Abstract:
A liquid measurement system including interface apparatus for sharing the excitation and signal processing of an ultrasonic transonic transducer and temperature measuring device remotely located at a liquid container is disclosed. The interface apparatus includes a common conduction path coupled to both of the ultrasonic transducer and the temperature device for conducting first excitation signals to the ultrasonic transducer and the corresponding echo signals therefrom, and for conducting second excitation signals to the temperature measuring device and the response signals therefrom, and an interface circuit for governing the conduction of the first and second excitation signals over the common conduction path from their generating sources and for providing a balanced interface for receiving both of the echo signals and response signals from the common conduction path. In one embodiment, the liquid measurement signal includes a processor that is coupled to the interface circuit for receiving the echo signals and the response signals therefrom for determining liquid level in the container based on the received signals.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed to liquid measuring systems in general, and more specifically, to a liquid measurement system including interface apparatus for sharing the excitation and signal processing of an ultrasonic transducer and temperature measuring device remotely located at a liquid container. 
     Present liquid measurement systems of the ultrasonic variety utilize an ultrasonic transducer disposed at the liquid container for measuring the level of liquid in the container. The ultrasonic transducer is excited to generate ultrasonic pulses directed at the surface of the liquid level and for receiving echoes from the surface of the liquid level that are converted into corresponding echo signals. These systems generally include a temperature measuring device, like a resistance temperature device or RTD, for example, disposed at the container in contact with the liquid in close proximity to the ultrasonic transducer. When the RTD is excited, it generates a response signal representative of the temperature of the liquid that is used along with the echo signals for the determination of the liquid level in the container. 
     While the ultrasonic transducer and its temperature measuring device are located at the liquid container, the apparatus for exciting and signal processing each such device is generally located remotely from the container. Where the liquid container is a fuel tank on-board an aircraft, the exciting and processing apparatus may be located anywhere from twenty to three hundred feet from the fuel tank. In addition, each device includes its own dedicated apparatus and cabling and on-board commercial aircraft in particular, there may be thirty to forty or more of these devices. Thus, in the aircraft industry, this individually dedicated apparatus and cabling represents a heavy burden in volume and weight as well as cost of labor, maintenance and fuel consumption. Accordingly, it is desirable, especially for aircraft applications, to reduce the dedicated apparatus and cabling for each such device. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a liquid measurement system includes interface apparatus for sharing the excitation and signal processing of an ultrasonic transducer remotely located therefrom at a liquid container and of a temperature measuring device disposed in close proximity to the ultrasonic transducer for measuring the temperature of tile liquid threat. The ultrasonic transducer is excited to generate ultrasonic pulses directed at the surface of the liquid level in the container and for receiving ultrasonic echoes from the liquid level that are converted into corresponding echo signals. The temperature measuring device is excited to generate a response signal representative of the temperature of the liquid in close proximity to the ultrasonic transducer. The interface apparatus includes a first means for generating first excitation signals for tile ultrasonic transducer, second means for generating second excitation signals for the temperature measuring device, a common conduction path coupled to both of the ultrasonic transducer and the temperature measuring device for conducting the first excitation signals to the ultrasonic transducer and the corresponding echo signals therefrom, and for conducting the second excitation signals to the temperature device and the response signals therefrom, and an interface circuit coupled between the first and second means and the common conduction path for governing the conduction of the first and second excitation signals over the common conduction path and for providing a balanced interface for receiving both of the echo signals and response signals from the common conduction path. In accordance with another aspect of the present invention, the liquid measurement system includes a processing means coupled to the interface circuit for receiving the echo signals and the response signals therefrom for determing liquid level in the container based on the received signals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram schematic of a liquid measurement system suitable for embodying the principles of the present invention. 
     FIG. 2 is a circuit schematic diagram of interface apparatus suitable for use in the embodiment depicted by FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a liquid container is shown at  10  containing liquid at a level  12 . By way of example for the purpose of describing the present embodiment, the container  10  may be an aircraft fuel tank containing aircraft fuel at the level  12 . In the present embodiment, an ultrasonic transducer  14  and a temperature measuring device  16  are included in a common transducer assembly  20  (refer to FIG. 2 for greater detail) that is disposed at the container  10 . While it is preferred that the ultrasonic transducer  14  and temperature measuring device  16  be contained in a common assembly, it is understood that the present invention need not be so limited. It is desired however to have the temperature measuring device  16  disposed at the container in contact with the liquid in close proximity to the ultrasonic transducer. 
     The ultrasonic transducer  14  which may be of the piezo ceramic variety including a PZT (lead zirconate titanate) crystal, for example, is excited by pulsed alternating current (AC) signals of a relatively high frequency that may be on the order of one (1) megahertz (MHz), for example, to generate ultrasonic pulses  22  directed at the liquid level  12 . In addition, the ultrasonic transducer  14  is operative to receive echoes  24  from the liquid level and convert them into corresponding echo signals. Ultrasonic transducers generally include a negative impedance component that acts to reduce power to the device at the operating excitation frequency thereof In the present embodiment, a tuning inductor  18  is coupled in series with the ultrasonic transducer  14  (shown in FIG. 2) and is of a positive reactive impedance value at the operating frequency to negate the negative reactive impedance component of the transducer  14  in order to maximize power thereto. 
     The temperature measuring device  16  which may be a resistance temperature device or RTD, for example, is disposed in parallel with the transducer  14  in the present embodiment. In this arrangement, it is preferred that the RTD be essentially non-inductive, that is less than five(5) microhenries, for example, in order not to introduce stray inductance to the circuit. For this purpose, a semiconductor-based RTD of the type manufactured by Kulite Semiconductor Products, Inc. would be suitable. The resistance of the RTD may be on the order of 1-10 KiloOhms for the present embodiment. In this parallel circuit arrangement, the RTD  16  may also function as a bleed resistor to the transducer  14  to prevent charge build-up on the capacitive component integral thereto. Another resistor (not shown) of a resistance on the order of one or more MegaOhms may also be coupled in parallel with the transducer  14  to ensure against such charge build-up. 
     A common conduction path  30  is coupled to both of the ultrasonic transducer  14  and RTD  16  for conducting first excitation signals to the transducer  14  and corresponding echo signals therefrom, and for conducting second excitation signals to the RTD  16  and response signals therefrom. In the present embodiment, the path  30  is a twisted pair of wires  32  and  34  that are coupled across the parallel circuit arrangement of the transducer  14  and RTD  16  at the container  10 . The path  30  may include a shield  36  covering the twisted wire pair which may be grounded at one end, preferably the end remote from the container  20 , or at both ends as the application dictates. 
     An interface circuit  38  that is shown in greater detail in the schematic of FIG. 2 is located remotely from the container  10  and coupled to the remote end  40  of the conduction path  30 . Also remotely located from the container  10  are a pulsed controlled oscillator circuit  42  for generating the first excitation signals for the transducer  14  and a precision current source  44  for generating the second excitation signals for the RTD  16 . Both of the circuits  42  and  44  that may be of a conventional design well known to those skilled in the pertinent art are coupled to the interface circuit  38 . The interface circuit  30  is operative to govern the conduction of the first and second excitation signals over the path  30  and to provide a balanced interface for receiving over the path  30  both of the echo signals and response signals from the devices  14  and  16 , respectively. The operation of the interface circuit  38  will become more apparent from the description of the circuit embodiment of FIG.  2 . 
     In the present embodiment, a digital signal processor  46  that may be of the type manufactured by Texas Instruments under the model number TMS320C32X, for example, operates to control the pulsed operation of the oscillator  42  and the operation of the source  44  through the interface circuit  38 . The processor  46  is coupled to the interface circuit  38  for receiving the echo signals over signal line  48  and the response signals over data lines  50  and is operative to determine Linder program control the level of the liquid in the container  10  based on the received signals. Since the operation of the processor  46  is digital, the analog response signals form the RTD  16  over signal line  52  are digitized by a conventional analog-to-digital converter (A/D)  54  that may also be controlled by the processor  46  in the present embodiment. The algorithms executed by the processor  46  for determing the liquid level from the echo signals and temperature response signals are well known to all those skilled in the pertinent art and for this reason need not be described in detail for the present embodiment. More specifically, in the present embodiment, the oscillator circuit  42  is gated to produce a pulse of one to sixteen sinusoidal cycles of one megahertz frequency at a burst frequency on the order of one to eight hertz. However, it is understood that the interpulse period and duration of the pulse may vary during operation based on the level of liquid in container  10 . 
     Referring now to FIG. 2, the oscillator circuit  42  is coupled to a primary side of a step up transformer  60  of the interface circuit  38 . The transformer  60  of the present embodiment is of the type having a ferrite torroidal core designed particularly for RF applications and has a winding ratio of typically one to eight. Accordingly, a differential pulsed AC signal having a high peak-to-peak voltage on the order of ninety to one-hundred and ten volts is induced across the secondary winding  62  of the transformer  60 . Back to back diodes D 1  and D 2  are coupled in series to one end  64  of the secondary winding  62 . In a balanced arrangement, back to back diodes D 3  and D 4  are coupled to the other end  66  of the secondary winding  62 . Capacitor C 1  is coupled in series with the diode pair D 1 , D 2  to form a circuit node  68  and for balance, capacitor C 2  is coupled in series with the diode pair D 3 , D 4  to form a circuit node  70 . For the present embodiment, the diodes may be 1N4148s and the capacitors may be of a value on the order of one microfarad. Wire  32  of the twisted pair is connected to the other side of capacitor C 1  at a circuit node  72  and for balance, the other wire  34  of the pair is connected to the other side of capacitor C 2  at a circuit node  74 . 
     A differential video amplifier  76  is coupled across the circuit nodes  68  and  70  through resistors R 1  and R 2 , respectively. Back to back diodes D 5 , D 6  are coupled across the inputs of the amplifier  76  to protect the input stage thereof against overvoltage damage from the pulsed high voltage AC excitation signal induced across the secondary winding  62 . The output of amplifier  76  may be coupled to the processor  46  over signal line  48  as shown in FIG.  1 . In the present embodiment, the amplifier  76  may be of the type manufactured by MAXIM bearing model number MAX 436, for example, that has a bandwidth around two hundred and seventy-five megahertz. In addition, an instrumentation amplifier  78  which may be of the type manufactured by Analog Devices bearing model number AD521, for example, is coupled across the circuit nodes  72  and  74  through the resistors R 6  and R 7 , respectively. A capacitor C 3  is coupled across the inputs of the amplifier  78  to filter out the high frequency first excitation signals and associated echo signals from interfering with the operation thereof. In the present embodiment, the resistors R 1  and R 2  may be of a value on the order of two KiloOhms, resistors R 6  and R 7  may be of a value on the order of ten KiloOhms, capacitor C 3  may be 0.1 microfarad and diodes D 5  and D 6  may be 1N4148s. 
     Moreover, to match the output impedance of the interface circuit  38  to that of the conduction path  30 , balanced impedance matching circuits are coupled from the nodes  72  and  74  to the ground reference of the circuit  38 . More specifically, a matching circuit comprising a series combination of capacitor C 4  and resistor R 4  in parallel with a capacitor C 4 ′ is coupled from node  72  to ground reference. And, a matching circuit comprising a series combination of capacitor C 5  and resistor R 5  in parallel with a capacitor C 5 ′ is coupled from the node  74  to ground reference. In the present embodiment, the values of C 4  and C 5  may be on the order of one microfarad, the values of the resistors R 4  and R 5  may be on the order of one Kilo Ohm, and the values of the capacitors C 4 ′ and C 5 ′ may be on the order of four hundred and seventy picofarads, for example. 
     Still further, the precision direct current (DC) current source  44  is coupled differentially to the nodes  72  and  74  through a high voltage differential switch  80 . The precision current signal of the source  44  which is the second excitation signal of the present embodiment is coupled to the pole of one switch of  80  and the ground reference or return path of the source  44  is coupled to the pole of the other switch. The differential switch  80  is controlled by the processor  46  in the present embodiment through a logic buffer circuit  82 . When operated in the open position., the second excitation signal is inhibited from exciting the RTD and no temperature response signal is produced thereby. In this state, the switch  80  offers a balanced high impedance to the pulsed AC excitation and associated echo signals. When operated in the closed position, the switch  80  enables excitation of the RTD by the source  44  via a low impedance path. In the present embodiment, the switch  80  may be of the type manufactured by Siliconix bearing model number DG507A, for example. 
     In operation, the processor  46  may control the oscillator  42  to generate pulsed AC excitation signals at around one megahertz, say on the order of five times a second to effect interpulse periods of two hundred milliseconds. Each burst or excitation pulse may include from one to sixteen cycles of the AC signal, for example. The transformer  60  steps up the excitation voltage to around ninety volts peak to peak which is passed along differentially through the balanced capacitors C 1  and C 2  and over the twisted wire pair  32  and  34  to excite the transducer  14 . While the pulsed AC excitation signals are being generated, the switch  80  is controlled to its open position to offer a balanced high impedance to the AC excitation signal. The diodes D 6  and D 7  pinch off and protect the input stage of amplifier  76  against the high voltage excitations signals. Also, the capacitor C 3  filters out the high voltage AC excitation signals to protect the input stage of the amplifier  78 . In response to the first excitation signals, the transducer  14  generates ultrasonic pulses directed at the surface of the liquid in the tank  10 . The echoes from the liquid surface are received by the transducer  14  and converted to echo signals that are conducted differentially back over the twisted wire pair of the common conduction path  30  and through the capacitors C 1  and C 2  to the amplifier  76 . The diode pairs D 1 , D 2  and D 3 , D 4  prevent a short circuiting of the echo signals by blocking out the low impedance path of the transformer secondary  62  up to at least two diode voltage drops which is an adequate voltage level to be detected and amplified by the amplifier  76 . In turn, the amplified echo signals are Output from the amplifier  76  over signal line  48  to the processor  46  for use in determining the level of the fuel in the tank  10 . 
     The aforementioned pulsed AC excitation of the transducer  14  may continue for a period of time to collect an adequate number of echo signals for signal processing thereof. Every so often, the generation of the first excitation signals is interrupted by the processor and switch  80  is controlled to its closed position to enable the second or DC excitation signal, which may be on the order of one milliamp, for example, generated from the source  44  to be conducted differentially over the twisted wire pair of the common conduction path  30  to the RTD  16 . Note that the return current path from the RTD is to the ground reference of the source  44 . The DC response voltage across the RTD that is representative of the temperature detected by the RTD is conducted back over the twisted wire pair to the interface circuit  38  wherein it is amplified by the amplifier  78  and conducted to the processor  46  via signal line  52  and A/D  54  for use along with the echo signals in determining the liquid level in the container  10 . The DC excitation signal and response signal are blocked from interfering with the AC excitation circuitry by the capacitors C 1  and C 2 . Once the processor  46  accepts the temperature measurement signal, it may return the switch  80  to its open position and continue controlling the oscillator circuit to generate the first excitation signals. In this manner, the interface circuit  38  is utilized to govern the conduction of the first and second excitation signals over the common conduction path  30  mutually exclusive of one another. 
     Thus, through the principles of the present invention, it is shown that an ultrasonic transducer and its temperature measuring device at a liquid container, like a fuel tank of an aircraft, for example, may share the same remotely located electronics on-board the aircraft for excitation and signal processing and share a common conduction path therebetween, thus affording a substantial savings in volume and weight as well as cost of labor, maintenance and fuel consumption. While the present invention has been presented here above in connection with a preferred embodiment, it is understood that modifications and equivalent substitutions may be made thereto without deviating from the broad principles thereof. Accordingly, the present invention should not be limited to any single embodiment, but rather construed in breadth and broad scope in accordance with the recitation of the appended claims hereto.