Date: Tue, 7 Apr 98 10:01:07 PDT
From: evans@medicine.wr.usgs.gov (John Evans)
Message-Id: <9804071701.AA01180@medicine.wr.usgs.gov>
To: lahr@gldage.cr.usgs.gov
Subject: Re: home-built hand-held accelerometer
John,
Finally solving the communications problems to get my prototypes into
the field. Took far longer than expected (doesn't it always?) and I
now know much more than I ever wanted to about TCP/IP/UDP and RS-232).
I like your idea of the spring tube--very direct and visual and easily
built. Could be calibrated against gravity easily too. Might want to
use a chunk of scrap TEFLON for the mass so it will slide easily (add a
bolt through the center to secure the springs and increase the mass).
Teflon (R) works very easily (too easily), but NEVER get it really hot
(in other words, don't try to melt it)--LETHAL gasses result (envision
breathing through Teflon-coated lungs). A good plastics supplier or Si
Valley surplus house could supply bulk Teflon (R) for minor cost (or
free with a little talking?) in the small amounts needed.
OSH has plenty of springs--you'll want sloppy ones.
On the electronic side, I have one accel built for outreach--it could
be borrowed. It is nominally 1 V/g, but has a 0-g offset and has a
temperature dependent gain (and a temperature sensor to help). It is
VERY quiet (far quieter than they need on a roller coaster) but is a
bit tricky to use because of the offset and temperature dependence.
Also gets nonlinear somewhere between 2 and 3 g, so the wilder rides
might not measure accurately.
For a more permanent (and less tricky) solution I suggest that they
talk to Analog Devices (contacts below) which I suspect would comp or
low-cost a few of their ADXL-05 or the newer ADXL-202 accelerometers.
These are 5-g devices. The latter may not be available yet--I have an
engineering sample sitting on my desk here. The -05 is a one-axis
accelerometer and the -202 a two-axis version, each with noise of a few
percent of g peak-to-peak over the frequencies of interest. Plenty
good enough for roller coaster physics demonstrations, and have been
used for that purpose. Would need power, a little wiring, and a
voltmeter to read the output. (Both my ICS-3028 box and the ADXLs are
micromachined silicon devices, and very tough--good for kids to use.
However, none would like being dropped onto concrete, since that type
of shock actually reaches thousands of gs, and these typically are
rated to 2000 g shock resistance (don't try this with an FBS-23!).)
Analog Devices:
804 Woburn St
Wilmington MA 01887-3462
617-937-1534 <Jim Doscher, Mkt Manager (Accels).
jim.doscher@analog.com
617-761-7436 <Christophe LeMaire, Business Devel. Eng.
christophe.lemaire@analog.com
(You could tell either one that I sent you. My most recent contacts
are with LeMaire, who gave me this -202 sample.)
Teflon (R):
Surplus house: HalTek Electronics, 1062 Linda Vista Av, Mtn. View
(650-969-0510) (101-->Shoreline-->toward mountains, away from
Bay--> left at first light-->down to Linda Vista Av-->left to
Haltek, which is on your left). They have scrap Teflon (R),
including some large chunks. Alternatively, try Commercial
Plastics and Supply Corp. in Santa Clara near 101 and De La Cruz
(408-988-6500). They might comp a scrap piece big enough (they
did comp part of the Teflon (R) I bought for the last Open
House--a urethane-foam fault model that needed a slippery
lower-crust and slide blocks).
Good luck,
John
evans@andreas.wr.usgs.gov
Cc: Burt C. Kessler
pinhole@exploratorium.edu