Understanding cable stress

A tic-toc test does not accurately demonstrate the signal degradation problem, because the test is localized and only a small section of the cable is stressed.

Flexing a cable places stress on all components found inside the cable. Conductors and shields twist and move during flexing and shields break down. Although special insulation and conductor materials can improve resistance to flexing stress, the cable's electrical performance will change as conductors move and shields change shape or become damaged by stress.

A shield is necessary for any differential pair used in an EMI/RFI environment, such as automated work cells. EMI will induce voltage and eddy currents that attenuate the signal. For protection, most cables have an aluminized polyester foil shield. A braid or served wire (spiral) shield is often used in combination with the aluminized polyester shield for increased effectiveness.

Although aluminized polyester is a flexible shield, it won't survive repetitive motion because the aluminum cracks and sheds particles. Shield resistance increases and the openings in the shield become windows for EMI, which induce voltage on the conductors and eddy currents on the shield wires. The result? A loss of differential signal due to increased attenuation. As the shield fails, the cable's electrical length grows shorter, and eventually the camera signal is completely lost. It's important to note that the conductors are still working at this point and the resistance of the wire has not changed significantly.

Designers should specify cable that will truly stand up to the rigors of high-flex applications. Often, it's necessary to find an alternative to the standard shield construction of aluminized polyester foil. In the foil shield family, some materials and methods of construction will provide longer-thanstandard operational life, but still won't achieve the milestone of at least one million cycles typically demanded by linear motion applications.

Testing, testing, 1-2-3

A flex test is an excellent method to judge cables in an "apples to apples" comparison, as it simulates a longitudinal stroke or rolling flex motion. One cycle is two complete strokes, a motion common in automation that stresses cable through the entire flex region. Flexing a long cable length under test conditions provides a functional "view" of impedance and attenuation characteristics. It's important to base cable selection on rolling flex testing, as opposed to tic-toc (or pivot) testing, to get a true picture of how the cable will perform in real-world conditions.