A range of AR display technologies have been used in studying the safety and efficiency of robot arm operations. For example, Hietanen et al. [4] used both projection-based AR and Microsoft HoloLens 1 in the context of a human–robot collaboration task involving a diesel engine assembly, utilising depth sensor information for work space monitoring in both cases. The results showed that AR interaction led to a reduction in task completion time by up to 21–24% and reduced robot idle time by up to 57–64%. Moreover, the user survey indicated that the projector–mirror setup was deemed more comfortable and easier to use than the HoloLens 1, which users found to be bulky and uncomfortable to wear for extended periods. Additionally, the limited field of view of the HoloLens 1 was identified as an issue. Lotsaris et al. [2] also used a HoloLens 1 in a system that enabled the display of robot sensor information and safety zones to a user. Feedback from the study revealed that, similar to previous research, the HoloLens 1 was considered uncomfortable to wear for prolonged periods, thereby rendering it unfit for extensive use in a work environment.

Hoang et al. [5] presented a virtual barrier system equipped with an AR interface to ensure safe human–robot collaboration (HRC). The system offers two distinct virtual barriers, namely a person barrier that surrounds and tracks the user and a user-defined virtual barrier for objects or areas. The study evaluated the performance of a pick-and-place task that simulated an industrial manufacturing procedure using the Microsoft HoloLens 2 compared to a standard 2D display interface. They concluded that the proposed configurable virtual barrier system, coupled with AR, enhances HRC safety compared to a conventional 2D display interface. Cogurcu and Maddock [7] considered user-configurable virtual geometric objects, such as cubes and cylinders, that enveloped a real Universal Robots 10 (UR10) robot arm. These objects were either a single shape that adapted dynamically to the robot arm’s movements or individual shapes that encapsulated the robot arm’s components. The virtual safety zone sizes or the objects are calculated based on a combination of ISO standards [11], hardware, and network latencies, which were not taken into account in prior studies.

Other work has considered the use of virtual and real environments for testing and optimising HRC. For example, [12] explored the use of virtual environments for testing safety systems in HRC, and [13] investigated whether VR technology used in manufacturing could provide faster design options. While these studies have provided valuable insights into the potential benefits of virtual environments, the overall picture is less clear. Some research suggests that virtual environments are not advanced enough to accurately simulate real-world conditions, and real-world testing is necessary for a comprehensive assessment of systems involving robots [9], while other research argues that virtual environments can provide a suitable approximation of real-world conditions, with some limitations [14]. For example, according to [15], the use of VR technology has been shown to provide users with a similar experience to physical interaction with a robot arm. In their study, the authors found that users were able to perform tasks with a virtual robot arm just as accurately and efficiently as they could with a physical robot arm.

A further aspect of the virtual robot arm experiment that could impact on transferability is the sense of touch. In the virtual robot arm experiment, virtual wooden blocks were manipulated by the user. Comments from some users suggested that they had difficulty grabbing the virtual blocks. This was not the case for the real wooden blocks. This could suggest a potential issue for the transferability of precision and efficiency skills in training situations. Overall, transferability remains an open research question [9,14,15].