US 10,641,663 B2

Sheet 18 of 22

May 5, 2020

U.S. Patent

8t “Dla

7]

by
aIneuty



US 10,641,663 B2

25

wherein @ near is the phase of Uyjeqs Alternatively, since U
and velocity V are in phase,

[UL = 1Wrneasl(e"?meas-V)) — [AU gle“®V — [AU ple2-W), )

wherein @, is the phase of the measured velocity. Thus,
determining the amplitude of U requires (equation 18)
knowing the phase of U,,.a¢ With respect to the current I, or
(equation 19) knowing the phases of U,,.,, and V with
respect to the current I. This would be provided for example
by synchronous sampling of U,,,, and I (and V in the case
of equation (19).

The expressions for the phase @,, of U corresponding to
equations (18) and (19) respectively are

[Cmneasl$iBPmeas ~ AU cl (20)

[GmeaslCOSPmeas

en

‘The equality of the right-hand sides of equations (20) and
(21) can be used as a check of the accuracy of measurements
and validity of assumptions. Equivalently to (21), the total
phase of the right-hand side of equation (19) should be zero.

At sharp anti-resonances in the body-armature relative 3

motion, the relative velocity and induced voltage vanish, and
large uncertainties result from using the fully in-situ Kibble
balance method. At such operating points the force can be
determined by the “quasi in-situ” approach of extracting the

ments of current, velocity and voltage at other frequencies,
and then using equation 1 to determine the force at and
around the antiresonance, with an uncertainty contribution
from the possible variation of the effective J integral with
operating point.

Extracting the amplitude of the induced voltage U from
the amplitude of the measured voltage Uy... requires a
phase measurement, the minimum requirement being a
measurement Of @jyoqq: the phase of the measured voltage
relative to the excitation current. In a force calibration of a
SUT, it will often be desired to know the phase of an output
signal from the SUT with respect to the force F,,,, applied
to it. It may appear that to determine the Lorentz force F
(amplitude and phase) applied to the armature by the mag-
netic field using equation (3) or (4), that the relative phases
of U, land V must be measured. However, since the current
Lis in phase with F, and the induced voltage U is in phase
with the relative velocity V, IFI=IUI/VI=IUJ]I/'VI and only
the amplitudes of the three input quantities are measured to
give the amplitude of F. The phase of F (with respect to the
current I) is zero. To determine the force F.,,,, applied to the
SUT, corrections to F for the armature inertia and parasitic
forces are applied as indicated by equation (6), and these
will in general include a phase correction.

In addition to measuring the phase of U,,,..; With respect
to the excitation current, additional phase measurement in
determining the phase response of the SUT output signal is
that of the SUT output signal with respect to the excitation
current. Analytical knowledge of the phase difference
between F and F,,,, can occur at some frequencies.

FIG. 21 shows the force F,,,,, applied to SUT, assuming
that the device is operated with a 10 N amplitude Lorentz

integral (using equations 2 and 5) from the measure- 3

S

20

2

0

60

65

26

force F. In the vicinity of resonances, the deviation in the
amplitude and phase of F,,,, from F due to the armature
inertia is significant. Away from such resonances, the phase
of F,,, tracks that of F closely. At frequencies above the
armature resonance at ~10 kHz, the Lorentz force goes
predominantly into the armature inertia and little force is
applied to the SUT. In this region the uncertainty in the
applied force using equation (6) is dominated by uncertainty
in the armature inertia F,,,,,. The parasitic force is modeled
as being due to a restraining stiffness of 3x10* N/m (pre-
dominantly due to support flexures), and is insignificant.

A location of resonant features can vary from one SUT to
another. The achieved force uncertainty as a function of
frequency can dependent on mechanical properties of the
SUT can be determined in the calibration as a function of the
electrical and mechanical measurements made.

With regard to uncertainty, equation (6) is repeated here
as equation (22) for convenience.

1 22)
Feagy = GU meas ~ AUR AU) ~ Fines = Foor

Quantities measured during operation of dynamic force
contactor 200 include voltage U,,,,s(t) across the coils,
voltage U,(t) across a current-measuring shunt resistance,
relative velocity V(t) of the armature with respect to the
body, acceleration a,(t) of the body, and operating frequency
. Quantities that are characterized offline are the residual
differential resistance AR and inductance AZ, the current-
measuring shunt resistance Rs, the armature and body
masses m,, m, and m,, and the stiffness K of the armature
support flexures. The current I(t) is determined as I=U/R,
The inertial offset F,,,,,(t) due to the armature acceleration is
determined as F,,,.,=m, a,4m, a,=m, (iwV+a,)-m,0°X(w).
The parasitic restoring force of the flexures is calculated as
K X(t), where the relative displacement X(t) of the armature
with respect to the body is determined as X(t)=V(t)/w. We do
not correct for additional parasitic forces (e.g. due to wires
and air), but treat these as an uncertainty SF,,,,. equal to an
estimated upper bound for such forces. Rewriting the mea-
surement equation in terms of these quantities gives

UsUneos  UsU eae (23)

VRs VRE

‘app =

(AR + iL) ~

[mi (itoV + ap) — mye” Xs]

Assuming these measurements to be uncorrelated, the
uncertainty in the applied force is given as

(24)

Poe a a
0B +(e Jour

( 8Fonp
a

BF Vs
Sat) oa

OF, Jo >
( amy me
8Fop

OK

Pe > (IF axp
oa + (7

(= a) ne +

OR,

(fe >
OXs

6x? +( OK? + Foon

In operation of dynamic force contactor 200, X <(«) can be
calculated from a model (X,() is determined from the


U.S. Patent May 5, 2020 Sheet 12 of 22

MEASURE QUANTITIES:

Unknown force signal 270 0 (t)
of force sensor 268

From dynamic force converter 200:

Alternating voltage 224: U(t)
Alternating current 226: /(t)
Reciprocating velocity 222° v(t)

Dynamic force 252

Favp

US 10,641,663 B2

CALCULATE CALIBRATION RESPONSE:
Ss

Previously uncalibrated force sensor 268 now SI

traceably calibrated

FIG. 12



US 10,641,663 B2

Sheet 22 of 22

May 5, 2020

U.S. Patent

66 “Old

(2H) Aquenbai4

301 000 0001 00S OOl OS

T

TEGRE  oe oe a8 iu
“ aaa

ss d Seed

L000

OL00

OOL'O

Quieyaour Jey


U.S. Patent May 5, 2020 Sheet 3 of 22 US 10,641,663 B2

200
234—~-
214 218 230
ro) ws
210-TTM__} €}----- > <-- Ay
x 9336702
224 226 ~~ 999
v v
! \
t
250~4 )
y 232

FIG. 8


US 10,641,663 B2

21
TABLE 1-continued
Quantity Value
DYNAMIC VARIABLES
Deployment Force Amplitude 10N
Current Amplitude (1) 04 A
Body Acceleration Amplitude (ay) 0.33 mis?

Armature-Body Relative Velocity 4.4 x 10° mis to 0.11 m/s
Amplitude (V)

Armature-Body Relative
Displacement amplitude (%,.)
Armature Back EMF (U)

Total Armature Voltage Drop (Umass)

4.6 x 107! mto 8.3 x 10° m

11x10 V to 28V
0.034 V to 28 V

Resistive Voltage Drop 0.68 V
Amplitude Across Each Coil

Differential Resistive Voltage 0.034 V
Drop Amplitude

Current Sense Voltage Amplitude av
,)

Inductive Voltage Drop Amplitude o1v

Across Each Coil
Differential Inductive Voltage
Drop Amplitude

1.3 x 10 V to 0.06 V

An example design of dynamic force contactor 200 is
shown in FIG. 17, and parameter values for dynamic force
contactor 200 that generates 10 N are listed in Table 1. Here,
dynamic force contactor 200 is a portable, hand-held trace-
able dynamic force for an exposed location on a system
under test. The magnetic field is generated using a perma-

nent magnet and guided by shaped pole-pieces. As discussed. 3

here, dynamic force contactor 200 provides simultaneous
measurement of coil current I, coil velocity V, and induced
voltage U. Current is measured with a calibrated sensing
resistor; voltage is measured with a calibrated external

voltmeter, and velocity is measured with a fiber-coupled 3

Fabry-Perot interferometer. Additionally, an accelerometer
measures absolute body acceleration used in calculating
absolute armature acceleration and correction for inertia of
the armature. The interferometer can be replaced by a
second accelerometer or a velocity or displacement trans-
ducer.

A resistive and inductive voltage drops across the coil can
occur. A bifilar coil can eliminate the resistive voltage drop
but not the inductive voltage drop. While the resistive and
induced voltages are 90 degrees out of phase, allowing
phase-sensitive detection to separate them, the inductive
voltage drop is 180 degrees out of phase with the induced
voltage and not separable. The dynamic force contactor 200
can include two nominally identical coils with approxi-
mately equal resistances R..,;;, Roo. and approximately
equal inductances £ 4, £ g,72- The two coils are arranged
in series so that the same current flows through each coil,
resulting in approximately equal resistive and inductive
voltage drops. Coil 1 is disposed proximate to the armature
of, is suspended by flexures, and oscillates under action of
Lorentz force on the current in the radial magnetic field,
developing induced voltage U. Coil 2 is fixed and develops
no induced voltage. Thus, the inductive and resistive voltage
drops across the coils are largely cancelled by a differential
measurement, leaving the induced voltage drop across coil
1

The induced voltage U is related to the measured voltage
Uneas bY

U=Upoas-AUp-AU

10

20

2:

40

45

65

22
and the applied force F is related to the coil current I and
measured voltage Usreae by

I (6)
Fagg = 3 Uneas 80g — AU) = Ft = Fore

wherein AUg=I(Rooin Roos) is the residual resistive volt-
age drop; AUr =2Af1(L coin -£ cons) is the residual induc-
tive voltage drop at excitation frequency f: F,,., is the force
required to accelerate the armature inertia, and F,,,, is the
parasitic force applied to the armature by support flexures,
connected wires, and air drag.

The coils are subjected to matched radial fields, achieved
by symmetrical construction and selection of matched per-
manent magnets: the application of the radial field to the
second inductor is done in order to match the operating
points on the nonlinear permeability curves of the inductor
cores. Furthermore, the coils are mounted together in the
housing of dynamic force contactor 200, which minimizes
temperature differences between the two core assemblies
and which would otherwise create differential magnetization
of the permanent magnets and permeability of the cores
Alternatively, the second coil could be disposed external to
dynamic force contactor 200 and connected to dynamic
force contactor 200 by a cable.

An electrical model of dynamic force contactor 200 is
shown in FIG. 17(5). The excitation current I is supplied by
a current source. I, and I, are the currents flowing in coils 1
and 2 respectively, and R,, £, and C,, represent “lead”
resistance, inductance and capacitance respectively. The
resistances R, and R, of the two coils are due to ohmic
resistance in the coil wire and to losses in the pole pieces and
permanent magnets due to eddy currents and magnetization
hysteresis. Capacitances C, and C, are parasitic capacitances
across each coil. Component values in the model are in
frequency-dependent quantities. The measured differential
voltage drop across the coils is

= M

Unreas =

1
u +il(wei + =(iwes + ) | =U+IAZ,

1 1
Rit To) Rr +iLw

wherein AZ is the impedance difference between the coils
and i=V=T. In the limit that C, and C, go to zero, AZ reduces
to AR+imAL where ARSR,-R, and AL #£,-L ,. In prac-
tice, a process for calibrating the impedance difference
AZ() includes holding coil 1 fixed (in addition to coil 2) so
that U=0 V, for which case AZ=U,,,,,,/I. If temporary clamp-
ing of coil 1 is less rigid than the permanent fixation of coil
2, a small current value can be used in this calibration to
minimize motion of coil 1 due to deformation of the clamp-
ing mechanism. The capacitances across the coils result in a
difference between the current I, in coil 1, which generates
the force F, and the total (measured) current I

1 (8)
Tt feo (Ry + iwoL1)"



US 10,641,663 B2

Sheet 19 of 22

May 5, 2020

U.S. Patent

70! 0005

6T DIA

(zy) Aquanbaig
G00 00S OO OS

uul Quawsseidsig saneiey

2

*
2
apnyidury

Loo 7/3
als

(ut

—A 99)


US 10,641,663 B2

5

246 disposed proximate to magnet 210 and that provides a
selected shape of magnetic field 212 in which electrical
conductor 214 is disposed and interacts with to produce
Lorentzian force 218. Armature guide 248 is disposed proxi-
mate to the armature 230 and that provides a linear motion
in the reciprocating direction 228 of the armature 230.
Housing 234,234 can include flange 260, rostrum 256,
caudal member 258, and handle 254. Armature guide 248
can be in contact with flange 260 so that armature guide 248
is linear motion along reciprocating direction 228. Also,
various components of dynamic force contactor 200 are
disposed in rostrum 256 and held in place by flange 260. It
is contemplated that magnetic field guide 246 contact and
inner wall of rostrum 256 and flange 260 so that magnetic
field guide 246 and magnet 210 remain stationary with
respect to housing 234. Enclosure 298 can be disposed on
dynamic force mediator 232 elements of dynamic force
contactor 200 is shown in FIG. 9.

Dynamic force contactor 200 produces dynamic force 252
and dynamic force signal 250. Dynamic force contactor 200
includes magnet 210 that produces magnetic field 212,
which interacts with alternating current 226 within electrical
conductor 214 to produce Lorentzian force 218. Lorentzian
force 218 moves armature 230 in reciprocating direction 228
and applies dynamic force 252 to structure 266. Dynamic
force 252 is measured using dynamic force mediator 232 by
measuring alternating voltage 224, alternating current 226,
and reciprocation velocity 222. In an embodiment, magnet
210 is a permanent or electric magnet. In an embodiment,
magnet 210 is a rare earth permanent magnet.

Magnet 210 produces magnetic field 212, also referred to
as magnetic field 212, that has a magnetic field strength from
anano-Tesla to tens of Tesla, specifically from 0.01 Tesla to
10 Tesla, and more specifically from 0.1 Tesla to 2 Tesla. In

an embodiment, magnetic field 212 is produced by a rare 3

earth permanent magnet.

Electrical conductor 214 is disposed in magnetic field 212
and communicates alternating current 226. In an embodi-
ment, electrical conductor 214 can be a single or multi-turn
coil of wire. Lorentzian force 218 is generated from elec-
trical conductor 214 interacting with magnetic field 212.
Secondary electrical conductor 220 is disposed in magnetic
field of secondary magnet 242 and generates a force that
moves armature 230 along reciprocating direction 228.
Reciprocation velocity 222 is a velocity of armature 230
along reciprocating direction 228. Alternating voltage 224 is
the voltage across electrical conductor 214. Alternating
current 226 is the current flowing through electrical con-
ductor 214. Reciprocating direction 228 is a primary direc-
tion along which armature 230 reciprocates. Armature 230
reciprocates and applies dynamic force 252 to structure 266.

Dynamic force mediator 232 can include a circuit that is
used to measure alternating voltage 224, alternating current
226, and reciprocation velocity 222. In an embodiment, with
reference to FIG. 10, dynamic force mediator 232 includes
current source 262 that supplies electrical current to elec-
trical conductor 214. Shunt resistor 264 is arranged in
parallel and in communication with current source 262 to
provide a measure of the alternating current 226. Here,
alternating voltage 224 is measured across terminals 300.

Housing 234 houses all or some components that gener-
ates or measures dynamic force 252. Accelerometer 236
measures acceleration of housing 234. The acceleration is
used to calculate displacement or velocity of housing 234.
Light source 238 can be used with light detector 240 to
determine relative motion of armature 230 and housing 234.
Light detector 240 can be used with light source 238 to

40

&

6

determine relative motion of armature 230 and housing 234.

Secondary magnet 242 can be used with secondary electrical
conductor 220 to generate motion of armature 230. Coupler
244 provides a physical link between secondary electrical
conductor 220 and secondary magnet 242 to armature 230

Magnetic field guide 246 is a high permeability material that
shapes magnetic field 212 so that it interacts with electrical
conductor 214. Armature guide 248 constrains motion of
armature 230 along reciprocating direction 228. Dynamic
force signal 250 is a signal that is proportional to dynamic
force 252, which is calculated from alternating voltage 224,
alternating current 226, and reciprocation velocity 222
optionally with a correction. Dynamic force 252 is a time-
varying force applied to structure 266. Handle 254 is mem-

s_ber by dynamic force contactor 200 is held with a hand or

mount. Flange 260 mechanically supports flexure 292 and is
attached to housing 234. Current source 262 generates
alternating current 226. Shunt resistor 264 measures alter-
nating current 226. Structure 266 can include force sensor
268, which can be calibrated with dynamic force contactor
200. Force sensor 268 is a force-indicating instrument
disposed on structure 266 and can be calibrated with
dynamic force contactor 200. Unknown force signal 270 is
a signal from force sensor 268 that is calibrated with

s dynamic force contactor 200. Support material 272

mechanically reinforces magnetic field guide 246. Optical
cavity mirror 278 is a mirror disposed on an optical cavity
that measures displacement and reciprocation velocity 222
of armature 230. Fiber optic 280 transmits light from optical
cavity mirror 278 to optical signal 286 output. Wire 282
communicates alternating voltage 224 and alternating cur-
rent 226. Supply current 284 generates alternating current
226. Optical signal 286 is an output of the optical cavity
from fiber optic 280. Strain gage 288 measures strain of
shear web 290 that indicates force applied to force sensor
268. Shear web 290 deflects when a force is applied to the
sensor. Flexure 292 constrains the primary motion of arma-
ture 230 along reciprocating direction 228. Direct light 294
is light emitted from light source 238 and can be detected
directly or indirectly by light detector 240. Reflected light
296 is light from light source 238 that is reflected from
armature 230 and detected by light detector 240. Enclosure
298 covers dynamic force mediator 232. Terminal 300 is an
access location wherein alternating voltage 224 is measured

It should be appreciated that various components of
dynamic force contactor 200 can be made from a metal,
plastic, glass, ceramic, polymer, composite, and the like or
a combination thereof. Materials of construction selected for
the components should not interfere with operability of
dynamic force contactor 200.

Dynamic force contactor 200 can be made in various
ways. In an embodiment, a process for making dynamic
force contactor 200 includes disposing electrical conductor
214 proximate to magnet 210 and in magnetic field 212 so

5 that alternating current 226 of electrical conductor 214 is

oriented perpendicular to magnetic field 212, wherein elec-
tric field 216 in combination with magnetic field 212 from
magnet 210 provide Lorentzian force 218: disposing arma-
ture 230 proximate to magnet 210 such that electrical
conductor 214 is disposed on armature 230, wherein arma-
ture 230 reciprocates in reciprocating direction 228 relative
to magnet 210 in response to Lorentzian force 218 and
wherein armature 230 produces dynamic force 252; and
disposing dynamic force mediator 232 in communication

5 with electrical conductor 214 and armature 230 such that

dynamic force mediator 232 monitors alternating voltage
224 across electrical conductor 214, alternating current 226


U.S. Patent May 5, 2020 Sheet 21 of 22

US 10,641,663 B2

1000, : T
(A)
10
2 +
2 meee Force applied to SUT, Faap. fo of
en ee Armature inertia, Fiaet Q tee, c
= 0100+ 4 formers Boot :
& wove PAASHUE Force, Fee 7
@ ‘Back ‘
5 2 :
re * ,
0.004 ; ° :
i
Lad i :
1078 it i
4 10 4100 1000 10°
Frequency (Hz)
s
aa Pid ‘ ——-
) | i
2 2 ppt
3 |
3 ° Y
£ nt Force applied to SUT, Fang
2 2h--+< Armature Inertia, Feet
8 ~The. Parasitic Force, Fu ‘ i i
& 4 40 400 4000 10000
Frequency [Hz]

FIG. 21


US 10,641,663 B2

23

and in a difference between the currents in the two coils, 1,
and 1,,

1 1
Tr ioCi(Ri + toLy) 1+ ioCa(R + woLs) | ~

9)
h-k ‘

iwl[C2{Ro + ito Lr) — C\(Ry + iwL)]-

With calibration of AZ(«), which includes the capacitances
C, and C,, this current difference is not a source of error in
the determination of U. However, capacitances C,, C;, and
C,, introduce error into the measurement of the force-
generating current I,. Additionally, £, will introduce a
(frequency-dependent) phase offset between the measured
current | and the force generating current I. For the remain-
der of this Example. neglect capacitances and £ , such that
Equation (6) becomes

UnreasUtTAR+OAE ), (10)

Mechanical behavior of dynamic force contactor 200 can
be determined by the mechanical system to which it is
coupled, e.g., a system under test (SUT). A mechanical
model of the device coupled to an SUT with an internal
degree of freedom is shown in FIG. 18. Deformability of the
armature and of the SUT are modeled by single degree of
freedom mass-spring-dampers. The body of the device is
modeled by a lumped mass supported by restraining force
F,. This model considers that the device is rigidly coupled

to the SUT. The armature is supported by flexures of total _
stiffness K, which exert a parasitic restraining force on the °

armature. Other sources of parasitic force include finite
stiffness of the wires connecting to the coils, and aerody-
namic drag. The directly measured motions in the device are
the motion of the body relative to an inertial reference frame

(x, in FIG. 18) and the motion of the armature relative to the *

body of the device (x,.=x,-x, in FIG. 18).

A number of degrees of freedom of SUT may vary from
zero up to tens or higher, and quantitative mechanical
properties (e.g., mode resonant frequencies and quality

0

factors) can vary from one SUT to another. The model *°

shown in FIG. 18 is sufficient for design and the uncertain-
ties of measured quantities to achieve a low-uncertainty
force output within range of SUT mechanical parameters.

The equations of motion for x,, the displacement of the
SUT, x,, the displacement of the armature, and x,, the
displacement of the device body, are

mk 4b 0-8, kX, IAF ay

(mgt ig bald

i pebtrk orth y.=0 (2)

mygy=Fp-P a3)

Solving these in the frequency domain gives

Xo) 1
Flo)

Oh + 2g tig 4)

my [Ont ndoP + (05 + 2iacaco) + NaC + PE cr600))

Lo? + of + 2igawaeol =

Xo) as)

Fo)

1 =n + Ju? + (w? + 21,2400) + aw? + 2ig,wsea)
my [q+ m0? + (a2 + Dig gcoq0d) + MB(O? + Dike)]

[-o? +e + 2iggcr960] — [eed + 2iaeroer}

Xp(o) __ Fel) F(a) = 1

mpcop

(16)

Flo)
with

50

60

65

24
-continued
he ns ay
one J oe fae a) .
my ms Wham,” Wham,
me mg
sma,
my

FIG. 19 shows dynamics of dynamic force contactor 200,
as given by equations (14-17), for a sinusoidal Lorentz force
of amplitude 10 N. The SUT mode stiffness k, is 10” N/m,
and the mass m, is 2 kg. The lumped masses m, and m, of
the armature are 40 g and 1.25 g respectively, and the
armature stiffness k, is 1.7x10* N/m. The armature param-

5 eters are based on a designed beryllium armature with an

aluminum wire coil. The damping ratios and of the armature
and SUT modes are 0.01. The mass m, of the device body
is 1.5 kg, and the restraining force F, is 95% of the force
applied to it by the magnetic field, i.e., a sinusoidal force of
amplitude 9.5 N.

The quantities plotted in FIG. 19 are the relative displace-
ment and velocity of the armature (m,) with respect to the
body of the device, as well as the absolute acceleration of the
body. A resonant frequency is at 350 Hz due to the SUT
degree of freedom and a resonant frequency at 10 kHz due
to armature internal degree of freedom. Intermediate
between the two resonances is an antiresonance in the
armature motion and in the relative motion between the
armature and to the body at nearly the same frequency. The
relative velocity can be below 0.1 m/s and can exclude the
immediate vicinity of antiresonances down to 10° m/s,
involving a dynamic range of 10° for 1% uncertainty. At the
lower resonance frequency, the relative armature-body dis-
placement reaches 50 jim that is larger with lower frequency
or more lightly damped resonances. With the model used for
the body support condition, the body acceleration amplitude
is 0.13 m/s”.

The induced voltage U is proportional to the relative
velocity V (equation (2)) plotted in FIG. 20(a). Also plotted
are amplitudes of the residual resistive and inductive voltage
drops and the total measured voltage Uyeas- These residual
voltage drops are remaining voltages after cancellation
between two matched coils. The inductances are modeled as
having a 1/f°° dependence, which are approximate for
off-the-shelf voice coils. The resistance of the coils is
modeled as frequency-independent. The coils differ in resis-
tance and inductance by 0.5% and 1%, respectively.
Residual resistive drop is larger than induced voltage U for
some of the operating frequency range. At the frequency
labeled f,, the residual resistive voltage drop IAR exceeds U
by a factor of 10. At this same frequency, the residual
inductive voltage drop is equal in magnitude to 0.68 U.

In FIG. 20(2), the phases of the different voltage terms
with respect to the excitation current I (and thus the Lorentz
force F) are shown. From equation (5), using the constant
phases (0, 7/2) of AU, and AU,, the amplitude of U is given
by

(18)

Waal? + ERP + LAU —
Imecs|[|AU ple0S(Pmeas) + AU ]e0S(Gmecs —/2)] >


US 10,641,663 B2

Sheet 16 of 22

May 5, 2020

U.S. Patent

AaTforyuo;
PIS OLS °

9T “Old

U

[U4

i
£9000-

——_—
oo

wy

66S

(x _ 8)

(a)

Iq

Aqddosy

sn CCE

ISN of

{ =
O00

(Hi
ai

ner — ery

|_ 987
PIE

P— 886

hm 06

vy)


U.S. Patent May 5, 2020 Sheet 8 of 22 US 10,641,663 B2

258 210

FIG. 8


US 10,641,663 B2

17

the same terminals that are used to supply the driving current
during use of the device or during force-based calibration.
Accelerometer 236 determines acceleration of the housing
xh, and an interferometer measures a difference between the
housing and the armature xa-xh. The corrections in equation
(5) can be calculated using this measure information, along
with ancillary measurements such as the flexure stiffness and
armature mass. A different type of motional sensor (for
example a second accelerometer, mounted on the armature)
can be used to measure the relative motion between the coil
and the magnetic field.

‘An advantage is the simplicity of the force measurement,
as the prior calibration of the BL product avoids making
measurements of voltage UE and velocity V during the force
realization. The value of the BL product can be characterized
as a function of coil position, as well as angular rotations,
and can contribute to uncertainty of the realized force. As
such, a magnet and coil geometry that subjects the coil to a
highly uniform and radial magnetic field, such as employed
in high precision Kibble balances, can be used. Likewise,
slower temporal variation of the BL product, e.g. due to the
effect of temperature changes, can contribute to uncertainty.

With regard to a dual coil with dual-mode (non-simulta-
neous) operation, a dynamic extension of Kibble balances is
used for static high-precision applications, wherein the mea-
surements of V (1), U(), and I(t) are performed in separate
phases, with a second coil being used to drive the force
generation coil at an optimal velocity in the BL-product

determination phase. This configuration has the advantage ,

0

8

that incorporating the drive mechanism into dynamic force ~

contactor 200 makes it easier to calibrate the BL product
closer in time and under similar conditions (e.g., the same
temperature) as when the force is realized. Moreover, the BL.
product can be measured before and after the force realiza-

tion to check for any changes. Although this mitigates slow ~~

temporal variations in the BL product, the spatial depen-
dence can be characterized. An armature structure is shown
in FIG. 15(6)

With regard to dual coil with single-mode (simultaneous)
operation, simultaneous measurement with a bifilar coil
reduces resistive voltage drop. Dynamic force contactor 200
can include two nominally-identical coils with approxi-
mately equal complex impedances Z,,..,;, and Z,.,;.- The two
coils are arranged in series so that the same current flows
through each coil, resulting in approximately equal resistive
and inductive voltage drops. Coil 1 includes the armature of
the device, is suspended by flexures, and oscillates under
action of the Lorentz force on the current in the radial
magnetic field, developing induced voltage UE. Coil 2 is
fixed and develops no induced voltage. Thus, the voltage
drops across the coil are largely canceled by a differential
measurement, leaving only the induced voltage drop across
coil 1. The induced voltage UE is determined from the
difference between the measured voltages U1 and U2 as,

UU UM, ©

where AUz3=IAZ,; corrects for impedance mismatch
between the two coils. The coils are disposed in matched
magnetic fields that are achieved by symmetrical construc-
tion and selection of matched permanent magnets. Applica-
tion of the magnetic field to the second coil matches oper-
ating points on the nonlinear permeability curves of the pole
pieces. Furthermore, both coils are mounted in the device
body, which minimizes temperature difference between the
two core assemblies that might cause differential magneti-
zation of the permanent magnets and differential permeabil-

3!

18

ity of the cores, An alternative design disposed the second
coil outside of dynamic force contactor 200, connected by a
cable.

With regard to estimated uncertainty, the force uncertainty
of dynamic force contactor 200 can depend on values of
measured quantities, which can depend on Kibble balance
design and mechanical properties of the system to which the
balance is coupled.

An uncertainty of the AC current measurement has an.
expected relative uncertainty of approximately 0.5% at 10
kHz for current amplitudes in the 0.1 Ato 1 Arange. Another
contribution is internal dynamics of the armature. The
armature can have a fundamental longitudinal internal reso-
nance above 10 kHz such that the force correction due to the
internal dynamics is below 10%. We can calculate such
corrections with approximately 5% uncertainty, resulting in
an overall uncertainty contribution of approximately 0.5%.
Spatial and temporal variation of the magnetic field will
contribute to the uncertainty for configurations in FIGS
15(a) and (6). Using an opposed-magnet design to maximize
field uniformity, the uncertainty contribution can be kept to
approximately 0.3%. Unexpectedly and beneficially, an
uncertainty less than 1% can be achieved up to frequencies
of 10 kHz, and conventional traceable dynamic force extend
to the few kilohertz frequency range and have uncertainties
in the range of a few percent.

Dynamic force transducers can be designed based off the
above description. Devices discussed here have the advan-
tage of being self-calibrating and can be installed in the host
measurement structure and then calibrated in that setting,
The dynamic force contactor 200 can be integrated into
conventional force transducers, as shown in FIG. 16(a) for
a shear-web style design to make the transducer self-cali-
brating. Conventional force transducers work by measuring
the deflection of an element. For the specific shear web force
transducer shown in FIG. 16(a), the indication of the force
is the voltage from a strain gage circuit USG. An SI-
traceable dynamic force therefore is described and by simul-
taneously measuring F and USG, the transducer sensitivity
can be determined within a host structure.

Additionally, dynamic force contactor 200 that includes
active feedback control to generate a current to balance or
compensate for an applied force by keeping the armature at
a fixed position is shown in FIG. 16(6). This also minimizes
accelerations of the armature and the need to correct for
inertia. This device is a high-frequency electromagnetic
compensation force sensor with Kibble-principle calibra-
tion. For both of these designs, a same mechanical correc-
tion can be used. Such design overcomes difficulties with
active feedback control in conventional devices where BL
(or similarly, a conversion from control current Ipyppiy. 0
force F) is calibrated by deadweights or with a known
external force. The dynamic force contactor 200 is calibrated
using Kibble’s approach in situ, which makes dynamic force
contactor 200 self-calibrating with a primary realization of
a SI-traceable force.

The dynamic force contactor 200 provides Kibble bal-
ances for dynamic force metrology that provide primary
dynamic forces directly to a system of interest. Uncertainties
on the dynamic force output from dynamic force contactor
200 are smaller than conventional force standards. The
dynamic force contactor 200 can be constructed at different
scales for applications at various force amplitude scales. For
dynamic force contactors 200 that have armatures and coils
with sufficiently small mass, e.g., less than approximately


US 10,641,663 B2

11

to dynamically calibrate force sensors in the application
setting in which they are used. This device overcomes
challenges in measuring dynamic forces, namely that a force
sensor’s dynamic calibration response changes depending
on the application setting in which it is used.

Dynamic force contactor 200 produces a Lorentz force
generated by current-carrying wire 214 in magnetic field 212
to apply dynamic force 252 to force sensor 268 that can be
used to dynamically calibrate force sensor 268 in its appli-
cation setting. As shown in FIG. 8, a wound coil electrical
conductor 214 is attached to armature 230, which sits in
magnetic field 212 that is generated by magnet 210 and
magnetic field guide 246. Armature 230 engages armature
guide 248 for linear motion. These components are disposed
in a portable housing 234. A current moving through elec-
trical conductor 214 in magnetic field 212 produces Lorent-
zian force 218 that moves armature 230. Lorentzian force
218 F(t) can be applied to force sensor 268 or structure 266.
Labels, A, B, and C correspond to time-varying responses
reciprocation velocity 222, alternating voltage 224, and
alternating current 226 in FIG. 8. To determine dynamic
force 252, reciprocation velocity 222 of armature 230 rela-
tive to magnet 210 is determined as well as alternating
current 226 through the electrical conductor 214 and alter-
nating voltage 224 across electrical conductor 214.

With reference to FIG. 12, to calibrate force sensor 268,
armature 230 of dynamic force contactor 200 is held against
force sensor 268 or structure 266 in contact with force sensor
268 to be calibrated. Dynamic force contactor 200 applies

dynamic force 252 to force sensor 268 or structure 266 and 3

unknown force signal 270 o(t) of tforce sensor 268 is
measured along with alternating voltage 224 U(t), alternat-
ing current 226 I(t), and reciprocating direction 228 v(t) of
dynamic force contactor 200, which are used to calculate

force dynamic force 252 F(t) that was applied from 3

dynamic force contactor 200. A dynamic calibration
response, also referred to as sensitivity S, of force sensor 268
can be calculated using unknown force signal 270 o(t) and
dynamic force 252 F,,p(t).

FIG. 9 shows dynamic force contactor 200 in a perspec-
tive view (panel A) and an exploded view (panel B). Here,
accelerometer 236 can be disposed on armature 230 or
disposed on a body of dynamic force contactor 200 to
measure acceleration of magnet 210. Enclosure 298 encases
many of the components of dynamic force contactor 200
such as dynamic force mediator 232 that measures voltage
and current, e.g., alternating voltage 224 and alternating
current 226. Accelerometers 236 determine reciprocation
velocity 222 of electrical conductor 214 relative to magnet
210. Additionally, accelerometer 236 on armature 230 can
used to make corrections to dynamic force 252 to make an
accurate measurement with lower uncertainty.

Dynamic force mediator 232, e.g., as shown in FIG. 10,
is connected to electrical metrological equipment, e.g., a
current source, voltage source, digital volt meter, and the
like, to measure alternating voltage 224 and alternating
current 226. It is contemplated that other circuitry or equip-
ment can be used to measure alternating voltage 224 and
alternating current 226. An AC or DC current source 262
produces a current in electrical conductor 214 in magnetic
field 212. Alternating voltage 224 across electrical conduc-
tor 214 is measured using electric metrological equipment.
Additionally, shunt resistor 264 can be used with electric
metrological equipment to determine alternating current 226
through electrical conductor 214.

In this manner, dynamic force contactor 200 determines
dynamic force 252. It is contemplated that electrical con-

S

2

40

&

6:

12

ductor 214 can be a coil disposed in a uniform radial
magnetic field 212 generated by magnet 210 that can be a
permanent magnet. Time-varying alternating current 226 I(t)
in electrical conductor 214 generates a time-varying axial
force on electrical conductor 214 according to

FQ=BLI(), ay

where B is the radial magnetic field and L is the length of
electrical conductor 214 in the field. The BL product is
determined by measuring alternating voltage 224 U({) across
electrical conductor 214 and reciprocation velocity 222 v(t)
of electrical conductor 214 relative to magnetic field 212,

_ vw 2)

BL

‘The voltage drop U,,,(t) across electrical conductor 214 is
the sum

Usd) -UD*URO, 8)

where Uz (t) is the drop due to the resistance of electrical
conductor 214 (including lead lengths to measurement
points) and can be corrected for or rendered negligible.
The force F given by [1] is applied to electrical conductor
214 and is applied to system-under-test (F,,,,,) less the force
F,,¢¢ used to accelerate the inertia of armature 230 (electrical
conductor 214 plus any additional material in which elec-
trical conductor 214 is embedded) and any parasitic forces
Far acting on armature 230 due to connected wires, sus-

‘par ;
pension, and air drag.

FE ayy nc E pe “

The force F,,., is given by the product of mass and
acceleration of armature 230,

Finefar ode, (5)

where a(r) is the local acceleration and p(r) is the local
density. The acceleration a can be measured using an accel-
erometer or interferometer. The mass of armature 230 (and
potentially mass distribution) can be known during use of
dynamic force contactor 200. Armature 230 can be con-
nected via flexures. Flexible wires can interconnect to elec-
trical conductor 214. For small-amplitude motion of arma-
ture 230, F,,,,. can be negligible.

With regard to SI traceable measurement or calibration
with dynamic force contactor 200, measurement of accel-
erometer output, voltage of shunt resistor 264, and alternat-
ing voltage 224 of electrical conductor 214 can be per-
formed by an external instrument or by a component
integrated into dynamic force contactor 200 such as dynamic
force mediator 232. Similarly, current source 262 can be an
onboard battery or capacitance.

It is contemplated that dynamic force contactor 200 can
include different geometries or types of electrical conductor
214 or magnet 210, different ways of measuring reciproca-
tion velocity 222 (including optical methods, such as inter-
ferometric measurements in different interferometer con-
figurations), determination of BL product by calculation,
different ways of measuring or estimating acceleration of
armature 230 and inertial correction, closed-loop operation
(e.g., feedback on force, velocity, displacement, accelera-
tion, current, voltage or strain amplitudes), handheld vs
mechanically-supported configurations, variations on the
force waveforms applied (sine, pulse, chirp, step, random,
etc.), various means of measuring or calculating F,,,,, using,

using

air-bearing support of armature 230 to minimize F,,..


U.S. Patent May 5, 2020 Sheet 5 of 22 US 10,641,663 B2
a) 200
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214 218 230
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252
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224-1996 og
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(B) 200
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210-;-_} <}----- > ope
= 252
224 F326 PA 94
H fl v
¥ :
240
Pm
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U.S. Patent May 5, 2020 Sheet 4 of 22 U

S 10,641,663 B2

200
234 —SL
214 218 230
5 7 5 228
210 ~~ TM~__] Se eee 5 ad See dae \ >
x 7 252
| 294 f
a 226 \_A 996
|__ 288 La 240
¥ ~ 1 299
lqennnnnne-e!
(
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v 232

FIG. 4


U.S. Patent May 5, 2020 Sheet 17 of 22 US 10,641,663 B2
a) 200
258 280 278 210 246 25
CP Ty %
286 aN 4 260
224 9000600 _| i fllscsaco 230
ae : — IM fey 52
i [
284 0000000—+ tot 47700000004 fee
(B) 200

th

FIG. 17

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U.S. Patent May 5, 2020 Sheet 7 of 22 US 10,641,663 B2

200
216 214
244 }
) v
v
242 220 210 214 230

FIG. 7


US 10,641,663 B2

1
DYNAMIC FORCE CONTACTOR,
PROVIDING A DYNAMIC FORCE, AND
CALIBRATING A FORCE SENSOR TO BE
TRACEABLE TO THE INTERNATIONAL
SYSTEM OF UNITS

CROSS REFERENCE TO RELATED
APPLICATIONS

The application claims priority to U.S. Provisional Patent
Application Ser. No. 62/558,101 filed Sep. 13, 2017, the
disclosure of which is incorporated herein by reference in its
entirety.

STATEMENT REGARDING FEDERALLY
SPONSORED RESEARCH

This invention was made with United States Government
support from the National Institute of Standards and Tech-
nology (NIST), an agency of the United States Department
of Commerce. The Government has certain rights in the
invention. Licensing inquiries may be directed to the Tech-
nology Partnerships Office, NIST, Gaithersburg, Md.,
20899; voice (301) 301-975-2573; email tpo@nist.gov; ref-
erence NIST Docket Number 16-033US1.

BRIEF DESCRIPTION

Disclosed is a dynamic force contactor to provide a

dynamic force, the dynamic force contactor comprising: a 3

magnet that provides a magnetic field; an electrical conduc-
tor that provides a current perpendicular to the magnetic
field, the current in the electrical conductor in combination
with the magnetic field from the magnet providing a Lorent-

zian force; an armature disposed proximate to the magnet, 3

the electrical conductor disposed on the armature such that
the armature reciprocates in a reciprocating direction rela-
tive to the magnet in response to the Lorentzian force and
that produces the dynamic force: and a dynamic force
mediator in communication with the electrical conductor
and the armature such that: the dynamic force mediator
monitors an alternating voltage across the electrical conduc-
tor; the dynamic force mediator monitors an alternating
current through the electrical conductor; and the dynamic
force mediator monitors a reciprocation velocity of the
armature.

Disclosed is a process for providing a dynamic force, the
process comprising: disposing the armature of the dynamic
force contactor in contact with a structure to which the
dynamic force is applied: providing electrical power to the
electrical conductor, the electrical power comprising a time-
varying amplitude: and reciprocating the armature at the
reciprocation velocity in response to providing electrical
power to the electrical conductor to provide the dynamic
force to the structure.

Disclosed is a process for calibrating a force sensor to be
traceable to the international system of units (SI), the
process comprising: disposing the armature of the dynamic
force contactor in contact with a structure to which the force
sensor under calibration is disposed; providing electrical
power to the electrical conductor, the electrical power com-
prising a time-varying amplitude; reciprocating the armature
at the reciprocation velocity in response to providing elec-
trical power to the electrical conductor; providing the
dynamic force to the structure: measuring the alternating
voltage; determining the alternating current through the
electrical conductor; determining the reciprocation velocity

S

2

40

&

3!

2

of the armature; determining the dynamic force from the
reciprocation velocity, the alternating voltage, and the alter-
nating current; and determining a calibration response for
the force sensor to calibrate the force sensor to be traceable
to the SI

BRIEF DESCRIPTION OF THE DRAWINGS.

The following description should not be considered lim-
iting in any way. With reference to the accompanying
drawings, like elements are numbered alike.

FIG. 1 shows a dynamic force contactor;

FIG. 2 shows a dynamic force contactor;

FIG. 3 shows a dynamic force contactor;

FIG. 4 shows a dynamic force contactor;

FIG. 5 shows, in panel A, dynamic force contactor in
which light from a light source is received by a photode-
tector when an armature is disposed at a first reciprocation
position, and panel B shows the dynamic force contactor in
which the armature is disposed at a second reciprocation
position such that light from the light source is not received
by the photodetector:

FIG. 6 shows a dynamic force contactor;

FIG. 7 shows a dynamic force contactor;

FIG. 8 shows a dynamic force contactor;

FIG. 9 shows a perspective view of a dynamic force
contactor in panel A and an exploded view of the dynamic
force contactor in panel B;

FIG. 10 shows a dynamic force mediator;

FIG. 11 shows a magnet disposed on a high permeability
material that is disposed on support material;

FIG. 12 shows providing a dynamic force to a structure
and calibrating a force sensor to be traceable to the inter-
national system of units (SI):

FIG. 13 shows a dynamic force contactor;

FIG. 14 shows a dynamic force contactor in panel A, a
mechanical model of the force contactor in panel B, the
dynamic force contactor disposed on an aerospace vehicle in
panel C, and a finite element model of the data make force
contactor disposed on the aerospace vehicle in panel D;

FIG. 15 shows a dynamic force contactor and dynamic
force mediator in panel A, a dynamic force contactor and
dynamic force mediator in panel B, and a dynamic force

5 contactor and dynamic force mediator in panel C;

FIG. 16 shows a Kibble balance in panel A and a dynamic
force contactor disposed on the Kibble balance;

FIG. 17 shows a dynamic force contactor in panel A and
dynamic force mediator in panel B;

FIG. 18 shows a mechanical model of a dynamic force
contactor;

FIG. 19 shows a graph of amplitude versus frequency for
a dynamic force contactor;

FIG. 20 shows a graph of amplitude versus frequency for
a dynamic force contactor in panel A and a graph of phase
difference versus frequency in panel B;

FIG. 21 shows a graph of force amplitude versus fre-
quency for a dynamic force contactor in panel A and a graph
of phase versus frequency in panel B; and

FIG. 22 shows a graph of relative uncertainty versus
frequency for a dynamic force contactor.

DETAILED DESCRIPTION

A detailed description of one or more embodiments is
presented herein by way of exemplification and not limita-
tion.


a2 United States Patent

Chijioke et al.

US010641663B2

US 10,641,663 B2
May 5, 2020

(10) Patent No.:
(45) Date of Patent:

(54) DYNAMIC FORCE CONTACTOR,
PROVIDING A DYNAMIC FORCE, AND
CALIBRATING A FORCE SENSOR TO BE
TRACEABLE TO THE INTERNATIONAL
SYSTEM OF UNITS

Applicant: Government of the United States of
America, as represented by the
Secretary of Commerce, Gaithersburg,
MD (US)

Akobuije D Chijioke, Silver Spring,
MD (US); Nicholas Vlajic, Frederick,
MD (US)

GOVERNMENT OF THE UNITED
STATES OF AMERICA, AS
REPRESENTED BY THE
SECRETARY OF COMMERCE,
Gaithersburg, MD (US)

Subject to any disclaimer, the term of this
patent is extended or adjusted under 35
USC. 154(b) by 20 days.

16/130,471
Sep. 13, 2018
Prior Publication Data
US 2019/0078945 Al Mar. 14, 2019
Related U.S. Application Data

Provisional application No. 62/558,101, filed on Sep
13, 2017.

Int. Cl.
GOIL 1/00
GOIL 1/12
HOIF 7/121
HOI 7/08
HOI 7/06

(71)

(72) Inventors:

(73) Assignee:

(*) Notice:
(21)
(22)
(65)

Appl. No.:
Filed:

(60)

GD)
(2006.01)
(2006.01)
(2006.01)
(2006.01)
(2006.01)

(Continued)

(52) US.CL

ve GOLL 1/122 (2013.01); HOLF 7/064
(2013.01); HOIF 7/066 (2013.01); HOIF
7/081 (2013.01);

(Continued)

(58) Field of Classification Search

CPC ace GOLL 1/122; HOIF 7/066; HOLF 7/081;
HOIF 7/121; HOF 7/122; HOIF 7/1844
(Continued)
(56) References Cited
U.S. PATENT DOCUMENTS
4,500,827 A * 2/1985 Merritt HO2K 35/04
290/1 R
5,587,618 A * 12/1996 Hathaway HO2K 31/02
310/102 A

(Continued)

OTHER PUBLICATIONS

Chijioke, R.L., et al., NIST 1-kilonewton sine force calibration
system, IMEKO International Conferences, 2014.

(Continued)

Primary Examiner — Max H Noori
(74) Attorney, Agent, or Firm — Oflice of Chief Counsel
for National Institute of Standards and Technology

(7) ABSTRACT

A dynamic force contactor includes: a magnet that provides
a magnetic field; an electrical conductor that provides an
electric field perpendicular to the magnetic field, the electric
field from the electrical conductor in combination with the
magnetic field from the magnet providing a Lorentzian
force; an armature disposed proximate to the magnet, the
electrical conductor disposed on the armature such that the
armature reciprocates in a reciprocating direction relative to
the magnet in response to the Lorentzian force and that
produces the dynamic force; and a dynamic force mediator
in communication with the electrical conductor and the
armature such that: the dynamic force mediator monitors an
alternating voltage across the electrical conductor; the
dynamic force mediator monitors an alternating current
through the electrical conductor; and the dynamic force
mediator monitors a reciprocation velocity of the armature.

16 Claims, 22 Drawing Sheets


U.S. Patent May 5, 2020 Sheet 10 of 22 US 10,641,663 B2

FIG. 10


U.S. Patent May 5, 2020 Sheet 9 of 22 US 10,641,663 B2

(A) 298

(B)

FIG. 9


US 10,641,663 B2

15

namic wind tunnel measurements, and automotive crash
testing, to name a few. Generally, standards with expanded
uncertainties at the level of approximately 1% are used for
such applications. Laboratory primary standards have been
developed, but uncertainties of such primary systems in the
few-percent range have so far been restricted to frequencies
less than 5 kHz. Furthermore, the dynamic sensitivity of an
assembled force measurement system is not the same as that
of the isolated force sensor, reducing the utility of ex-situ
calibration of force sensors for dynamic force measure-
ments. Portable or embedded dynamic force standards are
needed to calibrate force measurement systems in their
operational installations. One such portable dynamic stan-
dard is a calibrated force sensor that can be struck against the
force measurement system that is to be calibrated, resulting
in a common force applied to both devices. By measuring
the response of each device, the calibration of the portable
standard can be transferred to the application force mea-
surement system. A portable transfer standard of this sort,
namely an instrumented hammer, has been calibrated with
an uncertainty of less than 2.1% (k=2) up to 5 kHz. How-
ever, a portable calibrated force sensor is a secondary
standard, as it must itself be traceably calibrated against a
primary reference. The dynamic force contactor 200 here is
a portable primary standard based on the Kibble principle
although dynamic force contactor 200 can be a secondary
standard based on a calibration of magnetic field integral
against a force standard. Moreover, dynamic force contactor
200 can be integrated into a structure to generate known

dynamic forces at a location that is not externally accessible, 3

e.g., including integration with a force transducer as a
composite device.

In Kibble balances, a mechanical force (in balances
designed to measure mass or Planck’s constant this is

typically the local gravitational force on a mass) is balanced 3

by a Lorentz force
Th EF dh

a
where 1 is the uniform current flowing along a wire (usually
in the form of a coil) of length L and differential length
element dl, immersed in magnetic field of flux density B.
The accurate determination of the value of this BL integral
is the main challenge in applying the Lorentz force law for
accurate measurements. Kibble evaluated BL from induced
voltage UE generated across the wire when it is moved at
velocity V through the magnetic field,

{BE \xF (My Vb

Ue e

Thus, measuring voltage UE, velocity V, and current I
gives the force F in the direction of BL with magnitude F
according to:

= Ugh = |VilFleos8

<, Ul

4)
Fle oe"

where 0 is the angle between the BL and V vectors, and
BLsUE/V is the magnitude of BL. For a remainder of the
Example, vector notation is dropped and reference is made
to magnitudes F, U, V, and BL.

Demonstrated relative uncertainties of Sl-traceable
dynamic forces that have bandwidths in the 1 kHz to 5 kHz
range are a few percent. The dynamic force contactor 200
has a percent-level uncertainty and operates at frequencies
up to 10 kHz or above. This contrasts with static Kibble

S

2

40

45

3!

16

balances that determine Planck’s constant or realize mass
with relative uncertainties at the level of 1x10~

With the electrical current I(t) varying at kilohertz fre-
quencies, dynamic impedances, particularly coil induc-
tances, are critical. Similarly, the realized Lorentz force may
not be the force applied to the target structure because of
mechanical filtering effects. The difference between the
applied force F,,,,, and the force realized by equation (1) F
is shown in FIG. 14 with a two degree-of-freedom (DOF)
reduced order model. Summing the forces on the armature
yields

E app =F-m ark Sar¥s)-CfarHo)s co)

where F is the force realized by equation (1), Fapp is the
force applied to the structure, ma is the lumped-parameter
mass of the armature, kf is the stiffness of the flexures, and
xa and xh are the respective positions of the armature and
housing of the device. The second term on the right-hand
side is a correction of the realized force F due to the inertia
of the armature, the third term is a correction for the flexure
stiffness, and the last term accounts for dissipation from the
flexures. This model assumes that the armature has an
internal fundamental resonance frequency that is much
higher than the frequency range of interest. If the dynamics
of the armature significantly affect the response, then a
higher-order model, such as a three DOF system (shown in
FIG. 14(6)) ora finite-element model (e.g., FIG. 14(¢) & (d))
could be used to obtain an accurate correction. Thus, mea-
sured component motions, for example of the armature xa(t)
and housing xh(t) in equation (5), are inputs to determining
the generated force. In some applications, a location or
direction of the force applied by the Kibble standard may
differ from that where a known force is desired, involving a
model and possibly motional measurements in the applica-
tion structure as shown in FIG. 14(d) using a finite element
model to obtain the desired force.

A difference between static and dynamic operation is the
ability to operate in a simultaneous mode in the dynamic
case, wherein current, voltage. and velocity are measured at
the same time. Thus, BL can be measured at any instant of
time. Time variation of BL, due, e.g., to coil motion in a
non-uniform B field, is not a source error. The uncertainty
associated with BL can depend upon measured values of UE
and V, which are functions of time.

Configurations dynamic force contactor 200 as Kibble
balance dynamic force standards are discussed with regard
to a portable dynamic force standard, but configurations can
be inserted into a measurement system such as that shown in
FIG. 14(c).

With regard to a single coil with dynamically calibrated
BL configuration, a single coil disposed in a magnetic field
generates a known force. Here, BL is calibrated in a first
measurement, and during subsequent use of the device, the
current I is measured to determine force via equation (1).
The calibration of BL can be done by using an external
device to translate or oscillate the armature with the coil
open circuited, while measuring V and UE. The calibration
of BL can also be performed with the coil stationary, using
a statically calibrated force transducer or deadweights to
measure the applied force generated by a measured current
Tand using equation (1). Here, the Kibble principle plays no
role, and the device is just a calibrated voice coil actuator.

‘An exemplary design is shown in FIG. 15(A) with an
electrical circuit model. Current is determined from the
voltage drop across a sensing resistor (current shunt) incor-
porated into the device. During calibration based on the
Kibble principle, the induced voltage UE is measured using


US 10,641,663 B2

13

inductive or capacitive coupling of the current into electrical
conductor 214 to eliminate a wire link between electrical
conductor 214 and housing 234, type of wire used for a coil
of electrical conductor 214, measuring alternating current
226 other than a sensing resistor, and the like.

Alternating current 226 I(t) can be measured, e.g., by
placing an known resistance in series with electrical con-
ductor 214 and measuring alternating voltage 224 across
electrical conductor 214. Alternating voltage 224 U,,,(t) can
be measured by a voltmeter, and alternating current 226 v(t)
can be measured by an optical interferometer such as the
Fabry-Perot interferometer, or by accelerometers attached to
armature 230 and housing 234. This measurement may be
done simultaneously with application of dynamic force 252
and measurement of alternating current 226 or performed in
a separate step.

size of dynamic force contactor 200 can be selected for
a particular application and be small enough to be conve-
niently portable so that dynamic force contactor 200 can
apply known forces to calibrate force measurement systems
in various locations and configurations. In other words,
dynamic force contactor 200 is ported to a working mea-
surement system. It is contemplated, without limitation, that
a practical volume and mass for portability of dynamic force
contactor 200 is approximately 2 m* and 50 kg respectively
but dynamic force contactor 200 that are smaller in both
metrics by an order of magnitude or more are contemplated.
An amplitude of can be, e.g., from 1 mN to 1 KN. An applied
bandwidth of which can be from 1 mHz to 50 kHz. An

uncertainty within this amplitude and bandwidth can be 3

from 0.01% to 10%.

Example 2. Dynamic Force Contactor 200 as a
Portable Dynamic Force Standard

Here, equations are numbered starting again with equa-
tion 1, and dynamic force contactor 200 is a portable
dynamic force primary standard that uses Lorentz force on
a current-carrying wire in a magnetic field to generate a
known force. The device has a size that is effective (e.g.,
small enough) to be conveniently portable to be used to
apply known forces to calibrate force measurement systems
in various locations and configurations. The device is taken
to the working measurement system and not the other way
around although the working measurement system can be
taken to the device. With reference to FIG. 13, dynamic
force contactor 200 includes a coil as electrical conductor
214 in a uniform radial magnetic field 212 generated by a
permanent magnet as magnet 210. A time-varying alternat-
ing current 226 I(t) in the coil generates a time-varying axial
force on the coil according to

F()-BLIO), a)

wherein B is the radial magnetic field, and L is the length of
the coil in the field. I(t) is measured, e.g., by placing an
accurately-known resistance in series with the coil and
measuring the voltage drop across it. The BL product is
determined by measuring the voltage drop U(t) across the
coil and the velocity v(t) of the coil relative to the magnetic
field,

BL=Uiv() 2]
The voltage drop Utot(t) across the coil is the sum

Thot(s)-U+URO) BI
wherein UR(t) is the drop due to the resistance of the coil
(including lead lengths to measurement points) and can be
corrected for or rendered negligible.

S

2

40

&

60

14

U,pAt) can be measured by a connected voltmeter, and v(t)
can be measured by an optical interferometer such as the
Fabry-Perot interferometer shown in FIG. 13 or by accel-
erometers attached to the armature and body. This measure-
ment can be performed simultaneously with the force appli-
cation (and current measurement) or separately.

The force F given by equation [1] is applied to the coil and
applied to system-under-test (F.,,,) less the force F,,,,, used
to accelerate the inertia of the armature (coil plus any
additional material in which the coil is embedded) and any
parasitic forces F,,,,. acting on the armature due to connected
wires, suspension, and air drag

FFapy*F inetE par (4)

The force F,,., is given by the product of mass and
acceleration of the armature,

Fnelatr pdr (6)

wherein a(r) is the local acceleration and p(r) is the local
density. The acceleration a may be measured using an
attached accelerometer or an interferometer. The armature
mass (and potentially mass distribution) is a previously-
determined quantity taken to be known during use of the
portable standard. As shown in FIG. 13, the armature is
connected via flexures, and flexible wires are used to con-
nect to the coil; for small-amplitude motion of the armature
during use of the portable standard, these measures render
F,,,, negligible

With reference to FIG. 13, the measurements of acceler-
ometer output, sense resistor voltage, and coil voltage are
illustrated as being performed by external instruments but
can be integrated into the body of the force-applying unit.
Similarly, the power supply can be an onboard battery or
capacitance. On the other hand, the sensing resistance shown
in FIG. 13 as being incorporated can be external.

Example 3. High Frequency Kibble Balances for
Dynamic Force Metrology

Here, equations are numbered starting again with equa-
tion 1, and methods are described that generate high fre-
quency (approximately 10 kHz) Sl-traceable (International
System of Units) dynamic forces using Kibble (watt) bal-
ances. Differences between static and high-frequency Kibble
balances are discussed. Three potential configurations of
Kibble balances for dynamic force metrology, and their
relative benefits and drawbacks, are presented. The configu-
rations can be used to design primary dynamic force stan-
dards based on Kibble’s approach that can be used to
calibrate force measurement systems in situ. Such standards
can be embedded into engineering structures to provide
accurate reference force values.

The Kibble or watt balance provides an accurate experi-
mental method for relating macroscopic values of mechani-
cal quantities (e.g., force, mass, acceleration) to electrical
quantities (¢.g., voltage, current, resistance). Measurements
of Planck’s constant using such balances are a basis for the
proposed redefinition of the kilogram. A number of different
configurations of Kibble balances have been developed or
proposed for static measurements. Our device uses the
Kibble balance principle to realize SI-traceable (traceable to
the International System of Units) forces with frequencies
exceeding 10 kHz and uncertainties at or below the percent
level.

Traceable dynamic force standards are used for calibrat-
ing and testing the accuracy of force measurements in a wide
variety of applications, such as material testing, aerody-


U.S. Patent May 5, 2020 Sheet 6 of 22 US 10,641,663 B2

) ) )

220 242 210 214 230

FIG. 6


U.S. Patent May 5, 2020

Sheet 20 of 22

US 10,641,663 B2

410

0.100}-

=
@
Ss
= 0.010 _
an en oe cee? 7 .
© pana cee :
0.001 fe Measured voltage Upase! :
ora induced voltage U :
1044 : : |
sony ” i
vere Re: we i

10%

s BP CR) errr rrereeesereenerenenncensseen prowess eeev ener webarey ; En EE
3 mmm Seasined vOHEZC Upeas : :
ae jaduced vollage U : *
$
5 og c_tnatetive drop tants ) :
£ sees Resistive drop ARI : :
e H ‘
8 : ;
& : :

WE SO cn eee tate se “hams anaronar mel Y mommarel

2 ie i 7
1 10 100 1000 to
Frequency {Hz}

FIG.

20


US 10,641,663 B2

29

While one or more embodiments have been shown and
described, modifications and substitutions may be made
thereto without departing from the spirit and scope of the
invention. Accordingly, it is to be understood that the present
invention has been described by way of illustrations and not
limitation. Embodiments herein can be used independently
or can be combined.

Reference throughout this specification to “one embodi-
ment,” “particular embodiment,” “certain embodiment,” “an
embodiment,” or the like means that a particular feature,
structure, or characteristic described in connection with the
embodiment is included in at least one embodiment. Thus,
appearances of these phrases (e.g., “in one embodiment” or
“in an embodiment”) throughout this specification are not
necessarily all referring to the same embodiment. Further-
more, particular features, structures, or characteristics may
be combined in any suitable manner, as would be apparent
to one of ordinary skill in the art from this disclosure, in one
or more embodiments.

All ranges disclosed herein are inclusive of the endpoints,
and the endpoints are independently combinable with each
other. The ranges are continuous and thus contain every
value and subset thereof in the range. Unless otherwise
stated or contextually inapplicable, all percentages, when
expressing a quantity, are weight percentages. The
suffix “(s)” as used herein is intended to include both the
singular and the plural of the term that it modifies, thereby
including at least one of that term (e.g., the colorant(s)
includes at least one colorants). “Optional” or “optionally”

means that the subsequently described event or circumstance 3

can or cannot occur, and that the description includes
instances where the event occurs and instances where it does
not. As used herein, “combination” is inclusive of blends,
mixtures, alloys, reaction products, and the like

As used herein, “a combination thereof” refers to a 3

combination comprising at least one of the named constitu-
ents, components, compounds, or elements, optionally
together with one or more of the same class of constituents,
components, compounds, or elements

All references are incorporated herein by reference.

The use of the terms “a” and “an” and “the” and similar
referents in the context of describing the invention (espe-
cially in the context of the following claims) are to be
construed to cover both the singular and the plural, unless
otherwise indicated herein or clearly contradicted by con-
text. “Or” means “and/or.” Further, the conjunction “or” is
used to link objects of a list or alternatives and is not
disjunctive; rather the elements can be used separately or
can be combined together under appropriate circumstances.
It should further be noted that the terms “first,” “second,”
“primary,” “secondary,” and the like herein do not denote
any order, quantity, or importance, but rather are used to
distinguish one element from another. The modifier “about”
used in connection with a quantity is inclusive of the stated
value and has the meaning dictated by the context (e.g., it
includes the degree of error associated with measurement of
the particular quantity).

What is claimed is:

1. A dynamic force contactor to provide a dynamic force,
the dynamic force contactor comprising: a magnet that
provides a magnetic field; an electrical conductor that com-
municates an alternating current perpendicular to the mag-
netic field, the alternating current from the electrical con-
ductor in combination with the magnetic field from the
magnet providing a Lorentzian force; an armature disposed
proximate to the magnet, the electrical conductor disposed
on the armature such that the armature reciprocates in a

0

40

3!

30

reciprocating direction relative to the magnet in response to
the Lorentzian force and that produces the dynamic force;
and a dynamic force mediator in communication with the
electrical conductor and the armature such that: the dynamic
force mediator monitors an alternating voltage across the
electrical conductor; the dynamic force mediator monitors
an alternating current through the electrical conductor; and
the dynamic force mediator monitors a reciprocation veloc-
ity of the armature.

2. The dynamic force contactor of claim 1, further com-
prising a housing on which the magnet, the electrical con-
ductor, the armature, and the dynamic force mediator are
disposed.

3. The dynamic force contactor of claim 1, further com-
prising an accelerometer disposed on the armature.

4. The dynamic force contactor of claim 1, further com-
prising a light source disposed on the armature or the
housing, such that the light source provides light to monitor
the reciprocation velocity of the armature.

5. The dynamic force contactor of claim 1, further com-
prising a light detector disposed on the armature or the
housing, such that the light detector receives light from a
light source and monitors the reciprocation velocity of the
armature.

6. The dynamic force contactor of claim 1, further com-
prising a secondary electrical conductor disposed opposing
the electrical conductor such that the magnet is interposed
between the second electrical conductor and the electrical
conductor.

7. The dynamic force contactor of claim 6, further com-
prising a secondary magnet disposed opposing the electrical
conductor such that the magnet is interposed between the
secondary magnet and the electrical conductor.

8. The dynamic force contactor of claim 7, wherein the
secondary magnet is interposed between the magnet and the
secondary electrical conductor.

9. The dynamic force contactor of claim 7, wherein the
secondary electrical conductor is interposed between the
magnet and the secondary magnet.

10. The dynamic force contactor of claim 7, further
comprising a coupler in mechanical communication with
and interposed between the secondary electrical conductor
and the armature.

11. The dynamic force contactor of claim 1, further
comprising a magnetic field guide disposed proximate to the
magnet and that provides a selected shape of the magnetic
field.

12. The dynamic force contactor of claim 1, further
comprising an armature guide disposed proximate to the
armature and that provides a linear motion in the recipro-
cating direction of the armature.

13. The dynamic force contactor of claim 1, wherein the
dynamic force mediator produces a dynamic force signal
based on the reciprocation velocity, the alternating voltage,
and the alternating current.

14. The dynamic force contactor of claim 1, wherein the
reciprocation velocity, the alternating voltage, and the alter-
nating current are measured to be traceable to International
System of Units (SI, abbreviated from French language
Systeme international (d’unités)).

15. A process for providing a dynamic force, the process
comprising: disposing the armature of the dynamic force
contactor of claim 1 in contact with a structure to which the
dynamic force is applied; providing electrical power to the
electrical conductor, the electrical power comprising a time-
varying amplitude; and reciprocating the armature at the


US 10,641,663 B2

31

reciprocation velocity in response to providing electrical
power to the electrical conductor to provide the dynamic
force to the structure.

16. A process for calibrating a force sensor to be traceable
to the international system of units (SI), the process com-
prising: disposing the armature of the dynamic force con-
tactor of claim 1 in contact with a structure to which the
force sensor under calibration is disposed; providing elec-
trical power to the electrical conductor, the electrical power
comprising a time-varying amplitude; reciprocating the
armature at the reciprocation velocity in response to pro-
viding electrical power to the electrical conductor; providing
the dynamic force to the structure; measuring the alternating
voltage; determining the alternating current through the
electrical conductor; determining the reciprocation velocity
of the armature; determining the dynamic force from the
reciprocation velocity, the alternating voltage, and the alter-
nating current; and determining a calibration response for
the force sensor to calibrate the force sensor to be traceable
to the SI, wherein the reciprocation velocity, the alternating
voltage, and the alternating current are traceable to the SI.

+ ee ee

rt

0

32


US 10,641,663 B2

7

through electrical conductor 214, and reciprocation velocity
222 of armature 230. The process for making dynamic force
contactor 200 also can include disposing magnet 210, elec-
trical conductor 214, armature 230, and dynamic force
mediator 232 in housing 234. The process for making
dynamic force contactor 200 also can include disposing
accelerometer 236 on armature 230; disposing light source
238 on housing 234 or armature 230; disposing light detec-
tor 240 on housing 234 or armature 230; disposing second-

ary electrical conductor 220 opposing electrical conductor

214 such that magnet 210 is interposed between second
electrical conductor 220 and electrical conductor 214; dis-
posing magnetic field guide 246 proximate to magnet 210 to
provide a selected shape of magnetic field 212 in which
electrical conductor 214 is disposed; disposing armature
guide 248 proximate to the armature 230 to provide linear
motion in reciprocating direction 228 of armature 230;
providing housing 234 with flange 260, rostrum 256, caudal
member 258, and handle 254; or disposing enclosure 298 on
dynamic force mediator 232 elements of dynamic force
contactor 200 as shown in FIG. 9. Making individual
components of dynamic force contactor 200 can be accom-
plished using, e.g. additive manufacturing, machining,
manufacturing techniques, and the like. Components can be
joined together with a mechanical fastener, adhesives, and
the like. Alignment of armature 230 in magnetic field 212
can be performed by using alignment tooling,

Dynamic force contactor 200 can be made by combining:

a magnet (¢.g., an electromagnet or permanent magnet); an. 3

electrical conductor (e.g., metallic wire); light and rigid
material of high electrical resistivity, a resistor; a fast
voltage measuring circuit (e.g., a voltmeter); material of
high magnetic permeability; material of sufficient strength

and rigidity to support the magnet, electrical conductor, and 3

light rigid material; a flexure spring; an accelerometer; and
an AC electrical power source (e.g., a signal generator,
amplifier, and the like). With reference to FIG. 11, magnet
210 and magnetic field guide 246 can be arranged to produce
spaced apart regions of magnetic field 212. Support material
272 supports magnet 210 and magnetic field guide 246 and
shapes them into a selected shape.

Electrical conductor 214 is disposed to pass through
magnetic field 212 in series to conduct a current perpen-
dicular to magnetic field 212. Two sections of electrical
conductor 214 passing through magnetic field 212 can be
identical. Electrical conductor 214 can be two serially-
connected coils that are disposed in one of the two radial
magnetic fields 212. One sections of electrical conductor
214 in magnetic field 212 (e.g., magnetic field 212) is
supported on light and rigid material that supports electrical
conductor 214 and transmits force on electrical conductor
214 to external structure 266 or force sensor 268. This piece
of material move independently with respect to magnetic
field 212 except for support by flexure springs, which locate
electrical conductor 214 within magnetic field 212 and are
mounted for freedom along a selected axis of operation, e.g.,
reciprocating direction 228, and provide restraint with
respect motion transverse to reciprocating direction 228.
Accelerometer 236 can be disposed on the light rigid mate-
rial to measure reciprocation velocity 222. A portion of
electrical conductor 214 disposed in a second area of mag-
netic field 212 can be rigidly mounted to support material
272. A resistor can be connected in series with electrical
conductor 214 and the power source. The voltage measure-
ment circuits can be electrically connected across the resis-
tor to measure alternating current 226. Voltage measurement

3

40

&

g

0

8

circuits can be connected across each of the two sections of
electrical conductor 214 located in magnetic field 212 to
measure voltage.

The resistor, voltage measuring circuit, magnetic field
guide 246, or combination thereof can be mechanically
connected to remaining components to form a self-contained
device. Control electronics can be integrated in dynamic
force contactor 200 such that dynamic force signal 250 is an
analog or digital signal indicative of force. An integrated,
visible display, e.g., an LCD, or touch screen, can display
force. It is contemplated that the resistor or the second
magnetic field 212 and associated section of electrical
conductor 214 passing therethrough can be physically sepa-
rated and mechanically decoupled from dynamic force con-
tactor 200.

Dynamic force contactor 200 has numerous advantageous
and unexpected benefits and uses. In an embodiment, a
process for providing dynamic force 252 includes disposing
armature 230 of dynamic force contactor 200 in contact with
structure 266 to which dynamic force 252 is applied; pro-
viding electrical power to electrical conductor 214, the
electrical power including a time-varying amplitude; and
reciprocating armature 230 at reciprocation velocity 222 in
response to providing electrical power to electrical conduc-
tor 214 to provide dynamic force 252 to structure 266.

The process for providing dynamic force 252 further can
include measuring the alternating voltage 224, alternating
current 226, and the reciprocation velocity 222 which are
used to calculate the Lorentzian force 218. In an embodi-
ment, the velocity can be determined with the optical cavity
mirror 278 or a light source 238 and light detector 240.
Additionally, the acceleration of the housing can be mea-
sured with an accelerometer 236, which can be used to apply
necessary corrections to the measurements.

In the process for providing dynamic force 252, disposing
armature 230 of dynamic force contactor 200 in contact with
structure 266, and current source 262 generates alternating
current 226 in electrical conductor 214, which interacts with
magnetic field 212 to produce a Lorentzian force 218.

In the process for providing dynamic force 252, providing
electrical power to electrical conductor 214 can be provided
by a current source that is external or internal to dynamic
force contactor 200

In the process for providing dynamic force 252, recipro-
cating armature 230 at reciprocation velocity 222 can be
measured by an optical interferometric technique that can
include optical cavity mirror 278. The velocity can also be
measured using light source 238 and light detector 240.

In an embodiment, a process for operating dynamic force
contactor 200 includes connecting the movable piece of light
material supporting electrical conductor 214, e.g., to struc-
ture 266 or force sensor 268, such as by contacting armature
230 to structure 266 or fastening dynamic force contactor
200 to structure 266 (wherein a fastener such as a bolt is

5 included in the light material); energizing the electrical

power source and providing a selected time-varying wave-
form to electrical conductor 214; measuring time-varying
voltage U1(#) measured across resistor R; determining cur-
rent flowing in electrical conductor 214 as I(t)=U1(0)/R; and
from measurements of voltage across electrical conductor
214 disposed in magnetic field 212, U2(s) and U3(2), deter
mining alternating voltage 224 U(t) induced by motion of
first section of electrical conductor 214 as U(t)}=U2(1)-U3(9);
determining force generated by current flowing through

5 electrical conductor 214 as F(t)=I(t) V(t/U(®), wherein this

force is equal to the force applied to structure 266, Fayp(t).
plus any force F1(#) to accelerate electrical conductor 214


U.S. Patent May 5, 2020 Sheet 2 of 22

US 10,641,663 B2

(a) 200
234—~E
214 218 230
{ } 228
2107 -_} oo > + Sy
x x 252
i i
—
: ‘
224—~__ 99g F222
v
/ -
\ 250
232
(B) 200
234—~L
214 218 230
y + 228

210-7 ~_| to >

ope

252

* *
224 nog F222
v v
i \
250-~4 )
v 232


U.S. Patent

258

May 5, 2020

Sheet 13 of 22 US 10,641,663 B2

286 av \ 260
n
224) \ Ugoooo00 230
208 ii ye
oN Pee Fam
1 236
- 278
a
222 ‘\y
282

FIG. 13


U.S. Patent May 5, 2020
(a) 258 280 278 210 245 956
4 214
ia
286 \
3
Ct
284 pees Coe
tt i
222 5
9
264 282
254. 234
(B) *
258 280 278 210 726 56
( 214
74
rt
286 ae
=)
4 a i
it TTT Th
284 acco =
l |
C
222) 5S
264 ode
254 234
(©)
258 280 278 210 48 56
Cf ({ (24%
f
ose ir Tah
224 wasouee |_| hy Vrtdeee
226 IE Ee
284 ae:
; =} Eber,
5 S
282
264 282
254. 234

260

Sheet 15 of 22

230
252

236
278

260

* Fanp

Esappry (2)

230
252

* Fanp

236
278

260

Tagppiy (ee)

232

230
252

* Epp

236
278

. 15

Fsapply }

US 10,641,663 B2

232

(drive circuit not shown}

Rous)

[a |

Leona let)

Zevsizw)



U.S. Patent May 5, 2020 Sheet 1 of 22 US 10,641,663 B2

212 216
210 me 214 230
) ) )
v Xt is
‘ 252
l
“J b---- and €-----4----- y
21s] 228
—k zy
a
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H H
2244 H
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226~4 H
v y
en co
A 250
)
232

FIG. 1


US 10,641,663 B2

27

measured V(w) and a,(), to which the modeled X.()-X,
() given by equations (14) and (15) is added). Thus 8X, in
equation (24) has many contributions. The second column of
Table 2 lists expressions for sensitivity coefficients in equa-
tion (22), and the third column of Table 2 lists a range of the
corresponding force uncertainty contributions for the system
modeled. FIG. 22 shows uncertainty contributions and over-
all uncertainty as a function of frequency. From 10 Hz to 20
kHz, the combined uncertainty is less than 1% except for
frequencies near antiresonances at 80 Hz and 1.5 kHz. At
these frequencies, it may be advantageous to determine the
force based on a quasi in-situ calibrated BL integral rather
than from equation (6). The anti-resonance at 1.5 kHz is due
to the internal mode of the SUT, and the location and number
of such features will vary from one SUT to another.

TABLE 2

28

tance, this uncertainty contribution can be lowered by oper-
ating the device at a higher force level, and hence a higher
relative velocity V.

The dynamic force contactor 200 is a high-bandwidth
force standard that can be coupled to working force mea-
surement systems. Although an exemplary 10 N force ampli-
tude was used in this Example, dynamic force contactor 200
can operate at other frequencies, e.g., up to ~10 kHz, and
dynamic force contactor 200 can be sized for different force
amplitudes and frequency ranges. It is contemplated that
dynamic force contactor 200 can be disposed temporarily or
permanently into a mechanical system, e.g., a body that is
rigidly-coupled into the mechanical system in which the
force measurement is being made. Although this Example
describes operation of dynamic force contactor 200 at a

Contribution to
Force Rel. Uncertainty

Relative
uncertainty ae jox|
of Quantity Sensitivity Coefficient 7°
Quantity (3)
(x) x over range 10 Hz to 10 kHz
R, 2x 10% 15x10 to9x 107
STAR + iw LD)
Vv 1x 10% 9x 10° to 4.5 x 10
1 [UsUmeas nas
IF DV = — a] RT - (an +ioa om - >
Uneas 2x 104 Us WUsU meas 1.5 x 105 to 9 x 10
AFeaplDUnes = FR, ~ RP AR +WAL)
U, 1x10“ U, U2, 1x 10 to 4.5 x 107
OF opp/OUs = A — PHS AR 4+ iw L)
omlUs = TRE ype AR + ehh
m 2x 10% OF ypp/dMy = 0X5 1x 1077 to 8x 104
K 1x10? v <10 to 2.5 x 10
OF gplOK = ——
AL 20? 2 15 x 10° to 2x 104
UsU eat?
BFayplBM. ee
f VRE
4 2x 10-8 4
m, Tx 10 AF appl, = GOV +p) 12x 10% to 8 x 10
AR Sx 10% Us? 2x 10% to 7x 104
IsUineas
JBAR = - See
AF gplOAR = - 8
Xs 1x 104 to 5 x 107 OF jpp/OXs = m0" <10 to 3x 11
ap 1x 107 BF gpp/Say = -My 2x 10% to 2x 10%
Fear 1 BF app! Pyare = 1 5x 1077 to 8x 10°
© 1x 10% 2 <10°§ to 1 x 10°S
UsUeasAL
Se mV

VRE

The largest contribution to the force uncertainty over most
of the frequency range 10 Hz to 10 kHz is due to the
uncertainty in the sensing resistance used to measure the
current. The uncertainty in this resistance is estimated as
0.2%, including the effects of temperature-sensitivity of the
resistance. Apart from reducing the uncertainty of the resis-

fixed Lorentz force amplitude, achieved by holding the
excitation current amplitude fixed, alternatively, a different
45 quantity could be held constant. Feedback from SUT accel-
eration to the supply current could be used to keep the SUT
displacement amplitude constant to avoid damage.


US 10,641,663 B2

3

It has been discovered that a dynamic force contactor can
be a portable dynamic force standard that provides a
mechanical dynamic force so that the dynamic force applied
to a structure can be determined through electrical and
mechanical measurements. Advantageously, the dynamic
force contactor is a portable apparatus for dynamic force
calibration that dynamically calibrates a force sensor in an
environmental setting in which the force sensor resides or is
applied. The dynamic force contactor overcomes challenges
in measuring dynamic force such as a dynamic calibration
response of the force sensor that can change and that can
depend upon an application setting.

Trade, commerce, and test of materials and engineering
structures involve a determination of force in areas such as
commercial weight of goods (e.g., produce and raw mate-
rials), aerospace structures (e.g., aerodynamic drag forces
and reaction forces in landing gear), crash testing of
vehicles, machining and manufacturing processes, and the
like. The force can be measured with a conventional force
sensor that often outputs a voltage that represents an applied
force. The output voltage and a sensor calibration response
provide a force value for each value of the output voltage of
the conventional force sensor to determine the applied force.
The dynamic force contactor described herein provides
traceability of the calibration response of the conventional
force sensor through a measurement of voltage, current, and
reciprocation velocity to the International System (SI) of
Units.

The dynamic force contactor provides and determination

of dynamic force, wherein conventional national and inter- 3

national force calibration procedures that determine the
calibration response of conventional force sensor determine
static force where the applied force is constant and does not
change with time. Unexpectedly, the dynamic force contac-
tor dynamically calibrates the force sensor even when the
force changes in time. The dynamic force contactor over-
comes problems with calibrating a force sensor dynamically
that include a dynamic calibration response of the force
sensor that changes depending on the application setting in
which the force sensor is used. Moreover, the dynamic force
contactor provides accurate dynamic force measurements
and calibrates the force sensor for making accurate dynamic
force measurements.

Dynamic force contactor 200 provides dynamic force
252. In an embodiment, with reference to FIG. 1, dynamic
force contactor 200 includes magnet 210 that provides
magnetic field 212; electrical conductor 214 disposed proxi-
mate to magnet 210 in magnetic field 212 and that provides
alternating current 226 perpendicular to magnetic field 212.
Alternating current 226 from electrical conductor 214 in
combination with magnetic field 212 from magnet 210
provide Lorentzian force 218. Dynamic force contactor 200
also includes armature 230 disposed proximate to magnet
210 such that electrical conductor 214 is disposed on arma-
ture 230, and armature 230 reciprocates in reciprocating
direction 228 relative to magnet 210 in response to Lorent-
zian force 218, wherein armature 230 produces dynamic
force 252. In dynamic force contactor 200, dynamic force
mediator 232 is in communication with electrical conductor
214 and armature 230 such that dynamic force mediator 232
monitors alternating voltage 224 across electrical conductor
214, alternating current 226 through electrical conductor
214, and reciprocation velocity 222 of armature 230. As
used herein, alternating current and alternating voltage refer
to a voltage or current that varies with time such that a
frequency component for a frequency that can be 5 Hertz
(Hz) or greater is present. As used herein, reciprocating

S

2

40

&

4

refers to a mechanical motion that has a frequency compo-
nent at a frequency of 5 Hz or greater.

In an embodiment, with reference to FIG. 2, dynamic
force contactor 200 includes housing 234 on which features,
eg., magnet 210, electrical conductor 214, armature 230,
and dynamic force mediator 232, are disposed. According to
an embodiment, dynamic force mediator 232 is in commu-
nication with electrical conductor 214 and armature 230 to
produce dynamic force signal 250. Here, magnet 210 is
rigidly or monolithically attached to housing 234, and elec-
trical conductor 214 and armature 230 are movably disposed
on housing 234 to reciprocate in reciprocating direction 228
relative to housing 234. Dynamic force mediator 232 can be
disposed on housing 234 as in panel B of FIG. 2 or can be
remotely disposed to housing 234, as in panel A of FIG. 2

In an embodiment, with reference to FIG. 3, dynamic
force contactor 200 includes accelerometer 236 disposed on
armature 230. It is contemplated that accelerometer 236 can
be disposed on other parts of dynamic force contactor 200
besides or in addition to armature 230. Accelerometer 236
can provide an acceleration of armature 230, housing 234,
and the like depending on a location of disposal of accel-
erometer 236.

In an embodiment, with reference to FIG. 4 and FIG. 5,
dynamic force contactor 200 includes light source 238
disposed on housing 234 or armature 230 and light detector
240 disposed on housing 234 or armature 230. Light source
238 can produce direct light 294 that reflects from armature
230 as reflected light 296 that is detected by photodetector
240 is shown in FIG. 4. Here, as Armature 230 reciprocates
in reciprocating direction 228, an intensity of reflected light
296 from armature 230 temporally changes repeatably as a
function of a position of armature 230 relative to light source
238 so that reciprocation velocity 222 of armature 230 is
determined from the temporal variation of intensity of
reflected light 296. As shown in panel A of FIG. 5, direct
light 294 from light detector 240 is received by light source
238 when armature 230 is disposed at a first reciprocation
position, and panel B shows dynamic force contactor 200 in
which armature 230 is disposed at a second reciprocation
position such direct light 294 from light source 238 is not
received by light detector 240. Here, an amount of direct
light 294 received by light detector 240 temporally changes
repeatably as a function of a position of armature 230
relative to housing 234 so that reciprocation velocity 222 of
armature 230 is determined from the temporal variation of
intensity of direct light 294 as received by light detector 240
It should be appreciated that a mutual position of light
detector 240 and light source 238 can be changed between
housing 234 and armature 230.

In an embodiment, with reference to FIG. 6 and FIG. 7,
dynamic force contactor 200 includes secondary electrical
conductor 220 disposed opposing electrical conductor 214
such that magnet 210 is interposed between second electrical

5 conductor 220 and electrical conductor 214. Secondary

magnet 242 can be disposed opposing electrical conductor
214 such that magnet 210 is interposed between secondary
magnet 242 and electrical conductor 214. As shown in FIG.
6, secondary magnet 242 can be interposed between magnet
210 and secondary electrical conductor 220. As shown in
FIG. 7, secondary electrical conductor 220 can be interposed
between magnet 210 and secondary magnet 242. Here,
coupler 244 can be in mechanical communication with and
interposed between secondary electrical conductor 220 and
armature 230.

In an embodiment, with reference to FIG. 8 and FIG. 9,
dynamic force contactor 200 includes magnetic field guide


US 10,641,663 B2

Page 2
(51) Int. Ch

HOIF 7/18 (2006.01)

HOIF 7/122 (2006.01)
(52) US. CL

CPC veces HOLF 7/121 (2013.01); HOIF 7/122

(2013.01); HOLF 7/1844 (2013.01)
(58) Field of Classification Search
USPC. wesc soteseeesssssnseressesssssnnensee 13/779
See application file for complete search history.

(56) References Cited
U.S. PATENT DOCUMENTS

6,645,108 BI* 11/2003 Gradu BOOK 17/3462
180/249

8,182,433 B2* 5/2012 Leo AG61B 90/06
600/587

2011/0037327 Al* 2/2011 Denne HO2K 41/03
310/12.16

OTHER PUBLICATIONS

Fang, H., et al., “A watt balance based on a simultaneous measure-
ment scheme”, Metrologia, 2014, S80-S87, 51, IOP Publishing,
United Kingdom.

Robinson, LA., “Simplified fundamental force and mass measure-
ments”, Metrologia, 2016, 1054-1060, 53, IOP Publishing, United
Kingdom.

Sutton, C.M., “An oscillatory dynamic mode for a watt balance”,
Metrologia, 2009, 467-472, 46, IOP Publishing, United Kingdom.

* cited by examiner


US 10,641,663 B2

19
100 grams, bandwidths up to 10 kHz occur. For a one-
Newton sinusoidal standard, uncertainties at the percent
level or better occur.

Example 4. High-Bandwidth Kibble-Balance
Dynamic Force Standard

Here, equations are numbered starting again with equa-
tion 1, and dynamic force contactor 200 is described as
being a dynamic force standard based on principles of the
Kibble (watt) balance. The dynamic force contactor 200
provides low-uncertainty traceable dynamic forces over a
wide frequency range. The dynamic force contactor 200
determines force from simultaneous measurement of volt-
age, velocity and current. The dynamic force contactor 200
can operate with only a current measurement, based on a
calibrated BL integral. For a dynamic force contactor 200
that produces a 10-newton force amplitude, an uncertainty
below 1% can be achieved for operating frequencies exceed-
ing 10 kHz. This dynamic force contactor 200 provides an
accurately-known dynamic force for calibration of systems
for measuring dynamic forces such as forces varying as fast
as or faster than the settling time of the force measurement
system,

The dynamic force contactor 200 generates SI-traceable
forces up to high frequencies with uncertainties at or below
the percent level. At this level of uncertainty, it is believed
that measures and corrections taken in Kibble balances
designed for extreme-accuracy measurements are unneces-

sary, and high operating frequency achieves percent-level 3

uncertainty,
Traceable dynamic force standards are needed for cali-

brating and testing the accuracy of force measurements in a
wide variety of applications, such as material testing, aero-

dynamic wind tunnel measurements, and automotive crash ~

testing. Conventional primary standards are not easily
applied to force measurement systems in the field. A
dynamic sensitivity of an assembled force measurement
system is not the same as that of the isolated force sensor,
limiting the utility of external calibration of force sensors for
dynamic force measurements. The dynamic force contactor
200 overcomes these problems and provides a portable
dynamic force standard to calibrate force measurement
systems in their operational installations. A conventional
calibrated force sensor that can be struck against the force
measurement system that is to be calibrated results in a
common force applied to both devices, but this conventional
device is a secondary standard because it must be traceably
calibrated, allowing transfer of the calibration from a pri-
mary standard to a working system. In contrast, the dynamic
force contactor 200 herein advantageously is a primary
standard that can be used to directly calibrate so that it can
be a secondary standard,

In Kibble balances, a mechanical force in balances
designed to measure mass or Planck’s constant this is
typically the gravitational force is balanced by a Lorentz
force

LBR ord ya®

a)

where I is the current flowing in a wire (usually in the form
of a coil) of length L and differential length element <@,
immersed in magnetic field of flux density 4. The accurate

S

2

40

20
determination of the value of this F integral is the main
challenge in application of the Lorentz force law to accurate
calibration, and Kibble evaluated this from the induced
voltage U generated across the wire when it is moved at
velocity V through the magnetic field,

UV ABR od Pg FV, 2)

The dynamic force contactor 200 has an axial force
component, e.g., the component present ina geometry. Thus,
measuring voltage U, axial velocity component V and
current I gives the axial force magnitude F according to

von
Fo= a

where the time-dependence of the physical quantities has
been explicitly indicated. At an angular frequency ,

_ Hwy) @

Fo)
° Vw)

where F, U, J and V are complex amplitudes. The dynamic
force contactor 200 can operate based on these relations.

An alternative to using the dynamic force contactor 200 as
a Kibble balance (i.e., using equation 3 or 4 of this Example)
is to calibrate the Ed integral (e.g., using a static force
measurement) and then use equation (1) with only a current
measurement. Such a calibration can be performed at DC,
using a statically calibrated force transducer or deadweights.
While such a prior measurement of the @ integral removes
the need to make accurate measurements of voltage U and
velocity V, it introduces effects of spatial variation of the
integral as a function of coil position and of temporal
variation of the 2 integral due for example to the effect of
temperature changes. Nevertheless, with dynamic force con-
tactor 200 to provide a sufficiently spatially uniform
integral it can be an attractive mode of operation for
force uncertainty at the 1% level. Attention is paid to Kibble
balance operation, i.e., operation based on equations (3) and
(4). Furthermore, sinusoidal force waveforms are involved,
and the dynamic force contactor 200 can be a force calibra-
tion tool.

Table 1 provides values of device parameters and oper-
ating dynamic variables over the frequency range 10 Hz to
10 kHz for dynamic force contactor 200.

TABLE 1
Quantity Value
DEVICE PARAMETERS,

integral 25 NIA
Armature Mass (m3) alg
Body Mass 15 kg
Armature Stiffness (k,) 1.7 x 10° Nim
Armature Resonant Frequency 9.4 kHz
Flexure Stiffness (K) 3 x 104 Nim
Current Sense Resistance (R,) 10a
Coil Wire Resistance (R) 179

Resistance Difference Between
2 Coils (AR)
Coil Inductance (£)

0,005 R = 0,085 Q

1x 10° H + (2ale)4 0.05 H- s

Inductance Difference Between 2 0.01 x£
Coils (AL)
Mechanical Damping Ratio 0.01


US 10,641,663 B2

9

and supporting material and accelerometer 236 as well as
force F2(d) that deflects the mounting flexures such that the
force generated is F,,,,(t)=F(t)-F1(#)-F2(). When flexures
have negligible stiffness in the direction of motion, F2(¢) can
be ignored. The process also can include calculating F1(/)
from acceleration A(t) measured by accelerometer 236, and
known mass M of moving electrical conductor 214, support
structure, and accelerometer 236. The process can include
determining F,,,(t)=1() V(/U()-M A(t), wherein, if mass
M or acceleration A(t) is sufficiently small, F1(¢) is negli-
gible, and F,,,(0=1) VU.

Dynamic force contactor 200 also can be used to cali-
brating a force sensor to be metrologically traceable to the
international system of units (SI). As used herein, “metro-
logically traceable” refers to a property of a measurement
result whereby the result can be related to a reference
through a documented unbroken chain of calibrations, each
contributing to the measurement uncertainty. In this regard,
metrological traceability herein accords with the Interna-
tional Vocabulary of Metrology—Basic and General Con-
cepts and Associated Terms, VIM, 3rd edition, JCGM 200:
2008, also published as ISO Guide 99 by ISO (ISO/IEC
Guide 99-12:2007, International Vocabulary of Metrology—
Basic and General Concepts and Associated Terms, VIM
(2008). As used herein, “calibration” refers to an operation
that, under specified conditions, in a first step, establishes a
relation between the quantity values with measurement
uncertainties provided by measurement standards and cor-
responding indications with associated measurement uncer-

tainties and, in a second step, uses this information to 3

establish a relation for obtaining a measurement result from
an indication. It is contemplated that a calibration can be
expressed by a statement, calibration function, calibration
diagram, calibration curve, calibration table, and the like and

can include an additive or multiplicative correction of the 3

indication with associated measurement uncertainty.

In an embodiment, with reference to FIG. 12, a process
for calibrating force sensor 268 to be traceable to the SI
includes: disposing armature 230 of dynamic force contactor
200 in contact with structure 266 to which force sensor 268
under calibration is disposed; providing electrical power to
electrical conductor 214, the electrical power including a
time-varying amplitude; reciprocating armature 230 at
reciprocation velocity 222 in response to providing electrical
power to electrical conductor 214; providing dynamic force
252 to structure 266; measuring alternating voltage 224;
determining alternating current 226 through electrical con-
ductor 214; determining reciprocation velocity 222 of arma-
ture 230; determining dynamic force 252 from reciprocation
velocity 222, alternating voltage 224, and alternating current
226; and determining a calibration response $ for force
sensor 268 to calibrate force sensor 268 to be traceable to the
SI, wherein reciprocation velocity 222, alternating voltage
224, and alternating current 226 are traceable to the SI

In the process for calibrating force sensor 268 to be
traceable to the SI, disposing armature 230 of dynamic force
contactor 200 is placed in contact with structure 266 and
dynamic force 252 is applied.

In the process for calibrating force sensor 268 to be
traceable to the SI, providing electrical power to electrical
conductor 214 can be performed with current source 262
that generates alternating current 226 in electrical conductor
214, which interacts with magnetic field 212 to produce
Lorentzian force 218.

In the process for calibrating force sensor 268 to be
traceable to the SI, reciprocating armature 230 at recipro-
cation velocity 222 can be measured using optical interfero-

S

2

40

&

10

metric techniques that can include optical cavity mirror 278
The velocity can also be measured with light source 238 and
light detector 240.

In the process for calibrating force sensor 268 to be
traceable to the SI, providing dynamic force 252 to structure
266 can be done by using current source 262 to generate
alternating current 226 in electrical conductor 214, which
interacts with magnetic field 212 to produce Lorentzian
force 218.

In the process for calibrating force sensor 268 to be
traceable to the SI, measuring alternating voltage 224 can be
done across terminal 300 with a voltmeter that is traceable
to the SI, which can be integrated with dynamic force
contactor 200 or can be a separate instrument.

In the process for calibrating force sensor 268 to be
traceable to the SI, determining alternating current 226
through electrical conductor 214 can be done by using shunt
resistor 264 in dynamic force mediator 232.

In the process for calibrating force sensor 268 to be
traceable to the SI, determining reciprocation velocity 222
of armature 230 can be measured using optical interfero-
metric techniques that can include optical cavity mirror 278.
The velocity can be measured using light source 238 and
light detector 240.

In the process for calibrating force sensor 268 to be
traceable to the SI, determining dynamic force 252 from
reciprocation velocity 222, alternating voltage 224, and
alternating current 226 can be calculated using Kibble’s
method, described below.

In the process for calibrating force sensor 268 to be
traceable to the SI, determining a calibration response S for
force sensor 268 can be done by determining dynamic force
252 applied by dynamic force contactor 200 using Kibble’s
method and by measuring unknown force signal 270. The
calibration response is calculated as S=F/O, wherein F is the
Fourier Transforms of dynamic force 252, and O is the
Fourier transforms of unknown force signal 270.

Dynamic force contactor 200 and processes disclosed
herein have numerous beneficial uses, including providing
an SI traceable, dynamic force to be simultaneously realized
and measured in an environment inside or outside of a
laboratory setting, which transfers calibration to a force
transducer in its application setting.

Moreover, dynamic force contactor 200 and processes

5 herein have numerous advantageous properties. In an aspect,

dynamic force contactor 200 can have a smaller uncertainty
than conventional transfer standards. Moreover, dynamic
force contactor 200 can calibrate a force transducer to a
higher bandwidth than conventional products.

Dynamic force contactor 200 and processes herein unex-
pectedly provide and measure dynamic force using electrical
quantities as a primary standard up to high mechanical
frequencies in contrast to conventional transfer standards
that are secondary standards.

The articles and processes herein are illustrated further by
the following Examples, which are non-limiting.

EXAMPLES,

Example 1. Dynamic Force Contactor 200 as a
Portable Dynamic Force Standard

Here, dynamic force contactor 200 is a portable dynamic
force standard device that generates a mechanical force so

5 that an applied force can be determined through electrical

and mechanical measurements. Dynamic force contactor
200 is a portable, dynamic force calibrator that can be used


U.S. Patent May 5, 2020 Sheet 11 of 22 US 10,641,663 B2

210

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FIG. 11


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