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High-Speed Data
Acquisition System
Spectrometer
based chemical sensors with detector arrays produce very high data rates
that are beyond the capability of COTS data collection systems.
The U.S. Army Passive Standoff Detection group at Edgewood, MD.
contracted with MESH to develop a custom data system capable of collecting
180 Mbits/sec. The data
system was specifically designed to integrate with the Turbo FT
spectrometer, which has a 16 pixel detector array.
The Turbo FT produces 100 to 360 igrams/s x 16 pixels (actually a
2x8 detector array). At the
desired resolution of 16bits, and the desired igram length of 2048 points,
the effective data rate is 50 to 180 Mbits/sec.
MESH designed and fabricated a
custom data system. The
TurboFT Data System (TDS) was designed to capture and store up to 5760
igrams/sec (16 detectors x 360 scans/sec). The TDS is composed of three primary parts: the Data
Acquisition Server (DAS) with QNX, the Data Storage Servers (DSS) with
Linux, and a User Interface and Display (UID) with Windows 2000.
A system hardware schematic is shown below.
The system is divided into these components in order to handle the data
rate. The DAS digitizes the
igram data, packages the data, and then sends it to the DSS via four
100Mbit/s Ethernet lines. The
DSS preprocess the igram data and saves either igrams or spectra to 40GB
hard disks. The UID controls
the data acquisition (via commands to the DSS and DAS) and displays a
portion of the data in real time. The
DAS consists of 4 embedded nodes, which are each made up of a 266MHz PC104
CPU with 10/100 Ethernet, and 2 MESH designed PC104+ (i.e., PCI bus) A/D
cards. The DSS consists of 4
Linux servers. The TDS
altogether consists of 9 synchronized, networked computers.
To demonstrate
synchronization throughout the data system, a sine wave source from a
function generator was connected to all the system inputs in place of the
spectrometer. The digitized
sine waves from each “detector” (i.e., A/D channel) were overlaid to
show that all sine waves aligned within a fraction of a point spacing
throughout the 2048 point scan range as shown in Figures 1 and 2 below.
This
TDS development required software and hardware design.
MESH engineers developed custom software in Visual Basic, Visual C,
and embedded C for Linux and QNX. MESH
also designed (with Eagle PCB software) and programmed (Warp VHDL for
Cypress CPLDs) custom PCI bus A/D cards for the project.
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Figure
1
Overlaid sine waves. |
Figure
2
Overlaid sine waves show that all 16 A/D converters are
synchronized within 1 sample time period. |
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