Dytran Instruments 3256C User manual
Dynamic Transducers and Systems
21592 Marilla St. • Chatsworth, CA 91311 • Phone 818-700-7818
OG3256C.docx
REV A ECN 4930 11/29/07
REV B ECN 11835 05/22/15
OPERATING GUIDE
MODEL 3256C
CHARGE MODE ACCELEROMETER
TOP CONNECTOR, HERMETICALLY SEALED
NOTE:
Series 3256C features hermetically sealed construction.
Hermeticity is obtained by all-welded construction and
glass-to-metal sealed connector. Case material is titanium.
This guide contains:
1) Operating instructions, Model 3256C
2) Outline/installation drawing, Model 3256C
3) Specifications, Model 3256C
2
OPERATING INSTRUCTIONS MODEL 3256C
INTRODUCTION
The Dytran Model 3256C is a super
lightweight charge mode accelerometer with 15
pC/g sensitivity.
The self-generating seismic element,
utilizing piezoceramic crystals in planar shear
mode, converts acceleration to an analogous
electrostatic charge mode signal. This very high
impedance signal is fed to the center pin of an
axial mounted 10-32 connector mounted to the
hex base of the accelerometer.
Dytran in-line IEPE charge amplifiers
(4751, 4705) convert the charge mode signal to a
low impedance voltage mode signal. Coaxial
cables or twisted pair wire may be used to connect
charge amplifiers to constant current power units.
Power and signal are conducted over the same
two-wire cable from in-line amplifier to power unit.
Model 3256C may also be used with
conventional laboratory type charge amplifiers.
DESCRIPTION
Figure 1, below, is a representative cross
section of series 3256C.
Figure 1-Cross section, series 3256C
The seismic masses, made from a very
dense tungsten alloy, and are tightly preloaded
against the ceramic crystals by means of a special
preload screw, to a flat crystal support, under
hundreds of pounds of force. This is so there is
absolutely no relative motion between mass,
crystals and base keeping the non-linearity low
and the natural frequency high.
The force from acceleration (vibration or
shock) acting upon the mounting base is
transferred to the seismic mass through the
crystals, stressing the crystals in shear mode and
producing an electrostatic charge signal
analogous to the input acceleration.
Charge mode accelerometers are
intended to be used, for the most part, with a very
high input impedance amplifier called a charge
amplifier. The charge amplifier is essentially a very
high gain inverting amplifier with capacitive
feedback. See figure 2 below.
Figure 2-Functional schematic, the charge
amplifier
CHARGE AMPLIFIER, BASIC THEORY
A charge amplifier is a special high gain,
high input impedance inverting voltage amplifier
with capacitive feedback. The amplifier is usually
an operational amplifier (op-amp) with near infinite
voltage gain.
Referring to Figure 2, the input charge qin
is applied to the summing junction (inverting input)
of the charge amplifier and is distributed to the
input capacitance of the amplifier CAand the
feedback capacitor Cf. We may write the equation:
q
in = qA+ qfEq1
Using the electrostatic equation q = Cv and
substituting in equation 1:
q
in = vACA+ vfCf Eq 2
SEISMIC MASS
MOUNTING SURFACE
10-32 TAPPED HOLE
10-32 CONNECTOR
LO-NOISE CHARGE AMP
PIEZOCERAMIC CRYSTAL
C
in
CA
f
S1
-A
Rf
-
+
-v out
3
Using equation 2 and making the appropriate
substitutions and solving for the output voltage of
the amplifier in terms of input charge, amplifier
loop gain, and input and feedback capacitance we
have:
-qin -qin 1
Vout = --------------------------- = ----------- x ------------------------
CA / A + Cf(A + 1) Cf(1 + 1 / A) 1 + CA / Cf(A + 1)
Eq3
where A is the open loop gain of the op-
amp.
Now, letting gain A approach infinity, we have:
-qin
V
out = ------- Eq 4
Cf
This result (Eq 4) shows clearly that the
transfer function (gain) of a charge amplifier is a
function only of the value of the feedback
capacitor Cf. Notice that input capacitance CAhas
no effect on the sensitivity of the charge amplifier.
This means that cable capacitance, for example,
has no effect on the sensitivity, a significant find
when switching cable lengths and types.
INSTALLATION
(Refer to Outline/Installation drawing 127-
3256C) To install Model 3256C, is necessary to
prepare (or find) a flat mounting area of
approximately 0.5 inch diameter. Ideally, the
mounting surface should be flat to .001 in. TIR.
The flat mounting surface ensures intimate contact
between accelerometer base and mounting
surface for best high frequency transmissibility,
thus accuracy.
At the center of the mounting area, drill
and tap a 10-32 mounting port in accordance with
instructions on drawing 127-3256C. Clean the
area to remove all traces of machining chips,
burrs, etc.
Next, thread the Model. 6200 mounting
stud into the base of the 3256C. The stud should
enter easily and thread in up to the raised collar of
the stud by hand. This collar prevents the stud
from bottoming inside the tapped hole in the
3256C where it could possibly cause stresses in
the base structure which could, in turn, cause
anomalous behavior of the accelerometer at
higher frequencies.
After seating the stud, spread a light
coating of silicone grease, or other lubricant, on
either of the mating surfaces and thread the
accelerometer/stud combination into the tapped
hole by hand, until the accelerometer base seats
against the mounting surface. Check to see that
the mating surfaces are meeting properly, i.e., that
they are meeting flush and that there is not an
angle formed between the two surfaces indicating
that they are not co-planar. If this condition is
observed, torquing the accelerometer down will
strain the base causing possible poor frequency
response and even erroneous reference
sensitivity. Inspect the perpendicularity of the
tapped hole.
If the hand tight meeting between the two
surfaces is satisfactory, torque the 3256C to the
mating surface with 15 to 20 lb-inches of torque,
preferably measuring the torque with a torque
wrench torquing on the hex surface only.
Proper torque will ensure the best high
frequency performance from the instrument as
well as repeatability of sensitivity when mounting
and remounting.
Connect the cable (typically low-noise
Models 6013AXX or 6019AXX) to the
accelerometer snugging up the threaded lock ring
tightly by hand.
NOTE: Do not use pliers or vise grips on
the knurled lock ring. This could damage
the connector of the 3256C and/or the
cable connector.
To avoid stressing the cables which could
lead to early failure, especially under larger
excursions of the test object, it is good practice to
tie the cable down to a fixed surface near the
mounting area at a point approximately one inch
from the accelerometer.
If there is excessive motion between the
accelerometer and the nearest tie point, allow a
strain loop of cable to let relative motion occur
without stressing the cable.
Connect the other end of the cable to the
charge amplifier. Consult the operating manual for
the particular charge amplifier employed for
instructions on its use.
4
HIGH FREQUENCY RESPONSE
All piezoelectric accelerometers are
basically rigid spring mass systems, i.e., second
order systems with essentially zero damping. As a
result, these instruments will exhibit a rising
characteristic as the resonant frequency is
approached.
The frequency at which the sensitivity may
increases or decrease by 5% is approximately
5,000 Hz. The accelerometer is usable above this
frequency but to use it above 5,000 Hz, it must be
calibrated at the specific frequencies of intended
use because sensitivity deviations will increase
drastically as you greatly exceed this high
frequency calibration limit. Consult the factory for
special calibrations required above 5kHz.
CAUTIONS
1) Do not store or use the 3256C above
375 degrees F.
2) Do not allow cables to vibrate
unrestrained. This will eventually destroy the cable
and could lead to system inaccuracies and noise.
3) Avoid dropping or striking the
accelerometer, especially against rigid materials
such as concrete and metals. While Model 3256C
is intrinsically immune to shock induced overloads,
the very high trauma induced by dropping can do
permanent damage to the mechanical structure of
the accelerometer. This type of damage is not
covered by the warranty.
MAINTENANCE AND REPAIR
The welded construction of the Model
3256C precludes field repair.
Should the mounting surface become
distorted, nicked and otherwise distressed, it can
be redressed by CAREFULLY wiping on a new
sheet of 400 grit emery paper on top of a clean
surface plate. We stress "carefully" because if not
done properly, this procedure can do more harm
than good. Press the surface firmly against the
paper and draw directly toward you in several
short precise strokes making sure that the surface
remains in full contact with the paper and does not
"rock". Rotate the accelerometer 90 degrees and
repeat the procedure. When you observe the
bottom surface it should appear perfectly flat with
straight marks across it. If you cannot achieve
flatness with several attempts, return the
instrument to the factory for repair.
Since charge mode accelerometers are
high impedance devices, it is essential that the
connector insulation be kept at a high level.
Should the electrical connector become
contaminated with moisture, oil, grease, etc., the
entire instrument may be immersed in degreasing
solvents to remove the contaminants. After
degreasing, place the instrument in a 200 to 250
degree F oven for one hour to remove all traces of
the solvent.
Should a problem be encountered with the
operation of the instrument, contact the factory for
trouble shooting advice. Often our service
engineers may point out something which may
have been overlooked and which may save the
expense and time of returning the 3256C to the
factory.
If the instrument must be returned, the
service department will issue you a Returned
Materials Authorization (RMA) number to aid in
tracking the repair through the system. Do not
send the instrument back without first obtaining an
RMA number. At this time you will be advised of
the preferred shipping method.
A short note describing the problem,
included with the returned instrument, will aid in
trouble shooting at the factory and will be
appreciated.
We will not proceed with a non-warranty
repair without first calling to notify you of the
expected charges. There is no charge for
evaluation of the unit.
