Remote detection of rotating machinery with a portable atomic magnetometer

1.2 MB·

by  Luca Marmugi

Remote detection of rotating machinery with a portable atomic magnetometer
Luca Marmugi,∗ Lorenzo Gori, Sarah Hussain, Cameron Deans, and Ferruccio Renzoni
Department of Physics and Astronomy, University College London,
Gower Street, London WC1E 6BT, United Kingdom
(Dated: Compiled January 20, 2017)
Wedemonstrateremotedetectionofrotatingmachinery,usinganatomicmagnetometeratroom
temperature and in an unshielded environment. The system relies on the coupling of the AC mag-
netic signature of the target with the spin-polarized, precessing atomic vapor of a radio-frequency
opticalatomicmagnetometer. TheACmagneticsignaturesofrotatingequipmentorelectricmotors
appearassidebandsinthepowerspectrumoftheatomicsensor,whichcanbetunedtoavoidnoisy
bands that would otherwise hamper detection. A portable apparatus is implemented and experi-
mentallytested. Proof-of-conceptinvestigationsareperformedwithtesttargetsmimickingpossible
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applications,andtheoperationalconditionsforoptimumdetectionaredetermined. Ourinstrument
1
provides comparable or better performance than a commercial fluxgate and allows detection of ro-
0
tating machinery behind a wall. These results demonstrate the potential for ultrasensitive devices
2
for remote industrial and usage monitoring, security and surveillance.
n
a
J

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This is a preprint version of the article appeared in Appl. Opt. 56, 3, 743-749 (2017) DOI: 10.1364/AO.56.000743.
1

] One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, dupli-
et cation of any material in this paper for a fee or for commercial purposes, or modifications of the content of this paper
d are prohibited.
-
s
n I. INTRODUCTION tectionofrotatingmachinery,rotatingsteelsamples,and
i DC and AC electric motors, without any background
.
s Remote detection of illicit or hazardous activity inside noise screening. Through-wall detection is also demon-
c
i buildings has become increasingly important in security strated. Finally, comparison with a commercial fluxgate
s and defense. At the same time, the possibility of con- highlights the potential of the technique and the proof-
y
h tinuous monitoring of machinery would be valuable for of-concept demonstrator we describe in this paper.
p industrial activity and quality control. Therefore, the Thanks to the RF AMs’ high degree of scalability, po-
[ capability of noninvasively and remotely acquiring infor- tential for miniaturization [13] and wide range of fre-

mation about activities in concealed, harsh or inaccessi- quency tunability [14], the device we propose is a valid
1
v bleenvironmentswouldbeanimportantassetinvarious and concrete solution for remote detection of rotating
5 fields. Given the broad spectrum of potential applica- machinery for health, and usage and surveillance appli-
8 tions, including industry, health, and usage monitoring cations.
3 systems(HUMS),defenseandsecurity,earthscienceand
5
aerospace,manydifferentremotesensingapproaches,op-
0 erating either in a passive or active way, have been pro- II. EXPERIMENTAL SETUP
.
1 posedordeveloped[1,2]. Nonetheless,thelargeciviland
0 industrial use of air conditioning units, turbines, engines The setup is a table-top (0.40 m × 0.60 m) portable
7
and electric motors makes remote detection of rotating device, suitable for outdoor operation, based on an opti-
1
machineryanditsconditionmonitoringaveryimportant cally pumped RF AM [Fig. 1(a)].
:
v subject still to be fully addressed [3, 4]. The core of the sensor is a natural isotopic mixture
Xi Inthispaper,wedemonstrateremotesensingwithop- of Rb atoms contained in a cubic 25 mm glass cell at
ticalatomicmagnetometers(AMs)[5],alsoreferredtoas room temperature. 20 Torr of N are added as buffer
r 2
a optically pumped atomic magnetometers (OPMs). AMs gas, toincreasethelight-atominteractiontimeandlimit
are to date the state-of-the-art for DC and AC magnetic unwanted depolarization of the atoms.
field measurements, even in unshielded environments [6] A RadiantDyes NarrowDiode laser is locked to the D
2
and at room temperature [7]. line F = 3 → F(cid:48) = 4 hyperfine transition of 85Rb by
Here, we use a radio-frequency (RF) AM [8–12] for means of a Doppler-free dichroic atomic vapor laser lock
detecting the rotating target’s magnetic signature. In (DAVLL). To reduce costs and footprint, a single laser
particular, we demonstrate the feasibility of remote de- is used to obtain both the pump beam (for the prepa-
ration of the quantum state sensitive to magnetic fields)
andtheprobebeam(forinterrogationoftheatomicspin
precession induced by such fields). In detail, the σ+ res-
∗ [email protected] onant pump beam (δ = 0) aligns the atomic spins
pump
2
FIG.1. PortableRFAM(orOPM)forremotedetectionofarotatingmachinery. (a)Opticalsetup: apolarizingbeamsplitter
(PBS) splits the laser between the DAVLL and the core of the AM. A second PBS separates the light for the resonant and
circularly polarized pump beam and the linearly polarized probe beam, blue-detuned by means of a double-pass acousto-optic
modulator(AOM,δ =+190MHz). λ/2(HWP)andλ/4(QWP)waveplatesallowcontrolofthebeams’polarizations. (b)
probe
3D model of the sensor and principle of the detection technique. Atoms are spin-polarized by the pump beam along a DC
bias magnetic field (B ), produced by 136 mm diameter Helmholtz coils with 30 turns. B , provided by 54 mm diameter
DC RF
Helmholtzcoilswith30turns,coherentlydrivesatomicprecession,probedbythelinearlypolarizedprobebeam. TheACfield
generated by rotating objects or electric motors (B ) interacts with the precessing atoms, thus modifying the polarization
RM
oscillation of the probe beam. The cell’s and coils’ support is 3D printed in polylactide (PLA). Note that B is indicated
RM
here only for illustrative purposes. Its actual orientation depends on the specific target.
along a DC magnetic field, B = B ˆz, produced by sidebands at f ±mf , with m integer, in the RF
DC DC RF RM
a Helmholtz coil pair [Fig. 1(b)], via optical pumping to AM’s output spectrum. In this way, heterodyne mix-
the largest angular momentum projection Zeeman sub- ingimprintsinformationabouttherotatingtargetinex-
level of the ground state, |F = 3,m = +3(cid:105). An RF tra spectral components of the probe beam’s polariza-
F
field along yˆ, B , excites and periodically drives spin- tion plane oscillation, as a consequence of the non-linear
RF
coherences between the ground state Zeeman sublevels. atomic response. The latter allows mixing of oscillat-
The linear polarization plane of the off-resonant probe ing fields rather than the bare sum of their amplitudes.
beam (δ = +190 MHz) is modulated by the Larmor Consequently,theatomicmediumexhibitsnon-trivialre-
probe
precession of the atomic spins. The probe’s polarization sponse to AC fields, thus altering their spectral compo-
plane is detected by a balanced photodiode (BPD). nents (see, for example, Ref. [15]).
Inthisconfiguration,thetotalmagneticfieldcomprises Effectively, the perturbation of the forced trajectories
theexternalmagneticfieldgeneratedbytherotatingma- ofatomicspinsimposedbyB canbeseenasnutation,
RM
chineryorobjecttobedetected,BRM,andtheRFfield, which in turn modulates the probe beam’s polarization
BRF. plane rotation. As a result, fRF acts as a carrier for the
The output of the RF AM is fed to a spectrum ana- information stored in the sidebands f ±mf , thus
RF RM
lyzer,AnritsuMS2718B.Inabsenceofanyexternalfield, allowing the decoupling of the RF AM’s drive and the
thepowerspectrumisgivenbyasingle-narrowpeakcen- information storage and retrieval.
teredatthefrequencyofthedrivingRFfield,f ,which
RF B is used to tune f , so as to match the oper-
DC RF
can be chosen according to the specific operational con-
ational requirements. In this way, one can avoid noisy
ditions and measurement requirements.
bands to increase the detection capability. This feature
is of major importance in the case of low-frequency sig-
natures (<50 Hz), where 1/f noise and poor sensitivity
III. DETECTION TECHNIQUE of conventional magnetometers would prevent effective
detection. Thisprovidesarobustandsensitivedetection
The technique for the detection of rotating machin- methodforrotatingobjectsandmachineryinunshielded
ery is shown schematically in Fig. 1(b). The field B , and open environments.
RM
evolving at the frequency f , perturbs the precession Finally,wenotethat,evenwhentheobjectisnotrotat-
RM
of the atoms. As a result, a mixing is produced within ing(|B |=0),ourinstrumentcanprovideinformation
RM
the precessing atomic sample, which couples B and onconductivetargetsviatheactiveinductivecouplingof
RM
B . Inthetimedomain,theresultingACfieldismodu- magneticinductiontomography[16,17]. Thepresenceof
RF
lated by B . In the frequency domain, B produces a conductive object can be inferred from a variation of
RM RM
3
the amplitude or phase-lag of the detected field. the steel disk is rotating (continuous line) is compared
to the background spectrum, obtained when the object
is not rotating (dotted line).
IV. EXPERIMENTAL RESULTS The large central peak (0 Hz detuning) corresponds
to the operational frequency of the RF AM. Sidebands
In this section, results are arranged on the basis of at ±fRM=±(7.7±0.1) Hz, about 15 dB larger than the
thetargetunderevaluation,orofthespecificoperational background, reveal the rotation of the object. m=2 har-
parameters under investigation. We also report on the monics are also observed at ±(15.5±0.1) Hz. The width
direct comparison with a commercial fluxgate. of the spectral features is increased by small fluctuations
intherotatingspeedofthedisk. Atthecurrentstage,the
frequencyresolutionislimitedbythebandwidthandthe
A. Detection of Rotating Objects settings of the spectrum analyzer. Therefore, it ranges
from 0.1 Hz, as in the case of Fig. 2, to 1 Hz in the
broader sweeps.
First, we show the detection of rotating metallic ob-
Results obtained with ball bearings rotating at
jects, with residual magnetization. This is a challenging
f =(3.3±0.1) Hz, 0.80 m away from the RF AM, are
task for conventional sensors, given the small magnetic RM
also shown in Fig. 2(b). In this case, peaks about 35
signature and low rotational frequency, which would re-
dB above the background appear at ±f . In addi-
quirethesensorstooperateinregionwherethe1/f noise RM
tion, m=2 harmonics at ±(6.7±0.1) Hz and m=3 at
is dominant.
±(10.0±0.1)Hzareobservedwellabovethebackground.
Asteeldisk(diameter75mm,height15mm)issecured
at the center of a spinning gear. The system, realized
entirely in plastic to avoid spurious magnetic signatures
or eddy current induction, is hand-operated and allows 1. Steel Samples
frequencies of the order of f ≈10 Hz.
RM
Every test is performed at room temperature, in an Here, we briefly present results concerning small sam-
unshielded environment and without any compensation ples of commercially available steels, typically used in
ofthelocalstrayDCmagneticfields,soastomimicpos- manufacturing. We tested three squares of 25 mm × 25
sible field conditions. The AM’s frequency is arbitrarily mm×1.5mmofAISI420,EN19T,andEN24T.Samples
set to f =105 Hz. Detection of rotation is possible de- areconsiderablysmallerthanthepreviousonesandtheir
RF
spite f (cid:28)f , as demonstrated below. homogeneoussizeandshapeallowsfordirectcomparison
RM RF
of the system’s performance. Comparable detection per-
formance is observed in all cases.
m) -40 (a) Rotating Disk
B Background
d -60 -30
r ( RBoactaktginrogu AnIdSI420
e -80 -40
w
o
P-100 -50
-20 -10 0 10 20 m) -60
B
Bm) -40 (b) RBoeatartininggs Ball er (d -70
r (d -60 Background Pow -80
e -80
w -90
o
P-100
-100
-20 -10 0 10 20
Detuning from carrier (Hz) -110
-20 -10 0 10 20
Detuning from carrier (Hz)
FIG. 2. Detection of rotating objects. (a) Power density
spectrum of the RF AM obtained with a steel disk (75 mm
FIG. 3. Detection of rotating steel: 25 mm × 25 mm ×
diameter, 15 mm height) rotating around its main axis at
1.5 mm square of AISI420 steel, rotating at 0.40 m from the
f =(7.7±0.1)Hz. Thediskisplaced0.50mawayfromthe
RM sensor. The dotted trace marks the background, obtained
RF AM. Sidebands and m=2 harmonics are marked by the
without the rotating sample. Vertical dotted lines mark the
dotted vertical lines. (b) Power density spectrum of the RF
sidebandsproducedbytherotationatf =±(15.1±0.1)Hz.
AM obtained with ball bearings rotating at f =(3.3±0.1) RM
RM Traces are the result of 20 averages.
Hz. Sidebands and m=2 and m=3 harmonics are marked by
vertical dotted lines. Target is placed 0.80 m away from the
RF AM. Traces are averaged 20 times. Sidebands indicating the rotation of the samples ap-
pear in the spectra of all the tested samples, symmetric
In Fig. 2(a), a typical power spectrum obtained while withrespecttothecentralpeakatf =105 Hz. Forthe
RF
4
-30
(a)
-40 Fan ON
Fan OFF
m) -50
B
d -60
er ( -70
w
o -80
P
-90
-100
-400 -300 -200 -100 0 100 200 300 400
-30
(b)
-40 Drill ON
Drill OFF
m) -50
B
d -60
er ( -70
w
o -80
P
-90
-100
-400 -300 -200 -100 0 100 200 300 400
Detuning from carrier (Hz)
FIG. 4. Detection of rotating machinery powered by electric motors. (a) Power spectrum of the RF AM obtained with a 24
V DC fan at 1 m from the sensor. (b) Power spectrum of the RF AM obtained with an AC drill at 0.60 m from the sensor.
Continuous traces are taken when the motors are on, dotted traces when the motors are off. Vertical lines mark the magnetic
signatures of the motors, at f =±(170±1) Hz for the DC case and f =±(100±1) Hz plus m=2, 3 harmonics for the AC
RM RM
case. For consistency, all traces are averaged 20 times.
sake of brevity, we report in Fig. 3 the results concern- motorand,potentially,itsoperationalconditionsandin-
ing the AISI420 only. In all three cases, the peaks are tegrity [18]. This feature could be of major importance,
detected about 10 dB above the background spectrum for example, in the case of continuous monitoring indus-
level, measured when the samples are not rotating. trial plants and HUMS.
As an example of an AC motor, we present in Fig-
ure 4(b) results obtained with a 120 V AC drill (Du-
B. Detection of Electric Motors more,modelnumber37-021)withadjustablespeed(103-
1.6×104 RPM). Motor activity is revealed by the in-
Another important source of magnetic signatures are creased sidebands at 100 Hz and their m=2 and m=3
electric motors [18]. Here, we test our AM system with harmonics when the drill is in operation. Despite the
both DC and AC motors. overlap with the mainlines peaks, a clear difference is
Asatesttargetmimickingrotatingmachinerypowered seenwiththedrill-offspectrum(seeinparticularthe200
by DC motors, a 24 V DC fan (Sunon, model number Hzand300Hzsidebands). Thepeaks’frequencyisinde-
KDE2412PMB1-6AB)isused. Figure4(a)showsspectra pendent of the rotational speed of the drill. A pedestal
obtainedwiththefanplaced1mawayfromtheRFAM, appears at the carrier’s peak, due to low frequency noise
after performing an average of 20 measurements. produced by the rotating drill head.
When the fan is off, the spectrum in Fig. 4(a) exhibits
a set of sidebands either side of the peak of the carrier
at f =105 Hz. These sidebands, spaced by 50 Hz, are C. Response versus RF Power and Target Distance
RF
produced by the coupling between the powerline mag-
netic noise and the RF field. Our proof-of-concept system demonstrates that a ro-
Whenthefanisswitchedon,twoadditionalsidebands bust detection of the fan is possible at least up to 2 m,
appearatf =±(170±1)Hz. Thesepeaksarethemag- where - in the current testing environment - the signal
RM
netic fingerprint of this particular electric motor. The becomes comparable to the background noise, P ≈
BG
spectral signature of the DC fan was independently ver- −98 dBm. It is worth underlining that motors with a
ified with a commercial fluxgate (see also Fig. 7). It small size and small magnetic signatures were chosen
is noteworthy that this frequency is three times larger for the purpose of testing, whereas potential targets are
than the rotation frequency of the fan, 56±1 Hz. On the likely to exhibit much larger magnetic signatures.
one hand, this confirms that the observed signal is pro- We have also tested the feasibility of through-wall de-
duced by the coupling of B to the AC magnetic field tection. Therotatingmotorsareplacedbeyonda0.17m
RF
produced by the spinning motor. On the other hand, it thickwallcontainingconcrete,pipes,mainlines,andclut-
provides information about the internal structure of the ter, 0.70 m away from the RF AM. Results are shown in
5
Fig.5,whichreportstheheightofthecharacteristicpeak
dofistthaencfeanfraotmfrtehqeuesnenciseosr.fRAFs±defmRMonsatsraatfeudnbctyiotnheofcothne- 30 dBm)--6400 VVRRFF==05 VV
sistency between the two plots in Fig. 5, the wall does Bm) 25 ower (-80
not affect the detection. e (d P-100-500 -250 0 250 500
d 20 Detuning from carrier (Hz)
u
-40 plit V (V)
No Wall m 15 2 4 6 8 10RF12 14 16 18 20
Wall a 1.5
-50 k
a 1
Pe 10 ∆0.5
m) -60
0
B
d 5
r ( -70 2 4 6 8 10 12 14 16 18 20
e V (V)
w RF
o -80
P
FIG. 6. DC fan sidebands peak amplitude as a function of
-90 V suppliedtotheRFcoilsat105Hz. Thefanis0.75mfrom
RF
the RF AM. The uncertainties are the standard deviation of
-100 20measurements. Topinset: spectraat0V(dottedline)and
25 50 75 100 125 150 175 200 at 5 V (continuous line). For clarity, the spectrum at 5 V
Distance (cm)
has been shifted by +5 dB. The dotted black lines mark the
position of the f = ±(170±1) Hz sidebands produced by
RM
FIG.5. DCfansidebandspowerasafunctionofdistancefrom
thefan. Bottominset: relativechange∆ofthecarrier’speak
the RF AM, in free space (triangles) and beyond a 0.17 m
power in the corresponding cases. The dashed line is a guide
thick wall (open circles). The uncertainties are the standard
for the eye.
deviation of 20 measurements. Lines are guides to the eye.
f =105 Hz; f =170 Hz.
RF RM
a secondary frequency component (in this case f ) is
RM
InFig.6weshowtheeffectofthedrivingamplitudeof effectivelyamplifiedinasideband(f ±f )whenthe
RF RM
theRFfield(V )at105 Hz. Thespectrumisflatwhen power of the carrier frequency (f ) is increased.
RF RM
the RF supply is off (V =0), while the central peak As supporting evidence, we show in the bottom inset
RF
and sidebands appear as soon as the voltage increases of Fig. 6 the relative change ∆ = (P −P )/P of
i i min max
(see top inset). Given the proof-of-principle nature of the carrier’s peak at f as a function of V in the
RF RF
the present work and taking into consideration data re- same experiment. Here, P is the power of the peak at
i
portedinliteratureonatomicmagnetometers[8–12],one f ,andP andP aretheminimumandthemax-
RF min max
can anticipate a great potential for a further increase in imumP ,respectively. ∆ behaviorisconsistentwiththe
i i
detection capability and range. observed increase of the sidebands’ amplitude. Its rela-
WhenV (cid:54)=0,theheightofthefanpeakprogressively tivechangeisconsistentwiththeobservedincreaseofthe
RF
increases and saturates at about V =17 V, chosen as sidebands’amplitude. Inotherwords,asthecarrierpeak
RF
the RF amplitude for all the measurements presented increases, the sidebands’ amplitude increases, enhancing
here, unless otherwise stated. With this V , the pure the detection capability and extending its range.
RF
optical rotation signal as produced by the AM is about In summary, to maximize the amplitude of the side-
30 dB at f , where direct coupling of the RF to the bandsandhenceofthelikelihoodofdetection,oneshould
RF
photodiode’selectronicsproducesaspuriouscontribution over-drive the RF AM. In the present case, this implies
of 30 dB higher than the background level. VRF > 16 V. In usual conditions, however, this would
Thankstothedetectionmethod,thisisnotaproblem: effectively reduce the sensitivity at the carrier frequency
as a consequence of the decoupling of the information [19]. Nevertheless,becauseoftheapproachdemonstrated
storage in the sidebands from the sensor’s drive, extra here, this detrimental effect has no impact on the detec-
contributions at f will not have a direct detrimental tion of the rotating machinery’s magnetic signatures. In
RF
effect on the signals of interest. other words, the optimum sidebands detection can be
In addition, Fig. 6 demonstrates the role of B for safely pursued, even at the expenses of the absolute sen-
RF
the detection of B . It thus further confirms the anal- sitivity of the RF AM.
RM
ysis in terms of fields’ mixing inside the atomic sample.
In detail, given the identical conditions of the measure-
mentsandtheconstantaverageamplitudeof|B |,the D. Comparison with Commercial Fluxgate
RM
increaseofthef ±f peaks’amplitudeasafunction
RF RM
of V (Fig. 6) can be explained only by a direct depen- Finally, to validate our instrument and its operational
RF
dence of the sidebands on the carrier’s amplitude. This modality,wepresentacomparisonoftheresultsconcern-
is a well-known feature of heterodyne mixing, whereby ing the DC fan obtained with a commercial fluxgate and
6
with our RF AM.
-30
InFig.7weshowthespectrumobtainedbyconnecting -70 AM
tsD2ha0Cmeaeflfvauecnrxoagnigsafietgpse,ul’asracaoetsduiiontn0pg.ul5aet0sfttmRohMatath=weua(s1yse7pdf0er±cowtm1rit)uhtmHhoezuarflnpuaeRlxayFgkzaeAtirse,M.ciln.AeTafttrhhelyeer Bm) ---654000 Power (dBm)-----9988750505 Fluxgate
vanisdibiltes,hasarwmeollnaiscst.hTe5h0isHinzdneopiesnedpernotdluycceodnbfiyrmpsowtheratlinthees er (d -70 -100-18D0etun-1in7g0 from-160
w carrier (Hz)
peaks detected by the RF AM are produced by the DC o
P -80
fan.
-90
-20
-20 -100
Rotating AISI420
m)-40 Background -500 -250 0 250 500
dB-60 Detuning from carrier (Hz)
-40 wer (-80
m) Po--112000 FIG. 8. Comparison of the power density spectra produced
B -60 0 10 20 30 by the 24 V DC fan at f =3×104 Hz obtained with the
d Frequency (Hz) RF
r ( RF AM (continuous line) and a commercial fluxgate (dotted
we -80 line). SidebandsatfRM=±(170±1)Hzarevisibleonlyinthe
o atomic magnetometer trace. In the inset, a detailed view of
P
the −f =−170 Hz peak is shown. An offset is added to
RM
-100 both spectra in order to overlap the traces.
-120
0 50 100 150 200 250 300
bands are detected in the fluxgate’s output. This is a
Frequency (Hz)
consequence of the lack of frequency mixing in the latter
FIG. 7. DC fan signature monitored with a commercial and thus a further confirmation of the atoms’ role in the
fluxgate. Vertical dotted line marks the position of the creation of sidebands.
f =(170±1) Hz component produced by the rotating DC On the contrary, well-defined sidebands can be ob-
RM
motor, placed 0.50 m away from the sensor. Inset: AISI420 served in the AM trace, including 35 dBm high peaks
square rotating at fRM =(15.0±0.1) Hz, monitored with a at fRM=±(170±1) Hz. This is consistent with the pre-
commercialfluxgateplaced0.40mawayfromthesensor. No
vious observations, despite the change in f .
RF
signature is found at f , marked by a dotted vertical line.
RM
Traces are averaged 20 times.
V. CONCLUSIONS
Itisimportanttounderlinethatthef spectralcom-
RM
ponentisobtainedherebydirect detection. Thefluxgate
cannot support any frequency mixing, as demonstrated In conclusion, we have realized a proof-of-concept
below. TheDCfanpeakliesontopofanincreasingback- demonstration of remote detection of rotating objects
ground, which rapidly exceeds -90 dBm below 10 Hz, a and of both DC and AC electric motors with a com-
consequenceofthe1/f noiseofboththefluxgateandthe pact and portable optically pumped atomic magnetome-
spectrum analyzer. ter. The magnetic signatures of rotating targets appear
This is detrimental for low-frequency measurements. as symmetric sidebands of the carrier which drives the
Forexample,detectionofthesidebandsproducedbysteel RF AM and can be thus detected also in critical condi-
samples such as those shown in Fig. 3 would be impos- tions, such as low frequency, where conventional sensors
sible (see inset of Fig. 7 in the case of AISI420). First, usually have poor performance.
the lack of mixing with a carrier causes a reduction in The proposed instrument takes advantage of the per-
the peaks’ amplitude. Second, the peaks are comparable formanceofopticallypumpedatomicmagnetometersand
with the noise floor. the non-linear nature of atomic response to oscillating
In Fig. 8 the output spectra of both the fluxgate and fields. TheuseofanRFAM,inparticular,offersabroad
the RF AM are shown at f = 3×104 Hz. This fre- range of tunability, which is of major importance for op-
RF
quency was chosen in order to ensure operation of both erationinunscreenedscenariosanddifferentenvironmen-
sensors and, therefore, the possibility of direct compari- tal noise conditions. In this way, by using a less noisy
sonoftheirresponse. Forthistest,thefluxgateisplaced frequency band and frequency mixing amplification, our
in the same position as the vapor cell of the RF AM. proof-of-concept demonstrator exhibits comparable per-
Upon inspection of Fig. 8, it is clear how detection of formancetothatofcommercialdevicesinthecaseofhigh
f - in the current settings and working conditions - is frequency rotation (≥50 Hz), and increasingly better re-
RF
more effective in the case of the AM. More importantly, sponse in the case of low frequency rotation (<50 Hz).
in view of the detection of rotating machinery, no side- In particular, our instrument detects rotation of objects
7
and machinery below 15 Hz. miniaturization and power consumption containment.
Furthermore, the demonstrated detection of rotating
machinery through concrete walls makes this approach The work was supported by DSTL and CDE in the
suitable for harmless, noninvasive and nondisrupting re- framework of “What’s inside that building?” program.
mote sensing for security and surveillance, but also for L. G. is supported by Wellcome Trust. S. H. is sup-
continuouscontrolofcivilindustrialprocessesandhealth ported by DSTL – Defence and Security PhD – Sensing
and usage monitoring. and Navigation using Quantum 2.0 technology. C. D. is
The proposed system is therefore a promising can- supported by the Engineering and Physical Sciences Re-
didate for successful deployment in real-life scenarios, searchCouncil[grantnumberEP/L015242/1]. Wethank
thanks to its portability, its margin for further improve- D.HarveyandD.Tee(THALESUKResearchandTech-
mentdetectioncapabilityandrange,anditspotentialfor nology) for their support.
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