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Research Papers

Performance Indices for Collaborative Serial Robots With Optimally Adjusted Series Clutch Actuators

[+] Author and Article Information
Nicolas Lauzier

 Laboratoire de Robotique, Département de génie mécanique, Université Laval, Québec, Québec G1V 0A6, Canadanicolas.lauzier.1@ulaval.ca

Clément Gosselin

 Laboratoire de Robotique, Département de génie mécanique, Université Laval, Québec, Québec G1V 0A6, Canadagosselin@gmc.ulaval.ca

J. Mechanisms Robotics 4(2), 021002 (Mar 19, 2012) (11 pages) doi:10.1115/1.4005723 History: Received February 18, 2011; Revised January 02, 2012; Published March 12, 2012; Online March 19, 2012

Safety is the first priority when designing robots that are intended to physically interact with humans. New robotics standards state as a condition for collaboration that the robot should be designed so that it cannot exert forces larger than 150 N at its tool center point. An effective and reliable way of guaranteeing that this force cannot be exceeded is to place a torque limiter in series with each actuator, thus forming series clutch actuators (SCAs). Since the relationship between the joint limit torques and the achievable end-effector forces is configuration dependent, it is preferable to use adjustable torque limiters. This paper presents a method to optimally control the limit torques of a serial manipulator equipped with adjustable series clutch actuators. It also introduces two performance indices to evaluate the quality of the relationship between the joint limit torques and the achievable end-effector forces. The first one is the ratio of the minimum and maximum force thresholds. Even if it has a strong physical meaning, it is not differentiable everywhere in the workspace and is thus difficult to use in an optimization process based on its gradient. A second index, smooth, and expressed in a closed-form, is therefore introduced which is the determinant of the normalized Jacobian matrix postmultiplied by its transposed. Examples of redundant manipulator motion optimization and of collaborative robot architecture optimization using the second index are shown. The limitations of the proposed approach are that it is based on a static model—which is nevertheless valid under the current safety standards—and that gravity is neglected.

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Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Example of an implementation of series clutch actuators (pictures taken from Ref. [10])

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Figure 2

Achievable force limit imposed by one OASCA

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Figure 3

Examples of 2D achievable forces polytopes for manipulators with OASCAs

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Figure 4

The two indices μ and λ as a function of the orientation of the second column of the Jacobian for a planar 2-DOF manipulator

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Figure 5

Performance indices as a function of φ2 and φ3 for the case of a planar 3-DOF manipulator. It can be observed that the two indices behave similarly and that λ is differentiable everywhere whereas μ is not.

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Figure 6

Contour plots of the performance indices as a function of φ2 and φ3 for the case of a planar 3-DOF manipulator. It can be observed that the gradients of both indices are oriented similarly and that the global maxima (shown with markers) have the same locations for both indices.

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Figure 7

Relation between μ = Fmin /Fmax and γ, the largest angle between two subsequent (in terms of orientation) columns of the normalized Jacobian matrix

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Figure 8

Relationship between λ and μ during the optimization of the achievable force space using the gradient of λ for a spatial manipulator with n OASCAs

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Figure 9

Achievable force space which is optimal with respect to μ as obtained numerically for n = 6

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Figure 10

Achievable force space having the shape of a prism with a regular hexagonal base

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Figure 11

A 3-DOF redundant planar manipulator

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Figure 12

Comparison of the minimum force threshold at the TCP for the redundant manipulator and example trajectory with and without optimization

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Figure 13

Configurations and achievable force polygons after 75% of the trajectory for the least squares solution and the optimized case

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Figure 14

A 2-DOF planar manipulator with an additional optimally adjusted clutch

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