Rotation Controller

The rotation controller utilizes a system with sine-cosine voltages and is used for representing the command and response signals.

For utilizing only two conductors for the sine-cosine transmitter signals, a multiplexing system is used, where the current pulse's magnitude and polarity represent the two signals.

When turning the lever head, the rotation transmitter in the lever unit feeds the command signals, which are first sensed by the alarm circuits and then go via the multiplexer -de-multiplexer blocks that are connected to the rotation error calculator inputs.

The error calculator then compares the sine-cosine signals with the response signals, which are received from the rotation feedback transmitter in a similar way to the command one. The calculation of the control error is made in such a way that the thruster always rotates in a direction to achieve the ordered position in the shortest possible time.

The error signal is now used for control of the proportional valves, which regulate theoil flow to the two hydraulic turning servomotors.

As supervision of the valve command signal is also desirable, converter and alarm circuits are used in this signal path.

The interface circuits for the control valve are normally placed in the thruster room. In order to obtain good control stability, the hydraulic valves are equipped with an inner loop feedback transmitter connected to the valve interface circuit.

A signal to the rotation control deviation signal lamp in the back-up indicating unit is available in the circuit, sensing the control error in a similar manner to that in the thrust controller.

Tunnel Thruster

The stern of a vessel is controlled by means of a propeller and rudder, but the bow is left to itself. In order to overcome this, a tunnel thruster (also known as a lateral or bow thruster | may be installed in a transverse tunnel in the bow (Refer Figure 17.24). The need for such a system has already been explained in article 17.3.3 (A Manoeuvring System).

Thus a tunnel-type side thruster can be defined as a system in which propellers are installed inside a tunnel that passes laterally through the hull of the ship both fore and aft. By taking water in from one side and discharging it out at the other side, the ship can be turned or moved sideways. Such thrusters are used on a wide variety of ships including freighters, cargo barges, passenger liners, ferries, fishing boats, and work barges.

The thruster may have fixed blades of a helical shape but with equal pressure and suction sides, with no section camber (arch / curve). If it is of the controllable pitch type, the blades are made "plane" with zero design pitch. As the blades have to operate equally well in either direction, they must be symmetrical.

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