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Research Papers

A Novel Compact Torsional Spring for Series Elastic Actuators for Assistive Wearable Robots

[+] Author and Article Information
Giorgio Carpino

Postdoctoral Fellow
e-mail: g.carpino@unicampus.it

Dino Accoto

Assistant Professor
e-mail: d.accoto@unicampus.it

Fabrizio Sergi

Postdoctoral Fellow
e-mail addresses: fabrizio.sergi@unicampus.it
and fabs@rice.edu

Nevio Luigi Tagliamonte

Postdoctoral Fellow
e-mail: n.tagliamonte@unicampus.it

Eugenio Guglielmelli

Full Professor
e-mail: e.guglielmelli@unicampus.it
Laboratory of Biomedical Robotics
and Biomicrosystems,
Center for Integrated Research,
University Campus Bio-Medico di Roma,
via Álvaro del Portillo, 21, Rome, 00128 Italy

1Giorgio Carpino and Dino Accoto equally contributed to this work and should be considered co-first authors.

2Fabrizio Sergi is now with the MEMS Department, Rice University, Houston, TX 77005.

Contributed by the Design Innovation and Devices of ASME for publication in the JOURNAL OF MECHANICAL DESIGN. Manuscript received September 26, 2011; final manuscript received August 30, 2012; published online October 19, 2012. Assoc. Editor: Mary Frecker.

J. Mech. Des 134(12), 121002 (Oct 19, 2012) (10 pages) doi:10.1115/1.4007695 History: Received September 26, 2011; Revised August 30, 2012

The introduction of intrinsic compliance in the actuation system of assistive robots improves safety and dynamical adaptability. Furthermore, in the case of wearable robots for gait assistance, the exploitation of conservative compliant elements as energy buffers can mimic the intrinsic dynamical properties of legs during locomotion. However, commercially available compliant components do not generally allow to meet the desired requirements in terms of admissible peak load, as typically required by gait assistance, while guaranteeing low stiffness and a compact and lightweight design. This paper presents a novel compact monolithic torsional spring to be used as the basic component of a modular compliant system for series elastic actuators. The spring, whose design was refined through an iterative FEA-based optimization process, has an external diameter of 85 mm, a thickness of 3 mm and a weight of 61.5 g. The spring, characterized using a custom dynamometric test bed, shows a linear torque versus angle characteristic. The compliant element has a stiffness of 98N·m/rad and it is capable of withstanding a maximum torque of 7.68N·m. A good agreement between simulated and experimental data were observed, with a maximum resultant error of 6%. By arranging a number of identical springs in series or in parallel, it is possible to render different torque versus angle characteristics, in order to match the specific applications requirements.

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References

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Figures

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Fig. 1

General torque control scheme for a rotary SEA. τd represents the desired torque, ks the spring stiffness, Δθ the spring deflection, τl the output torque and θl the output angle.

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Fig. 2

Examples of compliant components for rotary SEA prototypes: [11] (a), [12] (b), [18] (c), [10] (d), [14] (e), and [16] (f)

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Fig. 3

Flowchart describing the spring design methodology

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Fig. 4

Elastic modules topologies taken into account. The solid circles represent the rigid inner and outer rings, while dotted ellipses represent the flexible elements.

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Fig. 5

An arbitrary morphology based on topology 3. Spring thickness is 3 mm.

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Fig. 6

Boundary conditions and swept mesh. The internal ring is fixed (double dashed line) while tangential forces (solid arrows) are applied to the outer ring. Dimensions are in [m].

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Fig. 7

Specific energy for the three investigated topologies. For each topology the best morphological implementation, obtained through the design refinement process, is considered. No morphology deriving from topology 2 reached the design objectives after the specified maximum number of design iterations (20).

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Fig. 8

von Mises stress in Pascal and 1:1 module deformation. Dimensions in [m].

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Fig. 9

Frontal view of the custom-made torsional elastic module. Diameter: 85 mm. Thickness: 3 mm. Weight: 61.5 g.

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Fig. 10

Schematic representation of the operating principle of the custom-made dynamometric test bed. (a) wire connected to Instron testing machine, (b) pulley, (c) primary shaft, (d) secondary shaft, (e) gear transmission, (f) external ring, (g) torsional elastic elements, (h) internal ring, and (i) encoder.

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Fig. 11

3D rendering of the dynamometric test bed (frame surfaces in transparency and wire connected to Instron not shown). (a) pulley, (b) primary shaft, (c) secondary shaft, (d) gear transmission, (e) torsional elastic module, (f) encoder, and (g) frame.

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Fig. 12

Elastic module in unloaded state (left) and in deformed state (right), applied torque: 7.68 N·m

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Fig. 13

Simulated data are represented with square dots, while experimental data are represented with circle dots. The stiffness (calculated as the inverse of the slope of the linear regression curves) is equal to 92 N·m/rad for the simulated elastic module and 98 N·m/rad for the real device.

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Fig. 14

(a) Section view of a parallel configuration of two basic elastic modules. 1, 2: elastic modules; 3: output shaft connecting the internal rings of the springs with the load; 4: flange connecting the external rings of the modules and the gearmotor. Using the basic element (specs reported in Table 4), the equivalent output stiffness is 196 N·m/rad and the maximum supported torque 15.36 N·m. (b) Section view of a design including the series connection of two parallel modules packs. 5, 6: parallel modules packs; 7: shaft connecting the internal rings of the two modules packs; 8: flanges acting as input and output ports of the elastic system and connect the external rings of the two modules packs. The equivalent stiffness between input and output is 98 N·m/rad and the maximum supported torque 15.36 N·m.

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