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New techniques slash test time and double test coverage. (Cable/Harness Test).

Find out how an automated cable/harness testing strategy helped one electric-rail car manufacturer reduce test and rework time and increase customer satisfaction.

Since 1975, Siemens Transportation Systems has received orders for more than 800 electric rail vehicles in North America, making the company the largest supplier on the continent. In fact, the Vehicle Division based in Sacramento, CA, has provided light rail service cars for more than half of the cities across the United States.

As demand for light rail grows, Siemens must find ways to increase the manufacturing capacity while reducing costs to remain competitive. With manufacturing economy in mind, Siemens' management continually explores new ways to achieve long-term cost savings in building the electric rail vehicles.

To begin, Siemens targeted the electrical testing of the vehicles, which required labor-intensive connectivity verification. For years, Siemens performed manual tests on all their electrical systems using inexpensive tools. The method was very primitive, using a hand-built bell box to check critical wiring systems, and encompassed only 50% of all vehicle wiring.

For that reason, Siemens needed a way to reduce the time, manpower, and associated costs required to perform these crucial tests while delivering a quality product.

Simple But Costly

Manual point-to-point ringing helps identify missing or incorrectly terminated wires but does nothing to find shorts between the wiring circuits. You can completely ring an assembly using this technique and still have problems from undetected errors.

The only way to assure zero faults in the wiring is to test each circuit for continuity and against all others for shorts. The time and manpower required to test for shorts against all other circuits renders such testing unfeasible with manual methods.

Even though only critical circuits were tested, it took two or three Siemens technicians days to identify and repair the faults. Another two or three days were required to perform an AC hipot test after the initial wire test. After realizing that too much time was spent checking out the vehicle, Siemens looked to DIT-MCO International for a test solution.

Designing the Test Solution

Once Siemens engineers decided to implement a test program, they had to determine how the system would be used and what results would be required. The process started by asking basic questions:

* How many test points are required?

* What type of tests must be performed?

* What access is appropriate for the unit under test (UUT)?

To calculate the number of test points, the number of end-point terminations for each wire must be counted. Frequently, the number of wires serves as a guide, but that can be deceptive if the wires are interconnected. A more accurate count would be the number of pins in all of the connectors on the assembly.

Once a count has been determined, it's good practice to add at least 15% to allow for unused test points due to the cable breakout from the test system. If the system has a 50-pin interface to mate with a 41-pin connector, nine of the system's test points might be wasted.

A modular interface connector helps minimize the number of unused test points by matching the connector size to the product. DIT-MCO's STAC Connector consists of modules that stack together to form the interface connector. With a five-pin increment, it will very closely match the product connector and eliminate wasted test points.

The type of test determines the functional capabilities of the test system. Siemens needed to measure continuity and insulation resistance of the wires in addition to testing components such as resistors, capacitors, diodes, and switches.

To test AC power circuits, an AC voltage generator must be included in the system in addition to the DC source used for testing passive devices. To activate relays or circuits, power sources are needed that can be connected during the test to simulate the operation of the UUT as well as the instruments and loads required to make the functional test measurements.

Product access determines the amount of time required to set up the test. Product access can range from simple interface cables to the UUT to highly automated schemes. The trade-off between highly engineered access schemes with fast and efficient handling must be compared to the time required for less sophisticated access methods.

For the high-volume, highly repetitive Siemens' UUT, a special-access mechanism was designed to interface with the product for testing. In this case, the test is dedicated to one or a few products with very little time required for the setup.

In some cases, it is not possible to bring the product to the tester. A portable solution can involve high-density electronic modules in portable cabinets or hand-carried distributed switching units. The Siemens testing demonstrates an application where the tester must move to the UUT.

Electronic Control Assemblies

Siemens declared the first target for improvement was the electronic control box (ECB). This assembly integrates 60 to 70 relays with multiple contacts and multiple operating states. Manually identifying and testing all of the operational states are monumental tasks requiring connection of power supplies in addition to the ring box.

The test platform selected for the ECB was the DIT-MCO Model 2500 Multiple Bus Analyzer (MBA) with a modular STAG interface. The MBA provides full functional testing of the control chassis. With the multiple system buses, it's easy to connect power supplies, loads, and instruments to any random point in the assembly, simulating the operating environment of the unit. This system effectively emulates the functional operation of the assembly.

The switching access system of the MBA differs from traditional wiring testers. With other systems, special adapter cabling connects power supplies and loads to the product under test. However, the MBA is a true random-access switching system with up to 10 buses that can connect to any device. It is shown testing an ECB in Figure 1.

From the drawings for the ECB, DIT-MCO's Special Products Group created a test program that would activate each relay and test all of the circuits contacting that relay. Sometimes several relays in series were required to be turned on to fully test the circuits.

Prior to implementation of the MBA, a test would require two to three days. Now, a full test of the unit only needs one minute.

The real proof of the MBA came after its installation. While demonstrating the test program, DIT-MCO asked for additional units to test. Several manually tested units were pulled from the storeroom. Within a short period of time, the MBA located faults in each of the units that would have been undetected until installed onto a vehicle and the operational testing started. Finding a single fault before it is installed into the vehicle will save hours of troubleshooting later.

The MBA also saves programming time. Since every test point has the full capabilities of the system, it is very easy to create the test program.

Vehicle Testing

For the vehicle test, a traditional test system was needed since there were thousands of wiring terminations and only a few points required power activation. Siemens specified the vehicle test system to include an AC voltage generator for testing of the power circuits in the vehicle.

DIT-MCO proposed a solution based on the Model 2508 Test System. The 2508 uses high-density switching modules so a great deal of testing can be packaged into a small system (Figure 2). With portability of the system as a requirement, the high-density configuration is necessary for successful testing since 5,000 test points were required.

A turnkey installation from the test-system manufacturer is especially important for new users of automated testing. And the manufacturer's test specialists are needed to fill the experience void by working closely with the users to ensure an on-time completion.

With the implementation of these new testing strategies, a 70% reduction in test time has been achieved, even including the time required for reworking the faults in addition to the testing. The primary savings come from identifying faults in the vehicle before the fully assembled car is powered up to perform functional test.

Results Analysis

Testing is not an isolated function but a part of a well-integrated process-control system. The test system finds the physical faults but cannot locate the cause of the faults.

The tester's error report provides vital information to repair the faults found in the product. The same information also can supply historical trending information. For even more improvements in the test process, in the future Siemens can implement such an error-tracking and diagnostic process using the DIT-MCO TestStats[TM] database.

By creating a test-error report database, all the errors from the tested products can be analyzed. When the test technician repairs the product, the database is updated with the repair information. Typically, the technician enters a fault code such as missing wire along with a description of the fault resolution.

The next time a technician repairs a product with the same fault, the database provides a historical reference of previous repairs. Knowing this information, the technician typically corrects the fault without further trouble-shooting.

The process-control engineer also can use the database to find the cause of the fault. Over the course of time, error information is analyzed to find the most common, recurring faults in a product or assembly line. The fault codes tell the engineer the source of the problem.

For example, if the fault code is a missing wire, the engineer may find that the manufacturing documentation is incorrect and missing a change-order instruction. This is the root cause of the problem and now can be fixed to prevent future missing wire faults.

The TestStats database access uses standard structured Query language (SQL) to retrieve the data into a spreadsheet application. Then it is very simple to perform analysis using the robust data and charting functions of the spreadsheet application.

No One Answer

As every product operates differently, there is no one answer for all applications. It is important to consider the test techniques available and the performance/cost trade-offs. If you do not possess the experience to make these decisions, it is important that you have a test-solution provider with the expertise to guide you through the process.

Siemens greatly benefited from the implementation of an automatic test strategy, and significant cost savings have been realized. Best of all, more tests are being performed, and all tests are documented and archived.

According to Siemens, a big challenge is proving to the customer that it is delivering a product that meets stringent requirements for the testing of the vehicle hardware. To that end, customers frequently are on site to watch the product testing procedure.

Siemens also was focused on test speed and fast product throughput while maintaining very high quality. This has been realized as Siemens continues to search for ways to provide quality products at competitive prices.

About the Author

Karl Sweers, the international technical marketing manager at DIT-MCO, has 20 years experience working with wire-harness testing. A graduate in electrical engineering, he has worked in DIT-MCO 's engineering department as both an electrical systems engineer and a software development manager. Mr. Sweers also earned a master's degree in business administration from the University of Missouri. DIT-MCO International, 5612 Brighton Terrace, Kansas City, MO 64130, 816-444-9700, e-mail: ksweers@ditmco.com

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Author:Sweers, Karl
Publication:EE-Evaluation Engineering
Date:Jun 1, 2003
Words:1885
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