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TECHNICAL PAPERS

Simultaneous Force and Stiffness Control of a Pneumatic Actuator

[+] Author and Article Information
Xiangrong Shen, Michael Goldfarb

Department of Mechanical Engineering, Vanderbilt University, Nashville, TN 37235

J. Dyn. Sys., Meas., Control 129(4), 425-434 (Jan 05, 2007) (10 pages) doi:10.1115/1.2745850 History: Received January 09, 2006; Revised January 05, 2007

This paper proposes a new approach to the design of a robot actuator with physically variable stiffness. The proposed approach leverages the dynamic characteristics inherent in a pneumatic actuator, which behaves in essence as a series elastic actuator. By replacing the four-way servovalve used to control a typical pneumatic actuator with a pair of three-way valves, the stiffness of the series elastic component can be modulated independently of the actuator output force. Based on this notion, the authors propose a control approach for the simultaneous control of actuator output force and stiffness. Since the achievable output force and stiffness are coupled and configuration-dependent, the authors also present a control law that provides either maximum or minimum actuator output stiffness for a given displacement and desired force output. The general control and maximum/minimum stiffness approaches are experimentally demonstrated and shown to provide high fidelity control of force and stiffness, and additionally shown to provide a factor of 6 dynamic range in stiffness.

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Copyright © 2007 by American Society of Mechanical Engineers
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Figures

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Figure 1

Pneumatic actuator controlled by pair of three-way valves

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Figure 2

Experimental setup for the second set of experiments, in which force and stiffness control were performed in the presence of motion control. The experimental setup for the first set of experiments was essentially identical, except the inertial load was clamped at x=0.

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Figure 3

Simultaneous force and stiffness tracking, each of 4.0Hz sinusoid, with piston rod constrained at x=0

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Figure 4

Force tracking of 0.25Hz sinusoid and simultaneous stiffness tracking of 4.0Hz sinusoid with piston rod constrained at x=0. The pressure trajectories (in chambers a and b, respectively) that generate the force and stiffness tracking are shown below the force and stiffness plots.

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Figure 5

Force tracking of 4.0Hz sinusoid and simultaneous stiffness tracking of 0.25Hz sinusoid with piston rod constrained at x=0. The pressure trajectories (in chambers a and b, respectively) that generate the force and stiffness tracking are shown below the force and stiffness plots.

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Figure 6

Force and stiffness tracking with force commands generated by the position control loop. The plots show actual and desired position tracking of a 0.5Hz sinusoidal motion and corresponding error, force tracking of the force command resulting from the position control loop and corresponding error, and simultaneous stiffness tracking for commanded sinusoidal stiffness variation of 0.5Hz, with corresponding stiffness error.

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Figure 7

Force and stiffness tracking with force commands generated by the position control loop. The plots show actual and desired position tracking of a 2.0Hz sinusoidal motion and corresponding error, force tracking of the force command resulting from the position control loop and corresponding error, and simultaneous stiffness tracking for commanded sinusoidal stiffness variation of 4.0Hz, with corresponding stiffness error.

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Figure 8

Force tracking with commands generated by the position control loop, while maximizing the output stiffness of the actuator. The plots show actual and desired position tracking of a 0.5Hz sinusoidal motion, force tracking corresponding to the force command resulting from the position control loop, and the corresponding stiffness of the actuator.

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Figure 9

Force tracking with commands generated by the position control loop, while minimizing the output stiffness of the actuator. The plots show actual and desired position tracking of a 0.5Hz sinusoidal motion, force tracking corresponding to the force command resulting from the position control loop, and the corresponding stiffness of the actuator.

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