Abstract:
A differential pressure sensor has a magnetized piston free to move in a cylinder. Magnetic endcaps seal the ends of the cylinder, forming chambers on either side of the piston. The moveable magnetic piston is constrained by the opposing magnetic fields of the endcaps. The piston is covered with magnetic ferrofluid, providing a low friction gas-tight seal around the piston. The cylinder has two pressure input lines, one being connected to the chamber on either side of the piston. The relative pressures of the input lines cause the piston to move to a position of equilibrium within the cylinder, with the magnetic fields of the endcaps holding the piston in place against the pressure. A magnetic field angle sensor detects flux lines on the outside of the cylinder, and the reading is correlated with the pressure differential between the two input lines.

Description:
BACKGROUND 
     Various embodiments of the present invention relate to sensors and measurement devices, and more specifically, to pressure sensors. 
     There are a number of different pressure related parameters that can be measured. An absolute pressure sensor measures pressure relative to perfect vacuum pressure of 0 pounds per square inch (PSI) or zero pressure. Atmospheric pressure is 101.325 kPa (14.7 PSI) at sea level with reference to vacuum. The pressure measurement taken on a car tire is sometimes called gauge pressure. A gauge pressure sensor measures the pressure relative to a given atmospheric pressure at the location of the measurement. For example, when the tire pressure gauge reads 0 PSI, there is actually around 14.7 PSI (or atmospheric pressure) in the tire. A third type of pressure measurement is differential pressure. A differential pressure sensor measures the difference between two pressure inputs to the sensing device. For example, a differential pressure sensor could be used to measure the pressure increase across an oil pipeline pump. 
     There are several conventional designs for pressure sensors operating on the basis of a number of different technologies. Piezoresistive strain gauge pressure sensors exploit the piezoresistive effect to detect strain due to pressure applied to a bonded strain gauge. A piezoelectric pressure sensor uses the piezoelectric effect of materials such as quartz to measure pressure induced strain. A capacitive pressure sensor has a diaphragm and pressure cavity designed to create a variable capacitor for detecting strain due to applied pressure. Electromagnetic pressure sensors measure the displacement of a diaphragm by exploiting changes in inductance, the Hall Effect, the eddy current principal, or using a Linear Variable Differential Transformer (LVDT). 
     However, such conventional pressure sensing technologies are not well suited to measuring low differential pressures, are expensive, and may be difficult to operate in a environments which require high withstand pressures. Accordingly, a robust, compact pressure sensor suitable for measuring low differential pressures is needed. 
     SUMMARY 
     Various embodiments disclosed herein involve a differential pressure sensor that has a hollow cylinder with magnetic endcaps sealing off each end. The cylinder contains a piston which also has a magnet. The piston is arranged such that its south pole faces the south pole of one endcap, and its north pole faces the north pole of the other endcap. The various embodiments feature a magnetic ferrofluid coating around the piston to provide a seal between the piston and said cylinder. The cylinder has a first pressure line input feeding into a first chamber of the cylinder formed between the piston and one of the endcaps. A second pressure line feeds into a second chamber of the cylinder between the piston and the other endcap. A magnetic flux angle sensor is positioned outside the cylinder to detect magnetic flux lines from the piston, thus determining its position. Based on the position of the piston, a calculating means develops a differential pressure reading between the two input lines. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute part of the specification, illustrate various embodiments of the invention. Together with the general description, the drawings serve to explain the principles of the invention. In the drawings: 
         FIGS. 1A-C  depict a cross-sectional view of a differential pressure sensor according to various embodiments of the present invention; 
         FIGS. 2A-C  depict flux diagrams for the three positions of the piston shown in  FIGS. 1A-C ; 
         FIGS. 3A-C  depict a cross-sectional view of a gravity compensated pressure sensor according to various embodiments of the present invention; and 
         FIG. 4  depicts a flowchart of activities for practicing various methods in accordance with the embodiments disclosed herein. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. 
       FIGS. 1A-C  depict a cross-sectional view of a differential pressure sensor  100  according to embodiments of the present invention. Various embodiments disclosed herein involve a differential pressure transducer that uses one or more permanently magnetized moving pistons contained within a cylinder and fixed magnets sealed into the ends of the cylinder. The cylinder may be made of a material with low magnetic permeability that is transparent to magnetic flux lines, yet able to withstand the pressures being measured. The moveable piston  101  is sealed into cylinder  103  using a ferrofluid  105  which provides a very low friction gas-tight seal. Piston  101  is configured to include a magnet. The piston  101  is constrained axially by opposing magnetic fields from fixed magnets in the two magnetic endcaps,  107  and  109 , positioned at both ends of cylinder  103 . The magnetic poles of magnetic endcap  107  and  109  are configured to be the same polarity as the respective adjacent magnetic poles of piston  101 , thus repelling each other. As shown in  FIGS. 1A-C , the south pole of magnetic endcap  107  faces the south pole of piston  101  magnet, and the north pole of magnetic endcap  109  faces the north pole of piston  101  magnet. In this way, the piston  101  is repelled from the magnetic endcaps  107  and  109  that oppose it from either end. 
     Conventional pressure sensing devices typically rely on a position sensor that breaches the walls of the pressure chambers. However, since the various embodiments disclosed herein use magnetic field position sensing, the walls of cylinder  103  can be configured to be quite thick, able to withstand very high absolute pressures. Furthermore, the use of magnetic fields to constrain the piston  101  provides a non-linear output, giving the device a relatively large dynamic range over which pressure differentials can be detected. The differential pressure sensor  100  tends to have the greatest sensitivity at lower pressure differentials, which is an advantage in certain circumstances. 
     The cylinder  103  is configured to have two pressure inputs  111  and  113 . The differential pressure sensor  100  measures the difference between the pressure in input  111  and input  113 . When each of the two inputs  111  and  113  provide equal pressure in chambers  115  and  117  above and below the piston, then piston  101  will remain in its nominally central position as shown in  FIG. 1A . Although the figures depict pressure chambers  115  and  117  of equal size, in some embodiments the pressure chambers may be designed to contain different volumes. This can be useful, for example, in order to tailor the desired sensitivity of the sensor system  100  to a range of expected pressures inputs from input lines  111  and  113 . Turning back to  FIGS. 1A-C , if the inputs  111  and  113  differ, then pressure above and below piston  101  from the inputs  111  and  113  acts to displace piston  101  from a nominally central position.  FIG. 1B  depicts input  113  having a relatively higher pressure than input  111 , thus driving piston  101  up, away from magnetic endcap  109  towards magnetic endcap  107 . In  FIG. 1B  the pressure in chamber  121  exceeds the pressure in chamber  119 . In  FIG. 1C  the input  113  has a lower pressure than input  111 , thus piston  101  is pulled downward, towards magnetic endcap  109  and away from magnetic endcap  107 . In  FIG. 1C  the pressure in chamber  125  is lower than the pressure in chamber  123 . 
     The piston  101  of pressure sensor system  100  may be either be made of a magnetic material, or may be configured to hold a magnet. In either case the piston  101  has a magnet that moves in response to pressure changes between the two inputs  111  and  113 . The moving magnet&#39;s external field allows the position of piston  101  to be detected using one or more magnetic field angle sensors  127  positioned outside the cylinder  103 . The field angle sensor  127  is typically positioned close enough to the outside of cylinder  103  to accurately resolve the position of the fixed magnet in moveable piston  101 . The sensor  127  may be implemented with any of several types of magnetic flux field angle sensors, including for example, a tunnelling magneto resistance (TMR) sensor, a giant magnetoresistance (GMR) sensor, anisotropic magneto resistance (AMR), Hall Effect devices, or other like types of magnetic field angle sensors known to those of ordinary skill in the art. The TMR sensors are capable of providing a large signal output of several hundred millivolts, and have two outputs, one proportional to the sine and the other to the cosine of the incident magnetic flux angle. TMR sensors are sensitive devices, and work in magnetic saturation so that the absolute magnitude of flux is immaterial above some small minimum value. As the pressure difference between the two inputs  111  and  113  changes the position of piston  101  within cylinder  103  also changes. This, in turn, alters the measured angle of the magnetic field flux lines. The effect on the magnetic field flux lines is detected by the magnetic field angle sensor  227 . This may be more clearly seen in  FIGS. 2A-C . 
     In various embodiments the angle sensor  127  outputs are connected to a microprocessor or other controller to provide a digital output, calibration factors, and the gravitational adjustment calculations. (The gravitational adjustment calculations are described below in conjunction with  FIGS. 3A-C ). Turning to  FIG. 1A , magnetic field angle sensor  127  is connected to a controller  149  or other calculating means which correlates the magnetic flux angle to the position of the piston  101  and calculates the pressure differential between two pressure inputs  111  and  113 . In various embodiments the controller  149 , or other calculating means, may be embodied as a microprocessor, control logic, circuitry, a computer, or other electronic device capable of being programmed to carry out instructions or routines. The controller  149  may contain, or have access to, memory or storage devices suitable for storing data, software instructions or routines for performing calculations related to magnetic flux angles, piston position, and pressure readings. The pressure sensor system  100  also has a display  153  and user input device  155  connected to the controller  149 . The display  153  may be embodied as an LCD or LED display, a computer screen, or other like type of display device suitable for visually conveying information such as the pressure readings and control codes for the system  100 . The user input device  155  may be embodied as a keypad, keyboard, buttons, computer mouse, or other like type of user input device. 
     Ferrofluid material  105  is a liquid which is attracted to magnets. The ferrofluid material  105  surrounding piston  101  provides a gas-tight seal with very low friction. Ferrofluid is a commercially available liquid comprising nanoscopic magnetic particles in suspension. Ferrofluids can be obtained from a number of sources, including Ferrofluidics Corporation of Nashua, N.H.; Ferrotec Corporation of Bedford, N.H.; 3M Specialty Chemicals Division, St. Paul, Minn.; and Liquids research Ltd of Bangor, Wales, United Kingdom. In various embodiments the type of ferrofluid is selected so that the fluid coming in contact with the ferrofluid seal  105  is immiscible with the ferrofluid, and does not react chemically with the ferrofluid. The magnet  101  prevents the ferrofluid  105  from floating away. In some implementations a kerosene-based ferrofluid has been used. Other ferrofluids of composed of differing materials may be used, depending upon the properties of the liquid undergoing pressure measurement. If the differential pressure sensor  100  is used for pressure measurements of an aggressive medium, a silicone oil barrier may be employed to keep the aggressive fluids away from bleeding through to the low pressure chamber. This piston magnet  101  is coated with ferrofluid  105  which produces a low friction seal between piston  101  and the walls of cylinder  103  as the piston moves up and down in response to pressure inputs  111  and  113 . The magnetic angle sensor  127  detects the resulting field angle of the magnetic flux from the three magnets, that is, from the magnet of the piston  101  and the magnetic endcaps  107  and  109 . 
       FIGS. 2A-C  depict flux diagrams for the three positions of the piston shown in  FIGS. 1A-C . In  FIG. 2A  the piston magnet  201  is located midway between the two fixed magnets  207  and  209  of the magnetic endcaps, causing the flux lines to be symmetrical about both axes in this configuration. It should be noted that the shape of the flux lines depends not only upon the distance between the magnets, but also upon a number of other factors and parameters of the design. For example, the shape of the magnets and the strength of the magnets are both parameters that affect the magnetic flux lines. In some implementations magnets of different shapes, and/or different strengths may be used, depending upon the design requirements of the system and the component features and constraints. 
     In the implementation and piston position depicted in  FIG. 2A  the flux lines at the sensor are substantially parallel to the central axis  251  of the cylinder. Therefore the magnetic field angle sensor  227  reads 0 degrees, which correlates to an equal pressure being received in the two inputs. In  FIG. 2B  the relative pressures of the two inputs has changed, causing piston magnet  201  to move to a position closer to fixed magnet  207  than fixed magnet  209 . This change of position causes a change in the magnetic flux lines which is detected by the sensor  227 . In the implementation and piston position depicted in  FIG. 2B  the magnetic field angle sensor  227  reads approximately 315 degrees (or negative 45 degrees). The system is able to correlate the 315 degree reading to a particular pressure differential between the two input pressures. The two pressures may be calculated based on the position of the piston which determines the volume of the two pressure chambers within the cylinder. Turning to  FIG. 2C , the two input pressures have again been changed, causing the piston magnet  201  to move to a position closer to fixed magnet  209  and farther away from fixed magnet  207 . In the implementation and piston position of  FIG. 2C  the magnetic field angle sensor  227  reads approximately 45 degrees. Once again the system will be able to correlate the 45 degree reading to a particular pressure differential between the two input pressures. 
       FIGS. 3A-C  depict a cross-sectional view of a gravity compensated pressure sensor according to various embodiments of the present invention. These embodiments compensate for errors introduced by gravity, acceleration or orientation—that is, errors caused by any of or more of the three factors including gravity, acceleration or orientation acting either singly or in combination. In these embodiments, the effects of gravity, acceleration or orientation on the moving magnet are compensated for providing two moveable piston magnets  301  and  335  suspended between fixed magnets contained in the magnetic endcaps  307  and  309 . Note that endcap  309  is oriented with its magnetic south pole facing inward towards pressure chamber  317  and the magnetic south pole of piston  301 . A pressure equalization path  333  is also provided to equalize the pressure in chambers  339  and  341 . In this way the pressure between each of the two moveable piston magnets  301  and  335  and their respective adjacent magnetic endcap  309  or  307  is equalized. In other words, the top and bottom cavities  317  and  331  of  FIG. 3A  are linked, and thus at the same pressure. Similarly, cavities  339  and  341  of  FIG. 3B  are linked, as are cavities  345  and  347  of  FIG. 3C . 
     In the embodiments of  FIGS. 3A-C  the effects of gravity, acceleration and orientation can be compensated for by adding 180 degrees to the upper magnetic field angle sensor  329 , reversing the sign (i.e., multiplying by negative one), and taking the average of the two readings. Turning to  FIG. 3B , magnetic field angle sensor  329  reads minus 45 degrees and magnetic field angle sensor  327  reads 225 degrees. The reading of sensor  329  is adjusted by adding 180 degrees (−45+180=135 degrees). Reversing the sign gives −135 degrees, which equals 225 degrees. For  FIG. 3A , sensor  329  reads 0 degrees and sensor  327  reads 180 degrees. Sensor  329  is adjusted by adding 180 degrees (0+180=180 degrees). Reversing the sign gives −180 degrees, which equals +180 degrees. 
     The explanation of the paragraph above uses the ideal readings taken from the figures, and so averaging the adjusted values would not result in any further accuracy (e.g., 225=225 degrees; and 180=180 degrees). In practice, however, the effect of gravity or acceleration could produce slight inconsistencies in the readings, which would be eliminated by averaging the adjusted values. Turning to  FIG. 3B  again, the magnetic field angle sensor  329  could read minus 46 degrees and magnetic field angle sensor  327  could be in error by the same factor, reading 224 degrees. The reading of sensor  329  is adjusted by adding 180 degrees (−46+180=134 degrees). Reversing the sign gives −134 degrees, which equals 226 degrees. Taking the average of the two sensor readings would yield (226+224)/2=225. 
     The various embodiments have been described in terms of magnets of the same polarity facing each other. This is depicted in  FIG. 1  where a south pole of encap  107  faces a south magnetic pole of the piston  101  and a north pole of endcap  109  facing the north magnetic pole of piston  101 . In this way the like poles push away from each other. Some embodiments, however, are configured so that differing poles face each other—that is, a south pole encap faces a north magnetic pole of the piston and a north pole endcap faces a south magnetic pole of the piston. In this way the opposite poles attract each other rather than pushing away from each other. 
       FIG. 4  depicts a flowchart of activities for practicing various methods in accordance with the embodiments disclosed herein. Many of the activities have been described in further detail above, in conjunction with  FIGS. 1-3 . The method begins at block  401  and proceeds to  403  where a magnetic piston is fitted within a cylinder. Typically, the outer surface of the magnetic piston closest to the walls of the cylinder is coated with a ferrofluid material that helps provide a seal between the piston and the cylinder wall. In block  405  of  FIG. 4  one open end of the cylinder is sealed with a first magnetic endcap, creating a first chamber between the piston and the first endcap. The first magnetic endcap is oriented so that it has a south magnetic pole facing the first chamber of the cylinder. The piston also has a south magnetic pole facing the first chamber of the cylinder. 
     The method proceeds to  407  for connection of a first pressure line to the first chamber. In block  409  the other end of the cylinder is sealed with a second magnetic endcap, forming a second chamber between the piston and the second magnetic endcap. The second magnetic endcap is oriented so that it has a north magnetic pole facing the second chamber of the cylinder. The north pole of the piston also faces the second chamber. In block  411  a second pressure line is connected to the second chamber. 
     Proceeding to block  413 , a magnetic flux angle detector positioned outside the chamber adjacent the piston detects the flux lines from the piston. In this way, by detecting the piston&#39;s magnetic flux lines, the position of the piston can be determined in block  415 . Once the position of the piston is known the differential pressure between the two cylinders can be calculated in block  417 . 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including” used in this specification specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The term “obtaining,” as used herein and in the claims, may mean either retrieving from a computer readable storage medium, receiving from another computer program, receiving from a user, calculating based on other input, or any other means of obtaining a datum or set of data. The term “plurality,” as used herein and in the claims, means two or more of a named element. It should not, however, be interpreted to necessarily refer to every instance of the named element in the entire device. Particularly, if there is a reference to “each” element of a “plurality” of elements. There may be additional elements in the entire device that are not be included in the “plurality” and are not, therefore, referred to by “each.” 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and gist of the invention. The various embodiments included herein were chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.