This is a multi-part message in MIME format. ---------------------- multipart/alternative attachment ----- Original Message -----=20 From: Delwin D Fandrich=20 To: pianotech@ptg.org=20 Sent: Thursday, January 17, 2002 12:21 PM Subject: Accelerometers, etc. Recently the following statement appeared in one of the posts on = soundboards: =93An accelerometer transduces variations in pressure to a=20 voltage.=94 =20 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. . 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. An accelerometer is basically a microphone. A specialized one that = first of all is a contact mic and usually calibrated for critical = measurement. 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. 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. 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. By the way, the magnetic pick up for the sat is a form of = accelerometer. The plate moves.=20 Carl Meyer Assoc. PTG Santa Clara, California cmpiano@attbi.com=20 principle accelerometers are quite simple. They consist of a base, a = slice of piezoelectric material=97usually quartz or an artificially = polarized ferroelectric ceramic=97and 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=97whether that be stationary or at some steady = velocity=97there is no electrical charge generated. 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. When appropriately mounted to the top of a vibrating body=97such as = the bridge of a piano as it is being played=97an 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. 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=92ve used beeswax to temporarily bond accelerometers = to the tops of bridges=97removing the strings from one adjacent unison = usually provides enough room=97and in various places around the rim and = structure. I=92ve 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=92ve not used = them. When looking at vibrations in plates I=92ve simply glued the = accelerometer to the plate surface using beeswax. Del Delwin D Fandrich Piano Designer & Builder Hoquiam, Washington USA E.mail: pianobuilders@olynet.com Web Site: www.pianobuilders.com ---------------------- multipart/alternative attachment An HTML attachment was scrubbed... URL: https://www.moypiano.com/ptg/pianotech.php/attachments/7c/5d/6e/26/attachment.htm ---------------------- multipart/alternative attachment--
This PTG archive page provided courtesy of Moy Piano Service, LLC