Stable dynamic interactions with the environment is necessary for any human-machine interface. The 'interaction controllers' for such machine interfaces need to satisfy requirements for 'coupled stability' (see Colgate_1988.pdf, attached). The approach for designing such 'interaction controller' is very different from the approach to servo design. A necessary and sufficient condition to guarantee the stability of a linear system coupled to a passive hamiltonian environment is that the driving point impedance of the system is positive real (see Colgate_1988.pdf, attached).

The interactions in such a coupled system can be analyzed in terms of transfer of energy between the human and the machine. For example, if the machine generates a disturbance and injects energy into the coupled system then human needs to act as a energy sink and absorb that energy within a certain time period such that the system states remain bounded. The objective of this project is to investigate such human-machine interactions during biped tasks such as standing and walking. The machine generates disturbance/perturbation via powered ankle-foot orthosis which is driven with a coupled bicore oscillator (Fig. 1).

Coupled Bicore
Fig. 1: Mark Tilden's Coupled Bicore

Nervous neurons and coupled bicore oscillators pioneered by Mark Tilden work on principles very similar to cellular automata. Each bicore follows very simple rules and when they are coupled together, they can generate complex behaviors. The coupled bicores can be connected to DC motors which act as a sensor (via back-emf) as well as an actuator. In theory, such a coupled system is capable of generating 'reflex behavior' similar to for example strech reflex. The investigation of such 'reflexes', learning, and group behavior in coupled bicore oscillators during interactions with a human and their applications in powered orthosis/prosthesis is the long-term goal of this project.

During three weeks at Telluride Neuromorphic Cognition Engineering Workshop 2010, we plan to investigate how healthy human adapts to bilateral ankle plantar-flexion/dorsi-flexion disturbances induced by a powered orthosis (Fig. 2) while s/he maintains standing balance. The coupled bicore oscillator produces cyclic plantar-flexion/dorsi-flexion moment (constant magnitude) at a different frequencies and modes (i.e. left ankle in-phase or out-of-phase with the right ankle). The center of pressure of the ground reaction forces is monitored with a Wii Balance Board, and the response of the human neuromuscular controller is monitored using Electromyogram from ankle plantar-flexor (Gastrocnemius) and dorsi-flexor (Tibialis Anterior) muscles. It is postulated that the human neuromuscular system will try to reject/damp the disturbances using feedforward and feedback control strategies, may be through causal inference/internal model of the disturbance (see Konrad_CausalInference.pdf, attached).

powered ankle-foot orthosis
Fig. 2: Powered ankle-foot orthosis driven by coupled bicore oscillator