Parts featuring built-in electrical wires
2 July 2018
AMAR relates to a novel AM-based design concept enabling the development of AM metal parts featuring built-in electrical wires. The concept is successfully applied and has been awarded with the innovation prize at AM Expo 18.
The following showcase describes the outcome of AMAR, a project related to the redesign of a SlipRing Assembly rotor (SRA rotor), based on additive manufacturing. This project was carried out by RUAG Space Switzerland Nyon (RSSN), jointly with CSEM, each partner bringing its own expertise, respectively the design of SRAs and the re-design of an existing product based on advanced manufacturing technologies. AMAR was funded by the SERI’s Swiss Space Office (SSO).
SRAs are electrical continuity devices intended to transfer electrical signals from a stationary member to a rotating member. SRAs are used on earth for a broad range of applications such as video surveillance, machine-tool, motion simulators and many others. In space, SRAs are recurring elements in satellites where they can be found in Solar Arrays Drive Mechanisms (SADMs), Antenna Pointing Mechanisms, Momentum Gyroscopes and other instruments. In its current state-of-the-art physical architecture, the rotor of an SRA consists of a stacking of high-precision insulating and conductive rings, each conductive ring being manually soldered to an electrical wire, itself routed to the extremity of the rotor. The stack is interfaced to a structural central shaft, and the whole assembly is mechanically stabilized by a matrix of casted resin.
The production process of SRA-Rotors
Today, the production of SRA rotors is a long and delicate process involving a large number of components and production operations. As a rule of thumb, the number of components increases with the number of electrical channels to be included in the rotor, following a multiplicative factor of three. In other words, each channel to be achieved involves three components: an insulating ring, a conductive ring and an electrical wire. Unsurprisingly, the manufacturing and assembly efforts tend to increase accordingly, as well as the probability of reliability issues. In this context, the motiva-tions to undertake a development project such as AMAR were obvious. SRAs being among RSSN’s flagship products, a set of strategic objective was defined, namely to reduce the manufacturing and assembly costs by more than 40%, whilst improving the overall reliability and repeatability of the end product. The redesign was also expected to enable a mass decrease of the rotor and to avoid the use of cables, which are part of
the current physical architecture.
Aims of the development project
Unsurprisingly, the manufacturing and assembly efforts tend to increase accordingly, as well as the probability of reliability issues. In this context, the motiva-tions to undertake a development project such as AMAR were obvious. SRAs being among RSSN’s flagship products, a set of strategic objective was defined, namely to reduce the manufacturing and assembly costs by more than 40%, whilst improving the overall reliability and repeatability of the end product. The redesign was also expected to enable a mass decrease of the rotor and to avoid the use of cables, which are part of the current physical architecture.
To fulfill these objectives, CSEM developed a new design and manufacturing concept, which makes it possible to merge the two essential features of the SRA rotor: the mechanical structure and the electrical conductors, including their electrical connection interfaces. The parts being produced can take various 3D shapes and therefore accom-modate to a broad range of product specifications. Thanks to the suppression of cables, monolithic designs of electromechanical components featuring built-in conductive wires can be envisioned, enabling significant simplifications of the assembly sequence.
The design concept
The design concept is illustrated in picture 6. It relies on the AM production of a monolithic structure comprising a structural hull (1) and a plurality of electrical wires (2) mechanically linked to the hull by means of sacrificial bridges (3). Various AM technologies can be applied, depending on the application requirements. As a second step, the structure is filled with insulating material (4). The insulating material is cured and finally, the sacrificial bridges are removed by means of a conventional subtractive process. The resulting component is a mechanical part featuring a built-in electrical conductor. The termination of the wires can take various shapes to achieve the function of electrical connection interfaces, such as pin, crimping, spring or slip ring contact. The shape of the wire terminations “A” and “B” can be directly achieved during the AM fabrication step or reshaped during the post-AM remachining already mentioned, when high precision is requested. The structural hull may comprise additional features such as mechanical interfaces, reference surfaces, flexure elements, lattice structure and many others, all of them being achieved “by design” during the AM fabrication or during the post-AM remachining.
The displayed figure describes the new physical architecture of the SRA rotor part in a simplified way. For more clarity, the built-in wires are all gathered within a 2D imaginary plane. The design includes 12 annular shape slip ring interfaces on the wire termination “A” and 12 soldering interfaces on the wire termination “B.” As depicted, the slip rings are intrinsically comprised into the monolithic structure, as built during the AM process. After casting and curing, the external shell of the structural hull is remachined and the separated rings appear encapsulated in the resin. The structural stiffness of the rotor is henceforth provided by the remaining metallic structure, or by the resin volume if the structural hull is completely removed.
The detail design was jointly elaborated between RSSN and CSEM. During this phase, the geometries were defined so that the use of support material could be completely avoided. Depending on the prototype versions, the diameter of the built-in wires was set in a range of 0.5 to 1 mm and two wire termination alternatives were implemented at “B”: axially oriented remachined soldering pin-holes and radially oriented AM-made soldering cradles. The external diameter of the rotor after final machining is a cylinder of 33 mm diameter and 44 mm height.
The final design was manufactured by means of laser melting, using the standard AlSi10Mg aluminum alloy. After the additive fabrication, the usual post-process steps of stress relief annealing, part separation and precision cleaning were executed. The part was then filled with an epoxy resin, cured and remachined in order to remove the external shell of the structural hull and the sacrificial bridges. After remachining, a gold layer is selectively applied to the surface of the slip rings in order to improve the tribological and electrical performances of the SRA rotor during operation. At the end of the sequence, cables were soldered to the final part which was then mounted on a performance test bench.
Properties of rotor types and further developments
The electrical properties of the rotor prototype were fully validated in terms of electrical continuity, insulation resistance and dielectric strength. The dynamic electrical noise and lifetime performances were also verified, showing satisfying results for SADM applications intended to Low Earth Orbit (LEO) and Geostationary Orbit (GEO) operated missions. A set of improvements was implemented to the next generation of proto-types in order to further improve some key characteristics such as dynamic electrical noise and rotor compactness, targeting a broader range of applications. Considering a number of 24 electrical tracks to be implemented, the new concept enables a reduction of the components from more than 70 parts to a single one, inducing a drastic decrease of the manufacturing and assembly efforts. The new concept also allows a significant reduction of the overall mass, since the central shaft can be removed or optimized. As wished, the new physical architecture of the rotor does not include cables anymore, which contributes to improving the reliability of the system. Based on a preliminary analysis performed at the end of the project, the cost reduction objective of 40% is considered as realistic. To consolidate this value, the development shall be further continued in order to fully define the design geometries and the process parameters. The final prototype shall then be fully qualified with respect to the application requirements foreseen. These very positive results pave the way for a continuation of the development, possibly in the framework of the European Space Agency (ESA) programs.
Evaluation of the project
During this 14-months project, a number of prototypes and design implementation tests were performed, thanks to the very short delivery time for the parts produced by laser melting. These multiple iterations allowed us to deeply understand the relevant parameters for a successful implementation of the design concept. The very successful outcome of AMAR does not only confirm the applicability of the concept to the SRA rotor application, but also to other products which could advantageously benefit from the same improved redesign. RSSN and CSEM would like to thank 3D Precision SA and ProtoShape GmbH for their active participation in the project.