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.
Oscillatory behavior is important for tasks such as walking and running. We are developing methods to add energy to enhance or vary the oscillatory behavior based on the system’s phase angle. We define a nonlinear oscillator using a forcing function based on the sine and cosine of the system’s phase angle that can modulate the amplitude and frequency of oscillation. The stability of the system is proved using the Poincaré-Bendixson criterion. Linear and rotational mechanical systems are simulated using our phase controller. The method is implemented and tested to control a pendulum. Lastly, we propose how to assist hip motion during walking using the phase-based forcing function.
As the robots are used in some of the extreme environments such as battle fields, disaster sites or extra planetary exploration, it is necessary to provide robust locomotive strategies that are optimal to each terrain. The hopping gait has been well studied in robotics and proven to be a potential method for locomotion in some of these areas. There have been some difficulties in developing terrain following controllers that maintain robust, steady state hopping gait which is disturbance resistant. This paper discusses controller strategies that have shown the ability to produce the robust hopping gait. An adaptive oscillator controller presented is this paper works remarkably well, both in simulation and experiments. In this paper, novel control methods based on the phase angle are used to control oscillating robotic systems. The phase oscillator uses the sine and cosine of the phase angle to change key properties of a mass-spring-damper system, including amplitude, frequency, and equilibrium. A hopping robot is used to show the applications of this phase oscillator. This paper presents the theoretical framework, simulations and experimental results.
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