Patent Application
20230328942
The present invention relates to a permalloy and its use. More particularly, but not exclusively, the present invention relates to a permalloy with extremely low magnetic noise which may be used in magnetic field instruments such as magnetometers or for magnetic shielding.
Although the present invention is not limited to magnetic instruments such as fluxgate magnetometers, for purposes of discussion, fluxgate magnetometers provide context of one application for which new alloys with improved characteristics are desirable. Fluxgate magnetometers are essential tools for geophysics, mineral exploration, space physics, and military sensing providing high precision magnetic field measurements. Despite their ubiquitous use, the origin of the intrinsic magnetic noise in a fluxgate is poorly understood and, in some cases, the key research appears to have been lost to history. Fluxgates form a measurement by modulating (gating) the local magnetic field by periodically magnetically saturating a piece of ferromagnetic material. The instrumental noise floor is therefore limited by the intrinsic magnetic noise of this material as it enters magnetic saturation.
Therefore, what is needed is a new material and use of the material in magnetometers or other applications.
Therefore, it is a primary object, feature, or advantage of the present invention to improve over the state of the art.
It is a further object, feature, or advantage of the present invention to provide a permalloy which has low noise performance when used as a sensor magnetometer core such as in a fluxgate magnetometer sensor.
It is a still further object, feature, or advantage of the present invention to provide a permalloy which has high power efficiency and thus lower power requirements when used as a sensor magnetometer core such as in a fluxgate magnetometer sensor.
Another object, feature, or advantage of the present invention is to provide an improved fluxgate magnetometer.
Another object, feature, or advantage is to provide an alloy which may be used for magnetic shielding.
One or more of these and/or other objects, features, or advantages of the present invention will become apparent from the specification and claims that follow. No single embodiment need provide each and every object, feature, or advantage. Different embodiments may have different objects, features, or advantages. Therefore, the present invention is not to be limited to or by any objects, features, or advantages stated herein.
According to one aspect a metal alloy is provided which includes by weight percentage of the metal alloy, 28 to 55 percent copper, 45 to 63 percent nickel, and 4 to 10 percent iron. The metal alloy may be weight percentage 28 percent copper, 62 percent nickel, and 10 percent iron. The metal alloy may be formed into a foil which may have a thickness of 100 μm or less. The metal alloy may be formed into a foil which may have a thickness of 50 μm or less. A magnetic field instrument may include a magnetometer core body formed from one or more layers of the foil. The magnetometer core body may be a ring core or a racetrack core or of other geometry. The magnetic field instrument may further include a sense winding and may further include a drive winding. The magnetic field instrument may be a fluxgate magnetometer.
According to another aspect, a magnetic field instrument may include a magnetometer core body formed from a permalloy comprising copper, iron, and nickel. The magnetic field instrument further includes a first set of coil turns around the magnetometer core body forming a sense winding. The permalloy may comprise a plurality of layers of permalloy foil. The permalloy may consist of by weight percent of the permalloy, 28 to 55 percent copper, 45 to 63 percent nickel, and 4 to 10 percent iron. Each of the plurality of layers of the permalloy foil may have a thickness of 100 μm or less.
According to another aspect, a magnetic field instrument is provided which includes a magnetometer core body formed from one or more layers of permalloy foil consisting of by weight percent of the permalloy, 28 to 55 percent copper, 45 to 63 percent nickel, and 4 to 10 percent iron. Each of the layers of the permalloy foil may have a thickness of 100 μm or less or 50 μm or less. The magnetic field instrument further includes a first set of coil turns around the magnetometer core body forming a sense winding. The magnetometer core body may be a ring core or a racetrack core. The magnetic field instrument may further include electronics operatively connected to the sense winding. The magnetic field instrument may be a fluxgate magnetometer.
According to another aspect, a method of manufacturing a magnetic field instrument includes forming metal alloy comprising by weight percentage of the metal alloy, 28 to 55 percent copper, 45 to 63 percent nickel, and 4 to 10 percent iron, rolling the metal alloy to form a foil, and constructing a magnetometer core using the foil. The method may further include heat treating the magnetometer core. The method may further include adding a sense winding to the magnetometer core. The method may further include adding a drive winding to the magnetometer core.
Illustrated embodiments of the disclosure are described in detail below with reference to the attached drawing figures, which are incorporated by reference herein.
Fluxgate magnetometers (Fornacon et al., 1999; Primdahl, 1979; Snare, 1998) provide sensitive and highly-stable measurements of the local DC and low frequency magnetic field and have a variety of applications ranging from geophysics, space-physics and space-weather monitoring, to marine and military sensing. The noise floor of a fluxgate is typically limited by the intrinsic magnetic noise of a magnetometer core that is periodically driven into magnetic saturation to modulate the local magnetic field. Much of the early work developing low-noise cores was carried out in the 1960s for military applications (e.g., Scanlon, 1966) and is not available in the scholarly literature. This work by the US Naval Surface Weapons Center (NSWC) White Oak (now a Department of Agriculture facility) and Infinetics Inc. (Scarzello et al., 2001) resulted in the standard S1000 fluxgate core. A one-inch ring-core manufactured from a thin (3 or 12 μm) foil of 6% molybdenum Permalloy with noise ranging from ˜4 to ˜20 pT/√Hz at 1 Hz. Müller (1998) and Musmann (2010) both describe more recent efforts achieving <5 pT noise using a 20 μm foil for similar Permalloy. However, none of these efforts provide enough details for the manufacturing process to be understood unambiguously or to fully understand the effect of various design choices on the performance of the resulting fluxgate core.
Recent results from (Narod, 2014) established a theoretical framework for the origin of magnetic noise and magnetic hysteresis in Permalloy foils. An initial effort (Miles et al., 2019) demonstrated and documented a preliminary manufacturing process for low noise (<10 pT) 1″ geometry ring-cores. This study expands on that work by examining the influence of foil thickness, heat treatment, geometry, and the Permalloy alloy with the goal of consistently producing lower-noise and lower-power fluxgate cores.
We present the results of an initial parameter sweep comparing the traditional 6% molybdenum alloy to a new 28% copper alloy. Each alloy is tested at both 100 and 50 μm foil thicknesses with thinner foils planned to be explored as our manufacturing capability improves. We explore the effect of the heat-treatment used to optimise magnetic noise by exploring six temperatures spanning 1000 to 1250° C. Fluxgate cores are manufactured in two geometries—the standard spiral-wound 1 in. ring-core and a new 6.45 mm×31.45 mm racetrack geometry using continuous foil washers. We explore the relationship between number of foil layers and both magnetic noise and power consumption. We examine the effect of an additional sub-Curie heat treatment at 100° C. on the intrinsic magnetic noise of the cores. We have standardised on using three layers of foil for most tests in both core geometries to simplify manufacturing. Consequently, we are not expecting exceptional noise performance in these rings; rather we want to understand trends and to optimize the manufacturing process.
We hypothesize that noise is influenced by their grain size developed in the permalloy in relation to the thickness of the foil. Specifically, that complete primary recrystallisation will result in lowering magnetic noise, whereas secondary recrystallisation will result in higher magnetic noise. We manufactured test coupons for every combination of material, thickness, and heat-treatment and characterized their grain size distribution to begin exploring this relationship.
All cores were manufactured from scratch at the University of Iowa. All permalloy alloys were melted and processed in house and all bobbins were manufactured and assembled based on in-house designs.
The fluxgate cores described here were based on permalloy manufactured at the University of Iowa. Casting small batches of permalloy in-house allows us to more easily and cost effectively explore new metallurgy such as the copper alloy described below. High-purity source powders were combined by ratio of weight, shaken to mix, and gently packed into an Alumina crucible (
The ingots were then homogenized for 7 d at 1100° C. under a slow purge of the same reducing atmosphere. A hydraulic press was used to flatten the ingots to ˜3 mm thickness (
Half of the fluxgates cores manufactured for this experiment were constructed from an alloy similar to the historical 6-81 Mo permalloy (English and Chin, 1967; Odani, 1964; Pfeifer, 1966; Pfeifer and Boll, 1969; Scanlon, 1966) combined by weight from 6% molybdenum, 81.3% nickel, and the remainder iron. The 6-81 alloy was first proposed for fluxgate use by Gordon et al. (1968) and has been used by several groups worldwide Including Infinetics Inc., Müller et al., (1998), and Musmann (2010). The historical 6-81 alloys included ˜0.5% manganese which is a common alloy additive to improve hot working, notionally by binding to sulphur contaminants. The historical 6-81 melts were hot rolled (Gordon and others, 1968), but our own melts were not, and therefore we did not include any manganese.
The second half of the fluxgate cores shown here were manufactured using a ferromagnetic alloy, mentioned in Narod (2014), that that we hypothesized has potential to produce high-quality fluxgate cores. This 28-62 Cu alloy was combined by weight from 28% copper, 62% nickel and the remainder iron. This ratio was derived from the theoretical framework established by Narod (2014) and prior experimental results from v. Auwers and Neumann (1935) who explored a wide range of iron-nickel-copper alloys searching for high initial permeability. V. Auwers and Neumann (1935) presented their work in a special printing from the Scientific Publications of the Siemens-Factory that was not circulated to the public.
The choice of 28-62 Cu for a first trial copper alloy resulted from an examination of v. Auwers and Neuman (1935)
Two styles of fluxgate cores were manufactured to explore the impact of geometry. The classic Infinetics S1000 1″ ring-core geometry was used as the standard design. We also manufactured cores in an alternative racetrack geometry (Gordon et al., 1965; Hinnrichs et al., 2000, 2001; Ripka, 1990, 1993, 2000) that combines the symmetry and closed flux path of a ring-core with the high single-axis geometric gain of a parallel rod core. The race-track geometry also allowed us to experiment with several design changes that were intended to increase reproducibility and reduce power consumptions.
The 1″ ring-cores were manufactured following the description in Miles et al., (2019) as shown in
The racetrack design makes several fundamental changes beyond the gross geometry of the core assembly as shown in
No insulating coating is applied; rather the racetrack washers are placed in the furnace bare and heat-treated using variations of the standard process heat-treatment as described below, prior to being assembled into a core. Heat-treated foils were then stacked into a non-conductive plastic bobbin (Delrin for prototypes, 30% glass filled PEEK for production) interleaved with insulating layers of Kapton of the same geometry. A plastic lid closed the core and supports a quasi-toroidal drive of AWG 32 magnet wire. Production cores have the foil wet-set into a polymer to prevent the foil layers from moving during the magnetizing drive pulses. We are investigating foil movement as a source of long-term offset shifts.
The stacked foil washers remove the need to spot-weld the ferromagnetic element, as is done in traditional spiral-wound sensors, avoiding the heat-effected area around the weld and the associated unpredictable magnetic properties. Heat-treating the foil washers individually removes the risk of undesired welding between layers and between the foil and the standard metal Inconel bobbin that can cause unintended shorting. This process also eliminates the differential strains between Inconel and Permalloy, that invariably happens when cooling the assembly from 1100° C. to room temperature. The race-track geometry aligns ferromagnetic mass on one axis, potentially producing lower noise; however, the race-track geometry cannot be double-wound like a ring-core to sample two orthogonal components. The quasi-toroidal drive windings are time-consuming to apply, but the closed flux path of the racetrack should reduce stray fields and offsets error compared to traditional parallel rod sensors.
The heat treatments used here are adapted from the profile (
The furnace was preheated to the dwell temperature at 300° C. per hour, the fastest rate suggested by the manufacturer to avoid cracking the Alumina work tube by thermal shock. Ramp (C-D) was completed at −300° C. per hour for the same reason. Ramp (D-E) was completed at −35° C. per hour as suggested by Gordon et al. (1968). The transition point (D) was set at 600° C. at the upper limited of the critical ordering range for 6-81 permalloy suggested by Gordon et al. (1968). To explore the effect of the heat treatment we standardised on a 4 h dwell at temperatures ranging from 1000 to 1250° C. as shown in
A common electronics package was used to drive each ring-core, measure the power consumption required, and characterize its power spectral density noise floor. In addition, foil coupons for each alloy and thickness were included in each heat treatment so that the grain size could be characterized optically.
The various fluxgate cores where characterised using a process similar to that described in Miles et al., (2019). A common, single-axis electronics package was used to drive and sample each fluxgate core. The 1 in. ring-cores were paired with a rectangular solenoidal sense winding similar to that used in Wallis et al., (2015) while the racetrack cores were paired with a tubular solenoidal sense coil matching their geometry (
A common electronics package was used to drive all cores using ±7.5V at 5.0 kHz. Both legs of the drive circuit used 1850 μH series inductors to create the resonant drive condition. For each core, the shunt capacitance was increased until symmetric current pulses were observed through the drive winding showing that minimum resonance had been achieved (
A common shorted-coil topology preamplifier was used for both sense windings. Several cores coupled large amounts of the 1 F current waveform into the output of the sensor (
A known-amplitude sinusoidal magnetic test signal was applied to the solenoid inside the five-layer MuMetal shield using a signal generator and a 10 k low-tempco resistor. The phasing of the direct digitization was adjusted to maximize the amplitude of the measured test signal. A linear scaling coefficient was then adjusted until the visualization software showed the test signal with the correct amplitude to calibrate the sensitivity of the complete single-axis magnetometer now formed around the fluxgate core under test. The 1 Hz test signal was then disabled, and 30 minutes of quiet data were taken with the core and sensor inside the magnetic shield. Welch's method of overlapped periodograms (100 sps, 4096-point fast Fourier transform (FFT), Hann window, 75% overlap) was used to estimate the power spectral density noise floor as shown in
Two figures of merit were established to simplify comparing core performance. Robust linear regression (MATLAB robustfit) was used to fit a linear trend to the noise floor from 0.05 to 1.0 Hz to exclude local and instrumental narrow-band noise. This trendline was evaluated at 1 Hz to produce the standard pT/√Hz at 1 Hz noise metric. We also evaluated the trendline at 0.1 Hz to reflect the updated INERMAGNET data requirement (Turbitt et al., 2013) for long period measurements, which has moved the noise requirement by a decade from 10 pT/√Hz at 1 Hz to 10 pT/√Hz at 0.1 Hz. The narrow bandwidth feature in
This fitting technique provides a robust, quantitative estimate of the intrinsic magnetic noise of the core despite instrumental noise at 1 Hz due to telemetry and several intermittent narrow-band magnetic noise sources in the test environment.
The drive circuit was powered by its own benchtop power supply allowing the ±0.5 mA resolution of the power required to drive each core. The drive frequency, drive voltage, and series inductance were held constant for all tests. The capacitance required to achieve resonant drive was determined empirically as described above. Power consumption for each core was measured after each core had stabilized by operating for at least 60 seconds.
Each heat treatment included 10×10 mm coupons for each combination of metallurgy and foil thickness. These coupons were used to estimate the grain size of the heat-treated material. Each coupon was photographed with a graticule for scale, using a widefield metallurgical inverted microscope under polarized visible light to emphasize the grain boundaries that had developed in the material as shown in
Grain size was estimated using the intercept method (Abrams, 1971) as implemented by the MATLAB linecut software package by Meister (2020). Lines are drawn across the sample and chord lengths are determined by eye each time a grain boundary is crossed. The distribution of chord lengths was fit to a log-normal curve (
For cross-comparison, each distribution was characterized by the mode of its probability density function (vertical blue line) plus or minus 10% up and down from that point on the cumulative density function (vertical lines).
Each point in
The ring-core noise at 1 Hz, noise at 0.1 Hz, and power consumption varied more than the equivalent racetracks performance. We cannot isolate the origin of this variation from the current dataset but speculate it may result from inconsistency in the welding process used to attach the spiral wound foil in the 1″ ring-core or due to inadvertent welding when the complete 1 in. ring core assembly is heat treated. Regardless, many of the same trends are observed in both the ring core and racetrack data.
The 50 and 100 μm foils produced broadly similar intrinsic magnetic noise. This was not expected since, as all the cores in
The measured noise as a function of heat-treatment dwell temperature, for all rings, geometries, and frequencies, are generally concave upward with minima around 1150° C. suggesting this may be the local optimum. The 100 μm foils require more power than the equivalent 50 μm foils. In the racetrack geometry cores, the thicker foils require between 1.5 and 2 times more power. The ring-cores show an equivalent trend in 6-81 Mo while the 28-62 Cu shows a much smaller effect. More generally, the racetrack cores require less power than their ring-core counterparts—despite containing more ferromagnetic material and generally providing lower noise. Both core designs have a comparable drive winding resistance of 1.5-1.9Ω, so it seems likely that the power difference results from eddy current losses in the conductive Inconel X750 bobbin used in the 1 in. ring core that have no equivalent in the insulating plastic bobbins used in the racetrack cores.
Optical grain size analysis of the foil coupons showed a consistent pattern of larger grain size and larger spread with increasing heat treatment temperature in the 50 μm foil consistent with accelerated grain growth at higher temperatures. The 100 μm foil cores show a maximum in grain size at 1150° C. dwell temperature but with wider variation. We speculate that the grains in the 100 μm foil may not span the entire thickness of the foil so the optical analysis of the surface is sampling various cross sections and providing less representative values. Couderchon et al. (1989) potentially saw a similar plateau in grain size.
The 28-62 Cu material provides surprisingly respectable noise performance for a first attempt at a new alloy. Compared to the traditional 6-81 Mo alloy, 28-62 Cu provides noise performance ranging from equal to twice as noisy. However, the power required to drive the 28-62 Cu cores is roughly 30% lower than comparable 6-81 Mo cores.
The race-track cores simultaneously provide lower noise and lower power consumption that their comparable ring-core. The power advantage will be partially offset by the need for at least three race-track cores per sensor whereas only two are required for a double-wound ring-core sensor.
3.6 Effect of the slow Quench and Sub-Curie Heat Treatment
One heat treatment was accidently programmed to skip the second slow cool at −35° C. per hour ramp suggested by Gordon et al. (1968). Rather, the furnace attempted to ramp down to room temperature at −300° C. per hour directly as shown in
The two resulting ring-cores (
A batch of four racetrack cores were manufactured to explore the relationship between magnetic noise, power consumption, and the number of foil layers. The four cores contained 1, 2, 6, and 9 layers of 50 μm 28-62 Cu alloy and had a common heat treatment.
We manufactured twenty notionally identical racetrack cores to test the noise variability of the manufacturing process. All rings were three layers of 50 μm 28-62 μm foil heat-treated with a dwell temperature of 1150° C.
The race-track washer design seems to offer several advantages over the traditional 1 in. spiral-wound ring core: more consistent yield in noise performance, significantly fewer high-noise outliers, and lower noise performance at equal or lower power consumption per core. These advantages will be partially offset by the need for at least one core per measurement axis (three cores per sensor) whereas ring-cores can be double wound (two cores per sensor). In general, the 50 μm foils outperform the 100 μm foils in terms of noise per drive power.
In this disclosure, we have introduced a novel, low-noise and low-power nickel alloy we have designated 28-62Cu permalloy. It comprises, by weight, 28% Cu, 62% Ni, Fe balance. This alloy is suggested by data published in von Auwers and Neumann (1935) and theory in Narod (2014). In 1935, high-Cu-content permalloys were investigated for more typical, high-saturation induction applications and fell out of favour. We believe that we are the first investigators since then to revisit Cu permalloys and specifically with regard to fluxgate sensors where lower saturation induction is advantageous.
The 6-81 Mo alloy generally produces lower noise than the 28-62 Cu alloy all other variables held equal. However, the 28-62 requires significantly lower power. The consistent grain size and evolution shown by the 50 μm coupon suggests that the grains developed in the foil may be spanning the entire thickness of the foil whereas in the 100 μm foils the grains may not penetrate the complete thickness. The 1150° C. dwell temperature appears to provide the lowest noise for all alloy and foil thickness combinations. Directly cooling at 300° C. per hour down to room temperature appears to significantly degrade noise performance in a way that cannot be improved by subsequent sub-curie heat treatment. However, the 100 hours at 100° C. sub-curie secondary heat-treatment offers a significant improvement in cores manufactured using the standard process suggesting it be added to the standard process or the second cooling rate be decreased. Additional layers of foil appear to reduce the magnetic noise until approximately six to nine layers. Each additional foil layer causes a linear increase in the power consumption.
These results suggest that thinner foils may yield superior noise and power performance which is consistent with 12.5 μm foil used in the best historical Infinetics cores. We are currently developing improved rolling mill capacity with the intention of investigating 25 and 12.5 μm foils. In general, a racetrack geometry core containing more layers of thinner copper alloy foil seems like a promising path to consistently manufacturable, low-noise and low-power fluxgate cores. Alloy 28-62Cu was chosen as the first copper alloy trial due to it having magnetic properties similar to those of 6-81Mo in the molybdenum permalloy composition range (Pfeifer and Boll, 1969). 28-62Cu has approximately zero magnetostriction and has minimum magnetocrystalline anisotropy, as evidenced by its local maximum initial permeability and minimum coercivity, with all these properties achieved by identical heat treatment.
The test of the alloy 28-62Cu was a preliminary study. Based on these results, we have begun an extensive survey of alloys of nearby compositions, guided by magnetic properties data presented in von Auwers and Neumann (1935).
In particular, alloy samples have been fabricated for material analysis following the zero magnetostriction Cu alloy regime from Auwers and Neumann (1935) and shown in the cells in
The alloys are currently being processed and prepared for analysis. However, three prototype fluxgate cores have been fabricated at 28-60 Cu, 33-58 Cu, 39-54 Cu, and 45-50 Cu. Twelve cores, three of each alloy, were compared to a matched trio of cores manufactured from traditional 6-81 Mo manufactured in the same batch (
We plan to cover a composition range from 28% to 55% copper and covering the zero magnetostriction range. We will be examining DC resistivity, coercivity, saturation moment, initial and maximum differential permeability, magnetostriction, Curie temperature, grain size, and grain growth fabrics with some of these properties examined at temperatures from room temperature to Curie temperature. These results suggest that thinner foils may yield superior noise and power performance which is consistent with 12.5 μm foil used in the best historical Infinetics cores. The present invention may be used with 25 and 12.5 μm foils. The 28-62 Cu alloy is promising in terms of noise relative to drive power. Generally, race-track geometry core containing more layers of thinner copper alloy foil may provide for consistently manufacturable, low-noise and low-power fluxgate cores.
Although various specific embodiments have been shown and described herein, it is to be contemplated that numerous options, variations, and alternatives are contemplated. This includes variation in the thickness of foils, the number of layers of foil used in a core, the manner in which the foil is used to construct a core, the ratio of elements in the alloy, the heat treatment, the geometry of the core, and other variations.
It is also contemplated that the alloy may be used for applications such as magnetic shielding. For example, foil or other thin layers of the composition may be provide for low field magnetic shielding in applications where shielding alloys based on molybdenum are insufficient.
The foregoing description has been presented for purposes of illustration and description. It is not intended to be an exhaustive list or limit any of the invention to the precise forms disclosed. It is contemplated that other alternatives or exemplary aspects are considered included in the invention. The description is merely examples of embodiments, processes or methods of the invention. It is understood that any other modifications, substitutions, and/or additions can be made, which are within the intended spirit and scope of the invention.
This application claims priority to U.S. Provisional Patent Application No. 63/164,045, filed Mar. 22, 2021, hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 63164045 | Mar 2021 | US |
