WTD 91 in Meppen is one of ten defense technology and defense science centers under the jurisdiction of the Federal Office of Defense Technology and Procurement. Its current tasks include technical investigations and testing of weapons, ammunition, armored vehicles and protective structures of all kinds. To this end, the extensive area in the Emsland region is used to conduct Sprinkling attempts on tanks, mine detection and medical vehicles, as well as on buildings. This naturally places special demands on the measurement technology used for this purpose.
In view of the reorientation of the Bundeswehr toward peacekeeping and peacekeeping "blue helmet" missions in crisis areas, the qualification and acceptance of the vehicles used there for mine protection is a particularly current task at WTD.
The possible effects on the occupants are examined. For this purpose, test dummies and seat structures are equipped with sensors to determine the forces and accelerations applied. The test dummies used are similar to the crash test dummies known from accident research, but are much more robust and even more elaborately designed.
The measured values can be used to calculate a so-called "Dynamic Response Index", which can be compared with ergonomic limit values. The metrological challenge in these tests lies in the extremely short duration of the processes and the large number of measurement data to be recorded simultaneously. The measurement duration is typically only 60 milliseconds, in exceptions sometimes up to 2 seconds. For example, 32 measurement channels with sampling rates of up to 1 MHz (corresponding to one million measurement values per second) are recorded per channel. Since the test objects are usually damaged after the test and the tests can therefore not be repeated, the greatest possible redundancy and data security of the measurements is required. The measuring instruments are located in a measuring container at a safe distance during the test, which requires complex sensor cabling of up to 100 meters in length.
With the LTT24, they have a measuring system that meets these high demands. This front-end system, developed and produced by LTT GmbH, the Würzburg-based specialist for ultra-fast measurement technology, expands the bandwidth of conventional PC measurement technology to previously unattainable dimensions. A single device offers 16 differential inputs. However, since the devices can be cascaded, synchronized acquisition of several hundred parallel channels is also possible. Separate A/D converters and amplifiers for each input allow simultaneous sampling of all channels and channel-specific amplification with input ranges between * 1 Volt and * 50 Volt. The possible sampling rate per channel ranges from 1 kHz to 2.5 MHz at 16 bits or up to 20 MHz at 12 bits, depending on the desired resolution. Each input has an adaptive anti-aliasing filter.
The measuring device itself does not require any drivers in the PC and is immediately ready for operation after being switched on. The internal hard disk of the device is automatically recognized by Windows as its own system drive and therefore allows direct access from the PC to the stored measurement data by any application programs.
The recorded measurement data is written either to a high-speed RAM of 512 megabytes or to the integrated 40-gigabyte hard disk, which is shock-proof up to 20 g. The data is stored on the hard disk. Due to the required redundancy, WTD always uses two systems independently of each other. This means, for example, that for 32 measuring channels, two LTT-186 units are operated synchronously twice. The first system runs in transient recorder mode and is automatically triggered via a TTL input. The other, on the other hand, is started manually by pressing a key or clicking the mouse. The stored signals are first analysed over the entire theoretical bandwidth of the accelerometers used (typically 40 kHz) in order to detect any oversteering. Only then is the bandwidth limited to the relevant range for the evaluation by means of digital filters.
For the test engineers at WTD 91, the use of the new systems means a significant improvement and rationalisation of their work.
Previously, magnetic tape recordings were used for such experiments.Handling the heavy tape drives was relatively cumbersome and archiving the sensitive tapes was costly.The analogue recorded signals first had to be digitised and then transferred to the evaluation computer before further processing.The sampling rate was also limited to 80 kHz per channel, which was close to the limit of the required bandwidth.Even PC plug-in card systems that had been tested in the meantime did not offer the required flexibility due to limited sampling rates and signal bandwidths.Only the LTT system provided the desired solution: thanks to the online functionality, the measurement data is now directly available in the PC and can be verified on site. This enables, for example, a system check (for cable breakage or similar) even before the start of the test and thus an increased redundancy in the recording. Data backup is now done on CDs, which is extremely convenient and space-saving, and also allows flexible playback of the data on any PC system.
Andreas Evelt from the responsible measurement technology group is very satisfied: "The compact and extremely robust LTT devices are easy and safe to transport and therefore allow us to use them much more flexibly.
They are also more economical than our old tape recorders, both in terms of purchase and maintenance, which is an important factor especially in times of tight budgets!"