Excel at data acquisition with easy-to-use software

Software that flows data directly into a spreadsheet makes data acquisition and instrument control simpler.

Dan Strassberg, Senior Technical Editor

The publicity that surrounds one approach to a problem can blind you to less glamorous ways that do the job faster, more easily, and at less cost. Simple to use and often inexpensive, data-acquisition and instrument-control software packages lack some of the power and much of the glamour of high-end application-development packages. However, the simple packages zero in on the needs of a large group of users.

The reasons for using computers to control instruments and manage data acquisition are almost always utilitarian. Few people perform these tasks solely for the joy of the exercise. Nevertheless, some users of data-acquisition and instrument-control software design complex applications. Such work can be both creative and challenging. Developers become quite involved in their work and in the tools they use. Successfully designing a complex instrumentation or data-acquisition application can be an exhilarating experience.

For most designers of complex applications, well-publicized high-end packages, such as Hewlett-Packard's VEE and National Instruments'(NI) LabView, LabWindows/CVI, and Component Works, are good choices. (VEE and LabView use graphical metaphors; LabWindows/CVI is for C programmers; and Component Works is for those who program in Visual Basic.) On the other hand, for people who are unfamiliar with the high-end packages and who just need to get the facts and get them quickly, simpler packages are often a better choice.

SEE THE DATA AS A GRAPH

In simple applications, users usually want to see the data in a form that makes unusual events or trends stand out. Displaying data as strip charts or simulated oscilloscope displays is the most basic way to satisfy this requirement. With this type of presentation, you can review the data either as it is acquired or later and select important segments to print out and retain in a lab notebook. Sometimes the data can form the basis for reports or presentations to co-workers. For such purposes, the ability to easily transfer data from the data-acquisition package to word processors and presentation-graphics packages is important.

The next step up in sophistication is to perform mathematical operations on the data. By presenting the data in a different form (say, by converting it from the time domain to the frequency domain), you can often make obscure facets stand out. Some packages include this type of mathematical-analysis capability (see sidebar "For more information..."). Others let you harness the power of Microsoft Excel, the top-selling spreadsheet program, to perform mathematical operations.

SOFTWARE IS EASY TO USE

Every software manufacturer says that its products are easy to learn and use. As a rule, though, the more a package can do, the more it costs and the greater the time and effort you must invest in learning to use it effectively. Even so, you needn't entirely give up on flexibility and capability to get data-acquisition and instrument-control software with which you can be productive just hours after opening the box.

You can segment data-acquisition and instrument-control software products into three tiers. Unfortunately, pricing doesn't always clearly indicate a product's position in the hierarchy, and you and the vendor won't necessarily agree on how to categorize a package.

Depending on what capabilities you want, middle-tier packages cost from approximately $500 to $2000. Despite the generally moderate prices, middle-tier products are quite capable and offer significant flexibility in application design. Moreover, most of these packages support hardware from multiple manufacturers. Examples of middle-tier packages are Capital Equipment Corp's TestPoint (Picture) (also sold by Keithley), DSP Development Corp's DADisp with GPIBLab, HEM Data Corp's Snap Master, Intelligent Instrumentation's Visual Designer, and NI's DasyLab.

Few users are yet aware that, to settle a patent dispute, NI recently acquired the company that published DasyLab. (The courts upheld NI's claim that DasyLab infringed on some of NI's LabView patents.) Several NI competitors have been distributing DasyLab. The future of those distribution agreements is uncertain because some distributors are uncomfortable providing sales data to a key rival. Nevertheless, NI expects to continue selling and supporting DasyLab because the package's capabilities complement those of the company's other software products. Although DasyLab can control IEEE 488-interfaced instruments, NI says that data acquisition is the package's forte.

FROM TRANSDUCER TO SPREADSHEET

Notwithstanding prices that range from "free" with a hardware purchase to approximately $200, products in the lowest tier aren't necessarily as limited as you might think. Many of these bottom-tier packages are add-ins to Microsoft Excel that let you transfer data in real time directly into an Excel worksheet. Examples are ComputerBoards' DAS Wizard, IO-tech's DAQView-XL (Picture), and NI's Measure (Picture).

Despite their low cost, these packages perform some basic functions that may pleasantly surprise you. For example, to get direct temperature readouts in IOtech's Excel add-in, DAQView-XL (and DAQView, upon which DAQView-XL is based), you need only specify the cold-junction channel, the thermocouple type, and whether you want readings in degrees Fahrenheit or degrees Celsius. The software automatically performs cold-junction compensation, linearization, and unit conversion. If you give the channels meaningful names, such as "IC-3 heat-sink temperature," those names appear as column headings in the Excel worksheet.

Although several Excel add-in packages work only with the vendor's hardware, very often the most important limitation on what you can accomplish with these packages is your proficiency with Excel. Be careful, however, of letting your familiarity with Excel entice you to use an add-in package when you really should choose a package from the middle tier. An Excel add-in may be inappropriate for your job if you need to create complex worksheets and write many macros in Visual Basic for Applications (VBA). VBA is the programming language that is part of newer versions of several Microsoft applications, including V7 and later versions of Excel.

For many users, there is another issue to consider: A simple approach that gets today's job done quickly may prove cumbersome or unworkable for next month's follow-on job. You must decide whether the likelihood of such follow-on work is great enough to justify choosing a more powerful package even though using that package involves a significant learning curve.

DATA-LOGGER PACKAGES

Fluke and Hewlett-Packard offer software that works with their own freestanding data-logger units. HP and Tektronix also offer software packages that work with their digital oscilloscopes.

The Fluke and HP data-logger packages highlight a philosophical difference between the design of data-acquisition boards and data loggers. Data loggers use internal µP hardware to perform such functions as linearizing transducer outputs, compensating for thermocouple cold-junction temperatures, and scaling into engineering units. Although data-acquisition boards with internal µPs are not uncommon, most data-acquisition boards delegate functions such as unit conversion to software that runs on the host PC.

A software vendor could, of course, use a layered architecture for drivers that interface various types of hardware units to the data-acquisition software. A data-acquisition board might communicate with a software module that converts ADC outputs to engineering units. That module could then communicate with a higher level module. A freestanding data logger could bypass the engineering-units module and communicate directly with the higher level module. This software-design approach would require standardized interfaces among the various modules. So far, though, software suppliers have demonstrated little enthusiasm for this sort of standardization.

DIFFERING PHILOSOPHIES

Similar philosophical differences extend to the design of software that supports only one category of hardware. One manufacturer may assign unit conversion to a driv-er, whereas another manufacturer may perform the same function in the software package with which the driver communicates and not in the driver itself. Yet another manufacturer might assign the unit-conversion task to an Excel worksheet. Although the internals of the software may seem unimportant, they can become significant under certain circumstances. If speed is important, for example in high-frequency vibration measurements, the software vendors insist that different approaches can result in significant perform-ance differences.

The most compact data format is the raw ADC data, which usually occupies only 2 bytes. After conversion to engineering units, the same data may occupy 6 bytes—or even more if you must add characters for an exponent and units. Consequently, sending data in engineering units through serial interfaces and the majority of parallel interfaces takes more time than sending raw ADC outputs. In addition, the data in engineering units consumes more memory. Even in this age of sub-$2000 PCs with 128 Mbytes of RAM, 13.6 Gbytes of hard-disk space, and 450-MHz µPs, such considerations matter.

The purpose of this discussion is to consider issues that may turn out to be important, even in selecting low-end software. If a package costs $99, the vendor is not likely to spend much time on the phone discussing characteristics that are important to you but unimportant to the average buyer. And the downloadable demo versions of the package may or may not answer your questions.

Therefore, you may find it worthwhile to invest in, install, and try out several packages to find out which one best meets your requirements. On the one hand, some of the packages are so inexpensive that you may feel that you can hardly go wrong. On the other hand, after you devote several days to learning the nuances of a package only to decide that it isn't suitable, you may change your mind.

You can reach Senior Technical Editor Dan Strassberg at 1-617-558-4205, fax 1-617-928-4205, e-mail ednstrassberg@cahners.com.

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