How to Select Data-Acquisition Systems for Mechanical Testing

Sept. 9, 1999
Data-acquisition systems have never been easier to use, now that all signal-conditioning circuits are provided and the integrated software needs no

Steve Lekas
Vice President
of New Products
IOtech Inc.
Cleveland, Ohio

The first step in selecting a data-acquisition system for mechanical testing involves analyzing the physical variables to be measured. The location of the item under test, the types of signals processed, and the dynamics of the measurement help determine the kind of data-acquisition (DA) system needed. Common measurements include strains found in tension, compression, shear force, torsional force, pressure, or weight. Other measurements often include temperature, displacement, linear and rotational speed, vibration and acoustics, humidity, liquid level, and true-false states.

DA hardware systems usually come in either plug-in cards for desk-top computers or stand-alone boxes that connect to a host computer. In either case, a transducer is necessary to convert these variables into an electrical signal, and signal conditioning to convert the transducers’ output into a higher voltage or another signal type to match a data acquisition system’s input. For example, plug-in boards and PC-card DA systems typically measure only voltage. They require additional external conditioning hardware to handle temperature, strain, and vibration, and sometimes cost more than the DA system itself.

PC-based DA products come in three general form factors: PC plug-in boards that install permanently inside a desktop PC, external modules or boxes that connect to the PC by a communication port, and PC cards (formerly called PCMCIA cards) that attach to the internal bus of the PC but can be inserted and ejected without opening or powering down the PC.

Most PC plug-in boards fall into one of two categories: ISA or PCI-bus. However, after 15 years, the 32-bit ISA bus is quickly disappearing in favor of the newer 64-bit PCI bus. The faster PCI slots are in most desktop computers made in the last few years. But, unlike the older ISA computers, PCI-based computers tend to have fewer slots. ISA computers were full of slots so users could add functions not integrated into the motherboard. But as more PCs come with built-in features such as sound, Ethernet, and modems, fewer slots are needed. It may be difficult finding a PC with enough slots today when a DA application requires more than two or three boards.

When DA boards are located outside the PC, the module or box-type DA systems connect to PCs by standard communication ports such as RS-232, parallel printer port, Universal Serial Bus (USB), IEEE-488, and Ethernet. Variables that influence the choice of a communication scheme are the required fan-out or number of channels connected, the distance between the DA device and the PC, and required speed of the acquisition or sampling rate. Measuring dynamic signals such as vibration, calls for fast sampling rates. Faster sample rates mean more data have to move through the communication port. Conversely, when measuring slow signals such as temperature, samples can be collected less frequently.

Plug-in boards have a speed advantage over external boxes. A PCI board can put data into the PC memory at 50 Msamples/sec. By comparison, external-box DA systems top out at about 1 Msample/sec, which is sufficient for most mechanical tests ranging from dc to 20 KHz in bandwidth.

Fast sample rates also are necessary when measuring signals such as accelerometer and displacement transducer outputs. To sufficiently represent the signal, the sample rate should be at least 10 times faster than the highest frequency component of the measured signal. In typical DA systems, the sample rate is aggregated over the number of channels being sampled. For instance, to sample five channels at 100 Hz requires a DA system that can sample at 500 Hz.

Fan-out refers to the number of DA devices that can be attached to one computer. Attaching multiple DA devices to the computer increases the number of channels in the system and allows distributing DA devices throughout a test area. To cut down on the electrical noise coupled into the signal connection cables and to reduce the amount of total cabling required, it’s wise to put the digitizer close to the signal source. In applications where transducers and voltage sources are physically separated, placing a box-style DA system at each group of signal sources may be the best system topology.

Different communication ports embody different fan-out methods. RS-232 and the PC’s parallel port are point-to-point connections, so when adding more devices, more ports are needed. IEEE-488 lets up to 31 devices and a controller connect in most combinations of daisy-chains and stars. USB is point-to-point as well, but it is possible to connect one port to a hub that provides multiple ports. This topology lets one USB port fan-out to 127 devices.

A serious drawback preventing USB from being a great standard for distributed systems is its maximum cable length of 5 meters (or 15 meters using hubs as repeaters). USB was designed to eliminate the plethora of connectors on the back of PCs in favor of one relatively high-speed port that runs all PC peripherals, including the mouse, keyboard, and audio I/O, none of which are far from the computer during normal use.

By far, Ethernet is the communication method that provides the widest fan-out. Most office and factory buildings are wired with some type of Ethernet. The same backbone used to run an accounting system can also collect distributed data from the factory floor. Moreover, a LAN routed to the Internet lets the DA collect data from instruments remotely located around the world.

As with all solutions that sound too good to be true, there is a major drawback to distributed systems: It’s synchronization. Because each DA device typically has its own timebase clock and trigger parameters, it’s difficult to satisfy applications that require multiple DA devices to collect data in lock-step. This is primarily a problem during high-speed data collection where skew of more than one msec makes a difference. When remote devices are collecting data slowly, say, less than once per second, then the time skew from device to device becomes insignificant.

For high channel-count applications where all signals are physically close together, some DA boxes allow expansion — channels are added without needing more attached digitizers. This method reduces costs and solves the synchronization problem because it uses only one digitizer.

External-box DA systems have a few advantages over plug-in boards. For one, the host computer need not be dismantled to install the DA box. Also, the box doesn’t consume internal PC resources like DMA channels and interrupts, nor does it need constant access to the rear of the computer. Further, plug-in boards typically connect to I/O with an external screw-terminal box that sits out on the bench top. But bench-top DA boxes come with all required screw terminals, so additional hardware is unnecessary.

On the other hand, plug-in boards work best for stationary, permanent installations, where the system is reconfigured infrequently. In these kinds of applications, the card is installed, the signals are connected, and the PC is reassembled only once.

The USB system is easy to use with true plug-and-play capability. While the computer is running, simply plug in the USB DA system. Windows software immediately recognizes the peripheral and installs the necessary drivers. A separate power supply to run low-power peripherals is not needed because the USB port supplies power. Just launch the data-acquisition application and perform the test. That’s it. When done, unplug the device.

Most Windows-based DA software can be grouped under one of these categories:
• Software that provides instant results for dedicated functions such as data-logging and display
• Icon-based point-and-click software for interactively developing more advanced custom test applications
• Comprehensive programming environments that provide flexibility for creating complex algorithms and custom operator interfaces

To develop a customized application, icon-based point-and-click software creates a block diagram of the desired setup without programming. Applications are altered by connecting a few icons which collect, analyze, and report the data. Icon-based data-acquisition software packages offer a variety of data display formats, mathematical and statistical analysis functions, and reporting formats to meet the demands of dynamic test environments.

For users who require even more control of DA applications, developing a program using a conventional or graphical programming language may be the answer. These languages let programmers develop elaborate graphical user interfaces that can be used by nontechnical operators. A conventional language is also well suited for producing applications that are sold with hardware as a data-acquisition solution.

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