Fluxgate Magnetometer - Geotech

1 1999-2002 carl W. MorelandRevision 1 Fluxgate Magnetometerby carl MorelandThe instrument of choice for the modern treasure hunter is no doubt the metal detector which has the capability of not onlyfinding precious metals but most can also provide some level of discrimination against unwanted targets such as pulltabs and metal detector operates by generating a dynamic electromagnetic field and looking for perturbations in the field due toconductive objects. Another instrument, called the Magnetometer , looks only at static magnetic fields (namely, earth s) forperturbations. Not all metals affect magnetic fields in an easily measurable way; the Magnetometer is sensitive only to ferroustargets such as iron and most cases the treasure hunter is trying to eliminate ferrous targets, or at least those that contain iron (such as nails), anddetect only gold, silver, and copper and their alloys.

2 So why would anyone want a Magnetometer that can only locate iron whiletotally ignoring gold? The answer lies in the things that sometimes accompany treasure. For example, a cache might be buried in asteel or iron container; placer gold is often found in highly magnetic black sand; and sunken treasure is often accompanied by ironartifacts. In fact, most of the sunken treasure ships found in the last 30 years were pinpointed with a Magnetometer , and the mag isan obvious choice in detecting steel-hulled ships. So while the mag is not the sort of instrument a weekend coin hunter might use,it is certainly a valuable tool for other treasure Basic MagThe basic Magnetometer consists of sensor which produces a signal that isproportional to the strength of the magnetic field around it.

3 One type, the protonprecession Magnetometer , uses a sensor which consists of a bottle of distilled water ora hydrocarbon such as kerosene around which a coil of wire is wound. The coil isperiodically energized to produce a strong magnetic field which aligns the spin ofthe hydrogen protons and effectively magnetizes the liquid. When the coil field isremoved the protons precess, sort of an atomic wobble that occurs with a frequencythat is dependent on the strength of the surrounding ambient magnetic type of Magnetometer uses a device called a Fluxgate sensor. Thefluxgate consists of a magnetic core which is periodically hard saturated by a first coil(much like the proton liquid), while a second coil senses the resulting electromagneticfield.

4 Figure 1 shows one such device which is a toroid core with the energizing coilwound toroidally and the sense coil wound flat around the outside. Under normalconditions, the sense coil will not detect the field that is generated by the toroidbecause it will be balanced, assuming the sense coil is symmetrically wound. However, an additional external magnetic field cancause an imbalance in the toroid s hysteresis which results in a net field that is detected by the sense coil. In this particularconfiguration, the sensitivity to the external field is sensitive to the orientation of the sense coil with respect to the Cheap and Easy SensorThe Fluxgate sensor is a little tedious to construct and the sensing circuitry israther difficult and requires an oscilloscope for proper adjustment.

5 Fortunately, aready-made Fluxgate sensor (Figure 2) that is incredibly easy to use is made by Speake& Co. in the UK and distributed by Fat Quarters Software in the Americas. Bothaddresses are given at the end of the article. The sensor is small - 62mm x 16mm (d) " x 5/8" - and has four 100 mil-spaced pins at one end. It can operate over a rangeof 50 microteslas ( T) with a resolution on the order of about 10nT. This makes itextremely sensitive to very small field anomalies. The pinout, shown in Figure 3,includes a feedback connection which is useful for making linearity corrections to thedevice. In this application, linearity is not a concern as we will use the sensor to detectthe presence of field anomalies, not to accurately measure their value.

6 Therefore, inthis design the Fluxgate sensor is truly a three-terminal device: 5 volt supply, ground,and SensorFIGURE & Co. FGM-3 1999-2002 carl W. MorelandRevision 2 The output of the sensor is an easy-to-use 5 volt pulse waveform whose frequencyvaries with the strength of the surrounding magnetic field, generally from about 50 KHz to120 KHz. As mentioned of Fluxgate sensors, the device is highly directional, meaning that itresponds differently as it is rotated in free space. This poses a problem because directionalvariations with respect to the earth s magnetic field can easily swamp out any smallanomalies that we might be looking for. One possible solution is to use two fluxgatesensors that are aligned in the same direction.

7 They will both see the same absolutemagnetic field regardless of orientation as long as they remain aligned to each other;however, if there is a local anomaly present it will affect the closer sensor by a greateramount. Technically, this type of arrangement is known as a gradiometer because is detects magnetic field gradients, but we willstill refer to it by the more general term that the output of the FGM sensor is a pulsetrain in the 100 KHz range. By using two sensors we canmix the two pulse waveforms and obtain a low frequencydifference signal. This type of arrangement is reminiscentof the BFO metal detector where two oscillators are mixedtogether and we look for a shift in one of the frequenciescaused by a metal target.

8 Like the BFO, we can directlymix the output frequencies of the two Fluxgate sensors toproduce an audible indication of a target. Figure 4 showsthe schematic for such a digital mixer . However, there isa better +5vFIGURE PinoutFIGURE MixerFGM-3 SchmittTriggerFGM-3 SchmittTriggerDCLKQF lipflopfoutFIGURE Schematic 1999-2002 carl W. MorelandRevision 3 The CircuitSpeake & Co. also makes a chip called the SCL007 gradiometer. It accepts the outputs from two Fluxgate sensors and does thedigital mixing for us. The output of the gradiometer chip is an 8-bit parallel word which corresponds to the mix frequency. A zerooutput means the sensors see the same field, a full-scale output means there is a large difference. The SCL007 also has a sign bitwhich can tell us which of the two sensors is detecting an increase in the magnetic field, making the total output a true offset binary9-bit word.

9 We are not particularly concerned with the sign bit, although it could be used to determine which sensor is closer to make immediate use of the 8-bit digital output, we must use a digital-to-analog converter (DAC) to produce an analogsignal, then use this voltage to produce an audible or visual signal. Since direct digital mixing will produce an audio frequencysignal without all of the intermediate mess, why bother using the gradiometer chip? Because it also provides a power-up calibrationof the two sensors which improves sensitivity and makes the sensors easier to use. The DAC can also directly drive a voltmeter andthe 8-bit parallel output is useful for data logging to a 5 shows the schematic for the final design. The gradiometer chip is basically connected per the Speake & Co.

10 Datasheet. The Analog Devices AD557 DAC converts the 8-bit word to an analog voltage with a range of volts. This voltage isthen used to drive a voltage-controlled oscillator (VCO) which produces an audible tone that drives a speaker. Note that the sensorsand gradiometer chip are powered by one regulator, while the DAC, VCO, and audio output have a separate regulator. This is anabsolute necessity as any supply fluctuations that get to the sensors will cause them to misbehave. Further isolation is provided ateach sensor with an L-C filter on the supply VCO is the only part of the circuit that requires any furtherexplanation. While there are integrated circuit VCOs available, it isdifficult to locate any that are very low power (<1mA) and readilyavailable from common sources ( , mail-order houses).