Wiring, Buses, and Boards
In our experience, producing the wiring looms that connect all the avionics components in a UAV airframe remains the most labor-intensive task in UAV manufacture. While 3D printing, numerical machining, and the purchase of off-the-shelf items allow the majority of UAV components to be gathered together for assembly, largely without human intervention, the design and manufacture of wiring looms for low-volume production runs tends to work against computer-aided design (CAD)-based design and automated manufacture. Although CAD software for setting up wiring looms is readily available, we rarely use it: instead, we rely on the use of “iron birds” to prototype cable runs around the airframe. These are plywood full-scale mock-ups of the aircraft in plan view on which all the airframe avionics components are laid out. Wire runs can then be cut to suit the configuration, and functionality tests carried out before flight-ready harnesses are produced. We adopt this approach, as against CAD-based methods, because of the very large amount design effort needed to fully specify cable harnesses in CAD systems, which even then rarely permit full functional testing and anyway take longer to complete than the making of a physical test harness. Figure 6.13 shows the “iron bird” for the SPOTTER aircraft, and Figure 6.14 shows one of the motor generator pairs used for functional testing. When the initial test harness is complete and functionally correct, we then use it as a template to specify the final flight-ready harnesses we need (which may
Figure 6.13 SPOTTER “iron bird” test harness layout. Note the full-size airframe drawing placed under the wiring.
Figure 6.14 Generator and drive motor for “iron bird” testing.
Figure 6.15 SPOTTER “iron bird” with resulting professionally built harness in place.
then be drawn in a CAD environment). Figure 6.15 shows one of the resulting professionally built, flight-ready harnesses that results from following this approach, laid out on the original “iron bird.”
Two key decisions in the design of wiring harnesses are the choice of plugs to be used for connections and the degree to which power and data buses are used as opposed to full wiring to each individual component. For simple airframes in lightweight aircraft, we tend to use individual three-part wires to connect each servo back to the control receiver and autopilot, with simple aero-modeler-style connections and safety clips, see Figure 6.16. For more complex and larger airfames, where a higher degree of integrity and redundancy is required, we use a power bus to supply all components and also adopt more sophisticated, self-locking plugs in which one-half can be bulkhead or baseboard mounted such as those that meet mil specs, see Figure 6.17. As yet, we have not routinely adopted one of the emerging proprietary standards for control data buses, largely to avoid committing to any particular supplier’s range of components, although we have built aircraft using them to gather aircraft diagnostic data.
To locate the avionics components in an airframe, two approaches can be adopted: first, a separate avionics baseboard can be used that is populated and wired before being placed in the airframe, as already seen in Figures 6.16 and 6.17 and also in Figure 6.18. Second, when 3D-printed fuselage components are in use, sockets and fittings can be designed into the printed structure to accept the various components to be used directly, see Figure 6.19. Of course, a combination of these two approaches can be adopted: we typically locate the servos that operate aerodynamic surfaces via designed sockets in adjacent 3D-printed SLS nylon structure where this is possible, while the main set of receivers, autopilots, batteries, and so on, are attached to a dedicated baseboard and wired and tested before insertion into the aircraft. Low-volume manufacture of baseboards is readily achievable: we use digital laser cutting machines to make these trays.
Figure 6.16 Decode-1 “iron bird” with harness that uses simple aero-modeler-based cable connections.
Figure 6.17 Baseboard with mil spec connections on left- and right-hand edges. Note SkyCircuits SC2 autopilot fitted top right with GPS antenna on top and switch-over unit in the center with very many wiring connections.
Figure 6.18 Laser-cut plywood baseboards.
Figure 6.19 Components located directly into 3D SLS nylon printed structure. The servo is screwed to a clip-in SLS part, while the motor is bolted directly to the fuselage.