Re: Accelerometers for >1000g measurements

On 2024-09-05 06:02, John R Walliker wrote:

On 05/09/2024 07:22, Jan Panteltje wrote:

On a sunny day (Wed, 04 Sep 2024 19:57:04 -0700) it happened john larkin
<jlarkin_highland_tech> wrote in <md7idj1plqodnthuqpcemaphbrtotlqveh@4ax.com>:
>

On Wed, 4 Sep 2024 18:30:46 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
>

So for this customer gig I need to measure the actual acceleration of a
parallel-rod transmission line that's being pounded into the ground with
a built-in slide hammer.  (It's for measuring soil moisture and salinity
by TDR.)
>
We're thinking about putting the TDR pulser and sampler in the part that
gets pounded (in a potted module obviously), so knowing how bad the
acceleration gets is going to be important.  I expect that it'll be
several hundred g in volcanic soil, so a full-scale range of 1000-2000 g
would be about right.
>
None of the MEMS IC accelerometers go anywhere near that high.
Measurement Specialties makes them, but they're $160 in onesies, e.g.

>
>
If you can make it emit some ultrasonic sound you can measure the received frequency shift from far away?
Same for RF likely...

 Attach an adhesive label printed with a black and white wedge to
the rod.  Illuminate with uniform light and view with a photodiode
lens slit combination.  Capture the velocity waveform on 'scope.
Calibrate at low speed.
I expect the acceleration peak might only last a few microseconds
but could be many thousands of g.
 John
 

It's not that hard to estimate.  The hammer part is about 130 mm tall, and the speed of a compressive wave in steel is 6 km/s or so.  Thus the impulse will last at least 22 us.   If the hammer and the electronics assembly have about the same mass, and everything is perfectly elastic, in the collision the hammer will go from speed v to zero, and the electronics will go from zero to v (the same speed as the hammer).
If the hammer speed is 10 m/s at impact (probably an overestimate), the acceleration will be about
10 m/s / 22 us = 450 000 m/s**2, or about 45000 g, quoted to about two more significant figures than I'm entitled to.
In real life the hammer won't be that fast, but the electronics assembly is heavier than the hammer and won't react as a unit.  It'll be that order of magnitude in the limit of very stiff material.  A compliant layer would reduce that, and so will the right choice of potting material, but the effects on the cables have to be considered.
Anecdotally it can take as much as 150 hammer blows to get the 60 mm tines fully inserted in the soil, so we're going to need this gizmo to survive at least 1E6 impulses over a 10-year life.
And then there's the ferrite-loaded shotgun balun to go from coax to the balanced line. ;)
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510
http://electrooptical.net
http://hobbs-eo.com