Very cool, thanks for the status update.
-george
Sent from my iPhone
On Mar 25, 2017, at 9:35 AM, Robert Watzlavick <rocket@xxxxxxxxxxxxxx> wrote:
For breakaway connectors on my rocket, I had originally planned to use DE-9
connectors and RJ-45 jacks w/o the tab. But I was having trouble figuring
out to keep the protuberances to a minimum and I didn't like the the off-axis
loads on the pins and shells. I remembered the MagSafe connectors for Apple
laptops but I couldn't find anything off the shelf that was: 1) orderable, 2)
had enough pins, and 3) was easy to integrate into the skin of a rocket.
It doesn't look too complicated so I decided to try making a custom version.
I found these Mill-Max spring loaded pins:
https://www.mill-max.com/engineering_notebooks/detail/78
Connectors with 8-pin arrangements are readily available from Digikey and
Mouser:
https://www.digikey.com/products/en?keywords=419-10-208-00-006000
https://www.digikey.com/products/en?keywords=818-22-008-10-001101
You can also get the loose pins and just press fit them into a block.
These magnets were the right size:
http://www.magnet4sale.com/n42-3-8x1-8-neodymium-rare-earth-disc-magnet-with-4-countersink/
Here are some photos of a prototype I made yesterday.
http://www.watzlavick.com/robert/rocket/rocket1/photos/IMG_3630m.JPG ;
(removable side w/ spring pins)
http://www.watzlavick.com/robert/rocket/rocket1/photos/IMG_3633m.JPG
http://www.watzlavick.com/robert/rocket/rocket1/photos/IMG_3631m.JPG ;(vehicle
side w/ target pins)
http://www.watzlavick.com/robert/rocket/rocket1/photos/IMG_3632m.JPG
http://www.watzlavick.com/robert/rocket/rocket1/photos/IMG_3635m.JPG ;(mated
assembly)
The white acetal blocks are 0.5 by 2.0 inches and the pin spacing is 0.1
inches. The parts cost is just under than $15.
I went with the concave target pins instead of the flat ones for the vehicle
side because I thought they would be better for the battery charging
connections. The contacts are rated at 2A continuous, 3A peak. The spring
loaded pins have a travel of 0.055 inches with a spring force of .15 lbf at
mid-deflection so a connector with 8 of them needs at least 1.1 lbf of
compression to hold it together. The magnets have a spec of 4.2 lbf but it's
not clear if that's with another magnet or a steel plate. I need to get a
fish scale or set up a load cell for a quick measurement.
To register the two assemblies together, I recessed the magnets by 0.02
inches on the vehicle connector and then let the magnets protrude and
equivalent amount on the removable connector. The magnets are strong enough
that if you get them anywhere close to each other, they just want to snap in
place. I used opposite polarity magnets on each end of each connector so you
can only connect the male and female portions in one orientation. I drilled
two holes in each end of the removable portion to attach a lanyard so when
the vehicle lifts off, it won't pull on the wires. The two halves separate
cleanly as expected.
The PC boards shown are just some proto boards I had laying around - for a
real installation, I would get some small PCBs made from Oshpark that either
terminate in pads or a connector like a Molex SL or RJ-45. Or, just use the
solder cup versions of the pins.
To test the signal integrity of the connector, I cut a network cable in two
and wired it to each side. I'm only going to use it at 100Base-T speeds (and
you only need 2 pair for that) but at GB speeds, iperf shows 894 Mb/s in one
direction and 938 Mb/s in the other. I didn't maintain perfect cable twist
up to the pins so maybe that's why it's not the same in both directions. The
original unmodified CAT-5E cable ran at 938 Mb/s. I don't have a fast enough
oscilloscope to look at the eye pattern.
The vehicle side with the target pins is almost completely flush so once I
decide how to do the skin around the avionics bay, this concept should work
well. For the removable side, I would provide strain relief for the cable
and use some potting compound to protect the exposed connections.
-Bob
