Edited by Robert Repas
• Data-acquisition systems connect to a PC using serial, USB, FireWire, or network connections.
NI USB-TC01 video, www.ni.com/usb-thermocouple
The earliest form of recording data involved manually taking measurements, recording them to a written log, and plotting them on graph paper. In the late 19th century, this process was automated with the use of strip-chart recorders that mechanically recorded measurements onto paper. Strip-chart recorders were a great leap over manual recording but still had drawbacks.
Today, the more widely used method of recording data is with a data logger (or paperless chart recorder). Data loggers are stand-alone instruments that measure signals, convert them to digital data, and store the data internally. Many data loggers include built-in displays and the ability to transfer the data to a PC for offline analysis, permanent storage, or report generation.
The use of PCs in data logging starts about the same time as the introduction of the PC. However, those early PC-based data loggers needed special hardware and software to carry out their function. The standard interface ports found on the typical PC from that period weren’t fast enough for high-speed data acquisition. Hewlett-Packard, a maker of test and data-acquisition instruments at that time, developed the Hewlett-Packard Interface Bus to get around the slow I/O of the early PCs. As more instrument makers adopted the HPIB, it soon became the general-purpose interface bus (GPIB), and was later adopted as an IEEE standard, IEEE-488. But limitations in PC-processing power kept the PC little more than a glorified data logger. The processing of raw data to create useful information came after the raw data was captured.
As the processing power in PC-based measurement devices grew, so did its capabilities. The PC now became part of the system with PC-based data loggers. The data logging software uses the higher performing PC processor, hard drive, display, and I/O bus to add features beyond just logging data. Today’s PC-based data loggers offer real-time visualization, in-line analysis, user-defined functions, terabytes of data storage, and network connectivity and interoperability.
Traditional stand-alone data loggers were capture-only devices that recorded data into their memory. But before that data could be used, technicians had to transfer it from the logger to a PC. In early systems, this meant manually rekeying data into the computer by hand. Later systems let the techs download the data through a communications cable. Once the data was in the PC, a spreadsheet program or other software provided graphing and visualization tools for analysis.
Because PC-based data-acquisition systems are always connected to the PC, live measurements can be displayed on the PC monitor in near real time making results instantly viewable. Some programming environments, such as National Instruments LabView, let users build custom interfaces to control the measurement device and display results. Creating a user interface with LabView is as simple as dragging and dropping controls and graphs in a programming window.
Stand-alone data loggers typically capture raw, unfiltered data. However, in most cases the data must be processed or conditioned before it is usable. Data conditioning can take many forms, from simple filtering to the application of Fast Fourier Transforms. The slow speed of early systems meant delays from the time of capture until the data was available for analysis.
Modern PCs have eliminated this bottleneck. Multicore processors and large memories let today’s PCs perform signal conditioning and analyze data on-the-fly. Analysis software includes many common math and signal processing functions such as differential equations, curve fitting, integration and differentiation, interpolation and extrapolation, and numerous other operations. Most of these tools use configuration wizards that make it easy to add analysis to measurements.
Generally, traditional data loggers are limited to the hardware and functions defined by the vendor. While good for accomplishing general-purpose tasks, they may not meet the needs of specialized uses. For example, technicians may want to log data only under certain conditions or to create custom alarms not built into the data logger.
PC-based data loggers are software-defined instruments, and this software is readily customized to meet specific needs. It’s quite possible to add alarms, logging conditions, report generation, data display, and other tools to the basic logging function, expanding beyond the basic capabilities of the software. Flexible file formats permit exporting data into other tools or to share with others.
Terabytes of data storage
Where PC-based data loggers really shine is in their data-storage capacity. Stand-alone loggers typically have limited memory usually based on Flash or some other long-term memory device. Once that memory is filled, the data logger cannot take additional readings.
But with a PC-based data logger, memory functions become virtually unlimited. First, it’s not uncommon to find a PC hard drive with terabyte capacity having ample space for measurements as well as permanent storage. But it’s also possible for the PC to off-load its memory to other devices like a CD, DVD, or memory stick, or transmit the data to another system on a network, freeing space for additional measurements while continuing to log new ones.
The PC’s ability to communicate over a network gives PC-based data logging its main advantage. First, as mentioned previously, a network can boost data-storage capacity to an unlimited degree. But even more so, it can permit access to logged data from any other networked machine, allowing remote monitoring of the data-logging operation from any location. Custom alarms can send e-mails alerting personnel to problems or link to a Web page that lets anyone with a Web browser look at the active data.
It may be difficult to continually check results on applications that need long-term monitoring over days or weeks. Remote monitoring via a network for remote viewing handles this problem. For example, NI LabView can be used to create custom alarm conditions that can send e-mails on an alarm trigger. It can even design Web services that permits visualizing data over Web-based applications.
By combining the acquisition and storage functions of stand-alone data loggers with the archiving, analysis, reporting, and display capabilities of modern PCs, PC-based logging systems have finally brought about full automation of the data-logging process.
Look Ma, no driver — temperature logging via USB
What makes the USB-TC01 noteworthy is that it needs no setup time or driver-software installation. Known as NI InstantDAQ technology, the device automatically loads everything the computer needs to take and record temperature measurements when plugged into a USB port. In addition, the module features a standard miniplug connector that lets the logger use different thermocouples to meet specific application needs.
The USB-TC01 offers temperature measurement and logging for applications in scientific labs; heating, ventilating, and air-conditioning (HVAC) units; cryogenics; industrial ovens and furnaces; engine-exhaust and combustion systems; building monitoring; and many other environments. In addition to saving time and resources in system setup, the device also exhibits the overall accuracy and reliability of more-complex measurement systems. With traditional stand-alone data loggers, measurements are made independently of the PC so data can only be viewed offline. The USB-TC01 is always connected to the PC so it can take live measurements with temperature readings displayed instantly on the PC monitor.
Optional free applications available at www.ni.com expand the basic functions of the USB-TC01. Or the USB-TC01 can be combined with NI LabView graphical-system-design software using the NI-DAQmx driver software.