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Research Papers

Pneumatic Variable Series Elastic Actuator

[+] Author and Article Information
Hao Zheng, Molei Wu, Xiangrong Shen

Department of Mechanical Engineering,
The University of Alabama,
359 H. M. Comer Hall,
Box 870276,
Tuscaloosa, AL 35487-0276

Contributed by the Dynamic Systems Division of ASME for publication in the JOURNAL OF DYNAMIC SYSTEMS, MEASUREMENT, AND CONTROL. Manuscript received June 22, 2015; final manuscript received May 6, 2016; published online June 15, 2016. Assoc. Editor: Zongxuan Sun.

J. Dyn. Sys., Meas., Control 138(8), 081011 (Jun 15, 2016) (10 pages) Paper No: DS-15-1283; doi: 10.1115/1.4033620 History: Received June 22, 2015; Revised May 06, 2016

Inspired by human motor control theory, stiffness control is highly effective in manipulation and human-interactive tasks. The implementation of stiffness control in robotic systems, however, has largely been limited to closed-loop control, and suffers from multiple issues such as limited frequency range, potential instability, and lack of contribution to energy efficiency. Variable-stiffness actuator represents a better solution, but the current designs are complex, heavy, and bulky. The approach in this paper seeks to address these issues by using pneumatic actuator as a variable series elastic actuator (VSEA), leveraging the compressibility of the working fluid. In this work, a pneumatic actuator is modeled as an elastic element with controllable stiffness and equilibrium point, both of which are functions of air masses in the two chambers. As such, for the implementation of stiffness control in a robotic system, the desired stiffness/equilibrium point can be converted to the desired chamber air masses, and a predictive pressure control approach is developed to control the timing of valve switching to obtain the desired air mass while minimizing control action. Experimental results showed that the new approach in this paper requires less expensive hardware (on–off valve instead of proportional valve), causes less control action in implementation, and provides good control performance by leveraging the inherent dynamics of the actuator.

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References

Figures

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Fig. 1

Double-acting pneumatic cylinder with two independently controlled chambers

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Fig. 2

Stiffness control of a pneumatic actuator: (a) Controller structure and (b) system diagram

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Fig. 3

The predictive pressure controller

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Fig. 4

Experimental setup

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Fig. 5

Piston position trajectory in the pneumatic VSEA experiment

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Fig. 6

Desired versus measured actuator force in the pneumatic VSEA experiment

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Fig. 7

Measured versus desired spring behavior in the pneumatic VSEA experiment

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Fig. 8

Solenoid valve commands for chambers a and b in the pneumatic VSEA experiment

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Fig. 9

Piston position trajectory in the experiment that involves the transition between two different sets of stiffness/equilibrium point

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Fig. 10

Desired versus measured actuator force in the experiment that involves the transition between two different sets of stiffness/equilibrium point

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Fig. 11

Piston position trajectory in the closed-loop implemented stiffness control experiment

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Fig. 12

Desired versus measured actuator force in the closed-loop implemented stiffness control experiment

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Fig. 13

Measured versus desired spring behavior in the closed-loop implemented stiffness control experiment

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Fig. 14

Proportional valve command (normalized to −1 to 1 range) in the closed-loop implemented stiffness control experiment

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