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September 2016 • The PCB Design Magazine 15 JOHN CARDONE ON DESIGNING FLEX FOR SPACECRAFT side coupled configuration is much more space efficient, and gives a better transition at the connector interface (less reflection) than using edge-coupled. And in a PWB design edge cou- pled is most common. This creates challenges that need to be resolved at the PFC-to-PWB in- terface (it's all about interfaces). Second, every guide you will see on PFC de- sign will warn that I-beam construction is a no- no. The problem is that a staggered design adds impedance where you may not want it, reduc- es common-mode noise rejection, and it uses more cable width or allows increased crosstalk between functions. To validate our designs, we conducted life tests with at least 2x needed cy- cles, under vacuum and temperature extremes. I have five rovers on Mars (one Pathfinder, two MERs and two MSL rovers) and each has far ex- ceeded its required life. Third, as the Mars rover designs have pro- gressed, the PFC challenges have gotten in- creasingly more strident. From Pathfinder to MSL, PFC cable lengths have increased to more than 10m, with full end-to-end cable lengths of ~15m. The longest cable runs transition from PFC inside the rover, to round wire outside, to flex over a 5 DOF (degree of freedom) robotic arm, back to round wire for a transition at the arm end, then to flex in a rolling loop on the drill mechanism, to a final wire segment to the motor. In this example of the drill rotation mo- tor, the requirement was <0.8 ohm one-way. We met the requirement, and for the upcom- ing M2020 project we're on track to improve upon it (and to reduce the trace-to-chassis ca- pacitance which was found to introduce some noise into the encoder reading). Oh, there's an- other challenge. How do you put noisy motors, heater, brakes (yes there are holding brakes) and quiet encoder, temperature, and data telemetry on the same cable? By the way, there are more than 4.5 miles of printed flexible circuitry in the rover's arm alone. Shaughnessy: Tell us a little more about your work on the MER. What were some of the unique issues you encountered on that project? Cardone: To tell you about MER, I'll need to start with MSR (Mars Sample Return). I was part of a small pre-planning team that was outlining the configuration for this mission. At some point NASA decided that returning a sample to Mars orbit, by a rover to be eventually picked up by another spacecraft, was too ambitious. As a side note, M2020 will be preparing and packaging samples, for eventual return to earth. I haven't looked into how they plan to do this but the plan might be up on jpl.nasa.gov. From that context MER began. A number of mechanical designers were co-located. Design- ers do little "engineering" and they generally work for many Cognizant Engineers. A CogE would be responsible for an element of a project, attend budget meeting, contract design, analy- sis, fabrication people, etc., while the designer drives the development of the CAD model and the documentation. I managed the design of all things inside of the Rover body, another man- aged the rover exterior, another the mobility system, robotic arm, mast cam, etc. Because I started with the MSR team, I came into the MER project with a viable con- figuration concept. The MER rover body is es- sentially an ice chest. Inside it is something like a 6U VME chassis. The front and back of this chassis supports stuff like the UHF and X- band components, the redundant batteries, inertial measurement units, and the cable tun- nels. The cable tunnels are insulated serpentine pathways that provided thermal isolation for the rover's wiring between this chassis and the