Abstract:
Liquid sensing apparatuses and liquid sensing methods are described. In some embodiments, a liquid sensing apparatus includes a magnetic element; a source magnetic field, which produces on the magnetic element a magnetic force with a component that opposes a gravitational force on the magnetic element; and a structure configured to prevent rotation of the magnetic moment of the magnetic element relative to the magnetic field, e.g., so that the magnetic force remains greater than the gravitational force. The apparatus is capable of sensing the level of a liquid using a combination of magnetic force and buoyancy force.

Description:
TECHNICAL FIELD 
     The invention relates to liquid sensing. 
     BACKGROUND 
     There are various ways to sense the level of a liquid in a tank, reservoir, or other liquid container. Some liquid level sensors use floats that have enough buoyancy to enable them to rise and fall with the surface of the liquid. Another type of liquid level sensor for small reservoirs is the thermister, which can be calibrated for use in high temperature fluids. Optical sensors, such as those that detect the difference between the index of refraction of air and liquid, can also be used. 
     Some liquid level sensors are point level sensors that indicate when a liquid has reached a predetermined height. Some liquid level sensors may indicate the level of a liquid over a continuous range. 
     SUMMARY 
     The invention relates to liquid sensing. 
     In one aspect, the invention features a liquid sensing apparatus, including a magnetic element, a source of magnetic field, producing on the magnetic element a magnetic force with a component that opposes a gravitational force on the magnetic element, and a structure configured to prevent rotation of the magnetic moment of the magnetic element to a point where there is no longer a magnetic force component opposing a gravitational force on the magnetic element. 
     Embodiments may include one or more of the following features. The structure is further configured to allow liquid to contact the magnetic element. The magnetic clement is acted on by a buoyancy force of a liquid. The apparatus further includes a sensor, such as a Hall effect sensor or a reed switch, that is responsive to the position of the magnetic element. The sensor is configured to feed back a signal that is responsive to the position of the magnetic element to the source of magnetic field. 
     The source of magnetic field can include an electro-magnetic coil or a magnet, such as a ferromagnetic material. The magnetic element can further include a non-magnetic material, such as one surrounding the magnetic material. The non-magnetic material can have density lower than the density of the magnetic material. The non-magnetic material can include a polymer. 
     In another aspect, the invention features a liquid sensing apparatus including a housing, a source of magnetic field associated with the housing, a sensor associated with the housing, and a magnetic element movably located within the housing, the magnetic element being between the source of magnetic field and the sensor. 
     In another aspect, the invention features a liquid sensing apparatus including a structure containing a post, a source of magnetic field located at a first end of the structure, a magnetic element slidably engaged with the post, and a sensor that is responsive to the position of the magnetic element at a second end of the structure. 
     In another aspect, the invention features a liquid sensing method. The method includes orienting a magnetic element in a magnetic field such that there is a magnetic force with a component that opposes a gravitational force on the magnetic element, and preventing the magnetic moment of the magnetic element from rotating relative to the magnetic field so that the magnetic force remains greater than the gravitational force. 
     Embodiments may include one or more of the following features. The method further includes contacting liquid to the magnetic element. The magnetic element is acted on by a buoyancy force of a liquid. The method further includes sensing the position of the magnetic element. The method further includes feeding back a signal that is responsive to the position of the magnetic element to the source of magnetic field. 
     The magnetic field can be provided by an electromagnetic coil or a magnet, such as a ferromagnetic material. The magnetic element can further include a non-magnetic material, for example, one surrounding the magnetic material. The non-magnetic material can have density lower than the density of the magnetic material. The non-magnetic material can include a polymer. 
     Other aspects, features, and advantages of the invention will be apparent from the description of the preferred embodiments thereof and from the claims. 
    
    
     DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic diagram of an embodiment of a liquid sensing apparatus. 
     FIGS. 2A,  2 B, and  2 C illustrate the liquid sensing apparatus of FIG. 1 during operation. 
     FIG. 3 is a schematic diagram of an embodiment of a liquid sensing apparatus. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a liquid sensing apparatus  100  is shown. Apparatus  100  includes a housing  106  (as shown, a tube) having one or more openings  108 , a magnet  110  positioned at a first (e.g., bottom) end of the housing, and a sensor  112  positioned at a second (e.g., top) end of the housing. Sensor  112  is interfaced with a detector (not shown), such as an operational amplifier or analog to digital converter, for detecting the output of the sensor. Within housing  106 , between magnet  110  and sensor  112 , apparatus  100  includes a capsule  102  that surrounds a magnet  104 . Magnets  104  and  110  are arranged such that the same magnetic poles face each other and the magnets repel each other. When apparatus  100  is placed in a liquid reservoir (not shown), openings  108  allow the liquid in the reservoir to contact capsule  102 . During use, capsule  102  (and magnet  104 ) can rise and fall according to the liquid level in the reservoir. Sensor  112  is configured to detect the position of magnet  104 , thereby providing an indication of the level of the liquid. 
     In particular, apparatus  100  is capable of continuously sensing the level of the liquid over a predetermined range using a combination of magnetic force and buoyancy force. When there is no liquid in the reservoir, capsule  102  floats within housing  106  due to the magnetic repulsion between magnets  104  and  110  (i.e., in preferred embodiments, the capsule does not contact the bottom of the housing, as shown in FIG.  1 ). Since the position of magnet  110  is fixed, capsule  102  is at an initial height, h 0 , which is dependent on the strength of magnets  104  and  110 , and the force of gravity acting on the capsule (which is proportional to its mass). In particular, the height of the capsule increases as its gravitational force (i.e. weight) decreases. With liquid in the reservoir, when capsule  102  contacts the liquid, the volume of liquid that is displaced is the same as the volume of the capsule that is submerged. Capsule  102  (and magnet  104 ) experiences a buoyancy force that changes its effective weight by the weight of the displaced liquid. As the level of the liquid increases capsule  102  (and magnet  104 ) displaces an increasing amount of liquid. As a result, an increasing buoyancy force acts on capsule  102  (and magnet  104 ). The effect of the decreased weight of capsule  102 , due to the increased buoyancy force, allows the magnetic force to increase the height of the capsule. This change in the position of capsule  102  can be detected by sensor  112  to provide an indication of the liquid level. 
     In addition, the combination of a magnetic force and a buoyancy force on capsule  102  can be used to make apparatus  100  relatively compact. For an object to be sufficiently buoyant such that it floats on a liquid, the object typically has a density less than that of the liquid. Due to the high density of magnetic materials, an object containing a magnet typically encloses a relatively large volume of air or a material less dense than the liquid in order to make the entire object float. For some small reservoirs, such as an ink reservoir used in ink jet printing, the size of an object that fits inside the reservoir is limited, so sensors that use a floating object may be too large. A buoyancy force does act upon an object that is denser than the liquid the object is in, but the buoyancy force alone typically does not make the dense object rise with the liquid. By using a magnetic force on a dense object, the object can rise with the liquid, as described above. 
     Still referring to FIG. 1, housing  106  is generally configured to position magnet  110 , capsule  102 , and sensor  112  in a predetermined arrangement. Housing  106  is configured to maintain the orientation of the magnetic moment of magnet  104  with respect to the magnetic field from magnet  110  such that there is a magnetic force (e.g., repulsion) between the magnets. In particular, while housing  106  can allow capsule  102  and magnet  104  a certain degree of rotation, the housing is configured to prevent rotation of the magnetic moment of the capsule and magnet  104  to the point where the magnetic field between magnets  104  and  110  can no longer oppose the gravitational force on magnet  104 . At the same time, housing  106  is configured to allow capsule  102  to move freely (e.g., vertically, as shown in FIG. 1) within the housing. Housing  106  can be made of any material that is appropriate for the environment in which apparatus  100  is used. For example, if apparatus  100  is used in a corrosive environment, housing  106  can be formed of a chemical resistant polymer. 
     Magnet  110  can be any material or device capable of providing a magnetic field. A suitable material is a magnet, for example, a ferromagnetic material such as neodymium-iron-boron or samarium-cobalt. Magnet  110  is positioned such that a predetermined pole (e.g., north or south) is directed to magnet  104 . To remove it from harsh environments, magnet  110  can be located outside of the reservoir, provided the reservoir is constructed of non-magnetic material. As shown in FIG. 1, magnet  104  can be encapsulated by housing  106 , for example, to protect the magnet from harmful environments. In other embodiments, magnet  104  can be exposed, or the entire capsule can be made up of the magnetic material. Sensor  112  can also be located on the outside of the reservoir. 
     In some embodiments, magnet  110  is an electromagnet. Since it can be turned on and off, an electromagnet can be used as a self-checking mechanism. When the electromagnet is turned off, the capsule, which is denser than the surrounding fluid sinks to the bottom. Sensor  112  (described below) can be used calibrate initial height h 0  by comparison with previous values. An electromagnet can also increase the operating range of apparatus  100  using feedback, as described below. 
     Sensor  112  can be any device capable of sensing the location of capsule  102 , and more specifically, magnet  104 . In some embodiments, sensor  112  is a Hall effect sensor, which operates by producing a voltage that is proportional to the strength of a magnetic field at its location. The voltage can provide a continuous indication of the position of magnet  104  (and capsule  102 ) over the operating range of the Hall effect sensor (which may be different from the operating range of capsule  102 ). In other embodiments, sensor  112  is a reed switch, which contains an electrical circuit that is opened or closed by magnetizable contacts. When the magnetic field (e.g., from magnet  104 ) is sufficiently strong, the contacts become magnetized, attract each other, and close the switch. When the magnetic field is sufficiently reduced, the contacts demagnetize to open the switch. A reed switch can provide a discrete indication of when the liquid reaches a particular level. As shown in FIG. 1, sensor  112  can be encapsulated by housing  106 , for example, to protect the sensor from harmful environments. 
     Capsule  102  and magnet  104  are configured as a float that is movable within housing  106 . Capsule  102  can have any configuration, such as a sphere, an egg shape, a rod, a ring, or a pill shape. Capsule  102  can be made of a material having any density including a low-density (e.g., lower than the density of the liquid) and/or non-magnetic material, such as aluminum or a polymer, or the capsule can be made completely of the magnetic material. The less dense the material, the more influence the buoyancy force has on the effective weight of capsule  102 . As a result, a larger operating range can be achieved. In some embodiments, such as when apparatus  100  is used in harmful environments, capsule  102  is made of a chemically resistant and/or heat resistant material, such as a fluoropolymer (e.g., Kynar™). Magnet  104  can be generally the same as magnet  110 . Magnet  104  is positioned in capsule  102  such that when the capsule is in housing  106 , the magnets  104  and  110  have the same poles facing each other. In some embodiments, magnet  110  is positioned on a surface (e.g., top or bottom surface) of capsule  102 , as shown in the Figures. Magnet  110  can be between two or more portions of capsule  102 . 
     FIGS. 2A-2C show apparatus  100  in operation. When the liquid is at a low level where it does not contact capsule  102  (e.g., line  200 ), the capsule is at initial height h 0 . Magnetic force (e.g., repulsion) and the force of gravity act on capsule  102 . As the level of the liquid increases such that capsule  102  is partially submerged (e.g., line  202 ), in addition to the magnetic force and the force of gravity, a buoyancy force acts on capsule  102 , which changes the position (height) of the capsule. As the height of capsule  102  rises with increasing liquid level, sensor  112  detects the change in position of magnet  104  and provides an indication of the capsule, and thus the liquid level. When the liquid reaches a high level (e.g., until capsule  102  is fully submerged under the liquid (line  204 )), there is no longer a change in buoyancy force if the liquid rises further. As a result, apparatus  100  has a finite operating range over which capsule  102  can continuously sense a change in liquid level. 
     In some cases, (for example, for reservoirs of limited size) the operating range of apparatus  100  can be increased. The operating range R is determined by the difference between the initial height, h 0 , of capsule  102  and its height when fully submerged, h s : R=h s −h 0 . To increase (e.g., maximize) R, h s  can be increased (e.g., maximized) and/or h 0  can be decreased (e.g., minimized), within practical limits. For a reservoir of a given size, the available space may determine a maximum value of h s . The value of h 0  is preferably sufficiently large so that capsule  102  does not contact, for example, the bottom of housing  106  which may determine a minimum value of h 0 . Sometimes, sensor  112  may have a limited range and not be able to sense the position of magnet  104  beyond a certain distance, which could determine a minimum practical value of h 0 . 
     There are several parameters that can be changed to set the values of h s  and h 0 , e.g., near their maximum and minimum values. Changing parameters such as the mass of capsule  102  and the strength of the magnets (which can be quantified as the product of the magnitudes of their effective magnetic moments) changes the values of h s  and h 0  in the same direction (i.e., they both increase or they both decrease). Since the values of h s  and h 0  may need to change in opposite directions, other parameters can be used. For example, without increasing the mass of capsule  102 , its volume can be increased (or its density decreased) to increase the value of h s  without a corresponding increase in h 0 . Reservoir size and material densities may limit the increase in range R. 
     Alternatively or in addition, to control the values of h s  and h 0  separately, some parameters can be made variable, e.g., dependent on the height of capsule  102 . For example, if an electromagnet is used for magnet  110 , then sensor  112  can be used to feed back a signal based on the position of magnet  104  to the electromagnet so that its strength increases with the height of magnet  104 . The current in the coil of the electromagnet can be set to be an increasing function of the height of magnet  104 . This approach increases the range R by allowing the magnetic strength to be stronger when capsule  102  is high to raise h s , and weaker when the capsule is low to lower h s . 
     In other embodiments, multiple sensing apparatuses can be placed at different levels in the reservoir, for example, to provide an indication of the level of a liquid in a reservoir over a large range. 
     Still other embodiments are possible. For example, referring to FIG. 3, a liquid sensing apparatus  300  includes a post  304  on which a magnet  302  inside a capsule  308  may slide. Post  304  maintains the orientation of magnet  302  and capsule  308 , and housing  306  allows free flow of liquid and air around the capsule. Additionally, a reservoir could be designed where the capsule is the only part of the sensor inside the reservoir. Magnet  110  can be located on the outside of the reservoir and sensor  112  can be located on the outside of the reservoir thus limiting the penetrations into the reservoir and decreasing the chances for leaks or contamination from outside. 
     Other embodiments are within the claims.