## 1 Introduction  
The currently emerging manufacturing paradigm, known as Industry 4.0, is behind the rethinking of how industrial processes are designed in order to increase their efficiency and flexibility, together with higher levels of automatization [1]. To this end, Industry 4.0 integrates multiple technologies such as the Internet of Things (IoT) [1], artificial intelligence (AI) [1], and cyber-physical systems. Accordingly, robotics researchers are proposing new solutions [2, 3] whereby collaborative robots, known as cobots, physically collaborate with well-qualified operators to achieve the goals of this revolution. Hence, the problem of the human subject's safety vs. the robot's high dynamic performances arises. This means that the next generation of widely used cobots should manifest human-friendly attributes. In the literature, two main approaches were proposed to tackle this problem: active impedance control (AIC) and passive compliance (PC). In the former, the impedance [4] is controlled to display safe behavior vis-a-vis the robot's environment, including the humans within it. In the case of a fast human-robot (HR) collision, this approach, because it cannot respond as quickly as within 200 ms [5, 6],can allow severe damage to the impacted human [7]. As a result, the PC approach attracted interest due to its instantaneous reaction to any potential impact. This latter, potential impact is defined as a quantification of the maximum impact force a robot can exert in a collision with a stationary object [8]. To emphasize, it is the combination of high mobile inertia and high velocity (i.e., the high kinetic energy) that makes robots dangerous [7,9]. With this in mind, achieving safety without compromising the desired dynamics boils down to reducing the reflected inertia, which is the key feature of the PC approach. Indeed, from a dynamic perspective, integrating passive mechanisms in robot joints decouples a certain colliding inertia from the rest of the robot. This reduces the overall kinetic energy absorbed by the impacted human. In line with this, the series elastic actuator [10] is among the first solutions that has a constant stiffness profile. A passive compliance system composed of purely mechanical elements often provides faster and more reliable responses to dynamic collisions [11]. To enhance the latter's design capacity to react to load variation, a series parallel elastic actuator was introduced by [PERSON] and co-workers [12]. To improve the safety of physical human-robot interaction (pHRI), [PERSON] et al. presented [13] a distributed macro-mini (DM [2]) actuation system that puts forward low-inertia actuators to interact with the human subject. This allows for both safety and a fast control reaction via the low inertia actuated part. In order to deal with any unsupervised collision while handling variable loads, the concept of the variable stiffness actuator (VSA) emerged. Through the years, several VSAs have been proposed, as discussed in previous studies [3,14]. Their design concepts resulted in different structural paradigms (e.g., serial or antagonistic), different stiffness profiles, and a wide range of power to mass ratios. Herein, a great emphasis is placed on a VSA's stiffness profile vs. the safety aspect. The proposed approach, leading to prototype V2 SOM (variable stiffness safety-oriented mechanism) [1], presents the following novelties compared to the literature:  
* The stiffness behavior, in the vicinity of zero deflection, is smoothened via a cam follower mechanism. * The stiffness sharply sinks to maintain, theoretically, as discussed in Section 2, a constant torque threshold in case of a collision. * The torque threshold, \(T_{max}\), is tunable according to the load variation. This paper presents the V2 SOM design as well as a simulation and experimental validations. V2 SOM is primarily conceived to enhance safety in normal working routines, as well as in the case of uncontrolled HR collisions.