Gyro Notes

NAVIGATIONAL EQUIPMENTS COURSE

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4. GYROCOMPASS GYROCOMPASS

ROOT OF ALL REFERENCES: The most basic natural reference is the earth’s axis of rotation, which gives rise to all other references and makes us feel a sense of directions. The direction towards which earth rotates is called east and the direction from which earth rotates is called west. In other words, earth rotates in counter clockwise direction when viewed from above. The axis of rotation appears to pass through earth’s surface at two points, called poles. The upper pole is called North Pole and lower pole is South Pole. All the meridians pass through these poles and make a good reference for our sense of direction. These meridians appear as vertical and parallel straight lines on a Mercator chart, the type used onboard onboard ships. ships. These These lines lines indicat indicatee True True North North and all direct directions ions or courses courses in degree degreess are


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measured from any of these lines in a clockwise direction. Compass rose on a chart uses this principle and aims at simplifying the measurements. Thus onboard ships, we need a device which always tells us about the direction of North so that we can readily know in which direction the ship is heading. Gyrocompass is the answer for that.

WHAT IS GYROCOMPASS? : A gyrocompass is a compass which finds true North by using an electrically powered fast spinning wheel and friction forces in order to use the rotation of the Earth. Gyro compass differs from magnetic compass in the sense that while magnetic compass indicates magnetic north gyrocompass points to true north.


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PRINCIPLE OF GYRO COMPASS: A gyrocompass is essentially a gyroscope or a fast spinning wheel mounted so that the wheel's axis is free to orient itself in any way. Because of the law of conservation of angular momentum, such a wheel maintains its original orientation. Since the Earth rotates, it appears to a stationary observer on Earth that a gyroscope's axis is rotating once every 24 hours; the axis always points to the same star. Such a rotating gyroscope cannot be used for navigation. The crucial additional ingredient needed for a gyrocompass is force of friction. This friction force results in a torque acting on the axis, causing the axis to turn in a direction perpendicular to the force. Over time, this has the effect that the axis will point to true North (to the North Star), at which point the axis will appear to be stationary and won't experience any more friction forces.


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Earth is the best example of free gyroscope in nature, because: It is freely suspended in space having no friction and thus having 3 degrees of freedom. It is heavy and well balanced with the equatorial mass corresponding to the plane o a rotor. Earth rotates at considerably high speed about its speed (equivalent to spin axis of rotor)

TWO IMPORTANT PROPERTIES OF A FREE GYROSCOPE: 1. 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 or moved. It resists any attempt to change its direction of spin. This property is called GYROSCOPIC INERTIA or RIGIDITY IN SPACE. The gyroscopic inertia of a rotor can be quantitatively expressed by its angular momentum (H). This will determine how much force is needed to change the direction in which the spin axis points at a given rate. H depends on ω (angular velocity) & I (moment of inertia) and is given by: H= ω I


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THIS IS DUE TO THE THREE DEGREES OF FREEDOM OF MOVEMENT: Freedom to spin about the axis of rotation Freedom to tilt in the vertical plane, about the horizontal axis (torque axis) Freedom to drift in the horizontal, plane about the vertical axis (precession

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axis)

2. PRECESSION: It is the movement of the spin axis when a force is applied to the gyroscope. When a couple is applied about its horizontal axis the spin axis will turn at right angles to the applied force in the direction of the spin of the wheel. Similarly couple applied about the vertical axis will make the spin axis turn about the horizontal axis in the direction of the spin axis of the wheel. This property is called PRECESSION.


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DESCRIPTION OF WORKING:

The gyrocompass is essentially a north-seeking gyroscope. It is encased in a housing fitted within various electronic components that keep the spin axis of the gyro aligned with terrestrial meridians, and sense the angle between the ship’s head and the gyro spin axis. The gyrocompass has three axes: the spin axis, torque axis, and precession

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axis. •

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The gyrocompass is usually located well down in the interior of the hull in order to minimize the effects of pitch and roll. The gyrocompass is connected by cables to gyrocompass repeaters located throughout the ship. These repeaters use electronic servo-mechanisms to reproduce the master gyrocompass readings at remote locations.


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A gyrocompass combines the action of two devices, a pendulum and a gyroscope, to produce alignment with the Earth's spin axis. The principle is demonstrated with the model shown in the illustration, which consists of a rapidly spinning, heavy gyro rotor, a pendulous case which permits the rotor axle to nod up and down (angle θ), and an outer gimbal which permits the axle to rotate in azimuth (angle ψ). For a gyroscope positioned at the Equator of the Earth, as the Earth rotates, the gimbal moves with it. So long as the rotor's spin axis is aligned with the Earth's axis, the gyro experiences no torque from Earth rotation. If there is misalignment, however, a sequence of restoring torques is initiated.

Gyrocompass Model.

Onboard the system is mounted in a complete set of gimbals to isolate it from rolling, pitching, and yawing motions of the ship. Friction must be minimized. The axle of the spinning wheel defines the spin axis. The inner gimbal possesses two degrees of rotational freedom and its axis possesses one. The rotor is journaled to spin about an axis which is always perpendicular to the axis of the inner gimbal. So, the rotor possesses three degrees of rotational freedom and its axis possesses two.


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ADVANTAGES OF GYROCOMPASS: THE GYROCOMPASS HAS SEVERAL ADVANTAGES OVER THE MAGNETIC COMPASS: • • •

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It is compact in size and highly reliable. It provides true North as opposed to magnetic North. It is easier to fit repeaters with gyrocompass and also to provide heading reference for other navigational equipment. It is not affected by metal in ship’s hull. Errors in readings are very small.

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DISADVANTAGES OF GYROCOMPASS: Being an intricate electronic instrument, the gyrocompass has some disadvantages: It requires a constant source of electrical power and is sensitive to power

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fluctuations. •

It requires periodic maintenance by qualified technicians

STARTING OF GYROCOMPASS: •

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Check the gyro heading and compare with magnetic compass after observing compass error. If the gyro axis is displaced by more than 20o then allow for about 6 to 7 hours of settling hours, Otherwise 4-5 hrs. must be allowed for settling time (check technical manual onboard) Disconnect or switch off all gyro repeater switches. Switch on the main switch. Check for oil or liquid level if window provided (refer to technical manual onboard) Adjust latitude and speed correctors- settings Switch on the alternator and wait for 10 seconds until it gains full speed. Switch ‘on’ the compass and azimuth motor switch. After heading is steady, switch on the repeater’s switch after aligning the repeater with master compass. Check compass error using astronomical body, leading lights, transit bearings etc. and compare repeater compass. If speed and position (latitude) input direct through GPS and /or log check that same is working.


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Check and verify compass error at regular intervals.

STOPPING OF GYROCOMPASS FOR LONG TIME: • • • •

Stop Repeaters Stop Azimuth/Compass Motors Switch Off The Alternator Lock The Rotor (Gimbals)

GYRO ERROR AND ITS CORRECTION: Charts are drawn with reference to true North. Any bearing read and picked up from chart is a true bearing. Conversely, any bearing, gyro or compass, needs to be corrected to true bearing before it can be plotted on a chart.


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Although the gyrocompass is a very accurate instrument and normally has a very small error associated with its readings (less than .10 to .20), the navigator is required to determine gyro error at least once a watch. This small amount of error is usually due to latitude in which ship is and the velocity of the vessel. Gyrocompass is usually provided with Latitude and Speed correctors, correct setting of which incorporates the desired corrections and the corrected reading is provided. Small amount of errors, however, can still creep into that need to be corrected mathematically. Gyrocompass error like magnetic compass error is measured in degrees as high (westerly) or low (easterly): –

If the gyrocompass bearing is higher than the actual bearing, the error is high and it has to be subtracted from the gyro reading to get true reading.

For example, in the figure above, the gyro bearing of the tank is 095 deg while the true bearing is 090 deg. Gyro reading being higher, the gyro error is 5 deg high and this amount needs to be subtracted from all gyro readings to get true readings. Gyro bearing = 095 deg True bearing = 090 deg Gyro error = 005 deg high

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If the gyrocompass bearing is lower than the actual bearing, the error is low and it has to be added to the gyro reading to get true reading


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For example, in the figure above, the gyro bearing of the tank is 085 deg while the true bearing is 090 deg. Gyro reading being lower, the gyro error is 5 deg low and this amount needs to be added to all gyro readings to get true readings. Gyro bearing = 085 deg True bearing = 090 deg Gyro error = 005 deg low

METHODS OF DETERMINING GYRO ERROR: Compass error should be checked at least once a watch and recorded in compass error book. There are several methods of determining gyrocompass error: • • •

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By taking azimuth of sun, moon, star or planet.. By comparing transit bearing of gyro with the transit bearing on chart.. If the ship is at a known location, such as a pier or an anchorage, a gyro error can be obtained by comparing a known bearing to an object ashore, as measured on a chart. Comparing the ship’s heading while pier side to the known heading of the pier will give gyro error.

Gyro Notes

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