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Research Papers

A Compact, Modular Series Elastic Actuator

[+] Author and Article Information
Jonathan P. Cummings

Department of Mechanical and Industrial
Engineering,
University of Massachusetts Amherst,
160 Governors Drive,
Amherst, MA 01003
e-mail: cummij3@gmail.com

Dirk Ruiken

College of Information and Computer Sciences,
University of Massachusetts Amherst,
140 Governors Drive,
Amherst, MA 01003
e-mail: ruiken@cs.umass.edu

Eric L. Wilkinson

College of Information and Computer Sciences,
University of Massachusetts Amherst,
140 Governors Drive,
Amherst, MA 01003
e-mail: ewilkinson@cs.umass.edu

Michael W. Lanighan

College of Information and Computer Sciences,
University of Massachusetts Amherst,
140 Governors Drive,
Amherst, MA 01003
e-mail: lanighan@cs.umass.edu

Roderic A. Grupen

College of Information and Computer Sciences,
University of Massachusetts Amherst,
140 Governors Drive,
Amherst, MA 01003
e-mail: grupen@cs.umass.edu

Frank C. Sup, IV

Department of Mechanical and Industrial
Engineering,
University of Massachusetts Amherst,
160 Governors Drive,
Amherst, MA 01003
e-mail: sup@umass.edu

1Corresponding author.

Manuscript received June 12, 2015; final manuscript received March 6, 2016; published online April 1, 2016. Assoc. Editor: Jun Ueda.

J. Mechanisms Robotics 8(4), 041016 (Apr 01, 2016) (11 pages) Paper No: JMR-15-1140; doi: 10.1115/1.4032975 History: Received June 12, 2015; Revised March 06, 2016

This paper presents the development of a compact, modular rotary series elastic actuator (SEA) design that can be customized to meet the requirements of a wide range of applications. The concept incorporates flat brushless motors and planetary gearheads instead of expensive harmonic drives and a flat torsional spring design to create a lightweight, low-volume, easily reconfigurable, and relatively high-performance modular SEA for use in active impedance controlled devices. The key innovations include a Hall effect sensor for direct spring displacement measurements that mitigate the negative impact of backlash on SEA control performance. Both torque and impedance controllers are developed and evaluated using a 1-degree-of-freedom (DoF) prototype of the proposed actuator package. The results demonstrate the performance of a stable first-order impedance controller tested over a range of target impedances. Finally, the flexibility of the modular SEA is demonstrated by configuring it for use in five different actuator specifications designed for use in the uBot-7 mobile manipulator requiring spring stiffnesses from 3 N · m/deg to 11.25 N · m/deg and peak torque outputs from 12 N · m to 45 N · m.

Copyright © 2016 by ASME
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Figures

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Fig. 1

Schematic of an SEA

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Fig. 2

Cross section view of the prototype SEA joint

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Fig. 3

The pulley configuration for a full-range absolute position sensor

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Fig. 4

Joint sensor positions (repeating sawtooth patterns) and phase shift between the sensors

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Fig. 5

Spring geometry used in SEA [2,16]. The discoloration is a result of the heat treatment process.

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Fig. 6

Spring deflection sensor configuration with beam length, r, and linear sensor displacement, d, over the deflection range of the spring

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Fig. 7

The sensor movement (indicated by arrows) is shown within the magnet field of a rectangular neodymium magnet. The shading corresponds to the simulated magnitude of the magnetic flux density |B| computed using femm software [21].

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Fig. 8

The normal magnetic flux density versus position from simulation and experimentation on micrometer stage change proportionally to the magnetic field both in simulation and experiment. Simulation and experimental units are different because experimental results did not quantify the actual magnetic field strength.

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Fig. 9

The magnetic flux density in the Z-direction (normal to surface of the sensor) at maximum displacement (4 mm) is plotted as a function of the size of the air gap

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Fig. 10

SEA model used to derive equations of motion assumes that the output link is locked

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Fig. 11

Joint-level impedance control structure

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Fig. 12

Bode plot of SEA prototype for sinusoidal torque inputs at three magnitudes (1.0 N · m,2.0 N · m, and 4.0 N · m) and the open-loop response

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Fig. 13

Torque Bode plot of SEA prototype with spring constants of 3.6 N · m/deg and 7.2 N · m/deg

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Fig. 14

Bode plot of actuator torque with 1.0 N · m magnitude sine wave centered at zero causing backlash and centered at 5.0 N · m in order to eliminate backlash

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Fig. 15

Trajectory following at 0.5 Hz (top) and 1.0 Hz (bottom) with high impedance (KZ = 360 mN · m/deg and DZ  = 28.1 mN · m s/deg) and low impedance (Kz = 36 mN · m/deg and Dz = 7.3 mNm s/deg)

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Fig. 16

The SEA prototype with a low impedance setting causes the link to be obstructed (left). Increasing the control impedance overcomes the obstruction (right), shown after the object has been moved and the arm retracted.

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Fig. 17

A 0.25 Hz reference trajectory with Kz = 360 mN · m/deg and Dz = 28.1 mN · m s/deg (left) and Kz = 36 mN · m/deg and Dz = 7.3 mNm s/deg with a collision at approximately 5 s

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Fig. 18

Step response of the impedance controller to a 30 deg step in the vertical plane against gravity at low (36 mN · m/deg) and medium (180 mN · m/deg) stiffnesses

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Fig. 19

The impact response of the impedance controller at medium and low stiffnesses, 180 mN · m/deg and 36 mN · m/deg. The link encounters a fixed metal rod at approximately 110 deg. Note that the gap between measured and desired torque for the medium impedance case is due to the limitations on current drawn from the motor.

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Fig. 20

uBot-7 SEA joints in shoulder flexion/extension, elbow flexion/extension abduction/adduction, and humeral rotate

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