Omega Engineering OMB-DBK-34A, User Manual

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DBK4, pg. 12
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DBK Option Cards and Modules
Sensitivity
The sensitivity of an accelerometer is defined as its output voltage per unit input of motion. The unit of
motion used is the “g”. One “g” is equal to the gravitational acceleration at the Earth’s surface, which is
32.2 ft/(sec)(sec) or 981 cm/(sec)(sec). The output is usually specified in millivolts per “g” (mV/g).
Sensitivity is usually specified under defined conditions (frequency, testing levels, and temperature), for
example: 100 mV/g at a frequency of 100 Hz, level +1 g, at 72°F.
While a given sensor model may have a “typical” sensitivity of 100 mV/g, its actual sensitivities may range
from 95 to 105 mV/g when checked under stated conditions. The manufacturer typically provides sensor
calibration values.
Transverse Sensitivity
An accelerometer is designed to have one major axis of sensitivity, usually perpendicular to the base and
co-linear with its major cylindrical axis. The output caused by the motion perpendicular to the sensing axis
is called the transverse sensitivity. This value varies with angle and frequency and typically is less than 5%
of the basic sensitivity.
Base-Strain Sensitivity
An accelerometer’s strain sensitivity is the output caused by deformation of the base due to bending in the
mounting structure. In measurements on large structures with low natural frequencies, significant bending
may occur. Units with low base-strain sensitivity should be selected.
Inserting a washer smaller than the accelerometer base under the base to reduce the contact surface area
can substantially reduce base-strain effects. This technique lowers the usable upper frequency range.
Acoustic Sensitivity
High-level acoustic noise can induce outputs unrelated to vibration input. In general, the effect diminishes
as the accelerometer mass increases. Using a light, foam-rubber boot may reduce this effect.
Frequency Response
An accelerometer’s frequency response is the ratio of the sensitivity measured at frequency f to the basic
sensitivity measured at 100 Hz. This response is usually obtained at a constant acceleration level, typically
1 g or 10 g. Convention defines the usable range of an accelerometer as the frequency band in which the
sensitivity remains within 5% of the basic sensitivity. Measurements can be made outside these limits if
corrections are applied. Care should be taken at higher frequencies because mounting conditions greatly
affect the frequency range (see
Mounting Effects below).
Dynamic Range
The dynamic measurement range is the ratio of the maximum signal (for a given distortion level) to the
minimum detectable signal (for a given signal-to-noise ratio). The dynamic range is determined by several
factors such as sensitivity, bias voltage level, power supply voltage, and noise floor.
Bias Level
Under normal operation, a bias voltage appears from the output signal lead to ground. There are two basic
MOSFET configurations commonly used. One exhibits a 7-8 V bias and the second a 9-12 V bias.
Operation of the two circuits is identical except for the available signal swing. The low-voltage version
typically exhibits 5-10 µVrms versus 10-20 µVrms for the high voltage.
Thermal Shock - Temperature Transients
Piezoelectric accelerometers exhibit a transient output that is a function of a temperature’s “rate-of-
change”. This “thermal shock” is usually expressed in g/°C and is related to:
•
Non-uniform mechanical stresses set up in the accelerometer structure
•
A pyroelectric effect in piezoelectric materials—an electrical charge is produced by the
temperature gradient across the crystal.