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PDFDeveloping both durable and flexible test systems to address the needs of today’s complex devices is critical.
National Instruments has collaborated with industry experts to create Designing Automated Test Systems, a five-step guide that features best practices for identifying your measurement needs, selecting hardware and software, and assembling and deploying your software-defined automated test systems. An overview of the five steps and a sample section are featured in this article.
Step 1 – Identifying Measurement Needs
Many test engineers choose their hardware based on instrument type rather than measurement need. You do not always need a digital multimeter (DMM) to make precision measurements, and you do not always have to use expensive tools to calibrate your devices. Step 1 of this guide describes best practices for identifying the measurement requirements of your automated test system so you can make cost-effective decisions when selecting your instruments.
Step 2 – Selecting Hardware
The next section of the guide provides specific suggestions as well as practical implementations for choosing your automated test system rack; selecting power distribution units; and designing switching frameworks, mass-interconnect solutions, and custom fixtures. One example in the guide highlights the importance of choosing a rack size based on the Electronic Industries Alliance (EIA) standard because most instruments are built according to this standard. Another best practice discusses the value of keeping enough spacing between instruments in your rack for ventilation.
Figure 1. This diagram in Step 2 of the guide illustrates a switching, mass-interconnect, and fixturing solution in an example automated test system.
Step 3 – Designing Software
The following step teaches techniques for building a scalable and reusable software framework and best practices in code module development. For instance, the guide discusses the importance of making your code “localization friendly” by using pictures to avoid language barriers and using words that translate easily. Additional topics covered include choosing a test executive, documenting code, and selecting an instrument driver paradigm.
Figure 2. Sleeves, wire ties, and strain relief are important for protecting cables in test systems.
Step 4 – Assembling the Test System
This section of the guide discusses considerations for assembling your automated test system. It provides suggestions on cable types and lengths to minimize measurement errors. For example, the guide recommends using copper conductors with silver plating for low-voltage measurements due to their low thermal characteristics. It also recommends using sleeving and/or wire ties to protect cable assemblies, hoses, and wire harnesses from chafing, cutting, and abrading. In addition to cabling considerations, Step 4 of the guide describes system grounding considerations, software activation and licensing, and test system validation techniques.
Step 5 – Deploying the Test System
The last step in the guide helps you deploy your software-defined automated test system and discusses various considerations for replicating your systems. Topics include creating a “deployment image” of your file directory structure and equipping facilities with requirements to run the test system.
A Practical Guide for Building Software-Defined Automated Test Systems
This five-step guide goes beyond theory and places a strong emphasis on presenting best practices in a practical and reusable manner. It features specific examples used by industry-leading test engineering teams to show how you can put theoretical concepts into action to save cost and time. In particular, the guide makes several references to the system that NI engineers built to test more than 50 I/O modules for the NI CompactRIO platform.
The following excerpt from Step 1 in the guide discusses how choosing hardware based on measurement need rather than instrument type can help reduce cost:
Best Practice: Test engineers often choose an instrument based on type rather than need. Such decisions often result in higher costs, so you should choose your instrument based on your measurement need rather than the instrument type.
Real-World Example: Following this practice was highly beneficial when NI engineers selected a method to calibrate the NI 9219 thermocouple module in the test system described in this guide. Typical calibration methods involve using expensive instruments that cost upwards of $50,000 USD. In this particular test system, however, the NI 9219 is calibrated using a Keithley source measure unit (SMU) and the NI PXI-4071 7½-digit PXI DMM.
Figure 3. This image shows the circuit for calibrating the NI 9219 voltage input module.
This is possible because the PXI-4071 DMM has an accuracy that is substantially higher than that of the NI 9219. In addition, because the PXI-4071 was already required for testing other CompactRIO modules, using it for calibrating the NI 9219 helped to substantially reduce the overall cost of the test system.
Table 1. The accuracy of the NI PXI-4071 is much greater than that of the NI 9219.
Download the Designing Automated Test Systems Guide
Review this content online to develop a stronger understanding of core software-defined automated test fundamentals as well as practical knowledge for applying test engineering best practices to your applications.
Jaideep Jhangiani is an automated test product manager at National Instruments. He holds a bachelor’s degree in computer engineering from Texas A&M University.
Download the Designing Automated Test Systems guide.
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This article first appeared in the Q4 2009 issue of Instrumentation Newsletter.
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