Real-time smooth task transitions for hierarchical inverse kinematics

Hierarchical inverse kinematics (HIK) is widely used for generating feasible velocity trajectories that serve as input references for highly redundant robots such as humanoid robots. To generate the velocity trajectories a set of prioritized tasks should be provided. For some applications, it is not necessary to change the priority order of the tasks in the stack of tasks (SoT) along the motion execution. However, complex tasks need a dynamic behavior of the SoT such that the insertion, removal or swap can be performed at running time. These task transitions may induce discontinuities in the joint velocities if they are not carefully handled. In this context, we propose an efficient strategy to manage task transitions through a simple procedure which smoothly interchange the priority of a couple of consecutive prioritized tasks. Furthermore, our method does not increase the computational cost of the HIK since neither any additional task should be added, nor parallel control laws should be computed. As a result our strategy may be used in real time to produce the velocity commands of real humanoid robots. The effectiveness of our strategy is verified at simulation level with the HRP-2 humanoid robot performing complex time-driven tasks.