The scores, hundreds, or thousands of wires that carry power, commands, and data around inside a complex electronic system are not high-technology components. But it’s those wires that make high-technology components operate. Testing the cables—those multilane electronic highways—before they are installed is an essential part of total system checkout.
Often the cable or harness to be tested has more complex elements than just single-conductor, twisted-pair, and coaxial cables. The tester must check those conventional elements to detect crimping, soldering, and welding errors. Beyond that, the assembly may include electrical components such as capacitors, resistors, and diodes. And some contain nonelectrical parts such as grommets and clips. All of these should be checked as part of a total cable or harness evaluation.
Flexibility of the Test System
Edward Luby, sales manager at Dynalab, outlined accuracy and user friendliness as the absolute necessities of a cable/harness test system. “After those characteristics, we see flexibility as the driving factor in cable testing systems. For example, one equipment manufacturer builds simple cable harnesses and performs basic tests; the next needs to verify the presence of specific nonelectrical components such as retainer clips, grommets, and other parts. Yet another must test electrical components such as resistors, diodes, capacitors, relays, and switches. Each company’s needs must be supported,” he explained.
Production-line cable assembly sequences also vary. One company may want wires added in a specific order, so the tester must give the operator step-by-step hookup details. In high-volume cable manufacture where each operator is responsible for just a small part of the assembly, the harness analyzer could travel on the assembly line with a cable. As a result, each case requires a flexible tester.
Many manufacturers want the tester to print a label when a cable harness passes the test so that no harness is shipped until the test results are satisfactory. With one cable assembly station, this can be straightforward. But where up to 30 cables are built and analyzed on one rotary assembly station, the customer wants one label printer to cover all the sets through a print server.
The Internet is another example of versatility, according to Karl Sweers, a technical marketing manager at DIT-MCO: “The world grows smaller every day as networks connect us to each other and the equipment under our control. Cable/harness test systems should be accessible via the Internet from any point in the world and controlled by a widely used software system.
“Obviously, security must be built into the system and controlled by a system administrator so unauthorized operators won’t use the wrong test data, edit files, or make other mistakes. The network also offers the capability to perform diagnostics from a central location to keep systems in operating condition,” he said.
In a variation of the versatility theme, Tim Horacek, a sales engineer at Weetech, suggests the need for two separate families of testers. “Low-end users want inexpensive units with easy-to-use, innovative interfaces, while high-end users demand flexibility, unique features, and accuracy,” he said. “While the technology train is speeding down the track of progress, these different forces are pushing test-station suppliers in opposite directions. Responsive manufacturers will offer solutions to both groups.”
Switching Throughput Rate
The throughput rate is another important characteristic of a cable tester. Thomas Neal, director of sales and marketing at Cabletest International, said that the key question is, “How many cables can I test in an hour?”. The answer involves knowing the actual measurement time plus the switching matrix speed and the settling time, the circuit-under-test charge and discharge times, measurement-method algorithms, and the connect/disconnect time.
Ken Rockwell, general manager of Cablescan, agreed, “Users are looking for faster setup and test capability. Computers and Windows/NT software are the tools to accomplish those objectives. The computer gives them a way to control tests and track results.”
The relays or solid-state switching devices in cable/harness testers also are factors in determining throughput speed. “The effective switching speed for relays in an electromechanical system is 10 to 50 ms because of contact bounce and settling,” according to Mr. Neal of Cabletest International. “The contact resistance is 0.01 W to 0.05 W . Solid-state switching systems are much faster than that, but limited to low-voltage applications.”
Contact resistances for solid-state switches typically are 0.25 W to 20 W . This could present a problem in testing some cables, but low resistances can be measured quite accurately with a four-wire Kelvin circuit.
The circuit-charge and discharge times on low-resistance continuity tests are not problems. However, when measuring high insulation resistance, the time constant can be more than 10 s. This could cause a serious slowdown in the test sequence. Where substantial capacitance is expected, a tester with high current capability will make the tests go much faster.
Another way to increase speed is to use look-ahead algorithms that compare instantaneous data with the operator-defined threshold. When a resistance measurement passes the threshold, the measurement is declared to be complete and the scanner moves on.
“In measuring insulation resistance, the linear test algorithm can make the greatest improvement in speed,” said Mr. Neal. “This means measuring from a wire to all the other wires together rather than to each of them separately. On a 50-wire cable, this improves the speed by a factor of almost 25 to 1.”
Composite Features of Testers
The cable/harness testers available today have a wide variety of features. For example, Brent Stringham, director of marketing and sales at Cirris Systems, noted, “Cable and harness manufacturers want to build and test at the same station and are looking for a test station that can guide the operator through both these steps. The equipment to satisfy this requirement must be easy to use.” Such a tester may accept a wire list, convert the information into a graphic wiring diagram, annotate the file, and save it.
The computer in some stations creates a searchable database on disk with descriptive notes, assembly instructions, or color codes. The system provides testing details on a removable cartridge for distribution to multiple test stations and stores programs for several cables on a single cartridge. You also can print cable schematics, complete with descriptive notes.
There is a wide degree of setup flexibility through interactive software. Some cable test systems enter cable identity automatically and accurately using a bar-code scanner. The most versatile systems create test procedures for batch testing of each type cable so an inexperienced operator can initiate an entire test by pressing one button.
Many of today’s systems give step-by-step wiring instructions to the operator. The systems perform in-process tests on harnesses so any error is detected as soon as it is made.
By definition, each station performs its basic function of testing cables and harnesses. It checks for continuity and scans for opens, shorts, and misconnected wires. Usually, it displays the results as an error list or on a schematic with any faulty connections highlighted. A versatile station has the capability to check up to 128 points and may be expanded by adding modules for a total of 1,024 or more points.
A versatile tester should measure electrical components that are built into the harness as well as verify the presence of nonelectrical components such as grommets and retainer clips. It also has the capability to check a cable by comparing it to a known-good model; compare cables graphically; and recognize extra, missing, or shifted wires instantly.
The typical station presents the test results in a useful format. It logs test results automatically on disk or prints them as each test is completed. It shows passes and lists each failure with the problem annotated in detail. It may print a label for each cable that passes the test and provide several interfaces to one printer.
The test equipment should be able to analyze the test results. This includes displaying the path of any given wire and tracing hidden wires. You must be able to view large cables with a high-resolution display by scrolling and display a summary showing connections of a highlighted wire to all its destinations.
And last, but far from least, you want cable/harness testers that can be easily upgraded. Then you can replace one or two blocks of a station but not the entire tester as technology moves forward.
Cable/Harness Test Equipment
Portable TesterThe 205 Universal Cable Tester is a portable, stand-alone cable/harness instrument for wired assemblies with up to 128 points. It detects an open, short, or miswire in <50 ns. Evaluation is based on manual entries or comparison with a good sample. Up to 50 configurations can be stored and retrieved. Test results are displayed on a 32-character LCD and can be sent to a separate printer. The tester weighs 6 lb and comes with a 12 to 14 VDC 500-mA adapter. $895. B&K Precision, (714) 237-9220.
The PC-based TestMate Continuity Tester can test up to 128 points, expandable to 256. Test programs are derived, stored, and entered from the host PC or learned from a sample harness. Hard copies of test results or wire lists can be fed to an optional printer. Up to 14 cable programs with labels or 56 cable programs without labels are stored. 128-point: $995. Cablescan, (626) 357-9269.
The MPT 1000-Series Wiring Analyzer is operated by an 80486®-powered computer with LabVIEW software and can be expanded to test up to 120,000 points. Setup is from a database, a spreadsheet, a wire list, or a CAD/CAM system. It tests resistors, capacitors, diodes, transistors, inductors, and switches. The maximum test voltage is 500 V or an optional 3,000 VDC/2,500 VAC. The system comes in a rolling cabinet, a benchtop unit, a portable or rack-mountable system, or a portable tester with a separate control unit. Starts at $17,500. Cabletest International, (905) 475-2609.
The CableEye M2-Expansion Cable Management System features 128 points and can be expanded to 1,024 points. An online software database defines test sequences and includes descriptive notes and label text for each cable type. The software assists in cable design and analysis of difficult problems. The system can be set up for one-button initiation of the test sequence and automatic logging of results to disk. The equipment links to a host computer by an RS-232-C port. Starts at $1,295. CAMI Research, (800) 776-0414.
The computer-controlled TR-4 Manufacturing Defects Analyzer checks continuity using DC current or AC/DC voltage stimulus. Opens and shorts will be detected and identified. Capacitance and inductance are measured with an autoranging circuit. Standard diodes, zener diodes, and LEDs can be tested for performance and orientation. The analyzer has eight bidirectional digital I/O lines, expandable to 96, that can sink 24 mA or source 2.6 mA. The basic tester checks 200 points, and expansion modules raise the total to 1,600 points. Starts at $6,140. CheckSum, (360) 435-5510.
The easy-wire™ System for building and testing electrical wiring harnesses and subassemblies has been expanded to add grid tiles. A board is assembled from several 3.85″ × 3.85″ tiles. A connector holder is mounted on a bracket and attached to a base that clips to the grid board. Harness components are assembled or modified on the board using prewired interchangeable adapters or dedicated discrete wiring from a standard transition board. 256-point: $1,745. Cirris Systems, (800) 441-9910.
Multiple Bus System
The 2500.MBA (Multiple Bus Architecture) Cable Tester performs point-to-point testing of cables and harnesses that contain components. With multiple instrument buses, it applies power, instruments, and loads to any test point. The system expands at the module level in 1,000-point increments or at the board level with two buses of 100 points per board. It supplies stimuli as high as 1,000 VDC or 750 VAC at 0 to 2 A. Four-wire continuity tests measure 0.01-W to 10-W resistances. Starts at $20,000. DIT-MCO, (800) 821-3487.
The 1024XL Circuit Analyzer has 128 bidirectional test points, expandable in increments to 1,024 points. It is PC-controlled under menu-based PASS® software. Error data and pin designations are displayed. Up to 32 programs can be stored on a small cartridge and called up as needed or transferred to another analyzer. Test voltages are individually programmable. Resistors, diodes, splices, bus bars, unused pins, and relays can be tested. Starts at $1,940. Dynalab, (614) 866-9999.
Automatic Test Set
The Automatic Test Set (ATS) performs routine tests on cables and harnesses under control of a PC with Windows 95 software. It has storage capability for test results and generates reports under program control. Up to five high-voltage switching assemblies can be installed, each with a 50-position or 100-position switch. The maximum test levels are 10 kV or 20 kV at 30 mA to 1.5 A. Contact company for price. Hipotronics, (800) 727-4476.
The CT-3200 Low-Voltage Cable/Harness Tester helps to build cables and serves as a high-volume production tester. It detects miswires, high-resistance continuity paths, intermittent defects, and opens or shorts. You also can perform sequential-step testing, auto-scan, and pin searches. Three versions provide 128-pin, 256-pin, or 512-pin capability. A port allows the tester to be set up and controlled remotely. A self-learning feature enables you to set up a test using a good cable as the model. Starts at $1,195. Slaughter, (800) 421-1921.
The Series 7 Tester has four buses to use with a variety of external IEEE 488 test equipment and power supplies. Normal continuity and high-voltage cable tests are run simultaneously. The tester also operates relays in the cable under test and makes related measurements with signal conditioners, spectrum analyzers, and other instruments. External test data can be read, verified, and printed under software control. Starts at $15,000. Weetech, (603) 465-3510.
Copyright 2000 Nelson Publishing Inc.