Measurement Computing WaveBook rev.3.0 - Manual (Page 46)

3-14 WBK14, WBK Expansion Options WaveBook User’s Manual
The MOSFET circuit will bias off at approximately 12 V in the quiet state. As the system is excited, voltage
is developed across the crystal and applied to the gate of the MOSFET. This voltage will cause linear
variation in the impedance of the MOSFET and a proportional change in bias voltage. This voltage change
will be coupled to the WBK14 input amplifier through the capacitor C. The value of R and the internal
capacitance of the piezoelectric crystal control the low frequency corner. Units weighing only a few grams
can provide high level outputs up to 1 V/g with response to frequencies below 1 Hz.
Accelerometer Specification Parameters
Noise in Accelerometers. The noise floor or resolution specifies lowest discernible amplitude
(minimum “g”) that can be measured. There are two main sources of noise as follows:
• Noise from the crystal and microcircuit inside the accelerometer. Some types of crystals, such as
quartz, are inherently more noisy than others. A good noise floor is 10 to 20 µV.
• Noise from electrical activity on the mounting surface. Since the signal from the accelerometer is a
voltage, 60 Hz or other voltages (ground loop, etc) can interfere with the signal. The best protection is
to electrically isolate the accelerometer.
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 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.