Limit Cycles to Enhance Human Performance Based on Phase Oscillators

Abstract

Wearable robots including exoskeletons, powered prosthetics, and powered orthotics must add energy to the person at an appropriate time to enhance, augment, or supplement human performance. This “energy pumping” at resonance can reduce the metabolic cost of performing cyclic tasks. Many human tasks such as walking, running, and hopping are repeating or cyclic tasks where assistance is needed at a repeating rate at the correct time. By utilizing resonant energy pumping, a tiny amount of energy is added at an appropriate time that results in an amplified response. However, when the system dynamics is varying or uncertain, resonant boundaries are not clearly defined. We have developed a method to add energy at resonance so the system attains the limit cycle based on a phase oscillator. The oscillator is robust to disturbances and initial conditions and allows our robots to enhance running, reduce metabolic cost, and increase hop height. These methods are general and can be used in other areas such as energy harvesting.

An admittance shaping controller for exoskeleton assistance of the lower extremities

“…Also, the view of human gait as a stable limit cycle has led to the emergence of oscillator-based exoskeleton control. Relevant methods include adding energy at resonance via a phase oscillator (Sugar et al 2015), and synchronizing exoskeleton torques to the user's lower-limb trajectory (Ronsse et al 2011) or to muscle torques (Aguirre-Ollinger 2015, 2013) using adaptive frequency oscillators (AFOs). One of the simplest strategies for exoskeleton control is to exploit the uniformity of the human gait cycle when walking at a constant speed.…”

“…In the human gait cycle, the swing phase takes advantage of pendulum dynamics of the leg (Kuo 2002;Sugar et al 2015). Therefore, for the present study we shall model the leg as a linear rotational pendulum.…”

Integral admittance shaping: A unified framework for active exoskeleton control

“…Current control methods include EMG feedback [29,15], linking exoskeleton torques to the phases of the gait cycle [30,20,31,17], facilitating a clinically correct gait via soft constraints [32] and modifying the dynamic response of the lower limbs by means of active admittance [33] or generalized elasticities [34]. Additionally, the view of human gait as a stable limit cycle has led to the emergence of oscillator-based controls [35,36,37].…”

Adaptive Natural Oscillator to exploit natural dynamics for energy efficiency

“…A sinusoidal joint trajectory is the simplest form that meets these properties and is the core of Phase Oscillators. Due to their simplicity, Phase Oscillators are extensively used in robotic applications [17][18][19][20]. Moreover, implementing Phase Oscillators using dynamical systems, such as Hopf and Van Der Pol oscillators [21], offers us stable limit cycles with advantageous properties such as smooth convergence behavior; see [22] for application in swimming robots.…”