Avoiding test system downtime with CableSense technology on the PXI platform
01 May 2019
New PXI products typically aim to meet a measurement challenge, whether that means higher-performance specifications or better system-level interfacing. The actual measurement itself, however, is only a small portion of the full workflow, and, as any test engineer can verify, plenty more can go wrong throughout development & maintenance of a test system.
This article was originally featured in the May 2019 issue of EPDT magazine [read the digital issue]. Sign up to receive your own copy each month.
From device-under-test (DUT) interfacing to the switch network to the long-term maintenance plan, the highest resolution voltage measurement is nothing without a reliable test system architecture in the first place. Over more than 20 years, we have seen countless examples of PXI platform innovation for the sake of the measurement, but as Michael Keane, Product Marketing Manager at National Instruments, the company that originally developed and introduced the PXI standard, asks, where is the innovation for the engineers themselves?
When intermittent failures start to increase, or test data suddenly becomes noisier than expected, or offset by an odd amount, the main question on an engineer’s mind is typically the same. What changed? Is the code, instrument, DUT or physical setup to blame? Too often, the issue comes down to one faulty electrical connection, after taking a prohibitively long time to diagnose. The key to mitigating risk here is to verify a system’s physical setup at the start of a test, to detect common ATE connection issues early, while remaining minimally disruptive to the test itself. To help do this, NI has introduced patent-pending CableSense technology to some of its PXI oscilloscope models. Using principles similar to a traditional time-domain reflectometer (TDR) on a real-time oscilloscope within your test system, you can detect changes from a known, golden setup, without having to alter the connections themselves.
Detecting changes in your setup
Automated test setups can range from simple, single coaxial cable connections to vastly complex mass interconnect systems, involving adapters, switches, receivers, interface test adapters (ITAs), and/or temperature and vibration chambers. To ensure the quality of a test, you need to maintain secure connections between the instrumentation and the DUT. However, as the size and complexity of a mass interconnect system grows, the number of potential failure points grows as well. Faults are increasingly likely to occur with connections closer to the DUT since those locations are the most exposed to insertions, changes and potential human error. For instance, connections in a temperature or vibration chamber are swapped between every DUT, but the cables between a PXI chassis and receiver are unlikely to be touched frequently, once they are situated.
Types of connection failures
Cables, connectors and relays unfortunately wear over time, and are susceptible to operational misuse. Cables can become loose or frayed, pins can get bent from misguided insertions, and relays may fail earlier than expected. During tester replication, an incorrect type or length of cable may also be used by mistake, resulting in unnecessary and avoidable measurement variation.
Characterising your setup using CableSense technology
Lab experts may be able to visually inspect systems in a high-level attempt to detect changes, but bent pins or internal cable failures are still too difficult to find this way. And tampering with a test station that should already be working fine is nerve-racking because noble intentions could cause new problems, especially when insertion counts are limited. To help you characterise your setup more efficiently, NI introduced CableSense technology to some of its PXI oscilloscope models, by incorporating a pulse generator behind each oscilloscope channel’s 50 O path.
Similar to a traditional TDR, the NI PXI oscilloscope sends a pulse along the entire passive electrical path, from the oscilloscope channel all the way to the DUT. When the pulse reflects between impedance changes, you can characterise impedance or reflection coefficient over time, correlate it with distance, and account for adapters, switches, receivers or ITA connections in the signal path. Critically, since the real-time PXI oscilloscope is already incorporated into the automated test system itself, you don’t need to alter any connections to perform this characterisation. External TDRs or wire testers require a combination of disconnections, cable swaps or equipment swaps for reactive debugging, so they’re not effective preventive solutions.
To apply this technology, you can create a limit mask from a known, golden setup to serve as the basis for future comparisons. The CableSense pulse is accessible through the NI-SCOPE API, so you can programmatically create these masks, and then verify against them later in an automated fashion, perhaps right at the start of your test sequence. Comparison logic detects both major failures, like loose cables or bad relays, and minor failures, like incorrect cable types or lengths. You can automate this check at your preferred frequency, for example, once every morning, or, for critical, longer-running tests, perhaps once per DUT. By tailoring the masks to your own tolerance for change, you ensure the repeatability you need in your specific application to prevent false failures.
Failure prevention alternatives
Test system downtime can be very expensive if a project schedule gets pushed back, or a DUT shipment is missed, so test organisations should do all they can to proactively avoid this and maximise their system uptime. When an issue does arise, engineers’ troubleshooting tools should help them pinpoint the source effectively, so time is not wasted troubleshooting down the wrong path.
The simplest, although least effective, route is to ignore the possibility of electrical path failures until necessary (or, until they’re already urgent). For additional peace of mind, some organisations may routinely replace cables before needed or schedule downtime for connection health verification using an external TDR. But frequent replacements require extra budget, and external TDRs take extra time, given that each test point must be disconnected from the test station and reassembled after verification.
Innovating beyond the measurement
While PXI analogue, digital and RF instruments are all continually growing in theirmeasurement performance, density or FPGA capabilities, CableSense technology as a hardware add-on is a tool specifically built to accelerate an engineer’s troubleshooting workflow. By leveraging an instrument already housed within an automated test setup, the PXI form factor allows the instrument to ‘see’ the rest of the passive electrical paths in a system’s switch network, without disrupting any connections. In this way, the proactive health check can be designed into the system as a whole and become part of an organisation’s operational routine geared toward minimising change – a PXI innovation meant for the engineers themselves.
Given its automation capability, CableSense technology can help you mitigate your overall risk by catching setup changes early, and qualifying the integrity of the measurements that follow – all without disrupting your typical workflow. Whether you’re trying to be first to market or just sticking to a strict schedule, chances are you don’t have time for downtime.
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