Troubleshooting Tips: Cabling Faults
| By: Kristin Lewotsky, MCA Contributing Editor
Putting as much engineering care into cabling design and installation as into the rest of your motion system will help ensure reliable long-term operation.
Cabling is the single most common point of failure in motion control systems. These faults can be quite difficult to diagnose, and yet all too often, cabling is the last thing that machine builders and OEMs think about when developing a design. The causes are myriad, ranging from faulty installation to poor configuration to improper choice of cable to begin with. The result is a host of unnecessary failures that could be avoided with proper techniques. We asked a panel of experts to talk about common mistakes and pitfalls, as well as solutions.
Kristin Lewotsky: What are the most common causes of cabling faults?
Improper cable selection for the application and/or environment. All cables are suitable for some application and environment—just not necessarily for the one needed. Next, improper installation methods, such as too much force. Poor termination is a very common source of faults. Lastly, flexing in motion control applications will significantly reduce the mean time between failures (MTBF). Cables installed statically should last years. Cables on robots often last months.
—Brian Shuman, Senior Product Development Engineer, Belden
Noise issues caused because the customer did not use properly shielded cable or because of improper grounding.
—Greg Hoell, Project Engineer, Minarik Applied Products & Systems
Vibration and loose wiring, as well corrosion and moisture from the environment. Also, not properly labeling the wiring and cables as far as what they’re connected to.
—Mario Mitchell, Product Manager for the electronics division, Parker Hannifin Automation Group
K.L.: How do you determine whether a fault in your system is caused by the cabling/wiring, especially when it is intermittent? What are your standard troubleshooting steps?
Inspect the cabling for any obvious signs of damage. Observe the system in use. Check any diagnostics from the controllers, the PLC computer, etc. Evaluate the cable disconnected from the system. Check continuity. Wiggle the connector at the interface and at the back shell! If all else fails, replace the cable, ideally from a different lot and validate that the problem goes away.
I use the GUI to determine if there is a cabling fault in a motion system. If there is something wrong with the feedback cable, the drive will give you a position feedback fault as you turn the shaft manually. If it’s related to the motor cable, the motor will run away if you give it a speed command. If it is I/O related, then turning ON/OFF the output or input and reading it in the controller (or PLC) helps troubleshoot the problem.
—Joe Figueroa, MAPS Project Engineer, Minarik Automation & Control
In a fast paced production environment downtime can be critical. If you think the fault may be in the cable, a spare is available, and routing a new cable is not difficult, swapping cables to get the machine back on line may be quicker. This can also depend on the availability of someone with the technical expertise to diagnose a fault using the GUI/HMI and being able to perform a repair with the cable installed.
—Tim McAdoo, System Application Engineer/Mechanical, Minarik Automation & Control
These problems often manifest as noise, and whether it’s from a broken cable or improper installation, they may all look the same. It can be an indication of poor practices—maybe you have high power lines running parallel to the digital lines, or cables coiled up to create antennas. Bonding the motor cable to the drive chassis can solve 80% of noise related problems. When problems seem to be intermittent, you really have to go back to basics and start breaking things down one by one. Whenever you have the opportunity to reduce to three elements in the system—say, motor, drive, and cable—if you have spares, sometimes you can replace one or more elements to try to isolate the problem. This often causes downtime and more work but it helps uncover the full problem. You can also “ring out” the cable by just putting a meter at either end and checking for shorts pin by pin, but that's a painstaking process.
—Jim Wiley, Application Engineering Manager, Parker Hannifine Automation
K.L.: How do you isolate the physical location of the fault on the cable, and how do you handle it? If the fault lies in the connector, for example, can you just replace it and keep the rest of the cable or do you have to swap out the entire cable no matter what?
If the cable has failed, we try to locate the fault area. This can help us determine the mode of the failure and how to prevent it in the future. If the cable caused the failure mode, we discard it and build a new cordset. If the connector turns out to be the cause, we most often replace the cable and the connector (new product).
—Lance Bredeson; Director, Connectivity; Turck
Check for obvious damage or problems such as the conductors are not terminated! Many cables can be spliced. However, unless the specific root cause of the fault can be absolutely identified, the entire cable should be replaced.
Visual inspection will show if there are any cuts or damaged cable/connectors, which could cause loss of/intermittent connections. If there is no damage to the cable, the connectors are the next culprits to check and typically where most problems lie. A contact could have been soldered or crimped incorrectly. Additionally when stripping the wire or cable, the installers may have accidentally cut extra wire strands or wire insulation.
I recommend replacing the entire cable and be done with it. I would rather focus on the system start-up (tuning, I/O checkout, etc.) rather than the cable.
K.L.: Over your career, what instance of a cabling fault did you find most difficult to isolate and correct? What could you have done differently to either isolate it more quickly or prevent the failure from happening?
The most difficult fault to address was an intermittent contact in a connector that was aggravated by vibrations from the machine and adjacent machines in operation. The contact actually showed continuity at 1 kHz. However, at DC (0 Hz) the contact was intermittent. The resolution took almost two months. In hindsight, I should have directly collected the sample and sent it to our lab for a full evaluation, including x-ray inspection. We ended up doing this after much finger pointing between the component vendors, assembly house, system integrators, electronics manufactures, and the end user.
A pin was crimped on backward and inserted into a connector. Because it was backward, the pin did not lock into place in the connector and would either have intermittent connection or no connection depending on the connector it was attached to. The difficulty here was finding the issue; with intermittent connections, if a visual inspection yields no findings then continuity testing can become arduous. Because the connection is intermittent you may get a good continuity reading on several passes before discovering the culprit.
Cabling faults related to the connector are the hardest ones to diagnose. Having a spare set of cables is worth the money.
A feedback connector on a motor had a pin that was not constrained. It looked okay, but when attaching the cable, the pin would be pushed back into the connector.
K.L.: What is the biggest mistake engineers make in specifying or installing cabling? Are there key design techniques they can use to minimize the possibility of failure?
Thinking they can save a few bucks by making the cable themselves instead of buying premade cable. Motor feedback cables can have as many as 15 or 20 leads within the cable. You have to get all those pins right, then you have to select the right gauge of wire, the right type of shielding, and if you make the wrong decisions, it can lead to problems.
Some may think it’s only cable, but there’s much more to it than transferring power and signal. In order for the cable/cordset to work properly for an extended time, the cable has to be specified correctly. We’ve had instances where the cable was pulled or stretched (small conductor size ~ 24AWG or smaller) and “ripped” the conductor. More often, we encounter [improperly applied] cable types. It’s surprising how many cable options are available in the market and how each market/application requires specific solutions (relating, as examples, to temperature, flexing, chemical exposure, washdown, agency requirements, etc.). Also, especially relating to flexing/motion applications, the guts of the cable are as, or more, important than the outer jacket. In motion/flex applications it’s all about minimizing friction (which causes failures). This is why it’s so important to specify the right cable type by application.
Under specification is a major cause of cabling faults. It is very common to spec in a standard product for an application that requires more to last. It is crucial to understand all of the challenges the cable will face in an application, including the type and frequency of movement, power requirements, temperatures, whether or not it will have an exposed installation, what types of weather, chemicals, or other liquids it may come in contact with, and if it will face abrasion or cutting hazards. All of these aspects can and should be accounted for in the cable’s construction.
—Chris Burke, Co-Owner and Key Account Representative, Allied Wire & Cable
Discounting the effects of the environment. Ambient temperature can significantly impact the electrical performance of the cabling. Exposure the mechanical or even chemical stressors on the cable can change the performance or even reduce the service life. I would recommend a thorough analysis of the environment and the application.
Ensuring the OD of the cable fits in the back shell of a connector, cord grip or conduit. Another common error is forgetting to order the appropriate tooling for specific connectors.
I think length (usually way too long or too short). Usually the location of the control panel is changed out in the field.
I have often seen cables with multiple connectors on one or both ends have the pig tails too short to allow proper routing and/or strain relief. Another common error is using cable carriers that are not sized large enough to allow passing of a connector beside other cables when routing/replacing cables, or using standard cables in high flex applications.
Failure to plan ahead. It comes down to how you route the cabling through the machine. If it wasn’t thought about ahead of time, you might wind up with tighter bend radii and improper routing. The best machine builders have engineers who specialize in cabling. They understand how to lay out the wiring in the machines, they do a good job of documenting it and putting together their work practices so it's easier for their installers to put together and they have a higher level of success.
Using the wrong type of cables—standard Ethernet cable that you use in an office setting as opposed industrial shielded Ethernet cable. It's very important to have industrial shielded Ethernet cable to reduce noise.
K.L.: What do you wish your customers knew about specifying/installing/troubleshooting cabling?
There are so many cable attributes you can adjust to meet the needs of an applications. For example, you can increase conductor strand counts to increase flexibility and flex life; change to more flexible materials; adjust the way the cable is laid up; add different armor or shielding to provide extra protection against physical damage or electromagnetic interference.
The different materials that are used in cables, even the different grades of a material. There are literally hundreds of grades of PVC for varying applications. Also certifications, standards and codes.
Customers should be aware on the influence of noise on cables, especially from high inductive loads. Customers need to know that running a feedback cable with motor leads can be very problematic.
On an axis where the motor may be moving, leave enough cable length from the strain relief to the motor connector to allow movement of the axis to each end of its travel limits and not put tension on the cable and/or bend sharply at the connectors.
K.L.: What do you wish you had known when you first started working with cabling?
How to quickly get to a solution for cabling faults instead of trying everything under the sun. Sometimes you don’t get to a root cause, but you fix the system. This is OK.
The importance and methodology of proper shielding and grounding. Although this is not necessarily specific to cabling, implementing the proper practices of shielding and grounding can save you years of trying to track down grounding/noise issues.
As a design engineer, I wish I had realized when designing and building my first CNC machine that I might one day be flying across the country to troubleshoot the prototype and replace some cables that I had made very difficult to route once the machine was installed.
K.L.: As system intelligence and bandwidth requirements rise, cabling becomes increasingly sophisticated, expensive, and heavy. Is a good trend or does it introduce new challenges? What do designers need to know about this trend to get the best possible results from their system?
Fieldbus is making systems easier to deploy. Rather than running many conduits and wires, all we need is a trunk. This is great. Also, the devices are getting smarter and offer diagnostic information to allow better troubleshooting. Customers need to understand proper installation techniques to prevent noise.
I have seen both the upside and the down side to the advancements in technology. The upside is the devices getting smarter. An example of the down side would be an end user wanting to expand their capacity, and the newer version of the same machine they purchased maybe five years ago now has a newer motion system/platform. This could mean that their maintenance technicians, programmers, support personnel must be updated as well.
I believe the increase in bandwidth and system intelligence is a great thing. Yes, there are consequences of progress, but I would contend these are far outweighed by the gains in productivity, efficiency, quality, and safety. I would recommend all motion control professionals develop a basic understanding of the existing and emerging technologies. Build relationships with technology leaders and solution providers. Sales people are great, especially at lunchtime, but solution partners are invaluable, especially when you need help the most.