Abstract:
A method for measuring a fluid level in a tank containing a fluid for a transportable temperature controlled space. The method includes providing a temperature control system for the transportable temperature controlled space, providing a fluid level sensor for sensing a fluid level in the tank, generating fluid level signals with the fluid level sensor indicative of the fluid level in the tank, providing a fluid level algorithm for receiving the fluid level signals from the fluid level sensor and computing the fluid level in the tank, and inhibiting nondeterministic fluid level signals from being introduced to the fluid level algorithm.

Description:
RELATED APPLICATIONS 
     This patent application is a continuation of U.S. patent application Ser. No. 13/078,620, filed Apr. 1, 2011, which claims priority to U.S. Provisional Patent Application No. 61/320,033 filed on Apr. 1, 2010, the entire contents of which are both incorporated herein by reference. 
    
    
     BACKGROUND 
     The present invention relates to an apparatus and method for making ultrasonic fluid level measurements in a transport refrigeration application. Particularly, the invention relates to the position of a fluid level sensor for detecting a fuel level in a fuel tank associated with a transport temperature control system. 
     In a transport temperature control system application, a temperature controlled space is transported over a road, rail, sea, air or the like. As a result, fuel in a fuel tank for the temperature control system is subjected to vibrations from turbulence resulting from movement of the temperature controlled space. Furthermore, periodic stopping and starting of the temperature control system while the temperature controlled space is in transit causes periodic electrical noise associated with cranking of an engine that drives a compressor. Thus, the fluid level sensor is simultaneously subjected to vibration noise and electrical noise, which causes errors in the fluid level reading. 
     SUMMARY 
     In one aspect, the invention provides a method for measuring a fluid level in a tank containing a fluid for a transportable temperature controlled space. The method includes providing a temperature control system for the transportable temperature controlled space, providing a fluid level sensor for sensing a fluid level in the tank, generating fluid level signals with the fluid level sensor indicative of the fluid level in the tank, providing a fluid level algorithm for receiving the fluid level signals from the fluid level sensor and computing the fluid level in the tank, and inhibiting nondeterministic fluid level signals from being introduced to the fluid level algorithm. 
     In another aspect, the invention provides a system for measuring a fluid level in a tank for a transportable temperature controlled space. The system includes a transport temperature control system for conditioning a load space of the transportable temperature controlled space, and a fluid level measurement system for measuring the fluid level. The fluid level measurement system includes a tank containing a fluid, a fluid level sensor for generating fluid level signals indicative of the fluid level in the tank, and a fluid level algorithm for receiving the fluid level signals from the fluid level sensor and computing the fluid level in the tank. The fluid level measurement system is configured to inhibit nondeterministic fluid level signals from being introduced to the fluid level algorithm. 
     In yet another aspect, the invention provides a method for measuring a fluid level in a tank containing a fluid. The method includes providing a fluid level sensor for measuring a fluid level in the tank, providing a temperature control system for a transportable temperature controlled space, providing a power source, powering the fluid level sensor and the temperature control system with the same power source, generating fluid level signals with the fluid level sensor indicative of the fluid level in the tank, and inhibiting nondeterministic fluid level signals. 
     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a fluid level measurement system for a tank in accordance with the present invention. 
         FIG. 2  is a plot of spacer height and near field distance vs. ring time for the fluid level measurement system of  FIG. 1 . 
         FIG. 3  is a schematic diagram of a container power system for powering the fluid level measurement system. 
         FIG. 4  is a schematic diagram of a trailer power system for powering the fluid level measurement system. 
         FIG. 5  is an image of a tractor and trailer having the tank of  FIG. 1 . 
         FIG. 6  is a schematic diagram of a temperature control system for the trailer of  FIG. 5 . 
         FIG. 7  is a schematic illustration of another construction of a fluid level measurement system for a tank in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Before any constructions of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other constructions and of being practiced or of being carried out in various ways. 
       FIG. 1  illustrates a fluid level measurement system  10  for use with a transport temperature control system  14  ( FIGS. 3-6 ). The fluid level measurement system  10  includes a fluid tank  18  for containing a fluid  20  and a fluid vapor  21 , such as a fuel tank containing a fuel and fuel vapor, and a fluid level sensor  22  having a face  23 . In the illustrated construction, the tank  18  is a cylindrical diesel fuel tank coupled horizontally to a vehicle ( FIG. 5 ) such that a longitudinal axis thereof lies substantially parallel to the ground. A spacer  26  is coupled with the tank  18  adjacent an opening in the tank  18  and extends from the top of the tank  18 . The spacer  26  is a tubular structure having a wall extending axially from an outer surface of the tank  18  and having first and second open free ends  26   a ,  26   b , respectively. The sensor  22  is coupled with the spacer  26  and positioned coaxially with respect to the spacer  26 . The face  23  of the sensor  22  is positioned adjacent the first free end  26   a  and the second free end  26   b  abuts the tank  18  and is in communication with an interior of the tank  18 . 
     In the illustrated construction, the fluid level sensor  22  is an ultrasonic fluid level sensor including a transducer that emits a sound, by way of an initial vibration, in the direction of a surface  30  of the fluid  20 , preferably in a direction perpendicular to the surface  30  of the fluid  20 . The sound is emitted from the sensor  22  at the face  23  of the sensor  22 . The sensor  22  includes a receiver that senses an echo of the initial sound reflected off the surface  30  and records a time between emitting the sound and receiving the echo. Based on the speed of sound through the medium through which the sound travels, e.g., air and fuel vapor  21 , and the recorded time, a separation distance A between the sensor  22  and the surface  30  of the fluid  20  is calculated. The system  10  includes a fluid level algorithm that is calibrated such that the distance A is associated with a fluid level of the tank. The fluid level is displayed on a display  24 . 
     The spacer  26  is sandwiched between the tank  18  and the sensor  22  to provide a minimum separation distance B between the sensor  22  and the surface  30  of the fluid  20  at a maximum fluid level  28 . The maximum fluid level  28  is a predetermined level that the tank  18  is designed to hold and need not be the absolute physical maximum of the tank  18 . For example, a fuel-dispensing nozzle typically shuts off automatically when the fuel reaches a fluid level that is less than the physical maximum of the tank  18 . In this case, the maximum fluid level  28  is the level at which the fuel-dispensing nozzle shuts off, and the fluid level algorithm is calibrated to associate a reading of “FULL” with the maximum fluid level  28 . The tank  18  includes a fill neck (not shown) for receiving the fuel-dispensing nozzle. The position of the fill neck (e.g., height) on the cylindrical tank  18  can also define the maximum fluid level  28 . The minimum separation distance B occurs when the fuel tank  18  is full and is the minimum distance necessary to inhibit nondeterministic signals, which lead to erroneous fluid level readings by the sensor  22 , as will be described in greater detail below. While the minimum separation distance B is constant and unique to a particular configuration of the fluid level measurement system  10 , the separation distance A is variable depending upon the level of the fluid  20 . 
     The minimum separation distance B is equal to, or in some constructions may be greater than, half the near field distance. The near field distance is calculated using the equation D nf =V sd *T t , where D nf  is the near field distance, V sd  is the speed of sound through the medium through which the sound travels, e.g., the fluid vapor  21  and air, and T t  is the ring period of the transducer. As described above, the transducer emits the sound by way of an initial vibration; however, the transducer continues to vibrate at a decreasing magnitude after the initial vibration. The ring period is the time for the vibrations of the transducer to settle, or decrease, below a threshold of the sensor&#39;s receiver, i.e., to reach a magnitude of vibration that the receiver of the sensor  22  can no longer detect. In other words, the near field distance is equal to the distance that the sound travels through the medium during the ring period. 
     The fluid level measurement system  10  is configured based on the near field distance such that the sensor  22  is positioned at a distance from the surface  30  of the fluid  20  that is equal to or greater than half the near field distance when the tank is full  22 , as indicated by the following equation: 
             B   ≥           V   sd     *     T   t       2     .           
That is, the minimum separation distance B is equal to or greater than half the near field distance.
 
     The spacer  26  is positioned vertically with respect to gravity and is sized to provide a spacer height C such that the sensor  22  and the surface  30  of the fluid  20  are separated by at least half the near field distance when the tank  18  is full.  FIG. 2  is a plot of the minimum required spacer height C and near field distance vs. ring time, or ring period. In the illustrated construction, the spacer height C is approximately equal to the minimum separation distance B. The spacer height C depends on the construction of the tank  18 . In the illustrated construction, the sensor  22  is employed with a diesel fuel tank  18  in which the spacer  26  extends between the maximum fluid level  28  and the face of the sensor  22 , as can be seen in  FIG. 1 . In other constructions, the sensor  22  may be employed with other types of tanks suited for holding fluid in mobile applications. In some constructions, other relationships between spacer height C and minimum separation distance B are possible depending on the geometry of the tank  18 . In other constructions, a spacer is not necessary to provide a sufficient minimum separation distance B between the sensor  22  and the maximum fluid level  28 . 
     In the illustrated construction of  FIG. 1 , the sensor  22  has a ring period of 500 microseconds which corresponds to a near field distance of approximately 6.5 inches. Therefore, the minimum separation distance B is approximately 3.25 inches. Inherently, the separation distance A is greater than half the near field distance when the tank  18  is not full. 
     The fluid level measurement system  10  is preferably employed with a transport temperature control system fuel tank  18 , such as for a truck, a trailer, a shipping container, a rail container, a van or another transport vehicle that stores and/or carries goods that must be maintained in a temperature controlled environment. However, in other constructions, other types of tanks for other applications may be used. 
     In one construction, illustrated in  FIGS. 3 and 5 , the fuel tank  18  and fluid level measurement system  10  are coupled to a transport vehicle  32  including a tractor  34  and trailer  38 . As shown in  FIG. 5 , the trailer  38  includes a frame  42  and an outer wall  46  supported on the frame  42  for substantially enclosing a temperature controlled load space  50 . Doors  54  are supported on the frame  42  for providing access to the load space  50 . In some constructions, the load space  50  can include a partition or an internal wall for at least partially dividing the load space  50  into sub-compartments, including two or more load space zones, each of which can be maintained at a different temperature or a different humidity. A plurality of wheels  58  are provided on the frame  42  to permit movement of the vehicle  32  across the ground. In some constructions, wheels and/or rails for a railroad or a boat vessel can be used for transporting temperature controlled containers. 
       FIG. 6  illustrates one construction of a temperature control system  14  that conditions the load space  50  of the trailer  38 . The temperature control system  14  includes a refrigeration circuit  62  having a compressor  66 , a condenser  70 , a receiver  74 , an evaporator  78  and an accumulator  82  connected in series, as is well understood in the art. The refrigeration circuit  62  may also include other components well known in the art, such as a three-way switching valve  86  for switching between a heating mode and a cooling mode. The temperature control system is fully described in U.S. Pat. No. 6,367,269 titled “ELECTRONIC THROTTLING VALVE DIAGNOSIS AND PREVENTATIVE SHUTDOWN CONTROL,” assigned to the same assignee as the present invention, the content of which is hereby fully incorporated herein by reference. 
     The compressor  66  is operatively coupled to an engine  92 . As shown schematically in the construction of  FIG. 3 , the engine  92 , such as a diesel engine, is coupled to the compressor  66  by a transmission  96  for driving the compressor  66 . The fuel tank  18  supplies fuel to the engine  92  by way of a fuel line  102 .  FIG. 3  further illustrates a power source, such as a battery  98 , for powering the fluid level sensor  22 , for powering a controller  104  for the temperature control system, and for powering an engine starter  94  for starting the engine  92  when cooling or heating is needed. Other components of the temperature control system  14  may also be powered by the battery  98 . 
     In other constructions, the engine  92  may include the vehicle engine or a gasoline engine. Other arrangements are possible and may be implemented, as desired. The fuel tank  18  may supply fuel to one or more of the engines employed. 
       FIG. 4  illustrates another construction in which the fluid level sensor  22  is employed with a temperature controlled transport container  90 . For shipments of perishable goods, the temperature control system  14  may be employed to heat and/or cool the container  90 . The transport container  90  may be transported by a variety of modes, such as by railcar, barge and truck. While the transport container  90  is stationary, such as while stored in a warehouse, on a dock, or near an airport, an external source of power such as utility electricity may be connected for powering the temperature control system  14 . If the container  90  is not provided with an external power source, a generator set  91  may be provided to power the temperature control unit  14 . For example, when the container is in transit by railcar, barge, or truck, the generator set  91  may be necessary. The generator set  91  includes the engine  92  to drive an alternator  93  which in turn provides electric power to the temperature control system  14 , specifically the compressor  66 . Thus, the engine  92  is operatively connected to the compressor  66 . The alternator  93  also provides electric power to the fluid level sensor  22 . 
       FIG. 7  illustrates another construction of a fluid level measurement system  110  having an internal spacer  126 . Elements of this construction that are similar to the construction of  FIG. 1  are given similar reference numerals. The internal spacer  126  is positioned vertically with respect to gravity. The internal spacer  126  provides the minimum separation distance B between the face  23  of the sensor  22  and a maximum fluid level  128  of the fluid within the internal spacer  126 , which is lower than the maximum fluid level  28  of the tank  16 , by trapping an air/vapor bubble  121  inside the spacer  126  to push the maximum fluid level  128  inside the spacer  126  away from the face of the sensor  23 , much like submerging a cup upside down in water. The internal spacer  126  is a tubular structure having a wall extending axially and having first and second open free ends  126   a ,  126   b , respectively. The face  23  of the sensor  22  is positioned adjacent the first free end  126   a  and the second free end  126   b  is in communication with the interior of the tank  18 . 
     The height C of the internal spacer  126  is at least equal to the minimum required spacer height shown in  FIG. 2 , as discussed above, and is preferably greater than the minimum required spacer height to account for possible tilting of the tank  18  or sloshing of the fluid within the internal spacer  126 . If the tank  18  and internal spacer  126  are tilted or if sloshing occurs, the air/vapor bubble  121  may escape from the internal spacer  126 , raising the level  128  closer to the level  28 . Therefore, the actual height C of the internal spacer  126  is preferably greater than the minimum required spacer height C. For example, if the ring time of the transducer is 350 microseconds, the minimum spacer height C is approximately 2.4 inches, as plotted in  FIG. 2 . The actual height of the internal spacer may be chosen to be 3.5 or 4 inches to reduce the chance of air/vapor escaping. Thus, similar to the construction described in  FIG. 1 , the minimum separation distance B is equal to or greater than half the near field distance. In some constructions, the internal spacer  126  may include a flange for inhibiting the air/vapor bubble  121  from escaping. 
     In operation, the fluid level sensor  22  is powered by the same power source that provides power to the temperature control system  14 . In the construction of  FIG. 4 , a generator set  91  provides power to the fluid level sensor  22  and the temperature control system  14 . In the construction of  FIG. 3 , a battery provides power to the fluid level sensor  22  and the engine starter  94 , which starts the engine  92  to drive the compressor  66  of the temperature control system  14 . In other constructions, other types of suitable power sources may be employed. 
     The output power, oscillator frequency, and analog circuitry of the fluid level sensor  22  depend on a constant input voltage. During startup of the compressor  66  of the temperature control system  14 , which occurs as needed while the trailer  38  or container  90  is in transit, the power source  98 ,  91  is subjected to a heavy load, e.g., cranking, causing the voltage supply to the fluid level sensor  22  to droop and be unstable (e.g., power supply noise). 
     While the trailer  28  or container  90  is in transit, vibrations from movement over the road or rail, or other turbulence, causes vibrations in the surface  30  of the fluid  20 . The combination of power supply noise and vibration noise in the fluid surface  30 , simultaneously, may result in a nondeterministic signal. A nondeterministic signal introduced to the fluid level algorithm results in multiple possible fluid levels being computed, causing glitches in the fluid level measurement, such as rapid changes in the fluid level reading in a short period of time, e.g., more than 4% in less than 10 seconds, when the algorithm selects the wrong fluid level out of the possible fluid levels. 
     When the sensor  22  is spaced from the surface  30  of the fluid  20  by at least half the near field distance, the nondeterministic signal is inhibited and glitches are avoided. Thus, the fluid level measurement system  10 ,  110  is configured such that the sensor  22  is spaced from the maximum fluid level  28 ,  128  of the tank  18  by at least half the near field distance such that the sensor  22  is spaced from the fluid surface  30  by a distance greater than half the near field distance when the tank  18  is not full. The spacer  26 ,  126  is sized accordingly to provide the necessary minimum separation distance B. 
     Thus, the invention provides, among other things, a fluid level sensor spaced from a maximum fluid level of the tank by a distance greater than or equal to half the near field distance of the sensor.