From: "Charles R. Patton" charles.r.patton@........

Date: Mon, 08 May 2000 15:23:27 -0700

Good work, John. A few comments. Ball bearings in today=92s hard disk drives are in the 10 u inch runout class or so. Hydrodynamic, gas or liquid, bearings are required below this. The trade is between viscous drag =96 power consumption =96 and start/stop performance. Gas, generally air, applications usually have a compressed air supply to run from. 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. 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. 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: * 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. 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. 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. 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. 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. Regards, Charles R. Patton, Owner Synergy Co. =93Creative, Cost Effective EMI, ESD and Analog Design Solutions=94 21490 Camino Arriba Murrieta, CA 92562 Phone: 909-698-9657 Fax: 909-698-0224 Email: charles.r.patton@........ __________________________________________________________ Public Seismic Network Mailing List (PSN-L)

Larry Cochrane <cochrane@..............>