Elsevier

Acta Astronautica

Volume 181, April 2021, Pages 40-51
Acta Astronautica

Research paper
Contact detection, isolation and estimation for orbital robots through an observer based on a centroid-joints dynamics

https://doi.org/10.1016/j.actaastro.2021.01.001Get rights and content

Highlights

  • Contact detection, isolation and estimation for orbital robots are addressed.

  • Two nonlinear observers for floating base robots are proposed and analyzed.

  • The observers are compared through simulations including realistic noise conditions.

  • The observer based on a centroid-joints dynamics provides superior performance.

  • Both observers are experimentally validated on real hardware.

Abstract

In this paper, the contact force detection, isolation and estimation problems for an orbital robot are addressed. First, an observer based on a base-joints dynamics is reviewed and analyzed considering the issues related to a space robotic application. Then, a new scheme is derived reformulating the dynamics in terms of the motion of the centroid of the whole robot and the joints. In both observers, three momentum-based residuals are defined, which can be used to reconstruct the external contact wrench. The reconstruction through the proposed observer turns out to provide superior performance with respect to the base-joints one thanks to the decoupling of the angular and joint momentum residuals from the spacecraft linear velocity, which is an inaccurate measurement in real space scenarios. The advantages of the proposed observer are shown through both simulative and hardware validation featuring a 7 degrees of freedom (DoF) arm mounted on a 6 DoF moving base.

Introduction

Orbital robotics is considered one of the key technologies for many future missions since it can provide higher levels of safety, reliability and performance [1]. Especially, space manipulators are considered one of the most promising technology for servicing, assembly and inspection in orbit. However, their use is still very limited due to the high complexity involved. One of the most challenging issue is dealing with physical contact between the robot and a target object.

Since the beginning, the dynamics of the contact phase has been extensively studied, considering free-floating robots [2], [3], [4], [5]. Many authors have addressed the problem of guaranteeing a safe interaction when the robot’s end-effector comes into contact with the target to perform grasping tasks [6], [7], [8], [9], [10]. In this context, effective control strategies can take advantage of the accurate knowledge of the contact force, which can be measured by employing a force–torque sensor (FTS) duly placed at the wrist. However, a strategy relying only on this sensor is not robust since a failure in the FTS could compromise the correct completion of the task, or even block the operations until repair. Furthermore, the FTS measures forces and torques acting only at the end-effector, and thus contacts that do not occur exactly there, e.g. a contact on links other than the one with the FTS, cannot be measured.

These motivations have pushed some researchers to propose a different approach, in which the contact force is estimated without using a dedicated sensor. In [11] the use of the disturbance observer is proposed, while in [6] the force is estimated through the target’s equations of motion. Both methods require quantities that are not measured directly, i.e., the joint accelerations, the linear velocity and the angular acceleration of the robot base for the former, and the target’s accelerations for the latter. These quantities could be obtained through numerical differentiation, but they would introduce nonnegligible noise in the estimation process. Moreover, these methods assume to know where the force acts, i.e. at the end-effector, and thus cannot be used for a more general situation of a contact on a generic link.

The problem of estimating a contact force involving whichever part of the robot has been extensively studied within the robotics community in the last years, especially for fixed-base robots [12], [13], [14], humanoids [15], [16], and flying robots [17]. On the other hand, few works have been carried out considering orbital robots. In [15] the well-known generalized momentum-based observer, originally presented in [12] for fixed-base robots, is extended to humanoids, featuring a floating-base dynamics similar to the one of space robots. This observer computes linear, angular and joint momentum residuals which turn out to be the estimates of the external generalized forces acting on the floating base and the disturbance joint torques due to a contact. Then, the residuals can be used to estimate the external wrench acting on the robot. The main drawback of the method is the need of a fast and accurate reconstruction of the base linear velocity, which is difficult to obtain in the case of a real space application.

In this paper, the residual-based observer [15] is reviewed and analyzed considering the issues related to a space robot. Then, a new observer is derived, which is based on a centroid-joints dynamics. The most important feature of the new observer is the complete decoupling from the base linear velocity of the angular and joints momentum residuals, which, therefore, result to be less noisy. This leads to a better-performing estimation of the contact wrench than [15]. Indeed, the wrench reconstruction can be performed using only these two residuals, and thus not requiring a measurement of the base linear velocity, but measuring solely the angular velocity, and the joint positions, velocities and torques, which can be acquired at high frequency and feature relatively low noise.

Furthermore, the proposed method allows reconstructing a contact on a generic point, and not only at the end-effector. It can be used to detect, locate and estimate unexpected collisions which may occur during the robot’s operations. The proposed strategy to isolate the collision uses only the information from the angular and joint momentum residuals of the presented observer. Consequently, the contact point can be found relying only on accurate sensors working at high frequency, and thus an accurate and fast wrench estimation is achieved.

The main contributions of this paper are: (1) the formulation of the observer [15] for application on an actuated space robot, the analysis of its limitations, and the development of a new observer based on a centroid-joints dynamics which solves these limitations; (2) a new method to identify a generic contact point along the robot, using only the angular and joint momentum residuals; (3) the experimental validation of both observers on the On-Orbit Servicing Simulator (OOS-Sim) hardware-in-the-loop facility [18] at the DLR. Note that, to the best of the authors’ knowledge, no experimental tests have been performed for the observer [15] before. The work in this paper is an extension of the authors’ preliminary analyses presented in [19].

The paper is structured as follows: in Section 2, the notation, the assumptions and the dynamic equations are introduced. In Section 3, the method in [15] is formulated for space robots and the proposed observer based on a centroid-joints dynamics is presented. In Section 4, the reconstruction of the external wrench is addressed. In Section 5, a strategy to isolate the generic contact point is presented. In Section 6, a simulation example is proposed to compare the performance of the observers. In Section 7, the results of the experimental validation of the observers are reported. Finally, in Section 8, the main conclusions are drawn and future works are discussed.

Section snippets

Problem statement and assumptions

A space robot is considered as a multibody system composed of n+1 rigid bodies connected with n revolute joints (see Fig. 1). An in-orbit proximity operation is taken as reference scenario. The robot is required to perform manipulation or inspection tasks, and thus to operate close to one or more other objects. In this context, contact situations may arise and they can be planned, e.g. grasping a target, or unexpected, e.g. a collision. In the former case, the accurate knowledge of the contact

Nonlinear force observer for space robots

In this section, the residual-based observer presented in [15] for humanoids is firstly adapted to space robots. This method is based on the momentum-based observer [13] in which a residual vector is defined as the difference between the generalized momentum of the fixed-base robot and its estimate. Under ideal condition, this residual vector turns out to be a filtered estimation of the external disturbance on the joints. Hereafter, the same idea is followed using the dynamics model (2) to

Reconstruction of the external wrench

Assuming to know the contact point along the space robot, the relation between the contact generalized forces fext,b, mext,b, and τext in (2) and the external wrench Fext can be expressed as in Eq. (3). Using the inverse of Eq. (14) and recalling that mext,c=Icaext,c, the relation between fext,c, mext,c, and τ̄ext, and Fext is given by: fext,cmext,cτ̄ext=ApcTJpTFext,where Jp=JvpJωp=RpbJˆv+RpbpcpJˆω+JvpRpbJˆω+JωpR6×n.Then, an estimate Fˆext of the external wrench at the end-effector can

Contact point isolation

Solving the isolation problem means locating the contact point p. In the following, first, a procedure to identify the contact point using all the residual vectors is presented. This procedure can be applied considering both the observer (5) and (20). Then, a different approach is proposed that relies only on mˆext,c and τ̄ˆext, avoiding the need of the base linear velocity knowledge. In the development of both strategies, it is assumed that the point of application of the external disturbance

Simulation example

In this section, the two observers are compared through numerical simulations including realistic noise models for the measurements. The linear velocity of the base is reconstructed using a kinematics-based Kalman filter. The superior performance and the advantages of the proposed method based on the centroid-joints dynamics are shown.

Experimental validation

Both observers (5) and (20) have been validated on the On-Orbit Servicing Simulator (OOS-Sim) hardware-in-the-loop facility [18] at the DLR. This is a robotic simulator reproducing the in-orbit dynamics of a space robot. Thanks to the OOS-Sim, it is possible to test the algorithms, which will run on the space robot, on ground, before the actual deployment in orbit. The facility is made up of two parts (see Fig. 6): a simulator arm and a test arm. The former one is a position-controlled KUKA

Conclusions

In this paper, the problem of estimating a contact wrench acting on a generic point along a space robot was addressed. The observer presented in [15] was reviewed and analyzed considering the issues related to a space robot. Then, a new observer was proposed introducing a new set of generalized velocities, including the linear velocity of the spacecraft-manipulator’s CM, the angular momentum around the CM and the joint velocities. This observer turned out to provide a more accurate estimation

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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