On Mon, 8 May 2000, Charles R. Patton wrote: > Good work, John. Thanks, just a half hour out of my day. I'll post the whole derivation som= e day soon when I can recall what "time" means. The point of my analysis was= to get only at the gradient, not the absolute field. The absolute field is no= ise for gradient determination. I think the idea though is that the gradient i= s far more useful in "resolving" sub-surface structures. I am still skeptica= l about the claim that these out perform seismic reflection techniques in ter= ms of time and quality though. Who knows, maybe they are right. I can think = of a million academic applications for this thing once it reaches the relative= ly poor and destitute realm of science (as compared to defense and oil of course). > Someone mentioned bumps cancel out. Not so. The bump looks just like > acceleration and acceleration is indistinguishable from gravity. The > difference is only in the frequency range, gravity is DC, while the bump > is transitory so can be integrated out with a sufficiently time. The noise and main component of the field only cancel in the summed signals from the two devices. But this requires a rigid frame and two acceleromete= rs acting at very precisely the same response and output sensitivity; a great mechanical feat in its own right. I think you could deal with a little bit= of play in the mathematical analysis, but it would be nice to build something that works right in the first place. A very small error in the two radii o= f the accelerometers would be devastating for these measurements, so some kin= d of control would be needed on this parameter as the instrument deforms over time. > A corollary of this is that the bearing defects =96 imperfections in the > balls and races =96 show up as a frequency spectrum in the accelerometers > also and would have to be removed in the same way, by integration. Or addition of the signals... A spectral analysis of each would provide mo= re information about the absolute field, but the summed signals would be neede= d for the gradients. =20 > John comments about lead niobate, and barium titanate strike to the > heart of the matter. Piezo accelerometers do not have DC response but > are stable and require high input impedance amplifiers to achieve low > frequencies. I touched on this subject in a note to PSN 8/17/97: This is great info. I figured somebody else around here would know more ab= out the actual instrumentation than I do. Another problem with lead niobate an= d other "perovskite structure" piezo-electrics is that they are "ferri-electr= ic" meaning that they act like iron in a magnetic field, only this is applied t= o an electric field. This will give them a definite phase shift in their response which is also a problem to be worked out mathematically. > * Re: Piezo accelerometers for seismic work. Highly unlikely. I have > used extensively piezo-ceramic accelerometers of approximately the size > in the web site (http://www.oceana.inter.net) illustration (cantilever > beam types in SMT 1206 like packages from TDK, Murata, and others.) > All of these have sensitivities of around 1 to 2 mV /g. They have > typical capacities of around 200 pF. So in order to bias the front end > preamp you have to parallel them with resistors of 10 M or more. This > then sets the low frequency rolloff, i.e.: > f =3D 1 / (2PiRC) =3D 1/(6.28*200e-12*1e7)=3D79.6 Hz > So you have an extremely high cutoff problem first of all. Couple that > with the fact you've got the 10 M resistor generating a noise voltage of > approximately 300+ nV, you are left with a minimum sensitivity of only > g*300nV/1mV=3D300 ug at 80Hz and falling from there. > If you want to fight the problem of the resistor versus cutoff frequency > the noise will only go up. So you're kind of in a catch 22. I don't > anticipate that this is a good approach. >=20 > So higher capacity can lead to lower frequency response. Today, SMT > (surface mounting) capacitors (caps) use multi-layer technology to > achieve high capacitance values. This technique is just now moving into > the piezo accelerometers mentioned above. The caps and accelerometers > use identical materials with the main differences being in the > construction to utilize a =93proof mass=94 to put stress into the ceramic > during acceleration and the =93poling=94 of the ceramic. Poling requires > taking the ceramic to its Curie point, applying a high voltage to its > electrodes, then cooling the part off while the voltage is applied. One > variations of the proof mass and higher strain (as it is the strain > which generates voltage) is the cantilever mentioned above. One other > point needs making. The ceramic materials have high piezoelectric > coefficients, but also have temperature and mechanical instability > problems in high resolution applications which is why the expensive > (repeatable, calibrated) applications use quartz accelerometers. But > those accelerometers are relatively low capacity and output. A proof > mass on a diaphragm to form a variable capacitance is a possibility and > would have DC response, but since the given was spinning disks, one has > to assume this avenue did not have the performance required, so AC > accelerometers were being used. >=20 > For those of an experimental bent. Take an SMT cap, 0.01 to 0.1 uF and > mount it on a PCB. Ground one end and feed the other end through a 1 > Mohm resistor from 5 to 10 VDC or so. This biasing supplies substitutes > in some measure for the poling. Monitor the junction with an > oscilloscope. Tap or bend the PCB and you should observe 10=92s of mV on > the scope. Practical hints on the cap selection: the higher the > capacity, the smaller the package, and the lower the voltage is > generally better. This is because this will tend to lead to caps with > the high K dielectrics, which are the more sensitive in this mode. Plug > the numbers here into the formula above and you=92ll find that low > frequency response becomes much easier to achieve with these high > capacities. Engineering folklore tells of unaware engineers who chased > microphonic boards for many hours due to this phenomenon. >=20 > Anyway, back to the spinning disk gravimeter. Piezo accelerometers are > stable and will still have repeatable output for delta G=92s in the midst > of the constant G=92s even in the several hundred range such as the > centripetal acceleration you could generate on a spinning disk. You > could use magnetic accelerometers, which after all, is what the > geophones are. But again, the standard magnetic units also don=92t have > DC response. So the use of the spinning disk to convert the DC gradient > to AC is an excellent way around the DC problems. Additionally it > provides two other benefits: by going to AC it helps move the amplifier > away from the 1/F noise problem and it makes possible sensing the > absolute value of the G field. >=20 > A thought experiment: In the following I=92ll use Ge for earth > acceleration and Ga for centripetal acceleration. With a reasonably low > rotation rate that yields a 1 G centripetal acceleration in the > accelerometers set the disk vertical so the accelerometer will be > subjected to a cycle of Ga (0) , Ga minus- Ge (90), Ga (180), Ga plus > Ge (270) in a sine wave fashion. If we subtract the 0 and 180 degree > values from the 90 and 270 degree positions we are left with a sine wave > representative of 2 times the Ge value. The maximum phase position is > the angle of the Ge component. The Ga value is an absolute calibration > because we can measure rotation rate and positions to PPM levels. The > bearings are probably air spindles, in order to get below the 10 u in > runout, which would show up as equivalent accelerations (G > variations.) It would be an interesting gadget. Indeed! John Hernlund E-mail: hernlund@....... WWW: http://www.public.asu.edu/~hernlund/ ***************************************************************************= *** __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>