Alexander Graham Bell was lucky. The few wires in his primitive phone circuit were correctly connected when he called for his assistant’s help. Watson heard him and, as they say, the rest is history.
Cable harnesses in vehicles and appliances are much more complex then Bell’s and must be thoroughly tested. For example, a car’s wiring is divided into several separate harnesses for the instrument panel, chassis, and engine. The chassis harness alone may contain more than 400 circuits; aircraft harnesses include thousands of connections.
Today’s cable harness materials are technically excellent. Conductors are made from very pure copper, the insulation is uniform and withstands high voltages and temperatures, and there are many types of terminations and proven terminating methods available.
According to Christopher Strangio, director of marketing at CAMI Research, “Problems in cables don’t usually come from the cable itself, but from the hand-assembled terminations at the ends. Over 95% of all the kinds of problems in cable assembly have to do with the terminations at the ends. As long as you have cables that have to be hand-assembled—which most of them are by crimping, soldering, or even welding—there’s a chance of human error.”
Because cable harness construction and testing have been labor-intensive, efforts are continuing to reduce time and cost. One approach is similar to guided assembly methods that identify component locations for PCB hand-inserted parts. It highlights the path of the next wire to be added to the harness.
Each wire is tested after it has been terminated and plugged into the appropriate connectors. In this way, any mistakes are found immediately and corrected. When the last wire has been completed, the cable harness also has been tested successfully.
If the time to install a completed cable assembly on the test fixture is significant, costs can be reduced by adding circuits to the harness for the tester to use. When the tester senses that all connectors have been correctly mated, the test is begun.
If the operator attempts to remove a harness before a test is completed, clamps built into the test fixture will not release the connectors. If clamps are not used and a connector has been removed, supervisory intervention may be required before the tester will resume operation. In the latter case, intelligent software repeats only those tests which directly involve the disturbed connector(s).
If many different types of harnesses are being handled in a repair or modification shop, it’s useful for the tester to check that the correct interfaces are being used. Some testers incorporate bar-code readers that the operator can use to verify that the cable harness type and the interfaces match the selected test and test setup.
However, wiring harness testers do much more than ensure correct connectivity. It’s important to know that twisted pairs are correctly terminated, specifically that the return paths of adjacent pairs are not intermixed. A miswiring of this type would not be detected by a continuity test, but would become readily apparent if the harness tester has capacitive measurement capabilities.
. Increasingly, cable harnesses include passive and active components that also need to be tested. And the tester no longer stands alone.
The Benefits of Networking
The cable tester is being included in enterprise networks. Program version control can be accomplished readily if all tests are always downloaded from a central data base maintained by test engineering. New programs can be installed and engineering change orders completed on one computer for the entire group of testers.
Immediate, read-only access to archived test programs is a benefit to companies that repair or modify a variety of cable harnesses, many of which may not be manufactured anymore. Some test-management software also can generate test programs for variations of a standard product. Compared to starting with a new program each time, debugging is much faster when most of the program is known to work correctly.
In many cases, material moving through a manufacturing process is controlled by bar codes. Within the manufacturing data base, the part number of an assembly refers to all the constituent parts and subassemblies. Ultimately, this information drives automated ordering and inventory control programs, so the capability for a tester to read or print bar codes allows it to directly influence the flow of material.
For example, different codes can be assigned to separate subassemblies of a complex harness. Unique codes, in turn, allow a better understanding of the value of work in progress and actual inventory levels.
Eric Harter, a product manager at Dynalab, explained, “One customer wanted to print a bar-code serialized label which would be applied to the harness after a successful test. After enough harnesses were tested to fill a shipping container, the customer wanted the analyzer to print a unique label to apply on the outside of the container. We only had to supply the instructions for writing the print command within the sequence file part of the program to accomplish what was needed.”
Manufacturing statistical process control (SPC) uses test failure data to determine weaknesses in design or assembly practices. For example, a common type of fault in the automotive industry results because cable assemblies cannot be mated with test plugs or sockets before installation in cars. Instead, special spring-loaded probe assemblies contact the harness connector pins and sockets without causing mechanical deformation.
Eventually, the probe contact mechanism can become unreliable, and operators may need to wiggle connectors before a test can be started. This kind of time-consuming fault can be identified by SPC programs that monitor test-time trends.
Involving a PC
Using the capabilities of a PC can make a tester much easier to use. For example, companies generally designate connectors and subassemblies according to their own system. Most cable testers have sufficient flexibility to accept this nomenclature directly: PA-1 on the customer’s cable = connection 127 in the tester. An equivalency table still exists in software, but it’s not necessary for the customer to have access to it, and he doesn’t think in terms of it.
The tester automatically copes with running the tests on the customer’s terms. This means that the tester can’t cause any confusion about what’s being tested because test references automatically agree with the customer’s documentation. This level of intelligence also allows the user to develop the required interface cabling in the most convenient manner.
Some testers recognize when a test has been reached that requires an operator action, such as turning a switch to a different position. Brian Laine, president of CheckSum, described two examples of more complicated cable testing. “We helped one customer test harnesses that incorporated switches. This required integrating multiple operator setup screens during the test in conjunction with multiple wiring topologies, one for each switch setting.”
“We also have provided test software for applications that allow multiple correct ways to wire a harness,” he continued. “Harnesses could be wired differently, but because of the nature of the application and the way the harnesses were later interconnected, they were equivalent. The test program specified the connections that could be swapped, and the system modified the remaining testing algorithm based on finding one of the allowed alternative wirings.”
You don’t have to have problems this difficult to benefit from having a PC associated with your cable tester. A PC provides a good user interface, generally a Windows-based GUI which is intuitive and easy to use.
Computational capabilities can provide analysis of failures and actual measurement data as part of the diagnostic information, not just pass/fail determination. For example, some testers display the value of a fault in the harness, its position from either end of a particular wire, and the color of that wire. A PC and good program-generation software will help you to produce a well-documented test program in the first place.
Testers are leveraging the networking capabilities of the PC as well. DIT-MCO has adopted a client/server architecture that has many benefits, explained Karl Sweers, the technical marketing manager. “While the client and server programs normally will exist on a single computer, they can be physically separated and connected through the user’s network or the internet. This means that DIT-MCO’s service engineers can monitor and even take control of a customer’s system to troubleshoot an equipment problem without leaving the office.”
On the other hand, there are some situations in which a PC may not be appropriate. “Most of the environments in which our equipment operates would be trouble for disk drives, CRT monitors, and keyboards,” said Dynalab’s Eric Harter. “Our products use battery-backed static RAM memory and PCMCIA type cards to solve the mechanical drive problem, vacuum fluorescent and liquid crystal displays, and minimal use of front, vertical surface switches to eliminate keyboard failure.”
Advances in Cable Testing
In addition to all the other things a modern cable tester does, it actually must test complete harnesses and wiring assemblies. Among the components being embedded in harnesses are relays, switches, resistors, capacitors, and diodes. Depending upon the types of components involved and the required tests, some customers use manufacturing defect analyzers (MDAs) as cable testers. MDAs offer guarding which aids low-level measurements. MDAs also can measure complex impedances such as parallel RC circuits.
If an assembly contains relays, solenoids, or lights, for example, they must be powered for their correct operation to be checked. Typically, this has been accomplished by using a Y-shaped adapter cable between the DUT and the tester. Power is supplied to the third leg of the Y from an external source. This approach works well, but requires special adapter cables for each type of assembly.
For very complex wiring assembly testing, Mr. Sweers said that DIT-MCO has developed a random access switching capability. Any test point can be connected to any of 10 random access buses and the output bus or input bus. This architecture provides simultaneous control of device power and circuit test. It is particularly well suited to testing control modules, avionics assemblies, and relay panels.
Cable harnesses used in cars often include components. For example, a large number of relays used for turn indicators, the horn, and lighting are plugged into sockets on the end of a cable. Some switch or relay contacts are provided with snubbing diode/capacitor/resistor networks. And, if the cable carries signals with fast edges, terminating resistors often are used.
An important thing to look for in a cable tester, especially if you need to test relay or switch contact resistance, is Kelvin four-wire resistance measurements. But, there are different levels of performance for Kelvin connections. Many systems offer 0.01 W minimum, but it can range from 0.001 W to 0.05 W . None of these values may make any difference in your test results. But, it depends on what you are doing, which is why most manufacturers don’t include Kelvin measurements as standard.
Some manufacturers offer options to extend the basic tester measurement capabilities, such as modules with additional channel capacity. Enhanced resolution or extended ranges also may be purchased. And software can be a very important purchase if a PC is used anywhere in the setup.
Thomas Neal, director of sales at Cabletest International, said, “Customers want easy integration of the continuity, isolation, and hipot test programs into more comprehensive program structures such as National Instruments’ LabVIEW. Cabletest MPT test programs can be developed on stand-alone stations and then launched from a high-level language.”
Some testers have extended the use of CAD data usually taken as the starting point for continuity tests. Additional information such as wire color, resistance or other component values, connector types, and pin designations has been incorporated to improve tester capabilities. For example, one tester can draw handbook-ready wiring diagrams for product documentation. The drawings show the type of connectors, the pin numbers, and wire colors.
As the cost and size of electronics continues to fall, Cirrus has introduced a tester with input modules that accept up to 256 points and cost less than $900. For large cable harnesses and backplane assemblies, several of these daisy-chained modules can be located close to groups of contacts to reduce test wiring.
Brent Stringham, marketing director at Cirrus, commented, “Size and speed continue to be important factors for users of cable and harness testers. Stand-alone testers compete for space on the workbench. The smaller and less noticeable the cable tester is, the better. Harness testers smaller than a shoebox are available today with custom Windows-based software and complex testing capabilities starting at less than $2,000.”
Cable/Harness Test Products
The Automatic Test Set (ATS) is a computer-controlled test system that can perform routine tests on multiconductor cables. Virtually any cable design can be accommodated within the maximum voltage and frequency limitations of the ATS. Features include a Windows 95 user interface, test data storage, and flexible report generation. Call company for price. Hipotronics, (800) 727-4476.
The ruggedized Model RWA1500 Transportable Wiring Analyzer comes with a built-in PC and both hard and floppy disk drives. Measurements include resistance from 0.5 WW , 4-wire resistance from 0.010 W to 400 W , and capacitance from 50 pF to 1 µF. Diodes and switches also are tested. High voltage is programmable in 1-V steps from 20 V to 1,500 V and can be switched to any of 256 points. Test results and control menus are displayed on a high-contrast backlit LCD touch screen. Separate LEDs indicate pass, fail, and high voltage. The 76-lb unit meets relevant MIL-STD-810E environmental specifications. Call company for price. Cabletest International, (905) 475-2607. to 10 M
Random Access Tester
The Windows-based Model 2500.MBA (Multiple Bus Architecture) Cable Harness Tester can switch power to active devices and relays. Adapter cables no longer have to be split to accept external power. Random-access connectivity supports power applied and measurements made to the same pin; for example, for simultaneous insulation and hipot testing. Measurements include resistance from 0.01 W to 99.9 kW , four-wire resistance from 0.01 W to 10 W , insulation resistance up to 1,000 MW , and capacitance from 10 nF to 1,000 µF. Voltage to 1,000 VDC and 750 VDC and currents up to 2 A can be provided. Optional digital comparator and capacitance comparator subsystems and additional system software are available. Call company for price. DIT-MCO International, (800) 821-3487.
Cable Tester Software UpgradeNew Windows 95/98/NT software for the PC-based CableEye® Cable Tester automatically generates color wiring graphics from continuity measurements. It is compatible with any data base, script, or log files created using the manufacturer’s previous DOS software. Using macro scripts, you can create a sequence to test a cable, print a label, and log the test results. Wiring diagrams can be produced directly on a laser printer. CableEye links to the PC through its serial port and may be used with laptop and notebook PCs. CableEye: from $1,295, software upgrade: $195. CAMI Research, (781) 860-9137.
Manufacturing Defect Analyzer
The Model TR-4 MDA System tests for shorts and opens using programmable thresholds. Measurements include resistance, capacitance, and inductance as well as complex impedances. Active guarding is provided to help isolate components during test. Biasing voltage and current supplies are provided so that three-terminal semiconductor devices and diodes can be measured while operating. The 200-point system comes with a computer and test executive software, programming software, a CAD conversion package, and a statistical process control package. Starts at $6,140. CheckSum, (360) 435-5510.
Touch-Screen Cable Tester
The Touch 1 is a cable/harness tester with hipot capability and a 3½” × 4½” graphical touch-screen control. A built-in Pentium-class PC runs the test program, displaying prompts as required. In addition to checking opens, shorts, miswires, high-resistance connections, and insulation resistance, the tester supports statistical reporting, file up/down load, networking, and test-point labeling. Low-level tests are run at 5-V, 6-mA maximum current. For hipot tests, voltage can be selected from 50 to 1,000 VDC. The standard capacity is 128 points, expandable to 512. Cirris Systems, (801) 973-4600.
The Dynalab Model 1536 Circuit Analyzer uses a 4-line × 20-character vacuum fluorescent display to present the user-defined block and pin designations, wire color, and circuit number. Once generated, test programs are downloaded to the analyzer via RS-232-C or a RAM cartridge. Model 1536 can execute programs written for the smaller Model 1024. Programmable items include test voltage 2 to 12 V, thresholds of 0 to 10 V, and constant current loads from 40 µA to 10 mA. The analyzer provides 1,152 test points minimum, expandable to 1,536 points maximum. From $4,139. Dynalab, (614) 866-9999.
Copyright 1999 Nelson Publishing Inc.