Turning in the Light

Rotary fiber-optic control provides EMI immunity in ship guidance systems

By Ulf Kapborg, Sales Engineer,
Litton Precision Products International
email: ulf.kapborg@littonppi.com

The Azipod system is now in use guiding cruise ships for several large cruise lines. Manufactured by ABB in Finland, it involves an electric propulsion drive with the motor installed inside a submerged nacelle or pod. This pod, together with its attached propeller, has the ability to pivot continuously about a vertical (azimuth) axis to facilitate ship steering in close quarters. The variable speed electric (AC/AC) drive produces smooth torque over the entire speed range down to a zero rpm propeller speed, and are built with up to 25 MW of power. Numerous advantages of this system include a significant reduction in vibration resulting from the direct drive and a highly maneuverable steering system.

The giant Azipod system on a cruise-liner

The pivoting nacelle incorporates a large electric AC motor directly driving a fixed-pitch propeller. The motor is speed-controlled by a frequency converter, with full torque available in either direction from zero to nominal speed. A fiber-optic control system from Litton Poly-Scientific, Blacksburg, VA, routes two full-duplex RS 422 optical channels for sensory feedback, enabling an addressable "command and interrogate" monitoring of such parameters as temperature and air flow at the propulsion motor. Two half-duplex channels deliver shaft speed measurement via encoder feedback to the motor control. Unaffected by magnetic fields, this optical control link provides immunity to interference and electrical cross-talk that can degrade signal quality in the high EMI environment around the giant AC propulsion motors.

The control system's transceivers convert incoming electrical signals to an optical format and condition the signal for transmission through the fiber-optic rotary joint (FORJ). The FORJ functions as an optical slip ring to enable the nacelle to rotate continuously about its azimuth axis unrestrained and without twisting fibers. This rotational flexibility provides a very versatile steering system for the ship while enabling critical sensor signals to be transmitted free of influences from electromagnetic interference.

Schematic of the Azipod propulsion system

Fiber-optic Rotary Joint

The patented Litton FORJ consists of adjacent optical waveguides, each of a "U" channel cross section, that accept an incoming optical beam and "form" it into a circular pattern around the circumference of the waveguide. Optical receptors, extending into the waveguide and rotating relative to it, pick-off the signal and route it to an optical detector where it is converted back into an electrical signal and amplified as required.

The FORJ is available with 2 to 7 single- or multi-mode channels. Operating temperature range is ­200° to 800°C. Off-axis units are also available. Other applications for the FORJ can be found in robotics, medical systems, milling machines, ROVs and cable reels.

For more information:

Circle 702 - Litton Poly-Scientific, or connect directly to their website via the Online Reader Service Program at http://www.OneRS.net/106df-702