1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-03-15a71 Center Point of Contact: JPLa71 Submitted by: Wil HarkinsSubject: Magnetic Design Control for Science Instruments Practice: Design flight subsystems with low residual dipole magnetic fields to maintain the spacecraft
2、s total static and dynamic magnetic fields within science requirements.Abstract: Preferred Practice for Design from NASA Technical Memorandum 4322A, NASA Reliability Preferred Practices for Design and Test.Benefit:Provided by IHSNot for ResaleNo reproduction or networking permitted without license f
3、rom IHS-,-,-Provides for a magnetically clean spacecraft, which increases the quality and accuracy of interplanetary and planetary magnetic field data gathered during the mission.Implementation Method:Because the dipolar portion of a spacecrafts magnetic field at its magnetometer experiment sensor l
4、ocation dominates the nondipolar part, each spacecraft subsystem is assigned a maximum allowable dipole magnetic field specification based on the magnetometer sensor sensitivity and the distance between the bulk of the subsystems and the sensor location. A typical maximum dipolar field allocation is
5、 10 nanoTeslas (gammas) at a distance of 1 meter from the geometric center of a spacecrafts subsystem, assuming the magnetometer sensor is mounted at the end of an 8-meter boom.To ensure that each subsystem will meet its respective dipole field specification, several design practices are observed du
6、ring the early stages of the subsystem design. These practices include:1. Magnetic Shielding of Magnetic Components A magnetic source can be enclosed in a high permeability material shield, which in effect confines the sources magnetic flux to within the walls of the shield enclosure. The shield sho
7、uld be completely enveloping, with the minimum number of holes and cutouts. The shield must be annealed after all machining and forming operations are completed. A general rule of thumb is to design the shield to operate within the linear range of the permeability curve between points A and B. Provi
8、ded by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionD 2. 3. Compensation of Magnetic Components A magnetic component can be neutralized by placing on or near its surface an equal but opposite field vector using compensation magnets or cu
9、rrent loops. Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionD 4. 5. Redesign of Circuit Board Current Paths to Reduce Loop Area Coverage Because a magnetic field B is proportional to loop area geometry A, number of loop turns N
10、, and current flow I through a circuit, a reduction in A produces a reduction in the magnetic field B, while still leaving I and N intact. refer to D descriptionD 6. 7. Replacement of Ferromagnetic Parts with Nonmagnetic Parts Another method for reducing magnetic fields is by simply replacing ferrou
11、s materials with nonmagnetic materials, preferably with relative permeability mrof approximately 1 so that the magnetic susceptibility cmis kept at approximately 0. refer to D descriptionD where m = momr, mr= 1 + cm, and mois the permeability of vacuum. Provided by IHSNot for ResaleNo reproduction o
12、r networking permitted without license from IHS-,-,-All spacecraft subsystems are individually subjected to a testing program aimed at fully characterizing each of the subsystems magnetic traits, as well as determining compliance with dipole field specifications. This testing program includes severa
13、l exposures to magnetizing fields of 25 Gauss and 3 Gauss to uncover easily permeable materials contained within the subsystem, and several exposures to demagnetizing fields of 50 Gauss and 40 Gauss to eliminate or reduce a subsystems residual magnetic field.Based on the above test program, a data b
14、ase is established containing subsystem information such as the X, Y, and Z spacecraft coordinates, maximum and minimum measured static magnetic fields, measured dynamic fields, and the calculated dipole moment components. From this data base, the total spacecraft static and dynamic dipole fields at
15、 the magnetometer sensor location are calculated using computer code, and are continually updated as new information becomes available. Results then are compared with the spacecrafts static and dynamic magnetic field science requirements.Technical Rationale:A spacecrafts total allowable magnetic fie
16、ld at the magnetometer sensor location r usually is determined by the sensors sensitivity level or by an agreed upon science requirement. The total field can be approximated by N number of dipoles, with N representing all of the spacecraft subsystems. To guarantee that the spacecrafts total magnetic
17、 field at r is within the desired allowable range, the individual moments due to N number of dipole sources must be kept to within predetermined dipole moment specifications. These individual specifications are derived by distributing the total allowable spacecraft moment amongst N number of spacecr
18、aft sources using a model that consists of a number of randomly oriented dipoles of strength Mj. The magnitude of the spacecrafts dipole magnetic moment is approximated by the Pythagorean sum of these individual subsystem dipole moments, with the radial part tending to be greater than either of the
19、transverse components for the dipole portion. The individual magnetic dipole field allocation, therefore, is determined from this model by the following equations:Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionD where BR,BQ, an
20、d BFare the field components of the spacecrafts magnetometer experiment sensor sensitivity or the science requirement levels at location r. Thus, Mjcan be determined for all spacecraft N sources assuming that Mjis the same for all j and the magnetic moments determining the far field are linear funct
21、ions of the vectors Mj. Because the dipolar portion of the spacecraft magnetic field dominates the nondipolar part and the spacecraft is dominated by the few largest sources, the general field allocation Bsfor a subsystem at a normalized distance of R meters is thus derived from Mjas follows: refer
22、to D descriptionD By ensuring that the dipole moment specifications of all spacecraft subsystems, as represented by N number of dipolar sources, are within their respective allocated dipole moment specifications, the overall spacecraft magnetic field at the magnetometer sensor location can be kept t
23、o within its science requirement or to below the magnetometers sensitivity level. Final verification is done by measuring the magnetic fields of all spacecraft subsystems and, subsequently, calculating the magnitude and orientation of their respective dipole moment components Mxj, Myj, and Mzjat spa
24、cecraft coordinates Xj,Yj, and Zj. The total spacecraft magnetic field at the magnetometer sensor location r then can be modeled using computer code to verify compliance with the specified science requirement or magnetometer experiment sensitivity.Impact of Non-Practice: Magnetometer experiment data
25、 will be corrupted with variable and unpredictable spacecraft residual magnetic field noise, thus limiting the accuracy of the magnetic field experiment.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Related Practices: N/AAdditional Info: Approval Info: a71 Approval Date: 2000-03-15a71 Approval Name: Eric Raynora71 Approval Organization: QSa71 Approval Phone Number: 202-358-4738Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
