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

May 8, 2008
Simple cable systems get the most out of robots.

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.

Long-life cables

In addition to the appropriate dress pack, it is imperative that six-axis robots use dynamic cables specifically designed for continuous flexing. Two important features to take into account are a cable’s torsion-resistance and shielding. Shielded cables face a greater risk of failure, as constant movements can easily compromise the cable jacket. Use unshielded, high-flex cables whenever possible to avoid problems. If this is not plausible, turn to special “rolling-flex” cables.

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

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Six-axis cable management

Properly specifying cable-management systems is an important part of every robotic design. Here are four installation approaches specifically for six-axis robots.

Standard modular systems are for robots that have normal work envelopes and sufficient space for cable movement within the work cell. It is specifically for basic, light-duty movements around the robot arm. The system attaches with intermediate, integrated strain-relief end brackets, so it requires no accessories or additional supports. Benefits include simple installation, ready access to cables, and it can be adjusted on the plant floor. Applications include palletizing machines, material handling, and pick-and-place robots.

Interior systems have structural fiber rods inside the cable carrier running parallel to the fourth axis that support cables and hoses to the sixth axis. This eliminates a cable loop or sagging cables. The system holds cables above the robotic workspace and away from the machine. It mounts on a robot or as a stand-alone unit. Fiber rods increase cable life and permit versatile positioning of cables and hoses. They are typically used in clean-room, spot-welding, and grinding applications.

Exterior systems or flex bars handle extreme robotic applications with a wide range of motion and reach. Made of high-tech composites, flex bars let cables and hoses move fluidly along with the robot while, at the same time, avoiding tangling and damage. High pullback force prevents loops from forming at the robot’s wrist. Flex bars handle a wide range of applications and require few cable-dress accessories to prevent undue stress and restriction. They feature low wear, are maintenance-free, and can be shortened by adjusting the length of mounting clamps.

Multiaxis carriers are compact and can be used universally. For instance, the Triflex RS System ships and installs as one piece and has an integrated fiber rod that returns cables to a home position after the robot completes a cycle. The system includes mounting plates and brackets, protection links, and a preengineered fiber-rod assembly. This results in a compact design with only one piece to install, and it provides end-of-arm tooling guidance. Multiaxis carriers are well suited for welding robots in tight areas, multiple-tool applications, material-handling jobs, and deburring operations. For added flexibility, versions of multiaxis carriers include fully enclosed designs; pull-through designs for easy cable access; and light, low-cost alternatives for applications that do not require completely enclosed cables.

Multiaxis cable carriers such as the Triflex RS include integrated supports and brackets, and typically ship and install as one piece.

Interior systems have fiber rods inside the carrier that support cables and hoses.

Smaller cable and hose segments help optimize motion and lessen the risk of failure.

Exterior systems provide support outside the cable carrier, permitting fluid movements without tangling or damage.

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