Message-Id: <3.0.32.19980408225937.00856540@pop3.1stlink.net>
Date: Wed, 08 Apr 1998 22:59:39 -0600
To: pinhole@exploratorium.edu
From: The Lahrs <john-jan@lahr.org>
Subject: accelerometers for Great America
Burt,
I posted your accelerometer question on the Public Seismic
Network (PSN) http://psn.quake.net/ , listserver and
some of the suggestions are repeated below. A lot of the PSN
members have built their own seismic stations, and so have
given some thought to this sort of measurement.
John Lahr
> For Great America, I would think that measuring acceleration
> to + or - 1 or 2 tenths would be sufficient. Electronic
> means would be great, but may be overkill, unless you
> have a hand held unit that they could borrow.
>
> Maybe just a clear plastic tube with two springs and a
> weight?
>
> _______
> | S |
> | P | - 1.g
> | R |
> | I |
> | N | - 0.5g
> | G |
> | ^ |
> | mass|
> | v |
> | S |
> | P | + 0.5g
> | R |
> | I |
> | N | + 1.0g
> | G |
> _______
>
> If the mass is close fit in the tube, then air will help to damp
> the motion, which is desirable.
"Charles R. Patton" <pattonKILLSPAM@dt.wdc.com> suggests:
Take a piece of plastic
strip like a 1" wide piece of a plastic divider from a notebook, or a
vertical slice from a plastic milk bottle or pop bottle. A "straw" the
diameter of pencil lead from a plastic broom is still another
possibility. In any case, use one of these as a horizontal spring.
Take a small cardboard box and cut a square hole in it. Tape one end of
the "spring" in the box, horizontally across the hole with the end of
the spring adjacent to the edge of the hole on the other side. Now
depending on the strength of the "spring" you can add weights, such as
pennies, marbles, lengths of wire, etc. one unit of weight is so much
deflection, and then is marked on the edge of the hole opposite the
"spring". 10 units is so much more. If you leave the 10 units attached
permanently, you have just marked a scale in 0.1's of a G. Turn upside
down and calibrate in other direction. Now you have a plus and minus G
meter.
Refinements are to cover the hole with Saran wrap in order to prevent
drafts from bothering it -- those 30+ MPH winds on the roller coaster
can play havoc with the calibration. Cutting the hole with a radius to
match the "spring" will improve accuracy. Use of a strip of plastic
rather than a "straw" will confine its reaction primarily to be
perpendicular to the plane of the "spring", or even better, two "straws"
spreading apart to the attachment point -- a triangle when viewed from
above. The side rigidity would be excellent this way so sine/cosine
vector resolution of forces could be attempted.
Cheap, fast, needs no machining, test equipment, electric power, simple
to calibrate, and uses materials found around the house.
--------------------------------------
| box |
| --------------------------- |
| | | |
| | | |
| S | plastic | |
| C | "spring" | tape|
| A |==X========================XXXX|
| L | X | |
| E | weight(s) | |
| | | |
| | | |
| --------------------------- |
| |
| |
--------------------------------------
Charles R. Patton
**********************************************************************
To which, John Evans <evans@andreas.wr.usgs.gov> added:
I also like your (Charles R. Patton's) idea--very accessible to school
kids. One refinement
probably is needed--damping. Without damping, a seismometer is nearly
useless, primarily demonstrating its own resonant frequency. The same
caveat applies here.
I suggest air damping by attaching a paperboard or mesh vane to the
mass-end of the spring, with the vane's normal vector parallel to the
direction of motion (i.e., so the vane must push broadside through a
lot of air when the mass moves). Indeed, a piece of aluminum sheet or
some aluminum window screen might make a good self-damping mass out at
the end of your spring.
Test for proper damping: (1) pull and release the mass; (2) it should
make about one cycle before coming to a rest:
*
*
*
*
..........*.....................................* * * * * * *
* *
* *
Time -->
Overdamping (where the mass just settles exponentially back toward zero
when released and never crosses zero) is better than underdamping in
this (dynamic) application. Of course, a plastic-wrap cover would now
be essential.
In John Lahr's version, there is inherent gas damping that can be
controlled by controlling the gap between the teflon mass and the wall
of the plastic tube. It is inherently protected from wind effects.
Might also be tougher. A down side with the tube design is the need to
anchor the ends of the springs. Not many hardware stores carry those
end-on spring anchors, though they are available.
--John
evans@andreas.wr.usgs.gov
-- * John C. and Jan H. Lahr * * john-jan@lahr.org * * 914 10th Street * * Golden, Colorado 80401 * * (303) 215-9913 * * http://www.lahr.org/john-jan * * http://giseis.alaska.edu/lahr *