Patent Publication Number: US-2007096857-A1

Title: Helmholtz coil system

Description:
GOVERNMENT LICENSE RIGHTS  
      The U.S. Government may have certain rights in the present invention as provided for by the terms of Contract No. DL-H-546270 awarded by the Charles Stark Draper Laboratory. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates generally to stray magnetic field testing, and more specifically, but not exclusively, to a gimbaled Helmholtz coil system that enables testing of components in a uniform magnetic field with precise and repeatable positioning and orientation of a device under test (DUT).  
     BACKGROUND OF THE INVENTION  
      A typical Helmholtz coil is a pair of similar coils, which are mounted on a common axis at a fixed distance apart. Essentially, passing equal currents through the two coils generates a highly uniform magnetic field within a limited space about the centroid between the coils. Thus, Helmholtz coils are ideal for use in stray magnetic field testing of a DUT, and can produce test results that are accurate and repeatable to an appreciable extent.  
      In this regard, a significant problem that arises with existing Helmholtz coil arrangements is that the test results are accurate and repeatable only as long as the position and orientation of the DUT can be maintained and repeated within the uniform portion of the magnetic field. To ensure maximum magnetic field uniformity across the DUT, the centroid of the DUT should be substantially positioned and maintained at the centroid of the magnetic coils. In other words, for maximum test accuracy and repeatability, the position and orientation of the DUT relative to the two coils generating the magnetic field have to be precisely maintained and repeated. Existing Helmholtz coil test arrangements provide no means for positioning and orienting a DUT between their coils. Additionally, the existing Helmholtz coil test arrangements are limited because the test wiring arrangements being used do not allow the DUT to be rotated for testing more than 360 degrees within the plane involved. Therefore, it would be advantageous to provide an improved Helmholtz coil system, which would allow testing of components in a uniform magnetic field with precise and repeatable centroid placement and angular displacement about any axis. As described in detail below, the present invention provides an improved Helmholtz coil system, which resolves the above-described DUT positioning accuracy and repeatability test problems of the existing Helmholtz coil arrangements and other related problems.  
     SUMMARY OF THE INVENTION  
      The present invention provides an improved Helmholtz coil test system, which allows testing of a DUT in a uniform DC or AC magnetic field with precise centroid placement and angular displacement about three independent axes. In accordance with a preferred embodiment of the present invention, a Helmholtz coil with a nonmagnetic 3-gimbaled positioning system is provided, which includes a base plate that supports two coils arranged perpendicular to the base, and a system of three nonmagnetic gimbals arranged in the magnetic field between the two coils. The gimbaled system includes an outer mount that is arranged perpendicular to the base plate and substantially in the center of the magnetic field. The gimbaled system includes three lockable gimbals, which can rotate on axes at right angles with respect to each other so as to allow a full 360 degrees of angular displacement within the x, y and z planes and also be locked for stabilization at any position therebetween. Thus, in accordance with teachings of the present invention, a DUT is mounted at the center of a test printed wiring assembly (PWA) that is attached to the inner-most or center gimbal, one or more of the three gimbals is moved and locked so as to position the DUT at a desired orientation, and power is applied to the Helmholtz coil system to generate a uniform stray magnetic field around the DUT. Also, in accordance with a second embodiment of the present invention, a set of slip rings can be provided with the gimbaled Helmholtz coil positioning system, which enables transmission of test measurement signals from the DUT to an external connection of the Helmholtz coil system and allows more than 360 degrees of displacement of the component in any of the x, y and z planes. In accordance with a third embodiment of the present invention, the coil currents and gimbal positions are driven under computer control and integrated with the DUT tester to further enhance the repeatability and automation of AC and DC stray magnetic field testing in terms of applied magnetic field strength, frequency, orientation, sequence and rates of change.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:  
       FIGS. 1A and 1B  are related drawings that show a pictorial representation of an example gimbaled Helmholtz coil test system, which can be used to implement a preferred embodiment of the present invention;  
       FIGS. 2A-2F  are related drawings that depict more details of the primary components of the example gimbaled Helmholtz coil test system shown in  FIGS. 1A and 1B ; and  
       FIGS. 3A and 3B  are related drawings that depict a right-side view and top view, respectively, of an example gimbaled Helmholtz coil test system with three displaced gimbals, which further illustrate the example embodiment shown in  FIGS. 1A and 1B .  
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT  
      With reference now to the figures,  FIGS. 1A and 1B  are related drawings that show a pictorial representation of an example gimbaled Helmholtz coil system  100 , which can be used to implement a preferred embodiment of the present invention. As shown,  FIG. 1A  depicts a perspective, front view of example gimbaled Helmholtz coil system  100 , and  FIG. 1B  depicts system  100  in a perspective, right side view. Referring to  FIG. 1A and 1B  for this example embodiment, gimbaled Helmholtz coil system  100  includes a base unit  102 . For clarity, a more detailed drawing of base unit  102  is depicted as base plate  202  in  FIG. 2A . In any event, for this example embodiment, base unit  102  can be made of an Aluminum material, but the present invention is not intended to be so limited and can be made of any suitable material (e.g., ceramic, plastic, non-magnetic material, etc.) that does not interfere significantly with the uniformity and/or strength of the magnetic field generated by gimbaled Helmholtz coil system  100 . As such, Aluminum or a similar material is preferable for base unit  102 , because the high thermal conductivity of the Aluminum material serves as a heat sink to draw away and help dissipate the heat generated by the magnetic coils of gimbaled Helmholtz coil system  100 . Also, as shown, a plurality of lengthwise slots  103  can be milled into base unit  102 , which effectively increases the surface area of base unit  102  and enhances its cooling effectiveness. At this point, it should be understood that the present invention is not intended to be limited to the particular material used for any component of gimbaled Helmholtz coil system  100 . As a practical matter, all of the major components of gimbaled Helmholtz coil system  100  may be made from the same type of suitable material (e.g., Aluminum, ceramic, plastic, non-magnetic material, etc.).  
      For this example embodiment, gimbaled Helmholtz coil system  100  also includes a plurality of coil ring base mount units  104   a ,  104   b . Again, for clarity, a more detailed drawing of one example of the coil ring base mount units  104   a ,  104   b  is depicted as coil ring base mount  204  in  FIG. 2B . The coil ring base mount units  104   a ,  104   b  are affixed to the upper surface of base unit  102 . As shown, each coil ring base mount unit  104   a ,  104   b  is mounted substantially at the center of the upper surface of base unit  102  and flush with a respective side of base unit  102 . Similar to base unit  102 , the coil ring base mount units  104   a ,  104   b  can be made from an Aluminum material or a material with similar heat transference and magnetic properties as Aluminum. Preferably, the coil ring base mount units  104   a ,  104   b  are affixed to base unit  102  with non-metallic screws (e.g., plastic screws).  
      Gimbaled Helmholtz coil system  100  also includes a plurality of coil ring units  108 ,  110  affixed to respective coil ring base mount units  104   a ,  104   b . A more detailed drawing of one example of the coil ring units  108 ,  110  is depicted as coil ring  208  (and  210 ) in  FIG. 2C . As shown, the outside, bottom portion of a coil ring unit  108 ,  110  is affixed (e.g., preferably with non-magnetic screws) to the inside surface of a respective coil ring base mount unit  104   a ,  104   b . Similar to the other components of gimbaled Helmholtz coil system  100 , each coil ring unit  108 ,  110  can be made of Aluminum or a similar material. In operation, each of the coil ring units  108 ,  110  includes one of the coils (not shown) that make up a Helmholtz coil. Thus, applying suitable currents to the coils wound around coil ring units  108 ,  110  functions to generate a uniform magnetic field in the space between coil ring units  108 ,  110 .  
      Notably, as a practical matter (but not intended as an architectural limitation to be imposed on the scope or coverage of the present invention), for the fabricated coils, small slots can be arranged uniformly around each of the coil ring units. These slots provide secure, accurate and uniform placement of a non-magnetic thread used in a preliminary characterization of a respective coil. Generally, it is difficult to accurately characterize the coils prior to inserting the gimbaled apparatus without such thread slots. Also, to facilitate winding of the coils and for accurate characterization of the field after the Helmholtz coils have been constructed, a bracing system can be utilized independent of the entire setup. If the coils are wound as a series connection, that is, as one long continuous piece of wire between both coils, the weight of the system and the tendency for the first coil to unravel and/or twist while winding the second coil makes winding difficult without the use of a brace. A part of the bracing system uses two small (Aluminum) rectangular pieces (not shown), which provide enhanced support during the winding process and characterization of the coils. Once the coils have been wound and are mounted on the base plate, the coils are preferably characterized prior to installation of the gimbaled apparatus. This same brace setup can be employed to support the coils during characterization.  
      For this example embodiment, a gimbal support unit  106  is also affixed to the upper surface of base unit  102  and arranged substantially midway between coil ring base mount units  104   a ,  104   b . A more detailed drawing of an example of the gimbal support unit  106  is depicted as gimbal support  206  in  FIG. 2D . Preferably, gimbal support unit  106  is affixed to base unit  102  with non-magnetic screws, and can be made of Aluminum or a similar material. For maximum stability, a plurality of coil supports (e.g.,  118   a - 118   d ) are affixed to each coil ring unit  108 ,  110  and the gimbal support unit  106 . Notably, referring to  FIG. 1B , although only four coil supports  118   a - 118   d  are shown in the right-side view, it may be assumed that two other coil supports are each affixed to a respective coil ring unit  108 ,  110  and the gimbal support unit  106  on the opposite side of gimbaled Helmholtz coil system  100  and would be seen in a left-side view. The coil supports  118   a - 118   d  are preferably affixed to the coil ring units  108 ,  110  and the gimbal support unit  106  with non-magnetic screws, and can be made of Aluminum or a similar material. Also, although two sets of holes for connections are shown at each end of the base of gimbal support unit  106 , one such set of holes may be provided, as long as the size of the holes is large enough to accommodate a suitably sized connector.  
      Notably, gimbaled Helmholtz coil system  100  also includes a plurality of gimbal units  112 ,  114 ,  116 . For this example embodiment, gimbal unit  112  (e.g., “outer” gimbal unit) is rotatably affixed to gimbal support unit  106 , gimbal unit  114  (e.g., “middle” gimbal unit) is rotatably affixed to gimbal unit  112 , and gimbal unit  116  (e.g., “inner” gimbal unit) is rotatably affixed to gimbal unit  114 . A more detailed drawing of an example of the outer and middle gimbal units  112 ,  114  is depicted as gimbal  212 ,  214  in  FIG. 2E . A more detailed drawing of an example of the inner gimbal unit  116  is depicted as gimbal  216  in  FIG. 2F . For clarity, gimbal  216  in  FIG. 2F  is shown without a circular plate or test PWA used for mounting a DUT (e.g., DUT  132 ), which covers the area circumscribed by the circumference of gimbal unit  116 . Thus, as shown in  FIGS. 1A and 1B , all of the gimbal units  112 ,  114 ,  116  are supported by gimbal support unit  106  and arranged substantially in the center of the uniform stray magnetic field generated by the Helmholtz coils arranged in coil ring units  108 ,  110 . The gimbal units  112 ,  114 ,  116  can be made from Aluminum or other suitable non-magnetic materials.  
      For this example embodiment, the rotational positions of the gimbal units  112 ,  114 ,  116  are controlled by a combination of pins and lock tabs. For example, referring now to  FIG. 2E  for clarity, a pair of recesses  213  are milled into the outer gimbal and middle gimbal (e.g., gimbal units  112 ,  114  in  FIGS. 1A and 1B ). Actually, as illustrated by  FIG. 1A , four such recesses are milled into the outer gimbal unit  112 , and two such recesses are milled into the middle gimbal unit  114 . In any event, a pin (e.g., only one pin  122  of two such pins is shown in the view of  FIG. 1A ) is disposed in the channel (e.g., channel  215  in  FIG. 2 ) of each of the recesses  213 . A lock tab  120   a ,  120   b  is disposed in a respective recess (e.g.,  213 ) and affixed to the outer gimbal unit  112  preferably with non-magnetic screws. One end of each pin is fixedly attached to the gimbal support unit  106 , and the other end of each pin is disposed in the channel (e.g.,  215 ) between the respective lock tab  120   a ,  120   b  and the outer gimbal unit  112 . Thus, the outer gimbal unit  112  can rotate (e.g., in two directions) about an axis formed by a straight line drawn between the two pins, and the rotational position of the outer gimbal unit  112  can be controlled by increasing or decreasing the pressure of the lock tabs  120   a ,  120   b  against the respective pins (e.g., by tightening the screws to lock the outer gimbal unit  112  in place).  
      Similarly, with respect to the middle gimbal unit  114 , a lock tab  124   a ,  124   b  is disposed in a respective recess (e.g.,  213 ) and affixed to the outer gimbal unit  112  (e.g., with non-magnetic screws). Each pin of a plurality of pins  126   a ,  126   b  is disposed in the channel (e.g.,  215 ) of a respective recess  213 . One end of each pin  126   a ,  126   b  is fixedly attached to the middle gimbal unit  114 , and the other end of each pin is disposed in the channel (e.g.,  215 ) between the respective lock tab  124   a ,  124   b  and the outer gimbal unit  112 . Thus, the middle gimbal unit  114  can rotate (e.g., in two directions) about an axis formed by a straight line drawn between the two pins  126   a ,  126   b , and the rotational position of the middle gimbal unit  114  can be controlled by increasing or decreasing the pressure of the lock tabs  124   a ,  124   b  against the respective pins  126   a ,  126   b . For example, the lock tabs can be tightened to lock the position of the middle gimbal unit  114  in place.  
      With respect to the inner gimbal unit  116 , a lock tab  128   a ,  128   b  is disposed in a respective recess (e.g.,  213 ) and affixed to the middle gimbal unit  114  (e.g., with non-metallic screws). Each pin of a plurality of pins  130   a ,  130   b  is disposed in the channel (e.g.,  215 ) of a respective recess  213 . One end of each pin  130   a ,  130   b  is fixedly attached to the inner gimbal unit  116 , and the other end of each pin is disposed in the channel (e.g.,  215 ) between the respective lock tab  128   a ,  128   b  and the middle gimbal unit  114 . Thus, the inner gimbal unit  116  can rotate (e.g., in two directions) about an axis formed by a straight line drawn between the two pins  130   a ,  130   b , and the rotational position of the inner gimbal unit  116  can be controlled by increasing or decreasing the pressure of the lock tabs  128   a ,  128   b  against the respective pins  130   a ,  130   b . For this example embodiment, the lock tabs can be tightened to lock the position of the inner gimbal unit  116  in place.  
      Notably, in accordance with a second embodiment of the present invention, a set of slip rings can be provided with the gimbaled Helmholtz coil system  100 , which enables transmission of test measurement signals from a test component mounted on the inner gimbal unit to an external connection of the gimbaled Helmholtz coil system and allows more than 360 degrees of displacement of the component in any of the x, y and z planes. For example, a suitable slip ring arrangement can be substituted for each of pins  122 ,  126   a , and  130   a , which enables the three gimbal units  112 ,  114 ,  116  to be rotated and also provides a suitable signal conduction path between the inner gimbal unit  116  and the gimbal support unit  106 . Thus, for this example embodiment, one or more test leads can be connected from a test component (e.g.,  132 ) to a suitable connector mounted on the rotatable inner gimbal unit  116 , and the slip rings will provide a signal conduction path from that (internal) connector via the rotatable middle and outer gimbals  114 ,  112 , respectively, to a second (external) connector mounted on the fixed gimbal support unit  106 .  
       FIGS. 3A and 3B  are related drawings that depict a right-side view and top view, respectively, of a gimbaled Helmholtz coil system  300  with three displaced gimbals, which further illustrate the above-described example embodiment shown in  FIGS. 1A and 1B . Referring to  FIGS. 3A and 3B , for this example embodiment, gimbaled Helmholtz coil system  300  includes a base unit  302 , two coil ring base mount units  304   a ,  304   b , a gimbal support unit  306 , two coil ring units  308 ,  310 , an outer gimbal unit  312 , a middle gimbal unit  314 , and an inner gimbal unit  316 . Notably, as shown, gimbaled Helmholtz coil system  300  includes three lockable gimbal units, which can rotate on axes at right angles with respect to each other to allow a full 360 degrees of displacement in the x, y and z planes and also be locked for stabilization at any position therebetween. Thus, in accordance with teachings of the present invention, a DUT can be secured to a plate or a PWA attached to the inner gimbal unit  316 , one or more of the three gimbal units  312 ,  314 ,  316  can be moved and locked so as to position the component at a point associated with a desired set of coordinates in the x, y and z planes in the space between the two coil ring units  308 ,  310 . Then, power can be applied to the coils (not shown) disposed in the coil ring units  308 ,  310  in order to generate a magnetic field between the two coils.  
      Note that the sizes of the gimbal support and gimbals of the present invention could be increased just up to the point where the coils would interfere with gimbal rotation. This action would provide more room for a larger test PWA to be attached to the inner gimbal.  
      It is important to note that while the present invention has been described in the context of a fully functioning gimbaled Helmholtz coil system, those of ordinary skill in the art will appreciate that the processes of the present invention are capable of being distributed in the form of a computer readable medium of instructions and a variety of forms and that the present invention applies equally regardless of the particular type of signal bearing media actually used to carry out the distribution. Examples of computer readable media include recordable-type media, such as a floppy disk, a hard disk drive, a RAM, CD-ROMs, DVD-ROMs, and transmission-type media, such as digital and analog communications links, wired or wireless communications links using transmission forms, such as, for example, radio frequency and light wave transmissions. The computer readable media may take the form of coded formats that are decoded for actual use in a particular Helmholtz coil system.  
      The description of the present invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. These embodiments were chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.