I. INTRODUCTION
Underactuation is a widely used design concept in many robotic systems [1], [2], [3], [4]. The number of control inputs (actuators) of an underactuated robot is less than the degree of freedom (DOF) of the system, leading to a reduction of both control and hardware complexity [5]. In the last few decades, we have seen an increased interest in the design of compliant robotic systems exploiting underactuation such as, robotic hands [6], grippers [7], [8] and exoskeletons [9], and walking robots [10], [11], most of which were based on tendon-driven mechanisms enabling lightweight, compact and more energy efficient system designs [12]. The dexterity of the prescribed systems can also be enhanced by their passive adaptability to the unknown environments and more robustness in handling external disturbances.