Abstract:
An apparatus for sensing rotary position that is particularly suited for electrical rotary actuators. The sensing apparatus comprises a magnet and a hall effect detector, both of which are arranged along the axis of rotation. The hall effect detector senses the angle of the return loop of magnetic flux lines from the north to the south pole of the magnet. When the magnet rotates relative to the hall effect detector, the angle of the magnetic flux lines changes which is detected by the hall effect detector. The arrangement of the magnet and the hall effect detector on the shaft axis of a rotary actuator avoids the magnetic flux line interference that is naturally produced by electrical actuators.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to apparatus and methods for sensing rotary or angular position, and specifically magnetic field sensors such as hall effect detectors with an arrangement particularly suited for electrical rotary actuators. 
     BACKGROUND OF THE INVENTION 
     There are a variety of known magnetic sensor technologies including magnetoinductive sensors, flux-gate sensors, magnetoresitive sensors and hall effect detectors. Hall effect detectors are the smallest and least expensive of these sensors. The operating theory of a hall effect detector is simple. If the magnetic flux lines of a magnetic field impinge pependicularly on a thin panel carrying a current, a voltage develops across the sides of the panel which can be measured. Due to compact size and cost considerations, hall effect detectors have been widely used in electrical rotary actuators for industrial applications. 
     One traditional method of employing a hall effect detector for sensing rotary position has been to offset the hall effect detector from the axis of rotation and employ a ring magnet (e.g. having two north poles and two south poles) about the shaft. The hall effect detector and ring magnet are arranged in a plane perpendicular to the shaft axis such that when the shaft rotates, the faces of the north and south poles cyclically pass directly in front of the sensing surface of the hall effect detector. One of the significant problems with this approach occurs when the sensor is closely coupled to an electromagnetic actuator. In this application, magnetic leakage fields develop due to the wire coil and emit out the end of the device. This can interfere with the sensor signal. This leads to accuracy problems. The sensor output is also sensitive to proper sensor and ring magnet positioning (e.g. providing the proper gap between the sensor and the ring magnet). 
     An attempt to solve this problem has been to mount a yoke to the end of the shaft. The yoke carries two magnets on opposite sides which are adapted to rotate around the hall effect detector which is mounted to the stator stationary on the axis. The yoke thus surrounds the hall effect detector such that the hall effect detector and the magnets are arranged in a plane perpendicular to the shaft axis. When the shaft rotates, the faces of north and south poles cyclically pass directly in front of the sensing surface of the hall effect detector. Again, this method is sensitive to proper placement and the gap between the magnets and the sensor. With the hall effect detector on the axis, the magnetic leakage fields do not substantial interfere with the sensor signal. However, the implementation of this method requires extra cost and parts of the yoke/magnet assembly. This method also requires extra space at the shaft end to accommodate the yoke which is undesirable for compact applications. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a rotary position sensor and method that overcomes these and other problems existing in the art, and that may be particularly suited for electrical rotary actuators. 
     An apparatus for sensing rotary position, comprising an in line magnet and a hall effect detector or other suitable magnetic sensor. The magnet is axially aligned with the hall effect detector in spaced relation along an axis. The magnet has radially spaced apart north and south poles with the imaginary line between the poles intersecting the axis. With this arrangement, the hall effect detector senses relative rotation between the hall effect detector and the magnetic about the axis. 
     It is a significant aspect of the present invention that the novel rotary position sensing apparatus is incorporated into an electrical actuator in a novel manner. According to this aspect, the electrical actuator includes a stator comprising a lamination stack and wire coils and a rotor adapted to be rotated by the stator. The rotor comprises an output shaft carried by the stator for rotation about an axis. A magnet is fixed to an end of the output shaft and rotates in unison with the shaft. The magnet has a north pole and a south pole which emit a magnetic field having magnetic flux lines traveling in a return loop from the north to the south pole. The return loop intersects the axis. A sensor is mounted in a stationary position on the stator and axially spaced from the magnet along the axis. The sensor senses an angle of magnetic flux lines traveling along the return loop. When the shaft rotates, the magnetic flux lines rotate with the magnet to impinge upon the sensor at different angles such that the sensor has an output related to the angular position of the shaft. In the preferred embodiment, the magnet and sensor are on the center axis so that the stator field interference is minimized. Given the sensing of the return field, it is a further advantage that the sensor is less sensitive to precise positioning of the sensor in the plane normal to the axis of the shaft. 
     Other objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention, and together with the description serve to explain the principles of the invention. In the drawings: 
     FIG. 1 is a cross section of an electrical actuator according to a preferred embodiment of the present invention. 
     FIGS. 2-4 are isometric views of the sensing apparatus according to a preferred embodiment of the present invention with different relative positions between the magnet and sensor among FIGS. 2-4 and with magnetic flux lines being schematically indicated. 
     FIG. 5 is a graph illustrating the sensor output based upon angular position of the magnet (including the positions shown in FIGS. 2-4) and therefore the shaft relative to sensor. 
     FIG. 6 is an enlarged cross section of a portion of FIG. 1 with the magnetic field of the sensor magnet and the magnet leakage field of the electrical actuator being schematically indicated. 
    
    
     While the invention will be described in connection with certain preferred embodiments, there is no intent to limit it to those embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents as included within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For purposes of illustration, and referring to FIGS. 1 and 2, a preferred embodiment of the present invention is illustrated as a magnetic sensor apparatus  10  incorporated into an electrical rotary actuator  12 . The rotary actuator  12  has a lamination stack/wire coils  14  secured within a stator housing  16  for a stator and an output shaft  18  having permanent magnets  20  for the rotor. The shaft  18  is journalled in spaced apart radial bearing sets  22  for rotation about an axis  40 . This particular rotary actuator  12  disclosed herein is of the variable position type adapted to rotate the shaft  18  between two angular positions (and position the shaft in discrete positions therebetween). As will be explained further below, the magnetic sensor apparatus  10  indicates angular position of the rotor, although the rotary actuator may be continuously rotating motor in which the magnetic sensor apparatus  10  would have and output to indicate the number of revolutions. Although one preferred application herein is disclosed, it will be appreciated to those skilled in the art that the magnetic sensor apparatus  10  may also have other applications in other rotary machines to which certain broader claims apply. 
     The stator includes integral electronics in the form of an electronics board  24  mounted in an electronics housing  26 . The electronics housing  26  is secured to the stator housing via vibrations isolators  28 . Further details the vibration isolators and other aspects of the disclosed electrical rotary actuator can be bad to U.S. application Ser. Nos. 09/793,225, 09/795,045 and 09/793,151, (now U.S. Pat. No. 6,467,587) assigned to the present assignee and filed on the same date as the instant application, the entire disclosures of which are hereby incorporated by reference. The electronics is operable to control the position of the shaft  18  as desired. The magnetic sensor apparatus  10  is connected to the integral electronics provide for closed loop control or position verifying feedback. 
     The magnetic sensor apparatus  10  includes a detector or sensor in the form of a hall effect detector  30  and a magnet  32 . The hall effect detector  30  is stationary having a mount  34  secured to the electronics board  24  and a thin film or sensing panel  36 . The hall effect detector  30  and more specifically the panel  36  is aligned on the rotational axis  40  and in a plane parallel to the axis  40 . The hall effect detector  30  detects magnetic flux lines of a magnetic field that impinge perpendicularly on the panel  36 . Specifically, impingement of the magnetic flux lines causes a voltage to develop across the sides of the panel  36  which can be measured and translated into an output representative of the sensed magnetic field. 
     The magnet  32  is mounted to the shaft end  42  by such means a gluing the magnet  32  into a formed recess  44 . The magnet  32  includes a north pole  46  at one face and a south pole  48  at the opposite face. The imaginary line between the north and south poles  46 ,  48  intersects the rotational axis  40  and runs perpendicular to the axis  40  in the disclosed embodiment. As shown schematically in FIGS. 2-4 and  6 , the magnet  32  creates a magnetic field  50  with magnetic flux lines  52  traveling from the north pole  46  to the south pole  48  in a return loop. With the magnet orientation of the disclosed embodiment, the magnetic flux lines  52  intersect the axis  40  perpendicularly. 
     In contrast to prior art arrangements of hall effect detectors and magnets, the magnet  32  and hall effect detector  30  of the disclosed embodiment are axially spaced apart and are both located on the rotational axis  40  as illustrated in the various figures. Instead of sensing the face of a magnet, the hall effect detector  30  of the disclosed embodiment senses the return loop of the flux lines  52 . As indicated above, the hall effect detector  30  detects magnetic flux lines of a magnetic field that impinge perpendicularly on the sensing panel  36 . 
     When the magnet  32  is aligned perpendicularly to the sensing panel  36  as shown in FIG. 2, the magnetic flux lines  52  intersect the sensing panel  36  at a perpendicular angle as shown in FIG.  2 . In this position, the hall effect detector  32  senses the maximum magnetic field emitted by the magnet  32  at the given axial spacing. This defines the maximum voltage differential across the sides of the panel which is indicated in FIG.  5 . For purposes of reference and differentiating different positions, the position of the shaft and magnetic illustrated in FIG. 2 have been designated as the home position or 0° degrees of rotation. 
     As the magnet  32  rotates from the home position and relative to the sensing panel  36 , the magnetic flux lines  52  no longer intersect the sensing panel  36  at a pure perpendicular angle, but instead at an inclined angle. For example as shown in FIG. 3, the magnet  32  has been rotated 45° relative to position of FIG.  2 . At the 45° position, the magnetic flux lines  52  intersect the sensing panel  36  at an inclined angle. At an inclined position, the hall effect detector  30  detects only the perpendicular vector component of the inclined magnetic flux lines  52  and does not sense any vector component running parallel to the sensing surface  36 . Using mathematical trig functions, the voltage magnitude across the sides of the sensing panel  36  at any angular position relative to the home or 0° position can be calculated and is equivalent to the SIN of the angle from the home position times the voltage magnitude at the home position. Thus, at the 45°, the magnitude of the voltage differential is the SIN of 45° times the voltage magnitude at the home position as shown in FIG.  5 . 
     As the magnet  32  continues to rotate, the perpendicular vector component continues to diminish ultimately until it becomes zero at the 90° position illustrated in FIG. 4 at which point the magnetic flux lines  52  run parallel to the sensing panel  36 . This point is also shown in FIG.  5  and the voltage differential across the sensing panel  36  becomes effectively zero or is otherwise negligible. As the magnet  32  continues to rotate, the magnetic flux lines  52  travel in the reverse direction through the sensing panel  36  which provides a negative voltage differential. At this point, it should be evident that angular position of the magnet  32  and therefore the angular position of the shaft  18  to which it is affixed is readily determined by the output of the hall effect detector  30 . As the shaft  18  and magnet  32  rotates a complete 360°, the hall effect detector  30  produces an electrical output in the form of a complete sin wave. By knowing the magnitude of output of the hall effect detector  30  at the home position, the angular position of the magnet  32  and shaft  18  can be determined in relation to the SIN wave illustrated in FIG.  5 . 
     In the disclosed embodiment, the hall effect detector  30  has also been arranged to avoid the magnetic leakage field  54  that is naturally produced as a byproduct of the action of the lamination stack/wire coils  14  of the electrical rotary actuator  12  during operation as indicated in FIG.  6 . As shown in FIG. 6, the magnetic leakage field  54  includes magnetic flux lines  56  that typically run generally parallel to the sensing panel  36  such that there is negligent effect on the output of the hall effect detector  30 . In addition, given the smaller gradient of the return field, the sensor is less sensitive to precise placement in the plane normal to the shaft. 
     The foregoing description of various preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments discussed were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.