The slip ring assembly comprises a spool, which rotates with the main body and fits snugly over the 3/8" IPT threaded pipe that serves as the "spine" for the payload, and a brush block mounted to a bearing housing which also supports the camera and compass platform. The brush block also supports a 1.25" OD nylon spur gear which is driven by a modified Futaba R/C servo fitted with a similar gear and modified to act as a free-rotating bidirectional speed servo. This servo is driven by the 7 Hz, 1 - 37 msec PWM mag bearing output signal from the compass, and is set for zero speed at 19 msec (180 degrees magnetic).
The spool is a stack of 6 Cu rings, 0.75" OD, 0.60" ID and 0.05" thick, alternated by 5 acetyl shoulder washers, also 0.75" OD and 0.05" thick, and a pair of acetyl end caps. The rings, washers and caps were lathe-turned from round stock on hand. Each ring has a slot 0.025" wide and 0.05" deep cut into its inner surface to which are soldered #26 AWG bus wire stubs for connection to a #28-6 ribbon cable routed along the inner surface of the hollow ring-washer stack. Mating slots to accept these stubs were cut into the shoulders of the adjacent washers.
The outer surface of each ring is turned with a 90-degree V groove to the full width of the ring, i.e. 0.025" deep.
After the ribbon cable conductors are soldered to their stubs, the rings and washers are pressed onto an acetyl tube 0.60" OD and 0.38" ID, having an axial groove 0.05" deep 0.30 wide on its outer surface to accommodate the thickness of the ribbon cable.
The end caps are also 0.75" OD x 0.55" L, and are drilled thru 0.38"
and counter bored at one end 0.60" x .16" D to fit over the tube. One cap has a radial milled slot 0.125" x 0.300" to pass the ribbon cable.
This stack is then fitted over a section of 3/8"-27 IPT pipe, of the type used for table lamps, and secured with 3/8" nuts at each end.
The brush block is also machined acetyl and is secured to the bearing/ spur gear mount with #2-56 screws. The block supports six U-shaped hairpin brushes formed from 0.015" steel music wire thru pairs of 0.05" holes on a 0.600" x 0.100" grid. After tinning with 5% Ag solder and acid flux, each brush is cleaned and soldered with 60-40 PbSn rosin core solder to one conductor of another #28-6 ribbon cable.
Another 0.125" x 0.300 axial slot is cut thru the brush block, spur gear web and bearing mount to pass this cable. After all the brushes are assembled into brush block, they are secured in place by a plate of 1/8" plex and 4 #2-56 screws.
Before final assembly, the spool and brush block were sent off to a fellow who does gold electroplating to put 200 micro inch of 14K Au on the outer surface of the rings and the contact surfaces of the brushes.
Each brush is manually "adjusted" for contact pressure necessary to yield under 0.5 ohm resistance over full rotation. Brush noise was checked using a 200 mAdc test current and an oscilloscope while the spool was manually rotated. The results were gratifying.
The bearing housing accepts the outer race of a stock 3/8" ID 7/8"OD single shielded ball bearing, the inner race of which fits snugly on the 3/8" pipe. The camera/compass platform was cut and bent up from 0.05" 5052 Al sheet and secured to the bottom of the bearing housing with 3 x #2-56 screws. This platform also carries a Tec R/C micro servo to provide elevation control for the camera, but we didn't have a command uplink in place for this feature in time for EOSS-63. This platform also serves to heatsink a 7805 regulator for the compass and the servo power; the camera runs from +7 - 15Vdc fed thru a slip ring.
The rest of the rings carry power ground, compass PWM, elevation PWM, camera video and a separate video shield ground.
The bottom of the payload module is formed from a 7" hemispherical acrylic dome from Edmund Scientific. The dome is protected from landing damage by a cage of 8 0.05" music wires running radially from the bottom center of the dome where the pipe passed thru out to each corner of the octagon-shaped foamcore housing.
Now, who said a picture is worth 1000 words!