4
to accommodate the integral stud. Clean the area to
remove all traces of machining chips, burrs, etc.
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 3049D 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. Excessive torque could damage the
ground isolation base.
Connect the cable, typically Models 6013AXX
(10-32 to 10-32) or 6019AXX (10-32 to BNC) to the
accelerometer snugging up the threaded lock ring
tightly by hand.
NOTE: Since this is a charge mode instrument, only
low-noise treated coaxial cable must be used to
connect this instrument to its charge amplifier. Both
cables that we recommend are low-noise treated.
NOTE: Do not use pliers or vise grips on the knurled
lock ring. This could damage the connector of the
3049D 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 input
of the charge amplifier and switch the power on.
THE CHARGE AMPLIFIER
Model 3049D is a piezoceramic instrument
and as such, it is usually used with an AC-coupled
charge amplifier rather than a direct-coupled
electrostatic charge amplifier such as those designed
for use with quartz sensors. Because of the reduced
insulation resistance of piezoceramic materials, direct
coupled charge amplifiers may drift when the 3049D is
connected to the input unless there is a provision for
AC coupling at the input or reduced time constant
such as featured in Dytran laboratory charge
amplifiers.
For this reason, it is best to use one of the in-
line series of amplifiers such as the 4705A, 4751B or a
true vibration type laboratory charge amplifier which is
AC coupled or has provision for piezoceramic sensor
inputs. These amplifiers convert to 3049D to 2-wire
IEPE operation. Again, remember that all of Dytran’s
charge amplifiers are inverting amplifiers which will
result in a reversal of the signal polarity of the 3149C.
HIGH FREQUENCY RESPONSE
All piezoelectric accelerometers are basically
rigid spring mass systems, i.e., second order
mechanical systems with essentially zero damping. As
a result, these instruments will exhibit a rising
characteristic as the resonant frequency is
approached. Some charge amplifier feature low-pass
filtering to compensate for this characteristic.
The upper frequency at which the sensitivity
may increase or decrease by 15% is approximately
10,000 Hz, the frequency to which the 3049D is
calibrated. The accelerometer is usable above this
frequency but to use it above 10,000 Hz, it must be
calibrated at the specific frequencies of intended use
because sensitivity deviations will increase drastically
as you exceed this high frequency calibration limit.
Consult the factory for special calibrations required
above 10,000 Hz.
