Dytran 3089A User manual

Dynamic Transducers and Systems
21592 Marilla St. • Chatsworth, CA 91311 • Phone [phone]
www.dytran.com • e-mail: [email protected]
OG3089A
Rev A ECN 4949
REV B ECN 8291 02/08/12
OPERATING GUIDE
MODEL 3089A QUARTZ SHEAR MODE
MINIATURE IEPE ACCELEROMETER
NOTE:
Model 3089A is a lightweight miniature piezoelectric
accelerometer. The accelerometer is adhesively mounted and
incorporates integral wires.
This guide contains:
1) Operating instructions, Model 3089A.
2) Outline/installation drawing, Model 3089A
3) Specifications, Model 3089A
4) Paper, "Low Impedance Voltage Mode (LIVM) Theory and
Operation
NOTE: IEPE is an acronym for Integrated Electronics Piezoelectric types of low impedance voltage mode sensors
with built-in amplifiers operating from constant current sources over two wires. IEPE instruments are compatible
with most other manufacturers’ comparable systems.

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SPECIFICATIONS
MODEL 3089A IEPE ACCELEROMETER
SPECIFICATION VALUE UNITS
PHYSICAL
WEIGHT 0.6 grams
SIZE, DIA x HEIGHT 0.19 x 0.25 inch
MOUNTING adhesive
OUTPUT WIRES integral
WIRE LENGTH variable
HOUSING MATERIAL 300 Series Stainless Steel
PERFORMANCE
SENSITIVITY, ± 5% [1] 10.0 mV/g
RANGE F.S. FOR ± 5 VOLTS OUTPUT ± 500 g
FREQUENCY RANGE, ± 10% 1 to 10000 Hz
RESONANT FREQUENCY >50 kHz
EQUIVALENT ELECTRICAL NOISE FLOOR .0014 g rms
LINEARITY [2] ± 1% % F.S.
TRANSVERSE SENSITIVITY, MAX. 5 %
STRAIN SENSITIVITY (@250µε).012 g/µε
ENVIRONMENTAL
MAXIMUM VIBRATION/SHOCK ±600/±3000 g PEAK
TEMPERATURE RANGE -60 to +250 °F
SEAL, HERMETIC welded/gtm header
COEFFICIENT OF THERMAL SENSITIVITY .03 %/°F
ELECTRICAL
SUPPLY CURRENT [3] 2 to 20 mA
COMPLIANCE VOLTAGE RANGE +18 to +30 V
OUTPUT IMPEDANCE, TYP. 100 Ω
BIAS VOLTAGE +7 to +9 VDC
DISCHARGE TIME CONSTANT, NOM. 0.3 to 0.6 Sec
POLARITY (SEE OUTLINE DWG) positive
ELECTRICAL ISOLATION case grounded
[1] Measured at 100 Hz, 1 grms per ISA RP 37.2.
[2] Measured using zero-based best straight-line method, % of F.S. or any lesser range.
[3] Do not apply power to this device without current limiting, 20 mA MAX. To do so will destroy the integral IC
amplifier.

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OPERATING INSTRUCTIONS MODEL 3089A
IEPE ACCELEROMETER
INTRODUCTION
The Dytran Model 3089A accelerometer
features IEPE operation. The self-generating seismic
element, utilizing pure synthetic Alpha quartz, converts
acceleration to an analogous voltage signal. This very
high impedance signal is fed to the gate of a tiny on-
board IC MOSFET amplifier which drops the output
impedance level ten orders of magnitude allowing this
instrument to drive long cables without appreciable
effect on sensitivity and frequency response.
Simple constant current type power units
supply power to operate the integral IC and separate
the signal from the DC bias of the internal amplifier.
Coaxial cables or even twisted pair wire may be used
to connect accelerometer to power units. Power and
signal are conducted over the same two-wire cable.
Model 3089A is a miniature accelerometer
designed to be adhesive mounted. The accelerometer
features an integral cable.
The sensitivity of Model 3089A is 10 mV/g.
DESCRIPTION
The 3089A incorporates a GTM header for
hermetic seal.
The seismic masses, made from a very dense
tungsten alloy, are tightly preloaded through the quartz
crystals to the element base in shear mode. 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 a voltage
exactly analogous to the input acceleration. This
voltage is impressed across the gate of the MOSFET
IC.
PULLDOWN
RESISTOR
CURRENT REG
DIODE
COUPLING
CAP
+
QUARTZ ELEMENT BIAS RESISTOR
POWER UNIT
BIAS MONITOR
METER
DC POWER
SOURCE
SENSOR
TOTAL SHUNT
CAPACITANCE
MOSFET
AMPLIFIER
Figure 1-Electro-mechanical schematic,
accelerometer and power unit system.
When constant current from the IEPE power
unit is applied to the accelerometer amplifier source
terminal, the amplifier "turns on" at approx. +10 Volts
DC quiescent bias level. When the accelerometer
senses acceleration, the resultant signal is
superimposed upon this bias voltage. In the power
unit, in its simplest form, a capacitor blocks the DC
bias and allows the dynamic signal voltage to be
separated and brought out to an "output" jack on the
power unit.
At this point the signal may be connected
directly to almost any type of readout instrument such
as DVM's, oscilloscopes, data collectors, spectrum
analyzers, etc. The approximate 100 Ohm output
impedance of the signal allows the driving of long
cables without adverse effects on sensitivity or
frequency response.
Referring to figure 2, the gate resistor R
serves to bias the gate of the IC to its proper operating
point and it also, in conjunction with shunt capacitance
C, forms a first order high-pass filter which sets the
low frequency response of the accelerometer in
accordance with the following equation:
.16
f
-3db = ------------------ (eq.1)
RC
where:
f
-3db = lower -3db frequency (Hz)

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R = resistance value R (Ohms)
C = total shunt capacitance C (Farads)
RC = discharge time constant TC (Seconds)
Equation 1 above, defines the frequency at
which the accelerometer sensitivity will be 3db down
when compared to the reference sensitivity measured
at 100 Hz.
The discharge time constant for Series 3089A
is 0.3 TO 0.6 Sec. yielding a lower -3db frequency of
0.32 Hz, from equation 1.
As rule of thumb, the lower -5% frequency is
three times the -3db frequency or 3 x .16 = 0.64 Hz.
INSTALLATION
(Refer to Outline/Installation drawing 127-
3089A) To install Model 3089A, is necessary to
prepare (or find) a flat mounting area of approximately
1/8 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.
Connect the integral cable to the power unit in
accordance with drawing 127-3089A..
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 to two inches 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.
If the installation is made with a Dytran IEPE
power unit, observe the monitor voltmeter located at
the front panel of each of the power units. If the meter
reads in the mid-scale region, (labeled "Normal"), this
tells you that the cables, accelerometer and power unit
are functioning normally and you should be able to
proceed with the measurement.
Check for shorts in the cables and connectors
if the meter reads in the "Short" region. Check for
open cables or connections if the meter reads in the
"Open" area. In this manner, the meter becomes a
trouble-shooting tool for the measurement system.
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
resonance is approached. A filter incorporated into
Model 3089A compensates for this rise.
The frequency at which the sensitivity may
increases or decrease by 10% is approximately 10000
Hz, the frequency to which the 3089A is calibrated.
CAUTIONS
1) Do not store or use the 3089A above
250°F. To do so can damage the IC amplifier.
2) Do not allow wires to vibrate unrestrained.
This may eventually destroy the wires and could lead
to system inaccuracies.
3) Avoid dropping or striking the
accelerometer, especially against rigid materials such
as concrete and metals. While Model 3089A is
protected against shock induced overloads, the very
high overloads induced by dropping can do permanent
damage to the MOSFET amplifier or 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 3089A
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

5
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.
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 3089A 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.