rww@esl.UUCP (Richard W. Webb) (08/02/88)
Hello Electronics experts,
I remember reading recently (in the last 3 months) about a design
of a small chip based accelerometer. The principle was that of a
variable capacitor, where the movable plate was actually a piece
of deposited metal that overhung another metal well.
^
| Acceleration causes bending of the metal overhang.
v
\__________ __________________________/
XXXXXXXXXX\___________________XXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
The amount of acceleration is detected as a change in the capacitance
between the plates of the air gap as the metal overhang bends. The
degree of bending is slight, but the device can be made more sensitive
by making the length of the overhang larger. Fast response is not a
vital factor for our application either, as the sampling rates are
at most 100 Hz.
Some of my friends and myself are interested in producing small
scale seismographs to monitor earthquakes in the California area.
We currently have two (2) classical mechanical seismographs. But
now we are looking into the possibility of using single chip
accelerometers as the sensing elements in a network of seismographs.
Any references to actual sensors like these, articles about such
sensors, or any other related types of sensors would be greatly
appreciated.
Thanks,
--
Richard W. Webb ecvax!decwrl!borealis!\
ESL Inc. MS/302 sdcsvax!seismo!- ames!esl!rww
495 Java Drive (408) 738-2888 x5729 ucbcad!ucbvax!/ /
Sunnyvale, CA 94088 SMAIL: rww@esl.ESL.COM ihnp4!lll-lcc!henry@utzoo.uucp (Henry Spencer) (08/03/88)
In article <740@esl.UUCP> rww@esl.UUCP (Richard W. Webb) writes: > I remember reading recently (in the last 3 months) about a design > of a small chip based accelerometer... I suspect that they are not sensitive enough for seismic work, although I could be wrong. It's likely that eventually they will be, mind you... One notion that's being pursued at the research stage is adding a scanning-tunneling-microscope probe to the bottom of the sensing mass, so that movements of a fraction of an atomic diameter can be detected; this would be several orders of magnitude more sensitive than the best accelerometers (of any kind) built to date. -- MSDOS is not dead, it just | Henry Spencer at U of Toronto Zoology smells that way. | uunet!mnetor!utzoo!henry henry@zoo.toronto.edu
commgrp@silver.bacs.indiana.edu (08/03/88)
> I remember reading recently (in the last 3 months) about a >design of a small chip based accelerometer. The principle was >that of a variable capacitor, where the movable plate was >actually a piece of deposited metal that overhung another metal >well. > > ^ > | Acceleration causes bending of the metal overhang. > v >__________ __________________________/ >XXXXXXXXXX\___________________XXXXXXXXXX >XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX > >The amount of acceleration is detected as a change in the >capacitance between the plates of the air gap as the metal >overhang bends... >Some of my friends and myself are interested in producing small >scale seismographs to monitor earthquakes in the California area. >We currently have two (2) classical mechanical seismographs. But >now we are looking into the possibility of using single chip >accelerometers as the sensing elements in a network of seismographs. >Thanks >-- >Richard W. Webb ecvax!decwrl!borealis!\ I don't know where to get silicon accelerometers; they're probably very expensive. I once worked with silicon strain- gauges, which are extremely sensitive but have enormous temperature coefficients. Seismographs usually require a LARGE mass to detect acceleration at extremely low frequencies. An electronic accelerometer could be made using a mass attached to an LVDT (linear variable differential transformer), which is a transformer in a bridge configuration which is unbalanced by slight movements of its core. The associated electronics can be complex but there's a New Chip made especially for LVDT interface; the chip includes the AC exciter. I can't remember its number or manufacturer; Don Lancaster told me about it last month when I met him at the National Speleological Society convention. He will describe it in an upcoming "Hardware Hacker" column in _Radio Electronics_ magazine. Don says this chip should also make a nice bridge- balance-type metal detector. Another LVDT configuration is the "force-balance" transducer: Displacement of the LVDT core produces an error signal for a negative-feedback circuit which drives a DC solenoid which opposes the displacement, driving the LVDT back to center. The DC output is proportional to the applied force. This system is used in electronic scales. A friend who used to work with these says that LVDT's are easy to build; I don't know the details. -- Frank reid@gold.bacs.indiana.edu
chris@spock (Chris Ott) (08/08/88)
Frank (reid@gold.bacs.indiana.edu) writes: > > > > [Stuff about silicon accelerometers deleted] > >Richard W. Webb ecvax!decwrl!borealis!\ > > [Stuff about LVDT's deleted] > > Frank > reid@gold.bacs.indiana.edu We are also using LVDT's for a project. I feel I should mention they are very accurate. We are using them to measure something to within 1/1000 of an inch. Anyway, here's where we got them: Schaevitz Engineering U.S. Route 130 & Union Avenue Pennsauken, NJ 08110 Tex (609) 662-8000 Relevant technical bulletins: #7003B, #7007, #1002E I don't know how much the LVDT's cost, but the driver circuit, which is available from the same company, costs about $125. If you know the theory, you could probably build one yourself. They explain how an LVDT works in one of those technical manuals. Hope that helps. ------------------------------------------------------------------------------- Chris Ott Internet: chris@spock.ame.arizona.edu Computational Fluid UUCP: {allegra,cmcl2,hao!noao}!arizona! Mechanics Lab amethyst!spock!chris University of Arizona -------------------------------------------------------------------------------