Toward Soft Robots You Can Depend On P hysical human-robot interaction (pHRI) represents one of the most motivating, challenging, and ambitious research topics in robotics. Many of the future and emerging applications of robotics, be they in service [14], assistance and care [23], rehabilitation [34], or in more traditional working contexts [34], will indeed require robots to work in close vicinity if not in direct contact with humans.A robot for pHRI applications must be regarded in all aspects as a safety-critical system, as it has been unfortunately proven several times in the past that conventional robots can be dangerous or even deadly machines [33]. Since the very beginning of industrial robotics, a great deal of attention has been paid to robot safety, the first line of defense having always been to take all measures to enforce segregation between robots and people [5], [40]. As market pressures together with ethical concerns are about to topple some of the barriers separating robots and people, safety standards are evolving. The 2006 revision of the ISO10218-1 standard [21], for instance, introduces more advanced concepts than in the past, such as the idea of a collaborative operation between humans and robots, and the replacement (albeit to a very limited, conservative extent) of fixed rules with risk assessment procedures. More generally, for applications involving pHRI, analysis tools are needed that are classical in the literature on critical systems [2], [38] but are still rather new in robotics [9], [10], [13], [42]. These tools focus on the attributes of 1) safety, i.e., the absence of damages and injuries; 2) reliability, the continuity of service; and 3) availability, the readiness of service; in a word, the comprehensive attribute of 4) dependability. The goal of this article is to begin an in-depth study of the dependability of robots for pHRI, starting with the analysis of an elementary, yet critical, robot component, i.e., the joint-level actuation subsystem.As an answer to the need to build robots that can provide useful performance while guaranteeing safety against all odds, engineers have proposed several innovative solutions to overcome the classical paradigm ''rigidity by design, safety by sensors and control,'' which is more suited for conventional industrial robotics, and are shifting toward a ''safety by design, performance by control'' philosophy [1], [14], [18]. In our own previous work [3], [4], variable stiffness actuation (VSA) and its generalization in variable impedance actuation (VIA) have been demonstrated to be effective in obtaining a safe yet performing robot motion by swiftly alternating stiff-and-slow and fast-and-soft motion modes. Indeed, in high-velocity impacts, low-joint impedance can effectively decouple the link's inertia from the actuator's reflected inertia, which is typically large due to the transmission gear ratio. Although the investigation of VIA, including variable damping [11], [25], [29], [31] and/or gear ratio, is a very promising research direction, as...
