Anti-Aliasing Filters

Anti-Aliasing filters were used on each differential channel to limit the vibration frequency response to the range of frequencies of interest for each mass body test. This keeps higher frequencies from being sampled at a lower rate and appearing in the data erroneously. One filter was required for each analog channel being sampled. The cutoff frequency is the frequency represented by the Nyquest rate of the time shared sample rate for an individual channel. (i.e. 100,000 samples / 9 channels = 11,111 samples. Divide by 2 (the Nyquest rate) equals 5,555 Hz.)

Test Sensors

The mass body study used accelerometers to provide differential analog signals on three axis in one package. The output voltage or each accelerometer is +/- 10 volts.

3-Axis Accelerometer

Sensor and Control Signals

The Marinus computer has two USB sockets allowing for two A/d Converters and associated accelerometers. The Marinus computer is fast enough to download the data buffers from each

Mass Body Overall Block Diagram

A/D converter and store the data in separate files for analysis. In actual practice, only one data acquisition program was written and associated to a unique A/D USB driver. The second instance of the program was associated to the second A/D USB driver. This allowed both instances of the data collection programs to write independent data to the BiTMICRO flash solid state hard drive with different file names. Both A/D converters monitored the same RECORD On/OFF switch.

The mass body sensor system major components are all approximately the same size and operate off of a single power source from the test fixture. This allows the mass body data collection system to occupy a small area and to operate with a minimal number of control lines (power and RECORD ON/OFF switch) to the outside.


A/D Converter Signal Lines

In the above figure, the Marinus top cover which included the speaker, power-on switch, and reset switch, has been removed to allow for the mounting of the BiTMICRO solid state flash drive. The remaining PC chassis is shock mounted to the mass body frame in such a way to allow for the connection of the Ethernet cable for control and downloading of data after a test.

The A/D converters are mounted separately and as close as possible to the accelerometers and aliasing filters. Care must be taken to ensure a good solid connection from the accelerometer cables through any connectors to the A/D converter inputs. Loose connector pins will cause noise in the sensor line that will be interpreted as vibration in the mass body.

Each of the discrete input lines from the accelerometer lines are further conditioned with a 10K ohm resistor to common ground. This greatly eliminated noise on the line and in the data.

PC Automatic Operation

The computer must be able to start from a power-off condition and initialize the operating system without human intervention. This means the operating system has to be set to not ask for a user name and password as well as autostart the data collection programs.

Unless it is selectable in the BIOS, the Windows 2000 operating system will check for a coherent response from the keyboard over the PS2 interface. This signal exchange must be created with a PS2 module designed to eliminate the presence of the keyboard during the boot sequence.

Also, the BIOS must also be set to allow the Marinus computer to defeat the APM (Automatic Power Monitor) function thus allowing the computer to start initializing when power is applied. This feature keeps the Marinus rebooting after each power cycle on the test body fixture.


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