NASA selected NI SC Express for accuracy and flexibility
04 September 2017
Replacing NEFF equipment in NASA’s largest human-rated environmental chambers is no easy task. NASA looked to National Instruments for help.
The data acquisition system is capable of accurately synchronising and measuring more than 500 analogue channels distributed throughout the chamber, without the need for additional external signal conditioning.
"For the first time, NASA will no longer need the custom signal conditioning equipment in addition to its measurement hardware. With NI SC Express hardware, NASA can achieve the ±3°F end-to-end accuracy requirements for thermocouples with commercial off-the-shelf technology." James Dean - Jacobs Technology
Simulating space on Earth
When NASA needed to test various repair techniques and materials that astronauts could use to repair damaged tiles on the space shuttle, it used the Chamber B facility at the Johnson Space Centre (JSC). The human-rated chamber allowed an astronaut to apply the “goo” repair candidates to the tiles in a simulated environment to test their performance at the extreme pressures and temperatures in space. At 10.7m wide and 13.1m tall, Chamber B is the smaller thermal vacuum chamber in building 32 at the NASA JSC, but it is no less complicated than its larger next door neighbour. Features such as the removable top to allow for the insertion of larger test articles, a traversing monorail to provide weight relief to one crew member at a time and dual crew locks to enable easy access and crew member entry all contribute to the complexity of the chamber.
To provide a space-like environment, Chamber B must reach temperatures of 90 kelvin at 10-3 micron (~1x10-9 atm). A nitrogen-based cooling system runs through the walls of the chamber to ensure the proper temperatures are reached. Thermocouple measurements are made around the chamber to characterise the thermal environment, ensure that the desired temperatures are reached and verify that nitrogen flow is achieved throughout the chamber cooling system. Certain test articles also require dedicated temperature measurements to ensure they are behaving as expected in these environments. Pressure and flow measurements are also made in the facility and brought into the DAQ system. Overall, Chamber B has more than 500 analogue signals, the majority of which are thermocouples.
Instrumenting NASA’s largest human-rated chamber is no easy task
High-channel-count DAQ systems always introduce new, complex challenges not seen in smaller systems, and the system at NASA JSC is no different. Accurate measurements are of the utmost importance in Chamber B. Furthermore, a sophisticated architecture must be defined for a distributed system that is capable of both logging and visualising data in a central location. Once a system is in place, it is not likely to change, because of budget constraints, the solution must be well planned and adaptable to suit the needs of future clients and tests.
The DAQ system in Chamber B is based on NEFF 470 and NEFF 620 systems with a custom OPC interface. To achieve the desired accuracy, all thermocouples had to be brought into a central reference oven kept at a stable temperature before going into the NEFF DAQ equipment. Furthermore, the custom OPC interface was designed for Windows NT and lacks the flexibility to add more systems or features. As the NEFF equipment became increasingly difficult and expensive to repair and maintain, NASA looked for a new solution for the Chamber B DAQ system.
Selecting the NI platform
For the new solution, NASA chose NI SC Express and PXI hardware for the DAQ system in Chamber B. The new setup will be composed of multiple NI PXIe-1075 chassis with NI PXIe-4353 input modules for thermocouples and an NI PXI-6682H timing module for synchronisation. Each NI PXIe-4353 is capable of acquiring from 32 thermocouples, so the entire system can measure more than 1,500 thermocouples. Given NASA’s accuracy requirements of ±3°F from end to end, achieving these levels has always been challenging without external signal conditioning equipment such as reference ovens or custom-built cold-junction compensation solutions.
However, with the NI system, for the first time, NASA will no longer need the custom signal conditioning equipment in addition to its measurement hardware. With NI SC Express, NASA can achieve the ±3°F end-to-end accuracy requirements for type T thermocouples with commercial off-the-shelf technology. By getting rid of the reference ovens, NASA can avoid wiring all the thermocouples to a central location before going to the data acquisition system, which gives NASA the option to create a more distributed system and remove expensive additional copper wiring. The accuracy and more distributed architecture will help NASA save on wiring costs, and the capabilities of the NI PXI platform and NI LabVIEW system design software make creating such a system possible.
High level synchronisation
With any distributed system, the level and method of synchronisation must be part of the initial planning for the system. Even though the Chamber B DAQ system predominantly consists of thermocouples, NASA still wants a high level of synchronisation between the measurements to accommodate requests for other types of high-speed measurements such as pressure or strain. To achieve this level of synchronisation, NASA uses a GPS time source to broadcast a 1588 signal to each of the PXI-6682H modules in the system. The 1588 signal allows for precise synchronisation with little configuration. Each PXI-6682H is given an IP address on the timing subnet and its priority is set so the module recognises the time source as the master clock and synchronises. From there, the configuration and handshaking are handled among the devices with no further configuration required. Now each PXI-6682H module is set to drive the 10 MHz clock on the PXI backplane as the acquisition clock for all of the modules in the system. With this architecture, NASA can achieve synchronisation within 100 µs between channels that may be in separate PXI chassis hundreds of feet apart.
Furthermore, the 1588 time server provides easy synchronisation with 1588-compliant control systems and the OPC clients to give the entire system a consistent time base for acquiring, viewing and logging all of the data.
For the Chamber B system, all of the data needs to be visible in real time in the control centre during the tests. Additionally, the data needs to be logged to a central database with backup copies on each PXI system. With the NEFF equipment, NASA used a custom OPC interface to talk to the Iconics OPC client. The interface was written more than 10 years ago for Windows NT. As NASA upgrades to new Windows versions, it’s been necessary to force the new operating system to recognise the NT-based SCSI drivers so that the NEFF data may be made available for the OPC clients. With the out-of-the-box OPC support of LabVIEW, developing an OPC server interface to the DAQ program was simple and the end result was more resistant to changes in the data server environment. Each of the PXI-based OPC servers provides data to the Iconics client where it is viewed in the control room and logged into an SQL database. For added dependability, each PXI system is logging data locally at a higher rate as a backup.
Creating a scalable and flexible solution
With any large facility such as NASA, effective re-use of both hardware and software can create large efficiency gains. The Chamber B data system software is drawn from a LabVIEW DAQ system developed for Chamber A. With this custom code library and standard architecture, additional channels may be easily added to the system for temporary or permanent use. New and supplementary systems may be quickly deployed on any NI-DAQmx-supported hardware. By using the 1588 synchronisation methods and the PXI-6682H, the additional channels can easily be synched to the rest of the control and data system.
Furthermore, with the OPC capability, the data is instantly available to any OPC client on the test network.
For example, when the needs of Chamber A exceed the channel counts of the current system, NASA engineers can move the PXI systems from Chamber B to Chamber A. Test support personnel simply choose a configuration database file, load this into NI Measurement & Automation Explorer (MAX) through a custom-made, LabVIEW configuration utility, and start acquiring data. To return to the old configuration, NASA engineers can reload the previous configuration file, and the DAQ system is ready to resume acquisition in Chamber B.
The PXI/SC Express system meets NASA’s strict accuracy and synchronisation requirements, and the LabVIEW software allows for quick deployment and data access. As older systems are phased out and upgraded, the PXI/SC Express hardware and LabVIEW custom libraries will be implemented in a similar fashion. This configuration forms a standard architecture for future use in all of NASA’s thermal and vacuum testing facilities.
The engineer’s guide to signal conditioning
Signal conditioning is one of the most important components of a data acquisition system because the accuracy of the measurement cannot be relied upon without optimising real-world signals for the digitiser in use. To get the accuracy, speed and performance you need to test with confidence learn more about signal conditioning and master the fundamentals by downloading NI’s Engineers Guide to Signal Conditioning. You can design a test system that will scale with your testing needs now and in the future.
Engineers Guide to Signal Conditioning
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