The less-is-more approach to six-axis robot design

Edited by Kenneth Korane

The phrase “less is more” might not immediately spark thoughts of automated machinery and six-axis robots, but these three little words are the basis for a best practice robotic engineers and integrators can and should apply.

The less-is-more approach centers on designing cable-management systems for six-axis robots — including cables, hose, tubing, carrier, and connectors — in three separate segments.

This differs from the current industry practice of using long, single-piece cables and hoses rigidly attached to the end-of-arm tooling. Smaller cable sections provide the same power, flexibility, and safety, but do a much better job of avoiding cable damage.

Cable management is in the limelight because machine reliability has increased dramatically in recent years even though robots have grown more complex. But the methods used to attach and guide cables have not followed suit. Since the 1960s, cable management on robots has not changed significantly. In fact, it is often altogether overlooked — perhaps because managing cables and hoses seems simple. In reality, it is a vital feature of any well-functioning robot.

Most experts agree one of the top blunders designers make is underestimating cable-management issues. For instance, during a recent conference hosted by the Robotic Industries Assn., a group of leading system integrators cited cable issues as the number one reason for downtime in robotics cells. Headaches range from tangled and corkscrewed cables to complete breaks that cause downtime, lost revenue, and damaged reputations.

New thinking

Current systems try to keep the cables static, while everything operating around them is dynamic. In essence, using one, long restrictive-cable package prevents movement in sync with the robot. Restrictions stress cables, and that accelerates failure. Often, technicians severely bind cables with excessive dress packs (protective coverings on cables and hose), cable ties, and even duct tape. The goal might be to minimize tangling and interference with the machine, but it can actually cause corkscrewing and failure.

Instead, engineers need to consider a six-axis robot as three separate segments: the sixth to third axis; the third to second axis; and the second to first axis. This breakdown is imperative to longer-lasting cables. Each cable segment should feature a minimal dress pack, strain relief with service loops, and a junction box that contains and protects the electrical connectors joining the cables. Follow these recommendations for best results.

From the sixth to third axis:

1. Use strain-relief cables (see “Long-life cables”) on the moving end (sixth axis) with a 1 to 2-ft service loop.
2. Protect cables and hoses with a modular, multiaxis cable carrier.
3. Segment cables at the third axis and install a junction box for quick diagnostics and cable replacement.

From the third to second axis:

1. Use strain-relief cables on the third axis with a 1 to 2-ft service loop.
2. Use a modular, multiaxis cable carrier.
3. Segment cables and install a junction box at the second axis.

Finally, from second to first axis:

1. Strain-relief cables on the second axis with a 1 to 2-ft service loop.
2. Install a multiaxis, reverse-bend cable carrier to protect and guide cables and hoses rotating around the robot.
3. Segment cables and install a junction box at the first axis.

Segmenting the dress pack into three shorter sections prevents it from wrapping, catching, or snagging on machines, and minimizes stress on cables and hoses. This approach applies to any six-axis robot, regardless of manufacturer or application. While other fixes such as duct tape and ties wraps might cost less and work temporarily, in the long run properly designed dress packs reduce unnecessary downtime and maintenance costs.

Additional tips

Another step that should extend cable life is to allow sufficient clearance inside the carrier for electrical cables, pneumatic hoses, and tubing for other media. This compensates for relative forces between cables and hoses. Carrier suppliers typically provide this data. For instance, general rules of thumb for an igus Triflex R carrier are:

1. Total cable and hose diameters must not exceed 60% of the carrier diameter.
2. Leave at least 10% clearance between any two cables and hoses.
3. Cables and hoses need to move freely inside the carrier.

Safety is also a major concern within robotics cells. With the less-is-more approach, designers can let cables and hoses move freely, but not to the point where they could potentially injure workers.

As six-axis robots evolve, cable-management systems need to develop along with them. Designers should consider the less-is-more approach for every robotic application, as it eliminates cable damage, expensive maintenance, and downtime. Of course, a number of other elements, including the robot’s function, space constraints, and budget also play a role. But for any combination, there is a suitable less-is-more approach that keeps vital cables away from harm’s way while letting them mimic the fluid movements of a six-axis robot. md

Make Contact

igus Inc., igus.com