Performance of serial underactuated mechanisms: Number of degrees of freedom and actuators

While underactuated mechanisms have become popular in robot-hand designs because of their passive adaptability, existing systems utilize only one actuator to produce motion in the multiple degrees of freedom in the serial chain of each finger. In this paper, we explore how the performance of an underactuated serial link chain changes as more actuators are added. The fundamental question of what extra capability an additional actuator provides to an underactuated system and how best to implement it has not yet been quantified in the literature. Using a simple linear underactuated mechanism, we show that the performance of a single-actuator system (measured as the average number of contacts made with the environment) quickly plateaus as the number of degrees of freedom of the mechanism is increased. Also, we show that as the number of actuators is increased, the system's passive adaptability improves as the mechanism implementation spreads the actuators across the joints.