Although the concept of industrial cobots dates back to 1999, most present day hybrid human-machine assembly systems are merely weight compensators. Here, we present results on the development of a collaborative human-robot manufacturing cell for homokinetic joint assembly. The robot alternates active and passive behaviours during assembly, to lighten the burden on the operator in the first case, and to comply to his/her needs in the latter. Our approach can successfully manage direct physical contact between robot and human, and between robot and environment. Furthermore, it can be applied to standard position (and not torque) controlled robots, common in the industry. The approach is validated in a series of assembly experiments. The human workload is reduced, diminishing the risk of strain injuries. Besides, a complete risk analysis indicates that the proposed setup is compatible with the safety standards, and could be certified. From the end user (PSA) viewpoint, two aspects are particularly noteworthy. First, since the operator load is reduced by approximately 60%, the proposed assembly cell can be reclassified in the PSA ergonomics scale. Second, a complete risk analysis by PSA indicates that the proposed setup is compatible with the safety standards, and could be certified. The article is organized as follows. Section 2 summarizes the state-of-the-art in collaborative manufacturing, and highlights our contributions in the context of current, related research. In Sect. 3, we present the targeted application: collaborative assembly of a homokinetic joint. The proposed framework is outlined in Sect. 4. The framework components (nominal trajectory generation, admittance control, and safety monitoring) are then detailed in the following sections (respectively, Sections 5, 6, and 7). Experimental results are reported in Section 8, and finally summarized in the Conclusion. This section summarizes the current state-of-the-art in collaborative manufacturing. We first review the more application-oriented research on humanmachine cooperation (Sect. 2.1), and then the academic research on physical human-robot interaction (Sect. 2.2). The authors of [3] provide a very rich overview of the emerging technologies in automotive assembly, including the supporting systems (mainly the information technologies). They show that mass customization requires high technological flexibility, and propose various designs to cope with this, by integrating both automated and human-based assembly. A similar perspective is that of the recent EU project , that aims at integrating assembly systems, and human capabilities. The main enablers, according to the authors, are: intuitive interfaces, safe strategies and equipment, proper methods for planning and execution, and the use of mobile robots, and of distributed computing. More recently, the U.S. Consortium for Automotive Research conducted a study on feasibility of fenceless robotic cells for automotive applications [5] . The study defines the levels of human-robot collaboration according to the cell complexity, to drive the probabilities of successful implementation. But as in the previously cited survey [2], the paper exposes the absence of high level human-robot collaboration, apart from "Intelligent Lift Assistants". Although some automotive manufacturers are gradually introducing robots in their human production line [6, 7] , a crucial question persists: how should a collaborative robotic cell be designed? Various researchers have looked into this. Papakostas et al. [8] discuss the key features of cooperating robotic cells in automotive assembly, and provide simulated comparisons of two scenarios: a conventional welding robotic cell, and one with cooperating robots. The authors of [9] assess five alternative safety designs, covering both hardware and control design, of a human-robot collaboration prototype cell for cable harness assembly. In [10], a new cell production assembly system, with human-robot cooperation is developed. The system consists of three key technologies; parts feeding by double manipulators on a mobile base, production process information support for the operator, and safety management for cooperation between operator and robot. The main target of [11] is safety of the shared work cell, in the absence of physical fences between human and robot. Since safety options provided by basic infrared sensors are limited, the authors design a network architecture of these sensors, for tracking user positions, while avoiding collisions. The authors of [12] propose a method for optimizing task distribution among workers and robots. The method is validated, using an ABB Dual Arm Concept Robot, in a PLC Input/Output module assembly scenario. Recent robotics research focuses on the study and characterization of physical human-robot interaction (pHRI [13, 14] ). The goal is to enable close collaboration between human and robot, in all service and industrial tasks, that require the adaptability of humans to be merged with the high performance of robots in terms of precision, speed and payload [15] . In this context, it becomes indispensable to define safety and dependability metrics [16, 17, 18, 19] . These can contribute to the definition of standards, such as the recent ISO 10218-1:2011
Regarding the first group of works, Cherubini et al. REF discussed a collaborative scenario where a human and a robot shared the task of Rzeppa homokinetic joint insertion.
Collaborative manufacturing with physical human–robot interaction
Robotics and Computer-integrated Manufacturing
Engineering