Polar Joystick Control System

The Polar Joystick System is the solution, when control of several rotatable thrusters with one lever is required. With the joystick lever - similar to the lever for a single thruster - anumber of thrusters can be manoeuvred with one hand. The joystick lever is located in conjunction with a Heading Control unit, which is connected to the gyro compass and by which an automatic heading control can be achieved both in positioning and transit control modes.

Manoeuvre Responsibility System

This is a common system for one or more rotatable thrusters and provides selection between control stations and also to be used if the Polar Joystick System or a Dynamic Positioning Control System is connected to the thruster controls.

Thrust Reduction Unit

This is an optional unit which monitors the generator plant (up to 8 generators) to prevent overload and a possible blackout. The generator load sensing transmitters consist of current transformers. The output from the thrust reduction unit is connected to all installed thrust controllers and proportionally reduces the ordered thrust command signals.

Features of the Electronic Control System for Rotatable Thrusters

* Thruster and rotation can be controlled by one lever and the lever position gives information of the thrust magnitude and direction.

* The built-in delay and load control functions permit fast handling of the command lever without risk of over loading the prime mover.

* Independent back-up control system for both pitch and rotation allows full manoeuvrability in case the main system is out of order.

* All modules of the system have front space dimensions of only 144 x 144 mm.

* Thus each control station can be tailor-made to meet a wide variety of requirements

* Each module has a cable connector and is therefore simple to install and replace.

* The system is designed to accept a Polar Joystick System, an Automatic Anchor, Assic System and can easily be adopted for connection of a Dynamic Positioning System.

Description of the Main Control System

Transmitter Alarm Circuit

In order to obtain reliable supervision of the input-output signals connected to the control amplifiers, all important signals are of the usual proportional current type - (4 to 20 mA). Only two conductors are used for each transmitter. The transmitters are therefore equipped with an electronic circuit, which converts the measured value into a proportional current consumption.

The controllers are provided with an alarm detector for each transmitter signal. This detector senses whether the transmitter signal is within correct levels and in case of failure, activates an alarm to the common sensing alarm circuit, which also checks that all supply voltages are correct. Each alarm channel is equipped with light indicators on the alarm circuit board.

Thrust Controller

The command signal from the control head is converted into a proportional +10V d.c. signal after it has passed the alarm detector. Then the command signal proceeds via a delay circuit with an adjustable time from 0 to 100 seconds.

For an electric motor drive, the delay function is normally not used. After this possible delay the command signal is connected to a multiplier, which operates only when a load reduction unit is installed and produces a decreasing signal.

The command signal is then compared with the response signal in the control error comparator. The output signal from the comparator is now used for activation of the proportional value, controlling the pitch hub servomotor.

In order to avoid increasing the pitch above an allowed level, the control error signal is processed in a pitch-limiting circuit. The valve command signal is converted into a proportional value of current and when it has passed the alarm detector, it is connected to the valve interface in the thruster room.

As mentioned earlier, thrust control means that the load shall be proportional to the lever position. Therefore the response signal mainly consists of the load value.

When an electric motor drives the thruster, a current transformer is used as the load transmitter and when a diesel engine drives the thruster, the load-controlling circuits are more complicated. After passing the alarm circuits, the pitch and load signals are converted into proportional d.c. signals. In the load calculator block, the load signal is multiplied by the pitch-size signal, which is saturated at 25% pitch. Above this limit the load signal dominates as response. As the load signal does not change polarity for ahead-astern pitch, the response signal must be given correct polarity by sensing the pitch direction.

The output from the load calculator block representing thrust is then connected to the comparator.

As thrust is shown in the indication unit, the response signal is also connected to the thrust-pitch indication instrument.

By a switchover relay, pitch alone can be used as a response indication signal. Thus a pitch control mode is possible, if desired. However, in this mode the load of the prime mover is not controlled.

After the comparator a detector block is connected for sensing the control error. An error signal above a determined level gives a signal to the thrust deviation signal lamp in the indication unit (during manoeuvring, this lamp is therefore illuminated briefly when pitch changes).

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