Abstract:
A device and method for signaling differential pressure change occurring during leak testing of an evaporative emission control space in a motor vehicle fuel system. A casing has sensing ports one of which is communicated to a reference pressure, such as atmospheric pressure, and another of which is communicated to sense pressure in the evaporative emission control space. As difference between the reference pressure and the pressure in the control space changes, the net magnetic flux acting on a magnetoresistive sensor changes. The sensor is electrically connected to the vehicle electrical system for signaling the differential pressure. The device may be used for both positive and negative pressure leak testing.

Description:
CROSS REFERENCE TO RELATED APPLICATION AND PRIORITY CLAIM 
   This application claims the benefit of U.S. Provisional Application No. 60/414,513 filed on Sep. 25, 2002 in the name of Gary Everingham and entitled DIFFERENTIAL PRESSURE SIGNALING DEVICE AND METHOD EMPLOYING A MAGNETORESISTIVE SENSOR, which is incorporated by reference herein in its entirety. 

   FIELD OF THE INVENTION 
   This invention relates generally to devices and methods for signaling differential pressure change. The inventive device and method are particularly advantageous for signaling leakage in evaporative emission control space of a motor vehicle fuel system. 
   BACKGROUND OF THE INVENTION 
   A known on-board evaporative emission control system for a motor vehicle comprises a vapor collection canister that collects volatile fuel vapors generated in the headspace of a fuel tank by the volatilization of liquid fuel in the tank and a purge valve for periodically purging fuel vapors to an intake manifold of the engine. A known type of purge valve, sometimes called a canister purge solenoid (or CPS) valve, is under the control of a microprocessor-based engine management system, sometimes referred to by various names, such as an engine management computer or an engine electronic control unit. 
   During conditions conducive to purging, the purge valve is opened by a signal from the engine management computer in an amount that allows intake manifold vacuum to draw fuel vapors that are present in the tank headspace and/or stored in the canister for entrainment with combustible mixture passing into the engine&#39;s combustion chamber space at a rate consistent with engine operation so as to provide both acceptable vehicle driveability and an acceptable level of exhaust emissions. 
   Certain governmental regulations require that certain motor vehicles powered by internal combustion engines which operate on volatile fuels such as gasoline have evaporative emission control systems equipped with an on-board diagnostic capability for determining if a leak is present in the evaporative emission control space. 
   One known type of vapor leak detection system for determining integrity of vapor containment space, i.e. evaporative emission control space, performs a leak detection test by positively pressurizing the evaporative emission control space using a positive displacement diaphragm pump. Associated valves, such as the purge valve and any vent valves are closed, and the diaphragm pump is reciprocated to create test pressure. Commonly owned U.S. Pat. No. 6,192,743, issued Feb. 27, 2001, discloses a module comprising such a pump. 
   Known test methods include operating the pump to create superatmospheric pressure in the closed space being tested and then detecting changes that are indicative of leakage. One method comprises measuring a characteristic of pump operation. An example of a time-based measurement is a measurement of how frequently the diaphragm pump must be cycled in order to maintain pressure. Other methods of measurement are pressure-based, such as measuring the rate at which pressure decays. 
   Another known type of vapor leak detection system for determining integrity of an evaporative emission control space performs a leak detection test by negatively pressurizing the evaporative emission control space. Negative pressurizing refers to creating vacuum, i.e. sub-atmospheric pressure. One way of creating negative pressure uses engine manifold vacuum. With the engine running, any vent valves are closed, vacuum is drawn through the purge valve which is left open, and after vacuum has been drawn, the purge valve is closed. Loss of vacuum after closing of the purge valve is an indication of leakage. Vacuum may also be created naturally in other ways, such as when the vehicle is parked and the fuel system cools. 
   SUMMARY OF THE INVENTION 
   The present invention concerns devices and methods for signaling differential pressure change, such as that which may occur during a leak test of an evaporative emission control space in a motor vehicle fuel system. 
   One advantage of the invention is that is may be used for both positive and negative pressure leak testing. Another advantage is that it can be embodied as a device that is well-suited for mounting in a motor vehicle in association with the vehicle fuel system. Such a device comprises a casing having sensing ports one of which is communicated to a reference pressure, such as atmospheric pressure, and another of which is communicated to sense pressure in the evaporative emission control space. The device comprises a magnetoresistive sensor electrically connected to the vehicle electrical system for signaling the pressure difference between the reference pressure and the pressure in the control space during a leak test. 
   One general aspect of the invention relates to a differential pressure signaling device comprising a casing divided by a movable wall to provide respective chamber spaces on opposite sides of the wall. A first sensing port communicates a first pressure to a first of the chamber spaces, and a second sensing port communicates a second pressure to a second of the chamber spaces. A signaling mechanism signals change in differential pressure between the two chamber spaces and comprises a first magnet, a second magnet, and a magnetoresistive sensor arranged such that movement of the wall that occurs in consequence of change in pressure differential between the chamber spaces changes the net magnetic flux from the magnets that act on the magnetoresistive sensor. 
   A further aspect of the invention relates to a differential pressure signaling device comprising a casing divided by a movable wall to provide respective chamber spaces on opposite sides of the wall. A first sensing port communicates a first pressure to a first of the chamber spaces, and a second sensing port communicates a second pressure to a second of the chamber spaces. A magnetic circuit exhibits change in a characteristic of magnetic flux in the circuit as differential pressure between the two chamber spaces changes, and a magnetoresistive sensor is arranged to respond to change in the characteristic of magnetic flux in the circuit as differential pressure between the two chamber spaces changes. 
   A still further aspect of the invention relates to a method for signaling change in differential pressure between first and second chamber spaces of a casing that are divided by a movable wall within the casing and to which respective pressures are communicated. The method comprises providing a magnetic circuit that exhibits change in a characteristic of magnetic flux in the circuit as differential pressure between the two chamber spaces changes, and magnetoresistively sensing change in the characteristic of magnetic flux in the circuit. 
   A still further aspect of the invention relates to a leak test device for a motor vehicle fuel system that holds volatile liquid fuel for operating the vehicle. The leak test device comprises a casing divided by a movable wall to provide respective chamber spaces on opposite sides of the wall. A first sensing port communicates a reference pressure to a first of the chamber spaces, and a second sensing port communicates pressure representative of pressure in evaporative emission control space of the fuel system to a second of the chamber spaces. A signaling mechanism signals change in differential pressure between the two chamber spaces and comprises a magnetic circuit that exhibits change in a characteristic of magnetic flux in the circuit as differential pressure between the two chamber spaces changes. A magnetoresistive sensor is arranged to respond to change in the characteristic of magnetic flux in the circuit as differential pressure between the two chamber spaces changes. 
   A still further aspect of the invention relates to a method for signaling leakage in a motor vehicle fuel system that holds volatile liquid fuel for operating the vehicle. The method comprises providing a magnetic circuit that exhibits change in a characteristic of magnetic flux in the circuit as differential pressure between a reference pressure and pressure in evaporative emission control space of the fuel system changes, and magnetoresistively sensing change in the characteristic of magnetic flux in the circuit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated herein and constitute part of this specification, include one or more presently preferred embodiments of the invention, and together with a general description given above and a detailed description given below, serve to disclose principles of the invention in accordance with a best mode contemplated for carrying out the invention. 
       FIG. 1  is a general schematic diagram of an exemplary automotive vehicle evaporative emission control system including a leak test device embodying principles of the invention. 
       FIG. 2  is a cross section view through an exemplary embodiment of the leak test device. 
       FIG. 3  is a graph plot related to operation of the device. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows an example of a portion of a motor vehicle fuel system  10 , including a leak test device  12 . A fuel tank  14  holds a supply of volatile liquid fuel for an engine  15  that powers the vehicle. Fuel vapors that are generated within headspace of tank  14  are collected in a vapor collection canister  16  that forms a portion of an evaporative emission control system. 
   At times conducive to canister purging, the collected vapors are purged from canister  16  to engine  15  through a purge valve  17 . For purging, purge valve  17  and a canister vent valve  18  are both open. Vent valve  18  vents canister  16  to atmosphere through a particulate filter  19 , allowing engine manifold vacuum to draw air into and through canister  16  where collected vapors entrain with the air flowing through the canister and are carried into the engine intake system, and ultimately into engine  15  where they are combusted. 
   From time to time, leak test device  12  is used for conducting a leak test for ascertaining the integrity of the fuel system, particularly the evaporative emission control system, against leakage. Such a test may involve either positive pressurization or negative pressurization of the evaporative emission control space. 
   For positive pressurization, purge valve  17  and vent valve  18  are operated closed to close off the evaporative emission control space that contains fuel vapors. That space is then positively pressurized by a pump (not shown) with subsequent decay in pressure being an indication of leakage. 
   For negative pressurization, purge valve  17  is left open while vent valve  18  is operated closed. AS engine  15  runs, intake manifold vacuum draws vacuum in the space being tested, and then purge valve  17  is closed. Subsequent decay in vacuum is an indication of leakage. 
     FIG. 2  shows an exemplary embodiment of leak test device  12  to comprise a casing  20  having a longitudinal axis  22 . Casing  20  is formed by a cylindrical tubular sidewall  24  and circular end walls  26 ,  28  that close opposite axial ends of sidewall  24 . A movable wall  30  divides the interior of casing  20  into first and second chamber spaces  32 ,  34 . A sensing port  36  in end wall  26  communicates chamber space  32  to vapor containment space of the evaporative emission control system. Another sensing port  38  in sidewall  24  communicates chamber space  34  to a reference pressure, typically atmospheric pressure. 
   A magnet  40  is disposed in a fixed location within the interior of chamber space  32 . Another magnet  44  is disposed centrally on movable wall  30 . A magnetoresistive sensor  46  is disposed in association with magnet  40  within chamber space  32  in an arrangement that enables it to sense the magnetic field surrounding magnet  40 . The magnets are disposed in general alignment along axis  22  with an imaginary line between each magnet&#39;s poles generally aligned with the axis. 
   Magnet  40  is poled to oppose magnet  44 . Hence, motion of magnet  44  toward magnet  40  will be met by increasing force opposing the motion. 
   Movable wall  30  comprises a diaphragm whose outer perimeter is held against and sealed to sidewall  24 , such as being clamped between upper and lower parts of the sidewall. The central region of the diaphragm that carries magnet  44  is displaced along axis  22  as pressure differential changes. 
   A spring  50  and a pre-set mechanism  52  pre-set a bias force that the spring exerts on wall  30  in a direction that urges magnet  44  toward magnet  40 . Mechanism  52  comprises an adjustment screw  54  threaded into end cap  28 , and a spring seat  56  that fits to one end of spring  50 . Turning adjustment screw  54  about axis  22  positions seat  56  along the axis, either increasing or decreasing the spring force exerted on wall  30 . Pre-setting the spring force serves to calibrate device  12  so that the signal given by device  46  is properly correlated with pressure differential between the chamber spaces. 
   As the pressure in chamber space  32  becomes increasingly positive relative to atmospheric pressure while atmospheric pressure is maintained in chamber space  34 , the movement of wall  30  away from the position shown in  FIG. 2  causes magnet  44  to move increasingly farther from magnet  40 , increasingly compressing spring  50  in the process. Likewise, as the pressure in chamber space  32  becomes increasingly negative relative to atmospheric pressure while atmospheric pressure is maintained in chamber space  34 , the movement of wall  30  away from the centered state of balance shown in  FIG. 3  will cause magnet  44  to move increasingly closer to magnet  40 , with spring  50  expanding in the process. 
   The magnetic field of magnet  44  interacts with that of magnet  40  to alter how the magnetic field of magnet  40  acts on magnetoresistive sensor  46 . Consequently, as the position of magnet  44  changes relative to magnet  40 , the magnetic flux acting on magnetoresistive sensor  46  also changes. 
   If chamber space  32  is communicated to positive pressure for a leak test, a decrease in positive pressure that is indicative of a leak equal to or exceeding a certain size will be signaled by device  46  at some point in the movement of magnet  44  toward magnet  40 . If chamber space  32  is instead communicated to vacuum, a decrease in vacuum during a leak test that is indicative of a leak equal to or exceeding a certain size will be signaled by device  46  at some point in the movement of magnet  44  away from magnet  40 . 
   Sensor  46  is powered by a small electric current, and is capable of providing a signal that distinguishes various amounts of magnetic flux acting on it. For example, the signal may be in the nature of a signal that distinguishes flux that equals or exceeds a threshold from flux that does not. Hence, by appropriate selection of magnets, area of wall  30 , and distance between magnets  40  and  44  for a certain pressure differential between chamber spaces  32 ,  34 , device  12  can correlate the signal of sensor  46  with pressure differential such that the signal will distinguish pressure differentials that equal or exceed a threshold pressure differential from pressure differentials that do not. The selection will typically be premised on an assumption that one chamber space will be communicated to atmospheric pressure while the other will be communicated either to positive pressure or vacuum. 
   The graph plot  60  of  FIG. 3  illustrates a representative characteristic of the signal provided by magnetoresistive sensor  46  as a function of pressure differential between chamber spaces  32 ,  34 . When pressure decay is used as an indicator of leak size, the inflection point marked  62  can serve to define the point of demarcation between an evaporative emission control space that has an effective leak size equal to or exceeding a pre-determined threshold and one that does not. When vacuum decay is used as an indicator of leak size, the inflection point marked  64  can serve to define the point of demarcation between an evaporative emission control space that has an effective leak size equal to or exceeding a pre-determined threshold and one that does not. 
   The invention provides a “non-contact” type device capable of reliably detecting relatively small amounts of change in pressure differential that may be sufficient to signal whether or not leakage exceeding a pre-determined effective size is present. The device is characterized by relatively little hysteresis, and can provide a signal level sufficiently strong for use by components forming a part of the vehicle electric system. 
   It is to be understood that because the invention may be practiced in various forms within the scope of the appended claims, certain specific words and phrases that may be used to describe a particular exemplary embodiment of the invention are not intended to necessarily limit the scope of the invention solely on account of such use. For example, a magnetoresistive sensor includes what is sometimes referred to as a giant magnetoresistive sensor.