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<< Figure 2:Model geometry showing the

coil distribution around the rotor. >>

The operating principle is relatively simple: an electric motor is attached to a flywheel. If the wheel accelerates, it builds up angular momentum in a certain direction, and the spacecraft rotates in the opposite direction due to the law of conservation of momentum. Note that such a device can only rotate a satellite around its own centre of mass and cannot be used to move the spacecraft to a different position.

The researchers at CSEM are operating under the assumption that the work of three reaction wheels can be done with one reaction sphere. That device is an iron ball covered with permanent magnets and held in position with magnetic levitation through magnetic fields generated by a number of electric coils. The sphere, acting as a rotor, is accelerated about any axis of rotation with a 3D motor. An attitude control system based on a reaction sphere would be smaller and lighter than those based on reaction wheels; even with its more complicated control electronics, it is estimated that the device can significantly increase the torque in the same volume. In addition, due to its magnetic bearing, the reaction sphere is expected to generate less micro vibrations due to the absence of ball bearings and lubricants. Finally, the possibility of using it as a multiple degree-of-freedom active vibration damper to absorb external disturbing forces is another attractive feature.

The concept of spherical actuator is not new and has been known for roughly 30 years. They have been used in robotics for spherical joints such as to mimic the wrist. However, this is believed to be the first application of spherical actuators in satellite technology.

728 Permanent Magnets A project funded by the European Space Agency was started in 2005 to investigate the viability of a reaction sphere for use in space. CSEM’s patented design is based on a 3D permanent magnet motor implemented with a multi-pole rotor and a 20-pole stator (figures 1 and 2). The rotor, manufactured and tested so far, is measuring roughly 20 cm in diameter, and consists of 728 permanent cylindrical magnets affixed to an iron sphere. To maintain as much symmetry as possible, the number of regularly distributed poles on the rotor follows the distribution of the eight vertices of a cube. The reaction sphere’s rotor can be accelerated about any desired axis and moved in any direction continuously without any disruption using a 20-pole stator that produces an 8-pole rotating field.

30 | commercial micro manufacturing international Vol 7 No.2

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