-
- News
- Books
Featured Books
- smt007 Magazine
Latest Issues
Current IssueComing to Terms With AI
In this issue, we examine the profound effect artificial intelligence and machine learning are having on manufacturing and business processes. We follow technology, innovation, and money as automation becomes the new key indicator of growth in our industry.
Box Build
One trend is to add box build and final assembly to your product offering. In this issue, we explore the opportunities and risks of adding system assembly to your service portfolio.
IPC APEX EXPO 2024 Pre-show
This month’s issue devotes its pages to a comprehensive preview of the IPC APEX EXPO 2024 event. Whether your role is technical or business, if you're new-to-the-industry or seasoned veteran, you'll find value throughout this program.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - smt007 Magazine
Matrix Maintenance in PXI with Relay Self-Test
December 31, 1969 |Estimated reading time: 7 minutes
Matrix switches are a key part of any test system. They allow users to connect the test equipment to the unit under test (UUT) in many different ways, enabling test equipment access to different parts of the UUT during the test process. The switching system is one of the most likely parts of the system to fail, not because the switching system itself is unreliable, but because it is in such a vulnerable position. The matrix can be subjected to abuse or accidents during development and in use due to programming errors in development, wiring errors, or unexpected UUT faults. David Owen and Bob Stasonis, Pickering Interfaces, explore the reasons for concern and a way that one test system manufacturer has found to diagnose these problems.Lifetime Factors
Matrix switches based on mechanical devices have a limited life, but the life of modern relays is very high. High-quality electro-mechanical relays (EMR) can often have life times quoted of the order of 100 million operations under light load conditions; instrument-grade reed relays have lifetimes in excess of 1 billion operations. The key factor that affects matrix life in test systems is the load characteristics and the conditions under which the relays are operated. Whatever the type of switching device, as soon as the relay has to close or open signal connections that have significant voltage or current present, the operation of the relay becomes a “hot switch” event. At the point that the contacts close or open, they carry a signal that generates an arc between the relay contacts. This erodes or stresses the precious metal contact materials. Relay life is strongly influenced by the load present during these hot switch events, a variation of three orders of magnitude is a common change in life time between a light load and a full load. System designers try to avoid hot switching, but in reality some tests require hot switching of the signals to keep test times low, avoid system restarts between tests, or simulate conditions such as intermittent faults or changing connectivity. Hot switching is a compromise that designers have to manage in their test systems. Hot switch events are also accidentally introduced when the load’s significant capacitive or inductive content generates high inrush currents or high back EMF voltages.
Many failures in test systems are not caused by the relays reaching their normal end of life. Some are due to premature mortality caused by manufacturing defects that are not initially detectable; many more result from accidental events that occur in the system. A common source of accidental events is the test system integration — cabling and software errors can connect parts that were never intended to be connected, for example shorts on power supplies or accidental hot switch events into capacitive loads. The relay may withstand these accidents with partial damage to its contacts, which will shorten its operational life. Even when the system is operating correctly, attempts to test a faulty UUT can stress the switching system, forcing operation beyond the relays’ specification.
Fault Diagnostics
The switching system’s vulnerability requires a system check that includes a way of testing the switching system. Some platforms, such as VXI, have historically offered a self test facility for the relays. The degree of coverage such a self test provides is patchy, sometimes it simply tests the control system and not the relay contacts (the most likely part to fail); some products test the relay contacts but are unable to test isolation relays. VXI products include this capability because the primary users (usually military or aerospace) demanded it and there is enough room in the product to allocate space to self test hardware.
Smaller footprint products, such as PXI, have not provided self test. The design burden of including self test has been high in terms of space, which reduces the density that can be achieved in the module and increases cost.
For these lower-cost and space-constrained products, test vendors have added perhaps one of the most misguided tools as a way of managing the issue: relay operation counting. In relay counting, the software counts how many times a relay has been operated, with the idea that a relay will be replaced when number of operations reaches some threshold. As a tool, this method is deeply flawed. Relay life changes by three orders of magnitude according to the load present. The software has little or no knowledge of the load, as it is simply a counting system. It takes no account of the accidents that happen in systems, such as UUT failures, that shorten relay life. Real relays are subject to significant variations by batch, so the data sheet estimates should reflect the worst batches rather than the best batches. The consequence is that real switching systems can have a much shorter or a much longer life than a relay operation counting system will indicate, and the error bands are measured in orders of magnitude rather than a few percentage points.
The situation gets worse if the user then performs maintenance based on relay operation counting. Modern devices work better and for longer if fewer rework operations are carried out on the assembly. Early “preventative” replacements risk disturbance of other parts and damage to tracks, particularly in the case of surface mount parts. There is much to be said for a philosophy of “if it’s working, leave it alone until you have good reason to suspect it is going to or has a problem.”
System Level Test
System integrators often include a degree of self test at the system level where the switching system is used to route signals back so a self test can be performed, for example by using a DMM to measure different paths. The investment required to do this can be quite high, and the more complex the system, the harder self test gets. The integrator has to understand the way the system routing is performed both at the cable interface level and within the switching parts themselves. Such self test routines will identify most gross faults in the connectivity, but they do not find intermittent paths easily or isolate where the fault is, for example which relay in the switching system or whether the fault is a relay or cable.
Relay Self Test in PXI
The status on self test is changing. A new generation of matrix solutions in PXI includes a built in self test facility, called built-in relay self test (BIRST). BIRST is implemented as a very compact hardware addition to the PXI modules that allows the supplied software to explore a matrix switch, measuring each path in the matrix for path resistance with a repeatability measured in a few milliohms (m?). Every relay is checked for its operation, making welded closed and open relays easily found. The measuring hardware has a high resolution capable of identifying variable contacts. The location of individual faulty relays is quickly identified, in most cases without ambiguity or to within a very few devices.
To perform self test, the user disconnects the PXI module from the test system and then runs the supplied program. The test process is fast, each relay requiring just a few 10s of milliseconds to fully test. The software then displays the test results as a graphical representation of the matrix, highlighting the relays that are faulty and allowing the user to quickly identify the physical position of the faulty device within the PXI module. The use of through hole relay components allows users to simply service the module with commonly available tools and get the module back in service after running BIRST again to check that all faults have been cleared.
Built-in self test at the relay level addresses the shortcomings of the older self test systems and relay counting methods. It allows users to get the maximum out their relays and the modules that contain them with little additional investment. It only identifies those relays that need attention for maintenance and avoids unnecessary system disturbance and downtime. The tool can identify relays with higher than expected path resistance, allowing users to change those relays before they completely fail.
The BIRST tool also compliments system-level tools. When running diagnostic tests, the system level tools can concentrate on testing cable interfaces and switching systems without relay test, leaving exploration of the BIRST-enabled matrices to the BIRST tool.
BIRST Solutions
The BIRST technology is being included in a new generation BRIC matrix solutions. These matrices feature extremely high crosspoint counts — up to 4406 — in a compact PXI module. This method does not affect the cost of the test systems.
In addition, a new range of EMR matrix solutions will have a BIRST capability, offering single-slot PXI matrix modules with matrix dimensions ranging from 64 × 2 to 16 × 16.The range of modules with BIRST support will continue to expand, marking a major improvement in the capabilities of PXI matrix solutions and improving their market acceptance in critical test applications.
David Owen and Bob Stasonis, Pickering Interfaces, www.pickeringtest.com
Join the PennWell SMT Group on LinkedIn
Become a Fan on SMT's Facebook Page
Post your electronics manufacturing, SMT-related material to the #SMT community on Twitter. Use the #SMT hashtag.