WO1995002177A1

WO1995002177A1 - Improvements in or relating to the measurement of bodies

Info

Publication number: WO1995002177A1
Application number: PCT/GB1994/001495
Authority: WO
Inventors: Robert Willi Schade
Original assignee: Robert Willi Schade
Priority date: 1993-07-06
Filing date: 1994-07-06
Publication date: 1995-01-19
Also published as: GB2279747A; AU7189994A; GB2279747B; GB9413581D0

Abstract

A method of measuring the relative or absolute density of a body comprising the steps of transmitting a signal of fixed wavelength between a transmitter (24) and receiver (26) and through said body and measuring the frequency (f2) of said signal and calculating the relative or absolute density from the measured frequency. The invention can be utilised in order to measure the level of a fluid in a container (40) e.g. a pipe, and also to the measurement of distances.

Description

Title: Improvements in or relating to the measurement of bodies

DESCRIPTION

This invention relates to improvements in or relating to the measurement of bodies.

There is need for a simple yet accurate method of measuring the absolute density value of a body and also the relative density of a body with respect to the density of a reference body, for example, air or water. Therefore, an aim of one aspect of the present invention is to provide such a method of measuring the absolute or relative density of a body.

A known method of measuring the depth of a liquid in a container is to use sonar techniques whereby the time taken for a signal to be transmitted to the surface of the liquid and return by reflection is measured and from the time period, the depth of the liquid can be calculated. However, in order to make the measurement it is necessary for a transmitter and receiver to be positioned within the container and this may lead to undesirable contamination of the liquid. A further problem experienced with this technique is that it can not be satisfactorily used for measuring the level of liquid in a sealed container, especially if the container is small.

As a consequence of the above, a further aim is to provide a method of measuring the level of a fluid in a container. It is known that the relation between the velocity of a sound wave and its wavelength is given by the following formula:

V = f.λ (1)

Where V = the velocity of the sound wave, = the wavelength of the wave and f = the frequency of the wave..

The velocity of the sound wave is also known to be directly proportional to the density of the medium through which the transmitted signal is passed through. Hence, the relationship can be shown by the following formula:

V OCD or V = KD (2) where D is the density of the medium and K is a constant. Substituting equation (2) into equation (1) it will be seen that the frequency of the wave is proportional to the density of the medium through which the wave is transmitted, ie. f <2< D or f = KD.

The above relationships are utilised in the various aspects of the present invention. In accordance with a first aspect of the present invention there is provided a method of measuring the density of a body comprising the steps of transmitting a signal of fixed wavelength between a transmitter and receiver and through said body, measuring the frequency of said signal and calculating the density from the measured frequentcy.

In one embodiment relative density is measured with respect to a reference body by comparing the measured frequency with a reference frequency obtained in respect of said reference body in order to determine a ratio of said frequencies and thereby obtaining the relative density of said body with respect to the reference body. In another embodiment, an absolute density value is measured by calculating the absolute density value from the measured frequency value, a reference frequency obtained in respect of a reference body and the density value of said body reference. It is preferred that the above-mentioned measuring methods include the step of obtaining a standing wave between said transmitter and receiver.

The standing wave maintained between the transmitter and receiver may have a fixed wavelength such that the distance of separation between the transmitter and receiver is a multiple of the wavelength. More preferably, said distance of separation is equal to a single wavelength.

It will be appreciated that said wavelength is fixed so that the frequency of the transmitted signal is the only variable other than the change of velocity of the signal when transmitted through a body of different density.

The above methods may include the further steps of modulating the signal with a high frequency carrier signal, transmitting said modulated signal between said transmitter and receiver and through the body and demodulating the received modulated signal in order to obtain the original signal.

It will be seen that the transmitting of a modulated signal through a body to be measured will overcome problems associated with acoustic interference which may be experienced by an audio signal transmitted without modulation.

In accordance with a further aspect of the present invention there is provided a method of measuring the level of fluid in a container comprising the steps of transmitting a signal of fixed wavelength between a transmitter and receiver and through the body of fluid, measuring the frequency of said signal and calculating the level of fluid from the measured frequency and calibrated measurement values. The term 'container is used broadly and is intended to include any structure which can contain a fluid and includes pipes.

It is preferred that the transmitter and receiver are positioned on external surfaces of the container and that they are placed on opposing surfaces of the container so as to be in a position to measure the change in the level of the fluid in the container.

Preferably, the calibration measurements are made by measuring the frequency of a signal transmitted through the container when the level of the fluid to be measured is zero and when the level of the fluid is at its maximum, ie. when the container is full of said fluid. The frequency measurements may be converted to density measurements.

It will be appreciated that if the inner dimensions of the container are known, then the precise value of the quantity of fluid in the container can be calculated from the measured level of fluid in the container.

In accordance with yet a further aspect of the present invention, there is provided a density measurement system comprising a signal generator, frequency measurement means, transmitter and receiver means and processing means for calculating the relative or absolute density value of a measured body from the measured frequency of the signal transmitted through the body, the frequency measurement of a reference body and it's density value. It is preferred that said signal generator is provided by a phase locked loop and that the frequency measurement means is position between the phase locked loop and the transmitter means.

In a preferred embodiment, the density measurement system further comprises modulation and demodulation means wherein said modulation means modulates a signal generated by the phase locked loop with a high frequency carrier signal and said demodulation means demodulates said modulated signal so as to obtain the signal generated by the phase locked loop.

In yet a further aspect of the present invention there is provided a method of measuring distance between a transmitter and a surface comprising the steps of transmitting a signal to a surface and receiving a reflected signal from the surface, adjusting the frequency of the transmitted signal until a standing wave having a signal path equal in length to a known number of wavelengths is obtained, measuring the frequency of the received signal and calculating the distance form said measured frequency value of the received signal.

Preferably, the distance is also calculated from a reference value calculated from a measurement of the frequency of a reference signal wherein the signal path length of the reference signal is known and the signal path length of the transmitted reference signal is equal to a known number of wavelengths.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, wherein:

Figure l is a schematic block diagram of a density measurement system according to one aspect of the present invention;

Figure 2 is a schematic illustration of a container and density measurement system;

Figure 3 is a calibration plot utilised in accordance with a further aspect of the present invention; and

Figure 4 is a schematic block diagram illustrating a distance measurement system according to a further aspect of the present invention.

With reference to Figure 1 in the first instance, a density measurement system is indicated generally by numeral 10. The system 10 comprises a phase locked loop (P.L.L.) 12, frequency measurement meter 14, processor unit 16, signal generator 18, modulator and demodulator 20, 22 and transmitter unit and receiver unit 24, 26. The P.L.L. 12 comprises a phase detector 28, low pass filter 30 and voltage controlled oscillator 32. It will be seen that the output of the demodulator 22 is fed directly into P.L.L. 12.

Initially, it is desired that the system is calibrated with respect to a reference body which is taken to be atmospheric air at a predetermined temperature. The transmitter unit and receiver unit are spaced apart and fixed at a predetermined distance of separation.

As there will initially be no input to the P.L.L, the voltage control oscillator (VCO) is allowed to run free at its so called free running frequency, for which is dependant upon the component of the oscillator. The VCO is adjusted such that the wavelength of the transmitted signal is set at slightly larger than the distance of separation between the transmitter and receiver. The signal from the P.L.L. is fed into the modulator 20 and modulated with a high frequency carrier signal generated by signal generator 18. The modulated signal is transmitted by the transmitter unit 24 to the receiver unit 26. The received modulated signal is demodulated by demodulator 22 and the demodulated signal is fed to the P.L.L.

A particular feature of the P.L.L. is that the frequency of the signal which is measured by the frequency meter 14 is driven to a frequency such that a standing wave is generated between the transmitter 24 and receiver 26. Once the frequency measured by the frequency meter 14 has settled to a steady value, ie. a value such that the corresponding wavelength is exactly the same as the distance of separation between the transmitter and receiver, a frequency reading fl is taken and the value transmitted to the processor unit 16.

In use, a body to be measured is positioned between the transmitter and receiver. As a result the medium through which the transmitted signal passes through is of a different density and as a consequence, the velocity of the signal will change and since the wavelength is held constant by virtue of the P.L.L., the frequency of the signal must change in order to maintain the relationship, V = f^.

The resulting frequency value f2 of the transmitted signal is measured by the frequency meter 14 and the value f2 is transmitted to the processor unit 16.

As described above, the relationship between frequency and density is given by the formula: f = K_D (1)

Since the wavelength is held constant throughout the measuring process, the above formula can be reduced to: f = K D.

The relationship between the frequency of the signal transmitted through the body f2 and its density D2 equates to the relationship of the frequency of the reference signal fl with the density of air Dl, by the following formula:

£2 = fl D2 Dl_ Therefore, D2 = f2.Dl fl

Since the frequencies are measured and density of air is known, the density of the body being measured can be easily calculated by the processor unit. If only the relative density is required then the ratio of the two frequencies provides this.

The above density measurement technique can be utilised in a number of applications and one such application is now described with reference to Figures 2 and 3.

The density measurement technique can be used to measure the level of liquid in a sealed container 40 of known inner dimensions. The container 40 comprises a base wall 42, top wall 44 and side walls 46. Firstly, the measuring system is calibrated and this is performed by positioning the transmitter unit 20 against the base wall 42 and the receiver unit 22 against the top wall 44. Density measurements are now made in order to determine the density of the container when filled with air, ie, D min and when filled with the liquid, ie. D max. From these two readings, it is possible to draw a calibration plot as shown in Figure 3. The plot indicates density measurement D along the vertical axis and the level of liquid in the container along the horizontal axis. It will be seen that when there is no liquid in the container then L = 0 and the density measurement is at its lowest, ie. D min. Similarly, when the container is full, L = max and the density measurement is at its highest, ie. D max. The calibration plot correlates density with the level of liquid in the container and hence it is possible to simply read off the level of liquid in the container for a given density reading.

In use, in order to determine the level of liquid in the container, a density measurement is made and the resulting density measurement is simply correlated with the calibration plot in order to determine the level of liquid in the container. In the preferred embodiment of the invention, the calibration results are stored in the processor unit 16 and the actual level of liquid in the container is calculated by the processor unit 16 from the measurement of the density of the container with the unknown level of liquid. It will be appreciated that with a fixed wavelength, frequency is proportional to the density and hence the above method of measuring liquid levels in a container can be made by simply measuring the various frequency values.

The above method not only finds application in the measuring of liquids stored in containers but can also be used to measure the level of a particular gas in a container. The above-mentioned measuring technique also finds application in the measuring of the level of liquid flowing through a pipe.

In accordance with a further aspect of the present invention there is provided a method of measuring distances and a further embodiment of the invention will now be described with particular reference to Figures 1 and 4 and initially to Figure 4.

A transmitter unit is indicated by numeral 50 and a receiver by unit 52. The transmitter and receiver units are aimed at a surface 54. The surface 54 could be the surface of a liquid stored in a container or simply the body of an object.

A signal is transmitted by unit 50 and the signal is reflected by surface 54 and received by unit 52, the path of the signal is indicated by broken line 56. The length of the signal path is designated by 'L' and if the distance of separation of the units 50, 52 is small then L is approximately equal to '2d' wherein 'd' is the distance of separation of the units from surface 54. If d is significantly greater in magnitude than the distance of separation of the units then 2d can be taken to be equal to L for practical measurement purposes.

If the wavelength of the transmitted signal is fixed as being equal to L then from above: d -λ 2

Hence, in order to obtain the distance d it is necessary to determine

From equation (1) above, it is known that:

_{± 2}

If the density of the body through which the measurements are made is the same, ie. D-^ = D₂, then: f ^ ⁼ ^2 2 -^-^e" ^₁ ⁼ —₂—2 ⁼ ^d ••••⁽2⁾ ^fl If a reference signal is used wherein the signal path 'L' is known then it is possible to calculate d since f_χ and f₂ can be measured.

System 10 described above with reference to Figure 1 can be utilised, with modification, in order to measure the distance d.

System 10 is simply shown as a block 58, transmitter unit 50 and receiver unit 52. A further transmitter unit 60 and receiver unit 62 are also shown and the direct signal path between the two units is set at a predetermined known length, ie. the distance of separation between the units is fixed at a predetermined convenient distance. Initially, a reference signal is transmitted between units 60, 62 in order to obtain a value corresponding to f₂Λ « The frequency of the transmitted signal is increased from an initial value until the phase locked loop locks onto a frequency whereby a standing wave is generated between the unit. The standing wave will have a wavelength equal to the distance of separation of the unit since the frequency has been increased from a low value and the first possible standing wave is generated. Since the distance of separation is known then ₂ can be calculated and the frequency of the received signal is measured such that reference value f₂.^₂ can be calculated.

In order to measure a distance 'd' from the transmitter unit 50 to a surface 54, the frequency of the signal is increased until a standing wave is generated which, in a similar as described above, will have a wavelength equal to the signal path length, ie. 2d. The frequency f--_ of the received signal is measured and with the measured reference value f₂-V ₂ and from equation (2), A ••_ can be calculated. Since )χ = 2d, the distance d can be calculated.

The operation of system 10 is similar to that described above and hence detailed explanation has not been provided again. Please note that in practice transmitter unit 50 and receiver unit 52 will be positioned as close together as feasible in order to ensure that the signal path 'L' can be approximated to 2d.

Claims

1. A method of measuring the density of a body comprising the steps of transmitting a signal of fixed wavelength between a transmitter and receiver and through said body, measuring the frequency of said signal and calculating the density from the measured frequency.

2. A method of measuring the relative density of a body comprising the steps of transmitting a signal of fixed wavelength between a transmitter and receiver and through said body, measuring the frequency of said signal and comparing said measured frequency with a reference frequency obtained in respect of a reference body in order to determine a ratio of said frequencies and thereby obtain the relative density of said body with respect to the reference body.

3. A method as claimed in claim 1 wherein said density is calculated from the measured frequency value, a reference frequency obtained in respect of a reference body and the known density value of said body reference.

4. A method as claimed in claims 1, 2 or 3 wherein said method includes the step of obtaining a standing wave between said transmitter and receiver.

5. A method as claimed in claim 4 wherein the distance of separation between the transmitter and receiver is a multiple of the wavelength of the standing wave.

6. A method as claimed in claim 5 wherein said distance of separation is equal to a single wavelength.

7. A method as claimed in any one of claims 1 to 6 wherein said method comprises the further steps of modulating the signal with a high frequency carrier signal, transmitting said modulated signal between said transmitter and receiver and through the body and demodulating the received modulated signal in order to obtain the original signal.

8. A method of measuring the level of fluid in a container comprising the steps of transmitting a signal of fixed wavelength between a transmitter and receiver and through the body of fluid, measuring the frequency of said signal and calculating the level of fluid from the measured frequency and calibrated measurement values.

9. A method as claimed in claim 8 wherein the transmitter and receiver are positioned on external surfaces of the container.

10. A method as claimed in claim 9 wherein said transmitter and receiver are placed on opposing surfaces of the container so as to be in a position to measure the change in the level of the fluid in the container.

11. A method as claimed in claims 8, 9 or 10 wherein the calibration measurements are made by measuring the frequency of a signal transmitted through the container when the level of the fluid to be measured is zero and when the level of the fluid is at its maximum, ie. when the container is full of said fluid.

12. A method as claimed in claims 8, 9, 10 or 11 wherein said frequency measurements are converted to density measurements.

13. A method as claimed in any one of claims 8 to 12 wherein said method includes the step of obtaining a standing wave between said transmitter and receiver.

14. A method as claimed in claim 13 wherein said standing wave maintained between the transmitter and receiver has a fixed wavelength such that the distance of separation between the transmitter and receiver is a multiple of the wavelength.

15. A method as claimed in claim 14 wherein said distance of separation is equal to a single wavelength.

16. A method as claimed in any one of claims 8 to 15 wherein said method further comprises the steps of modulating the signal with a high frequency carrier signal, transmitting said modulated signal between said transmitter and receiver and through the body and demodulating the received modulated signal in order to obtain the original signal.

17. A method of measuring distance between a transmitter and a surface comprising the steps of transmitting a signal to the surface and receiving a reflected signal from the surface, adjusting the frequency of the transmitted signal until a standing wave having a signal path equal in length to a known number of wavelengths is obtained, measuring the frequency of the received signal and calculating said distance from the measured frequency value of the received signal.

18. A method as claimed in claim 17 wherein said distance is calculated from said measured frequency and a reference value calculated from a measurement of the frequency of a reference signal wherein the signal path length of the reference signal is known and the signal path length of the transmitted reference signal is equal to a known number of wavelengths.

19. A method as claimed in claim 17 wherein said method comprises the further steps of modulating the signal with a high frequency carrier signal, transmitting said modulated signal and demodulating the received modulated signed in order to obtain the original signal.

20. A method as claimed in claim 18 wherein said method further comprises the steps of modulating the reference signal with a high frequency carrier signal, transmitting said modulated signal and demodulating the received modulated signal in order to obtain the original signal.

21. A density measurement system comprising a signal generator, frequency measurement means, transmitter and receiver means and processing means for calculating the relative or absolute density value of a measured body form the measured frequency of a signal transmitted through the body, a frequency measurement of a reference body and it's density value.

22. A system as claimed in claim 21 wherein said signal generator is provided by a phase locked loop.

23. A system as claimed in claim 22 wherein said frequency measurement means is position between the phase locked loop and the transmitter means.

24. A system as claimed in any one of claims 21, 22 or 23 wherein the density measurement system further comprises modulation and demodulation means wherein said modulation means modulates a signal generated by the phase locked loop with a high frequency carrier signal and said demodulation means demodulates said modulated signal so as to obtain the signal generated by the phase locked loop.