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Flux-Gate Magnetometer LEMI-008

Technical Description and Operation Manual
Lviv, 2000

DESIGNATION

LEMI-008  magnetometer for the measurement of the magnetic field variations in laboratory and land conditions is produced on the base of the flux-gate sensor, all three components of which are implemented in the same body. It consists of two units - sensor unit with the rotating support and electronic unit - both connected by a cable.

MAIN TECHNICAL PARAMETERS

Measuring ranges of variations *:
Range I
Range II


100 000 nT
3 200.0 nT
Resolution:
Range I
Range II

10 nT
0.1 nT
Digital outputRS-232
Analog output (inner only!) transformation factor2 mV/nT
Analog output noise at 1 Hz< 10 pT/Sqrt(Hz)   rms
Components orthogonality error<30 min of arc
Step offset ranging band by each axis 9 x10000 nT
9 x1000 nT
Smooth offset ranging band by each axis 1000 nT
Time of samples averaging1s, 2s, 5s, 10 s, 60 s
Volume of internal memory8 MB
GPS timing and co-ordinates determination 
Operating temperature rangeminus 5 to +40 C
Temperature sensor transformation factor7,6 mV/ C
Power supply, battery12 V, 0.2 A
Weight:
sensor with support
electronic unit

3.0 kg
3.5 kg
Length of connecting cable10 m

* The display shows measurement results in microtesla.

STRUCTURE AND OPERATION

 

Construction Description

The front panel of the electronic unit has the following control and commutation facilities. First from the right is SENSOR connector for sensor connection. Then decade switches OFFSET, microtesla for step offset ranging and smooth offset ranging regulators and the offset polarity switches for each of X, Y, Z axes are placed. Next is RANGE switch which switches the display scale either at 100.00 mcT (position I) or at 3.2000 mcT (position II). Then the switch RECORD is placed with two positions: data storage on (upper position) or data storage off (lower position). The RS-232 connector is intended for magnetometer operation modes control using external PC and also for data transmission to the external memory. The four-lines display shows one-second averaged output signals of each component, sensor temperature, electronics temperature real time and chosen samples averaging time of stored data and also the information about operation mode. Below the display the one-position switch backlight is placed: after it is depressed for a while the back-light of the display is on during 4 minutes. The GPS ANTENNA connector is under the display too. Last to the left are both POWER connector, fuse and switch ON-OFF.

The back panel has no connectors or switches. Non-magnetic housing and minimal magnetism of components enable the instrument to be used conveniently close to the sensor, if necessary. The electronic box is protected against dust and moisture but at any case has not to be immersed into water or exposed to strong rain!

The connecting cable between sensor and electronic unit is hand made and special measures are taken against the interferences. The total length of the connecting cable is 10 m and in the transportation position it is wound on the drum.

The magnetometer sensor, electronic unit and the cable on drum are placed alltogether in a special metallic case. This case is constructed in such a way that it is convenient to transportate in it the magnetometer to the operation point. At the point during the preparation for operation the sensor and cable are taken outside and the electronic unit remains in the case. In the closed position the case allows the additional protection against weather problems for electronic unit. To this, the free place arised now conveniently suits for the enough powerful battery installation inside the same case.

The sensor unit has three measuring components. Each component is formed by the flux-gate probe in the shape of racetrack 40 mm long, the core of which is made from 5 wraps of low-noise mumetal stripe 20 mcm thick. Each probe has three windings (see Fig. 1): excitation We (250 turns, 1 Ohm), output W0 (500 turns, 18 Ohms) and compensation Wc (500 turns, 33 Ohms). Both three components and housing for their fixation are made from thermocompensated quartz glass, having thermal dilatation factor much less than 1 ppm. Special means are applied to make the orthogonaglity of magnetic axes not worse than 30 minutes of arc.

The sensor housing is weatherproof but it is necessary to avoid its total immersion into water. It has also a thermal sensor (TS) inside.

 

Functional Diagram Description and Operation

The magnetometer functional diagram is given on Fig. 1. The electronic unit consists of 7 printed boards. The following units are situated on the main board: excitation channel (EC) of flux-gate sensor (FG), analog-to-digital converter (ADC), reference voltage source (RVS), clock generator (CG), frequency divider (D3), central processing unit (CPU), random access memory (RAM), programmable memory (PROM)*, internal real time clock (RTC), peripheric processor unit (PPU), two-channels serial port (RS-232), flash-memory (FM) with registers and control commutators, thermal sensor amplifier (TSA) of the TS inside FG and thermal sensor in the electronic unit (TE) with its amplifier (TEA) and power supply unit (PSU).

A set of other units is plugged in the special connectors on the main board. There are three subboards of measuring channels (MCx, MCy, MCz) and one subboard of global positioning system (GPS) receiver.

The board fixed to the front panel of the magnetometer has the step and smooth dividers of compensation control unit (CCU) for each of the three components and four-lines display (D).

For the magnetometer operation synchronization the quartz-stabilized CG together with D3 divider provide pulse signals with following frequencies: 22,11 MHz for CPU and PPU, 4422 kHz for ADC and 221.1 kHz for EC.

The EC itself has two frequency dividers (D1) and (D2), reset pulse former (RPF), pre-installed memory card (PMC), current amplifier (CA) and filter (EF). It provides high purity excitation currents into the We windings of each FG component, driving mumetal core in and out of saturation, so gating in and out the component of any external field intercepted by the sensor along the longitudinal axis of the race-track. The ferroresonance excitation mode of FG at the frequency 6,9 kHz is assured. PMC and RPF serve for the D2 preliminary setting in order to achieve the synchronous commutation of the phase detector (PD) of each MC with the Wo output signal.

When external field is applied to the sensor, corresponding W0 winding of FG gives the second harmonic signal (13,8 kHz) to its MC. The MC is composed from the input filter (IF), amplifier (A), voltage-to-current transformer (VCT), synchronous (phase) detector (PD), direct current amplifier (DCA). From the DCA output feedback signal via summing amplifier (SA) creates in the corresponding Wc winding of FG the compensation field which maintains the sensor in null field. In this way a precise linear measurement is obtained which is drift-free and stable over a wide range of field strength. Also on the SA input come the voltages from RVS and CCU via the step and smooth dividers and corresponding decade step amplifiers (DSA) and fine control amplifier (FCA), creating step zero ranging in the limits of  9 x 10 mcT (DSA1), 9 x 1 mcT (DSA2) and smooth ranging in the limits of 1 mcT (FCA). First order low pass filter LPF and matching amplifier (MA) on the MC output limit the highest signal frequency and MA output is the analog output of MC, connected to OUTPUT connector placed on the main printed board (outputs KT1, KT2 and KT3 on Fig. 1). Also the outputs of TSA and TEA arc in the same connector. The LPF cut-off frequency is equal to 1 Hz. From DCA output the signal via measuring channel commutator (MCC) goes to the sigma-delta type ADC input.

Each MC is interrogated by ADC each second in a following way. Sampling cycle for each MC takes 210 ms, from which 150 ms is a duty time and other 60 ms the MC output signal is sampled with the clock frequency of ADC and all these samples are averaged. The obtained average reading is then sent to the memory. Also the temperature of both sensor and electronics is measured: each first second - sensor temperature, each other second - electronics temperature. It is possible to make averaging longer if necessary: during 2, 5, 10 or 60 seconds. If more long than 1 s averaging time is selected, then the temperature is stored as described every second sample time. ADC is calibrated each time when switch record is on.

The CPU controls the ADC operation. A one-crystal 80C52 processor is used in CPU. It assures digital samples preliminary storage and averaging during given time interval, then data transmission to flash memory or external PC memory, files formation and transmission via RS-232 interface. Also it controls the four-lines display D and time marks transmission from inner energy-independent clock RTC and assures the link with peripheral processor unit PPU, taking from it the geographic co-ordinates of the magnetometer and exact GPS time for RTC correction.

By CPU command PPU switches on GPS at given time and makes GPS receiver output configuration in RMS format. When data about co-ordinates and time appears PPU transmits it to CPU. Also PPU function is to produce warning signals both on display and beep if the power supply voltage is lower than 10.5 V or if the switch RANGE is in the position I when the data registration is switched on.

The magnetometer control and initial setting are executed from external PC connected with the magnetometer via RS-232 (x3) connector using specially developed software mag-s.exe.

Power supplies +12 V, -12 V and +5 V with common line and insulated +5 Vi source are provided by corresponding pulse mode converters of PSU, powered from the battery 12 V (connector X4). The margins of battery voltage from 10,5 V to 14,5 V are admissible.

Attention! Small changes can occur from specimen to specimen, but the main conception is as described above.

 

IMPORTANT!

The overload of any MC (X, Y or Z) can cause disturbances in the other two MC.

That is why it is necessary to start operation in the survey mode (position I) of the switch RANGE at the front panel and to be at this mode all the time when any of components readings exceed 3200 nT (or 3.2 mcT at the position II). So, when compensating field values by X, Y or Z components, first start to do it in the position I of range switch. Only after achieving a residual field in each component less than 3200 nT it is possible to switch measuring range to the position II and make compensation of the field further to the desired value.

When the sensor is transported, make sure that it is in the vertical position. Then the transporting vibrations from any kind of transport are better supported. Avoid any falls and strong shocks more than 3g!

Guarantee

The term of guarantee is 1 year after delivery if all requirements of the present description as to applied voltage, weather conditions, vibrations and shocks are observed. During this term the manufacturer is responsible to repair the defects other than occurred through the fault of the consumer or force majeure, or if no possible, to change the magnetometer by other equivalent specimen.

The manufacturer maintains its obligations to make service and repair if necessary for still 2 years. By this the user has to cover the transportation/visit fees only.

Delivery date: ___

OPERATION MANUAL

1. PREPARATION FOR OPERATION

1.1. Prepare the place where the sensor has to be installed in such a way that the sensor was put on any rigid non-metallic support, preliminarily press firmly this support against the ground in order to harden the soil under it. Level the sensor using three levelling screws of the sensor basement and levels at the sensor basement as indicators.

1.2. Turn the sensor in such a way that the needle on its cover points the north.

1.3. Precise sensor orientation is made using Y-component reading: it shall be zero if the sensor X-component must be oriented to the magnetic north and readings shall be equal to mean Y-component value for given place (or to Bx magnetic sin D) if the orientation to geographic north is aimed.

1.4. Make following settings on the magnetometer front panel:

switch ON/OFF OFF;
switches OFFSET MICROTESLA - arbitrary positions; if installation is made for the first time, better to put all of them in extreme left position;
switch RANGE - 100.00 mcT (position I);
switch RECORD - lower position (off).

1.5. Make following connections:

couple other end of the sensor cable to the connector SENSOR (X1) on the front panel;
couple GPS antenna cable with the connector GPS ANTENNA (X2);
couple RS-232 connector with the computer.

1.6. Special precautions have to be taken when POWER connector is coupled to the 12 V battery. The wire with RED mark of the power cable has to be coupled to the positive terminal of the battery, the wire with BLACK mark - to the NEGATIVE one. If the polarity is wrong or the battery voltage is more than 18 V the fuse on the front panel is burning out immediately.

The magnetometer is ready for operation.

2. OPERATION

2.1. Switch the POWER switch in the position ON. If all connections are properly made, following information appears on the display.

Places 1 10 of first three lines - the values of magnetic field along X, Y ,Z components in microtesla.

Places 1 5 of the fourth line - sensor and electronic unit temperatures in degrees Celsium.

Places 9 16 of the fourth line - real time.

Place 13 of the first line:

“f” when the data registration into the inner flash memory is programmed, or

“F” when switch RECORD is switched to the upper position, i.e., data registration into the inner flash memory is on, or

"p" - when the on-line data registration into the external PC is programmed, or

"P" - when by this switch RECORD is switched to the upper position, i. e., on-line data registration into the external PC is on, or

“M” when the program “mag-s.exe” is operating.

Place 14 of the first line:

“*” appears in the end of each minute when the data from operative memory is stored in the inner flash memory, or

“>/<”” when flash memory data is transmitted to PC, or

“C” when flash memory is cleared.

Place 15, 16 of the first line - samples averaging time in seconds (01, 02, 05, 10, 60).

Place 14 of the second line - “G” appears when magnetometer is programmed to operate with GPS - receiver or is clear when magnetometer is programmed to operate without GPS.

Place 15 of the second line:

“c” when GPS is programmed to operate in the determination of local coordinates, time and date mode, or

“C” when GPS is synchronised and local coordinates are determined, or

clear when magnetometer is operating without GPS.

Place 16 of the second line - “T” appears when GPS is synchronized and exact time is determined.

Place 14 16 of the third line - the quantity of free memory blocks (one block = 64 KB) in the inner flash memory (127 maximum).

2.2. Compensate the offsets along X, Y, Z axes using step and smooth offset ranging switches OFFSET on the front panel. Switch RANGE put in the position II. Correct the offsets if necessary.

2.3. Activate mag-s.exe program in PC and using the PC keyboard and menu guide choose the necessary modes of magnetometer operation and data storage:

 

Set time of samples averaging

1 - 1 sec samples averaging.

2 - 2 sec samples averaging.

3 - 5 sec samples averaging.

4 - 10 sec samples averaging.

5 - 60 sec samples averaging.

 

Operation with FLASH memory

F - full FLASH memory reading.

B - last FLASH memory block reading.

C - FLASH memory clear.

A - running FLASH memory data address register.

 

GPS operation mode

H - GPS on. LEMI timing from GPS

K - GPS on. Time and coorinates.

L - GPS off. LEMI timing from internal clock.

 

PC service mode

T - LEMI time setting from PC

P - data output via RS-232 port in ASCII format

E - exit.

The installed modes are stored in the magnetometer’s constant memory and are independent from battery state. Only when other than installed modes is necessary to choose, the re-programing of the magnetometer is necessary.

2.4. Disconnect PC cable from the RS-232 connector.

2.5. Switch on data storage, swithching the RECORD switch in the upper position.

ATTENTION!

Data storage is carried out when RANGE switch is in the position II only! If this switch is in the position I, switching of the record switch to the upper position causes the warning information at the display and the beep warning signal.

The magnetometer is in operation in appropriate operation mode.

3. Data processing

3.1. After the registration time is expired put the RECORD switch in the lower position. Couple PC cable to the RS-232 connector of the magnetometer. Activate mag-s.exe program and using menu make data re-writing from magnetometer inner memory to the PC. It is necessary to mention that all inner operations and data transmissions are executed with the speed 9600 Baud. Only by flash-memory reading (B and F modes) data transmission via serial port is realised with the speed 115 kBaud. By this data file in the PC is formed in the hexadecimal system (bin index). The data file header contains the information about year, month, day, hour of the registration beginning (e.g., yy mm dd hh = 00 03 16 12.bin).

Two modes of data re-writing are possible: last data block storage or all flash memory volume storage.

3.2. Using bintotxt program the data file can be presented in text format (ASCII). Its header has the same information “yy mm dd hh” but with “mag” index, e.g., “00 03 16 12.mag”. In the text file the magnetic field data are in nanotesla units.

The text file header contains the installation place co-ordinates (if GPS operation mode with co-ordinates determination was installed) and day and time of registration beginning. Then 6 columns are formed in such a way: time, Bx, By, Bz, sensor temperature and electronic unit temperature.

Example:

sgprmc, 091846, a, 5150.1231, n, 02047.3338, e, 0.000, 0.0, 21.02.00, 3.5, e

Date: 21-2-2000
Time: 09:18:46

09.2000 -1,3 -18,6 14,3 +15 +22
09.2001 -1,3 -18,7 14,4 +15 +22

..................................................................................................

3.3. If whole flash memory volume is transmitted, a file with "fmb" index is formed into PC, the header of which has year, month, day, hour of the transmission beginning (e.g., 00 03 25 12 fmb).

The program bin8bin.exe serves for to divide the formed file (e.g., 00 03 25 12 fmb) into separate files in hexadecimal system and the corresponding catalogue is formed too (00 03 25 12). The files names reflect the beginning of the corresponding registration.

If more than one registration was made during one hour, first file has the name as shown upper (00 03 25 12.bin), the second and following files will have minute index (e.g., 00 03 25 12.15, e.g., 00 03 25 12.24 etc.). To transform hexadecimal files into the text format is possible using bintotxt.exe program which forms text files with .mag index.

If the on-line data transmission via RS-232 port ("P" mode) is necessary to stop, first press left ALT key on the connected to the RS-232 connector PC keyboard, then switch the RECORD switch to the lower position.

Imporant!

flash memory cannot be overwritten and when whole its volume is filled up the registration is stopped. That is why before the beginning of magnetometer operation make contain that the flash memory has enough of clear volume. The information about the available clear volume in blocks (maximum 127) is at the places 14-16 of the third display line. If "127" is displayed it means that all memory volume is clear.

4. OPERATION CESSATION

4.1. After all operations with the magnetometer including data re-writing, are over, put POWER switch in the OFF position. By this magnetometer stops to operate but all data including selected operation mode of the magnetometer, are conserved in the magnetometer’s inner memory.

4.2. Disconnect all cables both from the front panel including this of the sensor of the magnetometer. Close the cover of the electronic unit.

4.3. Wind the sensor cable on the drum and pack other cables. When transporting, to keep the sensor in the vertical position is highly desirable.

Avoid any falls of the sensor, especially by the lateral side!

 


Last modified: 10-01-2014

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