Abstract:
A non-contacting linear position sensor having bipolar tapered magnets. A pair of magnets are positioned adjacent each other and attached to a movable object. Each magnet has a central portion that is thinner than both ends of the magnets. A pair of pole pieces has ends that are arranged spaced apart in parallel relationship about the central portion. The other ends of the pole pieces are located spaced apart with a magnetic flux sensor located between. The magnetic flux sensor senses a variable magnetic field representative of the position of the attached movable object as the magnets move. The magnets have opposite polarities on either sides of the central portion.

Description:
CROSS REFERENCE TO RELATED AND CO-PENDING APPLICATIONS 
     This application is a Continuation in Part of U.S. patent application Ser. No. 09/208,296 filed Dec. 9, 1998 titled, Non Contacting Position Sensor using Bi-polar Tapered Magnets and is herein incorporated by reference. 
     This application is related to U.S. patent application Ser. No. 09/335,546 filed Jun. 18, 1999 titled, Non Contacting Position Sensor using Radial Bi-polar Tapered Magnets and is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     I. Technical Field 
     This invention relates, in general, to non-contacting position sensors. More particularly, this invention relates to the magnetic configuration of non-contacting position sensors utilizing Hall effect devices, particularly those used in automotive environments. 
     II. Background Art 
     Electronic devices are an increasingly ubiquitous part of everyday life. Electronic devices and components are presently integrated in a large number of products, including products traditionally thought of as primarily mechanical in nature, such as automobiles. This trend is almost certain to continue. To successfully integrate electronic and mechanical components, some type of interface between the two technologies is required. Generally this interface is accomplished using devices such as sensors and actuators. 
     Position sensing is used to electronically monitor the position or movement of a mechanical component. The position sensor produces an electrical signal that varies as the position of the component in question varies. Electrical position sensors are an important part of innumerable products. For example, position sensors allow the status of various automotive actuations and processes to be monitored and controlled electronically. 
     A position sensor must be accurate, in that it must give an appropriate electrical signal based upon the position measured. If inaccurate, a position sensor will hinder the proper evaluation and control of the position of the component being monitored. 
     A position sensor must also be adequately precise in its measurement. The precision needed in measuring a position will obviously vary depending upon the particular circumstances of use. For some purposes only a rough indication of position is necessary. For instance, an indication of whether a valve is mostly open or mostly closed. In other applications more precise indication of position may be needed. 
     A position sensor must also be sufficiently durable for the environment in which it is placed. For example, a position sensor used on an automotive valve will experience almost constant movement while the automobile is in operation. Such a position sensor must be constructed of mechanical and electrical components which are assembled in such a manner as to allow it to remain sufficiently accurate and precise during its projected lifetime, despite considerable mechanical vibrations and thermal extremes and gradients. 
     In the past, position sensors were typically of the “contact” variety. A contacting position sensor requires physical contact between a signal generator and a sensing element to produce the electrical signal. Contacting position sensors typically consist of a potentiometer to produce electrical signals that vary as a function of the component&#39;s position. Contacting position sensors are generally accurate and precise. Unfortunately, the wear due to contact during movement of contacting position sensors has limited their durability. Also, the friction resulting from the contact can result in the sensor affecting the operation of the component. Further, water intrusion into a potentiometric sensor can disable the sensor. 
     One important advancement in sensor technology has been the development of non-contacting position sensors. As a general proposition, a non-contacting position sensor (“NPS”) does not require physical contact between the signal generator and the sensing element. As presented here, an NPS utilizes magnets to generate magnetic fields that vary as a function of position and devices to detect varying magnetic fields to measure the position of the component to be monitored. Often, a Hall effect device is used to produce an electrical signal that is dependent upon the magnitude and polarity of the magnetic flux incident upon the device. The Hall effect device may be physically attached to the component to be monitored and move relative to the stationary magnets as the component moves. Conversely, the Hall effect device may be stationary with the magnets affixed to the component to be monitored. In either case, the position of the component to be monitored can be determined by the electrical signal produced by the Hall effect device. 
     The use of an NPS presents several distinct advantages over the use of the contacting position sensor. Because an NPS does not require physical contact between the signal generator and the sensing element, there is less physical wear during operation, resulting in greater durability of the sensor. The use of an NPS is also advantageous because the lack of any physical contact between the items being monitored and the sensor itself results in reduced drag upon the component by the sensor. Because the NPS does not rely upon an electrical contact, there is reduced susceptibility to electrical shorting caused by water intrusion. 
     While the use of an NPS presents several advantages, there are also several disadvantages that must be overcome in order for an NPS to be a satisfactory position sensor for many applications. Magnetic irregularities or imperfections may compromise the precision and accuracy of an NPS. The accuracy and precision of an NPS may also be affected by the numerous mechanical vibrations and perturbations likely be to experienced by the sensor. Because there is no physical contact between the item to be monitored and the sensor, it is possible for them to be knocked out of alignment by such vibrations and perturbations. A misalignment will result in the measured magnetic field at any particular location not being what it would be in the original alignment. Because the measured magnetic field will be different than that when properly aligned, the perceived position will be inaccurate. Linearity of magnetic field strength and the resulting signal is also a concern. 
     Some of these challenges to the use of an NPS have been addressed in existing devices, most notably the device of U.S. Pat. No. 5,712,561 issued to MsCurley, et al and assigned to the CTS Corporation, herein incorporated by reference. There remains, however, a continuing need for a more precise determination of physical location of an item based upon the measured magnetic field at a location. Most particularly, a new type of non-contacting position sensor is needed for use in linear motion applications which displays minimal deviations due to changes in temperature and maximum linearity of the magnetic field. 
     SUMMARY OF THE INVENTION 
     The present invention provides a sensor for sensing the movement of an attached movable object. The sensor includes a first and second magnet located adjacent each other and attached to the movable object. Each magnet has a central portion that is thinner than both ends of the first and second magnets. A first and second pole piece has a first end and a second end. The first ends are located spaced apart in parallel relationship about the central portion. The second ends are located spaced apart. A first and second air gap is formed between the first ends and the magnets. A magnetic flux sensor is positioned between the second ends for sensing a variable magnetic field representative of the position of the attached movable object as the first and second magnets move. The first and second magnets have a first polarity on one side of the central portion and a substantially opposite second polarity on the other side of the central portion. The first and second magnets each have a slot in the central portions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a linear position sensor using a bipolar tapered magnet. 
     FIG. 2 illustrates a side view of FIG.  1 . 
    
    
     It is noted that the drawings of the invention are not to scale. The drawings are intended to depict only typical embodiments of the invention, and therefore should not be considered as limiting the scope of the invention. The invention will be described with additional specificity and detail through the accompanying drawings. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1 and 2 illustrate a linear non-contacting position sensor (NPS) using a bipolar tapered magnet. The NPS of the preferred embodiment is particularly adapted for use in monitoring the linear position of a component. Sensor  10  includes a housing  12 . A shaft  14  is attached to a bow tie shaped magnetic assembly  20 . Shaft  14  is formed from a non-ferrous material such as a plastic. The magnetic assembly  20  includes an upper V-shaped tapered magnet  22  and a lower V-shaped tapered magnet  24 . The magnets  22  and  24  have thick ends that taper to a central portion  25  that is thinner than the ends. Magnets  22  and  24  are separated by a transition region  26  where the polarity of the magnets changes. The magnets are formed of bonded ferrite or other magnetic material. Magnetic assembly  20  can be attached to a shaft  14  by an adhesive or by other means. Upper magnet  22  has a north polarity region  22 A and a south polarity region  22 B. Lower magnet  24  has a north polarity region  24 A and a south polarity region  24 B. The north polarity regions  22 A and  24 A are seperated from the south polarity regions  22 B and  24 B by a transition region  27  where the polarity of the magnets changes. 
     Magnet  22  has an upper slot  28  and magnet  24  has a lower slot  30  formed therein. Slots  28  and  30  are located at the narrow part of magnetic assembly  20 . An upper air gap  32  is formed in the area between the V of upper magnet  22  above slot  28 . Similarly, a lower air gap  34  is formed in the area between the V of lower magnet  24  below slot  30 . As shall be described more fully below, the magnets create a magnetic field that varies in a substantially linear fashion as the magnets are moved along axis  60 . 
     An L shaped upper pole piece  40  and an L shaped lower pole piece  42  are held by housing  12 . Pole piece  40  has a first arm  40 A and a second arm  40 B. Pole piece  42  has a first arm  42 A and a second arm  42 B. Pole pieces  40  and  42  are made from a magnetically permeable material such as stainless steel and may be insert molded to the housing. Pole pieces  40  and  42  conduct magnetic flux  80  from the magnets in a loop. Flux  80  originating in magnet  22 A travels across gap  32 , through pole piece  40 , hall device  52 , pole piece  42 , gap  34  and magnet  24 B completing the loop. 
     A magnetic flux sensor, such as a Hall effect device  50  is located between second arms  40 B and  42 B. The Hall effect device  50  is carried upon a hybrid circuit substrate or printed circuit board (not shown). Wire leads  52  are connected to Hall effect device to connect with a hybrid circuit substrate or printed circuit board. The hall effect device is preferably be positioned toward the center of the arms  40 B and  42 B to avoid edge irregularities in the magnetic field created by the magnets. The Hall effect device  50  and pole pieces  40  and  42  are stationary while the magnets  22  and  24  move along axis  60 . The hall effect device and pole pieces are contained within housing  12 . 
     As the magnetic field strength generated by the magnets and detected by the Hall effect device varies with linear motion, the signal produced by the Hall effect device changes accordingly, allowing the position of the attached object to be monitored to be ascertained. 
     Magnets  22  and  24  produces a varying magnetic flux field as indicated by flux density vectors  80 . The polarity of the magnetic field generated by the magnet  22 A is indicated by the upward direction of the vectors  80 . Likewise, the strength of the magnetic flux field is indicated by the length of the vectors. The magnetic flux field generated by the magnet  22 A decreases in strength from the thick end to slot  28 . Magnets  22 B,  24 A and  24 B are similarly designed as illustrated. 
     Upper and lower slots  28  and  30  increase the linearity of the magnetic field within airgaps  32  and  34 . As a practical matter, the thin end of a magnet will always have a finite thickness and generate a non-zero magnetic field. If the thin ends of two magnets having opposite polarities are immediately adjacent, there will be a discontinuity of the combined magnetic field about the center of the air gaps. By providing a slot between adjacent thin ends of the tapered magnets, this discontinuity and other problems affecting linearity of sensor output may be avoided. Further, slots  28  and  30  allow for a consistent neutral zone, about the center of the slots, independent of magnetizing property variations, which aids linearity of sensor output. The slots  28  and  30  may be created during the molding of the magnet. If the magnets are formed individually, the gaps may be formed by appropriately positioning individual magnets. Alternatively, magnetic material may be removed to create the gaps after the magnets have been formed. 
     A magnetic flux sensor such as a Hall effect device  50  is positioned between arms  40 B and  42 B. Motion of shaft  14  causes relative movement between the magnets  22  and  24  and the pole pieces  40  and  42 . The magnetic field in the pole pieces is the sum of the magnetic fields generated by the magnet regions  22 A,  22 B,  24 A, and  24 B. The polarity and strength of the combined magnetic field varies along axis  60 . 
     The magnetic field detected by the Hall effect device  52  as magnets  22  and  24  move along axis  60  will be large and in an upward direction at the thick ends of magnet regions  22 A and  24 B and decrease substantially linearly as it approaches slots  28  and  30 , at which point the magnetic field will be substantially zero. As the magnets continue to move along axis  60 , the polarity of the magnetic field detected reverses with substantially linearly increasing magnitude. 
     This variance of magnetic field polarity and strength as a function of a linear position causes the electrical output signal from hall device  50  to vary. The signal changes substantially linearly from a large positive signal at the thick ends of regions  22 A and  24 B, passes through zero at slots  28  and  30  and becomes a large negative signal at the thick ends of magnet regions  22 B and  24 A. The signal produced by the Hall effect device  50  is proportional to the magnetic flux density carried by the pole pieces  40  and  42 . The magnet polarity directions could be reversed, if desired, resulting in a signal of opposite slope. The output could be offset to yield a positive voltage a both thick ends, from +0.5 to +4.5 volts for example. 
     The present invention is useful for measuring the linear movement of an attached object. The position sensor has improved linearity of magnetic field and the resulting signal as well as decreased signal variance due to temperature changes. This results from the movement of the magnets, about the center of the slots  28  and  30 . This is also called the zero Gauss, point. This improves the ability of the sensor to compensate for temperature changes by eliminating previously needed circuitry and additional processing complexities. The position sensor in accordance with the present invention may be affixed to the object to be monitored in any appropriate fashion. 
     It is to be appreciated that numerous variations from the example embodiments described herein may be made without parting from the scope of the invention. The magnets themselves may be individual magnets, or may be magnetic portions of larger magnets. The magnet gaps between the thin ends of adjacent magnets or magnet regions may be formed in any of a variety of ways. The precise type of apparatus the position sensor is attached to is immaterial to the present invention. Likewise, the particular type and variety of magnetic flux density sensor used in connection with a non-contacting position sensor in accordance with the present invention is immaterial. A variety of mechanisms may be used to connect the magnet assembly to the object to be monitored. The electrical connections and the methods of establishing them may vary from those shown in accordance with the preferred embodiment. One skilled in the art will likewise readily ascertain numerous other variations that may easily be made without departing from the spirit and scope of the present invention.