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Robotics 2
Prof. Alessandro De Luca
Collision detectionand robot reaction
Handling of robot collisionsn safety in physical Human-Robot Interaction (pHRI)n robot dependability (i.e., beyond reliability)
n mechanics: lightweight construction and inclusion of compliancen next generation with variable stiffness actuation devicesn typically, more/additional exteroceptive sensing neededn human-oriented motion planning (“legible” robot trajectories) n control strategies with safety objectives/constraints
n prevent, avoid, detect and react to collisionsn possibly, using only robot proprioceptive sensors
n different phases in the collision event pipeline
PHRIENDSFP6 STREP(2006-09)
Robotics 2 2
SAPHARI FP7 IP
(2011-15) SYMPLEXITYH2020 IP (2015-18)
European projects that have funded our research developments
Collision event pipeline
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S. Haddadin, A. De Luca, A. Albu-Schäffer: “Robot Collisions: A Survey on Detection, Isolation, and Identification,” IEEE Trans. on Robotics, vol. 33, no. 6, pp. 1292-1312, 2017
Collision detection in industrial robotsn advanced option available for some robots (ABB, KUKA, UR ...)n allow only detection, not isolation
n based on large variations of control torques (or motor currents)
n based on comparison with nominal torques on a desired trajectory
n based on robot state and numerical estimate of acceleration
n based on the parallel simulation of robot dynamics
n sensitive to actual control law and reference trajectoryn require noisy acceleration estimates or on-line inversion of the
robot inertia matrixRobotics 2 4
⇔𝜏 𝑡$ − 𝜏(𝑡$'() ≥ 𝜀 𝜏, 𝑡$ − 𝜏,(𝑡$'() ≥ 𝜀,, for at least one joint 𝑖
⇒𝜏0 = 𝑀 𝑞0 �̈�0 + 𝑆 𝑞0, �̇�0 �̇�0 + 𝑔 𝑞0 + 𝑓(𝑞0, �̇�0) 𝜏 − 𝜏0 ≥ 𝜀
⇒ ⇒𝜏: = 𝑀 𝑞 �̈�: + 𝑆 𝑞, �̇� �̇� + 𝑔 𝑞 + 𝑓(𝑞, �̇�) 𝜏 − 𝜏: ≥ 𝜀�̈�: =𝑑�̇�𝑑𝑡
⇒�̈�< = 𝑀'( 𝑞 𝜏 − 𝑆 𝑞, �̇� �̇� − 𝑔 𝑞 − 𝑓(𝑞, �̇�) �̇� − �̇�< ≥ 𝜀=̇, 𝑞 − 𝑞< ≥ 𝜀=
ABB collision detectionn ABB IRB 7600 video
n the only feasible robot reaction is to stop!Robotics 2 5
n robot model with (possible) collisions
n collisions may occur at any (unknown) location along the whole robotic structure
n simplifying assumptions (some may be relaxed)n manipulator is an open kinematic chainn single contact/collisionn negligible friction (or has to be identified and used in the model)
Collisions as system faults
with transpose of the Jacobianassociated to the contact point/area
control torque
joint torque caused by link collisioninertiamatrix
Coriolis/centrifugal(with “good” factorization):�̇� − 2𝑆 is skew-symmetric
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Analysis of a collision
in dynamic conditions a contact force/torqueon 𝑖th link produces
accelerationsat ALL joints
in static conditions a contact force/torqueon 𝑖th link is balanced
ONLY by torques at preceding joints 𝑗 ≤ 𝑖
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Relevant dynamic properties
n total energy and its variation
n generalized momentum and its decoupled dynamics
using the skew-symmetric property
NOTE: this is a vector versionof the same formula already encountered in actuator FDI
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Ex: prove this expression!
Monitoring collisions
Normal Modeof Operation
CollisionMonitor
Detection Isolation
Collision recognized
σ
|σ| ≥ σcoll
∥r∥ ≥ rcoll
r
NO
NOYES
YES
τ
without externalor contact sensors
usedeactivate/activate
continue
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.q, q
Reaction Strategy
Energy-based detection of collisions
n scalar residual (computable)
n … and its dynamics (needed only for analysis)
a stable first-order linear filter, excited by a collision!
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also via N-E algorithm!
Block diagram of residual generatorenergy-based scalar signal
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robot ∫+
+
∫+
−
++
scalarresidual generator
initializationof integratorB𝜎 0 = 𝐸(0)(zero if robotstarts at rest)
rigid robot with possible extra torque due to collision
B𝜎 0
Analysis of the energy-based methodn very simple scheme (scalar signal) n it can only detect the presence of collision
forces/torques (wrenches) that produce work on the linear/angular velocities (twists) at the contact
n does not succeed when the robot stands still…
Robotics 2 12
Collision detectionsimulation with a 7R robot
detection of a collision with a fixed obstacle in the work spaceduring the execution of a Cartesian trajectory (redundant robot)
signal back to zero after contact is over
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obstacle
Cartesian path
contact detectedwhen exceeding
a threshold
Collision detection experiment with a 6R robot
robot at rest or moving under Cartesian impedance control
on a straight horizontal line(with a F/T sensor at wrist for analysis)
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6 phases A: contact force applied is acting against
motion direction ⇒ detectionB: no force applied, with robot at restC: force increases gradually, but robot is
at rest ⇒ no detectionD: robot starts moving again, with force
being applied ⇒ detectionE: robot stands still and a strong force is
applied in 𝑧-direction ⇒ no detectionF: robot moves, with a 𝑧-force applied
≈ orthogonal to motion direction ⇒poor detection
Momentum-based isolation of collisions
n residual vector (computable)
n … and its decoupled dynamics
(diagonal)
𝑁 first-order, linear filters with unitary gains, excited by a collision!(all residuals go back to zero if there is no longer contact = post-impact phase)Robotics 2 15
in case, needs modified N-E algorithm!
Block diagram of residual generatormomentum-based vector signal
Robotics 2 16
robot ∫+
+
∫+
−
+ ++
residualgenerator
rigid robot with possible extra torque due to collision
�̂� 0initializationof integrators�̂� 0 = 𝑝(0)(zero if robotstarts at rest)
Analysis of the momentum method
n residual vector contains directional information on the torque at the robot joints resulting from link collision (useful for robot reaction in post-impact phase)
n ideal situation (no noise/uncertainties)
n isolation property: collision has generically occurred in an area located up to the 𝑖th link if
Robotics 2 17
Safe reaction to collisions
Normal Modeof Operation
CollisionMonitor
Collision recognized
Reaction Strategy
σ r
NOYES use
deactivatere-activate
continue
internal robot state and control command
τ
without external or contact sensors
𝝉𝑹
Robotics 2 18
Robot reaction strategy
n upon detection of a collision ( is over some threshold)n no reaction (strategy 0): robot continues its planned motion…n stop robot motion (strategy 1): either by braking or by
stopping the motion reference generator and switching to a high-gain position control law
n reflex* strategy: switch to a residual-based control law
n “zero-gravity” control in any operative mode
“joint torque command in same direction of collision torque ”(diagonal)
* = in robots with transmission/joint elasticity, the reflex strategy can be implemented in different ways (strategies 2, 3, 4)
Robotics 2 19
Analysis of the reflex strategy
“a lighter robot that can be easily pushed way”
n in ideal conditions, this control strategy is equivalent to a reduction of the effective robot inertia as seen by the collision force/torque
from a cow … ... to a frog!
Robotics 2 20
DLR LWR-III robot dynamicsn lightweight (14 kg) 7R anthropomorfic robot with
harmonic drives (elastic joints) and joint torque sensors
n proprioceptive sensing: motor positions and joint elastic torques
motor torques commands joint torques due to linkcollision
elastic torques at the joints
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friction at link side is negligible!
Exploded joint of LWR-III robot
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source of joint elasticity
Collision isolation for LWR-III robot elastic joint case
§ few alternatives for extending the rigid case results§ for collision isolation, the simplest one takes advantage of the
presence of joint torque sensors“replace the commanded torque to the motorswith the elastic torque measured at the joints”
n other alternatives use § link+motor position measures ⇒ needs knowledge also of joint stiffness K§ link+motor momentum + commanded torque ⇒ affected by motor friction
n motion control is more complex in the presence of joint elasticity n different active strategies of reaction to collisions are possible
Robotics 2 23
Control of DLR LWR-III robot elastic joint case
n general control law using full state feedback(motor position and velocity, joint elastic torque and its derivative)
n “zero-gravity” condition is realized only in a (quasi-static) approximate way, using just motor position measures
motorposition
error
elastic jointtorque ffwcommand
elastic jointtorque error
(diagonal) matrixof joint stiffness
motorposition
linkposition
Robotics 2 24
n strategy 2: floating reaction (robot ≈ in “zero-gravity”)
n strategy 3: reflex torque reaction (closest to the rigid case)
n strategy 4: admittance mode reaction (residual is used as the new reference for the motor velocity)
n further possible reaction strategies (rigid or elastic case) n based on impedance controln sequence of strategies (e.g., 4 + 2)n time scaling: stop/reprise of reference trajectory, keeping the pathn Cartesian task preservation (exploits robot redundancy by projecting
reaction torque in a task-related dynamic null space)
Reaction strategiesspecific for elastic joint robots
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Experiments with LWR-III robot “dummy” head
dummy head equippedwith an accelerometer
robot straighten horizontally,mostly motion of joint 1 @30o/sec
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Dummy head impact
strategy 2: floating reactionstrategy 0: no reactionplanned trajectory ends just afterthe position of the dummy head
impact velocity is rather low here andthe robot stops switching to float mode
Robotics 2 27
videovideo
Delay in collision detection
measured (elastic)joint torqueresidual r1
0/1 index fordetectiondummy head acceleration
impact withthe dummy head
2-4 msec!
gain KI = diag{25}
threshold = 5-10% ofmax rated torque
Robotics 2 28
Experiments with LWR-III robot balloon impact
possibility of repeatablecomparison of different
reaction strategies at high speed conditions
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Balloon impact
coordinatedjoint motion@90o/sec
strategy 4: admittance mode reaction
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video
Experimental comparison of strategiesballoon impact
n residual and velocity at joint 4 with various reaction strategies
impact at 90o/sec with coordinated joint motion
0: no reaction
1: motion stop
4: the fastest in stopping the robot
in post-impact phase
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Human-Robot Interaction ⎯ 1
n first impact @60o/sec
strategy 4: admittance mode strategy 3: reflex torque
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videovideo
Human-Robot Interaction ⎯ 2
n first impact @90o/sec
strategy 3: reflex torqueRobotics 2 33
video
“Portfolio” of reaction strategiesresidual amplitude ∝ severity level of collision
Stop Reflex Preserve
Cartesian trajectory(use of redundancy)
RepriseReaction
Taskrelaxation
Cartesian path(time scaling)
all transitions are controlled by
suitable thresholds
on the residuals
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Experiments with LWR-III robot time scaling
n robot is position-controlled (on a given geometric path)n timing law slows down, stops, possibly reverses (and then reprises)
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Reaction with time scaling
video
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Use of kinematic redundancyn collision detection ⇒ robot reacts so as to preserve as
much as possible (and if possible at all) execution of the planned Cartesian trajectory for the end-effector
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instant of collision detection
Task kinematics
§ task coordinates with (redundancy)
§ (all) generalized inverses of the task Jacobian
§ all joint accelerations realizing a desired task acceleration (at a given robot state)
arbitrary jointacceleration
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Dynamic redundancy resolution
§ all joint torques realizing a precise control of the desired (Cartesian) task
set for compactness
for any generalized inverse G, the joint torque has two contributions: one imposes the task acceleration control, the other does not affect it
arbitrary joint torqueavailable for reaction to collisions
projection matrix in the dynamic null space of J
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Dynamically consistent solutioninertia-weighted pseudoinverse
§ the most natural choice for matrix G is to use the dynamically consistent generalized inverse of J
§ in a dual way, denoting by H a generalized inverse of JT, the joint torques can in fact be always decomposed as
§ the inertia-weighted choices for H and G are then
§ thus, the dynamically consistent solution is given by
Robotics 2 40
Cartesian task preservation
n wish to preserve the whole Cartesian task (end-effector position & orientation)reacting to collisions by using only self-motions in the joint space
n if the residual (∝ contact force) grows too large, orientation is relaxed first and then, if necessary, the full task is abandoned (priority is given to safety)
spherical obstacle
simulation in Simulinkvisualization in VRML
De Luca, Ferrajoli @IROS 2008
video
Robotics 2 41
Cartesian task preservationExperiments with LWR4+ robot
video @IROS 2017
Robotics 2 42idle ⇔ relax ⇔ abort
Combined use6D F/T sensor at the wrist + residuals
n enables easy distinction of intentional interactions vs. unexpected collisionsn it is sufficient to include the F/T measure in the expression of the residual!Robotics 2 43
HRI/HRC in closed control architecturesKUKA KR5 Sixx R650 robot
Robotics 2 44
§ low-level control laws are not known nor accessible by the user: no current or torque commands can be used
§ user programs, based also on other exteroceptive sensors (vision, Kinect, F/T sensor) can be implemented on an external PC via the RSI (RobotSensorInterface), communicating with the KUKA controller every 12 ms
§ robot measures available to the user: joint positions (by encoders) and [absolute value of] motor currents
§ controller reference is given as a velocity or a position in joint space (also Cartesian commands are accepted)
motor currents measured on first three joints
Collision detection and stop
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high-pass filtering of motor currents (a signal-based detection...)
video @ICRA 2013
Distinguish accidental collisions fromintentional contact and then collaborate
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with both high-pass and low-pass filtering of motor currents - here collaboration mode is manual guidance of the robot
video @ICRA 2013
Other possible robot reactionsafter collaboration mode is established
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video@ICRA2013
collaboration mode:pushing/pulling
the robot
collaboration mode:compliant-likerobot behavior
video@ICRA2013
Bibliography
Robotics 2 48
n A. De Luca and R. Mattone, “Sensorless robot collision detection and hybrid force/motion control,” IEEE Int. Conf. on Robotics and Automation, pp. 999-1004, 2005.
n A. De Luca, A. Albu-Schäffer, S. Haddadin, and G. Hirzinger, “Collision detection and safe reaction with the DLR-III lightweight manipulator arm,” IEEE/RSJ Int. Conf. on Intelligent Robots and Systems, pp. 1623-1630, 2006.
n S. Haddadin, A. Albu-Schäffer, A. De Luca, and G. Hirzinger, “Collision detection and reaction: A contribution to safe physical human-robot interaction,” IEEE/RSJ Int. Conf. on Intelligent Robots and Systems, pp. 3356-3363, 2008 (IROS 2008 Best Application Paper Award).
n A. De Luca and L. Ferrajoli, “Exploiting robot redundancy in collision detection and reaction,” IEEE/RSJ Int. Conf. on Intelligent Robots and Systems, pp. 3299-3305, 2008.
n A. De Luca and L. Ferrajoli, “A modified Newton-Euler method for dynamic computations in robot fault detection and control,” IEEE Int. Conf. on Robotics and Automation, pp. 3359-3364, 2009.
n S. Haddadin, “Towards Safe Robots: Approaching Asimov’s 1st Law,” PhD Thesis, Univ. of Aachen, 2011.
n M. Geravand, F. Flacco, and A. De Luca, “Human-robot physical interaction and collaboration using an industrial robot with a closed control architecture,” IEEE Int. Conf. on Robotics and Automation, pp. 3985-3992, 2013.
n E. Magrini, A. De Luca, “Human-robot coexistence and contact handling with redundant robots,” IEEE/RSJ Int. Conf. on Intelligent Robots and Systems, pp. 4611-4617, 2017.
n S. Haddadin, A. De Luca, and A. Albu-Schäffer, “Robot collisions: A survey on detection, isolation, and identification,” IEEE Trans. on Robotics, vol. 33, no. 6, pp. 1292-1312, 2017.