| With the development of space technology,the requirement of the operation on the orbit is growing.In the new round of space exploration,the countries all over the world carried out the technology research of the operation on the orbit.Operational targets on orbit can be divided into two categories,cooperative target and non-cooperative target.Non-cooperative targets have uncertain patterns and grappling fixtures,which are often out of control and their movement characteristics might not be known beforehand.Non-cooperative targets cannot be captured in a stable environment.Thus,moving target capturing becomes the core issue of non-cooperative target capturing.There exists a relative motion between in moving target capturing.Collision will take place in the capture process,which may cause damage of the end-effector and failure of capture.Hence,the research on moving target capturing is meaningless.Hand gives human the ability to capture various objects,therefore the end-effectors similar to hand have been widely used in gripping tasks.With the improvement of the complexity of the operation on orbit,the application of the space robot arm(hand)is gradually increasing.Currently,most of robotic arms adopt the full-drive mechanism which needs the shape and position information of the objects to control the robot posture.A fully actuated rigid robotic arm typically uses ahybrid position and force controller to adapt to the movement velocity of the target.Nevertheless,because a collision occurs over an extremely short period of time,any time delay in the control system may prevent the arm from being able to respond to a collision event in a timely manner,ultimately resulting in a hard impact on the beam.This paper presents an underactuated robotic arm based on differential gears that incorporates a one-input dual-output lanetary gear system to distribute the driving torque.When the robotic arm collides with a moving object,a portion of the impact energy can be automatically transmitted to the passive joint through the planetary gear system.The analysis of dynamic characteristics and the control of moving target captueing are developed in this paper,which provides the technical support for moving target capturing.The underactuated arm may perform instability grasp so that the arm loses contact with the target in some grasping postures.In order to acquire high grasp stability,the proposed arm needs to be optimized.Therefore,a method is presented for analyzing the equilibrium contact force and the relative movement trend between the target and the arm to determine the work space in which stable grasping is possible.The structural configuration parameters,such as the length ratios among the three beams and the reduction ratio of the planetary gear system,are optimized to maximize the grasp stability work space.The robotic arm takes advantage of the underactuation to cope with collision.Based on an analysis of the movement relation of three joints,the driving torque is resolved into generalized force corresponding to coordinates of two underactuated joints for establishing a dynamic model of the underactuated robotic arm.Along with the dynamic equation,impulse-momentum equations involving the restitution coefficient of the system(arm and target)are used to establish the impact dynamic model which can extrapolate the impact momentum on the drive component.With the impact dynamic model,the impact-absorbing capability of the underactuated robotic arm is illustrated that the impact momentum on the drive component is reduced by a factor of three compared with a 2-DOF fully actuated rigid robotic arm.Impact experiments were carried out to validate the inference results presented in this paper by using of a prototype of the proposed arm.Due to the impact momentum obtained from the collision,the joint in collision may obtain backward velocity.In order to deal with the impact momentum,an impedance-based control is applied for adjusting input torque according to the velocity and the position.For reducing the rebound,the three-element model is used to take the place of the basic Voigt model in the impedance control,which makes the recovery process of the underactuated joint slow and soft after the collision. |
