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Research Papers

Optimal Design of Self-Adaptive Fingers for Proprioceptive Tactile Sensing

[+] Author and Article Information
Bruno Belzile

Robotics Laboratory,
Department of Mechanical Engineering,
Polytechnique Montreal,
Montreal, QC H3T 1J4, Canada
e-mail: bruno.belzile@polymtl.ca

Lionel Birglen

Robotics Laboratory,
Department of Mechanical Engineering,
Polytechnique Montreal,
Montreal, QC H3T 1J4, Canada
e-mail: lionel.birglen@polymtl.ca

1Corresponding author.

Manuscript received October 3, 2016; final manuscript received May 11, 2017; published online August 4, 2017. Assoc. Editor: Jun Ueda.

J. Mechanisms Robotics 9(5), 051004 (Aug 04, 2017) (11 pages) Paper No: JMR-16-1289; doi: 10.1115/1.4037113 History: Received October 03, 2016; Revised May 11, 2017

The sense of touch has always been challenging to replicate in robotics, but it can provide critical information when grasping objects. Nowadays, tactile sensing in artificial hands is usually limited to using external sensors which are typically costly, sensitive to disturbances, and impractical in certain applications. Alternative methods based on proprioceptive measurements exist to circumvent these issues but they are designed for fully actuated systems. Investigating this issue, the authors previously proposed a tactile sensing technique dedicated to underactuated, also known as self-adaptive, fingers based on measuring the stiffness of the mechanism as seen from the actuator. In this paper, a procedure to optimize the design of underactuated fingers in order to obtain the most accurate proprioceptive tactile data is presented. Since this tactile sensing algorithm is based on a one-to-one relationship between the contact location and the stiffness measured at the actuator, the accuracy of the former is optimized by maximizing the range of values of the latter, thereby minimizing the effect of an error on the stiffness estimation. The theoretical framework of the analysis is first presented, followed by the tactile sensing algorithm, and the optimization procedure itself. Finally, a novel design is proposed which includes a hidden proximal phalanx to overcome shortcomings in the sensing capabilities of the proposed method. This paper demonstrates that relatively simple modifications in the design of underactuated fingers allow to perform accurate tactile sensing without conventional external sensors.

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References

Figures

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Fig. 1

Typical closing sequence of an underactuated finger: (a) no contact, (b) first contact, and (c) second contact

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Fig. 2

Typical stiffness profile during grasping. The stiffness shown here is considered from the actuator’s perspective: (a) no contact, (b) first contact, and (c) second contact.

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Fig. 3

Geometric model of an underactuated finger

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Fig. 4

Geometry of the S-class linkage-driven finger: (a) only θ1 is unlocked; (b) only θ2 is unlocked; (c) only θn−1 is unlocked; (d) only θn is unlocked

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Fig. 5

Da Vinci’s mechanism

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Fig. 6

Optimal transmission factor for a 2DOF underactuated finger

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Fig. 7

Infinite instantaneous stiffness configurations

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Fig. 8

X2−θ2−θ3 relationship for an ideal 3DOF transmission (λ = 1)

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Fig. 9

X3−θ2−θ3 relationship for an ideal 3DOF transmission (λ = 1)

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Fig. 10

Normality conditions for approximate geometrical synthesis

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Fig. 11

Optimization procedure

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Fig. 12

Closing motion of an optimized 2DOF linkage-driven underactuated finger. The circles represent the location of the equilibrium point (infinite instantaneous stiffness).

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Fig. 13

Contact location relative error as a function of the instantaneous stiffness variation. The variables Δk2, ΔKc, and Kc¯ are, respectively, the error on the estimated contact location, the error on the instantaneous stiffness, and the exact instantaneous stiffness.

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Fig. 14

Closing motion of an optimized 2DOF da Vinci type finger (λ=1.2). The circles represent the location of the equilibrium point (infinite instantaneous stiffness).

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Fig. 15

Variation of the mean value of λ as a function of b1 for the complete closing sequence

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Fig. 16

Closing motion of an optimized 3DOF linkage-driven finger (λ = 1). As can be seen, the two conditions to obtain an infinite stiffness configuration are for all practical purposes always fulfilled.

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Fig. 17

Contact location relative error as a function of the instantaneous stiffness disparity for the distal phalanx

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Fig. 18

Underactuated finger with a hidden proximal phalanx

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Fig. 19

Comparison between the original and optimized fingers

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