# Control System Design and Methods for Collaborative Robots: Review
## 1 Introduction  
Human-robot collaboration is an innovative area aiming to construct an environment for safe and efficient collaboration between humans and robots to accomplish a specific task. This area introduces a new type of robot called collaborative robots or cobots. Unlike traditional robots, collaborative robots are robots that can work together with humans to perform tasks in several fields of life, including industry, education, agriculture, healthcare services, security, and space exploration. The term \"collaborative robot\" was first introduced in the academic literature by [PERSON] and [PERSON] in 1999 [1]. In the beginning, the collaborative robots were known as intelligent assist devices (IADs) until the standards of intelligent assist devices with personal safety elements were composed by the Robotic Industries Association (RIA) in 2003. Later, the term collaborative robot became standard. The first commercial collaborative robot that was capable of performing automated and streamlined repetitive industrial tasks was developed by KUKA in 2003 [2]. Figure 1 depicts the stages of development of collaborative robots since 2003. The considered cobots are single-arm and dual-arm and with six or seven degrees of freedom. The concept of collaborative robots is to combine the cognitive skills of humans with the precision and dexterity of robots to accomplish complex tasks. Unlike the classicalindustrial robots, cobots are simple, lightweight, reliable, with easy-to-deploy enhanced sensors. Thanks to these characteristics, they can detect a human's motion and are equipped with collision avoidance algorithms. Additionally, they can be programmed with mobile phones. Recently, with the development of Industry 4.0 standards, the vital role of cobots in automation, manufacturing in general and in small and medium enterprises (SMEs) in particular has become evident [3]. According to the recent survey report of Markets&Markets, the market share for collaborative robots is estimated to grow from USD 981 million to USD 7972 million during the period 2021-2026 [4]. The surge in demand for collaborative robots motivates researchers and manufacturers to develop advanced collaborative robotic systems that can integrate with the Internet of Things (IoT) [5; 6]. Human-robot (HR) interaction requires different levels of automation and human intervention. These interactions are commonly classified as HR coexistence, HR cooperation, and HR collaboration with various levels of automation (full, semi, collaborative). When humans and robots share a workspace but are working independently without sharing a task, this kind of interaction is known as HR coexistence. The terms HR collaboration and HR cooperation are used interchangeably. However, the two vary slightly. When the human and robot both seek to achieve the same goal and share the workspace, it is known as HR collaboration. However, HR-cooperation implies a situation in which robots and humans share a workspace and work simultaneously, but on separate sub-tasks. The collaborative robot (cobot) can be a manipulator or mobile robot that can work together on shared tasks in a shared workspace. In this article, the type of collaborative robot addressed are manipulator robots that can work with humans on shared work in the same workspace. However, collaborative robots are also known as multiple robots that can interact with each other to perform shared tasks, e.g., co-transporting objects with the help of multiple collaborative robots. However, this literature review focuses on the collaborative robot (manipulator) that has physical human-robot interaction and control methods relying on the motion-force control of the robot. While multiple collaborative robots use cooperative distributed control methods based, e.g., on game theory, that is out of the scope of our literature review presented in this paper.