Flex cable adds muscle to signal integrity
When designers are busy building machine vision systems, cable is often an afterthought. It shouldn't be this way. If your signals can't get from here to there in one piece, your application is running blind. To prevent that, think about cabling early in the design process, especially the metallic shields that stand guard against noise-induced signal error.
For the rolling flex test, a signal generator and vector network analyzer are connected to the cables under test using precision, controlled impedance cables isolated from flex testing motions. Cable conductors and shield resistance are also monitored.
The flex test used in this analysis is run on a high-speed linear motion test bench with a 0.5 m stroke length. The high-speed flex tester can run 90,000 cycles/day. Each cycle is two complete strokes. Acceleration is 4 g and the bend radius is 50 mm. Cables are placed in a cable carrier to control motion and bend radius. This test setup forms a loop with the cables that includes two bend sections. With this configuration, it's possible to stress approximately 540 mm of cable, simulating a 1 m stroke length.
In this test, FireWire cables from several manufacturers and up to four samples of each were used. After preliminary testing, only a few of the cables showed potential to last longer than 1,000 cycles of rolling flex. These cables employed a more robust shield construction, which enabled them to flex more than the other samples. One of the surviving cables used a combination of aluminized polyester foil and served wire shield on the twisted pairs, while another used a combination of copper foil and served wire shield.
Real-world results
To provide relevant results, any cable test should evaluate electrical performance characteristics under real-world flex conditions. For vision-based motion, the following should be analyzed: impedance, attenuation, shield resistance, and jacket durability. The graphs show how some FireWire cables do in a battery of flex tests. Each cable has two bend sections that are in the rolling flex area; impedance increases significantly in this area as cable shields and dielectric materials break down. Cables fail when the impedance changes because the signal reflections increase and the digital signals are attenuated. The camera can't communicate with the FireWire card when the signal is attenuated.
For more information,visit www.gore.com/highflex
Going the distance
Cross section of enhanced cable. Enhanced FireWire cable |
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© 2008 Penton Media Inc.
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