Abstract:
A liquid level measuring system for a personal hydration pack using capacitive sensing pads located outside of the reservoir. Variation in the liquid level due to movement of the pack is used advantageously to increase the resolution of the measurement. An indicator shows both the liquid level and the time remaining before reservoir depletion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
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   FEDERALLY SPONSORED RESEARCH 
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   SEQUENCE LISTING OR PROGRAM 
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   FIELD OF THE INVENTION 
   This invention relates to liquid level indicators, and more particularly to liquid level indicators for personal hydration systems. 
   BACKGROUND OF THE INVENTION 
   During strenuous physical activity, the human body requires a significant amount of fluid replenishment to replace lost fluids, in order to prevent dehydration. It is preferable for a person to be able to consume a small amount of fluid frequently while continuing to perform the activity, rather than having to stop the activity periodically to consume a larger amount of fluid. Hydration packs such as those disclosed in U.S. Pat. Nos. 6,364,168 and 6,422,439 have been developed to allow frequent replenishment. They contain a flexible reservoir with up to several liters of fluid, and a flexible hose allowing the wearer to drink without interrupting their activity. 
   There are a large variety of devices known in the art for carrying standard bicycle water bottles. These are typically worn on a person&#39;s belt or attached to a standard backpack. There are also other types of reservoir carriers, such as the the Beverage Container Belt disclosed in U.S. Pat. No. 6,598,770. For longer periods of activity, a hydration pack is often preferred over these devices, due to its larger capacity. Hydration packs are commonly available that contain up to 3 liters of fluid, whereas bicycle water bottles typically contain no more than 1 liter. 
   The hydration pack may be worn by a person hiking in a remote area without a nearby potable water supply, requiring the reservoir to contain enough fluid for the entire hike. The hiker must ration the fluid to insure it does not run out prematurely. This is typically accomplished by controlling how often, as well as how much fluid is consumed, each time a drink is taken. 
   A disadvantage of hydration packs of the current art is a lack of any indication of the amount of fluid remaining in the reservoir. The pack is normally worn on a person&#39;s back, with the reservoir contained entirely inside of the pack, requiring the person to stop their activity, then remove and open up the pack in order to visually examine the liquid level. Although the weight of the pack will change as liquid is consumed, this is not a reliable gauge of remaining liquid, especially for an inexperienced user of the hydration pack. 
   Another disadvantage is an inability to indicate how much time the remaining fluid is expected to last, or to provide advance warning when it is about to run out. 
   There is a broad range of techniques for measuring fluid level. Many require mechanical devices such as floats installed in contact with the fluid. Others sense properties of the fluid using probes that are in direct contact with it, such as the impedance sensing technique disclosed in U.S. Pat. No. 5,565,851. Still others use techniques not requiring direct contact with the fluid, such as the optical technique disclosed in U.S. Pat. No. 4,840,137, and the capacitance measurement methods of U.S. Pat. Nos. 4,295,370 and 6,472,887. 
   The capacitance method is frequently employed, due to the possibility of using sensors that are not in contact with the fluid and therefore not subject to contamination by the fluid. The current art includes methods with sensors exhibiting a continuous capacitance change proportional to fluid level, as disclosed in U.S. Pat. No. 4,383,444; as well as methods with multiple sensors to detect for the presence of fluid at several discrete levels, as disclosed in U.S. Pat. No. 4,003,259. 
   A disadvantage of continuous capacitance methods of the current art is their inability to compensate for changes in ambient capacitance, such as changes in location of objects in proximity to the sensor, without requiring electrostatic shielding or other methods which increase the cost of the system. 
   A disadvantage of discrete capacitance methods of the current art is the inability to resolve more liquid levels than the number of electrodes in the system. 
   Another disadvantage of discrete capacitance methods is the necessity of filtering out variations in measured liquid level due to movement of the reservoir, such as is disclosed in “QProx QT114 Charge-Transfer QLevel Sensor IC” Data Sheet. 
   A technique known in the art as dithering may be used to increase resolution of an analog signal, as disclosed in U.S. Pat. No. 6,016,113. 
   BRIEF SUMMARY OF THE INVENTION 
   One object of the present invention is to provide a system of measuring and displaying the fluid level contained in a hydration pack, to prevent a person from running out of fluid unexpectedly. 
   Another object of the invention is to allow the measurement set forth above to be made without requiring any objects to be placed inside or through the walls of the fluid reservoir. 
   Yet another object of the invention is to allow the system to be used with an existing hydration pack of any size without making any modifications to the reservoir. 
   A further object of the invention is to provide the measurement set forth above with increased resolution while the hydration pack is moving as it is being carried by a person. 
   A further object of the invention is to provide an estimate of the remaining time before the reservoir becomes empty. 
   The above objects are accomplished by providing a capacitance-based liquid level measuring system with multiple sensing locations. A circuit measures the capacitance between multiple sensing pads, which are positioned along a thin strip of flexible material in close proximity to the outside of one side of the reservoir. The capacitance measured between a selected pad and all other nonselected pads will vary depending on whether the liquid level is above or below the pad. The pads are equally spaced along the strip, and an additional pad is placed at the top of the strip above the maximum liquid level. 
   The level indication is determined by selecting each pad in sequence and measuring its capacitance, then computing the difference between each pair of adjacent measurements. The air/liquid boundary will be given by the pair of measurements showing the largest difference. If the boundary is very close to one of the sensing pads, the measurement will vary as the liquid moves around within the reservoir while the pack is being carried; otherwise, if the boundary is between a pair of pads, the measurement will stay constant. This information allows the number of possible boundary location indications to be twice the number of sensing pads. This indication is then provided to the person wearing the pack. 
   The change in level indication over time is used to compute an estimate of the time remaining before the reservoir becomes empty, and this information is also provided to the wearer. 
   Other features and advantages of this invention will be apparent from the following detailed description together with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of the preferred embodiment of the present invention. 
       FIG. 2  is a block diagram of the measurement circuit of FIG.  1 . 
       FIG. 3  is a plan view of the sensing strip showing the individual pads. 
       FIG. 4  is an isometric view of a reservoir with sensing strip attached. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a liquid level measurement system according to the present invention. Sensing strip  10  is connected through selector  11  to measurement circuit  12 , which measures the capacitance of the selected pad, and supplies the resulting value to a location in pad memory  13  corresponding to the selected pad. The values from memory  13  are fed through difference circuit  14  to temporal memory  15 , which stores several sets of values in sequence, acting as a shift register. Memory  15  supplies its data to comparator  16 , which then connects to timer  17  and divider  18 , as well as indicator  19 . 
     FIG. 2  shows the details of circuit  12 . Excitation source  20  supplies a sinusoidal waveform to the measurement node  21 . The output impedance of source  20  is sufficiently high so that the signal present at node  21  will vary depending on the externally presented impedance. This signal is fed through buffer  22  to shield node  23 . The signal is also fed through amplifier  24 , rectifier  25 , and low-pass filter  26 . This results in a DC value at node  27  which varies with the capacitance present at node  21 . 
     FIG. 3  shows strip  10  with sensing pads  31 A through  31 E formed out of conductive material placed on a piece of flexible insulating material  33 . The pads may be produced using any of various techniques known in the art, such as copper applied to a flexible printed circuit board or conductive ink printed on a flexible membrane. Each sensing pad is of the same size, and the spacing between pads is constant. A conductive trace runs from each pad to an extended area  34  of the insulating material, to provide for connection to selector  11 . The total area covered by each of the traces is the same, to minimize the difference in capacitance between the traces and the fluid. In order to reduce interaction between adjacent pads, a shield trace  32  is run between each pair of pads and traces. 
     FIG. 4  shows strip  10  affixed vertically along the outside of reservoir  41 , with the topmost pad above fill opening  43 , above the highest possible liquid level. One end of flexible hose  42  is secured near the bottom of reservoir  41 , and carrying straps  44  allow the reservoir to be carried on a person&#39;s back. 
   Selector 11 cycles through each of the pads  31 A through  31 E, connecting one at a time to measurement circuit  12 , and connecting all other pads to ground. Circuit  12  measures the capacitance between the selected pad and all other grounded pads, and the result is stored in a location in pad memory  13  corresponding to the selected pad. During the measurement, circuit  12  drives shield trace  32  with a voltage corresponding to the value measured at the selected pad. This reduces the capacitance effect of the traces, while not significantly affecting the measurement of the capacitance of the fluid. 
   After selector  11  has cycled once through all of the pads and filled up memory  13 , difference circuit  14  computes the capacitance difference between each pair of memory  13  values. For each pair of pads, if the liquid is either above or below both pads, the difference will be small. If the liquid is above only one of the pads, the difference will be large. The actual difference value will vary due to a number of factors, including the properties of the liquid and the reservoir material, movement and changes in shape of the reservoir, and variation of position of objects near the sensing pads. The largest of the differences represents the location of the air/liquid boundary. 
   Whenever the reservoir is being carried by a person, the water will be moving around, possibly producing varying indications of the location of the air/liquid boundary. The present location of the boundary is stored in temporal memory  15 , which is able to store the boundary locations for several past measurements, typically over the course of several seconds. Comparator  16  compares all of these past locations, to determine whether the location varies or is constant over those several seconds. 
   If the location is constant, comparator  16  assumes the actual liquid level is halfway between the corresponding pair of sensing pads. If it varies between two adjacent locations, the level is assumed to be centered on the pad that is a member of both pad pairs. If it varies between more than two locations, or two non-adjacent locations, then the locations of the highest and lowest pad pairs are averaged to determine the actual level. 
   As liquid is consumed and the level drops, timer  17  tracks the average time required to consume the fixed quantity of liquid represented by one increment of level indication. Divider  18  then divides the liquid level by this average time, to compute the estimated time remaining before the liquid is exhausted. 
   Indicator  19  displays the actual liquid level, as well as the estimated time remaining. The display is mounted in a location visible to the person wearing the hydration pack, such as on a shoulder strap. 
   While this invention has been described with reference to the described embodiment, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiment, such as using a different number of sensing pads, or integrating a majority of the functional blocks into a microcontroller, as well as other embodiments of the invention, which are apparent to persons skilled in the art to which the invention pertains, are deemed to lie within the spirit and scope of the invention.