NI PXI DMM and NI PXI multifunction DAQ module
National Instruments
Austin, TX

Larger than the Saturn V rocket and designed to eventually take crewed missions to Mars, the Space Launch System (SLS) will be the most powerful, versatile rocket ever created. Before its maiden launch in 2017, the rocket will go through testing at various facilities throughout the US, with one of the most impressive being the large structural tests scheduled for 2015 at NASA’s Marshall Space Flight Center in Alabama.

The Space Launch System is comprised of the engine, liquid hydrogen tank, an intertank, a liquid oxygen tank, and a forward skirt.
In structural testing, engineers verify that the mathematical software models that analysts have created are correct. To do this, loading and environmental conditions are simulated on a physical prototype while sensors measure the response. Typically, this is done using load frames to apply force while strain gauges, load cells, and displacement transducers characterize the test articles’ response. For the SLS structural tests, NASA will use more than 10,000 sensors to validate analysts’ models.

With tests of this scale, they are performed only once — rerunning a test is extremely expensive, if not impossible. Missing a data point during a test where the only physical prototype of a test article may be damaged is unacceptable. NASA’s entire structural test data acquisition solution is based on NI hardware, and has been designed to achieve the high reliability, flexibility, and accuracy demanded by these types of tests.

NASA’s software solution incorporates the following five main pieces, each designed to ensure they can meet the demanding requirements of these large tests:

  1. Test Database: Every detail about the test, from an individual channel’s calibration settings to the specific cables used, is recorded in this database. If NASA ever needs to duplicate a test, this database has all the information to do so.
  2. Sensor Database: To get the most accurate measurements possible, NASA generates and stores specific calibration information about each individual sensor in the sensor database, and uses this information during the test.
  3. Calibration Program: The entire test setup must be calibrated and verified before every test. This interactive program allows NASA to perform a calibration, verify all wiring, and troubleshoot any issues.
  4. Main Acquisition Program: This program supports multiple clients viewing data during tests so everything can be closely monitored as the test proceeds. There will be more than 72 individuals actively viewing the data, watching for any potential warning signs during the SLS tests.
  5. Local Remote Data Harvester (RDH) Program: Each node can connect through the network to the main acquisition program. In case of a loss of network connection, NI PXI embedded controllers store data locally on every RDH before transmitting it to the central hub. These RDHs are capable of measuring 256 channels of strain, voltage, or displacement sensors, as well as 64 channels of thermocouples.

As sensor types and channel counts change from test to test, NASA needed more flexible signal conditioning, and chose NI’s universal bridge input module. The SLS tests will require more than 10,000 channels of bridge-based measurements, all of which need to be synchronized. NASA goes to great lengths before every test to validate that all measurements are as accurate as possible and their setup is 100% correct. Within each RDH (in addition to the signal conditioning hardware), there are two NI PXI DMMs, an NI PXI multifunction DAQ module, and internal switching to route signals. These elements enable NASA to perform the following calibration routine.

Step 1 is the health test. In order to save time, NASA checks for any dead channels with a quick, coarse pre-scan of the amplifiers.

Step 2 is international calibration in which NASA does a two-point calibration using a simple analog output from an NI PXI multifunction DAQ module while an NI PXI DMM verifies the voltage levels. The gain and offset coefficients generated are used during the test to achieve the most accurate measurements possible.

Step 3 is the tolerance test. To verify that the internal calibration was valid for all input ranges, NASA performs a 16-point verification. This is verified by a second NI PXI DMM within the rack.

Step 4 is the excitation and balance test. NASA must also verify that the cabling from the RDH to the test stand is properly connected. The NI signal conditioning hardware is capable of measuring both the excitation of each channel and the balance (or offset) of the bridge. These values are also recorded for later use.

Step 5, channelization, is the most comprehensive and time-consuming test during the calibration routine. During channelization, each sensor is disconnected, replaced with a shunt, and measured to verify that all cabling is intact and that all channels are connected to the proper sensors. For SLS, this is estimated to take several days to complete.

Step 6 is the noise test. If a noisy channel is detected, this optional test takes 200 samples and applies a histogram. If there is too much variation, they may swap channels or cables to eliminate the noisy channel.

Step 7 repeats Steps 1 to 4. Since channelization may take several days, and many of the values obtained in early steps will be used during the test, NASA will repeat Steps 1 to 4 on the day of the test.

With their system architecture, NASA only needs to send back two DMMs for their yearly external calibration to maintain calibration for all of the data acquisition equipment in the RDH. This reduces downtime and saves NASA a significant amount of money every year.

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NASA Tech Briefs Magazine

This article first appeared in the November, 2014 issue of NASA Tech Briefs Magazine.

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