Conference Paper

Swing leg retraction helps biped walking stability

Delft Univ. of Technol.
DOI: 10.1109/ICHR.2005.1573583 Conference: Humanoid Robots, 2005 5th IEEE-RAS International Conference on
Source: IEEE Xplore


In human walking, the swing leg moves backward just prior to ground contact, i.e. the relative angle between the thighs is decreasing. We hypothesized that this swing leg retraction may have a positive effect on gait stability, because similar effects have been reported in passive dynamic walking models, in running models, and in robot juggling. For this study, we use a simple inverted pendulum model for the stance leg. The swing leg is assumed to accurately follow a time-based trajectory. The model walks down a shallow slope for energy input which is balanced by the impact losses at heel strike. With this model we show that a mild retraction speed indeed improves stability, while gaits without a retraction phase (the swing leg keeps moving forward) are consistently unstable. By walking with shorter steps or on a steeper slope, the range of stable retraction speeds increases, suggesting a better robustness. The conclusions of this paper are therefore two-fold; (1) use a mild swing leg retraction speed for better stability, and (2) walking faster is easier

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    • "For a simple biped model walking passively down a shallow slope, Wisse et al. [4] showed that all gaits without retraction are consistently unstable whereas a mild retraction speed improves the stability by reducing the size of the eigenvalues of the linearized Poincaré map. With smaller eigenvalues the transient response decays faster, implying a faster recovery after small disturbances [5]. "
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    ABSTRACT: In biped walking, swing-leg retraction can reduce the energy loss at heel-strike by reducing the foot speed relative to the ground at touch-down. However, it also takes extra effort to brake and then accelerate the swing leg in the rearward direction. This energetic trade-off of retraction is influenced by the mechanical coupling with the stance leg preemptive push-off, and changes with different step lengths and speeds. Previously it has not been clear under which circumstances a retracting hip torque has a net energetic benefit (if ever). Here, using a simple biped model probed with numerical and analytic methods, we show how the effectiveness of leg retraction is influenced by actuator efficiencies for positive and negative work. As the efficiency of negative work decreases, the energetic advantage of retracting hip torque is found (mainly) for longer steps, whereas at shorter steps the hip joint extends (not retracts) prior to heel-strike. For fast walking, however, a braking hip torque is still required at the end of swing phase to ensure heel-strike.
    Full-text · Conference Paper · Sep 2014
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    • "moving the swing foot closer to the stance leg) during the time period in which heel strike is expected to occur (Fig. 3). We believe that a slight swing leg retraction increases gait stability [15] because when the robot is traveling faster than normal, heel strike occurs sooner and the step length is longer, while heel strike occurs later and the step length is shorter for slower velocities. Because longer steps have impact angles further from vertical, they dissipate more energy during heel strike and slow the robot down. "
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    ABSTRACT: We describe three bipedal robots that are designed and controlled based on principles learned from the gaits of passive dynamic walking robots. This paper explains the common control structure and design procedure used to determine the mechanical and control parameters of each robot. We present this work in the context of three robots: Denise, the Delft pneumatic biped, R1, a highly backdrivable electric biped, and R2, a hydraulic biped. This work illustrates the application of passive dynamic principles to powered systems with significant control authority
    Full-text · Conference Paper · Feb 2005
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    • "We propose to make step length a function of step time, as step time is an indicator of the walker's kinetic energy content. The proposed function creates a negative relation between step length and step time, as longer step time indicates smaller kinetic energy content and longer step lengths induce larger energy loss at impact [5]. The same relation has previously been applied in a running model by Seyfarth et al. [6]. "
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    ABSTRACT: We propose that highly dynamic whole-body motions should be analyzed, realized and exploited for extending the capability of humanoid robots in the real world. Such motions are very different from the today's standard humanoid behaviors such as stable ZMP-based biped walking, and upper-body motion assuming the stability of the lower-body. The kind of motions we are interested are not locally stable states. Sometimes they include diverging trajectories. In this paper, we focus on one example of such motions: a roll-and-rise motion, in which the robot stands up in one action from lying state. It first swings up both of its legs high, swings them down, rolling forward and up on both feet, then extends the legs to achieve the standing posture. Analysis of the dynamics governing the motions is carried out, and some boundary conditions for successful motions are presented. Our current goal is to identify essential minimum control laws that assure the success of the task. In search of them, a series of systematic simulation experiments are carried out to plot the parameter regions which define success or failure. Experiments with real adult-size humanoid robot are also presented.
    Full-text · Conference Paper · Nov 2003
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