Abstract:
A rotary encoder includes a magnet disposed on a rotational axis of the encoder. The magnet is polarized transversely to the rotational axis. A first magnetic sensor is disposed on the rotational axis proximate the on-axis magnet. A magnet ring is disposed rotationally coaxially with the rotational axis and has a selected diametric distance from the axis. The magnet ring has a plurality of alternatingly polarized magnets. A number of pole pairs in the magnet ring is selected to match an angular resolution of the first magnetic sensor. A second magnetic sensor is disposed proximate the magnet ring.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Priority is claimed from U.S. Provisional Application No. 61/446,100 filed on Feb. 24, 2011. The foregoing application is incorporated herein by reference in its entirety. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The invention relates generally to the field of rotary orientation encoders. More specifically, the invention relates to rotary encoders having high angular resolution and absolute orientation determination capability. 
         [0005]    2. Background Art 
         [0006]    Rotary encoders provide information about the angular position of rotating device with respect to a selected reference. One type of such encoder uses one or more magnets rotatably mounted with respect to a magnetic field sensor. When the magnet is mounted to a shaft, for example, the sensor provides a signal related to the rotation angle of the shaft with respect to the sensor. There are two type of magnetic rotary encoders, on-axis and off-axis type. 
         [0007]    In on-axis magnetic rotary encoders, an example of which is shown at  10  in  FIG. 1 , a diametrically magnetized magnet  14  is placed on the end of a shaft (not visible in  FIG. 1 , but affixed to baseplate  12 ), with a sensor  16  mounted on the coaxially with the shaft and in close proximity to the magnet  14 . Analysis of the magnetic field from multiple sense elements within the sensor  16  is used to estimate orientation angle of the magnetic field from the magnet  14  with respect to the sensor  16 . On-axis rotary encoders typically provide 8 bits to 12 bits of resolution (2 8 =256 steps per revolution, 2 12 =4096 steps per revolution) and can provide absolute orientation information, i.e., rotary orientation information is directly related to the angle between a line connecting the north to south poles of the magnet  14  and the sensitive axis of the sensor  16 . Rotary orientation information is immediately available on application of power to the sensor  16 . 
         [0008]    In an on-axis sensor, however, the angular resolution however is limited by signal levels within the sensor  16  and the ability of associated signal processing circuitry to covert small voltages induced in the sensing elements to orientation information with sufficient accuracy. 
         [0009]    An off-axis encoder is shown generally at  20  in  FIG. 2 . In the off-axis encoder  20 , a magnet ring  22  with alternating north and south poles (not shown separately) may be mounted on a shaft (not shown for clarity of the illustration), with a sensor  24  mounted in close proximity to the magnet ring  22 . Analysis of the magnetic field from multiple sensing elements within the sensor  24  is used to estimate the sensor  24  position with respect to the closest pair of magnetic poles (not shown separately) on the magnet ring  22 . 
         [0010]    Using the encoder  20  configured as shown in  FIG. 2 , rotary orientation can be measured to a resolution determined by the sensor&#39;s resolution per pole pair and the number of poles pairs on the magnet ring  22 . For example, a magnet ring  22  with ten magnet pole pairs used with a sensor  24  providing 160 point resolution per pole pair, can effectively provide 1600 point resolution within a full rotation of the magnet ring  22 . 
         [0011]    The information available from an off-axis sensor  24 , however, is related only to its position with respect to the closest pole pair. Information as to absolute rotational orientation of the magnet ring is not available. The off-axis encoder  20  is therefore unable to provide absolute orientation with respect to selected reference. 
         [0012]    As a result, in order to obtain absolute rotational orientation information from an off-axis encoder, up on application of power the magnet ring  22  must first be rotated to a known reference position (i.e., “homed”), and the encoder reading recorded in memory. Then all subsequent encoder readings must be referenced against the reference position recorded in memory. The foregoing process is tedious. Further, in case the magnet ring rotational orientation changes during a subsequent power loss or power off condition, the change in orientation may not be recorded. Therefore, upon power up, orientation detected by the sensor  24  may be in error, necessitating another homing operation. 
         [0013]    There is a need for a rotary encoder having high angular resolution and absolute rotational orientation information available from a single device. 
       SUMMARY OF THE INVENTION 
       [0014]    A rotary encoder according to one aspect of the invention includes a magnet disposed on a rotational axis of the encoder. The magnet is polarized transversely to the rotational axis. A first magnetic sensor is disposed on the rotational axis proximate the on-axis magnet. A magnet ring is disposed rotationally coaxially with the rotational axis and has a selected diametric distance from the axis. The magnet ring has a plurality of alternatingly polarized magnets. A number of pole pairs in the magnet ring is selected to match an angular resolution of the first magnetic sensor. A second magnetic sensor is disposed proximate the magnet ring. 
         [0015]    Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  shows a prior art on-axis magnetic rotary encoder. 
           [0017]      FIG. 2  shows a prior art off-axis magnetic rotary encoder. 
           [0018]      FIG. 3  shows an example of a magnetic rotary encoder according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    A general representation of a rotary magnetic encoder is shown in  FIG. 3  at  30 . The encoder  30  includes an on axis magnet  32  and associated on axis sensor  36 . The encoder also includes an off axis magnet ring  34  with opposed pole magnets (not shown separately) and an associated off-axis sensor  38 . The magnet ring  34  may be rotationally coupled to the on-axis magnet  32  or to the rotary input thereto so as to rotate at same speed as the on axis magnet  32 . The encoder  30  uses the absolute orientation information from the on-axis sensor  36  and combines its resolution (multiplicatively) with the resolution available from the off-axis magnet ring  34  and off-axis sensor  38 . 
         [0020]    An important aspect of the structure shown in  FIG. 3  is to match the number of pole pairs on the off-axis magnet ring  34  with the resolution of the on-axis sensor  36 . By matching resolution to the number of pole pairs, the encoder  30  can provide absolute angular orientation information with the combined resolution of the two sensors  36 ,  38 . The realization of such combined resolution is only possible with matching as described above. 
         [0021]    Further, because the orientation information from the on-axis sensor  34  is absolute with respect to the on-axis magnet  32  and is not dependent on stored information, absolute rotary orientation information is available immediately when power to receiving/detecting circuits (not shown) is turned on. 
         [0022]    Two examples are described below, the first provides up to 18 bits of resolution (i.e. 360/2 18 =0.001373291 degrees) and the second provides up to 24 bits of resolution (i.e. 360/2 24 =0.0000214 or 2.14×10 −5  degrees). Lower resolution devices are also possible. 
       Example 1 
       [0023]    An encoder providing absolute position information with 18 bits of resolution combines an 6 bit resolution on-axis sensor (64 steps per revolution) with a 64 magnet pole pair magnet ring and an off-axis sensor with twelve bit resolution (4096 steps per pole pair). In the foregoing combination, the on-axis sensor provides absolute angular position information with 360/64=5.625° resolution. Because the magnet ring has 64 pole pairs, each pole pair precisely covers 5.625°. The off-axis sensor then provides absolute position information within each pole pair to a resolution 5.625/4096=0.001373291 degrees. This is equal to 360/2 18  degrees and may be represented as 18 bits of resolution. 
         [0024]    A high accuracy absolute encoder with 18 bit resolution as described in this example can be implemented using off-the-shelf sensors such as AS5145 for on-axis sensing and AS5311 for off-axis sensing, both from Austriamicrosystems, and off-the-shelf magnets such as p/n 2910041, a 0.236″ diameter×0.098″ thick diametrically polarized magnet from Dexter Magnetic Technologies for on-axis sensing, and 1 mm pole length multi-pole magnet strip (0.125″ wide×0.03″ thick×5.039″ length) also available from Dexter Magnetic Technologies, wrapped on a 1.6041 inch diameter ring. 
       Example 2 
       [0025]    An encoder providing absolute position information with 24 bits resolution combines a 12 bit resolution on-axis absolute sensor (4096 steps/revolution) with a 4096 pole pair magnet ring and a 12 bit resolution off-axis sensor. Following similar calculation as in Example 1, the present example encoder can provide absolute rotation position to 24 bits of resolution, i.e. 0.0000214 degree. 
         [0026]    Understanding the resolution numbers: The foregoing resolution numbers are small and their significance can be better understood by considering them in view of certain physical parameters. 
         [0027]    First, consider the resolution in terms of sensor voltage output pulses (counts) per degree of encoder rotation. A 20 bit resolution encoder provides 2094 encoder counts per degree of rotation, i.e., one degree divided 2094 times. A 24 bit resolution encoder provides 46728 counts per degree using the same form of calculation. 
         [0028]    By comparison, consider an 8 bit resolution encoder over a full encoder rotation (i.e., 256 steps over 360°). 8 bit resolution of the typical on-axis encoder cannot resolve one degree. A 10 bit resolution encoder (1024 steps over 360°) provides 2.8 counts per degree. A 12 bit resolution encoder provides 11.377 counts per degree of rotation. 
         [0029]    Next, consider the above resolution values in terms of resolving distance at the circumference of a 10 millimeter radius disc turning about its axis. A 20 bit resolution encoder resolves the circumference to 0.06 micrometers (μm). A 24 bit resolution encoder resolves the circumference to 3.73 nanometers (nm). By comparison, consider than an 8 bit resolution encoder resolves the circumference to 245.4 micrometers, and a 12 bit resolution encoder resolves the circumference to 15.3 micrometers. An average human hair of diameter of 0.05 mm held at a distance of 10 mm from the center of rotation covers and angle of 0.2865 degrees. With a 20 bit resolution encoder, the angle covered by the average human hair can be resolved into 834 divisions. With a 24 bit resolution encoder, the same angle can be divided into 13,351 divisions. 
         [0030]    Some non-limiting uses for a high resolution rotary encoder according to the invention include the following 
         [0031]    Medical: With increasing use of robotics for remote surgery techniques, extremely well controlled movement of remotely controlled implements have become essential. This may be in the areas such as ophthalmology or neurology where manipulation of retinal cells or nerve endings require movements with microscopic resolution. In order to effect these movements, which are far finer than is possible with a human hand with eye coordination, computers are used to move actuators in concert with feedback from suitable sensors. High resolution encoders can assist the computer, and therefore the surgeon, in effecting such very fine movements. 
         [0032]    Semiconductor fabrication: Systems for fabrication of semiconductor devices rely on of fine movement of the silicon wafer and manipulator arms. These movements are regulated by means of position feedback. High resolution encoders by virtue of their noncontact nature are suitable in these applications since they reduce contamination from wear products. 
         [0033]    Aerospace: High resolution angular position feedback can be used for precise targeting and for antenna positioning. 
         [0034]    Satellite telemetry: Satellite communication antenna dishes need to precisely track orbiting satellites. Satellite trajectory combined with precise angle feedback from a sensor mounted to the antenna and power spectrum from the antenna can assist precise tracking. 
         [0035]    While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.