Tech Briefs

An Automated Safeto- Mate (ASTM) Tester

This tester allows for fast, safe, and reliable checkout of connector interfaces for both critical flight hardware and companion ground support equipment (GSE).

Safe-to-mate testing is a common hardware safety practice where impedance measurements are made on unpowered hardware to verify isolation, continuity, or impedance between pins of an interface connector. Performing this on critical flight hardware under test and its associated GSE ensures minimal risk when the hardware is powered. Historically, safe-to-mate measurements are performed manually with data written into paper procedures. This laborious process potentially requires a large amount of time for connectors that could have as many as 104 pins. Risks include human error in performing the measurements on sensitive inputs or in recording the data as well as potential loss of hardcopy data.

A computer-based instrumentation solution has been developed to resolve all of these issues. The ASTM is connected to the circuit under test, and can then quickly, safely, and reliably safe-to-mate the entire connector, or even multiple connectors, at the same time. The operation is completely automated, so that personnel can devote time to other tasks. When the automated safe-to-mate is finished, data is retained as electronic records that can be saved for later review.

The ASTM marries off-the-shelf modular components such as a computer, multiplexers, and a digital multimeter (DMM) with a custom printed circuit board all in a chassis. This approach enables multiple, identical, automated safe-to-mate units to be constructed. Test engineers will not have to worry about finding and using the same DMM as the last time, as all of the units will contain an identical model. Safety has also been designed into this system by purposely selecting a DMM that outputs a very low voltage and current to make the measurements. A resistor has also been placed in parallel with the load being measured to further limit the current output of the digital voltmeter.

The ASTM software can read in schematic netlists from common tools such as Orcad or Mentor Graphics DXdesigner and can automatically determine the type of measurements. For example, it can determine which pins should be shorted together (e.g., power connections or ground connections), and will test that they are indeed shorted. The program also has a learning function that allows it to make impedance readings for those cases where a design netlist is not available (i.e., the hardware is essentially treated as a black box). Finally, the software saves all data into a file that it can use in the future to verify that the board does not change, or that identical boards that should have the same impedances actually do.

The system can handle pins with capacitive and diode loads. For example, the software senses the charge-up effect characteristic of capacitors to know that the load is capacitive. Finally, the ASTM employs a custom printed circuit board to handle the routing of signals between its front I/O connectors and its internal instrumentation multiplexers. This makes the system more reliable than other units that employ large, custom, internal harnesses. This also greatly reduces the size and the cost of assembly. Users simply develop test harnesses between the ASTM and their hardware under test.

This work was done by Phuc Nguyen, Michelle Scott, Alan Leung, and Michael Lin of Goddard Space Flight Center; and Thomas Johnson of Microtel LLC. GSC-16098-1

This Brief includes a Technical Support Package (TSP).

An Automated Safeto- Mate (ASTM) Tester (reference GSC-16098-1) is currently available for download from the TSP library.

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