Abstract
To control highly-dynamic compliant motions such as running or hopping, vertebrates rely on reflexes and Central Pattern Generators (CPGs) as core strategies. However, decoding how much each strategy contributes to the control and how they are adjusted under different conditions is still a major challenge. To help solve this question, the present paper provides a comprehensive comparison of reflexes, CPGs and a commonly used combination of the two applied to a biomimetic robot. It leverages recent findings indicating that in mammals both control principles act within a low-dimensional control submanifold. This substantially reduces the search space of parameters and enables the quantifiable comparison of the different control strategies. The chosen metrics are motion stability and energy efficiency, both key aspects for the evolution of the central nervous system. We find that neither for stability nor energy efficiency it is favorable to apply the state-of-the-art approach of a continuously feedback-adapted CPG. In both aspects, a pure reflex is more effective, but the pure CPG allows easy signal alteration when needed. Additionally, the hardware experiments clearly show that the shape of a control signal has a strong influence on energy efficiency, while previous research usually only focused on frequency alignment. Both findings suggest that currently used methods to combine the advantages of reflexes and CPGs can be improved. In future research, possible combinations of the control strategies should be reconsidered, specifically including the modulation of the control signal's shape. For this endeavor, the presented setup provides a valuable benchmark framework to enable the quantitative comparison of different bioinspired control principles.
Highlights
It is one of the longest standing goals of neuroscience to decode how the mammalian central nervous system (CNS) controls locomotion
By assessing the system’s stability and energy efficiency in simulations and hardware experiments, we quantified the contributions that reflexes and Central Pattern Generators (CPGs) have on highly-dynamic compliant movements under different environmental influences
Considering robotic applications, the key findings of our research extend previous knowledge about reflexes and CPGs, which suggests the need to reevaluate current methods to combine both strategies in one controller
Summary
It is one of the longest standing goals of neuroscience to decode how the mammalian central nervous system (CNS) controls locomotion. Until the beginning of the twentieth century, it was widely assumed that such locomotion was purely triggered and controlled by sensory feedback in the form of reflexes. Brown (1911) questioned this assumption and proposed instead the presence of what is known as Central Pattern Generators (CPGs): coordinated patterns of periodic activity that emerge without periodic input from sensory feedback from higher control centers (Ijspeert, 2008). Experiments have supported the notion that both control mechanisms, reflexes and CPGs, are important to control highly-dynamic compliant movements in all limbs (Ivanenko et al, 2006), even when they are not primarily used for locomotion (Zehr and Chua, 2000). The degree to which reflexes and CPGs influence the control of highly-dynamic compliant movements is still a fundamental open question in neurosience
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