The compass error should be checked each watch and on every new course to measure the error of the gyrocompass, which is the angle the gyro north makes with the True north to establish trends in deviation on different courses at varying latitudes.This is important because should the gyro compass fail, this information will be needed to set courses using the magnetic compass. It is worth noting that some modern coasters only have a magnetic compass (they do not have a gyrocompass).
This is important because should the gyro compass fail, this information will be needed to set courses using the magnetic compass. It is worth noting that some modern coasters only have a magnetic compass (they do not have a gyrocompass).
Different methods of finding compass error
- By Azimuth of a Celestial Body: This has been discussed in detail in a separate module. Sun, selected stars, moon and planets can be used.
- By Amplitude of a Celestial Body: Usually, the sun or moon is used for taking amplitude which is explained in detail in a separate module. The Amplitude is taken when the body is on the true horizon. Stars and planets being small and dim when are on the horizon are not suitable.
- When at Berth: The difference between the observed direction of the jetty and the charted direction when the ship is fully alongside will give the compass error. Caution: This will only be accurate if the vessel is close to the jetty throughout its length and the jetty extends to the full length of the vessel. This may not be accurate if either the bow or stern is away from the jetty or the jetty is not complete. In some ports, the ship may never be alongside the berth due to various reasons and hence this method of taking an error should be double checked as soon as possible after departure by either taking an azimuth of a celestial body or by taking a transit bearing.
- Transit bearings: This is a line on the chart where an observer would see two identifiable objects in line. A bearing taken when both the objects are in line can be used to determine the gyro and compass errors by comparing charted and observed bearings.
- Leading Lights: This is similar to transit bearings where two identifiable objects are used to draw a line of bearing on the chart. These bearing lines are used to indicate track to be followed when approaching a port or a channel. The difference between the charted bearing and the heading of the ship when on the leads can be used to determine the error on both gyro and magnetic compass. BEWARE OF PARALLAX ERROR WITH THIS METHOD.
- Horizontal sextant angles: When used to plot ship’s position, the observed bearing of one of the objects used for plotting the position when compared to the bearing line drawn on the chart from the observed ship’s position to the same object would give the compass error.
COMPASS ERROR
Содержание
- What are errors of Gyro Compass ?
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- What is gyro compass error
- GYRO COMPASS ( Construction of GYROSCOPE ,PROPERTIES OF A FREE GYRO , Operational errors of a gyrocompass )
- PROPERTIES OF A FREE GYRO
- Terms definition
- Operational errors of a gyrocompass
- How is the Gyro Compass System made North Seeking?
- Various Types of gyro-compasses used on Merchant Ships
- Procedure starting and stopping of gyro compass and routine maintenance of gyro compass
- Starting a Gyro Compass.
- Stopping gyro compass.
- Maintenance
What are errors of Gyro Compass ?
1) Latitude error
Latitude (or damping) error This error is present in a compass which is damped in tilt. A compass damped in tilt always settles east of the meridian and above the horizon in NH , and vice-versa. Its magnitude depends on the observer’s latitude, S in (error) α Tan (latitude), At equator, the error is nil.
The error is eastward in all northerly latitudes, and vice-versa. For the purpose of damping error, a latitude rider / adjuster is provided with the gyro compass which shifts the lubber line equal to the amount of error in the appropriate direction.
2) Speed Error (Course, Speed and Latitude 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 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.
3) BALLISTIC DEFLECTION ERROR
A temporary oscillatory error of the gyro compass introduced when the north-south component of the speed changes as by speed or course. Change, An accelerating force acts upon the compass, causing a surge of mercury from one part of the system to another in the case of the non-pendulous compass, or a deflection of a mass in the case of a pendulous compass.
4) BALLISTIC DAMPING ERROR
A temporary oscillatory error of a gyro compass introduced during changes of course and speed as a result of the means used to damp the oscillations of the spin axis.
5) GIMBALLING ERROR
This is due to the tilt of the compass rose. Directions are measured in the horizontal plane. If the compass card is tilted, the projection of the outer rim in the horizontal is an ellipse, and the graduations are not equally spaced. For normal angles of tilt, this error is small and can be neglected.
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Источник
What is gyro compass error
COMPASS ERROR
(Extracts courtesy of A.N.T.A. publications, Ranger Hope © 2008 www.splashmaritime.com.au)
The courses and bearings laid on a chart are true, but we steer courses and take bearings using a compass.
The compass used in small vessels is more commonly a magnetic compass, although some may be fitted with a gyro compass.
The magnetic compass and the errors involved
The difference between direction as measured by the compass and the true direction as measured on the chart is termed compass error, stated differently:
It is the angular difference between true north and compass north.В It is named east or west to indicate the side of true north on which the compass north lies.
Figure 1:В Direction of Compass Error
The Compass Error is a combination of two separate and distinct components, namely variation and deviation.
Variation
When influenced only by the earth’s magnetic field, a compass needle will point towards the earth’s north magnetic pole. This pole is located somewhere to the north of Canada and is slowly moving.
Examination of a globe will show that from a position on the East Coast of Australia the compass will point in a direction to the east of true north.В This is magnetic north, and the angle between it and true north is called variation.В In our case variation is east.В
To find the value of variation for any position simply consult the nearest compass rose on a marine chart.В The variation will be given for a specified year, together with the rate of change, allowing calculation of variation for any subsequent year.В See appendix for variation chart of the world.
Chart Aus 823 gives the following information on the compass rose to the south of St Bees Island:
Mag Var 8 ° 40’E (1979) Increasing about 2’ annually.
In 1997 the variation will have increased by 2’ each year for 18 years, a total of 36. Adding this to 8 ° 40’ we find that the variation for 1997 is 9 ° 16’E.
Deviation
In the unlikely event that a vessel is constructed entirely from non magnetic materials and has no electronics close to the compass, variation is the only error which will need to be accounted for. In all other cases the vessel and/or its equipment will create magnetic fields of their own. Some of these will be built into the vessel on the slip, others will change as the vessel moves around within the influence of the earth’s magnetic field.
The compass adjuster is usually able to reduce the effect of the vessel’s magnetic fields, but the causes are so complex that it is inevitable that some effects remain. For the ship’s compass to work at all the effect of the ship’s magnetism must be less that the force of the earth’s magnetic field.
To illustrate the effect of the vessel’s own magnetic field, imagine a vessel on which the compass needle is attracted towards the stern. When that vessel is heading towards magnetic north the effect of the pull towards the stern is to reduce the directive force at the compass but not to deflect it from magnetic north. As the vessel turns onto easterly headings the compass needle is deflected towards the stern i.e. towards west. When the vessel heads west the compass needle is deflected to the east. There will be no deflection when the vessel heads south, but an increase in directive force.
This deflection of the compass away from magnetic north is called deviation.В As with variation it is named East or West and the value will change according to the ship’s heading.В A deviation card is produced by the compass adjuster when the vessel is first commissioned and at intervals throughout its life.В It is displayed close to the compass position.В
The relationship between compass, magnetic and true courses and bearings is shown in the following diagram.
Figure 2:В Relationship between Compass, magnetic and truc courses and bearings.
Rules for applying Variation and Deviation.
To avoid drawing diagrams every time variation and deviation are applied, a number of memory aids have been developed to clarify the rules of application:
Television Makes Dull Company
(T V M D C) reminds us that to true we must apply the variation to find magnetic, and to this we apply deviation to arrive at compass (course or bearing).
If we start with a compass bearing and wish to convert it to true the order of operation is reversed (C D M V T).
Having decided the correct order in which to apply variation and deviation, we need to know whether the correction should be added or subtracted.В This may be decided using the word:
This simply indicates that to get from compass to true (the end points) we Add East.
Given that we add east (deviation or variation) it follows that we must subtract west (deviation or variation).
It also follows that if we add east to get from compass to true, we should add west when going from true to compass.
A simple layout for applying these rules is shown on the next page.
Compass Error
If we expect to take several bearings whilst steering the one steady course it makes good sense to arrive at a single correction to apply to all those bearings.В Remembering that the variation remains effectively the same whilst operating in one area, and that deviation only changes when we change course, we can find the compass error for the course being steered and apply that to all bearings taken whilst on that course.
We are steering 076 В° (C).В The deviation from the deviation card for 076 В° is 3 В° W and the variation from the chart is 11 В° E.В The compass error (combined variation and deviation) is therefore 8 В° E.В Since we are converting compass bearings to true we add East (CADET).
Therefore whilst steering 076 В° (C) add 8 В° to any compass bearing to convert it to true.
Simple layout for applying variation and deviation:
1.В Changing from compass to true.
Источник
GYRO COMPASS ( Construction of GYROSCOPE ,PROPERTIES OF A FREE GYRO , Operational errors of a gyrocompass )
- There is a thick wheel known as gyro wheel (which is made to rotate at high speed )
- It is mounted on a circular ring by its axil.
- To make the disc rotate in every direction 3 rings are used as shown in the above gif
- Basically, all the 3 discs are connected at 90 degrees, this allows a full 360 movement of a disc in any direction.
- It gives a 3-degree freedom which means
- It can spin on its axis .
- It can tilt about on its Horizontal plane .
- It can turn about its vertical axis .
PROPERTIES OF A FREE GYRO
1) rigidity of a space
when the disc is made to spin on its axis it is continuously pointing to a fix direction has the same direction as it was started it will not change the direction, although the supporting base is moved or tilted this unique property of a spinning free gyrascope is called rigidity of a space.
when the free gyroscope made to spin on its Axis it is continuously pointing to a fixed direction as the same direction as it was started. Assuming it is directed to a distance imaginary point in the space as referred to Cairo compass it is called gyrostar .
3) Precession
The axis of rotation has a tendency to turn at right angle to the direction of applied force.
Terms definition
1)Tilt -this is the angle by which the spin axis of gyro has apprently moved up and down from the horizontal plane.
2)Tilting- This is the apprent motion of gyro axel on the vertical plane. it may be upward and downward depending on factors which is causing it.
Rate of tilting per hour = 15 cos (latitude) * sin (azimuth).
3) Drift – This is the angle by which gyro axel has drifted and moved away from the original direction in the horizontal plane.
4) Drifting – This is the apparent motion of Gyro axel on the horizontal plane.
rate of drifting per hour = 15 *sin (Lat)
Operational errors of a gyrocompass
- Latitude error (Damping error or setting error).
- Course, Latitude & speed error (Steaming error).
- Ballistic Deflection
- Ballistic Tilt
- Rolling Error
- Inter Cardinal Rolling Error
- Rolling error
Gyro compass fitted on a ship also senses the accelerations present within the vessel at the time of rolling and pitching. The errors resulting due to that are known as rolling errors. - Speed and latitude error
It is caused by force generated by a combination of Earth’s rotation and Ships movement. - If the ship course is northerly the error is westerly and if the course is southerly the error is easterly.
- These errors are corrected by adjustment of latitude and speed corrector located on the gyroscope body.
- Ballistic deflection
This error occurs whenever the ship’s speed and the course is rapidly changed. If a vessel is going on N course and then changes its course by 90 degrees there will be a surge of mercury (Hg) from S part to N part ( As governed by Newton’s first law of motion).
How is the Gyro Compass System made North Seeking?
- 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 1/3 rd 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.
Various Types of gyro-compasses used on Merchant Ships
- Admiralty Sperry type
- Anschutz
- Arma Brown compass
Procedure starting and stopping of gyro compass and routine maintenance of gyro compass
Starting a Gyro Compass.
1) refer to the manufacturer’s manual and follow the procedure.
2)make a preparation of at least four hours before the compass is required for service
3) check that all supply switches are open
4) adjust the latitude and speed setting accordingly
5) Switch on an alternator and wait for 10 seconds until it gains full speed.
the compass will settle faster in Port then at sea also depends on how much the gyro axel was out of Meridian.
6) test the alarm switch on the alarm panel.
Stopping gyro compass.
1) open repeater switch.
2) open the azimuth motor switch.
3) switch off the alternator and lock the rotor.
Maintenance
Each watch
1) check repeater with a master compass to ensure that repeater is functioning properly if Power fails repeater may have to be reset.
2) check the compass error by azimuth
3) speed and latitude characters should be reset as necessary
4) inspect the compass to guard against any abnormal condition of operation.
Monthly
a) Check the alarm buzzer.
b) Clean and oil any part as indicated in the manufacturer’s manual.
c)General cleanliness should be checked repair and maintenance should only be carried out by a professional.
Источник
The total of the all the combined errors of the gyrocompass is called gyro error and is expressed in degrees E or W, just like variation and deviation. But gyro error, unlike magnetic compass error, and being independent of Earth’s magnetic field, will be constant in one direction; that is, an error of one degree east will apply to all bearings all around the compass.
The errors to which a gyrocompass is subject are speed error, latitude error, ballistic deflection error, ballistic damping error, quadrantal error, and gimballing error. Additional errors may be introduced by a malfunction or incorrect alignment with the centerline of the vessel.
Speed error is caused by the fact that a gyrocompass only moves directly east or west when it is stationary (on the rotating earth) or placed on a vessel moving exactly east or west. Any movement to the north or south will cause the compass to trace a path which is actually a function of the speed of advance and the amount of northerly or southerly heading. This causes the compass to tend to settle a bit off true north. This error is westerly if the vessel’s course is northerly, and easterly if the course is southerly. Its magnitude depends on the vessel’s speed, course, and latitude. This error can be corrected internally by means of a cosine cam mounted on the underside of the azimuth gear, which removes most of the error. Any remaining error is minor in amount and can be disregarded.
Tangent latitude error is a property only of gyros with mercury ballistics, and is easterly in north latitudes and westerly in south latitudes. This error is also corrected internally, by offsetting the lubber’s line or with a small movable weight attached to the casing.
Ballistic deflection error occurs when there is a marked change in the north-south component of the speed. East-west accelerations have no effect. A change of course or speed also results in speed error in the opposite direction, and the two tend to cancel each other if the compass is properly designed. This aspect of design involves slightly offsetting the ballistics according to the operating latitude, upon which the correction is dependent. As latitude changes, the error becomes apparent, but can be minimized by adjusting the offset.
Ballistic damping error is a temporary oscillation introduced by changes in course or speed. During a change in course or speed, the mercury in the ballistic is subjected to centrifugal and acceleration/deceleration forces. This causes a torquing of the spin axis and subsequent error in the compass reading. Slow changes do not introduce enough error to be a problem, but rapid changes will. This error is counteracted by changing the position of the ballistics so that the true vertical axis is centered, thus not subject to error, but only when certain rates of turn or acceleration are exceeded.
Quadrantal error has two causes. The first occurs if the center of gravity of the gyro is not exactly centered in the phantom. This causes the gyro to tend to swing along its heavy axis as the vessel rolls in the sea. It is minimized by adding weight so that the mass is the same in all directions from the center. Without a long axis of weight, there is no tendency to swing in one particular direction. The second source of quadrantal error is more difficult to eliminate. As a vessel rolls in the sea, the apparent vertical axis is displaced, first to one side and then the other. The vertical axis of the gyro tends to align itself with the apparent vertical. On northerly or southerly courses, and on easterly or westerly courses, the compass precesses equally to both sides and the resulting error is zero. On intercardinal courses, the N-S and E-W precessions are additive, and a persistent error is introduced, which changes direction in different quadrants. This error is corrected by use of a second gyroscope called a floating ballistic, which stabilizes the mercury ballistic as the vessel rolls, eliminating the error. Another method is to use two gyros for the directive element, which tend to precess in opposite directions, neutralizing the error.
Gimballing error is caused by taking readings from the compass card when it is tilted from the horizontal plane. It applies to the compass itself and to all repeaters. To minimize this error, the outer ring of the gimbal of each repeater should be installed in alignment with the fore-and- aft line of the vessel. Of course, the lubber’s line must be exactly centered as well
Gyro compass and compass error
COMPASS ERROR
(Extracts courtesy of A.N.T.A. publications, Ranger Hope © 2008 www.splashmaritime.com.au)
The courses and bearings laid on a chart are true, but we steer courses and take bearings using a compass.
The compass used in small vessels is more commonly a magnetic compass, although some may be fitted with a gyro compass.
The magnetic compass and the errors involved
The difference between direction as measured by the compass and the true direction as measured on the chart is termed compass error, stated differently:
It is the angular difference between true north and compass north.В It is named east or west to indicate the side of true north on which the compass north lies.
Figure 1:В Direction of Compass Error
The Compass Error is a combination of two separate and distinct components, namely variation and deviation.
Variation
When influenced only by the earth’s magnetic field, a compass needle will point towards the earth’s north magnetic pole. This pole is located somewhere to the north of Canada and is slowly moving.
Examination of a globe will show that from a position on the East Coast of Australia the compass will point in a direction to the east of true north.В This is magnetic north, and the angle between it and true north is called variation.В In our case variation is east.В
To find the value of variation for any position simply consult the nearest compass rose on a marine chart.В The variation will be given for a specified year, together with the rate of change, allowing calculation of variation for any subsequent year.В See appendix for variation chart of the world.
Chart Aus 823 gives the following information on the compass rose to the south of St Bees Island:
Mag Var 8 ° 40’E (1979) Increasing about 2’ annually.
In 1997 the variation will have increased by 2’ each year for 18 years, a total of 36. Adding this to 8 ° 40’ we find that the variation for 1997 is 9 ° 16’E.
Deviation
In the unlikely event that a vessel is constructed entirely from non magnetic materials and has no electronics close to the compass, variation is the only error which will need to be accounted for. In all other cases the vessel and/or its equipment will create magnetic fields of their own. Some of these will be built into the vessel on the slip, others will change as the vessel moves around within the influence of the earth’s magnetic field.
The compass adjuster is usually able to reduce the effect of the vessel’s magnetic fields, but the causes are so complex that it is inevitable that some effects remain. For the ship’s compass to work at all the effect of the ship’s magnetism must be less that the force of the earth’s magnetic field.
To illustrate the effect of the vessel’s own magnetic field, imagine a vessel on which the compass needle is attracted towards the stern. When that vessel is heading towards magnetic north the effect of the pull towards the stern is to reduce the directive force at the compass but not to deflect it from magnetic north. As the vessel turns onto easterly headings the compass needle is deflected towards the stern i.e. towards west. When the vessel heads west the compass needle is deflected to the east. There will be no deflection when the vessel heads south, but an increase in directive force.
This deflection of the compass away from magnetic north is called deviation.В As with variation it is named East or West and the value will change according to the ship’s heading.В A deviation card is produced by the compass adjuster when the vessel is first commissioned and at intervals throughout its life.В It is displayed close to the compass position.В
The relationship between compass, magnetic and true courses and bearings is shown in the following diagram.
Figure 2:В Relationship between Compass, magnetic and truc courses and bearings.
Rules for applying Variation and Deviation.
To avoid drawing diagrams every time variation and deviation are applied, a number of memory aids have been developed to clarify the rules of application:
Television Makes Dull Company
(T V M D C) reminds us that to true we must apply the variation to find magnetic, and to this we apply deviation to arrive at compass (course or bearing).
If we start with a compass bearing and wish to convert it to true the order of operation is reversed (C D M V T).
Having decided the correct order in which to apply variation and deviation, we need to know whether the correction should be added or subtracted.В This may be decided using the word:
This simply indicates that to get from compass to true (the end points) we Add East.
Given that we add east (deviation or variation) it follows that we must subtract west (deviation or variation).
It also follows that if we add east to get from compass to true, we should add west when going from true to compass.
A simple layout for applying these rules is shown on the next page.
Compass Error
If we expect to take several bearings whilst steering the one steady course it makes good sense to arrive at a single correction to apply to all those bearings.В Remembering that the variation remains effectively the same whilst operating in one area, and that deviation only changes when we change course, we can find the compass error for the course being steered and apply that to all bearings taken whilst on that course.
We are steering 076 В° (C).В The deviation from the deviation card for 076 В° is 3 В° W and the variation from the chart is 11 В° E.В The compass error (combined variation and deviation) is therefore 8 В° E.В Since we are converting compass bearings to true we add East (CADET).
Therefore whilst steering 076 В° (C) add 8 В° to any compass bearing to convert it to true.
Simple layout for applying variation and deviation:
1.В Changing from compass to true.
Источник
Gyro Compass Errors and Corrections — Nikos Karandinos [201x, PDF]
Gyro Compass Errors and Corrections
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| # | Тема | Форум | Автор |
|---|---|---|---|
| EN | Учебные материалы | paolina_3 | |
| EN | Навигация | NNT | |
| EN | Морская Астрономия | twicer | |
| EN | Chief Mates (Phase II) | gillnumil | |
| EN | Principles of Navigation — Prepared by : Ranjeet Singh [2018, PDF] | Навигация | TINGTONG |
| EN | Chief Mates (Phase II) | gillnumil | |
| EN | Библиотека судоводителя | twicer | |
| EN | Библиотека судоводителя | twicer | |
| EN | Miscellaneous | twicer | |
| EN | Морская Астрономия | SerTan |
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Источник
GYRO COMPASS ( Construction of GYROSCOPE ,PROPERTIES OF A FREE GYRO , Operational errors of a gyrocompass )
- There is a thick wheel known as gyro wheel (which is made to rotate at high speed )
- It is mounted on a circular ring by its axil.
- To make the disc rotate in every direction 3 rings are used as shown in the above gif
- Basically, all the 3 discs are connected at 90 degrees, this allows a full 360 movement of a disc in any direction.
- It gives a 3-degree freedom which means
- It can spin on its axis .
- It can tilt about on its Horizontal plane .
- It can turn about its vertical axis .
PROPERTIES OF A FREE GYRO
1) rigidity of a space
when the disc is made to spin on its axis it is continuously pointing to a fix direction has the same direction as it was started it will not change the direction, although the supporting base is moved or tilted this unique property of a spinning free gyrascope is called rigidity of a space.
when the free gyroscope made to spin on its Axis it is continuously pointing to a fixed direction as the same direction as it was started. Assuming it is directed to a distance imaginary point in the space as referred to Cairo compass it is called gyrostar .
3) Precession
The axis of rotation has a tendency to turn at right angle to the direction of applied force.
Terms definition
1)Tilt -this is the angle by which the spin axis of gyro has apprently moved up and down from the horizontal plane.
2)Tilting- This is the apprent motion of gyro axel on the vertical plane. it may be upward and downward depending on factors which is causing it.
Rate of tilting per hour = 15 cos (latitude) * sin (azimuth).
3) Drift – This is the angle by which gyro axel has drifted and moved away from the original direction in the horizontal plane.
4) Drifting – This is the apparent motion of Gyro axel on the horizontal plane.
rate of drifting per hour = 15 *sin (Lat)
Operational errors of a gyrocompass
- Latitude error (Damping error or setting error).
- Course, Latitude & speed error (Steaming error).
- Ballistic Deflection
- Ballistic Tilt
- Rolling Error
- Inter Cardinal Rolling Error
- Rolling error
Gyro compass fitted on a ship also senses the accelerations present within the vessel at the time of rolling and pitching. The errors resulting due to that are known as rolling errors. - Speed and latitude error
It is caused by force generated by a combination of Earth’s rotation and Ships movement. - If the ship course is northerly the error is westerly and if the course is southerly the error is easterly.
- These errors are corrected by adjustment of latitude and speed corrector located on the gyroscope body.
- Ballistic deflection
This error occurs whenever the ship’s speed and the course is rapidly changed. If a vessel is going on N course and then changes its course by 90 degrees there will be a surge of mercury (Hg) from S part to N part ( As governed by Newton’s first law of motion).
How is the Gyro Compass System made North Seeking?
- 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 1/3 rd 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.
Various Types of gyro-compasses used on Merchant Ships
- Admiralty Sperry type
- Anschutz
- Arma Brown compass
Procedure starting and stopping of gyro compass and routine maintenance of gyro compass
Starting a Gyro Compass.
1) refer to the manufacturer’s manual and follow the procedure.
2)make a preparation of at least four hours before the compass is required for service
3) check that all supply switches are open
4) adjust the latitude and speed setting accordingly
5) Switch on an alternator and wait for 10 seconds until it gains full speed.
the compass will settle faster in Port then at sea also depends on how much the gyro axel was out of Meridian.
6) test the alarm switch on the alarm panel.
Stopping gyro compass.
1) open repeater switch.
2) open the azimuth motor switch.
3) switch off the alternator and lock the rotor.
Maintenance
Each watch
1) check repeater with a master compass to ensure that repeater is functioning properly if Power fails repeater may have to be reset.
2) check the compass error by azimuth
3) speed and latitude characters should be reset as necessary
4) inspect the compass to guard against any abnormal condition of operation.
Monthly
a) Check the alarm buzzer.
b) Clean and oil any part as indicated in the manufacturer’s manual.
c)General cleanliness should be checked repair and maintenance should only be carried out by a professional.
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What are errors of Gyro Compass ?
1) Latitude error
Latitude (or damping) error This error is present in a compass which is damped in tilt. A compass damped in tilt always settles east of the meridian and above the horizon in NH , and vice-versa. Its magnitude depends on the observer’s latitude, S in (error) α Tan (latitude), At equator, the error is nil.
The error is eastward in all northerly latitudes, and vice-versa. For the purpose of damping error, a latitude rider / adjuster is provided with the gyro compass which shifts the lubber line equal to the amount of error in the appropriate direction.
2) Speed Error (Course, Speed and Latitude 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 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.
3) BALLISTIC DEFLECTION ERROR
A temporary oscillatory error of the gyro compass introduced when the north-south component of the speed changes as by speed or course. Change, An accelerating force acts upon the compass, causing a surge of mercury from one part of the system to another in the case of the non-pendulous compass, or a deflection of a mass in the case of a pendulous compass.
4) BALLISTIC DAMPING ERROR
A temporary oscillatory error of a gyro compass introduced during changes of course and speed as a result of the means used to damp the oscillations of the spin axis.
5) GIMBALLING ERROR
This is due to the tilt of the compass rose. Directions are measured in the horizontal plane. If the compass card is tilted, the projection of the outer rim in the horizontal is an ellipse, and the graduations are not equally spaced. For normal angles of tilt, this error is small and can be neglected.
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