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Research Papers

A Fixed Structure Gain Selection Strategy for High Impedance Series Elastic Actuator Behavior

[+] Author and Article Information
Kenan Isik

Department of Mechanical Engineering,
University of Texas at Austin,
Austin, TX 78712
e-mail: kisik@utexas.edu

Gray Cortright Thomas

Department of Mechanical Engineering,
University of Texas at Austin,
Austin, TX 78712
e-mail: gray.c.thomas@utexas.edu

Luis Sentis

Department of Aerospace Engineering and
Engineering Mechanics,
University of Texas at Austin,
Austin, TX 78712
e-mail: lsentis@austin.utexas.edu

1Corresponding author.

Contributed by the Dynamic Systems Division of ASME for publication in the JOURNAL OF DYNAMIC SYSTEMS, MEASUREMENT,AND CONTROL. Manuscript received February 6, 2018; final manuscript received August 24, 2018; published online October 10, 2018. Assoc. Editor: Dumitru I. Caruntu.

J. Dyn. Sys., Meas., Control 141(2), 021009 (Oct 10, 2018) (8 pages) Paper No: DS-18-1061; doi: 10.1115/1.4041449 History: Received February 06, 2018; Revised August 24, 2018

Series elastic actuators (SEA) are widely used for impact protection and compliant behavior, but they typically fall short in tasks calling for accurate position control. In this paper, we propose a simple and effective heuristic for tuning series elastic actuator controllers to a high impedance position control behavior, which compares favorably with previous publications. Our approach considers two models, an ideal model and a nonideal model with time delays and filtering lag. The ideal model is used to design cascaded proportional-derivative (PD)-type outer impedance and inner force loops as a function of critically damped closed-loop poles for the force and impedance loops. The nonideal model provides an estimate of the phase margin of the position controller for each candidate controller design. A simple optimization algorithm finds the best high-impedance behavior for which the nonideal model meets a desired phase margin requirement. In this way, the approach automates the trade-off between force and impedance bandwidth. The effect of important system parameters on the impedance bandwidth is also analyzed and the proposed method verified with a physical actuator.

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References

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Figures

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Fig. 1

Validation platform used in this paper: SA-SEA, an industrial grade series elastic actuator [1]

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Fig. 2

The cascaded impedance controller of SA-SEA

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Fig. 3

Simplified model of the SEA in a high-impedance environment (actuator output is rigidly grounded)

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Fig. 4

Phase margin surface

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Fig. 5

Phase margin surface (top view)

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Fig. 6

Impedance controller step response for selected feedback gains

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Fig. 7

Phase margin surfaces for system with time delay and derivative filters (lower) and without time delay and derivative filters (upper)

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Fig. 8

Top view of the phase margin surfaces for system with time delay and derivative filters and without time delay and derivative filters

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Fig. 9

Comparison of various cases on impedance behavior of the system (DF: derivative filtering and TD: time delay)

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Fig. 10

The effect of impedance control feedback time delay on step response. Force control loop time delay, Tf, is 0.5 ms for all cases.

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Fig. 11

Time delay effect on impedance bandwidth

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Fig. 12

Derivative filtering effect on impedance bandwidth

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Fig. 13

Joint position step responses with the automatically selected feedback gains for the SA-SEA

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Fig. 14

Bode plots of the gain selection experiments on the SA-SEA (f = 0.1–20 Hz)

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