Crystal Instruments Coco-80 - CoCo-80 Spectral Analysis Operations; Selecting a CSA Project for Analysis; Setting Spectral Analysis Parameters

CoCo-80 User Manual
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The CoCo instrument handles these three issues effectively so you can get reliable
velocity or displacement signals from the acceleration measurement, or
displacement signals from the velocity measurement. The CoCo hardware has a
unique design to provide 130dB dynamic range in its front-end measurement.
The signals with high dynamic range will create better results after digital
integration.
Since such build-in integration is conducted in the time domain before any other
data conditioning or spectral analysis, the time streams generated after the digital
integration can be treated in the same way as other time streams. They can be
analyzed or recorded.
CoCo also provides differentiation and double differentiation to calculate the
acceleration or velocity from velocity or displacement transducers. Differentiation
is not as common as integration.
It must be noticed that the displacement after double integration to the
acceleration is not the same as that measured by a proximity probe. A proximity
probe measures the relative displacement between an moving object to the fixed
coordinates seated by the probe. The accelerometer and its integration value can
only measure the movement of the moving object against the gravity field.
Sensor Consideration
Accelerometer signals that are non-dynamic, non-vibratory, static or quasi-static
in nature (low acceleration of an automobile or flight path of a rocket) are
typically integrated in the digital domain, downstream of the signal conditioner.
Piezoelectric and IEPE accelerometers are commonly used to measure dynamic
acceleration and, therefore, dynamic velocity and displacement. They should not
be used to measure static or quasi-static accelerations, velocities, or
displacements because the IEPE includes analog high pass filtering in the sensor
conditioning that cuts out any low frequency signal. At frequencies approaching 0
Hz, piezoelectric and IEPE accelerometers cannot, with the accuracy required for
integration, represent the low frequency accelerations of a test article.
When this slight inaccuracy is integrated in order to determine velocity and
displacement, it becomes quite large. As a result, the velocity and displacement
data are grossly inaccurate. A piezoresistive or variable-capacitance accelerometer
is a better choice for low frequency signals and for integration. These types of
sensors measure accelerations accurately at frequencies approaching 0 Hz.
Therefore the integration calculation of velocity and position can be used to
produce accurate results.