Abstract:
A torque sensor has a contoured magnetoelastic element that generates a magnetic field having a shape that exhibits gradual changes rather than sharp peaks in the axial direction, making the torque sensor less sensitive to positional changes between the magnetoelastic element and a magnetometer in the sensor. The element is contoured in any desired shape to modify the magnetic field generated by the element when it is deformed through applied torque. In one embodiment, the element is a magnetic material coating applied to a contoured shaft.

Description:
TECHNICAL FIELD  
         [0001]    The present invention is directed to torque sensors, and more particularly to magnetoelastic torque sensors that measure torque in a shaft by monitoring changes in a magnetic field generated by a magnetoelastic element coupled to the shaft.  
         BACKGROUND OF THE INVENTION  
         [0002]    Torque sensors known in the art rely on a magnetoelastic element attached to a component to sense torsion forces in the component. Deformation in the component caused by applied torque deforms the magnetoelastic element, resulting in a magnetic field that is proportional to the applied torque. A magnetometer disposed near the element detects the magnitude and polarity of the magnetic field, which indicates the magnitude and polarity of the applied torque.  
           [0003]    Magnetoelastic elements applied to cylindrical shafts are normally cylindrical as well to accurately reflect any changes in the shaft caused by applied torque. The magnetic field generated by a cylindrical element, however, tends to have a sharp spike at the measurement location, requiring precise alignment between the magnetometer and the element to obtain accurate readings of the spike. Normal mechanical mounting tolerances may cause slight axial and/or radial misalignments, however, making the magnetometer sensitive to the relative position of the element and the magnetometer. More particularly, if the magnetometer and/or element position shifts slightly and measures the magnetic field in an area other than the spike, the magnetometer will falsely indicate a change in the applied torque. Tighter tolerances may help reduce misreadings, but can be difficult to maintain during normal operations.  
           [0004]    There is a desire for a torque sensor that is less sensitive to the physical alignment between the element and the magnetometer.  
         SUMMARY OF THE INVENTION  
         [0005]    Accordingly, the present invention is directed to a torque sensor and a method of making a torque sensor having a magnetoelastic element that generates a magnetic field having a shape exhibiting a gradual change in the axial direction of the shaft, making the torque sensor less sensitive to positional changes between the element and a magnetometer in the sensor. The element is contoured in any desired shape to modify the magnetic field generated by the element when it is deformed through applied torque.  
           [0006]    In one embodiment, the magnetoelastic element is a magnetic material coating applied to a contoured shaft. Because the coating follows the contours of the shaft, the element formed by the coating will be contoured as well. Further, the coating is intimately connected with the shaft, ensuring that deformations in the element, and thus the resulting magnetic field, will accurately reflect deformations in the shaft. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a perspective exploded view of a shaft and torque sensor according to one embodiment of the invention;  
         [0008]    [0008]FIG. 2 illustrates one example of a shaft with an attached magnetoelastic element and a representative magnetic field according to one embodiment of the invention; and  
         [0009]    [0009]FIG. 3 illustrates one example of a shaft with a attached magnetoelastic element and a representative magnetic field according to another embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0010]    Referring to FIG. 1, a non-compliant magnetoelastic torque sensor  100  measures torque applied to a shaft  102  by sensing magnetic field changes caused by the applied torque. The sensor  100  includes a magnetoelastic element  104  disposed on the shaft  102  and preferably bonded to the shaft  102  so that any torsion force applied to the shaft  102  will also deform the element  104 .  
         [0011]    More particularly, the element  104  is disposed at a desired location on the shaft  102  and encircles the shaft  102 . In one embodiment, the element  104  provides a circumferential magnetic field that has no magnetic polarity in an axial direction if there is no torque applied to the shaft. If torque is applied to the shaft  102 , the resulting stress on the magnetic element causes the direction of the magnetic field to change, providing an axial component of the magnetic field corresponding to the amount of torque applied to the shaft  102 .  
         [0012]    A magnetometer  106  disposed near the element  104  senses the magnetic field change in the element  104  and determines the amount of applied torque from the change. For clarity, FIG. 1 shows a portion of the magnetometer  106  removed to expose the element  104 , but in practice the magnetometer  106  encircles the entire element  104 . In one embodiment, the magnetometer  106  detects the torque applied to the shaft  102  by measuring the axial component of the magnetic field, wherein the magnitude of the axial component represents the amount of torque applied to the shaft  102 . The magnetometer  106  preferably encircles the shaft  102  without touching the element  104 , leaving an air gap between the element  104  and the magnetometer  106 . The magnetometer  106  may be any magnetometer  106  that can generate an output corresponding to the detected magnetic field.  
         [0013]    [0013]FIGS. 2 and 3 illustrate two possible examples of the element  104  and their corresponding magnetic fields  108  according to the invention. As shown in the figures, the element  104  in the inventive torque sensor is contoured  110  rather than cylindrical with straight sides. The contouring  110  causes the magnetic field  108  generated by the element  104  to have a curved rather than spiked shape. Because the magnetic field  108  changes gradually rather than exhibiting a sharp spike, the magnetometer  106  will detect only slight changes in the magnetic field  108  if its position deviates axially from the correct measurement location. The slight changes are easily distinguishable from the larger field changes caused by applied torque. If desired, the magnetometer  106  may be designed to filter out the slight changes caused by misalignment and only output values above a selected threshold known to reflect changes caused by applied torque.  
         [0014]    The element  104  itself may be formed as a coating of magnetic material, such as nickel, nickel alloy, or other magnetic material applied directly to the shaft  102  via powder coating, spray, or other deposition methods. If the element  104  is manufactured by applying material directly to the shaft  102 , any number of known techniques may be used to align the magnetic field from the element  104  so that it can be sensed correctly by the magnetometer  106 . The magnetic field  108  may be oriented by creating a circumferentially directed magnetic flux through, for example, crystallizing the magnetic material in an aligning magnetic field, controlling the velocity and temperature of the material being deposited, applying stress on the shaft as the element is being deposited, or managing thermal expansion of the shaft during the deposition process.  
         [0015]    The specific method used to contour the element  104  depends on the structure of the element  104  itself and the way the element  104  is attached to the shaft  102 . For example, if the element  104  is a magnetic material applied directly to the shaft  102 , the shaft  102  itself may be machined to have the desired contour so that the magnetic material forming the element  104  will follow the contours of the shaft  102 . The specific shape of the contour  110  is not critical as long as the resulting magnetic field  108  has the desired shape characteristics for reducing the torque sensor&#39;s  100  sensitivity to relative positional changes between its components. Possible contour  110  configurations include, for example, an hourglass, a parabola, a hyperbola, two truncated cones, or an ellipse. Once the shaft  102  is contoured, the magnetic coating forming the element  104  will follow the contour of the shaft  102 . In other words, shaping the shaft  102  will, in turn, shape the element  104 .  
         [0016]    Alternatively, the element  104  may be a separate component that is pushed onto the shaft  102  to create a frictional fit. In this case, the contour  110  may be formed either before or after the element  104  is attached to the shaft  102 . Regardless of the specific structure of the element  104 , the element  104  should be intimately bonded with the shaft  102  so that the deformation in the shaft  102  will be accurately reflected in the deformation, and the resulting magnetic field, of the element  102 .  
         [0017]    [0017]FIG. 3 illustrates an alternative embodiment of the element  104 . In this embodiment, the magnetoelastic element  104  has a contour  110  that contains multiple curves, creating a magnetic field  108  having multiple peaks  112 . This example illustrates that the element  104  can be shaped to have any profile that generates a desired magnetic field  108  shape. Apply magnetic material to the shaft  102  to form the element  104  makes it easy for the element  104  to conform to even more complex contours  110  on the shaft  102 .  
         [0018]    Further, contouring the element  104  makes it possible to optimize the magnetic field  108  shape for any given magnetometer  106  by, for example, eliminating any spikes that would introduce position-sensitivity in the torque sensor  100  or improving hysteresis or magnetic field strength characteristics. For example, the element  104  may be contoured to reduce the air gap between the element  104  and the magnetometer  106  if there is an axial displacement between the centerlines of the two elements; the air gap reduction compensates for any reduction in the magnetic field strength caused by the displacement.  
         [0019]    Thus, by contouring the magnetoelastic element so that the magnetic field generated by the element has a more rounded profile, the invention desensitizes the torque sensor to changes in the relative position between the magnetometer and the element. As a result, there can be more relative movement between the magnetometer and the element without causing large changes in the magnetometer reading that falsely indicate a change in the applied torque.  
         [0020]    It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that the method and apparatus within the scope of these claims and their equivalents be covered thereby.