>-----Original Message-----
>From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU]
>On Behalf Of Alan B. Pearce


>talk to anyone involved in space electronics :)))
<snip>
>Essentially you will need to deal with someone who can do a vibration
>assessment for you. Then you will be able to decide if you really need PCB
>stiffeners. However if the PCB is less than about 4 inches square then a
>stiffener is probably not required, but in view of your particular use I
>would set out to have a mounting standoff support the middle of that area.
>I
>say 4 inches square as that seems to be about the size each side of a
>stiffener when we do our PCB design. However I do not know what the
>requirements were for items like the mars landers which would be needing to
>handle shock loadings similar to yours.

This is just a hobbyist project at this stage, so probably not worth the
expense of a full-up vibration test. The PCB will be approximately 2x3.5",
so I probably won't need stiffeners. I won't have any rigid mounting method
like standoffs, because there will be no chassis. Just a Nerf football
mounted on a wire.

>SMD components should be OK. larger components will need to be screwed down
>or staked. Typically capacitors like CKR06 size would be staked using an
>epoxy so they don't move under vibration. A shock from a landing like you
>are talking of could shear off the leads of such a component.

Cool. I'm planning to pot the whole board in epoxy, so hopefully this will
take care of securing most of the electronics.

>You should probably also look at using some form of conformal
>coating, but rather than epoxy I would look at a silicone coating that is
>removable to allow repair, or assessment of a failure at minimum.

Yeah... I was just reluctant to go to the expense of actually buying
something when I already have epoxy :O). Are there any cheap "hardware
store" solutions that you can suggest?

>Another possibility would be to put the electronics in a box which is
>enclosed with some form of shock absorber. This could be something like
>polystyrene or air filled bags. Essentially anything that will cushion your
>20mph sudden stop into a broader pulse. And do a few test drops before
>using
>the device in earnest. I would be tempted to fit an ADXL type accelerometer
>to monitor and log the landing shock on at least one unit if you are
>building multiples.

As I think I stated in a later post, I will be mounting the electronics in a
Nerf football, encased in fiberglass or something else hard but somewhat
flexible, so that the casing won't crack when it hits the ground.

BTW, do any of you have knowledge of the typical shock/acceleration limits
for load cells? I'm thinking about buying one from transducertechniques.com,
but I can't seem to talk any detailed technical specs out of them. Their
website only gives basic electrical and dimensional data. Do you think the
typical small load cell can take the kind of shock I'm proposing here?

Thanks!

TTYL

- Robert

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