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<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: =
black"><B>From:</B>
<A title=pianobuilders@olynet.com
href="mailto:pianobuilders@olynet.com">Delwin D Fandrich</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A =
title=pianotech@ptg.org
href="mailto:pianotech@ptg.org">pianotech@ptg.org</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Thursday, January 17, =
2002 12:21
PM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Accelerometers, =
etc.<BR></DIV>
<DIV><FONT color=#000080>Recently the following statement appeared =
in one of
the posts on soundboards:<BR>“An accelerometer transduces variations =
in
pressure to a </FONT></DIV>
<DIV><FONT color=#000080>voltage.” </FONT></DIV>
<DIV><FONT color=#000080></FONT> </DIV>
<DIV><FONT color=#000080><FONT face=Arial color=#000000 =
size=2>I agree that a
better understanding of the testing methods would be helpful to those =
who
don't have an electronic background. Thanks for that post. =
I'd
like to add to that understanding.</FONT></DIV>
<P class=MsoNormal>. When used to detect acceleration in a vibrating =
body or
system the accelerometers signal can also be electronically integrated =
to
indicate both the velocity and displacement of the vibrating =
object.</P>
<P class=MsoNormal><FONT face=Arial color=#000000 size=2>An =
accelerometer is
basically a microphone. A specialized one that first of all is a =
contact
mic and usually calibrated for critical measurement.</FONT></P>
<P class=MsoNormal><FONT face=Arial color=#000000 size=2>You =
alluded to this,
but let me stress that an accelerometer is generally an AC =
coupled
device and a strain gauge is a DC coupled device. AC coupled can =
also be
called capacitive coupled, so no static measurements can be =
made. For
example, if you mount a strain guage and an accelorometer to a =
beam and
then suddenly bend the beam to produce 1 volt output from each sensor, =
the
strain guage will stay at 1 volt as long as the bend remains, but the
accelerometer output will decay to zero at the rate of the rc time
constant.</FONT></P>
<P class=MsoNormal><FONT face=Arial color=#000000 =
size=2>Attaching
accelerometers to anything becomes problematic since the device =
affects the
resonance of the item under test and adds mass. The bonding is =
also
critical as you say. At some point errors can be introduced and =
its not
easy to identify the problem.</FONT></P>
<P class=MsoNormal><FONT face=Arial color=#000000 size=2>I =
hope this doesn't
add to the confusion. I have an accelerometer and a four trace
scope. I'll get some test made some day.</FONT></P>
<P class=MsoNormal><FONT face=Arial color=#000000 size=2>By =
the way, the
magnetic pick up for the sat is a form of accelerometer. The =
plate
moves.</FONT> </P>
<P class=MsoNormal><FONT face=Arial color=#000000 size=2>Carl =
Meyer
Assoc. PTG<BR>Santa Clara, California<BR><A
href="mailto:cmpiano@attbi.com">cmpiano@attbi.com</A> </FONT></P>
<P class=MsoNormal> principle accelerometers are quite simple. =
They
consist of a base, a slice of piezoelectric material—usually quartz =
or an
artificially polarized ferroelectric ceramic—and a seismic mass. The =
crystal
is placed between the base and the seismic mass. When the assembly is =
set in
motion the seismic mass (wishing to remain stationary) induces a =
mechanical
stress in the crystal which causes it to generate an electrical charge =
across
its pole faces. This electrical charge is proportional to the applied =
force.
Obviously this charge is generated only when there is acceleration. =
When the
accelerometer has achieved some steady state—whether that be =
stationary or at
some steady velocity—there is no electrical charge generated.</P>
<P class=MsoNormal>Accelerometers do not measure or indicate force =
or
pressure. Force or pressure applied to any part of an accelerometer =
will have
no effect on its output unless there is some acceleration involved. =
Or, I
suppose, if the force is great enough to cause physical damage to the
accelerometer housing in which case there will be some signal output =
but it
may be quite brief.</P>
<P class=MsoNormal>When appropriately mounted to the top of a =
vibrating
body—such as the bridge of a piano as it is being played—an =
accelerometer will
produce an electrical signal proportional to the physical vibratory =
motion of
the bridge at the mounting point of the accelerometer. Since =
accelerometers
are unidirectional they indicate acceleration (or velocity or =
displacement) in
only one direction. Most tests I have done on the mobility of the
bridge/soundboard assembly have involved studying the vertical motion =
(in a
grand) of the bridge. Usually this involved mounting the accelerometer =
just
beside the note in question. To measure the for-and-aft motion of the =
bridge
the accelerometer is simply turned on its side so its axis is aligned =
in the
direction you want to investigate. (Or you can spend a small fortune =
and
purchase a triaxial accelerometer.</P>
<P class=MsoNormal>There are several ways to mount an accelerometer =
on a test
object. The least destructive is to simply glue it in place. The glue =
can be
as benign as beeswax or as intrusive as some epoxy or CA adhesive. In =
trying
to figure out the motion of the agraffe, a CA adhesive was used to =
mount an
accelerometer to the top of a prepared agraffe (the top had been =
milled flat
to provide a good mounting surface for the accelerometer). I’ve used =
beeswax
to temporarily bond accelerometers to the tops of bridges—removing =
the strings
from one adjacent unison usually provides enough room—and in various =
places
around the rim and structure. I’ve also used studs screwed into =
holes drilled
into the bridge between unisons such that the accelerometer ended up =
locked
down tight with its base just above the strings. This lowered the =
resonant
frequency of the accelerometer some but (according to the =
manufacturer) not
enough to worry about for the tests I had in mind. Magnetic bases are
available from most accelerometer manufacturers but I’ve not used =
them. When
looking at vibrations in plates I’ve simply glued the accelerometer =
to the
plate surface using beeswax.</P>
<P class=MsoNormal>Del</P></FONT>
<DIV><FONT color=#000080>Delwin D Fandrich<BR>Piano Designer &
Builder<BR>Hoquiam, Washington USA<BR>E.mail: <A
=
href="mailto:pianobuilders@olynet.com">pianobuilders@olynet.com</A><BR>=
Web
Site: <A
=
href="http://www.pianobuilders.com">www.pianobuilders.com</A></FONT></D=
IV></BLOCKQUOTE></BODY></HTML>