This paper describes the main thermo-mechanical design features and performances of the Co-Alignment Sensor (CAS) developed by LIDAX and CRISA under ESA program with AIRBUS Defence & Space as industry prime.
The CAS can be generally described as a Focal Plane Assembly with integrated Optics, Detector, and Electronics.
The Co-Alignment Sensor (CAS) is a part of ATLID Instrument, whose mission responds to the need to provide a picture of the 3-dimensional spatial and temporal structure of the radiative flux field at the top of the Earth atmosphere, within the atmosphere and at the Earth’s surface.
The CAS is located on the ATLID Optical Bench and is part of the control loop that allows identifying the pointing direction of the Laser signal return used to control the Laser co-Alignment with Optical Bench. CRISA is the final responsible of the whole CAS project design and development
MODELS & PROJECT STATUS
The following deliverable models are considered:
• Structural and Thermal Model (STM see Fig. 1) with the following objectives:
• Protoflight Model (PFM)
Actually, the STM became a Qualification Model from thermo-mechanical point of view.
Currently, the STM qualification testing has been successfully completed.
Following main mechanical requirements are applicable:
Mass, Structural & Thermal
• Mass < 1.68kg
• Maximum Envelope: 198mm × 227mm × 130mm
• I/F loads on each interface point /fixation:
• The unit dimensioning shall consider sine, shock and vibrations loads
• Fail-safe damage tolerance design principles
• Non-operating Thermal Environment: 50ºC/-25ºC
• Conductive coupling to its Mechanical IF (Interface) < 0.06 W/K
• MCCD temperature<Optical Bench temperature +2K
• Dissipated power< 2.0W
• Interface flatness (bipods common plane): 10 μm
Stability & Alignment
MECHANICAL DESIGN DESCRIPTION
The CAS is shown in Fig. 1, and it is divided in three main components:
• Mechanical Bench Assembly (MBA) and main thermo-structural element developed by LIDAX
• Proximity Electronics (PE) that contains the required electronics connected to the MCCD (CFI manufactured by e2v) by means of a Flexible PCB (Printed Circuit Board) and developed by CRISA
• Optical Assembly (OA) that holds the optics (beam splitter, filter and lens) and developed by Bertin Technologies
The whole assembly is supported by means of three bipods to get the appropriate relation between stiffness and loads induced at the interface.
Mechanical Bench Assembly
Mechanical Bench Assembly shown in Fig. 2 is the main thermo-structural component of CAS and it is composed of different thermo-mechanical components such as:
The Main Bracket is a Ti6Al4V bracket of complex geometry, which is the main structural element of CAS, and its complex shape provides the following functionalities:
• It provides support and mechanical tolerances for relative positioning of optical sensitive elements:
• Stiffness and structural strength to support the above assemblies
Titanium choice (with respect to invar) is based on its lower mass density, high strength and better machining capabilities in spite of a higher thermal expansion coefficient (worst thermal performance).
Three identical Bipods also made of Ti6Al4V supports the Main Bracket (and all elements mounted on) providing the following functionalities:
• Overall stiffness and flexibility to assure a quasi-isostatic mounting
• Structural strength to support the whole CAS
• Thermal Isolation from Interface Bench
Manufacturing has become a complex issue due to the following:
• A extremely demanding overall flatness requirement (decision of avoiding any shimming process was taken early in the project)
• Complex shape of Main Bracket, needed to provide all required functionalities
Above points have driven to use Electrostatic Discharge Machining (EDM) manufacturing process for manufacturing the Main Bracket & Bipods Assembly, with the drawback of the exhaustive process controls required to assure the removal of alpha-case and hydrogen in the titanium surfaces.
A specific manufacturing process has been defined and successfully implemented for STM Model.
Thermal Straps with specific shapes due to AIV issues have been developed and submitted to qualification; they are based on piled sheets and terminals all made of golden copper.
Manufacturing process and correlation between design and measured conductance have been proven during the straps qualification campaign.
Comparison between specified and measured performances is shown in the below table.
|Mass||< 1.68kg||1.668 kg|
|Maximum Envelope||198 × 227 × 130 mm||188 × 220 × 127mm|
|I/F loads on each interface point /fixation||Forces: 14-55N Torques:0.7-1.45Nm||Forces<49N Torques<1.77Nm|
|Interface flatness (bipods common plane)||10 μm||25 μm||Exhaustive test campaign performed to evaluate this non-conformance resulted in compliance with the overall displacement & rotation errors due to flatness|
|MCCD stability with respect to Optics||< 1μm||1-3 μm||Limited by measurement accuracy|
|Overall equipment Stability (from mounting to flight operation)||Disp<30 μm Rot<100 μrad||Disp<17 μm Rot<65 μrad||Contributions: assembly, temperature, micro-setting & gravity release|
The following conclusions can be established:
• A CAS STM has been successfully qualified
• CAS STM is fully representative of the flight hardware from thermal and mechanical view
• Design has proved to provide response to the demanding structural and stability requirements
• Currently the PFM is under manufacturing
• Thermal Straps design and manufacturing process has been successfully qualified
• EDM manufacturing process has been proven to be suitable for flight hardware with appropriate process specification and controls
The activities described in this paper were made possible with the help and support of the LIDAX staff, and the following colleagues from different companies and institutions. We would like to thank, from INTA: Gonzalo Ramos & Tomás Belenguer and Christophe Delettrez & Philippe Lingot from Airbus Defence & Space for their collaboration and support.