PGB: Pico Gravity Box Enabling Vibration Free Activity on board the ISS

1. PGB: Pico Gravity Box Enabling Vibration Free Activityon board the ISS Universita’ di Pisa IFSI-CNR, Roma Laben, Dvisione Proel Tecnologie, Firenze Alenia Spazio, Torino DG Technology, Parma Galli&Marelli, Lucca

2. The PGB Team

5. 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….

6. 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 … )

7. 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.

8. 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)

9. The Case for Vibration Isolation on board the ISS The ISS is more noisy than a quiet site on the Earth !!!

10. 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

11. The 3 Goals of the PGB Project • To monitor the actual level of vibration noise on board of ISS • 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 • 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)

12. 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

13. PGB accomodation inside a Double Mid Deck Locker The PGB only (with 2 capacitance plates per face and 5 locking/unlocking mechanisms)

14. PGB accomodation inside a Double Mid Deck Locker • Section of the PGB with 4 capacitance plates • When two plates on the same side are used for compensation, the PGB is attracted in that direction • If a tension is applied toplates 1 and 4 the PGB rotates counter clockwise ( if 2 and 3 are used, it rotates clockwise)

15. PGB accomodation inside a Double Middeck Locker The locking / unlocking mechanism

16. 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)

17. Passive vibration isolation TF for translations TF for spring axial rotations TF for spring non-axial rotations ( All transfer functions computed with PGB data)

18. 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

19. Passive vibration isolation Two needs for the suspension springs: • Transfer electric power:  r2 (r radius of wire), large r desired • 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…)

20. Passive vibration isolation

21. 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

22. Capacitance sensors/actuators: the GGG experience

23. Capacitance sensors/actuators: the GGG experience Sensitivity obtained (on bench): 5 picometer in 1 sec integration time

24. The system and the control block diagram

25. The basic equations

26. The controlled system

27. 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 [Hz]

28. Micogravity environment: note that older official values were more optimistic…. Vibration noise expected (as of 1991) on the ISS

29. Expected PGB residual noise after passive/active isolation

30. ISA ELECTRONICS (SAGE inheritage…)

31. ISA ELECTRONICS (SAGE inheritage…)

32. ISA Passive thermal stabilization PGB alone transfer function MDL alone transfer function Combined transfer function

33. 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 

34. 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 

35. In summary, PGB will provide: • 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.

36. In summary, PGB will provide: • 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.

37. In summary, PGB will provide: • 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.

38. 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) nobili@dm.unipi.it