Virtual Energy Regulator: A Time-Independent Solution for Control of Lower Limb Exoskeletons

In this letter, we introduce a novel control strategy called Virtual Energy Regulator (VER) for lower limb rehabilitation exoskeletons. Unlike the conventional trajectory tracking controllers, VER, which is a time-independent controller, does not control the exoskeleton joints over a reference trajectory. Instead, it imposes a constraint to the state-space and consequently creates a limit cycle for each joint. The time-independent property of VER can resolve the human-exoskeleton coordination problem. The analytical perspectives of VER are studied in detail where we present the limit cycle existence conditions, considerations for the definition of the desired limit cycles, convergence proof, and limit-cycles synchronization. Finally, to support the presented mathematics, we apply the designed VER on Indego exoskeleton (without human) to perform a limit-cycle behavior similar to walking. The experimental and simulation results show that VER generates stable and synchronized limit-cycles at the joints. Our experimental and simulation results support our analytical findings and demonstrate the efficacy of VER for lower limb exoskeletons.