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PGB: Pico Gravity Box
Enabling Vibration Free Activity
on board the ISS
Universita’ di Pisa
IFSI-CNR, Roma
Laben, Dvisione Proel Tecnologie, Firenze
Alenia Spazio, Torino
DG Technology, Parma
Galli&Marelli, Lucca
The PGB Team
Anna M Nobili
Donato Bramanti
Erseo Polacco
Università di
Pisa
Università di
Pisa
Università di
Pisa
Giovanni Mengali
Università di
Pisa
Gian Luca Comandi
Università di
Pisa
Raffaella Toncelli
Università di
Pisa
Principal Investigator
Co-Investigator
Co-Investigator
Dynamics and Active
Control
Dynamics, Active
Control and Mechanical
Suspensions
Dynamics, Active
Control and Thermal
Analysis
Valerio Iafolla
IFSI (CNR) Roma
Principal Investigator of
ISA Accelerometer
Sergio Nozzoli
IFSI (CNR) Roma
ISA Electronics
Milyukov Vadim
Visiting Scientist
at IFSI (CNR)
Roma
Collaborator
Alfonso Mandiello IFSI (CNR) Roma
Giuseppe
Catastini
Alenia Spazio
(Torino)
Paolo Martella
Alenia Spazio
(Torino)
Alenia Spazio
ISA Electronics
ALENIA Study Manager
for PGB Transfer
Function and Active
Control
PGB Transfer Function
and Active Control
Elisabetta
Cavazzuti
Laben (Milano)
Laben Study Manager
Pietro Soravia
Laben (Milano)
PGB Electronics,
Transportation and
Accomodation
Roberto
Ronchi
Laben (Milano)
Thermal Analysis
Alberto Severi
Laben, Divisione
Proel (Firenze)
Supervisor for Laben/Proel
Contribution
Piero Siciliano
Laben, Divisione
Proel (Firenze)
Locking/unlocking
Mechanism
Lucio Zanin
DG Technology
Service Srl (Parma)
Carlo Galli
Galli&Morelli (Lucca)
Responsible for DG
Technology Contribution
(Mechanical Structure)
Responsible for
Galli&Morelli Contribution
The Case for Vibration Isolation on board the
ISS
• Weightlessness is a major advantage
for many activities which on Earth are
limited by local gravity. In some
cases, e.g. when the goal is to
understand the behavior of the
human body in absence of weight,
the ISS as such is the perfect
environment. But….
The Case for Vibration Isolation on board the
ISS
• Many scientific research activities
require, in addition to the absence of
weight, a low level of residual
disturbances, sometimes also in a
wide frequency range (Material and
fluid sciences; Crystal growth e.g.
crystals of Silicium with low levels of
impurities … )
The Case for Vibration Isolation on board the
ISS
• Material sciences are potentially destined
to take great advantage from the ISS, but
only provided that residual disturbances
on board the ISS are significantly reduced,
thus enabling what is widely known as:
science and applications in "microgravity",
which so far is not really available. This is
the main goal of the PGB (Pico Gravity
Box) vibration isolation system.
The Case for Vibration Isolation on board the
ISS
Vibration
noise
expected (as
of 1991) on
the ISS
Seismic
noise in
Cascina,
Pisa)
(VIRGO
site)
The Case for Vibration Isolation on board the
ISS
The ISS is more
noisy than a quiet
site on the Earth !!!
The advantages of passive vibration isolation
in space
• Absence of weight  weak suspensions,
hence low natural frequency of the system
above which vibration noise is reduced very
effectively
• Physical connection to the rack (easy transfer
of power and data, passive electric
discharging)
• Effectively combined with active isolation,
which is needed only at low frequencies
(around and below natural frequency) where
response time is long and active control is
The 3 Goals of the PGB Project
1. To monitor the actual level of vibration noise on board of ISS
2. To derive from measurements carried out, both outside and
inside the PGB (by means of 2 ISA accelerometers) the
actual transfer function provided by the device.
Demonstrating predictibility will make the PGB a natural
facility for "microgravity" space research
3. To test the sensitivity of ISA in an extremely quiet space
environment (i.e. inside the PGB), which is absolutely
impossible to achieve on Earth (due to seismic noise), and
would make ISA an even stronger candidate accelerometer
for dedicated space missions in fundamental physics (Goal:
10-11 g/Hz at a few Hz)
PGB accomodation inside a Double Mid Deck Locker
Net dimensions for
payload:
416.4x(229.2x2)
x516.1 mm
Maximum mass:
27.2 x 2 kg
PGB accomodation inside a Double Mid Deck Locker
The PGB only
(with 2
capacitance
plates per face
and 5
locking/unlocking
mechanisms)
PGB accomodation inside a Double Mid Deck Locker
Section of the PGB with 4 capacitance plates
A) When two plates on the same side are used for compensation,
the PGB is attracted in that direction
B) If a tension is applied to plates 1 and 4 the PGB rotates
counter clockwise ( if 2 and 3 are used, it rotates clockwise)
PGB accomodation inside a Double Middeck Locker
The locking
/ unlocking
mechanism
PGB accomodation inside a Double Middeck Locker
The following interfaces are available for a single MDL payload:
Net Dimensions for payload: 416.4 x 229.2 x 516.1 mm
Electrical power: +28Vdc +1,5/-3,0 Vdc 500 W max.
Thermal control/cooling:
- 200 W (by means of Air Avionics Assembly)
- or 500 W (by means of Moderate Temp. Water Loop)
Electrical. data I/F:
Serial RS422 (qty.1)
Ethernet (qty.1)
Analog +/-5V (qty.2)
Discrete 5Vdc (qty.3)
Video: NTSC/RS-170A (qty.1)
Waste gas vent: (resource shared, qty.1 for rack) 10-3 torr min. (125l/h)
Nitrogen: (shared resource, qty.1 for rack)
Maximum mass per unit: 27.2kg
OK for PGB (no further or specific request)
Passive vibration isolation
TF for
translations
TF for spring
axial rotations
TF for spring
non-axial
rotations
( All transfer functions computed with PGB data)
Passive vibration isolation
Spring made of 1 steel wire of 0.15 mm diameter, which provides
the stiffness, and 2 Cu wires (0.12 mm diameter each) for electric
connections from the spacecraft to the laboratory; each Cu wire
has a resistance of 1.5  and is insulated to better than 20 M. All
wires are glued with epoxy and made into a helical spring as
shown in the picture with a stiffness of about 10-2 N/m (10 dyn/cm)
in all directions. This is obtained by playing with the parameters
which determine the elastic properties of helical springs, namely
the thickness of the wire, the number of turns, the diameter of
each turn, the total length of the wire (45 cm in this case). The
measured mechanical quality factor of this spring is 90
Passive vibration isolation
Two needs for the suspension springs:
1. Transfer electric power:  r2 (r radius of wire), large r
desired
2. Provide soft connection to rack: stiffness  r4 (r
radius of wire), small r desired
We can play with number of wires in the spring and
their diameter (wide choice to satisfy our needs…)
Passive vibration isolation
SPRING CONSTANTS AND GEOMETRY
Steel
Cu X1
D
(cm)
d
(cm)
4
0.015
0.018
Cu X2
Steel
Cu X3
Steel
Cu X1
Steel
Cu X1
4.5
4.5
3.6
0.058
0.093
0.0195
0.012
0.017
0.019
5
5
0.02
0.0155
0.0155
0.024
4.1
4.1
5
Cu X2
ka
(N/m)
kt
(N/m)
0.233
0.059
0.066
0.059
0.069
0.11
0.047
0.083
0.117
0.0131
0.25
0.32
0.047
4.55
4.55
0.015
0.018
0.022
MTot
(gm)
0.041
0.012
Cu X2
Steel
Cu X1
M
(gm)
0.012
Cu X2
Steel
Cu X3
L
(cm)
0.128
0.086
0.077
0.207
0.386
0.388
0.052
4.55
0.012
0.104
0.174
0.118
0.0096
0.068
0.0578
0.068
0.0605
0.0092
0.0096
0.095
0.0187
0.095
0.0195
34
107
34
112
0.33
0.052
0.062
0.0758
0.0067
ktor
(Nmm/deg)
kfl
(Nmm/rad)
f
(mm)
n
0.15
8.210-4
9.710-4
0.01
0.012
1.33
3.25
1.33
3.25
1.33
3.25
1.33
3.25
1.33
3.25
1.33
3.25
0.0137
0.118
0.0092
0.0067
kTotal
(N/m)
1.910
-3
0.147
0.15
0.15
0.155
0.026
2.0610-3
1.210-3
1.110-3
0.015
0.013
-4
0.026
5.210-3
7.510-4
2.510-3
9.310-3
0.03
-4
-3
0.007
2.0610
-3
2.110-3
4.310-4
1.510
0.15
2.310
-2
2.110
1.710-4
1.710
0.007
0.062
0.0793
-4
1.3610
1.710-3
1.510
-4
-4
1.9610
-3
0.017
0.021
1.9610
-3
Steel
Cu X3
3.5
0.015
0.012
3.2
0.051
0.036
0.16
0.088
0.0196
0.088
0.0205
0.15
9.410
2.210-4
0.012
2.610-3
1.33
3.25
Steel
Cu X5
4
0.0165
0.012
3.64
0.070
0.041
0.275
0.086
0.0131
0.086
0.0137
0.15
1.210-3
1.910-4
0.015
2.310-3
1.33
3.25
Active isolation with capacitance sensors/actuators
2 capacitance plates
per face,
dimensioned to
provide required
control force (100pF
each, Fmax  3 milliN)
Use insulating
supports, ensure
symmetry and
balancing
Capacitance sensors/actuators: the GGG experience
Capacitance sensors/actuators: the GGG experience
Sensitivity obtained (on bench): 5 picometer in 1 sec integration
time
The system and the control block diagram
The basic equations
The controlled system
Microgravity Environment PSD Envelope:
recent official values
(NIRA 98-99, ESA-COF, US-Lab, JEM, CAM)
Acceleration PSD
[g/Hz]
Acceleration PSD
[m/s2/Hz]
Frequency
Micogravity environment: note that older official
values were more optimistic….
Vibration
noise
expected (as
of 1991) on
the ISS
Expected PGB residual noise after passive/active
isolation
ISA ELECTRONICS
(SAGE inheritage…)
ISA BOX
Thermal Control Board
Acquisition chain
& control Board
Acc Set
Microprocessor
& bus i/f Board
. memoria
Power Supply Board
ISA ELECTRONICS
(SAGE inheritage…)
CAP. SENSORS
PGB STRUCTURE
CAP. ACTUATORS
ISA1
ISA2
(P/L)
SERIAL
LINE
SYNCH
POWER
THER.
SERIAL
LINE
THER.
POWER
ELECTRONIC
UNIT
SYNCH
SERIAL
LINE
TEST &
MAINT.
POWER
MONITORS
ETHERNET
EXPRESS RACK
ISA Passive thermal stabilization
PGB alone transfer
function
MDL alone transfer
function
Combined transfer
function
ISA Passive thermal stabilization
Result from ISA experimental tests: 1 degree temperature
variation gives rise to an accleration disturbance of 510-7 g/Hz
(at all frequencies)
PGBTransfer
function for T=0
Double stage TF,
T=1C at all 
PGB alone TF,
T=1C all 
Double stage TF,
T=40C at all 
The radiometer effect
PGBTransfer
function for T=0
Double stage TF,
T=1C at all 
PGB alone TF,
T=1C all 
Double stage TF,
T=40C at all 
In summary, PGB will provide:
1. Measurement of vibration noise in 3 degrees of
freedom onboard the ISS at the location of the MDL.
The ISA instrument suitable for this purpose has
been manufactured and tested. It can work from very
low frequencies to several Hz and requires only
manufacturing of a space qualified version.
In summary, PGB will provide:
2. Significant passive/active vibration noise reduction by means of mechanical
suspensions (passive isolation) and capacitance sensors/actruators (active
isolation) at frequencies above a few 10-3 Hz. This noise reduction is
demonstrated with direct measurement performed by another ISA
instrument up to a few Hz, reaching a sensitivity of 10-11 g/Hz at about 3
Hz. At higher frequencies noise is also reduced (thanks to passive
attenuation), but it is no longer in the working range of ISA. Measurements
by the two ISA instruments up to several Hz provide a quantitative
measurement of the transfer function of the system and demonstrate the
prediction capability of the PGB noise attenuation system. As a result, this
validates the PGB as a facility for vibration isolation onboard of flying
structures. The main advantage of the PGB facility is that it can be easily
adjusted to the needs of the experimentalists because our prediction
capability allows us to choose the parameters of the system so as to
provide the required level of noise reduction in the required range of
frequency). The PGB mechanical structure, locking/unlocking system,
mechanical suspensions, capacitance sensors/actuators and electronics
have all been designed and are ready to initiate the construction design
and realization phase.
In summary, PGB will provide:
3. Demonstration of ISA sensitivity (so far limited by
seismic noise on the surface of the Earth) to the level
of 10-11 g/Hz (to be reached by the ISA instrument
located inside the PGB isolated system at a
frequency of about 3 Hz). This would be the best
sensitivity ever achieved by an accelerometer, better
than the sensitivity of the French accelerometers built
and flown by ONERA and CNES. This result would
make ISA a very competitive instrument for all space
missions that need an accelerometer. These missions
range from space geodesy and oceanography
missions, to planetary exploration missions (e.g. Bepi
Colombo mission to planet Mercury), to fundamental
physics missions.
Visit the PGB and GG Web Page
http://eotvos.dm.unipi.it/nobili
http://eotvos.dm.unipi.it/nobili/pgb
(80 MB of information available to
anyone in the world at any time)
[email protected]