F R December 2000 - unipi.it

PGB: Final Report, December, 2000

PGB: Enabling vibration free activity on board the ISSPI: Prof Anna Nobili, [email protected] tel (+39) 050 844252; mobile (+39) 0347 2522634

FINAL REPORTDecember 2000

ASI (AGENZIA SPAZIALE ITALIANA)UTILIZZAZIONE TECNOLOGICA DELLA STAZIONE SPAZIALE

PROGETTO USS-98056

PGB(PICO GRAVITY BOX)

ENABLING VIBRATION FREE ACTIVITY ON BOARD THE ISS

PGB: Final Report, December 2000

PGB: Enabling vibration free activity on board the ISS

TABLE OF CONTENTS

1 PGB: ENABLING VIBRATION FREE ACTIVITY ON BOARD THE ISS.............................................1

2 PGB STRUCTURE AND ACCOMODATION.............................................................................................2

2.1 TRANSPORTATION TO/FROM THE ISS. .........................................................................................................22.2 IN-ORBIT ACCOMMODATION: .....................................................................................................................22.3 MISSION PROFILE........................................................................................................................................32.4 PGB ACCOMMODATION. ............................................................................................................................42.5 LOCKING/UNLOCKING MECHANISM. ..........................................................................................................42.6 MDL INTERFACES: .....................................................................................................................................6

3 PASSIVE VIBRATION ISOLATION ...........................................................................................................6

3.1 MECHANICAL SUSPENSIONS. ......................................................................................................................63.2 SPRING CONSTANTS....................................................................................................................................83.3 SPRING GEOMETRY...................................................................................................................................103.4 FAILURE CRITERIA AND RISK FACTORS. ...................................................................................................103.5 GUIDELINES FOR SPRING CHOICE. ............................................................................................................123.6 PROCEDURE. .............................................................................................................................................133.7 FINAL CHOICE. ..........................................................................................................................................13

4 ACTIVE VIBRATION ISOLATION...........................................................................................................15

4.1 SENSORS AND ACTUATORS........................................................................................................................154.2 GOAL AND REQUIREMENTS OF THE ACTIVE CONTROL. ............................................................................194.3 SYSTEM MODEL........................................................................................................................................194.4 DERIVATION OF THE TRANSFER FUNCTIONS .............................................................................................21

5 ISA ACCELEROMETER.............................................................................................................................25

5.1 DESIGN AND CHARACTERISTICS ...............................................................................................................265.2 ISA CHARACTERIZATION..........................................................................................................................275.3 ISA ELECTRONICS. ...................................................................................................................................295.4 ISA ELECTRONICS BLOCK DIAGRAM AND FUNCTIONAL DESCRIPTION. ...................................................295.5 ACQUISITION CHAIN AND CONTROL BOARD.............................................................................................315.6 MICROPROCESSOR AND BUS INTERFACE BOARD. .....................................................................................315.7 THERMAL CONTROL BOARD AND POWER SUPPLY BOARD. ......................................................................33

6 ELECTRONIC UNIT....................................................................................................................................33

6.1 INTERFACES ..............................................................................................................................................336.2 MICROPROCESSOR BOARD........................................................................................................................356.3 MASS MEMORY BOARD. ...........................................................................................................................366.4 CONTROL BOARDS AND POWER SUPPLY BOARD. .....................................................................................366.5 OPERATIVE MODES...................................................................................................................................36

7 THERMAL ANALISYS................................................................................................................................37

7.1 DISTURBANCES DUE TO TEMPERATURE VARIATIONS...............................................................................377.2 RADIOMETER EFFECT. ..............................................................................................................................39

3 CONCLUSIONS AND FUTURE ACTIONS ..............................................................................................41

ANNEX 1 THE PGB STUDY TEAM ................................................................................................................43

ANNEX 2: PGB NATIONAL WORKSHOP.......................................................................................................44


PGB: Enabling vibration free activity on board the ISS 1

1 PGB: ENABLING VIBRATION FREE ACTIVITY ON BOARD THE ISS.

The ISS is a perfect environment to realize experiments which require absence ofweight; many activities of the applied sciences are potentially destined to take greatadvantage from the availability of space structures such as it. However, in some casesthe absence of weight is not sufficient and a low level of vibrational noise is required.The PGB laboratory is a passive/active vibration isolation system studied to reducethose kind of disturbances (Nobili et al. 1991a,b,c; Catastini et al. 1992, GG Phase AReport 1998). It is connected to the ISS by means of mechanical suspensions.Weightlessness allows us to use very soft suspensions and hence to have a lowthreshold frequency of the system and a good attenuation of vibration noise above it.Below and close to this frequency it is possible to use capacitance sensors/actuatorsin order to reduce noise actively. Passive noise attenuators rely on the fact that whenthe suspension point is forced at a frequency ω much larger than the natural frequencyωo of the pendulum, the oscillation amplitude of the suspended body is reduced by afactor (ωo/ω)2 with respect to that of the suspension point. A noise attenuator shouldwork in all 6 degrees of freedom, because both rotational and translational noise istransmitted by the ISS to the suspended laboratory. Since the torsion elastic constantof a helical spring is very small if compared with its translation elastic constant, therotational noise is reduced much more effectively than translational noise, as it isshown in Figure 1.1 . As a result, a passive attenuator must be essentially designedwith regard to reduction of translational noise.

We have planned to use two ISA accelerometers, one rigidly connected to the ISS andone inside the PGB. In this way it is possible to measure the level of the residual noisehence to demonstrate the capability of realizing a very effective noise attenuator.

10-6 10-4 10-2 10010-6

10-4

10-2

100

102

Frequency (Hz)

Noi

se re

duct

ion

fact

or

Figure 1.1: The transfer function for both rotational and translational noise. Blue line is the translationaltransfer function; red line is the torsional transfer function for rotations around the symmetry axis of thespring; green line is the flectional transfer function for rotations around an axis normal to the symmetryaxis of the spring. They have been evaluated for a system consisting of a box of side 25 cm and mass 40kg, suspended by a steel helical spring of quality factor Q=100, coil diameter D=4.5cm, wire diameterd=0.021cm, number of coils n=3.25. It is apparent that rotational noise is reduced much more effectivelythan translational noise.



The activity reported here has been carried out by the PGB Team (see Annex No.1)through internal collaborations among its members, in contact with ASIrepresentatives (Technological area, Italian projects for the ISS) and also with NASAofficials for what concerns the level of vibration noise (both at high frequency andquasi-stationary), which has to be expected on board the ISS. The PGB team has metfor a national Workshop in Pisa on June 19, 2000 (see Annex No. 2) and in separatemeetings at Laben/Proel, Firenze (September 2000) and at the University of Pisa(November 2000). Contact points with NASA official representatives for the Italianparticipation to the ISS have been established during the Workshop organized to thispurpose by ASI on May 17, 2000 (Rome, ASI HQ)

2 PGB STRUCTURE AND ACCOMODATION.

This Section contains the identification of possible accommodations during both thetransportation phase to/from the ISS and in-orbit operation.

2.1 TRANSPORTATION TO/FROM THE ISS.

A payload that must be active (powered) during transfer phase must be installed in theShuttle Mid-Deck. Once the payload is in orbit, it is removed from the Shuttle andmoved to the EXPRESS Rack. This operation will be performed to limit the power-down time (power-down time constraints shall be identified). Since PGB shall beoperated as an active payload, the transfer to/from the ISS shall be performed as apowered MDL in the Shuttle Mid-Deck with modified locker door (three windows).

2.2 IN-ORBIT ACCOMMODATION:The ISS provides International Standard Payload Rack (ISPR) locations for payloads.This standard rack provides a set of capabilities that may not be required by thefollowing types of payloads:

• Payloads (P/Ls) to be flown in the ISS which do not require interfaces and/orresources (i.e. mass and volume) of an entire ISPR location• P/Ls consisting of existing hardware that has already flown in the Shuttle Mid-Deck, in the Spacelab or in the Spacehab

These P/Ls are called “EXPRESS Rack payloads”. The EXPRESS Rack providesaccommodations to allow quick and simple integration for P/Ls of this type in the ISS.Payloads that can be accommodated inside the EXPRESS Rack can be categorisedfor integration purposes as: standard, non standard and unique. Considering theabove classification, PGB will be defined as Standard Express Rack Payload. In fact,PGB is supposed to use the standard interfaces and the standard cooling via AvionicAir Assembly (AAA) on rear panel.

As preliminary operative assessment, after installation, only pre-energizing checksand switch on/off procedures are envisaged to be executed by crew. The followingcrew time budget has been evaluated:



• crew time resources not exceeding 4 hours of cumulative, non-contiguous time onorbit during the Mission Increment

• crew training time not exceeding 5 to 12 hours in total.2.3 MISSION PROFILE.

In the expected configuration, the ISS receives a visit from the Shuttle every 92 days.This 92 days cycle is defined as Mission Increment. MI is subdivided in the followingphases:-19 days for Shuttle rendez-vous, docking and re-supply (acceleration levels of 10-3 g)-3 days for ISS re-boost to orbit (acceleration levels of 10-3 g)-30 days of undisturbed microgravity (acceleration levels of 1.8x10-6 g)-10 days of station maintenance (acceleration levels of 10-4 g)-30 days of undisturbed microgravity (acceleration levels of 1.8x10-6 g)Since the experiment execution is limited to the phase in which undisturbedmicrogravity is present, 60 out of 92 days are available for investigations.

Figure 2.1: PGB accommodated in a double Middeck Locker box. We can see the external container, thePGB with the springs and two capacitance plates for each face of the laboratory. The dark squarerepresnets the external ISA accelerometer fixed to the rack.



2.4 PGB ACCOMMODATION.

Figure 2.1 shows the system accommodation of the PGB in the volume of a doubleMDL with 12 capacitance sensors/actuators, 2 helical springs and the external ISAaccelerometer fixed to the MDL. The total mass of the system PGB + inneraccelerometer is about 40 kg. PGB's side is 25 centimeters.

2.5 LOCKING/UNLOCKING MECHANISM.The PGB shall be accommodated in a double Middeck Locker box (MDL) and locatedin the Express Rack of the US Laboratory. The locking system must be active in orderto prevent the movement of the box:1) during the Transportation phase to the ISS.2) during in-orbit operations when the acceleration level is larger than 1.8x10-6g.

The locking/unlocking mechanism of the PGB is shown in the Figure 2.2 . This systemis realised by means of an electromagnet which moves a lever connected with apiston. In this way the piston can go up and down and slide in a corresponding hole onthe external surface of the PGB. 5 such mechanisms are sufficient to sustain theweight (during launch) and lock the PGB. Figure 2.3 shows an overall view inside theMDL container.

PGB

Figure 2.2: The locking/unlocking mechanism. The electromagnet on the right (1) moves the lever (2) andthe piston (3) connected to it. This piston goes up and down and slides in a corresponding hole (4) on thesurface of the PGB.

1

2

3

4



Figure 2.3: Overall view with possible locations for the (5) locking/unlocking mechanisms. They actdirectly on the external surface of the B. Five mechanisms like the one shown in Figure 2.2 are sufficientto sustain the weight (during launch) and lock the PGB laboratory during in-orbit operations, when theacceleration level is high.

A second solution has been studied and is shown in Figure 2.4. The supports of thelocking system are such that they can slide along inclined planes (see Figure 2.5)which lift them to lock the PGB. The movement is provided by a feed screw, operatedfrom a little motor, which can slide along a track in the inclined plane. The supportsare brought back by a system of springs.

Figure 2.4: A second solution for the locking/unlocking mechanism. Six supports act on two oppositefaces of the PGB. The mechanism is moved by a little motor. On the left a section in the plane yz, on theright a section in the plane xz. 1) is the support, 2) the inclined surface.

X

Y

Z

1

2



Figure 2.5: a): the support (1) and the inclined surface (2). b): top view of the detail. This support ismoved along the inclined surface by means of a screw (3). This screw can slide along a track in theinclined plane. The springs are not shown. The motor must be placed on the right hand side of the pictureb).

2.6 MDL INTERFACES:The followings interface are available for a single MDL payload.Net Dimensions for payload: 416.4 x 229.2 x 516.1 mmElectrical 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

The air avionic assembly shall provide air in the range from 18.3 to 29.4 °C.The temperature perturbation on the MDL is evaluated to reach 11.1°K (worst case).

3 PASSIVE VIBRATION ISOLATION

3.1 MECHANICAL SUSPENSIONS.

Absence of weight in space allows very soft suspensions to be used even forsuspending a large mass, thus ensuring a low mechanical threshold frequency of the

12

a)

b)

2

3

1

Z

X

X

Y



passive attenuator. By means of the mechanical spring it is also possible an electricalgrounding of the PGB box (avoiding electrostatic disturbances) and data/powertransmission from/to the any instrument/experiment inside the PGB (in this case, theISA accelerometer). This is easily made by suspending the PGB laboratory with twohelical springs, acting on opposite faces of the box, each one made of four wires: onesteel wire, which provides the stiffens and 3 Cu wires for the required electricalconnections. This kind of springs have been proposed because they can easily satisfythe needs of the attenuator. One such spring has been manufactured as a prototypespring for the proposed GG experiment (see Figure 3.1 taken from "GG Phase AReport", 1998 ).

Figure 3.1: One suspension spring made of 1 steel wire and 3 Cu wires.

A configuration a little different from the one shown in Figure 3.1 can be studied inorder to supply the electric power. For PGB power line is requested at least anequivalent section of AWG32 (0.18−0.2 mm of diameter against 0.12 mm of diameterfor the Cu wire in the prototype made for the GG experiment). That can be reached bymanufacturing a spring with 3 Cu wires with different diameters (i.e. one steel wire forthe stiffness, one Cu wire of 0.18−0.20 mm of diameter for conduction and stiffnesstoo, two thin Cu wires only for conduction) or by splitting power on 2−3 AWG36 lines(0.12 mm of diameter, the same of the prototype) obtaining a spring with one steelwire for the stiffness and 4−5 thin Cu wires for conduction. Some possible designs arelisted below. Q=90 is the value measured in the laboratory for the quality factor of thespring shown in Figure 3.1 (all wires insulated and grouped together to form a singlewire). A value of Q higher than this (i.e. a lower dissipation) can be obtained bymanufacturing a spring with separate wires (not grouped together) insulated only atthe clamping; in this way, parts were deformations occur are not insulated andtherefore give rise to smaller losses.

Because of the many variables involved, a large number of springs can be designedthat can satisfy a given set of equations for stress and deflection.



Figure 3.2: One helical spring

We introduce the following parameters which characterize the helical spring (seeFigure 3.2): D= mean coil diameter, d= wire diameter, L= free length, n= total numberof coils, ν= Poisson's ratio, E= modulus of elasticity of the spring material (Young'smodulus), G= E/2(1+ν) (modulus of shear), F= working load (4⋅10-4N), τ= designstress at working load F, f= total deflection, p= coil pitch (distance between adjacentspring coils). These factors are related in some fundamental spring equations and arehereafter considered.

3.2 SPRING CONSTANTS.We can expand the spring's constants as functions of its geometry and the spring'smaterial characteristics. Doing so we find the formulas summarised below. So thelinear spring constant in axial direction (see Figure 3.3.a) is:

3

4

3

4

a D)1)(2n(16Ed

D)2n(8Gd)m/N(k

ν+−=

−= (3.1)

Assuming Hooke's law, the total deflection of the spring with a working load F is:

4

3

a GdFD)2n(8

)cm(f−

= (3.2)

The linear spring constant for a force applied in the plane of the coil (i.e. normal to thespring's axis; see Figure 3.3.b) is:

3

4

3t

t nD8Ed

2Dn

JE)m/N(k =

π

= (3.3)

where we have introduced:



π=

64d

tJ4

(3.4)

We can also distinguish two different kinds of rotation; for rotations around thespring's axis (see Figure 3.3.c) the constant is :

D)2n(2375Eddeg)/mmN(k

4

torr −=⋅ (3.5)

while for rotations in the plane containing the axis and a diameter of the helical spring(Figure 3.3.d) we have:

+π

=⋅

tp

fl

EJ1

GJ1

2Dn

1)rad/mmN(k (3.6)

where

π=

32d

pJ4

(3.7)

a) b)

c) d)Figure 3.3: Sketch of the elastic constants for the helical spring.



3.3 SPRING GEOMETRY.

The distance between adjacent spring coils (coil pitch) is:

( ) n/L~n/d2Lp −= (3.8)

The rise angle of the spring coils is:

π=ϑ

Dp

tanarc (3.9)

The free length of the spring is:

)tan(Dn~L ϑπ (3.10)

The length of the wire needed to make the spring is:

( )[ ])cos(/2n2DL w ϑ−+π= (3.11)

and its mass is:

4/Dnd~m 22 ρπ (3.12)

3.4 FAILURE CRITERIA AND RISK FACTORS.

If the working load F is much smaller than the critical one, the spring works in safeconditions. The maximum load P for a helical spring is equal to :

2

tor

fl

2

fl

L2π

kk1

L2πLk

P

+

= (3.13)

The condition F<P must be satisfied.

Compression spring buckling refers to when the spring deforms in a non-axialdirection. This is a very dangerous condition. As a result, it is important to design thespring in such a way that this risk is minimized. One way to check for buckling is tocompute the deflection height ratio f/L and see if it exceeds the maximum allowablevalue plotted in Figure 3.4.

The maximum shear stress occurs on the inner surface of the coils and it is equal to:

ϑ

ν+ν++−= )(tan

)1(23

D16d31

dπDF8

τ 22

2

3 (3.14)



The Soderberg Criterion provides a way to calculate a failure limit. The spring will fail ifthe following condition is satisfied:

fatiguemax

yield

fatiguemax σ2τ

σσ

2τ +

−> (3.15)

where σfatigue and σyield are tabulated (see Table 3.1). We can also plot the stress stateof our spring in the Soderberg diagram (Figure 3.5). For our choices the stress state isalways below the limit line.

f/L

L/D

Our Spring

Figure 3.4: Check for compression spring buckling.

AISI Type 302Category Copper Steel

Class Stainless SteelType Austenitic standard

Common Names Chromium-Nickel steel

Composition Cu Cr(17-19%), Ni(8-10%), Mn(2%),Si(1%), C, P, S(<1%)

Density 8.96 g/cm3 (T=25°C) 8 g/cm3 (T=25°C)Poisson’s Ratio 0.36 0.27-0.3 (T=25°C)Elastic Modulus 101 GPa 193 GPaTensile Strength 515 MPaYield Strength 205 MPa



Thermal Expansion 1.65⋅10-5/°K (T=298°K) 1.72⋅10-5/°C (T=0-100°C)Thermal Conductivity 401 W/(m⋅K) 16.2 W/(m⋅K)

Specific Heat 385 J/(kg⋅K) 500 J/(kg⋅K)Electric Resistivity 1.67⋅10-8 Ω⋅m (T=293°K) 720⋅10-9 Ω⋅m (T=25°C)

Table 3.1: Stainless Steel and Copper Properties.

OurSprings

Figure 3.5: Soderberg Diagram.

When springs are used in a mechanism, their dynamic behaviors must be analyzed.The first natural frequency of a helical spring is found to be:

ρG

n9Dd

mk

21ν 2

spring

aS == (3.16)

We need this frequency to be higher than the working frequency 3Hz.

3.5 GUIDELINES FOR SPRING CHOICE.

From equations (3.1) and (3.3) it is clear that ka can be equal to kt only if n=3.25 (n = 31/4). This choice is quite unusual since the number of active coils is very small (i.e. n-2 = 1.25). If we want a large number of active coils in the spring (for example tenactive coils ), from (3.1) and (3.3) we find that ka will be about 0.5kt. This difference isnot a problem from the point of view of the control laws. Once the material has beenselected, 3 free parameters (n,d,D) can be varied in order to satisfy a set ofconditions.



3.6 PROCEDURE.- To select the material.- To select the expected value for the linear constant.- To set a limit for the maximum deflection allowable (f<2mm) (The smaller is the

deflection, the smaller can be the gap between the capacitance plates and thePGB; hence, a lower voltage is sufficient to produce the required control force).

- To check that all failure criteria are well satisfied.- To minimize the total mass of the spring so that a zero-mass spring model can

realistically account for the energy dissipation process.

3.7 FINAL CHOICE.

Table 3.2 shows several possible solutions for the helical spring. Each choice perfectlysatisfies all the criteria typically used in order to establish if a spring will be in dangerof failing from fatigue. The total elastic constant for the spring made of 4−6 wires isevaluated as kTotal=kSteel+ΣkCu. We have evaluated some different designs; their elasticproperties both in the longitudinal and the transversal direction are very close to thenominal ka

Total= ktTotal=0.15N/m (note that k=0.3N/m is the value used in the control

laws. Since we plan to use two (equal) helical springs to suspend the PGB, each onemust have ka = 1/2k = 0.15 N/m), i.e.

%1k

kkkk

Totalt

Totala

Totalt ≈

−=∆

.



Table 3.2: SPRING 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)P

(N)PTotal

(N)τ

(Mpa) L/D f/L νTotal

(Hz)f

(mm) n ϑ(deg)

σ(Mpa)

Lwire(cm)

Steel 0.015 0.058 0.059 0.059 8.2⋅10-4 0.01 1.8⋅10-3 5.5 497

Cu X1 0.018 0.093 0.066 0.069 9.7⋅10-4 0.012 2.1⋅10-3 3.4

Cu X2

4

0.012

3.6

0.041

0.233

0.0131 0.0137

0.15

1.9⋅10-4 2.3⋅10-2 4.2⋅10-4

4.7⋅10-3

2.4

0.91 0.037 12.7 1.33 3.25 5.1 41

Steel 0.0195 0.11 0.118 0.118 2.1⋅10-3 0.026 4⋅10-3 5.7 505

Cu X34.5

0.0124.1

0.0470.25

0.0092 0.00960.147

1.7⋅10-4 2.06⋅10-3 3.3⋅10-4 4.9⋅10-3

1.90.91 0.032 12.2 1.33 3.25 5.1 46

Steel 0.017 0.083 0.068 0.068 1.2⋅10-3 0.015 2.3⋅10-3 4.9 505

Cu X1 0.019 0.117 0.0578 0.0605 1.1⋅10-3 0.013 2.1⋅10-3 3

Cu X2

4.5

0.012

4.1

0.047

0.32

0.0092 0.0096

0.15

1.7⋅10-4 2.06⋅10-3 3.3⋅10-4

5.1⋅10-3

1.9

0.91 0.032 10.8 1.33 3.25 5.1 46

Steel 0.02 0.128 0.095 0.095 2.1⋅10-3 0.026 3.6⋅10-3 4.7 501

Cu X35

0.01554.55

0.0860.386

0.0187 0.01950.15

4.3⋅10-4 5.2⋅10-3 7.4⋅10-4 5.8⋅10-3

20.91 0.032 9.86 1.33 3.25 5.1 51

Steel 0.0155 0.077 34 34 7.5⋅10-4 9.3⋅10-3 1.3⋅10-3 3.65 494

Cu X1 0.024 0.207 107 112 2.5⋅10-3 0.03 4.3⋅10-3 3.1

Cu X2

5

0.015

4.55

0.052

0.388

0.0067 0.007

0.155

1.5⋅10-4 1.96⋅10-3 2.6⋅10-4

6.1⋅10-3

1.5

0.91 0.032 10 1.33 3.25 5.1 51

Steel 0.018 0.104 0.062 0.062 1.36⋅10-3 0.017 2.4⋅10-3 4.2 501

Cu X1 0.022 0.174 0.0758 0.0793 1.7⋅10-3 0.021 3⋅10-3 2.8

Cu X2

5

0.012

4.55

0.052

0.33

0.0067 0.007

0.15

1.5⋅10-4 1.96⋅10-3 2.6⋅10-4

6 10-3

1.5

0.91 0.032 10.7 1.33 3.25 5.1 51

Steel 0.015 0.051 0.088 0.088 9.4⋅10-4 0.012 2.3⋅10-4 7.2 491

Cu X33.5

0.0123.2

0.0360.16

0.0196 0.02050.15

2.2⋅10-4 2.6⋅10-3 5.4⋅10-41.82⋅10-

33.1

0.91 0.042 15.3 1.33 3.25 5.1 36

Steel 0.0165 0.070 0.086 0.086 1.2⋅10-3 0.015 2.6⋅10-3 6.1 502

Cu X54

0.0123.64

0.0410.275

0.0131 0.01370.15

1.9⋅10-4 2.3⋅10-3 4.17⋅10-4 4.8⋅10-3

2.40.91 0.037 11.7 1.33 3.25 5.1 41

Note: We use the symbols X3 or X5 when the spring is made of 3 or 5 identical Cu wires, while X1 and X2 when it is used a wire with a diametermuch larger than the diameter of the other two Cu wires.



4 ACTIVE VIBRATION ISOLATION.

Passive and active vibration isolation are complementary and can be used incombination. In fact, while a passive system is very effective at high frequencies, anactive isolation system works better at low frequencies because the availableresponse time is longer. An active vibration isolation system can be realized by meansof capacitance sensors/actuators.

4.1 SENSORS AND ACTUATORS.

Capacitance plates, rigidly connected to the ISS, located in between the PGBlaboratory and the containing rack –2 capacitance plates for each face of thesuspended laboratory− form capacitance bridges (LC) capable to sense both smallrelative displacements and rotations. These bridges are similar to the ones being usedin the GGG experiment (see, e.g. GG Phase A Report, 1998) carried out in thelaboratories of Laben in Florence (see Figures 4.1 and 4.2). The sensitivity,demonstrated on bench, is of 5⋅10-2å displacements in 1 second of integration time.

The capacitance plates can also be used as actuators in order to provide the forcesrequired by the active control. The clearance (gap) between the fixed plates and PGBbox has been designed to be 3−5mm so that it is possible to produce forces of 2−4⋅10-

3N assuming a 300V maximum voltage. When the capacitance plates are employedas sensors, the reference sinusoidal signal has a frequency of hundreds of kHz, whilethe control is operated only at small frequencies. Some of the sensors/actuators willbe placed close to the walls of the container rack at a distance of 3−4 centimeters. Inthis case a capacitance is formed by the plates and the walls; it turns out to be about10 times smaller than the capacitance formed by the plates facing the PGB, but itmust be taken into account when the control is performed. Each plate will be made ofcopper, with a surface of 24x12 cm2 and a width of 2−3mm.

PGB: Final R

eport, Decem

ber 2000

PGB: Enabling vibration free activity on board the ISS

16

Figure 4.1 The capacitance bridge sensor circuit for the detection of relative displacements (as used so far in the GGG experiment.)



Figure 4.2: The new capacitance bridge sensor circuit currently in use in the GGG experiment. It is under testing in order to improve its sensitivity andstability.



Figure 4.3: Section of the PGB with 4 capacitance plates. A) When two plates on the same side are usedfor the compensation, the PGB is attracted in that direction. B) If a tension is applied to the plates 1 and 4the PGB rotates counter clockwise ( if 2 and 3 are used, it rotates clockwise).

Some care must be taken in the design of their supports so as to avoid vibrationalfrequencies in the range of interest (Figure 4.4). We actually plan to use 4 smallpedestals of non conductive material; but the solution adopted in the GGG is alsoavailable (construction of the isolating supports in one single piece, according to thenominal design, to be later cut in the required number of supports so as to ensure thebest of symmetry and balancing).

Figure 4.4: The most recent solution adopted in the GGG experiment (where there is the additionaldifficulty of a non-zero curvature of the capacitance plates) and a sketch of a possible design for thecapacitance plates of the PGB .



4.2 GOAL AND REQUIREMENTS OF THE ACTIVE CONTROL.The control equations have been written for the active control required. Numericalsimulations of the entire system have been carried out, including both passive andactive control, so as to obtain the transfer function to be expected. The transferfunction was constrained by the fact that the level of residual vibration noise inside thesuspended PGB must be well suited for the characteristics of the ISA accelerometerused to measure it, so as to be able to make the most sensitive measurementpossible. It is necessary that the residual noise achieved is in accordance with thebest sensitivity of the instrument located inside the PGB (i.e. the residual noise shouldnot exceed 10-11 g/√Hz at about 3 Hz) and that the clearance is compatible with thedesign of capacitive actuators (an error of ± 5 mm would provide a maximum force of3 mN assuming a 350 V maximum voltage). Official input data (simulated) have beenobtained by NASA representatives on the level of vibration noise expected on boardthe ISS. These data refer to vibration noise at high frequencies (from 0.01 Hz till 300Hz); they also give constraints on the expected quasi stationary noise (at lowfrequencies), particularly at the orbital frequency.

10-2

10-1

100

101

102

103

100

101

102

103

104

105

Microgravity Environment PSD Envelope(NIRA 98-99, ESA-COF, US-Lab, JEM, CAM)

Frequency [Hz]10

-210

-110

010

110

210

310

-5

10-4

10-3

10-2

10-1

100

Figure 4.5: PSD (power Spectral Density) of simulated disturbances

Considering the expected environmental disturbance on the ISS, the former of thespecified requirements corresponds to impose a rejection margin of at least 130 dB,while the passive system attenuation at this point is 90 dB. Therefore the activecontrol must be able to increase the rejection by at least 40 dB in the measurementbandwidth.

4.3 SYSTEM MODEL

The two-body system is presented in Figure 4.5 with its control block diagram. Let usassume that ISS and PGB are described by )x,x,x( 111 &&& , )x,x,x( 222 &&& and that FC and

Acc

eler

atio

n PS

D [µ

g/√H

z]A

cceleration PSD [m

/s 2/√Hz]



FD are the control and the disturbance forces. The equations of motion for the 2 bodiesare:

Figure 4.6: two-body system representation and control block diagram

1DCC22

CD11 m/FFkFkxm

FFkxmµ−−ξβ−ξ−=ξµ⇒

−ξβ−ξ−=

++ξβ+ξ= &&&&&&

&&&(4.1)

with 12 xx −=ξ the relative displacement, )mm/(mm 2121 +=µ the reduced mass ofthe system, k the spring stiffness, Q/kµ=β the damping coefficient and Q theoverall mechanical quality factor (including the internal spring damping and externalviscous damping acting on the PGB). All the simulations refer to the following valuesof m1, m2, k, Q:

m1 = 300000 Kgm2 = 60 Kgk = 0.3 N/mQ = 100



The control force is x)s(H)s(HF ACCPOSC &&+ξ= , with [ POSH ]=[N/m] and [ ACCH ]=[Kg].The relative displacement and absolute accelerations of ISS and PGB are so:

++µ

µ+

++µ

=

++ξµ=

++µ

µ+

−=ξ

ACC2

POS22

d

ACC2

POS2

1

Cd

11

ACC2

POS2

d

HmHGm

s

aHmHG

x

mF

aGm

x

HmHGm

s

a

2

&&

&& (4.2)

with skGµβ+

µ= . ξ and 2x&& are the relevant variables from the point of view of the

PGB/control performance, while the value of CF has to be within the capacitorcapability of generating force.

4.4 DERIVATION OF THE TRANSFER FUNCTIONS

Neglecting the effect of mass reduction due to location on ISS (the error of thisapproximation is order of 2⋅10-4 since m1~5000m2), the block diagram of the activelycontrolled system can be sketched as in Figure 4.7 below

Figure 4.7: Block diagram of the controlled system

where:adist is the acceleration of environmental applied on the ISS and then on theexternal box of the PGB;aPGB is the acceleration of the PGB in an inertial reference frame;

REFx

AccelerationController

)()(

sDenHsNumH

a

a

PositionController

Mass-Spring-Damper System

mks

ms

s

++ β2

2

- 1

21s

)()(

sDenHsNumH

p

p

aPGBξ&&

adist

ξ

acon

+-

+

+

- 1

+

++

+



acon is the control acceleration;xREF is the elongation of the idle spring.

Note that the clearance ξ is derived from:

distPGB aa −=ξ&& (4.3)

Ha is the acceleration filter. It must be able to augment the rejection permitted by thepassive system at the frequency of about 3 Hz.

HaOrder, freq.[Hz]

High-Pass Filter Numerator 1, ω=0High-Pass Filter Denominator 1, ω=1⋅10-4

Phase-Lead Filter Numerator 2, ω=1⋅10-3

Phase-Lead Filter Denominator 2, ω=1⋅10-4

Low-Pass Filter Numerator 3, ω= 7.Low-Pass Filter Denominator 4, ω= 2.5Gain 85.9

Table 4.1: Parameters used in the acceleration filter

Hp is the position filter. Its aim is to maintain the elongation of the spring below athreshold corresponding to the admitted clearance (i.e. ξ-xREF must be lower than theadmitted clearance between the internal and the external boxes.).

HpOrder, freq.[Hz]

Lead-Lag Filter Numerator 1, ω=5⋅10-5

Lead-Lag Filter Denominator 2, ω=1⋅10-4

Phase-Lead Filter Numerator 2, ω=1⋅10-3

Phase-Lead Filter Denominator 2, ω=3⋅10-2

Gain 0.226

Table 4.2: Parameters used in the position filter

The transfer function of the closed loop system from disturbance acceleration andreference spring length to the PGB acceleration is:

( )

+β+++

+

+β+

=)s(DenH

mks

m)s(NumH)s(DenHDenH)s(DenH)s(NumHs

x)s(NumH)s(DenHa)s(DenHmks

m)s(NumH)s(DenH

a

ppapaa2

REFpadistppa

PGB

(4.4)



The transfer function of the closed loop system from disturbance acceleration andreference spring length to the elongation is:

( )

( ) )s(NumH)s(DenH)s(DenH)s(DenHmks

m)s(DenH)s(NumHs

x)s(NumH)s(DenHa)s(DenH)s(NumH)s(DenH

papaaa2

REFpadistaap

+

+β++

++−=ξ

(4.5)

As there isn’t a reference profile for the spring elongation the attenuation of theexternal disturbance and the response in terms of distance between inner andexternal boxes can be studied considering only the first part of the reported transferfunctions.

10-5 10-4 10-3 10-2 10-1 100 101 102-140

-120

-100

-80

-60

-40

-20

0

20

40Passively vs Actively Controlled behaviour in frequency domain: Acceleration

Frequency (Hz)

Gai

n (d

B)

10-5 10-4 10-3 10-2 10-1 100 101 102-250

-200

-150

-100

-50

0

50

Frequency (Hz)

Pha

se (d

eg)

Figure 4.8: Transfer function acceleration/disturbance of the actively controlled system. The red line isreferred to the passive isolation only while the blue one is evaluated in the presence of active control too.



10-5 10-4 10-3 10-2 10-1 100 101 102-150

-100

-50

0

50

100Passively vs Actively Controlled behaviour in frequency domain: Position

Frequency (Hz)

Gai

n (d

B)

10-5 10-4 10-3 10-2 10-1 100 101 102-50

0

50

100

150

200

250

300

Frequency (Hz)

Pha

se (d

eg)

Figure 4.9: Transfer function displacement/disturbance of the actively controlled system. The red line isreferred to passive isolation only while the blue one is evaluated in the presence of active control too.

Figure 4.10 shows the expected vibration acceleration (RMS) on the PGB. Theresidual noise at about 3 Hz is at the level of 10-10m/s2 (i.e.10-11 g/√Hz), which is thegoal. It is obtained by combining the result plotted in Figure 4.8 with the expectednoise on the ISS shown in Figure 4.5.



10-4 10-2 100 10210-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3Expected vibration acceleration on the PGB

Frequency (Hz)

RM

S A

ccel

erat

ion

(m/s

2 )

Figure 4.10: Log/Log plot of the expected vibration acceleration (RMS) on the PGB. The red line isreferred to passive isolation only while the blue one is evaluated in the presence of active control too.

5 ISA ACCELEROMETER.

A prototype of the 3-axis ISA accelerometer (Italian Space Accelerometer) to be usedinside the PGB has been developed, manufactured and tested at IFSI (CNR) in Rome.Figure 5.1 a) shows the mechanical core of the accelerometer, namely a single sensorwith a low noise amplifier.

Figure 5.1: ISA accelerometer: a) Single sensor with a low noise amplifier. b) The sensor with two platesfor picking-up the signal and the electronics for the control.

a) b)



5.1 DESIGN AND CHARACTERISTICS

The spring restraint is due to the suspension torsion element. The sensitive axis isperpendicular to the face. Figure 5.1 b) shows a single sensor with two plates forpicking-up the signal and the electronics for the control. Four additional plates arefaced on the opposite sides of the central one forming capacitors. One couple of thesecapacitors provides the reading of the signal while the other one is used for thebalancing of the bridge, for lowering the natural frequency of the oscillator and like anactuator in order to excite the proof mass with a known electric signal.

The electromechanical parameters of the ISA accelerometer are given in Table 5.1 fortwo different level of resonance frequency. The total noise in acceleration, from thepoint of view of the mechanical oscillator, can be express by the formula:

fCZ4

TQT

mk4

)(a onn

r

ob2t ∆

β

ω+

ω≈ω (5.1)

t/1f ∆=∆ is the inverse of the acquisition time for each single measure and nnn i/eZ =is the amplifier noise impedance. The electromechanical factor β is given by:

2or

2

mC

ωα=β (5.2)

The electrical dissipation is expressed by:

βδ

Ωω

= tg4Q1

p

o

de(5.3)

where δtg is the angle of loss of the electrical part of transducer.

dem Q1

Q1

Q1 += (5.4)

3.5 Hz 10 Hzmina Sensitivity Hz/g 1210*3.3 − 1210*8.9 −

rm Proof mass (Kg) 0.22 0.22

of Frequency of resonance (Hz) 3.5 10

pf Polarisation frequency (KHz) 10 10P Pressure condition ( mbar ) < 410− < 410−

1C Sensing capacity (pF) 300 300

1Ctgδ Loss of 1C 410*4 − 410*4 −

aC External fixed capacity (pF) 300 300

Catgδ Loss of aC 410*3 − 410*3 −

Electronic device AD743/AD AD743/AD



nv Voltage noise of amplifier ( Hz/V ) 910*3 − 910*3 −

ni Current noise of amplifier ( Hz/A ) 1510*7 − 1510*7 −

nT Temperature noise of amplifier (K) 0.76 0.76α Transducer factor ( m/V ) 510 510β Electromechanical transducer factor 210*8.2 − 310*4.3 −

eQ Electric quality factor 410*6.6 310*6.2

mQ Mechanical quality factor 310*7.5 310*7.5Q Total quality factor 310*7.5 310*8.1

2bwa Brownian noise Hz/)sec/m( 22 2210*9.2 − 2110*9.2 −

2ela Electronic noise Hz/)sec/m( 22 2210*4.8 − 2110*8.6 −

Table 5.1: Electromechanical parameters of ISA accelerometer.

5.2 ISA CHARACTERIZATION.The ISA accelerometer can work in a frequency range from 10-4 Hz to 10 Hz. In aground laboratory it is therefore affected (and limited) by the presence of seismicnoise. The results of tests performed in the CNR-IFSI laboratory in October 2000 areshown below. The instrument was located inside an underground tunnel were seismicnoise was lower and thermal stability was better. Data were collected with a samplingtime of 1 second. Figure 5.2 shows the horizontal component of the solid tide of theEarth as measured by ISA (IFSI-CNR, Rome October 2000).

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

x 105

-1.5

-1

-0.5

0

0.5

1

1.5

2

x 10-7

Acce

lera

tion

y [g

]

Time [s]

ISA Caracterization

Figure 5.2: Solid tides as measured by ISA at CNR-IFSI, Rome, October 2000.

The measured power spectral density is shown in Figures 5.3 and 5.4 where thelimitation is due to the presence of seismic noise and not to the sensitivity of theinstrument.



10-5

10-4

10-3

10-2

10-1

100

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

PS

D A

ccel

erat

ion

x [g

/Hz1 /2

]

frequency [Hz]

ISA Caracterization

Figure 5.3: Power spectral density of residual noise as measured by ISA (x component in the horizontal plane) to alevel of 10-8g/√Hz.

10-5

10-4

10-3

10-2

10-1

100

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

PS

D A

ccel

erat

ion

y [g

/Hz1 /2

]

frequency [Hz]

ISA Caracterization

Figure 5.4: Power spectral density of residual noise as measured by ISA (y component in the horizontalplane) to a level of 10-8g/√Hz.



5.3 ISA ELECTRONICS.

ISA electronics as designed for flight is modular and allocated on printed circuit boardsin Single Eurocard format (100x160 mm2). Each board is made of FR4 PCBreinforced with Aluminum frame. Dedicated fasteners perform both structural andthermal functions. The boards are interconnected by a motherboard (backplane). Dueto the high density of components per board, the PCB’s are multilayer and two-sidedmounted. In general, the components, excluding connectors, are assembled in surfacemounting technology. The electronic boards are the followings:

- Acquisition Chain and Control Board (acquisition electronics),- Microprocessor and Bus interface Board (data processing and bus i/f),- Thermal Control Board (thermal control HW loop); if needed, see Section 7,- Power Supply Board (dc/dc converter and filter).

A possible mechanical accommodation of ISA box components is shown below(Figure 5.5). In the present configuration only 4 boards are defined but there is thepossibility to allocate 2 more spare boards. In total, 6 Single Eurocard boards can beaccommodated in the box.

ISA BOX

Acc Set

Thermal Control BoardAcquisition chain& control Board

Power Supply Board

Microprocessor& bus i/f Board

i

Figure 5.5 Accommodation of ISA box components.

5.4 ISA ELECTRONICS BLOCK DIAGRAM AND FUNCTIONAL DESCRIPTION.

The principle of the measure foresees 3 capacitive bridges, each one constituted by 2reference capacitors and 2 capacitances between the probe mass and the relatedaccelerometer assembly. The 3 bridges are assembled together in the AccelerometerSet (AS) and polarized by a reference Pump Voltage Generator at 10 kHz, 20 V (rms).The gravity perturbations are detected as variation of bridge capacitance and variationof the electrical signal that is sampled, acquired and processed. The microgravitymeasures are in the range from 0.0001 up to 10 Hz.



The scientific data are formatted, inserted in scientific telemetry and routed to theElectronic Unit (EU) for down-link (see Section 6 for a detailed description of EU). Asshown in ISA block diagram the functions split mainly in the followings parts:

- acquisition and conditioning electronics,- data processing and bus interface,- thermal control (if needed, see Section 7),- internal dc/dc power conversion and distribution.

Accelerometer PREAMPs

MPX

ADC

CHAINCONTROLLER

THERMALCONTROL s/s

CPUs/s

UART CommandData I/F

TemperatureMonitor

HousekeepingMonitors

TemperatureSensor(s)

Heater(s)

Pump Voltage

Calibration Voltages

Input pwrDC/DC

FIFORAM

EEPROM

PROC

Figure 5.6: ISA block diagram.

The acquisition chain performs a lock-in amplifier filtering by means of analog anddigital circuits. The acquisition chain split in the followings parts:

one preamplifier for each axis (3),a Multiplexer,an Analog to Digital Converter (16 bit),a First-in First-out buffer,a Chain Controller.

The chain controller is responsible to synchronise every part of the acquisition chainwith respect to the polarisation voltage of the 3 measurement bridges.

During a period of the polarisation voltage (100 us) 2 pairs of samples are acquired foreach axis; each pair of samples are acquired with a fixed phase difference (90°), andthis permits the reconstruction the amplitude of the signal. Two measures aregenerated per axis for every period of the pump generator. The ADC has to perform 4acquisition for 3 axis = 12 acquisition every 100 us (8us/acq.). It is requested tomeasure the gravity perturbation at the maximum rate of 10 Hz and so the samplingtime is assumed 100 ms min. In total there are:

12x1000=12000 acquisition samples (16 bit) for each measure of gravity.This is the data rate generated by the acquisition activity to be processed in real timeby the ISA cpu.



The processing unit for every pair of samples shall perform the followings floatingpoint operations to generate a measure: 2 multiplication + 1 addition + 1 square root.The average is calculated for each axis on 2000 measures.

The data are sent to EU via a serial line as scientific telemetry for datation andtemporary storage.

The thermal control (if needed, see Section 7) is performed by 2 loops:-one loop only by HW (short loop between thermal control hardware and Oven Box),-one loop by HW and SW (large loop with temperature monitor acquisition, cpuprocessing and commanding of the hw thermal control loop).A thermal control law is implemented for each loop. The law implemented by HW shallbe defined after the activities to be performed on Developing Model (DM) and shall befixed after the qualification tests on Engineering and Qualification Model (EQM). Thelaw implemented by SW shall be defined and fixed, but it can be modified during flightoperation as every part of the SW, by re-loading or patching the program RAM viatelecommand.

5.5 ACQUISITION CHAIN AND CONTROL BOARD.

On this board there are the complete acquisition electronic chain and the relatedcontroller, with the exception of the low noise preamplifiers that are allocated near thecapacitive bridges and the Accelerometer Set, in the Oven Box. The chain controller isable to control independently the DC bias of the measurement bridges with 3programmable DC voltage generators, by means of 3 Digital to Analog Converters(DAC) and the buffering circuitry. There is also a programmable generator for the lowfrequency reference stimuli to be injected, on user request, on the bridges forcalibration purposes. The multiplexer is able to selects the scientific and housekeepingsignals to be acquired with a sample/hold and the Analog to Digital Converter. Apreamplifier with programmable gain (1,2,4,8) shall be provided to increase thedynamic range if necessary.

The housekeeping acquisitions are relative to the followings channels:- the 3 thermistors of the HW thermal control (if needed, see Section 7),- the 4 Secondary Voltages,- the 3 Voltage DC bias of capacitive bridges,- the rms Voltage of the Pump generator,- the rms Voltages of the low frequency reference signal for calibration,Also for housekeeping measurements averaging technique shall be applied. Thesampling rate, requested for housekeeping monitors, is lower a that requested forscientific data. The complete acquisition cycle, started by the microprocessor, issynchronised with the external synch signal coming from EU.

5.6 MICROPROCESSOR AND BUS INTERFACE BOARD.

This board implements the following task:- management of EU telemetry/telecommand interface,



- processing in real time the data samples received form the acquisition chain,- thermal control board management and thermal control SW law implementation (if

needed),- acquisition chain synchronization, programming and management,- implementation of failure recovery. The block diagram and architecture of the board is shown below.

TSC21020DSP

CLOCKGEN.USART

PROMCTRL

8KbBOOTPROM

512KbEEPROM

SYSTEMBUS I/F

256K x 32SRAM

DUALPORTRAM

SYSTEM BUS

MEZZANINE CONNECTOR

RS-422SERIAL I/F DEBUG PORT

DATAMEM. BUS

PROGRAMMEM. BUS

JTAGINT.

INT.

INT.INT.

128K x 48SRAM

EDAC

EDAC

Figure 5.7: Block diagram and architecture of the board. The board is based on the space qualified Digital Signal Processor TSC21020 byTemic, that is a floating point processor tolerant to a total dose of 50krad (Si), SEUimmune to more than 30 Mev/mg/cm2 and SEL immune. DSP operating frequency is20 MHz and it may perform 20 million instructions per second and up to 40MFLOPS/s,thanks to separate data and program bus architecture. The main characteristics of this board are here summarized: - DSP TSC21020E clock a 20MHz (40 MOp/S) - Bootstrap Program Memory (non volatile) 8KB (PROM 8Kx8) - Program Memory (not volatile) 512KB (4 EEPROM 128Kx8 tbc) - Program Memory (volatile) 128Kx48 (6 SRAM 128Kx8 tbc) - Data Memory 128Kx32 (4 SRAM 128Kx8 tbc) - Serial interface controller (USART) - Synch input interface (from Lagrange). Note: a new version of DSP operating at 25 MHz is under qualification and will beavailable in the next future. At switch-on the bootstrap is performed by PROM and then the main program residentin EEPROM is loaded into static RAM, for a faster execution and to perform patchesand SW modification. A checksum routine running from PROM checks radiationeffects on program, and in case of checksum error, the program is re-load EEPROM



(the EEPROM selected, manufactured by SEI, are latchup free and SEU immunewhen they are read). In the present baseline the RAM are radiation hard, but it ispossible another solution with cheaper RAM protected by EDAC circuitry (ErrorDetection and Correction) to correct 1 bit error in a memory word. The use of EDACshall load the processor with the “scrubbing” activity and degrade the DSPperformances, so a trade-off will be performed to confirm/revise the present baseline.The DSP has also a JTAG interface that is used for in-chip emulation during SWdevelopment. The Serial interface controller is by USART IC 82C52 by HARRIS andRS422 differential drivers/receivers implementing a full duplex interface fortelemetry/telecommand management with EU. The synchronism pulse from EU, usedto start the chain controller acquisition cycle, is acquired by interrupt.

5.7 THERMAL CONTROL BOARD AND POWER SUPPLY BOARD.

If active thermal control will be necessary (see Section 7), this board shall implementthe thermal control of the Oven Box (OB) by an hardware circuit in closed loop withcommanding parameter setting by the microprocessor. The board monitors directly 3thermistors and drives the related heaters on the Oven Box (OB). At the moment, 5 Wmaximum are allocated for the power controlled with incremental step of 20 mW. The Power Supply Board is constituted by a space standard DC/DC converter whichproduces the output voltages needed to supply ISA.

6 ELECTRONIC UNIT.

The Electronic Unit (EU) will be the PGB controller and it will be in charge of:

• control and synchronize the two ISA accelerometers• manage the interface with rack controller• acquire time from rack to date the measures• acquire microgravity data from ISA accelerometers• date the measure and prepare the scientific telemetry packets• perform the active control of the PGB floating box• acquire data from PGB control sensors and drive control actuator

6.1 INTERFACES

The interfaces of the Electronic Unit are shown in the related PGB interface diagramFigure 6.1, and listed in the followings tables.



Figure 6.1: PGB interface diagram. ISA1 is the accelerometer fixed to the ISS, ISA2 the accelerometerinside the PGB.

ISA1 interfaces Type Number of linesPOWER +28Vdc 2DATA Serial RS422 2SYNCH Single end 1TEMP (thermistor) Single end 1

ISA2 interfaces Type Number of linesPOWER +28Vdc 2DATA Serial RS422 2SYNCH Single end 1TEMP (thermistor) Single end 1

ISA1 ISA2(P/L)

ELECTRONIC UNIT

POWER

SERIALLINE

POWER

EXPRESS RACK

SERIALLINE

MONITORS

POWERTHER.

THER.

SYNCHSYNCH

SERIALLINE

ETHERNET

TEST &MAINT.

PGB STRUCTURE

CAP. SENSORS

CAP. ACTUATORS



Express Rack interfaces TypePOWER +28VdcDATA EthernetTIME To be defined

Table 6.1: Interfaces of the Electronic Unit. ISA1 is the accelerometer fixed to the ISS, ISA2 theaccelerometer inside the PGB.

The data/telecommand interface between EU and ISA1−2 instruments are assumedserial line balanced standard RS422. The ethernet will be used for commanding andtelemetry communication link between EU and the Express Rack. The data ratesupported by this interface (2.5 Mbps maximum) is shared with the other ExpressRack payloads. Some analog and digital monitors (status information, power linevoltages/current and temperature of internal hot spot) of EU will be provided toExpress Rack for monitoring; the detailed list of EU monitors can be later defined.

0.12 mm diameter wires (AWG36) are too thin to supply power to the PGB payload.For PGB power line is requested at least an equivalent section of AWG30(diameter~0.25 mm) that can be reached by means of an appropriate design of thespring. To prevent possible perturbations generated by magnetic interaction betweenthe solenoid of the helical spring and the terrestrial magnetic field, the electrical powerline and the related return line have to be accommodated on the same spring.

The EU shall be a modular box able to allocate electronic printed circuit boards. TheEU PCB's format is D.E. (Double Eurocard = 233.4 mm x 160 mm). Each board will bemade of FR4 PCB reinforced with Aluminum frame. Dedicated fasteners will performboth structural and thermal functions. The boards are interconnected by amotherboard (backplane). Due to the high density of components per board, thePCB’s shall be multilayer and two-sided mounted. In general, the components,excluding connectors, are assembled in surface mounting technology. A preliminarylist of EU boards is the following:

- Microprocessor board with Ethernet and Serial interfaces- Mass memory board- Triaxial acquisition chain and control board- Miscellaneous board- Power supply board

6.2 MICROPROCESSOR BOARD.

The microprocessor module is the computing core of EU where the software, devotedto the management and control of PGB facility operations, is stored and executed .The CPU board is the space standard microprocessor board, based on ERC32chipset and provides the following main features :• SPARC V7 32 bit CPU• 10 MIPS at 14MHz clock (version at 25MHz is under developing)• 6 MB EDAC-protected RAM• 4 MB Flash EEPROM• 8 KB boot PROM



• 128 KB NVRAM• 4 serial communication interfaces RS422 drivers/receivers• 1 ethernet I/F (IEEE802.3, 10BaseT)• 1 Milbus 1553B I/F (option)• 2 mezzanine slots I/F (for future expansion)• VME bus I/F

6.3 MASS MEMORY BOARD.

The Mass Memory board is in charge of performing the PGB facility data recordingtask; in fact, in case of loss/interruption of ground link, the EU has to record thescience and housekeeping data acquired. Further a Mass Memory function isrequested to store SW and parameters. The following main characteristics are from aspace standard board:• non-volatile memory 85MBytes• VME bus I/F• Latch-up detection and current limiting circuitry• Microcontroller plus its EEPROM and RAM buffer.Access to the non-volatile mass memory takes place through a standardised ATAlogical interface. In the frame of cost reduction the Mass Memory board can besubstituted by a removable couple of hard disks assembly (redundancy is necessaryto recover latchup and single event upset, due at radiation on commercial item, bymeans of dedicated circuitry). The size of the mass memory will be increased, byusing commercial items, but also the power consumption/dissipation will be increased.This choice has to made before contruction.

6.4 CONTROL BOARDS AND POWER SUPPLY BOARD.

Two boards are foreseen to accommodate the acquisition sensor electronics and thedriver actuators electronics dedicated to the active control of PGB. The active controllaw will be implemented by software running on the main processor (microprocessorboard). Some service boards are necessary for EU box. In particular a Miscellaneousboard will provide the power switching/distribution function to switch-on/off the ISA1/2instruments.

6.5 OPERATIVE MODES.

The PGB payload will be controlled by Principal Investigators (PI) on ground, viatelecommand, or by crew, i.e Payload Specialist (PS), on board, by means of theExpress rack standard laptop.

EU is the main controller of PGB and it will switch-on/off ISA1 and ISA2 instrumentsby supplying the related power line.

After switch-on, EU shall execute the followings initialization tasks:bootstrap from non-volatile memory



execution of a series of automatic tests,copy the sw code from the non-volatile memory to RAM,start the sw process managing telemetry/telecommand with express rack,enter in stand-by operating state.

In stand-by it is possible to perform the generic patch and dump of the SW code of EUloaded in the program RAM, via up-link. It is also possible to perform software up-load, from ground into the EU mass memory, of program and data/parameter of EUand ISA instruments.

On user request nominal mode can be entered, and ISA1 and ISA2 instruments canbe switched-on. The following operating configuration are envisaged in nominal mode:• PGB locked (active and passive control off)• PGB unlocked (active and passive control on)• PGB unlocked and simulating a controlled vibration (active and passive control on)In principle, the last operative configuration can simulate a controlled perturbation onthe payload, with possibility to define the shape (sinusoidal, saw tooth, etc). Theconfiguration modes will be re-assessed.

7 THERMAL ANALISYS

7.1 DISTURBANCES DUE TO TEMPERATURE VARIATIONS

The ISA accelerometers need thermal stability of the environment in order to minimizethermal noise. This environment is the MDL which includes the PGB. It is necessary tothermally insulate the accelerometer from its environment both radiatively andconductively. A strong thermal de-coupling is needed between the ISA inside the PGB(ISA2) and the PGB environment. This can be done by minimising the radiativecoupling between the mobile (suspended) structure (the PGB) and the fixed structure(the MDL) by means of an aluminized kapton tape on all surfaces of the fixedstructure, which guarantees a very low thermal emissivity. The same tape can be usedalso for the internal surfaces of the mobile structure. The external surfaces of the ISAaccelerometer fixed to the MDL (ISA1) can be covered by MLI (Multi Layer Insulator)composed by about ten layers, reducing its emissivity below 0.1. The conductivecoupling between the ISA2 and the fixed structure is well reduced by the thin springs.All strategy to reduce thermal couplings between the ISA2 and the PGB environmenthas to be sized taking into account the power dissipated by the devices inside thePGB.

Assuming that thermal perturbations on the MDL are due only to the variation of theair temperature of the cooling system, the expected variation is of 11.1°K, i.e. thesame order of magnitude as in the SAGE proposed experiment (16°K; see SAGEReport on Phase A Study, ASI November 1998).

Here we report the results of a preliminary analysis of disturbances due to thermalvariations assuming passive thermal isolation only. We analyze how heat passesfirstly through the external environment (MDL) and secondly through the Pico GravityBox. Let us consider both the internal surface of the MDL and the internal surface of



the PGB covered by a Multi Layer Insulator composed by ten layers at least. Thesystem can be modeled by a block−diagram with two subsystems:

PGB ∆Ton ISA2∆Texternal MDL

Figure 7.1: Thermal Block Diagram.

MDL represents the rack of the international space station containing the PGB. Weare interested in temperature fluctuation at the level of the ISA2 accelerometer locatedinside the PGB. At the first block corresponds a thermal transfer function (TTF) whichis constant at low frequencies (i.e. frequencies lower than the critical one; we assumea critical frequency of about 10-4Hz, which is not an optimistic asumption) and whichdecreases at a rate of 20 db per decade at frequencies higher than the critical one. Atthe second block corresponds the thermal transfer function of the PGB. In this casethe transfer function is similar to the previous one, but the critical frequency is about10-3Hz (we have assumed a higher frequency because the mass of the body is smallerthan the mass of the Container). Instead of this model we could consider a thirdsubsystem in order to take into account the external box (a few kilograms) of ISA2.With this choice we would obtain a better rejection at high frequencies, but also the 2-stage model shown in Figure 7.1 is found to be enough to guarantee a residualthermal noise lower than the vibrational noise expected from active and passiveattenuation.

We obtain the total transfer function (for the full system of Figure 7.1) by multiplyingthe TTFs of each block. At high frequencies it decreases at the rate of 40 db perdecade (see Figure 7.2).

10-5 10-4 10-3 10-2 10-1 100 10110-10

10-8

10-6

10-4

10-2

100

Frequency (Hz)

Red

uctio

n fa

ctor



Figure 7.2: Thermal transfer functions. Red is the MDL's TTF, blue is the PGB's TTF and green is thetotal TTF.

Now, we must evaluate how temperature variations affect the accelerometer, startingfrom the experimental evidence that ISA’s sensitivity to temperature is of about 5⋅10-7

g/√Hz per degree of temperature variation, at all frequencies. As a consequence, inthe presence of a fluctuation of temperature ∆T(ν), the accelerometer will measure anacceleration 5⋅10-7⋅∆T(ν) g/√Hz per degree of temperature fluctuation. This noise mustbe lower than the residual vibrational noise obtained thanks to passive and activeattenuation assuming no temperature perturbation.

10-5 10-4 10-3 10-2 10-1 100 10110-16

10-14

10-12

10-10

10-8

10-6

10-4

10-2

Frequency (Hz)

Noi

se (g

/sqr

t(Hz)

)

Figure 7.3: The blue line represents the level of vibrational noise expected inside the PGB after passiveand active attenuation, assuming no temperature perturbation; the other three lines represent the level ofdisturbances that would be measured by the ISA2 instrument (inside the suspended PGB) because of thetemperature fluctuation ∆T(ν): the red line refers to a single stage system (PGB only) and a temperaturefluctuation ∆T(ν)=1°C at all frequencies; green and magenta lines refer to a double stage system (MDL +PGB; see Figure 7.1) and temperature variations of ∆T(ν)=1°C and ∆T(ν)=40°C respectively, at allfrequencies (at the level of the external surface of the MDL).

7.2 RADIOMETER EFFECT.

In the presence of temperature fluctuations ∆T(ν) inside the MDL, there can be temperaturegradients across the PGB with the same amplitude and frequency distribution as ∆T(ν).Temperature gradients and residual gas pressure around the PGB will give rise to theradiometer effect. The radiometer acceleration is given by:

L)(T

TV

M2p)(a ν∆⋅⋅=ν (7.1)



where p is the pressure of the residual gas, T its temperature, M the mass of the PGB, V itsvolume and L the length of its side. In the MDL the residual pressure is quite low (p≅ 10-3mbar)and the temperature is about 300K. The radiometer effect resulting from (7.1) is:

Hzg)(T107.2

Hz)(a 8 ⋅

νν∆⋅=ν − (7.2)

(ν and ∆T(ν) are adimensional). In Figure 7.4 the radiometer effect is plotted (expressed ing/√Hz) and compared to the residual vibrational noise inside the suspended PGB (resultingfrom active and passive isolation as reported in Sections 3 and 4), showing that theradiometer effect is not a matter of concern.

10-5 10-4 10-3 10-2 10-1 100 10110-20

10-15

10-10

10-5

100

Frequency (Hz)

Noi

se (g

/sqr

t(Hz)

)

Figure 7.4: The blue line represents the level of vibrational noise expected inside the PGB after passiveand active attenuation, assuming no temperature perturbation; the other three lines represent the level ofdisturbances measured by the ISA2 instrument inside the PGB because of the radiometer effect: the redline refers to a single stage system (PGB only) and temperature gradients ∆T(ν)=1°C at all frequencies;green and magenta lines refer to a double stage system (MDL + PGB; see Figure 7.1) and temperaturegradients of ∆T(ν)=1°C and ∆T(ν)=40°C respectively, at all frequencies. In all cases temperaturegradients are assumed across the PGB and the residual gas pressure is that inside the MDL, i.e. about10-3 mbar.

The previous analyses appear to indicate that temperature induced disturbances canbe kept below the expected level of noise reduction to be provided by the PGBpassive/active vibration isolation system without implementing active thermal controlof the ISA accelerometer. If this solution will turn out not to be adequate (e.g. becauseof conflicts between the required passive isolation and the level of internal heatdissipation) an alternative solution for active temperature control has already beendesigned (similar to that of the SAGE experiment; see SAGE Report on Phase AStudy, ASI November 1998) and can be implemented. It is described in detail inSection 5.



3 CONCLUSIONS AND FUTURE ACTIONS

At completion of this study we conclude that the PGB experiment is ready to berealized and flown onboard the ISS to be allocated in a double Middeck Locker (MDL).The PGB will provide the following results:

• Measurement of vibration noise in 3 degrees of freedom onboard the ISS at thelocation of the MDL. The ISA instrument suitable for this purpose has beenmanufactured and tested. It can work from very low frequencies to several Hz andrequires only manufacturing of a space qualified version.

• Significant passive/active vibration noise reduction by means of mechanicalsuspensions (passive isolation) and capacitance sensors/actruators (activeisolation) at frequencies above a few 10-3 Hz. This noise reduction is demonstratedwith 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 isalso reduced (thanks to passive attenuation), but it is no longer in the workingrange of ISA. The same ISA instrument can be used −at the level of the ISS andinside the PGB isolated system− because it has sufficient sensitivity and dynamicrange for both measurements. Measurements by the two ISA instruments up toseveral Hz provide a quantitative measurement of the transfer function of thesystem and demonstrate the prediction capability of the PGB noise attenuationsystem. As a result, this validates the PGB as a facility for vibration isolationonboard of flying structures. The main advantage of the PGB facility is that it canbe easily adjusted to the needs of the experimentalists because our predictioncapability allows us to choose the parameters of the system so as to provide therequired level of noise reduction in the required range of frequency). The PGBmechanical structure, locking/unlocking system, mechanical suspensions,capacitance sensors/actuators and electronics have all been designed and areready to initiate the construction design and realization phase.

• Demonstration of ISA sensitivity (so far limited by seismic noise on the surface ofthe Earth) to the level of 10-11 g/√Hz (to be reached by the ISA instrument locatedinside the PGB isolated system at a frequency of about 3 Hz). This would be thebest sensitivity ever achieved by an accelerometer, better than the sensitivity of theFrench accelerometers built and flown by ONERA and CNES. This result wouldmake ISA a very competitive instrument for all space missions that need anaccelerometer. These missions range from space geodesy and oceanographymissions, to planetary exploration missions (e.g. Bepi Colombo mission to planetMercury), to fundamental physics missions.

The PGB Project can be brought to completion by the PGB Team as in its presentstructure, i.e. with the University of Pisa leading the Project, in close collaboration withIFSI-CNR and with the contribution from well qualified national space industries(Alenia Spazio, Laben Milan, Laben-Prole Division, Florence) as well as from localprecision mechanics companies (DG Technology, Parma and Galli&Morelli, Lucca).



A costing report will be delivered to ASI in January 2000 for the entire PGB Project tobe completed in 2 and ½ years (30 months), allowing a flexibility of plus or minus 6months for adjustments depending on ISS planning.

ANNEX 1


ANNEX 1 THE PGB STUDY TEAM

PEOPLE INSTITUTION CONTRIBUTION TO PGB

Anna M Nobili Università di Pisa Principal Investigator

Donato Bramanti Università di Pisa Co-Investigator

Erseo Polacco Università di Pisa Co-Investigator

Giovanni Mengali Università di Pisa Dynamics and Active Control

Gian Luca Comandi Università di Pisa Dynamics, Active Control andMechanical Suspensions

Raffaella Toncelli Università di Pisa Dynamics, Active Control and ThermalAnalysis

Alberto Franzoso Università di Pisa Collaborator

Valerio Iafolla IFSI (CNR) ROMAPrincipal Investigatorof ISA Accelerometer

Sergio Nozzoli IFSI (CNR) ROMA ISA Electronics

Milyukov Vadim Visiting Scientist at IFSI (CNR)ROMA

Collaborator

Alfonso Mandiello IFSI (CNR) ROMA ISA Electronis

Giuseppe Catastini ALENIA SPAZIO, Torino ALENIA Study Manager for PGBTransfer Function and Active Control

Paolo Martella ALENIA SPAZIO, Torino PGB Transfer Functionand Active Control

Alberto Anselmi ALENIA SPAZIO, Torino Collaborator

Elisabetta Cavazzuti LABEN, Milano LABEN Study Manager

Pietro Soravia LABEN, Milano PGB Electronics, Transportation andAccommodation

Roberto Ronchi LABEN, Milano Thermal Analyses

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

Responsible for DG TechnologyContribution (Mechanical Structure)

Carlo Galli Galli&Morelli, Lucca Responsible for Galli&MorelliContribution (Mechanical Components)

ANNEX 2


ANNEX 2: PGB NATIONAL WORKSHOP

W o r k s h o p

PGB(Pico Gravity Box)

Enabling Vibration-Free Activityon board the

International Space Station

June 19, 200011 am — 6 pm

Università di Pisa, Dipartimento di MatematicaGruppo di meccanica Spaziale

Via Buonarroti 2Sala dei Seminari (1o Piano)

Contact point:Anna Nobili

39 050 844252; 39 0347 [email protected]

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F R December 2000 - unipi.it