Inclusion of the Actuators Dynamics for Soft Robot Control

Abstract

During the last decade there has been a growth of interest in the field of soft robots. The natural compliant motion of these systems promises a safer human-machine interaction and robust locomotion and manipulation in unstructured environments. The possibility to use its own body to absorb or conserve energy represents a fundamental advantage not found in traditional rigid robots. However, in order to achieve this desired level of performance, there are still many theoretical and technical advances needed, many of which are related to model-based control. So far, the control strategies are based on either point-to-point motion through the use of static models, or dynamic controllers formulated in end-effector or configuration space, which neglect the dynamics of the actuation system. This thesis analyzes the influence of such actuators dynamics in soft robots for trajectory tracking and investigates how this problem can be tackled with a singular perturbation approach for the case of a tendon-driven planar continuum mechanism. First, the equations of motion of the mechanism are obtained using the variable length constant curvature model and the Euler-Lagrange method. Then, the control laws for singular perturbation based controllers are proposed. A parameter identification phase and validation of the kinematic model are performed. Numerical simulations and real experiments in a testbed for different tendon stiffness are made and analyzed, comparing singular perturbation with baseline feedback controllers. Additionally, the closed loop frequency response for each control law is assessed. The results showed how low tendon stiffness imposes a great limitation in the control performance for the standard feedback controllers. On its turn, by considering the coupling dynamics and introducing a force tracking inner loop, singular perturbation controllers were capable to damp vibrations and achieve lower control errors, while not relying on high gains. The inclusion of the motors inertial and damping forces also proved to be fundamental for high frequency motion.