(Source: Westend61 /stock.adobe.com)
Engineers are often advised not to leave some aspect of their design to the end of the design process. While that is certainly sensible advice, it unfortunately often clashes with project realities, as something on the project list must be done later or be last; not everything can be figured into the design early on.
Still, what is important is at least to keep those design aspects in mind at the early stages, even if the details cannot be locked down. That is the case with power and signal cabling for robot assemblies and systems. Major mechanical considerations, motor issues, load capacity, precision and repeatability, performance objectives, cost, assembly concerns, and other factors usually have a higher priority during the initial design stages. Perhaps a sign of this is that many of the “glamour” photos of robots don't show their cable runs, as they might ruin the sleek and futuristic look associated with robots.
Nonetheless, as an engineering mentor taught me many years ago: cable assemblies—wires plus their connectors—are not innocent, passive bystanders in a design. Whether carrying power or signals, they are actually less visible yet still potential sources of electrical and mechanical problems. As such, they must be treated with consideration and respect when designing.
This admonition applies to basic AC- or DC-power cabling all the way up the electromagnetic spectrum to RF and even optical links. It also applies to low-level sensor and control-signal cabling, which is often collocated with the power runs (Figure 1). Further, robot cables have another additional challenge: they must be flexible enough to twist and move, while also able to endure repeated flexing, often in harsh environments. At the same time, these cables must not get in the way, get tangled, or fail. Designing for all of these challenges is a tall order.
Figure 1: The complete cable run for a robotic system includes power wiring for the motors and low-level signals for motor management, sensors, and connectivity, with rugged connectors at both ends of each cable as well. (Source: Arbystudio/stock.adobe.com; generated with AI)
The degree of difficulty posed by cabling is also affected by the robot-mechanism type and size. This structure can range from a small, tabletop-size, limited-function unit used to load medical vials into a test chamber to an overhead gantry with only Cartesian X/Y-axis motion to a fully articulated six-axis unit—also designated as six degrees of freedom, or 6DoF—that offers the most flexibility and motion capabilities that mimic the human arm (Figure 2).
Figure 2: The most complicated standard robot is the six-axis unit, which replicates the motion of a human arm: the six axes, each with its own motor, along with the designations of these axes and the motion each creates.[1] (Source: natatravel/stock.adobe.com)
While implementing reliable cabling in a motor-based robotic assembly may seem daunting, robotics designers should be thankful they are not dealing with the hydraulic-powered units that once dominated robots but have been supplanted by electric motor drives in most cases. The former had many hose-routing issues, caused an environmental mess if there was a small leak or large hose rupture, and required both an electrician and hydraulics technician to install and maintain, along with an external compressor for pressurization. It's no wonder robots powered by electric motors have taken over.
With the hurdles of hydraulic-powered robots mostly in the past, new issues affecting robotic cabling and placement have arisen. Primarily, these issues deal with the copper wires, cable protection, and cable management.
Copper wiring requires the appropriate choice of wire gauge, stranding, and connectors for motor power and low-level signals. The power wiring is determined by the voltage and current of the motor, while informal guidelines and formal standards define control- and communication-signal wiring (current loop, Ethernet, or other). There are also electrical standards for insulation performance. Most systems also require wiring for signals, like those from a motor’s feedback sensors, vision systems, pressure sensors, or strain gages.
In most cases, the overall wire assembly will need braided shielding to prevent electromagnetic interference (EMI) emissions and minimize interference from other sources. In addition, some of the cables within the bundle may have their own shielding to adhere to industry standards.
The use of shielding presents a dilemma. On one side, the repeated abrasion of the shield rubbing against wire insulation can make the combination a possible source of failure over the longer term. However, the simplistic solution of eliminating this add-on shielding layer brings two new problems: first, the robotic system is now more susceptible to incoming EMI and radio frequency interference (RFI); second, it makes the robotics itself into a source of outgoing EMI/RFI, which can adversely impact the electronics and functions of nearby equipment.
There is a solution when the use of unshielded, high-flex cables is not acceptable due to bi-directional interference considerations. Special cables are available from vendors as standard items with shielding that is specially wound so that any twisting is primarily absorbed by sliding and buffer elements between the sheath, shield, and insulation.
Beyond selecting the appropriate copper conductors and insulation based on electrical requirements, many mechanical issues for cable assembly must be addressed:
There are many demands on robotic cable carrier systems, including the following:[2]
Engineers have multiple solutions available for guiding and protecting cables in six-axis robots. Let’s focus on three well-known solutions: flexible tubing, enclosed dress packs, and robot cable-carrier systems.
Corrugated or flexible tubing, usually made of polyurethane (PU) or polyamide (PA), exhibits superior tear resistance at connection points and enables a long service life (Figure 3). Its downside is that it resists cable motion and can be fastened only at the two cable endpoints; further, its elasticity puts additional stretch and twist stress on the enclosed cable.
Figure 3: Braided cable housing is a basic way to protect the cable bundle while allowing flexibility for various cable types. (Source: Chepko Danil/stock.adobe.com)
Enclosing cables in a “dress pack” allows them to be attached directly to the robot (Figure 4) with corrugated plastic tubing that houses the cables installed inside. Again, there’s a downside: cable maintenance is difficult due to the completely enclosed, non-modular housing. Consequently, for modular designs, the entire assembly must be replaced if just one component breaks.
Figure 4: A more advanced protection is the cable dress pack, which uses corrugated plastic tubing as another housing. (Source: THINK b/stock.adobe.com)
A better solution is one that allows individual pieces to be replaced if needed, and carrier systems based on modular ball-and-socket links support that approach. As an added and important benefit, this solution ensures that the cable minimum bend-radius limit is maintained. It is also easy to add or remove links if the carrier system is too short or too long or if the robot is being re-deployed for another application (Figure 5).
Figure 5: The carrier system is more complicated than previous schemes but adds more protection along with other features and advantages, such as individualized repairs. (Source: Aleksandr Matveev/stock.adobe.com)
How you route and retain the cable is also an important decision. After all, if Hollywood celebrities can have their own dressers, why shouldn't a human-like multi-axis robotics installation also have theirs? You wouldn't want a “wardrobe malfunction” in a high-performance robot operating in a stressful environment.
While it’s admirable that installers and technicians try to minimize the likelihood of tangling and interference by adding additional dress packs, cable ties, and duct tape, these well-intentioned efforts can actually be counterproductive. The reason is that they can cause “corkscrewing” that leads to operational failure.
A better approach is to “divide and conquer” cable management by viewing a six-axis robot in terms of its three primary sections, each with a specific location and number of axes: the sixth to third axis, the third to second axis, and the second to first axis[3] (Figure 6).
Cable management product vendors offer guidelines based on experience and best practices for managing, restraining, and implementing strain relief for cables in each of these segments. Additionally, there are industry-standard and proprietary products to implement these ideas for cable management.
Figure 6: A more thorough examination of the robot cabling for six-axis units shows that cable management should be considered separately for each section. (Source: Four888/stock.adobe.com; generated with AI)
Although cables and their installation may seem mundane and routine, they are not—especially for robot systems and even more so for multiple-axis systems. There are considerations about the wires, shielding, insulation, and installation. Vendors of robotic cables and cable management products offer specialty products tailored to the unique aspects of these installations, helping engineers avoid costly and unnecessary mistakes.
Sources
[1] https://www.robot-store.co.uk/six-axis-robots [2] https://www.roboticstomorrow.com/article/2014/02/the-less-is-more-approach-to-robotic-cable-management/236 [3] https://www.igus.com/robot-dress-pack/robot-cable-management-solutions
Bill Schweber is a contributing writer for Mouser Electronics and an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical website manager for multiple topic-specific sites for EE Times, as well as both the Executive Editor and Analog Editor at EDN.
At Analog Devices, Inc. (a leading vendor of analog and mixed-signal ICs), Bill was in marketing communications (public relations); as a result, he has been on both sides of the technical PR function, presenting company products, stories, and messages to the media and also as the recipient of these.
Prior to the MarCom role at Analog, Bill was associate editor of their respected technical journal, and also worked in their product marketing and applications engineering groups. Before those roles, Bill was at Instron Corp., doing hands-on analog- and power-circuit design and systems integration for materials-testing machine controls.
He has an MSEE (Univ. of Mass) and BSEE (Columbia Univ.), is a Registered Professional Engineer, and holds an Advanced Class amateur radio license. Bill has also planned, written, and presented online courses on a variety of engineering topics, including MOSFET basics, ADC selection, and driving LEDs.