installing an atto-tesla magnetometer - intrel...competitor's domestic factory. without good...

Installing an Atto-Tesla Magnetometer © IEEE 2017, Slide 1SAS-2017

JaJames A. Kuzdrallmes A. Kuzdrall

IntrelIntrel Ser Service Companyvice Company

www.intrel.com/mea/www.intrel.com/mea/

Week of 13 March 2017Week of 13 March 2017

Installing an Installing an

Atto-Tesla Atto-Tesla

MagnetometerMagnetometer

Sensor Applications SymposiumSensor Applications Symposium

Rowan UniversityRowan University

Glassboro, NJGlassboro, NJ

Copyright IEEE 2017Copyright IEEE 2017

1) An atto-tesla is 10^-18 Teslas. The earth's magnetic field is 58e-6 Teslas, 58 trillion times larger.

2) The new technology is 120 times more sensitive than the most sensitive previous technology. This lecture discusses the new installation challenges at this extreme sensitivity.


How Does it Work

� MEA has 3 parts �

� � Bow decreases, strains

increase (�strip= 1.0)� Strain 1: 1.5/27.5= 0.05� Strain 2: 3.6/12= 0.30

1) The isometric drawing shows the 3 key parts.

2) In the lower drawing, the distance between the Stops is fixed at 100mm.

3) Two sets of strips, longer than 100mm, demonstrate the gain increase as the bowing gap decreases

4) Note: A mechanical strain is the induced fractional length change compared to the original total length. That is, the (original_gap-final_gap)/original_gap.

5) In both the upper two and lower two strips, the length change is the same, 1.0mm.

6) The calculated strain at the smaller 12mm gap is 6 times larger. Intuitively, as the gap approaches zero, the strain gain approaches infinity.

7) The units of gain are (�gap/gap)/(�gap/gap) = unity.

8) The amplifier output is strains. To continue, we need something that responds directly to strains and, with further processing, can be converted to volts.


Radiometer Block Diagram

1) An electrode under the bowing strip makes a capacitor.

2) Since the gap controls the sensitivity of the capacitor and of the MEA, it must be kept reasonably constant for consistent measurements.

3) The thermoelectric cooler/heater controls temperature of the whole MEA assembly. The gap responds to the temperature expansion coefficient difference of the substrate and bowing strip (about 25.6e-6(m/m)/°K - 7.2e-6(m/m)/°K in a typical radiometer design).

4) The Peltier thermoelectric module heats or cools its upper surface depending on the polarity of its drive voltage.

5) A phase sensitive detector (Synchronous Detector) tells whether the gap capacitance is larger or smaller than the reference capacitor. When it is larger, the polarity heats the assembly, expanding the strip, making the gap larger, and the capacitance smaller.


Magnetometer Adaptation

1) A magnetometer evolves from the radiometer design with these changes:

a) Use a magnetostrictive substrate, Terfenol-D.

b) Make the bowing strip non-magnetic, 304 stainless steel.

c) The stop separation changes; the strip length is constant.

2) A fixed magnet applies a magnetic bias to the Terfenol-D to increase the magnetostrictive sensitivity.

a) The bias field also distinguishes the magnetic field direction. Unbiased Terfenol-D shrinks the same amount for a north-south field as for a south-north field. With bias, the external field aids or opposes the bias, indicating its direction.

3) A wire coil balances the Blumlein bridge, replacing the thermoelectric heater/cooler. It subtracts out:

a) The earth's local magnetic field.

b) Thermal expansion changes.

c) Oscillator and transformer noise.


Benchmark Design

� Substrate: Terfenol-D

permanent magnet bias,

30mm by 10mm by

20mm

� Bowing Strip: 304 non-

magnetic stainless steel,

gold flash coat

� Strip size: 30mm by

0.254mm by 4mm

� Bowing gap: 1.0um, air

filled

� Bridge drive: 1200Vpp,

30KHz, breakdown at

4020Vpp

� Bridge amp: 2.5nV/�Hz,

2N4416A + AD797

� Op temperature: 300�K

� Ref. capacitor: 140pF

� Noise-equiv-strain:

�1.8e 19(m/m)/�Hz

� mea_app_mag_benchmark-

dsn-mfe.pdf

1) The low thermal emissivity gold flash on the bowing strip reduces the photon noise, if needed.

2) A 1.0 micron gap is about the smallest practical at this time.

3) The Paschen Effect explains why the gap breakdown voltage goes to a minimum then climbs rapidly as the gap size decreases. The variable is pressure times gap size. Each gas has its own plot.

4) The 2N4416A FET is ideal for the bridge: low gate leakage, small input capacitance, high gate breakdown voltage, high transconductance (low output resistance). If bought in the metal can (TO-72) avoid ultrasonic cleaning! It shakes off all the lead-bond wires. Use the SOT-23 package.

5) The "Magnetometer Benchmark Design" is being updated to include seismic noise sensitivity.


Inherent Sensitivity

Why use the inherent sensitivity?

1) The MEA-based sensitivity considers only instrument noises (photon, fluctuation, and bridge amplifier). However, larger environmental noises will determine the sensitivity limit in most installations.

2) The associated paper assumed seismic noise would need suppression by using back-to-back MEAs. A subsequent calculation found that not true. The large end force required to hold the 1 micron bowing gap greatly exceeds the likely natural seismic acceleration forces.


A Deployment Scenario

� Harbor traffic detection

1) Since installation is the subject of discussion, a specific site and purpose must be identified. Harbor intrusion detection is selected.

2) The black boxes firmly mounted to the harbor floor are the magnetometers. They are powered and controlled by cables running to the shore. A computer facility on the shore interprets the magnetic waveforms.

3) The small submarine is smuggling out goose down and chocolate. A corporate war is underway. The alien corporation wants to destroy the efficiency of the competitor's domestic factory. Without good sleep on goose-down pillows, the workers will doze off at work. Without their chocolate break, they will get grumpy. The sub must be stopped and captain made to return the contraband.


Some Environmental Disturbances

� Water temperature change

� tides, dam releases, wind, propellers, seasons

� Magnetic fields from support circuits

� Seismic acceleration

� natural and human activity

� Fish, frogmen, and frog-women

� false alarms, snoops

� Sea floor critters

� shark bites, octopus hugs

1) The list show some environmental "noises" that were not problems until very high sensitivities were considered.

2) The sensing scheme relies on the susceptibility difference between the object and the harbor water. The difference distorts the earth's local magnetic field enough for detection. The field's gradient changes when the object's susceptibility volume does not match that of water.

3) The distance computation needs only the perturbing magnetic anomaly, modeled by a single magnetic moment. The other magnetic fields can be ignored.


Temperature Change

� Ships are fast, temperature changes are slow

� 0.5ºC/hr seems OK

� How fast does water temperature really

change? Measure it in your harbor!

� Package thermometer/recorder for harbor

floor concealment

� 3-month autonomous data collection

� Slowest ship? Time-elapse photos

� Test insulated packaging, if needed

1) Expansion-coefficient-compensation and insulation should be enough to discriminate the relatively slow water temperature change from the faster-moving harbor traffic.

a) The relative temperature coefficients of the Terfenol-D and of the bowing strip should be the same from instrument to instrument, but not necessarily equal. Magnetometers are used in pairs to measure the field gradient. The temperature drift will subtract out.

b) If the thermal mass accompanying the magnetometer is large and the heat loss to the sea is small, time constants greater than 6 hours will suppress tidal temperature changes and any faster sea temperature changes.

2) If the local authorities agree that the harbor should be protected, a year-long study of the water temperature change rates at the proposed magnetometer locations should be started. If the survey shows need, insulated packaging should be designed.


Circuitry DC Current Variation

� Allowed at MEA: �I = 1.0pAdc, 100mm2 loop� Allowed at 1.0m: �I = 1.0uAdc, 100mm2 loop

1) Only the essential electronic parts mount in the MEA housing. Feedback requires the bias coil. The high transformer secondary voltage should be near where it is needed. The low noise, low output resistance FET buffer must be at the bridge balance node.

2) The magnetic field direction from twisted pair wires reverses at each turn, quickly canceling out with distance from the wire pair.

3) Magnetic field intensity drops off rapidly, 1/distance^3.

4) The area of current loops on printed circuit boards must be minimized.

5) DC current changes, rather than steady values, affect the magnetic detection.

6) Current sources and shunt regulators keep the total current constant.

7) Zeners shown in the drawing represent high precision voltage regulators.

8) See the associated IEEE paper for current and loop sizes.


Seismic Directional Sensitivity

� Length and width directions much less sensitive

(much stiffer) than thickness direction� Lateral tremors are 10X greater than vertical

1) The MEA bowing strip is very stiff in the width and length directions. The gap direction (thickness) is the most sensitive.

2) Lateral seismic accelerations are 10 times greater than vertical accelerations, so mount the MEA to the sea floor with the gap in the vertical direction. Mounting to deep rock may help.

3) A location less accessible to large ships will reduce accidental and deliberate anchor-drag damage.


Tremor Spectrum

� "Seismic Sensors and their Calibration", by Erhard Wielandt 2002

1) The graph shows the lowest seismic noise that can be expected at the most quiet locations found. Man-made noise is not present at the quiet locations.

2) Passing ships might be in range for 10 seconds to 1000 seconds.

3) The seabed noise is worst in the winter. At that time it is 50db worse than shown on the graph.


Seismic Noise Sensitivity

1) Surprise! Seismic noise is relatively small, 0.7e-21 compared to a total of 179.0e-21 strains/�Hz!

2) Why? Small gap requires a great pressure at the ends of the strip. The seismic acceleration force is small by comparison.

3) Define "Tolerable noise" as that which adds 10% or less to the RSS statistical average noise.

4) Tolerable vibration noise sources could add 256 times or 48db more noise to the seabed noise with only a 10% sensitivity loss.

5) Back-to-back MEAs suggested in the associated paper do not appear to be necessary for harbor traffic monitoring.


Towing and Flying

� Use vibration immunity for mobile applications:

� Boat-towed for geological mineral surveys

� Submarine-towed arrays for object location

� Inexpensive hobbyist drones for land mine

searches. Also avalanche, earthquake, drowning,

and quicksand victim searches.

1) Back-to-back cancellation of accelerations in the thickness direction is not needed for our harbor protection application. However, adding that attenuation to the design makes towing and flying practical.

a) towing in water, single or arrays: mineral surveys, ship and plane wrecks, harbor mines

b) suspended beneath a drone: safely locate land mines, find avalanche and drowning victims, passively detect tunnels, underground erosion, and nascent sinkholes


Measuring the Change

� Earth's field: 5.8e-5 Tesla� Detectable change: 2.4e-17 Tesla� Voltmeter resolution: 2.4 parts per trillion

(13 digits)� Oh - oh!

1) The flux-gate and Overhauser magnetometers require a resolution of 7 digits to read their noise level in the presence of the earth's magnetic field. Such meters are readily available.

2) No voltmeter comes close to the resolution needed by the MEA-based magnetometer, 13 digits. Observing the ultimate resolution in the presence of the earth's field requires a different approach.


Blumlein Bridge to the Rescue

� Blumlein bridge and its feedback subtract out all

but 1 part in 1.2e8 of the flux response� The DC left represents 5.8e-5/1.2e8= 4.8e-13

Tesla� A 1.9999 voltmeter range resolves the residual

DC to the equivalent of 2.4e-17 Tesla

1) Feedback again comes to the rescue. Precise subtraction removes the steady local magnetic field response. The relatively fast increases and decreases from the average are the signal caused by harbor traffic. (Just like AC coupling the oscilloscope.)

2) A 6-digit voltmeter can now resolve the magnetic field change down to the magnetometer's noise level.

3) The earth's local magnetic field is likely to change significantly with time-of-day and tides. Feedback also eliminates these variations.


Improvement: Measure the Gradient

� Gradient= Flux difference between two constant flux

contours separated by 1.0 meter� Balance 23Hz pickup

to 1part in 20000 via

synchronous detector� A-B near zero with no

shipping around� Detected field difference

increases with separation� Use bridge drive for

gain control

1) Subtracting two magnetometers adds east-west directional discrimination to the north-south provided by the bias.

2) Another pair of magnetometers, on or near the same magnetic contour but further north or south, give distance information.


Flux Gradient Equation

� b_earth= 5.8e-5 (T)

� object_radius (m),

object_length (m),

� sep= magnetometer

separation (m)

� disty= radial distance

from object (m)

� Susceptibility object, net

� Susceptibility of water=

-9.035e-6

� Procedure: adjust disty until

computed gradient is 10

times the magnetometer

noise (Tesla/�Hz)

� gradient units, Tesla/m

1) Put the equation in BASIC or MathCad where changes can be explored.


Waveform Exploration in the Lab

� What signal versus time appears as a ship passes?

� Try a scaled measurement:

� Two similar flux-gate magnetometers 20mm apart and

aligned with local north

� Flux-meter outputs to oscilloscope channel A minus

channel B

� Move small bar magnet around them in a circle to see

response

� Try the magnet aligned to and perpendicular to north

1) The closed-magnetic loops and third-power field strength decrease with distance make intuitive signal strength estimates difficult. Research papers reporting experimental or analytic estimates for the signatures of passing harbor traffic would be a good investment.

2) The susceptibility of various fish species and bottom dwelling creatures would produce valuable engineering data and interesting technical papers.


Review

� Magnetostrictive substrate: allows use of friable

Terfenol-D with non-magnetic stainless steel bowing strip� Thermal time constant: increase to make tides, prop-

wash, dam discharge, storms, etc., slower than ships� Magnetic flux changes: minimize with current

regulators, small current loop areas, and distance� Perturbation gradient: measure with two north-

aligned, contour-separated magnetometers� Seismic noise: suppressed with thickness axis vertical� False alarm study: measure susceptability "signatures"

of fish, octopus, frogmen, etc., for computer analysis� Passing-object waveform study: use flux-gates and a

bar magnet: distance, angle, relative axis alignments


Thank you for attending

Best wishes

for a successful career!

James A. Kuzdrall

installing an atto-tesla magnetometer - intrel...competitor's domestic factory. without good...

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