Abstract:
The present application discloses a method for measuring pressurized liquid fuel level in a fuel tank used on a vehicle comprising measuring the pressure of the fuel to generate a pressure signal and measuring the temperature of the fuel to generate a temperature signal. A fuel level signal is determined from the pressure signal and the temperature signal and changes in the fuel level signal are determined over time. A first point of the fuel level signal indicating approaching emptying of the tank is identified when changes in the fuel level signal exceed a given value and the approaching emptying of the tank is signaled to an operator of the vehicle in response to identifying the first point. Also disclosed is a fuel level meter system comprising a display comprising a series of at least three light sources arranged in a generally vertical column with a bottommost one of the at least three light sources being of a first color and light sources above the bottommost one of the at least three light sources being of a second color.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application No. 61/161,311, filed on Mar. 18, 2009 and entitled FUEL LEVEL METER FOR INDUSTRIAL VEHICLES, the contents of which are incorporated herein by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to industrial vehicles including forklift trucks and the like that are operated from liquid fuel contained within fuel tanks on the vehicles. More particularly, the present invention relates to measuring pressurized liquid fuel levels in fuel tanks used on such vehicles and indicating to operators of the vehicles when the tanks are approaching empty and approximately how much operating time remains. A common liquid fuel is liquid propane (LP) which is used to power many forklift trucks. Accordingly, the invention will be described with reference to forklift trucks which use LP as fuel. It is noted however, that the present invention is equally applicable to other industrial vehicles that use liquid fuel and liquid fuels other than LP including, for example, natural gas (NG). For ease of description, “propane,” “LP” or “fuel” will be used herein to refer to all currently available liquid fuels and liquid fuels that may become available in the future. 
         [0003]    Knowing when an energy supply is nearing exhaustion and the corresponding operating time remaining is important to operators since a vehicle may be operated well away from an area where fuel can be replenished. Typically, industrial vehicles which exhaust their fuel supplies are difficult if not impossible to move and, if not near or in a refueling area, it may be difficult and time-consuming to refuel them. Accordingly, the remaining fuel and run time is important information particularly to newer, less experienced operators. 
       SUMMARY OF THE INVENTION 
       [0004]    The present application discloses a method for measuring pressurized liquid fuel level in a fuel tank used on a vehicle comprising measuring the pressure of the fuel to generate a pressure signal and measuring the temperature of the fuel to generate a temperature signal. A processor determines a fuel level signal from the pressure signal and the temperature signal and monitors changes in the fuel level signal over time. A first point of the fuel level signal indicating approaching emptying of the tank is identified when changes in the fuel level signal exceed a given value and the approaching emptying of the tank is signaled to an operator of the vehicle in response to identifying the first point. 
         [0005]    The present application also discloses a fuel level meter system comprising a display comprising a series of at least three light sources arranged in a generally vertical column with a bottommost one of the at least three light sources being of a first color and light sources above the bottommost one of the at least three light sources being of a second color. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a perspective view of a lift truck powered by liquid fuel; 
           [0007]      FIG. 2  is a block diagram of a fuel system which can use the fuel level meter disclosed in the present application; 
           [0008]      FIG. 3  is a graph of a measured pressure signal of liquid fuel in a liquid fuel tank, a measured temperature of liquid fuel in a liquid fuel tank and a difference signal generated by taking the difference of the temperature and pressure signals; 
           [0009]      FIG. 4  is a graph of a measured pressure signal of liquid fuel in a liquid fuel tank, a measured temperature of liquid fuel in a liquid fuel tank, a difference signal generated by taking the difference of the temperature and pressure signals and a slope of the difference signal; 
           [0010]      FIG. 5  illustrates an improved fuel level display for the fuel level meter system disclosed in the present application; 
           [0011]      FIG. 6  summarizes various display levels showing a composite of a difference signal curve and fuel level displays for the fuel level meter system disclosed in the present application; 
           [0012]      FIG. 7  is flow chart of an embodiment for operation of the fuel level meter system disclosed in the present application; 
           [0013]      FIGS. 8A and 8B  together form a flow chart of an embodiment of a tank change test for operation of the fuel level meter system disclosed in the present application; 
           [0014]      FIG. 9  is a flow chart of an embodiment of a tank valve check for operation of the fuel level meter system disclosed in the present application; 
           [0015]      FIG. 10  graphically illustrates how the pressure, temperature and difference signal values change on replacing a first fuel tank with a second fuller fuel tank; and 
           [0016]      FIG. 11  graphically illustrates how the pressure, temperature and difference signal values change on replacing a first fuel tank with a second fuller full fuel tank and then on changing the second fuel tank back to the first fuel tank. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    The temperature and pressure of liquid fuel, commonly liquid propane (LP), inside a fuel tank  100  used on a vehicle, such as a lift truck  102  shown in  FIG. 1 , is monitored in the fuel level meter system of the present application. Since the fuel level meter system of the present application is equally applicable to other liquid fuels including natural gas (NG), for ease of description, propane, LP or fuel will be used herein to refer to any and all appropriate liquid fuels. Fuel temperature and pressure monitoring can be performed by using a sensor. Conveniently, the sensor  104  (see  FIG. 2 ) is a dual sensing (i.e., pressure and temperature) sensor, such as the GEMS 3202H200PG028000 sensor, that is commercially available from Gems Sensors &amp; Controls, Plainville, Conn.; however, any suitable temperature sensors, pressure sensors and combined temperature and pressure sensors are contemplated for use in the fuel level meter system of the present application. 
         [0018]    A fuel level signal, generated, for example, by a fuel level meter processor  103  by taking the difference between a measured pressure signal  108  and a measured temperature signal  106  representative of tank conditions, i.e., pressure-temperature (P−T) or temperature−pressure (T−P) is used for fuel level metering (see e.g.  FIG. 3 ). Alternately, the temperature signal and pressure signal can be divided by one another to generate the fuel level signal, i.e., P/T or T/P. The processor  103  can be a dedicated fuel level meter processor or another processor that is provided on the lift truck  102 . The present application is not to be limited by the fuel system block diagram as shown in  FIG. 2  since the position of components can be moved within the system, for example the fuel filter can be placed before the sensor  104 , or components can be moved and combined with one another, for example the fuel filter could be combined with the manifold block. 
         [0019]    Due to the properties of gases [Gas law: PV=nRT where P=absolute pressure of gas, V=volume of gas, n=number of moles of gas, R=universal gas constant ( 189  J/kg K for LP), and T=absolute temperature], if there is sufficient propane in the tank  100 , a normalized temperature signal  106  and a normalized pressure signal  108  are highly correlated to one another as shown in  FIG. 3 . In the illustrated embodiment, normalization is performed by using raw data from analog to digital (A/D) converters representative of an analog temperature signal and an analog pressure signal with the normalized values being used to generate the fuel level signal. The relationship between the temperature signal  106 , the pressure signal  108  and the fuel level signal  110  may be illustrated by graphing these signals versus time to generate curves representative of the signals so that signal and curve may be used interchangeably herein. 
         [0020]    In  FIG. 3 , the illustrated fuel level signal  110  is a difference signal, P−T, which embodiment will be described herein with the understanding that other relationships, such as T−P, P/T, T/P, can also be used in accordance with the teachings of the present application. The inventors of the present application have observed that when graphically illustrated, the resulting pressure curve  108  closely resembles the temperature curve  106  as long as the amount of propane in the tank is greater than about 9% to 12% of a full tank. While the pressure and temperature curves  108 ,  106  may appear jagged, the fuel level signal  110 , which may be referred to herein as a difference signal or difference curve, is much smoother than either the pressure curve  108  or the temperature curve  106  and is relatively flat and fluctuations which can occur in the pressure curve  108  due to ambient temperature variations are substantially eliminated from the difference curve as illustrated. During the majority of the time the tank is emptying, the fuel level signal  110  does not change substantially, see  110 A. It is only after the fuel in the tank has reached the final 9% to 12% of the fuel that the fuel level signal begins to change, see  110 B. At that time, a point  110 C in the difference curve indicating the emptying of the final portion of the fuel from the tank can be detected. 
         [0021]    During this stage of tank emptying, i.e., below the 9% to 12% full point, changes or reductions in the amount of gas remaining in the tank (changes in n, the number of moles of gas in the gas law equation noted above) have a noticeable effect on the pressure and temperature signals  108 ,  106 . The pressure and temperature signals  108 ,  106  become uncorrelated and the difference curve, the fuel level signal  110 , begins to fall sharply. Because at this time the pressure curve  108  falls and the temperature curve  106  rises, the fall of the difference curve, the fuel level signal  110 , is accentuated. 
         [0022]    An LP tank having a 30 pound (13.6 kg) capacity is the industry standard and the 9% to 12% full point for such a standard tank equals about 16 minutes of normal running time for the lift truck  102 . A variety of other tank sizes are available, however they are seldom used in the industry. If tank sizes other than the 30 pound (13.6 kg) capacity size are used, the fuel level meter system of the present application still works, but the final timing or remaining truck run time would change. That is, a smaller tank would result in less run time after the tank begins to run out of fuel while a larger tank would result in more run time after the tank begins to run out of fuel. Also, while normal run times for the final portion of fuel will be about the same for vehicles of a given vehicle model, the normal run times may vary for vehicles of different vehicle models so that normal run times may need to be determined for each vehicle model. 
         [0023]    Changes in the difference curve, the fuel level signal  110 , such as its slope and magnitude, are monitored to determine how much fuel remains in the tank, see the slope curve  112  in  FIG. 4  which illustrates the slope of the difference curve, the fuel level signal  110 , versus time. There is a first point in the difference curve, the fuel level signal  110 , where changes of a predefined amount over time occur as the difference curve starts to turn down. This first point defines a knee  114  in the difference curve, the fuel level signal  110 . In the illustrated embodiment, the knee  114  is defined as the point at which changes in the difference curve, the fuel level signal  110 , or the slope of the difference curve reach −11 A/D units in a 20 second time period. The knee  114  can be detected because it occurs when the pressure and temperature signals  108 ,  106  become uncorrelated and the difference curve, the fuel level signal  110 , begins to fall sharply. As noted above, these changes are due to the reduced amount of fuel in the tank. 
         [0024]    In the illustrated embodiment, the difference curve, the fuel level signal  110 , is sampled every 20 milliseconds with the samples being averaged every 2 seconds. The resulting value is compared to the value determined 20 seconds before to determine the change of the fuel level signal  110  or slope of the difference curve. It is noted that the knee can be defined earlier or later, i.e., at a slope of less than −11 A/D units/20 sec or at a slope or more than −11 A/D units/20 sec (e.g. between −9 and −11 A/D units/20 sec) or at a slope or more than −11 A/D units/20 sec (e.g. between −11 and −13 A/D units/20 sec), depending upon system requirements and preferences. 
         [0025]    A second point along the difference curve, the fuel level signal  110 , is a zero point  116 , i.e., the point at which the pressure curve  108  crosses the temperature curve  106  in A/D units in the illustrated embodiment. The inventors of the present application have observed that the zero point  116  normally is at about the midpoint of the portion of the downward sloping difference curve, the fuel level signal  110 , that extends between the knee  114  and the end point or tank empty point  118  of the difference curve. The zero point  116  also serves as a reference point for predicting how far down the difference curve, the fuel level signal  110 , will extend beyond the knee  114 , i.e., the difference curve generally extends as many A/D units beyond the zero point  116  to reach the tank empty point  118  as the difference curve extends from the knee  114  to reach the zero point  116 . 
         [0026]    Fuel level states exhibited by a fuel level meter or display are determined by using the knee  114 , the zero point  116  and the tank empty point  118  to calculate set points along the downward sloping portion of the difference curve, the fuel level signal  110 . The set points define trip locations along the difference curve at which the display is switched to a higher or lower fuel level indication. In currently used fuel level displays, two light sources, such as light emitting diodes (LEDs), are used to indicate fuel level. A green LED is illuminated until a pressure switch monitoring tank pressure falls below a given pressure, for example 60 pounds per square inch (PSI). Under normal ambient temperatures, a tank pressure of 60 PSI indicates that enough fuel remains in the tank for about 2 to 3 minutes of truck operation. When tank pressure goes below the given pressure, the green LED is extinguished and a yellow LED is illuminated. Unfortunately, the 2 to 3 minutes of remaining operating time can be reduced when the truck is operating in elevated temperatures and can be increased when the truck is operating in lowered temperatures. Thus, depending on ambient temperature, the truck runs out of fuel before it is expected or the truck is refueled before it is necessary to refuel it, possibly well before refueling is necessary, 15 minutes or more. 
         [0027]    The fuel level meter of the present application enables an operator to be alerted more accurately and over a longer period of remaining operating time to tank empty. While almost infinite alerting schemes using lights, analog meters, digital meters, LCD displays, and the like can be imagined, a novel fuel level display  200  is shown in  FIG. 2  as being connected to the fuel level meter processor  103  and illustrated in more detail in  FIG. 5 . The fuel level display  200  has four light sources, which are conveniently LEDs, although other light sources can be used. In the illustrated embodiment, the four light sources comprise three green LEDs  202 ,  204 ,  206  and one yellow LED  208 . As illustrated, the LEDs are arranged vertically in a column with the yellow LED being the bottommost one of the four LEDs. However, it will be appreciated that any arrangement of the lights may be used, such as a horizontal arrangement. Similarly, different colors of light may be used, provided an operator of the vehicle can understand what level of fuel is being signaled. In the embodiment depicted, prior to reaching the knee  114  of the difference curve, the fuel level signal  110 , all four LEDs  202 - 208  are illuminated to indicate that sufficient fuel is in the tank  100  to permit truck operation for an extended period of time. As illustrated, this LEVEL 4 display indicates that more than about 16 minutes of truck operating time remains. Upon reaching the knee  114  of the difference curve, the fuel level signal  110 , the top LED  202  (the top one of the three green LEDs  202 ,  204 ,  206 ) is extinguished and the remaining three LEDs  204 - 208 , two green LEDs and one yellow LED, remain illuminated. This LEVEL 3 display indicates that about 16 minutes of truck operating time remains. 
         [0028]    As noted above, a second point on the difference curve, the fuel level signal  110 , is defined when the difference curve reaches zero, i.e., the zero point  116 . While the second point could be used as the next change for the fuel level display  200 , to provide more increments in the display of run time remaining, four levels are used in the illustrated embodiment. To that end, a third point  120  is defined on the difference curve  110 . The third point  120  is positioned approximately midway between the knee  114 , i.e., the first point of the difference curve, and the midpoint or zero point  116  of the difference curve, i.e., the second point of the difference curve. When the difference curve, the fuel level signal  110 , reaches the third point  120 , the next to the top green LED  204  is also extinguished and the two bottom LEDs  206 ,  208 , the bottommost green LED and the yellow LED, remain illuminated. This LEVEL 2 display indicates that about 12 minutes of truck operating time remains. 
         [0029]    Upon reaching the second point, i.e., the midpoint or zero point  116 , of the difference curve, the bottommost green LED  206  is extinguished and the single bottom LED  208 , the yellow LED, remains illuminated. This LEVEL 1 display indicates that about 8 minutes of truck operating time remains. Upon reaching a fourth point  122  positioned between the midpoint or zero point  116  and the tank empty point  118  of the difference curve, the bottom yellow LED is flashed, an optional audible alarm is activated and “Out Of Fuel” may be scrolled on a combination numerical/text display  210 . This LEVEL 0 display indicates that about 4 minutes of truck operating time remains. The fourth point  122  is selected to be approximately midway between the second point  120  and the end point  118 . Advantageously, the fourth point  122  is selected to be approximately midway between the second point  120  and the end point  118  less a predetermined offset, in order to maintain even spacing in time of the set points along the difference curve, the fuel level signal  110 . In a working embodiment, an offset of 25 A/D units was used so that the fourth point  122  was spaced at the midpoint between the second point  120  and the end point  118  −25 A/D units. These levels and displays are summarized in  FIG. 6 . 
         [0030]    Operation of the illustrated embodiment of the fuel level meter of the present application will now be described with reference to a fuel level meter flow chart shown in  FIG. 7 . Upon key-on of the lift truck  102 , fuel level meter operation begins at  300 . At  302 , a determination of whether the fuel tank  100  was changed since the truck  102  was last operated is made so that an accurate representation of fuel level can be provided to the operator by operation of the fuel level display  200 , see  400  of  FIG. 8A . The last data stored just before key-off is retrieved from nonvolatile memory and if the fuel tank pressure is at least 35 PSI greater than the last stored fuel tank pressure, it is concluded that a full fuel tank has been installed on the truck  102 , the set points along the difference curve, the fuel level signal  110 , are determined for the full tank, the fuel level display  200  is set to LEVEL 4 and the tank change test is ended, see  402 ,  404 ,  406 ,  407 . 
         [0031]    If tank pressure is not 35 PSI greater than the last stored pressure, temperature and pressure data are taken and averaged for 2 seconds; an initial fuel level signal  110  (difference signal level value as illustrated) is calculated; and a fuel level signal  110  delta (difference value delta) is calculated by subtracting the ending fuel level signal  110  (ending difference value retrieved from nonvolatile memory) from the initial fuel level signal  110  (difference value),  408 ,  410 ,  412 . 
         [0032]    Pressure and temperature dynamics due to tank changes are illustrated in  FIG. 10  for a change to a full tank and  FIG. 11  illustrates pressure and temperature dynamics for a change from an original tank to a full tank and then back to the original tank. If the fuel level signal  110  delta (difference value delta) is greater than ±20 A/D counts of a 10 bit A/D converter count, an estimated fuel level signal recovery (difference value recovery) is determined and the fuel level signal  110  (difference value) is set equal to the initial fuel level signal  110  (initial difference signal) plus the estimated initial fuel level signal  110  recovery (difference value recovery), the resulting fuel level signal  110  (resulting difference value), and new set points are determined,  416 ,  418 ,  420 . The estimated fuel level signal recovery will be determined empirically and placed into a lookup table that can then be accessed based on fuel level signal  110  delta (difference value delta). The difference valve recovery accounts for an expected change in the difference curve as new fuel moves from the tank  100  to the sensor  104 . Transit times range from 80 to 100 seconds depending upon fuel demand and a tank change from empty to full could result in a difference curve increase of up to 70 over that transit period with a 30 pound tank. 
         [0033]    If the resulting fuel level signal  110  (resulting difference value) is less than the fourth point  122  (level zero set point), the fuel level display  200  is set to LEVEL 0,  422 ,  424 . If not, but the resulting fuel level signal  110  (resulting difference value) is less than the second point  116  (level one set point), the fuel level display  200  is set to LEVEL 1,  426 ,  428 . If not, but the resulting fuel level signal  110  (resulting difference value) is less than the third point  120  (level two set point), the fuel level display  200  is set to LEVEL 2,  430 ,  432 . If not, but the resulting fuel level signal  110  (resulting difference value) is less than the first point  114  (level three set point), the fuel level display  200  is set to LEVEL 3. If not, the fuel level display is set to LEVEL 4,  406 . 
         [0034]    If the fuel level signal  110  delta (difference value delta) is not within ±20 A/D counts of a 10 bit A/D converter count, then it is concluded that no tank change was made, the previous set points are used and the tank change test is ended,  438 ,  440 ,  407 . 
         [0035]    After the tank change test sequence is finished, see  407 , fuel level meter operation continues by averaging the pressure and temperature signals for 2 seconds and, in the illustrated embodiment, determining the fuel level signal  110  (difference value) by subtracting the temperature signal in A/D units from the pressure signal in A/D units,  304 ,  306 . After a tank change, it is possible that the tank valve will not be opened. To determine whether that has happened, a check is made for a tank valve closed condition at  308 . 
         [0036]    An illustrated embodiment of a tank valve check routine is begun at  500  in  FIG. 9 . A tank valve closed flag, which is false upon entering the tank valve closed routine, is checked to see if the flag has been set to true at  502 . If the tank valve closed flag is false and a timer indicating elapsed time within the tank valve check routine has not expired, a check is made to see if the pressure in the tank has dropped by greater than 23 PSI over the starting pressure at  506 . If so, the tank valve closed flag is set to true and the operator is advised that the tank valve is closed, for example by scrolling “Tank Valve Closed” on the combination numerical/text display  210  of the fuel level display  200 ,  508 ,  510 . An optional audible alarm can also be used. The tank valve check routine is then ended at  512 . If the tank valve closed flag is false, the timer indicating elapsed time within the tank valve check routine has not expired and the pressure in the tank has not dropped by greater than 23 PSI over the starting pressure at  506 , the tank valve check routine is ended at  512 . 
         [0037]    If the tank valve closed flag is true indicating that the tank valve is closed, the fuel pressure in the tank is checked to see if it has recovered by more than 12 PSI over the starting pressure,  502 ,  514 . If the fuel pressure has not recovered by more than 12 PSI from the starting pressure, the tank valve check routine is ended at  512 . If the fuel pressure has recovered by more than 12 PSI from the starting pressure, the fuel level display  200  is set to LEVEL 2, the optional audible alarm is disabled, the “Tank Valve Closed” message is stopped, the tank valve closed flag is set to false and the tank valve check routine is ended,  516 ,  518 ,  520 ,  522 ,  512 . 
         [0038]    When the tank valve check routine has ended  512  with an indication that the valve is closed, the fuel level display  100  is activated to display a LEVEL 5 alert where all four LEDs  202 ,  204 ,  206 ,  208  are flashed, an audible alert, if provided, is sounded and “Tank Valve Closed” is scrolled on the combination numerical/text display  210 ,  310 , and fuel level meter operation returns to  304 . 
         [0039]    If the tank valve closed flag is false, indicating that the tank valve is open, the fuel level signal  110  (difference value) is used to determine how to control the fuel level display  200 , for example as illustrated at  312  in  FIG. 7 . The display control illustrated at  312  is comparable to the display decisions made in  FIG. 8  as described above. 
         [0040]    Having thus described the invention of the present application in detail and by reference to embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims.