Thursday 12 November 2009

Steady satellite

Dynamometer keeps satellite on target

Most equipment manufacturers aim for maximum reliability but still maintain a team of service engineers for the few times something breaks down but sending an engineer to fix a fault is simply not an option, if the equipment is in space. This is the problem faced by Surrey Satellite Technology Ltd(SSTL), the most experienced small satellite company in the world, with over 32 successful launches since the first in partnership with NASA in 1981. Today’s SSTL satellites have evolved from earlier designs using the best available technology to minimise cost and maximise reliability.

The launch phase places huge vibrational loads on the payload making the need to over engineer and thoroughly test every component essential to ensure that these loads do not compromise the performance of the satellite. Typically, the “positioning wheels”, a type of gyro, that maintain the satellite in alignment with a ground station antenna when downloading data or a specific area when imaging, use precision ball race bearings that can become pitted in the launch phase resulting in electro-mechanical noise during in-space operation. This noise can interfere with the operation of the delicate electronics used for imaging and navigation systems on the satellite.

To simulate the operation of the “positioning wheels”, SSTL use a test rig based on a three-component dynamometer, manufactured by Kistler Instruments, which measures the forces produced by the “positioning wheels”. It is this force that, in space, maintains the satellite antenna in line with an earth station or a particular constellation depending on the mission objective. When the “positioning wheels” run at around 5,000 rpm to make a large change in the satellite position, noise is not a problem but when making small, repeated adjustments, noise can cause camera shake resulting in blurred imaging or loss of lock in the star camera positioning system.

“We chose the Kistler dynamometer and amplifier combination as we needed the long term stability and precision provided by the piezoelectric force sensors used in the instrument”,
says SSTL’s Andrew Haslehurst.

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