Real-Time Planner in the Operational Space for the Automatic Handling of Kinematic Constraints

Abstract

Planning problems in the operational space are characterized by implementation issues that do not occur in the joint space. For example, depending on the manipulator pose, relatively slow trajectories in the operational space could require unfeasible joint speeds, thus causing the degeneration of the system performances: Path tracking errors certainly increase but, in the worst situations, the manipulator must be stopped in order to prevent the system instability. This paper proposes a real-time planner in the operational space that is able to generate trajectories subject to dynamic constraints and devised according to the path-velocity decomposition approach. The feasibility is achieved by means of an automatic scaling system that, starting from a possibly unfeasible trajectory, modifies its longitudinal velocity in order to fulfill a given set of kinematic constraints, thus preserving an accurate path tracking. The scaling system promptly reacts to critical configurations through minimum-time transients. The proposed approach has been tested on an actual anthropomorphic manipulator by executing 6D trajectories. Note to Practitioners - The accurate path tracking must be guaranteed especially when trajectories are planned in the operational space. Unfortunately, path tracking worsens every time system limits are exceeded. The trajectory generator proposed in this paper is specifically designed for non-redundant manipulators and it is equipped with a scaling system that automatically modifies the speed of the end effector in order to guarantee an accurate path tracking. Several kinematic constraints are handled at the same time. Joint velocities are kept below the manufacturer's limits, while joint accelerations are bounded in order to achieve smooth movements. The system is also able to constrain the kinematics of the end effector. For example, in order to reduce the mechanical stress on the payload and to avoid the excitation of elastic modes, additional bounds on the velocities and accelerations of the end effector are considered and managed. The planner can also be used to generate minimum-time constrained trajectories in real-time. To this purpose, further constraints on the longitudinal velocities and accelerations have been introduced. Differently from alternative approaches, the proposed planning scheme does not require any interaction with the controller. This is an advantage, since controllers of industrial manipulators are typically not accessible or modifiable, while, in turn, proprietary trajectory planners can normally be replaced with ad-hoc implementations. The scaling system can be easily expanded in order to handle additional constraints. The trajectory smoothness, for example, can be improved by managing the jerk bounds, so that the ongoing research activity is currently focused on that target. In the same way, it could also be possible to handle some dynamic constraints, but this would impose the introduction of mutual interactions between the scaling system and the central control unit.