Create Date: January 21st, 2020

Introduction

ActiNav autonomously does all of the robot motion planning for transporting parts from a bin to their destination. Since parts may be anywhere in the bin, in any orientation, each path is different. Typical cable and hose management schemes, which rely on a predictable, contained path, are often insufficient for the wide variety of possible ActiNav movements. Standard cable management schemes are also too bulky and do not have enough flexibility. Cables may become tangled, or may catch on elements in the environment.
This article describes one way of managing electrical cables for bin picking using coiled cables.
See the note at the end about combined pneumatic/electrical systems.

Recommended Parts List

Prerequisites

• Coiled cable. The number of conductors depends on your end effector.
  • For bridging the last 3 wrist joints and the tightest connection to the robot, we recommend a coiled cable with an outer spiral diameter of 25 mm / 1 inch or less. Tensility (https://tensility.com) makes coiled cables containing three to eleven stranded wires from 18 to 28 AWG that are well suited (part numbers 31-00xxx).
  • It is also possible, but not preferred, to use a coiled USB cable. Most cables are too short:
    be sure you use a minimum 6 ft / 1850 mm extended length. USB cables are not as robust, stretch more, and will wear faster.
• Semi-flexible polyethylene tubing of a smaller inner diameter than the outer coil diameter of
  the cable. For a 25 mm / 1 inch coil, use 12.5 mm / ½” inner diameter tube. For a 15 mm / 0.6“
  USB coiled cable, use a 9.5 mm / ⅜” inner diameter tube. Exact dimensions are not important:
  what matters is that the cable needs to be wound tighter than its relaxed state to be forced in to
  the tube.
• Deformable PTFE tape. 0.0045” (0.11 mm) thick is easiest to handle, and seams don’t cause significant catch points. Use narrow tape for the rubber rings and wide tape for friction reduction, or use wide tape for both. https://www.mcmaster.com/76025A715
• Hose clamps: We recommend stainless steel clamps that are made for soft hose. These are smooth and have upturned edges, and are less likely to mar the robot finish:

UR5: Lower joints: https://www.mcmaster.com/5076K53 Lower tube https://www.mcmaster.com/5076K53

UR10: Lower joints: https://www.mcmaster.com/5076K54 Lower tube: https://www.mcmaster.com/5076K53

• (Optional) PVC tape to protect the robot
• Tools: A screwdriver for the hose clamps, fine toothed hacksaw or tube cutter, pointed (X-Acto)
  knife, scissors.

Determine your configuration

For ActiNav bin picking, the three wrist joints tend to have the greatest range of movements, and thus need the most flexibility in cable management. It is generally sufficient to use the coiled cable just to bridge these last three joints. You can transition back to regular cable with service loops for the base, shoulder, and elbow joints.

Prepare the cable

The coiled cable needs to be held close to the robot in a way that keeps it in place, but doesn’t create wear points or pinch points that will cause the cable to fail. The best way to do this is to attach it with the semi-flexible polyethylene tubing. Never clamp a cable directly to the robot: this will fail in just a few days.

1. Cut a piece of polyethylene tubing about 25 mm / 1 inch long. Use a fine-toothed saw or a tube cutter: if you can cut it with a knife, the tubing is too flexible.
2. Sand the edges of the tubing to remove cut marks.
3. Round off the inside corners of the tubing with the pointed knife.
4. Push the cable through the pieces of tubing. Generally, you can just pull it through. Depending on your end effector, there should be about 16 loops between one tube and the end; and about 14 loops between the two tubes for the UR5, and about 19 for the UR10.
5. Twist the coiled cable more tightly to pack it inside of the polyethylene tube. Ensure the right number of coils next to the tubes are still present as specified in step 4.

Prepare the robot

Tape the joints: The cable may rub against the rubber flat-rings that seal the robot joints, damaging the rubber. Also, the cable will abrade against the rough joint castings. At minimum, protect the flat-rings with a layer of PTFE tape. Note in the picture which side of the joint to place the tape on, so that the joint isn’t bridged by the tape. (A pneumatic end effector is shown, but the same principle applies). Do not use PVC tape. PVC deforms when the joint heats up and may “creep”.

Determine where to route the spiral tubing: Determine what the most likely robot pose is for picking and placing, and route the cable so it is least likely to interfere with the bin, placement target, or environment. This is generally best achieved by routing the tube over the top of the robot when it is positioned over the bin (see left top picture, below). You may want to run your ActiNav bin picking program first, to observe typical movements.

For difficult picks, especially when free spin around the tool Z axis is enabled, the robot may assume a wide variety of poses. Be sure that, whatever routing you choose, there is enough room for any wrist joint to rotate at least ±180°.

Use of coiled cabling is no guarantee that the cable will not interfere with the bin, environment, or placement. The amount of slack required for full joint rotations may protrude or sag.

Clamp down the tubes: Using the right size hose clamp, clamp one tube to wrist joint 2, near the connection with wrist joint 1. Clamp the other to the lower robot tube, near wrist joint 1. You may optionally use PVC tape under the clamps to protect the robot from scratches. When clamping, slightly crush (deform) the tube, to lock the coiled cable in place.  

Hose clamps and worm drives may have sharp edges. Account for this in your risk assessment! Also, locate the worm drive screws so they are unlikely to contact the cable. Occasional contact is generally OK.