Motion Control Resources
by Kristin Lewotsky, Contributing Editor
Motion Control & Motor Association Posted 08/27/2009
For robust, reliable motion systems, cabling must be properly designed, specified, and installed.
Despite ever-increasing functionality and sophistication, the most crucial aspect of a motion control system isn't components like controllers, drives, and even motors, it is the cabling. Integrators, systems houses, OEM machine builders, and component vendors all agree: Cabling is the single most common point of failure in motion control systems. They may not involve microprocessors or moving parts, but connectors are far more complex than simply a few conductors, some insulation, and a jacket. If a motion system is to perform as required and last, the wire harness requires informed design, specification, and installation.
The wiring of a motion control system performs two basic functions: power transmission and communications/control. Copper cable carries power to components ranging from motion controllers to motors. It can also pass communications and control signals, for example from HMIs to drives. Fiberoptic cable provides an alternative to copper for communications and feedback. It is lightweight, compact, precise, and immune to electrical noise. The downside is that it is more fragile than copper cable, adds the expense of optical-to-electrical converters, and requires specialized training for termination and installation.
Electrical cabling consists of copper conductors separated by insulation and jacketed to protect it (see figure 1). The insulating materials prevent arcing and allow a single cable to carry as many as 50 twisted pairs of conductors. Optical cabling consists of a silica glass core wrapped with a cladding that helps ensure the light propagates down the fiber. In addition, a fiberoptic cable will contain strength members and an outer protective jacket. These latter elements are particularly important because finely drawn silica lacks the tensile strength of copper wiring.
When cabling fails, the root cause is a compromised connection. Excessive bending or mechanical damage can fracture copper wire and optical fiber. Failures can be intermittent, as in the case of a cable subjected to repeated flexing. In the case of optical fiber, the silica can also suffer stress-induced micro cracks that cause gradual signal attenuation, eventually leading to failure.
The choice of strength members and jacketing materials can make or break - literally - a cable. Once these protective materials suffer damage from repetitive stress or isolated shock, the signal-carrying elements follow. “Typically your conductors are going to fail from bending and [other mechanical stress], and so much of that depends on what you use for the plastics that are molded around the core or extruded onto the wires,” says Lance Bredeson, the director of the connectivity division at Turck Inc. (Minneapolis, Minnesota).
Part of the inherent vulnerability of cable is the physical length of exposure. In fact, the rate of failure is directly related to the length of travel. "If the cable in a cable track is going to be running 10 m or longer, it's going to have a very hard time," says John Gavilanes, Director of Engineering, LAPP USA – (Florham Park, New Jersey). “Most of the time if you fall under that length, you're going to have a longer cable life.”
The choice of jacket material is critical to both performance and lifetime. In a high-flexible application, for example, the constant bending generates heat. The proper plastic can dissipate that heat while providing structural support that prevents fracturing (see figure 2). Applications very widely, however, so there is no one perfect solution.
By far the most common jacket material is polyvinyl chloride (PVC). It is economical and sufficiently durable to stand up to many high-flex applications, particularly when combined with other materials that add to its elasticity. Various grades of the material exist, from low cost and low performance versions up to high-performance grades that may even be rubberlike. In general, PVC becomes brittle when exposed to harsh environmental conditions such as sun, oil, and caustic chemicals, however. Although high-grade PVCs can provide some resistance, other materials provide better options for demanding applications.
One alternative is polyurethane. Unlike PVC, polyurethane does not dry out when exposed to hydraulic oils, cooling oils, and so on. It can survive some washdown environments. The material is also more tolerant of mechanical stress than PVC, although somewhat less flexible.
Although PVC can handle some chemical exposure, it is not the best solution for the type washdown environments experienced in food and beverage applications. In such a case, a more common choice is polypropylene.
As with much engineering, the choice of cable materials involves a trade-off between performance and cost. “A good oil-resistance PVC material is going to be $1.10 or $1.20 a pound but a good polyurethane material can fluctuate between $3.50 and maybe $5.00 a pound,” says Gavilanes. A factor of three or four cost increase is a hefty price to pay. Or is it? “If you have an application where you know if that cable breaks, you’ll have 20 people sitting around for eight hours, that's an easy justification.”
Of course, incorrect design or specification is not the only source of failure. The best specified table will not last for long if it is not properly installed. Ideally, cable should lie straight and untwisted (see figure 3). One common error is to install cable trays too short to accommodate the full cable run, so that it has to be forced into place. “So many of these failures are caused by how you dress the cables," says Bredeson (see figure 4). "How do you bend them when you're pressing them against the machine or against the side of the conveyor? What about the bend radius? A lot of times the cable is bent tighter than the tolerance we suggest and then they tie wrap them much too tightly, as well."
At the end of the track, the cable should be terminated with a clamp; otherwise, as the system operates, the cable will begin to walk. This will lead to fatigue, and, eventually, failure. Similarly, if a connector is located too close to a flex point, the contact pins in the connector will begin to telescope in and out and eventually break. In such a case, strain-relief elements are critical.
Bend radius is an important specification and varies from cable to cable. Bending the cable more than it is designed to tolerate is almost guaranteed to lead to trouble. The temptation to do so can arise during installation, however, for example, if an enclosure was not specified large enough to accommodate both the components and the bend radius of the cable connecting into them. It seems like an obvious mistake but it is all too common.
Thermal management is a perennial concern in engineering. Cold temperatures can make materials more brittle. This may not be a problem if they are static but by definition motion control systems involve movement. Consider a juice manufacturer whose packaging operations take place in a refrigerated unit that stays on over the weekend. “What happens is when you come in on Monday and start the machine up again, the PVC will crack on you,” says Gavilanes. “You have to be gentle on the cold temperature part of it.” A better choice for the cold is polyurethane, or polypropylene in food/beverage applications.
The trouble with cabling standards
If you're buying a motor for a washdown environment, you can choose an IP-64-rated motor and be reasonably confident you will get the performance you require. It seems as though something similar ought to be available for cabling, but the reality is not nearly so straightforward.
Standards that apply to cabling can be divided into safety and performance, with safety coming first. Safety standards vary by geographic location and must be followed stringently. Performance standards are rigorously defined for communications connectors such as the Category 5E and Category 6 cabling specified by the Telecommunications Industry Association (TIA). When it comes to connectors for power distribution and instrumentation cabling, however, the degree of standardization falls off quite rapidly.
The problem is that cable specifications are subject to systems-level requirements. Conditions vary widely for motion control systems, from the daily caustic washdown of a food application to the extreme high temperatures endured by systems in paper manufacturing. The TIA’s industrial cabling standards, for example, refer to a matrix of mechanical, ingress, climatic, and environmental considerations (MICE), focusing on factors like chemical exposure, temperature, moisture, etc.. “Unfortunately, that is based on a description of the environment and does not ensure conformance,” says Brian Shuman, senior product development engineer at Belden Inc. (Richmond, Indiana).
Sometimes the issue is not even one of environment or excessive mechanical strain, but of the motion itself. “In Ethernet cable, each pair is twisted differently to minimize crosstalk,” says Shuman. “If you were to take that cable and put it in a torsion motion application, the pairs could untwist. There would be no mechanical damage per se but you would have changed the physical geometry so that the signal couldn’t get through.”
There is no easy solution for the standards question. For the time being, the best approach is for engineers to communicate as many environmental and top-level performance details to their vendors as possible to ensure that they purchase a product that can address their needs.
Heat presents issues of its own, causing wavelength drift and noise in optical links and increasing resistance and signal attenuation in copper. In the case of power transmission, the solution involves assessing voltage drop to determine the required gauge increase. Communications and control-signal transmission can be much more sensitive to attenuation, however. In these cases, different materials may come into play for insulator, conductor, and jacket. Teflon, for example, adds thermal stability. Silver plating the conductor helps control oxidation and enhance signal. Again, all of these techniques carry associated costs, so it is important to determine whether your application requires them.
Another environmental concern is liquid. Water or other fluids can penetrate the cable and remain in the insulation, changing the impedance and shorting out the signal. The simplest solution is to take care to keep cable away from liquids, in particular not placing it in low elevations where water might settle into it. If a cable gets wet occasionally, it can dry out and operate fine. In the case of regular soakings such as in a washdown environment, however, a cable must be chosen that can protect against ingress.
The decision of whether to terminate a cable at installation or to buy factory terminated cables is an important one. Fiberoptic connections are highly vulnerable to contamination, so for all but the cleanest of environments, factory termination is key. When it comes to copper, the answer is less cut and dried. On-site termination allows the installer or OEM to choose the exact length of cable they need but it also requires a certain level of operator expertise. This may not be an issue during the initial installation but could pose a problem if a critical system fails in the middle of the night and the maintenance personnel are not able to cope. "It's good if your staff has the training and expertise for 2 a.m. field terminations to keep the line up and running but the more complicated the cable system, the more complicated the field installation,” says Brian Shuman, senior product development engineer at Belden Inc. (Richmond, Indiana). “As time passes, staying current is just one more skill set that would be needed from the maintenance personnel, whereas with terminated cords, it's plug and play.”
The use of factory terminated cables does impose some drawbacks as far as increased cost, limited choice of cable lengths, and need to stock a variety of spares. As for motors and drives, motion systems can be designed to limit the variety of cable lengths involved, reducing the amount of inventory required by both manufacturer and end-user.
No matter how wide the variety of stock cabling available, for some applications, only a custom solution will do. This approach should not be taken lightly, however. The cost of developing custom cables can run into the tens of thousands of dollars. The logistics of the process will likely impose minimum order sizes and longer lead times. The challenge is multiplied if the nature of a contract requires multiple vendors. These issues notwithstanding, if a wire harness is highly specialized and designed for a product line with a large production run, a custom solution may be best.
Cabling is a critical component of a motion system. Ultimately, the recipe for success comes down to careful assessment, planning, and communication. “You have to have your environment reasonably well-defined and convey that to your distributor or supplier," says Shuman. "If you say, ‘I need some 14 gauge with two conductors, and it's for an industrial installation,’ chances are you’ll get generic cable. But if you can tell me that this is going to be used in a pharmaceutical application where it's going to get a washdown with a mild bleach solution, then we’ll make sure the material is compatible with that environment. "
If you put in the engineering hours up front, plan for the future, and ensure proper installation, you should wind up with a cable harness that will be the strong rather than the weak element of your system.