Meridian Gyrocompass

7/17/2019 Meridian Gyrocompass

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A Division of TSS (UK) Limited

S G Brown

The information in this Manual is subject to

change without notice and does not represent

a commitment on the part of SG Brown

SG Brown

1 Garnett Close

Greycaine Industrial Estate

Watford

Hertfordshire WD2 4JL UK

Telephone +44 (0)1923 470800

Facsimile +44 (0)1923 470838

Meridian Gyrocompass

SystemManual

Document P/N 060070

Issue 2.0

© SG Brown May 2000





Contents

DPN 060070 Issue 2.0C © SGBrown Page 1 of 6

CAUTIONARY NOTICESYour attention is drawn to the following cautionary notices that apply throughout this Manual.

WARNINGThe Meridian Gyrocompass weighs 15.5kg. To avoid personal injury, take proper pre-

cautions if you lift or move the equipment.

CAUTIONThe Meridian Gyrocompass includes precision components and bearings. To avoidcausing damage to any part of the System, handle all items with care.

Retain the original transit cases so that you can use them to transport the system whennecessary. You will void the warranty if you use improper packing during transporta-tion.

CAUTIONSevere damage to the Meridian Gyrocompass can occur if you move the gyrocompasswhile the rotor is still spinning without the servo system in operation.

Note that the gyro rotor continues to spin for approximately ten minutes after youpower-off the system.

To avoid potential damage to the Meridian Gyrocompass, always allow a period of tenminutes after power-off for the gyro rotor to come to rest before you attempt to movethe gyrocompass.

CAUTIONDuring operation, the gyrocompass must remain level to within ±45°. If it experiencestilt greater than 45° in any direction, it will ‘topple’. Safety routines in the gyro softwarewill then power-off the gyro rotor and show alarm conditions on the RCU. To restorenormal operation, establish a level operating attitude and then power-on the gyrocom-pass normally.

Never apply a tilt of more than 45° with the gyro rotor spinning or during the gyrocom-pass initialisation procedure. Note that the gyro rotor continues to spin for approxi-mately ten minutes after you power-off the System.

CAUTIONIf you install the gyrocompass in an enclosed space, make certain there is sufficientventilation and circulation of free air to allow effective cooling.

CAUTIONDo not make any connections to the gyrocompass with power on the supply cable.




Page 2 of 6 © SGBrown Issue 2.0C DPN 060070

CAUTION You will void the warranty if you make any modifications to this equipment withoutprior permission from SG Brown.

DO NOT modify this equipment in any way without obtaining permission from SG

Brown.



Contents


CONTENTS

1 INTRODUCTION 1–11.1 System Description 1–31.1.1 Gyrocompass 1–3

1.1.2 Remote Control Unit 1–41.1.3 Auxiliary Inputs 1–51.1.4 Heading Outputs 1–5

1.2 Principle of Operation 1–5

2 INSTALLATION 2–12.1 Unpacking and Inspection 2–22.2 Physical and Electrical Installation 2–32.2.1 Selecting a location 2–32.2.2 Gyrocompass installation 2–42.2.3 External Remote Control Unit 2–92.2.4 Set the Gyrocompass DIP Switches 2–10

2.3 Alignment 2–122.4 Final Gyrocompass Installation Tests 2–132.5 Installation Drawings 2–14

3 OPERATING INSTRUCTIONS 3–13.1 Control Features 3–23.2 Initial Power-on 3–33.3 Operating Procedure 3–43.3.1 Latitude correction 3–4

3.3.2 Speed correction 3–43.3.3 DG operating mode 3–5

3.4 Error Modes 3–53.4.1 Loss of GPS 3–53.4.2 Loss of speed log 3–63.4.3 Gyrocompass failure 3–6

3.5 Operating Considerations 3–83.5.1 General Operating Considerations 3–83.5.2 Operating Considerations for High Speed Craft 3–8

4 TECHNICAL DATA 4–1

4.1 Specifications 4–14.1.1 Power Requirements 4–14.1.2 Performance (definitions as in ISO 8728) 4–14.1.3 Compensation 4–14.1.4 Environment 4–14.1.5 Signal Inputs 4–14.1.6 Signal outputs 4–24.1.7 Dimensions and Weight 4–24.1.8 Standards 4–2

4.2 Data Formats 4–3

4.2.1 NMEA 0183 Serial Data Formats – General information 4–44.2.2 Inputs 4–5





4.2.2.1 NMEA 0183 input signals 4–54.2.2.2 Pulsed input 4–9

4.2.3 Outputs 4–104.2.3.1 NMEA output signals 4–10

4.2.4 Course Recorder Output 4–14

4.2.5 NMEA 0183 sentence with Checksum 4–154.2.6 Other Output Formats 4–154.2.6.1 Synchro Output 4–154.2.6.2 Resolver Output 4–154.2.6.3 Stepper S-Code 4–164.2.6.4 Rate of Turn 4–16

5 MAINTENANCE 5–15.1 Built-in Test Equipment 5–25.1.1 Azimuth Drift Adjustment 5–35.1.2 Azimuth Bias Adjustment 5–3

5.2 Test Connector 5–4

A OPERATING THEORY A–1A.1 North-seeking Gyroscope A–2A.2 Gyrocompass Corrections A–5A.2.1 Latitude Correction A–5A.2.2 Gyro Damping A–5A.2.3 Speed Error A–6

A.3 Summary A–7



Contents


TABLE OF AMENDMENTS

Old Issue New Issue Date Details

– 2.0

2.0A2.0B

2.0C

11 May 2000

12 Dec 20001 Mar 2001

23 Apr 2001

New release. Corrected identification of Azimuth Bias potenti-

ometer and other details. Include DIP switch default settings.Modify power connection details.

Add notification to avoid product modif ications.







1 – Introduction

DPN 060070 Issue 2.0C © SG Brown Page 1 of 6

1 INTRODUCTION

The Meridian Gyrocompass is a master heading reference instrument that applies the charac-teristics of a dynamically tuned gyroscope and the effects of gravity and earth rotation to pro-vide a true north reference.

The Meridian Gyrocompass specification makes the System ideal for installation and opera-tion on board vessels of almost any size and in a wide range of applications.

Among the standard features of the Meridian Gyrocompass are:

A short settling time

Operation from a 24V DC electrical supply

This Manual is an important part of the Meridian Gyrocompass. It describes the System andcontains full installation and operating instructions. You should retain the Manual with the

System for use by personnel who will install and operate it.

Installation and operation of the Meridian Gyrocompass are not complex tasks. However, youshould spend time to familiarise yourself with the contents of this Manual before you start toinstall or use the System. Time spent in identifying the task sequence now will ensure yourSystem is operational in the minimum of time.

WARNINGSWhere appropriate, this Manual includes important safety information highlighted asWARNING and CAUTION instructions. You must obey these instructions:

WARNING instructions alert you to a potential risk of death or injury to users of theSystem.

CAUTION instructions alert you to the potential risk of damage to the System.

For your convenience, the Table of Contents section includes copies of all theWARNING and CAUTION instructions included in this Manual.

Throughout this Manual all measurements conform to the SI standard of units unless other-wise indicated.

For your convenience, this Manual includes several sections, each of which describes specificfeatures of the Meridian Gyrocompass:

You should read sections 1 and 2 before you attempt to install the System:

Section 1 contains introductory notes and describes those items supplied as standard.

Section 2 describes how to select a suitable location for the gyrocompass. This sectionincludes full instructions to install the System and connect it to external equipment.




Page 2 of 6 © SG Brown Issue 2.0C DPN 060070

You should read sections 3 and 4 before you use the System:

Section 3 describes how to operate the Meridian Gyrocompass.

Section 4 includes the System specifications and descriptions of the data formats.

You should read sections 5 if you suspect a fault on the System:

Section 5 describes how to use the internal 60-way test connector and explains how to con-duct simple adjustments with the gyrocompass housing removed.

This Manual also contains the following appendices:

Appendix A explains how a gyroscope can be made north seeking for use in a gyrocompass.

CAUTION You will void the warranty if you make any modifications to this equipment withoutprior permission from SG Brown.

DO NOT modify this equipment in any way without obtaining permission from SGBrown.



1 – Introduction


1.1 S YSTEM DESCRIPTION

The Meridian Gyrocompass comprises two sub-assemblies:

The gyrocompass housing

The Remote Control Unit (RCU)

Figure 1–1 shows the combined gyrocompass housing with the RCU included as an integralunit.

The Meridian Gyrocompass applies dynamic tuning to settle automatically to within 0.7° ofthe meridian within 45 minutes of power-on. Due to the physical principles of a north-seekinggyrocompass, achievable accuracy depends on the operating latitude and the vessel dynamics.To optimise its performance, the Meridian Gyrocompass uses information supplied by externalequipment, for example a GPS receiver and a speed log, to apply latitude and speed correc-tions.

Refer to Appendix A for a simplified explanation of the gyrocompass theory of operation.

1.1.1 GyrocompassFigure 1–1: Gyrocompass housing with

integral RCU

Figure 1–1 shows the gyrocompasshousing, which contains the followingitems:

True north seeking dynamicallytuned precision gyroscope andgimbal suspension assembly.

Power supply board.

Digital and analogue controlboards.

RFI filter and distribution board.

It is a relatively simple operation toinstall the gyrocompass and youshould be able to accomplish thisquickly without the need for special-ised personnel or equipment. How-ever, note that the gyrocompassweighs 15.5kg and you must take duecare when you lift and move it.

The care that you take when you align the gyrocompass housing with the surveyed fore-aftaxis of the vessel will have a direct impact on the accuracy of heading measurements delivered

by the System. Since the Meridian Gyrocompass is an ideal source of heading information foruse by other systems on board, such as radars and satellite communication antennas, the accu-





racy of its heading measurements will have a wide impact throughout the vessel. You shouldtherefore take care when you install and align the gyrocompass. Refer to Section 2 for fullinstructions to install, connect and align the Meridian Gyrocompass.

The only component available for user servicing is a 3A line fuse inside the gyrocompass

housing. In case of failure, refer to Section 5 for instructions to renew this fuse and check thePSU board supplies.

1.1.2 Remote Control Unit Figure 1–2: Remote Control Unit

The Remote Control Unit (RCU) provides all the functions and indicators necessary to controland operate the Meridian Gyrocompass.

The four-character LED can show a range of information:

Heading information, for example 012.3 when in the READY mode

Source of speed signal when in SPEED mode

Value of manually set speed when in SPEED mode with manual speed selected

Source of latitude signal when in LAT mode

Value of manually set latitude when in LAT mode with manual latitude selected

Status of gyro when in ALARM mode

Status of GPS when in ALARM mode

Status of ship’s log when in ALARM mode

Software version when in SOFTWARE VERSION mode

Refer to Section 3 for instructions to operate the Meridian Gyrocompass.



1 – Introduction


1.1.3 Auxiliary InputsAuxiliary inputs may be used for the Meridian Gyrocompass to apply latitude and speed cor-rections.

Ideally, the Meridian Gyrocompass should accept latitude and speed information from exter-

nal sources such as a GPS receiver or a speed log. However, you may supply this informationmanually if external sources are not available. The advantage of using GPS or a speed log toprovide correction signals is that they allow the Meridian Gyrocompass to apply correctionsautomatically.

Section 2 includes instructions to connect and configure the external sources of latitude andspeed information.

Section 3 includes instructions to set the latitude and speed manually.

1.1.4 Heading OutputsThe Meridian Gyrocompass is a self-contained precision navigation instrument that is capableof supplying heading reference information simultaneously to a wide range of equipment onboard the vessel. Throughout a typical vessel, applications that can use information suppliedby the Meridian Gyrocompass include:

Autopilot

Radars

GPS

Radio direction finder

Course plotter and recorder

Satellite communication systems

Satellite television

To support this wide range of equipment types, the Meridian Gyrocompass can supply headinginformation simultaneously through multiple channels using any of the common transmissionformats.

Refer to Section 3 for a description of the available output channels and their data formats.

1.2 PRINCIPLE OF OPERATION

In the absence of external influences, a free-spinning gyroscope will try to maintain a fixedorientation in space. The Meridian Gyrocompass exploits this property and uses gravity con-trol and earth rotation to align the gyroscope spin axis with the meridian, i.e. the true northdirection.

Refer to Appendix A for the general theory of gyrocompass operation.







2 – Installation


2 INSTALLATION

To obtain the best performance from the Meridian Gyrocompass you must take care when youinstall and connect it. This section includes all the information and instructions you will needto complete these tasks.

You should read this section carefully and understand the important instructions that it con-tains before you begin to install or connect the equipment.

2.1 Unpacking and Inspection Page 2

Explains the inspection checks that you should perform as you unpack the Meridian Gyrocom-pass.

2.2 Physical and Electrical Installation Page 3

Choose a suitable location to install the Meridian Gyrocompass. Connect the system to anelectrical supply and to external equipment.

2.3 Alignment Page 12

The care that you take as you align the Meridian Gyrocompass with the fore-aft datum on thevessel will have a direct influence upon its accuracy.





2.1 UNPACKING AND INSPECTION

WARNINGThe Meridian Gyrocompass weighs 15.5kg. To avoid personal injury, take proper pre-cautions if you lift or move the equipment.

CAUTIONThe Meridian Gyrocompass includes precision components and bearings. To avoidcausing damage to any part of the System, handle all items with care.

Retain the original transit cases so that you can use them to transport the system whennecessary. You will void the warranty if you use improper packing during transporta-tion.

CAUTION

Severe damage to the Meridian Gyrocompass can occur if you move the gyrocompasswhile the rotor is still spinning without the servo system in operation.

Note that the gyro rotor continues to spin for approximately ten minutes after youpower-off the system.

To avoid potential damage to the Meridian Gyrocompass, always allow a period of tenminutes after power-off for the gyro rotor to come to rest before you attempt to movethe gyrocompass.

The Meridian Gyrocompass undergoes a full series of electrical and mechanical tests during

manufacture and before dispatch. The packing case has a special design to protect the contentsagainst shock during transit so that the equipment should arrive without damage or defect.

As soon as possible after you have received the system, check all items against the shippingdocuments. Inspect all sub-assemblies carefully to check for any damage that may haveoccurred during transportation. If you see any damage file a claim with the carrier and imme-diately notify SG Brown.

To avoid loss or damage to any components of the system, store all sub-assemblies safely inthe transit case until you need to install them. Obey the storage temperature limits listed inSection 4.

Notify SG Brown immediately if there are any components missing from the shipment.

The title page of this Manual lists the contact details for SG Brown.



2 – Installation


2.2 PHYSICAL AND ELECTRICAL INSTALLATION

2.2.1 Selecting a locationThere are certain guidelines that you should follow to install the Meridian Gyrocompass suc-cessfully:

CAUTIONDuring operation, the gyrocompass must remain level to within ±45°. If it experiencestilt greater than 45° in any direction, it will ‘topple’. Safety routines in the gyro softwarewill then power-off the gyro rotor and show alarm conditions on the RCU. To restorenormal operation, establish a level operating attitude and then power-on the gyrocom-pass normally.

Never apply a tilt of more than 45° with the gyro rotor spinning or during the gyrocom-pass initialisation procedure. Note that the gyro rotor continues to spin for approxi-mately ten minutes after you power-off the System.

The gyrocompass weighs 15.5kg. Choose a mounting location that is level, flat and suffi-ciently strong to support the unit without flexing or experiencing extreme vibration. Themounting location can be open, as on a chart table, or enclosed within a cabinet.

CAUTIONIf you install the gyrocompass in an enclosed space, make certain there is sufficientventilation and circulation of free air to allow effective cooling.

Choose a location that protects the Meridian Gyrocompass from damage.

Do not install or operate the Meridian Gyrocompass where the ambient temperature couldfall below 0°C or rise above +45°C, or where rapid changes of temperature can occur.

Do not install the Meridian Gyrocompass close to strong mechanical or electrical noisesources, or in a location susceptible to vibration or shock.

Allow a minimum distance of 1.3m between the gyrocompass housing and any standardmagnetic compasses.

Choose a location that allows convenient access to install, connect and service the Merid-ian Gyrocompass. Refer to Figure 2–6 for clearance dimensions.





2.2.2 Gyrocompass installation You must align the Meridian Gyrocompass so that its fore-aft axis is parallel to the fore-aft datum on the vessel. Any misalignment between the gyrocompass housing and thevessel will have a direct effect on the accuracy of heading measurements delivered bythe system.

To install the Meridian Gyrocompass you will need the following tools:

Screwdriver 5.5mm × 150mm

Screwdriver 3mm × 75mm

Nut spinner 5.5mm

Combination spanner 10mm

Hexagon key 2mm

Adjustable spanner opening to at least 33mm

Suitable cables for the installation as indicated in Table 2–1.

There should be no need to remove the gyrocompass cover during installation. You may gainaccess to make power and signal connections by removing the gland plate assembly, compris-ing the Gland Plate and the Distribution Board. There is a removable panel on the top of thegyrocompass that allows access to the internal DIP switches and a 60-way test connector.

1. During installation you must align the Meridian Gyrocompass so that its fore-aft axis isparallel with the fore-aft datum on the vessel. It is not necessary for the gyrocompass to beon the vessel centre line. There are alignment marks on the base of the Meridian Gyrocom-pass to help you achieve the correct alignment.

2. Three elongated securing holes machined into the gyrocompass base allow you to makefine adjustments to alignment after installation. With the gyrocompass positioned accu-rately, mark the supporting surface with the centre positions for the three securing holes.Refer to Figures 2–5 and 2–6 for dimensions.

Table 2–1: Suitable cable types

Purpose Suitable cable

Power supply 7/0.5mm (1.5mm2) HOFR sheathed to BS6883

Synchro heading output

Resolver heading outputStepper S-code output

7/0.4mm (1.0mm2) butyl or EP rubber insulated, CSP

sheathed, wire braided and CSP oversheathed.

Serial data heading output

Serial data speed input

Serial data latitude input

1/0.85mm (0.6mm2) twisted pair, butyl or EP rubber insulated,

CSP sheathed, wire braided and CSP oversheathed.



2 – Installation


3. Remove the gyrocompass and drill three 8.5mm diameter holes, using the marks you have just made on the supporting surface as hole centres. Deburr the holes and remove anyswarf.

4. Reposition the gyrocompass and align it to the fore-aft datum. Use three M8 bolts with

washers and nuts to secure the gyrocompass in position.

5. Connect a 24V electrical supply (acceptable range 18V to 36V DC) to the Meridian Gyro-compass at J1, the three-pin power inlet on the Gland Plate. Figure 2–1 shows the GlandPlate.

Figure 2–1: Gyrocompass gland plate

CAUTIONDo not make any connections to the gyrocompass with power on the supply cable.

6. Connect the ship’s safety ground to the earthing stud adjacent to the power connector.

7. Make all necessary signal connections to the Meridian Gyrocompass at the DistributionBoard. The Distribution Board accepts open tails for all connections. Pass cables through

available glands in the Gland Plate. Glands A, D, E, F and J (identified in Figure 2–5)

Fuse

3.15AF

Compass Min. Safe Dist.

Build Standard No.

Serial No.

Mfg. Date

m

Watford, England.

CAUTION

Before removing the cover or the RCU

unit, remove the gland plate and

disconnect the RCU cable (TB1/17-22)

from the distribution board.

Gland plate release screws

Cover plate release screws





accept cables up to 18mm diameter, while all other glands accept cables up to 14mm diam-eter. Refer to Figure 2–3, and Tables 2–3 and 2–4 for cable connection details.

8. To maintain EMC compliance, connect all the wire braiding on the cables to the groundingposts on the inside surface of the Gland Plate as shown in Figure 2–2.

Figure 2–2: Termination of wire braided cables

9. Refit the Gland Plate, making certain there are no trapped wires or cables.

Figure 2–3: Gyrocompass distribution board



2 – Installation


Table 2–2: J1 – Power supply input pin details

Pin Description

1 Protective ground

2 +24V DC

3 0V

Table 2–3: Input signals

Signal description Signal type Distribution Board connector

DG TB1/1 TB1/2 (0V)

GPS input NMEA 0183 RS232 TB1/3 TB1/4 (0V)

GPS input NMEA 0183 RS422 TB1/5 (+) TB1/6 (–)

Log input NMEA 0183 RS232 TB1/7 TB1/8 (0V)

Log input NMEA 0183 RS422 TB1/9 (+) TB1/10 (–)

Log OK TB1/11 TB1/12 (0V)

Log TTL pulses TB1/13 (+) TB1/14 (0V)

Log relay Voltage free contact close TB1/15 (+) TB1/16 (0V)

RCU in TB1/17 (+) TB1/18 (–)

RCU On/Off TB1/19 (–) TB1/20 (+)

RCU Communications RS422 TB1/21 (A) TB1/22 (B)

Spare – TB1/23 TB1/24





Table 2–4: Output signals

Signal description Signal type Distribution Board connector

Channel 2 NMEA 0183 RS232 (all data) TB2/1 TB2/2 (0V)

Channel 1 NMEA 0183 RS232 (all data) TB2/3 TB2/4 (0V

Channel 2 NMEA 0183 RS422 (all data) TB2/5 (+) TB2/6 (–)

Channel 1 NMEA 0183 RS422 (all data) TB2/7 (+) TB2/8 (–)

Heading TTL S-type TB2/9 (5V) TB2/10 (L1)

TB2/11 (L2) TB2/12 (L3)

TB2/13 (0V)

Heading Synchro/resolver TB2/14 (36V) res

TB2/15 (26V) syn

TB2/16 (0V) syn/res

TB2/17 (S1) syn

TB2/18 (S1) resTB2/19 (S2) syn/res

TB2/20 (S3) syn/res

TB2/21 (S4) res

Gyro fail Voltage free contact closure TB2/22 (CC)

TB2/23 (NO)

TB2/24 (NC)

System Fail TTL TB2/25 TB2/26 (0V)

Gyro ready Voltage free contact closure TB2/27 (CC)

TB2/28 (NO)

TB2/29 (NC)

Gyro ready TTL TB2/30 TB2/31 (0V)

Channel 2 NMEA 0183 RS232 TB3/1 TB3/2 (0V)



Channel 2 NMEA 0183 RS422 TB3/7 (+) TB3/8 (–)









Rate of turn Analogue ±10V TB3/25 TB3/26 (0V)

Course recorder NMEA 0183 RS232 TB3/27 TB3/28 (0V)



2 – Installation


2.2.3 External Remote Control Unit The standard Meridian Gyrocompass has the Remote Control Unit (RCU) mounted integrallyand available for immediate operation.

There may be applications where you prefer to install the RCU at some distance from the

gyrocompass unit. A mounting kit, part number 929190, is available to use in these circum-stances. The kit includes the following items:

RCU housing

Mounting bracket

Blanking plate for the gyrocompass housing

There is no need to remove the gyrocompass cover to install the RCU externally:

1. Release and remove the four M3 screws at the corners of the RCU that secure it to thegyrocompass housing.

2. Remove the seven Gland Plate release screws marked! in Figure 2–1. Remove the GlandPlate. Note the connection sequence of the RCU cable at TB1/17–22 so that you canrestore the same connections through the extension cable.

3. Disconnect the RCU cable at TB1/17–22 .

4. Lift the RCU away from the gyrocompass and install it at the remote location.

The cable run between the RCU and the remote location must not exceed 100 metres.

5. Use the bracket with the mounting kit to fix the RCU to a desk or to a bulkhead. You mayalso flush mount the RCU in a panel. Choose a suitable location to mount the RCU:

The mounting surface can be vertical or horizontal according to requirements.

Avoid installing the RCU where it might experience severe shock or vibration.

Choose a location for the RCU that allows a clear view of the display in all conditions.

6. Use the two star knobs supplied to fit the RCU into the mounting bracket. Tilt the unit to aconvenient viewing and operating angle and then lock it in place by tightening both starknobs.

7. Supply and fit a cable to connect the RCU to TB1/17–22 on the Distribution Board . Thecable must have three screened twisted pairs and should not exceed 100 metres inlength.Route the cable through a vacant cable gland on the Gland Plate and make the cor-rect connections to the TB1 terminals.

8. Refit the Gland Plate and secure it in place using the seven release screws.

9. Fit the blanking plate to fill the gap left in the cover by the RCU.





2.2.4 Set the Gyrocompass DIP Switches1. Power-off the gyrocompass before you make any changes to the DIP switch settings.

2. There is a removable panel on top of the gyrocompass that allows access to the two inter-nal DIP switches (shown in Figure 2–4) without the need to remove the main gyrocompass

cover. Release and remove the three securing screws to lift off the panel.

3. Refer to Tables 2–5 and 2–6 set the DIP switches carefully for the specific requirements ofyour installation. Do not adjust the settings of other preset controls inside the gyrocom-pass.

4. Refit the access panel to the top of the gyrocompass cover

Figure 2–4: Location of DIP switches.

DIPswitches

Tilt biaspotentiometer

Azimuth bias potentiometer



2 – Installation


Table 2–5: SW1 DIP switch settings

Switch number

and setting

1 = on, 0 = off

Function

11

0

1

0*

21

1

0

0*

Speed Log Input100 pulses per nautical mile

200 pulses per nautical mile

400 pulses per nautical mile

LOG NMEA*

3

1*

0

Channel 1 Baud rate

4800*

9600

4

1*

0

Channel 2 Baud rate

4800*

9600

5

1

0*

Channel 1 update rate

10 Hz

1 Hz*

6

1

0*

Channel 2 update rate

10 Hz

1 Hz*

7

1*

0

NMEA sentence format for heading output

HDT*

VHW

8

1*

0

Directional gyro mode

DG mode not selected*

DG mode selected

Notes: 1. If you set a 10Hz update rate, the heading output defaults to HDT + ROT format only.

2. Factory default settings appear in bold and are marked with an asterisk in this table.





Table 2–6: SW2 DIP switch settings

2.3 ALIGNMENT

It is important to align the gyrocompass to the vessel accurately. Any misalignment betweenthe housing and the vessel will appear directly as a fixed error in heading measurements.

Because measurements from the Meridian Gyrocompass are available for use by diverse sys-tems around the vessel, any misalignment between the gyrocompass and the fore-aft datummight have a significant impact in many other areas of application.

There are several methods you may use to align the gyrocompass to the vessel fore-aft datum:

Align the gyrocompass to the fore-aft datum using a known reference line, such as a sur-veyed bulkhead or frame member. The marks on the gyrocompass base plate are precisionindicators of the gyrocompass alignment orientation.

Use the services of a marine surveyor to align the gyrocompass precisely with the fore-aft

datum.

Remove any residual misalignment by making minuscule adjustments to the gyrocompassmounting plate. When you have achieved perfect alignment, tighten the securing bolts fully tolock the gyrocompass in position.

Switch number

and setting

1 = on, 0 = off

Function

11*

0

Execute software versionOperational software*

Illegal setting

2

1

0*

Heading output resolution

Two decimal places

One decimal place*

3

1

0*

Checksum selector

Checksum NOT selected

Checksum selected*

4

10*

Channel 2 output selector

NMEA 0183 HDT onlyNMEA 0183 all data*

Switches 5 to 7 Not used

8

1

0*

Test mode

Factory test mode selected

Normal operational mode*



2 – Installation


2.4 FINAL G YROCOMPASS INSTALLATION TESTS

After you have installed the gyrocompass and power supplies are available to it, perform thefollowing installation tests:

1. Power-on the gyrocompass by following the instructions in sub-section 3.2. Wait for three

hours before you perform the following tests.

2. Check the vessel heading against a known reference mark on a chart. Typically this couldbe the alongside position of the fitting-out dock. Alternatively, accurately survey an objectat least five kilometres ahead of the vessel using the fore-aft line as a datum.

3. Check the displayed gyrocompass heading at intervals to make certain it is consistent withthe surveyed vessel heading.

4. If there is an error larger than ±0.5°, re-check the vessel fore-aft datum to confirm that it iscorrect.

5. Check that all the repeaters are accurately aligned with the gyrocompass heading and makecertain they maintain their alignment at all times while the gyrocompass is powered-on.





2.5 INSTALLATION DRAWINGSFigure 2–5: Gyrocompass installation – Sheet 1

3 0 3 3 0

3 4 0

1 7 0

1 1

0

1 6 0

S E 1 3 0 1 4 0

1 2 0

1 5 0

7 0

9 0

8 0

1 0 0

E

N E

6 0

4 0

5 0

2

5 0

2 3 0

1 9 0 1 8 0 2 0 0

S 2 1 0

2 2 0

S W 2 4 0

N

N W 3 2

0

2 7 0

2 6 0

W

2 8 0

3 0 0

2 9 0

3 1 0

2 0 10 0 3 5 0

F u s e 3 . 1 5 A F

C o m p a s s M i n . S a f e D i s t .

B u i l d S t a n d a r d N o .

S e r i a l N o .

M f g . D a t e

m

W a t f o r d , E n g l a n d .

C A U T I O N

B e f o r e r e m o v i n g t h e c o v e r o r t h e R C U u n i t , r e m o v e t h e g l a n d p l a t e a n d

d i s c o n n e c t t h e R C U c a b l e ( T B 1 / 1 7 - 2 2 ) f r o m t h e d i s t r i b u t i o n b o a r d .

2 4 V D C



2 – Installation


Figure 2–6: Gyrocompass installation – Sheet 2

S 2 1 0

2 2 0

S W 2 4 0

N

N W

3 2 0

2 7 0

2 6 0

W

2 8 0

3 0 0

2 9 0

3 1 0

2 0 10 0 3 5 0

F u s e

3 . 1 5 A F

C o m p a s s M i n . S a f e D i s t .

B u i l d S t a n d a r d N o .

S e r i a l N o .

M f g . D a t e

m

W a t f o r d , E n g l a n d .

C A U T I O N

B e f o r e r e m o v i n g t h e c o v e r o r t h e R C U

u n i t , r e m o v e t h e g l a n d p l a t e a n d

d i s c o n n e c t t h e R C U c a b l e ( T B 1 / 1 7 - 2 2 ) f r o m t h e d i s t r i b u t i o n b o a r d .

2 4 V D C

3 0 3 3 0 3 4 0

1 7 0

1 1 0

1 6 0

S E 1 3 0 1 4 0

1 2 0

1 5 0

7 0

9 0

8 0

1 0 0

E

N E 6

0

4 0

5 0

2 5 0

2 3 0

1 9 0 1 8 0 2 0 0





Figure 2–7: RCU installation – Table mount



2 – Installation


Figure 2–8: RCU installation – Flush mount







3 – Operating Instructions


3 OPERATING INSTRUCTIONS

This section explains how to power-on and configure the Meridian Gyrocompass after installa-tion. Refer to Section 4 for an explanation of the data formats relevant to the System.

3.1 Control Features Page 2

The RCU provides all the controls you will need to operate the Meridian Gyrocompass. It alsoincludes a four-character display panel that shows the heading indication and any alarm mes-sages and error codes.

3.2 Initial Power-on Page 3

Explains how to power-on the Meridian Gyrocompass after installation and describes the ini-tialisation sequence.

3.3 Operating Procedure Page 4

Explains how to select the latitude and speed correction sources, and how to set the latitudeand speed manually if necessary.

3.4 Error Modes Page 5

Identifies the system error modes. Use these indicators to identify a possible fault condition.

3.5 Operating Considerations Page 8

Includes general advice for operating the Meridian Gyrocompass on a vessel and on highspeed craft.





3.1 CONTROL FEATURESFigure 3–1: RCU front panel features

The RCU front panel includes all the operator controls for the Meridian Gyrocompass:

Table 3–1: RCU Control and Indicator functions

Control Function

!Power switch. This is a press-to-make, press-to-break switch, recessed to prevent accidental

operation.

"Selection button – Up. This is a press-to-make, release-to-break button.

#Selection button – Down. This is a press-to-make, release-to-break button

$ Latitude selection button. This is a press-to-make, release-to-break button. Use this button,together with the increase and decrease selection buttons to apply latitude correction manually.

%Speed selection button. This is a press-to-make, release-to-break button. Use this button,

together with the increase and decrease selection buttons to apply speed correction manually.

&Alarm. This is a press-to-make, release-to-break button. Use this button to cancel an audible

alarm.

012.3 Four-character display to indicate heading and system status.

Power lamp (red) Indicates 24V DC nominal power received by the Meridian Gyrocompass.

Ready lamp

(green)

Indicates that the Meridian Gyrocompass has settled to indicate the true heading.





3.2 INITIAL POWER-ON

The Meridian Gyrocompass starting cycle is fully automatic after power-on. All operator con-trols are on the RCU. In the following instruction sequence, the display description ‘G.Y.R.O. +. .O.N.’, for example, means that the four-character display alternates between showing ‘G.Y.R.O.’and ‘ . .O.N.’.

1. Check that there is a nominal 24V DC electrical supply available to the gyrocompass. Theacceptable supply range is 18V to 36V DC.

The gyrocompass draws 3A from the DC supply during start-up. To ensure continuousoperation, the power supply for this unit must have a 200W power rating.

2. To start the Meridian Gyrocompass press the power switch on the RCU.

3. Check that the red ‘Power’ lamp on the RCU illuminates. This lamp indicates only that theMeridian Gyrocompass is receiving power and does NOT indicate a settled condition.

Check that the instrument illumination is at maximum during the initialisation sequence.Even at its maximum setting, the instrument illumination may be difficult to see in brightambient lighting.

4. The RCU will sound an alarm briefly and the display indicates ‘TEST ’ while the systemperforms a series of self tests. On successful completion of the self-tests, the alarm willsilence and the display will flash the heading until the gyrocompass provides a stable northreference.

5. Wait for the gyrocompass to settle. This will occur automatically and will take between 24

and 45 minutes to complete. The Meridian Gyrocompass signifies its settled condition byilluminating the green ‘Ready’ lamp and by showing a stable heading on the RCU display.

6. Set the source of latitude information by following the instructions in sub-section 3.3.1.

7. Set the source of speed information by following the instructions in sub-section 3.3.2.

8. Use the increase and decrease selection buttons to adjust the RCU illumination level to acomfortable setting.

9. If necessary, you can view the software versions of the main and the control panel proces-

sors by pressing both the Up and the Down selection buttons simultaneously. The displaywill toggle between indications of the main processor software version (with prefix ‘M’)and the control panel software (with prefix ‘R’). The display will continue to toggle for afurther six seconds after you release the buttons and will then return to the heading indica-tion.





3.3 OPERATING PROCEDURE

The Meridian Gyrocompass will settle automatically after power-on, taking a maximum of 45minutes to provide a true north reference. The system requires only latitude and speed correc-tion, applied manually or from external sources, to perform to the specified accuracy.

Ideally, the Meridian Gyrocompass should accept latitude and speed information from exter-nal sources such as a GPS receiver or a speed log, which allow the System to apply correctionsautomatically.

3.3.1 Latitude correction1. Press and hold the Latitude selection button.

2. Use the up and down selection buttons"# to set the local latitude manually. The displaywill show the latitude in one-degree increments in the range 80°N to 80°S, for exampleL.70.N .

To select automatic latitude compensation from a GPS receiver, use the up or down selec-tion buttons"# to scroll beyond 80°N or 80°S until the display shows L.GPS . Disregardthe DG setting, which sets the directional gyrocompass operating mode explained in sub-section 3.3.3.

If there is no valid input available from a GPS receiver, the display will flash and theMeridian Gyrocompass will not accept the selection as an input source.

3. Release both buttons to set the latitude to the displayed value or to set the gyrocompass touse GPS as the source of automatic latitude correction. The display will indicate the lati-

tude setting for a further three seconds and will then return to the normal heading display.

If you input the operating latitude manually, remember to change the setting when nec-essary. Note that, in medium latitudes, a 10° error in setting the operating latitude willresult in a compass error of approximately 0.3°.

3.3.2 Speed correction1. Press and hold the Speed selection button.

2. Use the up and down selection buttons"# to set the speed manually in the range zero to90 knots.

To select automatic speed compensation from a GPS receiver or a speed log, use the upselection button" to scroll beyond 90 knots until the display shows S.GPS or S.LOG .

If there is no valid input available from a speed log or GPS receiver, the display willflash and the Meridian Gyrocompass will not accept the selection as an input source.

3. Release both buttons to set the speed to the displayed value or to set the gyrocompass touse GPS or a speed log as the source of automatic speed correction. The display will indi-cate the latitude source for a further three seconds and will then return to the normal head-ing display.





If you input the vessel speed manually, remember to set the average vessel speed andto change the setting when necessary. Return the setting to zero on completion of thevoyage. For a vessel steaming in a northerly direction, a 5-knot error in speed settingwill generate an error of approximately 0.5°.

3.3.3 DG operating modeFollow the instructions in sub-section 3.3.1 to set the latitude correction to the DirectionalGyro (DG) mode. In this mode you can use the Meridian Gyrocompass as a direction indicat-ing instrument all the way up to the poles. If the gyrocompass has settled on north immediatelyprior to entering the DG mode, it will continue to provide a useful indication of the northerlydirection for a period, but will not continue to seek north. The length of time that the directionindication remains valid depends entirely on the gyro drift characteristics.

Note that the gyrocompass will not north seek while operating in the DG mode.

3.4 ERROR MODES

The Meridian Gyrocompass has three possible Error modes:

1. Loss or corruption of GPS signal

2. Loss or corruption of speed log signal

3. Gyrocompass failure

3.4.1 Loss of GPS This failure mode can occur when you have selected GPS as the source of speed or latitude

information and the signal corrupts or becomes lost for a period of two minutes. You can rec-ognise this condition by the following indications:

The display flashes S.GPS + FAIL or L.GPS + FAIL at one-second intervals.

An audible alarm will sound.

Press the Alarm button to cancel the audible alarm. This will have two effects:

1. The audible alarm will cancel.

2. The RCU will automatically select the manual source for its speed or latitude input signal.It will use the most recent valid value supplied by the GPS source for the initial setting,although you can adjust this if necessary.

If a signal is available from a speed log, you may set this as the speed source.





3.4.2 Loss of speed log This failure mode can occur when you have selected the speed log as the source of speed infor-mation and the signal corrupts or becomes lost for a period of two minutes. You can recognisethis condition by the following indications:

The display flashes S.LOG + FAIL at one-second intervals.


Press the Alarm button to cancel the alarm. This will have two effects:

1. The audible alarm will cancel.

2. The RCU will automatically select the manual source for its speed input signal. It will usethe most recent valid value supplied by the speed log for the initial setting, although youcan adjust this if necessary.

If speed information is available from a GPS receiver, you may set this as the speed source.

3.4.3 Gyrocompass failureThe Meridian Gyrocompass has a built-in system that monitors operation of the gyrocompass.If it detects a fault condition, it will use three methods to alert you:

The display flashes GYRO + FAIL at one-second intervals.


The ‘Ready’ lamp will extinguish.

Press the Alarm button to cancel the audible alarm.

The RCU uses the decimal points at each display character position to identify the cause of thefailure.

Figure 3–2: Gyrocompass failure codes





If your system develops a fault condition as indicated by the display, refer to sub-section 5.1 for some simple checks that you can make. If necessary, contact SG Brown or an approvedlocal service agent for assistance.





3.5 OPERATING CONSIDERATIONS

3.5.1 General Operating Considerations You should leave the Meridian Gyrocompass running continuously. Power-off the system

only during long periods of lay-up, for example during vessel dry-docking. To power-offthe Meridian Gyrocompass, press the ‘Power’ button. The heading display will go blankand the front panel lamps will switch off. The gyro rotor will take approximately ten min-utes to come to rest.

If you intend to leave the system powered-off for an extended period, you should arrangeto run the gyrocompass for a period of at least thirty minutes at intervals of six months orless.

The Meridian Gyrocompass has full protection against interruption of its electrical supply.It will re-start and align itself automatically on restoration of electrical power. The headingindication will be accurate when the RCU ‘Ready’ lamp is on.

Monitor the Meridian Gyrocompass performance regularly. When functioning correctly,and provided the correct Speed and Latitude compensations are applied, the heading errorin latitudes up to 60° and for speeds up to 25 knots will normally be less than 0.75° regard-less of the vessel manoeuvres.

Never move the gyrocompass with the gyro rotor spinning unless you leave the servosoperational. Note that the gyro rotor continues to spin for a period of approximately tenminutes after you power-off the gyrocompass.

3.5.2 Operating Considerations for High Speed Craft The gyrocompass gravity control gives rise to errors whenever the gyrocompass acceler-

ates or decelerates along the north-south line, that is whenever the northerly speed orcourse changes. These errors are caused by the inertia of the pendulous element of thegyro, which produces a torque about the horizontal axis and therefore a precession in azi-muth. This effect, called ballistic deflection, causes an increase in error during accelera-tion.

The precession in tilt that arises from the damping component of gravity control is calledballistic tilt . The combined effects of ballistic tilt and ballistic deflection cause the gyro-compass to tilt downwards. Because of the factors that guide the behaviour of a dampedgyroscope, the gyro spin axis will return to the settled position by the normal anticlock-

wise spiral after the acceleration has ceased.

In the Meridian Gyrocompass, gravity control comes from an accelerometer (pendulum),which generates an electrical signal related to the tilt of the gyro spin axis. This deviceshas two important design features; it is heavily damped and the range of output is restrictedto a small angle.

The use of accelerometer damping by the Meridian Gyrocompass is of prime importancein the reduction of a particularly serious form of ballistic error called inter cardinal rollingerror. This type of error occurs most noticeably when the vessel steams on an inter cardinalheading while rolling simultaneously through a significant angle.





If the gyrocompass is installed at some distance above the vessel centre of roll rotation, asis usually the case on commercial vessels, the resulting lateral acceleration componentsalong the east-west and north-south axes of the gyrocompass combine to build an error inthe northerly settle point.

If the effect persists for long enough, this error might become as large as several degrees.However, by damping the accelerometer using a time constant several times larger than thevessel rolling period, inter cardinal rolling errors are significantly reduced.

Another form of ballistic error arises from north-south accelerations generated by vesselmanoeuvres. Such accelerations can arise from changes in speed and/or course. By limit-ing the angular output of the accelerometer, the Meridian Gyrocompass reduces the errorpotential typically to less than one degree.

It is also possible to eliminate the effects of acceleration by operating the gyrocompasstemporarily in the directional gyro (DG) mode. In this mode, the gyrocompass uses gravity

control for tilt corrections only, so that ballistic effects would cause negligible headingerror during short-term periods of acceleration.

You can set the DG mode manually from the RCU. The gyrocompass will automaticallyset the DG mode if the accelerometer signal exceeds a pre-set threshold. The gyrocompasswill remain in the DG mode until the accelerometer signal falls below the threshold valuefor more than 30 seconds. The alarm will activate if the accelerometer signal exceeds thethreshold setting for more than 30 minutes.

The Meridian Gyrocompass complies with all requirements of IMO Resolution A.821(19),Performance Standards for Gyrocompasses for High-Speed Craft.







4 – Technical Data


4 TECHNICAL DATA

4.1 SPECIFICATIONS

4.1.1 Power RequirementsVoltage 24V DC (acceptable range 18V to 36V DC)

Maximum power consumption 3A at power-on (The power supply capacityshould exceed 200W)

To comply with the requirements of IMO Resolution A.821(19), Performance Standards forGyrocompasses for High Speed Craft, power to the gyrocompass should be delivered by anuninterruptable power supply, for example the SGBrown UPS part number 929128.

4.1.2 Performance (definitions as in ISO 8728)Settle point error 0.25° sec latitude

Static error 0.1° sec latitude RMS

Dynamic accuracy 0.6° sec latitude (Scorsby andIntercardinal motion tests)

Settle point repeatability 0.25° sec latitude

Follow up speed 200°/s

Time to settle within 0.7° Less than 45 minutes

4.1.3 CompensationLatitude compensation range 80°N to 80°S

Speed compensation range 0 to 90 knots

4.1.4 Environment Operating environment EN 6095:1997 designated category ‘weather pro-

tected’

Operating temperature 0°C to +45°C (to ISO 8728)

–15°C to +55°C (with reduced accuracy)

Storage temperature –25°C to +80°C

4.1.5 Signal InputsLatitude NMEA 0183 via RS232 or RS422 from GPS

Speed Pulse or contact closure at 100, 200 or 400 pernautical mile from speed log.

NMEA 0183 via RS232 or RS422 from GPS or

speed log





4.1.6 Signal outputsS-type heading 1 × step-by-step, 6 steps per degree (TTL level)

Synchro heading 1 × 26V 400Hz (11.8V maximum line-to-line),sector value 360°

Resolver heading 1 × 36V 400Hz (8V maximum per phase),sector value 360°

Analogue rate of turn 1 × rate of turn ±20°/s (±10V)

Serial data outputs 6 × RS23211 × RS422

Serial data formats NMEA 0183 (all data) at 1Hz or 10Hz.Course recorder outputNMEA 0183 (heading,

date, time)

Status/alarm 5V TTL power/gyro failureVoltage free power/gyro failure contacts5V TTL system readyVoltage free system ready contacts

4.1.7 Dimensions and Weight Dimensions 344mm (H) × 267mm (W) × 440mm (D)

Weight 15.5kg

RCU size (when mounted externally) 96mm (H) × 192mm (W) × 108mm (D)0

RCU weight 0.75 kg

4.1.8 StandardsThe Meridian Gyrocompass is designed to meet the requirements of the following:

IMO Resolution A.424 (XI), Performance Standards for Gyrocompasses

IMO Resolution A.821 (19), Performance Standards for Gyrocompasses for High Speed

Craft BS EN 60945 (January 1997), General Requirements - Methods of testing and required

test results

BS EN ISO 8728:1999, Shipbuilding – Marine Gyrocompasses

BS 6217:1981, Graphical Symbols for use on Electrical Equipment

CE marking

Electromagnetic Compatibility (EMC) Directive

The Marine Equipment Directive 96/98/EC





4.2 DATA FORMATS

Set the DIP switches according to your specific input and output requirements. You will findthe instructions to do this in sub-section 2.2.4.

Inputs – Refer to sub-section 4.2.2

Acceptable input formats: Latitude information using serial NMEA 0183 GLL, GGA or RMC sentences. If more

than one of these formats is available, the Meridian Gyrocompass makes its selection inthe stated preference order. Refer to Figures 4–2, 4–3 and 4–4 respectively for a descrip-tion of these formats.

Speed information using serial NMEA 0183 VTG, VHW or RMC sentences. The sen-tences can contain speed information using knots and/or km/h. The Meridian Gyrocom-pass will use the speed in knots if available, and will calculate the speed in knots if thesentence contains only speed in km/h. Refer to Figures 4–8, 4–9 and 4–4 respectively for a

description of these formats.

TTL-compatible pulsed speed input with a TTL-level signal on contact closure.

Outputs – Refer to sub-section 4.2.3

Serial output formats:The Meridian Gyrocompass transmits all available information through RS232 and RS422serial lines using the NMEA 0183 format. The serial transmission rate can be either 4800 or9600 baud, with updates occurring at 1Hz or 10Hz as defined by the setting of the DIPswitches. The serial output contains all available NMEA sentences transmitted consecutivelyand in the following order:

Heading using either HDT or VHW sentence formats as selected by the DIP switches.Refer to Figures 4–10 and 4–9 respectively for a description of these formats.

Rate of turn using the ROT sentence format. Refer to Figure 4–12 for a description of thisformat.

Speed using VTG, VHW or RMC sentences determined by the available input format.Refer to Figures 4–8, 4–9 and 4–4 respectively for a description of these formats.

Latitude using GLL, GGA or RMC sentences determined by the available input format.

Refer to Figures 4–2, 4–3 and 4–4 respectively for a description of these formats.

Data and time using the ZDA sentence format. Refer to Figure 4–17 for a description ofthis format

Other output formats: Synchro Heading Output

Resolver Heading Output

Stepper S-code Heading Output

Rate of turn using a bipolar analogue voltage in the range ±10V





The following sub-sections describe each of the formats supported by the Meridian Gyrocom-pass.

4.2.1 NMEA 0183 Serial Data Formats – General informationThe Meridian Gyrocompass accepts and transmits asynchronous serial data using 8 data bits,

one stop bit and no parity through RS232 and RS422 transmit-only lines. The data bits occurin each packet with the least significant bit first. The most significant bit of the 8-bit characterwill always be zero.

Figure 4–1: Serial data format

All data is interpreted as ASCII characters that form NMEA sentences split into individualfields. All fields, including null fields, are separated by commas.

The NMEA format supports an optional checksum – if included, the checksum occurs as anadditional field immediately before the carriage return line-feed characters. It consists of anasterisk (*) followed by a checksum derived by exclusive OR-ing the eight data bits of eachvalid character preceding the asterisk, but excluding the $ symbol, in the sentence. The abso-lute value of the checksum is transmitted in ASCII characters representing the value in HEX.For circumstances where the Meridian Gyrocompass retransmits serial data using the sameNMEA sentence format supplied by an external source, it will recalculate any checksum and

insert the new value into the output sentence.

NMEA sentences are usually transmitted once per second, however you can set a DIP switchto select a transmission rate of 10 per second. The outputs are grouped into two channels thatcan be set independently to either 1Hz or 10Hz updates.





4.2.2 Inputs

4.2.2.1 NMEA 0183 input signalsIn the following descriptions of input sentences, the Meridian Gyrocompass uses the datafields marked ‘XXX’ in the NMEA sentence. The system does not use the fields marked ‘???’and their descriptions are included here for completeness only. The gyrocompass will recog-nise the arriving sentence format and will extract the required data from it automatically.

GPS Interface (see Table 2–3 for connection details)

The Meridian Gyrocompass can accept speed, latitude, date and time inputs at the GPS inter-face in NMEA 0183 format using GLL, GGA, RMC, VTG, VHW and ZDA sentences.

Figure 4–2: NMEA 0183 GLL sentence structure





Figure 4–3: NMEA 0183 GGA sentence structure

Notes for the NMEA 0183 GGA sentence:

Note 1: GPS quality indicator:0 = Fix not available or is invalid1 = GPS SPS mode, fix valid2 = Differential GPS, SPS mode, fix valid3 = GPS PPS mode, fix valid

Note 2: Geoidal Separation – The difference between the WGS-84 earth ellipsoid and meansea level (geoid). A negative character precedes the value if the mean sea level is belowthe ellipsoid.

Note 3: Time in seconds since the last SC104 Type 1 or Type 9 update. Null field when DGPSis not in use.





Figure 4–4: NMEA 0183 RMC sentence structure

Notes for the NMEA 0183 RMC sentence:

Note 1: Easterly variation (E) subtracts from true course. Westerly variation (W) add to truecourse.

Note 2: The positioning system mode indicator field supplements the positioning system statusfield (see Note 3), which will be set to V = invalid for all values of mode indicatorexcept for A = Autonomous and D = Differential. These fields will never be empty.

Note 3: Positioning system mode indicator:A = Autonomous mode

D = Differential modeE = Estimated (dead reckoning) modeM = Manual modeS = Simulator modeN = Data not valid

Figure 4–5: NMEA 0183 VTG sentence structure





Figure 4–6: NMEA 0183 VHW sentence structure

Figure 4–7: NMEA 0183 ZDA sentence structure





Log Interface (see Table 2–3 for connection details)

The Meridian Gyrocompass can accept speed inputs at the Log interface in NMEA 0183 for-mat using VTG and VHW sentences only.

Figure 4–8: NMEA 0183 VTG sentence structure

Figure 4–9: NMEA 0183 VHW sentence structure

4.2.2.2 Pulsed input The Meridian Gyrocompass can accept a speed input as a series of pulses or contact closures

occurring at a frequency of 100, 200 or 400 per nautical mile as selected by a DIP switch. Thegyrocompass determines the vessel speed by reference against the microprocessor timing cir-cuits. The speed pulses do not need to have a particular mark/space ratio, although they shouldbe TTL-level. Contact closures should be of good quality and electrically floating.





4.2.3 Outputs

4.2.3.1 NMEA output signalsThe Meridian Gyrocompass can output serial data through RS232 and RS422 transmit-onlyserial lines using the NMEA 0183 format. The output includes all the following information inthe order stated:

Heading information uses can use either the NMEA 0183 HDT or the VHW sentence for-mat as determined by the setting of a DIP switch. The resolution of the heading output canbe set to one or two decimal places at the DIP switches. Refer to Figures 4–10 and 4–11 respectively for a description of these output formats.

Rate of turn uses the NMEA 0183 ROT sentence format. Refer to Figure 4–12 for adescription of this output format.

Speed uses the NMEA 0183 VTG sentence format, described in Figure 4–13, if the gyro-compass is configured for a manual or a pulsed log speed input, or if this sentence is sup-plied by the external speed source. If speed information arrives at the gyrocompass usingeither the NMEA 0183 VHW or the RMC format then the gyrocompass will retransmitthis format, modifying the sentence to include the talker identifier ‘HE’. Refer to Figures4–11 and 4–14 respectively for a description of these formats.

If you have selected NMEA 0183 VHW as the heading output sentence and a valid NMEA0183 VHW format sentence is available from a GPS or speed log source, the MeridianGyrocompass inserts the heading information into the received sentence so that it trans-mits only one sentence containing both heading and speed. Note that the heading outputwill default to the NMEA 0183 HDT format if you set a 10Hz update rate for either Chan-

nel 1 or Channel 2.

Latitude output uses the NMEA 0183 GLL sentence format, described in Figure 4–15, ifthe gyrocompass is configured for a manual latitude input or if the GPS source uses thissentence format. If latitude information arrives at the gyrocompass using either theNMEA 0183 RMC or the GGA format then the gyrocompass will retransmit this format,modifying the sentence to include the talker identifier ‘HE’. Refer to Figures 4–14 and 4–16 respectively for a description of the NMEA 0183 RMC and GGA output formats.

Date and Time output uses the NMEA 0183 ZDA sentence format described in Figure 4–17. If this information is available from a GPS source, then the gyrocompass will retrans-

mit this format, modifying the sentence to include the talker identifier ‘HE’. If there is novalid date and time information from an external source, the gyrocompass will transmit theNMEA 0183 ZDA output sentence with empty data fields.

If you set a 10Hz update rate for either Channel 1 or Channel 2, the output format onthat channel will default to NMEA 0183 HDT + ROT only. Optionally you can set Channel2 to transmit NMEA 0183 HDT only.





Figure 4–10: NMEA 0183 HDT output sentence structure

Figure 4–11: NMEA 0183 VHW output sentence structure

Note: The field before ‘M’ is reserved for magnetic heading information. Magnetic heading isnot used, but the ‘M’ character is still transmitted.

Figure 4–12: NMEA 0183 ROT output sentence structure





Figure 4–13: NMEA 0183 VTG output sentence structure

Figure 4–14: NMEA 0183 RMC output sentence structure

Notes for the NMEA 0183 RMC output sentence:

Note 1: Easterly variation (E) subtracts from true course. Westerly variation (W) add to truecourse.

Note 2: The positioning system mode indicator field supplements the positioning system status

field (see Note 3), which will be set to V = invalid for all values of mode indicatorexcept for A = Autonomous and D = Differential. These fields will never be empty.

Note 3: Positioning system mode indicator:A = Autonomous modeD = Differential modeE = Estimated (dead reckoning) modeM = Manual modeS = Simulator modeN = Data not valid





Figure 4–15: NMEA 0183 GLL output sentence structure

Figure 4–16: NMEA 0183 GGA output sentence structure

Notes for the NMEA 0183 GGA output sentence:

Note 1: GPS quality indicator:0 = Fix not available or is invalid1 = GPS SPS mode, fix valid2 = Differential GPS, SPS mode, fix valid3 = GPS PPS mode, fix valid

Note 2: Geoidal Separation – The difference between the WGS-84 earth ellipsoid and meansea level (geoid). A negative character precedes the value if the mean sea level isbelow the ellipsoid.

Note 3: Time in seconds since the last SC104 Type 1 or Type 9 update. Null field when DGPSis not in use.





Figure 4–17: NMEA 0183 ZDA sentence structure

Note: Local time zone is the magnitude of hours plus the magnitude of minutes added, withthe sign of local zone hours, to local time to obtain UTC. Local time is generally neg-ative for East longitudes with local exceptions near the International Date Line.

4.2.4 Course Recorder Output The Meridian Gyrocompass transmits the following NMEA 0183 sentences, in the orderstated, at 4800 baud with a 1Hz update rate:

Heading – Transmitted as either the NMEA 0183 HDT or the VHW output sentence for-mat as determined by the setting of a DIP switch. If the DIP switch settings determine a10Hz update rate for either Channel 1 or Channel 2, then the heading output defaults to theNMEA 0183 HDT format. Refer to Figures 4–10 and 4–11 respectively for a descriptionof the NMEA 0183 HDT and VHW output formats.

Date and Time – Transmitted in the NMEA 0183 ZDA format defined in Figure 4–17. Ifdate and time information is available from a GPS source, then the Meridian Gyrocompassretransmits the information with the talker ID set to ‘HE’. If the checksum is present in theoriginal received sentence, it is recalculated and inserted in the output sentence. If a validsentence is not available from a GPS source, the Meridian Gyrocompass transmits theNMEA 0183 HEZDA sentence with empty fields.





4.2.5 NMEA 0183 sentence with ChecksumIf the optional checksum is to be sent with any of the above NMEA 0183 sentences, it appearsas an extra field inserted before the carriage return character as shown by example in Figure 4–18.

Figure 4–18: NMEA 0183 sentence with optional checksum

The checksum consists of an asterisk followed by the checksum calculated by exclusive OR-ing the eight data bits of each valid character preceding the asterisk, but excluding the ‘$’ sym-

bol, in the sentence. The Meridian Gyrocompass transmits the absolute value of the checksumin ASCII characters representing the value in HEX.

4.2.6 Other Output Formats

4.2.6.1 Synchro Output The synchro heading output is available continuously at TB2 on the Distribution Board whilethe gyrocompass is powered-on – refer to Table 2–4 for connector details. The output is at11.8V maximum line-to-line voltage derived electrically from a 1:1 resolver driven directly bythe gyrocompass azimuth gimbal. The synchro reference voltage is a nominal 26V 400Hz sup-ply generated internally.

Electrical loading specification:

Not less than 5k between any two S lines.

Not less than 1k between the two R lines.

4.2.6.2 Resolver Output The resolver heading output is available continuously at TB2 on the Distribution Board whilethe gyrocompass is powered-on – refer to Table 2–4 for connection details. The output is at 8Vmaximum voltage per phase signal from a 1:1 resolver driven directly by the gyrocompass azi-

muth gimbal. The resolver reference voltage is a nominal 36V 400Hz supply generated inter-nally.

The resolver sine and cosine outputs must be electrically isolated from each other.Contact the SG Brown Service Department for technical advice if necessary.

Electrical loading specification:

Not less than 5k between any two S lines.

Not less than 1k between the two R lines.





4.2.6.3 Stepper S-CodeThe stepper S-code output is available continuously at TB2 on the Distribution Board whilethe gyrocompass is powered-on – refer to Table 2–4 for connection details.

The stepper output is a TTL compatible S-encoded signal with a 10mA sink capacity.

4.2.6.4 Rate of TurnThe ROT output is calculated by the internal processor and made available continuously atTB3 on the Distribution Board while the gyrocompass is powered-on – refer to Table 2–4 forconnection details.

The ROT output is a bipolar analogue voltage in the range ±10V to represent rates of turn from–20° to +20° per second. Positive rates of turn are to starboard.



5 – Maintenance


5 MAINTENANCE

WARNINGThere is a danger of serious injury from voltages inside the Meridian Gyrocompass. Donot remove the gyrocompass cover unless you have the necessary skills and experi-

ence to perform maintenance work on a system of this nature. Always power-off thesystem before you remove the cover for maintenance work.

Observe all local safety regulations as you work on the equipment. Reconnect thesafety grounding straps and refit all safety covers to the equipment before you power-on the system.

CAUTIONPerform these simple maintenance instructions only if you have the skills and experi-ence required, and only when necessary. Inappropriate tampering with the internal con-trols and components of the gyrocompass can lead to damage or serious performancedegradation.

NEVER open the gyrocompass cover or make any adjustments inside the gyrocom-pass unless you are entirely confident in your actions.

There is very little need for user maintenance on the Meridian Gyrocompass and you shouldnever need to remove the covers.

The following sub-sections explain some very basic procedures that you may attempt if yoususpect the system has developed a fault. If you are in any doubt, contact SG Brown for advice

and technical assistance before you begin any maintenance work on the system.

5.1 Built-in Test Equipment Page 2

The Meridian Gyrocompass performs a self-test routine during the initialisation sequence andmonitors its status continually during normal operation. Any deviation from normal operationappears as an error message, with the cause declared as a sequence of decimal points on thefour-character display panel. This sub-section explains some very basic tests and adjustmentsthat you may perform on the system.

5.2 Test Connector Page 4

There is a 60-way test connector that allows you to measure critical voltages and signals.

Perform the tests described in this section of the manual and have the results available whenyou contact SG Brown for technical assistance.





5.1 BUILT-IN TEST EQUIPMENT

Figure 3–2 is a list of nine failure codes delivered by the built-in test equipment if it detects afault in the gyrocompass. In these fault conditions, the four-character display will show G.Y.R.O. + F.A.I.L. with the illuminated decimal points at positions 1 to 4 identifying the detected failure.

If the built in test equipment detects a fault, use the following table to investigate the cause.You can measure the voltages and signals on the pins of the 60-way test connector (refer toTable 5–2 for details of the test connector).

Table 5–1: Test measurements

Failure Code Measure Expected value Signal source

0001

Failure of DC

power supply

DC supply

42 (+ve) to 43

44 (+ve) to 43

17 (+ve) to 18

50 (+ve) to 5152 (+ve) to 51

45 (+ve) to 18

18V DC to 36V DC

+24V DC ±0.5V DC

–24V DC ±0.5V DC

+5V DC ±0.1V DC

+15V DC ±0.2V DC –15V DC ±0.2V DC

+5V DC (+0.2V/–0.7V DC)

Ship’s mains/PSU

DC/DC PSU

DC/DC PSU

DC/DC PSU

Control Board analogueControl Board analogue

Control Board analogue

0010

Failure of AC

power supply

19 to 43

20 to 43

21 to 43

58 to 43

57 (+ve) to 18

2.5V AC ±0.125V AC @ 19.2kHz

10V AC ±1V @ 480Hz (18V AC ±1.5V at start (1-min))

10V AC ±1V @ 480Hz (18V AC ±1.5V at start (1-min))

12V AC ±0.2V @ 400Hz

+5V DC ±0.2V DC






0100

High tilt pick-off

signal

10 (+ve) to 18

16 (+ve) to 18

37 (+ve) to 51

+5V DC ±0.2V DC (0V DC ±0.2V DC at start (1-min))


0V DC ±1V DC

Control Board digital



0101

High azimuth

pick-off signal

10 (+ve) to 18

16 (+ve) to 18

38 (+ve) to 51



0V DC ±1V DC




0110

Failure of

synchro-to-

digital converter

58 to 43

53 (+ve) to 51

54 (+ve) to 51

12V AC ±0.2V @ 400Hz

(5.4V DC ±0.5V DC) × sin heading

(5.4V DC ±0.5V DC) × cos heading




0111

High pendulum

signal

26 (+ve) to 51

28 (+ve) to 51

(–10V DC ±0.2V DC) × sin local latitude

(–8.5mV DC ±0.5mV DC) × speed(kts) × sin hdg × tan

local lat

North = +ve latitude; South = –ve latitude



Perform the azimuth drift test – see sub-section 5.1.1

1000 Check connections between:

Distribution PCB TB1/21 and RCU S+

Distribution PCB TB1/22 and RCU S–



5 – Maintenance


5.1.1 Azimuth Drift Adjustment You may use the following procedure to measure and, if necessary, adjust the azimuth drift:

1. Ensure that the gyrocompass is static and is operating in DG mode with the Speed input setmanually to zero and the Latitude set to local latitude. Use the DIP switches to set DG

mode – refer to Table 2–5. Refer to sub-sections 3.3.1 and 3.3.2 to set the latitude andspeed.

2. Note the initial heading (H1) shown on the RCU display.

3. Wait for one hour and then note the heading (H2) shown on the RCU display.

4. Calculate the azimuth drift rate (H2 – H1) degrees per hour.

5. Use a digital meter set to measure DC volts and monitor the Tilt Bias between pins 30 and51 of the 60-way test connector (with the positive test lead on pin 30).

6. Adjust the Tilt Bias potentiometer RV7 by 400mV × drift rate (°/hr). You must turn thepotentiometer anticlockwise to compensate for azimuth drift towards higher readings. Fig-ure 2–4 shows the location of the Tilt Bias potentiometer.

7. Repeat steps 1 and 2 above to ensure that the calculated drift rate is less than 0.2°/hr.

5.1.2 Azimuth Bias Adjustment You may use the following procedure to eliminate small angles of heading error from theMeridian Gyrocompass. Measure and, if necessary, adjust for azimuth drift as described in

sub-section 5.1.1 above before you adjust the azimuth bias.

Take care when you adjust azimuth bias – make only small adjustments each time andthen allow the gyrocompass to settle for three hours before you make any furtheradjustments. Note the original position of the azimuth bias control before you start sothat you can restore the starting condition if necessary.

1. Use a digital meter set to measure DC volts and monitor the Azimuth Bias between pins 29and 51 of the 60-way test connector (with the positive test lead on pin 29).

2. Adjust the Azimuth Bias potentiometer RV9 to cause a change in the azimuth bias voltage

that will produce the necessary change in compass heading. Figure 2–4 shows the locationof the Azimuth Bias potentiometer.

3. Turn the Azimuth Bias potentiometer anticlockwise to cause the heading to changetowards a lower reading. A 60mV DC change in Azimuth Bias will produce a 1-degreechange in heading





5.2 TEST CONNECTOR

There is a sixty-way test connector accessible behind the removable panel on the top of thegyrocompass cover. Release and remove the securing screws and lift off the panel to see thetwo DIP switches and the test connector. A test box (SG Brown part number 929195) is avail-able to facilitate connection to the 60-way test connector.

Table 5–2: Sixty-way test connector

Pin Name Function

1 S1_SYNCHRO 11.8V RMS 400Hz synchro S1 phase

2 S2_SYNCHRO_RES 11.8V RMS 400Hz synchro S2 phase

3 S3_SYNCHRO_RES 11.8V RMS 400Hz synchro S3 phase

4 26V_SYNCHRO_R1 26V RMS 400Hz synchro R1 reference

5 0V_SYNCHRO_RES 26V RMS 400Hz synchro R1 reference

6 GA_MODE Directional gyro mode control (+5V logic)

7 GC_MODE Gyrocompass mode control (+5V logic)

8 AA_MODE Auto alignment mode control (+5V logic)

9 LAT_NS Latitude north selection control (+5V logic)

10 SERVO_EN Tilt and azimuth servo enable control (+5V logic)

11 WHEEL_BOOST Gyro wheel supply boost control (+5V logic)

12 LOG_OK Speed log OK flag (+5V logic)

13 GPS_OK GPS OK flag (+5V logic)

14 SYS_FAIL System fail flag (+5V logic)

15 GYRO_RDY Gyrocompass ready flag (+5V logic)

16 PREPARE Prepare mode (servo nulling) (+5V logic)

17 VCC 5V DC supply

18 GND 5V DC supply return

19 PICK_OFF_SUPPLY_1 Gyro pick off supply 2.5V RMS 19.2kHz sine wave

20 WHEEL_SUPPLY_1 Gyro wheel supply 0 phase 10V (18V) RMS 480Hz square wave

21 WHEEL_SUPPLY_2 Gyro wheel supply 90 phase 10V (18V) RMS 480Hz square wave

22 PWMO Compass card illumination PWM control 5V 85Hz square wave

23 B Gravity control signal ±150mV DC/min T=60s

24 10V 10V DC positive voltage reference

25 _10V 10V DC negative voltage reference

26 LAT_TORQ Latitude torquing input signal (–10 sin[latitude])V DC

27 SPEED_N_TORQ Speed N torque i/p signal (7.3e –3 × speed(kts) × cos[heading])

28 SPEED_E_TORQ Speed E torque i/p signal (–8.5e –3 × speed(kts) × sin[heading] × tan[latitude]V DC



5 – Maintenance


29 AZ_BIAS Bias ad.j to azimuth torquer ±2.2V DC (60mV/deg heading)

30 TILT_BIAS Bias adj. to tilt torquer ±2.2V DC (400mV/deg/hour heading)

31 TILT_TEMP Bias adj. to tilt torquer proportional to temperature (400mV/deg/hr heading)

32 AZ_TEMP Bias adj. to azimuth torquer proportional to temperature (60mV/deg/hr heading)

33 T Temperature ref (non-inverted) from gimbal thermistor (DC V proportional to temp)

34 T_ Temperature ref (inverted) from gimbal thermistor (DC V proportional to temp)

35 TILT_TORQUER_LO Tilt torquer signal 0.013V DC/mA (torquer scale factor 10°/hr/mA)

36 AZ_TORQUER_LO Azimuth torquer signal 0.006V DC/mA (torquer scale factor 10°/hr/mA)

37 TILT_PICK_OFF_DC Demodulated gyroscope tilt pick-off signal

38 AZ_PICK_OFF_DC Demodulated gyroscope azimuth pick-off signal

39 NOT USED

40 NOT USED

41 NOT USED

42 24V 24V DC positive supply

43 0V Supply return for ±24V DC

44 –24V 24V DC negative supply

45 PSU_LO PSU (+5V DC and ±15V DC) under voltage flag (+5V DC logic)

46 NOT USED

47 NOT USED

48 NOT USED

49 ROT Analogue rate of turn output (0.5V DC/deg/s (10V DC max) CW +ve; ACW –ve

50 15V 15V DC positive supply

51 0Va Supply return for ±15V DC

52 –15V 15V DC negative supply

53 SIN_DC Analogue voltage proportional to sin(heading) (±0.1V DC/deg heading)

54 COS_DC Analogue voltage proportional to cos(heading) (±0.1V DC/deg heading)

55 RS232_RX_TEST RS232 receive port reserved for product testing

56 RS232_TX_TEST RS232 transmit port reserved for product testing

57 AC_OK AC supply (19kHz, 480Hz and 400Hz) OK flag (+5V DC logic)

58 400_REF_HI Reference supply 12V RMS 400Hz

59 AZ_MOTOR_HI Drive to azimuth follow-up DC servo motor

60 TILT_MOTOR_HI Drive to tilt follow-up DC servo motor

Table 5–2: Sixty-way test connector (Continued)

Pin Name Function





Figure 5–1: Simplified functional block diagram



5 – Maintenance


Table 5–3: Spares list for Meridian Gyrocompass 929060

SG Brown P/N Description

929033 Gimbal assembly

929066 Control Board Analogue

929083 Control Board Digital

929049 Control Board Remote

929045 Display Board

929074 DC/DC Power Supply

856000 Filter #1

929160 Filter #2

346808 Fuse link 3.15A 250V

929164 Gland Plate assembly

929190 RCU Mounting Kit

929194 Transit case

929195 Test box







A – Operating Theory


A OPERATING THEORY

A gyrocompass is a navigational instrument that provides a true north indication without refer-ence to the earth’s magnetic field. For its operation, the gyrocompass depends upon the fol-lowing:

The inertial properties of a freely spinning gyroscope.

The rotation of the earth about its own axis.

Gravity.

Figure A–1 shows a free-spinning gyroscope mounted in a balanced gimbal suspension. Thesuspension allows unrestricted movement about the vertical and horizontal axes so that thegyro rotor can adopt any orientation.

Figure A–1: Free-spinning gyroscope

With the gyro rotor stationary, it is easy to turn the gimbal suspension about either axis andallow it to remain there in a balanced condition.

However, when it is spinning, the gyro rotor exhibits a property called gyroscopic inertia. Thisproperty causes the spin axis of the rotor to remain pointing in the same arbitrary direction inspace and to resist any influence that tries to redirect that axis.

For simplicity, this explanation assumes the gyro rotor continues to spin perpetually at a con-stant speed. In a practical gyrocompass, the gyro rotor is the specially designed spindle of amotor that rotates at a constant speed.

To an observer on the surface of the earth, the free spinning gyroscope would appear to ‘tum-ble’ in its gimbal suspension once in every 24-hour period. This apparent deviation occursbecause, although the spin axis actually remains fixed in space, the earth rotates relative to it.





The apparent movement seen by the observer would depend on the location of the gyroscopeand the initial direction of the spin axis.

To an observer at the equator:

With an initial spin axis alignment level and parallel to a meridian (so that it points in a

true north-south direction), there would be no observable effect on the gyroscope duringthe 24-hour period.

With an initial spin axis alignment level and perpendicular to a meridian (so that it pointsin the east-west direction), the spin axis would appear to tumble about its horizontal axiswith the eastern end of the spin axis rising. After 24 hours, the gyroscope would completea single revolution in its gimbal suspension so that the spin axis would again point in theoriginal direction relative to the observer.

With an initial spin axis alignment somewhere between these two extremes, the gyroscopewould appear to tumble about its horizontal and its vertical axes to complete one single

revolution in 24 hours.

To an observer at one of the geographic poles:

With an initial spin axis alignment vertical (so that it aligns with the axis of earth rotation),there would be no observable effect on the gyroscope during the 24-hour period.

With the initial spin axis level, the gyroscope would appear to turn clockwise (at the northpole) or anticlockwise (at the south pole) about its vertical axis once in 24 hours.

To an observer at some intermediate latitude with the gyroscope oriented in some arbi-

trary initial direction:

The gyroscope would tumble about the north-south direction at and about the

east-west direction at , where Ω is the earth rotation rate (15° per hour) and λ is the

latitude of the gyroscope.

A.1 NORTH-SEEKING G YROSCOPE

Given a constant spin rate and frictionless gimbals, the gyroscope described above will alwaysmaintain its initial alignment relative to free space. To an observer on the surface of the earth,the revolutions that such a gyroscope performs every 24 hours would make it difficult to use asan instrument of navigation.

The ideal situation is for the gyroscope to align perfectly with the spin axis of the earth so thatit maintains a north-south orientation with no apparent tumbling during each 24-hour period.

To make the gyroscope north seeking, the gyrocompass uses gravity control and an effectcalled precession.

Ω λ cos–

Ω λ sin





Consider the example shown below where the gyroscope rotates about its spin axis in thedirection shown.

When an externally applied torque acts on the gyroscope suspension in the direction shown, apoint on the circumference of the rotor at 'O' will attempt to move in two directions simultane-

ously:

It will accelerate in the direction OA under the influence of the applied torque.

It will continue to move in the direction OB as the rotor spins.

The net result of these two movements actually starts to move the point on the circumferenceof the rotor in the direction OC, which is the resultant of the two perpendicular influences.

Since every point on the circumference of the rotor experiences the same effect as it passesthrough point O, it follows that the rotor will rotate about an axis that lies at right angles to theaxis of applied torque. This is the axis of precession. In this example, precession will act in thedirection shown.

Eventually, the spin axis of the gyroscope will turn sufficiently so that its spin axis coincides

with the axis of applied torque, at which point there will no further tendency for the gyroscopeto rotate about the precession axis.

Summary:If a free spinning gyroscope comes under the influence of a torque whose axis is perpendicu-lar to the spin of the gyro rotor, a precession results that tends to align the spin axis with theaxis of applied torque. The direction of this precession is such that, should alignment occur,the gyroscope spin direction will be the same as the direction of applied torque.

Precession





Consider the example shown in Figure A–2(a), which shows a free spinning gyroscope alignedso that its spin axis is level with the horizon. In this example, the spin axis of the gyroscopealigns with the local meridian so that its north end points north. In the balanced conditionshown, the weight suspended from the gyroscope bearings has no effect on operation and thegyroscope will maintain its alignment with the meridian.

Figure A–2: Gravity control of a gyroscope

It is more common for the initial alignment of the gyroscope to be at some angle away fromtrue north. Two conditions are therefore possible:

1. Gyroscope initially level and aligned to the west of north.

With the spin axis of the gyroscope initially level but with the north end pointing to the west oftrue north, the arrangement would briefly be balanced as shown in Figure A–2(a).

However, over time the north end of the spin axis would begin to tilt downwards. This isbecause the earth rotates while the gyroscope maintains a fixed orientation in space. In thisoff-balanced condition, shown in Figure A–2(b), the weight would try to return to its centrallocation and, in doing so, would apply an anticlockwise torque to the gyro suspension.

With the gyroscope spinning in the direction shown, precession arising from the anticlockwise

torque would move the north end of the rotor eastwards. Therefore, because this conditionarises from a westerly misalignment between the gyroscope and the meridian, the effect of thebottom weight is to drive the gyro rotor towards closer alignment with the meridian.

2. Gyroscope initially level and aligned to the east of north.

With the spin axis of the gyroscope initially level but with the north end pointing to the east oftrue north, the arrangement would briefly be balanced as shown in Figure A–2(a).

However, over time the north end of the spin axis would begin to tilt upwards with rotation ofthe earth. In this off-balanced condition, shown in A–2(c), the weight would apply a clockwisetorque to the gyro suspension.





Precession arising from the clockwise torque would move the north end of the rotor furtherwest and therefore into closer alignment with the meridian.

In practice, modern gyrocompasses, such as the Meridian Gyrocompass, exercise gravity con-trol by an indirect method. Tilt is detected by an electronic pendulum or accelerometer and the

resultant electrical signals are used to produce torques that have the same effect as a suspendedweight.

A.2 G YROCOMPASS CORRECTIONS

A.2.1 Latitude CorrectionSub-section A.1 explains how the addition of gravity control to the gyroscope gives it thenorth-seeking characteristic necessary for use in a gyrocompass.

When operating at the equator, such a simple gravity control would be sufficient to maintainalignment with the meridian with no further need for corrections.

However, as the operating latitude increases towards either of the poles, there is a greater ten-dency for the gyroscope to experience azimuth drift with time.

The rate of change of azimuth due to the earth’s rotation is constant for a given latitude. There-fore, the gyrocompass must apply a controlling correction torque, perpendicular to the spinaxis, to cancel the drift caused by the earth’s rotation. To generate this correction torque thegyrocompass needs to know the operating latitude.

A.2.2 Gyro Damping Whenever the gyroscope does not align perfectly with the meridian, precession caused by

gravity control and the horizontal component of the earth’s rotation will cause the north end ofthe gyro rotor to trace out an anticlockwise elliptical path.

The application of latitude correction causes this elliptical path to be symmetrical about apoint projected from the north end of the gyro rotor when horizontal and aligned with themeridian. For a given gyroscope, the ratio between the major and minor axes of this errorellipse is constant. The size of the ellipse depends on the initial displacement of the gyro axisaway from the meridian and the horizontal plane.

By reducing the amplitude of ellipsoidal excursion in one plane, it follows that the amplitudeof excursion in the other plane reduces proportionately to settle the gyro horizontally and in

the meridian.

In the gravity controlled gyroscope, a tilt of the rotor spin axis produced a torque about thehorizontal axis to drive the spin axis towards alignment with the meridian. However, to pro-duce a workable gyrocompass, there must be some form of damping.

One practical method for doing this is to include electrical feedback so that a tilt in the rotorspin axis also produces a torque about the vertical axis. The sense of this torque would be tocause a precession that would drive the spin axis towards the horizontal.

This would cause the vertical axis of the error ellipse to reduce progressively towards zero,

reducing the horizontal axis simultaneously. This process results in the north end of the gyro





rotor tracing a decreasing spiral path, eventually settling with the gyroscope horizontal andaligned with the meridian.

A.2.3 Speed Error Figure A–3 shows that the north end of a meridian-aligned free spinning gyro will appear to

rise as it moves northwards from the equator. This upward tilt is independent of the earth’srotation. If left uncorrected, this effect would interfere with the north-seeking properties of thegyrocompass because the compass would be unable to determine whether the tilt came from amisalignment or from the northward motion.

If left uncorrected therefore, the northward travel would cause an upward tilt that would causethe gyroscope to precess towards the west and then to go into a settling spiral. Eventually,given a constant speed of northward movement, the gyro would settle slightly to the west oftrue north.

Figure A–3: Gyrocompass speed error

Speed-related error is directly proportional to the north-south component of speed, andinversely proportional to the cosine latitude. This means that any error when the compass is onthe equator would become twice as large at latitude 60°, three times as large at latitude 70°,and nearly six times as large at latitude 80°. At latitudes above 80° the gyrocompass becomesvirtually unusable as a north seeking instrument.

The north-south component of speed is the product of actual speed and the cosine of thecourse made good. Speed related errors are therefore greatest when travelling in a northerly ora southerly direction.

To correct for these effects, the compass must know the direction and speed of travel. While ituses its own self-generated heading information to determine the direction of travel, speedinformation must come from an external source such as a speed log or a GPS receiver, or beapplied manually.





A.3 SUMMARY

A gyrocompass will indicate the true north direction after an appropriate settling period.

To maintain correct true north alignment, the gyrocompass must receive additional infor-mation concerning its operating latitude.

Also to maintain accuracy the gyrocompass needs to know its direction of travel, which itgenerates itself, and the speed of travel. Speed information must arrive from an externalsource, or be applied manually.

A gyrocompass becomes progressively less effective as a north seeking instrument athigher latitudes. Note that the Meridian Gyrocompass can be used successfully in high lat-itudes with the DG mode selected. In this mode, the gyrocompass acts as a directionalgyro. A directional gyro does not north seek, but can maintain a reference heading for ashort period. Follow the instructions in sub-section 3.3.3 to set the DG operating mode.







Contents


AAdjustments

Azimuth bias 5–3Azimuth drift 5–3Test connector 5–4

Alignment 2–4, 2–12Analogue rate of turn output 4–16Auxiliary inputs 1–5Azimuth bias adjustment 5–3Azimuth drift A–5Azimuth drift adjustment 5–3

CCable types 2–4Connections 2–7

Continuous operation 3–8Control unit. See RCUControls 3–2Correction

Speed A–6Corrections 3–4

Latitude 3–4, A–5

DData formats 4–3

Analogue rate of turn output 4–16Resolver output 4–15Serial NMEA 4–4Stepper S-code 4–16Synchro output 4–15

DIP switches 2–10Distribution Board connections 2–7

EError modes 3–5

GPS 3–5Gyro failure 3–6Speed log 3–6

FFault identification 5–2

GGPS 1–5GPS failure 3–5

Gravity control A–2Gyro damping A–5

Gyro failure 3–6Gyrocompass configuration 2–10Gyroscopic inertia A–1

I

InstallationAlignment 2–4, 2–12Cable types 2–4Choosing a location 2–3Connections to Distribution Board 2–7

LLatitude correction 1–5, A–5Latitude correction. See CorrectionsLoss of GPS 3–5

Loss of speed signal 3–6

MMaintenance

Error modes 3–5Test connector 5–4

Manual speed input 3–5

NNMEA data formats 4–4

North-seeking gyroscope A–2

OOperation

Continuous operation 3–8Lay-up 3–8Power failure 3–8Power-off 3–8Power-on 3–3

Operation during lay-up 3–8

PPower failure 3–8Power-off 3–8Power-on 3–3Precession A–2

RRate of turn analogue output 4–16RCU

Controls and indicators 3–2




External location 2–9Remote Control Unit. See RCUResolver output format 4–15

S

Speed correction 1–5, A–6Speed correction. See CorrectionsSpeed log failure 3–6Stepper S-code output 4–16Synchro output format 4–15

TTest connector 5–4Toppling 2–3

Documents

Meridian Gyrocompass