Critical considerations for improving robot productivity

A 7th axis, also referred to as cobot transfer unit (CTU) or range extender, consists of actuator-driven guide rails that carry the robot arm assembly from one task location to another. CTUs are becoming increasingly popular for operations requiring multiple tasks, such as milling, lathes, testing, inspection and welding. Adding a 7th axis of motion enables one robot to perform various operations across multiple pieces of equipment or workstations.

A robot equipped with a 7th axis, for example, could load one machine and then, while that machining cycle runs, move on to load another machine. This capability reduces the need to deploy multiple robots to accomplish different tasks.

CTUs are traditionally mounted horizontally on a table, wall or ceiling. Table mounting is the most common orientation and likely the least expensive option. The table is usually made of steel or extruded aluminium that is secured to the floor, providing a solid base that is relatively easy to relocate and accommodate power and control cabling.

The downside of a table-mounted solution is that the assembly can obstruct operator access when human intervention is required for setup, monitoring or other purposes. Mounting the CTU on the ceiling or wall solves that problem.

If wall space is available, mounting the CTU provides better accessibility to the application than floor mounting. The wall must have adequate support or the ability to be reinforced. Wall mounting does, however, limit strokes to 5m because the drive belt may sag beyond that stroke length. Table- or ceiling-mounted systems, in contrast, can usually accommodate strokes of up to 10m.

Mounting the CTU from the ceiling can provide operators with even greater access to the task area provided that steel stanchions, rigging or other structural supports are installed. You do, however, still have to figure out what to do with the cables, which could otherwise hang into the task area. Some manufacturers use cable management trays, which keep the cabling out of the way.

Material considerations

Although CTUs intended for light-duty operation might be mounted on extruded aluminium tables, steel tables provide the stiffest support with the least vibration. And, if the CTUs have collision detection capability on the 7th axis, like those from Thomson Industries, the controls will stop or slow the robot's movement upon contact with a human.

The stiffness and lower vibration of steel tables also afford greater sensitivity control, which helps to avoid nuisance trips. rom a control tablet, users can adjust sensitivity settings to designate the level of human contact the cobot joints can endure before the system shuts down, thus avoiding nuisance trips based on incidental contact.

Regardless of mounting orientation, the flatness of the surface is critical to system performance. Baseplates typically sit between the CTU and the mounting surface unless the contact area has minimal surface variability (ie less than 0.1mm/m). The system could interpret any misalignment as an unexpected load and halt operations immediately. Such misalignment can also contribute to excessive wear, which could negatively impact long-term life.

While single-rail linear guides may be acceptable for the smallest cobot sizes (3 and 5 kg), for robots with payloads of 7 kg or above, a dual-guide system provides the proper moment load and stiffness that is typically required from applications like welding, machine tooling and material handling.

By adding an additional axis to a traditional six-axis robot, cobots can support multiple process points with high efficiency. Whether installed on a table, wall or ceiling, the integrity of the installation affects the performance, durability and collision avoidance of the system. With less concern about collisions, either nuisance or real, manufacturers can deploy cobots to work more closely and flexibly with humans, minimising nuisance trips and related downtime.