5.2.1 The measured acceleration forces can be
collected and stored in either an analog or a digital format. The
primary components of either type of data acquisition systems are
shown in the block diagram presented in Figure 5.2. Following is a
brief description of the purpose and operation of each component.
A thorough discussion of data acquisition systems can be found in
the Shock and Vibration Handbook (Harris, 1988).
5.2 Primary Components of a Data Aquisition System
5.2.2 The output signal from the accelerometers
is an analog signal, i.e., it is a continuous signal that has a varying
voltage. The magnitude of the voltage can be correlated to the acceleration
force through use of a calibration factor. The signal from each accelerometer
is passed through a signal conditioner. Signal conditioners primarily
are used to amplify the accelerometer signal to a level that can be
recognized by the data storage device. If necessary, anti-aliasing
filters can be provided in the signal conditioning equipment. Signal
aliasing and the need for anti-aliasing filters are discussed later.
5.2.3 If an analog data acquisition system is
being used, the conditioned accelerometer signals are recorded on
either an FM (frequency modulation) recorder or a direct recorder.
With an FM recorder, the amplitude information of the signal recorded
as a deviation from a constant carrier frequency. The actual signal
is recorded when using a direct recorder. FM recorders are preferred
in analog systems for two reasons. First, the recorded signals are
less susceptible to change caused by poor storage conditions. Second,
the lowest frequency that can be recorded with a direct recorder is
about 25 hertz. With FM recorders, the signal is faithfully recorded
down to and including DC. A DC signal is effectively zero hertz. After
the accelerometer signals have been stored, they can be played back
through an analog-to-digital converter for processing and presentation.
Analog to digital converters change the continuous signal to discrete
values that have been sampled at uniform increments of time (Harris,
1988). Signal sampling will be discussed shortly.
5.2.4 When using a digital data acquisition system,
the conditioned accelerometer signals are individually selected at
each time-step with the multiplexer, converted to a digital value
of the analog signal at that time, and stored in digital form. At
every time-step, all of the signals are sampled. Digital recording
of the data eliminates many of the problems associated with analog
recording. These problems include wow, flutter, and limited dynamic
range (Harris, 1988). A problem with digital recording is managing
potentially large quantities of data.
5.2.5 Ideally, the data acquisition equipment
used to collect and record the measured acceleration force data should
be in the lifeboat during the launch. By placing the data acquisition
in the lifeboat, there are no external cables external which can interfere
with the behavior on the lifeboat. Equipment placed in the lifeboat
must be capable of operating on battery power and must be able to
withstand the forces exerted on it during water entry.
5.2.6 Excellent results also have been obtained
using data acquisition equipment that is located on the dock and connected
to the lifeboat with a waterproof umbilical cable. The mass of such
a cable should be small relative to the mass of the lifeboat and it
should be allowed to fall freely during the launch. By using the umbilical
cable in the manner, the forces exerted on the lifeboat by the cable
will be small. The headway of the lifeboat after water entry should
be restricted with a painter so that the umbilical cable will not
be stretched or become disconnected from the data acquisition equipment.