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Research Papers

Series Elastic Actuators for Small-Scale Robotic Applications

[+] Author and Article Information
Priyanshu Agarwal

ReNeu Robotics Lab,
Department of Mechanical Engineering,
The University of Texas at Austin,
Austin, TX 78712
e-mail: ashish@austin.utexas.edu

Ashish D. Deshpande

Mem. ASME
ReNeu Robotics Lab,
Department of Mechanical Engineering,
The University of Texas at Austin,
Austin, TX 78712
e-mail: ashish@austin.utexas.edu

1Corresponding author.

Manuscript received June 10, 2016; final manuscript received January 23, 2017; published online March 24, 2017. Assoc. Editor: Satyandra K. Gupta.

J. Mechanisms Robotics 9(3), 031016 (Mar 24, 2017) (12 pages) Paper No: JMR-16-1168; doi: 10.1115/1.4035987 History: Received June 10, 2016; Revised January 23, 2017

Torque control of small-scale robotic devices such as hand exoskeletons is challenging due to the unavailability of miniature and compact bidirectional torque actuators. In this work, we present a miniature Bowden-cable-based series elastic actuator (SEA) using helical torsion springs. The three-dimensional (3D) printed SEA is 38 mm × 38 mm × 24 mm in dimension and weighs 30 g, excluding motor which is located remotely. We carry out a thorough experimental testing of our previously presented linear compression spring SEA (LC-SEA) (Agarwal et al. 2015, “An Index Finger Exoskeleton With Series Elastic Actuation for Rehabilitation: Design, Control and Performance Characterization,” Int. J. Rob. Res., 34(14), pp. 1747–1772) and helical torsion spring SEA (HT-SEA) and compare the performance of the two designs. Performance characterization on a test rig shows that the two SEAs have adequate torque source quality (RMSE < 12% of peak torque) with high torque fidelities (>97% at 0.5 Hz torque sinusoid) and force tracking bandwidths of 2.5 Hz and 4.5 Hz (0.2 N·m), respectively, which make these SEAs suitable for our application of a hand exoskeleton.

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Figures

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Fig. 1

Schematic for the two SEA designs–linear compression spring SEA (LC-SEA) and helical torsion spring SEA (HT-SEA)

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Fig. 2

Two Bowden-cable-based SEA designs: (a) LC-SEA and (b) HT-SEA

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Fig. 3

Two Bowden-cable-based SEA prototypes as attached to the mechanical breadboard for testing: (a) LC-SEA and (b) HT-SEA

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Fig. 4

Schematic of HT-SEA miniature Bowden-cable-based SEA mechanism. Refer to Fig. 2 for details on the actual design including the attachment of the springs.

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Fig. 5

The schematic of the torque controller implemented on the SEA test rig. The inner loop represents the position control implemented in the motor driver. The outer loop refers to the control loop implemented for output torque tracking.

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Fig. 6

The joint torque tracking performance for the PID controller with sinusoidal torque input in simulation: (a) output joint torque trajectory, (b) motor angle trajectory, (c) motor side cable tension, and (d) joint side cable tension. An effective spring stiffness of 2000 N/m or 0.3 N·m/rad is used for these simulations.

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Fig. 7

The test rig developed to assess the torque tracking performance of the SEAs. A 1 -m long Bowden-cable sheath separated the motor side and joint side, which were mounted on two different mechanical breadboards.

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Fig. 8

The index finger module of a hand exoskeleton prototype used for experimentation. The module is controlled using Bowden-cable-based SEAs with remotely located motors. Magnetoresistive angle sensors are used for measuring the exoskeleton joint angles.

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Fig. 9

The joint torque tracking performance of the two SEAs with sinusoidal desired torque trajectory: (a) output joint torque trajectory comparison for LC-SEA and (b) output joint torque trajectory comparison for HT-SEA. The identified stiffness value used for evaluating the estimated torque for LC-SEA and HT-SEA was k = 1103 N/m and kj = 0.265 N·m/rad, respectively. The torque measured using the load cell, desired torque trajectory as available to the real-time controller and the torque trajectory as estimated by the real-time controller using SEA are referred to as measured, desired, and estimated torques, respectively.

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Fig. 10

Bode plot of the two SEAs: (a) magnitude and phase of LC-SEA and (b) magnitude and phase of HT-SEA

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Fig. 11

Comparison of the identified system response with the measured and desired torque trajectories for a portion of the applied chirp signal: (a) fifth-order system response for LC-SEA and (b) fourth-order system response for HT-SEA

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Fig. 12

The joint torque tracking performance of HT-SEA for a desired torque trajectory of sinusoidal nature

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Fig. 13

The joint torque tracking performance of LC-SEA under varying degrees of disturbance: (a) mild, (b) medium, and (c) severe

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Fig. 14

Results from stiffness control of the PIP joint of the index finger exoskeleton with low stiffness (kpip = 0.1 N·m/rad). Exoskeleton PIP joint: (a) relative angle (θpip,r) and (b) torque (τpip,exo).

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Fig. 15

Results from stiffness control of the PIP joint of the index finger exoskeleton with high stiffness (kpip = 0.6 N·m/rad). Exoskeleton PIP joint: (a) relative angle (θpip,r) and (b) torque (τpip,exo).

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