Abstract:
An ultrasonic fluid-gauging probe has a still well and two or more piezoelectric transducer elements mounted at its lower end. A drive and processor unit may separately energize each element and provide a separate output indicative of fluid height so as to provide redundancy. Alternatively, one element may be energized and the other used to receive the reflected signal in normal use but, when a fault is detected, one element may be used to both transmit and receive. The elements may be mounted side-by-side, such as on a common substrate, or one above the other in a stack.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to fluid-gauging probes. 
   Ultrasonic fluid-gauging probes, such as for measuring the height of fuel in an aircraft fuel tank, are now well established and examples of systems employing such probes can be seen in U.S. Pat. No. 5,670,710, GB2380795, GB2379744, GB2376073, U.S. Pat. No. 6,598,473 and U.S. Pat. No. 6,332,358. The probe usually has a tube or still well extending vertically in the fuel tank and a piezoelectric ultrasonic transducer mounted at its lower end. When the transducer is electrically energized it generates a burst of ultrasonic energy and transmits this up the still well, through the fuel, until it meets the fuel surface. A part of the burst of energy is then reflected down back to the same transducer. By measuring the time between transmission of the burst of energy and reception of its reflection, the height of fuel in the still well can be calculated. 
   Because the probe only has one transducer, failure or partial failure of the transducer or its associated electrodes or electrical circuit results in a complete loss of information from the probe. In order to provide sufficient reliability and redundancy in, for example, aircraft applications, it is usual to provide additional probes so that sufficient information can be provided in the event of probe failure. This leads to extra cost and weight. 
   BRIEF SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide alternative fluid-gauging probes. 
   According to one aspect of the present invention there is provided a fluid-gauging probe including a still well and at least two acoustic transducer elements mounted towards one end of the still well, the transducer elements being energizable to propagate acoustic energy into fluid within the still well and to receive acoustic energy reflected by a fluid interface within the still well. 
   Each of the transducer elements may be arranged to propagate energy and each of the transducer elements may be arranged to receive energy. Alternatively, the probe may be operable in a first mode where one element propagates energy into the fluid and another element receives the reflected energy, the probe being capable of being changed to operate in a second mode where one element both propagates and receives energy. The probe may be arranged to be switched from the first to the second mode when a fault is detected. The transducer elements may be provided on a common substrate. The transducer elements may have an electrode common to two elements. The transducer elements may be each operable at the same frequency or they may be operable at different frequencies. The transducer elements may be mounted side-by-side. Alternatively, they may be mounted one above the other and the elements may be selectively operable at different frequencies by using different combinations of pairs of electrodes. The transducer elements are preferably of a piezoelectric ceramic. 
   According to another aspect of the present invention there is provided a fluid-gauging system including a probe according to the above one aspect of the present invention. 
   Various different configurations of probe according to the present invention will now be described, by way of example, with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified cross-sectional side elevation view of a first form of probe; 
       FIG. 2  is a simplified cross-sectional side elevation view of a second form of probe; 
       FIG. 3  is a simplified cross-sectional side elevation view of a third form of probe; 
       FIG. 4  is a simplified cross-sectional side elevation view of a fourth form of probe; 
       FIG. 5  is a simplified cross-sectional side elevation view of a fifth form of probe; 
       FIG. 6  is a simplified cross-sectional side elevation view of a sixth form of probe; 
       FIG. 7  is a graph illustrating the system transfer function of the probe shown in  FIG. 6 ; 
       FIG. 8  is a simplified cross-sectional side elevation view of a seventh form of probe; 
       FIG. 9  is a simplified cross-sectional side elevation view of an eighth form of probe; 
       FIG. 10  is a simplified cross-sectional side elevation view of a ninth form of probe; 
       FIG. 11  is a simplified cross-sectional side elevation view of a tenth form of probe; and 
       FIG. 12  is a simplified cross-sectional side elevation view of an eleventh form of probe. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   With reference first to  FIG. 1  the fuel-gauging system comprises a probe  1  mounted projecting vertically, or substantially vertically, upwardly from the floor of a fuel tank (not shown). The probe  1  has a tubular still well  10  and an acoustic device in the form of a piezoelectric transducer assembly  11  mounted at the lower end of the still well so that it is immersed in any fuel  2  that is present. The transducer assembly is usually mounted in a housing that is acoustically-transparent at the frequency of operation so as to protect the piezoelectric ceramic from direct contact with fuel. A foam pad (not shown) or the like on the lower surface of the transducer provides damping. The transducer assembly  11  has two piezoelectric elements  12  and  13 , which may be two halves of a circular disc shape cut across its diameter. Electrodes  14 ,  15  and  16 ,  17  connected to the two elements  12  and  13  are connected to a drive and processing unit  3 . The unit  3  is arranged to apply bursts of alternating voltage to the electrodes  14  to  17  so as to energize both elements to resonate and produce bursts of ultrasonic energy that are propagated upwardly along the still well  10 . The processing unit  3  also receives electrical signals produced in the two elements  12  and  13  by the bursts of energy reflected from the fuel surface  4  and incident on the elements. The processing unit  3 , measures the time between transmission of the ultrasonic energy and reception of the reflection and calculates the height h of fuel within the still well  10  in the usual way from knowledge of the speed of transmission of the acoustic energy. It will be appreciated that in most systems there will be several probes distributed about the tank in order to measure the height at different locations. If either one of the elements  12  or  13  or their associated electrodes or circuits should fail or partially fail, the processor  3  simply uses the signal from the remaining element, thereby providing a degree of redundancy against failure. 
   Instead of operating the transducer elements  12  and  13  collectively in the manner described above, one element could be used in a first mode to produce the transmitted signal and the other element used to derive the reflected signal. In such an arrangement, if there was a failure, the processing unit  3  would revert to a second mode where it attempts to use one element both to propagate and receive the ultrasonic energy. If this failed, it would attempt to use the other element. 
   It will be appreciated that the probe could have different numbers of transducer elements, such as three, four or more and that they could be of various different shapes. 
     FIG. 2  shows an arrangement where the transducer elements  212  and  213  are physically spaced laterally and completely separate from one another. These elements  212  and  213  can be operated completely separately of one another and may have different frequencies. Where the transducer elements have matched characteristics, such as optimum frequency, they can be operated together, with one element receiving reflected signals produced by the other element. Again, a probe could have a cluster of any number of such transducer elements. 
   As shown in  FIG. 3 , the gap between separated transducer elements  312  and  313  could be filled by a filler material  300  that is electrically non-conductive and acoustically transparent so that energization of one element can energize the other or others of the elements. 
   The arrangement in  FIG. 4  shows how multiple transducer elements  412  and  413  can be provided on a common piezoelectric ceramic substrate  400  by physically separating the electrodes  414  to  417 . 
   The number of electrodes can be reduced in a probe  510  of the kind shown in  FIG. 5  where a transducer  511  with two elements  512  and  513  has three electrodes namely a single, common electrode  514  extending across the entire upper surface of both elements and two separate electrodes  515  and  517  on the lower surface disposed in the regions of the two elements. In general, for a probe having n transducer elements, this arrangement enables the number of electrodes to be reduced from 2n to n+1. 
   The different transducer elements need not be matched and operate at the same frequency.  FIG. 6  shows a probe  610  with two elements  612  and  613  having different thicknesses and hence different thickness mode resonant frequencies. The upper electrodes  614  and  616  and the lower electrodes  615  and  617  of these two elements are connected with one another as two output lines  618  and  619 .  FIG. 7  illustrates the different optimum operating frequencies f 2  and f 3  of the two elements  612  and  613  respectively. It will be appreciated that the different operating frequencies of the elements enables the processor to discriminate between the different elements so that individual ones of the elements can be selected by appropriate choice of frequency. 
     FIG. 8  shows a probe  810  having two transducer elements  812  and  813  of different thicknesses and operating frequencies formed on a common blank  800  of piezoelectric material by appropriately machining the blank with two regions  801  and  802  of different thickness. It will be seen that this arrangement has a common electrode  814  on the upper surface and two interconnected electrodes  815  and  817  on the lower surface. Instead of having two separate electrodes on the lower surface, there could be a single common electrode  915  as shown in  FIG. 9 . 
   The arrangements described above have the transducer elements disposed laterally of one another, they could, however, be stacked one above the other as shown in  FIG. 10 . In this configuration there are two elements  1012  and  1013  mounted one on the other and each having two separate electrodes  1014  and  1015 , and  1016  and  1017 . In such an arrangement, the upper element  1012  or elements are selected to be acoustically transparent to the frequencies of operation of the element  1013  or elements below. 
   The number of electrodes in a stack arrangement can be reduced in the manner shown in  FIG. 11  where a common electrode  1115  is used both for the lower surface of the upper piezoelectric element  1112  and the upper surface of the lower piezoelectric element  1113 . For a stack of n elements this enables the number of electrodes to be reduced from 2n to n+1. 
   Instead of operating the elements individually, they could be operated together by energizing non-adjacent electrodes. In the arrangement of  FIG. 11 , for example, the middle electrode  1115  would be held at a high impedance and the two elements  1112  and  1113  would be driven together by energizing the top and bottom electrodes  1114  and  1117 . The optimum frequency of operation is dependent on the combined thickness of the two elements  1112  and  1113 . Thus, with a stack of two elements the individual elements can each be operated separately at one frequency and the combined elements can be operated together at a different, lower frequency. 
   As the number of elements in the stack increases, so does the number of different operating modes.  FIG. 12  shows a stack of three transducer elements  1212 ,  1213  and  1214  with four electrodes  1215  to  1218 . This enables each of the elements  1212  to  1214  to be used individually, or the two different pairs of adjacent elements  1212  and  1213 , and  1213  and  1214  to be operated together, or all three elements  1212 ,  1213  and  1214  to be operated together. The unused electrodes would be set to a high impedance condition 
   It will be appreciated that there are various different arrangements in which a probe could be provided with more than one transducer element. Instead of using piezoelectric ceramics, it may be possible to use alternative acoustic generating materials.