GB2276727A - Magnetoresistive magnetometer - Google Patents

Abstract

A magnetometer includes three magnetoresistive sensing elements 30, 32, 34 in orthogonal orientation on a non-ferrous mounting block 10. Outputs 60, 62, 64 from the sensors are processed 70, 72 to generate field strength values for each direction, total field strength, and also peak value information in varying fields. A liquid crystal display output 74 is provided. <IMAGE>

Description

IMPROVEMENTS IN AND RELATING TO MAGNETOMETERS The present invention relates to magnetometers, and in particular to magnetometers capable of the measurement of magnetic field in each of three orthogonal axis directions.

During deperming and/or degaussing of large structures (e.g., ships), there is a requirement to measure the total field levels within the structure. In addition, where the magnetic susceptibility levels are known of particular equipment located within the vessel, it is also desirable to find the worst-case field direction.

For example, if a particular item of equipment is a salient working part of a vessel which cannot be removed during the deperming process, and it is susceptible in the vertical, or Z-direction, it will be necessary to monitor the actual vertical (Z-direction) field close to the device in order to limit the applied hull deperming field.

Magnetic field mapping or measurement is usually carried out using known flux gate type magnetometers.

Other types may be used, such as those employing the Hall effect. These magnetometers have a remote sensor head, which, in the case of the flux gate type, is attached by a flexible lead to a main unit which contains processing circuits and display apparatus. In the case of a threeaxis, wide range flux gate magnetometer (for example with a range of +5my), the sensor head is bulky and the main unit is typically larger than a briefcase. Such units are invariably expensive. The complex circuitry required to compensate for the non-linearity of the coils of the flux gate head necessarily contributes to the size of the unit.

Use of the equipment in confined spaces can be extremely difficult or impractical. In addition, use of the equipment where the flexible leads of the head cannot penetrate the confining walls of the space being measured without influencing the measurement being taken (for example by a door or access hatch being left open) can also be a problem.

Magnetometers employing Hall-effect type sensors are also disadvantaged in that the Hall-effect sensors tend to be sensitive to mechanical stress and difficult to arrange in a tri-axial arrangement. Single axis types presently available do not allow for directional testing, and are further disadvantaged in that the Hall effect sensor head is particularly temperature sensitive. High temperature sensitivity requires complex compensation circuits.

The flux gate magnetometer is generally available in a tri-axial arrangement, i.e. it can typically measure in the three-axis directions (X, Y and Z) and also can provide a total field strength measurement (T=ç[X2+Y2+Z2]). However, these units do not process the outputs to provide a peak value facility. If a peak value of any of the field directions is required, then it is necessary to record all of the output field directions and process them with a separate data acquisition system (e.g.

a data logger) to capture all of the peak values as required. These large and bulky units are unsuitable for extensive field mapping exercises.

It is an object of the present invention to provide a magnetometer capable of accurate measurement of magnetic field in the three-axis directions. It is a further object of the present invention to provide such apparatus capable of giving total field measurement, and peak field measurement. It is a further object to provide such a magnetometer in the form a small, portable and inexpensive device which may be used in confined spaces or those difficult of access. The provision of a peak hold facility particularly allows the magnetometer to be deposited inside a space to be measured without direct supervision by the user during measurement.

According to one embodiment, the present invention provides a magnetometer comprising: three magnetoresistive sensors mounted on a block of non-ferrous material, each of the sensors being orthogonally oriented with respect to the other two sensors, and adapted to provide an output proportional to an externally applied magnetic field; and processing means adapted to receive the output of each sensor and to compute the magnitude of the applied field in each of three mutually orthogonal axis directions.

The present invention will now be described in detail by way of example, with reference to the accompanying drawings in which: Figure 1 shows a perspective view of a mounting block adapted to accommodate sensors of a magnetometer according to the present invention; Figure 2 shows a schematic diagram of a magnetometer according to the present invention, using the sensor mounting block of Figure 1; Figure 3 shows a detailed circuit diagram of a part of the magnetometer of Figure 2; and Figure 4 shows an alternative embodiment of an amplifier circuit according to the present invention.

With reference to Figure 1, there is shown a presently preferred configuration for a sensor block 10, adapted to accommodate three magnetoresistive sensors of known type. A preferred sensor is the Philips KMZ10C. The sensor block 10 is constructed of a non-ferrous material, such as aluminium, and incorporates three housings 12,14,16 adapted to receive the sensors. The sensor block also incorporates three housings 18,20,22 each corresponding to one of the sensor housings, and each adapted to receive a permanent magnet.

The magnetoresistive sensors make use of a property of some current carrying materials to change resistivity in the presence of an externally applied magnetic field.

This change is brought about by rotation of the sensors magnetic dipoles relative to the direction of the current passing through the sensor. In the case of, for example, permalloy (a ferromagnetic alloy containing 20% iron and 80% nickel), a rotation of the magnetic dipoles of 900 normal to the current direction will produce a 2-3% change in the resistivity of the permalloy.

The preferred KMZ10C sensor is constructed of four permalloy strips arranged in a meander and connected to form the four arms of a Wheatstone bridge. It has an operational range of +7.5kA/m (+9.5mT), and a sensitivity of 1.5mV/Vbias per kA/m. Thus for an applied field of 5mT, and bias voltage of 5V, a full scale deflection output of in excess of 20mV can be achieved. An output substantially linear with respect to applied field can be obtained.

The preferred sensor incorporates specially trimmed resistors to give a zero offset at a temperature of 250C.

The sensors 30,32,34 (see Figure 2) are mounted in their respective housings 12,14,16 in a tri-axial arrangement such that each sensor is in orthogonal orientation to both of the other two sensors. The permanent magnets 40,42,44 are also mounted in their respective housings 16,18,20 in an arrangement such that each magnet and respective sensor are in similar mutual orientation.

The permanent magnets 40,42,44 are provided to generate a small auxiliary field for each sensor 30,32,34 which is necessary to prevent the sensors from reversing output, i.e. becoming re-oriented from a positive output to a negative output, or vice versa, upon application of an external field. This is also referred to as "flipping".

The magnet strength must be chosen with care since an auxiliary field which is too strong will result in loss of sensor sensitivity, but a magnet providing too small an auxiliary field may itself become demagnetised by the external field to be measured.

In a presently preferred embodiment, the sensor block measures approximately 50mm x 50mm x lOmm, and can be mounted directly onto a printed circuit board upon which also resides accompanying circuitry to be described hereinafter.

The sensor block shown in Figure 1 demonstrates various preferred features. It is desirable that the field produced by each permanent magnet influences only the respective sensor with which it is associated, and thus the distance between the magnets 40,42,44 is maximised in preference to the distance between the respective sensors 30,32,34. It is also desirable that each sensor 30,32,34 is positioned and aligned to have a clear "line of sight" for the orthogonal direction in respect of which it operates. Thus, sensor housings 12,14,16 are provided in the form of elongate recesses or channels in which, for example, housing 14 is aligned with the Y axis having a clear line of sight along its Y axis not conflicting with the Z axis alignment of sensor housing 16. It is also desirable to minimise and/or match the distance of each sensor from the printed circuit board to which the sensor block may be attached keeping connection lengths to a minimum, and to keep the overall dimensions as small as possible.

The sensor block is mounted away from any ferrous components such as a battery used as a power supply, since the field to be measured by the sensor can be modified by close proximity to ferrous material. Experimentation has shown that location of any battery at least 40mm away from the sensor block results in no significant problems relating to field modification.

Referring now to Figures 2 and 3, each sensor 30,32,34 is supplied with +5V dc from a voltage stabilised circuit 25 fed from a 9V battery 26. Each sensor 30,32,34 is electrically connected to a respective operational amplifier 50,52,54 also powered by battery 26. Each sensor output is proportional to the applied magnetic field and is amplified by the respective operational amplifier. Compensation for sensitivity drift with temperature is provided for each sensor amplifier by way of a negative feedback loop incorporating a KTY83-110 silicon temperature sensor 55 around each amplifier (see Figure 3). Offset adjustment is provided by respective potentiometers 56 connected between 0V and 5V dc to the non-inverting input of the respective amplifier 50,52,54.

Gain adjustment is provided in the feedback loop from the amplifier output 60,62,64 to the corresponding inverting input by potentiometers 57. An analogue output connector may be provided for calibration purposes. Each operational amplifier is arranged in a low pass filter configuration to reduce noise susceptibility.

The three analogue output signals 60,62,64 from amplifiers 50,52,54 are fed to an LTC1293 data acquisition integrated circuit 70. This device comprises a threechannel multiplexer unit, a sample-and-hold and a 12-bit analogue-to-digital converter. A serial link 71 is provided between the data acquisition IC 70 and a microcontroller IC 72 for control of the data acquisition functions and the transfer of digital data between the two devices.

The microcontroller 72 scales and processes the incoming data correctly and transforms it into a format appropriate to a liquid crystal display device 74.

Microcontroller 72 also monitors the status of any front panel control switches of the magnetometer, and the battery for low voltage condition. In the event of the microcontroller detecting a low battery condition, it may instruct the LCD 74 to flash intermittently, or provide other warning.

LCD 74 is driven directly from the output of the microcontroller 72 to indicate the value of each of the three axes measured (X, Y and Z) in mTeslas to two decimal places. In addition, the total field (T= /[X2+Y2+Z2]) is also displayed. A peak hold value and zero field (offset) are also displayed when selected, which is indicated by segments in the centre of the display.

Instantaneous indication of X, Y, Z and total (T) field is given on the LCD. By appropriate selection switch 75 mounted on a front panel of the device, the microcontroller 72 may be prompted to cause display of peak values (X, Y, Z and total) of a varying applied field. This is particularly useful when mapping a vessel where the applied magnetic field is varying in amplitude and direction.

A background zero option is also provided by way of ZERO switch 76 which may be used to cause the microcontroller to artificially zero any small background field values when taking differential measurements.

A further optional external data communication link 77 may be provided, for example as an RS232 standard interface.

The foregoing apparatus is sufficient small that it may be manufactured as a compact, hand-held unit containing all of the sensors, processing circuits, battery and output display. An overall size of 130mm x 140mm x 50mm has been achieved.

In use, alignment of the X, Y and Z axes of the unit may be readily achieved with external references. Directionality is effected by providing the unit with a flat base, and placing it on a floor or other horizontal reference, thus accurately placing the Z-axis sensor in the correct plane. A decal is provided on an outside panel of the unit to indicate alignment of the X- and Yaxes. During mapping of vessels, for example, on-board equipment is commonly mounted either in line with a known athwart bulkhead (Y-axis), or orthogonal to it, and it is thus a relatively simple matter to align the unit to it.

Various modifications to the circuitry may be made to further enhance the performance of the unit. For example, with reference to Figure 4, there is shown a circuit for the front end sensor output amplification stage which is an alternative to that described with reference to Figure 3, offering an improvement in the gain, offset and temperature compensation. Only a single channel is represented: two corresponding circuits would be provided for the Y-axis and Z-axis sensors. Where components provide a similar function to that described with reference to the circuit of Figure 3, the same reference numerals have been used.

The circuit provides offset adjustment by potentiometer 56 coupled to the inverting input of amplifier 82. Gain adjustment is provided by potentiometer 57 coupled between inverting inputs of amplifiers 84 and 86.

Compensation for sensitivity drift with temperature is provided by way of a negative feedback loop on amplifier 86 including a temperature sensor 55, in this case a thermistor. A constant current source 80 is provided to improve the operation of the sensor with varying battery voltage. Output amplifier 50 drives the analogue output 60 fed to data acquisition IC 70 via transistor 87. It will be clear that a number of other circuit designs will be suitable to achieve the desired functionality of the front end sensor output amplification stage.

Claims (10)

1. A magnetometer comprising: three magnetoresistive sensors mounted on a block of non-ferrous material, each of the sensors being orthogonally oriented with respect to the other two sensors, and adapted to provide an output proportional to an externally applied magnetic field; processing means adapted to receive the output of each sensor and to compute the magnitude of the applied field in each of three mutually orthogonal axis directions.

2. A magnetometer according to claim 1 further including: three permanent magnets each mounted on the block proximate to a respective magnetoresistive sensor and oriented with respect thereto such as to reduce the likelihood of the respective sensor from reversing output polarity.

3. A magnetometer according to claim 1 or claim 2 including: temperature sensing means adapted to compensate the sensor output for ambient temperature variations.

4. A magnetometer according to any preceding claim wherein said processing means includes means to derive the total magnitude of an applied field of the combined three axis directions.

5. A magnetometer according to any preceding claim wherein said processing means includes means to capture and store a peak value in a fluctuating applied magnetic field.

6. A magnetometer according to any preceding claim further including display means to display the computed magnitude of the applied magnetic field.

7. Apparatus substantially as described herein and with reference to the accompanying drawings.

8. A magnetometer according to claim 1 wherein said processing means includes: amplifier means coupled to the respective outputs of each of the magnetoresistive sensors, and including temperature compensation feedback adapted to compensate for magnetoresistive sensor temperature induced sensitivity drift.

9. A magnetometer according to claim 1 wherein: each of said magnetoresistive sensors includes four magnetoresistive elements arranged as the arms of a Wheatstone bridge circuit; a first output of said bridge circuit being coupled to one input of operational amplifier means connected in low-pass filter configuration; a second output of said bridge circuit being coupled to the other input of said operational amplifier means and to a gain control circuit.

10. A magnetometer according to claim 8 or claim 9, wherein outputs of said amplifier means are coupled to an analogue-to-digital conversion circuit, said conversion circuit being further coupled to a data acquisition circuit.