fifth-order, 10-Hz, Bessel filter at the input to the 64-channel data acquisition 

 system. Analog signals are digitized at 16 Hz, optically encoded, and trans- 

 mitted to the VAX 3400 server. Digitized records are serially recorded in 

 256 time unit blocks to random access memory (RAM) and transferred to an 

 800-MB hard drive. 



The data acquisition console (DAC) is a product of MTS Systems Corpora- 

 tion, Minneapolis, MN. It is based on the Series 499 control console design, 

 applied to a variety of engineering applications. The DAC is shared by the 

 directional wave generator, the spiral wave generator, and the two-dimension- 

 al channel wave generator. It is optically connected to remote stations, adja- 

 cent to the wave generator selected for a specific test series. Photographs of 

 the console appear in Figure 13-10. The DAC is comprised of three modules. 



The first module is a Model 499.45 analog-to-digital (A/D) module. It is 

 an intelligent, transputer-based module that provides channel scanning and 

 A/D conversion of up to 16 analog input channels. The board uses a scan 

 table RAM to specify the channels to be included in a scan and the order in 

 which they are to be converted. Four of these modules exist in the WRL 

 system. 



A second module is a Model 499.47 anti-aliasing filter, which is a 16- 

 channel, non-programmable low-pass filter. Each of the 16 channels can be 

 independently configured to support up to fifth-order Butterworth, Chebychev, 

 Linear-Phase Elliptic, or Bessel responses. The filters are configured for their 

 type and cut-off frequency by resistor networks contained in dual in-line pack- 

 age headers that are installed on the module circuit board. The filter module 

 is designed primarily for anti-aliasing purposes in a digital signal processing 

 application. The filters are also used to reduce unwanted high-frequency 

 noise. Four of these units exist in the WRL system. 



A third module is a Model 499.65 optical link, which provides the neces- 

 sary optical communications links between the VAX computer system, servo 

 boards, and data acquisition console. Two of these modules exist in the WRL 

 system. The optical/electrical interface appears on top of the VAX server 

 cabinet in Figure 13-8. 



Five other PC-based digital data acquisition systems were used to acquire 

 data in the foreshore and offshore. These systems were synchronized with 

 each other and the MTS data acquisition system using Atomic Standard Time 

 established in Fort Collins, CO, and broadcast as a radio signal (call letters 

 WWV). A 10-MHz radio receiver (RD-10) and digital signal processing card 

 (CTS-10) by Coordinated Time Link were used to decode the WWV time 

 signal, update the board clock, and then update the computer system clock. 

 This provided an accurate, reliable, and traceable common time base for all 

 data acquisition systems associated with the SUPERTANK project. The clock 

 interpreter and a local digital time readout are shown in Figure 13-11. 



Chapter 13 Wave Generation and Data Collection Systems 



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