Safe: shock attenuator for equipment

Compatibility problem with respect to shock environment is often discovered too late (i.e. when the spacecraft design is frozen, and hardware is already manufactured). Any design modification at this stage of the development would have a dramatic impact on the project.

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ABSTRACT

Compatibility problem with respect to shock environment is often discovered too late (i.e. when the spacecraft design is frozen, and hardware is already manufactured). Any design modification at this stage of the development would have a dramatic impact on the project. The aim of the SAFE (Shock Attenuator For Equipment) project is to develop and verify a product line of viscoelastic material dampers to be interposed between the equipment and the spacecraft structure, compatible with existing interfaces (existing hole in the equipment flange, existing threaded hole on spacecraft wall). Vibration tests at full specification on three dummy equipment from 0,6 kg up to 20 kg were successful and confirm that damped mount with SAFE damper could be set up in order to cancel shock without mechanical failure during launch sequence. This test sequence highlights strain rubber sensitivity and effect of rocking mode when damper location is under equipment Cog. But design trade off are possible to get correct performance.

 

1. INTRODUCTION

During the launching phase, various embedded electronic devices can submit critical shocks environment which can damage definitively them. In general, such mechanical incompatibility is discovered very late in the project, during the final shock tests. To solve the problem, one way is to develop a visco-elastic isolator to limit the propagation of the shock levels. But the time is limited for such development whose main technical constraint is to cannot change the interface configuration. To prevent such situation, the idea is to develop a qualified product line of standard shock attenuators based on visco-elastic materials interposed between the equipment and the spacecraft structure. This project, called SAFE (Shock Attenuator For Equipment) was driven under an ESA contract according to the following main scope of work: • Definition of a baseline requirements • Preliminary design of SAFE isolator coupled with preliminary validation tests • Final definition of the product lines • Qualification tests

 

2. SAFE DAMPER DEFINITION LINE

In order to limit the rocking mode effect, a concept based on individual damping elements (called “Pad”) and metallic brackets is proposed. The interest of the metallic bracket is to limit the distance between the different damping elements and the CoG location, and so to limit the rocking mode effects [1]. 5 types of damper are available. Each type is composed of an upper and lower damped Pads which will be screw on bracket.

Metallic parts are Aluminium 7075 T651 or 7175 T731 with an Alodine coating (or SURTEC). Lower damped Pads is equipped with two M3 stainless steel threaded insert. Rubber is a high damped silicon (SMACSIL VHD) with space heritage. As SMACSIL is low temperature sensitive rubber, shock cancelation is possible at negative temperature (down to -20°C).

 

3. DUMMY EQUIPMENT DESCRIPTION

3 dummies equipments were manufactured with bracket. Light equipment dummy is representative of a Gyro Measurement Unit

Weight (kg)    CoG (mm)   MOI at COG (kg.cm²) Z                     X         Y           Z 0,632             40,5               8,8       4,8        5,4

 

Medium equipment is representative of an On Board Computer. 2 Dummy Printed Circuit Board was also fitted inside the dummy frame

 

Weight (kg)     CoG (mm)     MOI at COG (kg.cm²) Z                    X         Y          Z 4,8                    99                262     366       220

Brackets are designed for SAFE Ø18 damper, CoG height versus damper is 80mm. Clearance between inner and outer bracket is higher than 2,5mm.

Heavy Equipment dummy is representative of a Modular Payload Interface Unit of 12 boards

Weight (kg)    CoG (mm)     MOI at COG (kg.cm²) Z                   X           Y             Z 19,5              105.5             3228      2537     3397

 

Brackets are designed for SAFE Ø23 damper, CoG height versus damper is 98mm. Clearance between inner and outer bracket is higher than 2mm.

For the three equipments guiding rules for selecting type number, positioning of dampers were : • First mode for a low sinus sweep less than 300Hz • Mode for qualification sinus upper than 120Hz • Modes for Random qualification between 120 and 300Hz Bracket were design to get a local stiffness at damper location higher than ten times damper stiffness.

 

4. LIGHT EQUIPMENT TESTS

A full vibration test campaign was run at room temperature on the 3 axes. Modal behaviour during OoP Random qualification sequence (24,5 gRMS) is presented on Fig 1.

Estimated maximum relative displacement (3s) is less than 1,2mm. Bracket has mode upper than 800Hz without impact on equipment level. Rubber is strain sensitive, Fig 2 presents transmissibly on equipment for several input levels. We can see a drift of mode when input level increase.

Modal behaviour during IPX Random qualification sequence (13,4 g RMS) is presented on Fig 3.

Estimated maximum relative displacement (3s) is less than 0,4mm on top. Mode at 400Hz is related test jig (corner mode). Because CoG height versus damper of 18mm, there is rocking mode effect with a second mode at 350Hz. Fig 4 presents impact of rocking mode with OoP (Z) displacement at damper location.

Modal behaviour during IPY Random qualification sequence (13,2 g RMS) is presented on Fig 5.

Estimated maximum relative displacement (3s) is less than 0,35mm on top. Mode at 400Hz is related test jig (corner mode). As foot print of damper along X axis is larger than for Y along Y axis, rocking mode effect is lower than for IPX test. A Shock tests Campaign was run thanks a “Ringing Plate” test jig developed by SMAC at Room temperature. Fig 6 to 8 present typical SRS on ringing plate and on equipment for each axis. We got a nice shock attenuation after 300Hz.

Time series Fig 9 indicate that on equipment, temporal signal is a damped sinus shape signal, at 165Hz for OoP and 180Hz for IP.

 

5. MEDIUM EQUIPMENT TESTS

A full vibration test campaign was run on the 3 axes at +10°C, 20°C and 30°C.

Modal behaviour during OoP Random qualification sequence (18,4 gRMS) at 20°C is presented on Fig 10. Estimated maximum relative displacement (3s) is less than 1mm. Bracket has mode at 1 KHz without impact on equipment level. SMACSIL rubber have a low temperature sensitivity Fig 11 presents transmissibly on equipment for +10°C, 20°C and 30°C. Frequency drift versus temperature is very low.

Modal behaviour during IPY Random qualification sequence (13,2 g RMS) is presented on Fig 12. Because CoG height versus damper of 80mm, there is rocking mode effect with a second mode at 280Hz. Estimated maximum relative displacement (3s) is less than 0,6mm on top. Mode at 700Hz is related equipment itself. Mode at 1200Hz is related to bracket, and has low impact on equipment.

Shock test Campaign was run thanks a “Ringing Plate” test jig developed by SMAC at -10°C, 20°C and 35°C Fig 13 and 14 present typical SRS on ringing plate and on equipment for OoP and IPY at -10°C. We got a nice shock attenuation after 300Hz.

As SMACSIL is low temperature sensitive, shock cancelation between -10°C to 35°C is very close, Fig 15 and 16.

 

6. HEAVY EQUIPMENT TESTS

A full vibration test campaign was run on the 3 axes at room temperature. Modal behaviour during OoP Random qualification sequence (18,4 gRMS) at 20°C is presented on Fig 17. Estimated maximum relative displacement (3s) is less than 0,7mm. Mode at 1800Hz is related to bending mode of dummy base plate.

Modal behaviour during IPY Random qualification sequence (12,8 g RMS) is presented on Fig 18. Because CoG height versus damper of 99mm, there is rocking mode effect with a second mode at 285Hz. Estimated maximum relative displacement (3s) is less than 0,45mm on top.

Shock test Campaign was run thanks a “Pendulum” test jig developed by ADETESTS at room temperature along OoP axis. We got a nice shock attenuation after 300Hz.

 

7. AEOLUS RMU SHOCK ISOLATION MOUNT

During late 2015, RMU equipment for AEOLUS program failed during shock test. We design a shock mount attenuation mount based on SAFE damper.

A full Mechanical campaign was run successfully. Figure 20 and 21 present shock attenuation performance measured on the “Ringing Plate” test jig.

 

8. CONCLUSIONS

 

Vibration tests at full specification on three dummies equipments from 0,6 kg up to 20 kg were successful and confirm that damped mount with SAFE damper could be set up in order to cancel shock without mechanical failure during launch sequence. Because of non-linear behaviour of rubber, dynamic response of damped mount versus input level and temperature is difficult to compute accurately. Thanks, SAFE project, we have in house some typical response of SAFE damped mount for different input load. We have also begun to built a stiffness data base of SAFE damper. Bracket should be rigid for damper efficiency. A target of local stiffness at damper location of a minimum of ten times damper stiffness is proposed. Brackets used during qualification task on dummy equipment were conform to this target. We observe some bracket mode during vibration test, these modes have a low impact on equipment level, but some analyse and modelling work should be welcome to get a better understanding. When a shock issue is found on an equipment during a development activity. A solution could be set up rapidly by design only the bracket and use the “on the shelf” damped Pads. No damper design activities are necessary. Load range could be extended by using softer or harder SMACSIL. For example, RMU shock isolation mount for AEOLUS was designed, manufactured and qualified in less than 4 months [1].

 

9. ACKOWLEDGEMENT

 

The authors would like to thank ESA in particularly Mr KIRYENKO Stefan for having granted the development of the SAFE line product, and the Airbus D&S Toulouse people for their support during all the project.

10. REFERENCES SMAC – Demerville T., Lamy P. (2016) SAFE:Shock Absorber For Equipment. ECSSMET 2016 ESA/ESTEC Stefan Kiryenko