Contact force observer for space robots

Abstract

In this paper, the problem of estimating a contact wrench at the end-effector for a space robot is addressed. To this aim, a generalized force observer based on a base-joints dynamics is first reviewed. Then, a different formulation is proposed, which is based on a centroidal-joints dynamics. The proposed observer features interesting decoupling properties from the base linear velocity that lead to a more practical and better-performing estimation when limitations in real space scenarios are considered. The two observers are compared and the advantage of the proposed one is shown through a simulation example featuring a free-floating robot composed of a 7 degrees-of-freedom (DOF) arm mounted on a 6DOF moving base.

Unexpected Collision Detection, Estimation, and Reaction for a Free-Flying Orbital Robot

“…However, in a space scenario, it is obviously more practical and convenient to detect, isolate and estimate a collision without using additional and dedicated sensors, which result in a higher cost and power demand. For this reason, in [25], the authors propose a nonlinear observer to estimate an external wrench acting at a generic point along the robot. This scheme is an extension to floating-base robot of the well-known and well-established momentum-based observer initially developed for fixed-base robot [26].…”

“…The three residuals can be used to detect and isolate the contact, and, moreover, they can be exploited to reconstruct the entire contact wrench. However, in [25], the linear momentum residual is shown to be sensitive to noise and uncertainties in the measurements, with consequent deterioration of the wrench reconstruction performance. This is mainly due to the dependence of this residual on the satellite linear velocity, which is difficult to acquire accurately at high frequency.…”

“…Indeed, they provide an accurate estimation since their computation relies on sensors featuring relatively high acquisition frequency and low noise, such as encoders, gyroscopes and joint torque sensors. Further details about this observer can be found in [25]. Also other researchers have proposed observer schemes to estimate a contact force for a space robot.…”

“…In this paper, the first three phases of the physical contact handling are addressed redesigning the nonlinear observer [25]. In particular, a different formulation of the system dynamics is used, in which the velocity of the CoM of the whole system, the body-frame angular velocity of the spacecraft and the joint velocities are introduced as new generalized velocities.…”

“…To summarize, the main contributions of the paper are: 1) the physical contact handling problem on a generic point is addressed for free-flying orbital robots for the first time; 2) a reaction control strategy to unexpected collisions is developed and proved to be input-to-state stable; 3) the observer presented in [25] is further developed and analyzed considering a more realistic actuation system, namely thrusters and VSCMG.…”