This is a multi-part message in MIME format. ---------------------- multipart/alternative attachment =20 Recently the following statement appeared in one of the posts on = soundboards: =93An accelerometer transduces variations in pressure to a voltage.=94 =20 This statement is somewhat misleading and has bothered me for a couple = of days now. There are a variety of sensors used in industry. Those = sensors commonly known as pressure sensors are transducers designed to = convert gas or liquid pressure into an electrical signal and this is = some removed from the function of an accelerometer.=20 Force sensors are transducers designed to convert force=97and the word = pressure could also be used here=97into an electrical signal. So are = load cells but these are typically designed to detect static loads. You = might find a load cell in your bathroom scale or in the truck scale = along side the road. Force sensors are designed to measure and monitor = changing levels of compression and tension and impact forces=97i.e., = dynamic loads. The electric piano I referred to some time ago used a = number of force sensors mounted between a more-or-less conventional = bridge and a slightly compliant base. The vibrating strings caused = variations in the force applied to the force sensors which then created = an electrical signal that was processed and sent to amplifiers and = speakers. To approximate the tone of the acoustic piano the compliant = base was necessary to allow the bridge assembly to move in response to = the motion of the vibrating strings in approximately the same way it = moves in the acoustical piano. And now we get to accelerometers. Accelerometers are transducers or = sensors that convert the acceleration aspect of motion into an = electrical signal for measurement, monitoring and control purposes. The = signal produced by the accelerometer is proportional (within the design = limits of the sensor) to the rate of acceleration. 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. In 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... 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