Leila Notash

On the design of fault tolerant parallel manipulators

Methodologies for the design of fault tolerant parallel manipulators are presented based on the failure analysis of manipulators. The investigations concentrate on the architecture and actuator/sensor distribution layouts of parallel manipulators. Failures of parallel manipulators are analyzed at the component level (link and joint failures), subsystem level (branch failure), and system level (parallel device failure). All potential joint and actuator distributions of parallel manipulators are identified, and the results for manipulators with the mobility of three and six are tabulated. The mechanical failures of parallel and serial manipulators are presented and classified based on the component, subsystem and system failures. In addition to the component failures, the failure of parallel manipulators as a result of the loss of degree of freedom, loss of actuation, loss of constraint, special configurations (singularities), as well as branch interference, are discussed. Redundant configurations of parallel manipulators are classified as redundant mobility, redundant sensing, and redundant actuation; all being considered for the performance improvement of these devices. Two criteria are defined for the identification of optimum configurations of parallel manipulators with one redundant actuation. For the considered classes of parallel manipulators, optimum fault tolerant configuration of each class is identified.

Uncertainty configurations of parallel manipulators

Abstract Uncertainty configurations of three-branch parallel manipulators are investigated. Considered is a broad class which includes all three-branch manipulators where each branch is comprised of a serial arrangement of three main-arm joints supporting a common payload platform through a passive spherical branch-end joint-group. The investigation is based on examining potential degeneracies of the screw systems formed by actuated-joint associated wrenches, identifying all potential uncertainties for the class. The characteristics of the unconstrained instantaneous degrees of freedom corresponding to each uncertainty configuration are discussed. Joint actuation layouts that eliminate the uncertainty configurations are determined through the consideration of all feasible cases of main-arm joint actuation (non-redundant and redundant). Furthermore, example configurations which result in the corresponding dependency case for an example three-branch parallel manipulator are identified. All potential uncertainty cases are shown to be eliminated by redundant actuation of all main-arm joints (3-3-3 actuation). The screw system spanned by the wrenches associated with the actuated joints of the branches and also by the screws reciprocal to all the joints of degenerate branch configurations can be concluded to always have an order of six for redundant actuation of all main-arm joints. In addition, 3-3-2 main-arm joint actuation is demonstrated to eliminate all except for one case of pottential uncertainty configuration types, and 3-2-2 is shown to be an advantageous seven actuator configuration in comparison to 3-3-1.

CAT4 (Cable Actuated Truss - 4 Degrees of Freedom): A Novel 4 DOF Cable Actuated Parallel Manipulator

The CAT4 (Cable Actuated Truss—4 degrees of freedom) robot is a novel, passively jointed, parallel robot utilizing six control cables for actuation. The architecture has been under development at the Queens University Robotics Laboratory. The robot utilizes a passive jointed linkage with 18 revolute joints to constrain the end effector motion and provide the desired structural stability, restricting the end effector to 3 translational degrees of freedom (DOF) and 1 DOF for end effector pitch. This central mechanism together with winched cable actuation gives a number of important benefits for applications where the advantages of a parallel robot are required in conjunction with light weight. Six electric motor driven winches control the length of the cable actuators that extend from the top frame to points on the end effector raft and jointed linkage to create a stiff, but lightweight, actuated robot. Simulation work on the robot is presented giving the kinematics, including a computational estimate of the workspace for a specific configuration. Results of computational simulation of the motion of the manipulator and a discussion of the advantages and potential difficulties are also presented.

Forward displacement analysis and uncertainty configurations of parallel manipulators with a redundant branch

The effect of adding a redundant branch in terms of reduction of the number of assembly modes and elimination of potential uncertainty configuration types is investigated for a class of parallel manipulators. Considered is a broad class that includes all three-branch manipulators where each branch is comprised of a serial arrangement of three main-arm joints supporting a common payload platform through a passive spherical branch end joint-group. The addition of a redundant branch effectively yields a four-branch manipulator class. Considered in particular is a 3-4 form of the manipulator where two branch ends meet at one point on the mobile platform. Symmetric main-arm joint sensing and actuation (two sensed/acutated main-arm joints per branch) is utilized. Synthetic geometry is used to study the number of assembly configurations of the resulting 3-4 four-branch parallel manipulators. It is presented that the number of assembly modes of three-branch parallel manipulators with passive spherical branch end joints can be reduced by utilizing a redundant branch. It is shown that there exist up to eight and up to four assembly modes when all unsensed joints are revolute and when all unsensed joints are prismatic, respectively. Combinations of unsensed prismatic and revolute joints are also investigated. It is determined that there are up to eight and up to four assembly modes when the unsensed main-arm joint of one of the concurrent branches is prismatic and when the unsensed joints of both concurrent branches are prismatic joints, respectively. Resolving the potential assembly modes require only the consideration of, at highest, second-order single-variable polynomials. In addition, kinematic design considerations allowing reduction of feasible assembly modes are discussed. The investigation of potential uncertainty configuration types is based on examining degeneracies of the screw systems formed by wrenches associated with the forces that the actuated-joints can apply. All linear dependency cases that could potentially cause uncertainties for the class of four-branch manipulators are identified. It is shown that while significantly reducing potential uncertainty configuration types, the addition of a redundant branch number cannot eliminate all potential dependency (uncertainty) cases completely. For the remaining potential uncertainty configuration types, the characteristics of the corresponding unconstrained instantaneous degrees of freedom are discussed.

On the forward displacement problem of three-branch parallel manipulators

Abstract Solutions for the forward displacement problem (FDP) are presented for three-branch parallel manipulators. Considered are all three-branch manipulators where each branch is comprised of a serial arrangement of three main-arm joints supporting a common payload platform through a passive spherical joint group at the branch end. All feasible combinations of revolute and prismatic joints and all feasible combinations of sensing of the main-arm joints are considered. Intersection of loci defining the feasible locations of the branch ends considering individual branches and branch combinations is utilized to identify FDP solutions. It is demonstrated that analytical FDP solutions can be found for all cases of redundant sensing (seven, eight or nine sensors) of the main-arm joint displacements. Furthermore, it is shown that with asymmetric distribution of non-redundant sensing (six sensors) an analytical solution can also be found. The maximum number of assembly modes and necessary conditions required for the maximum numbers are identified for all cases. Appropriate sensing redundancy is concluded to be an important consideration in the design of parallel manipulators since it facilitates an ability to have direct analytical FDP solutions; a reduction of the potential assembly modes; and the potential of sensor-failure-safe implementations.

Configuration engine for architecture planning of modular parallel robots

With the recent advances in research on parallel robots, the potential use of parallel manipulators has been expanding to both terrestrial and space applications including areas such as high speed manipulation, material handling, motion platforms, machine tools, medical fields, planetary exploration and so on. Therefore the need for methodologies for the systematic design of high performance parallel architecture manipulators increases. This article concentrates on the synthesis of parallel manipulators using a combination of genetic algorithm and simulated annealing optimization methods and a configuration engine capable of generating and designing different symmetric parallel manipulator architectures for a given task. Optimum parallel configurations are determined for a set of modular components, a specific task, and a set of criteria. The article also describes the application of the methodology as a design tool, and the results from a test case and a case study conducted with the program.

Extension of Antipodal Theorem to Workspace Analysis of Planar Wire-Actuated Manipulators

The wrench-closure workspace of planar wire-actuated parallel manipulators is investigated utilizing the antipodal method from multi-finger grasping and verified by the null space method from static analysis. The antipodal theorem is extended to analyzing planar manipulators with wires at distinct attachment points and external force/gravity as an additional wire. It is discussed that the null space method gives a more realistic workspace formulation as it takes into account wire tension limits. The antipodal method is superior for the workspaces analysis if large wire tensions are possible.

Failure recovery for wrench capability of wire-actuated parallel manipulators

Wire-actuated parallel manipulators and their failures are studied in this paper taking into consideration their failure modes. A methodology for investigating the effect of wire/actuator failures on the force/moment capability of manipulators is presented, and the criteria for full and partial recovery from these failures are established. The methodology is also applicable for the cases that the minimum norm solution for the vector of wire tensions gives a negative value for tension by treating the corresponding wire as failed. The proposed criteria are also valid for the manipulators that utilize hybrid actuation of wires and joints. Three planar wire-actuated parallel manipulators are used as the case study to illustrate the proposed methodology and criteria.

Optimal design and sensitivity analysis of flexible cam mechanisms

Abstract The optimum design of cam-follower mechanisms is considered in this paper. The design methodology proposed here leads to optimum values for the following design parameters: cam base-circle radius, follower roller radius, follower offset, cam thickness, return spring stiffness and initial compression for minimum cam volume, for the case in which structural flexibility effects of the cam mechanism are considered. It is shown that an optimal design based only on kinematic motion of the cam mechanism may be unacceptable when implemented on a real cam mechanism due to excitation of structural flexibility effects. These structural flexibility effects lead to large accelerations of the cam-follower which results in excessive surface stresses on the cam and follower roller. Such exceissve stress may lead to premature mechanical failure of the mechanism. To overcome this problem, a design methodology is proposed here to overcome this problem. The design process models the structural flexibility of the cam-follower mechanism and leads to a design which a sufficiently large margin of safety to ensure acceptable performance of the system. While a satisfactory approach to this design problem exists if structural dynamics stiffness and damping coefficients are known, variation in these parameters from their nominal values may lead to an unacceptable design when implemented. To investigate the nature of the effect of uncertainty in these parameters on the cam-follower performance, a sensitivity analysis is performed. The effect of variations in various structural dynamic parameters on the dynamic response of the flexible cam mechanism (displacement, velocity and acceleration of the follower) and also on the minimum and maximum values of the contact force are determined. Numerical simulation results support the results given in this paper.

Kinematic calibration of parallel manipulators

Identification objective functions considering end effector pose errors and loop closure (branch end distance) errors for the calibration of parallel manipulators are discussed. Based on the elimination of the need for fixturing devices, loop closure error based objectives are demonstrated preferred. Kinematic calibration models for the RSI 6 degree of freedom hand controller including device geometric parameters are introduced. Calibrations considering more complete models are demonstrated to yield improved calibrations in comparison to models considering only original length, angle, and potentiometer parameters. To achieve higher precision, it is concluded that noise-free joint displacement sensing, and accurate passive spherical branch end joints are required.