Low-jitter Reaction Wheel Prototype for Kestrel Eye

The Army is developing the Kestel Eye imaging microsatellite to provide ground imagery directly to the warfighter.    The goal of the program is to provide tactical grade images to forces on the ground at any time and deliver the images fast enough for use in fast moving ground operations. The satellite will provide battlespace awareness for rapidly evolving tactical situations on the ground, for example: the implanting of Improvised Explosive Devices (IEDs); perimeter security of forward operating locations; or movement of hostile motorized forces.

Princeton Satellite Systems is under contract to develop a control system to  meet the exacting standards of stability, satellite location, and pointing accuracy required to meet the needs of the Kestrel Eye satellite.  The objective of our work at PSS is to improve the pointing accuracy or the ground location accuracy of the Kestrel Eye imagery from 60 meters to 10 meters or less.

The features of the proposed control system that are critical to enabling this superb accuracy are:

• Ultra-precise star image centroiding with custom algorithms
• Miniature precision fiber-optic gyro for attitude base and high bandwidth control
• Low-jitter microsatellite reaction wheels utilizing Halbach array motors
• Nonlinear attitude filters incorporating star camera and nontraditional measurements • Composite structure to eliminate thermal distortion
• GPS orbit determination enhanced with two-way ranging

Recently, PSS has completed the design and fabrication of the first prototype reaction wheel.  The wheel is driven by a low-jitter axial flux brushless DC motor, the design of which is currently under patent review.   An important enabling technology is the Halbach array of magnets.  A Halbach array is sequence of permanent magnet segments, each with its magnetic axis rotated from the axis of its neighbor.  The resulting assembly concentrates almost all of the magnetic field on one side, with an almost negligible field on the other side.  This arrangement favors an axial flux motor with a single stationary stator holding coil windings sandwiched between two permanent magnet rotors, each of which has its Halbach field directed toward the stator.  The sketch shows the arrangement.  The stator is green, and the two rotors are red.

rwa_cutaway

We’ve gone through a number design iterations,  settled on a first prototype design, and fabricated it.  We also purchased a simple general-purpose motor driver in order to explore the operation of the motor before moving on to developing custom driver electronics.

We’re very pleased that our first iteration works.  Here’s a video showing the device in action.

We’re already at work on the second-generation wheel incorporating lessons learned in the first prototype.

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