## 3 Methodology
### Study Selection, Quality Assessment, and Data Extraction  
We limited the search to works published between 2015 and 2022 in order to focus on the most recent developments in this field. The search was performed in October 2022. A pilot search in the ACM and Elsevier databases indicated that after 2015 the interest of the robotics community in the use of game engines with robotic systems began to increase, surpassing a threshold of three papers per year. Keywords composing the search strings were defined using the PICO (Population, Intervention, Comparison, and Outcomes) method suggested in [31]. Several iterations and pilot searches were performed in order to determine an optimum set of primary studies. The strings and advanced settings defined for each database are shown in Table 2. \begin{table}
\begin{tabular}{l|l} \hline
**Database** & **Search String** \\ \hline IEEE Xplore & (Unity OR “Unreal engine”) AND Robot AND Human AND (Interaction OR Collaboration) \\  & AND(“virtual reality” OR “Augmented reality” OR “Most reality”) \\ \hline ACM Digital Library & [III: All; unity OR [All; “unreal engine”] AND IDTitle: robot AND [Abstract: human] AND [Abstract: interaction] OR [Abstract: collaboration] \\ \hline Springer Link & (Unity OR “Unreal engine”) AND Robot AND Human AND (Interaction OR Collaboration) \\  & AND(“virtual reality” OR “Augmented reality” OR “Mixed reality”) NOT Medicine NOT \\  & Surgery AND (Social OR Industry OR Manufacturing), Content type: Article \\ \hline Science Direct & All (Unity OR “Unreal engine”), Title and abstract: Robot AND Human AND (Interaction OR Collaboration OR Controlled), Subject areas: Engineering, Article type: Research articles \\ \hline Web of Science & Robot AND Human AND (Interaction OR Collaboration) AND (“Virtual reality” OR “Augmented reality” OR “Mixed reality”), Citation topics: Robotics, and Human-Computer Interaction, Document Types: Article \\ \hline \end{tabular}
\end{table} 
Table 2: Advanced settings used for each database. Figure 1: General system architecture of an extended reality application for Human–Robot interaction. In this article, the study selection is defined in two steps. In Step 1, we read the title and abstract of each article obtained from the search in the proposed databases. Then, an article was selected as a candidate to be included in the review if it met the following inclusion criteria:  
_The main focus of the article was to present an XR solution using a social or industrial robot and not other types of hardware artifacts such as drones, swarm robotics, autonomous vehicles, smartphones, intelligent speakers, gloves, or wearable devices._  
In Step 2, we skimmed the full text of the articles obtained in Step 1. An article was excluded from the review if it met any of the exclusion criteria:  
1. _The article focuses only on simulation (i.e., the robot is only displayed on a monitor) or there is no interaction between the user and a physical robot_
2. _The article is a previous version of a more recent study or system proposing the same software architecture or framework by the same authors_
3. _The article is a short paper, review article, thesis, technical report, or book chapter_
4. _The article is a repeated article in different databases_
5. _The article does not propose a solution using the Unreal Engine or Unity Engine_ Additionally, quality assessment (QA) was defined using the following questions:  
1. _Is the article published in a journal that is indexed on Web of Science?_
2. _Was the article presented at a conference in the top twenty Google Scholar list for Robotics, Engineering and Computer Science, Human-Computer Interaction, or Manufacturing and Machines?_
3. _Are the methods, hardware, and software architecture clearly described?_  
Articles that did not meet at least one of these quality criteria were excluded from the study. Table 3 shows the number of articles processed in the proposed steps. To answer the proposed research questions, we used an Excel spreadsheet to extract relevant data from each reviewed article.