Six axes of robotic motion usually provide sufficient freedom for most applications. However, mounting a robot arm on a movable base—known as a 7th axis—adds flexibility, enhances collaboration and provides collision protection. 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.

Traveling productivity improvement

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. (Figure 1) 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. A 7th axis is, however, a high-precision system, and maximizing productivity from it requires close consideration of where and how you mount it, and application-specific factors such as the availability of collision detection systems and the number of linear guides used.

Figure 1: The flexibility of a CTU, which can expand horizontal operating range up to 10 meters, significantly boosts productivity and output. (Image courtesy of Thomson Industries, Inc.)

Mounting orientation

CTUs are traditionally mounted horizontally on a table, wall or ceiling. (Figure 2) Table mounting is the most common orientation and likely the least expensive option. The table is usually made of steel or extruded aluminum that is secured to the floor, providing a solid base that is relatively easy to relocate, and accommodate power and control cabling.

Figure 2. Mounting the CTU on a table is the most common configuration, offering a cost-effective, solid base that simplifies installation and supports easy access to power and control cabling. Wall or ceiling mounting frees floor space. (Image courtesy of Thomson Industries, Inc.)

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 five meters because the drive belt may sag beyond that stroke length. Table- or ceiling-mounted systems, in contrast, can usually accommodate strokes of up to 10 meters.

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. (Figure 3)

Figure 3: Figure 3. Many 7th axis manufacturers often offer tray solutions to simplify cable management. (Image courtesy of Thomson Industries, Inc.)

Material considerations

Although CTUs intended for light-duty operation might be mounted on extruded aluminum 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. From 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.

Alignment issues

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 (i.e. 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.

Seventh axis vendor manuals usually provide guidance on the degree of misalignment a unit can tolerate. Some vendors, such as Thomson, offer laser-guided alignment systems that mount onto the base plate. The laser alignment system measures the flatness optically and transmit the measurements onto a digital application, which calculates and returns the exact number of provided shims to add to the corners of each baseplate to comply with recommended alignment tolerances.

Doubling down

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. (Figure 4)

Figure 4. Dual-rail linear guides, as shown on the Thomson Movotrak? CTU, help provide the moment load and stiffness needed for heavier loads. (Image courtesy of Thomson Industries, Inc.)

Optimizing return on robotics

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, minimizing nuisance trips and related downtime.

Companies without experienced, in-house cobot integrators would be wise to consult with professional integrators to ensure optimal performance. A professionally installed CTU will maximize the return on investment in cobots through higher productivity and optimized collaboration amongst humans and machines.