With this test system engineers monitor and record limit levels on hundreds of sensor channels in real time, while considerable mechanical stresses are applied to elements of the structure. The designers need to know how much loading the actual structure can accept before it starts to show signs of failure and loses design integrity.

In testing these physically large structures, the engineers were interested in minimizing cable runs for several reasons. As noted earlier, they were taking readings from low-level sensors and wanted to avoid the voltage drops and noise associated with long lead lengths or cable runs. In addition, proper shielded cable is expensive, so they sought a way to improve the system's electrical characteristics while trimming costs. Weight was also a concern because some of the sensor sites require that the data-acquisition systems be suspended from the ceiling, so cable weight also started to play an important role.

I/O Fits in Signal-Conditioning Chassis

Thus, although the engineers initially thought of using DAQ boards mounted in several PCs around the site, they decided that using an Ethernet-based system based on the PowerDNA Cube was far superior. The overall system measures signals from a variety of load cells and position potentiometers, but it will likely expand to include thermocouples, strain gages and accelerometers. To handle low-level signals, Visidaq adds signal-conditioning modules into a small chassis that also holds the PowerDNA Cube. Sold under the name Etherdaq, this compact solution does the entire job from accepting sensor inputs to transmitting digitized values over the network. The Etherdaq chassis situated at each measurement location supplies 32 signal-conditioning modules and one embedded PowerDAQ Cube. That Cube, in turn, is a model CM-8 and is equipped with two DNA-AI-201 analog-input layers, each with 24 channels; it also holds three DNA-DIO-403 digital-output layers with 48 points each.

A PowerDNA-based remote networked data-acquisition system with signal conditioning as designed by Visidaq Solutions, Inc.

Because it measures roughly 6 x 4 x 4" the PowerDNA Cube in the chassis takes up little space yet it holds its own CPU, network interface and still can accept hundreds of analog and digital I/O channels. Using local intelligence, the Cube performs the digitization locally and sends the results to a host PC over an Ethernet network using either copper or fiberoptic cable. What made the system especially attractive for this application is its high channel count and the ability to daisychain the Cubes on one Ethernet chain. In this way, the designers could string the units along one communication line, further minimizing cable runs.

In operation, four digital outputs from each one of the DNA-DIO-403 layers go to each of the 32 signal-conditioning modules. Two of the digital lines activate relays that shunt either of two precision resistors across the sensors to implement a resistance (R-Cal ) method for calibration. A third digital input initiates the autozero process, which uses circuitry to compensate for voltage losses in leads and other effects. This process allows the system to zero out the voltage regardless of the sensor position and thus get a true initial condition reading of the loads or deflection. A fourth digital signal from the PowerDNA Cube allows the operator to turn sensor excitation voltage on or off.

Distributed I/O in The True Sense

Once the system has completed this calibration routine, it can begin taking measurements. The existing configuration consists of seven Etherdaq systems, each with 32 channels sampling at 20 Hz, located throughout a test hanger-while some are located on the floor, some are suspended in the air so they can be close to the actual sensors. Although the system uses copper cabling for the Ethernet, Visidaq Solutions could convert to the PowerDNA's fiberoptic option if necessary.

The host software runs on a Visidaq PC-based Data Acquisition system. Because the system is recording, displaying and performing control on the signal-conditioning elements in real time, the designers chose Linux. The Visidaq Data Acquisition software is written in C because the firm started in the late 1980s using Motorola's Unix, then ported to Data General hardware and Unix, then to Hewlett Packard and HPUX, and most recently to Linux. Although the software for this application was a custom project, Visidaq Solutions added the new features to its standard application software, which additionally provided test operators with a variety of field proven general-purpose Visidaq data acquisition functions. What makes Etherdaq unique is how implements these special signal-conditioning features. Using UEI's PowerDNA product Visidaq Solutions could readily adapt Etherdaq for other special application requirements or remotely distribute industry standard signal conditioner products.

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