Acquiring and Analyzing Measurement Data in Real Time

Requirements for data acquisition (DAQ) systems now are far more demanding, particularly where complex metrological tasks such as those found in the aerospace industry are involved. Not only are sample rates increasing, but the number of channels and the amount of acquired data are growing. Also, the times per test and the amount of personnel with good training are decreasing. These changing requirements can be grouped into two main areas: handling more data and achieving this more effectively.

Review While Recording

It is primarily when handling and analyzing vast volumes of data that users need DAQ software that can acquire and display the measurement data in real time. For example, when conducting a long-term run-up test of a turbine, it is critical to review the beginning of this multiple-hour operation while the test continues. By reviewing and validating immediately, the user obtains confidence in the performance and proper behavior and is encouraged to continue the ongoing task.

However, there is a major shortcoming in many measurement data acquisition systems: Conventional PC measurement technology must be either in measurement mode or in playback mode. To emulate review while recording takes a complex function found only in high-end systems.

With this capability, some of the software display windows work in LIVE mode and display the measurement currently taking place in real time. Other windows, running in REVIEW mode, display earlier parts of the ongoing measurement or other measurements for comparative purposes. This unrestricted access to data while the measurement is still running can save a great deal of time, particularly with longer test runs.

The review-while-recording function is very computing-intensive. The process also must be programmed to ensure that read-and-write access is possible at the same time. As a result, HBM invested about 10 man years to implement and fine-tune this functionality.

Since HBM’s Genesis HighSpeed DAQ Systems support rates up to 25 MS/s, they are suited for vibration and acoustic testing of complex systems such as satellites. The systems can record up to several hundred channels simultaneously. Then, the measurement data is displayed in real time and analyzed with the Perception software while the measurement is still running. If the user zooms in on the data, subsets of ever-increasing size are used for the display until finally, for a zoom at sample level, the raw data sets are used.

Handling Large Data Sets

Another challenge, particularly with large data sets, is playing back acquired and stored data. For example, a single turbine run-up test (32 channels, 1 MS/s, 10 minutes) generates 19.2 GS or about 40 GB of data. Figure 1 illustrates a typical Genesis DAQ system configuration.

Figure 1. Block Diagram of Typical Genesis DAQ System

Many basic PC DAQ systems are limited because the data throughput to the hard disk is precisely the same as the data output from the hard disk. In practice, this means that a measurement that took 10 minutes to record at full hard-disk bandwidth also will take exactly 10 minutes to play back.

This problem is solved by HBM’s patented StatStream technology. Here, several parallel, reduced data streams are generated in real time during measurement and saved with the raw data. Even while acquisition of measurement data is still in progress, various reduced records with statistical information such as min/max or rms are stored in the measuring device in real time by the DSPs. Then these smaller records are accessed in compressed format during replay to produce an accurate representation of the recorded data.

This process allows for fast display of large records without loss of information. For example, a complete hard drive with 120 GB of data can be represented in less than 20 seconds.

This time saving in replay makes Genesis HighSpeed systems interesting for users who, until now, have recorded large quantities of data mainly with tape recorders. A tape recorder is roughly comparable in terms of bandwidth and memory depth, but a record captured at full speed for 12 hours requires another 12 hours to replay with a tape recorder. With Genesis, this takes 12 seconds.

In ballistics tests, measurement data must be recorded as part of a one-off measurement. This requires reliable triggering during data acquisition. In addition, increasingly small explosive charges are used in these test applications, which generate correspondingly high signal frequencies. This requires sampling rates up to 100 MS/s. With the integrated StatStream technology, the DAQ Perception software can calculate and display maximum values that occur in ballistics tests in a few seconds.

Reducing and Analyzing Data

Standard software is more suitable for analyzing measurement data if the user has a single, fixed application. In this case, the initial costs to adapt the software and the limited GUI do not pose any problems. With flexible applications, however, high-end DAQ software minimizes set-up times and eliminates user errors in these changing setups.

With high-end DAQ software, users can prepare the acquired measurement data for further analyses and reports quickly and easily. As with data analysis, two different solutions also are available here, both covered by Perception.

Users who want to create reports using standard Office packages can transfer their displays, indications, cursor tables, and calculation results to Word or Excel at any time. In this case, Office applications are used for the framework of the report which then is filled with displays and tables.

Users who need all layout options also can generate a complete report within the Perception software. This report then can be stored together with and as part of the measurement data.

By using the manufacturer’s own software, the functionality can be adapted to the specific hardware being used. This would be virtually impossible with standard software, especially with systems where the speed and size of the data sets recorded go far beyond normal performance. Figure 2 shows multichannel instrumentation associated with a large jet engine test facility.

Figure 2. Genesis Satellite

Reducing TCO

The purchase price of a high-end DAQ system is more than conventional PC measurement technology. However, when the total cost of ownership (TCO) is considered, a high-quality system often is several times more economical than PC measurement technology.

The implementation of a high-end DAQ system enables the user to reduce the training effort and set-up times significantly. As a result, users can diminish the time overhead for preparing, running, and analyzing measurements.

Application Example: Aerospace Power Control

Sensitive flight-control systems in large military and nonmilitary aircraft require very high quality power to meet the rigid safety standards that consumers and governments demand. Therefore, manufacturers use high-end products to monitor the power quality of the systems. Every major aerospace manufacturer has to provide power for the environmental control systems, electrical power generation, and power distribution systems to get aircraft off the ground and keep them in the air.

Environmental control systems provide management for cabin temperature, fresh airflow, pressurization control, heating and cooling, entertainment, lighting, and flight-control equipment. Manufacturers need to know the effect on the power system when any event occurs—from take-off to landing—to create the proper design and anticipate and eliminate any potential problems.

Hundreds of different AC and DC voltages pass through large aircraft. GEN series hardware with Perception can monitor these voltages. 230-VAC, 115-VAC, and 28-VDC bus systems run throughout the aircraft, powering every imaginable device. The electrical power-generation systems provide the power needed both in flight and on the ground. The quality of power from these systems is critical to the safety of those in the aircraft. To accurately determine the power quality, the power must be analyzed at high bandwidths.

HBM offers the bandwidth and throughput needed to capture data from hundreds of channels and the capability to interpret the results. Using the GEN series High-Voltage Probe option, up to ±1,000-V peak can be measured on more than 1,000 channels at bandwidths greater than 250 kHz with synchronization better than 100 ns across all channels.

Most aircraft are quite large and often require extensive and long cabling. To reduce the amount of coaxial cabling, the 1,000+ channels can be distributed among nine GEN series mainframes with up to 120 channels in a single mainframe. Each mainframe can be separated by up to 300 meters using a fiber-optic synchronization technique between all mainframes. This avoids grounding problems or noise/RFI issues while distributing absolute timing, trigger, and start/stop of recording within less than 100 ns. Using the IRIG time-code option allows the user to correlate data collected with other flight test information and high-speed video.

The GEN series offers a high-speed Ethernet option that provides additional transfer speed for streaming data to a high-performance PC. Up to 25 million data points can be transferred
through a 1-Gb Ethernet connection using a dedicated PC.

Data from multiple mainframes can be sent to the control PC through multiple network interface cards, allowing this data rate to grow to the capabilities of the PC bus. The design accommodates 10-GB communications that will be available soon.

Flexibility with system configuration is another key feature. Users can choose to acquire data continuously in a strip-chart mode during engine run-ups, as high-speed transient captures of specific short-term events, or a combination of both. Analog and
digital signals can be captured and triggered for fast analysis.

A system trigger can be a simple TTL pulse, a user command, or a complicated setup which includes pulse widths, hold-offs, sequences, or qualifiers. Analysis of the data then can be done quickly by searching for the triggered areas without manually searching through the data.

Displaying data from hundreds of channels can be difficult. Perception allows the user to monitor data through traces or digital meters. These meters can be set to display level alarms and be distributed over multiple monitors in a control-room environment. The user sets an acceptable limit, and anything outside that limit changes the display color. In addition, Perception uses StatStream technology to open huge data sets very quickly.

About the Author

Klaus Lang is head of International Product Marketing, DAQ Solutions at HBM GmbH. He has more than 25 years experience in data acquisition and previously held positions as sales and marketing manager and business development manager. e-mail: [email protected]

February 2010

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