OM2225. Mag-03RC Three-axis Marine Magnetometer

Transcription

1 OM2225 Mag-03RC Three-axis Marine Magnetometer

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3 Table of Contents How to use this Manual... 3 Symbols Glossary... 3 Introduction to the Mag-03RC... 4 Vector Measurements and Conventions... 5 Output Voltage Convention... 5 Output Voltage Scaling... 5 Signal Input and Cabling Recommendations... 6 Cabling... 6 Surge Protection... 6 Digitiser Recommendations... 6 Power Supply and Power Source Recommendations... 7 Marine cable considerations Magnetometer Connector Options... 8 Dry Mateable (Mag-03RC)... 8 Underwater Mateable (Mag-03RCU & RCUT)... 8 Installing the Mag-03RC... 9 Magnetometer siting... 9 Pre-installation testing... 9 Cable testing recommendations... 9 Installation guidance Insulation Test Using the Mag-03RC Testing And Field Monitoring Built-In Test Facility Page 1 of 16 ISSUE 2

4 Geomagnetic Field Reference Magnetic Hysteresis Environmental Precautions Troubleshooting Care and Maintenance Maintenance Magnetometer Cable Cleaning the Mag03-RC Calibration End of Life Disposal APPENDIX 1 - Typical Power Requirements Page 2 of 16 ISSUE 2

5 How to use this Manual This document describes the installation, operation and maintenance of the Mag-03RC Marine Magnetometer. Read this manual in conjunction with the data sheet DS Take the time to get well acquainted with your Mag-03RC(s) by reading this manual. Knowing and understanding your sensors will ensure you experience the most reliable operation. When service or maintenance is required, contact Bartington Instruments. Technical specifications for this product can be found in the appropriate datasheets. Details can be found on the Bartington Instruments website. Visit the Bartington Instruments website regularly for more information about any changes to our product range, for software downloads, datasheets and for support details. You can access all the information you need about your specific sensor, including service information at Symbols Glossary The following symbols used within this manual call your attention to specific types of information: WARNING: Indicates a situation in which serious bodily injury or death could result if the warning is ignored. Caution: Indicates a situation in which bodily injury or damage to your instrument, or both, could result if the caution is ignored. Identifies items that must be disposed of safely to prevent unnecessary damage to the environment. Note: A note provides useful supporting information and sometimes suggests how to make better use of your purchase. Page 3 of 16 ISSUE 2

6 Introduction to the Mag-03RC This document, which must be read in conjunction with the data sheet DS 2276 for this product, describes the function, testing, installation and operation of the magnetometer. The magnetometer consists of three vector fluxgates, with associated electronics, housed in a cylindrical pressure vessel with a marine compatible connector fitted at the top-end. The tri-axial magnetic sensors are located at the lower end of the cylinder which also contains the associated electronics needed to precisely convert the magnetic field strength at the sensors into analogue signal voltages. The tri-axial sensor consists of a cluster of three, feedback stabilised, fluxgate sensors. Each axis provides a highly linear magnetic response with virtually no hysteresis and no cross talk between axes. The magnetometer is ideally suited to the measurement of ship magnetic signatures. When used with a modern, high resolution digitiser (e.g. the Bartington Instruments DAS1 system) a ship s magnetic signature can be continuously recorded without recourse to the generation of a "backoff" field to compensate for the terrestrial magnetic field. A versatile centralising mechanism is provided to assist in seabed mounting within a suitably weighted, non-magnetic, seabed mounting tube. An internal built-in-test facility can be activated at any convenient time during use, to verify correct functioning of the magnetometer. The magnetometer can operate using either short or long (1 km) cables, without the need for a well-regulated power source. SURGE PROTECTION PWR+ PWR- ISOLATED PSU DRIVE ELECTRONICS X FLUXGATE Y FLUXGATE Z FLUXGATE X + - SENSE ELECTRONICS Y + - Z + - TEST SCREEN/0V TEST Figure 1 : Functional Schematic of the Mag-03RC sensor Page 4 of 16 ISSUE 2

7 Vector Measurements and Conventions Each tri-axial magnetometer channel produces three independent analogue output voltages in response to the magnitude and direction of the orthogonal components of a magnetic field. A "right-hand" co-ordinate system is adopted. In this system, the X,Y and Z axes correspond to the thumb, first and second finger respectively of the right hand. Conventional to geophysical measurements, the magnetometer should be installed so that the X axis is arranged to point North, the Y axis to point East and the Z axis to point down. A marker at the top of the magnetometer indicates the positive X (or North) direction. Each of the three vector sensors measures the magnitude and the direction of the component of the magnetic field that lies along its axis. The quantity B recorded for each axis will depend upon the magnitude and direction of the ambient magnetic field F and the angle θ between the sensing axis direction and the ambient magnetic field, where: B = F COS θ Output Voltage Convention Each of the three (XYZ) magnetometer analogue outputs is bipolar and provides a differential output voltage that is proportional to the magnitude and direction of the magnetic field component. The +/-10 volt differential output voltage refers to the voltage difference between each pair of output lines. The non-inverted output line will produce a positive output voltage response when the magnetic field component for that vector axis is in the positive direction (in the direction of the arrow marking for that axis); the inverted output line voltage will be the same magnitude, but with negative polarity. If the magnetic field component is reversed (opposite to the direction of the axis arrow), the inverted output will be a positive voltage and the non-inverted output will be negative. Each of the output lines, therefore, has a full-scale voltage range of +/- 5 volts. Output Voltage Scaling The constant of proportionality between the magnetic field strength and the differential analogue voltage is referred to as the scale factor. The scale factor is defined as the nominal full-scale analogue output voltage divided by the nominal full-scale magnetic field strength (the range ). The full-scale field strength is stamped upon the body of the magnetometer in microtesla (μt) units. Table 1 shows scale factors for the Mag-03RC models available. Model Full scale range (μt) Scale Factor (mv/ μt) Mag-03RC100 ± Mag-03RC200 ± Mag-03RC100 ± In practice, the nominal full scale range can be exceeded by 5% without loss of accuracy of scale factor. Note : This magnetometer can be supplied with a single ended ±10V analogue output as an alternative to the differential output. Page 5 of 16 ISSUE 2

8 Signal Input and Cabling Recommendations Cabling The standard magnetometer provides differential output lines for analogue signal transmission. The advantages of this differential arrangement are very high common-mode noise rejection and suitability of readily available cable types. Each of the anti-phase output lines has a low impedance at the signal source, damping the lines to prevent ringing. Each line has a bipolar swing of ±5 volts. The output drivers will drive a cable of up to 1km length. Cable inductance and capacitance considerations require that the cable be terminated with a differential amplifier having a circa 50kΩ input impedance. This arrangement will to provide some damping of undesirable high frequencies, but will also attenuate the signals over the full frequency range by circa 0.2% for a 1 km cable. Due to these effects: The cable pair loop resistance should not exceed 0.1 Ω per metre The pair loop inductance should not exceed 0.5 μh per metre The capacitance between conductors should not exceed 52 pf per metre The capacitance between conductors and shield should not exceed 120 pf per metre Additional capacitance across the line will reduce the bandwidth of the magnetometer signal. Surge Protection The signal lines are protected against electrical discharge by a cascade of high-energy protection circuits. The surge arrestor for each signal line consists of a bipolar 15 volt semiconductor shunt protection device close to the electronics followed by a 10 ohm series resistor followed by a 70 volt, high energy, gas discharge, shunt surge arrestor close to the connector. This is adequate to dissipate lightning induced currents. The common connection of the surge arrestors is to the cable shield connection. Note: A similar arrangement is recommended at the data acquisition end of the cable, typically within the system s digitiser or its interface circuitry, just before its differential amplifier. The ground termination should be to a low inductance conductor connected to the safety ground of the system. Digitiser Recommendations The analogue voltage signals produced by the magnetometer are normally converted into digital form at the shore base for data archiving and retrieval. The resolution of the digitiser must be adequate to resolve the minimum signal magnitude of interest and respond to the maximum frequency of interest. The number of bits and the sampling rate are therefore important. The available number of bits will primarily determine the resolution. For example, a 16 bit converter will resolve approximately one part in for a bipolar signal. This is equivalent to 3 nt in 100μT for a ±100μT full scale magnetometer. If the number of bits can be extended, each additional bit will double the resolution. The sampling rate is also an important consideration. The upper frequency limit of the magnetometer is 3 khz, therefore the absolute minimum (Nyqvist) sampling frequency must be twice this value for signal recovery. The magnetometer also produces an internally generated frequency component at around 15 khz and of a few mv magnitude. This unwanted frequency must be removed, prior to digitization, if aliasing due to the sampling rate is to be avoided. Aliasing is the production of unwanted frequencies in the band of interest due to the close proximity of the sampling frequency to some signal. Installation of a low pass filter with Page 6 of 16 ISSUE 2

9 a cut off frequency just above the 3 khz required band is therefore required between the magnetometer and the digitiser. A cut off slope of -12 db per octave minimum is suggested. Another desirable feature of a digitiser will be the ability to "back-off" the ambient (terrestrial) field value so that only the variations due to the vessel are recorded. A surge arrestor similar to that fitted within the magnetometer should be included in the digitiser or it's interface circuitry just before the differential amplifier. The ground termination should be to a low inductance conductor connected to the safety ground of the system. Note: The Bartington Instruments DAS1 Range Data Acquisition System is an ideal digitiser system for the Mag-03RC Magnetometers. Its modular design allows simply expansion to support any number of three-axis sensors up to 160. The DAS1 supplies power to the sensors, and provides 18-bit a-to-d resolution at the full 3 khz bandwidth of the Mag-03RC, with anti-aliasing filters. The system also features power supply and earth leakage current monitoring. Power Supply and Power Source Recommendations The power supply circuitry within the magnetometer unit provides galvanic isolation between the signal circuitry and the external power source. The power supply also regulates the internal magnetometer supply voltages giving immunity to the effects of voltage loss when very long telemetry cables are used for the shore connection. 70 volt bipolar gas discharge surge protection is provided between each of the supply lines and the cable shield connection. This ensures adequate protection, whilst not compromising the isolation of the power and signal paths. The magnetometer to power source connection should be via the recommended twisted and shielded pair cable (see datasheet DS2276) to give very high isolation of the power supply noise from the signal lines. The power supply switched-mode converter is synchronised to the master clock of the magnetometer to prevent the production of beat frequencies in the magnetometer signals. The input voltage range is 15 to 30 volts. The current requirement is 50 ma nominal Switch-on surge capability of 83 ma is required. The voltage loss over a long cable should take this value into account. For a 1km cable, the loop resistance will be circa 100 ohms resulting in a voltage loss of 8 volts at switch-on. In this situation, a minimum source supply voltage of 24 volts is required to ensure reliable operation. The supply circuitry is protected against reversed connection. Marine cable considerations. A water blocked cable is desirable if long operational life is to be assured. The polyurethane jacketed cable recommended in the brochure is of this type and the specification is matched to the requirements of the magnetometer. The connector must be moulded to the cable by a reputable vendor. A "dog's-leg" in the wire terminations braced by epoxy resin should underlie the polyurethane moulding. This arrangement will strengthen the joint and allow moderate stress to be tolerated safely during handling. This is particularly important during the operations of magnetometer deployment and recovery. The sea is a fairly benign environment, but the cable is particularly prone to wear and damage at the point where it emerges from the sea. Additional Page 7 of 16 ISSUE 2

10 protection in the form of a plastic hose or sleeve should be fitted here. On land, the cable can be "trenched-in" to the soil. Again, if mechanical damage is to be avoided then some protection here is also advised. Magnetometer Connector Options Caution: All connectors should be hand-tightened only, to prevent damage. Dry Mateable (Mag-03RC) This connector system is "dry-mateable" and must therefore be fully connected prior to deployment in water. The magnetometer connector contains an "O"ring face seal (BS 1806 size 015) which may come loose. It is therefore recommended that the protective cap supplied with the magnetometer connector remains fitted up to the time of use. A very light smear of silicone grease can be applied to the "O" ring to aid retention. The connector engagement is orientated by a raised spline on the cable connector and a keyway slot in the magnetometer connector. It is essential that this keyway system is accurately orientated prior to and during the engagement of the connectors. A very light smear of silicone oil can be applied to the cable connector piston-seal "O" ring to aid engagement and avoid abrading the "O" ring. Underwater Mateable (Mag-03RCU & RCUT) These sensors have a connector system that can be mated and unmated whilst underwater. This can significantly simplify system assembly, by allowing sensors to be connected and disconnected from the cables whilst submersed. Page 8 of 16 ISSUE 2

11 Installing the Mag-03RC Magnetometer siting Ideally the magnetometer will be mounted on the seafloor in a geological region that has consolidated sediment of several metres depth covering any magnetic base rock. Ideally, core sampling will reveal the structure of the marine sediment. The most magnetic rocks are the igneous types where the basaltic titanomagnetites dominate in the generation of seabed magnetic anomalies. The resulting induced and remanent magnetic moment will be of less than 50 to 100 nt above or below the ambient value at the most and will not have an adverse effect upon the accuracy of the magnetic measurements. A far greater problem will be due to magnetic debris such as metallic jetsam. Again, unless the magnetic contribution is a significant proportion of the desired measuring range it is unlikely to contribute a significant error to the measurement because the offset produced can be digitally removed. There exists a situation however for which corrections cannot be applied. This will occur where a large ferromagnetic object is within a few to tens of metres of the magnetometer. In this situation the magnetisation of the object could exhibit magnetic hysteresis as a result of either the ship signature or de-gaussing flux. It is for this reason that a magnetic evaluation of the intended site should be conducted using a resonance or total field magnetometer to establish that the seafloor is free from magnetic contaminants. The tidal variation will cause the height of the vessel above the magnetometers to alter therefore some means of monitoring this will be required (the Bartington DTS1 Depth/Tilt sensor is recommended). Siting the range away from estuaries will avoid the sea-current disturbances which accompany tide reversals. For the highest measurement resolution it is advisable that an additional magnetometer, that takes no part in the signature measurement, is situated elsewhere but in the vicinity of the range (perhaps on the shore) for the purpose of monitoring the background terrestrial field. The response of this reference magnetometer can be numerically subtracted from the results of the "active" magnetometers. In this way the measurement baseline is made substantially close to zero. Pre-installation testing Prior to installation the magnetometer, cable, power supply and data acquisition system must be fully tested on dry land to ensure that everything is functioning correctly. Cable testing recommendations A cabling test is recommended, using an electrical continuity tester (or ohm meter) connected between the respective contacts on the wet and the dry-end connectors to ensure that the correct pins have been allocated to the cable conductors. The test should also check for shortcircuit faults between conductors. The magnetometer should then be connected to cable connector and the power supply switchedon. With your data acquisition system active, rotate the magnetometer in the terrestrial field so that each axis in turn lies parallel to the approximate direction of the field. Observe that the data acquired is in accordance with the expected field value for each axis. Page 9 of 16 ISSUE 2

12 Activate the test function and observe that the approximate 1μT deviation is in accordance with the test document value. Installation guidance There are two possible ways of installing the magnetometers on the sea-floor. The first method requires that the magnetometers are pre-installed in the mounting tube together with the telemetry cable. The tube must be fitted with a heavy base and three legs suited to positioning on the sea floor. Two lanyards, held several metres apart, should be attached to the top of the tube and the assembly lowered to the seafloor in the correct orientation from the surface vessel. This method will result in less accurate alignment than the second method. For the second method a diver is required for fitting the magnetometers into their seabed mounting tubes. The magnetometer should be installed, with the connector at the top within a non-magnetic tube fixed to the sea-floor. It is assumed that the mounting tube is vertical to within one or two degrees. If the connector version is "dry-mateable", the magnetometer and cable must be pre-assembled and tested prior to lowering in to place in the mounting tube. Attaching a lanyard to the magnetometer body will assist in retrieval. The orientation stud at the top of the magnetometer body should be aligned so that it faces north. If this is to be accomplished by the diver who is fitting the sensor into the tube then the direction will be to Geomagnetic north as directed by a hand compass. An alternative method is possible where an acoustic link is established between the diver and the telemetry/data acquisition system. In this case the sensor should be approximately orientated as above and subsequently trimmed so that a null output is recorded for the Y axis at the data acquisition system. The magnetometer is supplied with a bow-spring arrangement that allows the fitting centrally within a non-magnetic tube mounted on the sea-floor. The tube can have an internal diameter D of between 102 and 250 mm. The bow-spring arrangement consists of two 100mm diameter rings separated by a flat, flexible spring. Each ring is fitted with two 12mm (M12 thread) nylon studs set at an angle of 120 degrees. The length of the studs determines the centralization of the magnetometer within the mounting tube. The stud length L in mm should be calculated using the formula: L = D/2 37 This allows 12 mm of thread to be engaged in the mounting ring and allows for some small adjustment. The supplied nylon lock nuts should be fitted. Important Note: All studs must be set to the same length. Adjust the deflection of the bow-spring so that the overall diameter of the studs and bow-spring is approximately 5 mm greater than the internal diameter of the mounting tube. A half-metre length of tube identical to the seafloor mounting tube can be used as a guide. To deflect the bowspring, clamp one of the 100 mm mounting blocks in a fixed position using a flat-bladed screwdriver to tighten the 1/4 inch UNC bronze screw. Next, compress the bow-spring to the required deflection and tighten the second mounting-block screw. Page 10 of 16 ISSUE 2

13 Insulation Test A simple electrical test can be applied to measure the effectiveness of the electrical insulation of the cable and magnetometer. With the magnetometer and cable submerged connect a 100 to 500 volt insulation resistance tester (Megger) between a sea-water electrode and the dry-end connector shield contact. The electrical resistance should be in excess of 1MΩ. If the cable has been invaded by a very small amount of water, indicated by a measurement of between 10 kω and 1MΩ, the magnetometer will probably continue to function correctly. If the problem is due however to invasion of water into the magnetometer or its connector, then a malfunction of the magnetometer is inevitable. This test should be carried out on a periodic basis at six monthly intervals. Page 11 of 16 ISSUE 2

14 Using the Mag-03RC Testing And Field Monitoring Built-In Test Facility The magnetometer is a precision measuring instrument. The very high temperature stability and low drift specifications make elaborate testing facilities unnecessary. The built-in test facility can be activated at any convenient time to give assurance that the magnetometers are functioning correctly. This test facility injects three precise currents, generated within the magnetometer into each of the three fluxgate sensors. This simple, internally generated test ensures that no externally generated noise is introduced into the magnetometer. Geomagnetic Field Reference Probably the most reliable method by which the performance of the range can be monitored is by reference to the diurnal and secular variation in the geomagnetic field. Most countries operate magnetic observatories that can be accessed via the internet using the international INTERMAGNET programme. By monitoring the response of each of the magnetometers in a range over a period of time, say several days, any differences in their response will become apparent. Comparison of the mean value recorded for all units and the observatory data will similarly give assurance that the range is functioning correctly. Some differences between range and observatory data must be expected due to lack of precise (interpolated) published values, geographical location and to a lesser extent magnetic induction in the deeper geology. The coordinating authority is IAGA and details of this organisation and a current list of participating observatories is provided by the World Data Centre Information on the daily activity of the geomagnetic field can be obtained from these observatories. A useful publication is the International General Reference Field (IGRF) that provides interpolated data on the field strength and direction (Inclination and Declination) information for the terrestrial field. Most observatories publish a K index which is a measure of the magnetic activity. Magnetic storms occur at around 22 year intervals due to solar activity. These need not adversely affect the range measurements because the sensors are configured as an array so all magnetometers are affected equally. The use of an additional reference magnetometer, outside of the range, will help with compensation of external noise. Page 12 of 16 ISSUE 2

15 Magnetic Hysteresis Caution: The Mag-03RC is designed to have an extremely low magnetic hysteresis. However, Bartington Instruments recommends your magnetometer is not subjected to magnetic fields greater than their stated measuring range for extended periods as this could alter the DC offset. If this occurs, the offset will exhibit drift as it returns to its original offset specification. Please consult the magnetometer datasheet for typical magnetic hysteresis (perming) data. NOTE: The magnetometer can be degaussed to reverse this effect. Please consult Bartington Instruments if you require advice regarding degaussing the magnetometer. Environmental Precautions Refer to the datasheet for maximum environmental electrical and mechanical ratings. Caution: Exceeding the maximum environmental ratings may cause irreparable damage to your sensor. Page 13 of 16 ISSUE 2

16 Troubleshooting Fault Possible Cause Solution No output signals from the sensor Power Source Fault Sensor fault Check your power source is supplying the correct voltage levels to the sensor. Contact the Bartington Instruments helpdesk One output signal is faulty Cable fault Sensor fault Check the cable connections Contact the Bartington Instruments helpdesk If the solutions suggested above do not fix the fault, please contact Bartington Instruments. Care and Maintenance Maintenance Magnetometer No routine maintenance of the Mag-03RC is required. Cable A cable insulation test, as described in Insulation Test, is recommended every 6 months. Cleaning the Mag03-RC If the magnetometer needs to be cleaned, use water and mild soap only. Caution: Never use chemicals, such as solvents, when cleaning the Mag-03RC. Caution: Take particular care when cleaning around electrical connections. Bent or damaged pins may cause the magnetometer to malfunction. Calibration High stability circuitry ensures that a minimum of ten years continuous use should be expected and no interim calibration should be required. Page 14 of 16 ISSUE 2

17 End of Life Disposal This symbol of the crossed-out wheelie bin indicates that the product (electrical and electronic equipment) should not be placed in municipal waste. Check local regulations for disposal of electronic products. Page 15 of 16 ISSUE 2

18 APPENDIX 1 - Typical Power Requirements Example of supply current profile at switch-on Current settles to quiescent value within 0.4 seconds following switch-on. Typical Quiescent Voltage and Current Voltage Current Power consumption V ma W Page 16 of 16 ISSUE 2

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