Re: like butta

Glen Walpert <nospam@null.void> wrote:

On Thu, 23 Jan 2025 03:43:03 -0000 (UTC), Phil Hobbs wrote:
 

Simon and I are working on a TDR based soil moisture sensor for
agriculture.  Like many such things, it uses two parallel tines made of
18/8 stainless, that form a balanced transmission line.
 
Ours has a slide hammer for pounding it into really difficult soil, e.g.
hardpan. The measured shock from that is around 1.6E5 m/s**2, i.e. 16000
gees, and over its lifetime it might see around 1E6 blows. Challenging.

 
Have you considered using a mechanical low pass filter on your slide
hammer, a pad on either the hammer or anvil striking surfaces?  Rawhide
faced iron hammers are very good at driving stakes into very hard soil
without the severe impulse of a metal on metal impact, and while rawhide
might not be good for 1E6 cycles you could try rubber sheet or heavy
gasket material, tune the response by changing thickness and hardness to
suit.  Presumably the same slide hammer is used for removal, so you would
want to pad the upper surface also.
 
Between padding the hammer and compliant mounting of your circuit (two
series low pass filters) you should be able to keep the acceleration of
your circuit module to something reasonable.
 
Hard epoxy potting compounds can put a lot of stress on parts from
differential thermal expansion, sometimes a compliant layer of a more
flexible material is used over sensitive components before potting with
hard epoxy.
 
Glen
 

Thanks, that’s interesting.
We’ve thought about a pad, but haven’t done any studies yet. The problem of
hard epoxy ripping things apart is pretty well known, I think—as you say,
the fix is a thin layer of RTV or something like that.

I didn’t know about rawhide-faced hammers, which sound cool. With hard
materials, the pulse width equals the length of the impactor divided by the
speed of sound in the material, which in this case is just about 20us.  The
force is the change of momentum divided by the pulse width.

If the pad compresses by 1 mm when the slide hammer arrives at 2 m/s, the
pulse width is about 500us, so the peak acceleration would be more like
1000 gees.

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics