Reprinted from Evaluation Engineering, December 2003
Copyright © 2003 Nelson Publishing Inc.
www.evaluationengineering.com

Although used in a wide variety of applications, data acquisition systems can be categorized into three basic groups: lab, distributed, and portable. Lab and distributed systems primarily are stationary; the UUTs are brought to the system. The exact opposite applies to portable systems. Usually, they function in harsh environments or while being transported, providing a unique set of application-specific considerations for electronic equipment that must be highly accurate and reliable.

Packaging
Data acquisition systems usually are a mix of data acquisition components from different suppliers that conforms to a standard format. Lab systems, for example, have a standard 19-in. rack form factor, and distributed systems comply with snap-track mounting standards. But since there is no standard for physically connecting portable devices, integrating devices from multiple suppliers in a portable system is difficult.

Because of the packaging constraints when selecting a portable system, it is important to anticipate future as well as immediate requirements. The system could be rendered useless if additional functionality required in the future is not available from a selected supplier. 

For example, if signal conditioning for accelerometers is required in a future application and not provided by the primary supplier, then you must use a separate signal-conditioning instrument connected to the main system. In a lab, this would not be a problem. But for use in a vehicle or a transportable system, it requires additional space, cabling, and mounting that could be impractical in many circumstances.

Another packaging consideration is the weight and volume of the system. In lab and distributed systems, the weight and size of a system generally are not critical as long as it fits in the allotted space. Portable systems, however, often have the added requirement of being transported, in which case weight and volume become especially important. 

Portable systems also are frequently used in vehicles where space constraints can be extreme. For example, a system that profiles the interior temperature of an automobile must fit into the trunk since its presence in the passenger compartment would affect the test results. In a compact car, the trunk often is very small, making the size of the data acquisition system critical. In general, smaller and lighter always are preferable for portable systems.

Signal Conditioning and Expansion
Portable systems almost always attach to sensors, so selecting a system that supports your sensor requirements is absolutely necessary. While packaging constraints are critical, there is another issue related to newer sensor technologies: Calibration information is stored on the sensor. 

If the data acquisition system is to benefit from this capability, then signal conditioning must be able to retrieve that information. Traditional signal conditioning can be obtained from the sensor supplier or a third party. But in the case of smart sensors, the data acquisition system must have built-in calibration retrieval capability since it must read digital data from the sensor. As the trend toward smart sensors continues, the necessity for the data acquisition supplier to provide the total signal-conditioning solution will only increase. 

Requirements for Stand-Alone Applications
Portable systems can further be categorized into stand-alone and PC-attached systems. Stand-alone systems, intended to operate without the presence of a PC, are more akin to a traditional datalogger. Stand-alone operation often is necessary because an operating environment with extreme temperature, vibration, or space constraints is unfriendly to a PC. 

For example, it would be impossible for a notebook PC and an operator to measure the strain on the hull of a jet ski while at sea. In that case, a PC may be used to configure the system before deploying the test and for retrieving and analyzing the acquired data. However, it would be impossible to interact with the data acquisition system during the test. 

Stand-alone systems often require a user interface so that the operator can call up predefined tests, initiate or arm the acquisitions, and verify that the acquisition occurred successfully. Since the environment often is too hostile for a PC to act in this capacity, a system must offer an alternative user interface. Because of the tight coupling between the user interface and the test hardware, it must be supplied by the data acquisition system supplier.

In an automotive test environment, for example, often there is too much sunlight and temperatures too extreme for a standard notebook PC to function correctly. Also, it may be necessary for the operator to be driving the car while interacting with the data acquisition system. 

One supplier of automotive test systems, ETAS, provides a steering wheel-mounted button mechanism that communicates with an electroluminescent heads-up display using Bluetooth wireless technology. The thumb switch is attached to the steering wheel with VELCRO^® and the display is mounted to the dashboard. Clicks on the steering-wheel switch communicate with the display that, in turn, communicates with the data acquisition equipment via Ethernet. The display is designed to be readable in bright sunlight and under extreme temperature conditions.

Another aspect of stand-alone systems is data retrieval. There are three basic methods: removable storage media, a PC attached via a standard interface such as USB or Ethernet, or wireless. 

In the case of removable media, PC-Card technology is used most often, offering storage in the gigabyte and above range particularly if rotating PC-Cards are supported. In the case of wireless data retrieval, cellular technology is most common since the application often involves driving vehicles into remote regions with extreme temperatures such as in northern Scandinavia or the deserts of Arizona. Since cell-modem technology changes rapidly, it is critical that the stand-alone system has generic support for a variety of cellular-based modems. 

Requirements for PC-Attached Applications
When compared to PC-based lab or distributed systems, PC-attached portable systems have unique system requirements. The link between the PC and the data acquisition system can take several forms including PC-Card, parallel, Ethernet, USB, and FireWire. 

One important consideration when selecting a system is the future availability of the link in newer PCs. Inevitably, the data acquisition system will enjoy a much longer life than the PC, and it is likely that two to four generations of PCs will attach to one data acquisition system over its useful life. For that reason, it is important to select data acquisition equipment with an interface likely to be available in newer generations of PCs. 

For example, parallel ports were the mainstay for linking notebook PCs to systems in the 1990s. Today, parallel ports are nearly impossible to find on new notebook PCs. It is, however, possible to add a parallel port to a newer PC via a PC-Card-to-parallel interface.

Most PC-attached portable systems use notebook PCs such as those from Dell, Toshiba, and Gateway. There also is a class of notebook PCs intended for more demanding portable environments, such as Panasonic’s TOUGHBOOK^® family.

Software for PC-Attached Systems
It is common practice to develop a data acquisition application in the lab before installing the equipment in the field. However, it usually is necessary to modify the application in the field after observing how the system performs.

For this reason, traditional programming languages that need extensive typing are less desirable than graphical environments that can be more easily modified in the field. In addition, many portable applications simply require logging of data. In these cases, the best environment may be one where you enter the acquisition parameters into a spreadsheet that can be easily modified with minimal typing or compiling.

Power
Lab and distributed systems always derive their power from a facility’s AC power. Portable applications rarely have AC power available and may need to operate from an internal battery. Portable systems must be able to adapt to a variety of power sources ranging from 12-V vehicle batteries to solar-powered cells. 

While some suppliers promote lab systems for portable use, in most cases, the user has to provide a DC-to-AC converter, which involves an additional piece of equipment and can increase overall power consumption due to losses in the converter. In general, it is more desirable to use equipment designed for DC operation than to convert AC-powered devices.

The amount of power consumed by a portable system is an additional consideration. Unlike lab and distributed systems, there generally is not an unlimited amount of energy available.

Data Storage and Retrieval
With disk space virtually unlimited, lab and distributed systems place almost no special demands on the way in which data is stored. In contrast, portable systems have several issues related to data storage. 

For starters, the vibration experienced by the portable system may preclude traditional rotating-media storage. Consequently, alternate forms of data storage such as solid-state PC-Card memory must be used. Solid-state memory generally is more expensive and has a smaller capacity than rotating disk memory, and you must pay special attention to the size of the acquired data files in portable systems.

In addition to vibration, temperature extremes experienced by portable systems may preclude rotating data storage and even some forms of nonvolatile solid-state storage. Pay close attention to the temperature ratings on the data-storage mechanism in applications where temperature extremes are possible.

Location Sensing
Since portable systems move, it often is necessary to record the location of the system in conjunction with other analog and digital measurements. With PC-based portable systems, it is relatively easy to attach a global positioning system (GPS) to the PC’s serial or USB port as long as the software environment supports acquisition of data from a GPS.

It’s more difficult with stand-alone systems since no PC is present to attach the GPS device. In that case, the stand-alone system must support the attachment of an external serial or USB device. This requires software support, which often involves sending configuration information to the GPS prior to an acquisition, and the physical connection via a serial or USB port. Without the support for generic external devices, stand-alone systems generally are unable to acquire location information, which can be a serious limitation in many applications.

Summary
Portable data acquisition applications place special demands on the test equipment that must be considered before you select a vendor. Since most portable systems do not mix components or software from different vendors, it is important that the selected vendor supports both the current and the future requirements of the system.

About the Author
At the time this article was published, Tom DeSantis was the president of IOtech, Inc.