Abstract:
The invention describes the use of an electrical pulse that is a single square wave to excite a piezoelectric crystal. This pulse is applied alternately to either side of the crystal at twice the nominal resonant frequency of the crystal. This method provides a significant increase in net drive power to the crystal without the need to provide a sine wave excitation of plus and minus voltage to the crystal.

Description:
REFERENCES CITED  
       [0001]     This application is based on and includes the information in provisional application 60/571835 filed on May 17, 2004.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a system for ultrasonically exciting the a piezoelectric crystal used in an ultrasonic ranging device.  
         [0003]     In North America and throughout the world, there are substantial numbers of propane fuel tanks installed outside residential and commercial premises to provide energy for heating, cooling and cooking. These tanks can range in size from ones which have a relatively small capacity (e.g. 50 gallons) to ones which have a relatively large capacity (e.g. 1000 gallons or more). Often, the propane fuel is sold by a propane distributor to its customers under a contract where the distributor provides the customer with the tank in return for the exclusive right to supply the customer with the fuel.  
         [0004]     Accordingly, there is a need in the propane industry for a direct method of monitoring customer inventory. In this respect, there are existing fuel tank mounted mechanical gauges which can be installed to provide an approximate indication of inventory. However, the readings they provide are typically quite unreliable (e.g. an error range of plus or minus 10% to 20%). An option that would provide improved accuracy would be to install on each tank a totalizing flow meter like that used by a fuel truck when delivering fuel to a tank, or comparable to that used by utilities that supply electricity, natural gas or water to utility customers. Then, the total volume of fuel removed from the tank then could be recorded and regularly compared with the volume when the tank was filled. However, apart from the cost of installing and maintaining such precision flow meters, the customers of a propane distributor are often located in non-urban areas. The distance between such customers can be too large to justify the repetitive manual collection of usage data.  
         [0005]     It is known to sense the level of liquid contained in a tank utilizing sonic measurement techniques; typically sonic or ultrasonic measurements made with the use of a piezoelectric crystal. If the level of liquid is known and the geometry of the tank is known, then the volume of liquid in the tank can be calculated based upon the measured level. However, when the liquid in question is propane and the tank in question is a propane tank, problems arise. One problem is the adequacy of acoustic coupling between the ultrasonic transducer and the fuel tank. If the coupling is poor, then it may not be possible to obtain a measurement, or any measurement made may be unreliable. Another problem is the voltage levels which are generated by the transducer. If relatively high voltages are involved, then regulatory approval may be difficult or impossible to obtain. Dominating any solution to such problems is the question of cost. A solution which enables direct measurement but which requires a costly installation on each fuel tank of each customer is an impractical solution.  
       BRIEF SUMMARY OF THE INVENTION  
       [0006]     In accordance with the present invention, there is provided a new and improved method of exciting a piezoelectric crystal used in an ultrasonic fuel level monitoring system for sensing the level of liquid in a liquid fuel tank. The system includes an ultrasonic transducer unit externally mountable on the bottom of the tank and a control unit operably connectable to the transducer unit. In a preferred embodiment where the tank is a propane tank, the control unit is preferably mounted atop the tank and is connected to the transducer unit by an electrical cable.  
         [0007]     The transducer unit includes a piezoelectric crystal (“piezo”) transducer, a piezo driver circuit, and preferably a temperature sensor. In cases where the propane tank is a steel wall propane tank, it has been found that the transducer unit advantageously further includes an aluminum disk normally interposed between the piezo and the tank wall to provide improved acoustic coupling.  
         [0008]     In response to command signals from the control unit, the piezo driver circuit causes the piezo to transmit ultrasonic pulse trains having a controlled pulse frequency through the wall of the tank (and the aluminum disk if included), and to listen for return echoes from the surface of liquid in the tank. Echo return signal information is sent back to the control unit from the transducer unit.  
         [0009]     The controlled pulse frequency determined by the control unit is an adaptively determined pulse frequency. More particularly, it has been found that the pulse frequency at which suitably strong echo return signals can be detected may not only differ from the resonant frequency of the piezo but also can depend upon the acoustical impedance of the “stack” (viz. the piezo, aluminum disk (if any), and tank wall thicknesses combined), the acoustical impedance of the bonding agent used to secure the transducer unit to the tank, and ambient temperature. It is possible that other factors may have an influential effect. All of these considerations and possible unknowns recommend against a fixed pulse frequency and in favor of a system where the pulse frequency is frequency agile, and is adaptively determined depending upon echo return signal strengths that can be obtained over a swept range of pulse frequencies.  
         [0010]     The process of transmitting ultrasonic pulse trains into a fuel tank and listening for return echoes is referred to herein as “pinging” the tank. As is well understood by those skilled in the art, the time that it takes for a transmitted signal to the reach the liquid surface and to be echoed back (viz. time-of-flight) will correspond to the liquid level. With knowledge of the tank geometry, a measurement of the volume of fuel in the tank then can be calculated.  
         [0011]     The temperature sensor mentioned above is desirable because, as is also well understood by those skilled in the art, tank geometry will vary with increasing or decreasing temperatures. Thus, the time-of-flight measured at one temperature may be the same as the time-of-flight measured at another temperature. Yet, the actual volume of fuel in the tank at the two temperatures may differ. With knowledge of the temperature and tank geometry as a function of temperature, a more accurate measurement of the volume of fuel in the tank can be calculated at any given temperature. When the transducer unit includes a temperature sensor, a signal corresponding to temperature is sent back to the control unit from the transducer unit.  
         [0012]     This invention describes a novel way to provide the electrical pulses required to excite the piezo crystal. The piezo driver circuit which forms part of the transducer unit advantageously comprises an analog switching circuit which has the effect of powering the piezo with either a single pulse, the voltage of which is twice that of the voltage source, or the same driving power as with alternating plus or minus voltages, the absolute values of which correspond to that of the voltage source.  
         [0013]     This technique significantly increases the net power delivered by the piezo to the fuel tank without having either a high voltage source in the transducer unit or the control unit, and without requiring a transformer to boost the source voltage. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a perspective view illustrating mechanical aspects of an ultrasonic fuel level monitoring system in accordance with the present invention when mounted to a propane fuel tank.  
         [0015]      FIG. 2  is a high level block diagram of the monitoring system shown in  FIG. 1 .  
         [0016]      FIG. 3  shows a working environment for the propane tank and the monitoring system shown in  FIG. 1 .  
         [0017]      FIG. 4  is a perspective view illustrating mechanical aspects of the transducer unit forming part of the monitoring system shown in  FIG. 1 .  
         [0018]      FIG. 5  is an exploded perspective of the transducer shown in  FIG. 4 .  
         [0019]      FIG. 6  consisting of  FIGS. 6A  to  6 C assembled in the manner shown in  FIG. 6D  is a diagram of circuitry the within transducer unit shown in  FIG. 2 .  
         [0020]      FIG. 7  is a set of waveforms illustrating the pulsing of a piezoelectric crystal in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     Referring to  FIG. 1 , there is shown an ultrasonic fuel level monitoring system for sensing the level of liquid in a propane fuel tank  500 . The system includes an ultrasonic transducer unit generally designated  10  externally mounted on the bottom of the tank and a control unit generally designated  100  externally mounted on the top of the tank. Control unit  100  is operably connected to transducer unit  10  by electrical cable  30 . As described below in more detail, the transducer unit transmits ultrasonic signals through the wall of tank  500  and listens for return echoes from the surface of propane liquid (not depicted) contained in the tank. The control unit directs the operation of the transducer unit and collects time-of-flight data and temperature data from the transducer unit.  
         [0022]     As shown in  FIG. 2 , transducer unit  10  comprises a transducer driver  11 , a piezoelectric crystal transducer or piezo  16 , and a temperature sensor  22 . Temperature sensor  22  is used to obtain a measurement of the temperature of tank  500  which is desirable in order to adjust readings taken for accuracy (e.g. within plus or minus 2%). Control unit  100  comprises a very low power microcontroller  110  connected to a satellite radio  120 , a battery power supply  150  and an ambient temperature sensor  160 . To minimize size and cost, radio  120  is preferably a simplex radio capable of transmitting but not receiving.  
         [0023]     As shown in  FIGS. 4-5 , transducer unit  10  additionally comprises a housing  12 , a printed circuit board (PCB)  14  for holding transducer circuitry which is normally mounted to the bottom of the board, a foam pad cushion set on PCB  14  and  18  atop which piezo  16  normally rests while connected to the board, and an aluminum acoustic coupling disk  20 . Temperature sensor  22  is mounted atop PCB  14 . Disk  20  is bonded (viz. with epoxy) directly atop piezo  16 . All components are secured within housing  12  by potting compound  24  with the upper surface of disk  20  left exposed for contact with the bottom outer wall of tank  500  when unit  10  is bonded to the tank.  
         [0024]     Referring to  FIG. 6 , transducer unit  10  includes a very low power microcontroller  40  operatively coupled to temperature sensor  22  and to an analog switching unit  48 , the latter of which comprises three analog switches  50  which receive gate and burst signals on lines  52 ,  54  from the microcontroller and pulse piezo  16  in the manner shown and described in  FIG. 7 . Echo return signals received by piezo  16  are output from switching unit  48  on line  56 . Such return signals are amplified by amplifier  60 , filtered by noise filter  64  and level detected with comparator  70 . The output of comparator  70  on line  72  is provided as an input back to microcontroller  40 .  
         [0025]      FIG. 3  illustrates a working environment wherein data collected by control unit  100  from transducer unit  10 , including time-of-flight data and temperature data, is transmitted by radio from control unit  100  via a satellite data link  600  to a computer  620 . Computer  620  includes a computer readable storage medium which receives the data input and, together with previously stored information on tank geometry and dimensions and a suitable computer program product, processes the received data to calculate a measure of the volume of fuel in tank  500 . The calculated measure is also stored in the storage medium and may be read-out and displayed by computer  620 . Via a further data link (e.g. the internet  640 ), the calculated measure may also be read-out and displayed from computer  620  by a second computer  660 .  
         [0026]     It will be appreciated that the foregoing working environment is one which can be expanded to a system where a centralized server computer (e.g. computer  620 ) receives data transmissions via satellite from numerous control units  100  each monitoring a different propane tank (e.g. tank  500 ). The propane tanks may be located at widely dispersed geographic locations. Some may be serviced by the same propane distributor and others may be serviced by different propane distributors. Each distributor could utilize its own computer  660  to receive data from computer  620  concerning the customers it services.  
         [0027]     In more detail, transducer unit  10  is silent and unpowered during all phases of operation except when a tank reading is being requested. Control unit  100  is timed to wake up at intervals (e.g. every 16 seconds) to determine the time of day, and to compare that time with a sensing and transmission schedule programmed in microcontroller  110 . If the system is not scheduled to either “ping” tank  500  or to transmit measurement information via satellite radio  120 , then unit  100  goes back to sleep.  
         [0028]     If the system determines that a sensed reading is required, then transducer unit  10  is powered on by the control unit activating a 3.3 volt power supply that feeds power to the transducer unit. The transducer unit then uses a very small, very low current drain, circuitry to generate 25 volts for use in the drive circuit for piezo  16  (viz. analog switching circuit  48 ).  
         [0029]     When transducer unit  10  wakes up, it is programmed to listen for a set of commands sent to it by control unit  100  over a synchronous serial data link including cable  30 . Control unit  100  will tell transducer unit  10  a lower frequency of piezo oscillation, an upper frequency of piezo oscillation, the number of pings to perform and the frequency difference between each sequence of pings.  
         [0030]     A ping is generated by microcontroller  40  flipping two signals from 0 to 3.3 volts and back to 0 volts at a frequency rate that is determined by command from control unit  100 . A pulse train of 36 pulses set for each ping has been found to be suitable, but it of course will be understood that a differing number of pulses also may be found suitable. Each of the 3.3 volt pulses is used to command the switching of the 25 volt power source into piezo  16  via analog switches  50 . As indicated in  FIG. 7 , the switching is done in such a way that each of the two sides of piezo  16  are pulsed alternately with a 25 volt spike. Advantageously, this has the equivalent effect of powering the piezo with either a 50 volt single pulse at the resonant frequency, or with alternating plus and minus 25 volt pulses.  
         [0031]     This technique significantly increases the net power delivered by transducer unit  10  to tank  500  without having either a high voltage source in the transducer unit or requiring a transformer to boost the voltage. There is also a significant savings in parts by using a square wave directly produced by microcontroller  40  instead of generating a sine wave to drive piezo  16 .