Abstract:
An apparatus measures a fluid level in a container. A light source emits a light beam. A light sensor can sense the light beam. An optical conduit is arranged in a container for holding fluid. The optical conduit is arranged between the light source and the light sensor along a path of the light beam, such that at least one part of the light beam passes through the optical conduit, and at least an other part of the light beam passes through the fluid when the container holds the fluid. The sensor senses the light beam when a level of the fluid coincides with the one part of the light beam passing through the optical conduit, and the sensor does not sense the light beam when the level of the fluid coincides with the other part of the light beam passing through the fluid due to internal reflection at the fluid level.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to an apparatus for measuring fluid levels in a container, and more particularly, to measuring fluid levels optically. 
     BACKGROUND OF THE INVENTION 
     There are numerous applications where it is necessary to measure an amount of fuel in a container. 
     U.S. patent application Ser. No. 10/955,485, “Method and system for encoding fluid level” filed by Holcomb et al. on Sep. 30, 2004 and issued as U.S. Pat. No. 6,992,757, describe a float riding on the surface of a fluid. The float is mechanically coupled to a rotating encoder disk which is segmented with optically transparent and opaque regions. A set of light emitting diodes (LEDs) are aligned with photo sensors on the other side of the disk So that the fluid level can be encoded as the disk rotates as the float moves up and down. 
     U.S. patent application Ser. No. 10/800,484, “Optical fluid level monitor” filed by David Corven et al. on Mar. 15, 2004, describes an optical sensor that includes a display, a light pipe optically connected to the display and extending to a level of interest in the reservoir, where the light pipe is formed from a material having a refractive index higher than air&#39;s refractive index and less than or equal to the liquid&#39;s refractive index; and a light optically connected to the light pipe. The light pipe can be a glass or plastic rod, or a bundle of optical fibers. 
     U.S. patent application Ser. No. 10/267,965, “Fluid container with level indicator, and fluid level indicator assembly for a fluid container,” filed by Lee et al. on Oct. 9, 2002, describes fluid level sensor that includes a visual display of a fluid level in a container using multiple capillary tubes terminating at different vertical levels from one another in the container. 
     U.S. patent application Ser. No. 10/265,954, “LCC-based fluid-level detection sensor” filed by Shi et al. on Oct. 7, 2002 and issued as U.S. Pat. No. 6,949,758, describes a fluid level sensor based on light communication channel (LCC) technology. One end of the LCC is connected to a signal source while another end is connected to a sensor. The LCC is dipped in a fluid container and a signal propagates and undergoes internal reflection through the LCC towards one of its ends which is connected to the sensor. The fluid level is detected by measuring an intensity of the signal reflected with the LCC that reaches a sensor. 
     U.S. Pat. No. 5,852,946, “Method and apparatus for detecting fluid level” issued Cowger on Dec. 29, 1998, describes a fluid level detector for providing a signal indicative of fluid level in a fluid container. The fluid level detector includes a first light conduit portion for providing light to fluid within the fluid container. A second light conduit portion is provided for receiving light provided by the first light conduit portion. Also included is a light path extending from the first light conduit portion to the second conduit portion. The light path has a light path length, which varies with an amount of fluid within the fluid container. The light path length variation produces light intensity variation at the second conduit portion which is indicative of fluid level in the fluid container. 
     U.S. Pat. No. 5,747,824, “Apparatus and method for sensing fluid level” issued to Jung et al. on May 5, 1998, describes an array of infrared LEDs and an array of photo sensors are positioned vertically in a cassette. A vertical line on which the LEDs are arranged is substantially parallel to a direction in which the fluid level is within the cassette. The LEDs are aimed upwardly at an angle of approximately 20 degrees from horizontal so the a beam of light does not penetrate the fluid/air interface. 
     The Jung system can be distinguished according to a number of characteristics. First, for each level to be measured that system requires a light source and sensor pair for each fluid level to be detected. Second, the system cannot detect how far below or above the fluid level is for a single source/sensor pair. Third, for accurate readings of multiple levels a baffle is required to block energy at various angles. For fluids that can scatter light, adjacent sensors need to be properly oriented. 
     SUMMARY OF THE INVENTION 
     The embodiments of the invention provide a fluid level sensor. Optical structures block transmission of light beam only when the fluid is within a certain level range. The structures can be serially stacked to construct encoder channels, which respond to fluid levels in multiple ranges. Multiple stacks can be combined to construct incremental, absolute, or any of a variety of standard encoder topologies. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1-6  are block diagrams of light pipes for measuring fluid levels according to an embodiment of the invention; 
         FIG. 7  is a block diagrams of a light pipe for measuring fluid levels according to an embodiment of the invention with a seven segment display; 
         FIG. 8  is a perspective diagram of the light pipes of  FIG. 6 ; and 
         FIG. 9  is a side view of a light pipe for an irregularly shaped container. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The embodiments of our invention provide an optical fluid level encoder for measuring a level of fluid in a container. 
       FIG. 1  shows a structure of a basic “building block” component of our encoder. A container  100  is partially filled with a fluid  101  and air  102 . A fluid level is  105 . 
     A light pipe  150  is arranged at an angle in the container. The light pipe  150  includes a light source  110 , e.g., a LED, a light sensor  120 , e.g., a photo detector. It should be noted that the positions of the source and sensor can be reversed. 
     The light pipe also includes one or more optical conduits  130 . In this embodiment, the two optical conduits are separated by a gap  135 . A length of the optical conduits and gap(s) can be precisely controlled. In the preferred embodiment, the optical conduits are constructed of cylindrical transparent acrylic rods of different lengths. The diameter of the rods is about 5 mm. 
     In the preferred embodiment, the diameter of the rod is made the same as the diameter of the LED  110  and the phototransistor  120  to facilitate assembly of the encoder, see  FIG. 9 . 
     As an advantage, any light beam entering the conduits at one end exits the conduits at the opposite end due to total internal reflection. Total internal reflection occurs when light beam is refracted at the medium boundary of the conduit to effectively reflect all of the light back into the conduit. Therefore, the conduits can be curved, see  FIG. 9 . 
     Optional means  160  for indicating or measuring a light intensity is connected to the light sensor. The encoded output value can be “0” (off) or “1” (on), or some continuous value as described below. The artisan skilled in the art will recognize that the means  160  can be any measurement component, e.g., electrical, optical, and mechanical. It should also be noted that the sensor  120 , can be passive, a translucent rod that is visible. In this case, the light beam will be visible in the sensor as long as the light beam penetrates the sensor. 
     The optical conduits is arranged between the light source and the light sensor along a path of the light beam, such that at least one part of the light beam passes through the optical conduit, and at least an other part of the light beam passes through the fluid when the container holds the fluid. It should be noted that the light beam can be any optical signal including visible light, infrared, ultraviolet, or in the form of a laser beam. 
     As shown in  FIG. 1 , the level of the fluid  101  is below the gap  135 . Therefore, light beam  111  emitted by the source  110  is sensed, and it can be deduced that the fluid level range  105  is either below the gap  135  or above the gap  135 , i.e., the container is almost empty or almost full. 
     As shown in  FIG. 2 , the level of the fluid  101  is above the gap  135 . Therefore, the light beam  111  emitted by the source  110  will be sensed, and it can be deduced that the fluid level range  105  is above the gap  135  or below the gap  135 ; again, the container is almost empty or almost full. 
     As shown in  FIG. 3 , the level of the fluid is in the gap. Therefore, the light is reflected at the fluid/air interface and no light is sensed, and it is possible that the fluid level range  105  is in the gap  135 . For an air/water interface, the critical angle for internal reflection is 48.75° or greater. 
     By precisely cutting the lengths of the optical conduits, it is possible to construct a fluid level encoder that can maintain the fluid level over a small range of values, e.g., only the values where the fluid level is in the gap. 
     During operation, as the fluid level rises, the level indicator can be incremented each time a gap is reached, and as the fluid level falls, the level indicator can be decremented. Thus, the configuration shown in  FIG. 3  can indicate three different ranges of levels of fluid. 
       FIGS. 4A ,  4 B, and  4 C show alternative arrangements with a single optical conduit. If the fluid level is in the range of the optical conduit, the output of the encoder is logical “1” or “on”, and logical “0” or “off otherwise. 
     The fluid level encoder will always be on when the container is almost empty in  FIG. 4A , half full for Figure B, and almost full for  FIG. 4C . 
       FIG. 5  shows an arrangement where the light pipe has multiple, e.g., six optical conduits, and five corresponding gaps to indicate eleven different fluid levels. 
     Stacked Light Pipes 
     In another embodiment of the invention as shown in  FIG. 6 , multiple light pipes  601  are “stacked” adjacently in the container, with the optical conduits and gaps being of different lengths. Thus, it is possible to construct an optical fluid level encoder. There is no necessity of stacking in any particular direction, as long as the liquid-air interface  105  covers and uncovers the optical conduits ends in an order needed to generate the desired output sequence. In one preferred embodiment, this output sequence is a Gray code. 
     Gray Code 
     A Gray code provide an encoding of 2 n  binary numbers such that only one bit changes from one value to the next. As an advantage, Gray codes are useful encoding fluid levels because a slight position change in the fluid level only affects one bit. In a conventional binary code, up to n bits can change as the fluid level rises or falls across a single dividing line, and a slight misalignments of the measuring device can cause extremely incorrect level readings. 
     For example, moving from level  7  to level  8 , i.e., that is, 0111 to 1000 in binary, can result in any of the 16 possible results from 0000 to 1111 as an intermediate state, depending on the slightest misalignment in the individual detectors for a 0 and a 1 in each of the four channels. Because a Gray code changes only one bit at a time, the worst case error is a single count in either direction, and that error only exists for the maximum permitted assembly misalignment of the assembly during manufacture. 
     A binary-reflected Gray code for n bits can be constructed by taking a Gray code for n-1 bits, and repeating it in reverse order, then prepending a zero to all values in the first half of the new code and a 1 to all values in the second half of the new code. 
     
       
         
               
             
               
               
               
               
               
             
               
             
               
               
               
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
               
               
               
             
               
             
               
               
               
               
               
             
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                   
               
             
             
               
                   For example, a 2-bit Gray code is: 
               
             
          
           
               
                   
                 00 
                 01 
                 11 
                 10. 
               
             
          
           
               
                 Repeating the code again, in reverse order, yields: 
               
             
          
           
               
                   
                 00 
                 01 
                 11 
                 10 
                 10 
                 11 
                 01 
                 00. 
               
             
          
           
               
                 Prepending a zero to each value in the first half yields: 
               
             
          
           
               
                   
                 000 
                 001 
                 011 
                 010 
                 10 
                 11 
                 01 
                 00. 
               
             
          
           
               
                 and prepending a 1 to each value in the second half yields: 
               
             
          
           
               
                   
                 000 
                 001 
                 011 
                 010 
                 110 
                 111 
                 101 
                 100, 
               
             
          
           
               
                 which is a valid three-bit Gray code. This process can be repeated 
               
               
                 indefinitely to yield Gray codes of any desired length and resolution. 
               
               
                   Note that the above Gray code is not the only possible one; for 
               
               
                 example, rotations of a valid Gray code yield other valid Gray codes. In 
               
               
                 the above example, we can rotate the 2-bit code 
               
             
          
           
               
                   
                 00 
                 01 
                 11 
                 10 
               
               
                 to 
               
               
                   
                 01 
                 00 
                 10 
                 11, 
               
             
          
           
               
                 which yields the 3-bit code: 
               
             
          
           
               
                   
                 001 
                 000 
                 010 
                 011 
                 111 
                 110 
                 100 
                 101. 
               
               
                   
                   
               
             
          
         
       
     
       FIG. 6  shows an optical fluid level encoder with three light pipes for encoding according to the above described three-bit rotated Gray code. For the fluid level shown in  FIG. 6 , the Gray code is code 011. 
     It should be understood that other types of codes can also be encoded; the codes need not be absolute codes such as binary or Gray codes. Quadrature codes can be used, as can virtual absolute codes, where a quadrature code provides high resolution, and a third code line provides a unique sequencing signature. As the fluid level changes slightly, the state of the unique sequencing code line yields a unique sequence that can only occur in one position, thus giving an absolute level with only three channels of data. 
       FIG. 8  shows how the light pipes can be stacked. A housing  810  is formed of, for example, plastic. The housing includes parallel channels  811 . A part  812  of the channels is slightly rounded so that the optical conduits  130 , light sources  110  and sensors  120  can be snapped into the channels. 
     Direct Digital Reading Fluid Level Device 
     Other embodiments are also possible as shown in  FIG. 7 . In one embodiment, the light pipes readout is entirely optical.  FIG. 7  shows a conventional seven segment numerical display device is often used to indicate numeric digits, e.g., 4. To generate a numeric display of the fluid level, we determine which segments should be lit for which fluid level range, and then stacks appropriately arranged light pipes to generate this pattern. The light emerging light from each stack can then be optically directed to the appropriate segment of the display. This arrangement gives a numeric reading of the fluid level using only light. No moving parts or electronics circuits are required, other than the power to the light sources. 
     Irregularly Shaped Containers 
     As an advantage, the fluid level encoder as described herein can also be used with irregular shaped containers as shown in  FIG. 9 . With such containers, it is impossible to use conventional mechanical sensors such as floats, or optical sensors that require a direct line of sight from the light source to the sensor. Here, the conduits  140  “bend” the light from the source  110 , around corners, to the sensor  120 . Note, in portions of the light pipe where the fluid level does not change much, the number of conduits can be sparse. 
     Although the invention has been described by way of examples of preferred embodiments, it is to be understood that various other adaptations and modifications can be made within the spirit and scope of the invention. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.