The
purpose of a bridge navigational watch alarm system is to monitor bridge
activity and detect operator disability which could lead to marine accidents.
The
system monitors awareness of the Officer of the Watch (OOW) and automatically
alerts the Master or another qualified person if, for any reason, the OOW
becomes incapable of performing OOW duties.
This
purpose is achieved by series of indications and alarm to alert first the OOW
and, if he is not responding, then to alert Master or another qualified person.
Additionally,
the BNWAS provides the OOW with means of calling for immediate assistance if
required.
Operational Modes of BNWAS:-
BNWAS should
incorporate the following 3 operational modes:
Automatic
Manual ‘ON’
Manual ‘OFF’
Automatic: The BNWAS is automatically activated when the vessel is navigating by means of heading or track control system (autopilot / trackpilot) and inhibited when this system is deactivated.
Manual ON: The BNWAS is always in operation.
Authorized person (Master) switches on system by single turning the key-switch to position “ON/OFF”.
Once dormant period (3 min, 9 min, 12 min) is set, the authorized person pulls out the key.
Dormant period starts from the moment the system has been switched on.
Manual OFF: The BNWAS is turned off completely.
ALERT SEQUENCE: – If dormant period is over without the OOW resetting,
the system activates all reset units by flash light.
ALERT STAGE 1:- If dormant period and flash light period (15 sec) are
over and the OOW has not yet resetted the button, the system activates
electronic buzzer on terminal board as well as additional buzzers located on
the bridge and wing area.
ALERT STAGE 2:- If dormant period, flash light period (15 sec) and alert stage 1 (15 sec) are over without OOW’s reset, system activates further alert via optic / acoustic alarm devices in officer’s area (cabins or staircase) as well as for VDR link.
ALERT STAGE 3:- If dormant period, flash light period (15 sec), alert
stage 1 (15 sec) and alert stage 2 (90 sec) are over without OOW’s reset, the
system activates the General Alarm as well as for VDR link.
EMERGENCY CALL:-
The OOW generates
an emergency call by pushing any reset unit longer than 5 sec.
That immediately
activates alert stage 2 and subsequently alarm stage 3.
After emergency
call has been released, reset is possible by pushing a reset unit.
Requirements of BNWAS:-
The bridge
navigational watch alarm system shall be in operation whenever the ship is
underway at sea.
System is powered
by ships main power and MUST have a battery back up giving a minimum of 6 hours
usage.
Carriage Requirements of Bridge Navigational Watch Alarm System (BNWAS):-
Cargo ships of 150 grt and upwards and passenger ships
irrespective of size constructed on or after 1 July 2011;
Passenger ships irrespective of size constructed before 1 July 2011, not
later than the first survey after 1 July 2012;
Cargo ships of 3000 grt and upwards constructed before
1 July 2011, not later than the first survey after 1 July 2012;
Cargo ships of 500 grt and upwards but less than 3000
grt constructed before 1 July 2011, not later than the first survey after 1
July 2013; and
Cargo ships of 150 grt and upwards but less than 500
grt constructed before 1 July 2011, not later than the first survey after 1
July 2014.
Short pulses of sound vibrations are
transmitted from the bottom of the ship to the seabed. These sound waves are
reflected back by the seabed and the time taken from transmission to reception
of the reflected sound waves is measured. Since the speed of sound in water is
about 1500 m/sec, the depth of the sea bed is calculated which will be half the
distance travelled by the sound waves.
Principles of Echo Sounder
The received echoes are converted into
electrical signal by the receiving transducer and after passing through to
stylus which burns out the coating of the thin layer of aluminum powder and
produces the black mark on the paper indicating the depth of seabed.
Components of Echo Sounder:
Components of Echo Sounder
Basically
an echo sounder has following components:
Transducer
– to generate the sound vibrations and also receive the reflected sound
vibration.
Pulse
generator – to produce electrical oscillations for the transmitting transducer.
Amplifier
– to amplify the weak electrical oscillations that has been generated by the
receiving transducer on reception of the reflected sound vibration.
Recorder
– for measuring and indicating depth.
CONTROLS:-
An
echo sounder will normally have the following controls:
Range
Switch – to select the range between which the depth is be checked e.g. 0- 50 m, 1 – 100 m, 100 – 200 m etc. Always check the lowest range first before
shifting to a higher range.
Unit
selector switch – to select the unit feet, fathoms or meter as required.
Gain
switch – to be adjusted such that the clearest echo line is recorded on the
paper.
Paper
speed control – to select the speed of the paper – usually two speeds
available.
Zero
Adjustment or Draught setting control – the echo sounder will normally display
the depth below the keel. This switch
can be used to feed the ship’s draught such that the echo sounder will display
the total sea depth. This switch is also
used to adjust the start of the transmission of the sound pulse to be in line
with the zero of the scale in use.
Fix
or event marker – this button is used to draw a line on the paper as a mark to
indicate certain time e.g. passing a navigational mark, when a position is
plotted on the chart etc.
Transducer
changeover switch – in case vessel has more than one switch e.g. forward and
aft transducer.
Dimmer
– to illuminate the display as required.
Working:
The
acoustic pulses of very short duration are transmitted vertically at the rate
of 5 to 600 pulses per minute having a beam width of 12 to 25°.
These
pulses strike the seabed and get reflected back towards the receiving
transducer as echoes.
These
received echoes are converted into electrical signals by the receiving
transducer and after passing through the different stages of the receiver, the
current is supplied to the stylus which bums out the coating of the thin layer
of aluminium powder and produces a black mark on the paper indicating the depth
of the seabed.
Principle used in the working of an Echo
Sounder:-
There are two
techniques:-
Ranging
Phasing
Ranging:-
In
echo sounder the stylus is mounted on circular belt driven by means of a stylus
motor which moves at certain speed and transmission takes place when the stylus
passes the zero marks.
A
magnet fixed on the stylus belt triggers the transmitter to transmit a pulse
every rotation of belt when stylus is at zero mark on the paper scale, the
transmission of the acoustic waves from the transducer is synchronized with the
stylus at the zero mark.
The
acoustic waves are reflected from the seabed and echoes are received by the
transducer and after passing through various stages eventually the current is
supplied to stylus which burns out the coating of the thin layer of aluminum
powder and produces the black mark on the paper indicating the depth of seabed.
This
cycle is repeated for every rotation so as the paper is pulled across the
display, the profile of seabed is obtained.
Suppose
the lowest range scale is 0 to 50 M, the transmission will take place when
stylus reaches at the zero mark.
When
the higher range is selected say 0 to 100 M, in order to cater for this range
scale, the speed of the stylus motor is reduced, in this process the scale
magnification is lost and as we switch over to higher ranges the scale becomes
more & more congested.
To
overcome this problem some of echo sounding machines work on phasing technique.
Phasing:-
In
phasing the speed of the stylus motor remains constant.
Instead
of changing the speed of the stylus, the transmission point is advanced.
If
the first range is 0 to 50 M the second range will be 50 to 100 M (instead of 0
to 100 M).
Various
sensors are positioned around the stylus belt, the magnet generates the pulse
when it passes the sensors which in turns activates the transmitter.
In
the below diagram, when we select the lowest range i.e. 0 to 50 M, the magnet
mounted on the stylus belt will activate sensor no. 1, transmission takes place
when the stylus exactly passes over the zero mark, when we switch over to
higher range, say 50 to 100 M, the magnet mounted on the stylus belt will
activate sensor no.2 and transmission will take place early, at the time of the
transmission, the stylus will not be passing over 50 M mark on display unit, in
other words there will be delay introduced by delay unit no.2 & the stylus
will reach the 50 M on display unit after delay of 0.067 seconds. (50 x 2 /
1500, where 50 correspond to the range, multiplied by 2 because double of
distance is covered by acoustic waves & the echoes and 1500 is the speed of
acoustic waves).
Likewise,
when we switch over to higher range say, 100 to 150 M, magnet mounted on the
stylus belt will activate sensor no. 3 & more delay will be introduced for
the stylus to pass over the 100 M.
Working of an Echo Sounder
Caution
when using phasing technique: – We must always start sounding at lowest
range and check for echoes, adjust the gain control if required and then only
switch over to higher range.
Errors of Echo Sounder:
Velocity of propagation in water:- The velocity changes with temperature salinity & pressure. The velocity of the acoustic wave assumed at the temperature of 16 degree C & Salinity of 3.4% is 1505 m/sec, but generally it is taken as 1500 m/sec for calculations. As velocity is varying hence depth recorded will be erroneous. Depth indicated in Fresh water can be about 3% higher than the actual depth. NP 139 can be referred in order to obtain the corrections. To compensate the error due to temperature variation, a component called “thermistor” may be mounted near the transducer & change in velocity of the acoustic wave through water from the standard value due to the change in sea water temperature is accounted for. Error due to pressure is not so significant.
Stylus speed error:- The speed of the stylus is such that the time taken by the stylus to travel from top to bottom on chart is same as the time taken by sound waves to travel twice the range selected, but due to fluctuation in voltage supplied to stylus motor, will cause error in the recorded depth.
Pythagoras error:- This error is found when two transducers are used, one for transmission and the other one for reception. This error is calculated using the Pythagoras principle. This error becomes prominent whenever distance between two transducer is more than 2 mtrs, manual should be referred in order to use the table for corrections.
Multiple echoes:- The echo may be reflected no. of times from the bottom of the sea bed, hence providing the multiple depth marks on paper.
The thermal and density layers:- The density of the water varies with temperature and salinity, which all tends to form different layers. The sound wave may be reflected from these layers.
Zero line adjustment error:- If the zero is not adjusted properly, it will give error in reading.
Cross noise:- If sensitivity of the amplifier is high, just after zero marking a narrow line along with the several irregular dots and dashes appear and this is called cross noise. The main reasons for the cross noise are aeration and picking up the transmitted pulse. If intensity of cross noise is high, it will completely mask the shallow water depths. This is controlled by swept gain control circuit.
Aeration:- When the sound wave is reflected from the reflected from the air bubbles, it will appear as dots, this is known as aeration.
Aeration can be due to pockets of bubble due to heavy weather.
Rudder hard over causing drastic alteration of course.
Pitching in light condition.
Whilst astern propulsion. (Switch over to forward transducer if available.)
Electrostrictive Transducer with respect to Echo Sounder:
This
type of transducer works on the basic principle of piezo-electric effect, i.e.,
certain crystals such as quartz, have a property that when pressure is applied
to the two opposite faces, a difference of potential is created which is
proportional to the applied pressure or when an alternating voltage is applied,
the crystals start vibrating or oscillating. This type of transducer is also
known as Piezostrictive transducer.
The
electrostrictive transducer uses the property of a crystal for transmission and
reception of acoustic waves in water. The crystal is firmly fixed between two
steel plates so that they act as a single unit.
The
purpose of the steel plates is to provide solid and robust housing for the
crystal as well as a suitable contact surface for seawater.
When
an alternating voltage is applied between the steel plates, the quartz and the
steel plates start vibrating together. The vibration will be of very high
amplitude, if the frequency of the alternating voltage is equal to the
resonance frequency of the crystal. The lower of the two steel plates is in
direct contact with the water, which will cause the vibration in the seawater.
The vibration is always perpendicular to the plate and hence always kept
horizontally.
Generally
only one transducer is used for transmission and reception of the signals and
this transducer is always mounted as pierced hull.
The electromagnetic speed log is based is upon the induction law, which states that if a conductor moves across a magnetic field, an electro motive force (e.m.f.) is set up in the conductor.
Alternatively, the e.m.f. will also be induced if the conductor remains stationary and the magnetic field is moved with respect to it.
The induced e.m.f. is directly proportional to the velocity.
Velocity when integrated with time gives distance
The induced e.m.f. ‘E’ is given by the following:
E = F X L X V
Where F = magnetic field
L = the length of the conductor
V = the velocity of the conductor through the magnetic field.
Constructional Details of an electromagnetic log sensor: This type of log consists of
Sensor
Amplifier
Indicator
Working of Electromagnetic Speed Log:
An
Electromagnet consisting of a coil carrying alternative Current (A.C.)
generates a vertical magnetic field in the water around the probe.
The
SW Conductor moving horizontally through this magnetic field has an
electromotive force induced into it proportional to the seed of the vessel. In
the EM log the ‘F’ and ‘L’ are maintained constant, therefore the induced
e.m.f. is directly proportional to the velocity ‘V’, which is the velocity
of the vessel through the water.
The
speed output from an EM log depends upon the water flow by way of the sensors.
This
type of log can give only speed through water and is greatly affected by the
current flowing under the ship.
The
induced e.m.f. and hence the speed indication will vary with the conductivity
of the water.
This
e.m.f. is picked up by 2 electrodes.
This
induced e.m.f. is very small hence the amplification is required.
The
amplified signal thus drives the mechanism which is connected to indicator.
Hence,
the induced e.m.f. which is directly proportional to the velocity is finally
displayed on the indicator.
More information:-
The
Log Extends up to about 20 cm outside the hull.
It
should be retracted in case of reduced UKC & before proceeding to dry dock.
Normally
retracted from the engine room.
If
sensors are also fitted athwartship, the speed in athwartship direction also
can be displayed.
Errors / Limitations of Electromagnetic Speed Log:
Siting of the probe is critical. This is so since if too close to the hull then due to the non-linearity of the hull form the speed of the water flow may give a wrong representation of the vessels speed. This is minimized by careful siting of the sensor as well as by calibrating the instrument while installation.
Pitching and Rollingalso give rise to errors however these are reduced by having an electrical time constant that is longer than a period of vessel motion. A well-adjusted log can have an accuracy of better than 0.1 percent of the speed range.
Sign of Speed, it can show astern speed as well, but without sign if AC current is used, if DC current is used to create the magnetic field it will show sign of speed range. This type of log can give only speed through water and is greatly affected by the current flowing under the ship.
While navigating in area with greater current, one must exercise precautions.
Advantagesof Electro-magnetic log with Doppler log:
No
moving parts
Less
affected by sea growth than Pit sword
Disadvantagesof Electro-magnetic log with Doppler log:
Salinity
and temperature of water affects calibration.
Measurements
affected by boundary layer, (water speed slowed down close to the hull by
friction).
Provides
boat/ship speed relative to water not ground. Current affects accuracy.
Difference between “Water Track Speed” and “Ground Track Speed“:
Water Track Speed:- In open seas, the sound pulse from the Doppler transducer may not reach the bottom, but get totally internally reflected from a layer of water in between. This is known as the echo from the ‘Water Track”. When the sound is bounced off a water layer, called a water track, speed indicated is the ‘Speed through Water”. or from a layer of water and the echo is at a higher frequency. The frequency of the echo from the water track will follow the same Doppler principles as the echo from the bottom track. However, the speed measured from the ‘water Track’ will not be ‘Speed over the Ground’, but it will be ‘Speed through Water’.
Ground Track Speed:- Speed over ground is the speed of the ship with respect to the ground or any other fixed object such as fixed buoy or island. Speed through water is the speed of the ship with respect to the water such as anything floating on water. A ship with her engine stopped in water with 2 knots currents will have zero speed through water but will have 2 knots speed over ground.
The
Doppler log is based on measurement of the Doppler effect.
The
Doppler effect can be observed for any type of wave – water wave, sound wave,
light wave, etc. we are most familiar with the Doppler effect because of our
experiences with sound waves. For instance, a police car or emergency vehicle
was travelling towards us on the highway. As the car approached with its siren
blasting, the pitch of the siren sound (a measure of the siren’s frequency) was
high; and then suddenly after the car passed by, the pitch off the siren sound
was low. That was the Doppler Effect – an apparent shift in frequency for a
sound wave produced by a moving source.
The Doppler Effect is a frequency shift that results
from relative motion between a frequency source and a listener.
If
both source and listener are not moving with respect to each other (although
both may be moving at the same speed in the same direction), no Doppler
shift will take place.
If
the source and listener are moving closer to each other, the listener will
perceive a higher frequency – the faster the source or receiver is
approaching the higher the Doppler shift.
If
the source and listener are getting further apart, the listener will perceive a
lower frequency – the faster the source or receiver is moving away the lower
the frequency.
So,
the Doppler shift is directly proportional to speed between source and
listener, frequency of the source, and the speed the wave travels.
Explanation of how ship’s speed is transmitted to remote displays:
Distance recording is achieved by using a constant speed motor (10) which drives the distance counter (11), via friction gearing.
The constant speed motor has been used in order that a distance indication may be produced that is independent of the non-linear characteristic of the system.
The motor is started by contact (5) as previously described.
The main shaft (7), whose angle of rotation is directly proportional to the speed of the ship, is fitted with a screw spindle (12).
The rotation of the shaft causes a lateral displacement of the friction wheel (13). At zero speed, the friction wheel rests against the apex of the distance cone (14), whilst at maximum speed the wheel has been displaced along the cone to the rim.
The distance indicator (11) is driven from the constant speed motor (10) via the cone.
The nearer to the rim of the cone the friction wheel rides, the greater will be the distance indication.
Revolutions of the distance shaft (15) are transmitted to the remote distance indicator via the servo transmission system (16 and 17).
The speed unit provides the following outputs to drive both speed and distance counters:-
An analogue voltage, the gradient of which is 0.1 V/knot, to drive the potentiometer servo-type speed indicators.
A pulse frequency proportional to speed.
The frequency is 200/36 pulses/s/knot. Pulses are gated into the digital counter by a 1.8-s gate pulse.
A positive/negative voltage level to set the ahead/astern indication or the B track/W track indication.
2000 pulses per nautical mile to drive the stepping motor in the digital distance indicator.
FORMULAof Doppler Log:-
Doppler effect can be further explained by following equations:
fr is the frequency received by observer.
ft is the transmitted frequency.
c is the speed of sound.
vO is Velocity of observer
vg is Velocity of source
If the source moves towards stationary observer, fr = c ft / (c – vg)
If the source moves away stationary observer, fr = c ft / (c + vg)
If the observer moves towards stationary source, fr = ft (c + vg) / c
If the observer moves away from stationary observer, fr = ft (c – vg) / c
If the observer & source moves away from each other, fr = ft (c – vg) / (c + vs)
If the observer & source moves toward each other, fr = ft (c + vg) / (c – vs)
Since,
in the case of the Doppler log, the source & observer are the same.
Hence,
vO is
equal to vS, is equal to v
fr = ft (c+ v) / (c – v)
fr =
ft (c+ v cos a) / (c – v cos a)
After Further
simplification
v = c (fr – ft) / 2 ft cos a
Given
a propogation angle of 60O, cos a = 0.5 (using single transducer
facing forward)
Graphs
of speed error caused by variations of the vessel’s trim:
It
follows that if the angle changes, the speed calculated will be in error
because the angle of propagation has been applied to the speed calculation
formula in this way. If the vessel is not in correct trim (or pitching in heavy
weather) the longitudinal parameters will change and the speed indicated will
be in error.
To
counteract this effect to some extent, two acoustic beams are transmitted, one
ahead and one astern. The transducer assembly used for this type of
transmission is called a ‘Janus’ configuration after the Roman god who
reputedly possessed two faces and was able to see into both the future and the
past.
After installing
transducer facing aft, the Doppler frequency shift formula now becomes:-
Frt
– fra – 4 vft
cos a / c
Hence, v = c (frt
– fra) / 4 ft cos a
Therefore
the transmission angle can effectively be ignored.
The
advantage of having a Janus configuration over a single transducer arrangement.
It can be seen that a 3O change of trim on a vessel in a forward
pointing Doppler system will produce a 5 % velocity error. With a Janus
configuration transducer system, the error is reduced to 0.2% but is not fully
eliminated.
The addition of a
second transducer assembly set at right angles to the first one, enables dual
axis speed (longitudinal speed and transverse speed) to be indicated.
Docking Operation:
The placing of the Janus configuration in a fore and aft direction is known as a single axis system and is used to calculate speed over ground in the forward and after direction. A dual axis system places a second grouping of Janus configured transducers in an athwart ships direction allowing for the calculation of a vessel’s speed when moving sideways through the water, as in docking. The beam width of the athwart ship installation is about 8 degrees to account for the possibility of a vessel’s rolling.
The Doppler system calculates speed to within an accuracy of about 0.5 percent of the distance traveled. It functions well for all speeds that modern vessels can attain and works from a minimum depth of about 1.5 feet to a maximum depth of about 600 feet. Frequencies employed are between 100 kHz and 600 kHz. There are primarily four errors to be aware of when using the Doppler system:
Transducer orientation error caused when the pitching or rolling of the vessel becomes excessive.
Vessel motion error caused by excessive vibration of the vessel as it moves through the water.
Velocity of sound errors due to changes in water temperature or density due to salinity and particle content.
Signal loss errors caused by attenuation of the vibrations during transit through the water or upon reflection from the bottom.
The Doppler system normally measures speed over ground to about 600 feet. This depth signals may be returned by a dense, colder layer of water located throughout the oceans called the deep scattering layer (DSL). Signals received off the DSL are not as accurate as signals received from bottom reflections but can still be used to provide an indication of speed through the water instead of speed over ground when bottom tracking. Your unit may have a manual or automatic system which will switch from bottom tracking to water tracking at increased depth.
The Doppler system can be connected with other electronic navigation systems providing generally accurate speed input. The navigator should be cautioned that precise speed should be determined not only by using the Doppler but also from careful calculations of distances between accurate navigational fixes.
Errors in a Doppler log & how are some of these errors overcome by the Janus Configuration:
ERRORS OF DOPPLER LOG:- The Log speed indicated is subject to various errors, spanning installation, equipment, data processing, varying propagation conditions and sea conditions.
Error in transducer orientation:- The transducers should make a perfect angle of 60° with respect to the keel or else the speed indicated will be inaccurate.
Error in oscillator frequency:- The frequency generated by the oscillator must be accurate and constant. Any deviation in the frequency will result in the speed showing in error.
Error in propagation:- The velocity of the acoustic wave at a temperature of 16°C and salinity of 3.2% is 1505 m/sec but taken as 1500 m/sec for calculation. This velocity changes with temperature, salinity and pressure. To compensate the error due to temperature change, a thermister is mounted near the transducer and change in velocity of the acoustic wave through the water from the standard value due to the change in sea water temperature is accounted for.
Error in ships’ motion:- During the period of transmission and reception, the ship may have a marginal roll or pitch and thereby the angle of transmission and reception can change and a two degree difference in the angle of transmission and reception can have a 0.10% error in the indicated speed, which is marginal and can be neglected.
Error due to rolling/pitching:- The effect of pitching will cause an error in the forward speed and not the athwartship speed. Similarly, rolling will have an effect on the athwartship speed, not the forward speed.
Actual
speed = Indicated speed/Cosß
Error due to inaccuracy in measurement of frequency:- The difference
in the frequencies received by the forward and aft transducers must be measured
accurately. Any error in this will be directly reflected in the speed of the
vessel.
Error due to side lobe:- When the side
lobe reception dominates over the main beam reception, there will be an error
in the speed indicated. The error is more pronounced on a sloping bottom as the
side lobe is reflected at a more favourable angle and will have path length
less than the main beam. This error can be eliminated with the help of the
Janus configuration and to reduce this error, the beam of the transmitted
acoustic wave is reduced.
THE ‘SPEED’ FORMULA WITH SHIP MOVEMENTS CORRECTION – JANUS CONFIGURATION:-
As the ship moves
forward, she also has an up and down motion in the vertical direction, called
‘heaving’. The vertical motion component is v sin α.
As this movement of
the ship has an effect on the frequency shift, it should be accounted for. This
is done by installing a second set of transducers (for transmitting and
receiving) in the aft direction at the same angle of 60º. (Refer figure). This
type of installation setup is called Janus Configuration.
The effect of frequency
shift due to vertical motion (the component v sin α ) of the ship gets
cancelled out in Janus Configuration and the resultant ship speed is calculated
by the formulae:
v = c
(Frf – Fra) / 4 Ft cos α
Where,
v= ship’s speed
c= speed of acoustic wave in water
Frf = Freq. of the received wave, from fwd direction
Fra = Freq. of the received wave, from aft direction
An auto pilot is the ship’s steering controller which automatically manipulates the rudder to decrease the error between the reference heading and actual heading.
Autopilot relieves the helmsman to great extent but definitely autopilot is not a substitute for helmsman.
Autopilot also reduces fuel consumption as the zig-zag course is avoided.
Working of Auto Pilot:-
Course
is selected by the course selector.
Present
heading is indicated by the compass.
The
output from the compass is fed to the comparator in the control unit. The
signal from the course selector is also fed to the comparator.
Difference
between the two signals is causing the output error signal detected by the
comparator.
Integrator
and differentiator also analyze the signal.
The
signals from the comparator, integrator and differentiator are fed to summing
amplifier (control unit).
The
summing amplifier in turn, passes the signals to error amplifier which also
receives feedback from the steering gear.
The
output of error amplifier is transmitted to steering gear via telemotor
transmitter and telemotor receiver.
A
torque motor may be fitted instead of a telemotor.
Controls available in Auto Pilot console:
The Autopilot Control Unit – The PID Control Unit:- In order to maintain the ship’s course accurately, the deviation signal has to be generated under the following conditions:
When the set course is changed (by the navigator).
When the ship deviates from the set course (due to external factors).
For this purpose,
the helm must be provided with data regarding the ship’s movement relative to
the course to steer line.
This is achieved by
electronic circuits with the help of the following:
Proportional
control
Derivative control
Integral control
Proportional
Control:-
Proportional Control – Auto Pilot
The
effect on steering, when only the proportional control is applied, causes the
rudder to move by an amount proportional to the off-course error from the
course to steer.
When
the ship has gone off-course to port, an error occurs and helm, proportional to
the deviation and hence error signal, is used to bring her back to the set
course.
As
the ship starts to return to the set course, the helm is gradually eased and
finally removed when the ship is back on the set course.
The
rudder will be amidships when the ship reaches its set course and then the
heading overshoots resulting in the vessel to go more to starboard. Correcting
helm is now applied causing the ship to return to port and back to the original
course.
The
vessel thus keeps on oscillating to port and starboard of the course line.
Derivative
Control:-
Derivative Control – Auto Pilot
In
derivative control, the rudder is shifted by an amount proportional to the rate
of change of the ship’s deviation from the course. Any deviation of course to
port will cause correcting rudder to be applied to starboard.
As
the rate of change of course decreases, the automatic rudder control decreases
and at a point X, the rudder will return to midships before the vessel reaches
its set course.
The
ship will now make good a course parallel to the required course.
Integral
Control:-
Integral Control – Auto Pilot
Certain errors due to the design of the ship (bow going to port due to transverse thrust, shape of the hull, current draft, etc.) have an impact on the steering capabilities of the ship and have to be corrected for effective overall steering performance.
In order to achieve this, signals are produced by sensing the heading error over a period of time and applying an appropriate degree of permanent helm. The rudder used to correct the course will now be about this permanent helm. That is, the permanent helm will now act as midships.
Additionally, there are various controls provided on the autopilot system along with a filter system for the action of the winds and waves which supply more data to the autopilot which optimizes the performance of integral control.
The output of these three controls is combined and the net resultant thus obtained drives the rudder maintaining the ship on the set course. This type of auto pilot is referred to as PID auto pilot.
Working of “Weather Control”in Auto Pilot System:
Rough weather and hostile sea conditions have adverse effects on the performance of the auto-pilot. Uncontrolled yawing of the ship can result in excessive rudder movement. Modern auto-pilot system has Weather control option in which the system automatically adjusts the setting to adapt to the changing weather and sea conditions. It also provides an option for the user to manual set a specific value.
Working of “Yaw Control” in Auto Pilot System:
The setting of the Yaw Control depends upon the wind and weather condition and their effect on the course keeping ability of the ship, in bad weather this setting should be set high and calm weather this should be set low. If Yaw Control is not set properly, the steering gear will over work & there will be excessive load on the system.
Working of “Off Course Alarm” in Auto Pilot System:
Off Course Alarm:- Usually an Off Course Alarm is fitted on the Autopilot. This can be set for the required amount of degrees. So that if at anytime the difference between the actual course and the Autopilot set course is more than the preset degrees, an alarm will warn the officer.
There is however, one limitation which should be noted. In case, the gyro compass itself begins to wander the Autopilot well steer so as to follow the wandering compass and the Off Course Alarm will not sound. It does not ring unless the difference between the course setting and gyro heading is more than the preset limit.
Working of “Rudder Limit” in Auto Pilot System:
Rudder Limit:- This setting specifies the maximum amount of rudder to be used when correcting the ship’s head or when altering course on autopilot. That is, if a setting of 10O is applied for rudder limit, when altering course the rudder will move to a maximum of 10O. This limit can be varied according to the requirements of the navigator.
Purpose of following settings in Autopilot: Rudder
This
control determines the amount of rudder to be used to correct the slightest
amount deviation from the set course.
The
higher is setting the larger the rudder angle is used to correct a course
deviation and this may result in over correcting.
But
if setting is less, the rudder angle is used to correct deviation may not be
sufficient and will take longer time to return to set course.
This
is proportional controller which transmits a signal which is proportional to
course error
Controller
output = constant (Kp) x Deviation
The
ratio can be changed by settings (i.e. the ratio between instantaneous heading
error and rudder command) also called rudder multiplier.
Lower
setting – Less rudder angle (Long time to return to set Co-Sluggish).
Therefore,
optimum setting required.
Purpose of following settings in Autopilot: Counter Rudder
This control determines the amount of counter action by the rudder to be used to steady the ship on the set course keeping the overshoot to minimum.
Too low setting will allow the ship to overshoot and too high setting will bring the ship back in long time.
This is Derivative control.
Purpose is to apply a relatively greater amount of helm at the beginning of a course alteration to get the ship turning. Once the ship is turning, just enough helm is applied in order to keep her coming around. When new heading is approached, opposite helm is applied to stop the swing. As the ship settles on new heading and the yaw rate disappears, the helm is removed.
Produces an output when course of vessel is changing.
Depends on rate of change of course:
Controller output = constant ( KD ) x change of error / time
Determines amount of counter rudder to steady the ship on set course.
Keeps over shoot to minimum.
Greater the ship’s inertia, greater the setting required. If ship has good dynamic stability, relatively small settings of counter rudder will be sufficient. If the ship is unstable, higher settings will be required.
Depends on ship’s characteristics, loaded/ballast conditions and rate of turn.
Too high setting will bring the ship to set Co slowly.
Too low setting allows overshoot.
As counter rudder settings increase, counter rudder increases.
KD – Counter rudder time constant (Calibration done at sea trial to set KD).
Purpose of following settings in Autopilot: Constant Helm
Constant / Permanent Helm:
This
is integral controller. (In NFU this control is out of action).
When
ship has known imbalance to one side, requiring a certain amount of bias helm
(e.g. TT of propeller) manual setting of the approximate bias speed up the
effect of the AUTOMATIC PERMANENT HELM calculator, because it started off
nearer to its target.
Whether
the control setting is estimated correctly or left at zero has no effect on the
final steering accuracy but only in the time it takes to reach this heading
accuracy.
If
not used as described above , the permanent helm should be left at ZERO and the
automatic permanent helm will function normally.
Produces
output as long a course error persists.
Used
when beam winds; couple formed causing ship to turn into wind.
Rudder
position required to counteract is permanent helm.
Continuous
control calibrated from 20 (P) to 20 (S).
Purpose of following settings in Autopilot: Weather
The setting of the yaw control depends upon the wind and weather condition and their effect on course keeping ability of the ship in bad weather this setting should be set high and calm weather this should be low.
Purpose of following settings in Autopilot: Rudder Limit
Rudder limit: This control specifies the maximum amount of rudder to be used, when correcting the ship’s head or altering the ship’s course.
Auto Pilot should not be used in the following conditions:
A freely suspended Magnet in a Magnetic Compass points towards the North:
Earth’s Magnetic Poles
A magnetic compass works because the Earth is like a giant magnet, surrounded by a huge magnetic field. The Earth has two magnetic poles which lie near the North and South poles. The magnetic field of the Earth causes a magnetized ‘needle’ of iron or steel to swing into a north-south position if it is hung from a thread, or if it is stuck through a straw or piece of wood floating in a bowl of water.
A compass works by utilizing the Earth’s magnetism in order to find directions. Its invention enabled people to perform navigation over long distances, opening up the sea for exploration
A compass points north because all magnets have two poles, a north pole and a south pole, and the north pole of one magnet is attracted to the south pole of another magnet.
The Earth is a magnet that can interact with other magnets in this way, so the north end of a compass magnet is drawn to align with the Earth’s magnetic field. Because the Earth’s magnetic North Pole attracts the “north” ends of other magnets, it is technically the “South Pole” of our planet’s magnetic field.
Notes on Earth’s Magnetism:
Earth’s magnetic field, also known as the geomagnetic field, is the magnetic field that extends from the Earth’s interior to where it meets the solar wind, a stream of charged particles emanating from the Sun.
Its magnitude at the Earth’s surface ranges from 25 to 65 microtesla (0.25 to 0.65 gauss). Roughly speaking it is the field of a magnetic dipole currently tilted at an angle of about 10 degrees with respect to Earth’s rotational axis, as if there were a bar magnet placed at that angle at the center of the Earth.
Unlike a bar magnet, however, Earth’s magnetic field changes over time because it is generated by a geodynamo (in Earth’s case, the motion of molten iron alloys in its outer core).
The North and South magnetic poles wander widely, but sufficiently slowly for ordinary compasses to remain useful for navigation.
However, at irregular intervals averaging several hundred thousand years, the Earth’s field reverses and the North and South Magnetic Poles relatively abruptly switch places.
These reversals of the geomagnetic poles leave a record in rocks that are of value to paleomagnetists in calculating geomagnetic fields in the past. Such information in turn is helpful in studying the motions of continents and ocean floors in the process of plate tectonics.
‘Variation’ and ‘Deviation’ of Magnetic Compass:
VARIATION:
Variation of Magnetic Compass
The true North Pole
and the magnetic north pole are not located at the same spot. This variation
causes a magnetic compass needle to point more or less away from true north.
The amount the needle is offset is called variation because the amount varies
at different points on Earth’s surface. Even in the same locality variation
usually does not remain constant, but increases or decreases at a certain known
rate annually.
The variation for
any given locality, together with the amount of annual increase or decrease, is
shown on the compass rose of the chart for that particular locality.
DEVIATION:
Deviation of Magnetic Compass
The amount a magnetic compass needle is deflected by magnetic material in the ship is called deviation.
Although deviation remains a constant for any given compass heading, it is not the same on all headings. Deviation gradually increases, decreases, increases, and decreases again as the ship goes through an entire 360° of swing.
The magnetic steering compass is located in the pilothouse, where it is affected considerably by deviation. Usually the standard compass is topside, where the magnetic forces producing deviation are not as strong.
Courses and bearings by these compasses must be carefully differentiated by the abbreviations PSC (per standard compass), PSTGC (per steering compass), and PGC (per gyrocompass).
The standard compass provides a means for checking the steering compass and the gyrocompass.
Notes on Ship’s Magnetism:
On a ship built up of wood, a magnetic compass would point to magnetic north knowing the variation at that place, the magnetic direction corrected and the true direction obtained, the ships built up of steel structures is of two types, soft iron magnetism and the hard iron magnetism.
Soft iron magnetism is the induced magnetism and hard iron magnetism is the permanent magnetism.
Sketch & label a wet card Magnetic Compass:
Wet Card Magnetic Compass
Necessity:- The dry card compass is too sensitive for steering purposes, especially in bad weather. Even small disturbances cause the dry card to oscillate. In the wet card compass oscillations are damped, without loss of accuracy, by immersing the card in a liquid. The card therefore has a ‘dead beat’ movement.
The card:- The wet card is made of mica and is only about 15 cm in diameter. The card is attached to nickel- silver float chamber that has a sapphire cap. The cap rests on iridium tipped pivot. The sapphire has a polishing effect on the iridium tip. This arrangement is practically frictionless.
The directive element:- In modern wet card compasses the directive element is a ring magnet fitted around the base of the float. The ring magnet offers less resistance to movement and causes less turbulence.
The bowl:- The diameter of the bowl is about 23 cm in order to reduce disturbances caused by turbulence in the liquid during rotation of the card. The top of the bowl is of transparent glass. The bottom is of frosted glass.
Allowance for expansion:- One method is to have a small accordion – like expansion chamber attached to the bowl. The chamber increases or decreases in volume, as necessary, as the liquid in the bowl expands or contracts due to changes in atmospheric temperature.
Suspension:- The bowl of the wet card compass is suspended by gimbals. This bowl, being considerably heavier than that of the dry card compass, does not have a glass hemisphere of alcohol and water attached to its underside. Instead, a ballast weight consisting of a ring of lead, enclosed in brass, is attached along with circumference of the underside of the bowl to bring its centre of gravity below the gimbals.
Care and Maintenance:- The care and maintenance required for a wet card compass and its binnacle is the same as that for a dry card compass. The only changes / differences are:-
The wet compass card, if found defective owing to stickiness of movement, has to be renewed by the manufacturer or his authorized agent. Hence, no spare wet card is carried. Instead, an entire bowl is carried as a spare.
In rare cases, a bubble may develop in the wet compass bowl. This has to be removed at the earliest opportunity.
Correctors on a compass binnacle and why are they required, various corrections to be applied to magnetic compass
The Binnacle – Magnetic Compass
The Binnacle:- The binnacle is a cylindrical container made of teak wood and brass. No magnetic materials are used in its construction. Even the screws are of brass and the nails, copper. The compass bowl is slung inside the top portion of the binnacle. The middle potion is accessible by a door and contains an electric bulb. Light from this bulk passes upwards through a slot, through an orange coloured glass fitted over the slot, through the bottom of the compass bowl, to illuminate the compass card from below. The orange colour ensures that the night vision of the observer is not adversely affected.
Corrector Magnets:- (See Figure) In the centre of the lower half of the binnacle, there are a number of horizontal holes, both fore & aft and athwartships, for ‘hard iron’ or ‘permanent’ corrector magnets which are meant to offset undesirable, disturbing, magnetic effects caused by the ship’s steel hull. The lower two-thirds of the binnacle has a vertical brass tube, at the centre, in which slides a ‘bucket’. This bucket has some magnets in it called ‘heeling error correctors’. The bucket is held in position by a brass chain.
Quadrantal
Correctors:-
(See Figure) These are two ‘soft iron’ spheres which are fitted in brackets,
one on either side of the binnacle. The brackets are slotted so that the
distance between the spheres can be altered as desired during compass
adjustment.
Flinders Bar:- (See Figure)
This is a soft iron corrector, (diameter about 7.5 to 10 cm) inserted in a 60
cm long brass case, fitted vertically on the forward or on the after part of
the binnacle. If the ship has more
superstructure abaft the compass, the Flinders bar is fitted on the forward
part of the binnacle and vice versa.
Remove an air bubble from the compass bowl
Removal of bubble: A bubble may form in the bowl owing to the fact that some of the liquid has somehow escaped from the bowl. This is a rare occurrence and must be remedied by following the manufacturer’s instructions. In most compasses:
Tilt the bowl until
the ‘filter hole’ comes uppermost. This hole is provided on the side of the
bowl.
Unscrew the stud/
screw provided.
Top up with ethyl
alcohol. If this is not available, distilled water would do.
Screw the stud/
screw back into place.
Gently let the bowl
return to upright.
In some modern compasses, small bubbles may be
removed as follows:-
Invert the bowl gently. This would cause the
bubble to enter a bubble trap provided for this purposes.
Gently return the bowl to upright.
The bubble should have disappeared.
Advantages of wet compass over Dry Compass Card
The dry card compass is generally used as a standard compass & the wet card compass as a steering compass. The dry card compass is very sensitive. Even a slight disturbance makes the dry card oscillate. In the wet card compass, the oscillation is damped in the liquid and hence more useful as a steering compass. In some ships, the wet compass is now used as a standard compass, mainly because of the availability of the gyro compass as the main direction indicating instrument.
Why the vessel is required to be swung once in a year to verify Magnetic Compass Deviation Card?
Explanation:-
Swinging the compass, or swinging the ship (as the operation is sometimes more accurately called as the ship swings around the compass card which, ideally, remains pointing north), involves taking the vessel to a suitable location in open water with plenty of room for maneuvering. With the vessel steady on each of the eight primary compass points, existing compass headings or bearings are compared with what we know the actual magnetic headings or bearings should be, the difference being the deviation.
During the process, any magnetic fields, created by the ship’s structure, equipment, etc, which cause the compass to deviate are reduced or, if possible, eliminated, by creating equal but opposite magnetic fields using compensating correctors. These are placed inside the compass binnacle or adjacent to the compass:
Magnets are aligned fore and aft and athwartships to create horizontal magnetic fields to compensate for the permanent horizontal components of the ship’s magnetism.
Soft iron correcting spheres or plates and the Flinders bar compensate for the induced magnetism caused by the effect the earth’s magnetic field has on the ship’s magnetism.
Heeling error magnets compensate for the vertical component of the ship’s magnetism.
The timing and logistics of this operation are often governed by the tide, the weather and other vessels in the vicinity. The time it takes to swing and adjust the compass is also influenced by the condition and accessibility of the compass and correctors, the manoeuvrability of the vessel, the skill of the helmsman and the complexity of, and reasons for, the deviating magnetic fields involved.
On successful completion of compass swing, a table recording any remaining residual deviation and a statement as to the good working order of the compass will be issued. A current deviation card / certificate of adjustment is a legal requirement on all sea going commercial vessels.
Deviation can be determined by a number of methods: the sun’s azimuth or known bearings of distant objects, such as a mountain peak or lighthouse are considered most accurate. In certain circumstances, such as poor visibilty, calibration is carried out by making comparisons with other navigation instruments, such as a gyro or GPS compass.
Using other navigation instruments to find deviation is only satisfactory if the absolute accuracy of these instruments has first been verified, or any known error is factored into the calculations. Most professionals prefer something tangible, such as a fixed landmark, with a known position and bearing to work with.
What is Dip? How compass is kept horizontal in varying latitudes?
Magnetic dip Error:-
Magnetic Dip error
Magnetic dip is the tendency of the compass needles to point down as well as to the magnetic pole.
Dip is greatest near the poles and least near the Magnetic Equator.
The compass card is designed to operate in the horizontal, therefore, any movement from the horizontal plane introduces dip error.
The needle of your magnetic compass will be parallel with Earth’s surface at the Magnetic Equator, but will point increasing downward as it is moved closer to the Magnetic Pole.
Northerly turning error is due to the mounting of the compass. Since the card is balanced in fluid, when the vessel turns, the card is also banked as a result of centrifugal force.
While the card is banked, the vertical component of the Earth’s magnetic field causes the north-seeking ends of the compass to dip to the low side of the turn. When making a turn from a northerly heading, the compass briefly gives an indication of a turn in the opposite direction.
When making a turn from the south, it gives an indication of a turn in the correct direction but at a faster rate.
Gyroscope having three degrees of freedom is called “FREE GYROSCOPE”
Free Gyroscope
Properties
of Free Gyroscope:-
Gyroscopic inertia
or rigidity in space
Precession
Gyroscopic Inertia:- A freely spinning gyroscope will maintain its axis of spin in the same direction with respect to space irrespective of how its supporting base is turned. It resists any attempt to change its direction of spin. Thus a free gyroscope has high directional stability. This property is called GYROSCOPIC INERTIA or RIGIDITY IN SPACE.
Gyro Precession:- Precession is the angular displacement of the spin axis of the gyroscope when a torque is applied to gyroscope. Hence, when a torque is applied to the spin axis the resulting movement will be in the direction at right angle to the applied torque. Earth is also a free gyroscope pointing north axis toward Polaris. (rigidity in space). We all are also aware that earth also possesses force of gravity.
Gyro Precession
First property of free gyro scope is useful. However, due to the placing of this gyroscope on the surface of the earth it will be moved along the direction of rotation of the earth. As such the gyroscope will have an apparent motion. For example, at night if the gyroscope is made to point in the direction of a star, then the gyroscope will follow the star as the earth rotates and the star apparently moves in the sky.
Tilt and Drift in a Gyro compass
Tilt is elevation or depression of the spin axis above or below the horizon.
Drift is the movement of the spin axis in the direction of azimuth.
Rate of tilting in degrees per hour = 15O sine Azimuth * cosine Latitude
Rate of Drift in degrees per hour = 15O sine Latitude
Tilt:-
If
a free gyroscope is situated on the equator and lies with its axis East West
and horizontal, it can be assumed of as pointing to a star with zero
declination and is about to rise.
The
East End of the gyroscope axis will follow the movement of this star and will
tilt upwards as the star rises.
After
nearly six hours the axis will be vertical and after nearly twelve hours the
gyroscope will have turned completely over with the axis again horizontal but
now the original East end of the axis would be pointing to the star setting due
West.
After
one sidereal day, the gyroscope would have tilted through 360O and
the star would again be rising.
This
rate of tilting of 360O in a day is a rate of 15O per
hour.
If
the gyroscope had been situated on the equator with its axis lying in the North
– South direction, then the North end would be pointing towards the Pole star
and would then have no apparent movement relative to the Earth.
The
rate of tilting thus varies from zero when the axis is lying North – South to a
maximum when it is lying East – West. That is the rate of tilting varies as the
Sine of the Azimuth.
A
free gyroscope situated at a pole with its axis horizontal would have an
apparent turntable motion due to the Earth’s rotation.
That
is it would follow a fixed star around the horizon but it would not rise or set.
The
rate of tilting thus varies from a maximum when the latitude is 0O
to zero when the latitude is 90O. That is the rate of tilting varies
as the Cosine of the Latitude.
Rate
of tilting in degrees per hour = 15O sine Azimuth * cosine Latitude
The
direction of tilting is such that the end of the gyroscope axis, which lies to
the East of the meridian, tilts upwards and the end of the axis, which lies to
the West of the meridian tilts downward.
Drift:-
Drift
is the apparent movement of a gyroscope in azimuth.
A
free gyroscope situated at the North Pole with its axis horizontal will have an
apparent movement, which is entirely in the horizontal plane.
Its
axis will appear to move in a clockwise direction when viewed from above. This
would be due to the real counter clockwise rotation of the earth beneath, this
circular motion causes the gyroscope to drift through 360O in one
sidereal day, that is at a rate of 15O per hour.
A
free gyroscope situated at the equator with its axis horizontal will not drift
at all, irrespective of whether its axis is set in the North – South or East –
West line.
The
rate of drift for a gyroscope with its axis horizontal thus varies from a
maximum at the poles to zero at the equator.
That
is the rate of drift varies as the sine of the latitude. For a free gyroscope
with its axis horizontal: Rate of Drift in degrees per hour = 15O
sin Latitude.
The
direction of drift depends upon hemisphere so that the North end of a
horizontal gyroscopic axis drifts to the eastwards in the Northern hemisphere
but to the Westwards in the southern hemisphere.
Tilting & Drifting of a Gyroscope
Course, Latitude and Speed Error in a Gyro Compass:
The
gyro compass settles in the N/S direction by sensing Earth’s spinning motion.
Same gyro compass when placed on a ship also senses the ship’s motion. And
therefore, the axis of gyro compass settles in a direction which is
perpendicular to the resultant of the Earth’s surface speed and the ship’s
velocity.
The
direction, in which the compass settles, is therefore, different to the
direction of the True North and depends on ship’s course, speed and latitude of
the observer.
This
error also increases as the observer’s latitude increases. The error is
westward on all Northerly courses and vice-versa.
In
exactly E-W courses, the error is nil. In exactly N-S courses, the error is maximum.
To
compensate for steaming error, a speed rider is provided, which in association
with the latitude rider, shifts the lubber line equal to steaming error in the
appropriate direction.
How is the Gyro Compass System made North Seeking?
North Seeking Gyro:-
In
order to damp unwanted oscillation, we need to achieve damping in tilt.
This
is done by means of offset slightly to the east of vertical, resulting in
component of the same force producing the required torque.
The
magnitude and direction of this force is pre-calculated to achieve the required
damping oscillation.
The
amplitude of each oscillation is reduced to 1/3rd of previous
oscillation.
The
spin axis reaches equilibrium and settles in a position at which drifting is counteracted
by control precession & the damping precession counteracts tilting.
Finally,
the gyro settles in the meridian & becomes north seeking.
Latitude course & speed error with respect to the Gyro CompassExplanation
Course, Speed and Latitude Error (Speed Error):-
The
gyro compass settles in the N/S direction by sensing Earth’s spinning motion.
Same gyro compass when placed on a ship also senses the ship’s motion. And
therefore, the axis of gyro compass settles in a direction which is
perpendicular to the resultant of the Earth’s surface speed and the ship’s
velocity.
The
direction, in which the compass settles, is therefore, different to the
direction of the True North and depends on ship’s course, speed and latitude of
the observer.
This
error also increases as the observer’s latitude increases. The error is
westward on all Northerly courses and vice-versa.
In
exactly E-W courses, the error is nil. In exactly N-S courses, the error is maximum.
To
compensate for speed error, a speed rider is provided, which in association
with the latitude rider, shifts the lubber line equal to speed error in the
appropriate direction.
This
error can be corrected automatically by a mechanism which moves the lubber line
by an amount equal to the error, or it can be found from correction tables or
from a portable correction calculator and then applied as necessary.
Three Degrees of Freedom:
As a mechanical device a gyroscope may be defined as a system containing a heavy metal wheel or rotor, universally mounted so that it has three degrees of freedom: spinning freedom, about an axis perpendicular through its center; tilting freedom, about a horizontal axis at right angles to the spin axis; and veering freedom, about a vertical axis perpendicular to both the other axes. The three degrees of freedom are obtained by mounting the rotor in two concentrically pivoted rings, called inner and outer rings. The whole assembly is known as the gimbal system of a free or space gyroscope. The gimbal system is mounted in a frame, so that in its normal operating position, all the axes are mutually at right angles to one another and intersect at the centre of gravity of the rotor.
Gyro Compass ‘Tangent Error’ Explanation
On
a non-pendulous gyrocompass where damping is accomplished by offsetting the
point of application of the force of mercury ballistic, the angle between the
local meridian and the settling position or spin axis.
Where
the offset of the point of application of mercury ballistic is to the east of
the vertical axis of the gyrocompass, the settling position is to the east of
the meridian in north latitudes and to the west of the meridian in south
latitudes.
The
error is so named because it is approximately proportional to the tangent of
the latitude in which the gyrocompass is operating.
The
tangent latitude error varies from zero at the equator to a maximum at high
northern and southern latitudes.
Starting a Gyro Compass:
A
gyro needs time to settle on the meridian, the time taken will depend on the
make, model & geographical location of the gyro.
The
settling time may be between one & several hours, manual provided by the
manufacturer has to be consulted before switching on the gyro.
If
compass has been switched off, it will take longer time to bring compass into
use.
Following
is the procedures for Sperry MK 37 digital.
At
power-up and prior entering the settling mode, system performs automatic procedure
to determine if the equipment is operating within specified parameters.
If
gyro is stationary the system opts for cold start, if rotating a hot start if
programmed.
During
a cold start, if no heading data is input to system when requested the gyro selects
automatic. Once the power is switched on, two bleeps prompts for heading input,
if the heading data is not entered within 5 minutes, the gyro switches to an
auto level process. (In some older make, the slewing is done manually, a
special key is provided for the same which is inserted into a slot).
If
heading data is fed the rotor is automatically slewed.
The
rotor is brought up to required speed within 14 minutes and the gyro will
subsequently settle within an hour.
If
heading data is not fed, the gyro will settle within 5 hrs.
Some more points:-
If entered heading is in error by more than 20 deg, gyro may take about 5 hours to settle.
Once gyro is settled, synchronize the repeaters (radar & ECDIS also need synchronization.)
If speed & latitude is fed manually, it should be done prior to starting the gyro.
Once settled, compass error should be checked & compasses should be checked more frequently.
