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  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    A Physically-BasedFault Detection and Isolation Methodand Its Uses in Robot Manipulators

    Alessandro De LucaDipartimento di Informatica e SistemisticaUniversit di Roma La Sapienza

    currently on leave atInstitute of Robotics and Mechatronics

    DLR Oberpfaffenhofen

    38. VDI/VDE Sitzung des FA 4.13Steuerung und Regelung von Robotern

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    Outline

    FDI problems Robot dynamics and physical properties Detection and isolation of actuator faults Adaptive scheme for actuator FDI Collision detection and reaction Extension to robots with joint elasticity

    collision detection/reaction + motor friction compensation

    Conclusions

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    FDI problems

    Fault Detection recognizing that a fault is affecting a dynamic system

    Fault Isolation discriminating the occurrence of a fault f from that of all other

    considered possible faults and disturbances

    FDI solution approach (model-based) design a residual generator system whose output

    is only affected by the fault f to be detected and isolated is not affected by any other fault or disturbance converges (asymptotically) to zero whenever f = 0

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    Robot dynamic models

    fully rigid case

    presence of transmission/joint elasticity

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    Relevant physical properties

    kinetic and potential energy

    relation between inertia and velocity terms

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    Relevant physical properties (contd)

    total energy and its variation

    generalized momenta and their decoupled dynamics

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    Robot actuators FDI

    faulted model

    fault types

    total failure power loss saturation bias possibly concurrent, intermittent, incipient, abrupt,

    commanded torque

    fault torque

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    Basic assumptions

    full state measurements implementation with available sensors (typically, position only)

    robot dynamic model accurately known adaptation might be included for uncertain parameters

    use of detection thresholds to handle noise (false alarms)

    only commanded torque available (no fault model is needed)

    any control input law open or closed-loop, linear or nonlinear model-based feedback

    no need of a specified reference motion

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    Early solutions

    1. : compare computed model-based torque(from measures) with commanded one

    2. : compare simulated acceleration(inverse robot dynamics) with those frommeasurements

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    and their limitations

    noisy acceleration (e.g., from double numericaldifferentiation of position measures)

    inversion of inertia matrix intrinsic delay (one or more digital steps) dependence on commanded input dynamics poor or no fault isolation (only detection)

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    Energy-based fault detection

    scalar detector

    and its dynamics (needed only for analysis)

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    Momentum-based FDI

    vector of residuals

    and its decoupled dynamics (a stable first-orderlinear filter)

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    Experimental setup

    Quanser Pendubot2nd link

    (passive)

    1st link(actuated)

    video swing-up Pendubot

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    Actuator FDI on Pendubot

    partially concurrent 10% power loss on actuator 1 and total failure on(missing) actuator 2

    PID control on first joint to 30

    commanded torques joint positions

    joint 1 joint 2

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    Actuator FDI on Pendubot (contd)

    thresholding and dynamic filtering of residuals

    residuals filtered residuals

    joint 1 joint 2

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    An adaptive FDI scheme

    include friction (difficult to estimate) in the model

    linear parametrization (may be extended togravity and inertia-related terms)

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    Adapt and detect

    using an estimate of friction parameters

    residual dynamics

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    Adapt and detect (contd)

    stability analysis via standard Lyapunov and LaSalletechniques (in absence of faults)

    parameter estimates converge to constant values (=correct ones for sufficient excitation)

    by overparametrization and suitable gain scaling, one may stilladapt also during faults

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    Adaptive actuator FDI on Pendubot

    situation as before, with power loss increased to 50% on actuator 1 on-line adaptation of both friction and gravity parameters

    commanded torques residuals

    joint 1 joint 2

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    Collision as a fault

    rigid robot model

    use only proprioceptive sensors possible contact at any point along the arm simplifying assumptions

    single contact robot as open kinematic chain unfaulted actuators

    transpose ofcontact point Jacobian

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    Analysis of collisions

    q1d1

    d2FK

    FK

    q2x1

    y1

    x0

    y0

    x2y2

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    Collision detection

    as before, scalar detector

    only contact forces (wrenches) that perform workon contact velocity (twists) can be detected

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    Directional detection and isolation

    as before, vector of residuals

    ideal situation (no noise)

    collision point is located up to link i

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    Choice of residual gains

    evaluation by simulation on 7-dof DLR-III arm(impact on last link)

    joint 2@30/s

    joint 4@200/s

    10 ms

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    Collision reaction strategies

    normal operation in zero-gravity

    once collision is detected ( above threshold) either stop the robot (braking) and then possibly

    reverse commanded motion (backtracking) or apply a reflex strategy with torque control using

    directional information of residual vector(move in the same direction of sensed force)

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    Dissipating energy

    when contact is lost, the residual decays until

    dissipate kinetic energy at highest rate (usingmaximum available torque) until robot stops

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    Operative robot states

    normal operationin zero-gravity reflex reaction

    energy dissipation

    collision = 0

    collision = 1

    || residual || > low

    || residual || low

    velocity 0

    velocity = 0

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    Robots with elastic joints (EJ)

    harmonic drives introduce joint elasticity effects motor friction and possible arm collisions

    DLR-III arm: motor position and joint torque sensors

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    Multiple detection for EJ robots

    it is simultaneously possible to compensate friction (a fault) on motor side detect collision at link side

    1. unmodeled motor friction detection and compensation(based on motor generalized momenta)

    decentralized linear observer(includes acceleration estimation)

    motor friction compensation

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    Multiple detection for EJ robots (contd)

    collision detection: several alternatives are possible forgeneralizing the rigid case analysis, the most simple is

    2.

    replace joint to motor torque

    robot control laws should be modified (e.g., in DLR-III arm) reflex strategies to contact detection include

    torque mode reaction admittance mode reaction

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2006

    DLR-III robot controller

    motor inertia reduction based on joint torque sensing

    leads to with general position/torque control law (depending on reference

    and gain values)

    obtaining a full state feedback law

    static gravity compensation (based on motor position)

  • 38. VDI/VDE SitzungABB, Ladenburg, Germany 25 January 2

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