Measurement Computing ZonicBook 618E rev.3.4 - Manual (Page 62)

6-14 Analog Signals 878595 ZonicBook/618E
Sensitivity
The sensitivity of an accelerometer is defined as its output voltage per unit input of motion. The unit
of motion used is “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 such as frequency, testing levels, and temperature.
An example: 100 mV/g at a frequency of 100 Hz, level +1 g, at 72°F. Note that, although a sensor may
have a “typical” sensitivity of 100 mV/g, its actual sensitivity could range from 95 to 105 mV/g (when
checked under stated conditions). Manufacturers usually provide 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 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 base-strain sensitivity is the output caused by a 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 in diameter than the accelerometer base) under the base reduces
contact surface area; and can substantially reduce the effects of base-strain. Note that 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. Use of 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, in upcoming text).
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 6 to 8 V bias and the second a 9 to12 V bias.
Operation of the two circuits is identical except for the available signal swing.
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, in which an electrical charge is produced by
the temperature gradient across the crystal.
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.