GB2319621A - Magnetic sensor - Google Patents

Abstract

The invention provides a high sensitivity and wide bandwidth detection system for magnetic fields using a miniature, low cost sensor device. Static and alternating magnetic fields, created for example by direct or alternating currents or a combination of both flowing in a conductor may be measured by non-contact means. The sensor comprises a high permeability magnetic core 10 or cores axially aligned within one or more miniature wound coils 12 to produce a device with a self resonant frequency above 10 MHz. Means is provided for monitoring the voltage developed across said sensor coils and means for providing alternating and direct current excitation to said coils. The output signal is derived from amplitude or frequency changes in the voltage developed across the sensor and from current feedback applied to the coil or coils to null the detected magnetic field.

Description

MAGNETIC SENSOR Field of the invention This invention relates to the monitoring of magnetic fields and electrical currents.

The invention provides a high sensitivity and wide bandwidth detection system for magnetic fields using a miniature, low cost sensor device. Static and alternating magnetic fields, created for example by direct or alternating currents or a combination of both flowing in a conductor may be measured by non-contact means. The sensor comprises a high permeability magnetic core or cores axially aligned within one or more miniature wound coils together with means for monitoring the voltage developed across said coils and means for providing alternating and direct current excitation to said coils. The output signal may be derived form amplitude changes in the voltage developed across the sensor and from current feedback applied to the coil or coils to null the detected magnetic field.

Background of the invention A number of sensors exist for measuring magnetic fields and electrical currents including Hall effect devices and fluxgate magnetometers. Hall effect sensors are small and low cost, often being fabricated on semiconductor wafers as integrated circuits, however these devices are relatively insensitive to weak magnetic fields. Furthermore, the frequency response of typical commercial Hall effect devices is less than a few hundred kilohertz and they suffer from thermal drift and noise. The fluxgate magnetometer is one of the most sensitive devices for the detection of magnetic fields which operates at room temperature. Such devices are based on saturable inductors and are used in marine and aviation navigation equipment as electronic compasses monitoring the Earth' s field, which is only a few tens of microtesla. State-of-the-art fluxgate magnetometers have picotesla resolution with very low drift, however, their typical bandwidth is limited to less than lkHz. Also, the size of a conventional fluxgate sensor is typically some centimetres across. Examples of prior-art devices are described in the commercial literature of Applied Physics Systems, Mountain View, California, USA and literature such as Sensors - A Comprehensive Survey, Volume 5: Magnetic Sensor ed. Gopel, Hess & Zemel published by VCH Publishers (UK) Ltd, Cambridge, UK (ISBN 0-89573-677-2). A number of saturable inductor magnetometers are described in the prior- art including single coil devices. For example US 5 239 264 describes a magnetometer system which uses the change in inductance of a sensor forming an L/R oscillator circuit such that its frequency of oscillation changes with the applied magnetic field. The sensor described was approximately 15mm in length and 0.5mm in diameter and operated at an excitation frequency of the order of tens of kilohertz. Whilst this is adequate for an electronic compass, the measurement of rapidly varying fields or currents is not possible using the system described. Further examples of single coil magnetometers are given in US 3 936 949 and US 5 124 648 where navigation systems employ saturable inductors, excited by a square wave and with means to provide dc feedback to the sensor coil to null any applied field. The circuits and sensors described in these examples relate to electronic compasses where an output bandwidth of the order of kilohertz is sufficient and where the sensors can have dimensions up to the order of centimetres.

It is an object of the invention described herein to provide a miniature, typically submillimetre dimensions, low cost magnetic field sensor with a wide signal bandwidth.

These features enable the sensor to be located in confined spaces and detect the magnetic field produced by small current carrying conductors and components, such as tracks on a printed circuit board or integrated circuit, or to read magnetic data from data storage media such as tape, cards and discs, or for use in position encoders where the relative position between a magnetic field source and sensor is to be determined or other applications where a small magnetic sensor is required.

In order to realise a sensor with a high frequency response it is necessary to for the device to have small electrical time constants relative to the period of the magnetic field to be measured. This may be effected by the construction of a sensor with low values of self inductance, capacitance and resistance which will result in a device with a high self resonant frequency. It is an object of the invention described herein to provide such a sensor together with interface electronics to provide a conditioned output signal responsive to a magnetic field being detected.

Description of the invention The sensor device, a method for its construction and suitable interface electronics will now be described.

The sensor comprises one or more wound solenoidal coils with a high magnetic permeability core or cores axially located within said coils. The dimensions of an example sensor are typically an axial length of less than 1 millimetre and an overall diameter of less than 0.3 millimetre. These dimensions result in typical values of self capacitance and inductance to produce a self resonant frequency above 10 MHz.

One aspect of this invention is that the sensor is a miniature device which is relatively small compared to conventional fluxgate and other inductive sensors and has correspondingly low values of inductance and capacitance. Furthermore, the sensor core has a small cross sectional area and is fabricated from a low loss magnetic material such that eddy current and hysteresis losses are minimised at high frequency.

Another aspect of the invention is that the excitation frequency for the sensor can be very high, typically above 1MHz, such that the consequent detection bandwidth is wide. Furthermore, the sensor construction is very simple, enabling it's low cost fabrication, and lead wires to the sensor itself are minimised in number simplifying packaging.

A further aspect of the invention is that the sensor excitation may be switched between a number of different methods according to the frequency, magnetic field range and sensitivity required. This allows flexibility in a measurement system using only a single sensor where the response of the device may be optimised to a particular application.

To fabricate the sensor, the coil or coils are formed from insulated wire wound onto a mandrel which is subsequently withdrawn to leave a freely supported coil wound in one or more layers, inside which a section of core material of approximately equal length to the axial dimension of the coil is positioned. The resulting assembly is then coated with wax, paint, varnish or similar adhesive or encapsulant to hold the whole together. With appropriate choices of wire diameter, coil length and mandrel diameter, the coil has sufficient stiffness to be handled for the core insertion and subsequent processes without the need for a bobbin to support the coil. This is a significant factor in the fabrication of miniature, preferably sub-millimetre scale, devices at low cost where reliable handling of small components is mechanically difficult. Also, the parasitic inductance due to the magnetic flux linking the air space inside the solenoidal windings is reduced as the central region only requires sufficient clearance around the core for the core to be positioned. Furthermore, the absence of a bobbin allows the overall sensor diameter to be minimised, thereby using less wire in its windings for a given number of turns and facilitating the positioning of the sensor in confined spaces.

Preferably, the windings comprise an even number of layers such that all the leads emanate from one end of the sensor which facilitates mechanical handling and packaging. For example it is found that a two layer winding of 50 turns may be produced reliably using 35 micrometre diameter insulated copper wire wound onto a tungsten mandrel of diameter 70 micrometres. The core comprises a long, thin piece or pieces of high permeability material such as a cobalt based amorphous metal alloy or nickel-iron alloy such as Permalloy. The cross section of the core or cores is small to reduce eddy current losses within it, allowing its operation at high frequency. Hence, for operation at very high frequencies, typically in the tens to hundreds of megahertz region, it can be advantageous for the core to comprise a number of smaller wires or laminations.

Examples of the invention will now be described with reference to the following accompanying drawings where: Figures 1 illustrates an example of the construction of a sensor element according to the invention.

Figure 2 shows a schematic diagram of an example sensor system where the sensor element is driven by an alternating current and the output signal is derived from the amplitude of the voltage developed across the sensor.

Figure 3 shows a schematic diagram of an example sensor system where the sensor is excited with an alternating current and the output is derived from the positive and negative amplitudes of the voltage across the sensor.

Figure 4 shows a schematic diagram of a sensor system where the sensor is excited by an alternating current and the output is derived from the dc bias current applied to the sensor by means of a closed loop to maintain the alternating voltage amplitude across the sensor at a constant value.

Figure 5 shows a schematic diagram of an example sensor system where the sensor is excited by an alternating current and the output signal is derived from the timings between consecutive positive and negative peaks in the voltage waveform across the sensor.

Figure 6 shows a schematic diagram of an example sensor system where the sensor comprises two separate coils, one for drive and one for detection, where the output signal is derived from the voltage peak timings or even harmonic (eg. second harmonic) content of the detection coil voltage waveform.

Figure 7 shows a schematic diagram of an example sensor system where the sensor is a passive component which develops an induced voltage signal across its winding as a result of an alternating magnetic field coupling with the sensor.

Figure 8 shows a schematic diagram of an example sensor system where a pair of sensor elements are used to monitor both the magnetic field magnitude and gradient which may be used to infer the current flowing in a conductor by non-contact means.

Figure 9 shows a sensor with a core shape optimised for detecting localised magnetic fields, such as those created by magnetic data storage media.

Figure 10 illustrates a sensor according to the invention incorporated into a current transducer using a toroidal flux concentrating core, with the sensor situated in the air gap of the toroidal core.

Figure 11 illustrates a sensor according to the invention incorporated into a resonant oscillator circuit such that a change in impedance of the sensor, caused by a magnetic field coupling to said sensor, causes a change in frequency of the oscillations form which change the output signals are derived.

An example of the sensor element construction is illustrated in Figure 1. The sensor 24 comprises a core 10 of soft magnetic material such as Permalloy, amorphous metal alloy or ferrite inside a solenoid winding 12 of insulated metal wire such as enamelled copper wire. The winding 12 may be wound in single or multiple layers with the ends of the windings comprising the connection leads 14 of the sensor. The assembly may be dipped in varnish, paint, adhesive or a similar compound to bond it securely together. The core 10 may conveniently be formed from strip, sheet or film of the material by photolithography and electro-chemical or chemical etching or may be shaped by mechanical processes such as cutting, grinding and polishing. Alternatively, the core may be formed by deposition of a layer on to a non-magnetic carrier such as copper wire, glass or polymer fibres by such processes as sputtering, thermal evaporation, electroless or electro-plating or a combination thereof. Typically the overall sensor 24 dimensions may be of the order of a millimetre in length and 100 200us across.

For example, fabrication of the core from an amorphous metal alloy ribbon such as the cobalt based alloy Vitrovac 6025, manufactured by Vacuumschmelze, Hanau, Germany, may be achieved by attaching a strip of the material to a glass substrate with adhesive, patterning a thin stripe of photoresist on the surface of the alloy and electroetching the exposed metal in a 10% dilute hydrochloric acid solution by applying a current density of about 500 mA per square centimetre between a carbon cathode and the alloy anode. The alloy ribbon which is about 25 micrometres thick is etched away in a few minutes, except for the thin photoresist covered area, leaving a thin strip suitable for use as the sensor core. The strip thus formed may be released from the glass by softening the adhesive in a solvent and peeling it off. The core is then cut to length and positioned inside the coils.

An example embodiment of the invention as a magnetometer will now be described with reference to Figure 2. The sensor winding 12 is driven through an impedance 18 by an alternating current from oscillator 14 and a direct current from dc supply 16.

The resulting amplitude of the voltage developed across the sensor 24 is monitored by means of a demodulator circuit 20 which is followed by a low pass filter 22 to reduce the fundamental and harmonics of the oscillator frequency present in the output signal at 26. A magnetic field coupling into the core 10 causes a shift in phase between the alternating current in the winding 12 and the voltage induced in said winding as the core is swept in and out of magnetic saturation. This phase shift results in a change in amplitude of positive and negative half cycles of the voltage waveform across the sensor 24 which change is monitored by the demodulator 20 and communicated to the output signal 26. Even at levels of excitation current insufficient to saturate the core, a change in amplitude on the application of a magnetic field to the sensor is still seen due to the change in impedance of the sensor as the applied magnetic field linking the core shifts its B-H loop operating region.

An alternative embodiment of the invention is shown in Figure 3 where the sensor 36 is excited through an impedance 34 by an ac oscillator 32 and a dc source 30. The positive and negative cycles of the voltage developed across the sensor 36 are rectified and low pass filtered separately by demodulators 38 and 40 and filters 42 and 44 to produce positive and negative voltages respectively. The output signal is then derived from the output of a summing amplifier 46 with inputs from 42 and 44.

In the context of the example arrangements illustrated in Figures 2 and 3, the sensor is considered to be operating in an open-loop mode. An alternative embodiment of the invention operating in a closed-loop mode is described with reference to Figure 4. In this embodiment the sensor 54 is driven through an impedance 52 by an oscillator 50 and the dc output from a differential amplifier 60. The amplifier 60 has one input derived from a variable dc voltage source 62 and the other input from a filter 58 and demodulator 56 acting on the voltage developed across the sensor 54. In this arrangement the dc bias current through the sensor is continually adjusted to compensate for the external magnetic field coupling with the sensor core such that the net time averaged value of the magnetic field within the core is not changing significantly. This mode of operation gives improved linearity and dynamic range of the measurement system.

A further embodiment is illustrated in Figure 5. In this example, the sensor 70 is driven with an alternating current by an oscillator 72 and voltage to current converter 74. A differential amplifier 78 subtracts a proportion of the drive voltage produced by attenuator and phase shifter 76 corresponding to the ohmic and reactive voltages across the sensor such that its output substantially comprises only induced voltage pulses derived from the core being swept in and out of saturation. The spacing between consecutive positive and negative pulses is dependent on the magnetic field detected by the sensor. The pulse stream from the differential amplifier 78 may be converted to a pulse width modulated (PWM) output by using a comparator with hysteresis (Schmitt trigger) 80 set and reset by positive and negative pulses respectively. The output from this comparator 80 may be used in digital form as a PWM signal or when filtered with a low pass filter 82, an analogue signal may be produced.

An alternative embodiment is shown in Figure 6 where the sensor 90 includes two coils around the core; a drive coil 85 and a pick-up coil 86. In this case the drive coil 85 is excited by an oscillator 92 through an impedance 91 to provide an alternating current through the coil. The change in magnetic flux through the core induces alternating polarity voltage peaks in the pick-up coil 86 which are input to a differential amplifier 99. The voltage waveform developed across the pick-up coil 86 also contains components due to mutual capacitive and inductive coupling between the coils not associated with the magnetic properties of the core, which can give rise to sub-optimal operation of the sensing system. To compensate for these effects a proportion of the drive voltage, its derivative and integral produced by attenuators 95, 97 ,98 integrator 96 and differentiator 94 may be subtracted from the pick-up coil signal by the differential amplifier 99. The resulting pulse train output 88 may then be processed by similar means to those described previously using a comparator 93 to generate a PWM signal 86 or a filter and synchronous rectifier 89 tuned to the second harmonic of the drive frequency may be used to derive an analogue signal 87 proportional to the detected magnetic field.

Figure 7 illustrates an alternative embodiment of the invention which utilises induction of a voltage across the sensor winding 100 created by an alternating magnetic field coupling with the core 101 and winding 100. In this case the amplitude of the induced voltage is related to the frequency and magnitude of the alternating magnetic field. In order to produce a constant output sensitivity, the induced voltage is passed through an amplifier 102 and filter 104 selected to give a uniform broad band response at the output 106.

With at least two sensor elements it is also possible to monitor field gradients in addition to field strength. An example of this application will be described with reference to Figure 8. A conductor 110 carrying a current I is monitored by a pair of sensors: a first sensor 112 at distance d and a second sensor 114 at distance (d+x) where x is a known dimension. The magnetic flux detected by the first sensor is given by B1 = uo Il (2nod) and by the second sensor is B2= = 1/ I / (27r(d+x)). These equations can be solved to give I = 2n B1 B2 / (Ro(BI - B2)/x) where Clo is the permeability of free space and (B i - B2)/x is the magnetic field gradient measured. Hence the current flowing in a conductor may be monitored by non-contact means, even if the distance to the conductor is not known precisely. The sensitivity of a sensing system according to the invention is such that milliampere currents flowing in printed circuit board (PCB) tracks may be measured.

An alternative example of the sensor construction is illustrated in Figure 9 where a coil or coils 154 are arranged around a core 152 which is shaped to create a gap 156.

This arrangement gives improved sensitivity if the sensor is detecting localised magnetic fields from an object 150, such as a magnetic tape, disc or card.

Sensors according to the invention may also be employed in the air gap of toroidal flux concentrators, through which a conductor carrying a current to be measured is looped one or more times, in a similar arrangement to conventional Hall effect devices configured as current transformers. The invention enables a current transformer with improved sensitivity and bandwidth to be realised. Figure 10 illustrates a sensor 24 according to the invention positioned in the air gap 120 of a soft magnetic toroid 122.

The wire 124, carrying the current to be measured, is looped through the toroid 122 one or more times. The sensor output may be monitored directly as a measure of the current or preferably, it is used to control a feedback current through a nuiling winding 126 also wound around the toroid such that the net time averaged flux within the toroid is maintained at a substantially constant level; this arrangement improves the dynamic range of the current sensor.

An alternative magnetometer utilising a sensor according to the invention is illustrated in Figure 11 where the change in inductance of the sensor 130 in the presence of a magnetic field influences the frequency of oscillation of an oscillator 132. The oscillator 132 may operate using an L/R time constant or LC resonant circuit, where the inductance term L is provided substantially by the sensor. The output signals comprise a frequency modulated output 138 derived from the oscillator 132 and an analogue voltage signal 136 derived from the oscillator by means of a frequency-tovoltage convertor 134.

The optimum drive and signal processing method for a particular application will depend on the sensitivity and bandwidth required. In realising a multi-purpose or multi-range magnetic field and current sensing system it may be appropriate to configure selectable drive and processing circuits with single or multiple sensor elements. For example, for very high frequency alternating fields where it is desired to reject a steady offset field, the arrangement illustrated in Figure 7 would be appropriate however, for combined alternating and steady measurements, the configurations illustrated in Figures 3 and 4 would be preferable. Thus a scheme offering interface circuits which are switchable between different modes can be advantageous, for example, in a general purpose laboratory instrument.

It will be appreciated by those skilled in the art that other combinations of sensor and interface electronics, beyond those illustrated by way of example herein, are possible.

The examples described herein are intended for illustration only and not to limit the scope of this patent specification, which is defined by the following claims.

Claims (17)

1. A miniature magnetic field sensor comprising one or more wound solenoidal coils inside which coils a core or cores of high permeability magnetic material are located with axes substantially coincident with those of the surrounding coil or coils and having a similar axial length to that of the coils, characterised in that said sensor has an electrical self resonant frequency exceeding 10 MHz.

2. A sensor according to claim 1 where the coil or coils have sufficient unsupported stiffness to maintain their shape prior to insertion of the core or cores and encapsulation or coating of the whole in wax, paint, varnish, adhesive or similar material.

3. A sensor according to claim 1 where the diameter of the wire forming the coils is less than 250 micrometres, the maximum cross-sectional dimension of the core is less than 250 micrometres and where the length of the devices less than 10 millimetres.

4. A sensor according to claim 1 where the coil or coils are formed by winding insulated wire onto a mandrel which is subsequently removed.

5. A sensor according to claim 1 where the core is made from an amorphous metal alloy, ferrite or crystalline or polycrystalline metal or other high permeability material.

6. A sensor according to any of the preceding claims where at least one of the core dimensions is defined by photolithographic and chemical or electro-chemical etching processes.

7. A magnetometer system comprising a sensor according to any of the preceding claims where at least one sensor coil is excited with a high frequency alternating current and a direct bias current where the change in amplitude of one or other or both of the positive and negative voltage half cycles across the sensor coil or coils is used to derive an indication of the magnetic field coupling with the sensor.

8. A magnetometer system comprising a sensor according to any of the claims 1 to 6 where at least one sensor coil is excited with an alternating current, a differential amplifier amplifies the induced voltage peaks as the core is swept from saturation through its linear region and removes the ohmic and reactive components of the excitation current due to the electrical impedance of the coil windings unrelated to the core, which amplified peaks are input to a Schmitt trigger which has appropriate hysteresis to change state at each consecutive positive and negative peak to produce a square wave at its output, the duty cycle of which square wave forms a digital output indication of the magnetic field being monitored by the sensor.

9. A magnetometer system according to claim 8 where an analogue output signal is produced by a low pass filter operating on the digital square wave output.

10. A magnetometer system comprising a sensor according to any of the claims 1 to 6 connected as part of an oscillator circuit such that a change in impedance of the sensor causes a change in oscillation frequency of said oscillator circuit and where at least one output signal is derived from the oscillation frequency.

11. A magnetometer system according to claims 7, 8, 9 or 10 where a direct nulling current is derived from the first output signal and fed back to one or more of the sensor coils to bring the first output signal substantially to zero or a constant offset value whereupon the second output signal, proportional to the measured magnetic field, is derived from the nulling current.

12. A magnetometer system comprising a sensor according to claims 1 to 6 where the output signal comprises the voltage induced in one or more of the sensor coils.

13. A magnetometer system comprising a sensor according to claims 1 to 6 where the output signal is derived from a filter and amplifier operating on the voltage induced in at least one of the sensor coils, where the filter operates to give a substantially flat or other prescribed frequency response of the system.

14. A current transducer, magnetic data recovery system or position encoder utilising a sensor or magnetometer according to any of the preceding claims.

15. A current transducer utilising a sensor according to claims 1 to 6 positioned in the gap of a flux concentrating toroid through which the current carrying conductor passes one or more times.

16. A current transducer according to claim 15 where the net flux in the toroid is maintained at a substantially constant level by means of a feedback coil linking said toroid.

17. A sensor, transducer or magnetometer substantially as herein described with reference to the accompanying descriptions and drawings.