PSN-L Email List Message

Subject: Re: Article on gravimeters
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 ( 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.


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@........


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