direct measure of the flow rate of liquid into or out of the vessel. Thus the invention in one form functions as a flow meter, without the disadvantages attendant upon the locating of a conventional flow meter actually within the liquid flow line.

Claims (11)

1. A device for determining the volume of liquid in a vessel, comprising a vibrator to vibrate the vessel and contents at its natural frequency, a sensor to detect the resulting vibrations and to deliver a signal dependent upon the frequency thereof, a source of a second signal at a predetermined fixed reference frequency higher than the frequency of the first signal, counting means for comparing the two said signals and presenting in digital form the number of cycles of said reference frequency equal to a predetermined number (including one) of cycles of said first signal frequency, and calculator means to derive therefrom a measure of the volume of liquid in said vessel.

2. A device as claimed in claim 1, wherein the sensor is connected to feed back a signal to the vibrator.

3. A device as claimed in claim 2, wherein the signal is fed from the sensor to the vibrator via an amplifier.

4. A device as claimed in any of the preceding claims, wherein the signal delivered to the counter by the sensor is first fed to a squaring amplifier.

5. A device as claimed in any of the preceding claims, wherein the counting means comprise a logic gate and a digital counter.

6. A device as claimed in any of the preceding claims, wherein the calculator means is a computer.

7. A device as claimed in any of the preceding claims, having latch and reset means to permit multiple successive observations.

8. A device as claimed in any of the preceding claims, wherein the vibrator and the sensor are supported at opposite ends of a sprung yoke.

9. A method of determining the volume of liquid in a vessel, comprising vibrating the vessel and its contents at its natural frequency, producing a signal dependent upon said frequency, comparing said signal with a reference signal of higher frequency and counting the number of cycles of said reference frequency in a predetermined number (including one) of cycles of said first signal frequency, presenting the result of said counting in digital form and calculating therefrom the volume of liquid in said vessel.

10. A method as claimed in claim 9, wherein a sensor is employed both to feed back the sensed signal to the vibrator and also to supply a signal for comparison with the reference signal.

11. A method as claimed in claim 9 or claim 10, wherein the two signals are compared in square-wave form.

1 2. A method as claimed in any of claims 9 to 11, wherein frequency determinations are made with the vessel empty and also with the vessel containing liquid and the volume of liquid is calculated by comparing said frequencies.

1 3. Apparatus for determining the volume of liquid in a vessel, substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawing.