Optimal Control for Clutched-Elastic Robots: A Contact-Implicit Approach

TL;DR

This paper presents a contact-implicit optimization method for controlling clutched-elastic robots, enabling automatic discovery of clutch sequences and improved dynamic motion control.

Contribution

It introduces a novel contact-implicit scheme that optimizes control inputs and clutch sequences simultaneously for clutched-elastic robots.

Findings

Effective control of a double pendulum with clutch-based actuators

Automatic clutch sequence discovery improves robot dynamics

Method reduces unnecessary clutch transitions

Abstract

Intrinsically elastic robots surpass their rigid counterparts in a range of different characteristics. By temporarily storing potential energy and subsequently converting it to kinetic energy, elastic robots are capable of highly dynamic motions even with limited motor power. However, the time-dependency of this energy storage and release mechanism remains one of the major challenges in controlling elastic robots. A possible remedy is the introduction of locking elements (i.e. clutches and brakes) in the drive train. This gives rise to a new class of robots, so-called clutched-elastic robots (CER), with which it is possible to precisely control the energy-transfer timing. A prevalent challenge in the realm of CERs is the automatic discovery of clutch sequences. Due to complexity, many methods still rely on pre-defined modes. In this paper, we introduce a novel contact-implicit scheme designed to optimize both control input and clutch sequence simultaneously. A penalty in the objective function ensures the prevention of unnecessary clutch transitions. We empirically demonstrate the effectiveness of our proposed method on a double pendulum equipped with two of our newly proposed clutch-based Bi-Stiffness Actuators (BSA).