Oscillation Damping, Collision Detection and Reaction for a Multi-Elastic-Link Robot Arm

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[IEEE CSS Video Clip Contest 2014 Submission]
In first place, elasticity in the links of robot arms and structurally comparable mechatronic systems such as construction machines, fire rescue turntable ladders, cherry pickers or automobile concrete pumps is a highly undesired effect. It prolongs settling times and deteriorates the positioning accuracy. Therefore substantial mechanical design efforts are commonly taken to avoid link elasticity in these mechanisms.

The presented work approaches from the contrary perspective and intentionally introduces intrinsic structural compliance in the links of an experimental robot platform. The motivation is to exploit the added intrinsic link compliance to reduce the overall robot weight, to cut costs, to add positioning tolerance as well as to add contact force sensing capabilities to the system. The video shows, how robust and rapid settling as well as disturbance rejection can still be accomplished by devising control algorithms [1,2] based on per link strain measurements.

In addition, the derivation and identification of mathematical models that accurately describe the load and joint configuration dependent static end effector deflections allows — through software — for the compensation of the inaccuracy of the mechanism [3]. The feasibility of time critical and precise end effector positioning for an elastic link arm has been exemplified with ball catching experiments before [4] (//www.youtube.com/watch?v=P4_i_kGt2jA).

With the mechanical imperfections compensated by the developed inner loop control software, the video demonstrates, how the intrinsic link compliance can be exploited to actively shape the apparent arm compliance, to sensitively sense contact forces, to safely react to accidental collisions as well as to enable intentional physical human machine interaction. The control scheme behind these features uses an identified model of the residual damped arm dynamics [5]. This model is way simpler to derive and identify than a holistic arm model including the oscillatory and actually infinite dimensional arm dynamics.
An identified linear mapping from the strain readings acquired close to the hubs on each passively compliant link and the motor torques turns each link into load side joint torque sensors. This way the video shows that collision detection and reaction techniques originally developed by other authors for rigid or elastic joint robots can be readily adopted for the use with elastic link robots [6].

The provided results imply that link elasticity is not necessarily just a problem. In contrast, the devised control concepts are able to compensate for the machine imperfections reveal promising new perspectives.

Time-line:
00:12 Introduction to the experimental setup
00:36 Exp I: Oscillation Damping: step motion
00:48 Exp II: Oscillation Damping: harmonic disturbance
02:10 Exp III: Collision detection and reaction: blunt impacts with a balloon
02:37 Exp IV: Collision detection and reaction: sharp impacts with a balloon
03:02 Exp V: Collision detection and reaction: sharp impacts with a Christmas ball
03:23 Exp VI: Collision detection and reaction: sharp impacts with a human arm
03:47 Exp VII: Interaction in zero gravity mode

For more information on the project please visit://www.rst.e-technik.tu-dortmund.de/cms/en/research/robotics/TUDOR_engl/index.html

References:
[1] Malzahn et al.: Vibration Control of a Multi-Flexible-Link Robot Arm under Gravity, IEEE ROBIO 2011.
[2] Malzahn, J. und T. Bertram: Fractional Order Strain Feedback for Oscillation Damping of a Multi-Elastic-Link Arm Under Gravity, ISR/ROBOTIK 2014
[3] Phung, A. S. et al.: Data Based Kinematic Model of a Multi-Flexible-Link Robot Arm for Varying Payloads, IEEE ROBIO 2011.
[4] Malzahn, J., et al.: A Multi-Link-Flexible Robot Arm Catching Thrown Balls. In ISR/ROBOTIK 2012.
[5] Malzahn, J., et al.: Dynamics Identification of a Damped Multi Elastic Link Robot Arm under Gravity, IEEE ICRA 2014.
[6] Malzahn, J. und T. Bertram: Collision Detection and Reaction for a Multi-Elastic-Link Robot Arm, IFAC World Congress 2014.

The Institute of Control Theory and Systems Engineering (RST) at TU Dortmund is engaged in fundamental research in the fields of robotics and computational intelligence. Furthermore the applied research focus is automotive systems and mechatronics. //www.rst.e-technik.tu-dortmund.de

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7 COMMENTS

  1. Hello Fotis Dimeas,
    in this video we don't use vision. However, we do that in the following works:

    Malzahn et al: Predictive Delay Compensation for Camera Based Oscillation Damping of a Multi Link Flexible Robot, ICIRA 2012.

    Malzahn et al: Scene Adaptive RGB-D Based Oscillation Sensing for a Multi Flexible Link Robot Arm in Unstructured Dynamic Environments, IROS 2012.

    There we compute a replacement signal for the individual strain measurements from the reconstructed camera motion.

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