Initialization

Each type of A/D converter has an associated API set of functions that control the converter operation. The functions control the number of storage buffers used, sample rates, channel assignments for unique sample rates, and methods of acquiring and downloading sample data. PCs with a newer BIOS can be set to boot without the presence of a keyboard.

The analog inputs were set for 2- wire differential inputs with a +/- 10V range. Since the same frequencies were of interest on each sensor, the sample rates for each input including the discrete inputs were the same. Therefore, each input was sampled at 1/9th of the total sample rate capacity of 100 kS/s. If higher vibration frequencies of interest on some sensors, then a higher sample rate for that input are needed.

Buffer Control and Data Storage

The A/D converters each used four buffers to rotate the input data to different buffers allowing others to be downloaded without missing sample data. During the mass body study, data was continuously sampled and each buffer was downloaded into the Marinus. This keeps the converter buffers current and downloaded. The size of the buffers depend on the number of channels sampled and the sample rates. The software API sets an interrupt to trigger the data acquisition software to download a buffer load of data.

Each buffer download was pre-pended with a marker bit pattern (DEAD BEEF in Hexadecimal) along with the time from the PC down to milliseconds. The PC time is also used to form the last part of the data file name. The first part of the name is the A/D converter designation. The file name is formed when the first data block is written to disk. This file name is kept throughout the test until the next time the power is cycled on the PC. This keeps the data file names unique and relevant to the testing done on a certain day and time.

Since power interruptions are always possible during a test, the data acquisition program always closed the data file after each data block save. The file was then reopened again at the start of the next data read. This action kept the hard disk directory current and kept the file available and readable if the test was prematurely terminated due to a power failure. The Marinus PC had plenty of time to write buffers from two A/D converters opening and closing each file during each read for the duration of the testing.

Data Retrieval

After each test, the power was removed from the Marinus PC leaving the accumulated buffers on the BiTMICRO flash solid state hard disk. A laptop computer with an Ethernet crossover cable was used to connect the Marinus PC to a network. The hard drive was previously set to Shared status so an external PC could access the drive and using the standard Copy command to copy the data to a remote location for analysis.

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