Omega Engineering OMB-DBK-34A, User Manual

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DBK Option Cards and Modules
958293
DBK4, pg. 13
This quasi-static effect produces a low-frequency voltage input to the MOSFET amplifier. This voltage is
usually well below the low-frequency corner, but the effect can reduce the peak clipping level and cause
loss of data. This effect does not affect the accelerometer’s basic sensitivity or the data unless the thermal
shift in the operation bias level results in clipping. Where drastic thermal shifts are expected, use 12 V bias
models. The effect’s severity is related to the mass of the accelerometer. In 100 mV/g industrial units, the
effect is usually negligible. Using rubber thermal boots can reduce the effect significantly.
Overload Recovery
Recovery time from clipping due to over-ranging is typically less than 1 ms. Recoveries from quasi-static
overloads that generate high DC bias shifts are controlled by the accelerometer input RC time constant that
is fixed during manufacture.
Power Supply Effects
The nominal power supply voltage recommended by most manufacturers is 15 to 24 V. Units may be used
with voltages up to 28 volts. Sensitivity variations caused by voltage change is typically 0.05%/volt.
Power supply ripple should be less than 1 mVrms.
Connector
This parameter specifies the connector type and size (4-48, 6-40, 10-32 coaxial etc) and the location on the
sensor, i.e., top or side (usually on the hex base). Where there is no connector on the sensor, an integral
cable is specified with the length and the connector, i.e., integral 6-ft to 10-32.
Physical Setup
Mounting Effects
Adding an accelerometer to a test object may influence the object’s mechanical resonance and/or reduce
the resultant vibration level. The mass of the structure and accelerometer as well as the position on the
structure can affect the measurement. For example, an accelerometer with a small mass relative to a car’s
mass will affect measurements on the hood when placed directly in the hood’s center. The same
accelerometer placed on the more massive frame will have little effect.
The resonance change in a simple spring-mass system caused by adding an accelerometer may be
approximated by:
∆
f = fn {1 - [m/(m - ma)]
½
}
Where:
∆
f = Change in Resonant Frequency
fn = Original Resonance Frequency without Accelerometer
m = Spring Mass
ma = Mass of Accelerometer
Threaded Stud Mount
The preferred method for attaching an accelerometer to the test surface is by the threaded stud mount. This
method fuses the accelerometer and test surface. When the stud mount is impractical, an adhesive
installation is usually a good alternative.
Adhesive Mounts
Some accelerometers can be glued directly to the test surface. Others must use mounting adapters (bases).
These adapters are normally glued to the test surface and the accelerometers are stud-mounted to them.
When applying a layer of adhesive, keep in mind that the adhesive creates a new spring-mass system with
lower high-frequency response. To avoid a thick glue line, Cyanocrylate adhesives work well because:
•
They cure quickly, within seconds.
•
Glue lines can be kept ultra-thin by using glue sparingly and pressing the accelerometer to the
surface with adequate force.
•
Cleanup is easy since these adhesives will dissolve with acetone.