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PGB: Pico Gravity Box Enabling Vibration Free Activity
on board the ISS
Universita’ di PisaIFSI-CNR, RomaLaben, Dvisione Proel Tecnologie, FirenzeAlenia Spazio, TorinoDG Technology, ParmaGalli&Marelli, Lucca
The PGB Team
Anna M NobiliUniversità di
PisaPrincipal Investigator
Donato BramantiUniversità di
PisaCo-Investigator
Erseo PolaccoUniversità di
PisaCo-Investigator
Giovanni MengaliUniversità di
PisaDynamics and Active
Control
Gian Luca Comandi
Università di Pisa
Dynamics, Active Control and Mechanical Suspensions
Raffaella ToncelliUniversità di
Pisa
Dynamics, Active Control and Thermal
Analysis
Alberto FranzosoUniversità di
PisaCollaborator
Valerio Iafolla IFSI (CNR) RomaPrincipal Investigator of ISA Accelerometer
Sergio Nozzoli IFSI (CNR) Roma ISA Electronics
Milyukov VadimVisiting
Scientist at IFSI (CNR) Roma
Collaborator
Alfonso Mandiello
IFSI (CNR) Roma ISA Electronics
Giuseppe Catastini
Alenia Spazio (Torino)
ALENIA Study Manager for PGB Transfer
Function and Active Control
Paolo MartellaAlenia Spazio
(Torino)PGB Transfer Function
and Active Control
Alberto AnselmiAlenia Spazio
(Torino)Collaborator
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 ZaninDG Technology
Service Srl (Parma)
Responsible for DG Technology Contribution
(Mechanical Structure)
Carlo GalliGalli&Morelli
(Lucca)
Responsible for Galli&Morelli Contribution (Mechanical
Components)
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
Seismic noise in Cascina, Pisa) (VIRGO site)
Vibration noise expected (as of 1991) on the ISS
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 easier
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 platesA) When two plates on the same side are used for
compensation, the PGB is attracted in that directionB) 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 LockerThe 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: 27.2kgOK 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 isolationSPRING CONSTANTS AND GEOMETRY
D(cm)
d(cm)
L(cm)
M(gm)
MTot
(gm)ka
(N/m)kt
(N/m)kTotal
(N/m)ktor
(Nmm/deg)kfl
(Nmm/rad)f
(mm)n
Steel 0.015 0.058 0.059 0.059 8.210-4 0.01
Cu X1 0.018 0.093 0.066 0.069 9.710-4 0.012
Cu X2
4
0.012
3.6
0.041
0.233
0.0131 0.0137
0.15
1.910-4 2.310-2
1.33 3.25
Steel 0.0195 0.11 0.118 0.118 2.110-3 0.026
Cu X34.5
0.0124.1
0.0470.25
0.0092 0.00960.147
1.710-4 2.0610-3 1.33 3.25
Steel 0.017 0.083 0.068 0.068 1.210-3 0.015
Cu X1 0.019 0.117 0.0578 0.0605 1.110-3 0.013
Cu X2
4.5
0.012
4.1
0.047
0.32
0.0092 0.0096
0.15
1.710-4 2.0610-3
1.33 3.25
Steel 0.02 0.128 0.095 0.095 2.110-3 0.026
Cu X35
0.01554.55
0.0860.386
0.0187 0.01950.15
4.310-4 5.210-3 1.33 3.25
Steel 0.0155 0.077 34 34 7.510-4 9.310-3
Cu X1 0.024 0.207 107 112 2.510-3 0.03
Cu X2
5
0.015
4.55
0.052
0.388
0.0067 0.007
0.155
1.510-4 1.9610-3
1.33 3.25
Steel 0.018 0.104 0.062 0.062 1.3610-3 0.017
Cu X1 0.022 0.174 0.0758 0.0793 1.710-3 0.021
Cu X2
5
0.012
4.55
0.052
0.33
0.0067 0.007
0.15
1.510-4 1.9610-3
1.33 3.25
Steel 0.015 0.051 0.088 0.088 9.410-4 0.012
Cu X33.5
0.0123.2
0.0360.16
0.0196 0.02050.15
2.210-4 2.610-3 1.33 3.25
Steel 0.0165 0.070 0.086 0.086 1.210-3 0.015Cu X5
40.012
3.640.041
0.2750.0131 0.0137
0.151.910-4 2.310-3 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
Sensitivity obtained (on bench): 5 picometer in 1 sec integration time
Acce
lera
tion
PS
D
[m/s
2/Hz]
Frequency [Hz]
Acc
ele
rati
on
PS
D
[g
/H
z]
Microgravity Environment PSD Envelope: recent official values
(NIRA 98-99, ESA-COF, US-Lab, JEM, CAM)
Micogravity environment: note that older official values were more optimistic….Vibration noise expected (as of 1991) on the ISS
ISA ELECTRONICS (SAGE inheritage…)
ISA BOX
Acc Set
Thermal Control Board
Acquisition chain& control Board
Power Supply Board
Microprocessor& bus i/f Board. memoria
ISA ELECTRONICS (SAGE inheritage…)
ISA1 ISA2(P/L)
ELEC TR O NIC UNIT
P O W E R
S E R IA LL INE
P O W E R
EXPR ESS R AC K
S E R IA LL INE
M O NIT O R S
P O W E RT HE R .
T HE R .
S YNC HS YNC H
S E R IA LL INE
E T HE R NE T
T E S T &M A INT .
PG B STR UC TUR E
C A P . S E NS O R S
C A P . A C T UA T O R S
ISA Passive thermal stabilization
PGB alone transfer function
MDL alone transfer function
Combined transfer function
ISA Passive thermal stabilizationResult from ISA experimental tests: 1 degree temperature variation gives rise to an accleration disturbance of 510-7 g/Hz (at all frequencies)
PGBTransfer function for T=0
PGB alone TF, T=1C all
Double stage TF, T=1C at all
Double stage TF, T=40C at all
The radiometer effect
PGBTransfer function for T=0
Double stage TF, T=1C at all PGB alone TF, T=1C all
Double stage TF, T=40C 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/nobilihttp://eotvos.dm.unipi.it/
nobili/pgb(80 MB of information
available to anyone in the world at any time)
nobili@dm.unipi.it
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