Locked Joint Failure Research Articles

A Fault‐Tolerant Approach for Modular Robots through Self‐Reconfiguration

Modular robots have unique advantages in handling faults and improving their robustness due to their self‐reconfiguration capacity and homogeneous interchangeability. When locked joint failure occurs in modular robots, the distribution of the failed modules will affect the manipulation capacity of the robot. In this article, a novel self‐reconfiguration method that utilizes only the remaining resources available to mitigate the damage caused by module joint failures is proposed. The key node configuration is searched by the particle swarm optimization (PSO) algorithm, and then the collision‐free reconfiguration path is planned by the rapidly exploring random trees (RRT) algorithm. The proposed method not only handles single‐module lockup failure but can also be expanded to multi‐module lockup failures, fully improving the fault tolerance of the modular robot. The method is deployed on the hardware, and the feasibility of the algorithm is verified by self‐reconfiguration experiments containing faulty modules.

Review of the Fault-Tolerant Strategy for Space Manipulators With Joint Failure

The fault-tolerant strategy of space manipulator is very important to guarantee the task completion and safety after locked joint failure (LJF) occurs. According to the different research stages, the fault-tolerant strategy of the space manipulator with locked joint failure (SMLJF) is divided into the fault-tolerant strategy in design stage and use stage. In view of the fault-tolerant strategy in the design phase, the system-level design and component-level design are reviewed, the fault-tolerant design methods of the structure and joints of the space manipulator are introduced, and the problems existing in these methods are pointed out. In view of the fault-tolerant strategy in the use phase, it can be divided into fault-tolerant strategy before and after LJF, respectively. The fault-tolerant strategy before LJF such as artificial joint limits, global fault-tolerant path planning, and the fault-tolerant strategy after LJF including online fault-tolerant strategy and offline fault-tolerant strategy is reviewed in detail. The current research status of fault-tolerant strategy for SMLJF is introduced systematically, and the problems of these strategies are also detailly analyzed in this paper. Finally, all the existing problems of fault-tolerant strategy for SMLJF are summarized, and the research direction of fault-tolerant strategy is prospected.

The kinematic design of redundant robots for maximizing failure-tolerant workspace size

It has been shown that one can guarantee a reachable workspace for a kinematically redundant robot after an arbitrary locked-joint failure if one artificially restricts the range of its joints prior to the failure. This work presents an algorithm for computing the optimal kinematic parameters and artificial joint limits for a robot to maximize this so-called “failure-tolerant workspace”. The proposed technique employs a genetic algorithm that incorporates a novel method for selecting an initial population that results in fast convergence to high-quality solutions. The algorithm is illustrated on multiple examples of kinematically redundant robots and is shown to be computationally tractable even for robots that perform tasks in 6D workspaces.

Fault Tolerant Control and Reconfiguration of Mobile Manipulator

Trajectory control of mobile manipulator under joint lock failure poses new challenges. In this work fault tolerant scheme is proposed for trajectory control under joint lock failure through reconfiguration while exploiting the existing redundancy of the mobile manipulator system. Closed loop inverse kinematics (CLIK) algorithm are employed to transform the tip trajectory control problem to joint space control problem. In this work, two kinds of reconfiguration schemes are proposed, one using the redundancy in the manipulator only and another using the redundancy available with complete mobile manipulator system. Both types of the reconfiguration schemes use CLIK algorithm to calculate mobile base position and manipulator joint configurations so that the tip follows the reference trajectory in spite of locked joint failure of manipulator. Simulation and experimental results are presented to validate the proposed fault tolerant schemes.

A Hybrid Approach for Estimating the Failure-Tolerant Workspace Size of Kinematically Redundant Robots

Previous work has shown that it is possible to guarantee a reachable workspace for a kinematically redundant robot after an arbitrary locked-joint failure if one artificially restricts the range of its joints prior to the failure. Identifying the optimal articial joint limits has been the subject of previous work to maximize this so-called “failure-tolerant workspace.” Unfortunately, these techniques are not feasible for a highly redundant robot operating in a spatial workspace. This work presents a novel hybrid technique for estimating the failure-tolerant workspace size for robots of arbitrary kinematic structure and any number of degrees of freedom performing tasks in a 6D workspace. The method presented combines an algorithm for computing self-motion manifold ranges to estimate workspace envelopes and Monte-Carlo integration to estimate orientation volumes to create a computationally efficient algorithm. This algorithm is then combined with the coordinate ascent optimization technique to determine optimal artificial joint limits that maximize the size of the failure-tolerant workspace of a given robot. This approach is illustrated on multiple examples of robots that perform tasks in 3D planar and 6D spatial workspaces.

Maximizing the failure-tolerant workspace area for planar redundant robots

Kinematically redundant robots have extra degrees of freedom so that they can tolerate a joint failure and still complete an assigned task. Previous work has defined the “failure-tolerant workspace” as the workspace that is guaranteed to be reachable both before and after an arbitrary locked-joint failure. One mechanism for maximizing this workspace is to employ optimal artificial joint limits prior to a failure. This current work presents a technique for determining these optimal artificial joint limits that is based on the gradient ascent method. The proposed technique is able to deal with the discontinuities of the gradient that are due to changes in the boundaries of the failure tolerant workspace. The technique is illustrated using two examples of three degree-of-freedom planar serial robots. The first example is an equal link length robot where the optimal artificial joint limits are computed exactly. In the second example, both the link lengths and artificial joint limits are determined, resulting in a robot design that has more than twice the failure-tolerant area of previously published locally optimal designs.

Trajectory optimization for inhibiting the joint parameter jump of a space manipulator with a load-carrying task

A trajectory optimization strategy is proposed in this paper to inhibit joint parameter jumps, which are unavoidable when a locked-joint failure occurs in a load-carrying task without shutting down for a space manipulator. By selecting objective functions and constraint conditions, a trajectory optimization model is established. To solve the model, global null-space correction terms containing compensation coefficients are introduced into the joint velocity and torque by utilizing the redundancy of the manipulator. Then by selecting the optimal compensation coefficients of all possible failure moments and fitting them as functions, which are used to correct the joint parameters during the whole task cycle, the inhibition of the joint parameter jump in a sudden failure is realized. Finally, simulations are carried out surrounding a 7-DOF space manipulator. It indicates the average inhibition ratios of the joint velocity and torque jump reach values of up to 95 and 93%, respectively, verifying the correctness and effectiveness of the trajectory optimization strategy. The strategy has two contributions: (1) both the joint velocity and torque jump can be optimally inhibited, and the average inhibition ratios reach 90% at least; (2) the trajectory optimization strategy is practical for a joint failure occurring at any moment.

Development of a compliant legged quadruped robot

This paper presents the detailed steps for design and development of a compliant legged fault tolerant quadruped robot where each leg has two links and two motorized revolute joints for locomotion. The body and upper links of legs are rigid whereas the lower link of each leg is compliant. Amble gait is demonstrated on the developed robot. Safety and reliability are the most critical issues for the quadruped robot. During the failure of any joint, performance of quadruped robot is affected. In this paper, locked joint failure is also discussed. Strategies for fault tolerant control of the quadruped are developed and experimentally validated. The developed robot can be used for various hardware-in-the-loop controller prototyping such as reconfiguration, fault tolerant control, and posture control, etc. pertaining to quadruped robots.

Failure tolerance strategy of space manipulator for large load carrying tasks

During the execution of large load carrying tasks in long term service, there is a notable risk of space manipulator suffering from locked-joint failure, thus space manipulator should be with enough failure tolerance performance. A research on evaluating failure tolerance performance and re-planning feasible task trajectory for space manipulator performing large load carrying tasks is conducted in this paper. The effects of locked-joint failure on critical performance(reachability and load carrying capacity) of space manipulator are analyzed at first. According to the requirements of load carrying tasks, we further propose a new concept of failure tolerance workspace with load carrying capacity(FTWLCC) to evaluate failure tolerance performance, and improve the classic A* algorithm to search the feasible task trajectory. Through the normalized FTWLCC and the improved A* algorithm, the reachability and load carrying capacity of the degraded space manipulator are evaluated, and the reachable and capable trajectory can be obtained. The establishment of FTWLCC provides a novel idea that combines mathematical statistics with failure tolerance performance to illustrate the distribution of load carrying capacity in three-dimensional space, so multiple performance indices can be analyzed simultaneously and visually. And the full consideration of all possible failure situations and motion states makes FTWLCC and improved A* algorithm be universal and effective enough to be appropriate for random joint failure and variety of requirement of large load carrying tasks, so they can be extended to other types of manipulators.

Fault accommodation in compliant quadruped robot through a moving appendage mechanism

Quadruped robots provide better stability and speed in comparison to other legged robots. However, its joint actuator or sensor failure severely affects locomotion. Strategies for actuator and sensor fault accommodation in a compliant legged quadruped are presented here. A pair of orthogonally mounted moving appendages mechanism is proposed here to accommodate locked joint failure. These appendages as rack mounted inertias perform controlled motion during actuator failure. A strategy for sensor fault accommodation is also presented. A three-dimensional multi-body dynamics model of quadruped robot and its fault accommodation strategies are developed using bond graph modeling approach. The control performance is validated both through simulations and experiments.

Designing a Failure-Tolerant Workspace for Kinematically Redundant Robots

Kinematically redundant manipulators are inherently more robust to locked joint failures than non-redundant manipulators. However, if poorly designed, performance degradation may still occur in the presence of a single locked joint. This paper presents a technique for designing a desired operating workspace for a kinematically redundant manipulator that can be guaranteed after the occurrence of an arbitrary single locked joint failure. The existence of such a workspace, called a failure-tolerant workspace, will be guaranteed by imposing a suitable set of artificial joint limits prior to a failure. Conditions are presented that characterize end-effector locations within the failure-tolerant region. Based on these conditions, an algorithm for computing the failure-tolerant workspace is presented. The algorithm is based upon identifying the boundaries of the failure-tolerant workspace. Examples are presented to illustrate the application of the proposed algorithm to various manipulator design problems.

A novel 4-UPU translational parallel mechanism with fault-tolerant configurations

This paper aims at designing a pure translational parallel mechanism constructed by UPU (universal-prismatic-universal joint) kinematic limbs. First, the typical problem of unexpected rotations is pointed out from analyzing the typical 3-UPU parallel mechanism, and the reason for unexpected rotations of parallel mechanism constructed by UPU kinematic limbs is analyzed. Then, in order to design a pure translational parallel mechanism constructed by UPU without the unexpected rotations, the 2-UPU single loop is chosen as the basic structure to construct the 4-UPU translational parallel mechanism. Each 2-UPU single loop can be used to constrain a rotation about an axis of the linear complexes, which defined the unexpected rotations. Therefore, a novel type of 4-UPU pure translational parallel mechanism with redundant actuations is proposed. Since the existence of redundantly actuated kinematic limb, this proposed parallel mechanism possesses analytical forward kinematics, and its singularity can be avoided completely. Finally, the workspace and fault-tolerant performance are analyzed. When the proposed 4-UPU parallel mechanism is located in a fault-tolerant configuration, the moving platform can still possess movable ability to realize the given task even if one kinematic limb is in locked-joint failure mode, and the fault-tolerant workspace is obtained.

평판 발을 가지는 사족 보행 로봇의 내고장성 걸음새

This paper proposes fault tolerant gaits of a quadruped robot with feet of flat shape. Fault tolerant gaits make it possible for a legged robot to continue static walking against a leg failure. In the previous researches, it was assumed that a legged robot had feet that have point contact with the surface. When the robot is endowed with feet having flat shape, fault tolerant gaits can show better performance compared with the former gaits, especially in terms of the stride length and gait stability. In this paper, fault tolerant gaits of a quadruped robot against a locked joint failure are addressed in straight-line motion and crab walking, respectively.

Reconfiguration of four-legged walking robot for actuator faults

This paper presents fault accommodation through the reconfiguration of a four legged (quadruped) walking robot. It is assumed that the walking robot is moving using a bounding gait. A bond graph model to represent the robot’s locomotion in the sagittal plane is developed. A novel and simple attitude control device based on a moving appendage is proposed. This appendage device serves as redundant hardware and is activated only when the robot experiences a locked joint failure. The bond graph model of the system is used to generate the analytical redundancy relations which are then evaluated with actual measurements to generate residuals. These residuals are used to perform structural fault isolation. Once the fault list is updated in the equipment availability database, an automaton selects the best option to reconfigure the system such that the given control objectives are achieved. The developed methodology is validated by considering the failure of a joint’s actuator.

고장이 존재하는 육족 보행 로봇을 위한 대수적 힘 분배

본 논문에서는 육족 보행 로봇의 새로운 힘 분배 알고리듬을 제안한다. 본 논문에서 고려하는 육족 보행 로봇은 다리 하나에 관절고착고장이 발생하여 내고장성 정적 세다리 걸음새로 보행한다. 제안되는 힘 분배 알고리듬의 핵심은 내고장성 걸음새가 가지는 안정여유도를 결정하는 지지 다리의 미끄러짐을 최소화시키는 것이다. 불연속적으로 움직이는 내고장성 세다리 걸음새는 정상 걸음새보다 안정도가 떨어진다는 약점이 있다. 본 논문에서 제안하는 힘 분배 알고리듬은 이러한 약점을 고려하여 내고장성 걸음새의 지지 다리가 세 개라는 성질과 Zero-Interaction Force 원리를 이용하여 최적화 기법을 쓰지 않고 대수적으로 모든 다리의 힘 성분을 구한다. 컴퓨터 시뮬레이션 사례 연구를 통해서 제안된 힘 분배 알고리듬과 기존 방법의 비교 분석을 실시하고 제안된 방법의 효용성을 입증한다. In this paper, a novel foot force distribution algorithm for hexapod walking robots is presented. The considered hexapod robot has fault-tolerant tripod gaits with a failed leg in locked-joint failure. The principle of the proposed algorithm is to minimize the slippage of the leg that determines the stability margin of the fault-tolerant gaits. The fault-tolerant tripod gait has a drawback that it has less stability margin than normal gaits. Considering this drawback, we use the feature that there are always three supporting legs, and by incorporating the theory of Zero-Interaction Force, we calculate the foot forces analytically without resort to any optimization technique. In a case study, the proposed algorithm is compared with a conventional foot force distribution method and its applicability is demonstrated.

Fault-Tolerant Gait Planning for a Hexapod Robot Walking over Rough Terrain

Multi-legged robots need fault-tolerant gaits if one of attached legs suffers from a failure and cannot have normal operation. Moreover, when the robots with a failed leg are walking over rough terrain, fault-tolerance should be combined with adaptive gait planning for successful locomotion. In this paper, a strategy of fault-tolerant gaits is proposed which enables a hexapod robot with a locked joint failure to traverse two-dimensional rough terrain. This strategy applies a Follow-The-Leader (FTL) gait in post-failure walking, having the advantages of both fault-tolerance and terrain adaptability. The proposed FTL gait can produce the maximum stride length for a given foot position of a failed leg and better ditch-crossing ability than the previous fault-tolerant gaits. The applicability of the proposed FTL gait is verified using computer graphics simulations.

Two-phase discontinuous gaits for quadruped walking machines with a failed leg

A fault-tolerant gait of multi-legged robots with static walking is a gait which can maintain gait stability and continue its walking against an occurrence of a leg failure. This paper proposes two-phase discontinuous gaits as a new fault-tolerant gait for quadruped robots suffering from a locked joint failure, which prevents a joint of a leg from moving and makes it locked in a known place. By comparing with previously developed one-phase discontinuous gaits, it is shown that the proposed gait has great advantages in gait performance such as the stride length and terrain adaptability. As an application of the proposed gait, the gait algorithm for ditch crossing and avoidance of quadruped robots with a failed leg is presented and illustrated by simulated figures.

Omnidirectional walking of legged robots with a failed leg

This paper studies omnidirectional walking of a hexapod robot with a locked joint failure by proposing crab gaits and turning gaits. Due to the reduced workspace of a failed leg, fault-tolerant gaits have limitations in their mobility. As for crab gaits, an accessible range of the crab angle is derived for a given configuration of the failed leg. As for turning gaits, the conditions on turning trajectories guaranteeing fault tolerance are derived for spinning gaits and circling gaits. Based on the principles of fault-tolerant gait planning, periodic crab gaits and turning gaits are proposed in which a hexapod robot realizes tripod walking after a locked joint failure, having a reasonable stride length and stability margin. The proposed fault-tolerant gaits are then applied to an obstacle avoidance problem of a hexapod robot with a locked joint failure. The kinematic constraints of fault-tolerant gaits should be considered in planning the robot trajectory.

Failure-tolerant path planning for kinematically redundant manipulators anticipating locked-joint failures

This work considers kinematic failure tolerance when obstacles are present in the environment. It addresses the issue of finding a collision-free path such that a redundant robot can successfully move from a start to a goal position and/or orientation in the workspace despite any single locked-joint failure at any time. An algorithm is presented that searches for a simply-connected, obstacle-free surface with no internal local minimum or maximum in the configuration space that guarantees the existence of a solution. The method discussed is based on the following assumptions: a robot is redundant relative to its task, only a single locked-joint failure occurs at any given time, the robot is capable of detecting a joint failure and immediately locks the failed joint, and the environment is static and known. The technique is illustrated on a seven degree-of-freedom commercially available redundant robot. Although developed and illustrated for a single degree of redundancy, it is possible to extend the algorithm to higher degrees of redundancy

Kinematic Constraints on Fault-Tolerant Gaits for a Locked Joint Failure

Fault-tolerant gaits in legged locomotion are defined as gaits with which legged robots can continue their walking after a failure event has occurred to a leg of the robot. For planning an efficient fault-tolerant gait, kinematic constraints and remaining mobility of the failed leg should be closely examined with each other. This paper addresses the problem of kinematic constraints on fault-tolerant gaits. The considered failure is a locked joint failure which prevents a joint of a leg from moving and makes it locked in a known place. It is shown that for the existence of the conventional fault-tolerant gait for forward walking on even terrain, the configuration of the failed leg must be within a range of kinematic constraints. Then, for coping with failure situations where the existence condition is not satisfied, the conventional fault-tolerant gait is adopted by including the adjustment of the foot trajectory of the failed leg. The foot trajectory adjustment procedure is analytically derived to show that it can help the fault-tolerant gait avoid dead-lock resulting from the kinematic constraint. To demonstrate its effectiveness, the proposed method is applied to the fault-tolerant gait generation for a quadruped robot walking with the wave-crab gait before a locked joint failure.