Closed loop docking with a nearly periodic moving target

Contact between two bodies is called "soft" if their relative speed impact is below a specified threshold. The objective of this research is to develop a control strategy that results in "soft" contact between a controlled body and an uncontrolled target whose motion is sinusoidal with slowly time varying amplitude and frequency. One approach is to completely synchronize the motion of the controlled body with the target and then gradually close the distance between them using a superimposed descent command. Another strategy results in tracking only during a short time before the contact. This "partial synchronization" approach is more power efficient than the complete synchronization strategy. The goal of this research is to develop a partial synchronization method that has the added requirement of having contact occur when the absolute velocity of the target is zero. This results in a safer, more robust situation since the system kinetic energy is low. The strategy developed here uses a reflection of the estimated target motion whose only common points with the target occur at zero speed. This requires real-time estimation of the target motion in conjunction with a nonlinear control law. The resulting closed-loop system is asymptotically stable with respect to the relative distance between the bodies. Simulation and experimental results indicate not only achievement of the soft landing goal, but also an 80% reduction in power consumption compared to a documented partial synchronization strategy. The contributions of this work are: stability analysis of an existing partial synchronization control strategy, development and validation of a new partial synchronization method which ensures landing at zero target velocity points, and development of an amplitude and frequency estimator that is robust to disturbances.