Lee Stephens
Systems Engineer
Danaher Motion
Wood Dale, Ill.

Engineers designing servo and stepper systems tend to focus on the obvious: the motor, controller, amplifier, and sensors. Signal and power cables, though no less important than the components they connect, are often overlooked until the project's end, or worse, handed off to an electrician without proper training. The result: lower-than-expected accuracy, frequent failures, low immunity from electromagnetic interference (EMI), and adverse effects on neighboring equipment.

Cable construction
A simple, fourconductor shielded cable may include four insulated, color-coded conductors, a foil shield with drain wire, and a PVC jacket. White is neutral, red and black are phase, and green represents ground.

Despite their apparent simplicity, cables are highly engineered components. They are designed and manufactured to let other components operate at peak levels. Cable features of note include insulation breakdown voltage, shielding (for individual conductors or pairs), jacketing (to protect the cable from mechanical, chemical, and environmental influences), and ampacity (the maximum current a conductor can carry before exceeding its temperature limit). Other considerations include a drain or grounding wire (used with foil shields), binding tapes, embedded steelsupport wires, and fillers (to give the cable a uniform circular, cross-section).

Selection criterion

Designers must consider several things when choosing cable for servo and stepper systems. Operating conditions — temperature, moisture, chemical exposure, abrasion, flexing, and expected life — are key. Other criteria, such as proper insulation type and thickness, depend on working voltages. The number of conductors and current ratings, in contrast, are often specified by the motor and drive manufacturer.

Some applications need separate feedback and power conductors. Others can make do with a single composite power and feedback cable. This is a decision based, in part, on how much interference (inductance) is acceptable between conductors and between the cable and its surroundings.

Servosystem
A typical servosystem includes a power supply (not shown), a servomotor with a builtin position sensor, and controller with a built-in amplifier. The controller receives the feedback signal through a feedback cable, and the motor receives power through the power cable.

If the area between conductors is too great, alternate paths for the signal will be found. Twisting the cable pairs (up to four turns per inch) decreases the likelihood of this sort of coupling. Additional measures to prevent coupling include the use of shielding.

Some applications require stationary cables, such as systems where the motor and drive are fixed relative to each other. In these situations, cable trays and conduits are frequently used to route the wiring. Other applications, like those involving a motor constantly moving in relation to the overall system (such as a robot arm), might require significant cable flexibility.

Inside and out

Two types of material are used in the conductor insulation and jacket, and each plays a different role in the cable's structure. One type electrically isolates individual conductors, or cable pairs. By comparison, jacket material provides the cable "skin" and protects the conductors, insulation, and shield from the environment, mechanical impact, and chemically aggressive substances. While some products, such as conventional hook-up wire and consumer-product power cords, have only a single layer of insulation that acts as a physical protective jacket, most industrial grade cables contain both. Jacket material is the major source of friction within moving tracks, and selection is tantamount to system success or failure. The environment, including ambient temperature and heat created by conductor current, determines the insulation material's maximum operating temperature. In general, temperature ratings can be interpreted as the maximum conductor temperature safely sustained by the insulation. However, if any cable (even a feedback cable) is routed near a heat-generating machine, ambient temperatures will influence the operating temperature.