Abstract:
A method of operating a pulse echo ranging system comprising a transducer assembly for providing transmission and reception of high frequency energy pulses at substantially different plural frequencies. The method uses signals received by the transducer assembly to generate an echo profile for signals received at at least a first of the frequencies to enhance the recovery of data beyond that obtained from the first signal alone.

Description:
FIELD OF THE INVENTION 
   This invention relates to pulse echo ranging systems, and more particularly although not exclusively to systems for monitoring the level of sludge in a body of liquid. 
   BACKGROUND OF THE INVENTION 
   In a number of applications, it is necessary to monitor liquids containing significant quantities of suspended materials, which materials may tend to separate or accumulate as a separate phase beneath a body of clearer liquid. A parameter which it is frequently desired to monitor is the level of the interface between settled material or sludge, and a supernatant liquid. 
   Suspended material in liquids tends to reflect or disperse acoustic energy, although the extent of this effect is frequency dependent. The effectiveness of a pulse echo ranging system in determining the position of an interface between a sludge layer and a supernatant liquid will depend upon the presence of suspended material, including gas bubbles, in the supernatant layer, and the concentration and nature of particles in the sludge layer. It is difficult to select a transducer frequency which will provide reliable detection of the interface under all circumstances. 
   Since transducers used in such systems will normally operate submerged, there is the possibility of build-up of material passing out of suspension and liquid and building up on the transducer assembly. Such build-up can be inhibited by ultrasonic insonification of vulnerable surfaces, particularly the radiating surface of the transducer assembly, but in practice, the most effective frequencies for operation of the transducer, if it is to be self-cleaning, often fall in a different range from those providing best detection of the sludge/liquid interface. 
   UK Patent Application 2054851A discloses a sonar system using multiple frequencies, one of which is generated by the beating of two closely spaced higher frequencies. UK Patent Application 2191055A dicloes a radar system operating at two substantially different frequencies in order to detect flocks of birds. 
   SUMMARY OF THE INVENTION 
   According to the invention, a method of operating a pulse echo ranging system comprises operating a transducer assembly to provide transmission and reception of pulses of high frequency energy at plural substantially different frequencies, and signals received by the transducer assembly are to generate an echo profile for signals received at at least a first of the frequencies, and utilizing the signal at another of the frequencies to enhance the recovery of data beyond that obtained from the first signal alone. 
   This enhancement may be achieved in various ways. Of two frequencies, one may be used to generate an echo profile and the other to clean the radiating surface of the transducer to maintain efficiency. If echo profiles are recovered for two or more frequencies, the additional profiles may be utilized to enhance the reliability of recovery wanted data. For example, if supernatant liquid contains large quantities of suspended material, acoustic energy at a frequency best suited to detecting a sludge interface may be so attenuated before reaching the interface that detection is not practicable. In this case, the lower frequency signal may provide additional penetration in order to detect the sludge interface and thus improve reliability. 
   Furthermore, if two or more echo profiles are recovered, the profile recovered from a signal which is not heavily reflected by a sludge interface or other interface which it is desired to detect may be utilized as a reference signal against which echoes occurring in the echo profile from a signal at a different frequency may be assessed. 
   Alternatively, the profiles from different frequencies may be summed, or differenced on the basis that the response from an interface to be detected to signals at substantially different frequencies will be substantially different. 
   If the echo responses at different frequencies are available, it may be possible to deduce characteristics such as quantities of suspended solids, layer density, presence of bubbles, and so on from the different responses to the different frequencies. 
   These various features may of course be used in combination, according to the manner in which a microcontroller controlling the system is programmed. 
   Further features of the invention will become apparent from the following description with reference to the accompanying drawings. 

   
     SHORT DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified schematic diagram of a pulse echo ranging system; 
       FIG. 2  is a section through a settling tank illustrating an exemplary operational environment for the mechanism; 
       FIG. 3  is a graph illustrating an exemplary echo profile produced by a signal component at one of the frequencies utilized; 
       FIG. 4  compares echo responses at two different frequencies in a primary clarifier; 
       FIG. 5  compares echo response at the two different frequencies in a secondary clarifier; 
       FIG. 6  is an annotated graph illustrating how echo data may be interpreted to provide additional data. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , there is shown a simplified schematic diagram of a pulse echo ranging system controlled by a central processing unit (CPU)  2 . Typically the CPU  2  will be incorporated in a microcontroller implementing peripheral functions used to implement some of the other blocks shown in  FIG. 1  as well as additional functions not described. The CPU operates under a control program stored in read-only memory (ROM)  4 , utilizing parameters stored in non-volatile random access memory (NOVRAM)  6 , and provided with working memory in the form of random access memory (RAM)  8 . An interface  10  provides for the export of data from the unit, and the import of operating parameters. Data may be exported in the form of a display, telemetry signals or alarm signals. The CPU  2  also controls a transmitter  12  which controls the timing, frequency and amplitude of high frequency pulses applied to a transducer  14 . A receiver  16  receives return echo signals from the same or a different transducer, amplifies them, usually logarithmically, and applies them to an analog to digital converter  18 , from where the digital echo profile is stored in RAM  8  for further processing. 
   The operation of such systems is described in more detail in our U.S. Pat. Nos. 4,596,144; 4,831,565; 4,890,266; 4,999,998 and 5,076,751, and only features of difference will be discussed further below. 
   Referring to  FIG. 2 , this shows an exemplary application of the invention, used for monitoring a sludge interface  20  in a tank  22 , of the invention, in which the transducer  14  is mounted with at least its radiating face  24  submerged in the liquid content of the tank. Above the interface  20 , the liquid content is liquid containing suspended material and possibly gas bubbles, while a denser sludge phase  28  settles out beneath the interface and above the bottom wall  30  of the tank. 
     FIG. 3  illustrates an exemplary echo response to a high frequency acoustic pulse emitted by the transducer  14 . In this instance there is a strong echo  32  from the bottom of the tank, while the weaker true echo  34  from the interface  20  shows little increase in amplitude relative to the immediately preceding portion of the profile which consists not only of a component due to ranging of the transducer, but also energy reflected by the suspended material in the liquid above the interface. This reflected energy tends to mask the wanted echo. 
   These phenomena are illustrated further in  FIGS. 4 and 5 , which show how the responses to different pulse frequencies can change in different environments. Graphs a and b in each Figure represent logarithmic plots of the amplitude of the echo profiles produced at 44 kHz and 150 kHz respectively in a tank acting as a primary clarifier and a tank acting as a secondary clarifier. In  FIG. 4 , the lower frequency (trace b) propagates well through liquid containing substantial amounts of suspended material, and clear echoes  32  and  34  can be seen both from the bottom of the tank and the sludge interface. At the higher frequency (trace a), absorption and scattering result in the echo  34  being much less marked, and the echo  32  being wholly absent. By contrast, behaviour in the secondary clarifier is quite different. In this case the suspended material is light and “fluffy”. It is penetrated well by the higher frequency (trace a), providing clear echoes  32  and  34  from the bottom of the tank and the sludge interface, whereas the lower frequency is severely attenuated and produces no recognizable echoes. 
   In a presently preferred embodiment, both frequencies are generated simultaneously by a common acoustic transducer. 
   Piezoelectric transducers used in acoustic ranging systems are commonly capable of resonating in different modes having substantially different resonant frequencies, and the transmitter  12  is configured to generate energy at two such widely spaced frequencies such as to cause the transducer to radiate at both frequencies during a pulse. For best results, the frequencies should have at least 2:1 ratio. Exemplary frequencies are 44 kHz and 150 kHz, or 50 kHz and 120 kHz. It is possible to use more than two frequencies, or to use a transducer assembly containing multiple piezoelectric elements having different frequencies, or to energize the transducer at different frequencies sequentially. It is also possible to utilize a transducer assembly comprising two separate transducers, for example two acoustic transducers operating at different frequencies. In some applications, it may be possible to combine an acoustic microwave transducer so as to exploit the different penetration and dispersion characteristics of the different types of radiation involved, for example, in applications where it is desired to detect both a foam interface and a liquid interface in a tank with the transducer assembly above both levels. 
   Regardless of the transmitter and transducer system used, there are various ways of using the invention to provide enhanced performance. At a lowest level, a higher frequency is used to generate an echo response for analysis, and acoustic energy at a lower frequency is used to inhibit deposition of solids on a radiating surface of the transducer assembly thus improving reliability. Preferably however, echo responses are obtained at each frequency and combined in some manner to provide enhanced data. 
   For example, the echo responses may simply be summed, providing a combined response in which echoes of energy occurring at any of the frequencies will normally be apparent. Even though one frequency fails to provide a substantial echo response from a feature of interest, the second frequency may result in a satisfactory response. An alternative or additional approach is to difference the responses at two frequencies. The resulting combined response will then represent the differences between the reflective behaviour of targets at the two frequencies and may enable distinct echo responses to be detected where the summary approach fails. A frequency which does not provide good echo responses may nonetheless be used as a reference against which to detect peaks or discontinuities in response at another frequency. 
   Comparing responses at different frequencies may permit further characteristics of liquid in a vessel to be determined, since the response to different types of suspended material at different frequencies is different. Referring to  FIG. 6 , a portion of the echo profile  36  consists of reverberation from suspended particles in the supernatant liquid, a portion  38  preceding the peak  34  from the sludge interface represents enhanced reflection from “fluff” of particle gathering above the interface, while the sharply depressed portion  40  of the response between the peak  34  and the peak  32  representing the reflection from the bottom of the tank represents high absorptive properties of the sludge beneath the interface. It will also be noted that the actual peak  34  due to reflection at the interface is quite small, but the peak is emphasized by the sharply contrasting reflective properties of the layers on either side of the peak. At a higher frequency, the reverberation from the suspended particles will typically be greater, and while the reflection from the interface may produce a more marked peak, this may be masked to a greater extend by reverberation. On the other hand, the reflection and dispersion by the fluff may be substantially less. These differing responses may permit substantial interpretation of systems whose characteristics have been the subject of empirical observation.