Motion Planning of Manipulators with Structural Safety as Priority


The production of lightweight robotic architecture has become a major issue due to increasing needs for high-speed motion, the reduction of energy use and the maintenance of safety. On the other hand, lightness tends to lower the structural strength of the architecture, which can lead to accidental destruction in a variety of situations. It is desirable for robotic components to become light and harmless and to be able to cope flexibly with various kinds of tasks and environments, particularly for those robots in the fields of welfare and life support, which must coexist with people. These multipurpose robots need appropriate structural design and also techniques to avoid structural damage by the external forces to which they are subjected, during their motion. Most past works on attitude determination and motion planning have been aimed at accomplishing the given tasks; such a focus is aimed at maintaining Asimovfs Second Law of Robotics, where robots are obliged to obey human orders as long as they do not conflict with the First Law (cf. do not harm humans). However, there are no works available on maintaining the Third Law, where robots are permitted to self-protect as long as such protection does not conflict with the First or Second Law.

In this research, a new strategy of motion planning concerning the Third Law of Robotics is discussed. The risk of structural damage is estimated in robotic architecture by the finite element method with Euler-Bernoulli beam elements, and the avoidance of structural damage precedes task accomplishment under certain circumstances. We define structural damage as the phase in which yielding occurs in structural members. The proposed strategy gives priority to two structural parameters. One is the risk of the structural member having a fully plastic section on a certain part due to subjected loads (the phase is generally described as gmember yieldingh), which is determined by calculating the yield function using resultant forces. The second is the total strain energy stored in the architecture, which is calculated using the stresses and strains occurring in all the structural members. Structural damage is avoided by changing the motion on the basis of the estimated parameters.

Three schemes with different combinations of structural parameters, as shown below, are developed and verified in this study.
EScheme A: Minimize total strain energy during motion.
EScheme B: Restrain resultant forces when the yield function exceeds a certain criterion.
EScheme C: Restrain total strain energy when the yield function exceeds a certain criterion.

Scheme A uses only one structural parameter, the total strain energy, which is information that expresses the condition of the whole architecture. Similarly, Scheme B uses only one structural parameter, the resultant force, which is information that expresses the condition of local parts in the whole architecture. Scheme C uses the latter local information for judging the structural danger, and uses the former global information as a restraining target. These three schemes are compared and numerically evaluated by assigning structurally severe tasks to manipulators.



Motion planning of three-link manipulator with structural safety as priority



Motion plan and experiments using robotic architecture



Changing motions due to mass of weight
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A motion-planning scheme concerning structural safety, using structural parameters as criteria and the restraining target, is implemented into a robotic arm interface. The numerical results show the difficulty of using only global information of the architecture, such as total strain energy, in seeking practical attitudes. However, the results also confirm that paths considering structural safety can be obtained effectively if local information such as the resultant forces is implemented as criteria. Furthermore, experiments on a robotic arm, particularly one with a low-stiffness link member, revealed the practicability of this scheme in avoiding structural damage to the constituent members. The scheme may be applicable in fields where robots must often perform irregular tasks.

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Related papers (Journals):

D.Isobe and Y.Ishii: Attitude Determination and Motion Planning of Robotic Architecture in View of Its Structural Strength, Journal of the Robotics Society of Japan, Vol.22, No.1, (2004), pp.75-82, in Japanese. abstract

D. Isobe and A. Komatsu: Motion Planning of Manipulators Regarding Structural Safety as a Prior Condition, Advanced Robotics, Vol.21, No.5-6, (2007), pp.533-554. abstract

D. Isobe and Y. Yamaguchi: A Motion Planning Scheme to Avoid Structural Damage of Robotic Arms, Transactions of the Japan Society of Mechanical Engineers, Series C, Vol. 78, No. 793, (2012), pp.3239-3254, in Japanese. abstract

Related papers (Proceedings):

Y.Ishii and D.Isobe: Attitude Determination for Restraining Strain Energy in Truss-Type Robotic Architecture, CD-ROM Proceedings of the JSME Annual Conference on Robotics and Mechatronics '01, No.01-4, (2001), in Japanese. abstract

Y.Ishii and D.Isobe: Attitude Determination and Motion Planning of Robotic Architecture for Restraining Strain Energy, Proceedings of the Conference on Computational Engineering and Science, Vol.7, No.2, (2002), pp.771-774, in Japanese. abstract

Y.Ishii and D.Isobe: Motion Planning of Manipulators for Restraining Strain Energy, CD-ROM Proceedings of the JSME Annual Conference on Robotics and Mechatronics '02, No.02-6, (2002), in Japanese. abstract

D.Isobe and Y.Ishii: Attitude Determination and Motion Planning of Mechanisms with Structural Safety as Priority |Proposal of Feeble Robotics|, Proceedings of the 8th Robotics Symposia, (2003), pp.533-538, in Japanese. abstract

A.Komatsu and D.Isobe: Motion Planning of Manipulators Using Structural Parameters, CD-ROM Proceedings of the JSME Annual Conference on Robotics and Mechatronics '04, No.04-4, (2004), in Japanese. abstract

A.Komatsu and D.Isobe: Motion Planning of Manipulators Considering Its Structural Risk, CD-ROM Proceedings of the 22nd Annual Conference of the Robotics Society of Japan 2004, (2004), in Japanese. abstract

A.Komatsu and D.Isobe: Motion Planning of Manipulators Considering Its Structural Risk, CD-ROM Proceedings of the 23rd Annual Conference of the Robotics Society of Japan 2005, (2005), in Japanese.abstract

A.Komatsu and D.Isobe: Motion Planning of Manipulators for Avoiding Structural Failure by Using Structural Parameters, Proceedings of the 18th JSME Computational Mechanics Conference, No.05-2, (2005), pp.711-712, in Japanese. abstract

A.Komatsu and D.Isobe: Motion Planning of Robotic Arms with Structural Safety as Priority, CD-ROM Proceedings of the JSME Annual Conference on Robotics and Mechatronics '06, No.06-4, (2006), in Japanese. abstract

Y. Yamaguchi and D. Isobe: Motion Planning of Robot Arm to Avoid Structural Damage - Introduction of Inertia -, CD-ROM Proceedings of the JSME Annual Conference on Robotics and Mechatronics '11, No.11-5, (2011), in Japanese. abstract

Y. Yamaguchi and D. Isobe: Motion Planning Scheme for Avoiding Structural Damage of Robotic Arms, CD-ROM Proceedings of the 29th Annual Conference of the Robotics Society of Japan 2011, (2011), in Japanese. abstract

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