Reducing Test Time for Fiber-Optic Voltage Controllers

A voltage controller in a fiber-optic switch provides an enormous testing challenge because it has 2,520 discrete channels that must be tested separately. Until recently, it took an operator about 30 seconds to test each channel manually with a voltmeter.

To reduce costs, a custom production testing system has been developed to slash the time needed to test the special voltage controller from three days to two hours. The new system is based on a standard rack-mounted computer with five data acquisition cards, each connected to eight 64-channel multiplexers (MUXs). It simultaneously tests all channels in a total cycle time of about two minutes.

The Fiber-Optic Switch

The complex opto-electronic conversion process required to manage traffic on provider networks creates bottlenecks in the current telecommunications network. This has prompted a major trend toward ultra-dense, high-performance, all-optical switches that offer greater flexibility, higher density, and higher switching capacity than electrical switch cores.

These all-optical switches provide for network growth while offering significant cost savings. Yet, major challenges must be overcome to make these switches practical: the development of highly dense mirror arrays that instantly change the path of light channels, for instance.

Micro electromechanical system (MEMS) methods are being used to fabricate microscopic moving structures that can switch beams of light. The MEMS fabrication technique results in high-aspect ratio structures for systems of capacitive sensors, electrostatic actuators, switch contacts, holes, and channels.

Voltage Measurement Problem

A critical issue that must be addressed by manufacturers of these devices is the application of precise voltages to each of the mirrors. One leading-edge product has 630 mirrors, each of which can be turned in two axes. In operation, each mirror requires four discrete voltage sources to turn it in the positive and negative direction in each axis. Consequently, a voltage controller with 2,520 channels is needed to control the mirror array.

To ensure reliable performance, the equipment manufacturer must test each of these channels before assembling the cross-connect switch. Previously, this involved a tedious manual process in which an operator connected a voltmeter to each of the channels and performed a series of tests. While it took less than a minute to test each channel, the large number of channels meant that three full days were required to complete the testing. This lengthy process prevented ramping up production quickly to meet increasing product demand.

The equipment manufacturer approached engineers at Network Test Solutions to build a system that would test all channels simultaneously. Network Test Solutions, a Silicon Valley-based test-engineering company, specializes in turnkey solutions from developing test plans and strategies to design, fabrication, and installation of automated production test systems for companies manufacturing networking and telecommunications products.

The design team considered multiplexing 40 single-channel data acquisition cards out to 64 channels each, for a total of 2,560 channels. But a data acquisition card only has one measurement input so it would have to switch the MUX one channel at a time, let it settle, make the measurement, and store the results.

The process would have taken 30 seconds per channel or a total of about 21 hours to scan all of the channels. That’s nearly as long as it took to do the job by hand. Also, purchasing 40 data acquisition cards would have amounted to $85,000 in hardware including the MUXs.

Designing a Solution

The design team opted to develop a rack-mounted computer with a PCI bus that can handle less expensive, off-the-shelf data acquisition cards. But conventional data acquisition cards do not handle the amount of throughput needed to meet the cycle time.

The company found that Microstar Laboratories offered a data acquisition processor (DAP) card for high-speed simultaneous processing: the DAP 4400a acquires data at a 3.2 MS/s rate. A configuration was developed using five DAP cards mounted on the PCI bus with each connected to eight 64-channel MUX cards.

With its high data rate, each data acquisition card can scan the 512 channels in about two seconds. It takes another 15 seconds to download the data to the host PC. The elapsed time for the entire operation is about 2 minutes.

The operator still needs to connect the cables and perform other tasks, but the resulting two-hour cycle time is a dramatic reduction from manual or other automated methods. In addition, the total data acquisition hardware cost, including DAP 4400a/446 Data Acquisition Processor Boards, multiplexers, and cables, is about $46,000.

An onboard microprocessor on the DAP 4400a runs on DAPL, a multitasking, real-time operating system that provides more than 100 commands optimized for data acquisition and machine control.

It took the design team only a few hours to write and test the DAPL commands required to measure each channel 10 times and send the results to the host PC. DAPL communicates directly with the test-executive operator interface running on the PC under Windows 98. 
Network Test Solutions wrote the operator interface using a National Instruments’ LabWindows test-development system. The interface leads the operator through the entire testing process. First, the operator sets the DUT on a shelf of the rack that contains a bar-code scanner and connects the multiplexer cables to the unit. After this, the operator hits a start button, and the test executive automatically scans the serial number of the unit and selects the right tests for that model.

The first iteration of the tester measures all channels at full voltages. A future upgrade will handle four different voltage levels: 40, 80, 120, and 160 V.

The key to the success of this application is the capability of the 4400a card to acquire samples at a high rate while operating totally independently of the central processor.

About the Author

George Atherton, vice president of marketing at Microstar Laboratories, has worked for the company since 1991. Before then, he had spent most of his career in the IT industry in support, sales, and marketing roles. Mr. Atherton has a master’s degree in economics from Cambridge University. Microstar Laboratories, 2265 116th Ave. N.E., Bellevue, WA 98004, 425-453-2345, e-mail: [email protected]


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Published by EE-Evaluation Engineering
All contents © 2003 Nelson Publishing Inc.
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October 2003


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