GB2100429A - Apparatus for discrimination against spurious elastic wave (e.g. ultrasonic) pulses in the detection of disturbed liquid interfaces or surfaces - Google Patents

Abstract

Where detection of reflected ultrasonic pulses is employed for indicating the level of a liquid surface 13a or interface which is subject to mechanical disturbance, erroneous indications can arise from signals reflected by entrained bubbles of gas 15a or drops of one liquid within another. In addition, the desired signal from a disturbed surface or interface may not always be recognised because the instantaneous amplitude of the signal can vary over a wide dynamic range. Such problems are eliminated or reduced by employing two or more transducers 14a, 16a and coincidence signal detection techniques (Fig. 4, not shown). <IMAGE>

Description

SPECIFICATION Apparatus for discrimination against spurious elastic wave (e.g. ultrasonic) pulses in the detection of disturbed liquid interfaces or surfaces The invention relates to the detection of liquid levels or interfaces between different liquids using pulses of elastic waves, typically at ultrasonic frequencies.

The general technique for using such ultrasonic pulses to detect liquid levels or interfaces is well known. The present invention is particularly concerned with a problem which arises when the liquid surface or interface to be detected is disturbed. Typically such disturbance give rise to formation of bubbles of gas in the liquid phase or drops of one liquid phase in another. Conventional electronic systems are designed to recognise only the first ultrasonic signal whose amplitude exceeds a preset threshold value and which is received within a predetermined time period after excitation of the transducer (or some other reference time). Thus, in general, any earlier recognition (within the time period) of a spurious echo prevents subsequent detection of the desired signal.Conversely, failure to detect the desired signal (because of insufficient amplitude) may result in false indentification of a later echo.

For the most general case of a liquid-liquid interface, therefore, two types of spurious indication can result from mechanical disturbance. Firstly, a bubble of gas or drop of another liquid phase located between the transducer and the interface may reflect sufficient ultrasonic energy that the corresponding echo is recognised in preference to the later signal from the interface itself. Secondly, the interface itself may instantaneously be sufficiently agitated that no recognisable signal is received from that location; if no spurious signals have been received from other locations nearer to the transducer, then a conventional electronic system will instead recognise any subsequent signal which may exceed the predetermined amplitude.Such a spurious signal could result from a bubble of gas or drop of another liquid phase located beyond the interface; alternatively any later signal from another liquid surface or due to spurious or multiple echoes will be recognised if that signal is received within the predetermined time period.

If the level of the disturbed surface of a single liquid phase is to be measured, false indications will usually result only from the first of the two processes described above.

Reflections from bubbles and drops and other spurious echoes are more likely to exceed the preset threshold amplitude if the electronic system is designed to detect a high proportion of the reflections from a disturbed surface or interface. An example of such a system is described in our Patent Specification No. 81 1 8335 Agents ref: 12867 LgH filed on the same day as the present application.

For most practical applications, the electronic system can be designed such that a reasonable proportion of individual excitations of the transducer result in recognition of the appropriate surface or interface. Nevertheless it is a problem that any mechanical disturbance will generally also result in a significant proportion of invalid measurements and there should therefore be incorporated in an automatic measurement system some means whereby invalid measurements may be recognised and discarded. It is an object of this invention to provide a solution to this problem.

The invention provides apparatus for detecting a surface or interface level of a liquid which apparatus comprises means for injecting elastic wave signal pulses into the liquid towards the surface or interface simultaneously or in predetermined time relationship from at least two spaced locations, the arrangement being such that the volumes of the liquid respectively traversed by the signal pulses from the two or more locations do not overlap, receiver means associated with each location for receiving elastic wave signal pulses reflected back from the surface or interface, and coincidence detection means for determining when such reflected signal pulses are received in coincidence or predetermined time relationship at the said receiver means.

Conveniently a transducer which performs the combined function of converting an electrical signal pulse into an elastic wave signal pulse for transmission and the converse for reception is positioned at each of the said spaced locations. However separate transducers may be employed for transmission and reception at each location.

In one arrangement according to the invention overlapping of the volumes within the liquid traversed by the signal pulses is prevented by one or more appropriately located physical barriers.

Alternatively such overlap is prevented by adequate spacing apart of the said spaced locations.

In order to accommodate the effect of small variations in the distance from the liquid surface or interface to the receiver means caused by disturbance in the liquid, electrical signal processsing means are included in the apparatus for processing the received signal pulses, whereby coincidence is registered even although disturbance of the surface or interface causes small variations in the respective times taken for signal pulses reflected therefrom to reach the respective receiver means at the said.

spaced locations.

Specific constructions of apparatus embodying the invention will now be described by way of example and with reference to the drawings filed herewith, in which: Figure 1 is a diagrammatic cross-section of an apparatus; Figure 2 is a diagrammatic cross-section of another apparatus; Figure 3 is a diagram representing the spatial extent of the elastic wave transmitted by a circular piezoelectric transducer; Figure 4 is an electrical circuit block diagram; and Figure 5a to 5d represent wave forms associated with Figure 4.

Referring to Figure 1 a vessel 11 contains a liquid 12 the level 13 of the surface of which is to be continuously monitored. For this purpose there if provided a transducer 14 for converting electrical signal pulses into elastic signal pulses which travel through the liquid 12 from the bottom of the vessel 11 towards the surface 1 3 from which they are reflected and can be detected by the transducer 14 operating in a receive mode. The principle of operation to provide an indication of the location of the liquid level 13 is known and depends upon the indication given of the time taken for the elastic signal pulse to travel from the transducer 14 to the surface 1 3 and back again to the transducer 14.

In a typical plant environment the surface 13 of the liquid may be disturbed for example by mechanical agitation or by flow of liquid or gas elsewhere within the vessel. A consequence is that the elastic signal pulses reflected from the surface 1 3 will have widely varying intensity. The electrical detection apparatus has therefore to be able to recognise a reflected signal pulse even though its intensity may be a fraction of that reflected by an undisturbed horizontal surface. One arrangement for meeting the problem of detecting reflected signal pulses with widely varying intensity is described in the Patent Specification No. (Agents ref: 12867 LgH) referred to above.

As illustrated in Figure 1 disturbance of the liquid 12 may also result in the formation of bubbles 1 5. Elastic signal pulses reflected from such bubbles may have an intensity comparable with the lowest intensity reflected signal pulses expected from the disturbed surface 13. In this way the bubbles 1 5 may give rise to false indications of liquid level. This problem is met in the example illustrated in Figure 1 by providing a second transducer 1 6 spaced apart from the transducer 14 but otherwise operating in exactly the same manner as transducer 1 4 to inject elastic wave signals into the liquid 12 and detect the returning elastic signal pulses returning from the surface 13.

The dotted lines 17 and 1 8 in Figure 1 indicate the volumes of liquid 12 respectively traversed by the elastic wave signal pulses from the transducers 14 and 1 6. A barrier 1 9 ensures that there is no part of the liquid 12 traversed by signals from both transducers 14 and 1 6.

The distance from transducer 14 to the liquid surface 1 3 is equal to the distance from the transducer 16 to the surface of the liquid 13 so that if the two transducers are excited in synchronism the reflected signal pulses from the liquid surface 1 3 should also arrive in coincidence. It is most unlikely, on the other hand, that signal pulses reflected from bubbles 1 5 will arrive in coincidence at the transducers 14 and 1 6. Thus by applying a coincidence detection technique described below with reference to Figure 4 it is possible to distinguish between reflected signal pulses from the surface 1 3 and reflected signal pulses from bubbles 1 5.

Determination of the appropriate location of transducers 14 and 16 and of the barrier 19 depends upon the characteristic shape, indicated by the dotted lines 1 7 and 1 8, of the volume of liquid traversed by elastic wave signal pulses projected by the transducers. The form of the volume, most simply described as that "illuminated" by a transducer is illustrated in Figure 3. It will be seen that there is a near field region of length Zm in which there is very little divergence of the elastic energy so that in the near field the volume illuminated is substantially a cylinder of diameter d and length zm. At distances z from the transducer greater than Zm known as the far field, the elastic wave beam diverges with a half angle 0.For elastic wave signal pulses of wavelength t and frequency f, the relationships between the various parameters are as follows:- A=v/f Zm = (d2 0 = sin1(1 .22.t/d) where v is the veiocity of ultrasound in the liquid (e.g. approximately 1 500 ms1 in water). At any range z in the far field (where z is greater than Zm) the illuminated volume will have the diameter given approximately by: D = d +2(z-zrn)tan O These equations enable one to calculate an appropriate separation for the transducers 1 4 and 1 6 and appropriate dimensions for the barrier 1 9.

Figure 2 illustrates a modified apparatus in which similar components to those described with reference to Figure 1 are marked with the same reference numerals but distinguished by the suffix a. In this example instead of a barrier 1 9 the transducers 1 4a and 1 6a are spaced sufficiently far apart for there to be no overlap of the illuminated volumes even at the expected maximum liquid level. The following Table shows the required minimum horizontal separation S between the two transducers 1 4a and 1 6a for a number of different choices of transducer frequency f and maximum liquid depth z assuming a transducer diameter d of 25 mm.

Separation S/mm required for maximum liquid depth z Frequency Wavelength Near Field Half-Angle f/MHz A/mm z/mm H z = 0.5m z=1m z = 2m z = 5m 0.5 3 51 8.40 158 306 602 1490 1 1.5 104 4.20 83 157 303 744 2.5 0.6 260 1.7 39 68 127 303 5 0.3 521 0.8 25 39 68 156 Very little separation is required between transducers if the reflecting surface lies within the near field. The Table shows that the separation required is reasonable in relation to typical plant dimensions even if liquid depths of up to 5 metres are to be monitored.

It should be noted that the same directivity applies to a planar transducer in both transmission and reception. Consequently scattering from a bubble or drop of one transmitted elastic wave pulse into the other receiver transducer will be virtually eliminated so long as the volumes of liquid illuminated by the transducers do not overlap as described above. However, it would be possible to achieve additional discrimination by selecting different resonant frequencies for each of the transducers whereby bandpass amplifiers in the detection circuitry would discriminate further against elastic wave signal pulses from one transducer being scattered into the other transducer.

Referring to Figure 4, received elastic wave signal pulses are converted by the transducers 1 4 and 16 operating in the receive mode to receive electrical signal pulses which are passed respectively on lines 21 and 22 to amplifiers 23 and 24. The gain of these amplifiers 23 and 24 is swept by controllers 25 and 26 respectively in a known manner for adjusting the sensitivity of the detection circuitry with time so that it matches the expected intensity of the reflected signal which will undergo a known attenuation with distance travelled through the liquid. Further amplification of the signals is provided by amplifiers 27 and 28 the outputs from which are applied to strobe generators 29 and 31.These strobe generators include a threshold discriminator which is controlled in a known way to be enabled during a predetermined period of time encompassing the range of possible arrival times of signal pulses reflected from the liquid surface. Each of the strobe generators (29 and 31) transmits a standard strobe pulse (typically of 75 nanosecond duration) immediately on detection of the first signal pulse which is received during the predetermined time period and whose amplitude exceeds the preset discriminator level. The independent strobe pulses from the strobe generators 29 and 31 are fed to monostable devices 32 and 33 respectively each of which provides a pulse output of predetermined duration th which can be adjusted by altering an adjustable control setting on the monostable devices.

Arrival of received signal pulses on lines 21 and 22 in coincidence is determined by feeding the output at X from monostable device 32 and the output at Y from monostable device 33 into a device which immediately performs a logical AND function on the binary signals X and Y to provide an output Z.

As indicated by Figures 5a to 5d there will be a signal output at Z for precise coincidence (Figure 5a) and also for near coincidences within the timescale set by the duration of the output pulses from the monostable devices 32 and 33. Accepted near coincidence pulses are indicated in Figures 5b and 5c.

Figure 5d illustrates reflected signal pulses arriving out of coincidence; under such circumstances, there is no output pulse at Z.

The setting of the duration At of the output pulses from the monostable devices 32 and 33 is adjusted to accommodate electronic limitations and real transient fluctuations in the local liquid level due to mechanical disturbance or other causes.

It will be appreciated that conventional electronic circuitry will be employed to provide an indication of the liquid level and operate alarms if and as required. The output at Z from the equipment illustrated in Figure 4 is used simply to control the conventional detection circuitry to ensure that this operates only upon reflected signal pulses which are in coincidence and not upon spurious reflected signal pulses deriving from bubbles or other spurious echoes. Associated electronic devices (the details of which are not relevant to this invention) must be designed in such a manner that excitation of the two ultrasonic transducers need not generate coincident received signals. Any invalid measurement which results in no pulse output at Z must be rejected and associated data should be neither displayed nor incorporated in any subsequent data processing.

The invention is not restricted to the details of the foregoing examples. For instance the apparatus may be adapted for use in detecting both a liquid-liquid interface and a liquid-gas surface where two immiscible liquids are present. In this case, of course, interfacial mixing may occur on both sides of the liquid-liquid interface as well as at the liquid-gas surface; therefore spurious signals can originate not only from gas bubbles entrained in the liquid but also from drops of one liquid entrained in the other. For such a dual interface detection apparatus two strobe generators would be employed, for example in the manner described in Patent Specification No. 811 8335 (Ref: 12867 LgH).

Whilst it is clearly simplest to arrange for the expected transit time for elastic wave signal pulses from transducer to interface and back again to be the same for each of the transducers it is, of course, possible for this to be different physically and for the difference to be compensated by appropriate and carefully synchronised timing delays in the electronic circuitry. Most conveniently, two vertically displaced transducers would be excited at different times but with such a time difference that received signals from any surface or interface would be coincident.

Alternatively, the timing measurements on the reflected signals can be digitised and testing for coincidence applied by an associated computer or by dedicated digital electronics. In that case any predetermined variations in the characteristics of the system (such as unsynchronised excitation of the plurality of transducers, or varying elastic wave signal path lengths) can be compensated in the digital processing.

It would be possible, of course, to extend the system to three or more channels so as to reduce further the possibility of false coincidence. However, two channels would probably be sufficient in most practical situations.

It is to be noted that the duplication of transducer and electronic circuitry has other advantages in that the system is inherently self-checking because mal-operation or failure of one channel will usually result in total loss of coincidence signals. On the other hand it would be readily possible, for example in an emergency, for the system to be operated with a single channel alone.

Claims (9)

1. Apparatus for detecting a surface or interface level of a liquid which apparatus comprises means for injecting elastic wave signal pulses into the liquid towards the surface or interface simultaneously or in predetermined time relationship from at least two spaced locations, the arrangement being such that the volumes of the liquid respectively traversed by the signal pulses from the two or more locations do not overlap, receiver means associated with each location for receiving elastic wave signal pulses reflected back from the surface or interface, and coincidence detection means for determining when such reflected signal pulses are received in coincidence or predetermined time relationship at the said receiver means.

2. Apparatus as claimed in Claim 1, wherein a transducer which performs the combined function of converting an electrical signal pulse into an elastic wave signal pulse for transmission and the converse for reception is positioned at each of the said spaced locations.

3. Apparatus as claimed in Claim 1, wherein at each location there is positioned a transducer for transmission and a separate transducer for reception.

4. Apparatus as claimed in any of the preceding claims, wherein overlapping of the volumes within the liquid traversed by the signal pulses is prevented by one or more appropriately located physical barriers.

5. Apparatus as claimed in any of Claims 1 to 3, wherein overlapping of the volumes within the liquid traversed by the signal pulses is prevented by adequate spacing apart of the said spaced locations.

6. Apparatus as claimed in any of the preceding Claims, wherein electrical signal processing means are included in the apparatus for processing the received signal pulses, whereby coincidence is registered even although disturbance of the surface or interface causes small variations in the respective times taken for signal pulses reflected therefrom to reach the respective receiver means at the said spaced locations.

7. Apparatus as claimed in any one of the preceding Claims, wherein means is provided for distinguishing reflected signal pulses injected into the liquid at one of the spaced locations from reflected signal pulses derived from signal pulses injected into the liquid at the other or another spaced location.

8. Apparatus as claimed in Claim 7, wherein the said means comprises in combination transducers and associated drive units tuned to different resonant frequencies respectively at each of the spaced locations and band pass amplifiers in the detection means for discriminating against detection at one spaced location of reflected signal pulses derived from signal pulses injected at a different spaced location.

9. Apparatus substantially as hereinbefore described with reference to, and illustrated in, Figure 1 or Figure 2 and Figures 5 and 6 of the drawings filed herewith.